The oxidation of $\mathrm{Cu}\mathrm{Ga}{\mathrm{Se}}_2$ crystals and polycrystalline thin films was investigated with electron paramagnetic resonance (EPR) and photoelectron spectroscopy (PES). An EPR signal assigned to ${\mathrm{Cu}}^{2+}$ evolves when the samples were stored under ambient conditions for a few months, independent of the morphology of the specimens. The formation of a native oxide layer consisting of ${\mathrm{Ga}}_2{\mathrm{O}}_3$ and $\mathrm{Se}{\mathrm{O}}_2$ was observed in PES during the initial stage of oxidation. Only after long term oxidation for several months an additional copper hydroxide phase, $\mathrm{Cu}{\left(\mathrm{O}\mathrm{H}\right)}_2$, was detected. When the $\mathrm{Cu}{\left(\mathrm{O}\mathrm{H}\right)}_2$ phase was reduced by annealing in vacuum at $200°\mathrm{C}$ or removed by etching in KCN also the ${\mathrm{Cu}}^{2+}$ EPR signal disappeared. It is shown that the ${\mathrm{Cu}}^{2+}$ EPR signal originates from the $\mathrm{Cu}{\left(\mathrm{O}\mathrm{H}\right)}_2$ phase and the activation energy for the thermal reduction of ${\mathrm{Cu}}^{2+}$ to nonparamagnetic ${\mathrm{Cu}}^{+}$ is $E_a=0.29\mathrm{eV}$. A model for the native oxidation of $\mathrm{Cu}\mathrm{Ga}{\mathrm{Se}}_2$ will be presented and implications for solar cell device performance discussed.

• Received 1 April 2004
• Corrected 23 November 2004

DOI:https://doi.org/10.1103/PhysRevB.70.205321