Investigation of a solar battery using the environmentally friendly semiconductor : beta-FeSi_2

太阳能电池通常由非晶硅或多晶硅制成，但转换效率有限。其原因之一是Si的吸收系数低。CuInSe_2/CdS通过沉积多晶可以提供更高的效率，但它含有有毒的Se和Cd。本项目的目的是研究一种环境友好的高吸收系数半导体，从而实现薄膜的高效率，其中一个候选材料是半导体硅化物β-FeSi_2，但目前还没有高质量薄膜的报道，其能带结构也不清楚。我们用多层膜法和分子束外延法成功地生长了高质量的β-FeSi_2薄膜，其载流子迁移率比目前报道的高一个数量级。实验证明，该材料的吸收系数高达10^5/cm。能带结构也很清楚：间接带隙为0.81eV，直接带隙为0.97eV。发现导电类型受沉积Si/Fe比的控制，但0.81eV的带隙太小，不能单独作为太阳能电池提供高效率。因此，另一个半导体硅化物具有更宽的带隙被搜索。BaSi_2是另一个候选者，因为据报道其带隙为1.3eV或0.7eV。首先，我们研究了多晶BaSi_2，发现其间接带隙为1.1eV，直接带隙为1.25eV，比β-FeSi_2稍宽。我们尝试用分子束外延（MBE）方法在Si（111）衬底上生长BaSi_2薄膜，并首次成功地获得了高质量的BaSi_2薄膜，其太阳能电池特性还不清楚，但这些结果表明半导体硅化物是一种很好的环境友好型太阳能电池材料。

Solar batteries are usually made of amorphous Si or poly-Si, but the conversion efficiency is limited. One of the reason is low absorption coefficient of Si. CuInSe2/CdS can give a higher efficiency by a deposited poly-crystalline, but it contains toxic Se and Cd. The purpose of this project is to investigate an environmentally friendly semiconductor with high absorption coefficient, therefore, high efficiency with a thin film.One of the candidate was a semiconducting silicide ; beta-FeSi_2, but there had been no report on a high quality thin film and details of the band structure had not also been clear. We succeeded to grow a high quality beta-FeSi_2 film by multi-layer method and MBE growth, which could give a higher carrier mobility of one order of magnitude than those reported so far. The absorption coefficient was proved to be as high as 10^5/cm. Energy band structure was also made clear: indirect band gap of 0.81eV and direct band gap of 0.97eV. It was found that conduction type was controlled by the depositing Si/Fe ratio.Band gap of 0.81eV is, however, too small to give a high efficiency as a solar battery by itself. Therefore, another semiconducting silicide with a wider band gap was searched. BaSi_2 was another candidate because it was reported that the band gap was either 1.3eV or 0.7eV. Firstly, we investigated poly-crystalline BaSi_2 and found it has an indirect band gap of 1.1eV and direct band gap of 1.25eV, a little bit wider than beta-FeSi_2. We tried to grow BaSi_2 thin film on Si (111) by a MBE method and succeeded to get a high quality thin film for the first time.Solar battery characteristics has not been clarified yet, but these results could indicate that semiconducting silicide is a good candidate of environmentally friendly solar battery materials.