Formation and Characterisation of Amorphous Gallium Nitride
Abstract
For this study, wurtzite GaN was epitaxially grown on a Al2O3 substrate. Four samples were implanted with 4.7MeV Au ions to doses below and above the amorphisation threshold. These are characterised relative to an un-implanted as-grown sample. The extent of amorphisation within the ion-implanted material is characterised by comparing the results of rigorous analysis using the complementary techniques of X-Ray Diffraction (XRD), Rutherford Backscattering Spectrometry (RBS), Transmission Electron Microscopy (TEM) and Extended X-ray Absorption Fine Structure (EXAFS). XRD experiments highlight disordering of the GaN structure in the highest dose sample, characteristic of amorphisation. This is contrasted by results of TEM analysis which show uniformly distributed but randomly orientated GaN nanocrystals within an amorphous matrix, suggesting that the material is not completely amorphous. EXAFS analysis suggests an unusually high second nearest neighbour (NN) coordination number (CN) in the highest dose sample further indicating the presence of residual crystallinity within the ion-implanted material. The random orientation of the GaN nanocrystals within the otherwise amorphous matrix is consistent with ion-induced reconstruction and suggests the nanocrystals were formed after the matrix was rendered amorphous. TEM analysis also yields images of N2 gas bubbles, characteristic of ion-induced disordering of GaN, within the amorphous and nanocrystalline GaN material. The results of studies performed on other III-V semiconductors suggests that this would indicate the presence of Ga-Ga homopolar bonding however no direct evidence of this was found.