Buffer-free epitaxial growth of ZnO(0001) thin films at room temperature by tetramethylammonium hydroxide pretreatment of ?-Al2O3(0001) substrates

In this study, heteroepitaxial ZnO(0001) thin films were grown directly on ?-Al2O3(0001) substrates at room temperature by laser-molecular beam epitaxy without buffer layers. The epitaxial films were grown on substrates pretreated with strongly alkaline tetramethylammonium hydroxide solution, while the substrates cleaned by hydrochloric acid resulted in uniaxially oriented growth with poor in-plane structural anisotropy. The surfaces of the thin films were ultraflat and composed of ultrafine nanocrystallites based on low-growth temperatures, which well reflected the atomically stepped morphology of the substrates. In addition, the ZnO thin films exhibited high transparency with a transmittance of >80 % in the visible region and optical bandgaps of -3.4 eV. The epitaxial and uniaxially oriented ZnO(0001) films were both n-type degenerate semiconductors and demonstrated relatively low resistivities of -3.1 & COPY; 10-3 and -2.1 & COPY; 10-3 3 cm, respectively, at room temperature. The epitaxial thin film indicated a relatively higher carrier concentration of -3.4 & COPY; 1020 cm-3 and lower mobility of -6.1 cm2/V s as compared with the uniaxially oriented film. This suggested a greater number of crystal defects were introduced, including oxygen vacancies based on relaxed epitaxial strain.& COPY;2023 The Ceramic Society of Japan. All rights reserved.