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The influence of working pressure and inter-electrode gap distance on the quality of microcrystalline silicon prepared by PECVD at 40 MHz in three different KAI™ reactors designs with inter-electrode gaps of 28 mm, 16 mm and 7 mm has been investigated. The microcrystalline devices were implemented and studied as bottom cells in MICROMORPH™ tandem modules or processed with an optical filter to simulate the absorption of a top cell on a module size of 1.43 m 2 . Increasing the working pressure from 250 Pa to 2000 Pa resulted in an improvement of the efficiency of 1% absolute due to improvement in the open circuit voltage V oc and the fill factor FF . The influence of the deposition rate on the quality of the intrinsic absorber layers has been investigated and optimized. An unconventional crystallinity profile throughout the intrinsic absorber layer has been developed to further improve either the current density J sc or V oc and FF . By controlling and tuning the Raman crystallinity a very high open circuit voltage V oc of >1.42 V in a tandem cell design in full size modules (1.43 m 2 ) could be realized.

Columnar growth was observed in the amorphous part of mixed phase layers deposited at very low substrate temperatures. The width of the columns and the layer thickness at which they are first distinguishable in a cross-sectional transmission electron microscope (X-TEM) image, about 120 nm, is similar for the substrate temperature range of 40-100 °C, but the columns are less well developed when either the substrate temperature is increased or the dilution ratio is lowered. This growth behaviour and the incubation layer are attributed to hydrogen-induced surface diffusion of growth precursors resulting in an amorphous-amorphous roughness transition.

A 15-nm lithium fluoride (LiF) thin film evaporated on glass substrate is shown to enhance the nucleation of microcrystalline Si grown by plasma enhanced chemical vapour deposition at the amorphous/microcrystalline boundary conditions. The effect is more pronounced at low substrate temperatures, nucleation density being 10 times higher at ∼ 80 °C. The effect is ascribed to the ionic chemical nature of LiF, the low work function material used in organic electronic devices, and we propose its use for micro patterning crystalline Si regions in otherwise amorphous Si film.

Four series of intrinsic thin Si films were prepared by plasma enhanced chemical vapor deposition at standard and high growth rate conditions. We suggest a simple ‘μc-Si:H layer quality factor’ based on the ratio of subgap optical absorption coefficient values: α (1.4 eV)/ α (1 eV). This ratio minimizes the light scattering effects for rough films and can serve as a reliable detection of the amorphous/microcrystalline structure transition and also as a figure of merit for the microcrystalline layer. The quality factor is evaluated for series of our samples with well known structure and also compared with samples from other laboratories with different deposition and measurement techniques.

We have proposed annealing in H 2 O vapor as a new effective and low-cost method for passivating polycrystalline silicon grain boundaries and for improving the performance of poly-Si based devices. The effect of H 2 O vapor treatment was experimentally found to differ from analogous anneal in nitrogen and hydrogen. Mechanism of the H 2 O vapor treatment was studied by Kelvin force microscopy, used to measure the potential change at individual grain boundaries and point defects. The potential change was dependent on the grain boundary character and it correlated with the crystalline disorder and internal stress observed by microscopic Raman spectroscopy. Effect of H 2 O vapor passivation was experimentally connected with the potential change at the grain boundaries before and after the treatment.

Preparation of thin film silicon at high growth rate is an important target for its application in solar cells. The properties of hydrogenated microcrystalline silicon, prepared with the help of PECVD multi-hole cathode in a high pressure and depletion regime in a wide range of thicknesses are described in detail. We illustrate the surprising result that we can prepare high growth rate microcrystalline silicon from 0.4 up to 30 μm thickness without great peel-off problems. The room temperature dark DC conductivity, as well as the crystallinity, increased up to 5 μm film thickness and then started to decrease again. These results are explained by the initial temperature profiling and a thickness-induced increase of the lateral inhomogeneity.

Lithographically prepared micrometer-sized permalloy ellipses were used to control the growth of nanocrystalline Si in otherwise amorphous Si film prepared by plasma enhanced chemical vapor deposition. Atomic force microscopy and micro-Raman spectroscopy were employed to study the surface structures before and after the deposition of the Si film. The possible applications of the controlled growth of nanocrystalline Si micro-patterns are discussed as well as the mechanisms leading to the growth of these patterns.

Microcrystalline silicon (μc-Si:H) thin films prepared by plasma enhanced chemical vapour deposition (PECVD) at 37 °C has been studied by cross-sectional TEM and ambient conductive AFM. We have succeeded in the combined measurement of topography and local conductivity under standard ambient conditions, overcoming the surface native oxide by more sensitive (pA range) current detection. We observed the columnar structure of the amorphous phase in the TEM micrograph and related it to the surface corrugation of the same size detected by AFM.

We present a complex characterization of thin silicon films, the growth of which has been influenced by permanent magnet, placed under the substrate. The pattern of microcrystalline regions in otherwise amorphous film varied with the orientation of the magnetic field. Study by atomic force microscopy and by micro-Raman spectroscopy revealed that the microcrystalline regions resulted from increased nucleation density of crystalline grains at the locations where the magnetron effect could be expected. This phenomenon allowed us to study in detail the transition between amorphous and microcrystalline growth. Moreover, it can be used as a kind of ‘magnetic lithography’ for preparation of predefined microcrystalline patterns in otherwise amorphous silicon films.

We have examined the common methods for determination of the crystallinity of mixed phase silicon thin films from the TO–LO phonon band in Raman spectra. Spectra are decomposed into contributions of amorphous and crystalline phase and empirical formulas are used to obtain crystallinity either from the integral intensities (peak areas) or from magnitudes (peak maxima). Crystallinity values obtained from Raman spectra excited by Ar+ laser green line (514.5 nm) for a special sample with a profile of structure from amorphous to fully microcrystalline were compared with surface crystallinity obtained independently from atomic force microscopy (AFM). Analysis of the Raman collection depth in material composed of grains with absorption depth 1000 nm in an amorphous matrix (absorption depth 100 nm), was used to explain reasons for systematic difference between surface and Raman crystallinities. Recommendations are given for obtaining consistent results.

Amorphous/microcrystalline transition was studied in the high growth-rate depositions of hydrogenated silicon films at a high pressure (700 Pa) in a depletion regime using a series of samples with the ratio of hydrogen to silane flows from 10 to 32. Results show the characteristic features of the amorphous/microcrystalline transition: abrupt change of dark conductivity and crystallinity accompanied by peaks of roughness and diffusion length, observed previously at standard growth rates.

Polycrystalline silicon films for solar cells grown by atmospheric pressure chemical vapour deposition require hydrogenation to passivate defects at grain boundaries. Passivation by remote plasma hydrogenation of 12-μm-thick poly-Si films increased Hall effect carrier mobility from 3 to 20 cm2/Vs, photoluminescence intensity at 0.98 eV band more than 2 times and decreased contrast of local electronic conductivity between grains, observed by combined AFM. However, excessive hydrogenation led to surface damage and defect creation evidenced by widths of Raman LO–TO peak at 520 cm− 1 and X-ray rocking curve of (220) diffraction line. Depth profile by repeated etching of the surface and following the signature of Si–H2 and H2 bonding in Raman spectra showed that the damage extended up to 100 nm from the surface.

Hydrogen passivation is essential for improving the properties of polycrystalline silicon thin films. We have observed that remote plasma hydrogenation with duration up to 30 min effectively passivated the defects and improved the Hall mobility, trap density and photoluminescence intensity. Over 60 min of hydrogenation caused the photoluminescence intensity to decrease. It seems that excessive hydrogenation not only passivated defects but also created new defects (Si–H2 bonds and hydrogen molecules) in the grains. Raman spectroscopy detected that hydrogen formed Si–H2 bonds in the poly-Si up to 100 nm from surface. Creation of these defects corresponded to a decrease of the photoluminescence intensity. These defects might be harmful to poly-Si-based devices.

Changes in the growth of thin silicon films at very low substrate temperatures (35 °C

Silicon thin films grown near the boundary between the amorphous/microcrystalline growth offer superior properties for industrial applications. Series of silicon samples, in which crossing of this transition region was achieved by changing a single technological parameter (dilution of silane in hydrogen, deposition temperature, sample thickness) were used to test our model of transport, connecting the macroscopically observed transport properties and the crystallinity, hydrogen content, grain size and grain boundaries. Microscopic study by AFM led to the formulation of the geometrical model of growth of mixed phase Si. The demand for research of microcrystalline or polycrystalline silicon prepared at low substrate temperatures is stimulated by the use of cheap plastic substrates. In addition to a direct deposition an alternative technology, such as metal-induced crystallization supported by the electric field is discussed. Possible future application of thin silicon films, for example in a “nanolithography”, is also shown. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A series of investigations were performed to study the influence of the substrate temperature on the structure and properties of silicon thin films. Substrate temperature was varied in the wide range of 35–200 °C. It has been shown that the films grown below 60 °C exhibit an unusual structural behavior. A sharp TO phonon peak at 520 cm −1 was detected in Raman spectra, which is associated with the crystalline structure. In contrast to these results, the same samples do not show any crystallite-related peak by X-ray diffraction and their optoelectronic properties (dark conductivity, activation energy and subgap absorption spectra) show amorphous features. A similar discrepancy was observed for a hydrogen dilution ratio ( r H =([SiH 4 ]+[H 2 ])/[SiH 4 ]) series of samples deposited at 60 °C. Hydrogen dilution ratio was varied from 25 to 170. It seems that at low substrate temperature a parameter window exists where the silicon thin films can be grown with the properties combining both crystalline and amorphous behavior.

A way in which thin films of hydrogenated amorphous silicon (a-Si: H) can be instantaneously crystallized at room temperature is reported. The metal-induced solid-phase crystallization (MISPC) method with nickel surface coverage is used. In comparison with previous reports on the MISPC of a-Si: H, the crystallization temperature is reduced by more than 350°C. This is achieved by introducing two novel technological steps: firstly, we use hydrogen-rich a-Si: H films (hydrogen content between 20 and 45at.% H) and, secondly, we apply a high transverse electric field. Polycrystalline silicon islands as large as 3 mm across appear instantaneously after having reached a threshold electric field of about 105Vcm−1. We report macroscopic visualization of the crystallization process as well as microscopic investigation (micro-Raman measurements and scanning electron microphotography) of the crystallized films. We have found that appropriate patterning of the nickel electrode helps to increase homogeneity of...

We have investigated the role of the sample thickness and silane dilution on the structure and electronic properties of protocrystalline silicon thin films deposited at very low substrate temperatures (∼80 ° C ) . Coincidence of the maxima in surface roughness and ambipolar diffusion length ( ≳100 nm ) with formation of the network of interconnected crystalline grain aggregates was observed. While the presence of the isolated grain aggregates improves the photoconductive properties before the percolation threshold is reached, further increase in crystallinity may have opposite effect due to detrimental role of increasing concentration of the defective grain boundaries.

Large complexity of microstructure in hydrogenated microcrystalline silicon and existence of at least two different sizes of crystallites is demonstrated by combined atomic force microscope topography/local current map. We correlate activation energy and prefactor of the simplest transport property – dark conductivity, measured parallel to the substrate – with the crystallinity and roughness in wide range of microcrystalline silicon samples. This allowed us to formulate a simple model of transport based on the idea that, contrary to small grains, the formation of their aggregates (large grains/columns) dramatically changes the mechanism of transport from band like to hopping.

We have used plasma enhanced chemical vapor deposition (PECVD) to deposit silicon thin films (∼0.2–1 μm) with different crystallinity fractions on Nanosensors PtIr5 coated atomic force microscopy (AFM) cantilevers (450 × 50 × 2 μm 3 ). Microscopic measurements of Raman scattering were used to map both internal stress and extrinsic stress induced in the films by bending the cantilevers using a nanomanipulator (Kleindiek Nanotechnik MM3A). Thanks to the excellent elasticity of the cantilevers, the films could be bent to curvature radii down to 300 μm. We observed the stress induced shift of the TO–LO phonon Raman band of more than 3 cm −1 for fully microcrystalline film corresponding to the stress ∼0.8 GPa. The shift of the similar film with amorphous structure was ∼2.5 cm −1 .

Hydrogenated amorphous silicon (a-Si:H) layers are prepared by plasma-enhanced chemical vapor deposition on metallized glass substrates. Ambient atomic force microscopy (AFM) is employed for both modification and characterization of a-Si:H films. Voltage pulses of up to 35 V are applied as a cantilever scans the amorphous silicon surface in contact mode AFM. Subsequent detection by Kelvin probe microscopy reveals a persistent negative charge stored in the a-Si:H layers. The stored charge is always negative independent of voltage polarity and results in an upward shift of the Fermi level by as much as 0.1 eV. Only at higher negative voltages (

Preparation of amorphous silicon/microcrystalline silicon superlattices allowed us a systematic study of transition from isotropic amorphous silicon to microcrystalline silicon with anisotropic (columnar) microstructure. The fact that just a few nm of amorphous interlayers are sufficient to interrupt columnar growth of crystallites is reflected in a clearly demonstrated isotropy of transport properties of all superlattice samples. Values of dark conductivity and diffusion length as well as grain size vary with changing crystallinity and so we can tailor the properties of the resulting material by adjusting thicknesses of amorphous and microcrystalline layers repeated to achieve a total desired thickness. Properly selected design of superlattice can lead to transport properties more suitable for solar cells than with pure microcrystalline silicon.

A series of silicon thin films was made by very high frequency plasma enhanced chemical vapour deposition (VHF PECVD) at substrate temperatures below 100 °C at different hydrogen to silane dilution ratios. The electronic properties of these layers were studied as a function of the surface crystalline fraction as determined accurately from a combination of microscope images at different length scales (gathered by using different types of microscopes). The results show that the electrical conductivity increases monotonously as a function of crystalline surface coverage and no discontinuity is observed at the percolation threshold. An increase in conductivity of four orders of magnitude for layers with a high crystalline content is observed after annealing at temperatures up to 170 °C. Combined with the information that oxygen is incorporated at Si-H surface bond sites, this suggests that doping of the intergrain boundaries by oxygen might be dominantly responsible for the electronic properties of mixed phase silicon.

Silicon thin films were grown by PECVD at high-pressure (700 Pa), high-power (4-8 W/cm2) depletion regime using a multi-hole powered electrode. Series of samples were deposited varying hydrogen/silane ratio, plasma power and film thickness to study evolution of film structure and transport properties around a-Si:H/muc-Si:H transition. We suggest a simple figure-of-merit for the films based on alpha(1eV)/alpha(1.4eV) ratio from CPM, which correlates well with the values of ambipolar diffusion length measured by surface photovoltage method (SPV) perpendicular to the substrate, i.e., the direction of the photogenerated carriers collection in solar cells

Layers of densely packed luminescent Si nanocrystals embedded in fused silica act as wavelength-specific planar waveguides that filter the wide-band spontaneous emission. The waveguides’ light output consists of two spectrally narrow (∼10 nm), orthogonally polarized, and spatially directed bands. This effect is shown to result from leaky modes of the lossy waveguides. The results have general applicability to lossy, asymmetric waveguides and show the way to produce spectrally narrow emission without the use of optical cavities.

A combined cross-sectional transmission electron microscope (XTEM) and atomic force microscope (AFM) study of a hydrogen to silane dilution series of thin silicon films deposited by very high frequency plasma enhanced chemical vapor deposition at a substrate temperature that is low enough (39°C) to neglect the role of the surface diffusion in the growth process is reported. XTEM images of a mixed amorphous/microcrystalline layer reveal a structure of isolated conically shaped crystalline conglomerates (surface diameter ∼570±75nm) embedded in an amorphous phase of columns with diameters of ∼51±3nm. Detailed closeups of these crystallites, combined with AFM images of the hydrogen dilution dependent evolution of the surface, reveal similarities between the nucleation of amorphous and crystalline columnar structures at this low substrate temperature.

Silicon thin films were grown near the microcrystalline/amorphous boundary at substrate temperatures TS = 35–200°C and dilutions [H2]/[SiH4] = 25–167. The conductivity percolation threshold occurred at crystallinity ~60%, above the random composite threshold 33.3%, due to amorphous tissue coating crystalline grains and limiting the electronic transport. Higher content of hydrogen at lower TS facilitates formation of ordered silicon phase even close to room temperature (the sample grown at 35°C had 30 at% of hydrogen and crystallinity ~60%), providing a technological window for deposition of silicon thin films on cheap polymer substrates with electronic properties suitable for solar cells.