Hydrogenated amorphous silicon films for photovoltaics and thin film transistors are deposited from silane containing discharges. The radicals generated in the plasma such as SiH3 and H impinge on the surface and lead to silicon film growth through a complex network of elementary surface processes that include adsorption, abstraction, insertion and diffusion of various radicals. Mechanism and kinetics of these reactions determine the film composition and quality. Developing deposition strategies for improving the film quality requires a fundamental understanding of the radical-surface interaction mechanisms. We have been using in situ multiple total internal reflection Fourier transform infrared spectroscopy and in situ spectroscopic ellipsometry in conjunction with atomistic simulations to determine the elementary surface reaction and diffusion mechanisms. Synergistic use of experiments and atomistic simulations elucidate elementary processes occurring on the surface. Herein, we review our current understanding of the reaction mechanisms that lead to a-Si:H film growth with special emphasis on the reactions of the SiH3 radical.

We present femtosecond time-resolved studies of the frequency-dependent photoconductivity in the far-infrared spectral range (∼ 1 – 10 meV) in PECVD a-SiGe:H and a-Si:H thin films. The experiments are carried out using an optical pump / terahertz (THz) probe technique, in which a femtosecond pump pulse excites carriers into the extended states and a time-delayed probe pulse measures the resulting change in the far-infrared optical properties, which are directly related to the ac photoconductivity, as the carrier distribution evolves in time. We find that the frequency-dependent conductivity measured on picosecond time scales shows a strongly non-Drude behavior, with components of the response fitting to a power-law frequency dependence, reflecting processes associated with localized states.

Hydrogen bonding in laser crystallized boron and phosphorous doped polycrystalline silicon is investigated using Raman spectroscopy and hydrogen effusion measurements. During laser crystallization the intensity of the local vibration modes near 2000 and 2100 cm−1 decreases. The intensity of vibration mode at 2000 cm−1 decreases faster than the one at 2100 cm−1. From H effusion measurements, the hydrogen density-of-states (H DOS) distribution is derived. For undoped amorphous silicon the H DOS exhibits two prominent peaks at hydrogen binding energies of E– μH = –1.1 and –1.5 eV. In B doped a-Si:H the peak at –1.1 eV is less pronounced while in P doped a-Si:H the H binding energy increases by about 0.1 eV. In all samples laser crystallization causes an increase of the H binding energy by about 0.2 – 0.3 eV. However, the peaks in the H DOS observed in B-doped samples are preserved during laser crystallization.

Photoluminescence (PL), PL excitation, Raman scattering, IR absorption spectra as well as PSi surface morphology, have been studied as a function of Si/SiOx interface formation during PSi ageing in ambient air with the aim to reveal a PL mechanism. It is shown that fresh- prepared PSi layers created at low values of the anodization current Ia is characterized by “red” emission band centered at 1.72 eV, while the samples prepared at higher values of Ia have “orange” PL band centered at 2.00 eV. During oxidation in ambient air two processes take place at the PSi ageing: the oxidation of small size Si nano-crystallites up to their disappearing and the change of silicon oxide composition at the Si/SiOx crystallite surface. These processes initiate changes in PL and PL excitation spectra. The peak position of “red” PL band shifts to the high-energy up to 1.80-1.85 eV while the “orange” band does not change essentially.

The excitation intensity dependency of nanocrystallite Si (nc-Si) related micro-photoluminescence (μ-PL) from the multi-recipe Si-implanted quartz is characterized. The μ-PL at 724 nm contributed by nc-Si with 3-4 nm diameter is maximized after annealing at 1100°C for 3 hours. By increasing the excitation intensity from 10 kW/cm2 to 300 kW/cm2, the μ-PLs of 1-hr and 3-hr annealed quartz significantly red-shift from 723 nm to 725 nm and from 724 nm to 735 nm, respectively. This can be explained by the anomalous quantum stark effect due to a surface electric field oriented from photo-ionized nc-Si dots to quartz surface. After 1-hr illumination at power of 300 kW/cm2, the μ-PL peak wavelength of 3-hr annealed sample is further red-shifted by 2.5 nm. By measuring the accumulated surface charges built up during optical excitation process, the correlation between excitation-intensity dependent PL wavelength red-shift and the photo-ionized nc-Si surface electric-field related quantum stark effect is primarily elucidated.

We present photocarrier time-of-flight measurements of the hole drift-mobility in microcrystalline silicon samples with a high crystalline volume fraction; typical room-temperature values are about 1 cm2/Vs. Temperature-dependent measurements are consistent with the model of multiple-trapping in an exponential bandtail. While this model has often been applied to amorphous silicon, its success for predominantly crystalline samples is unexpected. The valence bandtail width is 31 meV, which is about 10-20 meV smaller than values reported a-Si:H, and presumably reflects the greater order in the microcrystalline material. The hole band-mobility is about 1 cm2/Vs – essentially the same magnitude as has been reported for electrons and for holes in amorphous silicon, and suggesting that this magnitude is a basic characteristic of mobility-edges, at least in silicon-based materials. The attempt-frequency is about 109 s-1; this value is substantially smaller than the values 1011 - 1012 s-1 typically reported holes in amorphous silicon, but the physical significance of the parameter remains obscure.

Hydrogenated amorphous silicon photodiodes have been considered for use in array-based image sensors. They promise to significantly reduce the size and cost of CMOS image sensors, while offering the promise of improved pixel sensitivity. However, Staebler-Wronski Effect (SWE) based electrical crosstalk degradation has been a major concern in their acceptance, due to degraded spatial contrast and color fidelity. Since the SWE is a fundamental mechanism of a Si:H, solutions to this issue must look to ways of mitigating the SWE on diode array performance rather than elimination of SWE. In order to study electrical crosstalk, a novel device structure that inhibits light from reaching portions of the a-Si:H/dielectric interface was designed and fabricated to directly measure interpixel leakage currents. Results from these structures indicate that edge leakage can be a significant contributing component to the measured signal. In addition, a CMOS-compatible structure to suppress electrical crosstalk was designed and fabricated. Results from these structures demonstrate suppression of crosstalk up to lateral electric fields of at least 2 x 104 V/cm. Such suppression is adequate for densely packed minimum-size pixel arrays. Aspects of the design and implementation of the structure will also be discussed.

In this work, we have developed a method for accelerated stress testing of TFT driver circuits for AMOLED display backplane applications. Based on high current and temperature stress measurements, acceleration factors have been retrieved, which can be used to significantly reduce the testing time required to guarantee a 20000-hour display backplane lifespan.

Aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) in a conventional furnace with N2 protection has been studied at reaction temperatures ranging from 200 to 500°C by using optical microscopy, and transmission and scanning electron microscopy. The a-Si and Al layers were deposited with plasma-enhanced chemical vapor deposition (PECVD) and electron beam evaporation, respectively. The structures in the study are Al/a-Si and a-Si/Al on Si or glass wafers coated with 3000 Å PECVD SiO2. It was found that Al induces crystallization of a-Si for both Al/a-Si and a-Si/Al structures by exchanging positions of Al and Si layer through diffusion of Si into Al and the grain size of crystallized Si (c-Si) increases with the decrease of AIC temperature. AIC for Al/a-Si structures starts at a temperature as low as 200°C, which is 100°C lower than that for a-Si/Al structures. Kinetics analysis found that the activation energies are 1.76 eV and 1.65 eV for both Al/a-Si and a-Si/Al structures, respectively. The quality of AIC c-Si depends on the order, thickness and thickness ratio of a-Si to Al. Microstructural observations indicated that the c-Si for Al/a-Si structures is better and more suitable for use in fabrication of thin film transistors (TFTs) than that for a-Si/Al structures.

Advances in amorphous silicon solar cell and module development over the past two decades has led to widespread commercial application in consumer and building integrated photovoltaic applications (BIPV). The technology has taken two pathways: (i) superstrate and (ii) substrate. Both pathways have unique advantages over crystalline modules and have demonstrated promising stability and reliability with continuous improvement in performance. Multi-junction modules with amorphous and microcrystalline silicon have demonstrated initial conversion efficiencies in the range of 13%-13.5%.

The development of microcrystalline diodes grown at low temperature by PECVD techniques is reported. Current densities near 200 A/cm2 at + 2 V, and rectification ratios on the order of 105 at +/- 1V and 107at +/- 2V were obtained. The reverse currents were in the nano-ampere range. Correlations between deposition conditions and film quality are presented. The effects of mesa formation and subsequent treatments designed to reduce process damage are discussed: annealing conditions yield an increase in forward current, and a decrease in reverse current. Fabrication conditions are compatible with applications requiring low temperature processes (e. g., multi-layer structures, molecular layers, or plastic substrates and coatings).

A new back reflector comprised of an Al/(multi-layered stack)/ZnO structure is being developed to replace Al/ZnO used in manufacturing and boost conversion efficiencies with improved back reflector performance. Use of the multi-layered stack should lead to improved reflectivity which will in turn improve solar cell currents and efficiencies. The results from studies of different transparent conductive oxides (TCOs) which comprise the multi-layered stack are reported with emphasis on ZnO alloys. Alloying with Si or MgF2 and using moderately high substrate temperatures, TCOs with low indices of refraction between 1.6 and 1.7 have been fabricated. The Si, Mg and F contents for these alloys were near 14, 12 and 33 at.%. Structural analysis demonstrates that alloys with MgF2 have smother surfaces and finer morphologies than those for ZnO. The expected high values for multi-layered structures with these alloys have yet to be achieved but this is likely due to properties of layers in the structure other than the ZnO alloys which have yet to be fully optimized.

In this work, we report the composition, optical, and electrical properties of a- Si1-YGeY: H, F films to be used as sensing layer in uncooled microbolometers. The a-Si1-YGeY films where Y is Ge content in solid phase were deposited by low frequency PECVD from SiH4 and GeF4 feed gases, and H2 and Ar were used for dilution. The film composition, IR transmission and temperature dependence of conductivity were measured. The reduction of conductivity activation energy from 0.86 eV to 0.39 eV and the increase of room temperature conductivity from 1x10−9 to 2.1×10−3 Ohm−1cm−1 were observed with the change of Y from 0 (Si) to 1(Ge). These results demonstrate this material to be a good candidate as a sensing material in uncooled micro-bolometers, due to its high absorption in the range of λ = 10-13 μm, its relatively high activation energy, Ea=0.4 eV, consequently, a high temperature coefficient of resistance (TCR), and moderate resistivity at room temperature.

We have used a cascaded-arc expanding thermal plasma (ETP) to produce thin films of amorphous silicon at high growth rates (> 3 nm/s). Here, we present a study of the effect on material properties of hydrogen injection in the nozzle, i.e., at the exit of the arc where the plasma expands into the reactor chamber. The advantage of using extra H2 in the nozzle is that the plasma chemistry and pressure in the arc remain unchanged, whilst higher growth rates and a material with low defect densities can be obtained.

We observe that with an increase of substrate temperature the growth rate decreases due to densification of the material. This densification is accompanied by a reduction of the hydrogen content and of the microstructure parameter. Further we observe that hydrogen content decreases with higher growth rate. A strong relation is found between the light conductivity and the microstructure parameter indicating a large void fraction in samples grown at low temperature.

We have been able to grow a-Si:H material, with H2 in the nozzle, at 350°C and 3 nm/s with a light conductivity of 1.2 × 10−5 Ω1cm−1, which can be suitable for solar-cell application.

The effects of 3-MeV electron irradiation on a-Si:H have been studied using Time-Resolved Microwave Conductivity (TRMC). A Van der Graaff electron accelerator is used to generate the probe-beam pulses for the TRMC experiment as well as for the in-situ irradiation of the samples for the degradation of the material. Using several probe-beam pulse doses, TRMC transients were obtained on samples that have been subjected to various radiation fluences. These transients were later analyzed using a simple model based on the Shockley-Read-Hall capture and emission processes. Using these simulations we deduce a relationship between the radiation fluence and the defect density in the material.

We report on direct measurements of surface potentials on cross sections of a-Si:H and a-SiGe:H n-i-p solar cells using scanning Kelvin probe microscopy. External bias voltage (Vb)induced changes in the electric field distributions in the i layer were further deduced by taking the derivative of the Vb-induced potential changes. This procedure avoids the effect of surface charges or surface Fermi-level pinning on the potential measurement. We found that the electric field does not distribute uniformly through the i layer of a-Si:H cells, but it is stronger in the regions near the n and p layers than in the middle of the i layer. The non-uniformity is reduced by incorporating buffer layers at the n/i and i/p interfaces in the a-Si:H solar cells. For a-SiGe:H solar cells, the electric field at the p side of the i layer is much stronger than at the n side and the middle. The non-uniformity becomes more severe when a profiled Ge content is incorporated with a high Ge content on the p side. We speculate that the increase in defect density with increasing of Ge content causes charge accumulation at the i/p interface.

Over the last decade SiGe has been proposed as a structural material for low thermal budget microelectromechanical systems (MEMS) that can be post-processed on top of standard CMOS driving and controlling electronics [1-6]. There are several ways to decrease the deposition temperature of SiGe and at the same time preserve the desired physical properties for MEMS as low electrical resistivity, high quality factor, economical growth rate and low mean stress and strain gradient. The conventional approach to reduce the crystallization thermal budget is to increase the germanium content to 60%, or more, using conventional Low Pressure Chemical Vapor Deposition (LPCVD) [1-3]. In this case highly conductive polycrystalline films can be realized, but the strain gradient is relatively high. This can be eliminated by furnace annealing at 450°C [2], which might introduce damage to the underlying circuits such as in the case of Cu/low k CMOS. This problem can be alleviated using excimer pulsed laser annealing [7, 8], which has been attractive for low thermal budget applications such as thin film transistors (TFT) [9], solar cells fabricated on glass substrates [10] and for monolithic integration of MEMS devices on top of standard driving electronics using SiGe as an active material [8, 11]. Also the use of hydrogenated microcrystalline SiGe allows for a low thermal budget [12]. In addition, metal induced crystallization has recently been proposed to enhance the crystallization of silicon at temperatures as low as 500°C, and the realized devices had outstanding performance compared to those employing conventional solid-phase crystallization [13]. This technique enhances crystallization by two methods. First, it has been observed that depositing SiGe on top of a thin Al or Ni layer, has a polycrystalline micro-structure close to the metal/ SiGe interface [11]. Annealing this film for a long period (is determined by the annealing temperature), results in metal diffusion and a subsequent crystallization of the film. Finally, when the metal is diffused completely through out the film, it can be etched away. The main disadvantage of this approach is that the mean stress is highly compressive and this might affect the functionality of surface micromachined structures [13].

We report on transient and modulated photoconductivity experiments with undoped michrystalline silicon in which access to density-of-states information is limited because the Fermi level results in occupancy of localised states in the energy range which is scanned. Simulation results show that a defect peak will be masked if most of the distribution is occupied because of the Fermi level position and the density-of-states determined from the experimental data is not an image of the true distribution. Another difficulty with obtaining reliable density-of-states distributions in microcrystalline silicon is the metastability of samples with respect ot adsorption of gases. If dark-conductivity changes are large upon heat treatment in vacuum, the modulated and transient photocurrent response are also affected to a large degree and the density-of-states profiles apart form being influenced by the Fermi level position thus also depend on the thermal history of the sample.

A graphite catalyzer has been used to grow μc-Si:H films using the thermo-catalytic (HW) chemical vapor deposition (CVD) technique. The films grown in the amorphous-microcrystalline ‘transition regime’ have been found to exhibit a high photosensitivity of the order of 102-103 at a crystalline volume fraction of 0.2-0.4. The effects of deposition parameters such as silane concentration, pressure and substrate temperature on the microstructure and electrical properties of the films have been studied. It has been found that the graphite catalyzer offers a wider window of the deposition parameters for the growth of the ‘transition regime’ films as compared to the conventional W and Ta catalyzers. In addition, the graphite wires also exhibit significantly greater chemical as well as mechanical stability than the W and Ta wires, which results in improvement of the reproducibility of the technique. However, at the same filament temperature and other conditions, the deposition rates are about 10 times lower than for W or Ta filament. Increasing of the filament temperature, on the contrary, lead to radiative heating and carbon contamination of the growing film.

Structural and electronic properties of SiCl4-based microcrystalline silicon films were studied. A rather dense (non-porous) material structure is obtained near the transition to amorphous material, in particular at substrate temperatures of 250°C and above. Boron doping results in very high conductivity values while for phosphorus doping only lower values are reached. This latter effect is attributed to a different microstructure with lower crystalline fraction, higher hydrogen and chlorine content and increased porosity in highly phosphorus- doped material.