The present paper describes a new type of cylindrical X-ray detector based on thick amorphous silicon diodes. The authors report on the successful development of these detectors that have diameters as small as 3mm. The effect of silane dilution in hydrogen on deposition rate, powder formation, layer adhesion and detector performance is discussed. A set of deposition parameters is presented, that allows a reasonable compromise between the mentioned effects. First prototypes of this type of dosimeter have been characterised with the radiation from a 60Co source in a hospital. The corresponding results and interpretation are given in this paper.

In order to overcome the intrinsic speed limitation of amorphous silicon nipin color sensors we present an alternative way of achieving bias-controlled spectral sensitivity of two-terminal thin film devices, piin structures with appropriate band gap and thickness of their single layers can be used as photodetectors that are able to sequentially extract different color signals. Color separation is achieved by controlling the absorption and electric field profile across these piin devices, and thanks to the differences in electron and hole transport properties. Because in contrast to nipin devices there is no need for reverting readout voltages for color separation, this type of sensors can be operated at much higher readout frequencies. Spectral response and bias voltage transients have been analysed up to 20kHz, and preliminiary data are presented on the optimization of speed, dynamic range and color separation by varying bandgap and thickness of p- and i-layers. Furthermore a three-color sensor has been realized by introducing an additional intrinsic layer.

Bandgap and μτ - (carrier mobility x lifetime) - engineered multi-color nipin detectors with three linear independent curves of the spectral response in a range from 470nm to 670nm are successfully fabricated. The spectral response of nipin structures employed as two terminal devices shifts by the applied voltage. Switching transients after changing the color sensitive voltages under different monochromatic illumination conditions are investigated and can be classified in different cases. In order to improve the understanding of the photo current transients, we have characterized the influence of different illumination conditions on the voltage dependent capacitance behavior. To verify the capacitance measurements, a SPICE model has been developed, which describes the transient behavior of the detector. The simulations are in good agreement with the experimental data and support the explanation of the transient cases after bias switching. Based on these results further optimization criteria to accelerate the transient behavior are presented.

Amorphous silicon (a-Si:H) based n-i-p-i-n/p-i-n-i-p structures serve as two color detectors. These basic structures can be extended to obtain multi color detection. In this paper capacitance measurements in the dark and under various illumination conditions as well as photo and bias current transient measurements in the time range 10-6-10-2 s are presented. The transients have been investigated for different bias voltages and photon fluxes.

Device capacitance becomes strongly bias dependent if illumination is applied to the sensor. It is dominated by the reverse biased diode of the nipin/pinip structure. Bias and photocurrent transients are influenced by defect recharging due to injection. For high photon fluxes only defect states near the band edges contribute to the transient current. This behavior is confirmed by dispersion like transient phenomena. Under dark conditions deep defects influence the transient behavior.

This work presents the main static and dynamic performances showed by one dimensional thin film position sensitive detectors (ID TFPSD), based on a-Si:H technology, with a size of 80 mm × 5 mm. The results obtained show that the TFPSD is able to respond to light powers as low as 2μ W/cm2, presenting a detection accuracy, linearity and response frequency better than 10 gm, 2% and 2 KHz, respectively. These results are quite promising regarding the application of these sensors to a wide variety of optical inspection systems where continuous quality control is required.

The influence of different TCOs (SnO2 and ITO) on the photoelectrical properties of 1 -D position sensitive detectors based on p-i-n structures was studied. A strong cross-contamination in the p-layer and contamination in the i-layer reduce the quality of the device. Numerical analysis of TCO/p-i-n structure also revealed a strong increase in defect states at the p-layer surface which can be attributed to the reduction of TCO. ITO seems to be less appropriate for a front TCO, although the spectral response of the p-i-n structure under reverse bias is not significantly affected by the conditions at the TCO/p heterojunction.

Progress towards x-ray shot-noise limited imaging with resolution superior to film is reported for a large format amorphous silicon imaging system containing 1536x 1920 pixels. Improvements over previous work include the development of a complete system, the use of charge-sensitive amplifiers, and careful control of extrinsic noise. A charge-sensitive amplifier with 1000:1 dynamic range is used to detect the signal in the presence of a large parasitic capacitance, 95pF, which both increases the intrinsic noise in the amplifier and technical noise coupled from other sources. The RMS noise level, 4200 e, is within two octaves of single x-ray counting.

In the tandem photovoltaic-electrochromic (PV-EC) device, a wide-gap, semitransparent, amorphous silicon-carbon alloy (a-SiC:H) photovoltaic device and an electrochromic optical transmittance modulator (EC device) are deposited sequentially to form a monolithic device on a single substrate. This device can be used as a “smart” window for active control of daylighting and building cooling load without an external electrical connection.

Last year we reported preliminary results on our development of a semi-transparent PV cell incorporating an a-SiC:H i-layer. Here we report our recent progress on the semitransparent PV component of a PV-EC device and development of a Li-based EC device that colors at voltages below 0.9 V. Finally, we discuss both recent progress and difficulties in integrating the two devices on one substrate.

Photoaddressed spatial light modulators (PSLM) based on nematic liquid crystals using hydrogenated amorphous silicon-carbon films (a-Si1-xCx,:H) and p-i-n structures as photosensitive layers are designed and studied. It is shown that an application of a-Si1-xCx:H p-i-n diodes greatly improved the operation speed of a PSLM, increased the film transparency and decreased the absorption of the red readout light. As a result, the readout light intensity could become ≈ 20 times larger. The maximum diffraction efficiency of the PSLM was achieved for a write-in light intensity of 40μLW/cm2, with a resolution (FWHM) of the diffraction efficiency of 54 mm-1. The optical response of the PSLM was observed in the frequency range from 1 Hz to 100 Hz.

We have studied the light-induced degradation of amorphous silicon-germanium (a-SiGe:H) alloy single-junction solar cells with high initial performance deposited at high rates. The intrinsic layers were deposited using microwave (MW) glow-discharge technique at deposition rates between 10 and 40 Å/s. The results show that light-induced degradation of the cells is higher than that of cells deposited at low rates using RF glow-discharge technique, and it does not strongly depend on deposition rates over this range. The total hydrogen content and the ratio of Si-H2, Ge-H, and Ge-H2 to Si-H bonding estimated by infrared (IR) absorption in films are correlated with the cell degradation results. We have also investigated the effect of ionbombardment on film properties. Films with low ion-bombardment are more porous and have higher composition of Si-H2 and Ge-H2 bonding. Appropriate ion-bombardment makes denser structure in a-SiGe:H alloy films deposited at high rates. This improves the cell performance as well.

Modulated Electroabsorption (EA) measurements have been widely used to estimate built-in potentials (Vbi) in semiconductor devices. The method is particularly simple in devices for which the built-in potential is dropped in a single layer of the device. However, experimental results in amorphous silicon and organic devices can involve at least 2 layers. In the present paper we consider the information which can be obtained about 2-layer semiconductor devices from electroabsorption measurements. In particular we describe a 2-layer EA model appropriate to a-Si:H based pin solar cells, for which both the p+ and i layers contribute to the EA signal. We present an analysis of capacitance and second harmonic measurements which yields the EA coefficient for the p+ layer of the device, and we present measurements on a-Si:H pin devices which appear consistent with this analysis. Wavelength dependent EA then yields the built-in potential across the 2-layer device.

We have performed voltage-pulse stimulated capacitance transient measurements along with other junction capacitance measurements on a-Si:H/ a-Si,Ge:H heterostructures deposited on p-type crystalline silicon using the glow discharge technique. For a filling pulse that puts the c-Si/a-Si:H junction in forward bias, the capacitance transients consist of two components- a fast component corresponding to electron emission and a slow component corresponding to hole emission. For a fixed starting reverse bias, we have found that the density of trapped holes is proportional to the product of the filling pulse voltage and the filling pulse width and reaches a saturation value at a certain value of the product. These hole traps appear to reside at or very close to the a-Si:H/a-Si,Ge:H interface. The estimated trap concentration is around 1011/cm2 and is independent of temperature and applied bias, ruling out the creation of the traps during the filling pulse. We also report results on samples in which a-Si:H/ a-Si,Ge:H interface has been modified during growth to alter the concentration of the hole traps.

We study the effect of illumination intensity on solar cell performance in a-Si:H solar cells. We find that the fill factor strongly depends on light intensity. As we increase the illumination intensity from low levels to one sun we observe a decrease in fill factor of approximately 15% in as grown cells. We attribute this effect to electric field collapse inside the cell. We propose that photogenerated space charge (free and trapped charge) increases with light intensity and causes field collapse. We describe the origin of space charge and the associated capacitance - photocapacitance. We measure the photocapacitance as a barometer to probe the collapsed field. We obtain a good agreement between photocapacitance experiments and theory. We also explore the light intensity dependence of photocapacitance and explain the decrease of FF with the increasing light intensity.

Based on measured characteristics of degraded p-i-n structures, simulations of degraded structures were performed using our numerical simulator ASPIN in order to fit and explain pronounced hump-shaped voltage-dependent internal collection efficiency (ICE) characteristics under weak short-wavelength illumination. Agreement with measured hump-shaped ICE characteristics was obtained only if in addition to the introduction of light-induced dangling bond defect states, their capture cross-sections were also increased, in particular capture crosssection for the charged defect states. This causes a change in occupancy of defect states at the p-i interface and front part of the i-layer under forward bias. Consequently, it increases the electric field in the front part of the cell, which results in recovery of the ICE.

Using suitable simplifying approximations inside the particular regions of a p-i a-Si/n c-Si heterojunction solar cell, the analytical expressions for the solar cell current-voltage characteristics are derived showing clearly the dominating first-order effects on solar cell performance. The derived closed form solutions indicate that in the useful forward voltage range the largest dark current component of this cell is the interface recombination current and that the main contribution to the photocurrent comes from the light generated holes in the c-Si substrate layer. The transfer of holes across the intrinsic layer and over the ΔEv barrier is strongly suppressed resulting in an attenuation of solar cell dark and photocurrent.

Analysis of the current density-voltage (J-V) characteristics of sandwich a-Si:H n+-i-n+ structures made of the same materials but with varying thicknesses of the i-layer, has given rise to two somewhat contradictory speculations regarding the density of states (DOS) at the Fermi level of the i-layer. The first of these postulates that the DOS over the first 2000–4000A of the i-a-Si:H layer is larger by a factor of 3–10 than that in the rest of the film With this hypothesis however, the experimentally observed low activation energies (Ea) for thin n+-i-n+ devices remain unexplained. In the other analysis, a consideration of the n+-i space-charge regions enables a fair description of the experimental results assuming that the electron DOS is position independent in the i-layer. In the present report we use accurate computer modelling to calculate and analyse the J-V characteristics of a-Si:H n+-i-n+ structures in the low bias ohmic region as a function of device thickness, ambient temperature and the quality of the i-layer. We find that the n+-i interface is abrupt only for a device with a high dangling bond density. For good quality material however, the depletion width extends right across the i-layer for a thin device and the zone of charge neutrality is never reached, resulting in low values of Ea in the ohmic region. In the latter case Ea increases with i-layer thickness. Analysis by detailed computer modelling confirms that the experimental results may be explained by assuming a density of states independent of the thickness of the i-layer.

In the present paper we show results of dark current-voltage measurements performed on p+ a- SiC:H/n c-Si heterojunction diodes at various temperatures (100–400K). We investigated the voltage derivative of these J-V curves in order to the distinguish possible current transport mechanisms. It was found that for low temperatures (<300K), the current is determined by recombination of carriers in the crystalline silicon, whereas at high temperature (>300 K), by a tunnelling mechanism. At room temperature, both mechanisms contribute to the current. By using an equivalent circuit model and detailed numerical simulations we have interpreted our experimental characteristics. The simulations done at room temperature, show that at low forward bias voltage the current is controlled by recombination in the crystalline silicon and that at high forward bias voltage by a combination of multi-step tunnelling and a-SiC:H series resistance. For interface state densities equal to or higher than 1012 cm−2, the recombination was found to be dominated by the states at the amorphous-crystalline silicon interface.

We have measured the admittance (conductance and capacitance) of a-Si:H/c-Si heterostructure Schottky diodes as a function of frequency, temperature and voltage in the dark and under spectral illumination (in the wavelength range between λ=500nm and λ=1200nm). Thus, it is possible to observe the activation/deactivation of trapping-detrapping effects within the a-Si:H layer (near the a-Si:H/c-Si interface). We have determined the conduction band offset of the a-Si:H/c-Si heterostructure. The spectral behaviour of the admittance is dominated by the absorption of light in the c-Si and the valence band offset of the heterojunction. We have also developed an equivalent circuit of the a-Si:H/c-Si heterostructure Schottky diode in the dark, which is capable of describing the measured behaviour.

Charge collection, transient photocurrents and collection efficiency under additional bias illumination were used to characterize 3–4 micron thick a-Si:H pin-diodes. The wavelength dependent decrease or increase in the spectral response, depending on the bias flux and absorption depth, is related to the distribution of the electric field, recombination and majority carrier diffusion. At higher photon flux an overshoot in the transient photocurrent after switch-on of steady illumination indicates the time scale for the changes in internal variables. Collection efficiencies under large bias monochromatic photon flux well in excess of the maximum value of 100 % for probe beam generated carriers are observed with a large amplification ratio. These efficiencies sensitively depend both on the applied voltage and the defect density. Numerical modelling reveals the influence of internal variables on the transient and steady state photocurrents under the different illumination conditions.

We have examined the material properties and operation of bottom-gate amorphous silicon thin film transistors (TFTs) using temperature measurements of the subthreshold current. From the derivative of current activation energy with respect to gate bias, we have deduced information about the density of states for several different transistor types. We have demonstrated that, in TFTs with thin active layers and top nitride passivation, the current conduction channel moves from the gate insulator interface to the passivation insulator interface as the transistor switches off. Our 2D simulations clarify these experimental results. We have examined the effect of bias stress on the transistors and analyzed the resulting reduction in the subthreshold slope. Based on these results, we have extended our analytic amorphous silicon TFT SPICE model to include the effect of bias stress.