Conduction in Degenerately Doped Zn_1−x Al _x O Thin Films

Abstract

This chapter reports the electrical and optical properties of degenerately doped ${\mathrm{Zn}}_{{{1}_{-} }_{x}}{\mathrm{Al}}_{x}\mathrm{O}$ films. Thin films of Zn_{1 − x} Al _x O ($x = 0\mbox{ \textendash }0.1$) were grown on single crystal c-plane sapphire substrates by pulsed laser deposition (PLD) technique. From electrical and optical characterizations, all film compositions were found to be highly transparent over the visible spectrum and exhibit a metal-like temperature dependence of resistivity. The carrier concentration, conductivity, and optical band gap of these films were not observed to increase continuously with increased Al concentrations. Detailed characterization showed that these properties initially increased reaching a maximum at 4 at.% Al. Beyond this Al concentration a decrease in these properties was observed. Through temperature-dependent electrical transport measurements it was found that at higher Al concentrations, scattering due to electron–electron interactions increased. Electron–electron interactions arise due to the introduction of disorder in the crystal potential caused by high concentrations of Al dopants and oxygen vacancies. High levels of disorder can significantly disrupt the band structure of ZnO reducing the density of states at the Fermi level (N(E _F)). The decrease in N(E _F) directly corresponds to the decrease in carrier concentration affecting the optical band gap and conductivity.