Thermal degradation of electrical properties and morphology of bulk single-crystal ZnO surfaces

Abstract

Bulk $\mathrm{ZnO}$ substrates were rapid thermal annealed in either air or ${\mathrm{N}}_2$ at temperatures up to $500°\mathrm{C}$ . The root-mean-square roughness of the surface as measured by atomic force microscopy begins to increase even after $200°\mathrm{C}$ anneals in ${\mathrm{N}}_2$ or $300°\mathrm{C}$ in air. The Schottky barrier height, ${\Phi }_B$ , obtained from $\mathrm{Pt}∕\mathrm{Au}$ diodes fabricated on these surfaces shows a decrease in effective barrier height upon annealing from $0.35\mathrm{eV}$ in control samples to $\sim 0.30\mathrm{eV}$ in those annealed at $400°\mathrm{C}$ and an increase in the saturation current density (from $4\mathrm{A}{\mathrm{cm}}^{-2}$ in the control samples to $17–37\mathrm{A}{\mathrm{cm}}^{-2}$ in the annealed samples). These results show that the $\mathrm{ZnO}$ surface is degraded by quite low annealing temperatures and care must be exercised in designing the thermal budget for processing of $\mathrm{ZnO}$ devices. At much higher annealing temperatures $(1000°\mathrm{C})$ , the surface completely decomposes to leave metallic $\mathrm{Zn}$ droplets.