MOBILITY OF MODULATION-DOPED ALGAAS LOW-TEMPERATURE MBE-GROWN GAAS HETEROSTRUCTURES/

A study of the mobility of a novel modulation doped heterostructure in which the channel region is made of low-temperature molecular beam ep itaxially grown GaAs (LT-GaAs) and all other layers are grown at norma l temperatures is presented for the first time. The resistivity of the as-grown samples (in-situ annealed) is very high, as is that of singl e layers of bulk LT-GaAs. However, in the presence of light, the resis tivity of the LT-GaAs modulation-doped field effect transistor (MODFET ) is significantly lower, facilitating reliable Hall measurements. We speculate that the observed decrease in resistivity of the LT-GaAs MOD FET is due to the formation of a two-dimensional electron gas (2DEG) a t the heterointerface under illumination. A number of samples grown un der different growth conditions were investigated. Mobilities for thes e samples were found to be in the range of 250 to 750 cm(2)/Vs at 300K and similar to 3000 to 5500 cm(2)/Vs at 77K. A first-order computer s imulation was implemented to calculate the mobility of the 2DEG using the relaxation-time approximation to solve the Boltzmann equation, tak ing into account different scattering mechanisms. Scattering by the ar senic clusters and by ionized impurities in the LT-GaAs MODFET channel are found to be the two dominant mechanisms limiting the mobility of the LT-GaAs MODFET samples. Experimental values are in good agreement with theoretical results.