Effect of oxygen partial pressure on the behavior of Ga-doped ZnO/p-Si heterojunction diodes fabricated by reactive sputtering

Abstract

A systematic investigation of Ga-doped ZnO (GZO) films deposited by radio frequency reactive magnetron co-sputtering of Zn and GaAs has been carried out toward the realization of n-GZO/p-Si heterojunction diodes. X-ray diffraction and X-ray photoelectron spectroscopy studies of these films have shown a strong dependence of the c-axis orientation of crystallites, Ga/Zn ratio, oxygen vacancies and chemisorbed oxygen species on the partial pressure (percentage) of oxygen in the Ar-O2 sputtering atmosphere. The GZO films deposited at low O2 percentage (≤ 6%) exhibit Ga/Zn ratio ~ 0.03 and consequently high conductivity ~ 103 Ω−1 cm−1 with carrier concentration > 1020 cm−3, leading to the formation of non-rectifying contact with p-Si. A significant finding of the present study is that the doping level and conductivity of GZO layer can be controlled by O2 percentage in sputtering atmosphere, which has been utilized to fabricate heterojunction diodes with GZO films deposited above 6% O2, which possess low Ga/Zn ratio ~ 0.01 and carrier concentration ≤1019 cm−3. In particular, the diode fabricated with 8% O2 displays ideality factor ~ 7 and reverse saturation current ~ 2 × 10–6 A, along with improved turn-on-voltage and series resistance. The nearly complete c-axis orientation, absence of oxygen vacancies and presence of chemisorbed oxygen at the grain boundaries in GZO layer appear to facilitate improved diode performance.