Annealing Effects in LPCVD Grown Zinc Oxide Films

Vacuum annealing studies of boron-doped zinc oxide films grown by low pressure chemical vapor deposition (LPCVD) from diethylzinc and H2O vapour are reported. Electrical measurements show, depending on the substrate temperature of the films, a moderate increase in conductivity and Hall mobility up to annealing temperatures of about 200-300°C, followed by decreases at higher annealing temperature. Highest conductivity and mobility values (  ≈ 1000 S/cm and μ ≈ 35 cm2/Vs, respectively) were reached for material deposited at TS = 175°C and annealed at 250°C. Effusion measurements show the release of molecules like H2, H2O and CH4 starting from 200-300°C. Effusion of implanted inert gases helium and neon for structural analysis show that the microstructure of as-deposited materials depends strongly on TS. For TS =150°C material, migration through interconnected voids dominates He effusion while for material deposited at higher TS effusion from isolated voids limits the helium release. After annealing up to 400-500°C, the latter effusion mechanism dominates all types of investigated LPCVD ZnO material. The results suggest that a low concentration of interconnected voids and/or a reduction by annealing is a prerequisite for obtaining improved electrical properties.