Transport and Capacitance Measurements of Electron Multilayers in Wide Parabolic Quantum Wells.
Abstract
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 threedimensional 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 threedimensional 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 selfconsistent 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 capacitancevoltage (CV) 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 CV measurements in zero magnetic field show four welldefined 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 inplane magnetic field. We observe highfield oscillations in the magnetoresistance and the capacitance derivative profiles, which we tentatively attribute to commensuration effects between electron cyclotron motion and the periodic potential.
 Publication:

Ph.D. Thesis
 Pub Date:
 1994
 Bibcode:
 1994PhDT.......120B
 Keywords:

 Physics: Condensed Matter; Engineering: Materials Science