Peculiar near-band-edge emission of polarization-dependent XEOL from a non-polar a-plane ZnO wafer

• Published in: Optics express Vol. 26; no. 3; p. 2731
• Main Authors: Lin, Bi-Hsuan, Wu, Yung-Chi, Chen, Huang-Yeh, Tseng, Shao-Chin, Wu, Jian-Xing, Li, Xiao-Yun, Chen, Bo-Yi, Lee, Chien-Yu, Yin, Gung-Chian, Chang, Shih-Hung, Tang, Mau-Tsu, Hsieh, Wen-Feng
• Format: Journal Article
• Language: English
• Published: United States 05.02.2018
• ISSN: 1094-4087; 1094-4087
• DOI: 10.1364/OE.26.002731

Polarization-dependent hard X-ray excited optical luminescence (XEOL) was used to study not only the optical properties but also the crystallographic orientations of a non-polar a-plane ZnO wafer. In addition to a positive-edge jump and extra oscillations in the near-band-edge (NBE) XEOL yield, we observed a blue shift of the NBE emission peak that follows the polarization-dependent X-ray absorption near-edge structure (XANES) as the X-ray energy is tuned across the Zn K-edge. This NBE blue shift is caused by the larger X-ray absorption, generating higher free carriers to reduce the exciton-LO phonon coupling, which causes a decrease in the exciton activation energy. The extra oscillations in XANES and XEOL as the polarization is set parallel to the c-axis is attributed to simultaneous excitations of the Zn 4p - O 2pπ -bond along the c-axis and the bilayer σ-bond, whereas only the σ-bond is excited when the polarization is perpendicular to the c-axis. The polarization-dependent XEOL spectra can be used to determine the crystallographic orientations.