non-UHV Environment Research Articles

Crystal quality improvement of solid-phase crystallized evaporated silicon films by in-situ densification anneal

An in-situ densification anneal at 550 °C was applied to e-beam evaporated a-Si films on Si (1 0 0) wafers in a non-UHV environment. The c-Si films were then obtained by annealing the as-deposited a-Si films at 575 °C in a tube furnace. It is shown that the crystal quality of the c-Si films obtained from low-rate (50 nm/min) evaporated a-Si film is considerably improved by the densification anneal, whereas densification has no beneficial effect on c-Si films obtained from high-rate (300 nm/min) evaporated a-Si. However, the improvement of c-Si obtained from low-rate evaporated a-Si is not due to a significant difference in the oxygen content in the films. It is suggested that the improvement of the c-Si films is related to structural relaxation induced by the densification anneal of low-rate evaporated a-Si.

Homoepitaxial silicon growth in a non-ultra-high vacuum environment by ion-assisted deposition on Si wafer and seeded glass substrates

Eliminating the requirement of ultra-high vacuum (UHV) conditions and achieving high-rate crystalline silicon (c-Si) growth are important targets for cheap mass production of semiconductor devices such as thin-film solar cells. In this paper we report on the achievement of high-quality, high-rate, epitaxial Si growth in a non-UHV environment by ion-assisted deposition (IAD) on (100) Si wafer substrates as well as on glass substrates featuring a polycrystalline Si seed layer. This was achieved by high-rate growth to suppress the contamination in the growing Si film and by a sacrificial protective layer which protects the initial growth surface prior to start of epitaxy. On (100) Si wafer substrates good structural quality of the epitaxially grown Si films was achieved, resulting in 1-Sun open-circuit voltages of up to 550 mV. On planar seeded glass substrates the heating procedure had to be modified to take the large thermal mass and the thermal stress caused by the underlying glass substrate into account. It is shown that the pre-heating phase must be extended by more than a factor of 4 and that high growth temperatures have to be avoided to minimise thermal stress. Taking these considerations into account and using two post-deposition treatments (rapid thermal annealing and hydrogenation), open-circuit voltages of 420 mV and short circuit-current densities of about 10 mA/cm 2 under 1-Sun illumination are achieved on glass substrates.

High-quality homo-epitaxial silicon growth in a non-ultra-high vacuum environment by ion-assisted deposition

Eliminating the requirement of ultra-high vacuum (UHV) conditions and achieving high-rate crystalline silicon (c-Si) growth are important targets for cheap mass production of semiconductor devices such as thin-film solar cells. In this paper, we report on the achievement of high-quality, high-rate, epitaxial Si growth on Si wafer substrates in a non-UHV environment (base pressure about 1×10 −8 Torr) at low temperatures (440–830°C) by ion-assisted deposition. Impurity contamination in the growing Si film is reduced to tolerable levels by high-rate growth, whereas substrate surface related contamination problems are controlled by the use of a sacrificial protective surface film in combination with a carefully optimised sample heating procedure. Two types of removal of the sacrificial layer from the substrate surface (a high-temperature method and a low-temperature method) are investigated. Thin-film solar cells epitaxially grown on (1 0 0)-oriented Si wafer substrates are used as test vehicles to demonstrate that both substrate surface treatments enable the growth of device-grade Si material in a non-UHV environment.

Oxide surfaces and metal/oxide interfaces studied by grazing incidence X-ray scattering

Experimental determinations of the atomic structure of insulating oxide surfaces and metal/oxide interfaces are scarce, because surface science techniques are often limited by the insulating character of the substrate. Grazing incidence X-ray scattering (GIXS), which is not subject to charge effects, can provide very precise information on the atomic structure of oxide surfaces: roughness, relaxation and reconstruction. It is also well adapted to analyze the atomic structure, the registry, the misfit relaxation, elastic or plastic, the growth mode and the morphology of metal/oxide interfaces during their growth, performed in situ. GIXS also allows the analysis of thin films and buried interfaces, in a non-destructive way, yielding the epitaxial relationships, and, by variation of the grazing incidence angle, the lattice parameter relaxation along the growth direction. On semi-coherent interfaces, the existence of an ordered network of interfacial misfit dislocations can be demonstrated, its Burger's vector determined, its ordering during in situ annealing cycles followed, and sometimes even its atomic structure can be addressed. Careful analysis during growth allows the modeling of the dislocation nucleation process. This review emphasizes the new information that GIXS can bring to oxide surfaces and metal/oxide interfaces by comparison with other surface science techniques. The principles of X-ray diffraction by surfaces and interfaces are recalled, together with the advantages and properties of grazing angles. The specific experimental requirements are discussed. Recent results are presented on the determination of the atomic structure of relaxed or reconstructed oxide surfaces. A description of results obtained during the in situ growth of metal on oxide surfaces is also given, as well as investigations of thick metal films on oxide surfaces, with lattice parameter misfit relaxed by an array of dislocations. Recent work performed on oxide thin films having important physical properties such as superconductivity or magnetism is also briefly reviewed. The strengths and limitations of the technique, such as the need for single crystals and surfaces of high crystalline quality are discussed. Finally, an outlook of future prospects in the field is given, such as the study of more complex oxide surfaces, vicinal surfaces, reactive metal/oxide interfaces, metal oxidation processes, the use of surfactants to promote wetting of a metal deposited on an oxide surface or the study of oxide/liquid interfaces in a non-UHV environment.

UHV microscopy of surfaces

Results are reported using UHV electron microscopy to determine the role of background gases in influencing surface damage experiments and on the gold (001) surface prepared by ion-beam cleaning/thinning and annealing. In maximum valence oxides the end product is a higher-symmetry oxide or metal in UHV, but in a non-UHV environment secondary reactions take place. No evidence is found for electronic damage of non-maximal valence oxides, only sputtering and electron-stimulated reactions. For the gold (001) surface we have reproduced conventional surface-science preparations using ion-beam cleaning and annealing to produce the known reconstructed hexagonal monolayer on the surface.

Imaging by Sliding Planes in Scanning Tunnelling Microscopy

ABSTRACTScanning tunnelling microscope images of layered materials in a non-uhv environment exhibit various anomalous phenomena, including enhanced corrugation heights, periodicity over large areas and a marked absence of point defects. We have modified a precision indentation device to allow STM rastering of a tip across a surface, while simultaneously monitoring mechanical contact. Images we have obtained from this apparatus on an HOPG sample exhibit atomic scale resolution with contact areas much larger than a single atom. Contrast in the image results from periodic conductance fluctuations as the layers of the sample undergo shear in the region of the tip. We provide a model for this process, which explains a variety of curious, and otherwise unrelated phenomena occurring during STM imaging of these materials.

The mechanical design of thin beryllium windows for synchrotron radiation

Vacuum-tight windows can be used to isolate an ultrahigh vacuum (UHV) synchrotron beamline from the equipment of an experimenter operating in a non-UHV environment. A thin beryllium plate mounted on a vacuum flange can be used as a high transmission window/filter for experimenters using synchrotron radiation in the X-ray region. Synchrotron radiation absorbed in the center of a beryllium window is converted to heat. This paper investigates the mechanical and thermal stresses in a beryllium window when that window is used as both a vacuum seal and a synchrotron radiation filter. Thermal and mechanical stresses are calculated using ANSYS - a finite element engineering analysis program which allows inclusion of nonlinear stress factors.

Adsorbate deformation as a contrast mechanism in stm imaging of biopolymers in an aqueous environment

The Scanning Tunneling Microscope (STM) has already been applied to several problems in biology. Examples are imaging of both stained and unstained biopolymers adsorbed on substrates such as graphite in air and in vacuo, AFM images of the surface of a polypeptide crystal and STM images of carbon-platinum replicas. Useful though these methods are, they involve dehydration of the samples and, in the case of adsorbates on graphite, the experiments are not easy to repeat. Sonnenfeld and Hansma have invented a method for operating the STM in conducting solvents'. We have been investigating the mechanism of the STM operated in a non-UHV environment and are using the STM to study aggregates of biopolymers on a metal substrate submerged under a buffer solution. Although formation of the aggregates must modify hydrated biopolymers considerably, repeatable images are obtained relatively easily, and deposition from a biochemically useful environment is straightforward.