Intrinsic a-Si:H samples were grown with and without hydrogen (H2) dilution of silane at different growth rates. We find that the dilution leads to a considerable reduction in the defect density, in particular at high growth rates. The defect density is particularly low for samples grown using H2 dilution conditions at growth rates as high as 10 Å/sec. Using transient photocapacitance measurements we find evidence for a small concentration of microcrystallites embedded in the amorphous films. An increase in the microcrystalline fraction correlates with a decrease in the defect density.

The H2 dilution technique at a high deposition rate (RD) was investigated by depositing hydrogenated amorphous silicon (a-Si:H) under a high if power density of 750 mW/cm2, which is 20 times as large as that of conventional conditions. It was found that the H2 dilution ratio γ ( = [H2 gas flow rate] / [SiH4 gas flow rate]) tendency of the film properties, such as the H content (CH), optical gap (Eopt), SiH2/SiH and photoconductivity (σph) of a-Si:H is different for the high rf power (750 mW/cm2) and the medium rf power (75 mW/cm2) conditions. Under medium rf power, the CH, Eopt and SiH2/SiH decrease as γ increases. Under the high if power, on the contrary, the CH and Eopt, monotonously increase while maintaining a low SiH2/SiH and a high σph of 10-6 S/cm as γ increases. These results suggest that increasing the rf power enhances the H incorporation reactions due to H2 dilution. It is thought that a high rf power causes the depletion of SiH4 and hence the extinction of H radicals, expressed by SiH4 + H* → SiH3* + H2, is suppressed. A high H radical density enhances the incorporation of H into a-Si:H, resulting in very wide-gap a-Si:H with a high CH, Consequently, very wide-gap a-Si:H with device-quality (Eopt of 1.82 eV with an (αhv)1/3 plot, corresponding to > 2.1 eV with Tauc's plot, and σph of 10-6 S/cm) can be obtained at a high RD of 12 Å/s without carbon alloying.

We report on the development of helium diluted a-Si:H and a-SiGe:H solar cells with higher deposition rates and better feedstock utilization than devices made with hydrogen dilution. Both the initial and the stabilized efficiencies of the He-diluted single-junction aSi:H and a-SiGe:H cells are similar to those of hydrogen-diluted cells with state-of-the-art intrinsic materials. The total fabrication time for tandem cells has been reduced by 17% by using helium dilution without loss in initial and stabilized efficiency.

The production of amorphous silicon, e.g. for solar cells, requires large area, high-deposition rate plasma reactors. Increasing the radio frequency from the conventional 13.56MHz up to VHF has demonstrated higher deposition and etch rates and lower particle generation, a reduced ion bombardement and lower breakdown, process and bias voltages.

But otherwise the use of VHF leads to some problems. The non-uniformity of deposition rate increase due to the generation of standing waves (TEM wave) and evanescent waveguide modes (TE waves) at the electrode surface.

Increasing the frequency and/or the deposition area the plasma impedance, the capacitic stray impedance of the RF electrode and other parasitic capacitive impedances decrease. Increasing the frequency and/or the RF power, the phase angle of the discharge and of the impedance at every point at the lines between the RF matching network an the RF electrode tends more and more towards -90°. This results in increasing currents and standing waves with extremly high local current maximas. Increasing resistances of lines and contacts due to the skin effect and loss-caused heating up of the lines the power losses increase extremely, up to 90% and more. In spite of the increasing of the coupled power, the plasma power does not increase. Thermal destructions of the lines due to extreme expansion or melting are possible.

Some solutions to reduce the non-uniformity of the deposition rate like multipower feeding, central backside power feeding, electrode segmentation, use of load impedances, published in former publications, will be discussed in connection with several reactor types (coaxial, large area, long plasma source) in view of the efficiency of power coupling and the practical realization. Solutions to minimize the power losses at the lines will be presented.

We report progress in hydrogenated amorphous silicon n-i-p solar cells with the i-layer grown by the hot-wire chemical vapor deposition technique. Early research showed that we grew device-quality materials with low saturated defect density (2 × 106/cm3), high initial ambipolar diffusion length (~2000 Å) and low hydrogen content (<1%). One of the major barriers to implementing this material into solar cells is the high substrate temperature required (>400°C). We re-assess the effects of low substrate temperature on the property of the films and the performance of the solar cells as an alternative avenue to solving this problem. We find that the material grown at 300°C can have similar values of saturated defect density and ambipolar diffusion length as the one grown greater than 400°C. We also study the effect of i-layer substrate temperature ranging from 280° to 440°C for n-i-p solar cells. We now consistently grow devices with Fill Factor (FF) greater than 0.66, with the best close to 0.70 at lower substrate temperature. A collaboration with United Solar System, in where they grew the p-layer and top contact, produced devices with initial efficiencies as high as 9.8%. We produce n-i-p solar cells with initial efficiencies as high as 8% when we grow all the hydrogenated amorphous silicon and top contact layers. All these i-layers are grown at deposition rates of 16 to 18 Å/sec. We need to further improve our p-layer and transparent conductor layer to equal the collaborative cell efficiency. We also report light-soaking results of these devices.

Excimer laser annealing technique is proposed to increase the grain size and controlling the microstructure of polycrystalline silicon (poly-Si) thin film. Our method is based on the lateral grain growth during laser annealing. Our specific grid ion beam irradiation method was designed to maximize the lateral growth effect and arrange the location of grain boundaries. We observed well-arranged poly-Si grains up to micrometer order by transmission electron microscopy (TEM).

A low heat-capacitance substrate has been introduced to lateral grain-growth for Si thin-films. Grains as large as 13μm could be fabricated by irradiation of a single shot of an excimer-laser light pulse with intensity gradient. The film has an orientation in the (110) direction, different from (111) for the conventional excimer-laser-annealed film, and is applicable to a seed layer for solid-phase epitaxial-growth aiming at solar cell application.

The role and interaction of vacancies and dopants in the crystallization of amorphous Si (a-Si) by solid-phase epitaxy (SPE) was investigated. To this end, we studied: (i) the solid-phase epitaxy rate measured by time-resolved reflectivity (TRR), (ii) the dopant and carrier concentrations measured by secondary ion mass spectrometry (SIMS) and spreading resistance (SR) analysis, and (iii) the vacancy concentration measured by positron annihilation spectroscopy (PAS). Phosphorus was implanted into a-Si on Si (001), which was previously amorphized by 29Si+ implantation, to create a nonuniform P doping profile. Phosphorus doped samples compensated with a similar boron profile were also studied. Samples were vacuum annealed for various times so that the amorphous-crystal interface was stopped at various depths providing frozen frames of the SPE process. These samples were then studied with PAS to investigate the vacancy population and to identify the impurity-defect complexes. Using this method, we have observed a population of phosphorus-vacancy complexes in the epitaxial layer.

We have investigated the solid-phase crystallization of a-Si1-xGex:H (x=0 and 0.5) films by using electron spin resonance and x-ray diffraction. The films were deposited on Coming 1737 glass in a plasma-enhanced chemical vapor deposition system using SiH4 and GeH4 gases. The films were then annealed to be crystallized at 600°C. It was observed that, for the a-Si:H film, both the spin density and the g-value first increased with annealing time, and then rapidly decreased as the film was crystallized. For the a-Si0.5Ge0.5:H film, the Ge dangling bond spin density increased from 3 × 1018 cm-3 to 2 × 1019 cm3 for the first stage of annealing and then decreased to 3 × 1017 cm-3 after being crystallized; the Si dangling bond spin density just increased to about 2 × 1017 cm-3 and remained nearly constant for further annealing. It is thought that exodiffusion of hydrogen resulted in the increase of spin density in the beginning, and then some portions of amorphous components were converted into the crystalline phase by further annealing. And the keen correlation between the dependence of x-ray peak intensity and the dependence of Ge dangling bond spin density on the annealing time suggests that the Ge dangling bonds rather than Si dangling bonds play an important role in the crystallization of the Si0.5Ge0.5 film.

Porosity depth profiles with exponential or sinusoidal shape were fabricated electrochemically in crystalline silicon using time-variable current densities and studied employing variable angle spectroscopic ellipsometry. Since volume porosity in porous silicon depends on the current density, it was possible to electrochemically tailor porosity depth profiles, which in a first approximation resembled the time modulation of the applied current. Optical characterization of the samples were realized using multilayer optical models and the Bruggeman effective medium approximation allowing variations of the index of refraction according to the applied current density profiles. The analysis also revealed deviations from desired profiles in terms of in-depth inhomogeneities.

The segregation of phosphorus to grain boundaries in phosphorus implanted Si0.87Ge0.13 films, deposited by chemical vapor deposition (CVD), was directly observed by scanning transmission electron microscopy (STEM) with energy dispersive x-ray (EDX) microanalysis. The segregation was determined to be a thermal equilibrium process by measuring and comparing the average phosphorus concentrations at the grain boundaries in Si0.87Ge0.13 films subjected to 700, 750 or 800°C annealing, following the implantation and 1000°C annealing processes. The measured segregation energy was 0.28 eV/atom. No Ge segregation was found at grain boundaries in phosphorus implanted Si0.87Ge0.13 films by STEM x-ray microanalysis. Neither was evidence shown by STEM microanalysis that Ge segregated to grain boundaries in intrinsic Si1-xGex films with x = 0.02, 0.13 and 0.31. Secondary ion mass spectrometry (SIMS) analysis showed that these intrinsic Si1-xGex films contained 1019 to 4 × 1019/cm-3H, depending on the deposition temperature.

The random medium model for porous silicon (PS) is applied to a porous silicon layer to be used as a back reflector in a thin film silicon solar cell realized in an ultrathin epitaxially grown Si layer on PS. Three-layer-structures (epi/PS/Si) have been fabricated by RPCVD of 150-1000 nm epitaxial silicon layers on the porous surface of silicon wafers. The light reflection has been measured in the 300-1000 nm wavelength range. An excellent agreement is found between the experimentally measured reflectance curves and those calculated using the random medium model. The analysis shows that the epitaxial growth has led to an appreciable reduction in the porosity in the intermediate PS layer from about 60% to 20-30%.

The existence of a novel tetrahedral semiconductor Quasi-Single-Crystalline (QSC) phase is posited. In the QSC semiconductor phase, the films consist of grains with a diamond structure of tetrahedral elements, such as Si, Ge and C, and the grains have a preferred orientation, such as <111> or <100> normal to the film. The lattices perpendicular to the grain boundaries are quasi-matched with neighboring grains. The grains in the films form a regular array, and are distributed more uniformly than in conventional poly-crystalline semiconductor films. Because of the small-angle grain boundaries, a tetrahedral QSC semiconductor such as QSC Si films are expected to have extremely low energy barriers at the grain boundaries.

We have investigated a novel surface-seeded crystallization technique at low processing temperatures (≤ 550°C) and high pressures (10MPa~25MPa) using polished polycrystalline diamond seeds. By controlling the high pressure, the nucleation and growth of silicon can be controlled to obtain improved quality silicon films on amorphous substrates at low temperatures. Depending on the annealing temperature and applied pressure, the orientation of crystallized silicon thin films varies as seen by x-ray diffraction and transmission electron microscopy results. In addition, crystallization of amorphous silicon thin films has effect on their roughness.

Solid phase crystallization of Si1-xGex, have been investigated by means of change in crystallinity and crystallization temperature using the ellipsometric spectroscopy (2-5 eV). Dispersion analysis on the spectra revealed that the Si1-xGex alloys show both abrupt increase and rapid saturation of the crystallinities according to the structural transitions. The saturated values of crystallinities did not reach those of single crystals in all samples and rapidly decreased with increase of Ge concentration. Ion-implanted amorphous SilxGe, exhibited much lower activation energies of re-crystallization than those of the deposited amorphous samples. Furthermore, their crystallization temperature were strongly depend on the species of implanted ions such as As+ and BF2+. It was found that the crystallization velocity of the As+-implanted Si1-xGex was estimated as 4 times faster than that of the BF2+-implanted Si1-xGex. Itinerant property of Ge in Si may give rise to the decrease of the crystallinities as well as the decrease of the activation energy of solid phase crystallization which may be strongly affected by the catalytic effects of the implanted impurities.

The structural properties of the amorphous Si (a-Si), which was created from crystalline silicon by 2 MeV electron irradiation at low temperatures about 25 K, are examined in detail by means of transmission electron microscopy and transmission electron diffraction. The peak positions in the radial distribution function (RDF) of the a-Si correspond well to those of a-Si fabricated by other techniques. The electron-irradiation-induced a-Si returns to crystalline Si after annealing at 550°C.

One of the most effective techniques used to obtain high quality amorphous silicon alloys is the use of hydrogen dilution during film growth. The resultant material exhibits a more ordered microstructure and gives rise to high efficiency solar cells. As the hydrogen dilution increases, however, a threshold is reached, beyond which microcrystallites begin to form rapidly. In this paper, we review some of the interesting features associated with the thin film materials obtained from various hydrogen dilutions. They include the observation of linear-like objects in the TEM micrograph, a shift of the principal Si TO band in the Raman spectrum, a sharp, low temperature peak in the H2 evolution spectrum, a shift of the wagging mode in the IR spectrum, and a narrowing of the Si (111) peak in the X-ray diffraction pattern. These spectroscopic tools have allowed us to optimize deposition conditions to near the threshold of microcrystallinity and obtain desired high quality materials. Incorporation of the improved materials into device configuration has significantly enhanced the solar cell performance. Using a spectral-splitting, triple-junction configuration, the spectral response of a typical high efficiency device spans from below 350 nm to beyond 950 nm with a peak quantum efficiency exceeding 90%; the triple stack generates a photocurrent of 27 mA/cm2. This paper describes the effect of the improved materials on various solar cell structures, including a 13% active-area, stable triple-junction device.

Medium range order (MRO) and the formation of microcrystallites in a-Si:H prepared by plasma-enhanced chemical vapor deposition (PECVD) and hot-wire chemical vapor deposition (HWCVD) have been probed by systematic x-ray diffraction studies with films as thin as those used in solar cells. Effects of substrate temperature, hydrogen dilution, film thickness, and type of substrate have been examined. High-hydrogen-diluted films of 0.5 μm thickness, using optimized deposition parameters for solar cell efficiency and stability, are found to be partially microcrystalline (μc) if deposited directly on stainless steel (SS) substrates but are fully amorphous provided a thin (20 nm) n-layer of a-Si:H or μc-Si:H is first deposited on the SS. The latter predeposition does not prevent partially microcrystallinity if the films are grown thicker (1.5 to 2.5 μm) and this is consistent with a recently proposed phase diagram of thickness versus hydrogen dilution. Analysis of the first (lowest angle) scattering peak of the a-Si:H phase demonstrates that its width, directly related to MRO, is reduced by heavier hydrogen dilution in PECVD growth or by increased substrate temperature in HWCVD growth. The narrowest width of fully amorphous material correlates with better solar cell stability and this is not likely related to bonded hydrogen content since it is quite different in the optimized PECVD and HWCVD a-Si:H. A wide range of MRO apparently exists in the residual amorphous phase of the mixed a/μc material.

Studies have been carried out on the thickness dependent transition between the amorphous and microcrystalline phases in intrinsic Si:H materials (i-layers) and its effect on p-i-n solar cell performance [1]. P(a-SiC:H)-i(a-Si:H)-n(μcSi:H) cell structures were deposited with the intrinsic Si:H layer thickness and the flow ratio of hydrogen to silane, R=[H2]/[SiH4], guided by an evolutionary phase diagram obtained from real-time spectroscopic ellipsometry. The thickness range over which the fill factors are controlled by the bulk was established and their characteristics investigated with different protocrystalline i-layer materials (i.e., materials prepared near the amorphous to microcrystalline boundary but on the amorphous side). Insights into the properties of these materials and the effects of the transition to the microcrystalline phase were obtained from the systematic changes in the initial fill factors, their light-induced changes, and their degraded steady states for cells with i-layers of different thickness and H2 dilution.

Using infrared, H evolution and x-ray diffraction (XRD), the structure of high H dilution, glow discharge deposited a-Si:H films ‘on the edge of crystallinity’ is examined. From the Si-H wag mode peak frequency and the XRD results, we postulate the existence of very small Si crystallites contained within the as-grown amorphous matrix, with the vast majority of the bonded H located on these crystallite surfaces. Upon annealing at ramp rates of 8-15°C/min, a H evolution peak at ~400°C appears, and film crystallization is observed at temperatures as low as 500°C, both of which are far below those observed for a-Si:H films grown without H dilution using similar rates. While the crystallite volume fraction is too small to be detected by XRD in the as-grown films, these crystallites enable the crystallization of the remainder of the amorphous matrix upon moderate annealing, thus explaining the existence of the low temperature H evolution peak.