High Frequency Glow Discharge Research Articles

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.

Evolution of the microstructure in microcrystalline silicon prepared by very high frequency glow-discharge using hydrogen dilution

A series of samples was deposited by very high frequency glow discharge in a plasma of silane diluted in hydrogen in concentrations SiH4/(SiH4+H2) varying from 100% to 1.25%. For silane concentrations below 8.4%, a phase transition between amorphous and microcrystalline silicon occurs. Microcrystalline silicon has been characterized by transmission electron microscopy (TEM) and x-ray diffraction. The medium-resolution TEM observations show that below the transition, the microstructure of microcrystalline silicon varies in a complex way, showing a large variety of different growth structures. For the sample close to the phase transition, one observes elongated nanocrystals of silicon embedded in an amorphous matrix followed at intermediate dilution by dendritic growth, and, finally, at very high dilution level, one observes columnar growth. X-ray diffraction data evidence a (220) crystallographic texture; the comparison of the grain sizes as evaluated from TEM observations and those determined using Scherrer’s equation illustrates the known limitations of the latter method for grain size determination in complex microstructures.

Intrinsic microcrystalline silicon (μc-Si:H) deposited by VHF-GD (very high frequency-glow discharge): a new material for photovoltaics and optoelectronics

The development of μc-Si:H technology and the introduction of intrinsic 〈i〉 μc-Si:H as photovotaically active material is retraced. Special emphasis is laid on the use of very high frequency glow discharge as a particularly propitious deposition method for μc-Si:H. Thereby, the use of a gas purifier to reduce oxygen content and obtain intrinsic layers with ‘midgap’ character is described. Recent results obtained with single-junction μc-Si:H solar cells and a-Si:H/μc-Si:H tandem solar cells are given. The analysis of carrier collection in single-junction μc-Si:H solar cells is undertaken with the variable intensity measurements method. It yields effective mobility×lifetime ( μτ) eff products for the i-layer in p–i–n and n–i–p solar cells in the range 10 −7–10 −6 cm 2 V −1. Similar values have been found for μτ-products in individual layers based on photoconductivity and ambipolar diffusion length measurements. Transmission electron microscopy images for μc-Si:H layers are given. They display a complex microstructure not suspected before. On the other hand, atomic force microscopy data reveal a pronounced surface roughness that correlates well with the optical light scattering and with the pronounced enhancement of the apparent optical absorption coefficient, in the 1–2 eV region, as already observed before.

Microstructures of Microcrystalline Silicon Solar Cells Prepared by Very High Frequency Glow-Discharge

ABSTRACTThe microstructure of two different νc-Si:H solar cells was studied by Transmission Electron Microcopy (TEM). At the micrometer scale, a difference in the grain structure is observed close to the ZnO substrate: cracks are found in the cell deposited with the higher silane concentration. Surprisingly, the cell with the cracks close to the transparent conductive oxide shows the largest VOC (530 mV) and FF (68%). These first studies reveal that microstructures of fully microcrystalline silicon devices may show a quite large variation with corresponding effects in solar cell performance.

Microcrystalline silicon and micromorph tandem solar cells

“Micromorph” tandem solar cells consisting of a microcrystalline silicon bottom cell and an amorphous silicon top cell are considered as one of the most promising new thin-film silicon solar-cell concepts. Their promise lies in the hope of simultaneously achieving high conversion efficiencies at relatively low manufacturing costs. The concept was introduced by IMT Neuchâtel, based on the VHF-GD (very high frequency glow discharge) deposition method. The key element of the micromorph cell is the hydrogenated microcrystalline silicon bottom cell that opens new perspectives for low-temperature thin-film crystalline silicon technology. According to our present physical understanding microcrystalline silicon can be considered to be much more complex and very different from an ideal isotropic semiconductor. So far, stabilized efficiencies of about 12% (10.7% independently confirmed) could be obtained with micromorph solar cells. The scope of this paper is to emphasize two aspects: the first one is the complexity and the variety of microcrystalline silicon. The second aspect is to point out that the deposition parameter space is very large and mainly unexploited. Nevertheless, the results obtained are very encouraging and confirm that the micromorph concept has the potential to come close to the required performance criteria concerning price and efficiency.

Low temperature silicon deposition for large area sensors and solar cells

Low temperature deposition methods are needed for realizing large area electronics on alternative substrates like glass, polymer or steel foils. Reasonable electronic quality of low temperature deposited silicon, however, can only be achieved by employing additional, non-thermal sources of energy for promoting and optimizing the surface controlled silicon growth. Methods like very high frequency glow discharge or hot-wire chemical vapour deposition provide a wide range of control over the deposition process, enabling high growth rates, amorphous as well as crystalline silicon deposition and effective hydrogen etching. The current status of these deposition methods is reviewed and the resulting material properties are discussed. Applications like two-terminal colour sensors and photodiodes for retina implantation demonstrate the potential of joining amorphous and crystalline silicon technology into hybrid sensor systems and teach us how to use the building blocks for novel large area electronics.

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.

Stability of a-Si:H prepared by hot-wire and glow discharge using H2 dilution evaluated by pulsed laser degradation

The quality of intrinsic amorphous silicon films prepared by different deposition techniques was investigated. For very high frequency glow discharge, both the substrate temperature as well as the hydrogen dilution were varied. These layers were compared to hot wire material produced at comparable temperatures. To study the stability of these films, an optimised degradation method was developed in which a pulsed dye laser was used in combination with a monochromatic steady beam to achieve a relatively fast stabilisation of the light induced degradation. The film quality was monitored by the photoconductivity and by the ambipolar diffusion length from which the material parameter, μ0τ0, was extracted. Taking into account the transport properties after light soaking as well as the optical absorption we concluded that the hot wire material could lead to more stable solar cells in comparison with plasma enhanced chemical vapor deposition material.

Recent progress in micromorph solar cells

Recently, we have demonstrated that intrinsic hydrogenated microcrystalline silicon, as deposited by the very high frequency glow-discharge technique, can be used as the active layers of p–i–n solar cells. Our microcrystalline silicon represents a new form of thin film crystalline silicon that can be deposited (in contrast to any other approach found in literature) at substrate temperatures as low as 200°C. The combination of amorphous and microcrystalline material leads to a `real' silicon-based tandem structure, which we label `micromorph' cell. Meanwhile, stabilised efficiencies of 10.7% have been confirmed. In this paper, we present an improved micromorph tandem cell with 12% stabilised efficiency measured under outdoor conditions. Dark conductivity and combined SIMS measurements performed on intrinsic microcrystalline silicon layers reveal a post-oxidation of the film surface. However, a perfect chemical stability of entire microcrystalline cells as well as micromorph cells is presented. Variations of the p/i interface treatment show that an increase of the open circuit voltages from 450 mV up to 568 mV are achievable for microcrystalline cells, but such devices have reduced fill factors.

Low Defect Density Microcrystalline-Si Deposited by the Hot Wire Technique

ABSTRACTThe optical and electronic properties of a series of microcrystalline silicon (μ-Si) films, deposited by the hot wire (HW) technique, are reported. Preliminary results suggest, using moderate H2 /SiH4 dilution ratios and substrate temperatures (320°C), high filament temperatures, and no H gas purifier, that the subgap absorption for these films, measured using the constant photocurrent (CPM) method, can be as low as that obtained for films deposited by the very high frequency glow discharge (VHF-GD) technique. The film dark conductivities of the HW samples, ranging as low as 2.0 × 10−8 (ohm cm)−1, lend further credance to these low defect values. At the same time, the optical absorption in the region > 1.6 eV is higher than that previously observed for the VHF-GD deposited samples. The present results, discussed in the context of the film microcrystalline fraction, suggest that there is no unique, good quality, low defect density μ-Si material, and that different deposition techniques can be used to successfully deposit device quality gc-Si. We also present optical and structural data for films deposited at lower substrate temperatures and higher deposition rates, and suggest combinations of deposition parameters to be used that may further improve the electronic properties of these films.

Potential of VHF-plasmas for low-cost production of a-Si: H solar cells

Compared to the use of the standard glow discharge technique the production of amorphous silicon solar cells by the very high frequency glow discharge (VHF-GD) bears yet additional cost reduction potentials: Using VHF-GD at excitation frequencies higher than 13.56 MHz, a more efficient dissociation of silane gas is obtained; thus, higher deposition rates are achieved; this reduces considerably the deposition time for intrinsic amorphous and microcrystalline silicon layers. Furthermore, by itself and even more so, in combination with argon dilution, VHF-GD technique improves silane utilisation and leads, thus, to further cost reduction. Finally, by combining the VHF-GD technique and the “micromorph” concept “real” tandem cells (i.e. a superposition of two cells with distinctly different band gaps) can be deposited at low temperatures without the use of expensive germane gas.

Fast Deposition of μc-Si:H by Restrictive Dilution and Enhanced HF-Power

Note: IMT-NE Number: 258 Reference PV-LAB-ARTICLE-1997-009 Record created on 2009-02-10, modified on 2017-05-10

Optical and transport studies on thin microcrystalline silicon films prepared by very high frequency glow discharge for solar cell applications

The initial growth stage of phosphorus doped microcrystalline silicon films prepared by plasma enhanced chemical vapor deposition with different plasma excitation frequencies in the range 13.56–116 MHz was studied by Raman and infrared spectroscopy, optical transmission and reflection, and conductivity measurements. The sensitivity of Raman spectroscopy and optical reflection on Si crystallites in the initial growth regime is compared and optical reflection at 4.5 eV is proposed as an easy and reliable tool for this investigation. While the crystallite formation on amorphous silicon substrates at 13.56 MHz is delayed in comparison with glass, SiO2 and chromium substrates, nucleation of the crystalline phase on amorphous silicon is found to be greatly enhanced at higher plasma excitation frequencies. On the other hand, for deposition on glass, SiO2, and chromium at frequencies equal to or higher than 70 MHz, increased porosity is found in the initial growth region. The results are interpreted within a model that suggests a conelike initial formation of the silicon crystallites and a higher etching rate of disordered material at high plasma excitation frequencies. In addition, the extension of the process of crystallite formation from the film-plasma interface into a growth zone more than 10 nm deep is proposed. The application of the microcrystalline silicon layers prepared at high plasma excitation frequency is demonstrated in amorphous silicon based tandem solar cells.

A-Si:H/a-Si:H stacked cell from VHF-deposition in a single chamber reactor with 9% stabilized efficiency

In the present paper we present results on a-Si:H/a-Si:H stacked cells deposited in a single-chamber reactor by the very high frequency-glow discharge (VHF-GD) deposition technique at 70 MHz. Hydrogen dilution of the i-layer yields more stable amorphous p-i-n solar cells, similar to what is observed for RF deposition. High dilution ratios of the i-layer are found to enhance contaminations. This is, for the single-chamber reactor, due to the etching effect of the highly reactive H 2-plasma. Additionally, oxygen incorporation into the i-layer is favored by the high hydrogen dilution. Different means to suppress the contaminations are employed and discussed. Regarding the stacked cell design, we show by experiment and simulation that it is important to carefully adjust the current mismatch between the component cells such as to obtain a slight top-cell-limited behavior after degradation. We present an a-Si:H/a-Si:H stacked cell with an initial efficiency of 9.8% showing only 8% relative degradation which results in a stabilized efficiency of 9%. The deposition rate of the employed H 2-diluted i-layer material is 4 Å/s. It is therefore demonstrated that it is possible to make highly efficient stacked cells showing good stability also in a single-chamber system and employing the VHF technique to obtain higher rates.

CORRELATION BETWEEN THE POPULATION OF ACTIVE SPECIES IN LOW PRESSURE PLASMAS AND THEIR REACTIONS WITH ORGANIC COMPOUNDS

The oxidation of four alkanes, two olefins and twelve aromatic compounds in the liquid phase with either high voltage or radio frequency glow discharges has been studied. The results of the liquid-plasma interactions of this work considerably differ from those in homogeneous gas phase. Neither fragmentation products in the traps nor polymers on the reactor walls have been detected. The selectivity of product formation is far superior to plasma oxidations in gas phase, making these techniques attractive for preparative chemistry. Total conversion, i.e., mass transformed against initial mass of substrate, has been studied as a function of the temperature of the liquid and the oxygen flow rate in the reactor. Optical emission spectroscopy has been utilized to detect and monitor the active species in the plasma. At the conditions for maximum oxidation no evidence of O2(1Δg) has been observed. A correlation between the behavior of the O(3P) population in the discharge and the reactivity of the different compounds, as a function of the oxygen flow rate in the reactor, is presented.

Reactions of O(3 P) with aromatic compounds in the liquid phase

The oxidation of 12 aromatic compounds using either high voltage or radio frequency glow discharges has been studied. The reactions have been carried out by making the oxygen plasma reach the low vapor pressure substrate. This (1.5-3 ml) was placed into a double-walled glass vessel that was cooled down to temperatures close to its freeing point. Oxygen pressure was of the order of 0.05-0.30 torr, the ratio p(Oa) / (vapor pressure of the liquid) being in the range 10–120. The results obtained in the plasma-liquid interactions of this work considerably differ from those in the homogeneous gas phase. Neither fragmentation products in the traps nor polymers on the reactor walls have been detected. Product formation has proved to be more selective as well. For anisole and five monoalkylben:enes o-, m-, and p-alkvlphenols amounted to 65–86% of the total yield, ortho derivatives being the most important products. The oxidation of the alkyl side chain was observed at a lower level than the aromatic hydroxylation. No ipso substitution was detected. Diand trimethylbenzenes were also studied, di- and trirnethylphenols being the major products. For all substrates dihydroxy derivatives were the most important byproducts. Total conversion, i.e., mass transformed against initial mass of substrate, has been studied as a Junction of temperature of the liquid and oxygen fog, rate in the reactor, this ranging from 10 to 20 mMol/h. The optimum conversions were 7 to 40%. A correlation between these results and the behavior of the O(3P) population in the discharge has been /bund.

Enhanced optical absorption in microcrystalline silicon

An enhanced optical absorption, compared to crystalline silicon, was observed in the above gap region together with very low defect-connected absorption in microcrystalline silicon (μc-Si:H) prepared by very high frequency glow discharge technique at 70 MHz. As the μc-Si:H material has a very low fraction of amorphous phase, a ‘crystalline silicon like’ absorption model is proposed which associates most absorption properties of μc-Si:H to the crystalline phase in the material. Thereby the influence of internal strain, microcrystalline boundaries, hydrogen content and elastic light scattering on the absorption properties are discussed.

On the Way Towards High Efficiency Thin Film Silicon Solar Cells by the “Micromorph” Concept

AbstractRecently the authors have demonstrated that compensated or “midgap” intrinsic hydrogenated microcrystalline silicon (μc-Si:H), as deposited by the Very High Frequency Glow Discharge (VHF-GD) technique, can be used as active layer in p-i-n solar cells. Compared to amorphous silicon (a-Si:H), μc-Si:H was found to have a significantly lower energy bandgap of around 1 eV. The combination of both materials (two absorbers with different gap energies) leads to a “real” tandem cell structure, which was called the “micromorph” cell. Micromorph cells can make better use of the sun's spectrum in contrast to conventional double-stacked a-Si:H / a-Si:H tandems.The present study will show that the compensation technique (involving boron “microdoping”) used sofar for obtaining midgap μc-Si:H can be replaced by the application of a gas purifier. The use of this gas purifier has a beneficial influence on the transport properties of undoped intrinsic μc-Si:H. By this procedure, increased cell efficiencies in both, single microcrystalline silicon p-i-n as well as micromorph cells could be obtained. In the first case 7.7 % stable, and in the second case 13.1% initial efficiency could be achieved under AM1.5 conditions. Preliminary light-soaking experiments performed on the tandem cells indicate that microcrystalline silicon could contribute to an enhancement of the stable efficiency performance. Micromorph cell manufacturing is fully compatible to a-Si:H technology; however, its deposition rate is still too low. With further increase of the rate, a similar cost reduction potential like in a-Si:H technology can be extrapolated.

Origins of atmospheric contamination in amorphous silicon prepared by very high frequency (70 MHz) glow discharge

The authors have studied the effect of plasma power, reactor outgassing rates, and of silane purity on the oxygen, carbon, and nitrogen contents of amorphous silicon material prepared by the very high frequency (70 MHz) glow discharge technique. The silane purity could be optionally enhanced by the application of a getter-based silane gas purifier. It was found that oxygen incorporation was enhanced at lower deposition rates, whereas the nitrogen and carbon film contamination were unaffected. The deposition rate dependence of the incorporation is in excellent agreement with a proposed model. Apart from the effects of plasma power on the incorporation probability, the reactor outgassing rate and the purity of the silane gas itself were identified as the main contamination sources for the atmospheric contaminants in the deposited films. At the low outgassing rate, at least around one-half of the oxygen detected in the a-Si:H material originates from the silane gas. Due to the reduced outgassing rate and an enhanced purity of the silane gas used, the authors have deposited a-Si:H-material with the lowest concentrations of atmospheric contaminants reported to date. Furthermore, the present results exclude a contamination of the a-Si:H-material by a post- oxidation after air exposure.

Investigation on the sputtering effects on r.f. plasma processing

The effects of plasma bombardment variations at the substrace surface in hihg-frequency (13.56 MHz) r.f. glow discharges, as are commonly utilized in various PVD processes, were investigated by means of a series of sputter weight loss experiments. The weight losses on small substrates, removable from a larger cathode arrangement, mounted in various positions in the chamber were recorded after exposure to a series of glow discharges maintained at a range of argon pressures and power densities. Therefore, the relationship between ion bombardment intensity and sputter removal at the substrate could be determined at different locations in the deposition volume. The results were compared with previous work in which similar experiments were performed in d.c. and low-frequency (380 kHz) glow discharges and hence the degree of sputtering uniformity in each of these discharge systems could be ranked. The findings show that high-frequency glow discharges behave similarly to the d.c. diode system at low pressures and at elevated pressures the bombardment characteristics are similar to a low-frequency discharge. The work is significant for coating and plasma treatment applications because of the profound effects that ion bombardment has, for example, on coating thickness uniformity, coating structure and diffusional effects.