Enhancement-mode ZnGa2O4-based Phototransistor with high UV-visible rejection ratio grown by metalorganic chemical vapor deposition

In this study, a high-crystallinity zinc gallium oxide (ZnGa2O4) epilayer of about 80 nm thickness was successfully grown using metalorganic chemical vapor deposition. X-ray diffraction, atomic force microscopy, UV-visible spectroscopy measurements, and X-ray photoelectron spectroscopy were used to investigate the quality of the epilayer. Transmission line measurement analysis revealed the electrical properties of the ZnGa2O4 epilayer: a sheet resistance of 276 M Omega/square, a transfer length of 0.28 mu m, and a specific contact resistivity of 21.63 Omega/mm2. A phototransistor made of ZnGa2O4 was designed with a channel width of 250 mu m and a channel length of 20 mu m. The source-drain electrodes were fabricated using a Ti/Al/Ni stacking technique. The gate dielectric comprised 30 nm thick Al2O3, and the gate contact was made of Ni. The device possessed a higher threshold voltage value of about 4.8 V and an impressive On/Off current ratio of 106. Under 240 nm UV light exposure, the phototransistor exhibited a transition from enhancement mode to depletion mode, leading to a remarkable shift in the threshold voltage from 5 V to approximately -16 V. Moreover, the responsivity and rejection ratio at 240 nm was 128.5 A/W and 105 at a gate voltage of -10 V. The study shows that an increasing thickness of the ZnGa2O4 epilayer from 80 nm to 135 nm results in a depletion mode transistor. The higher concentration of about twenty percent oxygen vacancies resulted in a longer fall time for the phototransistor. These results underscore the significant potential of ZnGa2O4 in facilitating deep UV-based detection applications.