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We have used small-angle X-ray scattering (SAXS) and Doppler-broadening measurements of positron-annihilation radiation to study changes in the microvoid distribution in PECVD a-Si:H films during annealing. From a comparison of data on deuterium diffusion with information obtained from SAXS we conclude that changes, during annealing, in the dispersive character of deuterium diffusion are likely to be caused by void formation through clustering of smaller structural defects.
A series of hydrogenated amorphous silicon (a-Si:H) films was deposited by rf glow-discharge deposition using various processing conditions. We have studied microstructure in the films by means of infrared absorption spectroscopy. Small-angle X-ray scattering measurements were used to determine the microvoid fractions of a few selected samples. Our results show that both the void fraction and the amount of microstructure can be varied either by changing the substrate temperature or by H2 dilution. Bond-angle variation in the films was probed by Raman scattering measurements. The Raman data indicate that the substrate temperature is the main variable that determines the bond-angle variation. We conclude that the presence of microvoids in a-Si:H does not influence the structural disorder of the amorphous matrix surrounding the voids. Our results are in agreement with experimental work on microvoids in a-Si1-xCx:H, and model calculations on voids in a-Si.
A thin film of amorphous silicon (a-Si:H) has been developed for application as the charge retention layer in electrostatic loudspeakers. The vibrating diaphragm in this type of loudspeakers is usually coated with a thin film in order to assure homogeneous charge distribution across the surface. This thin film should be capable of maintaining a high surface charge density and possess a high lateral resistivity to prevent charge displacement. Furthermore, it should be mechanically stable, capable of accomodating mechanical deformation, and resistant against humidity. Conventionally, a graphite layer is used in these applications. However, the conventional layers are frequently unstable and suffer from charge displacement effects eventually leading to electric breakdown. Further requirements are that the film can be deposited continuously and homogeneously over large areas in excess of 1 m2 and that the deposition technique is compatible with the properties of the thin membrane. We investigated small-area prototypes of electrostatic loudspeakers with a-Si:H thin films deposited on polyimide substrates in the Utrecht deposition system, ASTER. Plasma-deposited amorphous silicon films fabricated under certain conditions are shown to meet all of the above requirements.
We have annealed PECVD a-Si:H films at 250, 300, and 350°C and measured the evolution of the infrared absorption spectrum. We observe that, during the initial stage of such a heat treatment, atomic hydrogen migrates from the isolated state to the clustered state. Thus diffusion of atomic hydrogen must occur around 300°C. Microvoids with internal surfaces covered with SiH bonds appear to be more stable than voids lined with SiH2 bonds and (SiH2)n polymers.
The measurement of mass resolved ion energy distributions at the grounded substrate in an RF glow discharge allows to determine the ion flux and the ion energy flux towards the surface of a growing hydrogenated amorphous silicon (a-Si:H) layer. This provides the means to study the influence of ions on the structural properties of a-Si:H. Here we focus on the α-γ’ transition as occurs in silane-hydrogen plasmas at an RF frequency of 50 MHz and a substrate temperature of 250 °C. The structural properties of the layers appear to depend on the kinetic energy of the arriving ions. This is supported by measurements of ion fluxes under other deposition conditions and by characterization of the corresponding layers. The influence of ions on the growth is discussed in terms of their flux, and the amount of delivered kinetic and potential energy to the growing film. The measurements suggest that a threshold energy of about 5 eV per deposited atom is needed for the construction of a dense amorphous silicon network.
Hydrogenated amorphous silicon thin films, co-doped with oxygen, are made using lowpressure chemical vapor deposition (LPCVD) or plasma-enhanced chemical vapor deposition (PECVD). The films are implanted with Er to a peak concentration of 0.2 at.%. Roomtemperature photoluminescence at 1.54 μm is observed in both amorphous materials, after thermal annealing at 300–400 °C. The PECVD films with low 0 content (0.3, 1.3 at.%) show a luminescence intensity quenching by a factor 7–15 as the temperature is raised from 10 K to room temperature. The quenching is well correlated with a decrease in luminescence lifetime, indicating that non-radiative decay of excited Er3+ is the dominant quenching mechanism as the temperature is increased. In the LPCVD films, with 31 at.% 0, the quenching is only a factor 3, and no lifetime quenching is observed. The results are interpreted in the context of an impurity Auger excitation model, taking into account the fact that oxygen modifies the Si bandgap and the Er-related defect levels in the gap.
For the first time ion energy distributions (IED) of different ions from silane-hydrogen (SiH4-H2 ) RF plasmas are presented, i.e. the distributions of SiH3+, SiH2+ and SiH2+. The energy distributions of SiH3+ and SiH3+ ions show peaks, which are caused by a charge exchange process in the sheath. A method is presented by which the net charge density in the sheath is determined from the plasma potential and the energy positions of the charge exchange peaks. Knowing the net charge density in the sheath and the plasma potential, the sheath thickness can be determined and an estimation of the absolute ion fluxes can be made. The flux of ions can, at maximum, account for 10% of the observed deposition rate.
A-Si:H p+-i-n+ solar cells have been made employing plasma enhanced chemical vapour deposition at frequencies between 30–80 MHz. Here, only the i-layer was fabricated at these very high frequencies (VHF). Both the p+- and n+-layer were made using 13.56 MHz. A previous study has shown the material quality to depend on mainly the applied rf-power, and only slightly on the frequency. It should be noted that for homogeneity reasons a certain optimized pressure is required for each frequency. There is a clear correlation between material quality and solar cell parameters. An initial efficiency of 10 % has been obtained for cells deposited at 65 MHz using a low power density, while the deposition rate still is 2–3 times higher than the one at 13.56 MHz. Light-soaking reveals stabilisation at 6 % for the best cell, which compares well to conventional 13.56 MHz cells.
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.
A systematic study of material quality has been performed for intrinsic a–Si:H layers deposited by plasma enhanced chemical vapor deposition at excitation frequencies between 30 and 80 MHz (VHF). The process conditions were optimized not only for “device quality” opto-electronic properties but also for a uniformity in layer thickness better than 5% over the 10 cm × 10 cm substrate area. We found optimized homogeneities at different pressures depending on the excitation frequency. The effect of frequency at these optimum conditions on the material quality is small. VHF-intrinsic layers have been used in a–Si:H p+-i-n+ solar cells, in which both the p+ and n+ layer were made using 13.56 MHz. There is a clear correlation between material quality and solar cell parameters. Material deposited at low power densities is of so-called “device quality,” which is confirmed by demonstrating an initial efficiency of 10% for cells deposited at 65 MHz using a low power density. The deposition rate still is 2–3 times higher than the one at 13.56 MHz. Light-soaking of the cell leads to stabilization at 6% for the best cells, which compares well to conventional 13.56 MHz cells.
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