Band alignment of wide bandgap NiO/MoO3 and NiO/WO3p-n heterojunctions studied by high-resolution X-ray photoelectron spectroscopy

Abstract

NiO is one of the few transition metal oxides exhibiting p-type conductivity and visible light transparency. On the other hand, MoO3 and WO3 are high work function, wide gap metal oxides which show predominantly n-type character and have been widely applied in carrier selective contacts, heterojunction diodes, electrochromic and sensing applications. Hence, detailed understandings of the optical properties and electronic band offsets of the NiO/MoO3 and NiO/WO3p-n heterojunctions are crucial for the design and application of transparent oxide-based optoelectronic devices utilizing these heterojunctions. In this work, we synthesized thin films of the three oxides and characterized their structural and optical properties. In particular, the band alignment of NiO/MoO3 and NiO/WO3p-n heterojunctions were studied by high-resolution soft x-ray photoelectron spectroscopy (XPS). We found that both heterojunctions have a type-II band offset with the NiO/MoO3 exhibiting a larger valence band offset of 2.02 eV compared to 1.71 eV for NiO/WO3. The band positions of the three oxides with respect to the vacuum level obtained by Ultraviolet-Ambient Photoemission spectroscopy suggest that MoO3 and WO3 have a propensity to be n-type due to the easy formation of native donor defects. In contrast, NiO has a p-type character due to the high position of its VBM which favors the formation of native acceptor defects.