This dissertation reports on the properties of a two-dimensional electron system in the presence of charge which is localised in the close vicinity. This is realised in a GaAs/Al_0.33Ga_0.67As semiconductor heterostructure containing a layer of InAs self-assembled quantum dots. A very important and unique feature of this material system is that the population of excess electrons stored in the quantum dots can be reduced via recombination with photo-excited holes. It may also be restored via the application of an appropriate front gate voltage. The presence of an AlGaAs barrier layer ensures that a population of electrons in the quantum dots is persistent. The results of experiments which are performed over a wide range of carrier concentrations in the 2DEG are presented, yielding insight into the transport mechanisms in both the diffusive and strongly localised regimes. In the diffusive transport regime, a treatment of the quantum dots as remote Coulomb scattering centres with a variable charge occupancy, allows a measurement of the contribution to the scattering exclusively from the dots themselves. When the quantum dots are large, the treatment of the dots as scattering centres breaks down and strong localisation is observed. In this insulating regime a transition from variable range hopping to weak localisation can be observed as a consequence of removing electrons from the quantum dots. Samples which are fabricated from this system and are of a length scale which is comparable to the average hopping length, are sensitive to single-photons. By removing an electron from an individual quantum dot, an incident photon modulates the transmission through a tunnelling barrier which is created by the repulsive potential from this dot. This results in a detectable conductance change. The statistics of such events reveal the transport mechanisms in a mescoscopic sample over three orders of magnitude in conductance as the localisation potential is tuned site-by-site, during a single experiment.

This thesis comprises two distinct pieces of work. The first piece involved the fabrication and study of induced one-dimensional hole gas. The second is based on molecular electronics and consisted of the development of nanogaps and measurements of nanocrystals and molecules using the gaps. GaAs/AlGaAs heterostructure was used to create the 1D hole gas. A 2D hole gas can be induced at the GaAs/AlGaAs interface by applying a negative voltage to a surface gate. Changing the gate voltage changes the density of the holes in the 2D system. At low densities, interaction effects become more prominent. The effective mass of holes is about eight times that of electrons in GaAs. Therefore holes are much more sensitive to interaction effects and may exhibit interesting behaviours not seen in electron systems. Unfortunately the quality of wafers were not good enough. Although induced 1D hole devices have been fabricated, they could not give us new insight into physics. For the molecular electronics project a technique was developed to fabricate nanogaps of size 5 nm. These gaps have been used to study molecules and nanocrystals. Both single-particle energy levels and Coulomb charging effects are important in affecting the tunnelling spectra of the nanocrystal devices. Measurements on molecular devices have proved less successful. The molecular systems are very complex, and hold great potential for device applications. The nanogaps developed can be used to measure various nano-elements, the only requirement on the elements being the ability to self-assemble a monolayer on gold surface, which is the way nano-elements are incorporated into the gaps.

This dissertation considers the physics of electron transport in novel, nanometer scale semiconductor pillar arrays, where conduction can be controlled by strong surface charging and single electron charging effects. Densely packed arrays were fabricated with a novel natural lithography process where a thin granular metal film acts as a mask for dry etching of double barrier heterostructure (DBHS) and single barrier heterostructure (SBHS) GaAs/AlAs material. A 3nm-7nm thick Au, AuGe, AuPd or Sn film thermally evaporated on GaAs consists of discrete grains less than 50nm in lateral size. Reactive ion etching of the film creates a GaAs pillar array where the individual pillars are 10nm-50nm in diameter. The array packing density is ˜60% over the entire area of the thin film. The pillars were individually characterised with a scanning tunnelling microscope (STM). Single pillar contact measurements demonstrate that each pillar acts as a novel transistor with strong gating of resonant tunnelling peaks in the I-V characteristics. This behaviour can be explained by considering the charge trapped in surface states along the pillar. The surface charge creates an asymmetric potential barrier in the conduction band which gates resonant tunnelling through the energy levels of the DBHS and the energy levels of a bias dependent potential well created by the barrier itself. The pillars were collectively characterised using 'multiple pillar' devices. In DBHS devices, single electron charging effects were observed up to 60K and clear Coulomb staircases up to 35K. Unusual bistable switching behaviour was also observed up to room temperature, where the device could be cycled between a high conductance and low conductance state. The bistability can be explained by considering the device conductivity when the surface states are mostly empty or filled. Empty states trap electrons injected from the contacts and the conductivity is low. With mostly filled states, electrons can move in the conduction band without trapping and the conductivity is high. The DBHS resonant level may switch the device between the bistable states.

Selective area growth of III-V semiconductor compounds using molecular beam epitaxy (MBE) has been envisaged as an <I>in situ</I> fabrication method for integrated circuits on a nanometer scale. However, conventional selective area growth techniques using MBE are limited to only two dimensional, template-like, pattering of the epilayer. The work presented in this dissertation describes the selective area growth of AlGaAs based structures using a Ga<SUP>+</SUP> focused ion beam (FIB) as one of the group III matrix element sources in a MBE growth chamber. Since stoichiometric epitaxy of a III-V semiconductor compound can be achieved with an excess supply of the group V element, supplying the Ga matrix element as a FIB, under standard MBE growth conditions, was shown to facilitate a maskless, <I>in situ,</I> lateral selective area growth technique for GaAs. Consequently, this FIB-MBE growth technique, FIMBE, has the potential of exploiting the precise control over the elemental composition afforded by MBE in the growth (<I>z</I>) direction with the high spatial resolution of FIB technology in the lateral (<I>xy</I>) plane. Moreover, it offers the unique facility of growing fully integrated three-dimensional structures into one as-grown epilayer structure. The necessary modifications required to a standard FIB column and MBE growth chamber to fully exploit the combination of these two technologies, along with the operational performance of the fully integrated FIMBE growth system are presented. A study of the effect of incident ion energy (E<SUB>ion</SUB>) on the film growth rate identified two growth rate limiting processes; (i) the inherent properties of the Ga<SUP>+</SUP> FIB (E<SUB>ion</SUB> <25eV) and (ii) material sputtering from the growing GaAs film (E<SUB>ion</SUB>>100eV). However, a systematic reduction in the surface roughness of the FIMBE grown GaAs films was observed with increasing incident ion energy.

This thesis focuses on the electronic and photophysical phenomena that occur at the heterojunction between two distinct organic semiconductor materials. The hetero junction luminescence is correlated to the photovoltaic performance in a polymer blend diode. Polymer semiconductor blends allow efficient operation of photovoltaic diodes when there is a large interfacial area of hetero junction between electron donor and acceptor polymers. In this thesis, we use electromodulation spectroscopy to investigate the luminescent and photovoltaic behaviour of electron- and hole-transporting polyfluorenes blends at a broad range of blend ratios, temperatures and electric fields. In the systems investigated, an exciton at the hetero junction produces either free charges or an exciplex (or a similar interfacially-bound charge-transfer pair). We find that an externally-applied electric field increases the number of free charges and quenches the exciplex luminescence with a one-to-one correspondence: the increase of the photocurrent internal quantum efficiency is equal to the reduction in the exciplex emission. We conclude that, independent of temperature and morphology, the photovoltaic quantum yield is predominantly limited by the dissociation of the geminate electron-hole pair intermediate at the hetero junction. Once the charges are fully separated they are transported across the material and collected at the external circuit with nearly unit efficiency.

No description available.

Off-axis electron holography promises to fulfill the requirements of the semiconductor industry for a technique that can be used to provide quantitative information about dopant potentials in semiconductors with nanometer spatial resolution. The technique involves using an electron biprism in a transmission electron microscope (TEM) to interfere a coherent electron wave that has passed through a specimen with a reference wave that has passed only through vacuum. The focused-ion-beam (FIB) miller is the preferred method of sample preparation for semiconductor TEM analysis. In the FIB, a 30kV Ga ion beam is used to thin the specimen in the region of interest. It is in­creasingly recognised that this method of TEM specimen preparation and subsequent treatments have a profound influence on the phase shifts measured from doped semiconductors. In addition to the effect on the phase shift of surface depletion resulting from the presence of the specimen surfaces, the electrostatic potential in the specimen may be affected by the combined effects of oxidation, physical damage and the implantation of ions such as Ar and Ga during preparation of the sample for electron microscopy, as well as by the effects of irradiation by high-energy electrons during examination in the TEM. Semiconductor specimens have been prepared for examination using electron holography by combining advanced sample preparation techniques such as annealing and Ar ion milling in order to assess the effect of these treatments on the measured phase shifts across the junctions.

This dissertation is concerned with the fabrication and physics of Molecular Beam Epitaxy (MBE) regrown semiconductor devices. I use the term regrown to refer to a structure which has been fabricated by MBE growth over a surface which has been <I>ex-situ </I>patterned. The structures presented in this dissertation can be subdivided into two broad categories: the narrow facet channels and the patterned back-gates. The narrow facet channels form part of a new class of devices, where the amphoteric nature of silicon dopant in the AlGaAs lattice is used to create lateral confinement. Growth of a GaAs A1GaAs heterostructure over a (311)<I>A</I> substrate, which is etched to expose a narrow (100) facet, results in the formation of a narrow two dimensional electron gas (2DEG) on the facet which is interspersed between two, two dimensional hole gases (2DHGs). The transport characteristics of the narrow 2DEG can be modulated by applying voltage to the adjacent hole gases, i.e. biasing either or both of the two dimensional p-n junctions formed between the 2DEG and 2DHGs. The actual width of the electron channel was found to be considerably smaller that the lithographic facet width. Theoretical modelling showed that this was a result of the fabrication and growth of the sample and not the electro-statics of the system. The inverse structure of a narrow 2DHG interspersed between two 2DHGs was also fabricated. The transport through the narrow 2DHG could be modulated in a similar manner. The patterned back-gated structures are the simplest illustration of the fabrication technique and are discussed first in the dissertation. The patterned back-gate is formed as islands of n<SUP>+</SUP> GaAs on an undoped GaAs substrate. Subsequently grown continuous layers are therefore guided in three dimensions over a wafer surface with regions of differing semiconductor compositions. This is a wafer scale fabrication technique, which completely dismisses the need for complicated lithography. The patterned back-gate was used to distinguish between the presence of an extended state formed between two 2DEGs and a second subband formed in one of the 2DEGs. The second subband was then used as a probe to characterise the regrowth interface. The series of back-gated long split gates showed quantisation plateaux illustrating the high quality of the samples fabricated using the regrowth technique.

This dissertation describes a project that brought together the physics of low-dimensional devices with the technology of low-temperature scanning probes. The low-dimensional devices were single or double quantum wires where electron motion is effectively restricted to a single dimension. Initially, the scanning probe functioned as an atomic force microscope (AFM) to locate a quantum wire. Without moving the AFM tip, the circuitry was changed to convert the scanning probe into a scanning charged tip. As the charged tip scanned over the device surface, the resistance of the device was recorded to build up an image. When the charged tip scanned over the quantum wire, the device resistance deviated. The images revealed broad structure which measured the electric potential of the tip itself, with additional small scale structure when the tip was positioned over the centre of the quantum wire. The small scale structure is a representation of quantum-mechanical electron modes resulting from modulations to the 1D eigenenergies. This experiment show electron density through low-dimensional devices. Both ends of the quantum wire were connected to a two-dimensional electron system (2DES) from which the resistance measurements were taken. When the tip scanned the 2DES region just outside the quantum wire, the tip's electric field scattered electrons. A small proportion of the electrons injected out of the quantum wire into the 2DES were scattered back through the quantum wire. This process is known as backscattering, which reduces the net electron flow through the quantum wire and increases the device resistance. The images revealed cones emanating from the quantum wires, to provide information on the angular distribution of injected electrons.

Recent advances in epitaxial growth technology have made the formation of high-quality, strained-layer heterostructures in the silicon-germanium material system possible. This thesis presents an overview of a range of low-temperature measurements of some of these structures. As the materials are relatively new, the processing techniques for making samples are not well-established, so the thesis discusses the methods used, and in particular, it describes a variety of attempts to fabricate gated devices. Transport measurements of a high-mobility two-dimensional hole gas at low temperatures are described and analysed. Effective mass, quantum lifetime and phase-coherence times are extracted, with the temperature-dependence of the latter following a power-law, the exponent of which indicates a relatively clean system. This exponent predicts the scaling exponents around quantum Hall effect to Hall insulator transitions which are also measured. Screening is shown to cause a strong temperature-dependence of the conductivity. Transport measurements of ungated and Schottky-gated samples of high-quality two-dimensional electron gases at low temperatures are also presented. General features are discussed, including a strong overshoot associated with odd-numbered Hall plateaux and an accompanying asymmetry in the valley-spilt Shubnikov-de Haas peaks. A possible explanation in terms of strong inter-valley scattering is put forward. A range of new behaviours is shown to arise when the carrier density is varied by means of a gate. An anomalous quenching of the valley splitting at a filling factor of 3, resistivity fluctuations at high fields and other effects are presented and discussed. Finally, the thesis describes far infrared measurements of a range of electron gas samples. Cyclotron resonance frequencies of ungated samples deviated from the expected proportionality to the magnetic field. This is explained as resulting from a disorder potential coupled with electron-electron interactions, leading to an apparent lateral confinement.