Surface texture and structure of ZnO films synthesized by off-axis sputtering deposition
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C-H. Su, and S.L. Lehoczky Microgravity Science and Applications Department, NASA/Marshall Space Flight Center, Huntsville, Alabama 35812
M.A. George Department of Chemistry, The University of Alabama, Huntsville, Alabama 35899
D.H. Lowndes Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6056 (Received 3 December 1999; accepted 11 February 2000)
Morphology and structure of ZnO films deposited on (0001) sapphire and glass substrates by off-axis sputtering were investigated at various temperatures and pressures. All films show highly textured structures on glass substrates and epitaxial growth on sapphire substrates. The full width at half-maximum of theta rocking curves for epitaxial films is less than 0.5°. In textured films, it rises to several degrees. The trend of surface textures in films grown at low pressures is similar to those grown at high temperatures. A morphology transition from large well-defined hexagonal grains to flat surface was observed at a pressure of 50 mtorr and temperature of 550 °C. The experiment results are explained by the transport behavior of depositing species. I. INTRODUCTION
A recent surge of interest to synthesize ZnO films and crystals has been credited to overgrowth of the GaN material because of the small lattice mismatch. With an energy gap of 3.3 eV at room temperature, the ZnO and GaN materials contain many similarities that make ZnO a very promising material for blue/green laser devices.1 Both materials have a wurtzite structure, a wide-band gap in the ultraviolet range, high melting temperatures, high hardness, and comparable piezoelectric and dielectric constants.2–4 Since ZnO has an excitonic binding energy (60 meV) that is strong enough to make excitonic lasing at room temperature possible, synthesizing high-quality epitaxial ZnO films for optoelectronic applications has regained much attraction.5–7 Therefore, the study of the film structure and surface morphology is needed for the development of device fabrications. Many techniques are used to synthesize epitaxial and textured ZnO films, including pulsed-laser deposition (PLD), molecular beam epitaxy, and chemical vapor deposition, as well as plasma and radio-frequency sputtering.8–12 A systematic morphologic study of ZnO films synthesized on various substrates using PLD with the on-axis configuration, in which the target and substrate surfaces face each other, indicates that films have a textured surface when they are grown at temperatures from 350 to 750 °C and at the pressure region of 10−5–1 torr.12 Two types of texture islands were observed on the film J. Mater. Res., Vol. 15, No. 5, May 2000
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surface on the (0001) sapphire substrates. One type of islands has sixfold symmetry, and the other islands are polygon shapes. Beyond this temperature and pressure region, the film surface is flat. An argument is that the competition between the surface diffusion/redistribution and the arriving rate of adatoms is responsible for the mor
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