Crack-Free Thick AlN Films Obtained by NH 3 Nitridation of Sapphire Substrates

We demonstrate that NH 3 nitridation of sapphire substrates effectively suppresses cracks in AlN epilayers. The sapphire nitridation promoted three-dimensional (3D) growth at the initial stage, in contrast to the 2D growth mode on non-nitrided sapphire. The coalescence of 3D columnar grains in the process created voids, which act as strain absorber and thus crack-free thick epilayers were obtained. The control of nitridation period is also found important. The optimum nitridation period realized an atomically-smooth epilayer with superior structural quality. On the other hand, the least nitrided sapphire exhibited high twist mosaic of the grains which complicate the epilayer smoothing process, and the longer nitridation promoted slow recovery of a smooth epilayer.