High responsivity self-powered photodetectors based on p-Si/ZnO heterojunction

Recently, there has been an increased interest in broadband and ultraviolet (UV) photodetectors (PDs) that operate in self-powered mode. Results for broadband PDs based on a p-Si/ZnO heterojunction are presented. The ZnO layers were deposited by radio frequency (RF) sputtering at room temperature from a ZnO target in Ar at three RF powers: 40, 45, and 50 W. XRD results showed that layers deposited at 45 and 50 W had crystallite sizes of about 7.9 and 10.06 nm, respectively. X-ray photoelectron spectroscopy indicated that Zn was mainly in the Zn2+ oxidation state. The O 1s spectra revealed the presence of hydroxyl groups, adsorbed water, and surface oxygen in the ZnO surface region, with concentrations that depended on RF power. Optical characterization showed that the optical bandgaps were between 3.20 and 3.26 eV. Electrical characterization of p-Si/ZnO PDs showed that the diodes with ZnO layers deposited at the three powers became charged after UV illumination. Low-temperature annealing at 90 °C for 10 min strongly reduced the deep traps in 45 W PDs. Annealing at a higher temperature of 120 °C for 10 min completely removed the deep traps in both 45 and 50 W PDs, while no effect was observed in the 40 W PDs. The annealed devices showed high responsivity and specific detectivity, which improved with increasing thermal process temperature. The preparation of high-quality ZnO layers by RF sputtering at room temperature and annealing at relatively low temperatures opens a pathway for fabricating electronic devices on flexible substrates.