Photoluminescence of Nominally Undoped Heavy n-Type ZnO Nanowires

We report the identification of a donor band and the correlation between n-type conductivity and the green emission in ZnO nanowires. Temperature-dependent photoluminescence is used to investigate nominally undoped ZnO nanowires with high n-type conductivity. Within the whole temperature range, a dominant free-to-bound transition with a donor band of about 150 meV below the conduction band minimum is observed. The nanowires show very strong green emission, which is quenched with activation energy of about 220 meV. The correlation between the high n-type conductivity and the strong green emission is discussed in detail, and we suggest that they may have different origins.