Damage buildup in GaN under ion bombardment

Abstract

The damage buildup until amorphization in wurtzite GaN films under keV light ${(}^{12}\mathrm{C})$ and heavy ${(}^{197}\mathrm{Au})$ ion bombardment at room and liquid nitrogen $({\mathrm{LN}}_{2})$ temperatures is studied by Rutherford backscattering/channeling (RBS/C) spectrometry and transmission electron microscopy (TEM). The effect of beam flux on implantation damage in GaN is reported. A marked similarity between damage buildup for light and heavy ion bombardment regimes is observed. The results point to substantial dynamic annealing of irradiation defects even during heavy ion bombardment at ${\mathrm{LN}}_{2}$ temperature. Amorphization starts from the GaN surface with increasing ion dose for both ${\mathrm{LN}}_{2}$ and room-temperature bombardment with light or heavy ions. A strong surface defect peak, seen by RBS/C, arises from an amorphous layer at the GaN surface, as indicated by TEM. The origin of such an amorphous layer is attributed to the trapping of mobile point defects by the GaN surface, as suggested by the flux behavior. However, in the samples implanted with light ions to low doses $(1\ifmmode\times\else\texttimes\fi{}{10}^{15}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}),$ no amorphous layer on the GaN surface is revealed by TEM. Damage buildup is highly sigmodal for ${\mathrm{LN}}_{2}$ temperature irradiation with light or heavy ions. Formation of planar defects in the crystal bulk is assumed to provide a ``nucleation site'' for amorphization with increasing ion dose during irradiation at ${\mathrm{LN}}_{2}$ temperature. For room-temperature bombardment with heavy ions, the damage in the GaN bulk region saturates at a level lower than that of the amorphous phase, as measured by RBS/C, and amorphization proceeds from the GaN surface with increasing ion dose. For such a saturation regime at room temperature, implantation damage in the bulk consists of point-defect clusters and planar defects which are parallel to the basal plane of the GaN film. Various defect interaction processes in GaN during ion bombardment are proposed to explain the observed, somewhat unexpected behavior of disorder buildup.