49 research outputs found

    Silicon surface passivation by hot-wire CVD Si thin films studied in situ surface spectroscopy

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    Silicon thin films can provide an excellent surface passivation of crystalline silicon (c-Si) which is of importance for high efficiency heterojunction solar cells or diffused emitter solar cells with well-passivated rear surfaces. Hot-wire chemical vapor deposition (hotwire CVD) is an attractive method to synthesize Si thin films for these applications as the method is ion-bombardment free yielding good quality films over a wide range of deposition rates. The properties of the interface between Si thin films and H-terminated c-Si substrates have been studied during film growth by three complementary in situ techniques. Spectroscopic ellipsometry has been used to determine the optical properties, film thickness and surface roughness whereas information on the H-bonding modes and H-depth profile has been obtained by attenuated total reflection infrared spectroscopy. Second-harmonic generation (SHG), a nonlinear optical technique sensitive to surface and interface states, has been used to probe two-photon resonances related to modified Si-Si bonds at the interface. The observations have been correlated with ex situ lifetime spectroscopy experiments. On the basis of the results, the growth and surface passivation mechanism of the films will be discussed, including the role of defect states, built-in electric fields, (nanometer-level) epitaxial growth, influence of the substrate temperature, etc

    Real-time study of α-Si:H/c-Si heterointerface formation and epitaxial Si growth by spectroscopic ellipsometry, infrared spectroscopy, and second-harmonic generation

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    The performance of many devices based on Si thin films deposited on crystalline Si (c-Si) is highly governed by interface quality. For many of these applications, only fully epitaxial films or fully amorphous films having an abrupt interface with the substrate are desired. However, the realization of these perfectly sharp interfaces and the mechanisms governing their formation are not fully understood yet. In this study, the interface formation between Si thin films and c-Si has been investigated by simultaneously applying three complementary optical techniques in real time during low temperature Si film growth. The films were deposited in a hot-wire chemical vapor deposition process by using both native oxide covered and H terminated Si(100) substrates. The formation of hydrogenated amorphous Si (a-Si:H), epitaxial Si, and mixed phase Si has been detected with spectroscopic ellipsometry by measuring the optical properties of the growing films. The evolution of the hydrogen content and hydrogen bonding configurations in the films has been monitored by attenuated total reflection infrared spectroscopy. A clear dependence of the hydrogen content on film morphology is observed with the amorphous films containing significantly more hydrogen. The surface and interface sensitive technique of second-harmonic generation (SHG) has been applied both spectroscopically and in real time. The SHG spectra of a-Si:H films on Si(100) obtained in the SHG photon energy range of 2.7–3.5 eV revealed a dominant contribution originating from the film/substrate interface related to E/E1 critical point (CP) transitions of c-Si. The real-time behavior of the SHG response is shown to strongly depend on differences in initial film morphology, which allows for identification of direct a-Si:H/c-Si heterointerface formation, nanometer-level epitaxial growth, and fully epitaxial growth at a very early stage of film growth. On the basis of the results obtained by the three optical techniques, the c-Si surface passivation mechanism by a-Si:H thin films is addressed and it is demonstrated that the combination of the techniques provides a profound method to control processes occurring during Si thin film growt

    Development of a heat flux gauge

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    Radial cataphoresis in a low-pressure neon-mercury positive column

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    The process of radial cataphoresis is investigated in the positive column of a low-pressure neon-mercury discharge. The emission spectrum and the electric field are measured for several discharge tube diameters, neon pressures, mercury pressures and electric currents. From the results of these measurements, the current from which the emission spectrum of the lamp starts to contain a significant amount of neon radiation is determined. At this critical current, the mercury density profile in the tube is assumed to be significantly depleted due to the process of radial cataphoresis. The measured trends in the critical current are compared to an approximate discharge model

    Real time spectroscopic ellipsometry on ultrathin (

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    Real time spectroscopic ellipsometry was used to det. the time evolution of the dielec. function, bulk thickness, and surface roughness during hot-wire chem. vapor deposition of hydrogenated amorphous silicon (a-Si:H). The amorphous silicon films were deposited on native-oxide-covered c-Si(100) and GaAs(100) substrates at temps. in the range from 70 to 350 DegC. Data anal. by a three layer optical model, consisting of substrate, bulk, and surface roughness layer, revealed that the dielec. function of the a-Si:H film changes in the initial growth regime (

    Real time spectroscopic ellipsometry on ultrathin (

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    Real time spectroscopic ellipsometry was used to det. the time evolution of the dielec. function, bulk thickness, and surface roughness during hot-wire chem. vapor deposition of hydrogenated amorphous silicon (a-Si:H). The amorphous silicon films were deposited on native-oxide-covered c-Si(100) and GaAs(100) substrates at temps. in the range from 70 to 350 DegC. Data anal. by a three layer optical model, consisting of substrate, bulk, and surface roughness layer, revealed that the dielec. function of the a-Si:H film changes in the initial growth regime (

    Attenuated total reflection infrared spectroscopic study of hydrogenated amorphous and microcrystalline silicon film evolution

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    Plasma breakdown is the process that occurs when a voltage is applied across an electrode gap and the neutral gas in the gap becomes ionized and electrically conducting. The goal of our research is to study breakdown processes experimentally on a sub-nanosecond timescale, so that features of breakdown can be observed with adequate time and spatial resolution. In this paper, we describe the measurement system and measurement methods for this study

    Hot-wire deposition of a-Si:H thin films on wafer substrates studied by real-time spectroscopic ellipsometry and infrared spectroscopy

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    Film growth of hydrogenated amorphous silicon (a-Si:H) by hot-wire chemical vapor deposition was studied simultaneously and in real-time by spectroscopic ellipsometry and attenuated total reflection infrared spectroscopy. The a-Si:H films were deposited on native oxide-covered GaAs(100) and Si(100) substrates at temperatures ranging from 70 to 350 °C. A temperature dependent initial growth phase is revealed by the evolution of the surface roughness and the surface and bulk SiHx absorption peaks. It is discussed that the films show a distinct nucleation behavior by the formation of islands on the surface that subsequently coalesce followed by bulk a-Si:H growth. Insight into a temperature-activated smoothening mechanism and the creation of a hydrogen-rich interface layer is presented
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