Electron transport in wide parabolic gallium arsenide/aluminum gallium arsenide wells

This thesis presents an experimental study of electron transport in remotely-doped wide({dollar}>{dollar}2000 A) parabolic GaAs/Al{dollar}sb{lcub}rm x{rcub}{dollar}Ga{dollar}sb{lcub}rm 1-x{rcub}{dollar}As wells at low temperatures (to 50 mK) and in strong magnetic fields (to 11 T). Data from three samples are shown which demonstrate that the electron gas layers in these wells are wide (up to 2000 A), of variable width, have high mobilities (to {dollar}sim{dollar}3 {dollar}times{dollar} 10{dollar}sp5{dollar} cm{dollar}sp2{dollar}/V-s), and are of approximately uniform three-dimensional density n{dollar}sb{lcub}rm 3D{rcub}{dollar} {dollar}sim{dollar}1 {dollar}times{dollar} 10{dollar}sp{lcub}16{rcub}{dollar}cm{dollar}sp{lcub}-3{rcub}{dollar}. These data also demonstrate that the 3D densities obtainable are below the Mott critical density n{dollar}sb{lcub}rm c{rcub} sim{dollar} 1.5 {dollar}times{dollar} 10{dollar}sp{lcub}16{rcub}{dollar} cm{dollar}sp{lcub}-3{rcub}{dollar} for the metal-insulator transition in doped bulk n-type GaAs, making wide parabolic GaAs/Al{dollar}sb{lcub}rm x{rcub}{dollar}Ga{dollar}sb{lcub}rm 1-x{rcub}{dollar}As wells (WPBWs) an attractive system for observing electron-electron interactions at low densities.; The parabolic wells of the MBE-grown WPBW samples were grown by the digital alloy technique. A series of three samples was prepared with a range of 3D densities spanning the critical density n{dollar}sb{lcub}rm c{rcub}{dollar} for doped GaAs: n{dollar}sb{lcub}+{rcub}{dollar} = 2.5 {dollar}times{dollar} 10{dollar}sp{lcub}16{rcub}{dollar}cm{dollar}sp{lcub}-3{rcub}{dollar}, n{dollar}sb{lcub}+{rcub}{dollar} = 7 {dollar}times{dollar} 10{dollar}sp{lcub}15{rcub}{dollar}cm{dollar}sp{lcub}-3{rcub}{dollar}, and n{dollar}sb{lcub}+{rcub}{dollar} = 5 {dollar}times{dollar} 10{dollar}sp{lcub}15{rcub}{dollar}cm{dollar}sp{lcub}-3{rcub}{dollar}. The measured temperature dependences from 300 K to 4 K of the Hall sheet density and mobility for all three samples are very similar and show no qualitative changes with decreasing n{dollar}sb{lcub}+{rcub}{dollar}. The sheet densities are temperature-independent below T = 70 K and the mobilities {dollar}mu{dollar} all approach plateaus with {dollar}mu >{dollar} 2 {dollar}times{dollar} 10{dollar}sp5{dollar}cm{dollar}sp2{dollar}/V-sec below T {dollar}sim{dollar} 30 K. The T = 50 mK Shubnikov-de Haas (SdH) resistance oscillations in low transverse magnetic fields ({dollar}<{dollar}0.3 T) show structure due to multiple occupied subbands. Fourier transforms of the SdH data show well defined peaks which determine the subband sheet densities n{dollar}sb{lcub}rm si{rcub}{dollar} and the energy splittings E{dollar}sb{lcub}rm F{rcub}-{dollar}E{dollar}sb{lcub}rm i{rcub}{dollar}. These quantities agree well with self-consistent calculations for all three samples, evidence that the actual electron layer widths and 3D densities are close to the design values.; Preliminary magnetoresistance measurements with fields (0 to 11 T) applied in the plane of the electron layer and parallel (R{dollar}sb{lcub}rm par{rcub}{dollar}) or perpendicular (R{dollar}sb{lcub}rm perp{rcub}{dollar}) to the current are shown for two samples over the temperature range 0.42 K {dollar}<{dollar} T {dollar}<{dollar} 4.2 K. Near B {dollar}sim{dollar} 1 T we observe peaks in R{dollar}sb{lcub}rm par{rcub}{dollar} at the same field positions as dips in R{dollar}sb{lcub}rm perp{rcub}{dollar}. These features seem to correlate with the depopulation of the subbands but are not understood; a possible explanation is the formation of a spin density wave state, although a single particle explanation appears likely.