Negative‐U Properties of the Deep Level E3 in ZnO.

The deep level E3 is a commonly observed defect in bulk crystals and thin films of ZnO independently of the method of growth. Its chemical origin is still not finally understood. In this paper, previous research on the E3 level is reviewed and recent experimental results acquired by optical deep level transient spectroscopic (DLTS) methods are presented for the temperature range from 4 to 350 K, for photon energies between 0.25 and 4 eV, and for emission rates ranging from 10−3 to 104 s−1. The capture cross section of the optical transition of E3‐trapped electrons into the conduction band is described. Additionally, a second optical transition occurs for photon energies higher than 1 eV, which accounts for the lowering of the E3 DLTS signal at low temperatures under illumination. Further, it is unambiguously shown that the E3 defect can bind up to two electrons and that it has the properties of a negative‐U center. Photoluminescence measurements reveal that the E3 defect is directly connected to a radiative recombination channel at 2.09 eV. Further, electron paramagnetic resonance (EPR) measurements were performed with optical excitation in dependence on the temperature and photon energy. Interestingly, the temperature‐dependent EPR signal of V O + is similar to that of the photo‐ionization cross‐section of the E3 defect. These experimental findings are discussed in comparison with published EPR results and results obtained by first‐principle calculations of native point defects in ZnO. [ABSTRACT FROM AUTHOR]