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状样品中观察到了光致发光，而在薄膜样品中没有观察到光致发光。首先要弄清楚这个问题，重要的是要阐明这两个样品的基本性质。本文主要研究了β-FeSi_2薄膜的高压光吸收特性。用模板法和SiO_2覆盖法制备了Si/Fe多层膜薄膜。结果表明，在带隙外约0.3 eV的光子能量范围内，实验吸收系数比第一原理计算的吸收系数大几个数量级。对于这种大的吸收系数，一个可能的解释是计算的能带结构所产生的鞍点激子效应。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3 eV带间跃迁的影响。TH…更多的计算表明，带间跃迁的线性压力系数均为正，对于1.6 eV以下的跃迁，约为8 meV/Gpa；对于大于1.6 eV的跃迁，约为16 meV/Gpa。这一特性表明，带隙附近的态密度由两个不同的带组成，分别具有大的和小的压力系数。研究了有意非掺杂的p型β-FeSi2薄膜的空穴迁移率。随退火温度的升高和时间的延长，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，如声学声子散射、极性光学声子散射、谷间散射、电离杂质散射和晶界散射，分析了观察到的空穴迁移率。各种光学测量结果表明，纳米二硅化铁(nc-FeSi2)制备的均匀薄膜的光吸收比单晶FeSi2的光吸收大约10%。此外，还发现nc-FeSi2的氢化作用强烈地改变了其光学特征能量。纳米晶态出现在-FeSi2介电常数的虚部，约为2~3 eV。较少

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