There are essentially two elements to this thesis, which investigates aspects of spin transport on the nanoscale. The first considers structural percolation and the ferromagnetic transition in Fe/GaAs. The second is an exploration of novel spin transport experiments. This has taken in investigations of spin-polarized scanning tunnelling microscopy, spin polarized electroluminescence and spin transport in quantum conductance in Cu nanowires. The growth of Fe on the GaAs(100)-2x6 reconstruction is studied using scanning tunnelling microscopy at room temperature. This reveals that the structural percolation occurs at a nominal thickness of 3 ML, thermal effects means that the ferromagnetic transition does not occur at room temperature until 4 ML of Fe have been deposited. This reveals that it is a combination of structural and thermal effects rather than magnetically dead layers that cause the delayed onset of ferromagnetism in Fe on GaAs. The exploration of novel spin transport effects had mixed success. Several methods of spin-polarized scanning tunnelling microscopy were tried, but no spin dependent contrast was seen. The efficiency of the spin injection process from an Fe film into GaAs was investigated, by measuring the degree of circularity polarization of electroluminescence from a buried quantum well. This method has yielded promising initial results, we have measured a net polarization from an Fe film and no polarization from a Au film. The quantum transport in Cu nanowires was investigated. The conductance of the Cu nanowires was measured by a break junction approach - tapping macroscopic wires together. This was done in the presence and absence of a magnetic field - which revealed a significant magneto-conductance effect which is totally unexpected for Cu.

Pulsed terahertz (THz) systems for spectroscopy and imaging are traditionally based on Ti:Sapphire femtosecond laser excitation, with low-temperature (LT)-GaAs photoconductive devices. A number of designs of GaAs photoconductive antennae were characterised in this thesis using 800 nm laser excitation. The THz power emitted was investigated as a function of specific antenna geometry, laser excitation position, laser power, and applied bias. The electric field profile of the devices under bias was investigated by electron beam induced current. These measurements give an indication of the device electric field profile, and can be applied to both pulsed and continuous-wave THz (cw-THz) antennas. There are, however, considerable advantages in moving to longer wavelength excitation. These include a step towards the optimum wavelengths for fibre-optic laser delivery, and potential reductions in the cost of the laser technology. In this thesis, the operation of a pulsed THz imaging system that uses, for the first time, a 1.06 <i>m</i>m femtosecond laser, and photoconductive devices fabricated from LT-In_0.3Ga_0.7As is demonstrated. Biological and non-biological images are presented to show the many potential applications of THz radiation, and the technology compared to existing 800 nm based systems. It is found that <i>ex-situ</i> post growth annealing plays a critical role in determining the properties of LT-InGaAs, and that material with a sub-500 fs carrier lifetime, and superior resistivity can be obtained. The dependence of the carrier lifetime and resistivity on growth and anneal temperature are discussed in detail. To characterise the material, x-ray diffraction is used to identify the presence of defects or traps. Time-resolved photoreflectance shows how the carrier lifetime varies, and electrical measurements are used to give the resistivity of the material. It was demonstrated that upon a mid-temperature anneal (400-600°C) the carrier lifetime actually falls, whilst the resistivity increases.

Molecular Beam Epitaxy (MBE) is a growth technique which allows a high level of control over the composition of single crystal epilayers deposited on a suitable single-crystal substrate. MBE growth of GaAs/AlGaAs heterostructures allows abrupt, defect-free interfaces to be fabricated, which, combined with modulation doping, provides a means of creating high mobility two-dimensional electron gases (2DEGs) at the heterointerface. This thesis is primarily concerned with the control and assessment of the level of disorder in the heterostructure. Two types of disorder over which the grower has little <i>direct</i> control have been considered. These are the interface morphology and the unintentional background impurities incorporated in the epilayer. The low temperature 2DEG mobility over a range of carrier densities is measured to quantify the level of disorder. The reliability and reproducibility of this measurement is discussed here in some detail. In order to compare different structures illumination should be used to vary the carrier density rather than a surface gate due to variability introduced by the device processing. However, illumination above the GaAs band-gap alters the depletion field which is shown to affect the scattering probability. This must be taken into account when determining scattering parameters. The effect of the anisotropic interface morphology on the 2DEG mobility is investigated. There is a large difference in mobility between the [110] and [?10] current flow directions, especially for high mobility samples. Interface roughness is also assessed by atomic force microscopy and low temperature photoluminescence. Reduction of the growth temperature from 585°C to 450°C increases the interface roughness, but reduces the anisotropy. Addition of atomic hydrogen over this temperature range did not result in any improvement in morphology. In a relatively low mobility regime the mobility is shown to be limited by background impurities in the undoped Al_xGa_l-xAs spacer, evaluated as 1.5x x 10¹⁶cm^-3.

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Here we present the first time measurements in two-dimensional (2D) hole gases of the intrinsic photo-induced anomalous Hall effect (Pl-AHE) and the circular photogalvanic effect (CPGE) in perpendicular magnetic field. Both these spin-transport effects are manifested in systems with broken space-time inversion symmetry. Their underlying physics may be explained in terms of the Berry phase in momentum space or by using symmetry considerations. We have used a number of techniques to characterise the samples and to study the spin-transport: magneto-transport measurements, such as Shubnikov - de Haas and quantum Hall revealed the samples' mobility and concentration; polarised and unpolarised photoluminescence and photoluminescence excitation at zero magnetic field gave information about energy levels in the quantum wells so that the samples could be resonantly excited, and in magnetic field these latter magneto-optical measurements were directly compared to the spin-transport data. To measure the photoexcited spin-dependent voltages, samples were illuminated with circularly polarised light which created a nonequilibrium spin-polarisation. The ion was modulated using an electro-optical modulator and the resulting voltage from the samples was detected in phase with the modulation signal. This voltage was then measured as a function of magnetic field, laser helicity and laser power density, sample temperature, current through the sample, etc. This study has shown that at zero magnetic field no intrinsic photoinduced anomalous Hall effect is measurable (within the measurement error, under interband excitation), but that a photoinduced spin dependent voltage is detectable in the perpendicular magnetic field. The measured voltage starts to be pronounced above 1T. The effect has also been found to exist only in 2D hole gases grown on low symmetry crystallographic planes, such as (311).

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