Temperature-dependent negative photoconductivity of undoped ZnO films

Undoped ZnO films of 100 nm thickness have been deposited on p-type Si (100) substrates at room temperature by radio-frequency magnetron sputtering and subsequently annealed in a rapid thermal annealing apparatus under a nitrogen ambient by varying annealing temperature (TA) from 973 to 1273 K in steps of 100 K for 3 min. Photoconductivity spectra in the UV and visible ranges are measured as a function of TA in the range of measurement temperatures (T) from 80 to 350 K under an applied bias of 1 V. For the samples annealed at TA = 973 and 1073 K, negative photoconductivity is observed below T = 300 K, and its absolute magnitude increases as TA increases. In contrast, the photoconductivity of the samples at TA ≥ 1173 K is always positive, irrespective of T, and its magnitude increases as TA increases. These PC behaviors are discussed based on the defect states at the grain boundaries of the ZnO films.