Fabrication of Mg2Si thermal-to-electric energy conversion device using die-casting sysnthesis method

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With the spread of solar-cell power generation, the demand for solar cells with higher conversion efficiency is increasing. It is necessary that the materials which can be used for energy-conversion applications should be environmentally-benign semiconductors, namely, they possess the advantages of abundance of their constituent elements and non-toxicity of their processing by-products. Sil-xGex alloys is known as a candidate material for producing carrier multiplication (Auger generation), which allows a conversion efficiency of 30% when the Ge amount is about 68%. Sil-xGex alloys are important materials not only for microelectronic devices but also for solid-state power generators such as solar cells and thermoelectric devices, because of their chemical stability, mechanical strength at elevated temperatures, and a close match of the n-/p-type alloys in terms of their thermal and electrical characteristics enable better device operation. However, since the Si-Ge system shows a comple … More te series of solid solutions with a phase relationship, it is not easy to precipitate crystals possessing a certain composition of silicon or germanium selectively by using conventional Bridgman, Czochralski and floating-zone methods.So far, we have performed bulk crystals growth of Sil-xGex using an advanced version of the Bridgman technique combined with a die-casting growth method, but precise and reproducible control of their compositional uniformity and thickness were not accomplished. In order to realize Sil-xGex wafer with an arbitrary composition, we have developed a new method of preparing Sil-xGex by a wafer-proximity method. This method enables to form bulk polycrystalline Sil-xGex with x=0.3~0.7, with slight deviation of ?}0.5% for the compositional uniformity. The grown Sil-xGex crystals obtained were 15x15 mm2 and 3 mm thick. The wafer-proximity method allows the control of the wafer thickness and composition x by simply changing the growth temperature and the duration. The microstructure of the samples was analyzed by using an optical microscope, and their composition was determined by electron-probe microanalysis. For the grown specimens room temperature photoconductivity measurement was carried out. The room temperature I-V characteristics of the samples, which possess pn junction by phosphorus doping using phosphor silicate glass (PSG) were measured using a solar-simulator. The obtained results imply that the newly developed wafer-proximity growth technique can fabricate bulk Sil-xGex (x~0.6-0.7) crystal for Auger generation in a reproducible manner. Less