Single- and dual-wavelength photodetectors with MgZnO/ZnO metal–semiconductor–metal structure by varying the bias voltage

Abstract

By varying the bias voltage of an MgxZn1−xO/ZnO metal–semiconductor–metal photodetector (MSM-PDs), the detection wavelength can be modulated from a single to a dual wavelength. A long-wavelength band response is caused by the ZnO absorption and a short-wavelength band response is caused by MgxZn1−xO. At a 0 V bias voltage, the photogenerated electrons in ZnO are confined to the MgxZn1−xO/ZnO interface, arising from the piezoelectric polarization. The accumulated electrons hop the MgxZn1−xO layer through the assistance of defects; however, the photogenerated electrons in MgxZn1−xO cannot cross over the large barrier height at the Au/MgZnO interface, resulting in a single-wavelength photodetector with a long-wavelength band (345–400 nm) having a peak wavelength of 370 nm. By increasing the bias voltage to 1–2 V, the barrier height is lowered, enabling the photogenerated electrons in MgxZn1−xO to easily cross over the low barrier height, leading to dual-wavelength photodetectors having peak wavelengths of 370 and 340 nm. On further increasing the bias voltage beyond 2 V, the photogenerated electrons in ZnO sink deeply in the hollow at the MgxZn1−xO/ZnO interface owing to the large applied voltage. These electrons are effectively confined at the MgxZn1−xO/ZnO interface, which retards the tunneling of the photogenerated electrons in ZnO through the MgxZn1−xO layer; hence the MSM-PDs revert back to single wavelength photodetectors; however, the detection wavelength is different from that of the MSM-PDs biased at 0 V. Instead of having a long-wavelength band (345–400 nm), the MSM-PDs demonstrate a short-wavelength band (320–345 nm) at a 3 V bias voltage.