Nonpolar nitrides have attracted considerable attention due to the potential for eliminating undesirable electric field effects and the possibility of producing devices with higher efficiencies than conventional c-plane nitrides [1]. The nonpolar nitride films with a-plane and m-plane have been grown on r- and m- plane sapphire, (302) and (100) LiAlO2, a- and m-plane SiC, and a- and m-plane GaN substrates [2]. The structural anisotropies of the nonpolar nitride films were expected due to the unequal growth rate [3] so there is a dependence of the full widths at half maximum (FWHMs) of X-ray rocking curves (XRCs) on the azimuth angle. In this work, we report on the anisotropic characteristics of a-plane GaN films that have an M-shape or W-shape dependence on the azimuth angle with minimum FWHM along the c-axis or m-axis directions of a-plane GaN. 
  Nonpolar a-plane GaN films were grown on 2 inch r-plane sapphire substrates by metalorganic chemical vapor deposition. With the control of nucleation and three-dimensional growth conditions, we obtained M-shape (sample A) and W-shape (sample B) a-plane GaN films depending on the azimuth angle. The surface morphology and the crystalline quality of samples A and B were characterized by high resolution X-ray diffraction and atomic force microscopy (AFM). For optical properties, photoluminescence (PL) measurements at room temperature were carried out using a 266 nm laser.
  1 x 1 μm² AFM images of the surface morphologies of samples A and B are shown in Fig. 1(a) and (b). Both samples had a smooth surface and the root mean square (RMS) of samples A and B was found to be 0.5 and 0.8 nm, respectively. The structure qualities of samples A and B were evaluated from the FWHM of XRC. Figure 2 shows the FWHMs of XRC of a-plane GaN films along with the azimuth angle. Azimuth angles of 0^o and 90^o were defined as the projection of the incident beam being parallel to the [0001] and [1-100] directions, respectively. The FWHMs of the (11-20) XRC for samples A and B were found to be dependent on the azimuth angle. Sample A had M-shape behavior and the FWHM values of the [0001] and [1-100] directions were 474 and 638 arcsec, respectively. The FWHMs of the [0001] and [1-100] directions for sample B with W-shape anisotropy were found to be 856 and 542 arcsec, respectively. The crystal quality of sample A along the [0001] direction was better than sample B and the reverse was found for crystal quality along the [1-100] direction. Room temperature PL spectra of samples A and B are shown in Fig. 3. Both spectra displayed the near band-edge emission at 363.4 nm and the FWHMs of the PL for samples A and B were 12.9 and 12.7 nm, respectively. The PL intensity of sample B was 1.7 times stronger than that of sample A although the average crystal quality of sample A was better than that of sample B. This indicates that the PL intensity of a-plane GaN layer is more affected by the crystal quality of the [1-100] direction rather than the [0001] direction. 

References: 
 [1] P. Waltereit, O. Brandt, A. Trampert, H. T. Grahn, J. Meinnger, M. Ramsteiner, M. Reiche, and K. H. Ploog, Nature (London) 406, 865 (2000).
[2] V. Darakchieva, M.-Y. Xie, N. Franco, F. Giuliani, B. Nunes, E. Alves, C. L. Hsiao, L. C. Chen, T. Yamaguchi, Y. Takagi, K. Kawashima, and Y. Nanishi, J. Appl. Phys. 108, 073529 (2010).
[3] H. Wang, C. Chen, Z. Gong, J. Zhang, M. Gaevski, M. Su, J. Yang, and M. A. Khan, Appl. Phys. Lett. 84, 499 (2004).  

 Figure 1. AFM images of a-plane GaN films: (a) sample A and (b) sample B. 

 Figure 2. Azimuth angle dependence of the FWHM of the (11-20) rocking curves for a-plane GaN films. 

Figure 3. PL intensities for samples A and B at room temperature.

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/28825 must be provided.