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Characteristics of hetero-junction diodes based on ion beam sputtered ZnO thin films

ZnO/p-Si hetero-structure photodiodes have been fabricated by ion beam sputtering technique. The sputtered ZnO films were identified to be polycrystalline nature with wurtzite structure of ZnO and n-type electrical conductivity. Several single and bi-layer metal contacts were produced on both sides of hetero-junction and IV characteristics were measured. It was found that Ni-Au bi-layer contacts were ohmic and best for the diode measurements, having high linearity and low resistivity on both p-Si and ZnO. The IV curve of the hetero-junction indicate that all of the samples have low forward resistance, around 30 ohms and a leakage current around 1 milliamp at 4 volts reverse bias. The photo response of these diodes was tested by shining an 8mW, 780-nm diode laser on to the diodes and measuring the resulting photo current. The responsivity of the hetero-junction was measured as 0.3A/W. This is a quantum efficiency of 48% at 780-nm. Measurement of the ZnO absorption at 780-nm and estimates of the first surface reflectivity of these samples suggests that quantum efficiencies up to 85% are achievable. This is comparable with the best silicon diodes reported in the literature to date which has quantum efficiencies up to 32% and responsivities up to 0.18A/W.