Transport and Capacitance Measurements of Electron Multilayers in Wide Parabolic Quantum Wells.

This thesis presents a study of coupled electron layers produced by superimposing a wide parabolic well (WPBW) with multiple square wells to produce a remotely -doped multiple quantum well structure (RMQWs). A series of RMQWs were produced using the digital alloy method with parabolic well curvatures adjusted to give three-dimensional densities 4.0 times 10^ {15} cm^{-3} to 4.6 times 10^ {16} cm^{-3} . The total number of superlattice periods was varied from 8 to 20, all having 200 A periods, and barrier widths and heights were chosen so the bandwidth of the lowest miniband was ~2meV. Magnetoresistance and Hall effect measurements of samples at temperatures to 50mK give Hall mobilities from 23,000 cm^2/Vs to 110,000 cm ^2/Vs and sheet densities from 1.4 times 10^{11 } cm^{-2} to 4.3 times 10^{11 } cm^{-2}, indicating 4 to 18 square wells are occupied. The integer quantum Hall effect was also examined. We concentrate on two structures A and (B) consisting of ten (eight) periods of 60A (80A) wide, 175 (83) meV high barriers and 140 A (120A) wide wells in a 2000A (1600 A) wide parabola with curvature chosen to give an average three-dimensional design density 2.5 times 10^{16} cm ^{-3} (4.8 times 10^{16} cm ^{-3}). Fourier analysis of Shubnikov -de Haas oscillations gives power spectra with several clearly defined peaks from which we obtain the energy separations E_{F} - E_{i} that are in excellent agreement with self-consistent calculations. Measurements of the capacitance C(V_ {rm g}) between a front gate and the electron gas and its derivative dC/V_ {rm g} are used to study quantum effects via capacitance-voltage (C-V) profiling. We consider the derivative of capacitance with respect to both voltage and magnetic field to differentiate quantum features from density profile effects. Low T temperature C-V measurements in zero magnetic field show four well-defined electron layers in each device. In perpendicular magnetic fields, we study the coupled electron layers subband structure: our results are in good agreement with theory. We measure the capacitance and magnetoresistance of sample A for current flow perpendicular to an in-plane magnetic field. We observe high-field oscillations in the magnetoresistance and the capacitance derivative profiles, which we tentatively attribute to commensuration effects between electron cyclotron motion and the periodic potential.