| Summary: | 博士 === 國立交通大學 === 材料科學與工程學系 === 99 === The structural properties of various non-polar ZnO thin films have been systematically characterized in detail by using electron microscopy. The structural properties include crystallography relationships, interfaces, crystal defects, and growth mechanism. Non-polar ZnO thin films have been grown on (001), (112), and (114)LaAlO3 (LAO) single crystal substrates by pulsed laser deposition method, respectively.
The crystallography relationships between non-polar ZnO films and LAO substrate are identified according to the selected area diffraction (SAD) pattern of cross-sectional transmission electron microscopy (XTEM). On (001)LAO, a-plane ZnO thin film is realized to consist of two types of domains perpendicular to each other. The crystallography relationships between a-plane ZnO domains and (001)LAO are determined to be [0001]ZnO-I//[110]LAO and ZnO-I// LAO, whereas [0001]ZnO-II// LAO and ZnO-II// LAO for the other domain. Non-polar ZnO epitaxial film grown on (112)LAO appears in pure m-plane orientation. The crystal orientation relationships between m-plane ZnO and (112)LAO are verified to be [0001]ZnO// LAO and ZnO// LAO. For non-polar ZnO epitaxial film on (114)LAO, the surface plane of ZnO exhibits near orientation. The -plane ZnO epitaxial film presents only one direction parallel to (114)LAO, which is confirmed to be [0001]ZnO// LAO.
For the microstructure of a-plane ZnO domains, XTEM images show that the domains form columnar structure. In a-plane ZnO domains, the threading dislocations extend straight perpendicular to ZnO/LAO hetero-interface. The results of plan-view TEM (PVTEM) reveal that a-plane ZnO domains appear L-like shape. The arms of L-like shape are verified to be parallel to ZnO and ZnO(±[100]LAO and ±[010]LAO). According to high-resolution PVTEM (HR PVTEM) images, domain boundaries are identified to be inversion domain boundary, near 45° or 45° boundary, and local r-plane twin boundary. In a-plane ZnO domains, the predominant crystal defects are dislocation, basal stacking fault (BSF), and stacking mismatch boundary (SMB). SMB is a planar defect consisted of m- and r-planes. The densities of dislocations, BSFs, and SMBs are 5×1010 cm-2, 5×105 cm-1, and 3×105 cm-1, respectively.
For non-polar ZnO epitaxial thin films grown on (112) and (114)LAO, the hetero-interfaces have been realized by the comparison of HR XTEM images and simulation HR XTEM images. The results of the simulation images conclude that a ZnO/LAO hetero-interface might be composed of more than one type of configuration. This phenomenon can be attributed to the defects in the hetero-interface, such as BSF and misfit dislocation. The HR XTEM image also shows that the BSF generated at the hetero-interface might be able to assist the relaxation of mismatch-induced strain. In non-polar ZnO epitaxial thin films on (112) and (114)LAO, the predominant crystal defects are identical with that in a-plane ZnO domains. In non-polar ZnO epitaxial films, SMBs can extend to have U-turn profile and become loops. The appearance of SMBs results from that two crystals with penetration BSFs coalesce to form a boundary with stacking mismatch. Based on the calculation in PVTEM images, the densities of dislocations and BSFs are 5.1×1010 cm-2 and 4.3×105 cm-1 in m-plane ZnO epitaxial film. For -plane ZnO epitaxial film, the densities of dislocations and BSFs are 3.8×1010 cm-2 and 3.1×105 cm-1.
The growth mechanism of non-polar ZnO films can be supported by the results of ZnO nucleation and growth experiments in home-built vertical chemical vapor deposition system. At the initial stage of thin film growth, m- and r-plane facets naturally appear on non-polar ZnO island-like growing grains. For a-plane ZnO domains, the growing grains would merge with accommodation along r-plane facets to form near 45° or 45° boundaries and local r-plane twin boundaries in order to reduce the total energy of domain boundaries. During the growth of non-polar ZnO epitaxy, the growing grains with penetration BSFs coalesce along pre-existent m- and r-planes to form SMBs. The formation of SMBs in a-plane ZnO domains is similar with that in non-polar ZnO epitaxy. After the growth of non-polar ZnO thin film, the surface exhibit stripe morphology parallel ZnO c-axis.
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