SiO2 thin films were prepared on p-type Si (100) substrates by atomic layer deposition (ALD) using SiH2Cl2 and O3(1.5 at.%)/O2 as precursors at 300. The growth rate of the deposited films increased linearly with increasing amount of simultaneous SiH2Cl2 and O3 exposures, and was saturated at about 0.35 nm/cycle with the reactant exposures of more than 3.6×109L. A larger amount of O3/O2 than that of SiH2Cl2 was required to obtain a saturated deposition reaction. The composition of the deposited film also varied with O3/O2 exposure at a fixed SiH2Cl2 exposure. The Si/O ratio gradually decreased to 0.5 with increasing amount of O3/O2 exposure. Finally, we also compared the physical and electrical characteristics of the ALD films with those of the films deposited by conventional chemical vapor deposition (CVD) methods. In spite of low process temperature, the SiO2 film prepared by the ALD method was in wet etch rate, surface roughness, leakage current and breakdown voltage superior to that by other several CVD methods.

This paper presents microcrystalline silicon (μ c-Si:H) p-i-n (superstrate-type) solar cells fabricated by 100 MHz plasma-enhanced chemical vapor deposition (PECVD) at i-layer deposition rates of >2 nm/s. Under high-rate conditions, in particular, the deposition pressure is found to play a dominant role in determining short circuit current (Jsc) of solar cell. With anincrease in deposition pressure from 3 to 7-9 Torr, Jsc increases by more than 50% due to a significant improvement in the long wavelength (>600 nm) responses, which essentially leads to high efficiency (∼8%) solar cells in the 2-3 nm/s deposition rate range. Further progress in solar cell efficiency has been made by the improvement of TCO/p and p/i interfaces. As a result, efficiency reaches 9.13% (Jsc=23.7 mA/cm2,Voc=0.528 V,FF=0.73) with a 2.3μm-thick i-layer grown at 2.3 nm/s. Transmission electron microscopy and secondary-ion mass spectroscopy studies reveal that samples prepared at lower pressure (∼4 Torr) comprise many grain boundaries due to disordered grain growth, which induces an anomalous incorporation of atmospheric impurities (predominantly oxygen) after exposing sample to air. In contrast, the high-pressure process (<7 Torr) provides denser grain columns coalesced with [110]-oriented crystallites, which in turn inhibits impurities from penetrating deeper in the film. Based on above results, we propose that the less post-oxidation behavior associated with the denser microstructure of high-pressure-grown μc-Si:H is responsible for the excellent charge collection in p-i-n solar cells.

Red/near-infrared photoluminescence (PL) of silicon irradiated by ions for wide dose range is investigated. The results are presented obtained for doses near amorphization threshold, where PL is associated with the formation of composite structure composed of nanocrystals (quantum dots) embedded into amorphized matrix and for doses that are strongly larger than amorphization threshold. The PL in this case is caused by formation of nanocrystals due to recrystallization of amorphous layer and penetration of stresses behind the border of this layer.

Metal induced lateral crystallization (MILC) of hydrogenated amorphous silicon (a-Si:H) was studied and a model was developed based on the resistance measurement of the films. Hydrogenated amorphous silicon films of 300 nm thickness were deposited using plasma enhanced chemical vapor deposition (PECVD) on oxidized p-type (100) silicon wafers. Thermally evaporated 200 nm thick aluminum layer was deposited over amorphous silicon and patterned using photolithography. The samples were annealed at different temperatures for different time periods. After annealing the resistance of amorphous silicon between aluminum pads was measured. Based on these measurements, a model was developed to predict the lateral crystallization velocity. In this model, the resistance change due to loss of hydrogen from the film was also taken into account. For this purpose, another set of experiments was conducted. In this set, hydrogenated amorphous silicon films of 300 nm thickness were deposited on Corning 7059 glass. The samples were annealed for different period of time at different temperatures. After annealing, parallel bars of silver paint were formed on the samples and the resistance of each sample was measured. The theoretically determined lateral crystallization velocity was verified using optical microscope observations and X-ray diffraction analysis and was found to be in close agreement.

The interchangeability of ion bombardment and deposition temperature during a-Si:H film growth at high deposition rates (10-42 Å/s) by means of the expanding thermal plasma has been studied. The ion bombardment is generated by applying an external rf bias voltage on the substrate. It is shown that the opto-electronic performance of the a-Si:H films improves considerably when a moderate rf substrate bias voltage (∼20-60 V) is applied, i.e. the photo response increases two orders of magnitude up to 106. Furthermore, it is also revealed that the additional energy supplied to the growth surface by the ion bombardment, makes a reduction of the deposition temperature by ∼100 °C possible, while preserving good material properties. On the basis of the results obtained, three effects caused by the rf substrate bias can be distinguished: creation of an additional growth flux, a reduction of the void incorporation, and an increase in the vacancy density.

Exponential distributions of tail states have been able, within the framework of a multiple-trapping transport model, to account rather well for the time-of-flight photoconductivity transients that are measured with ‘standard’ a-Si:H, i.e. material prepared by plasma-enhanced chemical vapor deposition at ∼250°C. A field-dependent carrier mobility in the dispersive transport regime is part of the observations. However, samples prepared in an expanding thermal plasma, although still exhibiting the dispersive transients, fail to show this field dependence. The presence of a Gaussian component in the density of valence-band tail states can account for such behavior for the hole transients. Nanoscale ordered inclusions in the amorphous matrix are thought to be responsible for the Gaussian density of states contribution.

This work discusses the new use of an old deposition technique: spray pyrolysis. The objective is the deposition of thin films of lead iodide and mercury iodide and their future use as photoconductors in medical X-ray digital imagers. We discuss the main advantages and limitations of the deposition process comparing both materials. The final thin films are structurally investigated using X-ray diffraction and microscopy. The deposition technique seems to be very promising for the future development of large area radiation detectors.

The last two decades have witnessed tremendous progress in the science and technology of amorphous and microcrystalline silicon-based photovoltaic. Advances in the understanding of materials and devices have led manufacturers to expand their production capacity; the production of solar panels based on this technology exceeded 25 MW in 2003. Hydrogen dilution in the active gas mixture during deposition has played a key role in improving the quality of the materials and the performance of the devices. In this paper I shall review the properties of the optimum material for device application, and discuss the production status. I shall also report on the new opportunities that are opening up for these products for space and stratospheric applications.

The temperature dependent H evolution from a-Si:H provides unique information on the H-bonding and microstructure. Traditional undiluted a-Si:H films show a high temperature H-evolution peak near 600°C. However device-quality compact nanocrystalline silicon films grown near the phase boundary of amorphous and microcrystalline growth show a new low temperature H- evolution peak near 400°C in addition to a second high temperature peak near 600°C. The origin of this peak cannot be attributed to microvoids or a substantial density of dihydride species typical of porous low-temperature films. We have simulated the H evolution using a molecular dynamics generated model of nanocrystalline silicon, where nano-crystallites reside in a background amorphous matrix. An excess density of H occurs at the crystallite surface. We find a low temperature evolution peak at 250-400°C, where the H-evolution starts from the surface of the nano-crystallite. In addition there is a higher temperature peak at 700-800°C providing good agreement with H-evolution measurements. The mobile H is found to exist in both the bond-centered type of species and H2 molecules – which has implications for H-diffusion models.

This paper presents results of the spatial and frequency detection limits of an integrated array of 32 one-dimensional amorphous silicon thin film position sensitive detectors with a nip structure, under continuous and pulsed laser operation conditions. The data obtained show that 0.45×0.06 cm arrays, occupying a total active area of about 1 cm2 have a spatial resolution better than 10 m m (modulation transfer function of about 0.2), with a cut-off frequency of about 6.8 KHz. Besides that, under pulsed laser conditions the device non-linearity has its minimum (about 1.6%), for a frequency of about 200Hz. Up to the limits of the cut-off frequency, the device nonlinearity increases to values above 4%.

The development of electronic paper has been a long-term goal and electrophoretic displays are a promising candidate for this application. Apart from developing the electrophoretic display medium there are important issues to solve regarding the overall system. The importance of the electronic addressing method increases with the demand for arbitrary images or text. We have developed and tested active-matrix backplanes based on amorphous silicon technology, as well as the driver electronics. The electrophoretic ink is combined with the backplane employing polymer MEMS cell structures. This system allows us to display high-resolution images and it is a good test bed for investigating various parameters of the electrophoretic display medium and of the electronics.

Films deposited by the Hot Wire CVD method were studied by means of dark conductivity, FTIR, Hydrogen Evolution, SEM and AFM surface characterization. Three types of metastability were observed: a) long term irreversible degradation due to oxidization processes on the film surface, b) reversible degradation determined by uncontrolled water adsorption, c) fast field switching effect in the film bulk.

Oxygen and hydrogen content and its bonding configurations have been analyzed by hydrogen evolution and infrared spectroscopy methods on the films deposited on glass substrates and silicon wafers subsequently. It has been found that metastable processes close to the film surface are stronger than in the bulk. The switching effect is the fast increase of charge carrier density observed on bottom chromium contacts under a condition of air admittance. We propose this effect is associated with morphology changes during film growth and electrical field induced by adsorbed atmospheric components on the film surface.

A study of the effects of light-soaking and atmospheric adsorption (aging) on the dark- and photo-conductivity of a series of microcrystalline silicon films of varying crystallinity is presented. Light-soaking in vacuum slightly reduces photoconductivity in films close to the amorphous – microcrystalline transition, and there is also a reduction in dark current. Aging increases the dark current, and thus unless due care is taken during light-soaking experiments to eliminate or compensate for aging, the apparent effect of light-soaking may be reduced or even reversed in sign. Transient photocurrent decays confirm the presence of a large density of metastable light-induced defects. A shift in the apparent distribution of defects occurs on prolonged aging, which may be due either to changes in the DOS or a shift in the Fermi level.

Inverters made of monolithically integrated p- and n-channel thin film transistors of nanocrystalline silicon were demonstrated on both Corning 1737 glass and Kapton E polyimide substrates. The TFT's geometry is staggered top-gate, bottom-source/rain. A nc-Si:H seed layer promotes the structural evolution of the nc-Si:H channel. Electron field-effect mobilities of 15 - 30 cm2V−1s-1 and hole mobilities of 0.15 - 0.35 cm2V−1s−1 were obtained. Slightly lower carrier mobilities were observed in the TFTs made on polyimide than on glass substrates. High gate leakage currents and offsets between the supply HIGH voltages and the output voltages in the inverters indicate that the low-temperature gate dielectric needs improvement.

We have prepared poly-Si1-xGex thin films with different germanium contents (Ge=5%∼40%) by reactive thermal CVD. In this study, the Ge content was controlled by varying the source gases GeF4 flow rate at a fixed Si2H6 flow rate. The effects of GeF4 flow rate on growth rate, film crystallinity, and electrical properties were investigated. The films were always polycrystalline when GeF4 was introduced even in a small amount, while only amorphous film deposited without GeF4. With an increase in GeF4 flow rate, Ge content and conductivity of the films increased and its activation energy decreased. When GeF4 flow rate over a certain value, the growth rate decrease and finally no film could be deposited. These behaviors are discussed in relation with a role of GeF4 for the crystal growth at a low temperature.

Polycrystalline CdTe thin-film solar cells have shown high potential for low cost, large-area module fabrication. But successful large-scale commercial production has been elusive. Fabrication of the basic n-CdS/p-CdTe heterojunction is possible by a wide variety of methods, including close-spaced sublimation, vapor-transport deposition, electrodeposition, chemical bath deposition, and magnetron sputtering. An overview of these methods is presented as well as the role of the postdeposition “activation” treatment using CdCl2 and issues related to the difficulty of obtaining low resistance back contacts to CdTe. We present some of our recent fabrication results using rf magnetron sputtering and discuss some of the advantages that appear possible from the use of sputtering methods in this class of materials. Some of these advantages are particularly relevant as the polycrystalline thin-film PV community addresses the challenges of fabricating tandem cells with efficiencies over 25%.

In this paper, we report on nc-Si:H thin films deposited by the pulsed PECVD technique at a temperature of 150C and TFTs made using this material. RF power and silane flow rate were varied in order to study the effect of different levels of crystallinity on the film. Electrical conductivity, Hall mobility, optical transmittance, and Raman backscattering were measured on films of two different thicknesses. From the Raman data we see that the 50 nm films with hydrogen dilution are mostly amorphous, indicating the presence of a thick incubation layer. The values obtained for the conductivity, mobility, and optical gap varied depending on the processing conditions and these results are discussed. Bottom-gate TFTs were fabricated using a pulsed PECVD channel layer and a SiN gate dielectric. The TFTs' extracted parameters are msat μsat ≤ 0.38 cm2=(V s), Vt,sat ≥7:3 V, Ion/off >106, and S < 1 V/decade. The TFT performance and material properties are presented and discussed.

A study has been carried out on the evolution of light induced defects in protocrystalline (diluted) a-Si:H films under 1 sun illumination. A room temperature reversal is observed in the photocurrents at 25°C, which is consistent with the relaxation in the recombination currents on corresponding p-i-n solar cells. It is also consistent with the pressure of “fast” states such as have been observed after high intensity illumination. Even with the limitations imposed by the relaxation in the light induced changes on the subgap absorption measurements, the evolution of distinctly different gap states centered around 0.9 and 1.15eV from the conduction band was identified. The kinetics of the electron occupied states, kN(E), at these two energies is compared with that of the neutral dangling bond (D0) densities as measured with electron spin resonance. Because of the similarity between the preliminary results of these kinetics it has not been possible to identify which states correspond to the D0 nor to draw any reliable conclusions about the nature of the different states.

Using a novel hollow cathode plasma-jet reactive sputtering system in which an intense plasma, ignited in an Ar/H2 flow, is directed through silicon and germanium nozzles, a series of a-SiGe:H thin films have been prepared on silicon and glass substrates. These films have been optically characterized by infrared (IR) spectroscopy and spectroscopic ellipsometry (335-1000nm). Total hydrogen concentrations, as determined by FTIR, varied with deposition conditions and ranged from 2.5 × 1021 to 1.6 × 1022 atom cm−3 and correlated with secondary ion mass spectrometry (SIMS) elemental analyses to within 10%. Conductivity measurements in the dark and under simulated AM1 solar illumination have indicated that the films properties are very good. The light to dark conductivity ratio has consistently been greater than 1000 for films with band gaps down to 1.3 eV.