We propose metastabilities in amorphous silicon fall into two classes. One class is the local changes of structure affecting a macroscopic fraction of sites. The other class is the metastable generation of dangling bonds with mid-gap states. The local metastability is explained by a new metastable state formed when H is flipped to the backside of the Si-H bond at monohydride sites. The dipole moment of this H-flip defect is larger and increases the infrared absorption. This H-flip defect accounts for large structural changes observed on light soaking including larger absorption and volume dilation. We propose a new model for the generation of metastable dangling bonds. The new ‘silicon network rebonding model’ involves breaking of weak silicon bonds and formation of isolated dangling bonds, through rebonding of the silicon network. Hydrogen motion is not involved in metastable defect formation. Defect formation proceeds by breaking weak silicon bonds and formation of dangling bond-floating bond pairs. The floating bonds migrate through the network and annihilate, producing isolated dangling bonds. This new model provides a new platform for understanding the atomistic origins of lightinduced degradation.

Using plasma growth sources with concurrent particle bombardment, silicon thin films can be deposited with various phases and microstructures. DC Reactive Magnetron Sputtering (RMS), in particular, can produce amorphous, mixed-phase, nanocrystalline, polycrystalline, porous columnar, and epitaxial Si films. In particular, a large flux of low energy, heavy ions strongly affects the phase and microstructure, and therefore the resulting film qualities. Lowpressure (1.6 mTorr) RMS is particularly suited for this type of plasma manipulation: we bias the substrate to produce the ion energy of choice, and use an external magnetic field to control the ion/neutral flux ratio, therefore decoupling the parameters of bombardment energy and flux. In this work, we study the influence of slow (<40eV), heavy (Ar+), ions in RMS deposition on the formation kinetics and microstructures of microcrystalline, mixed-phase, and poly crystalline Si films. The analytical methods are ellipsometry, Raman scattering, and AFM. We will show how ion bombardment influences the direct nucleation of extremely smooth polycrystalline Si on glass at temperatures below 400°C, enhances the crystallinity of this polycrystalline Si in the bulk, and produces nanocrystalline Si with tailored grain sizes at temperatures below 200°C.

We have developed a physically-based analytical model of the static current-voltage characteristics of hydrogenated amorphous silicon (a-Si:H) inverted staggered thin film transistors (TFTs) in the reverse (leakage) regime (VG<0,VD>0). We studied analytically (based on measurement data) the dependence of the leakage current on process parameters (i.e. the deposition-temperature-dependent phosphorus diffusion profile in the a-Si:H active layer), geometrical parameters (i.e. a-Si:H thickness, source/drain overlap areas), and operating conditions (i.e. VG, VD). The derived analytical model is implemented in HSPICE. The simulated and measured results are in good agreement with a discrepancy of less than 5%.

The change with time in the electrical conductivity of a hydrogenated-tritiated amorphous silicon film (a-Si:H:T) has been studied. The conductivity decreased with time after deposition. A model is developed to account for the decrease. The radioactive decay of tritium into helium produces energetic beta particles. Each β particle creates over 1500 electron-hole pairs in the film thereby increasing the conductivity of the film. The 3He atoms diffuse away leaving dangling bonds behind. We find that neutral dangling bonds (D0) are responsible for the decrease in conductivity by acting as recombination centers in the material.

We report an ITO/a-SiNx:H/a-Si:H MIS photodiode structure based on room temperature deposition of optically transparent polycrystalline ITO for applications in large area optical and x-ray imaging. The photodiode structure exhibits device characteristics with reduced leakage current and enhanced photosensitivity giving rise to a hundred-fold improvement in dynamic range. This notable improvement in performance is believed to be due to the reduced diffusion of oxygen from the ITO to the a-Si:H layer, and thus reducing the density of defect states inside the a-Si:H layer. The behavior of photo and dark current is consistent with an elaborate transport model for the Schottky barrier. The model agrees reasonably well with measurement data for the dark current and provides a consistent picture in terms of the photo current behavior in the MIS structure, where the insulating layer serves to reduce the oxygen diffusion.

Effects of thermal annealing in the properties of PECVD amorphous-Si0.8C0.2:H layers were studied. In order to reduce the density defects and increase the electrical conductivity, some samples were annealed: at 500 °C during 1 hour followed by 15 minutes at 800 °C. The results show that in the course of the thermal process, the hydrogen diffuse outside the film and the annealed Si0.8C0.2:H films tend to segregate in silicon clusters and, as a result, partially crystallize. Electrical dark conductivity shows an increase in more than six orders of magnitude, while the optical bandgap decreases from 1.9 eV to 1.4 eV. Annealed- Si0.8C0.2 films have been used as emitter in silicon bipolar transistors. Results indicate promising features such as low base currents and good emitter Gummel numbers.

In this work we report studies on the non linear dependence of the optical gain with the incident power in an amorphous silicon bulk barrier phototransistor based on a n-i-p-i-n structure. The optical gain shows a quasi-hyperbolic dependence on the illumination intensity. The non-linear behavior was predicted by an analytical device model which takes into account the properties of both material and structure, which lead to the amplification mechanism of the device.

We attain good quality hydrogenated silicon carbon films grown by plasma-enhanced chemical vapor deposition. Similar to hydrogenated silicon, we observe a characteristic edge of crystallinity at medium hydrogen dilution ratios of the feedstock gases. In the transition regime between amorphous and nanocrystalline phase, our thin films exhibit a remarkable ratio of photocarrier mobility-lifetime product to dark conductivity of 105... 106 cm3A-1 and minimum light-induced degradation. The static index of refraction increases and the resonance energy decreases for films below the onset of crystallinity which points towards a higher compactness of the protocrystalline material. Hence, alloying of hydrogenated silicon with small amounts of carbon leads to the formation of SiC:H layers that feature an optical bandgap of 2.0 eV and simultaneously maintain the superior optoelectronic properties of protocrystalline silicon.

An amorphous ZnO/p-i-n/Al imager that uses a small-signal scanning beam to read out the short circuit current signal is presented. An analysis of the image geometric distortion, restoration, and enhancement is performed. A simple image-processing algorithm is used to recover main features of projected images. Modifications of the transducer structure are proposed for improving the sensor performance.The effect of the doped layers and image intensity on the sensor output characteristics are analysed. Results show that a trade-off between the sensor structure and contact geometry is needed for a correct read-out. Algorithms and tools are proposed for image analysis.

Results are presented of a study on the mechanical stress dependence of the resistance of polycrystalline silicon (Poly-Si) films, doped with different atomic species. Two types of Poly-Si film implanted with boron and phosphorus ions were studied, namely, B-doped films of 400 nm and P-doped layers of 250 nm thickness, which were deposited by LPCVD at 620 °C on thermally oxidized silicon wafers. Film doping was done by ion implantation at 50 keV, with a dose of boron and phosphorus of 2 × 1014 and 5.3 × 1014 cm−2, respectively. The Poly-Si films were annealed in a H2 ambient at 1000 °C for 20 min to activate the implanted atoms. A controlled amount of external stress was applied to the silicon wafers in order to study the impact on the electrical performance of the implanted Poly-Si resistors. The resistance of the B-doped Poly-Si films is shown to increase by the mechanical stress, while the resistance of the P-implanted Poly-Si films remained unchanged. It is concluded that this difference is related to the structural differences between Poly-Si films implanted with boron and phosphorus, respectively.

Low temperature growth of thin crystalline Si films on amorphous glass substrates by Molecular Beam Epitaxy (MBE) as well as by two different plasma enhanced chemical vapour deposition (PECVD) systems both using a DC plasma was carried out and the mictrostructural properties were analysed. In particular, the control over the texture along [220] orientation and the resulting columnar nature of the films were studied. The TEM cross section of Si films grown by MBE shows grains with an average width of 750 nm and a length of 3 microns. Using the low energy PECVD (LEPECVD) technique microcrystalline Si films were obtained with a growth rate of up to 35Å/sec. For high quality Si films with larger grains one may grow Si films by MBE whereas for Si films with passivated grain boundaries and for increased deposition rate, it is preferred to grow Si films by the newly developed methods of DC-PECVD.

Up to now an in-line method for parameter determination of deposited thin polysilicon films is not available. In this paper a method for monitoring the polysilicon deposition process in device manufacturing by variable angle spectroscopic ellipsometry (VASE) is demonstrated. Therefore several polysilicon films are deposited on thermally oxidized [100] silicon wafers. These samples are characterized by VASE in the optical range of 450 - 850 nm. Parameters are determined by simulation using a multilayer model consisting of air, interface layer (surface roughness), polysilicon, SiO2 and silicon substrate. Different optical models representing the properties of polysilicon are tested. The free parameters are the oxide thickness, the composition and the thickness of the interface layer (air, polysilicon) as well as the thickness and the complex refractive index of the polysilicon layer. Results of the spectroscopic analysis are verified by AFM and SEM measurements. It can be shown that parameters of the deposited polysilicon films, which often could only be determined by complex and destructive off-line analysis methods are also accessible by non-destructive in-line VASE measurements.

Transverse charge collection in a p-i-n structure based on microcrystalline silicon and lateral transport in a single intrinsic layer were analyzed using transient photoconductivity (TPC), steady-state photocarrier grating analysis (SSPG), and the flying spot technique (FST). Photocurrents were excited either with HeNe laser light (633 nm, intensity about 20 mW/cm2) or with 5 ns pulses from a Nd:YAG laser (532 nm, peak power of about 500 kW/cm2 with 5 mJ pulses). The response time τR varied greatly from about 3 ms at low intensity down to a few μs under pulsed laser excitation. The minority carrier diffusion length LD measured by FST in diode structures was usually larger than values obtained from the SSPG method applied to intrinsic microcrystalline layers.

We systematically explore the relationship between gas-phase kinetics and film microstructure in the hot-wire CVD technique using diluted silane (1% in He) and additional hydrogen. Using a wire temperature of 2000°C, films were grown on Si (100) at 300°C using 1 mTorr SiH4 and 99 mTorr He at hydrogen pressures from 0-100 mTorr. Transmission electron microscopy and atomic force microscopy measurements indicated that continuous microcrystalline films had a columnar grain structure and that grain size increased from 40 nm using SiH4/He to 85 nm using SiH4/He/H2 with 20:1 H2:SiH4 ratio due to the etching of silicon by hydrogen. Etching rate measurements using a quartz deposition monitor show that, under the current deposition conditions, a transition from net film growth (0.17 nm/s using only SiH4) to net etching occurs at a H2:SiH4 ration of 80:1. The effect of atomic H on the nucleation density during the initial stages of growth has also been investigated, revealing a sublinear dependence of nucleation density with time and a decrease in nucleation density with increasing H2 dilution. High deposition rate growth with no H2 dilution has been achieved on a low-density array of seed nuclei produced using high H2 dilution.

The drift and diffusion in the presence of charged defects and photocarriers trapped in the tail states is re-examined. In continuity equations, diffusive and drift currents are related to free particles while the Poisson equation includes all charges. In order to make use of ambipolar diffusion approximation, the mobilities and diffusion coefficients should be attributed to the total electron and hole populations making them strongly particle-number dependent. Due to the asymmetry of the conduction and valence band tails, almost all trapped electrons reside in negatively charged defects (D−). A simple model of photocarrier traffic via tail and defect states allows to establish the effective mobility values and coefficients in Einstein relations. In a photocarrier grating experiment, grating of D− is counterbalanced by the grating of trapped holes. Nevertheless, electrons remain majority carriers, allowing the measurement of minority carrier diffusion length, but analysis is needed to relate the latter with μτ product.

Carrier transport property was studied in relation to microstructure for polycrystalline silicon (poly-Si) prepared from SiF4 and H2 gas mixtures at temperatures below 360°C. It was found that Hall mobility markedly increased with the increase in crystal fraction especially at crystal fractions larger than 80% and it exhibited good correlation with orientation fluctuation as well as with crystal fraction. In addition, dependence of Hall mobility on orientation fluctuation exhibited different tendency between (220) and (400) oriented poly-Si. It suggests that atomic configuration and electronic structure at grain boundaries depends on orientation structure, resulting in the different dependence of Hall mobility on orientation fluctuation. By applying hot-vapor annealing at 310°C to 1 μm-thick P-doped (400) oriented poly-Si, Hall mobility was improved from 10 cm2/Vs to 17 cm2/Vs.

Highly sensitive hydrogenated amorphous silicon (a-Si:H) microbolometer arrays have been developed that take advantage of the high temperature coefficient of resistance (TCR) of aSi:H and its relatively high optical absorption coefficient. TCR is an important design parameter and depends on material properties such as doping concentration. Ultra-thin (∼2000 Å) aSiNx:H/a-Si:H/ a-SiNx:H membranes with low thermal mass suspended over silicon readout integrated circuits are built using RF plasma enhanced chemical vapor deposition (PECVD) and surface micromachining techniques. The IR absorptance of the bolometer detectors is enhanced by using quarter-wave resonant cavity structures and thin-film metal absorber layers. To ensure high thermal isolation the microbolometer arrays are vacuum packaged using wafer level vacuum packaging. Imaging applications include a 120×160 a-Si:H bolometer pixel array IR camera operating at ambient temperature. Non-imaging applications are multi-channel detectors for gas sensing systems.

Electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) is used to grow thin epitaxial films on Si(100) wafers. We report on systematic variations of deposition parameters like substrate temperature, substrate dc bias voltage, and gas composition. The structural quality was significantly improved by increasing the substrate temperature from 325 to 500 °C. Simultaneously, compressive lattice strain tends to increase. A negative dc bias voltage resulted in highly disordered films and increased surface roughness due to enhanced ion damage. In contrast positive bias voltages decreased the defect creation by reducing the ion bombardment of the surface during growth. Under so far optimized conditions the remaining disorder is given by two-dimensional, extended defects running parallel to the growth direction and forming grain boundaries with a lateral spacing of 500–700 nm. The single grains are essentially free of one- and two-dimensional defects and show the same orientation as the substrate. By reducing the H2 dilution and adding Ar to the excitation gas the deposition rate increased from 5.3 to 16.2 nm/min. This resulted in inferior structural quality which might be attributed to the reduced etching effect, the enhanced ion bombardment and/or the increased growth rate.

Ultra-thin hydrogenated amorphous silicon thin films have been deposited by thermal chemical vapor deposition (CVD) to prepare smooth top surface of the films avoiding the ion bombardment. Rapid thermal oxidation of thermal CVD a-Si:H results in nanocrystalline dots in the ultra-thin silicon films. Spectroscopic ellipsometry (SE) and high resolution transmission electron microscopy (TEM) have been used to investigate the optical and structural properties of both ultra-thin a-Si:H and nanocrystalline silicon films. To analyze the ellipsometric data of ultra-thin a-Si:H films, a new parameterization i.e., the combination of Sellmeier law and four Lorentz peaks, has been successfully introduced. Width of the Lorentz peaks are directly related with the change of optical functions with the thickness of a-Si:H films. It has been certified that the dense Si matrix with smaller degree of disorder is formed when the thickness exceeds 8nm and the films with the thickness of less than 3.8 nm becomes voided. To interpret the ellipsometric data for nanocrystalline silicon films, three layer model (SiO2, poly-Si+a-Si+void and SiO2) has been adapted. It is inferred from SE and TEM analyses that the size and the density of nanocrystalline dots can be controlled by the morphology of initial ultra-thin a-Si:H films and RTO conditions.

We fabricated top-gate polycrystalline silicon thin film transistors on steel foil substrates with two processes: (1) Directly deposited n+ source/drain layers. (2) Ion-implanted n+ source/drains. The steel substrates enable reducing the crystallization time of amorphous silicon to polysilicon from 4 hours at 600°C to 20 seconds at 900°C. Thin film transistors were made from polysilicon channel material crystallized at 950°C for either 20 seconds or 20 minutes. The steel foil substrates were passivated with SiO2. The best thin film transistors to date have mobilities of above 30cm2/Vs in both the linear and saturated regions.