Reconstruction of the 2D hole gas spectrum for selectively doped p-Ge/Ge1-xSix heterostructures

The magnetic field (0less than or equal toBless than or equal to32 T) and temperature (0.1less than or equal toTless than or equal to15 K) dependences of longitudinal and Hall resistivities have been investigated for p-Ge0.93Si0.07/Ge multilayers with different Ge layer widths 12less than or equal tod(w)less than or equal to20 nm and hole densities p(s)=(1-5) x 10(15) m(-2). An extremely high sensitivity of the experimental data (the structure of magnetoresistance traces, relative values of the inter-Landau-level gaps deduced from the activation magnetotransport, etc.) to the quantum well profile is revealed in the cases where the Fermi level reaches the second confinement subband. An unusually high density of localized states between the Landau levels is deduced from the data. Two models for the long-range random impurity potential (the model with randomly distributed charged centers located outside the conducting layer and the model of the system with a spacer) are used to evaluate the impurity potential fluctuation characteristics: the random potential amplitude, the nonlinear screening length in the vicinity of integer filling factors nu=1 and nu=2, and the background density of states (DOS). The described models are suitable for explanation of the observed DOS values, while the short-range impurity potential models fail. For half-integer filling factors, a linear temperature dependence of the effective quantum Hall effect plateau-plateau (PP) transition widths nu(0)(T) is observed, contrary to the expected scaling behavior of the systems with short-range disorder. The finite T-->0 width of the PP transitions may be due to an effective low-temperature screening of a smooth random potential due to the Coulomb repulsion of electrons. (C) 2003 MAIK "Nauka/Interperiodica".