Microstructural, electrical and carrier transport properties of Au/NiO/n-GaN heterojunction with a nickel oxide interlayer
Nickel oxide (NiO) films are prepared on n-type GaN by an e-beam evaporation technique and its structural and chemical characteristics analysed by XRD, TEM and XPS measurements first at room temperature. XRD and TEM results reveal that the NiO films are oriented and that the NiO/n-GaN interface has a good quality. XPS analysis demonstrated that the NiO films clearly showed Ni 2p3/2 and 2p1/2 peaks at 854 eV and 872 eV along with the O 1s peak at ∼529.1 eV. Then, we fabricated an Au/NiO/n-GaN heterojunction Schottky diode with a NiO insulating layer and compared its electrical properties with the Au/n-GaN Schottky junction. The Au/NiO/n-GaN heterojunction presents excellent rectifying behaviour with a low reverse-leakage current compared to the Au/n-GaN Schottky junction. Calculation revealed that a higher barrier height is achieved for the Au/NiO/n-GaN heterojunction than for the Au/n-GaN Schottky junction, implying the barrier height was modified by the NiO insulating layer. Using Cheung's and Norde functions and an ΨS–V plot, the barrier heights are estimated and we found that the values are comparable with one another. The results suggest that the interface state density (NSS) of the Au/NiO/n-GaN heterojunction decreases compared to the Au/n-GaN Schottky junction, which indicates the NiO insulating layer plays a significant role in the reduced NSS. The results demonstrate that Poole–Frenkel emission governs the reverse leakage current in both junctions, which could be associated with structural defects and trap levels in the insulating layer.