By varying the bias voltage of an Mg x Zn1-x O/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 Mg x Zn1-x O. At a 0 V bias voltage, the photogenerated electrons in ZnO are confined to the Mg x Zn1-x O/ZnO interface, arising from the piezoelectric polarization. The accumulated electrons hop the Mg x Zn1-x O layer through the assistance of defects; however, the photogenerated electrons in Mg x Zn1-x O 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 Mg x Zn1-x O 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 Mg x Zn1-x O/ZnO interface owing to the large applied voltage. These electrons are effectively confined at the Mg x Zn1-x O/ZnO interface, which retards the tunneling of the photogenerated electrons in ZnO through the Mg x Zn1-x O 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.
MgZnO/ZnO two-dimensional electron gas (2DEG) structures with ZnO annealed at various temperatures (600-900 °C) and photodetectors (PDs) with and without a 2DEG structure were fabricated using a radio frequency magnetron sputtering system. It was found that the carrier concentration and mobility increase with the annealing temperature owing to the improved crystalline in ZnO; however, high-temperature (800 °C or higher) annealing can degrade the crystalline of the ZnO layer. Hall measurements showed that compared with that of bulk ZnO, the sheet carrier concentration of the 2DEG sample increased from 1.3 × 10 to 1.2 × 10 cm, and the mobility was enhanced from 5.1 to 17.5 cm/V s. This is because the channel layer is the total thickness (300 nm) in bulk ZnO, whereas the carriers are confined to a 45 nm region beneath the MgZO layer in the 2DEG sample, confirming the 2DEG behavior at the MgZnO/ZnO interface. The PDs with 2DEG structures demonstrate a higher ultraviolet (UV) response and a UV/visible rejection ratio that is six times larger than that of the PDs without a 2DEG structure. The 2DEG structure also induces a photocurrent gain, which results in a 240% quantum efficiency for the 310 nm incident wavelength. The related mechanism is elucidated with a band diagram.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.