The effect of Multi Quantum Well growth regime transition on MQW/p-GaN structure and light emitting diode (LED) performance

This work presents an interesting observation on a possible growth regime transition from diffusion-limited to desorption-limited at Multi-Quantum Well (MQW) growth structure. In common practices, this transition is normally observed by increasing the growth temperature. However, in this work, this phenomenon is noticed by increasing the V/III ratio during the Indium Gallium Nitride/Gallium Nitride (InGaN/GaN) MQW growth process. By increasing the nitrogen (N)-precursors, the V/III of MQW growth structure was varied at three different ratios of 5109, 6387 and 7664 respectively. The X-ray Diffraction (XRD) peaks measured on these three devices reveals the highest Indium (In) incorporation of ~11.2% is obtained at 5109 ratios followed by 6387 ratios with ~5.0% and ~0.0% incorporation for 7664 ratios. Additionally, the EDX mapping also discloses the presence of In element on the p-GaN surface and it reduces significantly with the increase of the MQW V/III ratios. This trend implies the MQW growth process was occurred under diffusion-limited regime, which also affects the p-GaN upper layer. However, XRD results shows that the increment of MQW V/III ratios depreciates the MQW thicknesses, which manifests that the growth condition changed to metal-limited or N-rich regime, where the important reactants start to desorb from the sample. This leads to the low growth rate of InGaN/GaN layer and degrades the devices performance. The blue shift of InGaN peaks in photoluminescence spectra has support the notion of In reduction at high MQW V/III ratios. At 20 mA, the devices of 5109 and 6387 ratios with a forward voltage of 3.57 V and 3.95 V produce electroluminescence peak at 443.74 nm and 487.45 nm, respectively. Despite the 5109 sample exhibits the highest In percentage, green speckles were produced at low optical threshold voltage due to the proliferation of localization states induced by the In clusters. The device also experiences the higher reverse current leakage compared to 6387 device due to higher threading dislocation density.