Reassignment of O-related infrared absorption peaks in CdSe

Abstract

Abstract Upon their observation in 2008, the IR absorption peaks at 1094.11, 1107.45, and 1126.33 cm−1 of O-doped wurtzite-CdSe sample have been proposed to associate with the local vibrational modes (LVM) of oxygen – vacancy Cd complex defect (OSe-VCd) (Chen et al., 2008) [8] . Later, first principles calculations by T-Thienprasert et al. reveal that the LVMs of the OSe-VCd complex defect are actually at 223, 507 and 517 cm−1 not in the 1000 cm−1 range (T-Thienprasert et al., 2012) [14] ; suggesting that the assignment has to be revised. Bastin et al. identified the source of the modes to be a sulfur-oxygen defect complex (Bastin et al., 2014) [11] by intentionally varied the amount of S and analyzed the isotope abundances of S and O from the measurements. In this work, sulfur-oxygen defect complexes in wurtzite-CdSe were studied by first principles calculations. It is found that SO2 energetically prefers to substitute on the Se site accompanied by a Cd vacancy (VCd); forming a SO2-VCd complex defect. According to the crystal symmetry of CdSe, there are two different VCd surrounding the Se site that SO2 occupied. In addition, there are various different orientations of SO2 relative to the VCd. The lowest energy configuration is identified. Its local vibrational modes along with the modes of other possible orientations can explain the observed values both in terms of the frequencies and the dipole orientations. Further, our nudged elastic band calculations showed that the rotation barrier of the SO2 in the complex is quite low; indicating the molecule can rotate at reasonable temperature. This is consistent with the experimental observation that the peaks are broadened and merged into one at high temperatures.