Using Remote PECVD we have deposited layers of “device quality” a-Si:H and Si-based dielectrics. We find that the formation of depletion or accumulation layers at a-Si:H/dielectric interface depends on the specific dielectric (SiO2 or Si3N4) and on the film deposition sequence. In addition, we have studied process-gas/substrate interactions by in-situ Auger Electron Spectroscopy (AES) and determined that Si-O or Si-N bonds are produced at a-Si and c-Si surfaces exposed to plasma excited He/O2, or NH3 mixtures, respectively. These process-gas/substrate interactions occur in parallel with film deposition and are correlated with the electronic properties of the interfaces, and the layer deposition sequence.

We investigated the temperature dependent characteristics of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFT's) at temperatures down to 20 K. With decreasing temperature, the threshold voltage increased, the field effect mobility and the on-current decreased. The measured on-currents versus inverse temperature above 80 K are represented as the sum of two exponentially varied currents. It is concluded that on-current is nearest-neighbour hopping between 120 K and 80 K. Below this temperature, the temperature dependence of on-current is explained by variable range hopping and below about 30 K on-current becomes nearly independent of temperature. At very low temperature hopping probability may be governed not by temperature but by temperature independent tunneling, depending on the overlap of the wave function. The explanation of threshold voltage increase at low temperature is given.

Amorphous Silicon TFT performance is shown to be strongly affected by an air leak into a plasma deposition system during film growth. The introduction of a controlled air leak into the system raises the oxygen content of the active layer. For both high and low voltage transistors (HVTFT & LVTFT) the drive current, FET mobility, and threshold voltage are degraded with increasing leak rate. Moreover, the stability of HVTFTs containing a space charge limited region is adversely affected by increasing leak rate.

Field effect transistors (TFTs) with different channel lengths L and widths W have been prepared to study the influence of the geometry on the characteristics of the transistors. The mobility μ and the threshold voltage Vth are determined from the √Ids (Uds) plot. Effective values of L and W can be determined by analyzing their dependence on geometry. It can be shown that parts of the amorphous silicon which are outside the area between the source and drain contacts contribute to the current. The reason for this is that the drain-source field spreads out into these regions. Taking this effect into account we obtain the values for the mobility μ. The part of the overlap contributing to the transistor current is smaller than 2 μm.

The contamination effects induced by B2H6 and B(CH3)3 in a-Si:H layers are compared, using in-situ Kelvin probe experiments as well as SIMS measurements. It is found that B(CH3)3 induces at least 50 times less contamination, even at 250°C. It is also found that materials containing CH3 radicals have a lower electron affinity.

The temperature dependence of the growth, structure and transport of plasma-deposited microcrystalline and polycrystalline silicon films have been investigated over the temperature range of 50°C to 350°C. While low substrate temperature yields small grain sizes as expected, high temperature also tends to suppress the grain growth, contrary to the normal behavior observed in thermal CVD (chemical vapor deposition) where crystallinity grows with temperature. In fact, the highest temperature of 350°C corresponds to the lowest degree of crystallinity. Furthermore, it is found that, unlike the polycrystalline Si prepared by thermal CVD, the transport properties of the Si films do not necessarily correlate with the grain size or structural defects. While the largest average grain size of 800–1000 Å was obtained at 200°C, the highest Hall mobility and conductivity were obtained near 250°C, which corresponds to a material with smaller grains and an abundance of voids.

Internal photoemission is used to investigate the mobility edges in high quality, undoped, hydrogenated amorphous silicon (a-Si:H). The detailed studies carried out on intimate metal/ a- Si:H Schottky barriers are described and results on the electron barrier heights of these contacts formed on a-Si:H having different bandgaps are presented. The observed dependence of the barrier heights on changes in the optical gap due to hydrogen incorporation indicates that not all of these changes are due to the shifts of the valence band edges. The first results on the injection of holes into the valence band of a-Si:H are reported.

A new amorphous Si1-x.Bx. alloy with composition of boron x ranging from 0.01 to 0.5 was produced in a low pressure chemical vapor deposition (LPCVD) system. We performed Raman scattering experiments on the a-Si1−xBx alloys and specifically monitored the Si-Si TO-like mode at around 480cm−1 and the TA-like mode at around 130cm−1. A pronounced broadening of the TO-like band as well as a decrease of the intensity ratio ITo/ITA are observed with an increase of boron concentration in the a-Si1−x Bx alloys. Based on the Raman spectra, the structural changes induced by boron incorporation in the a-Si network are discussed in the conceptual framework of the continuum random network (CRN) model. We conclude that boron incorporation enhances the structural disorder and induces topological changes in the amorphous network.

Recent experimental results on the low temperature drift mobility in amorphous silicon are examined on the basis of the approach to hopping transport developed by Silver and Bässler. It is shown on general grounds that the main features of the experimental results cannot be explained by a purely exponential tail state distribution, but are consistent with the distribution used by Spear and Cloude (1988) in model calculations.

The focus of our work is to investigate the role of defects introduced by particle irradiation on the electrical and optical properties of amorphous silicon based alloys. We have studied photovoltaic cells and intrinsic films and have found that irradiation produces electrical and optically active defects which anneal at temperatures less than 200 degrees Celsius. The defects produce sub-band-gap states with a peak 1.4 eV below the conduction band edge; the structure of the material does not appear to be altered as evidenced by the band-tail states. Significant room temperature annealing effects have been observed following 1.00 MeV proton irradiation of intrinsic films.

Gap state profiles in a-Si:H around the Fermi energy have been determined by phase shift analysis of modulated photocurrents. Measurements on different samples in various light-soaked and annealed states show a general strong correlation of the defect structure with the Fermi level position, with a minimum in the DOS at Ef and a peak above Ef. A model of the defect structure involving a peak of D+ states above Ef is outlined that accounts for the observed correlations.

Optical bias enhancement in transient photocurrent, by a factor of more than 10, is observed in a-Si:H films. The transient photocurrent is found to decay exponentially shortly after the light pulse is turned off. The exponential decay constant of the transient current is found to be proportional to (optical bias level)−0.38 at room temperature. A theory based on the multiple trapping model is developed to explain the experimental result. This effect is applied to study the density of state of the sample as well as the process of charge recombination.

A dense network structure with low defect and void densities in a-SiGe:H alloys are realized by remote deposition conditions (triode reactor configuration, H2-dilution of SiH4+GeH4 gas mixture). The material properties are characterized by optical transmission, photothermal deflection spectroscopy (PDS), scanning electron microscopy (SEM), and space charge limited currents (SCLC). A high refractive index n∞. (λ->∞) and a fine surface structure in SEM images indicate an optical dense network, moreover the defect density determined by SCLC is reduced in comparison to films deposited in a conventional diode reactor. The substrate temperature TS, one of the key parameters for high quality low gap material, sensitively influences the optical and electronic properties of the samples. The optimum substrate temperature (TS≍=225°C) is found to be lower than for the deposition of a-Si:H (TS≍250°C).

In an effort to enhance the electrical properties of silicon thin films, we have performed recrystallization experiments on a variety of amorphous silicon films using an excimer laser. The intense, pulsed UV produced by the laser (308nm, using XeCl gas) is highly absorbed by the amorphous material and thus provides intense localized heating in the near surface region. Two types of starting films were studied: plasma CVD a-Si:H and LPCVD a-Si. The subsequent modification produces crystallites whose structure and electrical characteristics vary due to starting material and laser scan parameters. The treated films have been characterized using Raman, x-ray diffraction, TEM, SIMS and transport measurements. The results indicate that crystallites nucleate in the surface region. The degree of crystallization near the surface increases dramatically as a function of deposited laser energy density and less so as a function of laser shot density. The hall mobility of the highly crystallized samples exhibit an increase of 2 orders of magnitude over the amorphous starting material. In the PECVD material, the rapid diffusion of hydrogen causes voids to be formed at intermediate laser energy densities and removal of film at higher energy densities. The LPCVD material withstands the high laser energies to produce well crystallized films with crystallite sizes greater then 1000Å.

Experimentally measured current-voltage measurements in amorphous silicon hydride pin diodes have been fit to computer simulations by Hack and den Boer [1] using relatively small values for the neutral and charged trap capture rates. Silver and Cannella [2] have proposed larger values for the capture rates, accompanied by larger electron and hole band mobilities. We discuss here the possible reasons for expecting the larger values for the capture rates, and their influence on current voltage measurements both with and without the higher mobilities. It is found that the simulated I-V results using only the larger capture rates are much smaller than those for Hack and den Boer's parameters. However, when the large values are used for both the capture rates and the mobilities, the I-V relationship is quite similar to that for the smaller capture rates and mobilities. We also show the differences in the simulated transient currents for the different sets of parameters, and we discuss the causes of these differences.

The standard time of flight technique is modified to analyse the internal field profiles in hydrogenated amorphous silicon or amorphous alloys devices. The spatial resolution of the field can be better than a thousand angstroms.

With this method the internal field profile in a Schottky structure, built in the sandwich geometry Pt/a-SiGe:H/Cr, was studied. The field profile is found to decrease exponentially with the distance to the Pt/a-SiGe. We also underline the non-ohmic behaviour of the a-SiGe/Cr interface. The shape of the internal field along the intrinsic zone in P-I-N devices is also studied by the same method.

The variations of the field profiles in both devices (Schottky and P-IN) with various applied DC biases is presented.

Several stationary and transient techniques are used to characterize a large number of intrinsic a-Si:H films prepared in a narrow range of production conditions. Correlation between stationary photoconductivity, the activation energy of the dark conductivity and the long-time range decay of the transient photoconductivity and photoinduced absorption is observed. The applicability of a-Si:H films for solar cells in view of these properties is discussed.

p- and n-type weakly absorbing highly conductive (σ>0.1(Ωcm)-1) SiC thin films with similar structural and optoelectronic properties have been prepared in a TCDDC reactor. These films contain Si microcrystals embedded in an amorphous Si:O:C:H matrix. C is preferentially incorporated as stoichiometric SiC, whereas O is present mainly as suboxide. Absorption coefficients smaller than cr-Si are observed for films containing about 20at% C and 25at% O. From diffraction studies there is no evidence for the presence of SiC crystallites. Photoluminescence studies show a peak at about 1.68eV, similar to gd a-SiC:H of comparable optical properties.

Plasma Enhanced Chemical Vapor Deposition CPECVD) is used extensively for the preparation of amorphous materials. However, to date we do not have a full description of the deposition process. By this we refer to the following steps ; the source gas decomposition [1], the gas phase reactions, diffusion within the plasma [a], adsorption of the various species, solid-gas reactions, nucleation and subsequent film growth [3–7]. To a large extent the diversity of the processes which are involved in film formation explain the observed variation in the characteristics of supposedly identical material made in different laboratories. Even with such variations certain trends relating the properties of the materials with the growth processes are apparent. In particular it is well established that hydrogen saturation of the dangling bonds is essential. Although how much hydrogen is optimum, and how it is incorporated in the growing film are questions of considerable importance.

Significant deployment of the promising option of photovoltaics for energy will require cost-effective technologies that compete effectively with conventional sources. One such option utilizes thin films of a variety of photovoltaic materials. These thin films must be manufacturable in large quantities, and they must have high performance and acceptable reliability. Amorphous silicon (a-Si) was the first successfully demonstrated thin film to be widely adopted by industry. This material is already used to power a larger number of such consumer products as calculators, watches, and battery chargers. Recently, a-Si solar cells have been scaled up to large-area modules for power applications. Large fields of these modules have been deployed by utility companies for their evaluation. Polycrystalline thin films such as copper indium diselenide (CIS) and cadmium telluride (CdTe) have recently shown promise in following the path of a-Si. High-efficiency, large-area submodules have been successfully tested. By combining these materials in hybrid combinations, researchers have demonstrated much higher efficiencies. Even higher efficiencies have been demonstrated with more conventional materials such as silicon and gallium arsenide in thin-film form. Such devices have a high degree of acceptability because of their successful application for power uses in their non-thin-film form. Extensive examples are given to demonstrate the technical viability of these photovoltaic approaches for possible use in utility-scale power plants and in other near-term, highvalue markets.