Effects of the Growth Environment on the Phase Stability of Sputter-deposited CdxZn1-xO Alloys

ZnO and CdO, having wurtzite (WZ) and rocksalt (RS) crystal structures, respectively, form alloys that have a structural phase transition window strongly dependent on deposition methods and conditions. WZ phase
$\text{Cd}_{\mathrm{x}}\text{Zn}_{1-\mathrm{x}}\mathrm{O}$
alloys exhibit a direct bandgap that is largely tunable from 3.3 eV to 1.8 eV. However, at high Cd concentration phase separation tends to occur and achieving single-phase
$\text{Cd}_{\mathrm{x}}\text{Zn}_{1-\mathrm{x}}\mathrm{O}$
alloys with
$\mathrm{x} > 0.4$
is challenging. In this work, we present the materials properties of
$\text{Cd}_{\mathrm{x}}\text{Zn}_{1-\mathrm{x}}\mathrm{O}$
alloys synthesized using radio frequency magnetron sputtering and investigated the effects of deposition conditions on the phase transition behaviors. At a nominal growth temperature of 230°C a mixture of both WZ and RS phases were found in alloys for a wide composition window of
$0.4 < \mathrm{x} < 0.7$
. With the addition of oxygen gases during sputter deposition, the WZ-to-RS phase transition composition was extended to
$\mathrm{x} > 0.6$
and becomes much narrower. Density functional theory calculations show a lower formation energy for the wurtzite phase for alloys up to x∼0.75 with the presence of excess O in the form of O interstitials. These results present a convenient way to extend the composition of wurtzite-
$\text{Cd}_{\mathrm{x}}\text{Zn}_{1-\mathrm{x}}\mathrm{O}$
alloys to bandgaps within the visible range for opto-electronic applications including photovoltaics.