We report for the first time the photo-quenching of defect related photoluminescence emission from amorphous Si:H. There is no shift in the position of the emission peak, although the magnitude of photoquenching depends on the material, through the density of the states of the gap. Photoenhancement is not observed, even when the band tail emission is increasing, suggesting a new recombination mechanism.

We have studied the photoinduced changes of the conduction bandtail states and dangling bond defects in a series of n-type doped hydrogenated amorphous silicon samples of different doping levels, using the techniques of drive level capacitance profiling and voltage pulse transient photocapacitance. Each film was examined in a series of partially annealed states of progressively higher anneal temperature following light soaking. A general decrease in the number of dangling bonds and an increase in the number of occupied bandtail states was observed with increasing anneal temperature. The observed changes are discussed with respect to previously proposed defect reactions for photoinduced changes in n-type doped films, and we conclude that none of them can explain the observed changes in the lower doped film. Second, we discuss the application of transient voltage pulse photocapacitance and transient junction photocurrent to the study of the energy distribution and trapping properties of the photoinduced dangling bond defects in undoped hydrogenated amorphous silicon. We observe that the hole capture cross section of the photoinduced deep defects is larger by a factor of 3.3 compared to that of the intrinsic deep defects.

Through sub-bandgap absorption spectra and photoconductivity measurements of high-quality hydrogenated amorphous silicon (a-Si:H), changes in the electronic density of states with light-soaking and subsequent annealing are qualitatively described. Included is a more complete understanding of the role of deep defects as recombination centers. In this way, the simple power laws of Stutzmann, Jackson, and Tsai for the light-induced degradation of defect density and photoconductivity are. retained in the modified forms Evidence is also presented that the states closest to midgap, those states that act as effective recombination centers, are the first to anneal.

A Kaufman ion-beam source has been used to study the rehydrogenation and postdeposition hydrogenation of amorphous silicon. In the rehydrogenation study, hydrogen atoms were implanted into glow-discharge-deposited amorphous silicon materials in which the hydrogen content had been driven out by heating. In the posthydrogenation study, amorphous silicon samples with no hydrogen content detectable by infrared absorption and no photoconductivity were used as the starting material. These materials were deposited by thermal CVD, magnetron sputtering, or RF glow discharge.

We have grown two series, ‘SLOW’ and ‘FAST’, of high quality a-Si:H films by the VHF-GD (Very-High-Frequency Glow-Discharge) technique, the first at a low deposition rate of R∼3.6 Å/sec and the other at a high rate of R˜17 Å/sec. Both series show the same bulk deep defect density Nb≈4×1015/cm3 in the annealed state ‘A’ as measured by PDS. From lightinduced degradation experiments (Staebler-Wronski effect, SWE) the following interesting features are observed : a) The SWE is a bulk effect. To demonstrate this result we propose the use of PDS in combination with variable film thicknesses as a powerful and unambiguous method, b) The increase of the deep bulk defect density ΔNb up to the surated state of the SWE (state ‘B’) is the same for series ‘SLOW and ‘FAST’, c) However, the kinetics of the SWE differ considerably.

These results are interpreted with the help of experimentally observed continuous variation of the Urbach energy E0 of the valence bandtail accross the film. The E0 profile is, in turn, related to the deposition rate R. The presence of this inhomofgenei yis also discussed in context with.the recently developed equilibration theories relating deep and shallow (bandtail) states. Preliminary results in this study seem also to support the model involving conversion of weak Si-Si bonds into dangling bond upon light soaking. Further the influence of internal film stress as well as the effect of impurities on the degradation behaviour are addressed.

A systematic study of the electronic and optical properties of a-Si,Ge:H,F alloys in the initial and light-soaked states is presented. The three alloys have optical gaps of 1.20, 1.34 and 1.40eV. Constant photocurrent method (CPM) spectra show that the higher gap samples begin to exhibit increased subgap absorption due to light soaking much sooner than the low gap sample. Dark arid photoconductivity remain essentially constant as functions of light soaking time up to approximately 100 hours of illumination, when the dark conductivity begins to increase and the photoconductivity begins to decrease simultaneously. The electron mobility-lifetime product measured by the time-of-flight technique shows much scatter but no significant change with light-soaking. The hole mobility-lifetime product on the other hand decreases immediately upon light soaking and continues to decrease gradually with increasing illumination time. A discussion of these effects and a possible explanation in terms of the density of states is presented.

The magnitude of metastable light-Induced degradation of intrinsic hydrogenated amorphous silicon films deposited by rf glow discharge from different selene-hydrogen mixtures wan studied. By stone of spacecharge-limited current measurements the Fermi-lava1 density of states was determined for as-grown and light-soaked n+-t-n+ devices. Films deposited using a mixture of 55 vol.% silane and 45 vol.% hydrogen and a relatively high rf power level of 62 mW/cm2 to obtain a high growth rate exhibited an improved stability with regard to light-Induced degradation of the film, compared with films grown under the conditions of using pure etlane and an rf power Just sufficient to seintaln the plasma, conditions known to ylald good quality films. The results are explained in terms of an improvement in the structural oraer of the film.

A new explanation is offered for the metastable changes caused by combined electric field and elevated temperature in properties of a-Si:H such as doping efficiency, solar-cell efficiency, and “structural relaxation.” This explanation is based on properties of the localized centers that are associated with the light-induced Staebler-Wronski effect and solar-cell degradation. The newfeature is recognition that the metastable state of these localized centers is more polarizable than the ground state, so that afield increases the energy difference between the two states. Then, when the temperature is high enough to permit transitions, the density of metastable defects decreases, producing the observed effects. This explanation is general in the sense that it does not depend on any atomic model of the centers involved.

The degradation of amorphous silicon (a-Si) pin solar cells under prolonged illumination (Stabler-Wronski-Effect SWE) is investigated. We first measured the spectral response due to low-intensity illumination. The external collection efficiency Q is a function of the light wavelength λ and the applied voltage U. The functions Q(U) saturate for increasing reverse bias. Normalization to the saturation values defines the internal collection efficiency q. The functions q(U) for different λ cross through a nodal point with coordinates Uf, qf The voltage Uf represents the flat-band voltage of the uniform-field model. Our first SWE aspect consists of a substantial qf reduction caused by enhanced photocarrier recombination near the pi- and in-junctions. We additionally measured the photocharacteristics due to white illumination at a variable intensity ϕ The photocurrents vanish at the transition voltage Ut which is independent of ϕ for an undegraded cell. With advancing degradation, Ut decreases but Ut(ϕ) becomes an increasing function. The latter effect represents our second degradation aspect, supplementing the first one.

Light soaking gives rise to the increase of active acceptors as well as to the creation of dangling bonds in boron doped hydrogenated amorphous silicon. The annealing out of the metastable active dopants is slow compared with that of the created dangling bonds, results in higher conductivity than annealed value during heating the sample upon light soaking. The activation energy of the relaxation time for the metastable active dopants is ˜0.1 eV higher than that for the annealing of the created dangling bonds. We ropose a new model to explain our results.

The temperature and light intensity dependences of photoconductivity(PC) and PC response time(tr) were investigated in undoped a-Si:H. The effects of light-soaking at 110K and 300K on PC and tr are compared. The experimental results suggest that two kinds of metastable centers are induced by lightsoaking: one corresponds to increase in the subband-gap absorption, the other corresponds to the degradation of μτ product.

An analysis of the phenomenology of a class of bistable charge-trapping defects is undertaken and it is proposed that such defects explain the dominant metastable effects observed in doped hydrogenated amorphous silicon (a-Si:H). The stable configuration of such a defect changes with its charge state and a substantial barrier retards changes in configuration. Reconfiguration is accompanied by carrier trapping or emission. Published data on metastabilities observed in quenched, depletion-bias annealed and light-soaked films of both P-doped and B-doped a-Si:H are reviewed. The experimental observations are consistent with the proposed model.

In this paper we describe the design and operation of two novel types of amorphous silicon thin-film transistors and outline their application in large-area microelectronics. We first consider a high voltage transistor that can modulate a source-drain voltage in excess of 400 volts by applying low voltages to a controlling electrode covering a small portion of the channel near to the source. Secondly, for high-current output and moderately high voltage applications, we have fabricated vertical amorphous silicon transistors with channel lengths much smaller than the lithographic minimum feature size used in their fabrication. We show the design of both these new transistors together with their physics of operation and give results of output characteristics.

The integration of low and high voltage transistors into large-area circuits has enabled us to develop new applications for amorphous silicon in printing, input scanning and electronic copying. Page-wide arrays of both ionographic and electrographic printers have been fabricated. By combining amorphous silicon photodiodes with transistor arrays, we have made high resolution document scanners and copiers with directly coupled print and sensor elements. The ability to fabricate short channel vertical transistors offers the potential to further increase the speed and resolution of these large-area circuits.

We propose a new structure of vertical a-Si FET with the intermediate layer between the source and drain consisting of the multi-layer structure. The mechanism of off-current in short-channel vertical-type a-Si FET is discussed. The multilayered structure is incorporated in an attempt to reduce the back gate effect. The preliminary experimental results are also presented.

Whereas one-dimensional models can adequately predict the current-voltage behavior of ideal thin-film transistors, a detailed study of current flow requires a comprehensive two-dimensional simulation. Such an analysis provides important information regarding effective series resistance, overlap capacitance, and currentcrowding near electrodes. Numerical simulations also allow the rapid development and optimization of new devices.

We have developed a two-dimensional finite-element device simulator (MANIFEST) which we have used to study the effect of source-to-gate misalignments on the performance of amorphous-silicon TFTs. We find that submicron source-togate gaps do not seriously impair TFT performance.

This paper investigates the creation rate of carrier-induced defects in hydrogenated amorphous silicon (a-Si:H) metal-insulator-semiconductor (MIS) devices. The results demonstrate that holes increase the rate of defect formation by over 3 orders of magnitude while electrons increase the rate by roughly an order of magnitude. The carrier-induced defect kinetics, light-induced defect annealing, and decay of excess carriers in doped a-Si:H are quantitatively consistent with dispersive diffusion of hydrogen involved in the metastable process. The results provide strong evidence for the involvement of hydrogen in metastable effects in a-Si:H.

In this paper it is demonstrated for the first time that the material properties of amorphous silicon films deposited from a pure silane plasma can be improved by an appropriate fluorine implantation. It is shown that the field-effect density of states is reduced by this treatment, and that the instability induced by different treatments is reduced. This technique is of particular interest for the fabrication of thin film transistors.

Elementary amorphous silicon logic integrated circuits have been manufactured on glass substrates by rf glow discharge. Circuits presented include two different inverters, a 6-transistor addressable static memory cell, a 3-transistor addressable dynamic memory cell, and a 9-stage ring oscillator. All of the circuits use n-channel enhancement type load TFTs instead of ohmic resistors. The channel length of the driver transistors is 10 μn and 15 μm, experimental geometry ratios range from β=5 to β=33 The influence of the geometry ratio on static and dynamic characteristics is examined. Circuits operating at supply voltages of less than 5 V have been demonstrated.

The performance of a thin-film transistor (TFT) driven linear image sensor is analyzed. By experiment and computer simulation the charge trans-fer is investigated and the results are applied to interprete the performance of the conventional readout circuit. Its main disadvantage are the large number of crossovers between signal lines. Therefore, an alternative readout circuit with reduced number of crossovers is proposed.

Passivation of the back channel of thin film a-Si:H FETs is discussed. A one-dimensional model is used to predict the effect of back surface interface state density on the threshold voltage and subthreshold slope. A passivation method is described which results in a very high density of interface states. Two methods based on dual gate FETs are used to determine the interface state density. The principal effect of this method of passivation is to make the threshold voltage and subthreshold slope dependent on silicon thickness. This dependence is verified experimentally. For silicon > 150 nm, the dependence is weak. Variations in the deposition temperature of the passivation dielectric and the use of SiNx and SiOx are shown to have only a small effect.