Crystal Orientation Dependent Conductivity Improvement of Pure NiO Epitaxial Thin-Film Surface by Irradiation of Excimer Vacuum-ultraviolet Light

Nickel oxide (NiO) with rock-salt type structure is a well-known p-type wide-bandgap semiconductor, which have attracted interests for device application in optoelectronics such as photodetectors, electrochromic layers, photovoltaics and so on ^[1,2]. It is important to modify conductivity, morphology, and structure of surface and interface of epitaxial thin films for utilization in such functional devices. Aliovalent ions such as Li⁺ have been doped to increase carrier density and enhance p-type conduction of NiO thin films. Meanwhile, modification of p-type conduction in NiO without impurity doping would advance development of multilayered optoelectronic devices with controlled interfacial properties. There have been a few reports about altered electronic states and properties of polycrystalline NiO films after UV-ozone treatment using mercury lamps ^[3,4]. Further research on properties and structures of epitaxial NiO thin films irradiated with UV light depending on planar ionic arrangement would help us understand the photo-induced phenomena. In addition, application of vacuum-ultraviolet (VUV) light with larger photon energy and efficient production of active oxygen species would also contribute to modify the properties. In this study, noticeable conductivity improvement of epitaxial NiO thin film surface was obtained by excimer VUV-light irradiation, and the effect of epitaxial crystal orientation on the modified properties and morphology was investigated by changing the substrate planes.<br/>The NiO epitaxial thin films with various orientations such as (111), (110) and (100) were grown on mirror polished SrTiO₃ single crystal substrates by pulsed laser deposition (PLD) technique using a KrF excimer laser (<i>λ</i>=248 nm, <i>E</i>~1.2 J/cm²) and a pure NiO sintered target. The thin film growth took place at room-temperature (not-heated intentionally) in an oxygen pressure of 1×10^–3 Pa. The NiO thin films were subsequently introduced to VUV-light treatment using a Xe₂ excimer lamp (<i>λ</i>=172 nm, <i>E</i>~65 mW/cm² at lamp surface). The VUV-light was irradiated to thin film surfaces in air at the distance between lamp and sample surface of 0.5 mm. The resistivity of the PLD-grown 40 nm-thick epitaxial NiO thin films were ~1×10⁴ Ωcm or higher, although VUV-light irradiation drastically reduced the value of NiO (111) and (110) films to ~6×10^–1 Ωcm and ~2×10^–1 Ωcm, respectively after the treatment for 120 minutes. The resistivity of NiO (100) film was remarkably reduced films by four-digits or more, that ~8×10^–2 Ωcm was obtained after the irradiation. The epitaxy of rock-salt type NiO remained even after VUV-light irradiation, while slight decrease of plane spacings suggested redox reaction near the surface. Thickness dependent structural and morphological change according to the VUV-light irradiation would also be discussed in detail.<br/>[1] M. R. Hasan et al., <i>APL. Mater.</i> <b>3</b> (2015) 106101.<br/>[2] S. G. Danjumma et al., <i>Int. J. Eng. Res. Technol. </i><b>8</b> (2019) 461–461.<br/>[3] G. H. Aydogdu et al., <i>J. Appl. Phys.</i> <b>108</b> (2010) 113702.<br/>[4] M. T. Greiner et al., <i>J. Phys. Chem. C</i> <b>114</b> (2010) 19777–19781.