In this paper we report on the production of thin film transistors using a novel photo-enhanced chemical vapour deposition process. Both the active layer, amorphous silicon, and the gate dielectric, silicon dioxide, have been deposited using a windowless internal lamp deposition system, from silane and silane:nitrous oxide gas mixtures, respectively. Using this technique we eliminate some of the problems of conventional photo-CVD. We report on the material properties and the manufacture and characteristics of large scale (0.5 mm2) TFT test structures. For these structures we obtain a threshold voltage of 25 V and a field effect mobility of 0.07 cm2/V/s.

The characteristics of inverted staggered hydrogenated amorphous silicon/silicon nitride (a-Si:H/a-SiNx:H) thin film transistors (TFTs) are reported between 80 K and 420 K. The TFTs are found to have three distinct transport regimes. Between 80 K to approximately 260 K, the transport in the TFT channel is dominated by electrons hopping between localized gap states of a-Si:H and is analyzed using Mott's theory of variable- range hopping. As the tem-perature is increased above ∼260 K the current becomes thermally activated with an activation energy which depends on the gate voltage. The effective field effect mobility, as determined from the TFT characteristics in saturation, is activated in this regime, with an activation energy 0.10 to 0.15 eV. The various activation energies are found to be sensitive to annealing which can be explained by a reduction in deep and shallow states in the a-Si:H active layer. When operated above ∼360 K the TFTs become unstable due to rapid changes in threshold voltage under the applied gate field. The behavior of the threshold voltage is described over the entire temperature range and possible mechanisms are discussed.

In doping-modulated a-Si:H multilayers metastable excess conductance states can be excited by brief illumination. In addition to these so-called persistent photoconductivities (PPC) we observe new metastable conductance states by bias-cooling the dopingmodulated multilayers from a temperature Ta≥130° C. These metastable states exhibit greatly different nonlinear current-voltage characteristics both parallel and perpendicular to the layers. Furthermore, the magnitude of the PPC as well as the relaxation of the PPC to equilibrium are strongly affected by applying a bias during illumination and during relaxa-tion, respectively.

Our previous thermal equilibrium approach, based on the assumption of an intrinsic relaxation mechanism, has been evaluated on a quantitative scale. The predictions of this model are compared to experimental defect density data and a microscopic description of the thermalization process is presen-ted.

The change of the hydrogen effusion spectra and of deuterium in-diffusion profiles in a-Si:H and a-Si alloys by boron doping is attributed to a Fermi level dependent change of the Si-H bond rupture energy. For low and high substrate temperatures additional effects occur due to structural heterogeneity.

Differential scanning calorimetry (DSC) studies of P-doped hydrogenated amorphous silicon (a-Si:H) show characteristixc behaviors of glass transition phenomena. The glass transition temperature (Tg) detected by DSC for each sample well agrees with the equilibration temperature (TE) determined by the temperature dependence of dc conductivity. Regarding the influence of hydrogen content, Tg decreases and heat energy absorbed in this reaction increases, as hydrogen content increases. The interpretation for these phenomena is successfully attempted by the simulation based on the excess heat capacity model caused by the equilibrium process of defects.

Contactless transient photoconductivity measurements in intrinsic and n-type a-Si:H were performed at 220 °C and at room temperature where also rapidly quenched films were investigated. In undoped samples the measurements suggest thermally induced structure changes which lead to an increase of the effective electron lifetime. In n-type samples the effective electron lifetime decreases with temperature, what may be explained by an increase of the hole emission rate with temperature.

After rapid cooling from 210°C a characteristic electron spin resonance (ESR) and a characteristic below-gap optical absorption have been observed to grow with time at room temperature in hydrogenated amorphous silicon (a-Si:H). Both the increased ESR and the increased optical absorption anneal at temperatures above about 100°C. These unexpected results may have important consequences for our understanding of the electronic properties of a-Si:H.

Thermal equilibration processes in diode structures of doped hydrogenated amorphous silicon (a-Si:H) have been studied. The fast cooling from above the thermal equilibrium temperature (TE) results in an increase in dark reverse current as well as in forward current. The reverse leakage currrent and the diode quality factor increase with quenching temperature at above TE. It is concluded that the dangling bond density increases upon fast cooling from above TE, even though the amount of the increase is small compared with that for the active dopants. On the other hand, the drift mobility changes little after fast cooling. We propose a new model to explain the experimental results.

It is proposed that hydrogen in a-Si:H diffuses via a defect-mediated process in which the supporting defects are produced by charge-induced bond breaking. As the efficiency of defect production scales with the availability of mobile charge carriers, our model – in agreement with experiment – predicts small diffusion rates in undoped and compensated and relatively large rates in heavily doped material.

In this contribution I review some of the salient results obtained on periodic multilayer structures made from amorphous semiconductors by the method of plasma assisted chemical vapour deposition. Emphasis is placed on evidence for quantum size effects in heterostructures, on the carrier recombination kinetics in amorphous doping superlattices, and on phonon diffraction phenomena.

Firstly, a general discussion on the crystallization of amorphous multilayer structures is presented from the viewpoints of thermodynamics, where both a nature of the heterointerface and a repeat distance of the layer are theoretically shown to be key factors for determining the crystallization temperature (Tc). Secondly, experimental observations are described of the effect of each layer thickness and the heterointerface on the structural stability of reactively-sputtered a-Ge:H(a-Ge)/a-GeNx multilayer films. It is demonstrated that Tc of a-Ge:H(a-Ge) increases with decreasing its layer thickness and/or increasing the thickness of a-GeNx layer, which is interpreted qualitatively within the framework of the macroscopic thermodynamics. Relevant phenomena observed by other groups as well as the present results are discussed in a unified manner.

A staircase structure reflecting quantum-size effect has been observed in differential optical absorption spectra in a-Si:H/a-SiC:H multilayer and a-Si:H ultrathin single layer. The threshold energies for the identified subband transition are found to be consistent with those expected from one-dimensional quantum-well model involving a conservation rule for the subband index. The experimental approaches introduced here will open up new possibilities for investigating the quantized band structure as well as for establishing the design concept of functional elements based on quantum size effects.

a-Si1−xCx:H superlattice structures were fabricated by photo-CVD and glow discharge deposition. The compositional abruptness of the heterojunction has been confirmed by X-ray diffraction and Auger electron spectroscopy. The optical bandgap of amorphous silicon-based superlattices increases as the well layer thickness decreases. The existence of quantized levels in a-Si:H wells is demonstrated by the observation of resonant tunneling current through the three-barrier two-well structure.

Multilayers of a-Si:H/a-Si1−xCx:H was fabricated by a glow discharge method. The barrier layer using a-Si1−xCx:H was typicallg 20 Å thick, and the thickness of the well layer was changed from 11 to 510 Å. Hydrogen incorporation scheme in the well layer of a-Si:H was investigated by infrared absorption measurements. Structures of the a-Si:H layer were turned out to be strongly influenced by its thickness. Electron transport properties across the multilayer were studied. Tunneling current was observed when the barrierlayer thickness became narrow. Using a novel structure, it turned out that tunneling electron has higher energy.

An integrated a-Si:H p-i-n thin film diode (TFD) substrate having an ring structure has been developed for driving active matrix liquid crystal display (LCD) panels. The TFD sub-trate developed is for a 5.5″ LCD panel with 115,200 (240×480) pixels. A series of technical data on the fabrication process and the electrical properties of the a-Si TFD are presented. It has been shown that the forward bias characteristics of TFD were mainly dominated by the carrier recombination in the i layer near the p/i interface, and control of the plasma etching condition for the a-Si TFD patterning was very important to reduce the leak current and to improve the production yield.

It has been confirmed that avalanche multiplication occurs in amorphous semiconductors, especially in amorphous selenium. The obtained hole and electron ionization rates, β and α, have single exponential dependence on the reciprocal electric field. This phenomenon has been successfully applied for the first time to a highly-sensitive image-pickup tube with quantum efficiency as high as 10. This avalanche vidicon has excellent S/N characteristics. This may be due to the large ratio of hole to electron ionization rates from the shot noise analysis.

An amorphous silicon position sensitive detector (PSD) has been developed by utilization of uniform wide area photovoltaic generation characteristics. A series of technical data on design rules and performance are firstly presented. Secondly an application to telephone terminal message board system is described.

The telephone terminal equipped with an amorphous silicon position sensitive detector for a keyboard as an input device implemented under a liquid crystal display (LCD) as an output device can transmit not only voice but also graphic information. This is one of the remarkable applications of an amorphous silicon position sensitive detector which makes a telephone a low-cost, easy-to-use and graphic information medium.

We demonstrate the operation of a high-speed optically addressed spatial light modulator utilizing a hydrogenated amorphous silicon photosensor and a ferroelectric liquid crystal modulator. The device has numerous optical parallel processing and interconnect applications. It combines desirable resolution, switching speed, size, and contrast characteristics. The devices are driven by a square-wave voltage, such that read and write operations take place under reverse bias, and an erase operation occurs under forward bias. The capacitance associated with the photosensor plays a critical role in the device performance.

A new type of stable and sensitive UV detector has been fabricated with amorphous silicon based alloys. The detector has 50% higher UV sensitivity than GaAsP Schottky photodiodes in the 200nm ∼ 400nm wavelength region. It shows excellent stability to prolonged AM1 and UV light exposures. It also has an extremely low reverse saturation current and can be used as an ultra low noise photodetector.