It is predicted that vast volumes of impacted mine water will be produced by mining activities in the Mpumalanga coalfields of South Africa. The potential environmental impact of this excess water is of great concern in a water-scarce country like South Africa. Research over a period of more than 10 years has shown that this water can be used successfully for the irrigation of a range of crops (Annandale et al. 2002). There is however continuing concern from the local regulators regarding the long-term impact that large scale mine water irrigation may have on groundwater quality and quantity. Detailed research has been undertaken over the last 3 years to supplement the groundwater monitoring program at five different pilot sites, on both virgin soils (greenfields) and in coal mining spoils. These sites range from sandy soils to very clayey soils. The research has included soil moisture measurements, collection of in situ soil moisture over time, long-term laboratory studies of the leaching and attenuation properties of different soils and the impact of irrigation on acid rock drainage processes, and in depth determination of the hydraulic properties of the subsurface al each of these sites. including failing head tests, pumping tests and point dilution tests. This has been supported by geochemical modelling of these processes to quantify the impacts. The results indicate that many of the soils have considerable attenuation capacities and that in the period of irrigation, a large proportion of the salts have been contained in the upper portions of the unsaturated zones below each irrigation pivot. The volumes and quality of water leaching through to the aquifers have been quantified at each site. From this mixing ratios have been calculated in order to determine the effect of the irrigation water on the underlying aquifers.

It is predicted that vast volumes of affected mine water will be produced by mining activities in the Mpumalanga coalfields of South Africa, The potential environmental impact of this excess water is of great concern in a water-scarce country like South Africa. Research over a period of more than 10 years has shown that this water can be used successfully for the irrigation of a range of crops (Annandale et al., 2002)(1). There is, however, continuing concern from the local regulators regarding the long-term impact that large-scale mine water irrigation may have on groundwater quality and quantity. Detailed research has been undertaken over the last three years to supplement the groundwater monitoring programme at five different pilot sites, on both virgin soils (greenfields) and in coalmining spoils. These sites range from sandy soils to very clayey soils. The research has included soil moisture measurements, collection of in situ soil moisture over time, long-term laboratory studies of the leaching and attenuation properties of different soils and the impact of irrigation on acid rock drainage processes, and in depth determination of the hydraulic properties of the subsurface at each of these sites, including falling head tests, pumping tests and point dilution tests. This has been supported by geochemical modelling of these processes to quantify the impacts. The results indicate that many of the soils have considerable attenuation capacities and that in the period of irrigation, a large proportion of the salts have been contained in the upper portions of the unsaturated zones below each irrigation pivot. The volumes and quality of water leaching through to the aquifers have been quantified at each site. From this mixing ratios have been calculated in order to determine the effect of the irrigation water on the underlying aquifers.

In this study, high-resolution diffraction has been used to investigate the strain state and uniformity of (001) and (112) oriented HgTe-CdTe superlattices grown by molecular beam epitaxy. A number of reciprocal space maps were taken over the surface of the grown wafer, and variations in the spread of lattice spacings and tilts were quantified and used to identify the presence of local defects. Though all growths were fully strained, those with a larger mismatch exhibited a greater spread of lattice tilts from the substrate to the superlattice layers in both orientations. Further, the variation in composition of the CdZnTe substrates resulted in a variation of strain over the surface of the superlattice, and evidence of areas of significant dislocation densities present in substrate layer corresponded with defect-rich areas in the superlattice layer.

The dilute nitride GaAsN has been grown by MBE using an ECR nitrogen plasma source. This has allowed growth at a substrate temperature of 600 C, which in combination with an ion trap, has produced higher quality as-grown material. Layer chemistry has been assessed by SIMS, XRD and optical quality measured using photoluminescence.

Lattice mismatch between substrates and epitaxial layers of different mole-fractions can create a variety of distortions and defects in HgCdTe epilayers, thus degrading the performance of infrared detectors fabricated from this material. X-ray diffractometry is a sensitive non-destructive technique, which allows in-depth characterisation of the crystal lattice prior to detector fabrication. We present results of triple-axis diffractometry (TAD) performed on single and double layer HgCdTe films grown on (211)B CdZnTe substrates by molecular beam epitaxy (MBE). The positions of both surface-symmetric and asymmetric diffraction peaks have been used to extract lattice spacings parallel and perpendicular to the (211) growth direction. The unstrained lattice parameter of each epilayer has been calculated assuming that the layers are elastically strained. The low-symmetry of the (211) plane, coupled with the anisotropic elasticity of zinc-blende semiconductors, results in monoclinic distortion of the lattice, as observed in these samples. In double layer samples, the mosaicity of both layers is greater than that observed in single epilayers. Annealed samples show greater lattice distortion than as-grown samples

After the closure of collieries, they naturally start to fill up with water. As a result, hydraulic gradients develop between them and different hydraulic pressures are exerted onto peripheral areas or compartments within mines. This results in water flow between mines, or onto the surface. This flow is referred to as intermine flow.The collieries in the Witbank Coalfield have geometries such that there are several areas where this intermine flow is possible. Since the Department of Water Affairs and Forestry has declared intermine flow as one of its greatest concerns for granting closure to South African mines, much research into this phenomenon is required.The challenges in determining intermine flow are numerous, and attention has focused on identification of areas where these flows can take place. The quantification of these flows is problematic due to the uncertainties in exact geometric configurations and the variation in site-specific hydraulic properties of the coal and overlying lithilogical layers.During this study, use has been made of numerical flow modelling and several analytic solutions to test the applicability of the flow models as well as to predict groundwater flow directions, filling times of voids and flow volumes. The numerical modelling methodology entailed a downscaling approach starting with a broad regional model covering the entire area, followed by modelling the interactions between interconnected mines, and finally looking in detail at the major areas of interaction.

We have used synchrotron-based high-resolution core-level photoemission and valence-band emission measurements to characterize hydrogen chemisorption on nonpolar GaAs(110) and polar GaAs(100) and GaAs(001) surfaces. Chemisorbed atomic hydrogen forms both Ga-H and As-H bonds on all three surfaces, causing chemical shifts of core-level binding energies and changing the valence-band emission. For low hydrogen exposures arsenic desorbs from all surfaces. However, at higher exposures, the (110) surface transforms into a Ga-rich structure with traces of metallic Ga, while the (100) surface transforms into an As-rich structure. We have also observed some additional changes in the binding energy of bulk components of Ga and As core levels as a function of hydrogen exposure, which may be explained by hydrogen-induced changes in band bending.

Photon-stimulated desorption of H(+) from hydrogenated GaAs (110) and (100) surfaces was studied as a function of photon energy. Distinct peaks, observed around As 3d core-level binding energy for desorption from the GaAs (100) surface and in the As 3d and Ga 3p region for desorption from the GaAs (110) surface, show a striking similarity with the fine structure (spin-orbit splitting) measured in the photoemission from As 3d and Ga 3p levels. These results provide clear evidence for direct desorption processes and represent a basis for selective modification of hydrogenated GaAs surfaces.

The studies reported here present data obtained from ZnSe epilayers grown on GaAs substrates by MBE. The native oxide and carbon were removed from the substrate surface in two different ways, by thermally cleaning up to around 600°C and by exposing substrates at 450°C to atomic hydrogen. As passivated GaAs epilayers have also been used as substrates to grow ZnSe epilayers. SIMS profiles showed the existence of oxygen at the interface for all substrate preparation methods. Depth profiles obtained from SIMS show the expected mixing of the two materials at the interface and evidence of interdiffusion across the interface. SIMS measurements are difficult to quantify due to the variations in the ionisability of different atoms and the effect of different matrices. Depth profiles can be broadened by curved craters and by intermixing due to ion collisions. The range of layer thicknesses and of interface widths available in this study enable the broadening effects to be quantified so that the atomic distributions in the different interfaces can be obtained

Atomic resolution STM images of the (2 × 4) reconstruction, are presented which show the existence of at least four distinct surface structures. Two of these correspond to the α and β phases previously modelled from RHEED experiments. The second layer structure within the missing row of the α-(2 × 4) phase is clearly resolved in these images Evidence is presented of dimer twist as part of the surface energy minimisation process of one of the α phases. We also show images of a previously unreported structure that displays (2 × 4) symmetry.

X-ray reflection topography for imaging misfit dislocations in highly strained systems with a small critical thickness is assessed. The samples often contain complex heterostructures such as capping layers, quantum wells or superlattices. For the first time the strain within the heterostructure is taken into account explicitly into simulation of X-ray reflection topographs. A new algorithm to simulate such heterostructures is proposed on the basis of a local kinematical approximation within one step of the numerical integration of the Takagi-Taupin equations. Dramatic effects of the heterostructure configuration on the contrast of dislocation images are predicted by simulation, based on three effects: firstly the depth of the dislocation (referred to the free surface) determines the effect of surface relaxation on the distortion field; secondly the composition around the dislocation determines which part of the dislocation field takes part in image formation; thirdly interference between different layers with identical composition modulates the rocking curve and hence the resulting contrast. This is confirmed experimentally for two InxGa1-As-x/GaAs structures, a capped layer with growing cap thickness and a single epitaxial layer on a substrate with a buried multiquantum well.

A strained ${\mathrm{In}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$As layer is incorporated adjacent to the emitter barrier of an ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As double-barrier resonant-tunneling structure (DBRTS), so that it forms a prewell for the electrons that accumulate prior to tunneling into the GaAs quantum well (QW). Observation of photoluminescence (PL) and photoluminescence excitation (PLE) from the prewell then enables a direct optical determination of the charging behavior in the emitter-accumulation region to be achieved. We show in addition that an optical probe of the prewell can, by consideration of the electrostatics, provide a reliable determination of the charge distribution in the whole DBRTS at the peak of the tunneling resonance. These results are shown to be in agreement with separate determinations of the charge in the GaAs QW by direct PL measurements, and of the charge in the emitter-accumulation layer from magnetotransport studies. The electron density in the prewell can be varied continuously over a wide range, from 0 to 9\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. The second subband is populated at high applied bias, with the density varying from 0 to 1\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. This structure is then very well suited to the study of the many-body nature of the excitonic enhancement near the absorption threshold, over a much wider range of electron densities than previously studied on an individual sample. Utilizing temperature-dependent PLE measurements, we have monitored the variation from an atomic exciton in n=1, to a Fermi-energy edge singularity (FEES) in n=1, through to a FEES in n=2, in the same sample.

Images of misfit dislocations as obtained from X-ray reflection topography with a low incidence angle are simulated. New algorithms for integrating numerically the Takagi-Taupin equations are presented with decoupled, variable and adaptively chosen step sizes. This reduces drastically the computation time required. The adaptive integration is faster by a factor of ten or more over integration with fixed steps, is more accurate and requires a minimum of free parameters. Rocking topographs are introduced to show the contrast variation over the Bragg peak. It is shown that the contrast of dislocations parallel to the surfaces vanishes as the depth goes to zero, for any anisotropic medium and any Burgers vector.

The ability of x-ray topography to image individual dislocations makes it an ideal tool with which to investigate the initial stages of lattice relaxation in strained-layer semiconductors. Topographs provide a useful insight into the origin of the first misfit dislocations, which define the critical thickness, and make possible quantitative analysis of the initial strain relaxation process. Examples are given for the InxGa1-xAs/GaAs and In1-xAlxSb/InSb materials systems. In the second case the rate of increase in misfit dislocation density with layer thickness is substantially lower. This is believed to be due to the different substrate dislocation densities of GaAs (> 10(4) cm-2) and InSb (< 10(2) cm-2). The early success of these experiments has led to the development of an MBE facility which allows x-ray topography during the growth and post-growth processing stages of the materials. The system is described and the potential benefits over ex situ studies are discussed.

Angle-resolved photoemission techniques have been used to study the occupied band structure of the As-terminated GaAs(001)-1 X 1 surface grown by molecular-beam epitaxy. The measurements were taken along the [100] azimuth and covered the photon-energy range 10-70 eV. Structure plots involving linear-muffin-tin-orbital (LMTO) calculated valence bands and free-electron-like final states (FELFS's) were used to interpret the acquired data, which included normal-emission spectra at various photon energies and off-normal-emission spectra at selected photon energies. A structure plot of initial energy (E(i)) versus polar-emission angle (theta) was employed to interpret the two-dimensional electronic structure E(i)(k(i)parallel-to, k(i)perpendicular-to) observed from off-normal-emission spectra, where k(i)parallel-to and k(i)perpendicular-to are, respectively, the parallel and perpendicular components of the wave vector of the valence electrons to the sample surface. It was found that the majority of strong transitions could be explained as direct transitions from LMTO valence bands to primary or secondary cones of FELFS's. The FELFS model was found to have mixed success over the entire photon-energy range investigated. We show, in particular, that the assignment of transitions can be misleading if only normal-emission spectra are considered.

Angle-resolved photoemission measurements from normal-emission spectra have been used to determine the strain-induced changes in shape of the valence bands of InxGa1-xAs alloys with In concentrations of 10%, 20%, 27%, and 30%. The data have been analyzed using an iterative technique in which the experimentally determined bulk bands are fitted by polynomials. Differences between the strained and unstrained bands show k-dependent shifts that are similar in all alloys studied and whose sign and magnitude have been reproduced in the calculations. We observe an increasing strain-induced shift with concentration as reported but the shift is of the opposite sign to that previously reported by other investigators.

We report results from an angle-resolved photoemission study of the electronic structure of the short-period superlattice (GaAs)2/(AlAs)2. These results support the first-principles linear-muffin-tin-orbital-method calculation of Gopalan et al. In particular, a 0.5-eV band pp caused by the reduced periodicity in the [001] growth direction is clearly seen at a binding energy (relative to the valence-band maximum) of 5.3 eV.