Toward III-N /spl lambda/-cavity vertical emitters: heteroepitaxy of GaN and AlN

We study the initial nucleation and evolution of strain relaxation during the alternating growth of AlN on GaN and GaN on AlN by MOCVD. Emphasis is given to the evolution of strained nucleation from a few monolayers up to a quarter of wavelength for DBR application. It is found that the AFM morphology of AlN (/spl sim/300 A) depends sensitively on V/III ratios, ranging from large 3D island/platelets (under a high V/III ratio), to two-dimensional step flow mode (V/III ratio) with distinct macroscopic cracks, to the occurence of microscopic cracks (under a very low V/III ratio) accompanied by a reduction of the macroscopic cracks. GaN on AlN exhibited a strong tendency for 3D islanding.