Physics of GaN Quantum Well Structures

Lead Research Organisation: University of Manchester
Department Name: Physics and Astronomy

Abstract

Gallium nitride (GaN) is a wide direct band gap semiconductor alloy. Production of ternary compounds of III-nitrides such as Indium gallium nitride (InGaN) allows the band gap to be tuneable between the band gaps of two III-nitride binary compounds by varying the ratio of the group III elements within the semiconductor. The wide direct band gap and ability to vary the band gap energy allows III-nitrides and their ternary alloys to emit and detect light over the entire visible spectrum and hence marks them as attractive semiconductors to be used as detectors and emitters. InGaN has lower band gap energy than GaN so a quantum potential well can be created by sandwiching a nanometres thick layer of InGaN between two GaN layers. The presence of the InGaN quantum well enhances light emission relative to GaN due to the localization of charge carriers.
InGaN/GaN quantum well structures are employed in blue LEDS and phosphor coated white LEDs currently on the market. These LEDs are replacing incandescent forms of lighting which have shorter lifetimes, lower efficiencies and less physical robustness. A more efficient white LED can be made without an energy-dissipating phosphor coating by combining green emitting InGaN/GaN quantum well structures with red and blue emitting quantum well structures. However, green InGaN/GaN quantum well structures do not emit light as efficiently as blue InGaN/GaN quantum well structures. This project seeks to understand the physics of why green InGaN/GaN quantum well structures are less efficient through experiment and analysis by using photoluminescence spectroscopy and other techniques.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509565/1 01/10/2016 30/09/2021
1804314 Studentship EP/N509565/1 19/09/2016 30/09/2018 Lydia Man Jim
 
Description Two sets of green light-emitting indium gallium nitride/gallium nitride quantum wells (InGaN/GaN QWs) samples were investigated using photoluminesence spectroscopy. InGaN/GaN QW structures are widely used in LED lighting, which is more energy efficiency than traditional incandescent and compact fluorescent lighting.
The first series of InGaN/GaN QW samples had a varying number of quantum wells and were investigated to determine if increasing the number of quantum wells in green light-emitting quantum well structure increases the efficiency of the structure at room temperature. This was previously shown to be true for blue light-emitting InGaN QW structures by Hurst et al., who attributed the increased efficiency to an increased rate of charge carrier recapture at room temperature. The loss of carriers from quantum well structures contribute to an decrease in efficiency as the carriers can no longer re-combine radiatively and produce light. Photoluminescence spectroscopy and time-correlated single photon techniques were used to determine if the quantum well samples were comparable and to measure the internal quantum efficiency of the samples at varying temperature and excitation power densities. Two structures, one with five QWs and one with three QWs were deemed comparable. The internal quantum efficiency of the two comparable structures at room temperature indicated that for green light-emitting quantum wells, increasing the number of QWs in a structure increases the internal quantum efficiency.
The second set of InGaN/GaN multiple QW structures had a varying growth temperature and varying defect density. This set of samples were studied with respect to their behaviour under very high excitation power densities. At high excitation power densities, the efficiency of InGaN/QW samples decreases with efficiency; this effect is known as efficiency droop. Low temperature excitation power varying photoluminesence spectroscopy was performed on the samples to determine if the efficiency droop of the samples were affected by the growth temperature and defect density of the samples. The sample with the highest growth temperature and the lowest defect density was observed to experience efficiency droop at the a higher excitation power density compared to the other samples in the series, suggesting that efficiency droop could be linked to defect density.
Furthermore, the samples were investigated with respect to their high energy band (HEB) behaviour. The high energy band is a spectral feature which appears on the high energy side of the light emission spectrum of some InGaN/GaN QW samples under very high excitation power densities; it has only been very recently observed. The HEB behaviour of the samples did not vary with increasing growth temperature and defect density. This shows that the HEB is unlikely to be related to defect-related carrier recombination.
To further study efficiency droop, one InGaN/GaN mulitple QW sample was investigated using time-correlated single photon-counting techniques to measure its carrier lifetime at varying excitation power densities and at varying emission energies at low temperatures. The lifetime of the charge carriers at a given emission energy was observed to be constant at low excitation powers densities and to decrease at high excitation power densities; this reduction in lifetime occurred at a higher excitation power density at at higher photoluminescence emission energies.
Exploitation Route The findings may be put to use by others to improve the efficiency of LED lighting by providing more information on efficiency droop and how the efficiency of green light-emitting quantum well structures vary with quantum well number. Additionally, the variation in lifetime with increasing power (and carrier) density across the photoluminescence spectrum to be further investigated; performing excitation power varying photoluminescence time decay measurements at varying emission energies at 10 K on a single QW sample could remove the possibility that the observed behaviour in the previous section is due to variations between QWs within a multiple QW sample.
Sectors Electronics,Manufacturing, including Industrial Biotechology