Formation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphire

This paper addresses the formation of freestanding GaN substrates by a natural separation mechanism, effectively eliminating the need for post-growth processes such as laser liftoff, chemical etching or mechanical lapping to form freestanding GaN substrates. A number of GaN thick films were grown onto sapphire substrates by the hydride vapor-phase epitaxy (HVPE) method with thickness varying from 200 μm to 3.8 mm using either a low-temperature GaN or an AlN buffer as the nucleation step. We have found that samples grown on a low temperature GaN buffer naturally delaminate from the sapphire substrate post-growth over the entire thickness range studied. Furthermore, we have observed that the thinner films have high crack densities leading to the delamination of several smaller freestanding pieces. As the GaN thickness increases, the area of the delaminated pieces also increases, ultimately leading to a 1-to-1 correlation between initial sapphire substrate area and freestanding GaN area. However, the GaN films grown on AlN buffers did not delaminate. These results were accounted for by calculating the thermal stresses in the GaN film and substrate as a function of film thickness using Stoney's equation and assuming that the GaN buffer undergoes decomposition at the growth temperature.