The Pulsed PECVD technique involves modulating the standard 13.56 MHz RF plasma, in the kHz range. This allows an increase in the electron density during the ‘ON’ cycle, while in the ‘OFF’ cycle neutralizing the ions responsible for dust formation in the plasma. In this work, we report the increase of i-layer growth rate and silane gas utilization rate (GUR) for amorphous Si p-i-n solar cells grown in a large area (30 cm × 40 cm) single chamber deposition system. The i-layer growth rate of 5.4 Å/sec with a GUR of >15% has been achieved, which shows a device efficiency of 8.3% (almost same as of our conventional PECVD grown a-Si:H solar cell with ilayer growth rate of ∼1 Å/sec). We also deposited microcrystalline Si p-i-n devices using the Pulsed PECVD technique. The crystallite orientation of the films changes from a random to a (220) orientation near the microcrystalline-to-amorphous transition. The effects of crystallite orientation, grain boundaries and ion bombardment during growth on the solar cell performances are investigated. An efficiency of 4.8% for single junction μc-Si:H p-i-n device has been achieved for the i-layer thickness of 0.9 μm.

When an a-Si:H p-i-n structure is locally illuminated by a light spot, the non uniformity of light causes the appearance of a gradient in the carrier concentration between the illuminated and the dark zone. Carrier start to flow in agreement with such gradients, and when equilibrium is reached, the lateral diffusion process is counterbalanced by the appearance of a lateral component of the electric field vector in addition to the transverse usual one. The lateral fields act as a gate for the lateral flow of the carriers and small lateral currents appears at the transition region between the illuminated and the dark zone. Known as lateral photo-effect, this phenomena depends on the incident light wavelength, light intensity and on the applied bias. Anyway, its intensity can be, depending on the foreseen application, alternatively enhanced or reduced by correct device engineering. We have used the 2D numerical simulator ASCA to analyze the behavior of a-Si:H p-i-n structures under local illumination with the goal of observing the appearance of the lateral components of the electric field and current density vectors. The dependence of the lateral potential redistribution on the doping density, density of defects in the intrinsic layer, and layer thickness have been analyzed. This study aims to show how material properties and device geometry can be combined in order to control the lateral photo-effect.

We study the amorphous to microcrystalline silicon phase transition in hot-wire chemical vapor deposition thin-film silicon by depositing a series of unique, thickness-graded, samples on a glass substrate at 200°C. By inserting or withdrawing a motor-driven shutter during growth, we make samples that vary from 200 to about 2000 Å thick across each 5-cm along stripe. Each stripe is grown at a different dilution ratio of hydrogen to silane in the source gas. The phase composition at various locations was determined by Raman and ultraviolet-reflectivity measurements. Atomic force microscopy (AFM) images of topology reveal that the surface changes from a rather smooth a-Si phase to more granular microcrystalline-Si (rms roughness increases from 10 to 47 Å).

We have examined the electronic properties of intrinsic amorphous silicon films as a function of deposition rate, and compared these with the performance of companion solar cell p-i-n devices. The device efficiency in the light-soaked state was strongly inversely correlated with growth rate. Film properties were evaluated in both the as-grown and light soaked states using drive-level capacitance profiling and transient photocapacitance spectroscopy. Although deep defect densities measured by drive-level capacitance profiling did not vary significantly between samples, the magnitude of the defect band deduced via transient photocapacitance spectroscopy was well correlated with device performance. Possible reasons for this discrepancy are discussed. Urbach energies were also correlated with film growth rate.

The Ni silicide-mediated phase transformation of amorphous to crystalline silicon (c-Si) was studied using transmission electron microscopy. Amorphous silicon (a-Si) films coated with very thin Ni layer (∼10-1Å) were annealed at various temperatures. Randomly oriented NiSi2 precipitates were observed in the Ni deposited a-Si film annealed at 400°C. The nucleation of the epitaxial c-Si has occurred on the {111} faces of the octahedral NiSi2 precipitate at 430°C and then caused to the variation in the shape of the NiSi2 precipitates. During the growth of c-Si, the needle-like morphology developed from the migration of NiSi2 precipitates through the a-Si matrix leaving a trail of c-Si. The collision of a migrating NiSi2 precipitate with a stationary NiSi2 precipitates in the a-Si film gave rise to the change in the morphology of the growing Si grain and the formation of additional c-Si needles on variants of the <111> direction.

As a first step toward an understanding of the chemical and structural role of hydrogen in hydrogenated amorphous silicon, we utilized electron beam evaporation in an ultra high vacuum environment to deposit films of amorphous silicon and systematically dosed these films with atomic hydrogen during deposition. Secondary Ion Mass Spectroscopy (SIMS) data indicated that hydrogen concentration can be varied from the detection limit of SIMS to a value in excess of 1021 atoms cm-3. The intentional addition of hydrogen caused the concentration to fall from in an excess of 1021 atoms*cm-3 to below 1018 atoms*cm-3.

Proton NMR studies on hydrogen in numerous plasma deposited a-Si:H thin film samples over many years have established that there are primarily two dipolar broadened lines, representing different distinct degrees of proton clustering involving most of the contained hydrogen. In addition, both of these configurations were thought to be hydrogen tightly bonded to silicon. However, deuteron magnetic resonance established long ago that a substantial fraction of the hydrogen (deuterium) population was a weakly bonded species, very different than the silicon bonded hydrogen typically seen in infrared measurements. Deuteron magnetic resonance can easily distinguish bond strength because deuterons have spin, an electric quadrupole moment, and are sensitive to electric field gradients. The much weaker deuteron dipolar coupling results in NMR line shapes and widths that are determined by distributions of electric field gradients and angular averages about the applied magnetic field direction. The primary features of the deuteron line shape are a sharp powder-pattern Pake doublet attributed to the tightly silicon bonded species and a broad central Gaussian component originally interpreted as a weakly bound component, but now recognized as molecular deuterium and HD trapped in the amorphous equivalent of T-sites. This latter result is confirmed by spin-echo double resonance experiments using 1H and 29Si nuclides, which reveal a very appreciable population of molecular hydrogen in T-like sites. NMR line shape subtractions also indicate that the majority of this species resides in the less-clustered hydrogen phase. Also a comparison of the T-site molecular hydrogen fraction with measured photoresponse, combined with light-induced changes in the DMR line shape, indicate that this species is in regions of a-Si:H that control the optoelectronic properties of the material.

Temperature and frequency dependencies of the electrical properties of SiO2/nanocrystalline Si (nc-Si)/SiO2 sandwich structures have been studied. A clear positive shift in capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics suggests electron trapping in nc-Si dots. The role of interface states and deep traps in these devices has also been examined, which shows that they have little effect on the overall device performance.

We present a new microscopic model for metastable Si dangling bond defect creation in hydrogenated amorphous silicon, which is applicable to both light-induced and carrier-induced defect creation. The key feature of our model is that hydrogen is always in the tedrahedral-like site, which is strongly bound in amorphous silicon, and never in the bond-centered site. Breaking of Si-Si bonds and successive stabilisation by bond-switching of nearby hydrogen from doubly hydrogenated Si-Si bonds (SiHHSi) results in two hydrogen-stabilised dangling bonds (SiHD). Since hydrogen is in the Td site in all configurations, this defect creation reaction is consistent with ESR, NMR, hydrogenation experiments and the mechanical stress dependence of defect creation.

Residual ion implantation damage at source/drain junctions of excimer laser annealed polycrystalline silicon (poly-Si) thin film transistor (TFT) was investigated by high-resolution transmission electron microscopy (HR-TEM). Cross-sectional TEM observation showed that XeCl excimer laser (λ=308 nm) energy decreased considerably at the source/drain junctions of top-gated poly-Si TFT due to laser beam diffraction at the gate electrode edges and that the silicon layer amorphized by ion implantation, was not completely annealed at the juncions. The HR-TEM observation showed severe lattice disorder at the junctions of poly-Si TFT.

We observe a significant light-induced increase in the open-circuit voltage, Voc, of thin-film silicon solar cells whose intrinsic (i) layer consists of an amorphous and microcrystalline mixed phase. The increase depends on the i-layer thickness, the i-layer deposition temperature, the initial Voc values, and the light-soaking intensity. An increase of as large as 150 mV is observed. The original Voc is restored after subsequent thermal annealing. In-situ photoluminescence (PL) spectroscopy is used to investigate this metastable phenomenon. We find that the PL intensity and peak-energy position associated with the amorphous component of the heterogeneous material increase upon light soaking, suggesting a structural change. We propose that a reduction of microcrystalline volume fraction or size is responsible for the Voc enhancement.

In this paper we present the results for boron-doped microcrystalline silicon deposited at low temperature (230°C) by PECVD method. The structural and electrical properties are investigated as a function of boron concentrations using Raman, dark conductivity, hydrogen effusion and Hall effect measurements. The results show that the incorporation of boron induced an amorphization in the μc-Si:H films structure. A structural change due to boron incorporation is also accompanied by a change in hydrogen bonding environment. High conductive film of our boron-doped μc-Si:H films is attained at boron concentration 2000 ppm which consists nearly the same fraction of crystalline and amorphous phase (XC ~55 %).

Optically induced electron spin resonance (LESR) studies on time scales in between the previously published PL and LESR results (approximately 10 ms to 10 s) allow one to examine the cross over between energy-loss (downward) hopping of carriers and carrier recombination via tunneling. In addition, data in this time regime are directly compared in the same sample with NMR data on the dipolar spin-lattice relaxation of the bonded hydrogen where light induced electrons and holes are responsible for dipolar relaxation of bonded hydrogen. The LESR results confirm the interpretation of the NMR measurements.

Transient photoconductivity in microcrystalline silicon thin films prepared by VHF PECVD over a range of [silane]: [silane + hydrogen] concentrations γ from 3 to 6.3% has been measured as a function of temperature (210 K - 390 K) and laser pulse density (1013 – 1016 cm3). The behavior of highly crystalline films (γ ∼ 3%) bears certain similarities to n-type amorphous silicon, where the power law decay at long times is controlled by hole rather than electron kinetics. As γ approaches 6% the decay begins to resemble that of intrinsic amorphous silicon, consistent with structural studies which indicate a transition from crystalline to amorphous character. Transient photocurrent density of states spectroscopy on transitional films suggests a similar deep defect density to high-quality amorphous silicon, but a larger conduction band tail energy of some 30 meV and a tenfold increase in localised state density at a depth of about 0.4 eV.

The morphological and compositional instabilities in the heteroepitaxial strained alloy films have attracted intense interest from both experimentalists and theorists. To understand the mechanisms and properties for the generation of instabilities, we have developed a nonequilibrium, continuum model for the dislocation-free and coherent film systems. The early evolution processes of surface pro.les for both growing and postdeposition (non-growing) thin alloy films are studied through a linear stability analysis. We consider the coupling between top surface of the film and the underlying bulk, as well as the combination and interplay of different elastic effects. These e.ects are caused by filmsubstrate lattice misfit, composition dependence of film lattice constant (compositional stress), and composition dependence of both Young's and shear elastic moduli. The interplay of these factors as well as the growth temperature and deposition rate leads to rich and complicated stability results. For both the growing.lm and non-growing alloy free surface, we determine the stability conditions and diagrams for the system. These show the joint stability or instability for film morphology and compositional pro.les, as well as the asymmetry between tensile and compressive layers. The kinetic critical thickness for the onset of instability during.lm growth is also calculated, and its scaling behavior with respect to misfit strain and deposition rate determined. Our results have implications for real alloy growth systems such as SiGe and InGaAs, which agree with qualitative trends seen in recent experimental observations.

The effect of the microstructure and bonded hydrogen on the optical properties of microcrystalline films (μc-Si:H) was investigated by Spectroscopic Ellipsometry (SE) and Photothermal Deflection Spectroscopy (PDS). On samples with a high crystalline volume fraction we studied the reason for a large deviation of absorption coefficient in the energy range between 1.6 eV and 3.2 eV from the value predicted by effective medium theory. This enhancement can be attributed to scattering by the inhomogeneity of μc-Si:H, which is investigated by the introduction of the dense medium radiative transfer formalism to an optical scattering simulation. Further, we suggest strain as a reason for the enhanced absorption in highly crystalline μc-Si:H.

Large area p-i-n a-SiC:H heterostructures are used as LSP color sensors. For reading out the color signals, three appropriated voltages have to be successively applied in order to combine afterwards the information to yield a color image. The highly resistive and wide band gap doped layers confine the photogenerated carriers at the illuminated regions and driven by the scanner extract information on the image shape and intensity. The bias voltage controls the potential profile across the main generation region leading to color sensitivity. As the positive applied voltage increases the reversed electrical field in the bulk shifts toward the main generation regions, and successively suppresses the ac component of the photocurrent at each primary color allowing color extraction. The device performance is analyzed and the scanning technique for color separation improved. The influence of the optical and electrical bias on image contrast, resolution and color extraction is discussed. A physical model for image and color recognition is presented and supported by a two dimensional simulation.

We have investigated the cause for continued changes in the silane related species observed using mass spectroscopy during i-layer deposition process lasting several tens of minutes. Silane decomposition is found to influence the mass spectrometer filament and therefore, pressure changes were compared to ascertain differences in pumping speed using a rotary, mechanical pump. The cathode voltage was used as a measure of effective hydrogen to silane ratio in real time and deliberate tailoring of this ratio is seen to result in a discernible difference in the degradation of single junction amorphous silicon solar cells. Some obvious consequences of this subtle, pump-type dependent transient behavior are discussed.

a-Si:H films were prepared by hot wire chemical vapor deposition. One group was deposited at a substrate temperature of Ts=250°C with varied hydrogen-dilution ratio, 0<R<10; the other group was deposited with fixed R=3 but a varied Ts from 150 to 550°C. IR, Raman and PL spectra were studied. The Raman results indicate that there is a threshold value for the microstructure transition from a- to μc-Si. The threshold is found to be R ≈ 2 at Ts = 250°C and Ts ≈ 200°C at R=3. The IR absorption of Si-H at 640 cm-1 was used to calculate the hydrogen content, CH. CH decreased monotonically when either R or Ts increased. The Si-H stretching mode contains two peaks at 2000 and 2090 cm-1. The ratio of the integral absorption peaks I2090/(I2090+I2090) showed a sudden increase at the threshold of microcrystallinity. At the same threshold, the PL features also indicate a sudden change from a- to μc-Si., i.e. the low energy PL band becomes dominant and the PL total intensity decreases. We attribute the above IR and PL changes to the contribution of microcrystallinity, especially the c-Si gain-boundaries.

Thin film microcrystalline silicon solar cells were prepared with intrinsic absorber layers by Hot Wire CVD at various silane concentrations and substrate temperatures. Independently from the substrate temperature, a maximum efficiency is observed close to the transition to amorphous growth, i.e. the best cells already show considerable amorphous volume fractions. A detailed analysis of the thickness dependence of the solar cell parameters in the dark and under illumination indicate a high electronic quality of the i-layer material. Solar cells with very high open circuit voltages Voc up to 600mV in combination with fill factors above 70% and high short circuit current densities jsc of 22mA/cm2 were obtained, yielding efficiencies above 9%. The highest efficiency of 9.4% was achieved in solar cells of 1.4μm and 1.8μm thickness. These cells with high Voc have considerable amorphous volume fractions in the i-layer, leading to a reduced absorption in the infrared wavelength region.