Hopping conductivity and activated transport in InxGa1-xAs quantum wells

We present measurements of the diagonal R-x,R-x and off-diagonal R-x,R-y magnetoresistance under quantum Hall conditions on several high electron mobility transistors (HEMT) based on InxGa1-xAs quantum wells. From the magnetoresistance tensor we obtain the longitudinal conductivity sigma(xx). We study the transport mechanisms near the sigma(xx) minima at temperatures ranging between 2 K and 35 K; activated transport is the dominant mechanism for temperatures above 7 K while variable range hopping conductivity is significant for lower temperatures. We show that electron-electron correlations should be taken into account to explain the conductivity vs temperature behaviour below 5 K. Finally, we study the behaviour of the localization length as a function of Landau level filling and obtain a critical exponent gamma = 3.45 +/- 0.15.