Characterization of Sturctural Disorder by means of Polarized Photoreflectance in Amorphous Semiconductors

极化电吸收技术（PEA）用于测定非晶半导体中载流子的平均自由程和带边迁移率。该方法基于外电场下光谱微观理论的结果，该结果表明，由于局域态跃迁时矩阵元素的场致变化，EA信号出现了“外在”极化依赖，因此它可以作为非晶半导体无序程度的度量。在场光偏振平行和垂直条件下测量的EA信号强度比转化为载波平均自由程和带边迁移率。我们已将这项新技术应用于非晶硅合金。本研究得出以下结论：1)器件质量未掺杂a-Si: H的电子（空穴）平均自由程约为12*(5*)，估计电子（空穴）迁移率为10 cm2/Vs （3cm2/Vs）。2)迁移率在特定的沉积温度下达到最大值，这取决于沉积条件的细节。3)低温沉积的a- si: H的退火处理通过结构无序的松弛提高载流子迁移率。4)与碳合金化导致迁移率的持续降低，当碳浓度约为10%时，迁移率下降幅度最大（15%），这与锗合金化的不太明显的效果形成鲜明对比。5)磷的掺杂使电子迁移率发生了明显的畸变，而空穴迁移率与未掺杂时基本保持不变。

Polarized Electroabsorption technique (PEA) has been developed for the determination of carrier mean free path as well as mobility near the band edge in amorphous semiconductors. The method is based upon the result of microscopic theory of optical spectrum in the presence of external electric fields, which indicates that an "extrinsic" polarization dependence of the EA signal appears due to the fieldinduced change in the matrix element for transitions involving localized states, so that it serves as a measure of the degree of disorder in amorphous semiconductors. Measured EA signal intensity ratio between field-light polarization parallel and perpendicular conditions is transrated into carrier mean free path and band edge mobility. We have applied this new technique to amorphous silicon alloys. The conclusions reached from the present work are as follows ;1) The electron (hole) mean free path is about 12* (5*) for device quality undoped a-Si : H,which yields estimate of the electron (hole) mobility of 10 cm2/Vs (3cm2/Vs).2) The mobilities attain maxima at a particular deposition temperature which would depend on details of deposition conditions.3) Annealing treatment on a-Si : H deposited at a low temperature improves carrier mobilities through the relaxation of structural disorder.4) Alloying with carbon leads to a continuous reduction of mobility, with the largest drop (15%) for a carbon concentration of about 10 at%, this being in sharp contrast to a less-pronounced effect by germanium alloying.5) Phosphorus doping ramarkably detoriorates the electron mobility, while the hole mobility remains almost unchanged from that in undoped material.