Thermally activated below-band-gap excitation behind green photoluminescence in ZnO

Abstract

We explore the temperature dependence of the photoluminescence (PL) and photoluminescence excitation characteristics of the green emission band in undoped ZnO crystals. We find that there exists a thermally assisted luminescence channel, which tends to dominate over the existing luminescence channel especially under below-band-gap excitation. Shallow donor electrons are likely to contribute to the thermally assisted luminescence process by being thermally excited from the donor levels to the conduction band. We hence suggest that the shallow donor centers, which are presumably attributed to zinc interstitials and will not act as emitting centers by themselves, are indirectly responsible for the green PL emission. The most probable candidate of the emitting center is the neutral oxygen vacancy, showing the green emission either via a direct internal emission process or an indirect thermally assisted emission process in combination with shallow donor centers.