Orientation-Mediated Luminescence Enhancement and Spin-Orbit Coupling in ZnO Single Crystals

Temperature-, excitation wavelength-, and excitation power-dependent photoluminescence (PL) spectroscopy have been utilized to investigate the orientation-modulated near band edge emission (NBE) and deep level emission (DLE) of ZnO single crystals (SCs). The near-band-edge emission of ZnO SC with <0001> orientation exhibits strong and sharp emission intensity with suppressed deep level defects (mostly caused by oxygen vacancies Vo). Furthermore, Raman analysis reveals that <0001> orientation has dominant E2 (high) and E2 (low) modes, indicating that this direction has better crystallinity. At low temperature, the neutral donor-to-bound exciton (DoX) transition dominates, regardless of the orientation, according to the temperature-dependent PL spectra. Moreover, free-exciton (FX) transition emerges at higher temperatures in all orientations. The PL intensity dependence on the excitation power has been described in terms of power-law (I~Lα). Our results demonstrate that the α for <0001>, <1120>, and <1010> is (1.148), (1.180), and (1.184) respectively. In short, the comprehensive PL analysis suggests that DoX transitions are dominant in the NBE region, whereas oxygen vacancies (Vo) are the dominant deep levels in ZnO. In addition, the <0001> orientation contains fewer Vo-related defects with intense excitonic emission in the near band edge region than other counterparts, even at high temperature (~543 K). These results indicate that <0001> growth direction is favorable for fabricating ZnO-based highly efficient optoelectronic devices.