Transparent $\mathbf{Ni}_{\mathbf{x}}\mathbf{Cd}_{1-\mathbf{x}}\mathbf{O}_{1+\boldsymbol{\delta}}$ alloy thin films with bipolar conductivity

Abstract

NiO-CdO alloy films over the entire composition range were synthesized by RT magnetron sputtering in both pure Ar (Ni x Cd 1-x O) and $\text{Ar} +\mathrm{O}2(\text{Ni}_{\mathrm{x}}\text{Cd}_{1-\mathrm{x}}\mathrm{O}_{1+\delta})$ environments. These alloy films are nanocrystalline with rocksalt structure. We demonstrated that in the alloy composition range of $0.38\leq \mathrm{x}\leq 0.52, \text{Ni}_{\mathrm{x}}\text{Cd}_{1-\mathrm{x}}\mathrm{O}_{1+\delta}$ films with bipolar (i.e., ${n-}$ or $p$ -type) conductivity can be achieved by controlling the oxygen stoichiometry. Within this composition range the band gap of the alloy is ∼3-3.3 eV with transmittance >50% in the visible range. The unusual electrical and optical properties of $\text{Ni}_{\mathrm{x}}\text{Cd}_{1-\mathrm{x}}\mathrm{O}_{1+\delta}$ alloy thin films can be explained by the modifications of the electronic band structure due to anticrossing interactions between localized Ni d levels and extended valence and conduction band states of the alloy. In addition, we studied the transport mechanism of $p$ -type $\text{Ni}_{\mathrm{x}}\text{Cd}_{1-\mathrm{x}}\mathrm{O}_{1+\delta}$ alloy films and found that the hole mobility increases with temperature in the range of 400-450 K. This is consistent with hole transport via a small polaron hopping process with an activation energy $Ea$ of 0.174 eV ( $\mathrm{x}=1$ ) to 0.331 eV ( $\mathrm{x}=0.47$ ). These results strongly suggest that the nanocrystalline $\text{Ni}_{\mathrm{x}}\text{Cd}_{1-\mathrm{x}}\mathrm{O}_{1+\delta}$ alloy system has great technological potential for applications in transparent optoelectronic device.