III-Nitride semiconductors (GaN, InN, AlN and their alloys) know an extremely rapid and extensive development in electronics and optoelectronics where lighting represents the first application. White LED lighting systems are based on indirect phosphor photoluminescence with nitride LED as an exciting source, although nitride materials offer theoretically the unique potential of a direct electroluminescence at any wavelength of the visible spectrum. However, the efficiency of LEDs needs to be highly enhanced at the middle of the visible spectrum. This efficiency issue known as the “green gap” (Fig.1) could be solved by the tunability in composition of high quality InGaN materials. High efficiency LEDs covering the whole visible range is a key issue for giving rise to a huge market for displays and micro-displays based on nitride mini and micro-LEDs, representing a potential multi-billion euro market essentially for virtual and augmented reality devices and beyond.
A key point for growing better quality InGaN with high In content concerns the lattice mismatch between substrate and epilayer. This mismatch results in a stress in the InGaN epilayer increasing with the In content and actually presenting a subtle influence by affecting the phase diagram of the ternary alloy. And over a certain critical value, stress may cause relaxation in InGaN quantum wells by formation of misfit dislocations. The In solubility limit has been recently studied by a partner of this project. And the effect of lowering stress by substrate modification has also recently been studied by another team of the project. Strain engineering in this high stake issue can thus be considered as the cornerstone. Numerous techniques compete for tuning the mismatch strain/stress: nonpolar substrate orientation, lateral overgrowth, buffer layer engineering, etc. But none of these has proven to reach out to a satisfying solution yet.
We propose the development of an innovative process protected by a patent to transfer thin relaxed InGaN epilayers on sapphire substrates. This process works through a double layer transfer on a temporary polymer substrate. The elastic properties of this temporary substrate allow the accurate relaxation of the full-sheet epilayer. As a result, a relaxed InGaN pseudo-substrate is ready for further epitaxial regrowth of the In-rich LED structure.