Ion-induced crystal damage during plasma-assisted MBE growth of GaN layers

Gallium nitride layers were grown by plasma-assisted molecular-beam epitaxy on (0001)-oriented sapphire substrates using an electron cyclotron resonan ce (ECR) and a radio frequency (rf) plasma source. An applied substrate bia s was varied from -200 to +250 V, resulting in a change of the density and energy of nitrogen ions impinging the growth surface. The layers were inves tigated by high-resolution x-ray diffractometry and high-resolution transmi ssion electron microscopy (HRTEM). Applying a negative bias during growth h as a marked detrimental effect on the crystal perfection of the layers grow n with an ECR plasma source. This is indicated by a change in shape and wid th of (0002) and (<20(2)over bar 5>) reciprocal lattice points as monitored by triple axis x-ray measurements, in HRTEM images, isolated basal plane s tacking faults were found, which probably result from precipitation of inte rstitial atoms. The crystal damage in layers grown with a highly negative s ubstrate bias is comparable to that observed for ion implantation processes at orders of magnitude larger ion energies. This is attributed to the impa ct of ions on the growing surface. None of the described phenomena was obse rved for the samples grown with the rf plasma source. [S0163-1829(98)00248- 3].