There is a need to understand carrier generation polysilicon TFTs. When the generation is stimulated by the depletion of the material, due to the field effect, carriers are drawn to the surface and move along it by a process akin to diffusion. At some gate voltage the supply of carriers to the channel is insufficient to maintain the current flow. This leads to a reverse bias across the drain junction. It is proposed that carrier generation that occurs mainly near to the drain is more likely to be stimulated by increases of drain rather than gate voltage. The temperature dependence of currents at the current minimum, and at current saturation, of the transfer characteristic are described and are used to justify the model for generation and flow.

We use real-time spectroscopic ellipsometry (RTSE) for in-situ characterization of the optical properties and surface roughness (Rs) of hydrogenated amorphous silicon (a-Si:H) grown by hot-wire chemical vapor deposition (HWCVD) with varying deposition rates (5 to 120 Å/s). Early time evolution of the Rs during growth is remarkably similar for all deposition rates. During the first few Ås of growth, there is a sharp increase in Rs as the a-Si:H nucleates in separate islands. This is followed by a reduction of Rs as these areas coalesce into a bulk film, which occurs at an average thickness of 100 Å. After coalescence the Rs rises to a stable value that is dependent upon growth conditions with a general tendency for the Rs to increase with growth rate. However, neither the Rs nor the material electronic properties are unique for a given deposition rate. Films grown under high silane flow and low pressure have a better photoresponse and a lower Rs than films grown at the same deposition rate but with low silane flow and high pressure. We observe a stronger correlation of film properties with Rs than with deposition rate; namely a monotonic decrease in photo-response, and increase in optical gap, with increasing Rs.

The bias voltage dependent spectral response (with and without steady state bias light) and the current voltage dependence has been simulated and compared to experimentally obtained values. Results show that in the heterostructures the bias voltage influences differently the field and the diffusion part of the photocurrent. The interchange between primary and secondary photocurrent (i. e. between generator and load device operation) is explained by the interaction of the field and the diffusion components of the photocurrent. A field reversal that depends on the light bias conditions (wavelength and intensity) explains the photocurrent reversal. The field reversal leads to the collapse of the diode regime (primary photocurrent) launches surface recombination at the p-i and i-n interfaces which is responsible for a double-injection regime (secondary photocurrent). Considerations about conduction band offsets, electrical field profiles and inversion layers will be taken into account to explain the optical and voltage bias dependence of the spectral response.

We present infrared charge-modulation absorption spectra on phosphorus-doped amorphous silicon (a-Si:H:P) with doping levels between 0.17%-5%. At higher doping levels (1% - 5%) we find a sharp spectral line near 0.75 eV with a width of 0.1 eV. We attribute this line to the internal optical transitions of a complex incorporating four fold coordinated phosphorus and a dangling bond. This line is barely detectable in samples with lower doping levels (below 1%). In these samples a much broader line dominates the spectrum that we attribute to uncomplexed dopants. The relative strength of the two spectral features is in rough agreement with a model proposed by Street that has not been previously tested experimentally.

We compare switching behaviour in Cr/a-Si:H(p)/Ag and c-Si(p)/a-Si:H(p)/Ag structures containing p-type hydrogenated amorphous silicon. The a-Si:H layer is made by hot wire chemical vapor deposition. We observed that the switching is polarity-dependent only in the sample on c-Si(p). Switching to a low-resistance state occurs at 0.4 mA/cm2 when any of the metal contacts are biased positive. When the c-Si(p) is biased positive holes are injected and no switching occurs even up to 4 A/cm2. We suggest that the switching requires a blocking metal/a-Si(p), possibly because local electrical breakdown initiates metal filament formation.

Absorber layers of microcrystalline silicon thin-film solar cells deposited by plasma-enhanced chemical vapor deposition are characterized regarding the recombination lifetime. The characterization is based on a comparison of experimentally determined solar cell characteristics with results from numerical device simulations. Evaluation of the dark reverse saturation current indicates a strong dependence of τ on the hydrogen dilution during the deposition. Close to the transition region to amorphous growth where the highest solar cell efficiencies are observed τ is maximum within the crystalline deposition regime and equals 30 ns.

Using real time spectroscopic ellipsometry to characterize the microstructure and evolutionary growth of Si:H materials deposited with and without hydrogen dilution, phase diagrams were developed which clearly defined and established growth in the protocrystalline regime. Guided by these phase diagrams thin films and intrinsic layers in p-i-n cell structures were grown which consist solely of the protocrystalline phase so that the bulk uniform properties of the material could be characterized with confidence. Studies were carried out on the light induced changes in these films and cell structures that include the annealing out of defects as well as their creation under 1 sun illumination at temperatures from 25°C to 100°C that include the attainment of a degraded steady states (DSS). Defect states were characterized in films with electron mobility lifetimes (μτ), and subgap absorption at 1.2eV (α(1.2)); and in the i material of the p-i-n cells by the bulk limited fill factor (FF). The contributions of the different gap states to SWE are identified and characterized. The absence of direct correlations between α(1.2) with μτ and FF present in undiluted and diluted materials also found in protocrystalline Si:H. Similarities, on the other hand, are found between the μτ products and the FFs including the striking changes in the kinetics that occur at ∼40°C. Direct correlations between the changes in μτ and FF at different temperatures are presented. The reason for this correlation and lack of it for α(1.2) are briefly discussed with direct correlation of the α(1.2) to cell characteristics being presented, be it not with the FF.

Rapid progresses are achieved in catalytic CVD (Cat-CVD), often called hot-wire CVD, in the past 3-years NEDO national project in Japan. Cat-CVD technology presents many advantages in thin-film formation processes; high-efficiency of gas use, large-area deposition, no ion bombardment and low-temperature deposition even below 200°C. All of the elemental techniques for the industrially applicable Cat-CVD apparatuses, such as the suppression of the metal contamination, the precise control of the substrate temperature, the life extension of the catalyzer, 1-m size uniform deposition and the chamber cleaning, have been completely developed. Sophisticatedly designed substrate holder with electrostatic chuck and showerhead equipped with catalyzers are both key technologies for these achievements. High reproducibility for film properties is also obtained by controlling the reaction between high-density radicals and chamber walls. Prototype mass-production apparatus for SiNx passivation films in GaAs devices has been already developed and this will be probably the first application of Cat-CVD in industry. These recent movements appear to promise the drastic revolution in semiconductor and flat-panel display industries by introducing Cat-CVD in very near future.

A novel method for analysing post-transit photocurrent-time data using Tikhonov regularization is presented. The multiple-trapping rate equations are solved exactly in the time domain, avoiding certain mathematical approximations and numerical inaccuracies associated with approaches based on Laplace or Fourier transformations. Photocurrent decays simulated from discrete levels and model density of states (DOS) distributions are used to assess performance and to compare accuracy and resolution with existing methods. The technique is also shown to be effective as a practical DOS spectroscopy by application to experimental post-transit decays obtained from an amorphous silicon pin diode.

TFA technology, using ASICs coated with amorphous silicon based photo detectors, opens new applications for CMOS image sensors. One particular field of interest is the detection of low light level images with pixel photocurrents in the femtoampere range and below. In this paper we describe the effects of the capacitances on the pixel amplifier, we derive noise values for the detector and the amplifier, and we estimate the gain fixed pattern noise level. An inverter circuit providing a 40-fold increase of the charge conversion gain is presented.

A quantitative study of the trap-dangling bond tunneling recombination in hydrogenated microcrystalline silicon (μc-Si:H) is presented. The transition coefficients were measured at various light exposures and temperatures between T = 10 K and T = 140 K using time-domain measurements of spin-dependent recombination (TSR). TSR is a new characterization method related to electrically detected magnetic resonance (EDMR). It combines the advantages of pulsed electron spin resonance with that of EDMR. In contrast to previous models, the experimental results can only be interpreted if the interaction between the spins of the trap and the dangling bonds as well as triplet recombination is considered.

Proton NMR near 200 MHz has been used to examine the dynamics of hole-burning in HWCVD a-Si:H films deposited from SiH4. Radiofrequency hole-burning is a tool to distinguish inhomogeneous broadening from homogeneous broadening. The 3 kHz FWHM resonance line from T-site-trapped H2 molecules shows a hole-burn behavior similar to that found for PECVD a-Si:H films, as does the 24 kHz FWHM line from clustered silicon-bonded hydrogens. However the ∼80 kHz FWHM superbroad line varies with tip angle uniformly across the line for hole-burns applied at any location within ∼50 kHz from the resonance center. The superbroad line is homogeneously broadened and apparently arises from superclose SiH clusters with inter-proton distances considerably less than 2 Å.

Germanium films deposited on glass by plasma-enhanced chemical vapor deposition from germane and hydrogen grow in the structure succession of amorphous-nanocrystallineamorphous-nanocrystalline as the substrate temperature is raised from 30°C to 310°C. We ascribe the phase formation, from low to high temperature, to a sequence of low to high mobility of Ge growth species on a surface that is hydrogenated at low temperature but not hydrogenated at high temperature. We report some structural, optical, and electrical transport properties of Ge films as a function of deposition temperature and film thickness.

Undoped microcrystalline silicon samples deposited in the transition regime between amorphous and microcrystalline growth have been investigated by dark conductivity measurement and Raman spectroscopy. From the latter, a semi-quantitative crystalline volume fraction Xc of the sample was deduced and correlated with dark conductivity data in order to reveal possible percolation controlled transport. No threshold was observed around the critical crystalline fraction value Xc of 33%, as reported previously, but a threshold in conductivity data was found at Xc≈50%. This threshold is interpreted here speculatively as being the result of postoxidation, and not constituting an actual percolation threshold.

We compare the predictions of several analytical models for conductivity fluctuations in a homogeneous semiconductor containing discrete and distributed traps, using a Monte-Carlo simulation of the relevant multi – trapping (MT) transitions. The simulation directly embodies the statistical features associated with such processes, in a simple ‘model - independent’ approach, free of approximations and assumptions. We compare the results with those of several analytical approaches. In one, the noise spectrum is assumed to reflect separately, the characteristic individual release time constants of the various trapping centers in the material. In another, the trapping time into the ensemble of electron traps is taken to be the dominant time constant, and hence, in a material such as a-Si:H, where the trapping time into tail sates is of order 1ps, this is taken to imply that this component of the conductivity noise spectrum is unobservable in practice. Our own analytical approach, incorporates coupling (albeit weak) between traps, which necessarily communicate via the extended states. Preliminary results of the simulation support our thesis, and verify that the same information is contained in the real part of the modulated photoconductivity (MPC) spectrum. A ‘full Monte’ – Carlo simulation incorporating all gap states and spatial inhomogeneities is now a priority.

Wire-desorbed radicals present during hot-wire chemical vapor deposition growth have been measured by quadrupole mass spectrometry. For wire temperatures in excess of 1500 K, Si is the predominant radical desorbed from a new wire, with a minor contribution from SiH3. Aged wires showed profoundly different radical desorption kinetics, consistent with evaporation of Si from liquid silicon. It is proposed that this aging is related to silicide formation at the surface of the wire.

In this paper we report on the use of the expanding thermal plasma (ETP) technique for the deposition of carbon-free SiO2 films by means of hexamethyldisiloxane (HMDSO)/oxygen mixtures, at a growth rate of 8-10 nm/s. Information concerning the film chemical properties and refractive index/growth rate have been obtained by means of FTIR measurements and in situ single wavelength ellipsometry, respectively. Because of its geometry, the ETP configuration has proven its suitability for studies concerning the fragmentation paths of the HMDSO molecule and the reactions occurring in the plasma phase. In this framework, very recent results obtained by coupling the SiO2 film-deposition set up with a very sensitive, high spectral resolution- absorption technique, Cavity Ring Down Spectroscopy, are presented.

Results from previous work indicate the feasibility of the amorphous silicon (a-Si) active pixel sensor (APS) readout circuit that performs on-pixel amplification for low-noise, real time imaging applications (e.g. fluoroscopy). In this paper, the noise contribution of the APS readout circuit is examined. In addition, due to the metastable nature of a-Si, the APS stability is discussed. Unlike conventional PPS systems with one thin film transistor (TFT) switch per pixel, the APS has three TFTs per pixel. The large number and sizes of a-Si TFTs can reduce the pixel fill factor if the TFTs are not embedded beneath the sensor as in continuous layer sensor architectures. In this paper, we present preliminary APS noise and stability measurements along with considerations of fully overlapped pixel architectures on APS performance.

Low temperature lateral growth of amorphous silicon films has been achieved on thin flexible glasses using ultra-violet assisted metal-induced crystallization technique. 125μm ordinary glass substrate is sputter-coated with 1500Å chromium and a 1000Å SiN layer, respectively. 1000Å Si film was deposited using e-beam evaporation at a temperature of 350°C. Equally spaced dots of nickel pads with 140μm separation were used as seed of crystallization of a-Si layer. Crystallinity of the samples was studied using XRD, SEM and optical microscopy. Some crystallographic etchants were used to develop the crystal orientations for SEM analysis. Based on this study, a lateral growth rate of 2μm/hr is obtained at a temperature of 380°C. Activation energy of 1.4 to 1.5eV is extracted for this UV-assisted MILC process.

Mechanism and growth rate of Metal-Induced-Lateral-Crystallization (MILC) with annealing temperature range from 550°C to 625°C were studied. Base on the MILC growth mechanism and effect of metal diffusion, a modeling on metal impurity distribution was developed. The modeling can be used to predict the distribution of metal impurity formed in the polysilicon layer after MILC annealing process. By applying the modeling, effects of annealing on the metal impurity distribution can be analyzed.