Hassan Ali, Khan Abbas Ahmad, Ahn Yeong Hwan, Azam Muhammad, Zubair Muhammad, Xue Wei, Cao Yu
China International Science & Technology Cooperation Base for Laser Processing Robotics, Wenzhou University, Wenzhou 325035, China.
Zhejiang Provincial Key Laboratory of Laser Processing Robotics, College of Mechanical and Electrical Engineering, Wenzhou University, Wenzhou 325035, China.
Nanomaterials (Basel). 2022 Jun 26;12(13):2192. doi: 10.3390/nano12132192.
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 (ILα). 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.
利用温度、激发波长和激发功率相关的光致发光（PL）光谱来研究ZnO单晶（SCs）的取向调制近带边发射（NBE）和深能级发射（DLE）。具有<0001>取向的ZnO单晶的近带边发射表现出强烈且尖锐的发射强度，同时深能级缺陷（主要由氧空位Vo引起）受到抑制。此外，拉曼分析表明<0001>取向具有占主导的E2（高）和E2（低）模式，这表明该方向具有更好的结晶度。根据温度相关的PL光谱，在低温下，无论取向如何，中性施主到束缚激子（DoX）跃迁占主导。此外，在所有取向中，自由激子（FX）跃迁在较高温度下出现。PL强度对激发功率的依赖性已根据幂律（ILα）进行了描述。我们的结果表明，<0001>、<1120>和<1010>的α分别为（1.148）、（1.180）和（1.184）。简而言之，综合的PL分析表明，DoX跃迁在NBE区域占主导，而氧空位（Vo）是ZnO中的主要深能级。此外，即使在高温（543 K）下，<0001>取向在近带边区域比其他取向包含更少与Vo相关的缺陷且具有强烈的激子发射。这些结果表明<0001>生长方向有利于制造基于ZnO的高效光电器件。
