High Temperature Reproducible Preparation of Mg 2 Si Films on (001)Al 2 O 3 substrates Using RF Magnetron Sputtering Met
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High Temperature Reproducible Preparation of Mg2Si Films on (001)Al2O3 substrates Using RF Magnetron Sputtering Method Atsuo Katagiri1, Shota Ogawa1, Takao Shimizu1, Masaaki Matsushima1, Kensuke Akiyama1, 2, and Hiroshi Funakubo1 1 Department of Innovative and Engineered Materials, Tokyo Institute of Technology, J2-43, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan 2 Kanagawa Industrial Technology Center, 705-1 Shimoimaizumi, Ebina-shi, Kanagawa 243-0435, Japan ABSTRACT A RF magnetron sputtering method was used to prepare Mg2Si films at 300-400oC on (001) Al2O3 substrates from a Mg disc target with Si chips. Mg deposition was not detected at 400oC from a pure Mg disc target without Si chips due to the high vapor pressure of Mg. However, the amount of Mg deposition increased with the increase in Si/(Mg+Si) area ratio of the target surface together with the increase of the Si deposition. The obtained films had a stoichiometric composition of Si/(Mg+Si)=0.33 that consisted of the well crystalline Mg2Si single phase regardless of Si/(Mg+Si) area ratio of the target surface. This showed the existence of a “process window” against supply ratio of Si/(Mg+Si) for Mg2Si single phase films with a stoichiometric composition. This is considered to be due to the vaporization of the excess Mg prepared under the Mg excess condition as reported by Mahan et al. for Mg2Si films prepared at 200oC by ultra-high vacuum evaporation. INTRODUCTION Magnesium silicide (Mg2Si) has been extensively investigated because of its great potential in a wide range of applications including thermoelectrics, photovoltaics, and optoelectronics. For example, Mg2Si is an attractive alternative to present solar cell materials because of its narrow band gap (ranging from 0.66 to 0.80 eV) [1 - 4] and high absorption coefficient of 3 × 105 cm-1 at 2.5 eV[5]. In addition, Mg2Si has been investigated as a potential high performance environment-friendly thermoelectric material [2, 6] because it consists of the non-toxic and inexpensive elements. Furthermore, it is lightweight and this is advantageous for use in mobile applications, such as for automobile use. [7] A key challenge of this material is the improvement of its thermoelectric performance, especially for p-type semiconductors, because its dimensionless figure of merit was much lower than that of an n-type one, although both n-type and p-type semiconductors with good thermoelectric performance are required for real applications. [8] A survey of the appropriate doping elements for Mg2Si is essential for attaining good performance, and high quality films, such as epitaxial films are a very powerful way for achieving this. This is also very useful for solar cell and optoelectronics applications because epitaxial Mg2Si films with both p- and n-type semiconductors are also requested. Films prepared below 200oC with following post-annealing processes have been widely used for preparing Mg2Si films [8-14]. However, film deposition at high temperature above
200oC enable direct crystallization, and it
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