Influence Of A Buffer Layer On The Properties Of The ZnO Films Prepared On Si And Glass Substrates

ZnO is a direct-wide bandgap semiconductor which possesses bright and coherent emission at room temperature and even at elevated temperatures. A high bandgap of 3.37eV and a free exciton binding energy of 60meV along with high absorption coefficient make ZnO attractive for optoelectronic devices such as the blue and UV emitters, UV detectors as well as non-linear optical devices, catalysis, transparent conducting oxides, optoelectronics, magnetic devices, solar cells, piezoelectric transducers and sensors.Radio-frequency magnetron sputtering (RF MS) is widely used in the industry for deposition of different semiconductor and metallic films. This technology of deposition is relatively low cost. In addition, among all methods for deposition of ZnO films, magnetron sputtering is characterized by several advantages: (i) process with low substrate temperature; (ii) good film adhesion; (iii) very good thickness uniformity and high density of the films; and (iv) deposition from metallic targets by sputtering in reactive gas mixtures. Thus, RF MS is a very prospective technology for deposition of ZnO films and designing devices on their basis.In most studies, ZnO epitaxial films are grown on sapphire substrates. Although both ZnO and sapphire exhibit the same hexagonal-type lattice structure, it has been shown that in-plane orientation of the ZnO film rotates by 30°or 90°with respect to (001) sapphire basal plane when growing ZnO on sapphire substrates. Such rotation might result in generation of dislocations. Also, since sapphire is an electrically insulating material, it introduces complexity to device fabrication processes when grown crystals are applied to injection-light-emitting devices. Electrically conductive Si substrates can relieve this problem. In addition, the crystal quality of the Si substrate is expected to be markedly higher than that of the sapphire substrate. Moreover, from the viewpoint of integration with Si-based circuits and devices, the growth of high-quality ZnO film on Si is very important. However, the growth of epitaxial ZnO thin films on Si is known to be a difficult task, because the Si substrate surface is easily oxidized and covered with a resulting amorphous SiOx layer when it is exposed to reactive oxygen sources. This amorphous SiOx layer degrades the crystal quality of ZnO grown on the Si substrate. Therefore, many researchers have used buffer layers between ZnO thin films and Si substrates to improve the quality of ZnO thin films. Some groups used the homogeneous buffer layer and still others used the heterogeneous buffer layers such as CaF₂, MgO, SiC, Ti, etc.1. Undoped ZnO films were deposited by radio frequency (RF) magnetron sputtering on amorphous buffer layers such as ZnO, Al and Al₂O₃ prepared on Si(111) substrates. The effects of various buffer layers on the crystallinity quality of the ZnO were studied. The results are summarized as follows:(1) The LT- ZnO buffer layer and the subsequent high temperature treatment relax the lattice strain due to lattice mismatching and may reduce dislocations and defects at the boundary with the substrate. No defects such as cracks and dislocations caused by interruption of deposition ZnO film were observed in its SEM image.(2) Al/Si (111) template is a very suitable candidate substrate for growing the ZnO thin films. The initial Al buffer layer can prevent exposure of the Si surface to the oxygen source during the initial growth stage of the ZnO film.(3) The Al₂O₃ buffer layer results in a large grain size by decreasing the density of nucleation sites and reduces the dislocation density, which might be more favourable for high-quality ZnO film.2. Multilayer thin films show different physical properties other than the conventional monolayer thin films. The quality of films deposited on buffer layer is found to be superior to those grown directly on a substrate. In this paper, effects of amorphous buffer layer (as SnO₂,SiO₂ and Al₂O₃ prepared on glass substrates) on the microstructural and optical properties of ZnO thin films were studied. The research results are as follows:(1) Based on the difference in the nucleation and growth modes of ZnO films on the buffer layer and the ?at substrate, ZnO films on the buffer layer might contain much lower density of crystalline boundaries and defects than that in the films on the ?at, meaning better crystalline structure and higher optical quality.(2) The physical mechanisms of observed emissions in photoluminescence (PL) measurement can be explained as following. The violet luminescence emission at 422nm is probably attributed to both aspects of interface trap density (grain boundary area) and preferential orientation; the 446 nm (2.78 eV) blue emission originated from the electron transition from the shallow donor level of oxygen vacancies to the valence band and electron transition from the shallow donor level of zinc interstitials to the valence band; the green emission with 484 nm wavelength (2.56 eV) may originate from the electron transition from the deep donor level of the ionized oxygen vacancies to the valence band; the 530nm(2.34eV)green emission is due to the electron transition from the donor level of zinc interstitials to the acceptor level of zinc vacancies.