In-depth study of semipolar (11-22) InGaN/GaN-based led towards efficient green emission / Gary Tan

Tan, Gary (2024) In-depth study of semipolar (11-22) InGaN/GaN-based led towards efficient green emission / Gary Tan. PhD thesis, Universiti Malaya.

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Abstract

Semipolar (11-22) InGaN/GaN-based MQW and LED structures were grown on m-plane sapphire, and three separate studies were conducted to explore the impact of various parameters on LED structure growth. The first study investigated the influence of ammonia partial pressure on MQW structures and LED emission wavelength. Results indicated that, at a fixed V/III ratio, ammonia and group III partial pressures affected InGaN/GaN MQW growth rate and indium incorporation in the quantum well. Moreover, the growth behaviour of InGaN quantum wells differed from GaN quantum barriers at low V/III ambient. A critical threshold of ammonia partial pressure, approximately 300 Pa, was observed for InGaN QW growth. Below this threshold, InGaN decomposition led to extremely low growth rates. However, as the ammonia partial pressure exceeded the threshold value, the InGaN QW growth rate sharply increased, resulting in improved indium incorporation and higher MQW abruptness, ultimately enhancing LED performance. The second study explored the effects of a new method called Low Temperature Ammonia Treatment (LTAT) on semipolar (11-22) LED structures. Two samples were prepared, one with LTAT and one without. The LTAT sample displayed a single crystalline XRD curve with distinct fringes, while the sample without LTAT exhibited a mixed phase. The LTAT-treated sample demonstrated better crystal quality, especially in terms of reducing BSF density. Additionally, the LTAT sample exhibited a single sharp peak near the 538.2 nm wavelength, indicating enhanced green light emission due to the LTAT application during growth. In the third study, the effect of AlN ML growth rate on semipolar (11-22) InGaN/GaN LED structures was examined. The sample with an AlN ML growth rate of 1.3 nm/s exhibited outstanding crystal quality with reduced defect densities, having the least BSF density of 1.60 x 104 cm-1. This sample showed a single peak profile in both PL and EL spectra with green wavelength emission, whereas other samples displayed multi-peak profiles, suggesting an inhomogeneous indium composition. Despite all three samples showing high forward voltage and on-resistance, the sample with an AlN ML growth rate of 1.3 nm/s exhibited lower values in both parameters. Lastly, the optimized sample was fabricated, exhibiting an EQE of 0.12% at 20 A/cm2 and an efficiency droop of 53% at 100 A/cm2. In summary, these studies shed light on the effects of various parameters on the growth of semipolar (11-22) InGaN/GaN-based MQW and LED structures. Ammonia partial pressure influenced growth rate, indium incorporation, LED structure's abruptness, and optical performance. LTAT enhanced green light emission. AlN ML growth rate influenced crystal quality and indium composition, with an intermediate growth rate showing improved structural, optical, and electrical performance. Overall, proper device fabrication can enhance electrical properties and enable precise analysis of device performance.

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