“On Pillars” Epitaxial Overgrowth of Freestanding AlN Layers – EPICEA

This project aims (i) to increase the crystal quality of AlN layers epitaxially grown by HT-HVPE (High Temperature Halide Vapor Phase Epitaxy) on patterned substrates and (ii) to assess HT-HVPE as an alternative process to produce freestanding AlN wafers with industrial grade. AlN layers will be grown by HT-HVPE in a cold wall reactor between 1200°C and 1500°C on patterned silicon (111) and sapphire (001) substrates. Two approaches will be proposed: a top down approach where the seed substrates are deeply etched to produce a dense forest of high aspect ratio micron-size pillars on which the growth will take place and a bottom-up approach where a dense forest of GaN pillars grown an sapphire are used as seed layer. The originality of the project is the use of a very high aspect ratio (>>10) dense pillar forest as an intermediate layer to grow thick continuous AlN layers (>100 µm). The presence of pillars, combined with optimal process parameters that promote the formation of voids at their bottom (diffusion limited growth at the length scale of the pillars), should decrease growth and cooling-down stresses in the AlN grown layer while ensuring that most of the AlN crystal volume grows freely by lateral overgrowth. The main idea is to decrease at most the mechanical interactions between AlN and the seed layer by using fragile pillars patterned in cheap, available and easy-to-process materials. The threading dislocations originating from lattice mismatch and primary AlN islands coalescence as well as the compressive stress during cooling-down will be reduced by this approach. If sufficiently thick, the grown layer would be easily removed from the pillars bed just by cooling down the assembly from process temperature to room temperature.