Electronic transport of low dimensional holes in induced p-type GaAs devices

摘要

In this thesis we study low dimensional hole systems fabricated on GaAs/AlGaAsheterostructures without any modulation dopants. The electrical transports of holes in twodimensions (2D), one dimension (1D) and zero dimension (0D) are studied. Two types of fieldeffect transistors are fabricated and studied. The first type is a semiconductor insulatorsemiconductor field effect transistor (SISFET) in which 1D hole wires and a hole quantum dotare studied. The second type is a metal insulator semiconductor field effect transistor(MISFET) in which devices with the ability to switch the type of charge carriers in theconduction channel between electrons and holes are fabricated (ambipolar devices).The 1D hole wires are fabricated on the crystal plane of (100). The 1D hole wires showstrong Zeeman splitting when the in-plane magnetic field is applied parallel to the 1D wires,and very small Zeeman splitting when the in-plane magnetic field is applied perpendicular tothe 1D wires, regardless of the crystallographic orientation ([ 011] or [011 ̅] ). This effect isdifferent compared to 1D hole wires fabricated on the crystal plane of (311)A, where there isan interplay between anisotropies due to the low crystal symmetry and 1D confinementresulting in different Zeeman splitting measured in wires oriented in differentcrystallographic orientations.We then move onto the fabrication and study of a single hole transistor in a GaAs/AlGaAsheterostructure. The Coulomb blockade oscillations resulting from single hole chargestunnelling on/off the quantum dot are observed and we also measure the charging energy ofthe quantum dot with source-drain bias spectroscopy. The quantum dot is found to be morestable and has less electrical noise compared to a single electron transistor fabricated onsilicon, and compares favourably with an electron quantum dot fabricated on a GaAs/AlGaAsheterostructure with modulation dopants.We also fabricated the first ambipolar devices on a GaAs/AlGaAs heterostructure with theMISFET design and characterised these devices at low temperatures. Firstly the chargetransport of electrons and holes are compared directly in a 2D ambipolar device bymeasuring the density dependence of the carrier mobility. It is observed that the electronmobility can be modelled with charge scattering theories taking into account backgroundimpurity scattering, interface roughness scattering and screening. However, when the samefitting parameters are used to model the hole mobility by switching the effective mass ofelectrons to holes, the theory cannot fit the experimental hole mobility. Several possibilitiesfor the deviation of the calculations are discussed but further experimental and theoreticalworks need to be conducted in order to determine the exact cause for this deviation.Finally we characterised ambipolar 1D wires where both electrons and holes can bemeasured. Ballistic transport of both types of charge carriers is observed and we comparethe 1D subband spacings of electrons to holes.