High work function (p-type NiO_1+<i>x</i>)/Zn_0.95Ga_0.05O heterostructures for transparent conducting oxides

Abstract

We report the growth and properties of heterostructure thin films consisting of a thin overlayer of p-NiO1+x on Zn0.95Ga0.05O (GZO) by pulsed laser deposition for transparent electrode applications. The GZO films with a thin p-type NiO1+x overlayer exhibited a higher work function. It is envisaged to facilitate hole injection across the heterojunction in a solid state device resulting in improved device efficiency. The crystalline quality of the bilayer films was investigated by x-ray diffraction. NiO1+x overlayers showed the preferred orientation along the [1 1 1] direction on Zn0.95Ga0.05O (0 0 0 1) films deposited on a glass substrate while they were epitaxial when the substrate used was sapphire. The effects of the NiO1+x overlayer thickness variation and Li doping on the electrical and optical properties of NiO1+x/Zn0.95Ga0.05O bilayer films were also investigated. The bilayer films with an optimized overlayer thickness showed good optical transparency (⩾85%) and low resistivity of ∼10−4 Ω cm up to temperatures as low as 100 K. Using x-ray photoelectron spectroscopy it has been established that nickel in NiO1+x exists in multiple oxidation states of Ni2+ and Ni3+. The presence of Ni3+ gives rise to p-type conductivity in non-stoichiometric NiO1+x. Additionally, ultraviolet photoelectron spectroscopy studies showed that the bilayer films have high work function values ranging from 5.2 to 5.3 eV. A correlation between the surface work function and Ni3+/Ni2+ ratio has also been established.