High quality AlN and GaN grown on Si(111) by gas source molecular beam epitaxy with ammonia

We describe the growth of high quality AIN and GaN on Si(1 1 1) by gas sour ce molecular beam epitaxy (GSMBE) with ammonia (NH3). The initial nucleatio n (at 1130-1190K) of an AlN monolayer with full substrate coverage resulted in a very rapid transition to two-dimensional (2D) growth mode of AlN. The rapid transition to the 2D growth mode of AlN is essential for the subsequ ent growth of high quality GaN, and complete elimination of cracking in thi ck (> 2 mum) GaN layers. We show, using Raman scattering (RS) and photolumi nescence (PL) measurements, that the tensile stress in the GaN is due to th ermal expansion mismatch, is below the ultimate strength of breaking of GaN , and produces a sizable shift in the bandgap. We show that the GSMBE AW an d GaN layers grown on Si can be used as a substrate for subsequent depositi on of thick AlN and GaN layers by hydride vapor phase epitaxy (HVPE).