Remote plasma deposition of microcrystalline silicon thin-films: Film structure and the role of atomic hydrogen

Microcrystalline silicon films grown in an expanding thermal plasma, i.e. in the absence of ion bombardment, are found to be porous and rich in nano-sized voids. By carrying out an extensive investigation on the material quality of films deposited in the amorphous-to-microcrystalline transition regime, on the microcrystalline silicon growth development, and on the influence of the substrate temperature, it is concluded that the inferior material quality is related to the lack of a sufficient amount of amorphous silicon tissue. As possible cause for the insufficient amount of amorphous silicon tissue, the interaction of atomic hydrogen with amorphous silicon films has been studied in order to highlight a possible competition between film growth and H-induced etching of amorphous silicon, and between film growth and H-induced surface/film modification. The etch rates obtained are too low to compete with film growth. Furthermore, atomic H cannot be considered responsible for the poor quality of amorphous tissue present in the microcrystalline silicon films, as the H up-take mainly takes place in divacancies. These results suggest that ion bombardment may be a necessary condition to provide good quality microcrystalline silicon films.     

New insights in microcrystalline silicon deposition with expanding thermal plasma chemical vapor deposition

We report on further insights in the microcrystalline silicon (μc-Si:H) deposition using expanding thermal plasma chemical vapor deposition. We have shown before that the refractive index at 2 eV of μc-Si:H layers increased if the silane (SiH₄) was injected close to the substrate, while the deposition rate remained the same. We argued that at high injection-ring position, the SiH₄ travels a long way to the substrate and therefore has a long interaction time with the plasma, in particular atomic hydrogen. In this way, the SiH₄ injection position influences the number of hydrogen atoms stripped from the SiH₄ as well as the consumption of atomic hydrogen. In this paper, we present an analysis of the growth flux of depositing particles as function of the radical production rate. The data suggest that there is no dependence on the SiH₄ injection position, implying that the mix of depositing radicals is not changed. However, the data also show the microcrystalline-to-amorphous transition shifts to higher SiH₄ flows for lower injection positions. We therefore now think that it is not the interaction time between the SiH₄ and the arc plasma determining the material properties, but the interaction of excess atomic hydrogen with the μc-Si:H growth surface.     

Transport properties of microcrystalline silicon,prepared at high growth rate

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.     

Deposition of microcrystalline silicon films at ultrafast rate by using a new microwave induced plasma source

Synthesis of microcrystalline silicon (μc-Si) film at an ultrafast deposition rate over 100 nm/s is achieved from SiH₄ + He by using a high density microwave plasma source even without employing H₂ dilution and substrate heating techniques. Systematic deposition studies show that high SiH₄ flow rate and working pressure increase film deposition rate while high He flow rate decreases the rate. On the other hand, crystallinity of deposited Si film decreases with increasing SiH₄ or He flow rate and working pressure. Enhancements of gas phase and surface reactions during film deposition process are responsible for the achievement of high deposition rate and high film crystallinity.     

The application of very high frequency inductively coupled plasma to high-rate growth of microcrystalline silicon films

High-rate growth of microcrystalline silicon films (μc-Si:H) from inductively coupled plasma (ICP) of H₂ diluted SiH₄ generated with a very high frequency (VHF: 60 MHz) power source has been studied from the viewpoint of efficient gas dissociation. From the VHF power and gas pressure dependences of the film growth rate and optical emission intensities, we have found that the Si and SiH emission intensities and the intensity ratio of H_α to SiH are good indicators for the film growth rate and crystallinity, respectively. The generation rate of film precursors is reflected by the Si and SiH emission intensities while the flux ratio of atomic hydrogen to film precursors, which plays an important role on the structural relaxation for the crystalline network formation, is characterized by the intensity ratio of H_α to SiH. An increase in SiH emission while keeping the intensity ratio of H_α to SiH at a certain level enables us to enhance the film growth rate without significant deterioration in the crystallinity. In this study, a growth rate as high as 10 nm/s was obtained for highly crystallized films.     

Substrate holder biasing for improvement of microcrystalline silicon deposition process

Application of a dual frequency plasma source for the deposition of microcrystalline silicon thin films from highly diluted SiH₄/H₂ was investigated in this paper. A positively or negatively biased low frequency voltage was applied on the substrate holder while the conventional frequency of 13.56 MHz was used for the powered electrode. The results show a significant increase of the deposition rate and an improvement of the film crystallinity in the case of the positive biasing. Plasma diagnostics and modeling were used to understand the beneficial effect of positive biasing on the deposition process. The results revealed that the observed changes are not only due to the variation of ion flux and ion bombardment but also depend on the changes in the production and distribution of neutral species in the discharge space.     

Preparation and properties of microcrystalline silicon films using photochemical vapor deposition

microcrystalline silicon films have been prepared through mercury photosensitized decomposition of monosilane at low gas pressures. The dark and light conductivities of the silicon films tend to increase at reactant pressures lower than 65 Pa and become 10^?2Ω^?1· cm^?1 at 26 Pa. From the Raman scattering and x-ray diffraction, silicon films were found to consist of a mixed phase structure including both microcrystalline and amorphous regions.     

Determination factor of <110> direction growth in microcrystalline silicon thin film deposition

We have proposed the mechanism of the <110> directional growth of microcrystalline silicon (μc-Si) thin films deposited by PECVD (plasma enhanced chemical vapor deposition) from SiH₄ and H₂ gas mixture, where dimeric radicals act a key role to form bridge nuclei for the ledge formation on the (110) facet. In order to look further into details of the mechanism, we investigated other important factors that influence the growth of μc-Si in <110> direction in terms of their impact on crystallinity with varying deposition temperature. The enhancement of surface diffusion length of radicals is inferred from the enlargement of the crystalline grain size accompanied with the increase of the deposition temperature. The growth in <110> direction is also promoted as the deposition temperature increases. Therefore, it is suggested that the surface diffusion length of radicals is another key factor that governs the crystalline growth in <110> direction. The growth mechanism of μc-Si thin films in <110> direction is discussed in terms of the relation between the surface diffusion length of monomeric radicals depending on the substrate surface temperature and the average space of bridges depending on the density of dimeric radicals on the growing surface.     

High-rate deposition of nanocrystalline silicon using the expanding thermal plasma technique

Nanocrystalline silicon films have been deposited at very high deposition rates using the expanding thermal plasma technique and their structural properties have been analyzed. The crystallinity and crystallite size and orientation have been determined for various hydrogen-to-silane dilution ratios and it is shown that films with a crystalline fraction of 60–80% can be deposited at deposition rates within the range 1.5–3.0 nm/s. The hydrogen concentration and atomic densities in the film have been investigated by infrared spectroscopy and elastic recoil detection/Rutherford backscattering revealing underestimation of the hydrogen content by infrared spectroscopy as well as a reduced atomic film density for the nanocrystalline silicon films.     

Influence of crystalline volume fraction on the performance of high mobility microcrystalline silicon thin-film transistors

The influence of the crystalline volume fraction of hydrogenated microcrystalline silicon on the device performance of thin-film transistors fabricated at temperatures below 200 °C was investigated. Transistors employing microcrystalline silicon channel material prepared close to the transition to amorphous growth regime exhibit the highest charge carrier mobilities exceeding 50 cm²/V s. The device parameters like the charge carrier mobility, the threshold voltage and the subthreshold slope will be discussed with respect to the crystalline volume fraction of the intrinsic microcrystalline silicon material.     

Substrate temperature dependence of microcrystalline silicon growth by PECVD technique

Undoped hydrogenated silicon films have been prepared from a gas mixture of silane and hydrogen, varying substrate temperature from 180–380 °C in an ultrahigh vacuum system using RFPECVD technique. XRD and Raman measurements enable us to know that the films are microcrystalline throughout the substrate temperature range. Bond formation of the SiH films at different substrate temperature is studied through different characterisation techniques like Fourier transform infrared spectroscopy and hydrogen evolution study. The infrared absorption spectroscopy and hydrogen evolution study reveal two types of growth: the formation of a void rich material at low T_s (∼180 °C) and a compact material at comparatively higher T_s.     

Electron spin resonance studies of microcrystalline and amorphous silicon irradiated with high energy electrons

Paramagnetic defects in μc-Si:H and a-Si:H with various structure compositions were investigated by electron spin resonance (ESR). The defect density was varied by high energy electron bombardment and subsequent annealing. The spin density increases by up to 3 orders of magnitude. In most cases the initial spin density can be restored upon annealing at 160 °C.     

Toward the fast deposition of highly crystallized microcrystalline silicon films with low defect density for Si thin-film solar cells

In this study, employing a high-density, low-temperature SiH₄–H₂ mixture microwave plasma, we investigate the influence of source gas supply configuration on deposition rate and structural properties of microcrystalline silicon (μc-Si) films, and demonstrate the plasma parameters for fast deposition of highly crystallized μc-Si films with low defect density. A fast deposition rate of 65 Å/s has been achieved for a SiH₄ concentration of 67% diluted in H₂ with a high Raman crystallinity of X_c > 65% and a low defect density of (1–2) × 10¹⁶ cm⁻³ by adjusting source gas supply configuration and plasma conditions. A sufficient supply of deposition precursors, such as SiH₃, as well as atomic hydrogen H on film growing surface is effective for the high-rate synthesis of highly crystallized μc-Si films, for the reduction in defect density, and for the improvement in film homogeneity and compactability. A preliminary result of p–i–n structure μc-Si thin-film solar cells using the resulting μc-Si films as an intrinsic absorption layer is presented.     

Thermodynamics of AlN deposition by means of the aluminium-trichloride-ammonia process

Equilibrium yields for CVD of AlN dependent on the input ratio AlCl₃/NH₃ and on temperature were computed (A) for AlCl₃ + NH₃ ⇋ AlN + 3 HCl; (B) for additional complexing AlCl₃ + NH₃ ⇋ AlCl₃ · NH₃ at substrate temperature which did not result in changes of yield above 900°C; (C) for AlCl₃ · NH₃ ⇋ AlN + 3 HCl corresponding to complete inhibition of dissociation near the substrate of the complex formed or preformed at lower temperature which resulted in considerably lower yields. A corresponding decrease of experimental results occurred within reactors having a long AlCl₃–NH₃-mixing zone at 350°C or AlCl₃ · NH₃-input with respect to a reactor with a short mixing zone near to the substrate. The dissociation of the complex present in the former two cases was inhibited considerably. – This investigations demonstrate the additional influence of homogeneous reactions, which has to be regarded for CVD especially with complexing reactants in connection with the role of reactor geometry, a problem being increasingly discussed at present.     

Low-temperature deposition of crystalline silicon nitride nanoparticles by hot-wire chemical vapor deposition

The nanocrystalline alpha silicon nitride (α-Si₃N₄) was deposited on a silicon substrate by hot-wire chemical vapor deposition at the substrate temperature of 700 °C under 4 and 40 Torr at the wire temperatures of 1430 and 1730 °C, with a gas mixture of SiH₄ and NH₃. The size and density of crystalline nanoparticles on the substrate increased with increasing wire temperature. With increasing reactor pressure, the crystallinity of α-Si₃N₄ nanoparticles increased, but the deposition rate decreased.     

On the oxidation mechanism of microcrystalline silicon thin films studied by Fourier transform infrared spectroscopy

Insight into the oxidation mechanism of microcrystalline silicon thin films has been obtained by means of Fourier transform infrared spectroscopy. The films were deposited by using the expanding thermal plasma and their oxidation upon air exposure was followed in time. Transmission spectra were recorded directly after deposition and at regular intervals up to 8 months after deposition. The interpretation of the spectra is focused on the Si-H_x stretching (2000-2100 cm⁻¹), Si-O-Si (1000-1200 cm⁻¹), and O_xSi-H_y modes (2130-2250 cm⁻¹). A short time scale (< 3 months) oxidation of the crystalline grain boundaries is observed, while at longer time scales, the oxidation of the amorphous tissue and the formation of O-H groups on the grain boundary surfaces play a role. The implications of this study on the quality of microcrystalline silicon exhibiting no post-deposition oxidation are discussed: it is not sufficient to merely passivate the surface of the crystalline grains and fill the gap between the grains with amorphous silicon. Instead, the quality of the amorphous silicon tissue should also be taken into account, since this oxidation can affect the passivating properties of the amorphous tissue on the surface of the crystalline silicon grains.     

Low temperature plasma deposition of silicon thin films: From amorphous to crystalline

We report on the epitaxial growth of crystalline silicon films on (100) oriented crystalline silicon substrates by standard plasma enhanced chemical vapor deposition at 175 °C. Such unexpected epitaxial growth is discussed in the context of deposition processes of silicon thin films, based on silicon radicals and nanocrystals. Our results are supported by previous studies on plasma synthesis of silicon nanocrystals and point toward silicon nanocrystals being the most plausible building blocks for such epitaxial growth. The results lay the basis of a new approach for the obtaining of crystalline silicon thin films and open the path for transferring those epitaxial layers from c-Si wafers to low cost foreign substrates.     

Metastability effects in hydrogenated microcrystalline silicon thin films investigated by the dual beam photoconductivity method

Metastability effects in microcrystalline silicon (μc-Si:H) thin films have been investigated using dark conductivity, σ_D, photoconductivity, σ_ph, and sub-bandgap absorption methods. Nitrogen and inert gasses can cause reversible aging effect in conductivities but not in the sub-bandgap absorption. However, DI water and O₂ gas treatment result in both reversible and nonreversible effects in conductivities as well as in the sub-bandgap absorption. Only oxygen affected the dark conductivity reversibly in amorphous silicon, a-Si:H, films, other results were unaffected from the aging and annealing processes applied.     

Total pressure dependence of doping element incorporation during the chemical vapour deposition of epitaxial silicon

Starting from the literature on doping element incorporation at atmospheric and lower pressures the partial pressure of the dopants available within the deposition system is selected as a uniform basis of reference for the total pressure dependence of the doping element incorporation. It is shown that also at reduced pressures the incorporation equilibria of phosphor and arsenic, the author previously derived from the temperature dependence of the doping element incorporation may be used.