Strain-Relaxed InGaN Buffer Layers for Long Wavelength Nitride Devices
The InGaN material system spans from the ultraviolet (363nm) to the near infrared (1.8µm). It has found tremendous success in solid state lighting, leading to huge improvements in both lifetime and efficiencies of white light sources. InGaN-based light sources can be found in most households, automobiles and displays today. However, these successes are mostly limited to low In-fraction devices emitting in the violet and blue wavelengths. This dissertation focuses on long wavelength InGaN devices emitting in the red, far-red and near infrared.
Because of the large lattice mismatch between InN and GaN, high composition InGaN necessary for long wavelength devices is highly strained when grown on a GaN template. A novel method of biaxially strain-relaxing an InGaN buffer across a planar substrate is demonstrated by leveraging the temperature sensitivity of high composition InGaN layers. The thermal decomposition of an InGaN underlayer to form voids is shown to relax an InGaN buffer across the entire substrate. Quantum wells grown on these buffers show red-shifted emission and a decrease in the strain-state of the active region. We then optimize the growth conditions of the relaxed InGaN buffers to demonstrate smooth, highly relaxed InGaN in a single MOCVD growth.
Next, we demonstrate devices on the strain-relaxed buffers that compare favorably with those using other relaxation methods. The devices show remarkably low forward voltage and high growth temperature compared to conventionally grown counterparts. Using this relaxation method, far-red LEDs emitting over 750nm are achievable. We show red 5 × 5 µm^2 micro-LEDs emitting at 640nm with > 0.45% EQE measured on-wafer.
Finally, we explore growth of InN quantum dots on semipolar (2021) and (2021) substrates, two orientations previously unexplored in InN or In-rich InGaN. We map out growth parameters and characterize the crystal growth on the two planes, demonstrating room temperature photoluminescence in the 1300nm range. An unconventional p-down device structure is proposed and progress is shown towards a first electrically-injected InN LED with photoluminescence shown from a full device. Key challenges are identified, namely the temperature sensitivity of InN and poor crystal quality from the resulting low temperature growth.