| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 94 === A 300 x 300 micro-meter square crack-free GaN/AlN multilayers of 2 micrometer thick has been successfully grown on the Si(111) substrate patterned with SixNy or SiO2 meshes by MOCVD. The cathodoluminescence (CL) and Raman results show that the better quality of GaN is obtained for the SixNy mesh patterned Si(111) as the substrate. And better quality of GaN is achieved for smaller mesh size.
The in-plane stress exhibits a U shape distribution across the “window” region, supported by the Raman shift of the GaN E2(TO) mode. This indicates a stress relaxation abruptly occurring near the edge of the “window” region due to the free standing surface (11-bar01) or (112-bar2). The in-plane stress is almost relaxed at the corner of the “window” region due to three free standing surfaces (11-bar01), (112-bar2), and (101-bar1). The maximum in-plane stress is located near the surface of the multilayers at the center of the “window” region, supported by the Raman measurements and the failure observations. The role of the SixNy mesh in the stress relaxation is discussed.
The band gap shift in the 80 x 80 micrometer square crack-free GaN/AlN multilayers on the mesh patterned Si(111) was characterized by cathodoluminescence (CL) and Raman techniques. The GaN band gap derived from CL spectra depends on the spatial point inside a mesh, which changes from 3.413 eV (at center), to 3.418 eV (at edge), and to 3.426 eV (at corner). The band gap shift is attributed to the variation of tensile stress inside the mesh, confirmed by Raman mapping. The shift of GaN band gap per unit stress is determined to be 0.03 eV/GPa.
Scanning photoelectron microscopy (SPEM) was applied to extract chemical images of the GaN/AlN multilayers within the mesh. The SPEM images study of the GaN/AlN multilayers on a mesh patterned Si(111) is dependent on the local charging. The V-defect on the surface of GaN can be observed by SPEM images and is determined to be Ga terminated surface.
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