Patterning of sapphire substrates via a solid state conversion process

Nanopatterned sapphire substrates offer the potential for improved performance of devices based on III-V nitrides, e.g., light-emitting diodes and laser diodes. Due to the chemical stability and hardness of sapphire, however, surface patterning is a time-consuming and expensive process. Therefore, a novel method was utilized, whereby a surface coating of Al was deposited on a sapphire substrate and patterned into an array of square mesas using e-beam lithography. The lateral dimensions of each mesa were approximately 400 x 400 nm, and the average height was approximately 100 nm. The metallic film was subsequently subjected to an oxidation treatment at 450 degrees C for 24 h (a heat treatment which had previously been shown to minimize hillock formation). For the second heat treatment, which is necessary to induce migration of the sapphire interface and hence achieve solid state conversion, a range of temperatures (800-1350 degrees C) was explored. Results showed that for a heat-treatment time of 1 h, pattern retention was achieved for annealing temperatures less than or equal to 1250 degrees C. Successful epitaxial conversion of the patterned mesas to sapphire was confirmed using electron backscatter diffraction.