Nicolas Wyrsch

Influence of plasma excitation frequency fora-Si:H thin film deposition

The effect of plasma excitation frequency on the deposition rate and on the optical and electrical properties of amorphous silicon film is studied over the range 25–150 MHz. Deposition rates as high as 21 Å/sec are obtained at ∼70 MHz, which is a factor of 5–8 larger than typical rates obtained for the conventional 13.56-MHz silane glow-discharge system. Only minor changes occur in the defect density (as measured by the photothermal deflection spectroscopy method), the optical bandgap, and the electrical conductivity over this frequency range. In a preliminaryinterpretation given here, the large variation of the deposition rate as a function of excitation frequency is explained in terms of changes in the electron energy distribution function.

How to reach more precise interpretation of subgap absorption spectra in terms of deep defect density in a-Si:H

Values for the deep defect density determined from PDS, CPM or ESR may vary significantly on the same sample depending on the method of data analysis used. Experimental conditions under which they yield comparable results are discussed. Procedures for determination of deep defect density from optical spectra are reviewed and compared on samples in light-saturated states. The measurement of the absorption coefficient at E=1.2 eV, with a new range for the calibration factor, is suggested as an easy and generally usable procedure for the determination of deep defect density.

Mobility lifetime product - a Tool for Correlating a-Si:H Film Properties and Solar Cell Performances

The missing correlation between film characteristics and a‐Si:H‐based p‐i‐n solar cells is still a controversial subject. The authors present a new parameter μ0τ0, evaluated from steady‐state transport measurements on a‐Si:H layers, which can indeed relate film quality and cell performance as far as the latter is limited by the quality of the intrinsic 〈i〉 layer. Thereby, two specific features of the evaluated μ0τ0 product can explain its successful role as a quality parameter for a‐Si:H: First, the computation of μ0τ0 takes into account the effects of the prevailing dangling bond occupation, which is very different in uniform films as compared to the occupation profile prevailing through the i layer of a p‐i‐n solar cell; second, the evaluated μ0τ0 product combines information about band mobility and defect density; furthermore it avoids some of the well‐known pitfalls of usual deep defect density measurements such as constant photocurrent method and photothermal deflection spectroscopy. Experimental dat...

Hole drift mobility in μc-Si:H

In microcrystalline hydrogenated silicon (μc-Si:H), the drift mobility dependencies of holes on electric field and temperature have been measured by using a method of equilibrium charge extraction by linearly increasing voltage. At room temperature the estimated value of the drift mobility of holes is much lower than in crystalline silicon and slightly higher than in amorphous hydrogenated silicon (a-Si:H). In the case of stochastic transport of charge carriers with energetically distributed localized states, the numerical model of this method gives insight into the mobility dependence on electric field. From the numerical modeling and experimental measurement results, it follows that the hole drift mobility dependence on electric field is predetermined by electric field stimulated release from localized states.

Microstructure and surface roughness of microcrystalline silicon prepared by very high frequency-glow discharge using hydrogen dilution

The microstructure of a series of silicon films deposited by very high frequency glow discharge (VHF-GD) with silane concentration in hydrogen varying from 100% down to 1.25% has been observed with transmission electron microscopy (TEM). The surface topography of the layers has been analysed by atomic force microscopy (AFM). At silane concentration below 8.6%, a phase transition between amorphous hydrogenated silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) is observed by TEM. After this transition, the further decrease of silane concentration leads to complex changes of the crystalline microstructure of the layers. AFM observations of the surface reveal that the film rms roughness increases with the decrease of the silane concentration. The surface morphology is not related simply to the microstructure of crystalline grains as observed by TEM.

Structural properties and electronic transport in intrinsic microcrystalline silicon deposited by the VHF-GD technique

A series of microcrystalline samples was deposited by the very high frequency glow discharge (VHF-GD) technique, with various input powers while keeping all the other parameters of deposition constant. The goal was to correlate transport and structural properties and avoid as much as possible the problem of a variation of the Fermi level between the samples. The observed decrease of the photoconductivity and of the product mobility-lifetime of hole (as measured by time of flight, TOF) with the increase of the power was surprisingly not connected to the structural properties, which remain approximately unchanged, but with a surface contribution to the transport properties.

VHF Plasma Deposition: A Comparative Overview

The use of plasma excitation frequencies f in the VHF band (30–300 MHz), and particularly of f=70 MHz, for the high-rate deposition of amorphous hydrogenated silicon (a-Si:H) is described. Deposition rates, using monosilane (SiH 4 ) as source gas, are thereby increased roughly five fold to over 10 A/s as compared with the conventional case of RF plasma enhanced chemical vapour deposition with f=13.56 MHz. This may possibly be attributed to an enhancement in the high-energy tail of the electron energy distribution function (EEDF) of the plasma. Thereby, no noticeable deterioration in film properties is found. Characteristics of VHF-deposited a-Si:H films are extensively reported, including properties like microstructure, hydrogen effusion behaviour and its low internal mechanical stress. High quality hydrogenated microcrystalline silicon (μc-Si:H) can be deposited at low substrate temperature and low plasma power densities thanks to VHF glow discharge. This can be linked to a reduction in sheath potential and to the energy of the ions arriving at the growing surface. Thereafter, use of VHF plasma in applications such as 100 μm thick a-Si:H layer for particle detectors and powder-free deposition of solar cells with efficiencies over 8% are reported.

Electronic transport in hydrogenated microcrystalline silicon: similarities with amorphous silicon

Undoped hydrogenated microcrystalline silicon (μc-Si:H) layers were grown by the very high frequency glow discharge (VHF-GD) technique under various deposition conditions. The electronic transport properties under illumination were investigated by means of steady-state photoconductivity and steady-state photocarrier grating methods. Similarly to hydrogenated amorphous silicon (a-Si:H), power law dependencies as a function of the generation rate are observed for the photoconductivity, for the ambipolar diffusion length, and for the parameter b (indicating the Fermi level). For μc-Si:H, as for a-Si:H, nearly constant product of (mobility × recombination time) of majority and minority carriers is observed as a function of the parameter b. Based on these similarities, we assume that the electronic transport model developed for a-Si:H remains valid for μc-Si:H.

Ambipolar diffusion length and photoconductivity measurements on ‘‘midgap’’ hydrogenated microcrystalline silicon

Hydrogenated microcrystalline silicon (μc‐Si:H) deposited by VHF plasma‐enhanced chemical vapor deposition has recently been proven to be fully stable, with respect to light‐induced degradation, when adequately used in p‐i‐n solar cells. Stable solar cells efficiencies of 7.7% have been obtained with single‐junction cells, using ‘‘midgap’’ microcrystalline i‐layers, having an optical gap of around 1 eV. In the present paper, the electronic transport properties of such microcrystalline layers are determined, by the steady‐state photocarrier grating method (SSPG) and steady‐state photoconductivity measurements, in a coplanar configuration. The conditions for the validity of the procedure for determining the ambipolar diffusion length, Lamb, from SSPG measurements (as previously theoretically derived in the context of amorphous silicon) are carefully re‐examined and found to hold in these μc‐Si:H layers, taking certain additional precautions. Otherwise, e.g., the prevalence of the ‘‘lifetime’’ regime (as opp...

Limiting factors in the fabrication of microcrystalline silicon solar cells and microcrystalline/amorphous (‘micromorph’) tandems

This contribution presents the status of amorphous and microcrystalline silicon solar cells on glass, and discusses some material/fabrication factors that presently limit their conversion efficiency. Particular attention is focused on recent results and developments at the Institute of Microtechnology (IMT) in Neuchâtel. The performances and stability of microcrystalline silicon single-junction and amorphous/microcrystalline (‘micromorph’) tandem solar cells are discussed, as a function of material properties. Recent results on the electrical effect of cracks in microcrystalline silicon material are presented. Degradation under the effect of illumination is a well-known limiting factor for amorphous silicon solar cells. As a comparison, studies on the stability of microcrystalline silicon with respect to light-induced degradation are commented upon. The importance of transparent contacts and anti-reflection layers for achieving low electrical and optical losses is discussed. Finally, efforts towards industrialization of micromorph tandem solar cells are highlighted, specifically (i) the development and implementation of an in situ intermediate reflector in a large-area industrial deposition system, and (ii) recent achievements in increasing the growth rate of microcrystalline silicon.