Developing InGaN Pseudo-Substrates for Tailoring Red and Longer-Wavelength Nitride-Based Devices

The tunable direct bandgap of nitride semiconductor alloys makes these materials perfect candidates for the design of full-color emitters. Emitters based on InGaN layers with moderate In content (up to 20%) operating in the near UV and blue spectral range are already widely used, providing energy efficient lighting solutions. However, increasing the In content required to realize longer wavelength emitters often results in poor structural crystal quality. Theoretical studies show that the introduction of high In contents above 30% in coherently grown InGaN layers requires substrates with lattice parameters larger than GaN. One of the obvious solutions is to use relaxed InGaN layers for this purpose. There are several approaches to obtain relaxed InGaN layers. While most of them are based on elastically relaxed layers obtained by preparing a compliant layer (e.g. porous GaN) and subsequent patterning, in our attempt we focus on InGaN layers plastically relaxed by introducing misfit dislocations. In this approach it is possible to obtain InGaN pseudo-substrates with a higher in-plane lattice constant up to 0.321 nm at the moderate cost of a higher dislocation density (TDD) in the range of 10⁸ - 10⁹ cm^-2. To keep the TDD as low as possible, we have focused on InGaN layers with In contents in the range of 17-20% deposited on ammonothermally grown GaN substrates with TDD in the range of 10⁴ cm^-2. This work focuses on recent developments in the technology of growing and processing InGaN pseudo-substrates. We have carried out detailed investigations on the nucleation and gliding of misfit dislocations, which are the key mechanisms governing the plastic relaxation of the InGaN layer. The surface quality of the InGaN films, which are rough after the relaxation process, can be greatly improved by chemical mechanical polishing. The average roughness Ra changed from 1.68 nm for the as grown surface to 0.21 nm for the polished InGaN. The polished InGaN buffers were used to deposit structures with InGaN QWs. The indium content in QWs deposited on relaxed InGaN films, as determined by X-ray diffraction, is the same as in reference structures deposited on bulk GaN substrates. However, the luminescence of QWs deposited on InGaN pseudo-substrates is shifted toward red compared to the reference structures. The reasons for the observed luminescence shift are discussed.