2-DIMENSIONAL ELECTRON-GAS IN GAAS/AL1-XGAXAS HETEROSTRUCTURES - EFFECTIVE MASS

We have measured the Shubnikov-de Haas (SdH) effect in the high-mobility GaAs/Al1-xGaxAs heterostructures of a two-subband populated two-dimensional electron gas. The carrier concentrations of first and second subbands were determined by the two-frequency SdH oscillations. Both carrier concentrations increased after the persistent photoconductivity effect, and the increments agreed with Smith's results and Ando's calculation. The effective masses for both subbands at the Fermi levels of 29.1 and 2.3 meV, evaluated from the temperature dependence of SdH amplitudes, were(0.066 +/- 0.001)m0 and (0.067 +/- 0.001)m0. Comparing with the band-edge mass, we found, quantitatively, that the nonparabolic band effect in the region near the minimum of the conduction band is insignificant.