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

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.