Professor Stephen Sweeney


Professor of Physics
PhD BSc CertEd FInstP CPhys FSPIE
+44 (0)1483 689406
12 ATI 01
Centre Secretary: Mrs Nicole Steward
+44 (0)1483 689859

Biography

Areas of specialism

Photonics; Semiconductor Physics; Semiconductor Materials; Semiconductor Devices; Laser Physics

My qualifications

1995
BSc Applied Physics
University of Bath
1995
CertEd (Education)
University of Bath
1999
PhD Semiconductor Laser Physics
University of Surrey

Previous roles

2015 - 2018
Head of Department of Physics
Department of Physics, University of Surrey
2010 - 2015
Head of Photonics group
Advanced Technology Institute, University of Surrey
2009 - 2015
EPSRC Leadership Fellow
Engineering and Physical Sciences Research Council
01 January 2015 - 31 December 2015
President of the Physics and Astronomy Section
British Science Association

Research

Research interests

Courses I teach on

Undergraduate

Postgraduate taught

My publications

Publications

Sweeney SJ, Lock DA, Adams AR, Menendez J, VanDeWalle CG (2005)Carrier recombination in InGaAs(P) quantum well laser structures: Band gap and temperature dependence, In: Physics of Semiconductors, Pts A and B772pp. 1545-1546
Sweeney SJ, Lyons LJ, Lock D, Adams AR (2002)Direct measurement of facet temperature up to the melting point and COD in high power 980 nm semiconductor diode lasers, In: Conference Digest - IEEE International Semiconductor Laser Conferencepp. 161-162
The high-energy emission from high power lasers was measured and the facet temperature was extracted. Severe heating was observed up to the onset of catastrophic optical damage (COD). The results showed that under high power operation, the laser facet heat-ups to the melting point of GaAs caused the facet to melt.
Marko I. P., Adams A. R., Sweeney S. J., Jin S. R., Murdin B. N., Schwertberger R., Somers A., Reithmaier J. P., Forchel A. (2004)Experimental investigations into the thermal properties of 1.5-1.8-/spl mu/m InAs/InP quantum dash laserspp. 61-62

We present what we believe to be the first ever high-pressure and spontaneous emission measurements on quantum dash lasers. The results show that temperature sensitivity of these lasers is caused by nonradiative processes, which depend on the lasing wavelength.

Knowles G, Fehse R, Tomić S, Sweeney SJ, Sale TE, Adams AR, O'Reilly EP, Steinle G, Riechert H (2002)The temperature and pressure dependence of 1.3 μm GaInNAs vertical-cavity surface emitting lasers (VCSELs), In: Conference Digest - IEEE International Semiconductor Laser Conferencepp. 139-140
The temperature and pressure dependence of the threshold current of GaInNAs based vertical-cavity surface-emitting lasers (VCSEL) were studied. The temperature variation of the main recombination processes measured in GaInNAs edge emitting lasers (EEL) was used with the same active regions for calculating the temperature and pressure dependence of the threshold current density of VCSELs. It was shown that the VCSEL has the cavity mode on the low energy side of the gain peak at room temperature by comparing the actual lasing photon energies.
Sweeney SJ, Jin SR, Fehse R, Adams AR, Higashi T, Riechert H, Thijs PJA (2002)A comparison of the thermal stability of InGaAsP, AlGaInAs and GaInNAs quantum-well lasers for 1.3 μm operation, In: Conference Digest - IEEE International Semiconductor Laser Conferencepp. 43-44
The thermal stabilities of InGaAsP, AlGaInAs and GaInNAs quantum-well lasers for 1.3 μm operation were compared. The optical properties and temperature characteristics of GaInNAs quantum-well (QW) lasers were investigated. It was found that defect-related non-radiative recombination made a significant contribution to the total threshold current in the GaInNAs system, while the Auger recombination process made an increasingly significant contribution at higher temperatures.
Jin SR, Sweeney SJ, Knowles G, Adams AR, Higashi T, Riechert H, Thijs PJA (2002)Optical investigation of recombination processes in GaInNAs, InGaAsP and AlGaInAs quantum-well lasers using hydrostatic pressure, In: Conference Digest - IEEE International Semiconductor Laser Conferencepp. 83-84
The recombination processes in GaInNAs, InGaAsP and AlGaInAs quantum well lasers were optically investigated using hydrostatic pressure. The lasing- energy dependence of carrier-recombination in these quantum-well lasers were compared. It was found that the defect-related mono-molecular current at threshold remains nearly constant as a function of lasing energy.
Lock D, Sweeney Stephen, Adams Alfred (2003)Wavelength dependence of catastrophic optical damage threshold in 980nm semiconductor diode lasers, In: 2003 IEEE LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2pp. 421-422

We investigate the wavelength dependence of the catastrophic optical damage current in 980nm lasers. Using high pressure and low temperature techniques, we find an intrinsic dependence of this threshold on wavelength.

Tomić S, Fehse R, Choulis SA, O'Reilly EP, Adams AR, Sweeney SJ, Andreev AD, Hosea TJC, Riechert H (2002)Experimental and theoretical analysis of the recombination processes in GaInNAs 1.3 μm Lasers, In: Conference Digest - IEEE International Semiconductor Laser Conferencepp. 41-42
The recombination processes in GaInNAs 1.3 μm lasers were analyzed theoretically and experimentally. The threshold current was determined by measuring the light emitted from the lasers. The variation of threshold current with temperature and pressure the for quantum well devices was also studied.
Fehse R, Sweeney Stephen, Adams Alfred, McConville D, Riechert H, Geelhaar L (2004)Influence of growth temperature on defect density in 1.3 mu m GaInNAs-based lasers, In: Messe K, (eds.), 2004 IEEE 19TH INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE, CONFERENCE DIGESTpp. 85-86

We show that the dramatic changes in threshold current density with changing active region growth temperature in 1.3mum GaInNAs-based lasers can be attributed nearly entirely to changes in the defect related monomolecular recombination current.

Marko I P, Adams A R, Sweeney S J, Jin S R, Murdin B N, Schwertberger R, Somers A, Reithmaier J P, Forchel A (2004)Experimental Investigations into the Thermal Properties of 1.5-1.8μm InAs/InP Quantum Dash Lasers, In: 2004 IEEE 19th International Semiconductor Laser Conference, 2004. Conference Digest.

We present what we believe to be the first ever high-pressure and spontaneous emission measurements on quantum dash lasers. The results show that temperature sensitivity of these lasers is caused by nonradiative processes, which depend on the lasing wavelength.

Phillips AF, Sweeney SJ, Adams AR, Thijs PJA (1999)The hydrostatic pressure dependence of the threshold current in 1.3 mu m InGaAsP quantum well semiconductor diode lasers, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH211(1)pp. 513-518
Walker AB, Thijs PJA, Sweeney SJ, Adams AR, Silver M, O'Reilly EP, Watling JR (1999)Dependence of threshold current on QW position and on pressure in 1.5 mu m InGaAs(P) lasers, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH211(1)pp. 525-531
Sweeney SJ, Higashi T, Adams AR, Uchida T, Fujii T (1998)Improved temperature dependence of 1.3 mu m AlGalnAs-based MQW semiconductor diode lasers revealed by hydrostatic pressure, In: ELECTRONICS LETTERS34(22)pp. 2130-2132 IEE-INST ELEC ENG
Sweeney SJ, Phillips AF, Adams AR, O'Reilly EP, Thijs PJA (1998)The effect of temperature dependent processes on the performance of 1.5-mu m compressively strained InGaAs(P) MQW semiconductor diode lasers, In: IEEE PHOTONICS TECHNOLOGY LETTERS10(8)pp. 1076-1078 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Jin S. R., Ahmad C. N., Sweeney S. J., Adams A. R., Murdin B. N., Page H., Marcadet X., Sirtori C., Tomić S. (2006)Spectroscopy of GaAs/AlGaAs quantum-cascade lasers using hydrostatic pressure, In: Applied Physics Letters221105(2006)
Adams AR, Silver M, OReilly EP, Gonul B, Phillips AF, Sweeney SJ, Thijs PJA (1996)Hydrostatic pressure dependence of the threshold current in 1.5 mu m strained quantum well lasers, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH198(1)pp. 381-388
Knowles G, Tomic S, Jin S, Fehse R, Sweeney SJ, Sale TE, Adams AR (2003)Gain-cavity alignment profiling of 1.3 mu m emitting GaInNAs vertical cavity surface emitting lasers (VCSELs) using high pressure techniques, In: PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS235(2)pp. 480-485
Adams Alfred, Marko Igor, Mukherjee J, Sweeney Stephen, Gocalinska A, Pelucchi E, Corbett B (2014)Semiconductor quantum well lasers with a temperature insensitive threshold current, In: Conference Digest - IEEE International Semiconductor Laser Conferencepp. 82-83
Sweeney Stephen, Fehse R, Adams Alfred, Riechert H (2003)Intrinsic temperature sensitivities of 1.3 mu m GaInNAs/GaAs, InGaAsP/InP and AlGaInAs/InP-based semiconductor lasers, In: 2003 IEEE LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2pp. 39-40 IEEE

The apparent temperature stability of GaInNAs-based lasers is-attributed to significant defect current. By removing this current, GaInNAs devices have a similar temperature dependence to InGaAsP devices whilst AlGaInAs devices are more thermally stable.

Tan SL, Soong WM, Green JE, Steer MJ, Zhang S, Tan LJJ, Ng JS, Marko IP, Sweeney SJ, Adams AR, Allam J, David JPR (2013)Experimental evaluation of impact ionization in dilute nitride GaInNAs diodes, In: APPLIED PHYSICS LETTERS103(10)ARTN 1pp. ?-? AMER INST PHYSICS
Fehse R, Jin S, Sweeney SJ, Adams AR, O'Reilly EP, Illek S, Egorov AY, Riechert H (2001)The temperature dependence of the recombination processes in 1.3 mu m GaInNAs-based edge emitting lasers, In: LEOS 2001: 14TH ANNUAL MEETING OF THE IEEE LASERS & ELECTRO-OPTICS SOCIETY, VOLS 1 AND 2, PROCEEDINGSpp. 330-331

We report on the pressure dependence of the threshold current in 1.3 mum InGaAsP and 1.5 mum InGaAs quantum-well lasers measured at low temperatures similar to100 K. It was found that the threshold current of both devices slowly increases with increasing pressure (i.e., increasing band gap) at similar to100 K consistent with the calculated variation of the radiative current. In contrast, at room temperature we observed a reduction of the threshold current with increasing pressure. Our low-temperature, high-pressure data confirm the results of previous atmospheric pressure measurements on the same devices which indicated a transition in the dominant recombination mechanism from radiative to Auger as the device temperature is increased from similar to100 to 300 K.

Fehse R, Sweeney SJ, Adams AR, McConville D, Riechert H, Geelhaar L (2004)Influence of growth temperature on carrier recombination in GaInNAs-based lasers, In: IEE PROCEEDINGS-OPTOELECTRONICS151(5)pp. 447-451
Adams AR, Marko IP, Mukherjee J, Sweeney SJ, Gocalinska A, Pelucchi E, Corbett B (2014)Semiconductor Quantum Well Lasers with a Temperature Insensitive Threshold Current, In: 2014 24TH IEEE INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE (ISLC 2014)pp. 82-83
Quantum well lasers have been extremely successful in a wide range of applications, with optical fibre communications being of particular importance. However, in spite of their success, their performance is not ideal, for example, the threshold current of semiconductor lasers is often very sensitive to temperature. This has led to the need for thermoelectric coolers and associated control electronics to stabilize the laser temperature, however, such coolers often consume more energy than the laser they are controlling and also add to the overall heat dissipation of the system. Such coolers also tend to have far less long-term reliability than the laser diode itself. There are consequently many circumstances where it would be advantageous and far cheaper to simply compensate for temperature variations by mechanisms built into the epitaxial structure of the laser chip itself. This paper focuses on a new design and demonstration of a MQW laser structure which can overcome the intrinsic temperature sensitivity of the laser.
Higashi T, Sweeney SJ, Phillips AF, Adams AR, O'Reilly EP, Uchida T, Fujii T (1999)Experimental analysis of temperature dependence in 1.3-mu m AlGaInAs-InP strained MQW lasers, In: IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS5(3)pp. 413-419 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Higashi T, Sweeney SJ, Phillips AF, Adams AR, O'Reilly EP, Uchida T, Fujii T (1999)Observation of reduced nonradiative current in 1.3-mu m AlGaInAs-InP strained MQW lasers, In: IEEE PHOTONICS TECHNOLOGY LETTERS11(4)pp. 409-411 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
O'Brien K, Sweeney SJ, Adams AR, Murdin BN, Salhi A, Rouillard Y, Joullie A (2006)Recombination processes in midinfrared InGaAsSb diode lasers emitting at 2.37 mu m, In: APPLIED PHYSICS LETTERS89(5)ARTN 0pp. ?-? AMER INST PHYSICS
Sweeney SJ, Jin SR, Ahmad CN, Adams AR, Murdin BN (2004)Carrier recombination processes in 1.3 mu m and 1.5 mu m InGaAs(P)-based lasers at cryogenic temperatures and high pressures, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH241(14)pp. 3399-3404
Soong WM, Ng JS, Steer MJ, Hopkinson M, David JPR, Chamings J, Sweeney SJ, Adams AR, Allam J (2008)Dark current mechanisms in bulk GaInNAs photodiodes, In: Proccedings of 20th International Conference on Indium Phosphide and Related Materials
We have grown a series of bulk GaInNAs p-i-n diodes and identified some of the dark current mechanisms present in our devices. With a nitrogen composition of ~4 %, the band gap can be reduced to 0.94 eV. We also demonstrate that low dark current density is achievable without compromising the absorption and hence quantum efficiency up to 1.4 mum.
Sweeney SJ, Higashi T, Adams AR, Uchida T, Fujii T (2000)A comparison of AlGaInAs and InGaAsP-based 1.3 mu m semiconductor lasers using high pressure, In: HIGH PRESSURE RESEARCH18(1-6)pp. 49-55
Sayid SA, Marko IP, Adams AR, Sweeney SJ, Barrios P, Poole P (2010)Thermal behavior of 1.55 μm (100) InAs/InP-based quantum dot lasers, In: IEEE Poceeedings of 22nd International Semiconductor Laser Conferencepp. 75-76
Unlike InAs/GaAs quantum dot lasers, in 1.55μm InAs/InP devices, non-radiative recombination dominates device behavior from very low temperature (~40K) and accounts for ~94% of Jth at room temperature with a To of ~72K from 220K-290K.
Ng JS, Tan SL, Goh YL, Tan CH, David JPR, Allam J, Sweeney SJ, Adams AR (2010)InGaAsN as absorber in APDs for 1.3 micron wavelength applicationspp. 187-190
Two issues with using InGaAsN as absorber in avalanche photodiodes (APDs) for 1310nm wavelength applications are addressed here. Firstly, we demonstrated InGaAsN p-i-n diodes with stable photoresponse around 1310nm but reverse leakage current density slightly above the acceptable limit of ~0.2mA/cm2 at 150kV/cm. We also investigated whether or not InGaAsN as absorber is compatible with Al0.8Ga0.2As (the proposed avalanche material in our separate-absorption-multiplication APD design) in terms of the relationship between α and β in InGaAsN. Our observations suggest α ~ β in InGaAsN, making it compatible with Al0.8Ga0.2As.
Sweeney SJ, McConville D, Masse NF, Bouyssou RX, Adams AR, Ahmad CN, Hanke C (2004)Temperature and pressure dependence of recombination processes in 1.5 mu m InGaAlAs/InP-based quantum well lasers, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH241(14)pp. 3391-3398
Sale TE, Sweeney SJ, Knowles G, Adams AR (2001)Gain-cavity alignment in efficient visible (660 nm) VCSELs studied using high pressure techniques, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH223(2)pp. 587-591 WILEY-V C H VERLAG GMBH
Fehse R, Sweeney SJ, Adams AR, O'Reilly EP, Egorov AY, Riechert H, Illek S (2001)Insights into carrier recombination processes in 1.3 mu m GaInNAs-based semiconductor lasers attained using high pressure, In: ELECTRONICS LETTERS37(2)pp. 92-93 IEE-INST ELEC ENG
Phillips AF, Sweeney SJ, Adams AR, Thijs PJA (1999)The temperature dependence of 1.3-and 1.5-mu m compressively strained InGaAs(P) MQW semiconductor lasers, In: IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS5(3)pp. 401-412 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Tan SL, Tan LJJ, Goh YL, Zhang S, Ng JS, David JPR, Marko IP, Allam J, Sweeney SJ, Adams AR (2010)Reduction of dark current and unintentional background doping in InGaAsN photodetectors by ex situ annealing, In: Proceedings of SPIE - Posters Session7726
InGaAsN is a promising material system to enable low-cost GaAs-based detectors to operate in the telecommunication spectrum, despite the problems posed by the low growth temperature required for nitrogen incorporation. We demonstrate that InGaAsN p+-i-n+ structures with nominal In and N fraction of 10% and 3.8%, grown by molecular beam epitaxy (MBE) under non-optimal growth conditions, can be optimized by post growth thermal annealing to match the performance of optimally grown structures. We report the findings of an annealing study by comparing the photoluminescence spectra, dark current and background concentration of the as-grown and annealed samples. The dark current of the optimally annealed sample is approximately 2 μA/cm2 at an electric field of 100 kV/cm, and is the lowest reported to date for InGaAsN photodetectors with a cut-off wavelength of 1.3 μm. Evidence of lower unintentional background concentration after annealing at a sufficiently high temperature, is also presented.
Ikyo AB, Marko IP, Adams AR, Sweeney SJ, Bachmann A, Kashani-Shirazi K, Amann M-C (2009)Gain peak-cavity mode alignment optimisation in buried tunnel junction mid-infrared GaSb vertical cavity surface emitting lasers using hydrostatic pressure, In: IET OPTOELECTRONICS3(6)pp. 305-309
Fehse R, Jin S, Sweeney SJ, Adams AR, O'Reilly EP, Riechert H, Illek S, Egorov AY (2001)Evidence for large monomolecular recombination contribution to threshold current in 1.3 mu m GaInNAs semiconductor lasers, In: ELECTRONICS LETTERS37(25)pp. 1518-1520 IEE-INST ELEC ENG
Tan S, Hunter CJ, Zhang S, Tan LJJ, Goh YL, Ng JS, David JPR, Marko Igor, Sweeney Stephen, Adams Alfred, Allam Jeremy (2012)Improved optoelectronic properties of rapid thermally annealed dilute nitride GaInNAs photodetectors, In: Journal of Electronic Materials41(12)pp. 3393-3401 IEEE
We investigate the optical and electrical characteristics of GaInNAs/GaAs long-wavelength photodiodes grown under varying conditions by molecular beam epitaxy and subjected to postgrowth rapid thermal annealing (RTA) at a series of temperatures. It is found that the device performance of the nonoptimally grown GaInNAs p-i-n structures, with nominal compositions of 10% In and 3.8% N, can be improved significantly by the RTA treatment to match that of optimally grown structures. The optimally annealed devices exhibit overall improvement in optical and electrical characteristics, including increased photoluminescence brightness, reduced density of deep-level traps, reduced series resistance resulting from the GaAs/GaInNAs heterointerface, lower dark current, and significantly lower background doping density, all of which can be attributed to the reduced structural disorder in the GaInNAs alloy.© 2012 TMS.
Chamings J, Adams AR, Sweeney SJ, Kunert B, Volz K, Stolz W (2008)Temperature dependence and physical properties of Ga(NAsP)/GaP semiconductor lasers, In: APPLIED PHYSICS LETTERS93(10)ARTN 1pp. ?-? AMER INST PHYSICS
Ng JS, Soong WM, Steer MJ, Hopkinson M, David JPR, Chamings J, Sweeney SJ, Adams AR (2007)Long wavelength bulk GaInNAs p-i-n photodiodes lattice matched to GaAs, In: JOURNAL OF APPLIED PHYSICS101(6)ARTN 0pp. ?-? AMER INST PHYSICS
Lock D, Sweeney SJ, Adams AR, Robbins DJ (2003)Auger recombination in InGaAs/AlGaAs-based MQW semiconductor lasers emitting at 980 nm, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH235(2)pp. 542-546
Fehse R, Adams AR, Sweeney SJ, Tomic S, Reichart H, Ramakrishnan A (2003)Carrier recombination processes in MOVPE and MBE grown 1.3 mu m GaInNAs edge emitting lasers, In: SOLID-STATE ELECTRONICS47(3)pp. 501-506
Adams AR, Fehse R, Tomic S, O'Reilly EP, Andreev A, Knowles G, Sale TE, Sweeney SJ, Steinle G, Ramakrishnan A, Riechert H (2002)Characterisation of 1.3pm wavelength GaInNAs/GaAs edge-emitting and vertical-cavity surface-emitting lasers using low-temperature and high-pressure, In: ChangHasnain CJ, Xia YX, Iga K, (eds.), APOC 2002: ASIA-PACIFIC OPTICAL AND WIRELESS COMMUNICATIONS; MATERIALS AND DEVICES FOR OPTICAL AND WIRELESS COMMUNICATIONS4905pp. 183-197
Fehse R, Tomic S, Adams AR, Sweeney SJ, O'Reilly EP, Andreev A, Riechert H (2002)A quantitative study of radiative, Auger, and defect related recombination processes in 1.3-mu m GaInNAs-based quantum-well lasers, In: IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS8(4)pp. 801-810 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Jin SR, Fehse R, Sweeney SJ, Knowles G, Adams AR, O'Reilly EP, Reichert H, Illek S, Egorov AY, Thijs PJA, Uchida T, Fujii T (2002)Modal refractive index of 1.3 mu m InGaAsP, AlGaInAs and GaInNAs semiconductor lasers under high hydrostatic pressure, In: ELECTRONICS LETTERS38(7)pp. 325-327 IEE-INST ELEC ENG
Jin SR, Sweeney SJ, Tomic S, Adams AR, Riechert H (2003)Unusual increase of the Auger recombination current in 1.3 mu m GaInNAs quantum-well lasers under high pressure, In: APPLIED PHYSICS LETTERS82(14)pp. 2335-2337 AMER INST PHYSICS
Knowles G, Sweeney SJ, Sale TE, Adams AR (2001)Self-heating effects in red (665 nm) VCSELs, In: IEE PROCEEDINGS-OPTOELECTRONICS148(5-6)PII 10.104pp. 256-260 IEE-INST ELEC ENG
O'Reilly EP, Fahy S, Lindsay A, Tomić S, Fehse R, Adams AR, Sweeney SJ, Andreev AD, Klar PJ, Grüning H, Riechert H (2003)Novel electronic and optoelectronic properties of GaInNAs and related alloys, In: OSA Trends in Optics and Photonics Series88pp. 523-525
We overview how the novel electronic structure of dilute nitride alloys modifies the gain characteristics of GaInNAs lasers. Optimised devices should have comparable or better characteristics than InP-based emitters, enabling GaAs-based 1.3 μm vertical emitting lasers. ©2000 Optical Society of America.
Jin SR, Sweeney SJ, Adams AR, Higashi T, Riechert H, Thijs PJA (2003)Wavelength dependence of the modal refractive index in 1.3 mu m InGaAsP, AlGaInAs and GaInNAs lasers using high pressure, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH235(2)pp. 491-495
Jin SR, Sweeney Stephen, Tomic S, Adams Alfred, Riechert H (2003)High-pressure studies of recombination mechanisms in 1.3-mu m GaInNAs quantum-well lasers, In: IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS9(5)pp. 1196-1201 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

The pressure dependence of the components of the recombination current at threshold in 1.3-mum GaInNAs single quantum-well lasers is presented using for the first time high-pressure spontaneous emission measurements up to 13 kbar. It is shown that, above 6 kbar, the rapid increase of the threshold current With increasing pressure is associated with the unusual increase of the Auger-related nonradiative recombination current, while the defect-related monomolecular nonradiative recombination current is almost constant. Theoretical calculations show that the increase of the Auger current can be attributed to a large increase in the threshold carrier density with pressure, Which is mainly due to the increase in the electron effective mass arising from the enhanced level-anticrossing between the GaInNAs conduction band and the nitrogen level.

Adams AR, Thijs PJA, Jin SR, Sweeney SJ (2003)Coupling of large optical loss with Auger recombination in 1.3 mu m InGaAsP lasers investigated using hydrostatic pressure, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH235(2)pp. 547-551
Masse NF, Adams AR, Sweeney SJ (2007)Experimental determination of the band gap dependence of Auger recombination in InGaAs/InP multiple quantum well lasers at room temperature, In: APPLIED PHYSICS LETTERS90(16)ARTN 1pp. ?-? AMER INST PHYSICS

The band gap dependencies of the threshold current and its radiative component are measured using high pressure techniques. Detailed theoretical calculations show that the band gap dependence of the internal losses plays a significant role in the band gap dependence of the radiative current. Temperature dependence measurements show that the radiative current accounts for 20% of the total threshold current at room temperature. This allows us to determine the pressure dependence of the non-radiative Auger recombination current, and hence to experimentally obtain the variation of the Auger coefficient C with band gap. (c) 2007 American Institute of Physics.

O'Brien K, Adams AR, Sweeney SJ, Jin SR, Ahmad CN, Murdin BN, Canedy CL, Vurgaftman I, Meyer JR (2006)Analysis of the major loss processes in mid-infrared type-II "W" diode lasers, In: Conference Digest - IEEE International Semiconductor Laser Conferencepp. 43-44

The results from high-pressure and low-temperature measurements on mid-infrared type-II W-structure lasers suggest that Auger recombination is the major loss process that prevents their continuous-wave operation at room temperature.

Jin SR, Sweeney SJ, Tomic S, Adams AR, Riechert H (2003)Quantifying pressure-dependent recombination currents in GaInNAs lasers using spontaneous emission measurements, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH235(2)pp. 486-490
Crutchley BG, Marko Igor, Adams Alfred, Sweeney Stephen (2013)Investigating the efficiency limitations of GaN-based emitters, In: 2013 Conference on Lasers and Electro-Optics Europe and International Quantum Electronics Conference, CLEO/Europe-IQEC 2013
In this study low temperature and high pressure techniques have been used to investigate the recombination processes taking place in InGaN-based quantum well light emitting diodes (LEDs) which have emission across the blue-green region. Despite relatively high peak efficiencies of the GaN-based emitters, there remain issues relating to the strong efficiency reduction at higher currents that are required for normal operation in most applications. It is observed that there is a relative reduction in efficiency as injection current is increased in a phenonmenon which is known as efficiency droop. There are three main arguments for the cause of efficiency droop that are discussed in the literature: non-radiative Auger recombination, carrier leakage and a defect-related loss mechanism. In spite of extensive research to date, there is little agreement on the cause of efficiency droop as most experiments can only measure the overall efficiency behaviour leading to difficulties in determining the individual contributions from the different loss mechanisms. © 2013 IEEE.
Jin S. R., Sweeney S. J., Ahmad C. N., Adams A. R., Murdin B. N. (2004)Radiative and Auger recombination in 1.3 µm InGaAsP and 1.5 µm InGaAs quantum-well lasers measured under high pressure at low and room temperatures, In: Applied Physics Letters357(2004)

We report on the pressure dependence of the threshold current in 1.3 µm InGaAsP and 1.5 µm InGaAs quantum-well lasers measured at low temperatures ~100 K. It was found that the threshold current of both devices slowly increases with increasing pressure (i.e., increasing band gap) at ~100 K consistent with the calculated variation of the radiative current. In contrast, at room temperature we observed a reduction of the threshold current with increasing pressure. Our low-temperature, high-pressure data confirm the results of previous atmospheric pressure measurements on the same devices which indicated a transition in the dominant recombination mechanism from radiative to Auger as the device temperature is increased from ~100 to 300 K

Masse NF, Marko Igor, Sweeney Stephen, Adams Alfred, Hatori N, Sugarawa M (2005)The influence of p-doping on the temperature sensitivity of 1.3 mu m quantum dot lasers, In: 2005 IEEE LEOS Annual Meeting Conference Proceedings (LEOS)pp. 603-604 IEEE

We find that non-radiative recombination plays an important role in p-doped quantum-dot lasers. Along with carrier thermalisation effects, this is responsible for the temperature insensitive operation as observed around room temperature in these lasers.

Sweeney Stephen, McConville D, Jin SR, Ahmad CN, Masse NF, Bouyssou RX, Adams Alfred, Hanke C (2004)Temperature and wavelength dependence of recombination processes in 1.5 mu m InGaAlAs/InP-based lasers, In: 2004 International Conference on Indium Phosphide and Related Materials, Conference Proceedingspp. 738-741

The improved thermal stability of 1.5 mu m InGaAlAs- compared with InGaAs-based lasers is investigated using a combination of low temperature and high pressure techniques. The results indicate that this is due to lower nonradiative Auger recombination in the InGaAlAs devices because of the higher conduction band offset made possible with the InGaAlAs system which results in a lower hole density in the quantum wells at threshold.

Sweeney SJ, Phillips AF, Adams AR, O'Reilly EP, Thijs PJA (1998)Determination of the influence of Auger recombination on the threshold current of 1.3 μm and 1.5 μm InGaAs(P) strained-layer lasers and its variation with temperature, In: Conference Digest - IEEE International Semiconductor Laser Conferencepp. 63-64
We investigated the influence of Auger recombination from 90 K to above room temperature and found its contribution to the threshold current at 300 K to be about 80% and 50% at 1.5 μm and at 1.3 μm respectively.
Higashi T, Sweeney SJ, Phillips AF, Adams AR, O'Reilly EP, Uchida T, Fujii T (1998)Observation of reduced non-radiative recombination current in 1.3-μm AlGaInAs/InP multiple-quantum-well lasers, In: Conference Digest - IEEE International Semiconductor Laser Conferencepp. 61-62
We investigated experimentally the temperature dependence of the threshold current in 1.3-μm AlGaInAs/InP MQW lasers, and found that compared with GaInAsP/InP devices the higher characteristic temperature was caused by a reduction of the non-radiative recombination current.
Adams Alfred, Marko Igor, Mukherjee J, Stolojan Vlad, Sweeney Stephen, Gocalinska A, Pelucchi E, Thomas K, Corbett B (2015)Semiconductor Quantum Well Lasers With a Temperature-Insensitive Threshold Current, In: IEEE Journal of Selected Topics in Quantum Electronics21(6)150080pp. ?-? IEEE
This paper proposes and demonstrates a new multiquantum well (MQW) laser structure with a temperature-insensitive threshold current and output power. Normally, the mechanisms that cause the threshold current (Ith) of semiconductor lasers to increase with increasing temperature T (thermal broadening of the gain spectrum, thermally activated carrier escape, Auger recombination, and intervalence band absorption) act together to cause Ith to increase as T increases. However, in the design presented here, carriers thermally released from some of the QWs are fed to the other QWs so that these mechanisms compensate rather than augment one another. The idea is in principle applicable to a range of materials systems, structures, and operating wavelengths. We have demonstrated the effect for the first time in 1.5 μm GaInAsP/InP Fabry-Perot cavity edge-emitting lasers. The results showed that it is possible to keep the threshold current constant over a temperature range of about 100 K and that the absolute temperature over which the plateau occurred could be adjusted easily by redesigning the quantum wells and the barriers between them. TEM studies of the structures combined with measurements of the electroluminescent intensities from the wells are presented and explain well the observed effects.
Sweeney SJ, Phillips AF, Adams AR, O'Reilly EP, Silver M, Thijs PJA (1998)Transition from radiative to nonradiative recombination in 1.3-μm and 1.5-μm InGaAs(P) multiple quantum well semiconductor diode lasers, In: Conference on Lasers and Electro-Optics Europe - Technical Digestpp. 304-?
Measurements of the threshold current, Ith as a function of temperature, T were performed on 1.3 μm and 1.5 μm compressively strained lasers from 90 K to 370 K and the temperature sensitivity parameter, To. In addition, L, the integrated spontaneous emission emanating from the side of the devices was collected. By measuring L at the threshold as a function of temperature, it was verified that the relationship To(IRad)= T holds true even to above room temperature.
Marko IP, Masse NF, Sweeney SJ, Adams AR, Hatori N, Sugawara M, Jantsch W, Schaffler F (2007)Recombination, transport and loss mechanisms in p-doped InAs/GaAs quantum dots, In: Physics of Semiconductors, Pts A and B893pp. 837-838
Sweeney SJ, Adams AR, O'Reilly EP, Silver M, Thijs PJA (2000)Effect of auger generated hot-holes on 1.5-μm InGaAs(P)-based quantum well semiconductor lasers, In: Pacific Rim Conference on Lasers and Electro-Optics, CLEO - Technical Digestpp. 391-392
Using a combination of experimental and theoretical techniques, the direct-CHSH process, that produces hot-holes, was found to be the most important Auger process in 1.5-μm semiconductor lasers. Results of the study clearly highlight the design implications of both quantum well placement and total waveguide thickness on laser performance.
Marko IP, Adams AR, Sweeney SJ, Teissier R, Baranov AN, Tomic S (2008)Gamma-L scattering in InAs-based quantum cascade lasers studied using high hydrostatic pressure, In: 2008 IEEE 21ST INTERNATIONAL SEMICONDUCTOR LASER CONFERENCEpp. 47-48
Marko IP, Adams AR, Sweeney SJ, Masse NF, Krebs R, Reithmaier JP, Forchel A, Mowbray DJ, Skolnick MS, Liu HY, Groom KM, Hatori N, Sugawara M (2007)Band gap dependence of the recombination processes in InAs/GaAs quantum dots studied using hydrostatic pressure, In: PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS244(1)pp. 82-86
O'Brien K., Sweeney S. J., Adams A. R., Murdin B. N, Salhi A., Rouillard Y., Joullié A. (2006)Recombination processes in midinfrared InGaAsSb diode lasers emitting at 2.37 µm, In: Applied Physics Letters051104(2006)

The temperature dependence of the threshold current of InGaAsSb/AlGaAsSb compressively strained lasers is investigated by analyzing the spontaneous emission from working laser devices through a window formed in the substrate metallization and by applying high pressures. It is found that nonradiative recombination accounts for 80% of the threshold current at room temperature and is responsible for the high temperature sensitivity. The authors suggest that Auger recombination involving hot holes is suppressed in these devices because the spin-orbit splitting energy is larger than the band gap, but other Auger processes persist and are responsible for the low T0 values.

Tomic S, O'Reilly EP, Fehse R, Sweeney Stephen, Adams Alfred, Andreev Aleksey, Choulis SA, Hosea Thomas, Riechert H (2003)Theoretical and experimental analysis of 1.3-mu m InGaAsN/GaAs lasers, In: IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS9(5)pp. 1228-1238 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

We present a comprehensive theoretical and experimental analysis of 1.3-mum InGaAsN/GaAs lasers. After introducing the 10-band k . p Hamiltonian which predicts transition energies observed experimentally, we employ it to investigate laser properties of ideal and real InGaAsN/GaAs laser devices. Our calculations show that the addition of N reduces the peak gain and differential gain at fixed carrier density, although the gain saturation value and the peak gain as a function of radiative current density are largely unchanged due to the incorporation of N. The gain characteristics are optimized by including the minimum amount of nitrogen necessary to prevent strain relaxation at the given well thickness. The measured spontaneous emission and gain characteristics of real devices are well described by the theoretical model. Our analysis shows that the threshold current is dominated by nonradiative, defect-related recombination. Elimination of these losses would enable laser characteristics comparable with the best InGaAsP/InP-based lasers with the added advantages provided by the GaAs system that are important for vertical integration.

Sweeney SJ, Knowles G, Sale TE, Adams AR (2001)Quantifying the effect of indirect carrier leakage on visible Al(GaInP) lasers using high pressures and low temperatures, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH223(2)pp. 567-572 WILEY-V C H VERLAG GMBH
Sweeney SJ, Higashi T, Andreev A, Adams AR, Uchida T, Fujii T (2001)Superior temperature performance of 1.3 mu m AlGaInAs-Based semiconductor lasers investigated at high pressure and low temperature, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH223(2)pp. 573-579 WILEY-V C H VERLAG GMBH
Marko IP, Adams AR, Sweeney SJ, Teissier R, Baranov AN, Tomic S (2009)Evidence of carrier leakage into the L-valley in InAs-based quantum cascade lasers under high hydrostatic pressure, In: PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS246(3)pp. 512-515
Adams AR, Marko IP, Sweeney SJ, Teissier R, Baranov AN, Tomić S (2009)The effect of hydrostatic pressure on the operation of quantum cascade lasers, In: Proceedings of SPIE - Quantum Cascade Lasers and Applications I7222
Quantum Cascade Lasers (QCLs) have been very successful at long wavelengths, >4μm, and there is now considerable effort to develop QCLs for short wavelength (2-3μm) applications. To optimise both interband and QC lasers it is important to understand the role of radiative and non-radiative processes and their variation with wavelength and temperature. We use high hydrostatic pressure to manipulate the band structure of lasers to identify the dominant efficiency limiting processes. We describe how hydrostatic pressure may also be used to vary the separation between the Γ, Χ and L bands, allowing one to investigate the role of inter-valley carrier scattering on the properties of QCLs. We will describe an example of how pressure can be used to investigate the properties of 2.9-3.3μm InAs/AlSb QCLs. We find that while the threshold current of the 3.3μm devices shows little pressure variation even at room temperature, for the 2.9μm devices the threshold current increases by ~20% over 4kbar at 190K consistent with carrier scattering into the L-minima. Based on our high pressure studies, we conclude that the maximum operating temperature of InAs/AlSb QCLs decreases with decreasing wavelength due to increased carrier scattering into the L-minima of InAs.
Jin SR, Ahmad CN, Sweeney SJ, Adams AR, Murdin BN, Page H, Marcadet X, Sirtori C, Tomic S (2006)Spectroscopy of GaAs/AlGaAs quantum-cascade lasers using hydrostatic pressure, In: APPLIED PHYSICS LETTERS89(22)ARTN 2pp. ?-? AMER INST PHYSICS
Lock D, Sweeney SJ, Adams AR (2004)Pressure induced wavelength dependence of catastrophic optical damage in 980 nm semiconductor diode lasers, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH241(14)pp. 3416-3419
Jin SR, Sweeney SJ, Ahmad CN, Adams AR, Murdin BN (2004)Radiative and Auger recombination in 1.3 mu m InGaAsP and 1.5 mu m InGaAs quantum-well lasers measured under high pressure at low and room temperatures, In: APPLIED PHYSICS LETTERS85(3)pp. 357-359 AMER INST PHYSICS
Knowles G, Fehse R, Tomic S, Sweeney Stephen, Sale TE, Adams Alfred, O'Reilly P, Steinle G, Riechert H (2003)Investigation of 1.3-mu m GaInNAs vertical-cavity surface-emitting lasers (VCSELs) using temperature, high-pressure, and modelling techniques, In: IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS9(5)pp. 1202-1208 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

We have investigated the temperature and pressure dependence of the threshold current (I-th) of 1.3 mum emitting GaInNAs vertical-cavity surface-emitting lasers (VCSELs) and the equivalent edge-emitting laser (EEL) devices employing the same active region. Our measurements show that the VCSEL devices have the peak of the gain spectrum on the high-energy side of the cavity mode energy and hence operate over a wide temperature range. They show particularly promising I-th temperature insensitivity in the 250-350 K range. We have then used a theoretical model based on a 10-band k.P Hamiltonian and experimentally determined recombination coefficients from EELs to calculate the pressure and temperature dependency of I-th. The results show good agreement between the model and the experimental data, supporting both the validity of the model and the recombination rate parameters. We also show that for both device types, the super-exponential temperature dependency of I-th at 350 K and above is due largely to Auger recombination.

Fehse R, O'Reilly EP, Sweeney SJ, Adams AR, McConville D, Riechert H, Geelhaar L (2005)Investigation of carrier recombination processes and transport properties in GaInAsN/GaAs quantum wells, In: AIP Conference Proceedings772pp. 985-986 American Institute of Physics
It is shown that the dramatic changes in threshold current density with changing active region growth temperature in 1.3μm GaInNAs-based lasers can be attributed almost entirely to changes in the defect related monomolecular recombination current in the optically active material. In addition, growth temperature dependent changes in the QW morphology are shown to have a significant influence on the transport properties of the structure. © 2005 American Institute of Physics.
Lock D, Sweeney SJ, Adams AR (2003)Fundamental limitations of high power 980nm InGaAs/GaAs pump lasers, In: 2003 IEEE LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2pp. 427-428
Sweeney SJ, Jin SR, Tomic S, Adams AR, Higashi T, Riechert H, Thijs PJA (2003)Hydrostatic pressure dependence of recombination mechanisms in GaInNAs, InGaAsP and AlGaInAs 1.3 mu m quantum well lasers, In: PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS235(2)pp. 474-479
Sweeney Stephen, Lyons LJ, Adams Alfred, Lock DA (2003)Direct measurement of facet temperature up to melting point and COD in high-power 980-nm semiconductor diode lasers, In: IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS9(5)pp. 1325-1332 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

The authors describe a straightforward experimental technique for measuring the facet temperature of a semiconductor laser under high-power operation by analyzing the laser emission itself. By applying this technique to 1-mm-long 980-nm lasers with 6- and 9-mum-wide tapers, they measure a large increase in facet temperature under both continuous wave (CW) and pulsed operation. Under CW operation, the facet temperature increases from similar to25 degreesC at low currents to over 140 degreesC at 500 mA. From pulsed measurements they observe a sharper rise in facet temperature as a function of current (similar to 400 degreesC at 500 mA) when compared with the CW measurements. This difference is caused by self-heating which limits the output power and hence facet temperature under CW operation. Under pulsed operation the maximum measured facet temperature was in excess of 1000 degreesC for a current of 1000 mA. Above this current, both lasers underwent. catastrophic optical damage (COD). These results show a striking increase in facet temperature under high-power operation consistent with the facet melting at COD. This is made possible by measuring the laser under pulsed operation.

VanDeWalle CG, Marko IP, Andreev AD, Sweeney SJ, Adams AR, Krebs R, Deubert S, Reithmaier JP, Forchel A, Menendez J (2005)The influence of auger processes on recombination in long-wavelength InAs/GaAs quantum dots, In: Physics of Semiconductors, Pts A and B772pp. 681-682
Marko P, Adams AR, Sweeney SJ, Whitbread ND, Ward AJ, Asplin B, Robbins J (2007)The influence of carrier density non-pinning on the output power of 1.55 μm lasers at high temperature, In: Optics InfoBase Conference Papers
Crowley MT, Marko IP, Masse NF, Andreev AD, Tomic S, Sweeney SJ, O'Reilly EP, Adams AR (2009)The Importance of Recombination via Excited States in InAs/GaAs 1.3 mu m Quantum-Dot Lasers, In: IEEE J SEL TOP QUANT15(3)pp. 799-807 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
The temperature dependence of the radiative and nonradiative components of the threshold current density of 1.3 mu m InAs/GaAs quantum-dot lasers have been analyzed both experimentally and theoretically. It is shown that the weak temperature variation measured for the radiative current density arises because the optical matrix element for excited state transitions is significantly smaller than for the ground state transition. In contrast, nonradiative Auger recombination can have a similar probability for transitions involving excited states as for those involving ground state carriers. The sharp increase in the threshold current density at high temperatures follows the temperature variation of the cubed threshold carrier density confirming that Auger recombination is the dominant recombination mechanism in these devices at room temperature.
Masse NF, Marko IP, Adams AR, Sweeney SJ (2009)Temperature insensitive quantum dot lasers: are we really there yet?, In: JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS20pp. 272-276 SPRINGER
Twenty five years ago Arakawa suggested that by confining carriers in three dimensions (in quantum dots) a temperature insensitive threshold current (I-th) could be achieved in semiconductor lasers. In this paper we discuss investigations on state-of-the-art 1.3 mu m InAs/GaAs undoped and p-doped quantum dot lasers for telecommunication applications and discuss the extent to which this original hypothesis has been verified. In this study, the threshold current and its radiative component (I-rad) are measured as a function of temperature and pressure. The results show that although the radiative component of the threshold current can be temperature insensitive in undoped quantum dot lasers, a strong contribution from non-radiative Auger recombination makes the threshold current highly temperature sensitive. We find that p-doped devices can have a temperature insensitive I-th over a limited range around room temperature resulting from an interplay between an increasing non-radiative Auger current and decreasing radiative current. The decrease in I-rad, also observed below 200 K in undoped devices, is attributed to an improvement in the carrier transport with increasing temperature. Gain measurements show that even if p-doping is successful in reducing the effect of gain saturation, the modal net gain of p-doped devices is less than in undoped lasers due to increased non-radiative recombination and non-thermal carrier distribution.
Massé NF, Sweeney SJ, Marko IP, Andreev AD, Adams AR, Hatori N, Sugawara M (2006)Intrinsic limitations of p-doped and undoped 1.3 μm InAs/GaAs quantum dot lasers, In: 20th IEEE International Semi-conductor Laser Conference - Conference Digestpp. 143-144
The temperature dependencies of the recombination and gain processes reveal intrinsic limitations on the performances of quantum dot lasers. Controlling the transport of the carriers using the inhomogeneous broadening makes temperature stable threshold current possible
Marko Igor, Adams Alfred, Sweeney Stephen, Sellers IR, Mowbray DJ, Skolnick MS, Liu HY, Groom KM (2004)Recombination and loss mechanisms in low-threshold InAs/GaAs 1.3 mu m quantum dot lasers, In: Messe K, (eds.), 2004 IEEE 19TH INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE, CONFERENCE DIGESTpp. 57-58
We show that even in quantum dot lasers with very low threshold current density (Jth=740-50 A/cm(2) at 300 K) the temperature sensitivity of the threshold current arises from nonradiative recombination which comprises similar to60-70% of Jth at 300 K.
Marko IP, Aldukhayel AM, Adams AR, Sweeney SJ, Teissier R, Baranov AN, Tomić S (2010)Physical properties of short wavelength 2.6μm InAs/AlSb-based quantum cascade lasers, In: Conference Digest - IEEE International Semiconductor Laser Conferencepp. 95-96 IEEE
We used high hydrostatic pressure techniques to understand the deteriorating temperature performance with decreasing wavelength of short wavelength quantum cascade lasers. Influence of inter-valley scattering and distribution of the electron wave functions will be discussed.
Eales Timothy, Marko Igor, Ikyo BA, Adams Alfred, Arafin S, Sprengel S, Amann M-C, Sweeney Stephen (2017)Wavelength dependence of efficiency limiting mechanisms in Type-I Mid-infrared GaInAsSb/GaSb lasers, In: IEEE Journal of Selected Topics in Quantum Electronics23(6) IEEE
The efficiency limiting mechanisms in type-I GaInAsSb-based quantum well (QW) lasers, emitting at 2.3 μm, 2.6 μm and 2.9 μm, are investigated. Temperature characterization techniques and measurements under hydrostatic pressure identify an Auger process as the dominant non-radiative recombination mechanism in these devices. The results are supplemented with hydrostatic pressure measurements from three additional type-I GaInAsSb lasers, extending the wavelength range under investigation from 1.85-2.90 μm. Under hydrostatic pressure, contributions from the CHCC and CHSH Auger mechanisms to the threshold current density can be investigated separately. A simple model is used to fit the non-radiative component of the threshold current density, identifying the dominance of the different Auger losses across the wavelength range of operation. The CHCC mechanism is shown to be the dominant non-radiative process at longer wavelengths (> 2 μm). At shorter wavelengths (< 2 μm) the CHSH mechanism begins to dominate the threshold current, as the bandgap approaches resonance with the spin-orbit split-off band.
O'Brien K, Sweeney S J, Adams A R, Murdin B N, Salhi A, Rouillard Y, Joullie A (2006)Recombination Processes in Midinfrared InGaAsSb Diode Lasers Emitting at 2.37 mu m, In: Applied Physics Letters89(5)

The temperature dependence of the threshold current of InGaAsSb/AlGaAsSb compressively strained lasers is investigated by analyzing the spontaneous emission from working laser devices through a window formed in the substrate metallization and by applying high pressures. It is found that nonradiative recombination accounts for 80% of the threshold current at room temperature and is responsible for the high temperature sensitivity. The authors suggest that Auger recombination involving hot holes is suppressed in these devices because the spin-orbit splitting energy is larger than the band gap, but other Auger processes persist and are responsible for the low T-0 values.

Marko IP, Sweeney SJ, Adams AR, Jin SR, Murdin BN, Schwertberger R, Somers A, Reithmaier JP, Forchel A (2004)Recombination mechanisms in InAs/InP quantum dash lasers studied using high hydrostatic pressure, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH241(14)pp. 3427-3431
Murdin B N, Page H, Marcadet X, Sirtori C, Tomic S, Jin S R, Ahmad C N, Sweeney S J, Adams A R (2006)Spectroscopy of GaAs/AlGaAs Quantum-Cascade Lasers Using Hydrostatic Pressure, In: Applied Physics Letters89(22)

The authors have measured the output spectrum and the threshold current in 9.2 mu m wavelength GaAs/Al0.45Ga0.55As quantum-cascade lasers at 115 K as a function of hydrostatic pressure up to 7.3 kbars. By extrapolation back to ambient pressure, thermally activated escape of electrons from the upper lasing state up to delocalized states of the Gamma valley is shown to be an important contribution to the threshold current. On the other hand leakage into the X valley, although it has a very high density of states and is nearly degenerate with the Gamma band edge in the barrier, is insignificant at ambient pressure.

Ng JS, Soong WM, Steer MJ, Hopkinson M, David JPR, Chamings J, Sweeney SJ, Adams AR (2007)Long wavelength bulk GaInNAs p-i-n photodiodes lattice matched to GaAs, In: JOURNAL OF APPLIED PHYSICS101(6)ARTN 0pp. ?-? AMER INST PHYSICS

We report bulk GaInNAs p-i-n photodiodes lattice-matched to GaAs substrates, grown by solid source molecular beam epitaxy with photoresponses out to similar to 1.3 mu m. The as-grown samples were characterized optically, structurally, and electrically. A low background doping concentration in the range of 10(14)-10(15) cm(-3) was obtained in the samples. One of the samples with a 0.5 mu m thick GaInNAs absorbing layer gave a responsivity of 0.11 A/W for a band edge of 1.28 mu m at reverse bias of 2 V.

Lock D, Sweeney SJ, Adams AR, Deubner S, Klopf F, Reithmaier JP, Forchel A (2004)Carrier leakage suppression utilising short-period superlattices in 980 nm InGaAs/GaAs quantum well lasers, In: PHYSICA STATUS SOLIDI B-BASIC RESEARCH241(14)pp. 3405-3409
McConville DG, Sweeney SJ, Adams AR, Tomic S, Riechert H (2007)Temperature and pressure dependence of the recombination mechanisms in 1.3 pm and 1.5 pm GaInNAs lasers, In: PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS244(1)pp. 208-212
Adams AR, Sweeney SJ (2006)The physics controlling the sensitivity of semiconductor lasers to high temperatures, In: 2006 OPTICAL FIBER COMMUNICATION CONFERENCE/NATIONAL FIBER OPTIC ENGINEERS CONFERENCE, VOLS 1-6pp. 1631-1633
Chamings J, Adams AR, Sweeney SJ, Kunert B, Volz K, Stolz W (2008)Thermal properties of Silicon compatible GaNAsP SQW lasers, In: 2008 IEEE 21ST INTERNATIONAL SEMICONDUCTOR LASER CONFERENCEpp. 61-62
Marko Igor, Adams Alfred, Sweeney Stephen, Mowbray DJ, Skolnick MS, Liu HYY, Groom KM (2005)Recombination and loss mechanisms in low-threshold InAs-GaAs 1.3-mu m quantum-dot lasers, In: IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS11(5)pp. 1041-1047 IEEE

We show that even in quantum-dot (QD) lasers with very low threshold current densities (J(th) = 40-50 A/cm(2) at 300 K), the temperature sensitivity of the threshold current arises from nonradiative recombination that comprises similar to 60% to 70% of J(th) at 300 K, whereas the radiative part of J(th) is almost temperature insensitive. The influence of the nonradiative recombination mechanism decreases with increasing hydrostatic pressure and increasing band gap, which leads to a decrease of the threshold current. We also studied, for the first time, the band gap dependence of the radiative part Of Jth, which in contrast increases strongly with increasing band gap. These results suggest that Auger recombination is an important intrinsic recombination mechanism for 1.3-mu m lasers, even in a very low threshold QD device, and that it is responsible for the temperature sensitivity of the threshold current.

Marko IP, Masse NF, Sweeney SJ, Andreev AD, Adams AR, Hatori N, Sugawara M (2005)Carrier transport and recombination in p-doped and intrinsic 1.3 mu m InAs/GaAs quantum-dot lasers, In: APPLIED PHYSICS LETTERS87(21)ARTN 2pp. ?-? AMER INST PHYSICS

The radiative and nonradiative components of the threshold current in 1.3 mu m, p-doped and undoped quantum-dot semiconductor lasers were studied between 20 and 370 K. The complex behavior can be explained by simply assuming that the radiative recombination and nonradiative Auger recombination rates are strongly modified by thermal redistribution of carriers between the dots. The large differences between the devices arise due to the trapped holes in the p-doped devices. These both greatly increase Auger recombination involving hole excitation at low temperatures and decrease electron thermal escape due to their Coulombic attraction. The model explains the high T-0 values observed near room temperature. (c) 2005 American Institute of Physics.

Marko Igor, Masse N, Sweeney SJ, Adams AR, Sellers IR, Mowbray DJ, Skolnick MS, Liu HY, Groom KM (2005)Effect of gain saturation and nonradiative recombination on the thermal characteristics of InAs/GaAs 1.3 mu m quantum dot lasers, In: The 18th Annual Meeting of the IEEE Lasers and Electro-Optics Society, 2005. (LEOS 2005)pp. 401-402 IEEE
Gain saturation increases the radiative component, J(rad), of the threshold current density, J(th), and its contribution to the thermal sensitivity of J(th) in short cavity or low QD density devices. However, the main cause of their thermal sensitivity is a strong non-radiative recombination.
Sharpe Matthew (2020)Development of novel semiconductor based photodetector devices University of Surrey
This thesis investigates novel range semiconductor materials and structures for use in optoelectronic devices operating from UV to mid-IR. These devices have an abundant variety of applications, including optical communications and imaging. The studies primarily focus on the electronic band structure of the materials and how this impacts device performance. Dilute bismide III-V alloys are a material of great interest for IR applications, especially with the band structure engineering that can be carried out to fulfil the condition where the spin-orbit splitting energy, ∆
Waché Rémi, Fielder Tim, Dickinson Will E.C., Hall Joe L., Adlington Peter, Sweeney Stephen J., Clowes Steven K. Selective light transmission as a leading innovation for solar swimming pool covers, In: Solar Energy207pp. 388-397 Elsevier
An innovative, extrudable material with the ability to filter the sun’s energy has been developed for the mass manufacture of high performance swimming pool covers. Solar radiation in the visible spectrum ( nm) is absorbed by the material so that minimal visible light enters the pool water which inhibits photosynthesis to prevent algae growth. Furthermore, the material has high transmission properties in the near infrared that can be efficiently absorbed by the water allowing for a higher temperature increase compared to a standard non-selective opaque cover. We have developed a model to enable the cover efficiency to convert solar energy to heat a swimming pool, calculated based on the wavelength dependent absorption and transmission properties of the cover. We have validated this model using dedicated full-scale test-facility. Our results demonstrate that a selective transmission cover can increase the absolute heating efficiencies by approximately 12% compared to the fully opaque equivalent.
The advances in semiconductor technology coupled with the potential of lab-on-chip spectroscopy, long wavelength telecommunications and small-scale optical interconnects has invigorated academic and commercial interest in mid-infrared optoelectronics. However, significant challenges remain. This thesis explores two approaches for mid-infrared optoelectronic lasers. Type-I GaSb quantum well lasers exhibit some of the highest performance metrics of any semiconductor laser system in the 2 μm – 3 μm wavelength range. However, the threshold current density increases substantially with increasing wavelength and temperature, impacting component reliability and the overall efficiency of a laser-based optoelectronic system. Through a combination of temperature and hydrostatic pressure techniques, Auger recombination is identified as the dominant cause for the performance degradation of type-I mid-infrared laser with increasing wavelength and temperature. Using hydrostatic pressure measurements, the wavelength dependence of the Auger coefficient over the 2 μm - 3 μm range is constructed, revealing two important regimes. At wavelengths < 2 μm the CHSH Auger process dominates the room temperature non-radiative threshold current. For wavelength > 2 μm, the CHSH process is effectively suppressed due to the energetic separation between the lasing energy and the spin-orbit split-off band. In this regime another Auger process, such as CHCC or CHLH recombination begins to dominate, increasing exponentially with wavelength. The temperature dependence of the radiative and non-radiative threshold current density indicates that this Auger process has an activated character and is sensitive to the intrinsic properties of the quantum well band structure. To leverage the advanced manufacturing capabilities and high yields of the Si-microelectronics industry, there is intense research activity to realise CMOS compatible optoelectronics. One emerging strategy is to augment the optical properties of group-IV materials through band structure engineering, such as incorporating Sn into the Ge lattice. Hydrostatic pressure measurements of the GeSn absorption edge exhibit an intermediate pressure coefficient between that of the Γ and L conduction band critical points. This is indicative of strong band mixing effects in the GeSn alloy. In the presence of band mixing the conventional distinction between the indirect and direct band gap breaks down. Instead it is more appropriate to discuss the nature of the band gap in terms of the fractional Γ-character of the conduction band states at the band edge. The pressure coefficient of the absorption edge for samples with Sn content between 6% – 10% reveal a continuous evolution in the Γ-character with increasing Sn-concentration. High Γ-character is observed even at low Sn concentrations of 6%, when the GeSn alloy is expected to exhibit a fundamentally indirect band gap. These band mixing effects have important implications for designing efficient photonic and electronic devices utilizing GeSn and related material systems.
Sweeney SJ, Jha A (2015)Preface, In: Journal of Physics: Conference Series619(1)
Cheetham KJ, Krier A, Marko IP, Aldukhayel A, Sweeney SJ (2011)Direct evidence for suppression of Auger recombination in GaInAsSbP/InAs mid-infrared light-emitting diodes, In: APPL PHYS LETT99(14)141110 AMER INST PHYSICS
Bushell Zoe L., Broderick Christopher A., Nattermann Lukas, Joseph Rita, Keddie Joseph L., Rorison Judy M., Volz Kerstin, Sweeney Stephen J. (2019)Giant bowing of the band gap and spin-orbit splitting energy in GaP1−χBiχ dilute bismide alloys, In: Scientific Reports9(1)6835 Nature Research
Using spectroscopic ellipsometry measurements on GaP1−χBiχ/GaP epitaxial layers up to χ = 3.7% we observe a giant bowing of the direct band gap (EΓg) and valence band spin-orbit splitting energy (ΔSO). EΓgSO) is measured to decrease (increase) by approximately 200 meV (240 meV) with the incorporation of 1% Bi, corresponding to a greater than fourfold increase in ΔSO in going from GaP to GaP0.99Bi0.01. The evolution of EΓg and ΔSO with χ is characterised by strong, composition-dependent bowing. We demonstrate that a simple valence band-anticrossing model, parametrised directly from atomistic supercell calculations, quantitatively describes the measured evolution of EΓg and ΔSO with χ. In contrast to the well-studied GaAs1−χBiχ alloy, in GaP1−χBiχ substitutional Bi creates localised impurity states lying energetically within the GaP host matrix band gap. This leads to the emergence of an optically active band of Bi-hybridised states, accounting for the overall large bowing of EΓg and ΔSO and in particular for the giant bowing observed for χ ≲ 1%. Our analysis provides insight into the action of Bi as an isovalent impurity, and constitutes the first detailed experimental and theoretical analysis of the GaP1−χBiχ alloy band structure.
Eales Timothy D., Marko Igor P., Schulz Stefan, O’Halloran Edmond, Ghetmiri Seyed, Du Wei, Zhou Yiyin, Yu Shui-Qing, Margetis Joe, Tolle John, O’Reilly Eoin P., Sweeney Stephen J. Ge1-xSnx alloys: Consequences of band mixing effects for the evolution of the band gap Γ-character with Sn concentration, In: Scientific Reports Springer Nature
In this work we study the nature of the band gap in GeSn alloys for use in silicon-based lasers. Special attention is paid to Sn-induced band mixing effects. We demonstrate from both experiment and ab-initio theory that the (direct) Γ- character of the GeSn band gap changes continuously with alloy composition and has significant Γ-character even at low (6%) Sn concentrations. The evolution of the Γ-character is due to Sn-induced conduction band mixing effects, in contrast to the sharp indirect-to-direct band gap transition obtained in conventional alloys such as Al1-xGaxAs. Understanding the band mixing effects is critical not only from a fundamental and basic properties viewpoint but also for designing photonic devices with enhanced capabilities utilizing GeSn and related material systems.
Sweeney SJ, Marko IP, Jin SR, Hild K, Batool Z, Ludewig P, Natterman L, Bushell Z, Stolz W, Volz K, Broderick CA, Usman M, Harnedy PE, Oreilly EP, Butkute R, Pacebutas V, Geiutis A, Krotkus A (2014)Electrically injected GaAsBi quantum well lasers, In: Conference Digest - IEEE International Semiconductor Laser Conferencepp. 80-81 IEEE
GaAsBi QWs have the potential to remove inherent recombination losses thereby increasing the efficiency and reducing the temperature sensitivity of near-infrared telecommunications lasers. GaAsBi QW lasers are reported and prospects for 1550nm operation are discussed.
Liebich S, Zimprich M, Beyer A, Lange C, Franzbach DJ, Chatterjee S, Hossain N, Sweeney SJ, Volz K, Kunert B, Stolz W (2011)Laser operation of Ga(NAsP) lattice-matched to (001) silicon substrate, In: APPLIED PHYSICS LETTERS99(7)ARTN 0pp. ?-? AMER INST PHYSICS
Bushell ZL, Florescu Marian, Sweeney Stephen High-Q photonic crystal cavities in all-semiconductor photonic crystal heterostructures, In: Physical Review B95(23)235303 American Physical Society
Photonic crystal cavities enable the realization of high Q-factor and low mode-volume resonators, with typical architectures consisting of a thin suspended periodically patterned layer to maximize confinement of light by strong index guiding. We investigate a heterostructure-based approach comprising a high refractive index core and lower refractive index cladding layers. While confinement typically decreases with decreasing index contrast between the core and cladding layers, we show that, counterintuitively, due to the confinement provided by the photonic band structure in the cladding layers, it becomes possible to achieve Q factors > 10 4 with only a small refractive index contrast. This opens up opportunities for implementing high-Q factor cavities in conventional semiconductor heterostructures, with direct applications to the design of electrically pumped nanocavity lasers using conventional fabrication approaches.
Hossain N, Hosea TJC, Sweeney SJ, Liebich S, Zimprich M, Volz K, Kunert B, Stolz W (2011)Band structure properties of novel BxGa1-xP alloys for silicon integration, In: JOURNAL OF APPLIED PHYSICS110(6)ARTN 0pp. ?-? AMER INST PHYSICS
Bastiman F, Ng JS, Sweeney SJ, David JPR, Mohmad AR (2011)Photoluminescence investigation of high quality GaAs1-xBix on GaAs, In: APPLIED PHYSICS LETTERS98(12)ARTN 1pp. ?-? AMER INST PHYSICS
Batool Z, Chatterjee S, Chernikov A, Duzik A, Fritz R, Gogineni C, Hild K, Hosea TJC, Imhof S, Johnson SR, Jiang Z, Jin S, Koch M, Koch SW, Masnadi-Shirazi M, Kolata K, Lewis RB, Lu X, Millunchick JM, Mooney PM, Riordan NA, Rubel O, Sweeney SJ, Volz K, Thomas JC, Thränhardt A, Tiedje T (2013)Bismuth-containing III-V semiconductors: Epitaxial growth and physical propertiespp. 139-158
Sharpe M. K., Marko I. P., Duffy D. A., England J., Schneider E., Kesaria M., Fedorov V., Clarke E., Tan C. H., Sweeney S. J. A comparative study of epitaxial InGaAsBi/InP structures using Rutherford backscattering spectrometry, X-ray diffraction and photoluminescence techniques, In: JOURNAL OF APPLIED PHYSICS126(12)125706 AMER INST PHYSICS
In this work, we used a combination of photoluminescence (PL), high resolution X-ray diffraction (XRD), and Rutherford backscattering spectrometry (RBS) techniques to investigate material quality and structural properties of MBE-grown InGaAsBi samples (with and without an InGaAs cap layer) with targeted bismuth composition in the 3%–4% range. XRD data showed that the InGaAsBi layers are more homogeneous in the uncapped samples. For the capped samples, the growth of the InGaAs capped layer at higher temperature affects the quality of the InGaAsBi layer and bismuth distribution in the growth direction. Low-temperature PL exhibited multiple emission peaks; the peak energies, widths, and relative intensities were used for comparative analysis of the data in line with the XRD and RBS results. RBS data at a random orientation together with channeled measurements allowed both an estimation of the bismuth composition and analysis of the structural properties. The RBS channeling showed evidence of higher strain due to possible antisite defects in the capped samples grown at a higher temperature. It is also suggested that the growth of the capped layer at high temperature causes deterioration of the bismuth-layer quality. The RBS analysis demonstrated evidence of a reduction of homogeneity of uncapped InGaAsBi layers with increasing bismuth concentration. The uncapped higher bismuth concentration sample showed less defined channeling dips suggesting poorer crystal quality and clustering of bismuth on the sample surface.
Hild K, Sale TE, Sweeney SJ, Hirotani M, Mizuno Y, Kato T (2004)Modulation speed resonant-cavity and leakage current in 650 nm light emitting diodes, In: IEE PROCEEDINGS-OPTOELECTRONICS151(2)pp. 94-97 IEE-INST ELEC ENG
The authors have investigated red-emitting (650 nm) resonant-cavity light emitting diodes for use with polymer optical fibres. The small signal modulation response is characterised by a single order roll-off. The -3 dB bandwidth is found to be determined solely by the differential carrier lifetime, T, in the active region and hence dependent on current density, J, alone with no intrinsic size effects. The tau(J) relation allows the calculation of the active region carrier density and hence the recombination parameters (mono- and bimolecular) in the regime where the leakage is small. It is shown that the leakage is insignificant for currents below 200 A/cm(2) at 20degreesC. Above 40degreesC the leakage rises rapidly with temperature, and is evident from a dramatic fall in tau and an accelerated rise in the current required to maintain constant light output.
Hossain N, Hild K, Jin SR, Sweeney SJ, Yu S-Q, Johnson SR, Ding D, Zhang Y-H (2010)Influence of device structures on carrier recombination in GaAsSb/GaAs QW lasers, In: Proceedings of Photonics Global Conference
We investigate the temperature and pressure dependence of carrier recombination processes occurring in various GaAsSb/GaAs QW laser structures grown under similar growth conditions. Thermally activated carrier leakage via defects is found to be very sensitive to the strain induced interface imperfections. Nonradiative recombination is found to be sensitive to the number of QWs. A strain compensated MQW structure leads to a reduced contribution of non-radiative recombination to the threshold current density (Jth) and a high characteristic temperature (T0) of 73K at room temperature.
Marko IP, Sweeney SJ (2015)Optical and electronic processes in semiconductor materials for device applications, In: Springer Series in Materials Science203pp. 253-297 Springer
In this chapter we consider the important optical and electronic processes which influence the properties of semiconductor photonic devices. Focussing on a number of material systems, we describe semiconductor materials and structures used for light-emitting applications (lasers and LEDs) operating in a wide spectral range from visible to mid-infrared. The main carrier recombination mechanisms in semiconductor devices are discussed and experimental methodologies for measuring and analysing these mechanisms are introduced. Near infra-red (IR) quantum well (QW) lasers are discussed in depth considering several new approaches to overcome fundamental performance issues. Different approaches for the longer wavelength (mid-IR) semiconductor devices are reviewed showing the benefits of different approaches to material and device design where energy efficiency and high temperature operation are the principal concerns. Finally, semiconductor lasers and LEDs for the visible spectral range are briefly introduced in terms of the most important issues related to their performance.
Bushell Z, Stolz W, Volz K, Broderick CA, Usman M, Harnedy PE, O'Reilly EP, Butkute R, Pacebutas V, Geizutis A, Krotkus A, Jin SR, Sweeney SJ, Marko IP, Hild K, Batool Z, Natterman L, Ludewig P (2014)Electrically injected GaAsBi Quantum Well Lasers, In: 2014 24TH IEEE INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE (ISLC 2014)pp. 80-81
GaAsBi QWs have the potential to remove inherent recombination losses thereby increasing the efficiency and reducing the temperature sensitivity of near-infrared telecommunications lasers. GaAsBi QW lasers are reported and prospects for 1550nm operation are discussed.
Constant SB, Tomic S, Lock D, Sale TE, Sweeney SJ, Hosea TJC (2003)Spectroscopic characterization of 1450 nm semiconductor pump laser structures for Raman amplifiers, In: JOURNAL OF APPLIED PHYSICS93(12)pp. 9446-9455 AMER INST PHYSICS
Jarvis S, Mukherjee J, Perren M, Sweeney SJ (2013)On the Fundamental Efficiency Limits of Photovoltaic Converters for Optical Power Transfer Applications, In: 2013 IEEE 39TH PHOTOVOLTAIC SPECIALISTS CONFERENCE (PVSC)pp. 1031-1035 IEEE
Hossain N, Jin SR, Sweeney SJ, Liebich S, Ludewig P, Zimprich M, Volz K, Kunert B, Stolz W (2011)Physical properties of monolithically integrated Ga(NAsP)/(BGa)P QW lasers on silicon, In: 8th IEEE International Conference on Group IV Photonicspp. 148-150
This paper reports the lattice matched monolithic integration of novel direct band-gap dilute nitride Ga(NAsP) QW lasers on an (001) silicon substrate using novel (BGa)P strain compensating layer. Lasing operation up to 165K is verified with a threshold current density of 1.6kAcm-2 and a characteristic temperature of 73K for a SQW device, which is a positive step towards a commercial solution for the monolithic integration of long term stable laser diodes on silicon substrates.
Sayid SA, Marko IP, Sweeney SJ, Poole P (2010)Temperature sensitivity of 1.55μm (100) InAs/InP-based quantum dot lasers, In: Proceedings of 22nd International Conference on Indium Phosphide and Related Materialspp. 23-24
Semiconductor lasers with quantum dot (QD) based active regions have generated a huge amount of interest for applications including communications networks due to their anticipated superior physical properties due to three dimensional carrier confinement. For example, the threshold current of ideal quantum dots is predicted to be temperature insensitive. We have investigated the operating characteristics of 1.55 μm InAs/InP (100) quantum dot lasers focusing on their carrier recombination characteristics using a combination of low temperature and high pressure measurements. By measuring the intrinsic spontaneous emission from a window fabricated in the n-contact of the devices we have measured the radiative component of the threshold current density, Jrad. We find that Jrad is itself relatively temperature insensitive (Fig. 1). However, the total threshold current density, Jth, increases significantly with temperature leading to a characteristic temperature T0~72 K around 220 K-290 K. From this data it is clear that the devices are dominated by a non-radiative recombination process which accounts for up to 94% of the threshold current at room temperature (Fig. 1).
Sweeney Stephen, Broderick CA, Jin S, Marko Igor, Hild Konstanze, Ludewig P, Bushell Zoe, Stolz W, Rorison JM, O’Reilly EP, Volz K (2017)GaAs1−xBix/GaNyAs1−y type-II quantum wells: novel strain-balanced heterostructures for GaAs-based near- and mid-infrared photonics, In: Scientific Reports746371 Nature Publishing Group
The potential to extend the emission wavelength of photonic devices further into the near- and midinfrared via pseudomorphic growth on conventional GaAs substrates is appealing for a number of communications and sensing applications. We present a new class of GaAs-based quantum well (QW) heterostructure that exploits the unusual impact of Bi and N on the GaAs band structure to produce type-II QWs having long emission wavelengths with little or no net strain relative to GaAs, while also providing control over important laser loss processes. We theoretically and experimentally demonstrate the potential of GaAs1−xBix/GaNyAs1−y type-II QWs on GaAs and show that this approach offers optical emission and absorption at wavelengths up to ~3 μm utilising strain-balanced structures, a first for GaAs-based QWs. Experimental measurements on a prototype GaAs0.967Bi0.033/GaN0.062As0.938 structure, grown via metal-organic vapour phase epitaxy, indicate good structural quality and exhibit both photoluminescence and absorption at room temperature. The measured photoluminescence peak wavelength of 1.72 μm is in good agreement with theoretical calculations and is one of the longest emission wavelengths achieved on GaAs to date using a pseudomorphically grown heterostructure. These results demonstrate the significant potential of this new class of III-V heterostructure for longwavelength applications.
Hossain N, Jin SR, Sweeney SJ, Liebich S, Ludewig P, Zimprich M, Kunert B, Volz K, Stolz W (2010)On the temperature dependence of monolithically integrated Ga(NAsP)/(BGa)P/Si QW lasers, In: Optics InfoBase Conference Papers
Lasing operation up to 120K is reported in novel direct band-gap Ga(NAsP)/(BGa)P lasers grown monolithically on a silicon substrate. A carrier leakage process is found to dominate the temperature dependence of the laser threshold current. © 2010 OSA /FiO/LS 2010.
Wang J-B, Johnson SR, Zhang Y-H, Bueckers C, Blume G, Thraenhardt A, Schlichenmaier C, Klar PJ, Weiser G, Koch SW, Hader J, Sweeney Stephen, Moloney JV, Hosea Thomas (2007)Microscopic electroabsorption line shape analysis for Ga(AsSb)/GaAs heterostructures, In: JOURNAL OF APPLIED PHYSICS101(3)ARTN 0pp. ?-? AMER INST PHYSICS

A series of Ga(AsSb)/GaAs/(AlGa) As samples with varying GaAs spacer width are studied by electric-field modulated absorption (EA) and reflectance spectroscopy and modeled using a microscopic theory. The analysis of the Franz-Keldysh oscillations of GaAs capping layer and of the quantum-confined Stark shift of the lowest quantum well (QW) transitions shows the strong inhomogeneity of the built-in electric field indicating that the field modulation due to an external bias voltage differs significantly for the various regions of the structures. The calculations demonstrate that the line shape of the EA spectra of these samples is extremely sensitive to the value of the small conduction band offset between GaAs and Ga(AsSb) as well as to the magnitude of the internal electric field changes caused by the external voltage modulation in the QW region. The EA spectra of the entire series of samples are modeled by the microscopic theory. The good agreement between experiment and theory allows us to extract the strength of the modulation of the built-in electric field in the QW region and to show that the band alignment between GaAs and Ga(AsSb) is of type II with a conduction band offset of approximately 40 meV.

Chamings J, Ahmed S, Sweeney SJ, Odnoblyudov VA, Tu CW (2008)Physical properties and efficiency of GaNP light emitting diodes, In: APPLIED PHYSICS LETTERS92(2)ARTN 0pp. ?-? AMER INST PHYSICS
Marko Igor, Sweeney Stephen (2017)Progress towards III-V-Bismide Alloys for Near- and Mid-Infrared Laser Diodes, In: IEEE Journal of Selected Topics in Quantum Electronics23(6)1501512 Institute of Electrical and Electronics Engineers (IEEE)
Bismuth-containing III-V alloys open-up a range of possibilities for practical applications in semiconductor lasers, photovoltaics, spintronics, photodiodes and thermoelectrics. Of particular promise for the development of semiconductor lasers is the possibility to grow GaAsBi laser structures such that the spin-orbit-splitting energy (ΔSO) is greater than the bandgap (Eg) in the active region for devices operating around the telecom wavelength of 1.55 μm, thereby suppressing the dominant efficiency-limiting loss processes in such lasers, namely Auger recombination and inter-valence band absorption (IVBA). The ΔSO > Eg band structure is present in GaAsBi alloys containing > 10% Bi, at which composition the alloy band gap is close to 1.55 μm on a GaAs substrate making them an attractive candidate material system for the development of highly efficient, uncooled GaAs-based lasers for telecommunications. Here we discuss progress towards this goal and present a comprehensive set of data on the properties of GaAsBi lasers including optical gain and absorption characteristics and the dominant carrier recombination processes in such systems. Finally, we briefly review the potential of GaAsBiN and InGaAsBi material systems for near- and mid-infrared photonic devices on GaAs and InP platforms, respectively.
Marko I, Sweeney S, hild K (2016)Temperature stable mid-infrared GaInAsSb/GaSb Vertical Cavity Surface Emitting Lasers (VCSELs), In: Scientific Reports619595 Nature Publishing Group
GaInAsSb/GaSb based quantum well vertical cavity surface emitting lasers (VCSELs) operating in mid-infrared spectral range between 2 and 3 micrometres are of great importance for low cost gas monitoring applications. This paper discusses the efficiency and temperature sensitivity of the VCSELs emitting at 2.6 μm and the processes that must be controlled to provide temperature stable operation. We show that non-radiative Auger recombination dominates the threshold current and limits the device performance at room temperature. Critically, we demonstrate that the combined influence of non-radiative recombination and gain peak – cavity mode de-tuning determines the overall temperature sensitivity of the VCSELs. The results show that improved temperature stable operation around room temperature can only be achieved with a larger gain peak – cavity mode de-tuning, offsetting the significant effect of increasing non-radiative recombination with increasing temperature, a physical effect which must be accounted for in mid-infrared VCSEL design.
Mohmad AR, Bastiman F, Hunter CJ, Richards RD, Sweeney SJ, Ng JS, David JPR, Majlis BY (2014)Localization effects and band gap of GaAsBi alloys, In: PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS251(6)pp. 1276-1281 WILEY-V C H VERLAG GMBH
Fox NE, Sharma TK, Sweeney SJ, Hosea TJC (2009)Room temperature characterisation of InGaAlAs quantum well laser structures using electro-modulated reflectance and surface photovoltage spectroscopy, In: PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE206(5)pp. 796-802
Hossain N, Chamings J, Jin SR, Sweeney SJ, Liebich S, Reinhard S, Volz K, Kunert B, Stolz W (2010)Recombination and loss mechanisms in GaNAsP/GaP QW lasers, In: Proceedings of Photonics Global Conference
In this paper the authors present a comprehensive study of the threshold current and its temperature dependence in novel direct band-gap Ga(NAsP)/GaP QW lasers which provide a potential route to lattice matched monolithic integration of long term stable semiconductor lasers on silicon. It is found that near room temperature, the threshold current is dominated by nonradiative recombination accounting for ~87% of the total threshold current density. A strong increase in threshold current with hydrostatic pressure implies that a carrier leakage path is the dominant carrier recombination mechanism.
Sayid SA, Marko IP, Cannard PJ, Chen X, Rivers LJ, Lealman IF, Sweeney SJ (2010)Thermal performance of 1.55μm InGaAlAs quantum well buried heterostructure lasers, In: IEEE Proceedings of 22nd International Conference on Indium Phosphide and Related Materialspp. 265-268
We have investigated the threshold current Ith and differential quantum efficiency as the function of temperature in InGaAlAs/InP multiple quantum well (MQWs) buried heterostructure (BH) lasers. We find that the temperature sensitivity of Ith is due to non-radiative recombination which accounts for up to ~80% of Jth at room temperature. Analysis of spontaneous emission emitted from the devices show that the dominant non-radiative recombination process is consistent with Auger recombination. We further show that the above threshold differential internal quantum efficiency, ηi, is ~80% at 20°C remaining stable up to 80°C. In contrast, the internal optical loss, αi, increases from 15 cm-1 at 20°C to 22 cm-1 at 80°C, consistent with inter-valence band absorption (IVBA). This suggests that the decrease in power output at elevated temperatures is associated with both Auger recombination and IVBA.
Hossain N, Hosea J, Liebich S, Zimprich M, Volz K, Kunert B, Stolz W, Sweeney S (2013)Band Structure Properties of (BGa)P Semiconductors for Lattice Matched Integration on (001) Silicon, In: Ihn T, Rossler C, Kozikov A, (eds.), PHYSICS OF SEMICONDUCTORS1566pp. 47-48
Pearce JV, Ongrai O, Machin G, Sweeney SJ (2010)Self-validating thermocouples based on high temperature fixed points, In: METROLOGIA47(1)pp. L1-L3 IOP PUBLISHING LTD
Jarvis SD, Mukherjee J, Perren M, Sweeney SJ (2014)Development and characterisation of laser power converters for optical power transfer applications, In: IET OPTOELECTRONICS8(2)pp. 64-70 INST ENGINEERING TECHNOLOGY-IET
Sweeney S (2011)Selected papers from the Semiconductor and Integrated Optoelectronics (SIOE'10) Conference, In: IET OPTOELECTRONICS5(3)pp. 99-99 INST ENGINEERING TECHNOLOGY-IET
Sweeney SJ (2010)Bismide-alloys for higher efficiency infrared semiconductor lasers, In: 22nd IEEE International Semiconductor Laser Conferencepp. 111-112
The incorporation of Bismuth in III-V alloys, such as GaAsBi/GaAs provides a preferential semiconductor band structure to suppress non-radiative recombination and optical losses, improving the efficiency and temperature stability of infrared semiconductor lasers.
Hild K, Batool Z, Jin SR, Hossain N, Marko IP, Hosea TJC, Lu X, Tiedje T, Sweeney SJ (2013)Auger Recombination Suppression And Band Alignment In GaAsBi/GaAs Heterostructures, In: Ihn T, Rossler C, Kozikov A, (eds.), PHYSICS OF SEMICONDUCTORS1566pp. 488-489
Using a combination of experimental and theoretical techniques we present the dependence of the bandgap Eg and the spin orbit splitting energy so, with Bi concentration in GaAsBi/GaAs samples. We find that the concentration at which so,> Eg occurs at 9%. Both spectroscopic as well as first device results indicate a type I alignment.
Mohmad AR, Bastiman F, Hunter CJ, Ng JS, Sweeney SJ, David JPR (2011)The effect of Bi composition to the optical quality of GaAs1-xBix, In: APPLIED PHYSICS LETTERS99(4)ARTN 0pp. ?-? AMER INST PHYSICS
Liebich S, Zimprich M, Ludewig P, Beyer A, Volz K, Stolz W, Kunert B, Hossain N, Jin SR, Sweeney SJ (2010)MOVPE growth and characterization of Ga(NAsP) laser structures monolithically integrated on Si (001) substrates, In: 22nd IEEE International Semiconductor Laser Conferencepp. 143-144
In this work we focus on the MOVPE growth of Ga(NAsP) laser structures for electrical current injection lattice matched on exactly orientated Si substrates and their structural characterization.
Hossain N, Jin SR, Sweeney SJ, Yu S-Q, Johnson SR, Ding D, Zhang Y-H (2010)Improved performance of GaAsSb/GaAs SQW lasers, In: Proceedings of SPIE: Materials + Mode-Locking7616
This paper reports the improvements and limitations of MBE grown 1.3μm GaAsSb/GaAs single QW lasers. At room temperature, the devices show a low threshold current density (Jth) of 253 Acm-2, a transparent current density of 98 Acm-2, an internal quantum efficiency of 71%, an optical loss of 18 cm-1 and a characteristic temperature (T0) = 51K. The defect related recombination in these devices is negligible and the primary non-radiative current path has a stronger dependence on the carrier density than the radiative current contributing to ~84% of the threshold current at RT. From high hydrostatic pressure dependent measurements, a slight decrease followed by the strong increase in threshold current with pressure is observed, suggesting that the device performance is limited to both Auger recombination and carrier leakage.
Sweeney SJ, Hild K, Marko IP, Yu S-Q, Johnson SR, Zhang Y-H (2008)Thermal characteristics of 1.3 mu m GaAsSb/GaAs-based Edge- and Surface-emitting Lasers, In: 2008 IEEE 21ST INTERNATIONAL SEMICONDUCTOR LASER CONFERENCEpp. 83-84
Hild K, Sweeney SJ, Marko IP, Jin SR, Johnson SR, Chaparro SA, Yu S, Zhang Y-H (2007)Temperature and pressure dependence of carrier recombination processes in GaAsSb/GaAs quantum well lasers, In: PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS244(1)pp. 197-202
Sale TE, Knowles GC, Sweeney SJ, Onischenko A, Frost JEF, Pinches SM, Woodhead J (2000)-180 to +80 degrees C CW lasing in visible VCSELs, In: 2000 IEEE 17TH INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE, CONFERENCE DIGESTpp. 15-16 IEEE
Pearce JV, Machin G, Sweeney SJ, Ongrai O (2013)Self-calibration of a W/Re thermocouple using a miniature Ru-C (1954 °c) eutectic cell, In: AIP Conference Proceedings1552 8pp. 504-509
Previous successful investigations of miniature cobalt-carbon (Co-C, 1324 °C) and palladium-carbon (Pd-C, 1492 °C) high temperature fixed-point cells for thermocouple self-calibration have been reported [1-2]. In the present work, we describe a series of measurements of a miniature ruthenium-carbon (Ru-C) eutectic cell (melting point 1954 °C) to evaluate the repeatability and stability of a W/Re thermocouple (type C) by means of in-situ calibration. A miniature Ru-C eutectic fixed-point cell with outside diameter 14 mm and length 30 mm was fabricated to be used as a self-calibrating device. The performance of the miniature Ru-C cell and the type C thermocouple is presented, including characterization of the stability, repeatability, thermal environment influence, ITS-90 temperature realization and measurement uncertainty.
Hossain N, Jin SR, Sweeney SJ, Liebich S, Reinhard S, Volz K, Kunert B, Stolz W (2010)Physical properties of Ga(NAsP)/GaP QW lasers grown by MOVPE, In: 23rd Annual Meeting of the IEEE Photonics Societypp. 65-66
We are reporting for the first time, lasing operation at room temperature (RT) with a low threshold current density (Jth) in novel direct band-gap Ga(NAsP)/GaP QW lasers. A carrier leakage process is found to dominate the temperature dependence of the laser threshold current.
Sweeney SJ, Hild K, Jin S (2013)The potential of GaAsBiN for multi-junction solar cells, In: 2013 IEEE 39TH PHOTOVOLTAIC SPECIALISTS CONFERENCE (PVSC)pp. 2474-2478 IEEE
Marko Igor, Read GW, Hossain N, Sweeney Stephen (2015)Physical Properties and Characteristics of III-V Lasers on Silicon, In: IEEE Journal of Selected Topics in Quantum Electronics21(6)1502208 IEEE
The development of laser technology based on silicon continues to be of key importance for the advancement of electronic-photonic integration offering the potential for high data rates and reduced energy consumption. Progress was initially hindered due to the inherent indirect band gap of silicon. However, there has been considerable progress in developing ways of incorporating high gain III-V based direct band gap materials onto silicon, bringing about the advantages of both materials. In this paper, we introduce the need for lasers on silicon and review some of the main approaches for the integration of III-V active regions, including direct epitaxial growth, hybrid integration, defect blocking layers and quantum dots. We then discuss the roles of different carrier recombination processes on the performance of devices formed using both wafer fusion and direct epitaxial approaches.
Sweeney SJ, Batool Z, Hild K, Jin SR, Hosea TJC (2011)The Potential Role of Bismide Alloys in Future Photonic Devices, In: 13TH INTERNATIONAL CONFERENCE ON TRANSPARENT OPTICAL NETWORKS
In a similar manner to the dilute nitrides, the incorporation of Bismuth in semiconductors such as GaAs is predicted to lead to a band-anti-crossing effect (in the valence band) causing a large band gap bowing. In addition, the large size of Bismuth atoms gives rise to a large spin-orbit splitting. This opens-up interesting new possibilities for efficient photonic devices, such as near- and mid-infrared lasers which are more thermally stable and less susceptible to losses compared to conventional InP-based devices. Since Bismuth principally influences the valence band, while nitrogen influences the conduction band, combining Bismuth and Nitrogen in III-V alloys offers huge potential for engineering the conduction and valence band offsets, the band gap and spin-orbit splitting, with wide scope for the design of photonic devices.
Prins AD, Lewis MK, Bushell ZL, Sweeney SJ, Liu S, Zhang Y-H (2015)Evidence for a defect level above the conduction band edge of InAs/InAsSb type-II superlattices for applications in efficient infrared photodetectors, In: APPLIED PHYSICS LETTERS106(17)ARTN 1 AMER INST PHYSICS
We report pressure-dependent photoluminescence (PL) experiments under hydrostatic pressures up to 2.16 GPa on a mid-wave infrared InAs/InAs0.86Sb0.14 type-II superlattice (T2SL) structure at different pump laser excitation powers and sample temperatures. The pressure coefficient of the T2SL transition was found to be 93 ± 2 meV·GPa-1. The integrated PL intensity increases with pressure up to 1.9 GPa then quenches rapidly indicating a pressure induced level crossing with the conduction band states at ∼2 GPa. Analysis of the PL intensity as a function of excitation power at 0, 0.42, 1.87, and 2.16 GPa shows a clear change in the dominant photo-generated carrier recombination mechanism from radiative to defect related. From these data, evidence for a defect level situated at 0.18 ± 0.01 eV above the conduction band edge of InAs at ambient pressure is presented. This assumes a pressure-dependent energy shift of -11 meV·GPa-1 for the valence band edge and that the defect level is insensitive to pressure, both of which are supported by an Arrhenius activation energy analysis.
Jha A, Sweeney SJ (2015)Optical, optoelectronic and photonic materials and applications Preface, In: SEMICONDUCTOR SCIENCE AND TECHNOLOGY30(4)ARTN 04030 IOP PUBLISHING LTD
Maspero Ross, Sweeney Stephen, Florescu Marian (2016)Unfolding the band structure of GaAsBi, In: Journal of Physics: Condensed Matter29(7)075001 Institute of Physics
Typical supercell approaches used to investigate the electronic properties of GaAs(1−x)Bi(x) produce highly accurate, but folded, band structures. Using a highly optimized algorithm, we unfold the band structure to an approximate $E____left(____mathbf{k}____right)$ relation associated with an effective Brillouin zone. The dispersion relations we generate correlate strongly with experimental results, confirming that a regime of band gap energy greater than the spin–orbit-splitting energy is reached at around 10% bismuth fraction. We also demonstrate the effectiveness of the unfolding algorithm throughout the Brillouin zone (BZ), which is key to enabling transition rate calculations, such as Auger recombination rates. Finally, we show the effect of disorder on the effective masses and identify approximate values for the effective mass of the conduction band and valence bands for bismuth concentrations from 0–12%.
Bushell ZL, Ludewig P, Knaub N, Batool Z, Hild K, Stolz W, Sweeney SJ, Volz K (2014)Growth and characterisation of Ga(NAsSi) alloy by metal-organic vapour phase epitaxy, In: JOURNAL OF CRYSTAL GROWTH396pp. 79-84 ELSEVIER SCIENCE BV
Broderick CA, Usman M, O'Reilly EP, Broderick CA, O'Reilly EP, Sweeney SJ (2012)Band engineering in dilute nitride and bismide semiconductor lasers, In: Semiconductor Science and Technology27(9)
Highly mismatched semiconductor alloys such as GaN As and GaBi As have several novel electronic properties, including a rapid reduction in energy gap with increasing x and also, for GaBiAs, a strong increase in spin-orbit-splitting energy with increasing Bi composition. We review here the electronic structure of such alloys and their consequences for ideal lasers. We then describe the substantial progress made in the demonstration of actual GaInNAs telecommunication (telecom) lasers. These have characteristics comparable to conventional InP-based devices. This includes a strong Auger contribution to the threshold current. We show, however, that the large spin-orbit-splitting energy in GaBiAs and GaBiNAs could lead to the suppression of the dominant Auger recombination loss mechanism, finally opening the route to efficient temperature-stable telecomm and longer wavelength lasers with significantly reduced power consumption. © 2012 IOP Publishing Ltd.
Blume G, Hosea TJC, Sweeney SJ, de Mierry P, Lancefield D (2005)AlGalnN resonant-cavity LED devices studied by electromodulated reflectance and carrier lifetime techniques, In: IEE PROCEEDINGS-OPTOELECTRONICS152(2)pp. 118-124 IEE-INST ELEC ENG
Buaprathoom S, Pedley S, Prins AD, Sweeney SJ, Berghmans F, Mignani AG, DeMoor P (2012)High concentration measurement of mixed particle suspensions using simple multi-angle light scattering system, In: OPTICAL SENSING AND DETECTION II8439ARTN 8pp. ?-? SPIE-INT SOC OPTICAL ENGINEERING
Batool Z, Hild K, Hosea TJC, Lu X, Tiedje T, Sweeney SJ (2012)The electronic band structure of GaBiAs/GaAs layers: Influence of strain and band anti-crossing, In: JOURNAL OF APPLIED PHYSICS111(11)ARTN 1pp. ?-? AMER INST PHYSICS
Hossain N, Hild K, Jin SR, Yu S-Q, Johnson SR, Ding D, Zhang Y-H, Sweeney SJ (2013)The influence of growth conditions on carrier recombination mechanisms in 1.3 mu m GaAsSb/GaAs quantum well lasers, In: APPLIED PHYSICS LETTERS102(4)ARTN 0pp. ?-? AMER INST PHYSICS
Blume G, Hosea TJC, Sweeney SJ, Johnson SR, Wang JB, Zhang YH (2005)Spectroscopic investigations of GaAsSb/GaAs based structures for 1.3 mu m VCSEL applications, In: IEE PROCEEDINGS-OPTOELECTRONICS152(2)pp. 110-117 IEE-INST ELEC ENG
Pal J, Migliorato MA, Li C-K, Wu Y-R, Crutchley BG, Marko IP, Sweeney SJ (2013)Enhancement of efficiency of InGaN-based light emitting diodes through strain and piezoelectric field management, In: Journal of Applied Physics114(7)
We report calculations of the strain dependence of the piezoelectric field within InGaN multi-quantum wells light emitting diodes. Such fields are well known to be a strong limiting factor of the device performance. By taking into account the nonlinear piezoelectric coefficients, which in particular cases predict opposite trends compared to the commonly used linear coefficients, a significant improvement of the spontaneous emission rate can be achieved as a result of a reduction of the internal field. We propose that such reduction of the field can be obtained by including a metamorphic InGaN layer below the multiple quantum well active region. © 2013 AIP Publishing LLC.
Sweeney SJ, Jin SR (2013)Bismide-nitride alloys: Promising for efficient light emitting devices in the near- and mid-infrared, In: JOURNAL OF APPLIED PHYSICS113(4)ARTN 0pp. ?-? AMER INST PHYSICS
Mohmad AR, Bastiman F, Hunter CJ, Richards R, Sweeney SJ, Ng JS, David JPR (2012)Effects of rapid thermal annealing on GaAs1-xBix alloys, In: APPLIED PHYSICS LETTERS101(1)ARTN 0pp. ?-? AMER INST PHYSICS
Hossain N, Jin SR, Liebich S, Zimprich M, Volz K, Kunert B, Stolz W, Sweeney SJ (2012)Efficiency-limiting processes in Ga(NAsP)/GaP quantum well lasers, In: APPLIED PHYSICS LETTERS101(1)ARTN 0pp. ?-? AMER INST PHYSICS
Hossain N, Marko IP, Jin SR, Hild K, Sweeney SJ, Lewis RB, Beaton DA, Tiedje T (2012)Recombination mechanisms and band alignment of GaAs1-xBix/GaAs light emitting diodes, In: APPLIED PHYSICS LETTERS100(5)ARTN 0pp. ?-? AMER INST PHYSICS
Chai GMT, Hosea TJC, Fox NE, Hild K, Ikyo AB, Marko IP, Sweeney SJ, Bachmann A, Arafin S, Amann M-C (2014)Characterization of 2.3 mu m GaInAsSb-based vertical-cavity surface-emitting laser structures using photo-modulated reflectance, In: JOURNAL OF APPLIED PHYSICS115(1)ARTN 0pp. ?-? AMER INST PHYSICS
Buaprathoom S, Sweeney SJ, Pedley S (2012)Dual wavelength multiple-angle light scattering system for cryptosporidium detection, In: Progress in Biomedical Optics and Imaging - Proceedings of SPIE8427 Society of Photo-Optical Instrumentation Engineers
A simple, dual wavelength, multiple-angle, light scattering system has been developed for detecting cryptosporidium suspended in water. Cryptosporidium is a coccidial protozoan parasite causing cryptosporidiosis; a diarrheal disease of varying severity. The parasite is transmitted by ingestion of contaminated water, particularly drinking-water, but also accidental ingestion of bathing-water, including swimming pools. It is therefore important to be able to detect these parasites quickly, so that remedial action can be taken to reduce the risk of infection. The proposed system combines multiple-angle scattering detection of a single and two wavelengths, to collect relative wavelength angle-resolved scattering phase functions from tested suspension, and multivariate data analysis techniques to obtain characterizing information of samples under investigation. The system was designed to be simple, portable and inexpensive. It employs two diode lasers (violet InGaN-based and red AlGaInP-based) as light sources and silicon photodiodes as detectors and optical components, all of which are readily available. The measured scattering patterns using the dual wavelength system showed that the relative wavelength angle-resolved scattering pattern of cryptosporidium oocysts was significantly different from other particles (e.g. polystyrene latex sphere, E.coli). The single wavelength set up was applied for cryptosporidium oocysts'size and relative refractive index measurement and differential measurement of the concentration of cryptosporidium oocysts suspended in water and mixed polystyrene latex sphere suspension. The measurement results showed good agreement with the control reference values. These results indicate that the proposed method could potentially be applied to online detection in a water quality control system. © 2012 SPIE.
Sayid SA, Marko IP, Sweeney SJ, Barrios P, Poole PJ (2010)Efficiency limiting processes in 1.55 mu m InAs/InP-based quantum dots lasers, In: APPLIED PHYSICS LETTERS97(16)ARTN 1pp. ?-? AMER INST PHYSICS
Mukherjee J, Wulfken W, Hartje H, Steinsiek F, Perren M, Sweeney SJ (2013)Demonstration of Eye-Safe (1550 nm) Terrestrial Laser Power Beaming at 30 m and Subsequent Conversion into Electrical Power Using Dedicated Photovoltaics, In: 2013 IEEE 39TH PHOTOVOLTAIC SPECIALISTS CONFERENCE (PVSC)pp. 1074-1076 IEEE
Lyytikainen J, Dumitrescu M, Cripps SA, Hosea TJC, Sweeney SJ, Lock D, Leinonen T (2005)High temperature operation of 760 nm vertical-cavity surface-emitting lasers investigated using photomodulated reflectance wafer measurements and temperature-dependent device studies, In: IEE PROCEEDINGS-OPTOELECTRONICS152(2)pp. 103-109 IEE-INST ELEC ENG
Marko IP, Jin SR, Hild K, Batool Z, Bushell ZL, Butkute R, Ludewig P, Stolz W, Volz K, Pacebutas V, Geizutis A, Krotkus A, Sweeney SJ (2015)Properties of hybrid MOVPE/MBE grown GaAsBi/GaAs based near-infrared emitting quantum well lasers, In: SEMICONDUCTOR SCIENCE AND TECHNOLOGY30(9)ARTN 0pp. ?-? IOP PUBLISHING LTD
Crutchley BG, Marko IP, Sweeney SJ, Pal J, Migliorato MA (2013)Optical properties of InGaN-based LEDs investigated using high hydrostatic pressure dependent techniques, In: Physica Status Solidi (B) Basic Research250(4)pp. 698-702
High pressure electroluminescence (EL) measurements were carried out on blue and green emitting InGaN-based light emitting diodes (LEDs). The weak pressure coefficient of the peak emission energy of the LEDs is found to increase with increasing injection current. Such behaviour is consistent with an enhancement of the piezoelectric fields under high pressure which become increasingly screened at high currents. A subsequent increase in the quantum confined Stark effect (QCSE) is expected to cause a reduction of the light output power as pressure is applied at a fixed low current density (∼10Acm). A similar proportional reduction of light output power as pressure is applied at a fixed high current density (260Acm) suggests that there is a non-radiative loss process in these devices which is relatively insensitive to pressure. Such behaviour is shown to be consistent with a defect-related recombination process which increases with increasing injection. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Aldukhayel A, Jin SR, Marko IP, Sweeney SJ, Zhang SY, Revin DG, Cockburn JW (2013)Investigations of carrier scattering into L-valley in λ=3.5μm InGaAs/AlAs(Sb) quantum cascade lasers using high hydrostatic pressure, In: Physica Status Solidi (B) Basic Research250(4)pp. 693-697
In order to identify the performance limitations of InGaAs/AlAs(Sb) quantum cascade lasers, experimental investigations of the temperature and pressure dependencies of the threshold current (I) were undertaken. Using the theoretical optical phonon current (I) and carrier leakage (I) to fit the measured threshold current at various pressures, we show that the electron scattering from the top lasing level to the upper L-minima gives rise to the increase in I with pressure and temperature. It was found that this carrier leakage path accounts for approximately 3% of I at RT and is negligible at 100K. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Mukherjee J, Jarvis S, Sweeney SJ, Perren M (2013)Efficiency limits of laser power converters for optical power transfer applications, In: Journal of Physics D: Applied Physics46(26)
We have developed III-V-based high-efficiency laser power converters (LPCs), optimized specifically for converting monochromatic laser radiation at the eye-safe wavelength of 1.55 m into electrical power. The applications of these photovoltaic cells include high-efficiency space-based and terrestrial laser power transfer and subsequent conversion to electrical power. In addition, these cells also find use in fibre-optic power delivery, remote powering of subcutaneous equipment and several other optical power delivery applications. The LPC design is based on lattice-matched InGaAsP/InP and incorporates elements for photon-recycling and contact design for efficient carrier extraction. Here we compare results from electro-optical design simulations with experimental results from prototype devices studied both in the lab and in field tests. We analyse wavelength and temperature dependence of the LPC characteristics. An experimental conversion efficiency of 44.6% [±1%] is obtained from the prototype devices under monochromatic illumination at 1.55 m (illumination power density of 1 kW m) at room temperature. Further design optimization of our LPC is expected to scale the efficiency beyond 50% at 1 kW m. © 2013 IOP Publishing Ltd.
Usman M, Broderick CA, Batool Z, Hild K, Hosea TJC, Sweeney SJ, O'Reilly EP (2013)Impact of alloy disorder on the band structure of compressively strained GaBixAs1-x, In: PHYSICAL REVIEW B87(11)ARTN 1pp. ?-? AMER PHYSICAL SOC
Ludewig P, Knaub N, Hossain N, Reinhard S, Nattermann L, Marko IP, Jin SR, Hild K, Chatterjee S, Stolz W, Sweeney SJ, Volz K (2013)Electrical injection Ga(AsBi)/(AlGa)As single quantum well laser, In: APPLIED PHYSICS LETTERS102(24)ARTN 2pp. ?-? AMER INST PHYSICS
Duan G-H, Le Liepvre A, Jany C, Lamponi M, Make D, Lelarge F, Fedeli JM, Messaoudene S, Keyvaninia S, Roelkens G, Van Thourhout D, Liu S, Hu Y, Reed GT, Thomson DJ, Gardes FY, Owens N, Yang X, Petropoulos P, Debnath K, O'Faolain L, Krauss TF, Myronov M, Lever L, Ikonic Z, Kelsall RW, Leadley DR, Marko IP, Sweeney SJ, Cox DC, Brimont A, Sanchis P (2012)High performance silicon optical modulators, In: Proceedings of SPIE - The International Society for Optical Engineering8564
In this work we present results from high performance silicon optical modulators produced within the two largest silicon photonics projects in Europe; UK Silicon Photonics (UKSP) and HELIOS. Two conventional MZI based optical modulators featuring novel self-aligned fabrication processes are presented. The first is based in 400nm overlayer SOI and demonstrates 40Gbit/s modulation with the same extinction ratio for both TE and TM polarisations, which relaxes coupling requirements to the device. The second design is based in 220nm SOI and demonstrates 40Gbits/s modulation with a 10dB extinction ratio as well modulation at 50Gbit/s for the first time. A ring resonator based optical modulator, featuring FIB error correction is presented. 40Gbit/s, 32fJ/bit operation is also shown from this device which has a 6um radius. Further to this slow light enhancement of the modulation effect is demonstrated through the use of both convention photonic crystal structures and corrugated waveguides. Fabricated conventional photonic crystal modulators have shown an enhancement factor of 8 over the fast light case. The corrugated waveguide device shows modulation efficiency down to 0.45V.cm compared to 2.2V.cm in the fast light case. 40Gbit/s modulation is demonstrated with a 3dB modulation depth from this device. Novel photonic crystal based cavity modulators are also demonstrated which offer the potential for low fibre to fibre loss. In this case preliminary modulation results at 1Gbit/s are demonstrated. Ge/SiGe Stark effect devices operating at 1300nm are presented. Finally an integrated transmitter featuring a III-V source and MZI modulator operating at 10Gbit/s is presented. © 2012 SPIE.
Simmons RA, Jin SR, Sweeney SJ, Clowes SK (2015)Enhancement of Rashba interaction in GaAs/AlGaAs quantum wells due to the incorporation of bismuth, In: APPLIED PHYSICS LETTERS107(14)ARTN 1 AMER INST PHYSICS
This paper reports on the predicted increase in the Rashba interaction due to the incorporation of Bi in GaAs/AlGaAs heterostructures. Band structure parameters obtained from the band anti-crossing theory have been used in combination with self-consistent Schrödinger-Poisson calculations and k.p models to determine the electron spin-splitting caused by structural inversion asymmetry and increased spin-orbit interaction. A near linear seven fold increase in the strength of the Rashba interaction is predicted for a 10% concentration of Bi in a GaAsBi/AlGaAs quantum well heterostructure.
Sweeney Stephen, Thijs PJA (2003)Origin of the high temperature performance degradation of 1.5 mu m InGaAs(P)/InP quantum well lasers, In: 2003 IEEE LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2pp. 977-978 IEEE
High temperature degradation of the efficiency of 1.5pm InGaAs(P) lasers is shown to be due to strong coupling between Auger recombination and internal absorption. This is explained using a simple analytical model.
O'Reilly EP, Sweeney SJ, Wang S, Zide JMO (2015)Dilute bismides and related alloys Preface, In: SEMICONDUCTOR SCIENCE AND TECHNOLOGY30(9)ARTN 09030 IOP PUBLISHING LTD
Hild K, Marko IP, Johnson SR, Yu S-Q, Zhang Y-H, Sweeney SJ (2011)Influence of de-tuning and non-radiative recombination on the temperature dependence of 1.3 mu m GaAsSb/GaAs vertical cavity surface emitting lasers, In: APPLIED PHYSICS LETTERS99(7)ARTN 0pp. ?-? AMER INST PHYSICS
Chai GMT, Broderick CA, O'Reilly EP, Othaman Z, Jin SR, Petropoulos JP, Zhong Y, Dongmo PB, Zide JMO, Sweeney SJ, Hosea TJC (2015)Experimental and modelling study of InGaBiAs/InP alloys with up to 5.8% Bi, and with Delta(so) > E-g, In: SEMICONDUCTOR SCIENCE AND TECHNOLOGY30(9)ARTN 09401 IOP PUBLISHING LTD
Hild K, Sweeney SJ, Jin SR, Healy SB, O'Rellly EP, Johnson SR, Wang J-B, Zhang Y-H, Jantsch W, Schaffler F (2007)Band alignment and carrier recombination in GaAsSb/GaAs quantum wells, In: Physics of Semiconductors, Pts A and B893pp. 1431-1432
Sayid SA, Marko IP, Cannard PJ, Chen X, Rivers LJ, Lealman IF, Sweeney SJ (2010)Thermal Characteristics of 1.55-mu m InGaAlAs Quantum Well Buried Heterostructure Lasers, In: IEEE JOURNAL OF QUANTUM ELECTRONICS46(5)pp. 700-705 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Hild K, Sweeney SJ, Wright S, Lock DA, Jin SR, Marko IP, Johnson SR, Chaparro SA, Yu S-Q, Zhang Y-H (2006)Carrier recombination in 1.3 mu m GaAsSb/GaAs quantum well lasers, In: APPLIED PHYSICS LETTERS89(17)ARTN 1pp. ?-? AMER INST PHYSICS
Coote J, Reddy S, Sweeney SJ (2007)Optimisation of distributed feedback laser biosensors, In: IET OPTOELECTRONICS1(6)pp. 266-271
Blume G, Hosea TJC, Sweeney SJ (2005)A study of the low-energy interference oscillations in photoreflectance of GaAsSb/GaAs quantum well structures, In: PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE202(7)pp. 1244-1254
Jin S, Sweeney SJ (2013)InGaAsBi alloys on InP for efficient near- and mid-infrared light emitting devices, In: JOURNAL OF APPLIED PHYSICS114(21)ARTN 21310 AMER INST PHYSICS
Maspero R, Sweeney SJ, Florescu M (2013)Modelling the Auger Recombination rates of GaAs(1-x)Bi x alloys, In: 13th International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 2013pp. 81-82
We calculate the Conduction, Heavy Hole (HH) - Split-off Hole (SO), HH (CHSH) Auger Recombination rates for GaAs(1-x)Bix alloys, which are candidates for highly efficient telecommunication devices. A ten-band, tight-binding method, including spin-orbit coupling, was performed on a 9×9×9 strained supercell in order to generate an accurate band structure to perform the calculation on. This band structure was then unfolded to give a true E-k relation. As predicted by experiment, there should be a decrease in the Auger recombination rate as the concentration of Bismuth increases ending in a suppression at greater than ∼11% Bismuth. © 2013 IEEE.
Hild Konstanze, Sweeney Stephen, Lock DA, Wright S, Wang JB, Johnson SR, Zhang YH (2005)On the thermal stability of 1.3 mu m GaAsSb/GaAs-based lasers, In: 2005 IEEE LEOS Annual Meeting Conference Proceedings (LEOS)pp. 330-331 IEEE
In spite of the almost ideal variation of the radiative current of 1.3 mu m GaAsSb/GaAs-based lasers, the threshold current, J(th), is high due to non-radiative recombination accounting for 90% J(th) near room temperature. This also gives rise to low T-0 values similar to 60K close to room temperature, similar to that for InGaAsP/InP.
Thomas T, Mellor A, Hylton NP, Fuehrer M, Alonso-Alvarez D, Braun A, Ekins-Daukes NJ, David JPR, Sweeney SJ (2015)Requirements for a GaAsBi 1eV sub-cell in a GaAs-based multi-junction solar cell, In: SEMICONDUCTOR SCIENCE AND TECHNOLOGY30(9)ARTN 09401 IOP PUBLISHING LTD
Calibrations of Standard Platinum Resistance Thermometers (SPRTs), the defined interpolating instrument of the International Temperature Scale of 1990 (ITS-90), are performed using fixed-point cells containing nearly pure materials (of the order of 99.9999%). The effect of impurities at the parts per million levels on the measured temperature during fixed point realisations is often the most significant contribution to the calibration uncertainty budget. The recommended method for evaluating this impurity effect relies on chemical analyses whose uncertainties and detection limits are often too large at the small quantities of impurities present, and as such cannot inform a meaningful correction; rather, this result informs the impurity contribution to the uncertainty budget. In order to elucidate the effects of the myriad experimental parameters which may affect the cell conditions, and hence the measured freezing curve, a 2D-axisymmetric coupled heat and impurity transport model utilising the Phase-field Method is developed. A broad qualitative study of tin point realisations illustrates a range of phenomena—including thermal gradient interface instabilities and solid-state de-wetting—which have effects on the measured temperature of the order of 100-200 μK. The interfacial instability has not, to our best knowledge, been predicted elsewhere with respect to the realisation scheme studied. The instability wavelength observed in the model is λ=3.1±0.7 mm, which compares favourably with the growing Fourier mode of shortest wavelength predicted by Mullins-Sekerka instability theory (λ≈4 mm). The solid-state de-wetting phenomenon offers a potential mechanism which agrees well with several experimental trends noted in the literature; the existence of a ‘critical’ film thickness during freeze initiation and the presence of a variable temperature depression over the freeze duration. This work provides a basis for tailoring of experimental techniques so that reliable corrections for the effect of impurities can be determined, thus leveraging a step decrease in potential calibration uncertainties. Since this exercise is performed with the UK national temperature standards, this work directly benefits all fields that rely on the precise measurement of temperature. Also, there is significant potential for further development of the model; some avenues for research are suggested.
This thesis describes an experimental study of exciton recombination in isolated semiconducting single-wall carbon nanotubes synthesised by different methods and wrapped in different polymers PFO (9,9-dioctylfluorenyl-2,7-diyl), PFO-BPy (9,9-Dioctyfluorenyl-2,7-diyl-Bipyridine) and P3HT (poly 3-hexylthiophene). We present a comprehensive study using femtosecond transient absorption measurements of the kinetics of exciton recombination, where diffusion of excitons in the confined one-dimensional system significantly affects their optical and electronic properties. In all studied samples of isolated nanotubes, an exciton-exciton process dominated the recombination under high excitation, and exhibited a distinct crossover to a diffusion-limited regime with anomalous kinetics at late times. We attribute the reaction-diffusion crossover to a finite reaction probability per exciton-exciton encounter. We have demonstrated a methodology to determine the microscopic parameters controlling reaction and diffusion processes, based on measurements at high initial density where the optical absorption is fully saturated. In studies of the same nanotube species synthesized by both HiPCO (high pressure catalytic decomposition of carbon monoxide) and CoMoCAT (cobalt and molybdenum catalysts based chemical vapour deposition technique) methods and wrapped by different polymers, the exciton reaction probability was approximately constant, corresponding to on average one in five exciton-exciton interactions resulting in an exciton recombination. On the other hand, there was significant variation in the diffusive hopping time for samples synthesized by different processes or subject to different processing. This is consistent with the wide range of values for the diffusion coefficient reported in the literature. We have found that excitons in nanotubes synthesized by the HiPCO are more mobile than CoMoCAT nanotubes, by a factor of ~2 for nanotubes wrapped in P3HT. This may be associated with a lower defect concentration in HiPCO nanotubes compared to CoMoCAT. The nanomaterials studied in this thesis are promising for nanotube-organic hybrids based light emitting and harvesting devices. Our findings will not only facilitate the selection of materials in these applications but also represent experimental data that challenges existing theoretical models for kinetics of non-equilibrium stochastic systems.
Optoelectronic integration on silicon is an area of increasing interest for both physicists and the microelectronics industry. Due to the limitations of silicon as an optical gain medium, the integration of III-Vs with silicon microelectronics has become a prominent area of research. However, the fundamental physical differences between these materials has caused such lasers to be strongly limited by non-radiative recombination. Studies of these mechanisms are therefore essential for solutions to be developed that will allow commercially viable III-V/Si lasers to be fabricated. This thesis presents such studies for three of the four leading approaches to producing III-V/Si lasers (quantum dots on silicon are not studied), with conclusions on the relative performance of each presented in the final chapter. AlGaInAs/InP laser active regions wafer bonded onto pre-processed silicon-on-insulator waveguides have exhibited strong performance, with electrical injection lasing demonstrated at room temperature. However, large and temperature sensitive threshold current densities of 3.4 6.16 kAcm
In this thesis we discuss the manufacture and characterisation of micro-optical elements, for guiding light into sub-wavelength beams & spots, and for use in super-resolution imaging. A physical limit exists in microscopy where it is impossible to view object smaller than half the illuminating wavelength, via conventional means. In white light microscopy this creates an resolution limit of 321nm (at a wavelength of 500nm, in air). This places a limit on the smallest objects a researcher can study using optical microscopy. We present a method for fabricating plano-convex lenses which, when placed in near proximity to the samples, boost magnification of conventional microscopes by up-to 2.5x and resolve features below 200nm, with white light illumination. We also demonstrate a curved axicon Bessel-beam former, that produces long (17 micrometer) non-diffracting beams of light, that can be sub-wavelength in width, down to 2/3rds the wavelength. In this thesis we contribute the following to current knowledge: We describe a focused ion-beam milling technique to form bespoke geometry of parabolic & spherical curvature, including reflective dishes, of diameter 1-10 microns, with a surface roughness of 4.0-4.1nm. As part of this work, we calculate the efficiency of a new technique for removing ion-beam induced damage, using wet-chemical etching. Here we show that increasing the ion-dose above 3000 µC/cm^2 allows a higher percentage of the implantation and amorphisation damage to be removed, and leaves less than 0.5% of the gallium remaining in the surface. We use the ion-milled dishes to form lens moulds; we double-replicate the brittle silicon mould, to create a hard wearing rubber mould. As multiple rubber moulds can be created per silicon mould the process becomes industrially scalable. A thin-film of polymer lenses is then formed from the mould. We characterise these lenses, demonstrating 1.2-2.5x magnification and resolution of 200nm. We demonstrate their use by imaging two biological samples, one fixed & stained, and one unlabelled in water. Additionally, using computer simulations alongside the focused ion-beam manufacturing technique, we demonstrate a curved axicon lens structure, that forms long, non-diffracting beams of intense light. We model and experimentally analyse how the lens profile and high-to-low refractive index change forms the beam, and show that increasing the refractive index change decreases the beam width but at a loss of light transmission.
This thesis investigates a range of novel photonic devices and their constituent semiconductor materials with emission in the infrared (IR) region of the spectrum. These have a variety of potential applications, including in telecoms, sensing and defence systems. Studies focus on aspects of the electronic and photonic band structures, and how these impact upon device performance. Type-II interband cascade lasers and LEDs emitting in the mid-IR region of 3 – 4 μm are characterised using temperature and hydrostatic pressure dependent techniques. The key finding is that the threshold current density exhibits a minimum for emission around 0.35 eV (~3.5 μm), in both the pressure dependent results and data for many devices with different design wavelengths. The increase in threshold current density towards lower energies can be explained by an increase in CHCC Auger recombination. The increase in threshold current towards higher energies cannot be well explained by an Auger process, and it is concluded that this may be evidence of defect-related recombination. Dilute bismide alloys are an interesting new material system for IR applications. The electronic and optical properties of several dilute bismide alloys are determined by spectroscopic ellipsometry. Key findings include the first experimental measurements of the spin-orbit splitting in GaNAsBi, which show that it is approximately independent of N content, and the first evidence for a decrease in the direct band gap of GaP with the addition of bismuth, reducing by 130 ± 20 meV/%Bi. The refractive index was determined for all the materials and in the transparency region the real part of the refractive index was found to decrease approximately linearly with increasing band gap. In addition to modifying the electronic properties, photonic effects can be used to develop new IR devices. Finite difference time domain simulations of photonic crystal cavity structures within thick multi-layer slabs were carried out. These showed that it is possible to achieve high Q-factors, > 10^4, in slabs with refractive indices corresponding to typical semiconductor heterostructures. This opens up possibilities for designing photonic crystal lasers that do not require the thin suspended membranes typically found in the literature, with applications in integrated photonic circuits and on-chip sensors.
This work describes the design, development and characterisation of high efficiency photovoltaics (laser power converters) for the conversion of monochromatic light from a laser source into electrical energy. The technology provides a means of transmitting power wirelessly through free-space, for applications in the remote powering of electrical devices and systems. It also provides a means of efficiently transmitting power though fibre-optic cables, allowing electrical power to be delivered free from electromagnetic interference. The design of the laser power converter is considered for efficient conversion of monochromatic light at a target wavelength of 1550nm. This wavelength was chosen based on its ability to transmit through the atmosphere and silica-based fibre-optics with minimal losses. It also allows for the maximum exposure limit of 1kWm^-2 to be transmitted in free-space, which is eye- and skin-safe. Various semiconductor materials were explored for this design in terms of their maturity, band-gap tunability and lattice matching to common substrates. The laser power converter was then developed based on the material system InGaAsP/InP with a band-gap tuned to match the incident target wavelength. These cells were then characterised using a tunable laser source and the best cell achieved a conversion efficiency (at 20 degrees) of 38.9% at an irradiance of 0.73kWm^-2 at the target wavelength. However, earlier field tests conducted by Dr. Jayanta Mukherjee demonstrated an efficiency of 45% at 1kWm^-2, which is much higher than conventional single-junction solar cells and currently holds the record for monochromatic PVs operating at 1550nm. The various carrier recombination mechanisms that limit the efficiency are then investigated by measuring the cell performance down to temperatures of 100K. In this measurement the efficiency at 39Wm^-2 is shown to increase from 28.6% to 72% over the temperature range 300-100K and approaches the theoretical detailed-balance limit. An advanced temperature-dependent diode and resistance model is then formulated to predict the dominant carrier loss mechanisms at room temperature. It was found that (to a first approximation) defect-related carrier recombination dominates over the temperature range with a lifetime of 5us at room temperature. The model also determined a carrier mobility at room temperature of 12.4 cm^2V^-1s^-1 in the emitter layer, which results in a high sheet resistance and limits carrier transport to the contacts. Finally, the effects of non-uniform illumination (due to the Gaussian laser beam profile) on the device performance is investigated. A detailed carrier transport model is devised to understand the implication of non-uniform illumination on the diffusion and recombination of carriers generated in the top emitter layer. A light-beam-induced-current scan and a carrier-time-of-flight scan across the cell surface is then conducted to determine local changes in the device performance and obtain the carrier transport properties. From this the emitter diffusion coefficient and SRH lifetime (to a first approximation) were found to be 3.96cm^2s^-1 and 5us, which is in good agreement with the temperature-dependent illumination study. This work then proposes a new top contact design, which overcomes the impact of non-uniform illumination and sheet resistance.