The heterogeneity of a-Si:H and a-Si:D films has been probed on the nano-scale by small-angle neutron scattering (SANS). Films were deposited by two techniques, plasma-enhanced chemical-vapor deposition (PECVD) and hot-wire chemical-vapor deposition (HWCVD) using conditions that yield high-quality films and devices. Four samples were examined in a light-soaked state (AM1.5, 300 h) and then re-examined after annealing (190°C, 1 h) in-situ to look for any change in SANS associated with the Staebler-Wronski effect. No changes were observed in the SANS intensity to a precision that could have readily detected the 25% change reported in 1985 (Chenevas-Paule et al). Significant differences are observed in hydrogenated and deuterated films, as well as in the PECVD versus the HWCVD materials.

The Si-H vibration in IR spectra of our device quality poly-Si films grown by hot-wire chemical vapour deposition (HWCVD) made at low wire temperature (Tw=1800 °C) is at 2000 cm−1 whereas in a poly-Si film made at high wire temperature (Tw=1900 °C) both 2000 cm−1 vibrations as well as 2100 cm−1 are observed. On the other hand, the Raman spectra (probing the upper part of the film) of Si-H stretching vibration measured for both these samples show only 2000 cm-1 mode. XTEM micrographs of these films show that whereas the low Tw film has a structure made of closely packed crystalline columns, the high Tw film has conical crystalline structures with amorphous region between them. The crystal cones meet each other towards the top of the film and form a closed structure. This is confirmed by Raman spectrum at 520 cm−1. We attribute the 2100 cm−1 mode to the Si-H bonds at the surface of the cones touching the amorphous regions. The Si-H vibration shifts to 2000 cm-1 when the crystalline cones coalesce with each other, as is the case in the upper part of both types of films.

The microstructure of two different νc-Si:H solar cells was studied by Transmission Electron Microcopy (TEM). At the micrometer scale, a difference in the grain structure is observed close to the ZnO substrate: cracks are found in the cell deposited with the higher silane concentration. Surprisingly, the cell with the cracks close to the transparent conductive oxide shows the largest VOC (530 mV) and FF (68%). These first studies reveal that microstructures of fully microcrystalline silicon devices may show a quite large variation with corresponding effects in solar cell performance.

Amorphous hydrogenated silicon films deposited by Plasma Enhanced Chemical Vapour Deposition (PE-CVD) using standard rf-glow discharge at 13.56 MHz were used to produce amorphous silicon heterostructures. Junction properties were studied from current-voltage (IV), capacitance-voltage (C-V) and spectral response measurements. The photosensitivity of these structures was investigated for different amorphous film thicknesses and different applied bias voltages. It was shown that the output device characteristics could be improved by plasma hydrogen treatment before the deposition of the amorphous layer. The results show that ITO/a-Si:H/c-Si structures present high internal gain in the visible infra-red region and high collection efficiency in the blue range. They can be used as visible/near-IR photodiodes or for current amplifications proposes.

An hydrogenated amorphous silicon junction field effect transistor suitable for analog and digital applications is presented. The device is constituted by a p+ - i - n− junction, with the drain and source contacts patterned on the n-doped layer and the gate electrode patterned on the p+ doped layer. As in the crystalline case, the device is a voltage-controlled resistor, and its drain-source resistance can be varied, with a voltage applied to the gate electrode, by modulating the width of the depletion layer extending into the n-type channel.

The doping value of this layer has been chosen as a trade-off between high value of channel conductivity and a relatively low defect density in the material. The manufactured device, with W/L=5000/200 μm, shows the typical current-voltage curves of a JFET. In particular, at low VDS, the current presents the linear behavior of the triode zone, where the JFET operates as a linear resistance whose value is controlled by the gate voltage. At higher VDS the JFET works in the pinch-off region as a dependent current generator.

First results are very encouraging, since we have achieved transconductance values of 10−6 V/A, which are comparable to those of state of the art TFT.

A systematic study has been made of the growth of both hydrogenated amorphous silicon (a-Si:H) and silicon nitride (a-SiN) by electron cyclotron resonance plasma enhanced chemical vapour deposition (ECR-PECVD). In the case of a-SiN, helium and nitrogen gas is injected into the system such that it passes through the resonance zone. These highly ionised gases provide sufficient energy to ionise the silane gas, which is injected further downstream. It is demonstrated that a gas phase reaction occurs between the silane and nitrogen species. It is control of the ratio of silane to nitrogen in the plasma which is critical for the production of stoichiometric a-SiN. Material has been produced at 80 °C with a Si:N ratio of 1:1.3 a breakdown strength of ∼6 MV cm−1 and resistivity of >1014 Ωcm. In the case of a-Si:H, helium and hydrogen gas is injected into the ECR zone and silane is injected downstream. It is shown that control of the gas phase reactions is critical in this process also. a-Si:H has been deposited at 80 °C with a dark conductivity of 10−11 Ω−1 cm−1 and a photosensitivity of just below 4×104. Such materials are suitable for use in thin film transistors on plastic substrates.

We present femtosecond studies of carrier dynamics in hydrogenated amorphous silicon-germanium alloys grown by the recently developed hot-wire assisted chemical vapor deposition (HWCVD) technique, which is promising for producing high-quality device-grade materials. We have used wavelength-resolved femtosecond pump-probe techniques, in which an intense pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in optical properties as a function of time delay following the pump pulse, to study the dynamics of photoexcitations in these materials. Femtosecond dynamics measurements have been carried out on thin film samples under experimental conditions with varying sensitivity to carriers in extended states or in band tail states. The relaxation dynamics of carriers associated with extended states show a strong dependence on excitation density and follow a bimolecular recombination law, consistent with a number of earlier studies on related amorphous materials. In contrast, measurements involving carriers excited directly into band tail states reveal significantly altered dynamics, characterized by a marked deviation from simple bimolecular recombination at short times.

It has been shown that useful current gains can be obtained in hot-electron device structures containing very thin chromium disilicide layers of nanometer dimensions in hydrogenated amorphous silicon [1]. The a-Si:H/a-CrSi2/a-Si:H device structure made using PECVD and sputtering techniques naturally forms a hot-electron transistor device where the electrons are emitted across a high potential barrier on one side of the silicide and are collected over a low barrier on the other. Recent results [2] have shown that current gains can be in excess of 40 in structures having a-CrSi2 bases ∼1 nm thick.

Here we outline the relatively simple technology used to make these devices and examine their performance as phototransistors in which the photo-current is amplified by hot-electron transistor action. The speed of response can be maximised by operating the phototransistor with high electric field across the collector since it is the transit time of the photo-induced carriers that determines the response time. We show that these devices provide a useful new active element for large area amorphous silicon electronics.

We report top-gate n channel μc-Si:H TFTs with saturated electron mobilities up to 40 cm2V−1s−1 and ON/OFF ratios up to ~106. The μc-Si:H was grown from silane, dichlorosilane, and hydrogen. The glow discharge was excited at a frequency of 80 MHz to raise the growth rate to ∼1Å/sec, which is above that achievable with 13.56 MHz. Deposition temperatures were 230°C for the i-layer and 280°C (the highest temperature in the process) for the n+ source and drain layers. The TFTs were fabricated from 340-nm thick μc-Si:H films, and with a 300-nm thick gate insulator of plasma deposited SiO2.

In depositing a-Si:H by HWCVD using W filaments, one issue common to this technique is that of filament lifetime. When using undiluted silane as the source gas, a buildup of silicon at the colder ends of the filament is routinely observed (thickening), and it is here that filament breakage usually occurs. Less well understood is the effect of filament alloy formation on a-Si:H electronic properties. In this work we combine ambipolar diffusion length (SSPG) measurements on consecutively deposited a-Si:H films with SEM surface topography and sputter (SP) Auger depth profiling of the filament Si/(W+Si) composition to track film electronic properties as a function of the Si buildup on short filaments entirely exposed to the growing film surface. We find that with increasing exposure time of the filament to silane, appreciable Si exists everywhere along the filament length, even in the non-thickened central regions. We discuss the effect of this alloying on the film deposition rate and electronic properties, and suggest that the nature of the filament surface must be carefully considered when optimizing a- Si:H film electronic properties. Finally, we discuss possible ways to minimize this alloying by post deposition treatments, which include different filament ‘run’ temperatures.

The floating body induced transient characteristics in polycrystalline silicon TFTs is reported. An obvious current surge, arising from the floating body effect, is observed. Using a 2D numerical simulator and comparisons to SIMOX FETs, the insight into the mechanisms governing the experimentally observed switching behavior of poly-Si devices is obtained. It is found that the defect states in poly-Si cause the current surge and exert an effect equivalent to doping in SIMOX FETs.

Non-infrared-active hydrogen bonding species were investigated by analyzing the infrared spectra and the calibrated temperature desorption spectroscopy (CTDS) spectra of hydrogen released during degassing of hydrogenated amorphous silicon thin films. Samples were degassed gradually using a linear temperature ramp (0.5°C/s). Each stage corresponds to a temperature at which the hydrogen effusion peaks can be found (~ 340°C, ~ 500°C and ~ 610°C). Differences in the amounts of hydrogen obtained from the FTIR spectra and the CTDS measurement correspond to the non-infrared-active, occluded hydrogen.

Relatively thick amorphous silicon films for solar-cell applications were prepared by metal-induced-crystallization (MIC). Then, the thickness-dependent characteristics of micro-Raman spectra from a cross section of the prepared polycrystalline silicon (poly-Si) films were analyzed. It was found that Ni-induced crystallized films have a uniform composition that is 80% polycrystalline and 20% nanocrystalline. Also, the x-ray diffraction data show that a sub-mono-layer of Ni is sufficient for MIC of 6-μm-thick amorphous silicon (a-Si) films.

From investigations on the SiH3 and SiH radical density and the surface reaction probability in a remote Ar-H2-SiH4 plasma, it is unambiguously demonstrated that the a-Si:H film quality improves significantly with increasing contribution of SiH3 and decreasing contribution of very reactive (poly)silane radicals. Device quality a-Si:H is obtained at deposition rates up to 100 Å/s for conditions where film growth is governed by SiH3 (contribution ∼90%) and where SiH has only a minor contribution (∼2%). Furthermore, for SiH3 dominated film growth the effect of the deposition rate on the a-Si:H film properties with respect to the substrate temperature is discussed.

Recent developments in the area of cost effective 2D direct x-ray sensor arrays on the base of a-SiN:H back-to-back Schottky diodes and no switching devices per pixel are presented. Discussion focuses on two major aspects: (i) x-ray sensitivity of the sensor itself and possibility of its improvement; (ii) the overall performance of the sensor array.

Studies have been carried out on the kinetics of light induced changes in thin films and solar cells fabricated in different laboratories. Unlike the many studies on undiluted a-Si:H films the kinetics reported here are for 1 sun illuminations carried out to the degraded steady state (DSS). The light induced changes at temperatures from 25 to 100°C were characterized with electron mobility lifetime products and subgap absorption in films and fill factors in solar cells. The significant changes in the degradation kinetics that occur in these materials in this t mperature range have further confirmed the importance of charged defects, not only in the annealed state, but also in the degraded steady states. Results are also presented on corresponding solar cell structures, which also indicate that defects other than neutral dangling bonds have to be taken into account if any meaningful evaluation is to be made of their solar cell properties.

The early work on amorphous silicon in Europe was dominated by the activity of the Dundee group, who demonstrated the feasibility of substitutional doping, in contradiction with previous theories which denied the possibility of doping disordered semiconductors. Also, the role of hydrogen in this new “good” disordered semiconductor was not immediately accepted, and the controversy was finally settled by the crucial experiment of post-hydrogenation by D. Kaplan. It is little known that this process of post-hydrogenation, currently used for the improvement of devices, was covered by a patent, which turned out to be quite inapplicable!

The high hopes raised by this “new” material, in particular for photovoltaic applications, rendered the field highly competitive, resulting in a surprising neglect of the basic principles of physics. A striking example is the sweeping under the rug of the effect of band bending at the surface of intrinsic a-Si:H. This effect makes the surface much more conducting than the bulk, rendering a large number of published transport measurements in planar geometry completely meaningless.

Research in Europe has been less application-oriented than research in USA and Japan, but on a small scale it was not completely absent from industrial applications to photovoltaics; a start-up adventure “a la French” is described. The problem of disordered materials is one of the timely solid-state topics, to continue to be a major subject of materials research in the near future. In that respect, amorphous silicon is an exemplary system, and the “hydrogen glass” picture, pioneered by R. A. Street et al., is an open field of research for the improvement of disordered semiconductors.

The post-initial growth dynamic scaling exponent β, which describes the surface roughness evolution in time, is determined for a-Si:H growth using in situ single wavelength (632.8 nm) rotating compensator ellipsometry. β is measured as function of the substrate temperature for three different growth rates 2, 5 and 22 Ås-1 under conditions where SiH3 dominantly contributes to growth. β (≤ 0.5) decreases with increasing substrate temperature and does not strongly depend on the growth rate within the range of growth rates. A roughness evolution model is proposed, based upon a random generation of active growth sites and a subsequent site dependent surface diffusion process. The measured β temperature dependence can be simulated with an activated site hopping activation energy of about 1.0 eV. This activation energy is much higher than what would be expected from a model based upon the diffusion of physisorbed silyl SiH3 radical and suggests therefore another mechanism which is responsible for the surface smoothening during a-Si:H growth.

The microstructural and phase evolution of silicon films (Si:H) prepared by low temperature (200°C) rf plasma-enhanced chemical vapor deposition (PECVD) with hydrogen dilution of silane has been studied using real time spectroscopic ellipsometry (RTSE) and atomic force microscopy (AFM). Both RTSE and AFM support the concept of an evolutionary phase diagram that describes different regimes of bulk layer thickness and H2-dilution ratio R within which predominantly amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) films are obtained. In this study, the evolutionary phase diagram has been extended to identify four separate growth regimes: (i) a-Si:H with a smooth surface and a stable roughness layer thickness, (ii) a-Si:H with a rougher surface and an unstable roughness layer thickness, (iii) mixed-phase (a+μc)-Si:H, and (iv) fully-coalesced (single-phase) μc-Si:H. Based on this framework, the effect of increased rf power to achieve higher deposition rates in the rf PECVD process was investigated.

Ion-assisted deposition (IAD) enables low temperature (≥ 435°C), high-rate (≤ 0.5 μm/min) epitaxial growth of silicon films. Therefore, IAD is an interesting deposition technique for microelectronic devices and thin film solar cells. The Hall-mobility of monocrystalline epitaxial layers increases with deposition temperature Tdep and reaches values comparable to those of bulk Si at Tdep ≥ 540°C. Polycrystalline epitaxial layers exhibit inhomogeneous electrical properties, as shown by Light Beam Induced Current measurements. Recombination within the grains dominates over recombination at grain boundaries. Secco etching identifies an inhomogeneous density of extended structural defects in the polycrystalline epitaxial layers and in the substrate. A major part of the extended defects in the epitaxial layers originates from defects in the substrate.