Alf Adams

Professor Alfred Adams


Distinguished Professor of Physics
FRS, BSc, PhD, DSc (Leicester) FInstP, CPhys, LFIEEE, HFIET, CEng, DUniv
+44 (0)1483 689310
21 ATI 02

Biography

Research

Research interests

My publications

Publications

Marko Igor, Adams Alfred, Sweeney Stephen, Sellers IR, Mowbray DJ, Skolnick MS, Liu HY, Groom KM (2004) Recombination and loss mechanisms in low-threshold InAs/GaAs 1.3 mu m quantum dot lasers, 2004 IEEE 19TH INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE, CONFERENCE DIGEST pp. 57-58
We show that even in quantum dot lasers with very low threshold current density (Jth=740-50 A/cm(2) at 300 K) the temperature sensitivity of the threshold current arises from nonradiative recombination which comprises similar to60-70% of Jth at 300 K.
Masse N, Marko I, Sweeney S, Adams AR, Hatori N, Sugarawa M (2005) The influence of p-doping on the temperature sensitivity of 1.3 mu m quantum dot lasers, 2005 IEEE LEOS Annual Meeting Conference Proceedings (LEOS) pp. 603-604 IEEE

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

Sweeney S, Fehse R, Adams AR, Riechert H (2003) Intrinsic temperature sensitivities of 1.3 mu m GaInNAs/GaAs, InGaAsP/InP and AlGaInAs/InP-based semiconductor lasers, 2003 IEEE LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2 pp. 39-40 IEEE

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

Tomic S, O'Reilly E, Fehse R, Sweeney S, Adams AR, Andreev AD, Choulis S, Hosea TJC, Riechert H (2003) Theoretical and experimental analysis of 1.3-mu m InGaAsN/GaAs lasers, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 9 (5) pp. 1228-1238 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

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

Sweeney S, Lyons L, Adams AR, Lock D (2003) Direct measurement of facet temperature up to melting point and COD in high-power 980-nm semiconductor diode lasers, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 9 (5) pp. 1325-1332 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

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

Eales Timothy, Marko Igor, Ikyo BA, Adams Alfred, Arafin S, Sprengel S, Amann M-C, Sweeney Stephen (2017) Wavelength dependence of efficiency limiting mechanisms in Type-I Mid-infrared GaInAsSb/GaSb lasers, IEEE Journal of Selected Topics in Quantum Electronics 23 (6) IEEE
The efficiency limiting mechanisms in type-I GaInAsSb-based quantum well (QW) lasers, emitting at 2.3 ¼m, 2.6 ¼m and 2.9 ¼m, 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 ¼m. 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 ¼m). At shorter wavelengths (
Jin S, Sweeney S, Tomic S, Adams AR, Riechert H (2003) High-pressure studies of recombination mechanisms in 1.3-mu m GaInNAs quantum-well lasers, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 9 (5) pp. 1196-1201 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

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

Tan S, Hunter C, Zhang S, Tan L, Goh Y, Ng J, David J, Marko I, Sweeney S, Adams AR, Allam J (2012) Improved optoelectronic properties of rapid thermally annealed dilute nitride GaInNAs photodetectors, Journal of Electronic Materials 41 (12) pp. 3393-3401 IEEE
We investigate the optical and electrical characteristics of GaInNAs/GaAs long-wavelength photodiodes grown under varying conditions by molecular beam epitaxy and subjected to postgrowth rapid thermal annealing (RTA) at a series of temperatures. It is found that the device performance of the nonoptimally grown GaInNAs p-i-n structures, with nominal compositions of 10% In and 3.8% N, can be improved significantly by the RTA treatment to match that of optimally grown structures. The optimally annealed devices exhibit overall improvement in optical and electrical characteristics, including increased photoluminescence brightness, reduced density of deep-level traps, reduced series resistance resulting from the GaAs/GaInNAs heterointerface, lower dark current, and significantly lower background doping density, all of which can be attributed to the reduced structural disorder in the GaInNAs alloy.© 2012 TMS.
Fehse R, Sweeney S, Adams AR, McConville D, Riechert H, Geelhaar L (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 pp. 85-86

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

Crutchley B, Marko I, Adams AR, Sweeney S (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
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.
Knowles G, Fehse R, Tomic S, Sweeney S, Sale T, Adams AR, O'Reilly P, Steinle G, Riechert H (2003) Investigation of 1.3-mu m GaInNAs vertical-cavity surface-emitting lasers (VCSELs) using temperature, high-pressure, and modelling techniques, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 9 (5) pp. 1202-1208 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

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

Marko Igor, Adams Alfred, Sweeney Stephen, Mowbray DJ, Skolnick MS, Liu HYY, Groom KM (2005) Recombination and loss mechanisms in low-threshold InAs-GaAs 1.3-mu m quantum-dot lasers, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 11 (5) pp. 1041-1047 IEEE

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

Adams Alfred, Marko Igor, Mukherjee J, Sweeney Stephen, Gocalinska A, Pelucchi E, Corbett B (2014) Semiconductor quantum well lasers with a temperature insensitive threshold current, Conference Digest - IEEE International Semiconductor Laser Conference pp. 82-83
Adams Alfred, Marko Igor, Mukherjee J, Stolojan Vlad, Sweeney Stephen, Gocalinska A, Pelucchi E, Thomas K, Corbett B (2015) Semiconductor Quantum Well Lasers With a Temperature-Insensitive Threshold Current, IEEE Journal of Selected Topics in Quantum Electronics 21 (6) 150080 pp. ?-? IEEE
This paper proposes and demonstrates a new multiquantum well (MQW) laser structure with a temperature-insensitive threshold current and output power. Normally, the mechanisms that cause the threshold current (Ith) of semiconductor lasers to increase with increasing temperature T (thermal broadening of the gain spectrum, thermally activated carrier escape, Auger recombination, and intervalence band absorption) act together to cause Ith to increase as T increases. However, in the design presented here, carriers thermally released from some of the QWs are fed to the other QWs so that these mechanisms compensate rather than augment one another. The idea is in principle applicable to a range of materials systems, structures, and operating wavelengths. We have demonstrated the effect for the first time in 1.5 ¼m GaInAsP/InP Fabry-Perot cavity edge-emitting lasers. The results showed that it is possible to keep the threshold current constant over a temperature range of about 100 K and that the absolute temperature over which the plateau occurred could be adjusted easily by redesigning the quantum wells and the barriers between them. TEM studies of the structures combined with measurements of the electroluminescent intensities from the wells are presented and explain well the observed effects.
Sweeney S, McConville D, Jin S, Ahmad C, Masse N, Bouyssou R, Adams AR, Hanke C (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 pp. 738-741

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

Marko I, Andreev AD, Adams AR, Krebs R, Reithmaier J, Forchel A (2003) The Role of Auger Recombination in InAs 1.3-/mu m Quantum-Dot Lasers Investigated Using High Hydrostatic Pressure, IEEE Journal of Selected Topics in Quantum Electronics 9 (5) IEEE

InAs quantum-dot (QD) lasers were investigated in the temperature range 20-300 K and under hydrostatic pressure in the range of 0-12 kbar at room temperature. The results indicate that Auger recombination is very important in 1.3-mum QD lasers at room temperature and it is, therefore, the possible cause of the relatively low characteristic temperature observed, of T-0 = 41 K. In the 980-mn QD lasers where T-0 = 110-130 K, radiative recombination dominates. The laser emission photon energy E-las increases linearly with pressure p at 10.1 and 8.3 meV/kbar for 980 nm and 1.3-mum QD lasers, respectively. For the 980-mn QD lasers the threshold current increases with pressure at a rate proportional to the square of the photon energy E-las(2). However, la the threshold current of the 1.3-mum QD laser decreases. by 26% over a 12-kbar pressure range. This demonstrates the presence of a nonradiative recombination contribution to the threshold current, which decreases with increasing pressure. The authors show that this nonradiative contribution is Auger recombination. The results are discussed in the framework of a theoretical model based on the electronic structure and radiative recombination calculations carried out using an 8 x 8 k(.)p Hamiltonian.

Lock D, Sweeney S, Adams AR (2003) Wavelength dependence of catastrophic optical damage threshold in 980nm semiconductor diode lasers, 2003 IEEE LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2 pp. 421-422

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

Jin S. R., Sweeney S. J., Ahmad C. N., Adams A. R., Murdin B. N. (2004) Radiative and Auger recombination in 1.3 µm InGaAsP and 1.5 µm InGaAs quantum-well lasers measured under high pressure at low and room temperatures, Applied Physics Letters 357 (2004)

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

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

O'Brien K., Sweeney S. J., Adams A. R., Murdin B. N, Salhi A., Rouillard Y., Joullié A. (2006) Recombination processes in midinfrared InGaAsSb diode lasers emitting at 2.37 µm, Applied Physics Letters 051104 (2006)

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

Jin S. R., Ahmad C. N., Sweeney S. J., Adams A. R., Murdin B. N., Page H., Marcadet X., Sirtori C., Tomi? S. (2006) Spectroscopy of GaAs/AlGaAs quantum-cascade lasers using hydrostatic pressure, Applied Physics Letters 221105 (2006)
Jin S R, Ahmad C N, Sweeney S J, Adams A R, Murdin B N, Page H, Marcadet X, Sirtori C, Tomic S (2006) Spectroscopy of GaAs/AlGaAs Quantum-Cascade Lasers Using Hydrostatic Pressure, Applied Physics Letters 89 (22)

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

O'Brien K, Sweeney S J, Adams A R, Murdin B N, Salhi A, Rouillard Y, Joullie A (2006) Recombination Processes in Midinfrared InGaAsSb Diode Lasers Emitting at 2.37 mu m, Applied Physics Letters 89 (5)

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