Anisotropy of Surface Electronic Mobility in Germanium

Abstract

The anisotropy of the surface mobility of electrons p for carriers moving in thin layers of the surface space charge (SSC) and diffuse scattering on the surface was predicted for multivalley semiconductors of the Ge and Si types in (1 to 4). In (4) the anisotropy of ps has been analysed theoretically for different crystallographic orientations assuming absence of intervalley scattering and zero normal component of current for every valley (in (1, 2) the latter was not taken into account).It turned out, particu€arly, that for (100) and (111) planes degenerates into scalars, though for great surface potentials Ys these magnitudes may differ from each other rather noticeably. For other planes the mobility turns out to be anisotropic.The magnitudes calculated are given in Fig. 1. They show the anisotropy degree K = psl/ps2 a s a function of Y using the theory developed in (4). The calculation has been made on a computer f6r intrinsic Ge using the exact dependence Y(z). S S S Experiments aiming at detecting and investigating the anisotropy of have S been carried out. Measurements of the integral field effect (FE) have been made on the same sample of intrinsic Ge by passing a current along different crystallographic directions. FE-curves in the SSC-accumulation region were then compared, and from the ratio of conductivity changes (for fixed electric fields) K(YJ was obtained. Samples cut along the (loo), (lll), and (115 planes were used.To detect the anisotropy of )1 using this method it is convenient to utilize the S ( 1 1 : ) plane: the mutually perpendicular directions on these planes correspond to the main crystallographic axes (il0) and (111) for which a considerable K (% 1.2) is predicted. Thus, the experiment may be carried out on a plate of a simple square form, which essentially simplifies the manufacturing of samples and reduces the masking factors in the utilized method (irreproducibility of the surface properties for different samples as well as inhomogeneity).