Ballistic transport in two-subband two-dimensional carrier systems at GaAs/AlGaAs heterojunctions

We have investigated ballistic transport phenomena of two-dimensional carrier systems that are confined to AlGaAs/GaAs heterostructures and occupy two electric subbands. Two types of two-subband systems, classified by the origin of their subband splitting, are studied in this thesis. They are the confinement-subband and spin-subband systems. The confinement-subband system studied here is a two-dimensional electron system (2DES) confined to a wide GaAs quantum well with two electric subbands occupied. As for spin-subband systems, a two-dimensional hole system (2DHS) is confined to either an AlGaAs/GaAs interface or a GaAs quantum well with a single occupied confinement-subband. The spin-degeneracy of the single confinement-subband is then lifted, at zero-magnetic field, by the crystal as well as the structural inversion asymmetry of the system. In this thesis, we report the first quantitative study of the 2DHSs evolving from spin-degenerate systems to spin-split systems. The measured spin splitting agrees reasonably well with the results of calculations that employ essentially no adjustable parameters.; We use both the transverse magnetic focusing (TMF) and commensurability oscillations (COs) measurements to study the ballistic transport properties of these two-subband systems in this thesis. We find that a standard TMF device is not able to detect the ballistic transport of upper subband electrons. By adding a tunable barrier, we are able to improve the energy resolution of the simple TMF device and use it to observe the USE ballistic transport. We then demonstrate that COs provide for a more powerful technique for studying ballistic transport. We are able to use COs measurement to deduce the ballistic scattering times for both the upper and the lower subband electrons. We find that the lower subband electrons have longer scattering time and we discuss the dominant scattering processes for both the upper and lower subband electrons. By studying the COs in a spin-subband system, we find clear evidence for spin-resolved ballistic transport.