Silane Fraction Research Articles

Role of H3 + ions in deposition of silicon thin films from SiH4/H2 discharges: modeling and experiments

A 1D fluid model including a detailed chemistry for silane-hydrogen discharges as well as surface reactions to account for deposition and etching processes has been implemented to study the effects of gas pressure (1 to 3.5 Torr) and silane concentration (2 to 10%) on the deposition rate of silicon thin films in a standard RF-PECVD reactor. The thickness of the films and their deposition rate as functions of the process conditions were determined from optical modelling of UV-visible spectroscopic ellipsometry measurements. The experimental values of the deposition rate were compared with results from the 1D fluid model. SiH3 radicals are found to be the main contributor to the computed deposition rates, while H3 + ions play the main role in the etching process. The study reveals that etching by hydrogen ions must be taken into account to reproduce properly the experimental deposition rate. In particular etching by H3 + ions must be taken into account to achieve a good agreement between the experimental and modelled values of the deposition rate as a function of the total gas pressure and the silane fraction in the discharge.

Ignition Delay Time for Hydrogen–Silane–Air Mixtures at Low Temperatures

A modified physicomathematical model of ignition of hydrogen–silane–air mixtures is applied to calculate the ignition delay time for these mixtures at low initial temperatures (300–900 K) and pressures (0.4–1 atm) of the mixture. It is shown that the diagram of the ignition delay time as a function of temperature contains a region of the so-called negative temperature coefficient. The influence of the pressure in the mixture and of the silane fraction on the length of this region is studied. It is found that an increase in both factors (silane concentration and pressure in the mixture) leads to an increase in the length of the negative temperature coefficient region.

Chemical sputtering by H2+ and H3+ ions during silicon deposition

We investigated chemical sputtering of silicon films by Hy+ ions (with y being 2 and 3) in an asymmetric VHF Plasma Enhanced Chemical Vapor Deposition (PECVD) discharge in detail. In experiments with discharges created with pure H2 inlet flows, we observed that more Si was etched from the powered than from the grounded electrode, and this resulted in a net deposition on the grounded electrode. With experimental input data from a power density series of discharges with pure H2 inlet flows, we were able to model this process with a chemical sputtering mechanism. The obtained chemical sputtering yields were (0.3–0.4) ± 0.1 Si atom per bombarding Hy+ ion at the grounded electrode and at the powered electrode the yield ranged from (0.4 to 0.65) ± 0.1. Subsequently, we investigated the role of chemical sputtering during PECVD deposition with a series of silane fractions SF (SF(%) = [SiH4]/[H2]*100) ranging from SF = 0% to 20%. We experimentally observed that the SiHy+ flux is not proportional to SF but decreasing from SF = 3.4% to 20%. This counterintuitive SiHy+ flux trend was partly explained by an increasing chemical sputtering rate with decreasing SF and partly by the reaction between H3+ and SiH4 that forms SiH3+.

A hybrid kinetic Monte Carlo method for simulating silicon films grown by plasma-enhanced chemical vapor deposition

We present a powerful kinetic Monte Carlo (KMC) algorithm that allows one to simulate the growth of nanocrystalline silicon by plasma enhanced chemical vapor deposition (PECVD) for film thicknesses as large as several hundreds of monolayers. Our method combines a standard n-fold KMC algorithm with an efficient Markovian random walk scheme accounting for the surface diffusive processes of the species involved in PECVD. These processes are extremely fast compared to chemical reactions, thus in a brute application of the KMC method more than 99% of the computational time is spent in monitoring them. Our method decouples the treatment of these events from the rest of the reactions in a systematic way, thereby dramatically increasing the efficiency of the corresponding KMC algorithm. It is also making use of a very rich kinetic model which includes 5 species (H, SiH3, SiH2, SiH, and Si2H5) that participate in 29 reactions. We have applied the new method in simulations of silicon growth under several conditions (in particular, silane fraction in the gas mixture), including those usually realized in actual PECVD technologies. This has allowed us to directly compare against available experimental data for the growth rate, the mesoscale morphology, and the chemical composition of the deposited film as a function of dilution ratio.

Simulation of gas phase reactions for microcrystalline silicon films fabricated by PECVD

We present a numerical gas phase reaction model for hydrogenated microcrystalline silicon (μc-Si:H) films from SiH4 and H2 gas mixtures with plasma enhanced chemical vapor deposition (PECVD). Under the typical μc-Si:H deposition conditions, the concentrations of the species in the plasma are calculated and the effects of silane fraction (SF=[SiH4]/[H2+SiH4]) are investigated. The results show that SiH3 is the key precursor for μc-Si:H films growth, and other neutral radicals, such as Si2H5, Si2H4 and SiH2, may play some roles in the film deposition. With the silane fraction increasing, the precursor concentration increases, but H atom concentration decreases rapidly, which results in the lower H/SiH3 ratio.

Low-temperature synthesis of homogeneous nanocrystalline cubic silicon carbide films

Silicon carbide films are fabricated by inductively coupled plasma chemical vapor deposition from feedstock gases silane and methane heavily diluted with hydrogen at a low substrate temperature of300 °C. Fourier transform infrared absorption spectroscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy analyses show that homogeneous nanocrystalline cubic silicon carbide (3C-SiC) films can be synthesized at an appropriate silane fraction X[100%×silane flow(SCCM)/silane+methane flow(SCCM)] in the gas mixture. The achievement of homogeneous nanocrystalline 3C-SiC films at a low substrate temperature of300 °C is a synergy of a low deposition pressure (22 mTorr), high inductive rf power (2000 W), heavy dilution of feedstock gases silane and methane with hydrogen, and appropriate silane fractions X (X≤33%) in the gas mixture employed in our experiments.

Study of amorphous nanocrystalline thin silicon films by grazing-incidence small-angle X-ray scattering

Thin Si films, with thicknesses between 100 and 400 nm, were deposited by radio frequency plasma enhanced chemical vapour deposition in silane gas (SiH4) highly diluted by hydrogen. The growing conditions were varied to obtain different degrees of crystal fractions and a variety in individual crystal sizes. The crystalline to amorphous volume fraction, as estimated by Raman spectroscopy, varied from 5 to 45% while the individual crystal sizes varied from 2 to 8 nm. The average density of the samples was estimated by using near infrared spectroscopy and the effective medium approximation. All samples were porous and contained void volume fraction between 15 and 25%. Grazing-incidence small-angle X-ray scattering has been performed at the ELETTRA synchrotron radiation source (Trieste, Italy). The scattering patterns of all examined samples indicate the presence of `particles' in the `bulk' of the thin films with gyration radii in the range of 2 to 5 nm. The higher values were found for the samples with a higher crystalline fraction. The size and the size distribution of `particles' depend upon the deposition conditions. The samples which had been deposited with a higher discharge power and a lower silane fraction had larger particles and the roughness of their surface was higher.

Exploration of the deposition limits of microcrystalline silicon

Abstract The effect of various process parameters on the deposition rate of microcrystalline hydrogenated silicon is presented. The various pathways leading to high deposition rates involve the optimization of a combination of parameters, while maintaining the crystalline character of the film and avoiding the presence of particles. The deposition rate increases in conditions that enhance and at the same time “cool down” the electron population. Such conditions are: moderately higher frequencies, higher pressures, and the presence of larger molecules (like disilane) even in small quantities. There is an optimum pressure, determined by primary dissociation, for a certain silane fraction. In addition, silane fraction and power density must also increase up to the limit of transition to amorphous growth and before attachment becomes too important. In all of these cases, there is a need for optimizing the distance of the deposition substrate to the source of radical generation, especially at higher pressures, to further increase the already important contribution of higher silicon radicals. It is shown that similar deposition rates can be obtained via radical fluxes with very different compositions. In every case, there is a need for sufficient H atom fluxes to ensure crystalline growth.

Plasma emission diagnostics for the transition from microcrystalline to amorphous silicon solar cells

Abstract The possibility to employ spatially resolved optical emission spectroscopy (SROES) as a diagnostic tool for the prediction of the transition from microcrystalline to amorphous silicon growth was investigated. The transition was achieved by increasing the silane fraction in the mixture and was identified through the solar cell performance. A drastic change of the shape of the emission profiles, characterized by an enhancement of the production of species closer to the substrate, was observed in the transition region when increasing the silane fraction. Calculations of the probability of various species to reach the surface have shown that the change of the shape of the radical generation distribution in space finally leads to an increase of the contribution of highly reactive, highly sticking radicals like SiH2 to the film growth. On the other hand less reactive species like H atoms are less affected by the shape of their generation profiles. Their probability to reach the surface drops because of the increase of the collision frequency. Both these factors can explain the transition to amorphous silicon growth and the relation between emission profiles and the transition indicating a clear potential for using SROES in thin film solar cell performance optimization.

Absolute silicon monoxide density measurement by self-absorbed spectroscopy in a non-thermal atmospheric plasma

In this study we have estimated the density of SiO molecules produced in atmospheric dielectric barrier discharges by means of spectroscopic investigations based on the self-absorption diagnostic. The (2, 0), (1, 0) and (0, 0) ro-vibrational bands of the SiO(A 1Π–X 1Σ+) electronic transition were investigated. The effective values of Einstein coefficients and absorption cross sections for the considered rotational transitions were calculated. The SiO concentration during the plasma phase was estimated by comparisons between experimental and computed spectra for the three explored bands. For example, the estimated value was (5 ± 1) × 1013 cm−3, for (Ar/SiH4/N2O) = (98.43%/0.38%/1.19%). The conversion rate from silane to silicon monoxide was deduced as a function of silane fraction. The influence of the molecular fraction on the rotational temperature was investigated. A simple technique based on the ratio of band heads was presented as a useful tool, in order to roughly estimate the density of SiO.

Influence of catalyzer area and design on the growth of intrinsic hot-wire CVD thin-film silicon for photovoltaic applications

ABSTRACTRecently, hot-wire deposited microcrystalline silicon has attracted increasing attention. The use of hot-wire deposited intrinsic μc-Si:H for high efficiency solar cells was demonstrated by Klein et al. [1]. Integration of high-quality intrinsic μc-Si:H into all-hot-wire nip solar cells, prepared close to the transition to amorphous growth using a tantalum catalyzer, resulted in initial and stable efficiencies of 5.4 % on simple stainless steel substrates [2]. However, the deposition rates for the absorber material in both cases remained low, at values around 1 Å/s.In the present study we report on the dependence of deposition rate and material quality on the design and area of the tantalum catalyzer. It was found that different filament geometries require considerable changes in certain deposition conditions to optimize material properties. So, for example, enlarging the catalyzer surface made it necessary to decrease the hydrogen dilution of the process gas, in order to obtain the desired microcrystalline material close to the phase transition. These changes might be understood in terms of alterations of the gas decomposition relations on the catalyzer surface. For these modified conditions, deposition rates in the range of 2.5-10 Å/s could be achieved for μc-Si:H due to the fact that a higher silane fraction of the process gas could be used. For different wire geometries, the optimized intrinsic layers were incorporated into solar cells. Using a catalyzer with modified geometry and enlarged surface area, conversion efficiencies of ν = 4.4 % could be achieved for all-μc-Si:H, all-HWCVD solar cells at a rate of about 3 Å/s.

Studies on the formation of microcrystalline silicon with PECVD under low and high working pressure

The formation of microcrystalline silicon films has been studied under both low and high pressures with plasma enhanced chemical vapor deposition (PECVD). Under low pressure, the growth rate of films may be enhanced by increasing the silane fraction in the reaction gas, or by increasing the input power density. However, both of these methods tend to inhibit the growth of microcrystalline films and promote the growth of amorphous phase. Under high pressure, the microcrystalline can be formed as the silane is depleted by increasing the power density. In this paper, the mechanism of microcrystalline formation of the films has been analyzed based on the observations of the optical emission intensities of H α and Si* in the deposition plasma. It has also been found that the crystallinity near the interface between the deposited film and the substrate can be significantly improved for films deposited under high pressure.

Structure and optical properties of Si incorporated diamond-like carbon films deposited by r.f. plasma-assisted chemical vapor deposition

Silicon incorporated diamond-like carbon (Si-DLC) films were deposited by 13.56 MHz r.f. plasma assisted chemical vapor deposition (RF-PACVD), and the optical properties of the films were examined by spectroscopic ellipsometry analysis. In particular, the dispersion model developed by Jellison and Modine was used to determine the optical constants by fitting the measured ellipsometric spectra in the 300–1200 nm range. Various mixtures of benzene and diluted silane (SiH 4:H 2=10:90) were used as reaction gases, and the Si concentration in the film was found to increase monotonically up to 17 at.% as the diluted silane fraction in the reaction gas was increased from 0 to 95%. However, the variation of the optical gap exhibited distinctive silicon-concentration dependence; the minimum optical gap was found from the film with 5 at.% silicon concentration. The observed changes of the optical properties were attributed to the modification of the atomic bonding structures induced by silicon incorporation. In particular, the change in SP 2-bonded carbon cluster sizes and the interlinking between SP 2 clusters are discussed.

Structural dependence of mechanical properties of Si incorporated diamond-like carbon films deposited by RF plasma-assisted chemical vapour deposition

Mechanical properties and atomic bond structure of Si incorporated diamond-like carbon (Si-DLC) films were investigated. The films were deposited by 13.56 MHz r.f.- plasma-assisted chemical vapour deposition (r.f.-PACVD), using mixtures of benzene and diluted silane (SiH 4/H 2 10:90) as the reaction gases. Si concentration in the film was varied from 0 to 17 atomic (at.)% by increasing the diluted silane fraction from 0 to 95%. It was observed that the mechanical properties of the film changed significantly when the Si concentration was less than 5 at.%. In this concentration range, hardness, residual stress and elastic constants increased with increasing Si concentration. For higher concentrations of Si, the mechanical properties showed saturated behaviour. The changes of the mechanical properties are discussed in terms of the content of three-dimensional inter-links of the atomic bond network.

Tribochemical Reactions of Si Incorporated Diamond-Like Carbon Films During the Initial Transient Period

ABSTRACTTribochemical reactions between Si incorporated diamond-like carbon (Si–DLC) films and steel ball were investigated during initial stage of tribo-test. The films were deposited by r.f.–PACVD using mixtures of diluted silane (SiH4:H2=10:90) and benzene gases. Si concentration in the film was varied from 0 to 10 at.% by adjusting the diluted silane fraction in the reaction gases. A rotating type ball-on-disk wear rig was employed for the tribo-test in ambient atmosphere. When the Si concentration was less than 5 at.%, initial transient period of high friction coefficient was commonly observed. After the transient period, the friction coefficient becomes lower with increasing contact cycles. The initial transient period becomes shorter and the starting and maximum friction coefficients in transient period decreased with increasing Si concentration. Composition of the debris on the wear scar surface was analyzed by Auger spectroscopy at various stages in the initial transient period. We observed that when the friction coefficient increased in earlier stage of the transient period, iron and oxygen was observed in the debris. However, decrease in the friction coefficient in the later stage of the transient period was associated with the formation of silicon rich oxide debris. This result supports the friction mechanism of Si-DLC films that the formation of Si rich oxide debris results in low friction coefficient in ambient atmosphere.

Optical Properties of Amorphous-Microcrystalline Mixed-Phase Si:H Films

Amorphous-microcrystalline mixed-phase hydrogenated silicon (µc-Si:H) films have been prepared at a substrate temperature of 100°C by glow discharge decomposition of mixed SiH 4 / H 2 gas. We present the results of optical properties of these films as a function of silane fraction, i.e., optical gap energy, the characteristic energy representing the slope of the exponential tail of optical absorption coefficient and the refractive index in the infrared region and photoluminescence properties characterized by photoluminescence peak energy and the value of the full width at half maximum in photoluminescence spectra. It is also suggested from the light-induced effects in a-Si:H and µc-Si:H that those effects in photoluminescence is intimately associated with amorphous network.

Effect of hydrogen dilution on the growth of hydrogenated amorphous silicon studied by in-situ phase-modulated ellipsometry

We have conducted a study of the growth of hydrogenated amorphous silicon (a-Si:H) thin films, deposited in SiH 4H 2 r.f. discharges, through spectroscopic and real-time phase-modulated ellipsometry. The study focused on the microstructure of the a-Si:H-crystalline Si interface and the a-Si:H bulk layer. Real-time data were analysed using a theoretical growth model, previously proposed, which assumes cylindrical nucleation, coalesence and long-term growth phases. For high silane fractions (above 60%) the results are compatible with the existence of an abrupt interface, whereas for low silane fractions (20%) the real-time ellipsometric data can be modelled assuming the existence of an interface, 1.2 nm thick, composed of SiO 2 (95%) and a-Si:H (5%). The ellipsometric results indicate an optimal degree of silane fraction for which the bulk void fraction is minimized. The results are discussed in terms of growth kinetics.

Deposition of Polycrystalline Silicon thin Films by Plasma Enhanced CVD

ABSTRACTPolycrystalline silicon films have been grown using high frequency (110 MHz) rf plasma enhanced chemical vapor deposition with a hydrogen diluted silane gas. Polycrystalline growth was obtained when the silane fraction was less than 10% of the total gas mixture for all rf powers, in marked contrast to growth at 13.56 MHz where high power and more dilution are typically required for microcrystalline growth. Grains with [111] and [220] orientations were observed to have grain sizes up to 900 Angstroms. Room temperature free carrier concentrations as low as 5 × 1015 cm-3 for undoped films were determined by capacitance voltage measurements. Boron doped microcrystalline films were grown with conductivity as high as 8 (Ωcm) V

Simulation of the bulk and surface properties of amorphous hydrogenated silicon deposited from silane plasmas

A Monte Carlo simulation for the growth of amorphous hydrogenated silicon (a-Si:H) thin films by plasma enhanced chemical vapor deposition is presented. The goal of the model is to predict the bulk and surface properties of films (e.g., hydrogen content, deposition rate, buried hydride/dihydride ratios, porosity, and surface roughness) having thicknesses of ≲2000 Å. The effects on the film properties of the composition of the radical flux incident on the surface are examined. Film properties were found to be critically dependent on the ratio of SiH3/SiH2 in the radical flux. High values for this ratio results in film properties resembling chemical vapor deposition. Film properties obtained with small values resemble physical vapor deposition. Rough films (roughness >10’s of Å) accordingly result from radical fluxes having high SiH2 fractions. We find that surface roughness and hydrogen fraction increase with increasing growth rate and increasing film thickness, though thin films (<10’s of layers) have large hydrogen fractions due to there being a hydrogen rich surface layer with a thickness approximately equal to the surface roughness. We also find a correlation between porosity (subsurface voids) and hydrogen fraction, implying that the inner surfaces of voids are lined with 3/4 SiH configurations. In simulating films grown from Ar/SiH4 gas mixtures, we find that a decrease in the SiH3/SiH2 ratio in the radical flux, and an increase in the ion/radical ratio incident on the surface are largely responsible for the degradation of film properties observed with decreasing silane fraction.

Effect of Hydrogen Dilution on Structure of a-Si:H Prepared by Substrate Impedance Tuning Technique

The effect of hydrogen dilution of a source gas, SiH4, on structural inhomogeneity has been studied for hydrogenated amorphous silicon films prepared under the mode involving a high deposition rate by the substrate impedance tuning technique. As the silane fraction decreases from 100% to 1% with hydrogen dilution, the SiH bonding configuration gradually changes into the dihydride (SiH2) group; finally, the films become microcrystalline. On the other hand, the state density in the mid gap and the spin density, related to the structural disorder, have minima at silane fractions of 100% and 10%. These results seem to be consistent with a two-phase model suggested for high rate films, with an emphasis on a different medium range disorder between the films prepared at high and low deposition rates.