Microstructural study of ZnO grown on semipolar GaN templates

• Main Authors: Shih, Yi-Sen, 施議森
• Other Authors: Chang, Li
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/asjnj6

博士 === 國立交通大學 === 材料科學與工程學系所 === 103 === ZnO/GaN heterojunction has been suggested as a strong candidate for many device applications due to their wurtzite structure with small lattice mismatch and thermal mismatch. Nonpolar and semipolar ZnO/GaN can avoid and reduce the polarization to raise the light emission efficiency. This thesis work focus on understanding of growth of semipolar ZnO films on GaN from microstructural characterization. ZnO and Al-doped ZnO (AZO) were epitaxially grown on faceted a-oriented GaN template on r-plane sapphire. The faceted GaN was grown by metal-organic chemical vapor deposition (MOCVD), while ZnO and AZO were grown by chemical vapor deposition. The MOCVD-grown GaN template consisted of facets of a-plane and semipolar planes ({10-11} and {11-22}) on which simultaneous growth of nonpolar- and semipolar-oriented ZnO and AZO were explored. The surface morphology and the relation between microstructure and growth were identified by using scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS), focused ion beam (FIB), high resolution X-ray diffraction (HRXRD), and cross-sectional transmission electron microscopy (XTEM). On GaN {10-11} facets, the surface exhibits an L-shape or corner-like morphology which consists of connected stripes along [11-2-3]GaN and [1-210]GaN , whereas on GaN {11-22} facet surface, striation or stripe-like morphology can be seen along [11-2-3]GaN . The SEM observations show that the ZnO grown on semipolar GaN {10-11} facets exhibit the island growth mode with striation morphology along [1-210]GaN, and similar morphology on GaN{11-22} facets. The symmetric XRD pattern shows that ZnO grown on the faceted GaN exhibit (11-20) reflections only, the in-plane orientation relationship of [0001]ZnO//[0001]GaN and [-1100]ZnO//[-1100]GaN . On semipolar GaN (11-22), the epitaxial ZnO can form in two different semipolar orientations as evidenced by TEM and XRD. One orientation relationship is shown to be ZnO(11-22)// GaN(11-22) and [-1100]ZnO//[-1100]GaN, the other is ZnO(-101-1)// GaN (0002) and [5-72-3]ZnO// [-1100]GaN. On semipolar GaN(10-11) facet, it is also found that there exist two kinds of ZnO microstructure: one is ZnO (10-11) // GaN (10-11) and [1-210]ZnO//[1-210]GaN, and the other is ZnO (0002) // ZnO (10-11) and [-2110]ZnO//[1-10-1]ZnO. The morphologies of AZO films on nonpolar a-plane and semipolar GaN{11-22} facets exhibit similar to those of ZnO. However, it is smooth on GaN{10-11} different from ZnO striation morphology. The XRD analyses of AZO show the same result as the ZnO. Further XTEM observations, however, reveal that the AZO grown on semipolar GaN(10-11) facet is in the orientation relationship of AZO (0002) // GaN (10-11) and [-2110]ZnO//[1-101]GaN.