We have investigated the annealing effects of ultraviolet (UV) irradiation on the characteristics of hydrogenated amorphous silicon (a-Si:H) thin films and hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFT's) using XeCl excimer laser. The pulse duration of laser was 30nsec/shot and its intensity was varied from 10mJ/cm2 to 80 mJ/cm2 . By irradiating the excimer laser on the a-Si:H TFT's degraded by the electrical stress and/or the visible light illumination, the characteristics of TFT's, such as the on- current, threshold voltage, field-effect mobility and subthreshold sharpness, have been effectively restored to those of virgin sample, while the leakage current changed little. It should be noted that the a-Si:H channel layer was not crystallized by varying the laser intensity.

A new self-aligned a-Si TFT has been proposed. The TFT has the highly conductive n+ poly-Si source and drain with self-aligned manner. The key process technique employed to this TFT is of excimer-laser selective recrystallization. Two ways have been successfully demonstrated to acheive the selectivity of recrystallization. The fabricated TFT had the field- effect mobility of 0.8cm2/Vs and high on/off current-ratio more than 107.

We describe amorphous silicon alloy addressable photo-sensitive elements whose operation has been experimentally verified in 2-dimensional image sensors. The investigated structures consist of p-i-n photodiodes (PD) stacked and back-to-back connected to n-i-p blocking diodes (BD). A first developed device is based on a TCO-p+(Si)-i(Si)-n+(Si)- i(SiC)-p+(SiC)-Metal structure. Due to an optimization of the thickness and of the energy gap of the layers, we obtained a rectification ratio between the current levels in forward and reverse bias conditions If/Ir=106, under AM1.5 illumination, filtered at 500 nm. A better rectification ratio If/Ir>106, under white AM 1.5 light, has been achieved by introducing a metallic film inside the n+ layer, for light shielding; this further technological step allows both the independent optimization of BD’s and PD’s structures, and the increase of the operation speed.

2D image sensor arrays made with a-Si devices on glass over large area are of considerable interest as document scanners and in medical applications. We have made a test array containing a-Si NIP diodes for both the sensors and the active matrix switching devices. The issues of vertical crosstalk and image lag are discussed in relation to the device performance of the switching diode. The vertical crosstalk is controlled by the diode capacitance and the image lag by the high transient current in the device. We speculate that the transient current is a trap filling current in the deep states of the switching diode.

Large area, two-dimensional, amorphous silicon arrays are under development for x-ray imaging applications. Theoretical limits on frame rates imposed by array design and operational requirements are examined. Measurements of image lag as a function of frame rate are reported.

Amorphous silicon imaging arrays with ∼3×105 pixels have recently been developed and x-ray images of low contrast anatomical phantoms have been demonstrated. This paper reports on the linearity of response of these a-Si:H imaging pixels as a function of reverse bias voltage. The fraction of the imaging pixel's full signal range that maintains a linear response has been found to increase with increasing voltage.

The first page-sized a-Si:H sensor array intended for use as a contact imager is described. The array collects light very efficiently, making it well suited to high speed document input and medical x-ray imaging (by the addition of a phosphor). The array structure, control electronics and imaging properties are discussed. Data is presented from a 13" diagonal array with 512⋗560 pixels of size 0.45 mm, although the technology is capable of higher resolution. The image is read-out using 8 ICs containing 64 voltage sensitive amplifiers. The intrinsic dynamic range of the imaging array is about 105, but is limited to <1000 by the read-out amplifiers. The minimum read-out time of about 25 msec allows real time imaging.

Amorphous silicon based n-i-p-i-n structures may be used as color detectors. A simulation program has been developed which allows the examination of the spatial distribution of carrier concentrations, electric field and current densities under different illumination conditions. Furthermore current/voltage- and monochromatic response curves are presented. The results of the simulation point out that the defect density in the p-layer has a major influence on device performance.

Two-dimensional arrays of amorphous silicon photodiodes can be used as position-sensitive radiation detectors when they are coupled to an appropriate phosphor. We have developed signal readout schemes from amorphous silicon photodiode arrays utilizing one or two switching diodes attached to each pixel photodiode. Individual cells and prototype arrays of amorphous silicon photodiodes with single- and double-diode switching readout were fabricated and tested. A charge storage time and a readout time were measured. The measurement results were analyzed by simple circuit theory.

Pixelated imaging arrays consisting of hydrogenated amorphous silicon (a-Si:H) photodiode sensors and field effect transistors are under development for x-ray imaging. For such arrays it is important to quantify the sensor noise characteristics as these may, in some cases, limit the array performance for certain applications. The current-noise-power-spectra of ∼1 nm thick a- Si:H p-i-n sensors of various areas are presented. The power spectra were measured for different reverse bias voltages over a frequency range of ∼0.01 to 1.0 Hz. The power spectra revealed the noise to be composed primarily of flicker noise. The flicker noise showed a l/fbdependence where b ranged from ∼1.1 to 1.2. The magnitude of the flicker noise as a function of the sensor leakage current and the sensor area has been investigated and is presented.

We have developed a rectangular dual-axis large area Position Sensitive Detector (PSD), with 5 cm × 5 cm detection area, based on PIN hydrogenated amorphous silicon (a-Si:H) technology, produced by Plasma Enhanced Chemical Vapor Deposition (PECVD). The metal contacts are located in the four edges of the detected area, two of them located on the back side of the ITO/PIN/A1 structure and the others two located in the front side. The key factors of the detectors resolution and linearity are the thickness uniformity of the different layers, the geometry and the contacts location. Besides that, edge effects on the sensor's corner disturb the linearity of the detector. In this paper we present results concerning the linearity of the detector as well as its optoelectronic characteristics and the role of the i-layer thickness on the final sensor performances.

Metal-semiconductor-metal diodes have been fabricated using a range of dilute a - SiNx:H alloys. Current transport in unstressed devices was dominated by thermionic-field emission of electrons through the reverse biassed metal-semiconductor contacts with a tunnelling effective mass ≈0.1me. The barriers to electrons were strongly pinned by bulk states and scaled almost linearly with optical band gap.

Under high current stress, defect states were created below the Fermi level forming a -ve charge that changed the effective barrier height and produced drift in the current voltage characteristic. Unipolar stress measurements showed that this effect was related to hole transport. After prolonged stress the electrical characteristics were dominated by bulk effects due to the high density of metastable defects in the band gap.

The operation of image-subtraction is useful in optical processing. We report the first asynchronous image-subtracting optically addressed spatial light modulator incorporating a ferroelectric liquid crystal and a color-sensitive a-Si:H p-i-n-i-p photodetector. We present experimental results characterizing the sensitivity of the photodetector current-voltage characteristics to wavelength, and demonstrate image subtraction. Finally, we propose a model that explains the operation of the image-subtracting OASLM.

We have studied the transport mechanisms in thin film (≈ 2000 Å ) hydrogenated α-Si structures used as programmable antifuses in Field-Programmable-Gate-Arrays (FPGA) The antifuse was simulated using a back-to-back Schottky model for a metal/Si/metal thin film incorporating the thermionic-emission diffusion model for the metal-semiconductor contacts. The model predicts the current transport in the low voltage and high voltage regions. In the intermediate voltage range a linear field dependence of the barrier lowering is observed in the experimental results which leads to higher currents than predicted with the image force barrier lowering in the simulations. An important observation is that carrier multiplication is essential for breakdown which is evidence of impact ionization in α-Si. The permanent breakdown condition is modelled thermally, based on the steady- state solution to the heat equation.

A visible-light Thin Film Light Emitting Diode (TFLED) having a-SiN:H as a luminescent active layer has been developed. The TFLED has a structure of glass substrate/ITO/p a-SiC:H/i a-SiN:H/n a-SiC:H/Al. The emission color could be changed from red to orange, yellow, green and white-blue by varying the optical energy gap of the i a-SiN:H layer in the TFLED. The brightness was in the order of 0.1-1 cd/m2 . A series of systematic investigations on the basic properties of a-SiN:H films carrier injection and recombination mechanism in TFLEDs is described.

Because of the lower density of interface states in a-Si:H/a-SiN:H than that in a-Si:H/a-SiC:H, an a-Si:H/a-SiN multilayer reach-through avalanche photodiode is fabricated on an ITO/glass substrate by plasma-enhanced chemical vapor deposition (PECVD) . In order to improve the performance of the a-Si:H/a-SiN:H APD'S, a novel structure is used. By controlling the deposition ratio of silicon and nitrogen of amorphous SiN,the valence band top of a-Si:H is deeper than that of a-SiN:H, that is, the a-Si :H/a-SiN: H system has the electron potential well in a-Si:H, while the hole well is in a-SiN:H, thus we can successfully suppress the hole impact ionization, correspondingly enhance the electron impact ionization effectively.

The measurement of current versus voltage is employed to study the multiplication factors and the impact ionization coefficients. The characteristics of a-Si:H/a-SiN:H APD's,such as I-V curves, optical gains, impact ionization rates, excess noise factors, the relative response and the relationship between the breakdown voltage and wavelength, are studied. The electron multiplication factor is Mc=4.5 at reverse bias V=12v. An optical gain of 3.7 at reverse bias VR=12v and an incident light power Pin=3μw is obtained. Homo junction a-Si:H reach-through APD's and homojunction a-Si:H APD's are also fabricated for comparison.The results show that the novel a-Si:H/a-SiN:H APD's is promising in high-gain, low-noise photodetectors.

The ac conductivities of non-volatile analogue memory states are measured in electro-formed Cr/p+/V amorphous silicon structures for a broad frequency range (from 0.1 Hz to 32 MHz). The results suggest that the memory action is associated with electronic processes.

Using the high light absorption properties of amorphous silicon, we developed a new device configuration that can detect photons and store the induced charges for relatively long time. This device, coupled to a scintillator such as CsI(Tl) in an array form, could be used as a scintillation camera, or for long-term photo-detection such as radionuclide labeled chromatography. The detector has a simple sandwich structure consisting of a scintillator followed by a top metal layer, p-i-n layers of hydrogenated amorphous silicon (a-Si:H), a second metal layer, a thin insulating layer and a bottom metal layer. The electron-hole pairs generated in the i-layer by the interaction with the incident light will be separated by the imposed electric field and be stored in the central metal-insulator interface. Readout will be done by switching the external bias to ground after the storage time, which depends on the needs for the specific application. Prototype devices were fabricated and tested. The performances of the devices were analyzed in connection with the storage time and the background signal produced by the thermally generated charges.

Hydrogenated nanophase silicon powders prepared by low pressure and low temperature rf plasma using pure silane gas, have been characterized by transmission electron microscopy (TEM), Fourier transform IR spectroscopy (FTIR) and thermal desorption spectrometry (TDS) of hydrogen. By means of these analysis, the evolution of the hydrogen bonds and the oxidation processes as a consequence of annealing of the silicon powders under vacuum or atmospheric conditions have been investigated. The TDS results reveal the fundamental differences between the concentrations of hydrogen weakly and strongly bonded in silicon powders as compared to amorphous silicon films, and the FTIR spectra evidence the oxidation process taking place in the silicon powders as a consequence of the annealing. These results along with the TEM analysis show that silicon powder particles present intergrain linkage. We also study the silicon powder particles considering their microstructure and vibrational characteristics as well as their degree of polymerization.

The microstructure and integrity of thin silicon films on SiO2 after immersion in 10:1 buffered HF solution were investigated by AES, AFM and TEM measurements. Silicon films of 30 nm - 50 nm thicknesses were deposited on 15 nm of thermal oxide by LPCVD using SiH4 at 550 °C, 580 °C and 625 °C. These films were then treated with the HF for 15 seconds and characterized systematically. The data provided direct physical evidence for etching of gate oxide by the HF solution through defects (holes) in the 580 °C Si film.