Transport properties of GaAs/InGaAs double quantum wells and graded InGaAs heterostructures

In the first structure presented a single two-dimensional electron gas is positioned in a region of graded (0 ≤ x ≤ 0.1) InGaAs composition. Through a series of MBE grown wafers the technique of successfully growing InGaAs as an InAs/GaAs superlattice was demonstrated. Varying the period of the superlattice was used to achieve the graded InGaAs region in the final device design. The exchange-enhanced g-factor was measured via thermal excitation seen to increase with the application of positive back-gate voltages is. One-dimensional conductance is observed in a graded alloy system for the first time, a stepping-stone to the implementation of single electron devices. Through analysis of the low-field resistivity, the appearance of a second subband in the two-dimensional electron gas, attributed to the zero-field spin-splitting from the Rashba interaction, was seen, and a dependence on back-gate voltage observed. Gate-voltage control of the spin-orbit interaction has only previously been observed in much higher indium concentration samples. A second structure investigated consists of two two-dimensional electron gases and forms a new kind of double quantum well device. Two-dimensional electron gases are located in separate GaAs and InGaAs quantum wells, separated by an AlGaAs barrier. Devices presented in this thesis allow two different gating schemes to be investigated. Firstly large-area front- and back-gates allow the isolation of a two dimensional electron gas in either well. This means two-dimensional conduction can be limited to either the GaAs or InGaAs layer. Secondly through use of a split-gate midline device it is possible to select the conduction pathway through the device with quasi-one-dimensional channels. This technique uses surface gates only, and again it gives the ability to select the material composition in which the electron wavefunction is situated. Such a double quantum well system gives the possibility of investigating the effect of the local g-factor and spin-orbit coupling on various low-dimensional spin-related phenomena.EThOS - Electronic Theses Online ServiceGBUnited Kingdo