Stable p-type ZnO thin films on sapphire and n-type 4H-SiC achieved by controlling oxygen pressure using radical-source laser molecular beam epitaxy

Stable p-type ZnO thin films on sapphire (0001) and n-type 4H-SiC (0001) substrates were achieved in low-ionized oxygen pressure using radical-source laser molecular beam epitaxy system. The p conduction type of ZnO films originates from not only zinc vacancy, but also interstitial oxygen acceptor defects, which was investigated by X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, and photoluminescence measurements. The electrical properties were tested 180 days after deposition by Hall measurement. For the ZnO thin film grown on sapphire, a stable mobility of 17.0 cm² V^-1s^-1, a resistivity of 1.08 Ωcm, and a hole concentration of 3.4 × 10¹⁷ cm^-3 were achieved. The selection of 4H-SiC substrates improved the crystalline quality of ZnO films confirmed by the X-ray diffraction patterns. An intrinsic p-type ZnO film on n-type 4H-SiC, with a stable mobility of 44.6 cm² V^-1s^-1, a resistivity of 1.02 Ωcm, and a hole concentration of 1.4 × 10¹⁷ cm^-3 were achieved. The current-voltage curve of the p-n ZnO homojunction shows typical diode characteristics, which also confirmed the achievement of the p-type ZnO. The current-voltage curve of the p-ZnO/n-4H-SiC heterojunction also shows p-n junction rectifier features. These results suggest the possibility of ultraviolet photodetectors and light-emitting devices.