The effect of micro-doping, defect creation, and non-steady state occupation through optical transitions on the electron spin resonance signals found in undoped and weakly doped microcrystalline silicon with a high degree of crystallinity is investigated. The experimental results are in agreement with the assignment of the resonance at g=1.9983 to conduction electrons in the crystalline grains and the resonanccs around g=2.0052 to dangling bonds in the remaining amorphous phase and at the grain boundaries. The simultaneous presence of both resonances can result from a large conduction band offset between crystalline grains and grain boundaries or the amorphous phase. The presence of conduction electron spin resonance in compensated and even p-type material points also to potential fluctuations. Free electrons in interconnected crystalline grains are in agreement with the weakly activated transport found in μc-Si:H at low temperatures.

Microcrystalline silicon with various crystalline volume fractions was prepared by plasma enhanced chemical vapour deposition. The material was studied by steady state and transient electron spin resonance in the dark and under light illumination. The observed resonances at g-values of 2.01, 2.0052, 2.0043, 1.998 can be attributed to the amorphous and microcrystalline constituents, and their respective intensities change as the ratio of amorphous to crystalline volume is varied. The origin of a fifth resonance at g = 1.995 remains unclear. Smaller crystalline volume fractions lead to lower spin densities and affect the recombination behaviour of photogenerated charge carriers. The recombination behaviour in highly crystalline material is also influenced by moderate Fermi level shifts, where differences show up between n-type (or undoped) and p-type samples. The differences are attributed to trapping of photo-generated holes in deep states within the disordered regions.

Highly doped μc-Si:H samples with a wide range of crystalline volume fractions and grain sizes have been investigated by Hall-effect experiments. We present an experimental set-up with a current modulation technique and a 6-pole contact geometry which allows the measurement of the Hall-effect on highly doped μc-Si:H down to 10K. The experimental results exhibit a clear correlation between the mobility μ and the grain size δ. Further, the results show that the transport in μc-Si:H can not be described by thermal emission over grain boundaries alone, additional transport paths, e.g. tunneling processes through the barriers have to be taken into account.

The growth mechanism and material properties of -type µc-Si:H prepared with plasma enhanced chemical vapour deposition in the very high frequency range is investigated. By increasing the plasma excitation frequency the grain size, deposition rate and Hall mobility can be simultaneously increased without having to adjust other deposition parameters in particular the temperature. This effect is explained by an enhanced selective etching of amorphous tissue and grain boundary regions together with a sufficient supply of growth species at high frequency plasmas.

The low temperature hydrogen induced phase transformations are the basis for the layer-by-layer micro-crystalline silicon growth technique which involves the sequential deposition of thin amorphous silicon layers followed by hydrogen treatment. Very high frequency-PECVD layer-by-layer microcrystalline film growth was studied as a function of frequency. The highest crystalline content films were prepared on glass substrates at 49.7 MHz with deposition cycle times of 12 seconds and hydrogen treatment times of 120 seconds or longer. At lower frequencies, 13.56 and 27 MHz, no micro-crystalline content was observed. At higher frequencies (94.7 MHz) crystalline content was reduced compared to that of the 49.7 MHz case. Crystalline content is not correlated with the hydrogen etch rate as the highest etch rates occurred at 94.7 MHz while etch rates at 49.7 and 27 MHz were lower. It is possible to prepare thin micro-crystalline films using the VHF layer-by-layer growth technique.

The growth of microcrystalline silicon prepared by plasma-enhanced chemical vapour deposition depends on the deposition conditions and yields films with variable content of crystalline grains, amorphous network, grain boundaries and voids. The changes in the structural properties of a series of films grown under a variation of the dilution of the process gas silane in hydrogen, which induces a transition from highly crystalline to amorphous growth, were investigated. The evolution of the crystalline volume fraction was quantitatively analysed by Raman spectroscopy and X-ray diffraction. The results confirm the need for proper correction of the Raman data for optical absorption and Raman cross-section. Transmission electron microscopy was used to investigate the characteristics and the variation in the microstructure. Upon increasing the silane concentration the strong columnar growth with narrow grain boundaries degrades towards the growth of small irregularly shaped grains enclosed in an amorphous matrix. Simultaneously, the initial film growth becomes predominantly amorphous. Spherical voids with a diameter up to 14 nm and 'crack-like' cavities (which accumulate at the film-substrate interface in the highly crystalline growth regime) were observed. Relaxation reactions of strained bonds within a growth zone of the order of a few tens of nanometres and preferential etching are proposed to govern the growth process.

Acupuncture has become an accepted and validated part of Western mainstream medicine and is increasingly used by clinicians, midwives, and acupuncturists for reproductive care, induction of labor, and analgesia. Most studies of the effects of obstetrical acupuncture are descriptive, many in foreign languages. Only a few have evaluated efficacy. Results suggest that acupuncture ripens the cervix, initiates labor, reduces labor pain, and shortens the first stage of labor. There is some evidence suggesting that certain acupuncture points have very specific effects on the fetus and uterus, which may be mediated through the hypopituitary-thalamic axis or by local neurovascular stimulation. The substantial maternal hormonal changes occurring just before and during labor offer a unique opportunity to clarify the mechanisms of action of acupuncture. Using a conceptual model based on possible mechanisms of action of the use of acupuncture in obstetrics, the authors propose specific research questions into the physiology of acupuncture administered before and at parturition.

Microcrystalline silicon (μc-Si:H) prepared by plasma-enhanced chemical vapor deposition (PECVD) has been investigated as material for absorber layers in solar cells. The deposition process has been adjusted to achieve high deposition rates and optimized solar cell performance. In particular, already moderate variations of the crystalline vs. amorphous volume fractions were found to effect the electronic material – and solar cell properties. Such variation is readily achieved by changing the process gas mixture of silane to hydrogen. Best cell performance was found for material near the transition to the amorphous growth regime. With this optimized material efficiencies of 7.5% for a 2μm thick μc-Si:H single solar cell and 12% for an a-Si:H/μc-Si:H stacked solar cell have been achieved.

Low-temperature epitaxial growth of Si thin films on Si(100) was studied by transmission electron microscopy. The films were prepared by very-high frequency plasma-enhanced chemical vapour deposition at 200 degreesC. Large-scale epitaxial growth, which is restricted to pyramidal regions, is followed by a columnar growth with column diameters approximate to 50 nm, similar to the growth on non-crystalline substrates. The appearance of columnar growth is accompanied by the formation of {111}-facets, microtwins and stacking fault tetrahedra. These structural features bear a close similarity to those observed in low temperature MBE, which indicates similar mechanisms in the formation of growth defects.

The formation of microcrystalline silicon was investigated employing the layer-by-layer technique with very high frequency plasma excitation. Etching of the deposited layers was found to be a dominant effect during hydrogen plasma treatment. Microcrystalline growth occurred when the thickness of the deposited layer was reduced to a certain value in each cycle. The etching stopped when a noticeable crystalline volume fraction was produced; for long hydrogen treatment times no film growth was observed. (C) 1999 American Institute of Physics. [S0021-8979(99)06404-X].

For the application in thin film solar cells intrinsic microcrystalline silicon was prepared by very high frequency plasma enhanced chemical vapour deposition in a high purity system. Discharge powers between 5 and 50 W and silane concentrations of 2 - 8 % in hydrogen were used to influence the deposition rates and the structure of the material. At high powers and silane concentration, material with high crystallinity at maximum deposition rates of 4.8 Angstrom/s was obtained. The performance of the different materials was investigated in pin and nip type solar cells. In addition the suitability of magnetron sputtered and texture etched ZnO films as light scattering substrates for this type of solar cell is investigated and compared with a commercial transparent conductive oxide substrate. The ZnO substrate shows excellent light scattering performance yielding solar cell efficiencies of up to 7 % for an active layers thickness of 1 mu m. Maximum efficiencies of 7.5 % are obtained for both the pin and nip structures.

Intrinsic microcrystalline silicon (μc-Si:H) was prepared with plasma enhanced chemical vapor deposition (PECVD) from silane/hydrogen mixtures at 200°C with the aim to increase the deposition rate. Using a plasma excitation frequency of 95 MHz we obtain an increase of the deposition rate by a factor of 25 from that of our standard PECVD process at 13.56 MHz. This increase is obtained by the combination of a higher plasma excitation frequency, an increased silane concentration (SC) and larger discharge powers. Material prepared under these conditions at a deposition rate of 0.46 nm s−1 maintains crystallinity and electronic properties with dark conductivities, σD≈10−7 S cm−1, and spin densities in the range of 1016 cm−3.

The device performance of color sensors based on a p-i-n structure with modified absorption layers is discussed. The detectors are developed with regard to the μτ-(carrier mobility×lifetime)-product of the different layers. To detect the red part of the visible spectrum, detectors are produced with a-SiGe:H or μc-Si:H material introduced in the rear part of the device. Both detectors exhibit a moderate color separation and a bias voltage controlled shift of the spectral response from 490 nm to 600 nm. We have analyzed the differences of these two structures measuring reflectance, spectral response, as well as the transient behavior during light exposure and voltage switching. In comparison to the p·i·i·i·n-structure with an a-SiGe:H-layer in the rear part, the μc-Si:H-based p·i·i·i·n-diode has an improved transient behavior of the photocurrent after switching on illumination and an increased rise time after bias voltage switching.

Amorphous Si films prepared by plasma-enhanced chemical vapor deposition (PECVD) have been fully crystallized by pulsed Nd-YAG laser excitation at wavelengths of 1064 and 532 nm. Crystallite quality was observed by Raman spectroscopy showing a line width of ∼5 cm −1 at 520 cm −1 . Transmission electron microscopy (TEM) micrographs show crystallite sizes of several hundred nm and crystalline reflection peaks in X-ray diffraction (XRD) are observed. The quality and size of the crystallites prepared at a wavelength of 1064 nm improved with increasing laser energy density and are most sensitive to the energy density near the threshold energy for crystallization. The optimum laser energy densities are sensitive to the film thickness due to interference effects. A comparison of crystallization at wavelengths of 1064 and 532 nm shows no difference in the crystallite quality, but at 532 nm smoother surfaces are obtained. The laser energies for crystallization at 1064 nm are higher but not as much as expected by comparison of the absorption coefficients at 1064 and 532 nm.

Microcrystalline silicon prepared by plasma-enhanced chemical vapour deposition consists of variable volume fractions of amorphous phase, grain boundaries, cavities and crystalline grains. In this paper the structural properties, which strongly depend on the growth conditions, were investigated in detail by transmission electron microscopy and by Raman spectroscopy. A columnar structure parallel to the growth direction is observed for all conditions investigated. By increasing the plasma excitation frequency the crystalline volume fraction and the grain sizes are enhanced. Simultaneously an increase in the growth rate can be achieved, which is accompanied by an increasing etch rate of amorphous material. In addition, spherical voids were found predominantly in samples prepared at a low plasma excitation frequency. The growth of a porous initial layer containing a high density of 'crack-line' voids is observed when high plasma excitation frequencies are applied. These results suggest that the microcrystalline growth is governed by the preferential etching of the amorphous phase. In addition, chemical reactions have to be taken into account to explain the formation of the spherical voids.

We report on electroabsorption spectra for plasma deposited thin films of hydrogenated silicon ranging from amorphous (a-Si:H) to microcrystalline (μc-Si-H) structures. The EA spectrum of a-Si:H deposited from silane with low hydrogen dilution were consistent with previous works; material prepared with high hydrogen dilution showed a 0.07 eV blue shift of the spectrum and somewhat stronger electroabsorption. μc-Si-H specimens have a sharp peak at 1.19eV; the spectrum is blue shifted by 0.03 eV and is significantly stronger than electroabsorption reported in single crystal silicon. Spectral features which have no correspondence to single crystal silicon were also observed in μc-Si-H. Specimens deposited using “cyclic” deposition and chemical annealing had electroabsorption spectra with both the 1.19 eV, crystalline feature and a band peaking at 2.02 eV which we attribute to strongly hydrogenated a-Si:H. We discuss applications of electroabsorption to determining the crystal fraction of microcrystalline material and to determining grain size distributions.

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.

The electronic and optical properties of microcrys tall ine silicon films prepared by plasma enhanced chemical vapour deposition are investigated with Hall-effect, electrical conductivity, photothermal deflection spectroscopy and photoluminescence measurements. In particular, the influence of the grain size and the crystalline volume fraction on the conductivity, the carrier density and the Hall mobility is investigated in highly doped films. A percolation model is proposed to describe the observed transport data. Photoluminescence properties were studied in un-doped films. It is proposed that the photoluminescence is due to recombination at structural defects similar to those observed in crystalline silicon.

The diffusion and effusion of hydrogen in hydrogenated microcrystalline silicon films deposited in an electron cyclotron resonance reactor were studied for various deposition temperatures Ts. For deposition temperatures below 250°C, hydrogen effusion is found to be dominated by desorption of hydrogen from internal surfaces followed by rapid out-diffusion of H2. Higher substrate temperatures result in an increased hydrogen stability suggesting the growth of a more compact material. For this latter type of samples, a hydrogen diffusion coefficient similar as in compact plasma-grown a-Si:H films is found despite a different predominant bonding of hydrogen according to infrared absorption.

Microcrystalline silicon was prepared with glow discharge deposition from silane/hydrogen mixtures at plasma excitation frequencies in the range 13.56 MHz - 116 MHz. The influence of the plasma excitation frequency on the growth and the structural properties of the material is investigated. At high excitation frequencies, higher growth and etching rates, larger grain sizes with less disorder within the grains, higher crystalline volume fractions, a reduced amorphous but more porous interface layer on glass and quartz substrates, and faster nucleation on amorphous silicon substrates are obtained. The results are discussed within a schematical growth model.