Formation Of Microcrystalline Silicon Research Articles

Effects of H2 and Ar flow rates on the deposition of hydrogenated silicon thin films by an inductive coupled plasma-chemical vapor deposition system

Amorphous hydrogenated silicon films were deposited on quartz substrates in an inductive coupled plasma-chemical vapor deposition system with four internal low inductance antennas units. Different Ar and hydrogen flow rates were tested for their influences on the structures of deposited films. For monitoring purposes, Langmuir probe and optical emission spectrometer were installed to detect the variation of electrical field in plasma during deposition. Data from Langmuir probe and optical emission spectrometer were analyzed subsequently. After deposition, the films were examined by X-ray diffraction and Raman spectrometer for their microstructures. Results indicate that under the supply of pure Ar flow, the deposition rate can be expedited to 3.5nm/s and amorphous films were formed on quartz substrates. With the supply of mixed hydrogen and argon (Ar 15sccm+H2 50sccm+SiH4 50sccm), the deposition rate can reach 4.5nm/s. Although it is well known that a high supply of H2 helps the formation of micro-crystalline silicon, these deposited hydrogenated Si films, confirmed by X-ray diffraction patterns and Raman spectroscopy, all maintained their amorphousness under various range of Ar and H2 flow rates.

Microcrystalline silicon formation by silicon nanoparticles

Thin silicon films are of great importance for large-area electronic applications, for example, as the basis for switching electronics in flat-panel display devices or as the active layer of solar cells. In this paper, we show that silicon nanoparticles have the potential to be used as raw material for further processing toward a microcrystalline silicon film. This can be done by thermal treatment with a reduced thermal budget because the melting point of the nanoparticles is much lower with only 60% of the equilibrium melting temperature of silicon. Coagulation processes of liquid droplets then lead to the growth of microcrystalline silicon in agglomerated nanoparticles. We demonstrate by in situ transmission electron microscopy (TEM) and differential thermal analysis that silicon nanoparticles with a size of approximately 20nm start melting at around 1000K; furthermore, the TEM observations directly demonstrate the details of the coagulation process leading to microcrystalline silicon.

Formation of microcrystalline silicon and SiN x films by electron-beam-induced-chemical vapor deposition at ultra low temperature

We synthesized silicon films (hydrogenated microcrystalline silicon in hydrogenated amorphous silicon) and silicon nitride films by electron-beam-induced-chemical vapor deposition combined with low-temperature H tunneling reactions on cooled substrates, which adsorb source gases (SiH 4 or Si 2H 6). The photoconductivity of the silicon film was 0.7 × 10 − 5 Ω − 1 cm − 1 and the dark conductivity was 3.3 × 10 − 5 Ω − 1 cm − 1 . The dielectric constant of the silicon nitride film was estimated to be 6.5–7.0.

Influence of hot wire on the formation of microcrystalline silicon in MWECR–CVD system

The high-crystallinity and low-defect-density microcrystalline silicon films ( μc-Si:H) was prepared by using a new hot-wire-assisted microwave electron cyclotron resonance–chemical vapor deposition (HWAMWECR–CVD) system. In this system the hot wire plays an important role in suppressing the growth of a-Si:H in favor of μc-Si:H, thus improving the physical properties. The experimental results show that the μc-Si:H film prepared by using this new system, the crystalline volume fraction is increased from 16.4% to 63.2%, the photoconductivity is increased by two orders of magnitude, the optical band gap is decreased to 1.59 eV, and the light-induced degradation keeps almost constant compared to that prepared by conventional system.

Amorphous solar cells, the micromorph concept and the role of VHF-GD deposition technique

During the last two decades, the Institute of Microtechnology (IMT) has contributed in two important fields to future thin-film silicon solar cell processing and design: (1) In 1987, IMT introduced the so-called “very high frequency glow discharge (VHF-GD)” technique, a method that leads to a considerable enhancement in the deposition rate of amorphous and microcrystalline silicon layers. As a direct consequence of reduced plasma impedances at higher plasma excitation frequencies, silane dissociation is enhanced and the maximum energy of ions bombarding the growing surface is reduced. Due to softer ion bombardment on the growing surface, the VHF process also favours the formation of microcrystalline silicon. Based on these beneficial properties of VHF plasmas, for the growth of thin silicon films, plasma excitation frequencies f exc in the range 30–300 MHz, i.e. clearly higher than the standard 13.56 MHz, are indeed scheduled to play an important role in future production equipment. (2) In 1994, IMT pioneered a novel thin-film solar cell, the microcrystalline silicon solar cell. This new type of thin-film absorber material––a form of crystalline silicon––opens up the way for a new concept, the so-called “ micromorph” tandem solar cell concept. This term stands for the combination of a microcrystalline silicon bottom cell and an amorphous silicon top cell. Thanks to the lower band gap and to the stability of microcrystalline silicon solar cells, a better use of the full solar spectrum is possible, leading, thereby, to higher efficiencies than those obtained with solar cells based solely on amorphous silicon. Both the VHF-GD deposition technique and the “micromorph” tandem solar cell concept are considered to be essential for future thin-film PV modules, as they bear the potential for combining high-efficiency devices with low-cost manufacturing processes.

Formation of microcrystalline silicon at low temperatures and role of hydrogen

Changes in the growth of thin silicon films at very low substrate temperatures (35 °C < T S < 100 °C) near the a-Si:H/μc-Si:H transition region are illustrated using RMS roughness, crystallinity and hydrogen content measurements together with the corresponding evolution of the optoelectronic properties: dark conductivity σ D, its activation energy E a and prefactor σ 0 as well as the ambipolar diffusion length L diff. The role of hydrogen content, which increases at lower T S, is studied in relation to grain boundary formation.

Plasma deposition of thin film silicon: kinetics monitored by optical emission spectroscopy

The optical emission spectroscopy technique is used to characterise the temporal behaviour of a pure silane plasma in the first 90 s after ignition of a static closed-chamber very high frequency glow discharge. Special interest is drawn to the formation of microcrystalline silicon (mc-Si:H) in absence of any hydrogen feedstock gas dilution. The kinetics of the emission lines of SiH n and Ha is reported. The deposited films are characterised by photothermal deflection spectroscopy, Fourier transform infra red (FT-IR) absorption and show typical microcrystalline fingerprints; for the first time, such material is used as absorber layer in n–i–p type solar cell devices.

Growth of microcrystalline silicon films using deuterium dilution

The role of atomic hydrogen in microcrystalline silicon formation by plasma enhanced chemical vapor deposition is studied by deuterium dilution, SiH 4/D 2, in comparison with hydrogen dilution. The ratio of SiD* and SiH* emission intensities, SiD*/SiH*, observed in the plasma increases with increasing input power, indicative of increasing number of H–D exchange events. The ratio of deuterium to hydrogen content, D/H, in the film increases with increasing SiD*/SiH* in amorphous phase, while D/H remains constant in microcrystalline films. This implies the difference in hydrogen surface reaction in microcrystalline silicon growth from amorphous silicon growth. The mechanism of these reactions is discussed by investigating temperature dependence of D/H.

Effect of oxygen contamination on the properties of the silicon hydrogen alloy materials deposited under conditions near the microcrystalline silicon formation region

Two sets of films of silicon hydrogen alloy (Si:H) were deposited by plasma enhanced chemical vapor deposition (PECVD) method from silane mixed with hydrogen or argon under the deposition conditions suitable for producing microcrystalline silicon (μc-Si:H) growth. The sets were repeated in two different chambers: one with ultra high vacuum capability having a load lock chamber and the other a single chamber unit without any load lock. The samples deposited in the single chamber unit were found to be contaminated with oxygen. It was observed that the incorporation of unintentional oxygen in the film affects the material properties in a different way for hydrogen or argon dilution. In the case of hydrogen dilution the oxygen contamination results in a shift of the stretching mode frequencies of SiH and SiO bonds and the microcrystalline grain formation is also inhibited by the oxygen impurity. With argon dilution the first effect is present but μc-Si:H growth is not much affected by the oxygen contamination.

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.

In situ observation of low temperature growth of crystalline silicon using reflection high-energy electron diffraction

In situ observations of reflection high-energy electron diffraction (RHEED) have been performed to investigate the microcrystalline silicon formation mechanism in plasma-enhanced-chemical-vapor-deposition (PECVD) using a mixture of SiH 4 and H 2 at a temperature T s between 27°C and 560°C. In low temperature epitaxy (LTE) on Si(0 0 1), we found that the epitaxial structure of films as a function of temperature had, at 120°C, a surface structure Si1 × 1 and at 430°C Si2 × 1, depending on hydrogen dilution. This bistable epitaxy is mediated by the surface hydrogen mode. In polycrystalline silicon growth on a ZnO coated glass substrate, we observed in situ the change in the order and disorder of the structure and the change in the orientation during the growth. These results can be utilized to control crystal growth by a real-time feed-back to deposition parameters. Thus, we have demonstrated that in situ reflection high-energy electron diffraction observation is useful for the study of microcrystalline silicon growth.

The role of hydrogen in the formation of microcrystalline silicon

The growth mechanisms of microcrystalline silicon thin films at low temperatures (100–250°C) by plasma CVD are still a matter of debate. We have shown that μc-Si:H formation proceeds through four phases (incubation, nucleation, growth and steady state) and that hydrogen plays a key role in this process, particularly during the incubation phase in which hydrogen modifies the amorphous silicon network and forms a highly porous phase where nucleation takes place. In this study we combine in-situ ellipsometry and dark conductivity measurements with ex-situ high resolution transmission electron microscopy to improve our understanding of microcrystalline silicon formation.

Connection between hydrogen plasma treatment and etching of amorphous phase in the layer-by-layer technique with very high frequency plasma excitation

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.

Role of Hydrogen for Microcrystalline Silicon Formation

ABSTRACTThe role of hydrogen atoms in the formation process of hydrogenated microcrystalline silicon (μc-Si:H) by plasma enhanced chemical vapor deposition method has been investigated. Under the present conditions, the etching and the permeation of hydrogen atoms in the subsurface region do not cause the crystallization. The kinetics study of surface morphology and structure in the initial growth of μc-Si:H on an atomically flat substrate indicates that the onset thickness of island coalescence reduced under μc-Si:H formation condition. The results support the ‘surface diffusion model’ in which the surface diffusion of film precursors is enhanced by the sufficient hydrogen coverage of surface and by hydrogen atom recombination energy on the growing surface of the film.

Substrate selectivity in the formation of microcrystalline silicon: Mechanisms and technological consequences

We report the results of an in situ spectroscopic ellipsometry study concerning the substrate dependence of the evolution of microcrystalline silicon films deposited by alternating amorphous silicon deposition and hydrogen plasma treatment. The evolution of the composition of the films during growth, up to thicknesses of ∼100 nm, indicates that besides etching, the diffusion of atomic hydrogen efficiently promotes the growth (and/or nucleation) of buried crystallites. Moreover, the evolution of the films strongly depends on the nature of the substrate. This substrate selectivity is discussed in terms of initial growth processes. The effect of the hydrogen plasma well below the film surface, which produces the thickness-dependent film composition, along with the substrate selectivity, may be of prime importance in technological applications of microcrystalline silicon.

Role of Hydrogen Plasma during Growth of Hydrogenated Microcrystalline Silicon: In Situ UV-Visible and Infrared Ellipsometry Study

We have applied in situ UV-visible and infrared phase-modulated ellipsometry to investigate the role of hydrogen plasma during the growth of hydrogenated microcrystalline silicon ( µc-Si:H) by plasma-enhanced chemical vapor deposition (PECVD). The results of the deposition of µc-Si:H from the SiH4 highly diluted in H2, layer-by-layer (LbL) technique and post-hydrogenation experiments showed that the 3-dimensional cross-linking and relaxation of a Si network near the growing surface were essential for the formation of microcrystalline silicon. The major role of hydrogen plasma is the creation of the free volumes on the growing surface due to the inhomogeneous etching of the Si network and the promotion of the cross-linking reactions.

Hydrogen incorporation in silicon thin films deposited with a remote hydrogen plasma

Results are presented on the effects of growth conditions on hydrogen incorporation in Si thin films deposited with a remote hydrogen plasma. Oxygen contamination of the films was significantly reduced by replacement of the quartz tube that is commonly used to contain the hydrogen plasma with an alumina tube, with a concomitant increase in the electrical conductivity of P-doped a-Si:H films. Hydrogen incorporation was examined with a remote deuterium plasma and downstream injection of SiH4. As the gas flow ratio D2:SiH4 increases, the ratio D:H in the film changes as a consequence of the increasing flux of D at the growing surface. High silane dilution also promotes the formation of microcrystalline silicon, which itself affects H incorporation.