Abstract
This thesis is primarily concerned with the growth and characterisation of thin gallium nitride films by reactive sputtering. The deposition parameters, namely the substrate electrode temperature, pressure, relative N2:Ar flow rates and target bias are varied, and the trends in the structural, optical and electrical properties of the resulting films examined as functions of the different parameters. The structural characteristics of the films are studied using profilometry as well as scanning electron, transmission electron and atomic force microscopy. The elemental composition is measured by energy dispersive and electron energy-loss spectroscopy. The optical characterisation is done by ellipsometry, visible and infrared optical absorption, and photoluminescence. The films are found to be polycrystalline with columnar grains tens to hundreds of nanometres in diameter, despite the low substrate electrode temperatures of 18 to 450°C. The level of unintentional oxygenation is significant (3-30 atomic %) and usually increases as the deposition rate is lowered. Oxygen has a substantial effect on the optical properties, increasing the band gap values from 3.02 to above 4.00 eV and decreasing the refractive index from 2.46 to 2.03. High-field conduction in the bulk of the films is found to be single-centre Poole-Frenkel in nature, with characteristics corresponding to both semicrystalline and amorphous materials. In Cr-GaN-Al metal-semiconductor-metal structures, rectifying barriers are formed at the Cr-GaN interface at medium temperatures, high pressures and low target biases. The barrier-controlled conduction is not accurately described by either the thermionic-emission or diffusion theories. A number of variations on the basic GaN deposition process are investigated. These include post-deposition laser and thermal annealing, incorporation of rare earths, hydrogenation, deposition of GalnN and the fabrication of thin film transistor structures. It is shown that the best conditions for growing material having yellow photoluminescence are low target biases and growth-incorporated hydrogen.