It has been shown recently that there are two distinct types of recovery during annealing of amorphous Silicon after degradation due to light soaking. It has been postulated that the two different kinetics of annealing point to the existence of two different types of states, with perhaps one state being charged dangling bonds and the other state being neutral dangling bonds. To see if two kinds of states exist, in this paper, we study the kinetics of degradation within the first 100 seconds, and also study the entire absorption curve at all degradation times. An analytical model is derived for early time degradation based on the conversion of a D- state into a neutral dangling band by absorption of a light generated ( the trap-to-dangling bond conversion model of Adler) and the experimental data of degradation versus light intensity fit the predictions of the model very well. The model also predicts that the Adler-type negatively charged defect states, which have a negative correlation energy, upon conversion will transform into Do states at a higher energy, and therefore, there should be a decrease in absorption corresponding to states closer to the valence band, and an increase in absorption corresponding to states near the mid-gap. For the films where such D- states are deliberately introduced by using a small oxygen (a donor atom) leak, we see strong evidence for such a behavior in absorption, with a decrease in the 1.3-1.4 eV photon energy range, and an increase in the 1.1 eV photon energy range. The increase in Do corresponds well with the decrease in photo-conductivity, even at the earliest times.

The structure of germanium thin films prepared on glass by plasma enhanced chemical vapor deposition was characterized by Raman spectroscopy, atomic force microscopy (AFM) and field emission scanning electron microscopy (SEM). Crystallinity, surface roughness, and grain size were measured as functions of film thickness and deposition temperature. Grain nucleation was apparent for films as thin as 10 nm. Over the thickness range studied, grain size increased with film thickness, whereas average surface roughness started to increase with film thickness, but then remained fairly constant at approximately 1 nm for a film thickness greater than 25 nm.

The incorporation of hydrogen in vacancies and at void surfaces during hydrogenated amorphous silicon growth from a remote expanding thermal plasma (ETP) is systematically studied by variation of the mass growth flux Γa-Si:H and substrate temperature Tsub. An evident relation between the void incorporation and the growth parameters Γa-Si:H and Tsub has been observed. We speculate on a possible relation with the surface diffusion processes during deposition. An activation energy for surface diffusion during a-Si:H growth of 0.8-1.1 eV is obtained using this assertion, similar to the activation energy deduced from surface roughness evolution studies. For compact films hydrogen is predominantly present at vacancies, and a possible relation with the hydrogen removal mechanism during deposition is discussed.

We report on discrepancies in the absorption spectrum of a-Si:H measured by DC and AC constant photocurrent methods (CPM). Our measurements reveal discrepancies in the absorption coefficient alpha, of up to an order of magnitude. DC measurement gives the higher value for alpha at photon energies below the Urbach tail. In this paper we examine free carrier generation paths in AC CPM, and the influence of these paths on the photocurrent frequency response to modulated sub-gap illumination. A simple kinetic model is used to attempt to explain quantitatively the differences in the photocurrent frequency response for sub- and super-gap excitation. At first sight the basic AC technique is of doubtful validity, even at exciting frequencies as low as 1 Hz, since maintaining a constant AC photocurrent does not guarantee a constant carrier lifetime. On the other hand, the DC method involves several parallel excitation paths, obfuscating attempts to extract a density of states from the absorption spectrum. We demonstrate that a simple variation of AC CPM can provide a more accurate means of determining the density of states than DC CPM.

In the D.C. saddle field glow discharge deposition the transition from amorphous to microcrystalline silicon thin films occurs when the silane concentration in the gas phase drops below 10%. We report here the results of Raman spectroscopy, SEM, TEM, and HRTEM studies of the film morphology. We estimate the average crystallite size to be in the range of 5 to 7 nm and the crystalline volume fraction of 25 to 35%.

We have been investigated p- and n-type microcrystalline Si1-xCx (μc-SiC) films fabricated by a conventional rf (13.56 MHz) plasma CVD method with a use of a new carbon source of C2H6 gas at a low substrate temperature on a glass substrate. The Si crystallites incorporated in μc-SiC films retain with a carbon content up to 9 at.%. Both of p- and n-type μc-SiC films show relative high dark conductivities of on the order of 10-3 S/cm with optical energy gaps, E04, of~2eV. In infrared spectra, any pronounced features due to C-Hn vibration mode are not found in C2H6-based μc-SiC films, which is different from the case of CH4-based μc-SiC films.

A compact annealing machine with all solid-state green lasers has been developed, which has the advantage of widely adjustable solidification rate through the delay time control of two long pulses (pulse width ~100ns). Advanced lateral crystal growth (ALCG) process has been proved by the double-pulsed all solid-state laser annealing. The laser beam has a line shape 0.1mm wide and 17mm long, and the beam profile on the short axis is quasi-Gaussian (FWHM 0.1mm). Scanning the line beam along the short axis at the 86% overlapping ratio, the lateral crystal growth area of width 14μm, parallel to the long axis, is sequentially formed at the pitch of 14μm towards the scanning direction. The advanced lateral growth mechanism is easily explained as follows: (1) At the first irradiation, twin seed lines of width 4μm, parallel to the long axis, generates at a boundary between a near-complete melting region and a complete melting region. (2) At the second irradiation of scanning step 14μm, the front seed line in the scanning direction grows symmetrically toward both sides. (3) At the third irradiation of scanning step 2x14μm, the seeds laterally grow until stopped by the growing of seeds on both sides. Finally the ALCG process by the scanning line-beam technique like the current ELA enables us to produce the laterally grown Si thin-films sequentially arranging the belt-shaped texture at the pitch of 14μm. The quality of the laterally grown Si films is quite well except for the projections generated by the bump of lateral growing seeds.

Amorphous silicon (a-Si) films prepared on oxidized silicon wafer were crystallized to a highly textured form using contact printing of rolled and annealed nickel tapes. Crystallization was achieved by first annealing the a-Si film in contact with patterned Ni tape at 600°C for 20 min in a flowing forming gas (90 % N2, 10 % H2) environment, then removing the Ni tape and further annealing the a-Si film in vacuum for2hrsat600°C. An array of crystalline regions with diameters of up to 20 μm could be formed. Electron microscopy indicates that the regions are essentially single-crystalline except for the presence of twins and/or type A-B formations, and that all regions have the same orientation in all 3 directions even when separated by more than hundreds of microns. High resolution TEM analysis shows that formation of such orientation-controlled, nearly single crystalline regions is due to formation of nearly single crystalline NiSi2 under the point of contact, which then acts as the template for silicide-induced lateral crystallization. Furthermore, the orientation relationship between Si grains and Ni tape is observed to be Si (110) || Ni (001)

We reported Si nanostructured films formed by pulsed-laser deposition (PLD) in both inert Ar gas and reactive O2 gas. The as-deposited nanostructured films with visible photoluminescence (PL) show a transition from a film structure to a porous cauliflowerlike structure, as the ambient gas pressure increases from 1 mTorr to 1 Torr. The film consists of small crystals with size from 1 to 20 nm. The oxygen composition of SiOx increases with increasing O2 gas pressure, while Si 2p peak of the Si dioxide also becomes dominate. At 100 mTorr O2 gas, almost complete SiO2 structure is formed. The PL at 1.8–2.1 eV is attributed to the quantum confinement effect (QCE) in Si nanocrystal core, while the PL band at 2.55 eV can be explained by the light emission from the localized surface states at SiOx/Si interface. Laser annealing was applied to the as-deposited nanostructured films. The PL intensities are increased by about two to three times of magnitude after annealing. High laser fluence causes damages in the films and optimal laser fluence exists before film damages or laser ablation occur.

We describe a model for a-Si:H based pin solar cells derived primarily from valence bandtail properties. We show how hole drift-mobility measurements and measurements of the temperature-dependence of the open-circuit voltage VOC can be used to estimate the parameters, and we present VOC(T) measurements. We compared the power density under solar illumination calculated with this model with published results for as-deposited a-Si:H solar cells. The agreement is within 4% for a range of thicknesses, suggesting that the power from as-deposited cells is close to the bandtail limit.

We report the temperature dependence of the growth and decay of the optically induced electron spin resonance (LESR) on short and long time scales (10-3 s < t < 2500 s). This range of times spans the region between previously published photoluminescence and the LESR data. In addition, we examine the steady-state density of optically excited charge carriers as a function of temperature. These measurements lead to a better understanding of the band tail structure of amorphous silicon as well as the kinetics of the excitation and recombination processes.

Integrated imaging and particle sensors have been developed using thin-film on ASIC technology. For this purpose, hydrogenated amorphous silicon diodes, in various configurations, have been optimized for imaging and direct particle detection. These devices were first deposited on glass substrates and later on CMOS readout chips. With an optimization of the material properties and of the diode, a dark current of 1 pA/cm2 could be achieved on p-i-n structures at reverse bias voltage of 1 V. CMOS imagers, incorporating these optimized diodes were then fabricated and characterized. Very thick diodes (with thicknesses up to 50 μm) were also optimized and deposited on glass and on CMOS readout chips. Particle detectors in TFA technology with 12 and 30 μm a-Si:H n-i-p diodes have been fabricated and characterized using light pulse illumination. Direct detection of single low-energy beta particles has been demonstrated.

Expanding thermal plasma CVD (ETP CVD) has been used to deposit thin microcrystalline silicon films. In this study we varied the position at which the silane is injected in the expanding hydrogen plasma: relatively far from the substrate and close to the plasma source, giving a long interaction time of the plasma with the silane, and close to the substrate, resulting in a short interaction time. The material structure is studied extensively. The crystalline fractions as obtained from Raman spectroscopy as well as from X-ray diffraction (XRD) vary from 0 to 67%. The average particle sizes vary from 6 to 17 nm as estimated from the (111) XRD peak using the Scherrer formula. Small angle X-ray scattering (SAXS) and flotation density measurements indicate void volume fractions of about 4 to 6%. When the samples are tilted the SAXS signal is lower than for the untilted case, indicating elongated objects parallel to the growth direction in the films. We show that the material properties are influenced by the position of silane injection in the reactor, indicating a change in the plasma chemistry.

We have investigated the changes in the cathode potential in a dc discharge of silane and hydrogen used to deposit the intrinsic layer of p-i-n type solar cells at deposition rates from 1 to 10Å/s with the superstrate temperature at 200°C and 250°C. Under plasma conditions that lead to higher deposition rates (5-10Å/s), fluctuations of the cathode potential which are suggestive of the formation and de-trapping of particulates in/from the plasma, are observed at 200°C but disappear at 250°C. Improvement of the temperature uniformity over the plasma region from 1.7°C/cm to 0.7°C/cm removes the fluctuations of the cathode potential even at 200°C, indicating that the particulates are formed predominantly at the plasma boundary. Consequently, the stability of solar cells with i-layers deposited at ~10Å/s in the center of the plasma region at the same superstrate temperature improved by 26% suggesting that multiple silicon containing molecules diffuse from the edge to the center of the plasma region.

We have studied the influence of substrate temperature and hydrogen dilution ratio on the properties of silicon thin films deposited on single-crystal silicon and glass substrates. We varied the initial substrate temperature from 200° to 400°C and the dilution ratio from 10 to 100. We also studied the effectiveness of the use of a seed layer to increase the crystallinity of the films. The films were analyzed by atomic force microscopy, X-ray diffraction, Raman spectroscopy, and transmission and scanning electron microscopy. We found that as the dilution ratio is increased, the films go from amorphous, to a mixture of amorphous and crystalline, to nanocrystalline. The effect of substrate temperature is to increase the amount of crystallinity in the film for a given dilution ratio. We found that the use of a seed layer has limited effects and is important only for low values of dilution ratio and substrate temperature, when the films have large amounts of the amorphous phase.

This paper reports on the fabrication and characterization of microelectromechanical bridge resonators on glass substrates using thin-film technology and surface micromachining. All the processing steps are performed at temperatures below 110°C. The microbridges consist of either a single layer of heavily doped n-type amorphous silicon (n+-a-Si:H) or bilayers of aluminum (Al) and intrinsic a-Si:H. The bridge is suspended over a gate electrode with a 1 μm air-gap. Applying a voltage between the bridge and an underlying Al gate electrode electrostatically actuates the microstructures. The resulting deflection is monitored optically. The resonance of the microbridges is measured in air and in vacuum. Resonance frequencies up to 70 MHz and quality factors up to 3000 are obtained at pressures below 1 Torr. The energy dissipation mechanisms of the resonators are discussed.

μc-Si has traditionally been deposited by Hot Wire CVD at a low filament temperature. At these temperatures, silicides rapidly form on the filament surface, leading in the case of a tungsten filament to both film reproducibility and filament lifetime issues. By depositing films consecutively using identical deposition parameters, these issues are chronicled for a filament temperature of ∼ 1750°C. Upon increasing the filament temperature to ∼ 1825-1850°C, these reproducibility and lifetime issues disappear and, by lowering both the substrate temperature and chamber pressure, device quality μc-Si is deposited at high deposition rates in a filament regime where tungsten silicide formation is minimal. Both single junction and tandem solar cells are fabricated using this material, confirming the validity of this approach.

Fabrication of a novel TFT-LCD panel, using amorphous silicon oxycarbide (a-SiCO:H) films as a passivation layer, was successfully demonstrated for the first time. The a-SiCO:H low-k films were deposited using a standard PECVD (plasma-enhanced chemical vapor deposition) reactor from a gas mixture of trimethylsilane[Si(CH3)3H] and N2O. The resulting films have a dielectric constant between 2.7 and 3.5 and high optical transmittance in the range of visible light

The transfer characteristics of the TFT's having a-SiCO:H as a passivation layer were comparable with that of a conventional TFT with a PECVD-grown SiNx passivation layer. Stability of the resulting TFT was performed under prolonged bias conditions, and the source-drain current was fairly constant over the test period. The LCD panel with the a-SiCO:H passivation layer showed 30% higher brightness than that of the standard panel.

Materials grown close to the phase boundary of amorphous and microcrystalline growth have the best electronic properties for solar cells. Systematic molecular dynamics methods have generated such nano-crystalline silicon, consisting of a mixed phase of nano-crystallites in an amorphous matrix, using an embedding method. An excess density of H resides on the surface of the nanocrystallites. The structure of this heterogeneous phase will be characterized by atomic distribution functions and structure factors. The electronic band structure of smaller models of nanocrystalline silicon reveals no midgap states and is similar to a-Si:H. There is a highly strained region surrounding the crystallites. The presence of localized strain region may increase the stability of the material.