This paper outlines recent results from work on non-volatile, analogue switching effects in amorphous silicon devices. The implication for the switching mechanism of these results and also the technological applications of such analogue devices are discussed. Particular reference is made to their use as re-programmable, non-volatile weighting elements in neural network synapses.

A new type of the contact linear image sensor array using amorphous silicon photodiodes has been developed. The new device, namely “Full Contact” type, linear image sensor, has a pinhole of 60 µm in diameter at the center of the amorphous silicon photodiode pixel. With this structure, the focusing lens can be removed and the optical path can be reduced by about one half as compared with an ordinary facsimile scanner. Eventually the device becomes quite compact in volume and production process are rather simple, result in low cost. LED driven 1728 pixels for A4 size and 2048 pixels for B4 size devices with the resolution of 8 dots/mm have been developed so far. A series of technical data for the development of devices and their operational performance are presented and discussed.

We have been investigating the dark current and quantum efficiency of nip and pin photodiodes for document scanning applications. Alloying the top doped layer with carbon or making it microcrystalline was found to enhance the spectral response without causing significant deleterious effects to dark current. We also observed lower dark currents in both pin and nip(μc-Si:C:H) sensors compared to nip(a-Si:C:H) sensors, which we attribute to a more graded p-i interface in the former devices.

This paper reports the effects of a-SiC:H buffer layers on a-Si:H photodiode properties, especially the dark current and decay image lag. The photodiode has an ITO/p-type a-SiC:H/a-Si:H/i-type a-SiC:H/metal structure.

Using two heterojunctions–p-SiC:H/a-Si:H at the surface contact and a-Si:H/i-SiC:H at the back contact–significantly decreases the dark current. The p-SiC:H layer blocks electron injection, and the i-SiC:H layer suppresses the reaction between the a-Si:H layer and metal electrode. The use of both a-SiC:H layers may increase decay image lag. Excess boron doping to the p-SiC:H layer induces boron diffusion into the a-Si:H photo-electric conversion layer, which increases trap levels in the a-Si:H layer. The i-SiC:H layer, because of its high resistivity, decreases the electric field applied to the a-Si:H photo-electric conversion layer. Optimizing the amount of boron in the p-SiC:H layer and the resistance of the i-SiC:H layer enabled us to produce the characteristics required for high-definition television (HDTV) image sensors–a dark current on the order of 10−10 A/cm2 and a 1% level decay image lag.

It has been found that photosensitivity of amorphous silicon (a-Si) photoreceptor fabricated by g1ow-discharge decomposition of SiH4 – B2H6 – Ar has specific relation to peak-to-peak value of Rf (13.5 MHz) voltage (Vp - p) and DC bias voltage (Vdc) on the Rf powered-electrode. The lower the Vp - p value is depressed, the higher the photosensitivity is improved. Dependences of the Vp - p value on deposition parameters such as gas pressure, distance between Rf powered-e1ectrode and aluminum substrate, Rf electric power and dilution ratio of SiH4 to Ar seem to indicate correlation of Vp - p with electron energy and electron density in the plasma.

Furthermore, the Vp - p and Vdc values have been measured in plasma of He, Ne (rare-gases with no Ramsauer effect), Ar, Kr, Xe (rare-gases with Ramsauer effect), and H2, N2, CH4, SiH4 (molecular-gases): Specific behaviors in Vp - p and Vdc seem to have correlation with collision cross - sections of electrons (with the energy of 2 ∼ 10 eV) in these gases.

Large two-dimensional a-Si:H imaging arrays have applications in imaging of X-ray, gamma-ray and charged particle fluxes. Radiation imaging using a-Si:H p i n sensors together with a phosphor to transform the radiation into visible light is described, and some examples of images are shown. These arrays are very efficient in imaging large area diffuse sources, and the a-Si:H sensors have little radiation damage. The main limitation to the sensitivity of the array is the leakage current of the sensors, which is a source of noise and limits the integration time of the arrays. The contributions to the leakage current from bulk thermal generation and contact injection are discussed

Photocurrent multiplication has been observed in a-Si based p−i/SiNx/i−n junction cells under reverse biased high electric field. An apparent external quantum efficiency exceeds 20. A systematic investigation on electric and optoelectronic properties has been made to clarify the mechanism of photocarrier multiplication. The results indicate the possibility of inter-band tunneling via localized states in the a-SiN layer, which is induced by field-redistribution due to the built-up of trapped charges at the a-SiN/a-Si interface.

The design and fabrication of novel, thin-film continuous dynodes for a new generation of channel electron multipliers (CEMs) and microchannel plates (MCPs) are described. In particular, we demonstrate the feasibility of forming such dynodes by low pressure chemical vapor deposition (LPCVD) of amorphous Si films in capillary channels of macroscopic to microscopic dimensions. Finally, we discuss potential performance advantages of thin-film dynodes over conventional reduced lead silicate glass dynodes for CEMs and MCPs and implications for new applications.

We characterize the effects of light soaking on optically addressed spatial light modulators (OASLMs) incorporating a-Si:H photodiodes and relate these effects to the light-induced degradation observed in the photodiodes. After illuminating the OASLMs with white light for up to 100 hours, the rise time of the OASLM optical response decreases, the fall time increases, and the undesirable modulation in the absence of write light (electrical modulation) also increases. We were able to compensate for these effects and approximately restore the original optical transfer characteristics by applying a dc offset voltage across the device. The results can be interpreted by considering series resistance changes in the a-Si:H photodiode.

This paper examines near-equilibrium and non-equilibrium film formation processes in the plasma deposition of Si and their effects on network propagation, defect generation and interface quality based on the concept of film formation as a balance between deposition and ‘etching.’ Using non-F reactant gases and conventional PECVD, epitaxial Si growth was achieved at 250°C. By adjusting the extent of ‘etching’, one can selectively obtain amorphous, microcrystalline, polycrystalline, and epitaxial Si using similar process gases and reactors.

A close study was conducted on microcrystalline Silicon (μc-Si) prepared by PE-CVD (Plasma Enhanced CVD) from SiF4 with the assistance of atomic hydrogen. The atomic hydrogen played a major role in either making precursors, SiFnHm (n+m=3), by gas phase reactions with the fragments, SiFn (≤3), or constructing Si-network in the vicinity of the growing surface. Proper conditions of nucleation were markedly different from those of growth with respect to parameters, flow of atomic hydrogen and substrate temperature.

Highly conductive B-doped hydrogenated amorphous Si (a-Si:H) as well as amorphous SiC alloys (a-SiC:H) have been prepared from (SiH4) / (B2H6/SiH4) and (SiH4/CH4)/(B2H6/SiH4) plasmas, respectively by a novel surface-temperature-modulation method. Films produced by this technique exhibit a higher conductivity as compared to the conventionally prepared films, i.e., 7.0×l0−3scm−l for p-type a-Si:H with an optical gap of 1.7eV and 5.5×l0−5Scm−l for p-type a-SiC:H of optical gap 1.9eV.

High density, non-porous, highly photoconductive films of undoped a-Ge:H showing minimal microstructure have been prepared out of an r.f. glow discharge of GeH4 on substrates mounted on the powered electrode of a diode reactor. These films, prepared under the systematic variation of substrate temperature, discharge power and dilution of the plasma by H2 are markedly different from those produced on the unpowered electrode. An optimum level of applied power is found which does not differ significantly from that used to prepare state-of-the-art a-Si:H. For the reactor geometry used, dilution of the plasma with H2 is found to be essential to the preparation of high quality a-Ge:H. These films contain similar amounts of bound hydrogen to that found in good a-Si:H, and also relatively large quantities of unbound hydrogen. Transport is activated with a Fermi level near mid-gap, the dangling bond spin density is 5 × 1016 spins/cm3, and the samples exhibit photoluminescence. The electron μÄ has been measured using the TOF technique. The conditions for preparation of optimized material are quite different for those found to optimize the preparation of high quality a-Si:H in this reactor. This is asserted to be a principal cause for our finding, along with other laboratories, of inferior properties for a-SiGe:H alloys when compared with those of a-Si:H.

Hydrogenated amorphous silicon-germanium (a-SiGe:H) films are prepared by the catalytic chemical vapor deposition (Cat-CVD) method using a SiH4, GeH4 and H4 gas mixture. Properties of the films are investigated by the photo-thermal deflection spectroscopy (PDS) and electron spin resonance (ESR) measurements, in addition to the photo-conductive and structural studies. It is found that the characteristic energy of Urbach tail, ESR spin density and other photo-conductive properties of Cat-CVD a-SiGe:H films with optical band gaps around 1.45 eV are almost equivalent to those of the device quality glow discharge hydrogenated amorphous silicon (a-Si:H).

A-Si:H films were deposited by direct decomposition of Si2H6 using vacuum ultra-violet light (147nm) emitted from a microwave-excited Xe resonance lamp. The substrate temperature was varied between 200 and 300 °C . The hydrogen content in the films is estimated to be comparable or lower than that in high-quality glow discharge (GD) films based on results from infrared absorption measurements and hydrogen effusion measurements. Hydrogen atoms are incorporated in the form of Si-H bonds (2000 cm−1 mode), and Si-Hn bonds of the 2090 mode In a unit volume was one or two order less than that of Si-H bonds (2000 cm−1 mode). In spite of the lower hydrogen content, the defect density was lower than that of GD films. The mobility gap of the photo-CVD film was closer to the optical gap. For both n- and p-type doping, the dark conductivity increased with the dopant gas composition monotonously, and reached the order of 10−3 S/cm.

Wide-gap a-Si:H films with device quality (Tauc’s optical gap > 1.9eV, σph under AMI.5, 100mW/cm2 illumination ≥ 10−5, Ω−1cm−1, a σph/σ a≥106) have been fabricated. These films are deposited at low substrtate temperatures (TS≤80°C ) either by diluting SiH4 with H2 or optimizing the plasma parameters in a capacitively–coupled RF plasma–CVD reactor. Reduction in the SiH2 bond density and the ESR spin density are also observed. In this study, good film quality is always accompanied by a small deposition rate. Furthermore, σph is nearly the same if the deposition rate and Ts is the same, regardless of other deposition parameters. This suggests that the surface reactions or structural relaxations at the film-growing surface can produce high–quality a–Si:H films even at low TsS, if the deposition rate is low. Results in thermal annealing, light exposure, and solar cell performance confirm that these films have device quality and wide bandgap.

The ratio of Si-H2 bonds to hydrogen content in hydrogenated amorphous silicon films, prepared by mercury-sensitized photochemical vapor deposition, depends on the deposition conditions, in particular on the distance between the substrate and the light-transparent window.

The ratio is reduced from 20 % to 8 % by decreasing the distance from 30 mm to 8 mm. On the other hand, the hydrogen content remains constant at 15 at.%. Decreasing the distance has been found to be almost equivalent to increasing the light intensity, especially 254 nm-light intensity.

The boron doped p-layer is a critical part of a-Si:H solar cells. Trimethylboron (B(CH3)3) has been suggested to be a better doping gas and has a better thermal stability than B2H6. Single junction a-Si:H solar cells and a-Si:H/a-Si:H tandem cells with the p-layers deposited using B(CH3)3 have resulted in conversion efficiencies of 11.4% and 10.4%, respectively. Using these new p+-layers, we also reached 10% efficiencies in single junction a-Si:H solar cells with the i-layer deposited at a high deposition rate of ∼ 2 nm/s from either SiH4 or Si2H6 as a source gas.

Amorphous SiC was prepared by firing the polycarbosilane to various temperatures ranging from 500°C to 1200°C. However the l3c CPMAS NMR and ESR studies show that the carbon rich SiC has two major structural defects, C=C bonds and C-dangling bonds. Both defects concentrations vary with different sample firing conditions. Hydrogen has dramatic effect on decreasing the defect concentrations when used in the sample firing.

In an earlier study, we deposited ¼c-Si thin films by reactive magnetron sputtering (RMS). Here we extend our studies to the deposition of both undoped and high conductivity N-type and P-type ¼c-Si thin films by a remote PECVD. We show that ¼c-Si films can be deposited by bringing hydrogen, H2, into the source gas mixtures. The H2 could introduced by either upstream in a He/H2 mixture and directly plasma excited, or downstream, and be remotely excited along with the silane, SiH4, feed gas. The degree of crystallinity is shown to depend on the hydrogen dilution, the substrate temperature and the substrate material.