Optimizing n-ZnO/p-Si heterojunctions for photodiode applications

N-ZnO/p-Si heterojunction photodiodes have been fabricated by sputter deposition of n-ZnO films on p-Si substrates. The substrate temperatures of 300, 400, 480 and 550°C were taken for the n-ZnO film deposition using an Ar/O₂ ratio of 6:1. All the diodes show typical rectifying behaviors as characterized by the current-voltage (I-V) measurement in a dark room and their photoelectric effects from the diodes have been observed under illumination using monochromatic red light with a wavelength of 670 nm. Maximum amount of photo-current or responsivity is obtained under reverse bias conditions from a n-ZnO/p-Si heterojunction when the ZnO film was deposited at 480°C while the ZnO films deposited at 550°C show the best stoichiometric and crystalline quality. Junction leakage or dark current is much higher in the diode with n-ZnO deposited at 550°C than in the other diodes. It is thus concluded that for a photodiode the quality of the diode junction is as important as that of the n-ZnO film deposited on p-Si.