D. Knoesen

A study of the NiSi to NiSi2 transition in the Ni–Si binary system

Abstract The growth mechanisms of the transition from polycrystalline NiSi to single-crystal NiSi2 have been investigated. Samples were prepared by depositing Ni on 〈100〉 Si substrates by ultra high vacuum electron-beam deposition followed by vacuum annealing to form a silicide. Experiments were carried out at the transition temperature of 750°C for different annealing times. Rutherford backscattering spectroscopy (RBS), cross-sectional transmission electron microscopy (XTEM), scanning electron microscopy (SEM), Auger emission spectroscopy (AES) and X-ray diffraction (XRD) were employed with special emphasis placed on scanning and transmission electron microscopy. Cross-sectional examination in the transmission electron microscope reflects the coexistence of nickel monosilicide and nickel disilicide for samples annealed at 750°C. Scanning electron microscopy shows interesting surface morphology of samples in which island-like NiSi2 growth takes place in an NiSi thin film. Grain boundaries in the initial and final stages of NiSi2 formation are characterized by holes which extend from the surface to the silicon substrate. A mechanism is proposed to account for these observations by involving the following: lattice diffusion of Ni into the Si substrate, the conversion of NiSi grains to NiSi2 via nucleation and diffusion processes and lateral grain growth resulting in accumulation of vacancies on grain boundaries resulting in holes.

Thermally Induced Nano-Structural and Optical Changes of nc-Si:H Deposited by Hot-Wire CVD

We report on the thermally induced changes of the nano-structural and optical properties of hydrogenated nanocrystalline silicon in the temperature range 200–700 °C. The as-deposited sample has a high crystalline volume fraction of 53% with an average crystallite size of ~3.9 nm, where 66% of the total hydrogen is bonded as ≡Si–H monohydrides on the nano-crystallite surface. A growth in the native crystallite size and crystalline volume fraction occurs at annealing temperatures ≥400 °C, where hydrogen is initially removed from the crystallite grain boundaries followed by its removal from the amorphous network. The nucleation of smaller nano-crystallites at higher temperatures accounts for the enhanced porous structure and the increase in the optical band gap and average gap.

Influence of growth temperature on the microcrystallinity and native defect structure of hydrogenated amorphous silicon

The microstructure of hydrogenated amorphous silicon grown by hot-wire chemical vapour deposition (HW-CVD) on glass substrates, at different substrate temperatures ranging from 300 to 500 °C, has been studied using X-ray diffraction and positron annihilation techniques. In previous studies it has been shown that recrystallization is accompanied by a relaxation of the defect structure with an increase in the free volume at the positron annihilation site. The object of this work is to relate the initial defect configuration to the degree of order in the structure, which has been characterized through its radial density function giving accurate estimates of the nearest-neighbour separation and bond angles.

Stress in hydrogenated amorphous silicon determined by X-ray diffraction

Abstract The residual strain in an a-Si:H layer has been directly determined with X-ray diffraction techniques from variations in the diffraction angle of the first amorphous peak using CuKα radiation. The layer was deposited by HW-CVD on glass substrates at a growth temperature of 300 °C, and is known from previous studies to be highly disordered. It was found to have an average compressive stress of 750 MPa, using the c-Si lattice parameter as a reference, and typical values of the elastic constants for a-Si:H, increasing strongly towards the surface.

Structural characterisation of hydrogenated a-Si using slow positron beam techniques

Abstract Hydrogenated amorphous silicon (a-Si:H) grown by hot wire chemical vapour deposition is a promising candidate for robust inexpensive solar cells. However, prolonged exposure to light is known to lead to a reduction in efficiency of a-Si:H devices. The causes for this ageing effect are still unclear, but may be related to a structural relaxation or change in hydrogen content. In this work, results are presented for positron beam studies of the defect structure, using both lifetime and Doppler-broadening spectroscopy, of a-Si:H grown under different conditions.

Light induced changes in the defect structure of a-Si:H

The effect of light-soaking, using a simulated daylight spectrum (colour temperature 5800 K) on the crystallinity, defect structure, and total hydrogen concentration of a-Si:H grown by HW-CVD is being investigated in an ongoing project. In this article, positron beam based electron momentum spectroscopy is applied to monitor the evolution of the open-volume defect structure after each illumination stage. The results indicate an initial increase in free volume at dangling-bond complexes on illumination, with no indication of a change in defect concentration or generation of larger microvoids. On further illumination, there is a reduction in the low electron momentum fraction, which is not accompanied by a similar change in the free-volume. This can be interpreted as a reconfiguration of the hydrogen in the dangling-bond complex, followed by its release into a mobile state.

Plasma induced changes to TCO/a-Si:H interfaces

The active layers of an amorphous silicon solar cell are, in a superstate configuration, deposited on a Transparent Conductive Oxide (TCO). The effect of ion bombardment on the plasma enhanced chemical vapor deposition of a-Si:H and on TCO/a-Si:H interface reactions has been studied. An external DC-bias voltage is applied to the deposition plasma in order to change the ion flux and ion energy. The deposition rate increases with the applied bias-voltage, i.e., with the plasma potential. Hydrogenated amorphous silicon is grown on natively rough transparent conductive oxide. With cross-sectional transmission electron microscopy lower density regions can be observed in the a-Si:H in all sharp valleys of the TCO. The appearance of the lower density regions changes under influence of the ion bombardment. The observed changes are in agreement with the changes observed when no ion bombardment is present at all, like in a hot wire chemical vapor deposition process. The increased ion bombardment did not give rise to observable chemical reactions at the TCO/a-Si:H interface.

Structural evolution of a Ta-filament during hot-wire chemical vapour deposition of Silicon investigated by electron backscatter diffraction

We report on the application of electron backscatter diffraction to investigate the structural changes of a tantalum filament operated at typical hot-wire chemical vapour deposition conditions for the synthesis of hydrogenated nanocrystalline silicon. Various tantalum-silicides, identified by electron backscatter diffraction, form preferentially along the length of the filament. The filament has a recrystallized Ta inner core and a TaSi2 layer encapsulated with a Si layer at the cooler ends. The αTa5Si3, metastable Ta5Si3 and Ta2Si phases formed in addition to recrystallized Ta and TaSi2 at the centre regions. Cracks and porosity were prevalent throughout the length of the filament. The microstructural evolution of the aged tantalum filament can be ascribed to the thermal gradient along the filament length, recrystallization of Ta and the variation of silicon content within the filament.

Amorphous and nc-Si:H Intrinsic Thin Films for Solar Cells Applications

This contribution discusses the deposition process and properties of intrinsic silicon thin films processed by the hot wire chemical vapour deposition technique. We review some fundamental characterization techniques that are used to probe into the quality of the material and thus decide its susceptibility to be used as the intrinsic layer in solar cells industry. This paper covers the optical, structural and electrical properties of the material. Results from UV-visible and IR spectroscopy, XRD and Raman scattering, X-section TEM as well as dark and photo-currents are given. It is shown that the thermal activation energy is a good measure of the quality of the sample.

Growth Temperature, Microstructural Differences and Light-Induced Changes in a-Si:H Deposited on Glass Substrates

The influence of growth temperature on the microstructure of hydrogenated amorphous silicon (a-Si:H) layers deposited on glass substrates at different temperatures has been examined by means of X-ray diffraction and positron annihilation techniques in order to relate the initial configuration of the defect and the degree of ordering in the material to the growth temperature and the effects of light-soaking. The effect of light soaking, using a simulated day light spectrum, on the crystallinity, defect structure and total hydrogen concentration is being investigated in an ongoing project. Introduction Amorphous silicon and hydrogenated amorphous silicon are particularly important due to their potential technical applications such as in an inexpensive solar cell, thin film transistor, optical sensor etc [1,2]. The ideal structure of amorphous material was seen to consist of a random network, where atoms of silicon are linked covalently to four neighbours with only distortion in the bond angles and lengths [3]. In practice a-Si:H only approximates this perfect amorphous structure with the presence of point and large open volume defects [4]. It is also metastable and photodegrades after a moderate light illumination [5]. In this work we will relate the initial defect configuration as shown by positron annihilation spectroscopy and the degree of order in the structure, which has been characterized through its radial distribution density function, of a-Si:H samples deposited on glass substrate at different temperatures, under otherwise identical conditions. Light soaking conducted on one sample provides information on the light induced changes in the defect and microstructure of the material. Sample characteristics and experimental details The a-Si:H layers were deposited by hot wire chemical vapor deposition in a pure silane atmosphere on barium borosilicate glass (corning 7059). Except the substrate temperature which was varied from 300 0 C to 500 0 C in steps of 50 0 C, the other conditions were kept constant during deposition. Electron momentum spectroscopy was carried out using the continuous positron beam at UCT [6], using a conventional analysis in terms of the lineshape parameters S and W (corresponding to the fraction of annihilations with low and high momentum electrons respectively). The results presented are averages over a positron energy range of 3 9 keV, corresponding to implantation solely in the a-Si, over which the lineshape parameters were generally constant. The X-ray diffraction measurements were performed in Bragg-Brentano geometry using CuK radiation from a Cu tube operating at 40kV and 40mA. The diffraction patterns were recorded over a 2 range from 10 0 to 70 0 with a step size of 0.04 0 . The effect of the substrate and sample holder was removed from the front signal and the radial distribution function was calculated by the numerical sine transform of the scattering factor as a function of scattering vector k. Materials Science Forum Online: 2004-01-15 ISSN: 1662-9752, Vols. 445-446, pp 147-149 doi:10.4028/www.scientific.net/MSF.445-446.147