Study of the correlation between GaN material properties and the growth conditions of radio frequency plasma-assisted molecular beam epitaxy

The material properties of GaN thin films grown by radio frequency (RF) nitrogen plasma source molecular beam epitaxy (MBE) on (0001) Al2O3 substrates have been correlated to the V/III flux ratio during GaN growth and to the type and thickness of the buffer layer. The most remarkable observation is the change in the sign of the residual strain, from tensile to compressive as the V/III ratio alters from N-rich to stoichiometric (or slightly Ga-rich) conditions for GaN layers with a 17 nm AlN buffer layer. The residual strain was significantly reduced for a thinner 5 nm AlN buffer and it was zero for a 20 nm GaN buffer. A reduction of the rms surface roughness from 20 to 3 nm was achieved by decreasing the V/III ratio. Finally, stacking faults were observed only for significantly N-rich growth conditions.