The thermally-induced decay of light-induced, paramagnetic neutral silicon dan-gling bonds(K centers ) in hydrogenated amorphous silicon nitride thin films is monitored using electron spin resonance. The nitride films are of gate-quality and nitrogen-rich and are deposited at two different temperatures (250 and 400 °C). The kinetics for isothermal annealing of the light-induced K° states is dependent upon sample deposition temperature and is observed to follow a stretched exponential dependence, exp {— (t/τ)β}, upon annealing time (t). The stretched exponential factor, β, shows a non-linear dependence upon annealing temperature including temperature independent regimes. Thermal annealing is thermally activated with an apparent activation energy of ∼ 0.4 eV and is independent of deposition temperature. These results indicate that annealing is a dispersive process which involves hopping and multiple trapping or trap controlled hopping in the thermal annealing of light induced K centers in amorphous SiN1.6:H.

Metastable defects are induced in a-Si:H p-i-n devices by a forward bias current. The defect density increases approximately as the square root of time, reaching saturation at long inducing times, and with a weak temperature dependence. Current-induced defect annihilation is observed, in which the current causes a reduction in the previously induced defect density. Calculations of the changes in the forward bias current for different bulk defect densities are able to account for the measured results.

The paper presents the experimental data of single a-Si:H cells degraded under high intensity illumination and compares them to the numerically calculated J-V characteristics of degraded a-Si:H solar cells. The cells were deposited at two different substrate temperatures. The current voltage characteristics were calculated using our simulation program which allows for accurate determination of cell response under monochromatic or global AM1.5 spectrum.

We have simulated the light-induced metastable defects (Staebler-Wronski degradation) by increasing the density of localized states. In order to test the validity of this approach we compared the calculated fill factors under AM1.5, blue and red illumination with the measured fill factors for the same light.

From the experimental data we observed mat at near room temperature (∼35°C) both cells reach saturation in degradation of fill factors under high intensity illumination. The saturation level corresponded to the minimum density of localized states (gmin) of 1.2×1017 cm-3eV-1 and is independent of the light intensity used in experiment.

Metastable defects are created in hydrogenated amorphous silicon alloys presumably by the same mechanism as in a-Si:H. We find metastable defects created in a-SiCx:H and a-SiOx:H by hopping injection of photocarriers from adjacent a-Si:H layers. In a-SiCx:H the defects can be created only below T=150K, they anneal at TE=400K. In a-SiOx:H they are created at or below T=300K, they anneal at TE=480K. The anneal temperature is nearly independent of the creation temperature. The defects are detected by their charge exchange with adjacent a-Si:H layers whose conductance is thereby changed.

A successful application for a-Si:H is as the photosensor in a liquid crystal optically addressed spatial light modulator (OASLM). We analyze the response time of an a-Si:H p-i-n photodiode in a “pseudo-OALSM,” in which the liquid crystal is replaced by an equivalent capacitor, under both forward and reverse bias. Under reverse bias the two important effects are the photocurrent response time, and residual trapped charge. Under forward bias the mechanism shifts from double injection regimes to ohmic transport as a function of voltage. We relate these characteristics to the operation of an OASLM.

Using the combination of a novel amorphous silicon (a-Si) photosensing element and four a-Si thin-film transistors (TFTs) we have developed a circuit which has been incorporated into a two-dimensional image sensor to provide gain at the pixel level. Our circuit only requires two global connections and the complete array is fully process compatible with standard a-Si TFT fabrication procedures. This contrasts with conventional a-Si imaging arrays which have no gain at the pixel level, require an additional global power line, and necessitate a separate sensor deposition.

The dynamic response of pixels from an a-Si:H imaging array are investigated and a model is developed to explain their observed behavior. Such arrays are under development for radiotherapy and diagnostic x-ray applications.

A-Si : H and a-Si : H/a-SiGe : H heterojunctions have been prepared as the photoconductors in the liquid crystal light valves. The impedance matching conditions of the a-Si : H and the heterojunction in the valves have been carefully analysed, which has shown that the adjustment for the capacitance of the photoconductor is the key procedure for the matching, but the low photocapacitance effect in the a-Si : H limits the further improvements on the matching. The formation of the a-Si : H/a-SiGe : H diode promotes the photocapacitance effect in the photoconductor, and the matching behavior in the valve is greatly improved.

The experimental results for different structures of contact image sensor are reported in this paper, which includes Schottky barrier, p-i junction, p-l-n junction, and MIS structures. The J-V characteristics of p-i-n a-Si: H contact image sensor under dark and illuminated conditions have been simulated by solving the Poisson's equation and the continuity equations, and the results are correlated with the experiments. The dependence of the dark and photo currents on the parameters such as the density of states in the gap, intrinsic layer width, dopant concentrations of p* layer and n+ layer are discussed.

Csl(TI) layers 100–1000 μm thick were evaporated on glass substrates from a crystal Csl(TI). When they were exposed to calibrated X-ray pulses, their scintillation properties were found to be comparable to those of a crystal Csl(TI). Single p particles from radioisotopes were successfully detected by these layers coupled to a crystalline Si photodiode. The light spread inside evaporated Csl(TI) was measured by an amorphous Si (a-Si:H) photodiode array coupled to evaporated Csl(TI) layers. Monolithic X-ray detectors were fabricated by evaporating Csl(TI) on a-Si:H photodiodes directly. The signal yield and noise of this prototype were 1.5×10+4 electrons/MeV and 3×10+4 electrons FWHM, respectively. Larger signal size and lower noise are expected by optimizing the photodiode design.

An ITO/a-Si:H Schottky Photodiode(PD) with Mo as a barrier metal at Al/ITO contact has been made and it shows its useful characteristics. The PD without Mo had high dark current proportional to the contact-via-hole area and quite a large resistance at the contact area. The XPS analysis of the Al/ITO interface indicated that Al and O in ITO mutually diffused around the interface, resulting in the degradation of ITO/a-Si:H Schottky barrier. Most of the diffused Al formed aluminum oxide (AI2O3) at the Al/ITO interface, and which caused high resistance of the contact area.

Inserting Mo barrier metal 500Å thick, the dark current got less dependent on the contact area and decreased by 2/3 on 254×227 μm PD with 106×106//m contact-via-hole. The contact resistance also decreased significantly to 1/105 of those of the conventional ones. From the XPS analysis of interface, it was also found that Mo barrier metal prevented Al diffusion into the ITO and formation of aluminum oxide at the interface.

A new type of ferroelectric liquid crystal light valve (FLCLV) is presented. The design of the FLCLV is based upon the linear equivalent circuit analyses. A photosensor in the FLCLV consists of a metal-insulator-semiconductor (MIS) photodiode. A-Si:H doped with boron and nitrogen [a-Si:(:N:B)] is used in the MIS diode. The a-Si:H(:B:N) film has a dark-conductivity of less than 1×1012 S/cm and a high photosensitivity.

Consequently, the writing characteristics of the FLCLV for a two dimensional (2D) image are evaluated. Using writing light of 630 nm and 1 mW/cm2, a high resolution capability of 120∼140 1p/mm is obtained.

We have measured the optical absorption edge spectra for absorption coefficients between 1 and 105 cm-1, the photoluminescence spectra at 77 K, the conductivity-temperature relations, and the photoconductivity magnitude and spectral dependence for a series of r.f. glow discharge films of a-Ge:H. The films were deposited at the powered cathode and unpowered anode of a diode capacitive reactor having very different electric fields and plasma conditions, while substrate temperature, H2 dilution of a GeH4 plasma and applied r.f. power were varied. The structure of the films are radically different, with the anode-deposited films displaying a microstructure of low density material (voids) and the cathode-deposited films displaying homogeneity similar to that of device-quality a-Si:H. A self consistent explanation of the differences in measured optical and electronic properties is given, taking full account of the structural observations. From this analysis the conditions required for the production of a-Ge:H of good photoelectronic quality may be inferred. Preliminary structural, optical and photoelectronic data for a-Si1.x Gex :H of large x, prepared under conditions extrapolated from the a-Ge study, indicate significant improvements from current data in the literature.

Deuteron magnetic resonance (DMR) resolves structures such as tightly bound D, weakly bound D, molecular HD and D2 in microvoids and trapped on nanovoid internal surfaces in plasma-deposited a-Si:D,H and a-Ge:D,H. The relative populations of these structures correlate with photovoltaic quality as characterized by photo response function ημτ in a series of deuterated a-Ge and a-Si films. It is found that photo-illumination produces DMR-detected changes in these populations. The changes, which are reversible upon a 150°C dark anneal, include a quadrupolar doublet from strained-bond configurations and a shift from metastable electronic species. The spin-lattice relaxation of most of the components follows an error-function behavior characteristic of magnetization transport under extreme inhomogeneity.

In the energy range below mid-gap, absorption spectra obtained by photothermal deflection spectroscopy (PDS) show much higher values than data evaluated from photo-current measurements. This difference can now be explained as an additional absorption arising from combined vibrational excitation of molecular hydrogen and Si-H stretching modes, which can occur when the H2 molecule is deformed in a collision with Si-H. Down to lower energies, our samples absorb even stronger than in mid-gap. Extension of spectral scans down to photon energies of 0.4 eV shows that this results from absorption of the high energy wings of the collision induced H2 absorption, which is superimposed by the first overtone of the Si-H and Si-H2 stretching modes. The analysis of the H2 absorption profile and strength on samples as grown and annealed gives evidence that molecular hydrogen is enclosed in small cavities and polarized by anisotropic environment in as-grown material while it is concentrated in larger voids under high pressure after annealing above 350°C.

Measurements of co-planar current fluctuations in n-type doped hydrogenated amorphous silicon (a-Si:H) find that the spectral density of the noise accurately obeys a 1/f frequency dependence over the frequency range of 1 Hz to 1 kHz for temperatures ranging from room temperature to 450K. The noise displays a power law dependence on the d.c. curent passing through the sample, with a temperature dependent power law exponent. In addition, the resistance of the a-Si:H as a function of time displays switching phenomena; a surprising result given the effective volume ( ∼10-6 cm3) of the sample.

A new technique based on measurements of photoinduced current noise in amorphous silicon solar cells is reported. A 1/fn (n-1) component, superimposed to a frequency independent one due to simple shot noise, is attributed to the presence of defects limiting the conversion efficiency of the cell. In particular large defects can easily be detected since they are characterized by a very large increase of the l/fn component. In general a correlation is found between the intensity of the l/fn component and short circuit current density, fill factor and cell conversion efficiency.

We report on some recent results of first principles molecular dynamics simulations on a-Si. These simulations yield interesting results that challenge the standard beliefs about what constitutes a defect, how light induced defects arise, and about the origins of conduction band tailing.

It is proposed that hydrogen reactions near the growing surface control the structure of plasma-deposited amorphous silicon and its alloys. The model explains how the plasma influences the dangling bond density and silicon weak bond distribution, which are the main parameters influencing the electronic properties. The hydrogen interactions account for the dependence of the electronic structure on growth conditions, the transition to microcrystalline growth in hydrogen-rich plasmas, and the different electronic properties of a-Si:H alloys. The proposed growth mechanisms are closely related to the metastability phenomena which occur in bulk a-Si:H films.

We have studied the structural and transport properties of a-Si1−x Gex:H alloys made by DC glow discharge of SiH4 and GeH4 with substantial amount of H2 dilution. The dilution ratio, i.e. H2/(SiH4 + GeH4), was varied up to 40:1. IR spectroscopy revealed that H2 dilution greatly suppresses the tendency of forming microvoids due to Ge incorporation. Photoconductivity and ambipolar diffusion length, which represent transport properties of electrons and holes respectively, are also significantly improved by using H2 dilution. At a dilution ratio of 20:1, the electron υτ product reached 10-6 cm2/V at a bandgap 1.55 eV which is even larger than that of a device quality a-Si:H. Solar cells using alloy i-layers made with H2 dilution were fabricated. The stability of the devices under light exposure was found to be much improved with increasing H2 dilution, suggesting a strong correlation between the stability and the density of microvoids.