We present measurements on the decline of the open-circuit voltage VOC in a-Si:H solar cells during extended illumination (light-soaking) at 295 K. We used a near-infrared laser that was nearly uniformly absorbed in the intrinsic layer of the cell. At the highest photogeneration rate (about 2x1021 cm-3), a noticeable decline (0.01 V) occurred within about 10 minutes; VOC stabilized at 0.04 V below its initial value after about 200 hours. We found that both the kinetics and the magnitudes of VOC are reasonably consistent with the predictions of a calculation combining a bandtail+defect picture for recombination and a hydrogen-collision model for defect generation. The version of the hydrogen-collision model that we used assumes that only bandtail recombination drives the hydrogen collision processes. Within this picture, the crossover between bandtail and defect recombination occurs on the same timescale as the “light-induced annealing” process that accounts for stabilization of the optoelectronic properties for long lightsoaking times.

We have developed high current density thin-film silicon n-i-p diodes for low cost and low temperature two-dimensional diode-logic memory array applications. The diodes are fabricated at temperatures below 250°C on glass, stainless steel, and plastic substrates using hot-wire chemical vapor deposition (CVD). The 0.01-mm2 standalone diodes have a forward current-density (J) of near 10 kA/cm2 and a rectification ratio over 107 at ±2 V. The 25 μm2 array diodes have J > 104 A/cm2 and rectification of 105 at ±2V. On plastic substrates, we have also used plasma-enhanced CVD to deposit 10-μm diameter diodes with J ˜ 5 x 104 A/cm2. We found that the use of microcrystalline silicon (μc-Si) i- and nlayers results in higher current-density diodes than with amorphous silicon. Reducing the diode area increases the forward current density by lowering the voltage drop across the external series resistances. A prototype diode array memory based on 10-micron devices was successfully demonstrated by monolithically integrating diodes with a-Si:H switching elements. High current density diodes have potential applications in a variety of large area, thin-film electronic devices, in addition to a-Si:H-based memory. This could widen the application of thin-film silicon beyond its present industrial applications in thin-film transistors, solar cells, bolometers and photo-detectors.

Undoped nanocrystalline silicon (nc-Si:H) films were prepared by conventional 13.56 MHz plasma enhanced chemical vapor deposition (PECVD) at 260 °C, using highly H2-diluted SiH4 plasma. The nc-Si:H films were evaluated using electrical, structural, and chemical measurements. The optimized nc-Si:H film showed an oxygen concentration (CO) of ˜1.5 X 1017 at./cm3 and a dark conductivity (σD) of ˜10-6 S/cm, while the Raman crystalline volume fraction (XC) was ˜85 %. Top-gate staggered TFTs with a ˜100 nm nc-Si:H channel layer and an amorphous silicon oxide (a-SiOx) as the gate dielectric were fabricated. The TFTs showed a field effect mobility (μFE) of ˜150 cm2/Vs, a threshold voltage (VT) of ˜2 V, a subthreshold slope (S) of ˜0.25 V/dec, and an ON/OFF current ratio more than 106. To the best of our knowledge, the TFT mobility reported here is the highest achieved to date using state-of-the-art nc-Si:H films prepared by direct PECVD.

We have proposed low hydrogen concentration (CH) silicon nitride (SiNX) as a dielectric for flexible display application. The fabrication temperature on plastic substrate is limited below Tg (glass transition temperature, typically 130˜180 °C) and it was reported that CH in thin film is strongly depends on fabrication temperature. As the fabrication temperature is decreasing, hydrogen concentration is increasing. SiNX deposited in ultra low temperature (< 150 °C) has high CH which is porous, low density. Our experimental results using SiH4, He, N2 gas mixture shows that in the SiNX CH is less than 15 at.%. Breakdown voltage of proposed SiNX dielectric is 5 MV/cm. In the wet etch rate test using a nitride etching solution, He dilution is more dense than NH3 dilution. This process approach is useful for flexible display application.

We used X-ray diffraction (XRD), Raman scattering and photoluminescence (PL) spectroscopy to characterize structural and electronic properties of nc-Si:H films made with different hydrogen dilution ratios and hydrogen dilution profiling with continuously reduced hydrogen dilution during the deposition. The XRD results show that the crystalline volume fraction (fc) is in the range of 60-70% with grain size of 22-26 nm for the nc-Si:H films studied. Comparing the sample made using hydrogen dilution profiling to that with constant hydrogen dilution, the hydrogen dilution profiling promotes the (220) preferential orientation due to a very high hydrogen dilution in the initial growth. The Raman results show that the fc is in the range of 60-90%, depending on the sample and excitation wavelength. For the samples with constant hydrogen dilution, the fc measured by Raman increases along the growth direction. The hydrogen dilution profiling reverses this trend, which affirms that the hydrogen profiling controls the nanocrystalline structure evolution along the growth direction. The PL results show only one peak around 0.8-0.9 eV for the samples made with constant hydrogen dilution, but an additional peak at 1.4 eV appears in the sample made with the hydrogen dilution profiling.

Aluminum-doped hydrogenated microcrystalline cubic silicon carbide (μc-3C-SiC:H) films deposited by the hot wire chemical vapor deposition technique at a low substrate temperature of about 300 °C were annealed at various temperatures in vacuum atmosphere. The increase in conductivity was observed on annealing the films over 400°C. The onset of hydrogen desorption occurred in the undoped films at about 650°C, while the onset was shifted towards lower temperatures in the case of Al-doped films. These results indicate that hydrogen plays an important role on the conductivity of the Al-doped μc-3C-SiC:H films.

Hydrogenated amorphous silicon (a-Si:H) solar cells incorporating high deposition rate (8-10Å/s) intrinsic layers were deposited using modified very high frequency (MVHF) plasma. We have monitored the light scattered from powder generated in the plasma using an Ar-laser and a silicon photodiode. This simple, non-invasive technique allows us to make measurements on the same reactor used to make the solar cells. First, we have varied the total flow rate and observed a maximum in the scattered light intensity from powder in the plasma during the deposition of the intrinsic layer, and correlated this with the degradation, as well as the stabilized performance of the solar cells. Then, we have studied the effects of varying the deposition temperature and/or the addition of germane to the gas mixture on the scattered light intensity due to powder in the plasma.

The poly crystalline Ge films were prepared by the RF sputtering method with Ar and Ar-H2 mixture gases. The crystallization temperature increases from 400 °C to 500 °C due to the H2 introduction into the sputtering gases. On the other hand, the H2 introduction decreases the absorption coefficients in the long wavelength region corresponding to lower energy values than the energy gap of Ge, although much higher absorption coefficients are observed in the case of the Ar sputtering. Probably, the gap state density decreases due to the hydrogen termination of dangling-bonds in the grain-boundaries. The (220) preferential orientation is stressed and the highest Hall mobility is obtained with 75% of the gas flow ratio [Ar/ (Ar+H2)]. The preferential growth probably causes higher quality Ge films.

We present temperature-dependent measurements of the open-circuit voltage VOC(T) in hydrogenated amorphous silicon nip solar cells prepared at United Solar. At room-temperature and above, VOC measured using near-solar illumination intensity differs by as much as 0.04 V for the as-deposited and light-soaked states; the values of VOC for the two states converge below 250 K. Models for VOC based entirely on recombination through deep levels (dangling bonds) do not account for the convergence effect. The convergence is present in a model that assumes the recombination traffic in the as-deposited state involves only bandtails, but which splits the recombination traffic fairly evenly between bandtails and defects for the light-soaked state at room-temperature. Recombination mechanisms are important in understanding light-soaking, and the present results are inconsistent with at least one well-known model for defect generation.

This paper presents a two-dimensional a–Si:H/a-SiC:H n–i–p photodiode array with switching diode readout, developed specifically for fluorescence-based bio-assays. Both device structure and fabrication processing has enabled enhancement of the external quantum efficiency of the encapsulated device up to 80%, reduction of the photodiode leakage down to 10 pA/cm2 at -1V reverse bias, and increase of the rectification current ratio of the switching diodes up to 109. The critical fabrication issues associated with deposition of device-quality materials, tailoring of defects at the i–p interface, device patterning with dry etching, junction passivation, and contact formation will be discussed. Both sensing and switching diodes were characterized. While the observed dark current in the photodiodes at low reverse bias voltages is primarily due to carrier emission from deep states in the a–Si:H bulk, the leakage in the small switching diodes stems from peripheral defects along junction sidewalls. Optical losses in the photodiodes with ITO/a–SiNx:H antireflection coating were evaluated using numerical modeling, and the calculated transmission spectra correlated well with the spectral response characteristics. Measurements of the charge transfer time and output linearity demonstrated the efficiency of the single-switching diode readout configuration. The response of the array to optical excitation was also investigated. The observed long term retardation in the signal rise and decay at illumination levels less than 1010 photons/cm2-s can be associated with charge trapping in the undoped layer.

Activation of polycrystalline silicon (poly-Si) thin films doped as n-type using selective ion implantation of phosphorous was performed employing field-enhanced rapid thermal annealing where rapid thermal annealing of halogen lamps is combined with alternating magnetic fields. The ion activation was evaluated using Hall effect measurements incorporating the resistivity, the charge carrier concentration, and the mobility. Statistical design of experiments is attempted in order to clarify the effects and interactions of processes variables on field-enhanced rapid thermal annealing towards ion activation: the three processing variables are furnace temperature, power of halogen lamp, and the alternating magnetic field. Hall effect measurements indicate that the furnace temperature and RTA power are found to be dominant in activating the doped polycrystalline Si in dose. The activation process results from the competition between charge carrier concentration and mobility: the increase in mobility is larger than the decrease in charge carrier concentration.

In this work, an attempt of full color discrimination is presented using as sensitive devices multilayer stacked structures (p(SiC:H)/i(SiC:H)/n(SiC:H)/p(SiC:H)/i(Si:H)/n(Si:H)) sandwiched between two transparent conductive contacts. The thickness and the absorption coefficient of the front p-i-n cell is optimized for blue collection and red transmittance and the thickness of the back one adjusted to achieve full absorption in the green and high collection in the red spectral ranges. The current-voltage characteristics and the spectral sensitivity under different electric and optical bias conditions and light fluxes are analyzed. Results show that color selectivity is achieved by tuning the spectral sensitivity at different applied voltages. A physical model supports the results.

We have employed a triode-type plasma CVD system to fabricate highly stabilized hydrogenated amorphous silicon (a-Si:H) solar cells. The p-i-n type solar cells were fabricated on a textured SnO2/glass substrate (ASAHI VU type). By applying a triode system, the Si-H2 bond density in the film decreased to about one third (from 1.7 at.% for conventional parallel-plate-electrode to 0.6 at.% for a triode configuration), and correspondingly the degradation ratio decreased from 13 % to 10 %. We have achieved the degradation ratio of 5 % by optimizing the player deposition conditions. In case of a triode system, there were minor effects of higher hydrogen dilution in the stabilized efficiency. We have experimented the effects of the substrate temperature for a higher stabilized efficiency. Further improvement in solar efficiency has been made by applying antireflection layers to air/glass and TCO/p interfaces. As a result, we have achieved the stabilized efficiency of 9.22 % (Jsc = 15.9 mA/cm2, Voc = 0.863 V, FF = 0.672) with a degradation ratio of 7.8 %. We have also employed the triode-deposited a-Si:H solar cell to a tandem type solar cell with a structure of a-Si:H/hydrogenated microcrystalline silicon (μc-Si:H). We have achieved the stabilized efficiency of 10.9 % (Jsc = 12.0 mA/cm2, Voc = 1.31 V, FF = 0.691) with a degradation ratio of 7.3 %.

The light-intensity dependence of the photocreation of dangling bonds (DBs) were investigated for a-Si:H films with increasing density of defects before light soaking. Samples in which the density of neutral DBs had been increased by annealing at 400 C for 1 h exhibited a weak light-intensity dependence of the photocreated DBs compared to that for the as-deposited sample. Furthermore, the sample which had been illuminated with a light intensity of 1 W/cm2 for 1 h also showed a weak dependence. The results can be qualitatively explained by using rate equations for the densities of DBs and floating bonds (FBs) based on the FB-mediated photocreation of DBs. When both the densities of DBs and FBs before illumination increase, the light-intensity dependence of the DB density for a moderate value of the illumination time becomes weaker, qualitatively consistent with the observed results.

Technical issues for the performance characterization of silicon-based thin-film multi-junction devices, such as the a-Si/thin-film c-Si structure, are discussed. The measured spectral response of each component cell is affected by the shape of the I-V curves of the component cells under color bias light, as well as the applied bias voltage. This paper describes procedures for correcting the measured spectral response. High-fidelity solar simulators, which incorporate Xe lamp(s) and halogen lamps that simulate AM1.5G standard sunlight, are very useful for characterizing the I-V curves of the multi-junction devices. They can accurately reproduce the current generated in each component cell and the I-V curve of the devices under standard sunlight, with a simple spectral adjustment procedure, thanks to their basic good spectral fidelity to standard sunlight. The irradiance dependence of the I-V curve is important for translating the I-V curve under different irradiance conditions. Although translation is not straightforward for multi-junction devices, the experimental results of the present study suggest that it is possible under spectral conditions where the relative value of the photocurrent among the component cells is constant. Determination of the I-V curve of each component cell based on the I-V curves of the multi-junction devices is also mentioned.

Polycrystalline Si thin films and single NiSi crystalline nanowires were made by the metal induced growth method. The different growth mechanisms for poly-Si and nanowires lie in the metal silicide formation. Poly-Si growth is based on metal disilicide formation and the growth of nanowires depends on metal monosilicide formation. The metal silicide formation depends on catalyst species and sputtering power. Following catalyst coating, Si was deposited in a DC magnetron sputtering system to grow poly-Si or nanowires. Several metals (Ni, Co, Co/Ni and Pd) were used as catalysts to confirm the nanowire growth mechanism as well as to fabricate solar cells. Poly-Si thin films were about 5 μm thick with up to 1 μm crystallite size. Nanowires were up to 10 μm long with about 50 nm diameter.

Thin silicon films deposited using the Hot-Wire Chemical Vapor Deposition (HWCVD) technique are studied here for the effect of XeCl excimer laser crystallization on their structural, optoelectronic, and electron field emission properties. Excimer Laser Annealing (ELA) of the silicon thin films indicated increased dark conductivity and reduced optical gap. Encouraging Field Emission (FE) results were obtained from XeCl excimer laser processed HWCVD films on Cr, V, Mo, and Ti backplanes. Geometric field enhancement factors from surface measurements and Fowler-Nordheim Theory (FNT) were compared. FE thresholds were also found to be particularly influenced by the backplane material.

Formation of polycrystalline silicon (poly-Si) thin films on inexpensive glass substrates is of great interest for large area electronic devices. Large grain sizes are desirable to reduce grain boundary effects. In the aluminum-induced layer exchange process Al/a-Si bi-layers exchange their positions with a concurrent crystallization of the amorphous Si (a-Si) in a simple annealing step. The process is characterized by the self regulated suppression of nucleation by existing grains resulting in large grain sizes above 10 μm. This paper elucidates the process within the Al Si phase diagram. The change in Si concentration within the Al is shown to cause the nucleation suppression.

We have studied silicon nitride films a-SiN:H deposited at a substrate temperature of 350°C by means of the Low frequency (LF) PECVD from silane and nitrogen as stock gases. Film properties as hydrogen bonding and content, nitrogen content, refractive index and etch rate are reported and analyzed. Our deposited films show physical properties similar to those that are obtained deposition temperatures of 700°C by the low pressure chemical vapor deposition (LPCVD) technique. An investigation of bonding structures for the deposited films was performed, and quantitative results for hydrogen bonding based on Fourier Transform Infrared (FTIR) analysis are presented. It was observed that low hydrogen content in the films is in good correlation with low etch rate in 10% buffered HF solution, therefore these films present a material with good etch selectivity in respect to others materials (as phosphosilicate glass PSG, Al etc). Selectivity which makes these films very promising in surface micromachining for fabrication of sensors and device structures, e.g. microbolometers. Additionally, the Si-N bond at 830-840cm-1 was analyzed because of its big absorption produced at 12μm; therefore these films can be used as absorber layers in uncooled microbolometres.

Hydrogenated polymorphous silicon (pm-Si:H) has steadily emerged as a potential replacement of hydrogenated amorphous silicon. Possible changes in the density of gap states due to the presence of crystallites is of central importance in understanding steady state and dynamic characteristics of devices using these materials. We have studied a-Si:H and pm-Si:H grown by PECVD at optimized conditions through the measurement of the steady state reverse current and their transients in PIN devices. The transients are analyzed using isothermal spectroscopic techniques such as Time Analyzed Transient Spectroscopy (TATS), and high resolution Laplace DLTS as a function of temperature. In case of a-Si:H, we obtain the expected signature of emission from a broad density of states in the form of stretched exponentials. In contrast the corresponding spectra for pm-Si:H are dominated by nearly exponential fast current decay processes with discrete energies between 0.20 and 0.26 eV. It is shown that the study of the density of states by dynamic methods such as transient techniques reveal features not accessible to steady state measurements.