A microwave-based technique has been used to prepare microcrystalline Si (µc-Si) materials rates near 15 Å/s. The use of these materials as intrinsic layers (i-layers) for single and multi-junction devices has been assessed. Since the high deposition rates allow for fabrication of the required thicker µc-Si i-layers in a similar amount of time to that used for high quality a-SiGe i-layers (rates of 1-3 Å/s), the materials are attractive, low cost replacements for a-SiGe bottom cell i-layers in a-Si/a-SiGe and a-Si/a-SiGe/a-SiGe multi-junction cells. Single-junction nip, a-Si/µc-Si and a-Si/a-SiGe/µc-Si devices have been fabricated. For these devices, the doped and amorphous layers were deposited using conventional rf glow discharge processes and deposition equipment separate from that used to fabricate the µc-Si materials. 7.0% efficiencies have been achieved for single-junction devices while pre-light soaked 9.8 and 11.4% efficiencies have been achieved for the tandem and triple-junction devices, respectively. The single-junction devices exhibit a degradation of only 0-2% after long term (1000 hrs.) of light soaking demonstrating a high degree of stability. Based on the present status, the µc-Si material prepared at high rates qualifies as a reasonable candidate for the i-layer of a bottom cell of a triple-junction device. Improvements in the performance, particularly the FF, will be needed before use in single-junction and tandem devices can be considered.

In this work, we present the properties of n-type silicon films obtained by hot wire plasma assisted technique produced at different rf power and gas flow rate. The films were produced at a filament temperature of 2000°C and the rf power was varied from 0W to 200W while gas flow rate was varied from 15 to 100sccm keeping rf power at 50W. In this flow rate range, the growth rate of the films varied from 5Å/s to 250Å/s and the corresponding electrical room dark conductivity varied from 10−2 to 10(Ωcm)-1. On the other hand, we observed that the electrical conductivity increased from 2 to 6(Ωcm)-1, and the Hall mobility from 0.1 to 2cm2/V.s as rf power change from 0W to 200W. The infrared, EDS and XPS analyses revealed the existence of oxygen incorporation, which is not related to post-deposition oxidation. The X-ray diffraction and [.proportional]Raman data show the presence of Si crystals in the films structure and the SEM micrographs reveal a granular surface morphology with grain sizes lower than 60nm.

High quality low bandgap a-(Ge,Si):H alloys were prepared using low pressure, reactive ECR plasma deposition with high H dilution and subtle (sub-ppm) B-doping. The charge transport and optical properties of these low bandgap materials as a function of alloy composition have been investigated by employing the microwave photomixing technique and optical absorption spectroscopy. From the measurements of electric field dependence of the drift mobility, we have found strong evidence for the existence of long-range potential fluctuations in a-(Ge,Si):H alloys, and determined the depth and range of the potential fluctuations, and subsequently the charged defect density, as a function of alloy composition. It was found that at ∼30% Si in Ge, the photoresponse begins to decrease rapidly with increasing Si content due to the decreases in the mobility and lifetime, and meanwhile, both the charged defect density and the Urbach energy increase significantly. The latter indicates an increase in the compositional disorder. It is the potential fluctuations whose effect can be also enhanced by incorporating Si to the alloy system that result in the deterioration of the electronic properties of a-(Ge,Si):H alloys, similar to the case of the incorporation of Ge at the Si end. This enhanced effect accompanies with an increase in the depth, and a decrease in the range of potential fluctuations, leading to a decrease in the mobility, and consequently in the photoconductivity. Our present results demonstrate that the increased charged scattering centers and compositional disorder upon adding Si to the alloys play an important role in the potential fluctuations.

Spectroscopic ellipsometry (SE) analysis of so-called “specular” (macroscopically smooth) and “textured” (macroscopically rough) thin film amorphous silicon (a-Si:H) based solar cell structures demonstrates the need to incorporate interface layers into the multilayer stack in order to simulate the observed Stokes vector of the specularly-reflected beam. In most cases, these layers can be attributed to microscopic roughness (e.g., at the SnO2/p-layer/i-layer interface in a-Si:H p-i-n solar cells), as verified by atomic force microscopy (AFM). In limited cases, the layers may include regions wherein chemical intermixing also occurs (e.g., at the ZnO/Ag interface in back-reflectors), particularly for overlying films prepared by sputtering. In spite of the clear evidence for the existence of interface layers, they have been neglected in previous simulations of the optical quantum efficiency (QE) of the solar cells. In this study, we incorporate the experimentally- observed characteristics of interface layers as input into optical models for the p-i-n solar cell structure. In this way, we demonstrate the beneficial effects of SnO2/p/i interface microroughness as an anti-reflector and the detrimental effects of the ZnO/Ag interlayer as a parasitic absorber.

The first competitive a-Si/poly-Si multibandgap tandem cells have been made in which the two intrinsic absorber layers are deposited by Hot Wire Chemical Vapor Deposition (HWCVD). These cells consist of two stacked n-i-p type solar cells on a plain stainless steel substrate using plasma deposited n- and p-type doped layers and Hot-Wire deposited intrinsic (i) layers, where the i-layer is either amorphous (band gap 1.8 eV) or polycrystalline (band gap 1.1 eV). In this tandem configuration, all doped layers are microcrystalline and the two intrinsic layers are made by decomposing mixtures of silane and hydrogen at hot filaments in the vicinity of the substrate. For the two layers we used individually optimized parameters, such as gas pressure, hydrogen dilution ratio, substrate temperature, filament temperature, and filament material. The solar cells do not comprise an enhanced back reflector, but feature a natural mechanism for light trapping, due to the texture of the (220) oriented poly-Si absorber layer and the fact that all subsequent layers are deposited conformally. The deposition rate for the throughput limiting step, the poly-Si i-layer, is ≍ 5-6 Å/s. This layer also determines the highest substrate temperature required during the preparation of these tandem cells (500 °C). The initial efficiency obtained for these tandem cells is 8.1 %. The total thickness of the silicon nip/nip structure is only 1.1 µm.

Hydrogenated amorphous silicon thin films deposited from SiH4 containing plasmas are used in solar cells and thin film transistors for flat panel displays. Understanding the fundamental microscopic surface processes that lead to Si deposition and H incorporation is important for controlling the film properties. An in situ method based on attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy was developed and used to determine the surface coverage of silicon mono-, di-, and tri-hydrides as a function of deposition temperature and ion bombardment flux. Key reactions that take place on the surface during deposition are hypothesized based on the evolution of the surface hydride composition as a function of temperature and ion flux. In conjunction with the experiments, the growth of a-Si:H on H-terminated Si(001)-(2×1) surfaces was simulated through molecular dynamics. The simulation results were compared with experimental measurements to validate the simulations and to provide supporting evidence for radical-surface interaction mechanisms hypothesized based on the infrared spectroscopy data. Experimental measurements of the surface silicon hydride coverage and atomistic simulations are used synergistically to elucidate elementary processes occurring on the surface during a-Si:H deposition.

The microstructure of microcrystalline silicon p-layers on zinc oxyde (ZnO) has been studied by Transmission Electron Microscopy (TEM). ZnO has been characterised by X-Rays diffraction and by Scanning Electron Microscopy (SEM). These reveal that the ZnO surface topography consists of regular pyramids with a (110) preferential crystallographic growth axis. High Resolution TEM observations indicate that the p- layer growth starts at the top of the ZnO pyramids. This results, for fully microcrystalline p-layers, in a decreased thickness at the bottom of the ZnO pyramids. When the p-layer is fully microcrystalline, no amorphous incubation layer could be observed between ZnO and p-silicon layer. Moreover, the high crystallinity of the p-layers in microcrystalline p-i-n devices is accompanied by an increased fill-factor in the I-V characteristics.

Proton nuclear magnetic resonance (NMR) is an important tool for characterizing the structure of a-Si:H over a wide range of length scales including short- and medium-range order as well as nanostructures. Some of the structural information obtained by NMR is based directly on proton NMR characteristics such as the lineshape and dipolar interactions that probe distances between hydrogen atoms and hydrogen environment. Based on such information it is shown that hydrogenated multi-vacancy model for Si-H clusters is fully consistent with both experimental and theoretical observations. It is demonstrated thatchanges of short- and medium-range order can be detected by measuring the magnetic susceptibility χ. Here ï is not directly related to NMR characteristics and NMR is used merely as a sensitive and accurate magnetometer. The result indicates that a-Si:H prepared by hot-wire CVD (HWCVD) and plasma-enhanced CVD (PECVD) with high H-dilution have higher structural order compared to conventional a-Si:H. By carrying out NMR measurements on single a-Si:H thin film new NMR features were observed such as the orientation dependence of the proton NMR spectrum with respect to the magnetic field. Based on such orientation dependence, strong evidence of aligned nano-channels was obtained in some device quality a-Si:H films.

Even under the conventional device-quality discharge conditions, large amount of clusters are found to exist in silane radio frequency (RF) discharges. The size and density of clusters near the substrate on the grounded electrode are 2 nm and 1011 cm-3, respectively. In order to prevent such clusters from depositing on hydrogenated amorphous silicon (a-Si:H) films, we have developed a cluster-less plasma CVD reactor which can significantly suppress cluster amount by using gas viscous and thermophoretic forces exerted on clusters, and by reducing gas stagnation region. Using the reactor, we have demonstrated deposition of ultra high quality a-Si:H films which have a significantly less concentration of Si-H2 bonds compared to conventional device-quality films.

The effect of an electric field on germanium-seeded lateral crystallization of a-Si is studied for the first time and compared to this effect in Ni-induced lateral growth. While the crystallization rate is lower when Ge is used as the nucleation seed and annealing should be done at higher temperatures, filed-aided crystallization shows a similar behavior to that observed for Ni-induced crystallization. Optical microscopy results indicate that grain growth starting from the negative electrode occurs in Si films at annealing temperatures higher than 480°C, while the applied electric field ranges form 200 to 1400V/cm. SEM was also used to confirm the crystallinity of the films.

Electronic properties of thin epitaxial silicon films deposited at temperatures below 650°C by means of ion-assisted deposition strongly depend on substrate orientation as well as on deposition temperature: In (100)-oriented epitaxial films we find a low density of structural defects, and the minority carrier diffusion length is only limited by the presence of point defects or point defect complexes. These investigations also show an improvement of the electronic quality with increasing deposition temperature. Epitaxy on non-(100)-oriented substrates results in a significantly higher density of structural defects. The electronic properties of films deposited on stable flat surfaces, such as (111)- and (113)-oriented substrates are inferior as compared to (100)-oriented films, but are still superior to those of films deposited on faceted surfaces, as shown by light beam induced current and electron back-scattering diffraction measurements of polycrystalline thin films.

Charge transport in hydrogenated microcrystalline silicon (µc-Si:H) is determined by structure on several size scales: i) local atomic arrangement (<1 nm), ii) crystalline grains and their boundaries (1-10 nm), iii) grain aggregates or columns (0.1-1 µm) and finally iv) features comparable to layer thickness (0.1-10 µm). We first summarize our experimental results concerning these effects: differences of conductivities of grains and amorphous tissue measured locally by conductive AFM, transport anisotropy observed by comparing dark conductivity and ambipolar diffusion length parallel and perpendicular to the substrate, and finally thickness dependence of transport parameters (e.g. dark conductivity activation energy and prefactor). Most of these phenomena can be described by using a novel model of the µc-Si:H growth leading to a structure known as Voronoi adjacency network. The model is based on the nearest neighbor constrained growth. To our knowledge, the Voronoi structure is the first structural model able to predict structure and transport properties of the µc-Si:H and it may become a basis for the future predictive model of µc-Si:H based solar cells.

BP Solar started commercial production of amorphous silicon tandem solar cells in Toano, Virginia in 1997. The scale-up process has involved overcoming technology challenges in several areas. It was necessary to develop high speed, multiple-beam laser scribing systems that could pattern photovoltaic modules over an area of 0.8 square meters with close dimensional tolerances. In addition, the deposition rate of amorphous silicon was doubled while production was being ramped up, and the deposition system was modified to meet capacity requirements. The utilization of the germane feedstock gas was increased by about 25% in order to reduce material costs. In addition, the dimensional tolerances of the deposition system geometry were determined and controlled in order to assure uniform deposition of the amorphous silicon alloys. Effects of contaminants (such as pump oils and residual dopants) on device performance were quantified. Sources of debris that could cause shunts and shorts in the devices were identified and minimized. Current research efforts are focused on further increases in the amorphous silicon deposition rate, improvements in device performance and the development of in-situ diagnostic tools to monitor and control the manufacturing process.

Glass/ZnO:Al/p (SixC1−x:H)/i (Si:H)/n (SixC1−x:H)/Al (0<x<1) heterojunction cells were produced by PE-CVD at low temperature. Junction properties, carrier transport and photogeneration are investigated from illuminated current- and capacitance- voltage characteristics and spectral response measurements, in dark and under different illumination conditions. For the heterojunction cells high series resistance around 106 ω and atypical J-V characteristics are observed leading to poor fill factors, also it was observed that the responsivity decreases with the increase of the light bias intensity. For the homojunction the behaviour is typical of a non optimised p-i-n cell and the responsivity varies only slightly with the light bias conditions. A numerical simulation gives insight into the transport mechanism suggesting that in the heterojunctions and in dark conditions the transport mechanism depends almost exclusively on field-aided drift while under illumination it is dependent mainly on the diffusion of free carriers.

We report a new excimer laser annealing method by employing selectively floating a-Si active layer structure in order to increase the grain size of poly-Si film. The floating a-Si region blocks the heat conduction into the underlying substrate due to high thermal-insulating property of an air so that the lateral temperature gradient is successfully induced by the proposed simple air-gap structure. Our experimental results show that large grains were grown in the lateral direction from the edge to the center of the floating active region. The large grains exceeding 4 m were successfully obtained with only one laser irradiation.

Improvement of electrical properties by heat treatment with high-pressure H2O vapor was discussed for 7.4×1017cm−3 phosphorus-doped pulsed laser crystallized silicon films. The analysis of the electrical conductivity resulted in that 1.3×106-Pa-H2O vapor annealing at 270°C for 3 h reduced the density of defect states from 5.5 ×1018 cm−3(as crystallized) to 5.0×1017 cm−3 for tail states and from 1.0×1018 cm−3(as crystallized) to 3.0×1017 cm−3 for deep level defect states. The potential barrier height at grain boundaries decreased from 0.34 eV (as crystallized) to 0.05 eV by the heat treatment. The combination of oxygen plasma with high-pressure H2O vapor annealing effectively reduced the densities of defect tail sates as well as deep level defect states. It achieved a high performance of thin film transistors with a threshold voltage of 1.3 V and an effective mobility of 160 cm2/Vs.

We report on improvement in stability of a new type of amorphous silicon films, synthesized (growth rate > 0.1 nm/s) by driving the plasma condition close to the powder regime (or “γ-regime”) of rf PECVD. These films exhibit high mobility-lifetime products [(μτ)annld ∼ 10−4 cm2/V, σph/σd ∼ 5-10×105, Ea ≍ 0.7 - 0.9 eV ], compact network structure [CH ≍ 7 to 8 at%, nanovoid density < 0.01 %, ρ ≍ 2.23 ± 0.01 gm/cm3], new features of optical properties and density-of-state (DOS) above EF is significantly lower than that of state-of-the art films. The kinetics of light-induced (AM 1.5) degradation of μτ is very fast and saturated μτ ∼ 10−6cm2/V, a value similar to that of conventional a-Si:H films at annealed state. The improved stability of “new” a-Si films, henceforth it will be denoted as “quasi-amorphous silicon (qm-Si) thin- film”, will be correlated with its specific nanostructure.

Monte Carlo simulations of hot-wire chemical vapor deposition (HWCVD) gas ambients indicate different flux ratios (SiH3/Si and H/SiHx) under conditions for amorphous or polycrystalline silicon growth. Gas-phase reactions of Si with ambient SiH4 studied using abinitio methods reveals that collisional stabilization of the adduct (H3SiSiH) is unlikely under typical HWCVD growth pressures, but an energetically favorable, low-pressure pathway has been identified that leads to the formation of Si2H2 and H2. Threshold ionization mass spectrometry has revealed significant quantities of the radical SiH2 at HWCVD growth pressures, indicative of heterogeneous pyrolysis. Mass spectrometry at low pressures suggests that incident silane dissociatively adsorbs at the wire and undergoes sequential H elimination to produce subhydrides. Disilicon species were not detected in significant quantities at HWCVD growth pressures. Finally, hot wire operation in a pure H2 ambient yields SiH4 as the dominant etching product from the silicon-coated walls of the growth chamber.

Defect creation by electron beam irradiation is compared with that by light soaking in a-Si1-xNx:H films. For the film with x=0.06, the ESR spin density increases by 20-keV electron beam irradiation without changes in the g-value. However, for the film with x=0.47, the ESR spin density increases with accompanying the decrease in the g-value from 2.0042 to 2.0034. The decrease in the g-value can be explained by increasing number of N atoms at the backbond site of the Si atom having the dangling bond. Light soakings does not change the g-value of the ESR signals of the films.

The effusion of the rare gases neon and helium, as well as of hydrogen, was studied for plasma deposited (boron-doped and undoped) hydrogenated amorphous silicon films, grown at various substrate temperatures. Rare gas atoms were incorporated into the material during the growth process or by ion implantation. The results suggest that helium and neon effusion spectra give information on the material microstructure.