# Professor Stephen Sweeney

## Research Interests

Stephen's primary research interests lie in the area of semiconductor materials for photonic devices. He has produced >300 journal papers and conference proceedings in this area including several invited papers. He has also published >10 patents and 5 book chapters in the field of photonics.

Recent research topics include:

• novel semiconductor alloys such as bismides and nitrides for high efficiency photonic devices
• silicon compatible lasers for future optical computing
• semiconductor lasers for temperature insensitive operation (quantum dots, dilute-nitrides etc)
• LED sources for solid-state lighting
• vertical cavity surface emitting lasers and LEDs for plastic fibre systems
• semiconductor lasers for optical pumping (EDFA, Raman) and printing and data storage applications
• using photonic devices to sense chemical and biological samples (liquids and gases)

Information about currently available PhD projects can be found here

### Research Collaborations

Stephen collaborates with a wide range of international partners in both academia and industry, as can be seen from his publications list.

### Teaching

#### Current:

Level 2 Solid State Physics
Level 2 Laboratories

Module Leader for PHY2068 Solid State Physics

#### Past:

Level 0 Foundation Year Tutorials
Level 1 Principles of Physics (1PP)
Level 1 Electronics (1EL)
Level 1 Physics Laboratories (1LAB)
Level 3 Physics in Education (3PIE) [assessor]
Level M Photonics and Nanotechnology (3PNT)
Level D LabView

### Departmental Duties

• Head of Department of Physics (2015-)
• Head of Photonics group (2010-2015)
• Departmental Management group (2010-)
• Departmental Policy & Strategy group (2008-)
• Departmental & Faculty Research Committee (2010-)
• Chair, Physics Publicity & Marketing group (2010-13)
• Chair of Physics Department Board of Studies (2008-10)
• Chair of Physics Department BoS sub-committee (2008-10)
• MPhys/BSc Physics Programme Coordinator (2008-10)
• Physics with Foundation Year Co-ordinator (2003-2006)
• Physics Schools Liaison Officer (2004-2008)
• Physics Marketing group (2006-2008)
• Physics MPhys research placements co-ordinator (2006-2010)
• FEPS PG Research committee (2002-2008)

### Affiliations

• Fellow of the Institute of Physics, Chartered Physicist and a Senior Member of the Institute of Electrical and Electronic Engineers.
• Editorial Boards of the Journal of Quantum Electronics and the Journal of Materials Science: Materials in Electronics.
• Proposal Reviewer/advisor: EPSRC College Member, EU Photonics 21 Member, US Dept of Energy, Singapore A* Research Agency, Lithuanian Research Council, Humbolt Foundation, Austrian research agency, EU-Framework projects and NSF (USA).

### Research Grants

Stephen currently has ~£3M of research funding from sources including EPSRC, EU, TSB and commercial organisations.

## Contact Me

E-mail:
Phone: 01483 68 6814

Find me on campus
Room: 07 BB 03

My office hours

07 BB 03 (Monday-Thursday)

12 ATI 01 (Friday)

(Email Joanna Moore for appointments)

## Publications

### Highlights

• . (2016) 'Optical gain in GaAsBi/GaAs quantum well diode lasers'. Nature Publishing Group Scientific Reports, 6

#### Abstract

Electrically pumped GaAsBi/GaAs quantum well lasers are a promising new class of near-infrared devices where, by use of the unusual band structure properties of GaAsBi alloys, it is possible to suppress the dominant energy-consuming Auger recombination and inter-valence band absorption loss mechanisms, which greatly impact upon the device performance. Suppression of these loss mechanisms promises to lead to highly efficient, uncooled operation of telecommunications lasers, making GaAsBi system a strong candidate for the development of next-generation semiconductor lasers. In this report we present the first experimentally measured optical gain, absorption and spontaneous emission spectra for GaAsBi-based quantum well laser structures. We determine internal optical losses of 10–15 cm−1 and a peak modal gain of 24 cm−1, corresponding to a material gain of approximately 1500 cm−1 at a current density of 2 kA cm−2. To complement the experimental studies, a theoretical analysis of the spontaneous emission and optical gain spectra is presented, using a model based upon a 12-band k.p Hamiltonian for GaAsBi alloys. The results of our theoretical calculations are in excellent quantitative agreement with the experimental data, and together provide a powerful predictive capability for use in the design and optimisation of high efficiency lasers in the infrared.

• . (2016) 'Temperature stable mid-infrared GaInAsSb/GaSb Vertical Cavity Surface Emitting Lasers (VCSELs)'. Nature Publishing Group Scientific Reports, 6 Article number 19595

#### Abstract

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.

• . (2014) 'Development and characterisation of laser power converters for optical power transfer applications'. IET Optoelectronics, 8 (2), pp. 64-70.
• . (2013) 'Electrical injection Ga(AsBi)/(AlGa)As single quantum well laser'. Applied Physics Letters, 102 (24)

#### Abstract

The Ga(AsBi) material system opens opportunities in the field of high efficiency infrared laser diodes. We report on the growth, structural investigations, and lasing properties of dilute bismide Ga(AsBi)/(AlGa)As single quantum well lasers with 2.2% Bi grown by metal organic vapor phase epitaxy on GaAs (001) substrates. Electrically injected laser operation at room temperature is achieved with a threshold current density of 1.56 kA/cm2 at an emission wavelength of ∼947 nm. These results from broad area devices show great promise for developing efficient IR laser diodes based on this emerging materials system. © 2013 AIP Publishing LLC.

• . (2013) 'Impact of alloy disorder on the band structure of compressively strained GaBixAs1-x'. AMER PHYSICAL SOC PHYSICAL REVIEW B, 87 (11) Article number ARTN 115104
• . (2013) 'Bismide-nitride alloys: Promising for efficient light emitting devices in the near- and mid-infrared'. Journal of Applied Physics, 113 (4)
• . (2012) 'Temperature and Bi-concentration dependence of the bandgap and spin-orbit splitting in InGaBiAs/InP semiconductors for mid-infrared applications'. Applied Physics Letters, 101 (22)

#### Abstract

Replacing small amounts of As with Bi in InGaBiAs/InP induces large decreases and increases in the bandgap, Eg, and spin-orbit splitting, ΔSO, respectively. The possibility of achieving ΔSO > Eg and a reduced temperature (T) dependence for Eg are significant for suppressing recombination losses and improving performance in mid-infrared photonic devices. We measure E g(x, T) and ΔSO (x, T) in In0.53Ga 0.47BixAs1-x/InP samples for 0 x 0.039 by various complementary optical spectroscopic techniques. While we find no clear evidence of a decreased dEg/dT (≈0.34 ± 0.06 meV/K in all samples) we find ΔSO > Eg for x > 3.3-4.3. The predictions of a valence band anti-crossing model agree well with the measurements. © 2012 American Institute of Physics.

• . (2012) 'Efficiency-limiting processes in Ga(NAsP)/GaP quantum well lasers'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 101 (1) Article number ARTN 011107
• . (2012) 'The electronic band structure of GaBiAs/GaAs layers: Influence of strain and band anti-crossing'. AMER INST PHYSICS JOURNAL OF APPLIED PHYSICS, 111 (11) Article number ARTN 113108
• . (2011) 'Modulation of the absorption coefficient at 1.3 mu m in Ge/SiGe multiple quantum well heterostructures on silicon'. OPTICS LETTERS, 36 (21), pp. 4158-4160.

#### Abstract

We report modulation of the absorption coefficient at 1.3 μm in Ge/SiGe multiple quantum well heterostructures on silicon via the quantum-confined Stark effect. Strain engineering was exploited to increase the direct optical bandgap in the Ge quantum wells. We grew 9 nm-thick Ge quantum wells on a relaxed Si0.22Ge0.78 buffer and a contrast in the absorption coefficient of a factor of greater than 3.2 was achieved in the spectral range 1290–1315 nm.

• . (2011) 'Laser operation of Ga(NAsP) lattice-matched to (001) silicon substrate'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 99 (7) Article number ARTN 071109
• . (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'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 99 (7) Article number ARTN 071110
• . (2011) 'Temperature dependence of 4.1 mu m mid-infrared type II "W" interband cascade lasers'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 99 (2) Article number ARTN 021102
• . (2011) 'Photoluminescence investigation of high quality GaAs1-xBix on GaAs'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 98 (12) Article number ARTN 122107
• . (2010) 'Efficiency limiting processes in 1.55 mu m InAs/InP-based quantum dots lasers'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 97 (16) Article number ARTN 161104

### Journal articles

• . (2017) 'High-Q photonic crystal cavities in all-semiconductor photonic crystal heterostructures'. American Physical Society Physical Review B, 95 (23) Article number 235303

#### Abstract

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.

• . (2017) 'GaAs1−xBix/GaNyAs1−y type-II quantum wells: novel strain-balanced heterostructures for GaAs-based near- and mid-infrared photonics'. Nature Publishing Group Scientific Reports, 7 Article number 46371

#### Abstract

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.

• . (2017) 'Wavelength dependence of efficiency limiting mechanisms in Type-I Mid-infrared GaInAsSb/GaSb lasers'. IEEE IEEE Journal of Selected Topics in Quantum Electronics, 23 (6)

#### Abstract

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.

• . (2017) 'Progress towards III-V-Bismide Alloys for Near- and Mid-Infrared Laser Diodes'. Institute of Electrical and Electronics Engineers (IEEE) IEEE Journal of Selected Topics in Quantum Electronics, 23 (6) Article number 1501512

#### Abstract

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.

• . (2016) 'Unfolding the band structure of GaAsBi'. Institute of Physics Journal of Physics: Condensed Matter, 29 (7) Article number 075001

#### Abstract

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%.

• . (2016) 'Relationship between Human Pupillary Light Reflex and Circadian System Status'. Public Library of Science (PLoS) PLoS One, 11 (9) Article number e0162476
• . (2016) 'Optical gain in GaAsBi/GaAs quantum well diode lasers'. Nature Publishing Group Scientific Reports, 6

#### Abstract

Electrically pumped GaAsBi/GaAs quantum well lasers are a promising new class of near-infrared devices where, by use of the unusual band structure properties of GaAsBi alloys, it is possible to suppress the dominant energy-consuming Auger recombination and inter-valence band absorption loss mechanisms, which greatly impact upon the device performance. Suppression of these loss mechanisms promises to lead to highly efficient, uncooled operation of telecommunications lasers, making GaAsBi system a strong candidate for the development of next-generation semiconductor lasers. In this report we present the first experimentally measured optical gain, absorption and spontaneous emission spectra for GaAsBi-based quantum well laser structures. We determine internal optical losses of 10–15 cm−1 and a peak modal gain of 24 cm−1, corresponding to a material gain of approximately 1500 cm−1 at a current density of 2 kA cm−2. To complement the experimental studies, a theoretical analysis of the spontaneous emission and optical gain spectra is presented, using a model based upon a 12-band k.p Hamiltonian for GaAsBi alloys. The results of our theoretical calculations are in excellent quantitative agreement with the experimental data, and together provide a powerful predictive capability for use in the design and optimisation of high efficiency lasers in the infrared.

• . (2016) 'Temperature stable mid-infrared GaInAsSb/GaSb Vertical Cavity Surface Emitting Lasers (VCSELs)'. Nature Publishing Group Scientific Reports, 6 Article number 19595

#### Abstract

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.

• . (2015) 'Enhancement of Rashba interaction in GaAs/AlGaAs quantum wells due to the incorporation of bismuth'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 107 (14) Article number ARTN 142401
• . (2015) 'Requirements for a GaAsBi 1eV sub-cell in a GaAs-based multi-junction solar cell'. IOP PUBLISHING LTD SEMICONDUCTOR SCIENCE AND TECHNOLOGY, 30 (9) Article number ARTN 094010
• . (2015) 'Properties of hybrid MOVPE/MBE grown GaAsBi/GaAs based near-infrared emitting quantum well lasers'. IOP PUBLISHING LTD SEMICONDUCTOR SCIENCE AND TECHNOLOGY, 30 (9) Article number ARTN 094008
• . (2015) 'Dilute bismides and related alloys Preface'. IOP PUBLISHING LTD SEMICONDUCTOR SCIENCE AND TECHNOLOGY, 30 (9) Article number ARTN 090301
• . (2015) 'Experimental and modelling study of InGaBiAs/InP alloys with up to 5.8% Bi, and with Delta(so) > E-g'. IOP PUBLISHING LTD SEMICONDUCTOR SCIENCE AND TECHNOLOGY, 30 (9) Article number ARTN 094015
• . (2015) 'Evidence for a defect level above the conduction band edge of InAs/InAsSb type-II superlattices for applications in efficient infrared photodetectors'. Applied Physics Letters, 106 (17)

#### Abstract

© 2015 AIP Publishing LLC.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.

• . (2015) 'Physical Properties and Characteristics of III-V Lasers on Silicon'. IEEE IEEE Journal of Selected Topics in Quantum Electronics, 21 (6) Article number 1502208

#### Abstract

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.

• . (2015) 'Optical, optoelectronic and photonic materials and applications'. Semiconductor Science and Technology, 30 (4)
• . (2015) 'Semiconductor Quantum Well Lasers With a Temperature-Insensitive Threshold Current'. IEEE IEEE Journal of Selected Topics in Quantum Electronics, 21 (6) Article number 150080

#### Abstract

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.

• . (2015) 'Preface'. Journal of Physics: Conference Series, 619 (1)
• . (2015) 'Optical and electronic processes in semiconductor materials for device applications'. Springer Series in Materials Science, 203, pp. 253-297.
• . (2014) 'Physical properties and optimization of GaBiAs/(Al)GaAs based near-infrared laser diodes grown by MOVPE with up to 4.4% Bi'. Journal of Physics D: Applied Physics, 47 (34)
• . (2014) 'Growth and characterisation of Ga(NAsSi) alloy by metal-organic vapour phase epitaxy'. ELSEVIER SCIENCE BV JOURNAL OF CRYSTAL GROWTH, 396, pp. 79-84.
• . (2014) 'Localization effects and band gap of GaAsBi alloys'. Physica Status Solidi (B) Basic Research, 251 (6), pp. 1276-1281.
• . (2014) 'Characterization of 2.3 mu m GaInAsSb-based vertical-cavity surface-emitting laser structures using photo-modulated reflectance'. AMER INST PHYSICS JOURNAL OF APPLIED PHYSICS, 115 (1) Article number ARTN 013102
• . (2014) 'Electrically injected GaAsBi quantum well lasers'. Conference Digest - IEEE International Semiconductor Laser Conference, , pp. 80-81.
• . (2014) 'Effect of non-pinned carrier density above threshold in InAs quantum dot and quantum dash lasers'. IET Optoelectronics, 8 (2), pp. 88-93.
• . (2014) 'Development and characterisation of laser power converters for optical power transfer applications'. IET Optoelectronics, 8 (2), pp. 64-70.
• . (2013) 'InGaAsBi alloys on InP for efficient near- and mid-infrared light emitting devices'. Journal of Applied Physics, 114 (21)
• . (2013) 'The influence of temperature on the recombination processes in blue and green InGaN LEDs'. Physica Status Solidi (C) Current Topics in Solid State Physics, 10 (11), pp. 1533-1536.
• . (2013) 'Experimental evaluation of impact ionization in dilute nitride GaInNAs diodes'. Applied Physics Letters, 103 (10)

#### Abstract

The anomalous behavior of impact ionization in dilute-nitride GaInNAs photodiodes with a range of nitrogen content below 4% is investigated. The ratio of hole- and electron-initiated ionization coefficients, k = β/α, is enhanced by a factor up to ∼4 with increasing nitrogen content. The absolute ionization coefficients are suppressed by up to two orders of magnitude at low electric fields in samples with 3% N. The narrow band gap, suppressed impact ionization, and increased breakdown voltage characteristics of GaInNAs make it a suitable material for use as part of a composite collector in GaAs-based heterojunction bipolar transistors. © 2013 AIP Publishing LLC.

• . (2013) 'Enhancement of efficiency of InGaN-based light emitting diodes through strain and piezoelectric field management'. Journal of Applied Physics, 114 (7)
• . (2013) 'Efficiency limits of laser power converters for optical power transfer applications'. Journal of Physics D: Applied Physics, 46 (26)
• . (2013) 'Electrical injection Ga(AsBi)/(AlGa)As single quantum well laser'. Applied Physics Letters, 102 (24)

#### Abstract

The Ga(AsBi) material system opens opportunities in the field of high efficiency infrared laser diodes. We report on the growth, structural investigations, and lasing properties of dilute bismide Ga(AsBi)/(AlGa)As single quantum well lasers with 2.2% Bi grown by metal organic vapor phase epitaxy on GaAs (001) substrates. Electrically injected laser operation at room temperature is achieved with a threshold current density of 1.56 kA/cm2 at an emission wavelength of ∼947 nm. These results from broad area devices show great promise for developing efficient IR laser diodes based on this emerging materials system. © 2013 AIP Publishing LLC.

• . (2013) 'Optical properties of InGaN-based LEDs investigated using high hydrostatic pressure dependent techniques'. Physica Status Solidi (B) Basic Research, 250 (4), pp. 698-702.
• . (2013) 'Investigations of carrier scattering into L-valley in λ=3.5μm InGaAs/AlAs(Sb) quantum cascade lasers using high hydrostatic pressure'. Physica Status Solidi (B) Basic Research, 250 (4), pp. 693-697.
• . (2013) 'Impact of alloy disorder on the band structure of compressively strained GaBixAs1-x'. AMER PHYSICAL SOC PHYSICAL REVIEW B, 87 (11) Article number ARTN 115104
• . (2013) 'The influence of growth conditions on carrier recombination mechanisms in 1.3 μm GaAsSb/GaAs quantum well lasers'. Applied Physics Letters, 102 (4)

#### Abstract

We investigate the temperature and pressure dependence of the threshold current density of edge-emitting GaAsSb/GaAs quantum well (QW) lasers with different device characteristics. Thermally activated carrier leakage via defects is found to be very sensitive to the growth conditions of GaAsSb QWs. An optimization of the growth conditions reduces the nonradiative recombination mechanisms from 93% to 76% at room temperature. This improvement in carrier recombination mechanisms leads to a large improvement in the threshold current density from 533 Acm-2/QW to 138 Acm-2/QW and the characteristic temperature, T0 (T1), from 51 ± 5 K (104 ± 16 K) to 62 ± 2 K (138 ± 7 K) near room temperature. © 2013 American Institute of Physics.

• . (2013) 'Bismide-nitride alloys: Promising for efficient light emitting devices in the near- and mid-infrared'. Journal of Applied Physics, 113 (4)
• . (2013) 'On the fundamental efficiency limits of photovoltaic converters for optical power transfer applications'. Conference Record of the IEEE Photovoltaic Specialists Conference, , pp. 1031-1035.
• . (2013) 'LED junction temperature measurement using generated photocurrent'. IEEE/OSA Journal of Display Technology, 9 (5), pp. 396-401.
• . (2013) 'Modelling the Auger Recombination rates of GaAs(1-x)Bi x alloys'. 13th International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 2013, , pp. 81-82.
• . (2013) 'Self-calibration of a W/Re thermocouple using a miniature Ru-C (1954 °c) eutectic cell'. AIP Conference Proceedings, 1552 8, pp. 504-509.
• . (2013) 'Introduction to the issue on semiconductor lasers'. IEEE Journal on Selected Topics in Quantum Electronics, 19 (4)
• . (2013) 'Demonstration of eye-safe (1550 nm) terrestrial laser power beaming at 30 m and subsequent conversion into electrical power using dedicated photovoltaics'. Conference Record of the IEEE Photovoltaic Specialists Conference, , pp. 1074-1076.
• . (2013) 'Investigating the efficiency limitations of GaN-based emitters'. 2013 Conference on Lasers and Electro-Optics Europe and International Quantum Electronics Conference, CLEO/Europe-IQEC 2013,

#### Abstract

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.

• . (2013) 'The potential of GaAsBiN for multi-junction solar cells'. Conference Record of the IEEE Photovoltaic Specialists Conference, , pp. 2474-2478.
• . (2013) 'The influence of temperature on the recombination processes in blue and green InGaN LEDs'. Physica Status Solidi (C) Current Topics in Solid State Physics, 2 (12)
• . (2013) 'HYBRID SILICON DEVICES FOR ENERGY-EFFICIENT OPTICAL TRANSMITTERS'. IEEE COMPUTER SOC IEEE MICRO, 33 (1), pp. 22-31.
• . (2013) 'Bismuth-containing III-V semiconductors: Epitaxial growth and physical properties'. , pp. 139-158.
• . (2012) 'Improved optoelectronic properties of rapid thermally annealed dilute nitride GaInNAs photodetectors'. IEEE Journal of Electronic Materials, 41 (12), pp. 3393-3401.

#### Abstract

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.

• . (2012) 'Temperature and Bi-concentration dependence of the bandgap and spin-orbit splitting in InGaBiAs/InP semiconductors for mid-infrared applications'. Applied Physics Letters, 101 (22)

#### Abstract

Replacing small amounts of As with Bi in InGaBiAs/InP induces large decreases and increases in the bandgap, Eg, and spin-orbit splitting, ΔSO, respectively. The possibility of achieving ΔSO > Eg and a reduced temperature (T) dependence for Eg are significant for suppressing recombination losses and improving performance in mid-infrared photonic devices. We measure E g(x, T) and ΔSO (x, T) in In0.53Ga 0.47BixAs1-x/InP samples for 0 x 0.039 by various complementary optical spectroscopic techniques. While we find no clear evidence of a decreased dEg/dT (≈0.34 ± 0.06 meV/K in all samples) we find ΔSO > Eg for x > 3.3-4.3. The predictions of a valence band anti-crossing model agree well with the measurements. © 2012 American Institute of Physics.

• . (2012) 'Band engineering in dilute nitride and bismide semiconductor lasers'. Semiconductor Science and Technology, 27 (9)
• . (2012) 'Cavity mode gain alignment in GaAsSb-based near-infrared vertical cavity lasers studied by spectroscopy and device measurements'. Journal of Applied Physics, 112 (3)

#### Abstract

We present a combination of spectroscopy and device measurements on GaAsSb/GaAs vertical-cavity surface-emitting laser (VCSEL) structures to determine the temperature at which the wavelength of the VCSEL cavity mode (CM) aligns with that of the quantum well (QW) ground-state transition (GST), and therefore the gain peak. We find that, despite the achievement of room temperature (RT) continuous wave lasing in VCSEL devices, the QW transition and the CM are actually slightly misaligned at this temperature; room temperature electroluminescence measurements from a cleaved edge of the VCSEL wafer indicate that the 300 K QW GST energy is at 0.975 ± 0.005 eV, while the CM measured in the VCSEL surface reflectivity spectra is at 0.9805 ± 0.0002 eV. When the wafer sample is cooled, the CM and QW GST can be brought into alignment at 270 ± 10 K, as confirmed by temperature-dependent electro-modulated reflectance (ER) and edge-electroluminescence spectroscopic studies. This alignment temperature is further confirmed by comparing the temperature dependence of the emission energy of a fabricated VCSEL device with that of an edge-emitting laser structure with a nominally identical active region. The study suggests that for further device improvement, the room temperature CM and QW GST energies should be more closely matched and both designed to a smaller energy of about 0.95 eV, somewhat closer to the 1.31 μm target. The study amply demonstrates the usefulness of non-destructive ER characterisation techniques in VCSEL manufacturing with GaAsSb-based QWs. © 2012 American Institute of Physics.

• . (2012) 'Effects of rapid thermal annealing on GaAs1-xBix alloys'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 101 (1) Article number ARTN 012106
• . (2012) 'Efficiency-limiting processes in Ga(NAsP)/GaP quantum well lasers'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 101 (1) Article number ARTN 011107
• . (2012) 'The electronic band structure of GaBiAs/GaAs layers: Influence of strain and band anti-crossing'. AMER INST PHYSICS JOURNAL OF APPLIED PHYSICS, 111 (11) Article number ARTN 113108
• . (2012) 'Recombination mechanisms and band alignment of GaAs1-xBix/GaAs light emitting diodes'. APPLIED PHYSICS LETTERS, 100 (5) Article number ARTN 051105
• . (2012) 'High concentration measurement of mixed particle suspensions using simple multi-angle light scattering system'. Proceedings of SPIE - The International Society for Optical Engineering, 8439

#### Abstract

A simple multiple-angle light scattering system was developed for the differential measurement of particle concentrations in suspension even in high concentration where multiple scattering effects are significant based on size. The system combines multiple-angle detection to collect scattered angle dependent light intensities, and Partial Least Square Regression method (PLS-R) to compose the predictive models for analyzing scattered signal obtain concentrations of samples under investigation. The system was designed to be simple, portable and inexpensive. It employs a diode lasers (red AlGaInP-based) as a light source and a silicon photodiode as a detector and optical components, all of which are readily available. The technique was validated using 1.1 μm and 3.0μm polystyrene latex beads in both mono-dispersed and poly-dispersed suspensions. The measurement results showed good agreement between the measured results and reference values. Their deviations from the reference values are 2.4% and 1.5% relating to references' concentrations of 1.3×10 8 and 1.2×10 7 particles/ml for 1.1 μm and 3.0 μm in mono-dispersed solutions and 2.3 % and 3.5% relating to references' concentrations of 1.1×10 8 and 4.4×10 5 particles/ml for 1.1 μm and 3.0 μm in mixed solutions, respectively. This system is a compact but high performance system allowing multiple particle sizes in high concentration to be measured simultaneously. © 2012 SPIE.

• . (2012) 'Room temperature photoluminescence intensity enhancement in GaAs 1-xBi x alloys'. Physica Status Solidi (C) Current Topics in Solid State Physics, 9 (2), pp. 259-261.
• . (2012) 'Absorption characteristics of GaAsBi/GaAs diodes in the near-infrared'. IEEE Photonics Technology Letters, 24 (23), pp. 2191-2194.
• . (2012) 'Dual wavelength multiple-angle light scattering system for cryptosporidium detection'. Society of Photo-Optical Instrumentation Engineers Progress in Biomedical Optics and Imaging - Proceedings of SPIE, 8427

#### Abstract

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.

• . (2012) 'Non-stoichiometric GaAsBi/GaAs (100) molecular beam epitaxy growth'. Elsevier Journal of Crystal Growth, 338 (1), pp. 57-61.
• . (2012) 'Bismide alloys for photonic devices: Potential and progress'. 2012 IEEE Photonics Conference, IPC 2012, , pp. 602-603.
• . (2012) 'Improved Optoelectronic Properties of Rapid Thermally Annealed Dilute Nitride GaInNAs Photodetectors'. Journal of Electronic Materials, , pp. 1-9.
• . (2012) 'Bi incorporation in GaAs(100)-2×1 and 4×3 reconstructions investigated by RHEED and STM'. Elsevier Journal of Crystal Growth, 341 (1), pp. 19-23.
• . (2012) 'High performance silicon optical modulators'. Proceedings of SPIE - The International Society for Optical Engineering, 8564

#### Abstract

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.

• . (2011) 'Modulation of the absorption coefficient at 1.3 mu m in Ge/SiGe multiple quantum well heterostructures on silicon'. OPTICS LETTERS, 36 (21), pp. 4158-4160.

#### Abstract

We report modulation of the absorption coefficient at 1.3 μm in Ge/SiGe multiple quantum well heterostructures on silicon via the quantum-confined Stark effect. Strain engineering was exploited to increase the direct optical bandgap in the Ge quantum wells. We grew 9 nm-thick Ge quantum wells on a relaxed Si0.22Ge0.78 buffer and a contrast in the absorption coefficient of a factor of greater than 3.2 was achieved in the spectral range 1290–1315 nm.

• . (2011) 'Direct evidence for suppression of Auger recombination in GaInAsSbP/InAs mid-infrared light-emitting diodes'. APPLIED PHYSICS LETTERS, 99 (14) Article number ARTN 141110
• . (2011) 'Laser operation of Ga(NAsP) lattice-matched to (001) silicon substrate'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 99 (7) Article number ARTN 071109
• . (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'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 99 (7) Article number ARTN 071110
• . (2011) 'Reduced threshold current dilute nitride Ga(NAsP)/GaP quantum well lasers grown by MOVPE'. INST ENGINEERING TECHNOLOGY-IET ELECTRONICS LETTERS, 47 (16), pp. 931-933.
• . (2011) 'The effect of Bi composition to the optical quality of GaAs1-xBix'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 99 (4) Article number ARTN 042107
• . (2011) 'Temperature dependence of 4.1 mu m mid-infrared type II "W" interband cascade lasers'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 99 (2) Article number ARTN 021102
• . (2011) 'GaInNAsSb/GaAs Photodiodes for Long-Wavelength Applications'. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC IEEE ELECTRON DEVICE LETTERS, 32 (7), pp. 919-921.
• . (2011) 'Selected papers from the Semiconductor and Integrated Optoelectronics (SIOE'10) Conference'. INST ENGINEERING TECHNOLOGY-IET IET OPTOELECTRONICS, 5 (3), pp. 99-99.
• . (2011) 'Photoluminescence investigation of high quality GaAs1-xBix on GaAs'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 98 (12) Article number ARTN 122107
• . (2011) 'Electrical pumped integrated III/V laser lattice-matched to a Silicon substrate'. Device Research Conference - Conference Digest, DRC, , pp. 257-258.
• . (2011) 'A miniature high-temperature fixed point for self-validation of type C thermocouples'. MEASUREMENT SCIENCE & TECHNOLOGY, 22 (10) Article number ARTN 105103
• . (2011) 'Miniature Co-C eutectic fixed-point cells for self-validating thermocouples'. IOP PUBLISHING LTD MEASUREMENT SCIENCE AND TECHNOLOGY, 22 (1) Article number ARTN 015104
• . (2011) 'Band structure properties of novel BxGa1-xP alloys for silicon integration'. JOURNAL OF APPLIED PHYSICS, 110 (6) Article number ARTN 063101
• . (2010) 'Efficiency limiting processes in 1.55 mu m InAs/InP-based quantum dots lasers'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 97 (16) Article number ARTN 161104
• . (2010) 'Thermal Characteristics of 1.55-mu m InGaAlAs Quantum Well Buried Heterostructure Lasers'. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC IEEE JOURNAL OF QUANTUM ELECTRONICS, 46 (5), pp. 700-705.
• . (2010) 'Self-validating thermocouples based on high temperature fixed points'. IOP PUBLISHING LTD METROLOGIA, 47 (1), pp. L1-L3.
• . (2010) 'Comparative Study of Pt/Pd and Pt-Rh/Pt Thermocouples'. Springer International Journal of Thermophysics, 31 (8-9), pp. 1506-1516.
• . (2010) 'Physical properties of short wavelength 2.6μm InAs/AlSb-based quantum cascade lasers'. IEEE Conference Digest - IEEE International Semiconductor Laser Conference, , pp. 95-96.

#### Abstract

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.

• . (2010) 'On the temperature dependence of monolithically integrated Ga(NAsP)/(BGa)P/Si QW lasers'. Optics InfoBase Conference Papers,
• . (2009) 'The Importance of Recombination via Excited States in InAs/GaAs 1.3 mu m Quantum-Dot Lasers'. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 15 (3), pp. 799-807.
• . (2009) 'Editorial: Selected papers from the second international conference on optical, optoelectronic and photonic materials and applications, 2007'. Springer Journal of Materials Science: Materials in Electronics, 20 (SUPPL. 1), pp. S1-S2.
• . (2009) 'Why not do both?'. Physics World, 22 (12), pp. 44-45.
• . (2009) 'Selected Papers from the Second International Conference on Optical, Optoelectronic and Photonic Materials and Applications, 2007'. SPRINGER JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, 20, pp. 1-2.
• . (2009) 'The development of a compact free spectral range semiconductor laser biosensor'. EUROPEAN OPTICAL SOC JOURNAL OF THE EUROPEAN OPTICAL SOCIETY-RAPID PUBLICATIONS, 4 Article number ARTN 09013
• . (2009) 'Temperature insensitive quantum dot lasers: are we really there yet?'. SPRINGER JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, London, ENGLAND: 20, pp. 272-276.
• . (2008) 'An accurate determination of the electronic transitions of InAs/InGaAs/InP quantum dots for midinfrared lasers using simultaneous complementary spectroscopic techniques'. AMER INST PHYSICS JOURNAL OF APPLIED PHYSICS, 104 (8) Article number ARTN 083109
• . (2008) 'Temperature dependence and physical properties of Ga(NAsP)/GaP semiconductor lasers'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 93 (10) Article number ARTN 101108
• . (2008) 'Physical properties and efficiency of GaNP light emitting diodes'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 92 (2) Article number ARTN 021101
• . (2007) 'Temperature and pressure dependence of the recombination processes in 1.5 mu m InAs/InP (311)B quantum dot lasers'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 91 (13) Article number ARTN 131113
• . (2007) 'Semiconductor lasers as integrated optical biosensors: Sensitivity optimisation'. Journal of Physics: Conference Series, 76 (1)
• . (2007) 'Experimental determination of the band gap dependence of Auger recombination in InGaAs/InP multiple quantum well lasers at room temperature'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 90 (16) Article number ARTN 161113
• . (2007) 'Long wavelength bulk GaInNAs p-i-n photodiodes lattice matched to GaAs'. AMER INST PHYSICS JOURNAL OF APPLIED PHYSICS, 101 (6) Article number ARTN 064506
• . (2007) 'Microscopic electroabsorption line shape analysis for Ga(AsSb)/GaAs heterostructures'. AMER INST PHYSICS JOURNAL OF APPLIED PHYSICS, 101 (3) Article number ARTN 0

#### Abstract

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.

• . (2007) 'The influence of carrier density non-pinning on the output power of 1.55 μm lasers at high temperature'. Optics InfoBase Conference Papers,
• . (2007) 'GaInNAs lattice-matched to GaAs for photodiodes'. Conference Proceedings - International Conference on Indium Phosphide and Related Materials, , pp. 347-349.
• . (2006) 'Spectroscopy of GaAs/AlGaAs quantum-cascade lasers using hydrostatic pressure'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 89 (22) Article number ARTN 221105
• . (2006) 'Temperature dependence of the gain in p-doped and intrinsic 1.3 mu m InAs/GaAs quantum dot lasers'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 89 (19) Article number ARTN 191118
• . (2006) 'Carrier recombination in 1.3 mu m GaAsSb/GaAs quantum well lasers'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 89 (17) Article number ARTN 173509
• . (2006) 'Recombination processes in midinfrared InGaAsSb diode lasers emitting at 2.37 mu m'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 89 (5) Article number ARTN 051104
• . (2006) 'Analysis of the major loss processes in mid-infrared type-II "W" diode lasers'. Conference Digest - IEEE International Semiconductor Laser Conference, , pp. 43-44.
• . (2006) 'Band structure and high-pressure measurements'. Springer Series in Optical Sciences, 118, pp. 93-127.
• . (2005) 'Carrier transport and recombination in p-doped and intrinsic 1.3 mu m InAs/GaAs quantum-dot lasers'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 87 (21) Article number ARTN 211114
• . (2005) 'Investigation of carrier recombination processes and transport properties in GaInAsN/GaAs quantum wells'. American Institute of Physics AIP Conference Proceedings, 772, pp. 985-986.

#### Abstract

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.

• . (2005) 'Spectroscopic investigations of GaAsSb/GaAs based structures for 1.3 mu m VCSEL applications'. IEE-INST ELEC ENG IEE PROCEEDINGS-OPTOELECTRONICS, 152 (2), pp. 110-117.
• . (2005) 'AlGalnN resonant-cavity LED devices studied by electromodulated reflectance and carrier lifetime techniques'. IEE-INST ELEC ENG IEE PROCEEDINGS-OPTOELECTRONICS, 152 (2), pp. 118-124.
• . (2005) 'High temperature operation of 760 nm vertical-cavity surface-emitting lasers investigated using photomodulated reflectance wafer measurements and temperature-dependent device studies'. IEE-INST ELEC ENG IEE PROCEEDINGS-OPTOELECTRONICS, 152 (2), pp. 103-109.
• . (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'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 85 (3), pp. 357-359.
• . (2004) 'Modulation speed resonant-cavity and leakage current in 650 nm light emitting diodes'. IEE-INST ELEC ENG IEE PROCEEDINGS-OPTOELECTRONICS, CARDIFF, WALES: 151 (2), pp. 94-97.
• . (2004) 'Novel experimental techniques for semiconductor laser characterisation and optimisation'. IOP PUBLISHING LTD PHYSICA SCRIPTA, T114, pp. 152-158.
• . (2003) 'Direct measurement of facet temperature up to melting point and COD in high-power 980-nm semiconductor diode lasers'. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 9 (5), pp. 1325-1332.

#### Abstract

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.

• . (2003) 'High-pressure studies of recombination mechanisms in 1.3-mu m GaInNAs quantum-well lasers'. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 9 (5), pp. 1196-1201.

#### Abstract

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.

• . (2003) 'Theoretical and experimental analysis of 1.3-mu m InGaAsN/GaAs lasers'. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 9 (5), pp. 1228-1238.

#### Abstract

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.

• . (2003) 'Investigation of 1.3-mu m GaInNAs vertical-cavity surface-emitting lasers (VCSELs) using temperature, high-pressure, and modelling techniques'. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 9 (5), pp. 1202-1208.

#### Abstract

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.

• . (2003) 'Spectroscopic characterization of 1450 nm semiconductor pump laser structures for Raman amplifiers'. AMER INST PHYSICS JOURNAL OF APPLIED PHYSICS, 93 (12), pp. 9446-9455.
• . (2003) 'Unusual increase of the Auger recombination current in 1.3 mu m GaInNAs quantum-well lasers under high pressure'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 82 (14), pp. 2335-2337.
• . (2003) 'Putting nanocrystals to work: from solutions to devices - Discussion'. ROYAL SOC LONDON PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON SERIES A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, 361 (1803), pp. 340-343.
• . (2003) 'Novel electronic and optoelectronic properties of GaInNAs and related alloys'. OSA Trends in Optics and Photonics Series, 88, pp. 523-525.

#### Abstract

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.

• . (2002) 'A quantitative study of radiative, Auger, and defect related recombination processes in 1.3-mu m GaInNAs-based quantum-well lasers'. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 8 (4), pp. 801-810.
• . (2002) 'Modal refractive index of 1.3 mu m InGaAsP, AlGaInAs and GaInNAs semiconductor lasers under high hydrostatic pressure'. IEE-INST ELEC ENG ELECTRONICS LETTERS, 38 (7), pp. 325-327.
• . (2002) 'Experimental and theoretical analysis of the recombination processes in GaInNAs 1.3 μm Lasers'. Conference Digest - IEEE International Semiconductor Laser Conference, , pp. 41-42.
• . (2002) 'The temperature and pressure dependence of 1.3 μm GaInNAs vertical-cavity surface emitting lasers (VCSELs)'. Conference Digest - IEEE International Semiconductor Laser Conference, , pp. 139-140.
• . (2002) 'Optical investigation of recombination processes in GaInNAs, InGaAsP and AlGaInAs quantum-well lasers using hydrostatic pressure'. Conference Digest - IEEE International Semiconductor Laser Conference, , pp. 83-84.
• . (2002) 'A comparison of the thermal stability of InGaAsP, AlGaInAs and GaInNAs quantum-well lasers for 1.3 μm operation'. Conference Digest - IEEE International Semiconductor Laser Conference, , pp. 43-44.
• . (2002) 'Direct measurement of facet temperature up to the melting point and COD in high power 980 nm semiconductor diode lasers'. Conference Digest - IEEE International Semiconductor Laser Conference, , pp. 161-162.
• . (2001) 'Evidence for large monomolecular recombination contribution to threshold current in 1.3 mu m GaInNAs semiconductor lasers'. IEE-INST ELEC ENG ELECTRONICS LETTERS, 37 (25), pp. 1518-1520.
• . (2001) 'Self-heating effects in red (665 nm) VCSELs'. IEE-INST ELEC ENG IEE PROCEEDINGS-OPTOELECTRONICS, 148 (5-6) Article number PII 10.1049/ip-opt:20010843 , pp. 256-260.
• . (2001) 'Evaluating the continuous-wave performance of AlGaInP-based red (667 nm) vertical-cavity surface-emitting lasers using low-temperature and high-pressure techniques'. AMER INST PHYSICS APPLIED PHYSICS LETTERS, 78 (7), pp. 865-867.
• . (2001) 'Insights into carrier recombination processes in 1.3 mu m GaInNAs-based semiconductor lasers attained using high pressure'. IEE-INST ELEC ENG ELECTRONICS LETTERS, 37 (2), pp. 92-93.
• . (2001) 'Gain-cavity alignment in efficient visible (660 nm) VCSELs studied using high pressure techniques'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, 223 (2), pp. 587-591.
• . (2001) 'Superior temperature performance of 1.3 mu m AlGaInAs-Based semiconductor lasers investigated at high pressure and low temperature'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, 223 (2), pp. 573-579.
• . (2001) 'Assessing the performance of visible (665 nm) vertical cavity surface emitting lasers using high pressure and low temperature techniques'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, 223 (2), pp. 581-585.
• . (2001) 'Quantifying the effect of indirect carrier leakage on visible Al(GaInP) lasers using high pressures and low temperatures'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, 223 (2), pp. 567-572.
• . (2000) '-180 to +80 degrees C CW lasing in visible VCSELs'. IEEE 2000 IEEE 17TH INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE, CONFERENCE DIGEST, MONTEREY, CA: , pp. 15-16.
• . (2000) 'Effect of auger generated hot-holes on 1.5-μm InGaAs(P)-based quantum well semiconductor lasers'. Pacific Rim Conference on Lasers and Electro-Optics, CLEO - Technical Digest, , pp. 391-392.
• . (1999) 'Experimental analysis of temperature dependence in 1.3-mu m AlGaInAs-InP strained MQW lasers'. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 5 (3), pp. 413-419.
• . (1999) 'The temperature dependence of 1.3-and 1.5-mu m compressively strained InGaAs(P) MQW semiconductor lasers'. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 5 (3), pp. 401-412.
• . (1999) 'Observation of reduced nonradiative current in 1.3-mu m AlGaInAs-InP strained MQW lasers'. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC IEEE PHOTONICS TECHNOLOGY LETTERS, 11 (4), pp. 409-411.
• . (1998) 'Improved temperature dependence of 1.3 mu m AlGalnAs-based MQW semiconductor diode lasers revealed by hydrostatic pressure'. IEE-INST ELEC ENG ELECTRONICS LETTERS, 34 (22), pp. 2130-2132.
• . (1998) 'The effect of temperature dependent processes on the performance of 1.5-mu m compressively strained InGaAs(P) MQW semiconductor diode lasers'. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC IEEE PHOTONICS TECHNOLOGY LETTERS, 10 (8), pp. 1076-1078.
• . (1998) 'Observation of reduced non-radiative recombination current in 1.3-μm AlGaInAs/InP multiple-quantum-well lasers'. Conference Digest - IEEE International Semiconductor Laser Conference, , pp. 61-62.
• . (1998) 'Transition from radiative to nonradiative recombination in 1.3-μm and 1.5-μm InGaAs(P) multiple quantum well semiconductor diode lasers'. Conference on Lasers and Electro-Optics Europe - Technical Digest,
• . (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'. Conference Digest - IEEE International Semiconductor Laser Conference, , pp. 63-64.

### Conference papers

• . (2015) 'Selectively reflective transparent sheets'. SPIE-INT SOC OPTICAL ENGINEERING ACTIVE PHOTONIC MATERIALS VII, San Diego, CA: Conference on Active Photonic Materials VII 9546
• . (2014) 'Semiconductor quantum well lasers with a temperature insensitive threshold current'. Conference Digest - IEEE International Semiconductor Laser Conference, , pp. 82-83.
• . (2014) 'Electrically injected GaAsBi Quantum Well Lasers'. IEEE COMPUTER SOC 2014 24TH IEEE INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE (ISLC 2014), Palma de Mallorca, SPAIN: 24th IEEE International Semiconductor Laser Conference (ISLC), pp. 80-81.
• . (2013) 'The effect of hole leakage and auger recombination on the temperature sensitivity of GaInAsSb/GaSb mid-infrared lasers'. Optics InfoBase Conference Papers,
• . (2013) 'Band Structure Properties of (BGa)P Semiconductors for Lattice Matched Integration on (001) Silicon'. AMER INST PHYSICS PHYSICS OF SEMICONDUCTORS, Zurich, SWITZERLAND: 31st International Conference on the Physics of Semiconductors (ICPS) 1566, pp. 47-48.
• . (2013) 'Auger Recombination Suppression And Band Alignment In GaAsBi/GaAs Heterostructures'. AMER INST PHYSICS PHYSICS OF SEMICONDUCTORS, Zurich, SWITZERLAND: 31st International Conference on the Physics of Semiconductors (ICPS) 1566, pp. 488-489.
• . (2013) 'Investigating the efficiency limitations of GaN-based emitters'. Optics InfoBase Conference Papers,
• . (2012) 'InGaBiAs/InP semiconductors for mid-infrared applications: Dependence of bandgap and spin-orbit splitting on temperature and bismuth content'. ICP 2012 - 3rd International Conference on Photonics 2012, Proceedings, , pp. 154-158.
• . (2011) 'Strain engineering of the electroabsorption response in Ge/SiGe multiple quantum well heterostructures'. IEEE 8th IEEE International Conference on Group IV Photonics, London, UK: Group IV Photonics, pp. 107-108.
• . (2011) 'Physical properties of monolithically integrated Ga(NAsP)/(BGa)P QW lasers on silicon'. IEEE 8th IEEE International Conference on Group IV Photonics, London, UK: Group IV Photonics, pp. 148-150.
• . (2011) 'The Potential Role of Bismide Alloys in Future Photonic Devices'. IEEE 13TH INTERNATIONAL CONFERENCE ON TRANSPARENT OPTICAL NETWORKS, Stockholm, Sweden: 13th ICTON
• . (2010) 'Recombination and loss mechanisms in GaNAsP/GaP QW lasers'. Proceedings of Photonics Global Conference, Orchard, Singapore: PGC 2010

#### Abstract

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.

• . (2010) 'Comparative Study of Pt/Pd and Pt-Rh/Pt Thermocouples'. SPRINGER/PLENUM PUBLISHERS INTERNATIONAL JOURNAL OF THERMOPHYSICS, Portoroz, SLOVENIA: 1st TEMPMEKO and ISHM Joint International Symposium on Temperature, Humidity, Moisture, and Thermal Measurements 31 (8-9), pp. 1506-1516.
• . (2010) 'Temperature sensitivity of 1.55μm (100) InAs/InP-based quantum dot lasers'. IEEE Proceedings of 22nd International Conference on Indium Phosphide and Related Materials, Kagawa, Japan: 22nd IPRM, pp. 23-24.

#### Abstract

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).

• . (2010) 'Thermal performance of 1.55μm InGaAlAs quantum well buried heterostructure lasers'. IEEE Proceedings of 22nd International Conference on Indium Phosphide and Related Materials, Kagawa, Japan: 22nd IPRM, pp. 265-268.

#### Abstract

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.

• . (2010) 'Temperature sensitivity of mid-infrared type II "W" interband cascade lasers (ICL) emitting at 4.1μm at room temperature'. IEEE 22nd IEEE International Semi-conductor Laser Conference, Kyoto, Japan: ISLC 2010, pp. 41-42.
• . (2010) 'Bismide-alloys for higher efficiency infrared semiconductor lasers'. IEEE 22nd IEEE International Semiconductor Laser Conference, Kyoto, Japan: ISLC 2010, pp. 111-112.
• . (2010) 'InGaAsN as absorber in APDs for 1.3 micron wavelength applications'. Kagawa, Japan: IPRM 2010, pp. 187-190.

#### Abstract

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.

• . (2010) 'Influence of device structures on carrier recombination in GaAsSb/GaAs QW lasers'. Proceedings of Photonics Global Conference, Orchard, Singapore: PGC 2010

#### Abstract

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.

• . (2010) 'Physical properties of Ga(NAsP)/GaP QW lasers grown by MOVPE'. IEEE 23rd Annual Meeting of the IEEE Photonics Society, Denver, USA: Photonics 2010, pp. 65-66.
• . (2010) 'Improved Performance of GaAsSb/GaAs SQW Lasers'. SPIE-INT SOC OPTICAL ENGINEERING NOVEL IN-PLANE SEMICONDUCTOR LASERS IX, San Francisco, CA: Conference on Novel In - Plane Semiconductor Lasers IX 7616
• . (2010) 'Reduction of dark current and unintentional background doping in InGaAsN photodetectors by ex situ annealing'. SPIE Proceedings of SPIE - Posters Session, Brussels, Belgium: Optical Sensing and Detection 7726

#### Abstract

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.

• . (2010) 'Role of growth temperature on the physical characteristics of GaAsSb/GaAs QW lasers'. IEEE 23rd Annual Meeting of the IEEE Photonics Society, Denver, USA: Photonics 2010, pp. 59-60.
• . (2010) 'Efficiency limitations of green InGaN LEDs and laser diodes'. 22nd IEEE International Semiconductor Laser Conference, Kyoto, Japan: ISLC 2010, pp. 27-28.
• . (2010) 'MOVPE growth and characterization of Ga(NAsP) laser structures monolithically integrated on Si (001) substrates'. 22nd IEEE International Semiconductor Laser Conference, Kyoto, Japan: 22nd IEEE ISLC, pp. 143-144.
• . (2010) 'Lasing properties of monolithically integrated Ga(NAsP)/(BGa)P QW lasers on a Silicon substrate grown by MOVPE'. 22nd IEEE International Semiconductor Laser Conference, Kyoto, Japan: 22nd IEEE ISLC, pp. 109-110.
• . (2010) 'Thermal behavior of 1.55 μm (100) InAs/InP-based quantum dot lasers'. IEEE Poceeedings of 22nd International Semiconductor Laser Conference, Kyoto, Japan: ISLC 2010, pp. 75-76.
• . (2009) 'Gain peak-cavity mode alignment optimisation in buried tunnel junction mid-infrared GaSb vertical cavity surface emitting lasers using hydrostatic pressure'. INST ENGINEERING TECHNOLOGY-IET IET OPTOELECTRONICS, Cardiff, WALES: Semiconductor and Integrated Optoelectronics Conference (SIOE 2009) 3 (6), pp. 305-309.
• . (2009) 'Band-structure and gain-cavity tuning of 2.4-μm GaSb buried tunnel junction VCSELs'. Proceedings of European Conference on Lasers and Electro-Optics 2009 and the European Quantum Electronics Conference, Munich, Germany: CLEO Europe - EQEC 2009

#### Abstract

Low-cost, continuous-wave GaSb-based vertical cavity surface emitting lasers (VCSELs) operating at ~ 2.4 mum up to 50degC have been demonstrated recently. In this work we have used high pressure techniques to investigate ways to improve their performance and extend their working temperature range. Since the band-gap and energy of the gain peak (Ep) increase with pressure at 0.126 meV/MPa at constant temperature, when applied to edge emitting lasers (EEL) we can use pressure to determine the radiative and non-radiative recombination processes occurring. In the VCSEL the pressure dependence of the threshold current, is much more complicated. At the higher temperature the decreasing Auger recombination initially dominates. Therefore we predict that either increasing the band gap or increasing the operating wavelength will allow an improved temperature performance of these GaSb-based VCSELs.

• . (2009) 'Room temperature characterisation of InGaAlAs quantum well laser structures using electro-modulated reflectance and surface photovoltage spectroscopy'. WILEY-BLACKWELL PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, Wroclaw Univ Technol, Wroclaw, POLAND: 3rd International Workshop on Modulation Spectroscopy of Semiconductor Structures 206 (5), pp. 796-802.
• . (2009) 'Evidence of carrier leakage into the L-valley in InAs-based quantum cascade lasers under high hydrostatic pressure'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, Fortaleza, BRAZIL: 13th International Conference on High Pressure Semiconductor Physics (HPSP-13) 246 (3), pp. 512-515.
• . (2009) 'Band anti-crossing and carrier recombination in dilute nitride phosphide based lasers and light emitting diodes'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, Fortaleza, BRAZIL: 13th International Conference on High Pressure Semiconductor Physics (HPSP-13) 246 (3), pp. 527-531.
• . (2009) 'Dark current mechanisms in InxGa1-xAs 1-yNy'. IEEE IEEE Proceedings of LEOS Annual Meeting Conference, Belek-Antalya, Turkey: LEOS '09, pp. 233-234.
• . (2009) 'Band-structure and gain-cavity tuning of 2.4-μm GaSb buried tunnel junction VCSELs'. Optics InfoBase Conference Papers,
• . (2009) 'The effect of hydrostatic pressure on the operation of quantum cascade lasers'. SPIE Proceedings of SPIE - Quantum Cascade Lasers and Applications I, San Jose, USA: Quantum Sensing and Nanophotonic Devices VI 7222

#### Abstract

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.

• . (2008) 'Dark current mechanisms in bulk GaInNAs photodiodes'. IEEE Proccedings of 20th International Conference on Indium Phosphide and Related Materials, Versailles, France: IPRM 2008

#### Abstract

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.

• . (2008) 'Thermal characteristics of 1.3 mu m GaAsSb/GaAs-based Edge- and Surface-emitting Lasers'. IEEE 2008 IEEE 21ST INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE, Sorrento, ITALY: IEEE 21st International Semiconductor Laser Conference, pp. 83-84.
• . (2008) 'Reliable 1550nm SI BH lasers fabricated using an improved Ru precursor'. 20th International Conference on Indium Phosphide and Related Materials, Versailles, France: IPRM 2008
• . (2008) 'Thermal properties of Silicon compatible GaNAsP SQW lasers'. IEEE 2008 IEEE 21ST INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE, Sorrento, ITALY: IEEE 21st International Semiconductor Laser Conference, pp. 61-62.
• . (2008) 'Gamma-L scattering in InAs-based quantum cascade lasers studied using high hydrostatic pressure'. IEEE 2008 IEEE 21ST INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE, Sorrento, ITALY: IEEE 21st International Semiconductor Laser Conference, pp. 47-48.
• . (2008) 'Materials and light-emitting diode properties of dilute-nitride GaNP/GaP heterostructures'. Device Research Conference - Conference Digest, Santa Barbara, USA: 66th DRC, pp. 299-300.
• . (2008) 'The importance of recombination via excited states in InAs/GaAs 1.3μm quantum dot lasers'. IEEE IEEE Proceedings of 21st International Semiconductor Laser Conference, Sorento, Italy: 21st ISLC 2008, pp. 117-118.
• . (2008) 'Novel heterostructure design for increased spectral width of superluminescentdiodes and dual-wavelength laser diodes'. Device Research Conference - Conference Digest, Santa Barbara, USA: 66th DRC, pp. 309-310.
• . (2007) 'Optimisation of distributed feedback laser biosensors'. INST ENGINEERING TECHNOLOGY-IET IET OPTOELECTRONICS, Cardiff, WALES: 21st Conference on Semiconductor Integrated Optoelectronics (SIOE) 1 (6), pp. 266-271.
• . (2007) 'Recombination, transport and loss mechanisms in p-doped InAs/GaAs quantum dots'. AMER INST PHYSICS Physics of Semiconductors, Pts A and B, Vienna, AUSTRIA: 28th International Conference on the Physics of Semiconductors (ICPS-28) 893, pp. 837-838.
• . (2007) 'Temperature and pressure dependence of carrier recombination processes in GaAsSb/GaAs quantum well lasers'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, Barcelona, SPAIN: 12th International Conference on High Pressure Semiconductor Physics (HPSP-12) 244 (1), pp. 197-202.
• . (2007) 'High pressure studies of mid-infrared type-II "W" diode lasers at cryogenic temperatures'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, Barcelona, SPAIN: 12th International Conference on High Pressure Semiconductor Physics (HPSP-12) 244 (1), pp. 224-228.
• . (2007) 'Temperature and pressure dependence of the recombination mechanisms in 1.3 pm and 1.5 pm GaInNAs lasers'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, Barcelona, SPAIN: 12th International Conference on High Pressure Semiconductor Physics (HPSP-12) 244 (1), pp. 208-212.
• . (2007) 'The influence of carrier density non-pinning on the output power of 1.55 μm lasers at high temperature'. Conference on Lasers and Electro-Optics Europe - Technical Digest,
• . (2007) 'Band gap dependence of the recombination processes in InAs/GaAs quantum dots studied using hydrostatic pressure'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, Barcelona, SPAIN: 12th International Conference on High Pressure Semiconductor Physics (HPSP-12) 244 (1), pp. 82-86.
• . (2007) 'Carrier recombination mechanisms in mid-infrared GaInAsSb quantum well lasers'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, Barcelona, SPAIN: 12th International Conference on High Pressure Semiconductor Physics (HPSP-12) 244 (1), pp. 203-207.
• . (2007) 'Band alignment and carrier recombination in GaAsSb/GaAs quantum wells'. AIP Conference Proceedings, 893, pp. 1431-1432.

#### Abstract

Using a combination of experimental and theoretical techniques, we investigated the band alignment and the carrier recombination processes occurring in GaAsSb/GaAs structures. We find that for Sb fractions ∼30%, the band alignment is slightly type II. From studies on lasers based upon this material we show that at the high carrier densities required to achieve threshold, at room temperature, the devices are dominated by carrier leakage and non-radiative Auger recombination. © 2007 American Institute of Physics.

• . (2006) 'The physics controlling the sensitivity of semiconductor lasers to high temperatures'. OPTICAL SOC AMERICA 2006 OPTICAL FIBER COMMUNICATION CONFERENCE/NATIONAL FIBER OPTIC ENGINEERS CONFERENCE, VOLS 1-6, Anaheim, CA: Conference on Optical Fiber Communications/National Fiber Optic Engineers Conference, pp. 1631-1633.
• . (2006) 'Intrinsic limitations of p-doped and undoped 1.3 μm InAs/GaAs quantum dot lasers'. Conference Digest - IEEE International Semiconductor Laser Conference, , pp. 143-144.
• . (2005) 'Recombination and loss mechanisms in low-threshold InAs-GaAs 1.3-mu m quantum-dot lasers'. IEEE IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, Matsue, JAPAN: 19th IEEE International Semiconductor Laser Conference 11 (5), pp. 1041-1047.

#### Abstract

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.

• . (2005) 'A study of the low-energy interference oscillations in photoreflectance of GaAsSb/GaAs quantum well structures'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, Wroclaw, POLAND: International Workshop on Modulation Spectroscopy of Semiconductor Structures 202 (7), pp. 1244-1254.
• . (2005) 'On the thermal stability of 1.3 mu m GaAsSb/GaAs-based lasers'. IEEE 2005 IEEE LEOS Annual Meeting Conference Proceedings (LEOS), , pp. 330-331.

#### Abstract

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.

• . (2005) 'The influence of p-doping on the temperature sensitivity of 1.3 mu m quantum dot lasers'. IEEE 2005 IEEE LEOS Annual Meeting Conference Proceedings (LEOS), Sydney, Australia: 2005 IEEE LEOS Annual Meeting Conference (LEOS), pp. 603-604.

#### Abstract

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.

• . (2005) 'The influence of auger processes on recombination in long-wavelength InAs/GaAs quantum dots'. AMER INST PHYSICS Physics of Semiconductors, Pts A and B, Flagstaff, AZ: 27th International Conference on the Physics of Semiconductors (ICPS-27) 772, pp. 681-682.
• . (2005) 'Carrier recombination in InGaAs(P) quantum well laser structures: Band gap and temperature dependence'. AMER INST PHYSICS Physics of Semiconductors, Pts A and B, Flagstaff, AZ: 27th International Conference on the Physics of Semiconductors (ICPS-27) 772, pp. 1545-1546.
• . (2004) 'Carrier recombination processes in 1.3 mu m and 1.5 mu m InGaAs(P)-based lasers at cryogenic temperatures and high pressures'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, Berkeley, CA: 11th International Conference on High-Pressure Semiconductor Physics (HPSP-11) 241 (14), pp. 3399-3404.
• . (2004) 'Carrier leakage suppression utilising short-period superlattices in 980 nm InGaAs/GaAs quantum well lasers'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, Berkeley, CA: 11th International Conference on High-Pressure Semiconductor Physics (HPSP-11) 241 (14), pp. 3405-3409.
• . (2004) 'Pressure induced wavelength dependence of catastrophic optical damage in 980 nm semiconductor diode lasers'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, Berkeley, CA: 11th International Conference on High-Pressure Semiconductor Physics (HPSP-11) 241 (14), pp. 3416-3419.
• . (2004) 'Recombination mechanisms in InAs/InP quantum dash lasers studied using high hydrostatic pressure'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, Berkeley, CA: 11th International Conference on High-Pressure Semiconductor Physics (HPSP-11) 241 (14), pp. 3427-3431.
• . (2004) 'Temperature and pressure dependence of recombination processes in 1.5 mu m InGaAlAs/InP-based quantum well lasers'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, Berkeley, CA: 11th International Conference on High-Pressure Semiconductor Physics (HPSP-11) 241 (14), pp. 3391-3398.
• . (2004) 'Influence of growth temperature on carrier recombination in GaInNAs-based lasers'. INST ENGINEERING TECHNOLOGY-IET IEE PROCEEDINGS-OPTOELECTRONICS, Strasbourg, FRANCE: Symposium on Dilute Nitride and Related Mismatched Semiconductor Alloys held at the 2004 Spring Meeting of the EMRS 151 (5), pp. 447-451.
• . (2004) 'Influence of growth temperature on defect density in 1.3 mu m GaInNAs-based lasers'. 2004 IEEE 19TH INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE, CONFERENCE DIGEST, Matsue, JAPAN: 19th IEEE International Semiconductor Laser Conference, pp. 85-86.

#### Abstract

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.

• . (2004) 'Experimental investigations into the thermal properties of 1.5-1.8-mu m InAs/InP quantum dash lasers'. IEEE 2004 IEEE 19TH INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE, CONFERENCE DIGEST, Matsue, JAPAN: 19th IEEE International Semiconductor Laser Conference, pp. 61-62.
• . (2004) 'Recombination and loss mechanisms in low-threshold InAs/GaAs 1.3 mu m quantum dot lasers'. 2004 IEEE 19TH INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE, CONFERENCE DIGEST, Matsue, JAPAN: 19th IEEE International Semiconductor Laser Conference, pp. 57-58.

#### Abstract

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.

• . (2004) 'Temperature and wavelength dependence of recombination processes in 1.5 mu m InGaAlAs/InP-based lasers'. 2004 International Conference on Indium Phosphide and Related Materials, Conference Proceedings, Kagoshima, JAPAN: 16th International Conference on Indium Phosphide and Related Materials, pp. 738-741.

#### Abstract

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.

• . (2003) 'Carrier recombination processes in MOVPE and MBE grown 1.3 mu m GaInNAs edge emitting lasers'. PERGAMON-ELSEVIER SCIENCE LTD SOLID-STATE ELECTRONICS, STRASBOURG, FRANCE: Spring Meeting of the European-Materials-Research-Society (E-MRS) 47 (3), pp. 501-506.
• . (2003) 'Wavelength dependence of the modal refractive index in 1.3 mu m InGaAsP, AlGaInAs and GaInNAs lasers using high pressure'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, UNIV SURREY, GUILDFORD, ENGLAND: 10th International Conference on High Pressures in Semiconductor Physics (HPSP-X) 235 (2), pp. 491-495.
• . (2003) 'Coupling of large optical loss with Auger recombination in 1.3 mu m InGaAsP lasers investigated using hydrostatic pressure'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, UNIV SURREY, GUILDFORD, ENGLAND: 10th International Conference on High Pressures in Semiconductor Physics (HPSP-X) 235 (2), pp. 547-551.
• . (2003) 'Auger recombination in InGaAs/AlGaAs-based MQW semiconductor lasers emitting at 980 nm'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, UNIV SURREY, GUILDFORD, ENGLAND: 10th International Conference on High Pressures in Semiconductor Physics (HPSP-X) 235 (2), pp. 542-546.
• . (2003) 'Quantifying pressure-dependent recombination currents in GaInNAs lasers using spontaneous emission measurements'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, UNIV SURREY, GUILDFORD, ENGLAND: 10th International Conference on High Pressures in Semiconductor Physics (HPSP-X) 235 (2), pp. 486-490.
• . (2003) 'Gain-cavity alignment profiling of 1.3 mu m emitting GaInNAs vertical cavity surface emitting lasers (VCSELs) using high pressure techniques'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, UNIV SURREY, GUILDFORD, ENGLAND: 10th International Conference on High Pressures in Semiconductor Physics (HPSP-X) 235 (2), pp. 480-485.
• . (2003) 'Hydrostatic pressure dependence of recombination mechanisms in GaInNAs, InGaAsP and AlGaInAs 1.3 mu m quantum well lasers'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, UNIV SURREY, GUILDFORD, ENGLAND: 10th International Conference on High Pressures in Semiconductor Physics (HPSP-X) 235 (2), pp. 474-479.
• . (2003) 'Intrinsic temperature sensitivities of 1.3 mu m GaInNAs/GaAs, InGaAsP/InP and AlGaInAs/InP-based semiconductor lasers'. IEEE 2003 IEEE LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2, TUCSON, AZ: 16th Annual Meeting of the IEEE Lasers and Electro-Optics Society, pp. 39-40.
[ Status: Accepted ]

#### Abstract

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.

• . (2003) 'Fundamental limitations of high power 980nm InGaAs/GaAs pump lasers'. IEEE 2003 IEEE LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2, TUCSON, AZ: 16th Annual Meeting of the IEEE Lasers and Electro-Optics Society, pp. 427-428.
• . (2003) 'Origin of the high temperature performance degradation of 1.5 mu m InGaAs(P)/InP quantum well lasers'. IEEE 2003 IEEE LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2, TUCSON, AZ: 16th Annual Meeting of the IEEE Lasers and Electro-Optics Society, pp. 977-978.

#### Abstract

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.

• . (2003) 'Wavelength dependence of catastrophic optical damage threshold in 980nm semiconductor diode lasers'. 2003 IEEE LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2, TUCSON, AZ: 16th Annual Meeting of the IEEE Lasers and Electro-Optics Society, pp. 421-422.

#### Abstract

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.

• . (2002) 'Characterisation of 1.3pm wavelength GaInNAs/GaAs edge-emitting and vertical-cavity surface-emitting lasers using low-temperature and high-pressure'. SPIE-INT SOC OPTICAL ENGINEERING APOC 2002: ASIA-PACIFIC OPTICAL AND WIRELESS COMMUNICATIONS; MATERIALS AND DEVICES FOR OPTICAL AND WIRELESS COMMUNICATIONS, SHANGHAI, PEOPLES R CHINA: Conference on Asia-Pacific Optical and Wireless Communication (APOC 2002) 4905, pp. 183-197.
• . (2001) 'Influence of leakage and gain-cavity alignment on the performance of Al(GaInP) visible vertical-cavity surface emitting lasers'. IEE-INST ELEC ENG IEE PROCEEDINGS-OPTOELECTRONICS, CARDIFF, WALES: Conference on Semiconductor Optoelectronics 148 (1), pp. 55-59.
• . (2001) 'The temperature dependence of the recombination processes in 1.3 mu m GaInNAs-based edge emitting lasers'. IEEE LEOS 2001: 14TH ANNUAL MEETING OF THE IEEE LASERS & ELECTRO-OPTICS SOCIETY, VOLS 1 AND 2, PROCEEDINGS, SAN DIEGO, CA: 14th Annual Meeting of the IEEE Lasers-and-Electro-Optics-Society, pp. 330-331.
• . (2000) 'A comparison of AlGaInAs and InGaAsP-based 1.3 mu m semiconductor lasers using high pressure'. GORDON BREACH SCI PUBL LTD HIGH PRESSURE RESEARCH, UNIV MONTPELLIER, MONTPELLIER, FRANCE: XXXVIIth Meeting of the European-High-Pressure-Research-Group (EHPRG) 18 (1-6), pp. 49-55.
• . (1999) 'Dependence of threshold current on QW position and on pressure in 1.5 mu m InGaAs(P) lasers'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, THESSALONIKI, GREECE: 8th International Conference on High Pressure Semiconductor Physics (HPSP-VIII) 211 (1), pp. 525-531.
• . (1999) 'The hydrostatic pressure dependence of the threshold current in 1.3 mu m InGaAsP quantum well semiconductor diode lasers'. WILEY-V C H VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, THESSALONIKI, GREECE: 8th International Conference on High Pressure Semiconductor Physics (HPSP-VIII) 211 (1), pp. 513-518.
• . (1996) 'Hydrostatic pressure dependence of the threshold current in 1.5 mu m strained quantum well lasers'. AKADEMIE VERLAG GMBH PHYSICA STATUS SOLIDI B-BASIC RESEARCH, SCHWABISCH GMUND, GERMANY: 7th International Conference on High Pressure Semiconductor Physics (HPSP-VII) 198 (1), pp. 381-388.

### Patents

• . (2009) Optoelectronic Devices. Article number GB0911134.5

### Theses and dissertations

• . (2017) Towards high efficiency photovoltaics for applications in laser power beaming..
[ Status: Approved ]

#### Abstract

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 tran

• . (2017) Development of novel infrared photonic materials and devices..
[ Status: Approved ]

#### Abstract

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.

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