Optical and electrical properties of β-FeSi_2 under pressure

环境友好型半导体β-FeSi_2的基本问题是在亚微米球形样品中观察到光致发光，而在薄膜样品中却没有。为了弄清楚这个问题的第一步，阐明两个样本的基本性质是很重要的。我们继续进行的高压研究总结如下。本文研究了在Si（001）上采用模板和SiO_2封盖的Si/Fe多层膜制备β-FeSi_2的高压光吸收。在光子能量约为0.3 eV的带隙外，实验所得的吸收系数比第一性原理计算的吸收系数大几个数量级。对这种大吸收系数的可能解释是计算出的能带结构的鞍点激子效应。通过测量高达16 GPa的反射率光谱，研究了压力对β- fesi_2高达3ev的高带间跃迁的影响。结果表明，1.6 eV以下带间跃迁的线性压力系数均为正，约为8 meV/GPa， 1.6 eV以上带间跃迁的线性压力系数约为16 meV/GPa。这一特征表明，能隙附近的态密度由压力系数大、压力系数小的两个不同带组成。研究了有意掺杂p型β- fesi_2薄膜的空穴迁移率。随着退火温度和时间的增加，空穴迁移率在室温下增加到约450 cm^2/Vs。通过考虑各种载流子散射，如声声子散射和极光声子散射、谷间散射、电离杂质散射和晶界散射，对观测到的空穴迁移率进行了分析。迁移率与实验结果吻合较好，表明极光声子散射决定了室温下的空穴迁移率。各种光学测量证实，半径为3-5nm的纳米二硅化铁（nc-FeSi_2）均匀薄膜的光吸收比单晶□-FeSi_2的光吸收大10%左右。研究还发现，nc-FeSi_2的氢化反应使其光学特征能发生了强烈的变化。在□-FeSi_2介电常数的虚部（约2 ~ 3 eV）出现纳米晶态的特征。少

Fundamental questions of environmentally friendly semiconductor β-FeSi_2 is that photoluminescence has been observed in sub-micron ball samples but not in thin-film sample. For the first step to try to make this question clear it is important to elucidate basic properties of both samples. Our continued high-pressure studies are summarized in the following.We have investigated the high-pressure optical absorption of β-FeSi_2, thin films prepared from Si/Fe multilayers on Si(001) with template and SiO_2 capping. It is found that the experimental absorption coefficient in the range of photon energy of about 0.3 eV beyond the band gap is a few orders magnitude larger than the first-prinsicples calculated absorption coefficient. A possible explanation for this large absorption coefficient is the saddle-point exciton effect by the calculated band structure. The effect of pressure on high interband transitions up to 3 eV inβ-FeSi_2 is studied by measuring reflectivity spectra up to 16 GPa. Th … More e evaluated linear pressure coefficients for the interband transitions are all positive and about 8 meV/GPa for the transitions below 1.6 eV and about 16 meV/GPa for those beyond 1.6 eV. This characteristic indicates that the density of states near the energy gap consists of two different bands, with large- and small-pressure coefficients.The hole mobility of intentionally undoped p-typeβ-FeSi_2 thin films was investigated. With increasing annealing temperature and time, the hole mobility increased to approximately 450 cm^2/Vs at room temperature. The observed hole mobility was analyzed by considering various carrier scatterings such as acoustic-phonon and polar-optical-phonon scattering, intervalley scattering, ionized impurity scattering, and grain-boundary scattering. The nice fit of the mobility to the experimental results reveals that the polar-optical-phonon scattering determines the hole mobility at room temperature.Various optical measurements confirm that optical absorption in uniform thin films made from nanocrystalline iron disilicide(nc-FeSi_2) with a 3-5nm radius is larger by about 10 % than that of single crystalline □-FeSi_2. It is also found that the hydrogenation of nc-FeSi_2 changes strongly its optical characteristic energies. The nanocrystalline state appears characteristically in the imaginary part of dielectric constants of □-FeSi_2 around 2 to 3 eV. Less