A series of multilayer structures consisting of alternating layers of hydrogenated amorphous silicon (a-Si:H) and amorphous silicon nitride (a-SiNx:H) was fabricated using plasma enhanced chemical vapor deposition. The overall thickness and a-Si:H-to-a-SiNx:H ratio was kept constant for the different multilayer samples fabricated. A blue shift of the optical bandgap was observed with decreasing a-Si:H well layer thickness. High-Resolution Transmission Electron Microscopy was used to estimate the abruptness of the layer-to-layer transitions. The thickness of the interface mixing layer for transitions from a-Si:H to a-SiNx:H was estimated to be 0.5 − 1 nm, while for the reverse transition a thickness of 2-2.5 nm was found. Results from Fourier Transform Infrared Spectroscopy support the found thickness range.

The microcrystalline silicon (μc-Si:H) intrinsic layer for application in micromorph tandem photovoltaic solar cells has to be optimized in order to achieve cost-effective mass production of solar cells in large area, radio frequency, capacitively-coupled PECVD reactors. The optimization has to be performed with regard to the deposition rate as well as to the crystallinity uniformity over the substrate area. The latter condition is difficult to achieve since the optimal solar grade μc-Si:H is deposited at the limit between a-Si:H and μc-Si:H material, where the film crystallinity is very sensitive to the plasma process. In this work, a controlled RF power nonuniformity was generated in a large area industrial reactor. The resulting film uniformity was studied as a function of the deposition regimes. Results show that the higher the input silane concentration, the more the uniformity of the crystallinity is sensitive to the RF power nonuniformity for films deposited at the limit between a-Si:H and μc-Si:H. The effect of the input silane concentration on the microstructure uniformity could be explained on the basis of an analytical plasma chemistry model. This result is important for reactor design. In reactors generating nonuniform plasma the input silane concentration has to be limited to low values in order to deposit films with uniform microstructure. To benefit from the high silane flow rate utilization fraction encountered only for higher input silane concentration, the RF power distribution has to be as uniform as possible over the whole substrate area.

We have developed a fabrication process for amorphous-silicon thin-film transistors (a-Si:H TFTs) on free-standing clear plastic substrates at temperatures up to 300°C. The 300°C fabrication process is made possible by using a unique clear plastic substrate that has a very low coefficient of thermal expansion (CTE < 10ppm/°C) and a glass transition temperature higher than 300°C. Our TFTs have a conventional inverted-staggered gate back-channel passivated geometry, which we designed to achieve two goals: accurate overlay alignment and a high effective mobility. A requirement that becomes particularly difficult to meet in the making of TFT backplanes on plastic foil at 300°C is minimizing overlay misalignment. Even though we use a substrate that has a relatively low CTE, accurately aligning the TFTs on the free-standing, 70-micrometer thick substrate is challenging. To deal with this immediate challenge, and to continue developing processes for free-standing web substrates, we are introducing techniques for self-alignment to our TFT fabrication process. We have self-aligned the channel to the gate by exposing through the clear plastic substrate. To raise the effective mobility of our TFTs we reduced the series resistance by decreasing the thickness of the amorphous silicon layer between the source-drain contacts and the accumulation layer in the channel. The back-channel passivated structure allows us to decrease the thickness of the a-Si:H active layer down to around 20nm. These changes have enabled us to raise the effective field effect mobility on clear plastic to values above 1 cm2V−1s−1

The microstructure of hot-wire microcrystalline silicon films prepared at a wide range of deposition conditions was characterized by both the microstructure parameter from infrared absorption data (analyzing the Si-H stretching modes) and the effusion spectra of (low dose) implanted He and Ne. Parameter ranges leading to the growth of a dense material are identified. A (relatively) high silane flow at rather high filament temperature is found to result in a dense material at high deposition rate. The microstructure data obtained by the two microstructure characterization methods are found to be largely correlated.

Charge and discharge phenomena of the floating-gate amorphous silicon thin film transistor have been studied under dynamic operation conditions. The charge storage capacity decreases with the increase of the drain voltage because it is easier for electrons to be transported to the drain electrode than to be injected into the gate dielectric layer. The discharge efficiency with respect to the drain voltage has been investigated using three different discharge methods: negative gate bias voltage, light exposure, and thermal annealing, separately. The channel length affected both the charge capacity and the discharge efficiency due to the charge storage mechanism and the channel resistance. Majority of the stored charges were removed with the above method through various mechanisms. The low temperature favors the charge storage but the high temperature favors the discharge. This study revealed key parameters for the optimum operation of the low temperature fabricated nonvolatile memory device.

Production technologies of a-Si/a-SiGe flexible film solar cells have been established. The film solar cells having a unique monolithic device structure using through-hole contacts are continuously fabricated on flexible plastic films. Production technologies, such as (1) low temperature and high speed textured electrode deposition method, (2) high speed amorphous silicon deposition method, (3) modified roll-to-roll deposition apparatus technology, (4) large-area high quality transparent electrode deposition method, and (5) very high speed laser patterning method were developed. Increasing deposition rate of textured metal electrode was developed for improving production capacity. We developed and proposed low-temperature textured electrode deposition technique named “selective reactive sputtering”. In the deposition method, Ag-based electrode is deposited using small amount of Al containing Ag alloy targets. Ar/O2 mixed gas is used for reactive sputtering. In the deposition conditions, Ag is not oxidized and maintains low resistivity, whereas Al is oxidized selectively and changes the surface morphology drastically. The oxide incorporation affects on the surface reaction and enhances surface roughness formation. We also developed an amorphous silicon deposition control method which can increase deposition rate without deteriorating film properties. We found peak-to-peak voltage at the cathode (Vpp) had information of the plasma and could be used for optimizing the deposition conditions. Vpp has a strong relationship with stabilized efficiency of solar cells. Very high laser patterning method was also developed. The laser patterning method using a galvano scanner improves the pattering speed remarkably. Our new factory in Kumamoto started commercial production in the last year. We can produce 12MW/Y of solar cells using the new production line that can apply 1m-wide and 2000m-long film substrates at this time. We are planning to install more production lines and have 40MW/Y production capacity in FY 2008.

Silicon nanocrystals with diameters up to 30 nm are used as nucleation seeds for fast solid phase crystallization of amorphous silicon films. Purely amorphous films required an incubation time of up to 12 hours at 600°C prior to the onset of nucleation, while films with nanocrystals embedded between layers of amorphous silicon grew immediately upon annealing in a quartz tube furnace. Structural characterization was performed by heated-stage transmission electron microscopy and Raman spectroscopy.

The surface states of silicon nanocrystals (Si-ncs) considerably affect quantum confinement effects and may determinate final nanocrystals properties. Colloidally dispersed Si-ncs offer larger freedom for surface modification compared to common plasma enhanced chemical vapor deposition or epitaxial synthesis in a solid matrix. The Si-ncs fabrication and elaboration in water by pulsed laser processing is an attractive alternative for controlling and engineering of nanocrystal surface by environmentally compatible way. We report on the possibility of direct silicon surface ablation and Si-ncs fabrication by nanosecond pulsed laser fragmentation of electrochemically etched Si micrograins and by laser ablation of crystalline silicon target immersed in de-ionized water. Two nanosecond pulsed lasers (Nd:YAG, and excimer KrF) are successfully employed to assure fragmentation and ablation in order to produce silicon nanoparticles. Contrary to the fragmentation process, which is more efficient under Nd:YAG irradiation, the laser ablation by both lasers led to the fabrication of fine and room temperature photoluminescent Si-ncs. The processing that has natural compatibility with the environment and advanced state of fabrication technologies may imply new possibilities and applications.

Nanocrystalline silicon (nc-Si) and SiO2 films have been synthesized at a low substrate temperature using inductively coupled plasma chemical vapor deposition (ICP-CVD) methods with internal low-inductance antenna (LIA) units. The synthesis of these materials was performed in the separate vacuum containers where LIA units were installed so that the induced electric field from an antenna could be used effectively. Radio frequency (13.56 MHz) power was supplied through the matching circuit units. H2 and SiH4 gases were used for nc-Si synthesis, and O2 and SiH4 gases were used for SiO2 deposition. The gas flow ratios were 15 for H2/SiH4 and 4.0 for O2/SiH4. A substrate temperature, gas pressure, RF power and process time were varied in order to investigate optimum conditions for nc-Si synthesis. Silicon oxide films were deposited under conditions of 300°C, 0.2 Pa and 24 mW/cm3. A sample was prepared by SiO2 deposition and subsequent nc-Si synthesis after removing the natural oxide on silicon substrate by buffered 1%-HF (BHF) solution. In some cases, plasma treatments were inserted before or after nc-Si synthesis. The diameter and number density of nc-Si were determined by a high-resolution transmission electron microscopy (HR-TEM). Plan-view TEM images of nc-Si showed that spatially isolated nc-Si was synthesized and that the diameter and the standard deviation of nc-Si could be controlled not only with a substrate temperature, gas pressure, RF power and process time but also with pre/post plasma treatments. The resultant trend suggests that radical precursors and reactive nucleation sites on the SiO2 surface have an important role in the synthesis of nc-Si. The diameter of almost all nc-Si under the present conditions was less than 10 nm. In particular, under conditions of the substrate temperature of 200°C and 4.0 Pa with oxygen plasma pretreatment and hydrogen plasma posttreatment, the mean diameter and number density of nc-Si were 2.7 ± 0.5 nm and 6.5 × 1011 cm−2, respectively. This result is suitable for quantum effect device applications. In addition, electronic properties of a single SiO2 film were examined with the fabricated metal oxide semiconductor (MOS) capacitor. Breakdown voltage was 7.5 MV/cm at 1.0 × 10−6 A/cm2 and leakage current was 1.0 × 10−9 A/cm2 at 2.0 MV/cm for a SiO2 film with a thickness of 12 nm. This result clearly supports the present SiO2 film is capable of the thin dielectric layer of nc-Si devices.

The interaction of amorphous silicon and aluminum films to achieve polycrystalline silicon has been investigated using transmission electron microscope equipped with in-situ heating holder. Carbon coated nickel grids were used for TEM studies. An ultra high vacuum cluster tool was used for the deposition of a ∼50nm a-Si films and a vacuum deposition system was used to deposit a ∼50nm Al films on a-Si film. The microstructural features and electron diffraction in the plain view mode were observed with increase in temperature starting from room temperature to 275 °C. The specimen was loaded inside TEM heating holder. The temperature was measured and kept constant for 5 minutes during which the microstructure at fixed magnification of X63K was recorded and the electron diffraction pattern of the same area was also recorded. The temperature was then increase and fixed at desired value and microstructure and EDP were again recorded. The temperatures used in this experiment were 30, 100, 150, 200, 225, 275°C. A sequential change in microstructural features and electron diffraction pattern due to interfacial diffusion of boundary between Al and amorphous Si was investigated. Evolution of polycrystalline silicon with randomly oriented grains as a result of a-Si and Al interaction was revealed. After the in-situ heating experiment the specimen was subjected to high resolution TEM and EDS investigations after removing the excess Al. The EDS analysis of the crystallized specimen was performed to locate the Al distribution in the crystallized silicon. These studies show that the Al induced crystallization process can be used to prepare polycrystalline as well as nanocrystalline silicon by controlling the in-situ annealing parameters. The investigations are very useful as the nanocrystalline silicon is being investigated for its use in developing high efficiency silicon solar structures.

We have investigated the localized electronic states in mixed-phase hydrogenated nanocrystalline silicon thin films (nc-Si:H) with electron-spin-resonance (ESR). The dark ESR signal most likely arises from defects at the grain boundaries or within the crystallites. With illumination with photon energies ranging from 1.2 eV to 2.0 eV, there is no evidence of photo-induced carriers trapped in the bandtail states within the amorphous region. Dependence of the light-induced ESR (LESR) upon the exciting photon energy reveals that, at different excitation photon energies, different regions dominate the optical absorption. This behavior may have potential consequences for understanding the light-induced degradation in nc-Si:H.

This paper studies the impact of LTPS (low temperature polycrystalline silicon) TFTs with fluorine implantation under NBTI (Negative bias temperature instability) stress. The fluorinated TFTs' devices can obtain better characteristics with samller threshold voltage shift, lower trap states and lower subthreshold swing variation. Therefore, the fluorine implantation does not only improve initial electrical characteristics, but also suppresses the NBTI-induced degradation.

Large-area amorphous silicon (a-Si:H) sensor arrays are widely used for medical x-ray imaging, nondestructive testing and security screening. Most of the commercially available detectors are of the indirect conversion type, in which an x-ray phosphor screen is optically coupled to an array of a-Si:H sensors. The a-Si:H PIN photodiode and the MIS photoelectric converter are two alternative sensing elements used in these detectors. The major advantage of the MIS structure over PIN is fact that this device has the same layer sequence as the a Si:H TFT switch and therefore, they can be fabricated simultaneously resulting in an effective reduction in the lithography mask count. The main disadvantage of the MIS structure is the higher noise level due to transient dark current. The transient dark current originates from traps at the semiconductor-insulator interface and i-layer bulk defects. In this work we analyze the transient current transport in segmented-gate/SiN/a Si:H/n+/ITO structures under different biasing conditions and temperatures. Using a home-made setup the dark current decay was measured within an interval of 1 second in the temperature range from 294 to 353K. It is found that the dark current component associated with charge trapping at the insulator-semiconductor interface can be largely eliminated by adjusting the bias voltage during the refresh period. Under optimized biasing conditions and elevated temperatures the bulk current component becomes dominant.

By using a high density microwave plasma source, an ultrafast deposition rate over 1000 nm/s has been achieved for polycrystalline silicon (poly-Si) film deposition. We find that crystalline structure of the deposited film evolves along the film growth direction, i.e. large grains in surface region while small grains in the bottom region of the film. Systematic study of the deposition process has been performed as a function of the deposition duration. Based on the observed results, a possible mechanism, the annealing-assisted plasma-enhanced chemical vapor deposition, is proposed to describe the film growth process.

Deposition trend of μc-Si was investigated using microwave (2.45GHz) plasma enhanced CVD. μc-Si films with the preferential orientation of (111) and (220) were deposited and compared. Raman scattering results show that the (111) preferentially oriented film has higher crystallinity while ESR measurements result in the fact that the (220) preferentially oriented film has smaller dangling bond density. Bottom gate thin film transistors (TFT's) were fabricated using these μc-Si films as channel layer and evaluated. H2 plasma post-treatment has been found to be effective to improve the TFT characteristics. Mobility of about 1.4cm2/Vsec and on/off ratio of more than 105 have been achieved.

We have demonstrated the blue and yellow electroluminescence of MOSLEDs made on Si-rich SiOx film with buried Si nanoclusters of different sizes. The situation of dehydrogenation of Si nanocrystals within the SiOx film becomes more pronounced then the re-growth of SiO2 matrix along with the prolongation of annealing time period. A linear variation on the O/Si composition ratio of the Si-rich SiOx film related to the deposition recipe is reported, giving rise to the precipitation of Si nanocrystals with different size. With such synthesis conditions, the SiOx films result in relatively strong photoluminescence at blue and yellow colors. From the comparison of the I–V curves we can conclude that there is a linear decrease on the threshold voltage of the SiOx based MOSLEDs by decreasing the thickness of the SiOx layer. According to EL pattern, we could demonstrate that the yellow- and blue-light pattern can be observed at 5.5 and 7.25 MV/cm, respectively.

Recently, a new hydrogen NMR signal has been observed in a number of PECVD prepared hydrogenated amorphous silicon (a-Si:H) films of varying quality. It is speculated that the signal is the consequence of a dipolar-coupled hydrogen pair separated, on average, by 1.8 ± 0.1 Å. To elucidate the possible bonding configurations responsible for the NMR data of ref. [1], we have used ab initio simulation methods to determine a set of relaxed structures of a-Si:H with varying void sizes and H-concentrations. Models containing two isolated hydrogen atoms indicate a preferred H-H distance of approximately 1.8 Å when the two atoms bond to nearest neighbor silicon atoms. This separation also occurs for models containing small, hydrogenated voids, but the configurations giving rise to this H-H distance do not appear to be unique. For larger voids, a proton separation of about 2.4Å is seen, as noted previously [2]. There appears to be consistency between the computed structures and the NMR data for configurations consisting of isolated hydrogen pairs or for clusters of an even number of hydrogen atoms with the constraint that the average H-H distance is 1.8 Å. In this paper, we will discuss the most probable bonding configurations of clustered hydrogen based upon the extent of the NMR data and simulated structures.

Grain boundaries (GBs) in polycrystalline Si thin-film solar cells are believed to limit the photovoltaic efficiencies. In this paper, we report on a nanometer-resolution measurement of the carrier depletion at the GBs, using scanning capacitance microscopy (SCM). The SCM images exhibit the following features: (1) Carrier concentrations are lower at locations around the GBs than on center regions of the grains; (2) The depletion width at the GBs varies considerably, between 0 and 100 nm, depending on individual GBs; (3) Intra-grain carrier depletion was also observed at point and line defects; and (4) The faceted features that were observed on the topography of the as-grown film surface appeared on the SCM images even after the film surface was polished flat. The direct measurement of the carrier depletion on the GBs demonstrates that the GBs in Si thin films indeed create charged gap states. The nonuniformity of the carrier depletions suggests that the gap states depend on specific GB structures, which should relate directly to the grain orientations and grain facets adjacent to the GB. The depletion around the intragrain defects indicates that the defects are charged and can be recombination centers, and thus, harmful to device performance. This paper reports the first step of our studies toward understanding the relationships between the electronic and structural properties on specific GBs.

Conical microstructures with nanoscale sharpness form on silicon films as a result of single-pulse, localized UV irradiation using a solid-state, Q-switched Nd:YAG laser. Projection imaging of pinhole apertures was employed to obtain micron-sized irradiation spots on the surface of silicon-on-insulator samples. The formation of these structures requires melting of the silicon film and was followed at different laser fluence levels and irradiation spot sizes. Atomic force microscopy (AFM) was used to characterize the structures. After fabricating small arrays of such micro-cones, the silicon top layer was selectively etched away in order to understand the role of the underlying silicon oxide. AFM images of such etched samples revealed that the topography of the oxide material below the cones had been significantly modified: bumps with heights that represent a significant fraction of the original Si cone height have formed. This suggests that substrate melting plays an important role in the mechanism of formation of the silicon cones.

The ‘cone kinetics’ model, which explains development of cone-shaped inclusions during nanocrystalline silicon film growth and during low-temperature silicon epitaxy breakdown, is extended to protocrystalline (‘edge’) amorphous silicon, Si heterojunctions and other Si film morpholologies. We generalize the physics underlying cone formation and present diagrams that delineate the deposition regimes giving rise to the different film morphologies; these regimes are determined by the nucleation rate of the second phase and the relative growth rate of the phases present. The model predicts cone growth during thin-film deposition by plasma-enhanced and other chemical vapor deposition techniques when isotropic growth is coupled with the isolated nucleation of a second phase with a higher growth rate. Protocrystalline amorphous silicon and other embedded crystallite phases are formed when a second phase successfully nucleates but has a smaller growth rate than the surrounding amorphous film. The cone kinetics phase diagram provides a simple explanation of the various nanocrystalline film morphologies observed when silane precursors are diluted with different concentrations of hydrogen gas.