Magnetoresistance in low-temperature grown molecular-beam epitaxial GaAs

Conductivity, Hall effect as well as "physical" and "geometrical" magnetore sistances were measured at 290-440 K in molecular-beam epitaxial GaAs layer s grown at 200-400 degrees C. The experimental data were analyzed taking in to account the combined band and hopping conductance regime. Positive hoppi ng magnetoresistance parameters (Delta rho/rho(0)B(2))(h)approximate to 10( -4) T-2 and hopping Hall mobilities lower than 1 x 10(-4) m(2) V-1 s(-1) we re determined in the as-grown layers. A transverse-to-longitudinal hopping magnetoresistance ratio of about 2, consistent with hopping transport theor ies, was obtained. In the annealed layer grown at 200 degrees C (J200a) the band mobility determined from the geometrical magnetoresitance (GMR) mobil ity was found to be significantly higher than the band Hall mobility. It is related to a mixed band conductivity regime with the hole concentration p exceeding the electron one n. The difference between GMR and Hall mobilitie s decreases with increasing growth temperature as far as a typical single-c arrier band conductivity regime (n > p) is present in the layer grown at 40 0 degrees C. In contradiction to the layers grown at higher temperatures, t he J200a layer showed the opposite (positive) sign of the hopping Hall coef ficient as well as the largest hopping magnetoresistance parameter (approxi mate to 3 x 10(-2) T-2). (C) 1999 American Institute of Physics. [S0021-897 9(99)09022-2].