Probing the p-Ge_(1-x) Si_x/Ge/p-Ge_(1-x) Si_x quantum well by means of the quantum Hall effect

摘要

We have measured the temperature (0.1 <= T <=15 K) and magnetic field (0 <= B <= 32 T) dependences of longitudinal and Hall resistivities for the p-Ge_(90.93)Si_(0.07)/Ge multilayers with different Ge layer widths 10 <= d_w <= 38 nm and hole densities Ps = (1-5) X10~(15) m~(-2). An extremely high sensitivity of the experimental data (the structure of magnetoresistance traces, relative values of the inter-Landau-level (LL) gaps deduced from the activation magnetotransport etc) to the quantum well (QW) characteristics has been revealed in the cases when the Fermi level reached the second confinement subband. The background density of states (5-10) X 10~(14) m~(-2) meV~(-1) deduced from the activation behaviour of the magnetoresistance was too high to be attributed to the LL tails, but may be accounted for within a smooth random potential model. The hole gas in the Ge QW was found to separate into two sublayers for d_w > ~ 35 nm and Ps ≈ 5 X 10~(15) m~(-2). Concomitantly the positive magnetoresistance emerged in the weakest fields, from which different mobilities in the sublayers were deduced. A model is suggested to explain the existence of the plateaux close to the fundamental values in a system of two parallel layers with different mobilities.