Heavily doped GaAs : Se. II, electron mobility

A study of the mobility μ of Se-doped n+-GaAs grown by metalorganic chemical vapor deposition is presented. A significant decrease in μ is observed for n>1×1018 cm−3, which is a general characteristic of n+-GaAs. Previous explanations that the low values of μ are the result of autocompensation by the dopant are unsatisfactory in view of the universality of the decline in μ. A new formula is derived for the ionized impurity mobility μI for degenerately doped material which accurately predicts the experimental μ using no compensation and no adjustable parameters. The formula takes into account the increase of the effective mass m* due to nonparabolicity of the conduction band and due to distortion of the band by the donor atoms. For degenerate material, μI is inversely proportional to the square of m* at the Fermi energy EF. For uncompensated GaAs with n=1×1019 cm−3, m* at EF is 2.4 times m* for pure GaAs, and μ is only 1000 cm2/V s. Previous theories, which use the smaller optical effective mass m*opt in place of m*, predict higher mobilities and temperature variations of μ for n+-GaAs. The new formula correctly predicts that μ does not change with temperature when n>2×1018 cm−3.