In order to improve light trapping in thin-film silicon solar cells two novel approaches are investigated in this article: angle-selective management of light scattering inside the solar cell and wavelength-selective manipulation of high reflectance or transmittance of light. Diffraction gratings are analyzed as a representative of the first approach. Haze and angular distribution function of scattered (diffracted) light in reflection are measured for aluminum-based rectangular periodic gratings with different period and height of the rectangles. High haze values in specific wavelength region and scattering angles of the investigated gratings measured in air and water agree very well with the theoretical predictions. Considering the actual optical situation in microcrystalline silicon solar cells, optimal period and height of the rectangular gratings applied as a back reflector are calculated for obtaining the total reflection at the front interfaces. In the frame of the second approach, photonic-crystal-like structures are introduced. By means of optical simulations photonic-crystal-like structures are investigated for two possible applications: an intermediate reflector in a micromorph silicon solar cell with wavelength-selective reflectivity and a dielectric back reflector with a high reflectance in the long-wavelength region. The photonic crystal structure consisting of sequences of n-doped amorphous silicon and ZnO layers is designed for the efficient intermediate reflector. For the back reflector with a high reflectance the structures with intrinsic amorphous silicon, SiO2, MgF2 and TiO2 are proposed.

Solid state crystallization of hydrogenated amorphous silicon (a-Si:H) prepared by hot-wire CVD is studied in solid phase epitaxy mode. By using a novel optical method combined with cross-sectional TEM and SIMS, a reduction of epitaxial growth speed is observed with increase in a-Si:H film thickness. Namely, in films thinner that 0.5 micron, solid phase epitaxy velocity depends linearly on film thickness. As the film thickness increases beyond 1 micron, the average velocity of solid phase epitaxy decreases considerably with respect to that in thinner films. In this regime, its velocity becomes also time-dependent: initial slow propagation of crystallization front gets considerably accelerated after the front has traveled above 400nm. SIMS thickness profiles of hydrogen shows considerably more residual hydrogen in thicker films after the start of solid phase epitaxy. In addition, prolonged pre-dehydrogenation at lower temperatures results in the increase in the average epitaxy speed in thicker films. These phenomena are likely related to delayed hydrogen outdiffusion in thicker films, which also leads to time-dependent speed of the solid-phase epitaxy front. We conclude that excess residual hydrogen reduces the rate of solid-phase crystalline growth.

The agglomeration of a Silicon On Insulator (001) film during a thermal annealing at 900°C and 950°C under hydrogenated atmosphere has been characterized by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). At this temperature, the Si faceting along {110}, {111} and {113} planes is interpreted by a surface energy anisotropy. The anisotropy of the diffusion coefficient is also revealed by the formation of Si line along the <510>, <110> and <100> directions. A mechanism of the pearling instability of the <510> lines is proposed.

Transient reflectivity of amorphous Si (a-Si) films during thermal plasma jet (TPJ) irradiation has been measured to characterize the phase transformation in millisecond time domain. The a-Si films first transform to crystalline by solid phase crystallization (SPC) followed by melting of the film, and then solidifies to the final crystalline state. By increasing the SPC temperature from about 1100 K to 1300 K, the duration of phase transformation decreases from about 1 ms to 100 μs. The crystallinity of the SPC films is improved not only by annealing the films at a high temperature but also annealing them with longer duration.

Location-controlled grains with a diameter of 4 μm are successfully prepared by excimer laser crystallization of sputtered α-Si with μ-Czochralski (grain filter) process at a maximum processing temperature of 100°C. By a pulsed DC magnetron sputtering, α-Si film is deposited firstly on non-structured oxidized wafers for a test. It is found that a-Si film is easily ablated even with a low laser fluence when it is deposited with a substrate bias. From a non-biased sputtered α-Si precursor, grains with 1.8 mm in diameter can be prepared with excimer laser crystallization at room temperature. α-Si is then sputtered on the SiO2 with narrow holes (grain filters) and crystallized at room temperature. The location-controlled grains can be successfully prepared in a large energy density window. These location-controlled grains with a low temperature process are promising for single-grain thin film transistors (TFTs) on plastic substrate for an application to system integration on flexible microelectronics.

In this paper, location-controlled Silicon crystal grains are fabricated by a novel excimer laser crystallization method. An array of 1.8-μm-sized disk-liked grains are formed by this method, and the high-performance n-channel LTPS TFTs with field-effect-mobility reaching 308 cm2/Vs can be fabricated owing to the artificially-controlled lateral grain growth. This position-manipulated Silicon grains are essential to high performance and good uniformity thin film transistors.

In this study the light induced defect state creation under 1 sun illumination is investigated through changes in sub-threshold slope, Δ, and threshold voltage, VT, in a-Si:H TFT structures. Threshold voltage changes can be due to either charge trapping or defect state creation near the interface of gate insulator and a-Si:H active layer. By incorporating high quality thermal silicon dioxide in the a-Si:H TFT, the charge trapping effects have been minimized and the changes in threshold voltage can be attributed to induced defect states in the a-Si:H. Since the sub-threshold current is closely related to the band gap defect states, both VT and Δ can be used as a new probe to study the light induced defect creation in the a-Si:H. The information obtained about the nature of gap defect states from the analysis of changes in VT and Δ under light illumination is presented. The results obtained show that under 1 sun illumination there are very fast increases (within 10 seconds) in VT with subsequent slow increases and no observable changes in Δ, similar to those observed in the light induced photoconductivity changes in a-Si:H thin films. The increase in VT under the initial light illumination are associated with the Fermi level in a-Si:H active layers moving downward and causing band bending near the interface region of the gate insulator and a-Si:H active layer. Results are also presented on changes in VT under gate-bias stressing. Comparisons are then made between the results obtained under 1 sun light illumination and those under gate-bias stressing. The information obtained in this study about the nature of the gap defect states derived from characterization of the sub-threshold regimes is discussed.

We present a finite difference scheme for solving the variable coefficient heat equations with Dirichlet boundary conditions on irregular domains. A quadtree data structure is used to represent the non-graded adaptive Cartesian grids, and the interface is represented by the zero value points of the level set function. Numerical results in two spatial dimensions demonstrate second order accuracy for both the solution and its gradient.

The effects of RF-Plasma hydrogenation and applied mechanical strain on the crystallization of silicon layers deposited on plastic substrates have been investigated where the maximum temperature remained below 170 °C for the entire process. The structural properties of the samples have been studied by optical, scanning-electron and transmission-electron microscopies where the nano-crystallinity of the silicon layers has been confirmed. The maximum average diameter of the silicon grains was 4.5 nm and occurred for an applied tensile strain of 4 %. In addition, a thin-film transistor on a plastic substrate has been fabricated and found to possess an electron mobility of 2.4 cm2/Vs.

We report on directly deposited plasma-enhanced chemical vapor deposition (PECVD) nanocrystalline silicon (nc-Si:H) ambipolar thin-film transistors (TFTs) fabricated at 260 °C. The ambipolar operation is achieved adopting Cr metal contacts with high-quality nc-Si:H channel layer, which creates highly conductive Cr silicided drain/source contacts, reducing both electron and hole injection barriers. The n-channel nc-Si:H TFTs show a field-effect electron mobility (meFE) of 150 cm2/Vs, threshold voltage (VT) ~ 2 V, subthreshold slope (S) ~0.3 V/dec, and ON/OFF current ratio of more than 107, while the p-channel nc-Si:H TFTs show a field-effect hole mobility (mhFE) of 26 cm2/Vs, VT ~ -3.8 V, S ~0.25 V/dec, and ON/OFF current ratio of more than 106. Complementary metal-oxide-semiconductor (CMOS) logic integrated with two ambipolar nc-Si:H TFTs shows reasonable transfer characteristics. The results presented here demonstrate that low-temperature nc-Si:H TFT technology is feasible for total integration of active-matrix TFT backplanes.

We grow epitaxial silicon films onto (100) silicon wafers from pure silane by hot-wire chemical vapor deposition (HWCVD). The films grow epitaxially for a thickness hepi before a Si:H cones nucleate and expand. We study the dependence of hepi on growth rate and the differences between Ta and W filaments. The surface morphology of thin but completely epitaxial films are studied in order to correlate the surface roughness during growth with the eventual epitaxial breakdown thickness. Surface roughness, strain and H at the wafer/film interface are not likely to cause the observed breakdown.

Silicon cluster formation is known to occur within silane plasmas when a capacitively-coupled deposition reactor is operated at high gas chamber pressures. These clusters are sensitive to the thermophoretic forces that will, depending on the sign of the thermal gradient, direct them toward or away from the silicon film's growing surface. We have developed a dual-chamber deposition system that produces nanocrystalline silicon particles (roughly 3-5 nm in diameter) in a flow-through reactor, and injects these particles into a separate capacitively-coupled plasma chamber where the amorphous film is produced. The structural, optical and electronic properties of these mixed-phase materials are investigated as a function of the controllable thermal gradient applied across the silane plasma during deposition.

In this research, we have employed scanning capacitance microscopy (SCM) to image 2D carrier profiles of a MEMS device. Multiple device states are exercised and examined for changes in carrier response as a function of applied bias. First, experimental and simulated dopant profiles of the source and drain pn junctions were measured with device contacts grounded. Their comparison revealed good agreement. Scanning capacitance microscopy was then used to image changes in carrier distribution within the channel of the device while independent bias voltages were applied to the source, gate, drain, and well regions. Device operation was confirmed by simultaneously measuring the drain current. The SCM image contrast directly beneath the gate was observed to change as a function of applied gate bias voltage.

We have applied an advanced model to analyze the dielectric functions e = e1 + ie2 of amorphous silicon-germanium alloys (a-Si1-xGex:H) (i) as a function of alloy content × by varying the flow ratio G = [GeH4]/{[SiH4]+[GeH4]} in plasma-enhanced chemical vapor deposition (PECVD), and (ii) for the first time as a function of the measurement temperature Tm by cooling the newly-deposited film. All e spectra (1.5 − 4.5 eV) have been measured by spectroscopic ellipsometry (SE) either in real time during deposition or in situ post-deposition in order to avoid surface contamination. From the resulting extensive database, the optical properties of the alloys can be predicted for any value × and Tm within the ranges of the database. Such a capability is expected to be useful, for example, in real time control of optical gap in the PECVD process and in predicting the quantum efficiency of multijunction a-Si:H-based solar cells versus operating temperature. The effect on the database of other deposition parameters such as the electrode configuration and the H2-dilution ratio R = [H2]/{[SiH4]+ [GeH4]} have also been explored. The latter two studies provide useful insights into materials properties that can be extracted from a single spectroscopic measurement performed in real time during PECVD. For example, the energy width of the resonance in e correlates closely with the precursor surface diffusion characteristics observed throughout growth -- both determined from real time SE. This result indicates that short-range ordering in the film is improved when surface diffusion is promoted.

This paper presents a measurement technique for studying of the interface between a nanocrystalline silicon (nc-Si) film and a crystalline silicon (c-Si) substrate using microwave photoconductivity decay (MWPCD). The nc-Si films were deposited using plasma enhanced chemical vapor deposition of highly hydrogen-diluted silane. The films were deposited on both sides of the high purity float-zone (FZ) Si wafers. The high resolution transmission electron microscope (HRTEM) analysis of the interface and the characterization of the effective excess carrier lifetime of the samples using MWPCD revealed the following results: (i) The crystallinity of the deposited nc-Si films is very high. The nc-Si film follows the crystal orientation of the substrate such that not a well-defined boundary between nc-Si film and the c-Si substrate is observed. (ii) A surface recombination velocity of less than 10 cm/s was measured for the interface region of the nc-Si/c-Si junctions. (iii) A small discontinuity in the band-energy diagram of the interface region was observed.

We use conductive atomic force microscopy (C-AFM) to measure the local current flow in the mixed-phase hydrogenated silicon n-i-p solar cell structure without the top ITO contact. The forward biased C-AFM images reveal that for the fully amorphous region the current is very low on the entire surface. However, high current spikes appear in the mixed-phase region, where the current spikes are correlated to the formation of nanocrystallite aggregations with a diameter of ~500 nm. Furthermore, the density of the current spikes increases from the mixed-phase to the substantially nanocrystalline regions. The nanocrystallite aggregation supports our previously proposed parallel-connected two-diode model for Voc drops with crystalline volume fraction and light-induced Voc increase in the mixed-phase solar cells. Adding a 50-nm thick a-Si:H buffer layer between the p and i layers significantly reduces the magnitude of the high current spikes, even the top morphology appears unaffected. This result is also consistent with the previously proposed two-diode model for explaining the carrier transport in the mixed-phase solar cells.

The resonance of electrostatically actuated thin-film a-Si:H microbridges immersed in de-ionized water is detected and characterized. When the operating medium changes from vacuum to air, a small decrease of 5% of the resonance frequency occurs and the quality factor decreases from approximately 1000 to 100. The operation of the microresonators in deionized water produces a 60% shift in resonance frequency to lower values and the quality factor decreases to 10. Appropriate actuation conditions at resonance in water are used to avoid electrolysis and electrode field screening. The detection of the resonance frequency of a microbridge operating in solutions with high conductivities, up to 8 mS/cm, and viscosities up to 0.2 Pa.s is demonstrated.

Defect states were quantitatively evaluated in single-grain (SG) Si thin-film transistors (TFTs), prepared by micro-Czochralski (grain filter) process with excimer-laser crystallization, by means of isothermal charge deep-level transient spectroscopy with a high sensitive charge/voltage converter. Its sensitivity reaches 10-16 C and it operates in the range of 2 microseconds - 10 ms. Measurements were performed on the SG-Si TFTs with various energy densities of laser crystallization, various channel areas, and positions in the grain. Our results indicate a direct correlation of fabrication parameters, parameters of the TFT determined from its transfer characteristics, and parameters of defect states (energy position in the band gap, concentration) induced by coincidence site lattice boundaries inside the location-controlled grains and by defects in the grain filter.

Nanocrystalline Silicon is an important electronic materials for solar cells, for display devices and for sensors. In this paper, we discuss the influence of ions on the growth and properties of thenanocrystalline Si:H material. Using a unique growth geometry, combination of hot wire and ECR plasma growth, we show that low energy ion bombrdment is beneficial for growing high quality materials. Ion bombardment by H is shown to etch the films during growth and also promote crystallinity. The results on film growth are compared with simulations of growth using the SRIM program. The electronic properties measured include mobilities of both electrons and holes in device-type structures, carrier lifetimes, diffusion lengths, defect densities and capture cross-sections for holes. Electron mobility is found to increase with grain size, with a minimum mobility being in the range of 1 cm2/V-s. The hole mobility is also in this range, and three different methods of measuring it give approximately the same value. The capture cross-section for holes is of the order of 1-2 × 10-16 cm2. The lifetime of carriers is found to depend inversely on the defect density, implying that the recombination is trap controlled.

Thin-film transistors (TFTs) in active-matrix organic light emitting diode (AMOLED) displays are required to supply high and stable driving current to OLEDs. Top-gate TFTs with nanocrystalline silicon (nc-Si) active layer have shown promise to render high mobility and stable driving current. However, to be compatible with current production facilities, bottom-gate TFTs are demanded. Currently, bottom-gate nc-Si TFTs show insufficient field effect mobility and exhibit driving current instability due to presence of amorphous incubation layer at the interface with gate dielectric. Our research is motivated by the need to eliminate the incubation layer. In order to do so, we studied nc-Si deposition process to find the RF PECVD deposition regimes which lead to minimum incubation layer.

We have deposited a set of undoped nc-Si films by 13.56 MHz PECVD at 250°C by varying RF power, reactor pressure, silane and hydrogen flow rates. Raman spectroscopy, constant-photocurrent method (CPM) and optical absorption have been used to measure film crystallinity, defect density and optical bandgap, respectively. Carrier transport in the films has been studied using dark conductivity, photoconductivity and conductivity activation energy measurements.

Our results reveal that silane and hydrogen flow rates are the most contributing factors to film characteristics. The results also indicate that the reactor pressure does not have a significant effect on the film crystallinity. However, CPM data confirm that to obtain lower defect density, medium or high deposition pressures are preferred. We obtained films with dark conductivity and Raman crystallinity in the order of 10-6-10-7S/cm and 60-80 %, respectively. Furthermore, we have deposited nc-Si films as thin as 20nm with 60% crystallinity, which is crucial for bottom-gate TFTs. Finally, four different sets of bottom-gate TFTs have been fabricated by changing gate dielectric compositions and changing [SiH4]/([SiH4] + [H2]) gas flow ratio. The device performance, relationship to the film structure and deposition process, and future improvements will be discussed in details.