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Embargoed until 2013-02-28
Copyright: Chen, Jason Chao-Hsuan
Embargoed until 2013-02-28
Copyright: Chen, Jason Chao-Hsuan
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Abstract
In this thesis we study low dimensional hole systems fabricated on GaAs/AlGaAs
heterostructures without any modulation dopants. The electrical transports of holes in two
dimensions (2D), one dimension (1D) and zero dimension (0D) are studied. Two types of field
effect transistors are fabricated and studied. The first type is a semiconductor insulator
semiconductor field effect transistor (SISFET) in which 1D hole wires and a hole quantum dot
are 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 the
conduction 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 show
strong 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 to
the 1D wires, regardless of the crystallographic orientation ([ 011] or [011 ̅] ). This effect is
different compared to 1D hole wires fabricated on the crystal plane of (311)A, where there is
an interplay between anisotropies due to the low crystal symmetry and 1D confinement
resulting in different Zeeman splitting measured in wires oriented in different
crystallographic orientations.
We then move onto the fabrication and study of a single hole transistor in a GaAs/AlGaAs
heterostructure. The Coulomb blockade oscillations resulting from single hole charges
tunnelling on/off the quantum dot are observed and we also measure the charging energy of
the quantum dot with source-drain bias spectroscopy. The quantum dot is found to be more
stable and has less electrical noise compared to a single electron transistor fabricated on
silicon, and compares favourably with an electron quantum dot fabricated on a GaAs/AlGaAs
heterostructure with modulation dopants.
We also fabricated the first ambipolar devices on a GaAs/AlGaAs heterostructure with the
MISFET design and characterised these devices at low temperatures. Firstly the charge
transport of electrons and holes are compared directly in a 2D ambipolar device by
measuring the density dependence of the carrier mobility. It is observed that the electron
mobility can be modelled with charge scattering theories taking into account background
impurity scattering, interface roughness scattering and screening. However, when the same
fitting parameters are used to model the hole mobility by switching the effective mass of
electrons to holes, the theory cannot fit the experimental hole mobility. Several possibilities
for the deviation of the calculations are discussed but further experimental and theoretical
works 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 be
measured. Ballistic transport of both types of charge carriers is observed and we compare
the 1D subband spacings of electrons to holes.