Microcrystalline silicon (μc-Si) p-layers have been widely used in amorphous silicon (a-Si) solar cell research and manufacturing to achieve record high solar cell efficiency. In order to further improve the solar cell performance and achieve wider parameter windows for the process conditions, we studied the deposition of high quality μc-Si p-layer material using a very high frequency (VHF) plasma enhanced CVD process. A design of experiment (DOE) approach was used for the exploration and optimization of deposition parameters. The usage of DOE leads to a quick optimization of the deposition process within a short time frame. In addition, by using a modified VHF deposition process, we have improved the solar cell blue response which leads to a 6–10% improvement in the solar cell efficiency. Such an improvement is likely due to an improved microcrystalline formation in the p-layer.

Hydrogenated amorphous silicon (a-Si:H)/ crystalline silicon (c-Si) type heterodiodes in solar cell structures have been studied by rf glow discharge, dc magnetron sputtering, and a remote plasma deposition of a-Si:H onto p type c-Si. Carrier transport and photogeneration in such structures have been investigated by current-voltage-temperature, thermally stimulated capacitance (TSCAP), and spectral response experiments. Dark carrier conduction is found to be a combination of tunneling and interface recombination, but is dominated by either one depending on the deposition/sputtering conditions. The conditions investigated include energy of the plasma species, type of plasma cleaning, and substrate preparation techniques. For each of the conditions, the trap type, energy and concentration have been identified by TSCAP. Solar cells fabricated by the optimized fabrication scheme routinely yield 10.5% efficient devices having a short circuit current density (Jsc) of 30 mA/cm2, a open circuit voltage of 0.55 volts and a fill factor (FF) of 0.64, without an AR coating, over 0.3 cm2 area.

The emitter of amorphous/crystalline silicon heterojunction (HJ) solar cells is normally very thin. Consequently, the metal used as a front contact can produce a partial or even total depletion of this layer. As a result, the diffusion potential of the p-n junction deviates from its maximum value. In this paper, we report the results concerning HJ in which either metal dots (Au, Al), semitransparent metal layers, or indium tin oxide (ITO) dots or layers were used as front contact on the same HJ structure, namely (p)a-Si:H / (i)a-Si:H / (n)c-Si / Al. We show that, for thin p-layers, the dark and light J-V characteristics of HJ solar cells depend on the material used as front contact. In particular, we found that the dark saturation current increases if a low work function material is used. This increase is interpreted in terms of p-layer depletion, and is shown to directly influence the J-V characteristics under illumination, producing a reduction of the open circuit voltage of solar cells.

We report on the fabrication and stability tests of single junction a-Si:H, and tandem junction a-Si:H/a-Si:H solar cells using the ECR process under high hydrogen dilution (H-ECR process). We show that devices with high fill factors can be made using the H-ECR process. We also report on the stability studies of the solar cells under 1 and 2-sun illumination conditions. The solar cells show very little degradation even after 500 hours of illumination under 2 x sunlight illumination.

Time-delay problems will become extremely severe in active-matrix-addressed ultra-large liquid-crystal display-panels. The problem caused by large crossover capacitance can be solved by moving the signal bus-line to the facing glass plate. The problem caused by large gate capacitance can be solved only by reducing the TFT channel much shorter than the conventional value. This requirement can be satisfied by a novel vertical TFT fabricated by using a selective excimer-laser crystallization method. Switching characteristics have been evaluated for the prototype devices, and good on-characteristics have been confirmed for the O.5μm-long gate device. Improved device structures are also presented.

We have developed analytic SPICE models for hydrogenated amorphous silicon (a-Si:H) and polysilicon (poly-Si) thin film transistors (TFTs) which accurately model all regimes of operation, are temperature dependent to 150°C, and scale with device dimensions. These models have been presented in [1, 2]. In this work, we compare the current-voltage characteristics predicted by our models with the measured characteristics from TFTs fabricated at different foundries. We compare the extracted device parameters in order to evaluate the robustness of our models and to determine a suitable default parameter set. We also use the models to examine the effects of device scaling for short channel TFTs. The models can be accessed using the circuit simulator AIM-Spice [3], which is available at http://nina.ecse.rpi.edu/aimspice.

We review the status of active matrix display fabrication on plastic substrates, specifically active matrix liquid crystal displays (AM LCDs). Research efforts in the U.S. and Europe are surveyed. Currently available transparent plastics, and active programs to improve these materials are described. Our work on thin film transistor (TFT) fabrication at 125°C is then reported, with a comparison of reactive sputtering and plasma enhanced CVD methods.

Defects and carrier transport processes in silicon based thin-film transistors (TFTs) are investigated by spin-dependent transport (SDT). The resonance signal arising from less than 106 defects in the hydrogenated amorphous silicon (a-Si:H) TFT is detected with a sufficient signal-to-noise ratio. The leakage current mechanism in a-Si:H under high source-drain fields is identified by SDT as electron hopping via defect states located at the interface between undoped a-Si:H and the passivation silicon nitride layer. At temperatures below 100K, spin-dependent hopping of electrons in conduction band tail states is observed. The change of the dominant transport path from extended states conduction to variable range hopping with decreasing temperature is confirmed. SDT measurements on polycrystalline silicon (poly-Si) TFTs having silicon nitride and silicon dioxide as the gate dielectric films reveal differences in the defect structure in these devices. The overall results demonstrate that SDT is a powerful method to probe paramagnetic defects and carrier transport in TFTs.

We have developed a new amorphous silicon image sensor technology using a matrix array of amorphous silicon thin film transistors and photodiodes, where the amorphous silicon nip photodiode is fabricated on top of a thick insulating layer, on top of the thin film transistor array. We call this ‘diode on top’ technology or DOTTY. The active diode area can be as high as 93%, compared to 50% for our conventional photodiode-TFT technology. This leads to a higher signal to noise performance, which is important for medical X-ray applications.

We demonstrate a new technology for RC gate delay reduction, by fabricating an array of amorphous silicon thin-film transistors (a-Si:H TFTs) on a thin glass substrate provided with via holes. All gates are connected through via holes to a metal line that is run on the back side of the substrate. We opened via holes with a diameter of 35 to 50 μm in 50 μm glass foil. For the first time, all TFT pattern definition steps used a process which employs electrophotographic toner masks.

We have investigated the effect of depositing amorphous silicon (a-Si:H) under α and γ (or dusty) plasma conditions on hydrogen bonding and TFT performance. By infrared measurements three deposition regimes could be identified due to their distinctive absorption curves in the bending mode range of 800 to 900 cm'1. These corresponded to a films deposited below 300°C, γ films deposited below 300°C, and films deposited in either plasma condition at temperatures of 300°C and above. There was a correlation between the a-Si:H material regimes and TFT performance. When the deposition temperature was below 300°C the a-TFTs had a higher field effect mobility than γ-TFTs, but lower stability. For deposition temperatures of 300°C and above the films had more similar properties regardless of whether they were deposited under a or γ conditions. TFT mobilities were the same, but TFTs containing a-Si:H deposited under γ conditions were still more stable. These results show that the mobility and stability of TFTs are optimised for different growth conditions, and that the overall best conditions for TFT manufacture depends upon the specific application.

The large number of localized energy states in amorphous and polysilicon thin film transistors causes non-crystalline effects in both the DC and AC transistor characteristics. The observed frequency dispersion of the device capacitances is linked to the characteristic times of electron trapping and emission from localized thin film transistors and is modeled analytically by introducing effective RC time constants, which are proportional to the electron transit times, determined by the field effect mobility. The small-signal gate-to-source and gate-to-drain capacitances have been derived using Meyer's approach, which takes into account the non-zero drain-source voltage to achieve a partitioning of the channel capacitance. We have verified the model for amorphous silicon thin film transistors for varying gate lengths and frequencies.

Analog circuits using amorphous silicon thin film transistors offer significant advantages for in situ signal processing in large-area optical and x-ray imagers. However such circuits are susceptible to gate-bias-induced shifts in the threshold voltages of the constituent transistors. In this work, the change of threshold voltage for devices undergoing cycles of stress, relaxation and reverse bias is measured in order to determine the feasibility of resetting the threshold voltage electrically. It is concluded that, although the reverse bias does assist the recovery of the threshold voltage, the process is still not sufficiently rapid. An analog amplifier circuit is then described which uses negative feedback to achieve a gain that is stable to within 6% over a period of 8 hours.

Inverted-staggered thin film transistors (TFTs) incorporating hydrogenated microcrystalline silicon for both contact and channel regions have been fabricated by plasma enhanced chemical vapour deposition (PECVD) using the high hydrogen-dilution method. The deposition parameters for the channel region were chosen to yield near-intrinsic material with a dark conductivity activation energy of 0.7 eV and a Tauc gap of 1.98 eV, while the doped contact layer was optimised to produce a high dark conductivity of 10 S/cm.

These devices exhibit a low off-current but the field effect mobility is found to be lower than that of similar devices incorporating an optimised amorphous silicon channel region. The mobility activation energy in these devices is similar to those incorporating an amorphous channel, but the mobility pre-factor is reduced. We propose that this is due to inhomogeneous conduction through a microcrystalline region with a smaller grain size at the dielectric/channel interface.

The Semi-Insulating field plated High Voltage TFT (SI HVTFT) is a new poly-Si HVTFT structure with a much improved blocking capability and enhanced on-state performance [1][2]. The unique feature of the SI HVTFT is the semi-insulating amorphous oxygen-doped Si (SIAOS) field plate which connects the gate to the drain and reshapes the potential distribution in the offset region. The leakage current, flowing through the SIAOS field plate during device operation, determines device performance. In this paper, both experimental and 2-D simulation results for devices with different field plate conductivities are used to investigate the impact of the field plate properties on the performance of the SI HVTFT structure.

High-quality thin film transistors (TFT) with hydrogenated amorphous silicon (a-Si:H) deposited by hot-wire (HW) chemical vapor deposition as the active layer at growth rates above 20 Å/s are compared to TFTs with a-Si:H deposited by RF glow discharge at 1 Å/s. The subgap absorption measured by the constant photocurrent method and steady-state photoconductivity measured between source and drain are used to characterize the a-Si:H in the TFT. The activation energy of the dark conductivity is measured as a function of the gate voltage to obtain the position of the Fermi level. The effect of a bias stress on the TFT transfer curve is obtained.

An ambitious neurotechnology program has been started in Germany in 1995, in the long run aiming at the realization of visual prostheses for blind people. A broad technological approach has been chosen which besides crystalline silicon microelectronics also involves amorphous silicon photodiodes for subretinal implantation, because thin film technology may be able to offer better solutions to some of the complex problems. Special topics we report on here include the development of low temperature deposition techniques for enabling the use of flexible plastic or bio-degradable substrate foils, the study of protective and bio-compatible coatings, as well as novel contacting and energy supply schemes. The key issue for stimulating retina cells by the use of technically generated photocurrents is an optimum capacitive coupling to these cells. For this purpose we study several contact layers (p-doped a-Si:H, microcrystalline Si, metal-induced crystallization) which provide high perpendicular but at the same time low lateral conductivity, thereby greatly reducing parasitic losses to the surrounding tissue. The photovoltaic mode of operation of the implanted photodiodes may be limited due to shortcomings in the charge transfer to the nerve cells, in which case additional infrared energy has to be coupled into the devices. Local light-induced stimulation can then be realised by using an a-Si:H i-layer as a photoresistor on top of the IR-sensitive solar cell.

This paper deals with performance aspects of a selenium photoconductor on a thin film transistor array when used in x-ray imaging, e.g. for medical applications. We present the effects of doping structures within the Se on the spatial resolution, temporal response and dark current characteristics. Measurements were done on a 1922 pixel array with 200 μm pitch and a collecting electrode covering 50 % of the pixel area. Without additional measures the detector has a prohibitively slow response. With properly chosen doping structures the lateral conductivity can be engineered in a manner that the speed of the Se-TFT detector is increased to a level acceptable for video rate imaging. There is also a critical relationship between doping structures in the photoconductor and the spatial resolution of the detector. An idea of how these doping structures work in terms of resolution and speed of the detector is presented, together with proposals for an optimized doping scheme.

We present measurement results of the time-dependent reverse current in amorphous silicon Schottky diodes for a broad range of bias voltage stress conditions. The resultant behavior can be divided into three regimes, depending on the bias. At low biases, the reverse current exhibits a power-law dependence attributable to dispersive electron transport. In the medium bias regime, the current shows a dramatic increase which may be due to enhanced thermionic emission and tunneling of electrons across the barrier, enabled by hole transport to the metal-semiconductor interface. At high biases, the effects of prolonged stress were found to be irreversible. Here, an eventual decrease in reverse current was observed, with an associated loss of rectifying characteristics.

The detection of the fundamental components of the visible light (blue, green, red) is achieved with two terminal photo diodes based on amorphous silicon. By changing the bias voltage the preferential carrier collection region is shifted which leads to a color sensitivity. In order to obtain a high dynamic range, independent voltage controlled spectral response curves as well as a linear response of the photocurrent on the incident light intensity the μτ-product and the bandgap in the device have to be specially designed to deconvolute the optical signal and generate an RGB-signal. Since the light intensity can strongly influence the spectral sensitivity by recharging of defect states, an optimized design of the multi-layer structure is necessary. Therefore, an improved concept for the design of nipiin- and piiin-detectors is presented which results in a good suppression of these non-linearities. Our concept is based on a decreasing bandgap profile from the front to the back contact and an increasing μτ-product of the individual i-layers in direction of the p-layer.