We demonstrate a novel type of ZnO self-powered photodetector based on the asymmetric metalsemiconductor-metal (MSM) structure: one Au interdigitated electrode with wide fingers and the other one with narrow fingers. These ZnO photodetectors exhibit attractive photovoltaic characteristics at 0 V bias. More interestingly, with increasing the asymmetric ratio (the width of wide fingers : the width of narrow fingers) of the interdigitated electrodes, the responsivity of the ZnO self-powered UV photodetectors was enhanced obviously, reaching as high as 20 mA W À1 when the asymmetric ratio was 20 : 1. A physical model based on band energy theory was developed to illustrate the origin of the photoresponse at 0 V in our device. Our findings provide a new route to realizing self-powered photodetectors.
Mixed-phase ZnMgO (m-ZMO) thin films with a single absorption edge tuning from ∼3.9 to ∼4.8 eV were realized on a-face sapphire (a-Al2O3) by plasma-assisted molecular beam epitaxy. The small lattice mismatch of both ZnO and MgO with a-Al2O3 should be responsible for the single and controllable absorption edge. Metal-semiconductor-metal (MSM) photodetectors were fabricated based on these m-ZMO films, and the devices have the single cutoff wavelength, which can be tuned from 335 to 275 nm. These devices possess low dark current (78 pA for m-Z0.67M0.33O, 11 pA for m-Z0.59M0.41O, and 4 pA for m-Z0.39M0.61O at 40 V) and high responsivity (434 A/W for m-Z0.67M0.33O, 89.8 A/W for m-Z0.59M0.41O, and 3.7 A/W for m-Z0.39M0.61O at 40 V). Further response study reveals that the 90-10% decay time of m-Z0.67M0.33O, m-Z0.59M0.41O, and m-Z0.39M0.61O is around 37, 30, and 0.7 ms, respectively. Large amounts of heterojunction interfaces between wurtzite ZMO and cubic rock-salt ZMO could be responsible for the low dark current and high responsivity of our mixed-phase devices. The excellent comprehensive performance of m-ZMO UV photodetectors on a-Al2O3 suggests that m-ZMO UV photodetectors should have great applied potential.
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