Abstract:
Crystalline GaN has attracted wide interest due to its optical activity in the UV. Calculations of the band structure of amorphous GaN [1,2] predict that it too will have a defect-free band gap and therefore have potential in devices. We have grown nanocrystalline GaN (nx-GaN) by ion assisted deposition. Mn incorporation up to 12 % has been achieved in the nx-GaN and O incorporation of more than 12 % produces fully amorphous films. I have investigated the structure of nx-GaN, nx-GaMnN and amorphous GaON films via extended x-ray absorption fine structure (EXAFS) measurements and the optoelectronic properties of the nx-GaN and the GaON films via photoconductivity measurements. EXAFS measurements at the gallium edge reveal that the Ga atoms in these disordered GaN films have the same local heteropolar tetrahedral structure as crystalline GaN. There is no evidence for a separate phase in the oxygenated films, and Mn-edge EXAFS confirms that the Mn are also incorporated substitutionally (on the Ga site). Photoconductivity measurements reveal persistent photoconductivity in the GaON and a fast response in the nx-GaN. The time, intensity, wavelength, temperature and spatial dependencies of the photoconductivity can be successfully modelled by considering the effect of strong trapping of the majority carrier into band tail states.
