Matthew J. Davies

Structural, electronic and optical properties of m-plane (In,Ga)N/GaN quantum wells: Insights from experiment and atomistic theory

In this paper we present a detailed analysis of the structural, electronic, and optical properties of an

The impact of trench defects in InGaN/GaN light emitting diodes and implications for the “green gap” problem

m

The effects of Si-doped prelayers on the optical properties of InGaN/GaN single quantum well structures

-plane (In,Ga)N/GaN quantum well structure grown by metal organic vapor phase epitaxy. The sample has been structurally characterized by x-ray diffraction, scanning transmission electron microscopy, and 3D atom probe tomography. The optical properties of the sample have been studied by photoluminescence (PL), time-resolved PL spectroscopy, and polarized PL excitation spectroscopy. The PL spectrum consisted of a very broad PL line with a high degree of optical linear polarization. To understand the optical properties we have performed atomistic tight-binding calculations, and based on our initial atom probe tomography data, the model includes the effects of strain and built-in field variations arising from random alloy fluctuations. Furthermore, we included Coulomb effects in the calculations. Our microscopic theoretical description reveals strong hole wave function localization effects due to random alloy fluctuations, resulting in strong variations in ground state energies and consequently the corresponding transition energies. This is consistent with the experimentally observed broad PL peak. Furthermore, when including Coulomb contributions in the calculations we find strong exciton localization effects which explain the form of the PL decay transients. Additionally, the theoretical results confirm the experimentally observed high degree of optical linear polarization. Overall, the theoretical data are in very good agreement with the experimental findings, highlighting the strong impact of the microscopic alloy structure on the optoelectronic properties of these systems.

Low temperature carrier redistribution dynamics in InGaN/GaN quantum wells

The impact of trench defects in blue InGaN/GaN light emitting diodes (LEDs) has been investigated. Two mechanisms responsible for the structural degradation of the multiple quantum well (MQW) active region were identified. It was found that during the growth of the p-type GaN capping layer, loss of part of the active region enclosed within a trench defect occurred, affecting the top-most QWs in the MQW stack. Indium platelets and voids were also found to form preferentially at the bottom of the MQW stack. The presence of high densities of trench defects in the LEDs was found to relate to a significant reduction in photoluminescence and electroluminescence emission efficiency, for a range of excitation power densities and drive currents. This reduction in emission efficiency was attributed to an increase in the density of non-radiative recombination centres within the MQW stack, believed to be associated with the stacking mismatch boundaries which form part of the sub-surface structure of the trench defects. Investigation of the surface of green-emitting QW structures found a two decade increase in the density of trench defects, compared to its blue-emitting counterpart, suggesting that the efficiency of green-emitting LEDs may be strongly affected by the presence of these defects. Our results are therefore consistent with a model that the “green gap” problem might relate to localized strain relaxation occurring through defects. V C 2014 AIP Publishing LLC .[ http://dx.doi.org/10.1063/1.4896279]

A comparison of the optical properties of InGaN/GaN multiple quantum well structures grown with and without Si-doped InGaN prelayers

In this paper, we report on the effects of including Si-doped (In)GaN prelayers on the low temperature optical properties of a blue-light emitting InGaN/GaN single quantum well. We observed a large blue shift of the photoluminescence peak emission energy and significant increases in the radiative recombination rate for the quantum well structures that incorporated Si-doped prelayers. Simulations of the variation of the conduction and valence band energies show that a strong modification of the band profile occurs for the quantum wells on Si-doped prelayers due to an increase in strength of the surface polarization field. The enhanced surface polarization field opposes the built-in field across the quantum well and thus reduces this built-in electric field. This reduction of the electric field across the quantum well reduces the Quantum Confined Stark Effect and is responsible for the observed blue shift and the change in the recombination dynamics.

Comparative studies of efficiency droop in polar and non-polar InGaN quantum wells

We have studied the carrier recombination dynamics in an InGaN/GaN multiple quantum well structure as a function of emission energy and excitation density between temperatures of 10 K and 100 K. Under relatively low levels of excitation, the photoluminescence (PL) intensity and decay time of emission on the high energy side of the luminescence spectrum decrease strongly between 10 K and 50 K. In contrast, for emission detected on the low energy side of the spectrum, the PL intensity and decay time increase over the same temperature range. These results are consistent with a thermally activated carrier redistribution process in which the (temperature dependent) average timescale for carrier transfer into or out of a localised state depends on the energy of the given state. Thus, the transfer time out of shallow, weakly localised states is considerably shorter than the arrival time into more deeply localised states. This picture is consistent with carriers hopping between localisation sites in an uncorrelat...

Inner-city gunshot wounds - 10 years on

In this paper, we report on a detailed spectroscopic study of the optical properties of InGaN/GaN multiple quantum well structures, both with and without a Si-doped InGaN prelayer. In photoluminescence and photoluminescence excitation spectroscopy, a 2nd emission band, occurring at a higher energy, was identified in the spectrum of the multiple quantum well structure containing the InGaN prelayer, originating from the first quantum well in the stack. Band structure calculations revealed that a reduction in the resultant electric field occurred in the quantum well immediately adjacent to the InGaN prelayer, therefore leading to a reduction in the strength of the quantum confined Stark effect in this quantum well. The partial suppression of the quantum confined Stark effect in this quantum well led to a modified (higher) emission energy and increased radiative recombination rate. Therefore, we ascribed the origin of the high energy emission band to recombination from the 1st quantum well in the structure. Study of the temperature dependent recombination dynamics of both samples showed that the decay time measured across the spectrum was strongly influenced by the 1st quantum well in the stack (in the sample containing the prelayer) leading to a shorter average room temperature lifetime in this sample. The room temperature internal quantum efficiency of the prelayer containing sample was found to be higher than the reference sample (36% compared to 25%) which was thus attributed to the faster radiative recombination rate of the 1st quantum well providing a recombination pathway that is more competitive with non-radiative recombination processes.

High excitation carrier density recombination dynamics of InGaN/GaN quantum well structures: Possible relevance to efficiency droop

We report on a comparative study of efficiency droop in polar and non-polar InGaN quantum well structures at T = 10 K. To ensure that the experiments were carried out with identical carrier densities for any particular excitation power density, we used laser pulses of duration ∼100 fs at a repetition rate of 400 kHz. For both types of structures, efficiency droop was observed to occur for carrier densities of above 7 × 1011 cm−2 pulse−1 per quantum well; also both structures exhibited similar spectral broadening in the droop regime. These results show that efficiency droop is intrinsic in InGaN quantum wells, whether polar or non-polar, and is a function, specifically, of carrier density.

A study of the optical and polarisation properties of InGaN/GaN multiple quantum wells grown on a-plane and m-plane GaN substrates

INTRODUCTION In 1997, Injury published one of the first research papers to document the incidence and characteristics of civilian gunshot wounds in a UK urban environment. Since then there has been concern that firearm deaths and injuries have increased despite little published clinical evidence. METHODS We carried out a retrospective survey ten years on from the initial study. All patients presenting to the Emergency Department (ED) of Kings College Hospital with gunshot wounds from 1st January 2003 to 31st December 2004 were identified. Information regarding incidence, patient and injury characteristics and outcome was determined. RESULTS 46 patients presented with firearm injuries. 44 were male and the average age was 24 years. The majority were from minority ethnic groups. 38/46 presented out of hours and the police were documented to be involved in 36 cases. All injuries were due to assault by low energy projectiles. Of the 32 patients admitted the mean length of stay was 12.4 days. The majority of injuries were to the musculo-skeletal system. Six patients died from their injuries–5 from head/neck or chest injuries and 1 from intra-abdominal injury. DISCUSSION There appears to be little increase in firearm injuries seen over this 10 year period at our hospital and predominately young, black males continue to be the victims. Most present out of hours,potentially placing considerable challenges on junior medical staff. Most wounds were to the musculoskeletal system perhaps reflecting the desire to maim rather than kill and the absence of high velocity injuries may reflect the UKs stringent gun control legislation. The importance of high quality clinical audit is necessary to effectively plan training, service provision and violence prevention efforts. CONCLUSION Despite public, political and media concerns that deaths and injuries caused by firearm shave increased dramatically, this study finds little change in incidence or characteristics of those injured and attending an urban ED over a ten year period.

Theoretical and experimental analysis of the photoluminescence and photoluminescence excitation spectroscopy spectra of m-plane InGaN/GaN quantum wells

We report on the optical properties of InGaN/GaN quantum well structures measured at 10 K as a function of excitation density. At high excitation power densities we observe a component in the spectra that decays more rapidly than the localised carrier emission observed for low excitation power densities. We attribute this component to recombination involving weakly localised or delocalised carriers. At the high excitation power densities there is a reduction in the recombination internal quantum efficiency, so called efficiency droop. These observations are compatible with the model whereby efficiency droop is explained in terms of the non radiative loss of delocalised carriers.