Mobility Anisotropy of Electrons in Inversion Layers on Oxidized Silicon Surfaces

Abstract

Measurements of the field-effect mobility of electrons in $n$-type inversion layers of Si as functions of the crystalline orientation of the surface and of the azimuthal direction of the current path within the layers are reported. Significant mobility anisotropy observed over a range of strong electric fields perpendicular to the surface at temperatures from 77 to 320 \ifmmode^\circ\else\textdegree\fi{}K contrasts substantially with that in the case of $p$-type inversion layers reported previously. The anisotropy at room temperature can be successfully interpreted in terms of the effective-mass anisotropy calculated on the basis of Stern and Howard's formula for the two-dimensional carriers. For a full understanding of the experimental results, particularly at low temperatures, theoretical investigations including the anisotropy of the relaxation time are required, but have not been explored enough. Extensive data on the dependences of the mobility on the electric field and temperature as well as bulk-acceptor concentration are presented and discussed in the light of existing scattering theories. Some hole-mobility data are included for comparison.