Properties of GaN-Related Epitaxial Thin Films Grown on Sapphire Substrates as Transparent Conducting Electrodes

Abstract

Thin films of gallium nitride (GaN) and related nitride materials were prepared, and their properties as transparent conducting electrodes were investigated. GaN thin films were directly grown on sapphire single crystal substrates by the molecular beam epitaxy. Heavy doping of germanium was employed to reduce resistivity of the films, with sufficient reduction found to be possible while maintaining their epitaxial growth state. Optical transmission spectra of the films in the short wavelength region were slightly deteriorated by the heavy doping; however, this was successfully improved by growing GaN films under metal-rich conditions to increase the electron mobility and suppress unwanted increase of the carrier densities. In addition, the optical transmission spectra in the short wavelength region was improved also by alloying GaN with aluminum nitride, though the resistivities of these films were relatively higher than those of the unmodified GaN films. The prepared nitride thin films exhibited sufficiently suitable properties as transparent conducting electrodes for use in applications such as full-spectrum nitride-based solar cells.