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

极化电吸收技术（PEA）已发展用于测定非晶半导体中载流子的平均自由程和带边附近的迁移率。该方法是基于光谱的微观理论的结果，在外部电场的存在下，这表明，一个“外在的”的EA信号的偏振依赖性出现由于fieldinduced的矩阵元素的变化涉及到本地化状态的过渡，使它作为一个措施的无序程度在非晶半导体。将场光偏振平行和垂直条件下的EA信号强度比转换为载流子平均自由程和带边迁移率。我们已经将这种新技术应用于非晶硅合金。结果表明：（1）器件级非掺杂a-Si的电子（空穴）平均自由程约为12 ×（5 ×）; H、其产生10 cm 2/Vs（3cm 2/Vs）的电子（空穴）迁移率的估计。2）迁移率在特定的沉积温度下达到最大值，这将取决于沉积条件的细节。Si：在低温下沉积的H通过结构无序的弛豫来改善载流子迁移率。4）与碳的合金化导致迁移率的连续降低，其中对于约10原子%的碳浓度具有最大的下降（15%），这与锗合金化的不太明显的效果形成鲜明对比。5）磷掺杂显著地降低了电子迁移率，而空穴迁移率与未掺杂材料中的空穴迁移率相比几乎保持不变。

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.