Ion implantation in GaN at liquid-nitrogen temperature: Structural characteristics and amorphization

This paper deals with the results of a systematic investigation of damage generation and accumulation until amorphization induced by 180 keV ${\mathrm{Ca}}^{+}$ and ${\mathrm{Ar}}^{+}$ implantation in GaN films at liquid-nitrogen temperature. The structure of GaN films before and after implantation was characterized by Rutherford backscattering/channeling, cross-sectional transmission electron microscopy, and high-resolution x-ray diffraction. The as-implanted GaN films exhibits an expanded lattice. Its texture was determined by pole figure measurement. An amorphous component has been found after ${\mathrm{Ca}}^{+}$ implantation at doses not less than $3\times {10}^{14}{\mathrm{cm}}^{\mathrm{-}2}.$ This suggests that ${\mathrm{Ca}}^{+}$ implantation for $p$-type doping be carried out below this dose, in order to avoid unrecoverable structural damage and to achieve better transport properties. On the other hand, implantation with higher doses is generally needed to compensate for the native electron background of GaN and to realize $p$-type reversal. This conflict uncovers the essential difficulty for $p$-type doping of GaN by ion implantation. The maximum damage concentration exists in a depth of 100 nm below the surface, which corresponds to the mean projected range, and broadens gradually towards surface and greater depth with increasing ion fluence. The thresholds for the amorphization of GaN films are revealed to be $6\times {10}^{15}{\mathrm{cm}}^{\mathrm{-}2}$ for both ${\mathrm{Ca}}^{+}$ and ${\mathrm{Ar}}^{+}$ implantation. The amorphization mechanism is discussed and the accumulation of amorphous clusters seems to be the reason for the collapse of GaN crystalline.