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Doped zinc oxide films are of high interest in thin film solar cell technology for application as transparent conducting oxide. Rapid and detailed characterisation of ZnO thin film properties is required for quality control and optimisation of the deposited films. In the present work, a new model of dielectric functions based on the effective medium approximation (EMA) is developed and is applied for characterisation of polycrystalline boron doped zinc oxide (ZnO:B) films, deposited by low pressure chemical vapour deposition (LPCVD) technique onto glass substrates. The model takes into account that polycrystalline ZnO is considered to consist of crystal grains surrounded by depletion layers. Using this model and Fourier Transform Infrared Spectroscopy (FTIR) performed in reflection configuration over a wide mid-infrared spectral region (from 2 μm up to 25 μm), the properties of depletion layer and the bulk of the grains in ZnO can be rapidly characterised in detail, and the volume fraction of the depletion layer can be extracted. The results are in good agreement with previously presented theories of electron transport in polycrystalline materials. Using electrical measurements like conductivity and Hall techniques in addition to the optically determined parameters, predominant electron scattering mechanisms in polycrystalline films for different doping levels are identified. The measurements show the impact of the doping level on depletion layer of the crystallites. It is shown, furthermore, that under a water vapour rich environment the volume fraction of the depletion layer may increase up to 5 times and more, while the mobility of the charge carriers in the depletion layer drops drastically from about 31 cm2V-1s-1 to about 8 cm2V-1s-1. This indicates that water vapour exposure causes an increase of the potential barrier in the grain boundary depletion layer, limiting the electron transport across the grain boundaries to a classical thermionic emission mechanism. [ABSTRACT FROM AUTHOR]

The influence of oxygen and nitrogen impurities on the performance of thin-film solar cells based on microcrystalline silicon (μc-Si:H) has been systematically investigated. Single μc-Si:H layers and complete μc-Si:H solar cells have been prepared with intentional contamination by admitting oxygen and/or nitrogen during the deposition process. The conversion efficiency of ∼1.2 μm thick μc-Si:H solar cells is deteriorated if the oxygen content in absorber layers exceeds the range from 1.2×1019 to 2×1019 cm-3; in the case of nitrogen contamination the critical impurity level is lower ([N]critical=6×1018–8×1018 cm-3). It was revealed that both oxygen and nitrogen impurities thereby modify structural and electrical properties of μc-Si:H films. It was observed that the both contaminant types act as donors. Efficiency losses due to oxygen or nitrogen impurities are attributed to fill factor decreases and to a reduced external quantum efficiency at wavelengths of >500 nm. In the case of an air leak during the μc-Si:H deposition process, the cell performance drops at an air leak fraction from 140 to 200 ppm compared to the total gas flow during i-layer deposition. It is demonstrated that oxygen and nitrogen impurities close to the p/i-interface have a stronger effect on the cell performance compared to impurities close to the n/i-interface. Moreover, thick μc-Si:H solar cells are found to be more impurity-sensitive than thinner cells. [ABSTRACT FROM AUTHOR]

The influence of atmospheric contaminants oxygen and nitrogen on the performance of thin-film hydrogenated amorphous silicon (a-Si:H) solar cells grown by plasma-enhanced chemical vapor deposition at 13.56 MHz was systematically investigated. The question is addressed as to what degree of high base pressures (up to 10-4 Torr) are compatible with the preparation of good quality amorphous silicon based solar cells. The data show that for the intrinsic a-Si:H absorber layer exists critical oxygen and nitrogen contamination levels (about 2×1019 atoms/cm3 and 4×1018 atoms/cm3, respectively). These levels define the minimum impurity concentration that causes a deterioration in solar cell performance. This critical concentration is found to depend little on the applied deposition regime. By enhancing, for example, the flow of process gases, a higher base pressure (and leak rate) can be tolerated before reaching the critical contamination level. The electrical properties of the corresponding films show that increasing oxygen and nitrogen contamination results in an increase in dark conductivity and photoconductivity, while activation energy and photosensitivity are decreased. These effects are attributed to nitrogen and oxygen induced donor states, which cause a shift of the Fermi level toward the conduction band and presumably deteriorate the built-in electric field in the solar cells. Higher doping efficiencies are observed for nitrogen compared to oxygen. Alloying effects (formation of SiOx) are observed for oxygen contaminations above 1020 atoms/cm3, leading to an increase in the band gap. [ABSTRACT FROM AUTHOR]

Substantial effects of boron and phosphorus doping on the kinetics of laser-induced crystallization (LIC) in hydrogenated amorphous silicon (a-Si:H) are reported. A kinetic nanoscopic electron-related LIC model that suggests predictions and explanations of observed effects of B and P doping on the LIC temperatures and crystallite size in a-Si:H is presented. The LIC is considered to be the integral effect of a huge number of nanoscale picosecond material reconstructions, each of which is generated by a nanoscopic short-lived (picosecond) large-energy fluctuation. The LIC in doped a-Si:H occurs at temperatures substantially lower than those found in the crystallization in a furnace. Crystallite size in B-doped a-Si:H is half of that in P-doped and undoped material. [ABSTRACT FROM AUTHOR]

A nanoscopic kinetic model of controlled plasma-assisted microcrystallite formation (PAµCF) of Si in pre deposited a-Si:H films at low temperatures is proposed. The model suggests mechanisms for enhancement of the Si crystallization in a-Si:H films at low temperatures by treatment of the films in plasma. The model reveals certain kinetic advantages of hydrogen plasmas for the formation of Si crystalline nuclei in a-Si:H compared to other plasmas (Ar plasma, etc.). These advantages make the hydrogen plasma substantially more efficient in the PAµCF of Si in a-Si:H films. The proposed mechanism for PAµCF of Si is associated with the formation on the surface of the a-Si:H film and in the adjacent nanometer material layer of nanoscale (picosecond) short-lived hot spots of high energy density (or effective temperature). The hot spots are generated in the material by energetic plasma ions of energy ε[sub is] = 20-100 eV accelerated by the electrical field in the thin plasma layer near the solid surface. The hot spots promote Si crystallization in a-Si:H. It is shown how the plasma composition, energy, mass, and fluxes of the plasma ions impinging on the surface of the a-Si:H film determine the Si nucleation rate and density of Si microcrystallization. [ABSTRACT FROM AUTHOR]

The structure of a-Si:H, deposited at rates in excess of 100 Å/s by the hot wire chemical vapor deposition technique, has been examined by x-ray diffraction (XRD), Raman spectroscopy, H evolution, and small-angle x-ray scattering (SAXS). The films examined in this study were chosen to have roughly the same bonded H content C[sub H] as probed by infrared spectroscopy. As the film deposition rate R[sub d] is increased from 5 to >140 Å/s, we find that the short range order (from Raman), the medium range order (from XRD), and the peak position of the H evolution peak are invariant with respect to deposition rate, and exhibit structure consistent with a state-of-the-art, compact a-Si:H material deposited at low deposition rates. The only exception to this behavior is the SAXS signal, which increases by a factor of ∼100 over that for our best, low H content films deposited at ∼5 Å/s. We discuss the invariance of the short and medium range order in terms of growth models available in the literature, and relate changes in the film electronic structure (Urbach edge, as-grown defect density) to the increase in the SAXS signals. We also note the invariance of the saturated defect density versus R[sub d], measured after light soaking, and discuss possible reasons why the increase in the microvoid density apparently does not play a role in the Staebler–Wronski effect for this type of material. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Discusses a study which detailed gas evolution studies for structural characterization of hexamethyldisilazane-based a-Si:C:N:H films. Factor similar to the hydrogen evolution spectra; Influence on the thermal stability; Act widely termed as plasma polymerization.

Presents a study that determined the long-term stability of hydrogenated amorphous germanium (GE) measured by infrared absorption. Components of typical deposition conditions; Infrared absorption spectra of hydrogenated and deuterated amorphous germanium; Oxidation process of hydrogen-covered GE surfaces.

Investigates the hydrogen effusion spectra of oxidized silicon samples. Experiment; Information on the general effusion features; Data on the penetration depth and passivation properties; Desorption of molecules with higher mass numbers.

Presents a study that characterized the structure of superlattices based on amorphous silicon. Analysis of the optical electron micrographs of the amorphous silicon superlattices; Examination of the scanning electron microscopic images of doping superlattices; Evaluation of the hydrogen profiles in doping superlattices.

Examines the amorphous-to-crystalline transition of amorphous silicon thin films containing fluorine and hydrogen using the transmission electron microscopy. Experimental procedures; Experimental results; Discussion of findings.