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The influence of oxygen induced gap states on the device operation and the stability of polycrystalline pentacene thin film transistors (TFTs) was investigated by electrical in situ measurements. Unexposed devices are stable, whereas devices exposed to dry oxygen exhibit a shift of the threshold voltage upon prolonged device operation. Stressing the transistor in the on-state (negative bias) leads to a shift of the threshold voltage towards negative gate voltages, whereas prolonged bias stress in the off-state causes a shift of the threshold voltage in the opposite direction. The gap states are formed 0.18 eV (acceptor-like states) and 0.62 eV (donor-like states) above the valence band maxima. The charge carrier mobility and the on/off ratio of the transistor are not affected by the gap states. A simple density-of-states model will be presented, which allows for the explanation of the experimental results.

Polymer semiconductors are of increasing interest for thin film transistors (TFT) and active matrix arrays, since their mobility is up to ∼0.1 cm 2 /V s, and they offer simple fabrication by solution-based printing. To understand the role of disorder and localized states in the polymers, we measure p-channel polythiophene and polyfluorene TFTs, using both spin-coated and jet-printed films, and infer localized states properties from the TFT characteristics. Bias stress effects occur in the polymer TFTs, which exhibit a threshold shift without significant change in mobility. Holes are removed from the channel at a rate proportional to the square of their concentration, suggesting the slow formation of hole bipolarons.

The properties of polycrystalline pentacene thin film transistors (TFT) are reported, showing the influence of the deposition conditions with different inorganic and organic dielectrics. Dielectrics compatible with large area fabrication were explored to facilitate low cost electronics on glass or flexible plastic substrates. Transistors were realized on plasma-enhanced chemical vapor deposited (PECVD) silicon nitride, silicon oxide and spin casted poly vinyl phenol (PVP). The physical and chemical properties of the dielectric have a distinct influence on the morphology and the structural properties of the pentacene films. Polycrystalline films with large crystals and high structural order can be grown on smooth dielectrics. For rougher dielectrics the crystal size is reduced and the films exhibit no structural order. Furthermore, the growth is affected by the surface wetting properties of the dielectric. The surface chemistry can be controlled by a treatment of the dielectric with self-assembled monolayers (SAMS). The impact of the different dielectrics on the device parameters like mobility and subthreshold slope will be discussed.

Pulsed excimer-laser processing of amorphous silicon on glass substrates enables the fabrication of high-quality polycrystalline silicon (poly-Si) thin-film transistors. Here we describe the fabrication and testing of prototype imagers fabricated in CMOS poly-Si using a low-temperature, laser-crystallization process on glass substrates. Integrated shift registers and buffers for driving the gate lines of the array and integrated multiplexers on the data lines have been produced and are shown to run the imagers successfully without using external gate-line electronics. In addition, a three-transistor poly-Si circuit has been fabricated at each pixel, providing a charge gain of 11 for the small photo signal and a low noise level of 1200 electrons RMS.

Phase-change wax-based printed masks, in place of conventional photoresist masks, were used to fabricate a-Si:H thin-film transistors (TFTs). Printed wax-mask features with a minimum feature size of ∼20 μm were achieved using an acoustic-ink-printing process. Both discrete and matrix addressing structured bottom-gate TFTs with source-drain contacts overlapping the channel were created using a four-mask process. The TFTs had I–V characteristics comparable to photolithographically patterned devices, with mobility of 0.6–1 cm2/V s, threshold voltage of 2–3 V, and on/off ratios exceeding 107 for devices with channel lengths below 50 μm . The wax-mask process was also used to fabricate self-aligned TFT devices, eliminating the source-drain contact overlap constraint.

The growth of and the electronic transport in pentacene films on organic and inorganic dielectrics were studied. The morphology and structural properties of pentacene films are clearly correlated with the deposition parameters and substrate properties. To study the electronic properties we have formed inverted staggered transistors. The mobility in the transistors is correlated with the structural properties of the films and increases with crystal size. The TFTs exhibit typical mobilities of 0.4 cm 2 / V s and on/off ratios >108 on thermal oxide and smooth silicon nitride. The dielectric's influence on device performance will be discussed.

Organic semiconductor thin films offer new opportunities for large area electronics. In addition to having good electrical properties in thin film transistors and sensors, these materials can often be deposited by simple techniques such as dip-coating or jet-printing. We have fabricated two-dimensional imagers with organic light sensors, and report good imaging properties. The 512×512 element array has pixel size of 100 μm, and is addressed using amorphous silicon thin film transistors. The design allows a continuous layer of the organic sensor material without the need for pixel-level patterning, and so can take advantage of simple coating techniques. The sensors are bi-layers comprising a thin generator layer and a thicker hole transport layer. Three different sensors have been studied with generator layers comprising evaporated benzimidazole perylene (BZP) and dispersions of hydroxygallium phthalocyanine (HOGaPc) and titanyl phthalocyanine (TiOPc).

Laser processing allows the fabrication of self-aligned amorphous Si thin film transistors (a-Si:H TFTs). These devices have much smaller parasitic capacitance between gates and source/drain contacts and can have much shorter channel lengths compared to the conventional a-Si:H TFTs. We have fabricated matrix-addressed, optical imager arrays using these new a-Si:H TFTs as pixel switches. We also have demonstrated that four-phase dynamic shift registers using short channel a-Si:H TFTs can be operated at a clock speed of 400 kHz (less than 0.625 μs for each clock phase), indicating the possibility of integrating some of the peripheral circuits based on a-Si:H TFT technology. The advantages of using self-aligned a-Si TFTs as pixel switches in large-area, flat-panel imagers are discussed. Improved noise performance is expected for large area imager arrays.

Two-dimensional amorphous silicon image sensor arrays are used for medical imaging and document scanning. Our high-resolution arrays use a continuous p + –i–n + photodiode sensor layer with patterned n + -doped contacts. We report measurements of the lateral conduction phenomena that can arise with this sensor layer design, particularly when different illumination conditions give a non-zero voltage between adjacent pixels. The lateral conduction is affected by the sensor bias and by the potential of buried metal contacts. We also investigate the photocurrent collection properties by shining a focused laser beam in the gap between sensors. Collection at the contacts is evaluated, together with losses at the center of the gap. The effect of sensor bias on the collection is evaluated as well.

We discuss approaches to improve the performance of amorphous silicon (a-Si) X-ray imagers. Imager size has been increased to 30×40 cm and the number of pixels to >7 million. Processing improvements have nearly doubled the sensor fill factor (and hence sensitivity) and have reduced the data line capacitance which is the principal contribution to the electronic noise of the system. With new thin film transition (TFT) technology both amorphous and polysilicon TFTs made with the identical structure, using laser recrystallization are produced. The hybrid approach allows smaller leakage current a-Si:H devices to be used for the pixel TFT, and high mobility polysilicon devices for peripheral electronics. Arrays incorporating a lead iodide X-ray photoconductor have promise for higher sensitivity provided the leakage current can be reduced.

Large two dimensional amorphous silicon image sensor arrays offer a new approach to electronic document input and x-ray imaging. The sensor array technology is now capable of image capture at greater than 10 frames/sec and with resolution of 200–400 spi. We describe our new high resolution imaging system, comprising a page-sized sensor array with nearly 3 million pixels, and the accompanying high speed read out and processing electronics. The key technological issues of pixel resolution, sensor fill factor, leakage currents and noise are reviewed. Measurements of a new array architecture are described, in which the sensor is formed as a single continuous film on top of the matrix addressing components.

Dipolar spin lattice relaxation time, T 1D , measurements of 1 H in a-Si:H have been used to probe the local motion of hydrogen in a-Si:H. Variations of T 1D with doping level reveal a trend similar to the changes in hydrogen diffusion constants measured in similar samples using secondary ion mass spectroscopy (SIMS). The temperature variations of T 1D indicate that, if the local motion is thermally activated, the activation energies are at least an order of magnitude smaller than those measured using SIMS. The ‘diffusion constants’ for local hydrogen motion inferred from T 1D measurements are many orders of magnitude faster than the diffusion constants obtained from SIMS. The T 1D measurements are made on the timescale of milliseconds while the SIMS measurements are performed on a scale of several hours to days. A phenomenological model provides a plausible connection between these two measurements, which are made on very different timescales.

Measurements of photoluminescence in a-Si:C:H alloys are reported. The alloys are grown at different levels of hydrogen dilution in the plasma and the measurements are made to study how H dilution influences the defect and recombination properties. It is found that strong H dilution yields samples with low defect density and high photoluminescence efficiency. However, high H dilution also causes changes in band gap energy, which is partially responsible for the improved electronic properties.

Thermodynamic equilibrium kinetics of phosphorus (P) and boron (B) doped hydrogenated amorphous silicon (a-Si:H) are explored using dark conductivity measurements. Small variations in temperature of the semiconductor cause variations in active dopant concentration, which affects the conductivity. The equilibration kinetics of phosphorus- and boron-doped a-Si:H are different and there is a strong dependence on the deposition temperature (T s ). The transformation of P into the electronically active phase (activation) follows a stretched exponential time dependence with a temperature-independent dispersion parameter, β, of 0.85, whereas the transformation into the electronically passive bonding configuration (passivation) is also stretched exponential but with β ⩽ 0.8. The kinetics of boron metastability are similarly a stretched exponential, but with equal β values for activation and passivation. The time constant, τ, to achieve equilibrium for both transformations is thermally activated with energies E act ≈/ 1.1 eV for P and B. An increase in T 8 leads to an increase of E act .

1 H NMR dipolar echo measurements have been performed on a series of B-doped, P-doped, compensated and intrinsic a-Si:H samples. At room temperature the dipolar spin lattice relaxation time, T 1D , correlates with the macroscopic diffusion constants as determined by SIMS experiments. The local jump frequencies determined from these values of T 1D are orders of magnitude faster than those inferred from the macroscopic diffusion results and the associated activation energies are much less (∼ 0.2 eV). At 150 K and 100 K the dipolar echo in 10 −3 P-doped a-Si:H exhibits both a narrow line and a broad line. The full widths at half maximum of the narrow line and the broad line are the same as those of the narrow and broad components of the free induction decay, respectively. Because the narrow line exhibits a T 1D with a slightly different temperature dependence from the T 1D for the broad line the local motion is slightly different for hydrogen atoms in the clustered and dilute phases.

The relationship between electronic carriers metastable defects, and hydrogen motion in a-Si:H is investigated by photoconductivity and hydrogen diffusion experiments at temperature above 200°C. The photoconductivity decays with illumination time due to defect formation. The intensity dependence of the photoconductivity transients indicates that the defect formation mechanism is similar to that at lower temperatures. The photoconductivity data are used to analyse the dependence of the diffusion coefficient on continous and pulsed radiation.

We have performed hydrogen diffusion experiments in a-Si:H p-i-n diodes subjected to a constant and to a pulsed reverse electric field. Under a reverse field, the mean-free-path for diffusion of interstitial hydrogen is enhanced by a factor of at least ∼4. The results presented here indicate that the enhancement is not due to the field-drift of charged hydrogen atoms, but rather to the reduction in electronic carrier density under reverse bias. The carriers control the capture rate of interstitial hydrogen into Si-H traps and, therefore, the lifetime and the mean-free-path for diffusion through interstitials.

Hall data obtained on intrinsic, phhosphorus and boron doped a-Si:H and a-SiC:H in the temperature regime 200 K ⩽ T ⩽ 400 K are introduced and discussed. The data confirm double sign anomaly. The Hall mobilities, μ H , are significantly smaller than the drift mobilities, decreasing with increasing with increasing doping and/or carbon content. μ H of holes is about half that for electrons which indicates that μ H scales approximately with 1/E s , where E s is the tail slope. The presented interpretation of the Hall coefficient, that is introduced to be a function of Hall-and drift-mobility, enables the accurate determination of the carrier density in intrinsic and doped amorphous silicon and alloys from Hall data.

Thermally induced defects in a-Si:H are studied by time-of-flight transient photoconductivity. The increase in defect density upon quenching from elevated temperatures is observed as a decrease in the mobility-lifetime product. The defect densities agree well with ESR data and also show that defects are induced approximately uniformly in the bulk. No thermally induced changes in the drift mobility are observed, from which it is deduced that there is no detectable change in the band tail states by the structural equilibration, by contrast with the situation in chalcogenides.

We describe the characteristics of thin(1μm) and thick (>30μm) hydrogenated amorphous silicon p-i-n diodes which are optimized for detecting and recording the spatial distribution of charged particles, x-rays and γ rays. For x-ray, γ ray, and charged particle detection we can use thin p-i-n photosensitive diode arrays coupled to evaporated layers of suitable scintillators. For direct detection of charged particles with high resistance to radiation damage, we use the thick p-i-n diode arrays.

Experimental evidence for light-enhanced hydrogen diffusion in hydrogenated amorphous silicon is presented. Diffusion enhancement increases with illumination intensity and is suppressed in doped and in compensated material. The enhancement is attributed to an increased release rate of hydrogen from silicon-hydrogen bonds in the presence of photo-generated carriers.

A model based on hydrogen equilibration relates the growth of a-Si:H alloys to their electronic properties. The alloys are distinguished by the strength of the hydrogen bonding. The model explains why a-Si:C:H, a-Si:N:H and a-Si:O:H, have a large disorder broadening of the band tails and silicon dangling bonds, while a-Si:Ge:H has Ge dangling bonds and low disorder broadening. Possible growth conditions to improve the alloys are suggested.

The electronic stability of a-Si:H deposited in a remote hydrogen plasma reactor at high temperatures (e.g., T D =400 o C) and hydrogen dilution (e.g., H 2 to SiH 4 ratio of 20:1) is significantly different from that of optimized electronic-grade films from rf glow discharge deposition. This is a previously unexplored «parameter space» that should be accessible to other deposition techniques

The defect density in a-Si:H is obtained from the reverse bias current and transient forward current in pi-n devices. The kinetics of current-induced and light-induced metastable defect creation are compared, and current-induced defect recovery is reported. Computer modelling identifies the delay in the onset of the forward current as a suppression of the double injection current due to charge trapped in defects.

Experimental evidence is presented that the photoconductivity decay in a-Si:H can be dominated by contact effects rather than bulk properties. Two decay times are observed, one of which is the RC time constant associated with the contact depletion layer. The second, longer, decay is related to electron injection at the contact.

Doping in amorphous silicon is reviewed, with particular focus on the localized states introduced by doping, and the physical origin of the effects. Many experiments find that doped a-Si:H contains a large density dangling bonds and that the doping efficiency is low and concentration dependent. Recent experiments have given much new information about shallow states near the band tails. Neutral donors are observed directly by their hyperfine structure in ESR. A doping model is described which accounts for most of the data. However, many aspects of the doping process remain to be resolved.

Photoluminescence studies of melt-quenched GexSe1−x (0.2 < x < 0.43) glasses reveal two unresolved components with broad linewidths and large Stokes shifts. The lines have differing excitation spectra, composition dependences and decay lifetimes. Two distinct singly occupied localized states are also observed by optically induced ESR, and are distinguished by different composition dependences, microwave saturation, excitation wavelength dependences and annealing temperatures. These states are interpreted as defect states with negative correlation energy, originating from Ge and Se atoms. The annealing properties of luminescence fatigue and induced ESR indicate that at least one of the defects is common to both experiments. The annealing kinetics of ESR is reported and supports a model of recombination by tunnelling between singly occupied sites. A large increase in the luminescence linewidth is observed at the composition Ge37Se63. It is suggested that the reason is an anomalously large disorder potential that originates from the introduction of GeGe bonding states at the valence band edge.

Recent evidence that the electronic properties of doped hydrogenated amorphous silicon are influenced by thermal equilibrium processes are reviewed. The slow relaxation of the localized state distribution follows a stretched exponential time decay which we attribute to the dispersive diffusion of bonded hydrogen.

In this paper we investigate equilibrium changes in n-type amorphous silicon by means of capacitance-voltage measurements at varying frequencies. We show that both the density of shallow band tail charge as well as the dopant density are temperature dependent. Our results have been interpreted by means of a computer program enabling us to calculate the capacitance of thick Schottky barrier diodes based on a realistic density of states model for doped amorphous silicon. The computer modeling agrees well with the experimental data together with previously published results from sweep-out experiments.

The drift mobility and charge collection of compensated a-Si:H are measured using time-of-flight photoconductivity. The mobility of both electrons and holes decreases rapidly as the doping level is increased. From the results we suggest that shallow traps arising from boron-phosphorus complexes are introduced. The charge collection measurements demonstrate that compensated samples of a-Si:H have a low dangling bond density.

We report measurements of the trapping rates of carriers at charged and neutral dangling bonds in a-Si:H, using time-of-flight photoconductivity. The temperature dependence of the trapping rate is also measured from 150K to 340K.

Electron and hole time-of-flight measurements in glow-discharge a-Si: H have been made over a wide range of temperatures and electric field. Recently-developed techniques are used to identify the energy distributions of states within the conduction and valence band tails. Free carrier mobilities are also estimated, and whilst the value for electrons is closed to that normally accepted, the figure for holes is about 10 cm 2 V −1 sec −1 , which is an order of magnitude larger than the value often presently assumed.

Measurements of luminescence, optical absorption and infrared vibrational spectra are reported for thin films plasma-deposited from SiH4/O2/Ar mixtures. The addition of oxygen causes the optical absorption edge and luminescence peak position to shift monotonically to higher photon energy. Increases in the luminescence peak width and in the temperature of thermal quenching result in room temperature luminescence in the visible at high oxygen contents.

The nature of the deep recombination centers in a-Si:H is still a matter of considerable uncertainty. We present results here of transient ESR arising from dangling bonds in doped and undoped material at temperatures, defect levels and/or excitation energies for which direct band edge recombination is negligible. The efficiency for spin creation by sub- and near-band edge light is determined in the linear response regime in doped samples and is found to be of order unity. Thus, almost all recombination proceeds via a dangling bond, directly for sub-gap excitation or by a Shockley-Read mechanism for band edge excitation. The rate-limiting process for recombination is capture by neutral dangling bonds, D 0 , of electrons in undoped and n-type samples and of holes in p-type material.

The origin and nature of electrically active defects created by annealing are studied by luminescence, electron spin resonance, infrared absorption and hydrogen manometry. The initial defect density decreases similarly for all samples in annealing to ∼250°C. The temperature dependence of the subsequent increase in defect density is dominated by the effects of diffusion and sample microstructure and depends only indirectly on hydrogen bonding configurations. The peak in the rate of hydrogen evolution near 420°C is assigned to immediate release from isolated ≡SiH and/or =SiH 2 sites; this peak is found to shift to > 700°C because of diffusive delays in thick, homogeneous samples. Peaks at temperatures below 420°C are ascribed to hydrogen bonding sites on or near microstructural surfaces.

A theory of the temperature variation of the photoluminescence decay in plasma-deposited amorphous Si:H is presented. We use the low temperature lifetime distribution (neglecting diffusion) to obtain the radial distribution function of thermalized electron-hole pairs. Neglecting the effect of temperature on the distribution function, we use the exact solution of the diffusion equation, including tunnelling, to fit the luminescence decay data at higher temperatures. The Coulomb interaction of the electron-hole pair is treated in the prescribed diffusion approximation. We also obtain an estimate of the quantum efficiency vs. electric field at room temperature.

Light induced ESR (LESR) and photoluminescence measurements on doped and undoped a-Si:H are presented. From the temperature dependences it is shown that the same excited states are observed in both experiments. This result allows us to identify the origin of three LESR bands, and two luminescence transitions. From the data we also deduce that the primary defects in a-Si:H are dangling bonds with a positive correlation energy.

This paper clarifies the role of interfaces on amorphous silicon/nitride superlattice stuctures using TEM, SIMS and ESR measurements. Contrary to the common assumption of atomically sharp interfaces, the interface width is determined to be of the order of ∼10 A , arising from the reactive nature of the plasma. Moreover, our results indicate that there is a high density of charge residing at the interface, causing significant band bending in the multilayers.

Solid sources were used to evaluate potentially-interesting dopants for a-Si:H that as gaseous hydrides either are not readily obtainable or are of high toxicity when used in large volume. After a description of the remote hydrogen plasma reactor and deposition technique, results are presented which demonstrate the use of solid arsenic and antimony to dope a-Si:H for n-type electrical conductivity.

A chemical equilibrium model for PECVD of a-Si:H is used to interpret the irregular incorporation of As and P. The model shows that the irregularity occurs only for non-doping impurities; their distribution coefficient is inversely proportional to the rf plasma power, which leads to the large As concentration enhancement observed under low power growth conditions. A configuration energy diagram is proposed to account for this enhancement based on the preferential formation of stable, inactive As in a configuration other than the usual interconverting 3- and 4-fold coordinated sites.

The influence of the growth conditions on the equilibrium states in a-Si:H is explored by studying samples of different deposition temperature, rf power and gas dilution. A hydrogen-mediated weak bond model is used to calculate the defect density and to explain the correlation between defect density and relaxation times.