GaN growth on sapphire

Most of the work on GaN since 1968 [H.P. Maruskas and J.J. Tietjen, Appl. Phys. Lett. 15 (1969) 327] [1] etc. has used GaN grown on sapphire substrates. The most frequently chosen crystallographic planes have been the r-, c-, and a-plane of sapphire. The tremendous mismatch between the lattices of GaN and sapphire can be partly overcome by the use of thin buffer layers of AlN or GaN. In spite of the lattice-matching problem, many useful optoelectronic and electronic devices can be made.