Current-voltage, light I-V and internal quantum efficiency characteristics have been investigated in specular TCO/n+(a-Si:H)/i(a-Si:H)/Nickel Schottky barrier cell structures with protocrystalline intrinsic layers. The studies were carried out on structures with different thickness i layers after a degraded steady state had been reached with AMI.5 illumination at 27°C. These characteristics were modeled using the Analysis of Microelectronic and Photonic Structures(AMPS) and a gap state distribution, which includes charged defects, used in the analysis of results of detailed studies on thin films[1]. Fits are obtained to these characteristics for the different thickness cell structures using the same parameters as those used to fit the results on the corresponding intrinsic thin films. The results obtained from this study offer an approach to more reliable modeling of solar cells at their “end of life”.

The preparation of amorphous silicon films and solar cells using SiH2Cl2 source gas and electron cyclotron resonance assisted chemical vapor deposition (ECR-CVD) was investigated. By using buffer layers to protect previously deposited layers improved a-Si:H(Cl) solar cells were prepared and studied. The high quality a-Si:H(Cl) films used in this study exhibited low defect densities (~1015cm-3) and high stability under illumination even when the deposition rate was increased to ~15A/s. The solar cells were deposited in the n-i-p sequence. These solar cells achieved VOC values of ~ 0.89V and ~ 3.9% efficiency on Ga doped ZnO (GZO) coated specular substrate. The a-Si:H(C1) electron and hole μτ products were ~10-8cm2/V.

Surface micromachining is used with amorphous silicon, microcrystalline silicon, silicon nitride and aluminum films as structural materials to form bridge and cantilever structures. Low temperature processing (between 110 and 250 °C) allowed fabrication of structures and devices on glass substrates. Two processes involving different materials as the sacrificial layer are presented: silicon nitride and photoresist. The mechanical integrity of the fabricated structures is discussed. As examples of possible device applications of this technology, air-gap thin film transistors and the electrostatic actuation of bridges and cantilevers are presented.

Amorphous silicon large area sensor arrays are in production for x-ray medical imaging. The most common pixel design works very well for many applications but is limited in spatial resolution because the available sensor area (the fill factor) vanishes in small pixels. One solution is a 3-dimensional structure in which the sensor is placed above the active matrix addressing. However, such high fill factor designs have previously introduce cross talk between pixels.

We present data for a design in which the a-Si:H p-i-n photodiode sensor layer has a continuous i-layer and top p+-layer, and a patterned n+-layer contact to the pixel. Arrays of 64 μm and 75μm pitch have been fabricated and are the highest resolution a-Si:H arrays reported to date. The resolution matches the pixel size, and sensitivity has been improved by the high fill factor. Comparison is made between arrays with standard TFTs and TFTs with self-aligned source and drain contacts. Data line capacitance is improved by use of the self-aligned contacts.

Measurements are included on the contact to bias capacitance. The high fill factor design greatly suppresses lateral leakage currents, while retaining ease of processing. Provided illumination levels remain below saturation, the resolution matches expectation for the pixel size.

An image sensor with enhanced sensitivity for near ultraviolet radiation (UVA) has been fabricated in TFA (Thin Film on ASIC) technology. The device employs an amorphous silicon pin detector optimized for UV detection by carbonization and layer thickness variation. The front electrode consists of an Al grid or TCO. Measurements show a peak responsivity of 90 mAW-1 at 380 nm. The UV Imager prototype consists of 128 × 128 pixels with a size of 25 μm × 25 [tim each, fabricated in a 0.7 μm CMOS process. Global sensitivity control serves to achieve a dynamic range in excess of 80 dB. The sensor can be used in fields such as chemical, medical and astronomical applications. Furthermore, a UV monitor has been developed, suited to warn of excessive sunlight exposure, considering skin type and sun protection factor.

Practical methods for directly patterning hydrogenated amorphous silicon (a-Si:H) films have been developed. Direct patterning involves selectively oxidizing the hydrogen passivated a-Si:H surface or laser crystallization of the bulk. The oxide or polycrystalline layer formed in this way then becomes a mask for subsequent hydrogen plasma etching. Methods for selective oxidation of the a-Si:H surface have been extensively studied. Examination of the pattern generation threshold dose for excitation wavelengths from 248 to 633nm provides indirect evidence for electron-hole recombination breaking of the silicon-hydrogen bond. An additional hydrogen removal mechanism was observed whereby simple proximity of a tapered fiber optic probe less than 30nm from the sample surface resulted in pattern generation. Patterns were generated in both intrinsic and doped a-Si:H films by several means, including contact printing with a mask aligner, in situ projection lithography with an excimer laser, and direct writing with a near-field scanning optical microscope (NSOM). Direct patterning of a-Si:H films has a wide range of potential applications. We have demonstrated a-Si:H as an in situ photoresist material for patterning HgCdTe infrared detector arrays with all process steps done in vacuum. We have also demonstrated 100nm line widths using NSOM writing with a photolithography goal. Direct patterning of a-Si:H could simplify the manufacturing of thin film transistors, or other devices that require patterned silicon films.

The growing interests for real-time applications of our amorphous silicon X-ray image sensors requires a better understanding of transient behavior in imaging characteristics. In our pixel structure, which consists of a p-i-n photodiode and an addressing thin film transistor (TFT), the image lag can stem from a number of mechanisms, including charge retention on the photodiode, TFT charge transfer efficiency, and data-line charge sharing. The exact prevailing mechanism depends on the array design and the operating conditions of the sensors. This paper describes the results of our systematic studies on image lag, and its dependence on various operating conditions of the image sensors, such as applied bias, light intensity, frame time, and applied gate voltages. We find that the charge retention in the intrinsic region of the photodiode is an important source of the image lag, and it is correlated to the forward transient characteristics of the diode. This correlation can be explained by the common underlying mechanisms leading to depletion region formation and charge transport through the diode. Furthermore, we notice that the time dependent decay of the image lag, originated mainly from charge retention in the diode, presents power-law time dependence, which is consistent with the release of trapped charges from a continuous distribution of defect states. We also find only a small variation in the image lag, <%3, for a wide range of gate on-periods, 10μs-80μs, and gate on-voltages, 15V- 30V, which indicates an efficient charge transfer by the switching TFT. These results demonstrate that in our devices, the image lag is controlled by the intrinsic defect levels in the a-Si:H photodiode.

Nipiin structures with controlled bandgap and mobility-life-time product exhibit excellent stationary properties. However, the photocurrent transients of these devices show reasonable delay times in the range of tens of milliseconds before reaching steady state. Hence, the transient behavior after light switching of three color detectors based on a nipiin structure was studied by a detailed analysis of experimentally determined and simulated data. The numerical simulation reproduces well the characteristic features. The delayed current onset and the sign reversal of the transient photocurrent response are explained by trapping of holes in the vicinity of the central p-layer.

In this work we demonstrate that radiation up to 2 μm induces photocurrent in a single junction amorphous silicon structure at room temperature. The absorber layer is a microcompensated film deposited using very low concentrations of dopant species. Device operation is based on optical excitation of thermal generated carriers from trap states toward valence and conduction band in the high electric field region of the structure. Transient and frequency response under different bias voltages and illuminations conditions are presented. The possibility to use the infrared sensor in low bit rate communication systems has been demostrated by including our detector in a front-end system and measuring its frequency responce.

Quantum efficiency measurement have been reproduced with a numerical model, able to take into account sub-band gap absorption into single films. Model results indicate the presence of a large valence band tail and a high number of dangling bonds and shallow defects ascribed to the presence of dopant atoms.

In this work we study the effects of material properties on the reading process of color detectors by using a two dimensional simulator for the transient regime. In particular, starting from the potential and charge distribution in the device, we describe effects of the density of defects on the self-bias process. Results show the possibility to engineer materials in order to optimize response speed of the device.

The performance of amorphous silicon p-i-n diodes as radiation detectors in terms of signal amplitude can be greatly improved when there is a built-in signal gain mechanism. We describe an avalanche gain mechanism which is achieved by introducing stacked intrinsic, p-type, and n-type layers into the diode structure. We replaced the intrinsic layer of the conventional p-i-n diode with i1-p-i2-n-i3 multilayers. The i2 layer (typically 1~3 μm) achieves an electric field > 106 V/cm, while maintaining the p-i interfaces to the metallic contact at electric fields < 7 × 104 V/cm, when the diode is fully depleted. For use in photo-diode applications the whole structure is less than 10 μm thick. Avalanche gains of 10~50 can be obtained when the diode is biased to ~500 V. Also, dividing the electrodes to strips of 2 μm width and 20 μm pitch reduced the leakage current up to an order of magnitude, and increased light transmission without creating inactive regions.

This paper gives the first report of a directional breakdown phenomenon present in specially constructed amorphous silicon / crystalline silicon heterojunction structures. After breakdown, the forward current can be greatly increased, but the reverse leakage current remains unchanged or even lowered. We use the Analysis of Microelectronic and Photonic Structures (AMPS) [1] device simulation tool to study this phenomenon and show that the structures can provide a new approach to producing Programmable Read Only Memory (PROM).

In this paper we present an innovative diode based on the heterojunction between amorphous silicon and porous silicon grown on crystalline silicon. The device architecture gives several advantages. Deposition of amorphous silicon on porous material realises high performance junction at temperature less than 250°C and it passives the porous layer against the natural oxidation due to ageing in the environment. Porous technology allows to obtain a controlled textured silicon surface independently from crystalline silicon orientation just to give the opportunity to reduce surface reflectivity and the blue shift of the absorption spectra in solar cell application. Solar cells were characterised by I-V dark/light and quantum yield measurements. Under standard AM 1.5 light we obtained photovoltaic conversion efficiency greater than 10%. Change in photoluminescence in different gas environments showed for gas sensor applications give rise encouraging results. In dark condition we found the typical diode behaviour.

The noise current spectral density of an a-Si:H pin diode can be calculated with experimentally determined flicker noise coefficients by superposition of the shot and flicker noise spectra of photocurrent and dark current. The dependence of the flicker noise current spectral density in pin diodes on the pixel area is calculated with our expansion of Hooge's law for flicker noise. We propose a new method for the calculation of dynamic range (DR) and signal-to-noise ratio (SNR) in pin diode pixels as a function of pixel area, dark current, photocurrent and the integration time of the imager. DR and SNR of the pin diodes are calculated for Thin Film on ASIC (TFA) image sensors.

We have investigated charge collection in thin amorphous silicon solar cells under light bias illumination, both experimentally and by numerical simulation. In such charge collection experiments, space charge due to trapped bias-light generated carriers leads to an enhancement of a small signal probe beam charge collection. It is found that this enhancement of the small signal charge collection is strongly dependent on the diode thickness and the defect density in the samples. In particular for thin diodes (d < 0.5 microns) the charge collection enhancement can be shown to increase with light-induced degradation of the devices. The effect of these material parameters as well as other experimental parameters, such as light bias and probe beam photon flux, will be demonstrated by means of numerical simulation.