The Preparation And Properties Of MgZnO:Al Transparent Conducting Films

Transparent conducting oxide (TCO) is a kind of important optoelectronic material. It has high electrical conductivity and high optical transparence in the visible region. Because of their excellent optoelectronic characteristics, TCO films have been widely used in photo-electronic devices such as solar cell and displays. In recent years, many research workers have become interested in studying the TCO films and developing various deposition techniques, for instance, magnetron sputtering, evaporating, chemical vapor deposition (CVD) and so on.In this dissertation, the preparation process of the MgZnO:Al transparent conductive films deposited by radio frequency magnetron sputtering on glass substrates were reported in detail. The structural, electrical and optical properties were also investigated in terms of the preparation conditions including sputtering pressure, sputtering power, substrate temperature and annealing temperature.As a new-style wide band-gap compound semiconductor material, MgZnO was firstly prepared by Ohtomo in 1998. It is a kind of mixed-crystal consisted of ZnO and MgO, and the Mg ions and Zn ions replace each other in the lattice. ZnO has hexagonal wurtzite structure while MgO has cube crystal structure. The replacement of Zn²⁺ by Mg²⁺ does not induce a significant change in lattice constant due to the similarity of their ionic radius.The band gap energy of Mg_xZn_1-xO can rise from 3.3eV to 7.8eV through increasing the Mg content (x), and the structure can be changed from hexagonal wurtzite to cube. Doped Mg_xZn_1-xO has better electrical conductivity. Like ZnO, the optical transparence of Mg_xZn_1-xO in the visible region is as high as 85%. It has been used widely based on its great characteristics.The Mg_0.1Zn_0.9O:Al films prepared by radio frequency magnetron sputtering on glass substrates were polycrystalline with hexagonal wurtzite structure and had a preferred orientation (002) with the c-axis perpendicular to the substrates. The diffraction angle 2θ was about 34.6°. The experiment indicated that there was no MgO phase in Mg_0.1Zn_0.9O:Al films, and the magnesium atoms replaced the zinc atoms in the hexagonal lattice.With the increase of the sputtering pressure, the XRD intensities of diffraction peaks decreased and the peak width at half height (Wh/2) became larger. Meanwhile, the resistivities of the Mg_0.1Zn_0.9O:Al films went down from 2.5(?)cm to 0.5(?)cm. After the pressure was more than 2.5Pa, the resistivity increased slightly. The transmittances of the films in the visible region were over 90%. The band gap energy was 3.62eV.The increase of the sputtering power made the average growth rate of the Mg_0.1Zn_0.9O:Al films faster, the crystallinity better and the grain size larger. The resistivities of the films decreased with increasing the sputtering power, but it increased a little when the sputtering power exceeded 100W. In addition, the optical transparency of them became worse, for the reason that the films turned thicker as the power became larger.When the glass substrate temperature increased, the XRD diffraction peak intensity became more intensive and sharper, the crystallinity became better and the crystalline grain size became larger. The Mg_0.1Zn_0.9O:Al films had the best crystallinity when the substrate temperature was 180°C, and the conditions became worse when the temperature exceeded 180°C. The resistivity of the films decreased with the rise of substrate temperature. The change was caused by the variation in carrier concentration and the mobility. In accord with the crystallinity, the films had the lowest resistivity with the substrate temperature of 180°C, and its electrical conductivity became worse after the temperature became higher. Likewise, the carrier concentration and the mobility became larger as the glass substrate temperature went up. The substrate temperature had little influence on optical transparency of films, and the transmittance still remained over 90%. The band gap energy was 3.62eV.Vacuum annealing can improve the crystallinity of the Mg_0.1Zn_0.9O:Al films remarkably. Accordingly, the resistivity of the films became lower with the annealing temperature increased. The mobility increased obviously and the carrier concentration also became larger. The lowest resistivity was 1.5×10^-2(?)cm and the largest mobility was 7cm²V^-1s^-1. After annealing, the optical transmittances of the films were over 90% in average, and the band gap became wider. The band gap energy was 3.65eV, and this increase is due to Burstein-Moss effect.The following parameters were found to be optimal and were used in Mg_0.1Zn_0.9O:Al film preparation: the sputtering pressure being 1Pa; the sputtering power being 100W; sputtering at room temperature; annealing the films at 400°C. Under these conditions, the lowest electrical resistivity was about 1.5×10^-2(?)cm, the carrier concentrations was 6.5×10^1-cm^-3, and the Hall mobility was 7cm²V^-1s^-1. The transmittance of the Mg_0.1Zn_0.9O:Al films in the visible range was over 90%. The band gap energy was 3.65eV.