The properties and deposition process of GaN films grown by reactive sputtering at low temperatures

Polycrystalline gallium nitride films, 100 nm to 1 mu m thick, were deposited under a range of conditions. Substrate electrode temperatures during sputtering were varied from room temperature to 450 degrees C, the pressure from 0.15 to 6.0 Pa, the nitrogen fraction of the deposition atmosphere from 10% to 100% and the target bias from -400 to -1800 V. The deposition rates as functions of these conditions are in the range 0.5-25 nm/min. The growth rate is considered to be controlled respectively by the thermally activated desorption from the substrate, changes in the mean free path and concentration of gas particles, differences between the sputter yields of Ga and GaN in Ar and N-2, and changes in the ion current and sputter yields. The films are generally columnar, with the grain size increasing with film thickness. The most crystalline films were grown at mid range temperatures, low N-2 concentrations, and low target biases, and the most disordered were grown at low pressures. The latter two cases suggest that decreasing the energy of particles incident on the film during deposition results in a more ordered film. The biaxial stress is compressive and shows an increasing trend with the target bias and N-2 concentration, reaching 4.7 GPa at 75% N-2. Oxygen contamination of 3-30 at. % has a major effect on the optical properties of the films, increasing the band gap values from 3.02 to > 4.0 eV and the Urbach tail energies from around 150 to 840 meV and decreasing the refractive index from 2.46 to 2.03. At a 40% N-2 deposition fraction, the N:Ga ratio is more or less constant at 1:1. Since the absolute oxygen incorporation rate changes very little, it is the relative film deposition rate which determines the final oxygen concentration. Excess Ga at low N-2 concentrations causes a decrease in the band gap and an increase in the Urbach tail energy.