MgZnO/ZnO Two-Dimensional Electron Gas Photodetectors Fabricated by RF-Sputtering.

MgZnO/ZnO two-dimensional electron gas (2DEG) structures with ZnO annealed at various temperatures (600-900 ºC) 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 ºC or higher) annealing can degrade the crystalline of the ZnO layer. Hall measurements showed that compared with bulk ZnO, the sheet carrier concentration of the 2DEG sample increased from 1.3×1013 to 1.2 × 1014 cm-2, and the mobility was enhanced from 5.1 to 17.5 cm2/V-s. This is because the channel layer is the total thickness (300 nm) in bulk ZnO, whereas the carriers are confined to about 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 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.