Lateral and Longitudinal Surface Superlattices on Shallow GaAs Heterostructures

[摘要] Longitudinal and lateral surface superlattices were fabricated on GaAs heterostructures. Most devices were made on shallow materials to exploit the proximity of the two dimensional electron gas to the surface, although some devices were fabricated on deeper material to compare their behaviour with the results obtained from devices fabricated on shallow material. The superlattices were fabricated on Hall-bars enabling four probe measurements to be made. Measurements were made at temperatures of 20 K, 4.2 K or 1.5 K depending on the requirement of the experiment and the measurement system used. Longitudinal superlattices were fabricated using negative resist techniques. In this process a periodic array of resist strips was deposited along the channel of a Hall- bar parallel to the direction of current flow. An overlying Schottky gate was deposited on the array. Bias was applied to confine the electrons electrostatically under the resist. As negative bias is further increased the channels of electrons become squeezed and should show evidence of one dimensional quantum confinement, although no evidence of one dimensional confinement was seen in the samples fabricated in this work. The values of negative voltage which define the points at which electrons are depleted from the ungated areas (the cut-off voltage) and gated areas (the threshold voltage) are compared to theory. It was found that, due to complicated factors inherent in the fabrication process, the data did not agree well with models which predict the cut-off voltage. Lateral surface superlattice samples were fabricated using positive resist. This technique leaves strips of metal which cross the Hall-bar channel perpendicular to the direction of current flow. Superlattices were fabricated with periods down to 60 nm, the smallest period yet reported. Using lateral superlattices allows an estimation of the potential in the 2DEG induced by the surface gate, from the analysis of a series of oscillations in the longitudinal magneto-resistance measurement which are known as commensurability oscillations. The variation of this induced potential with gate bias was also studied. This was found to vary depending on the type of barrier material used. Shallow AlAs barrier material has a layer of screening electrons around the donors. This screening layer reduces the induced potential in the 2DEG. This layer of screening electrons is not present in shallow AlGaAs barrier material so larger induced potentials are observed using these samples. A secondary aim, measuring the smallest period superlattices, was to enter a purely quantum regime where new physical effects are predicted. The devices with the smallest periods did not show any evidence of quantum effects and the possible reasons for this absence are discussed. After the failure of the longitudinal negative resist samples to give consistent values of cut-off voltage, a third type of gate structure was fabricated, called a finger superlattice. This finger structure is similar to a lateral superlattice but was designed so that the strips of metal did not completely cross the channel, which allows access to the bulk regions between the metal fingers. These samples were fabricated using positive resist techniques and were used to compare the cut-off voltages with two theoretical models. The theoretical models differ in the boundary conditions assumed at the ungated areas, which lead to quite different results. The longitudinal superlattices did not give conclusive evidence of which model was correct because of the unquantifiable effect of gate bias through the resist ribs. The finger superlattices were fabricated to overcome this problem, and offer strong evidence that one model, the 'frozen surface' model, correctly describes the behaviour of ungated GaAs at low temperatures.

[发布机构] University:University of Glasgow

[关键词] Condensed matter physics