Microstructures of microcrystalline silicon thin films prepared by hot wire chemical vapor deposition

Undoped hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared at low temperature by hot wire chemical vapor deposition (HWCVD). Microstructures of the μc-Si:H films with different H₂/SiH₄ ratios and deposition pressures have been characterized by infrared spectroscopy X-ray diffraction (XRD), Raman scattering, Fourier transform (FTIR), cross-sectional transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). The crystallization of silicon thin film was enhanced by hydrogen dilution and deposition pressure. The TEM result shows the columnar growth of μc-Si:H thin films. An initial microcrystalline Si layer on the glass substrate, instead of the amorphous layer commonly observed in plasma enhanced chemical vapor deposition (PECVD), was observed from TEM and backside incident Raman spectra. The SAXS data indicate an enhancement of the mass density of μc-Si:H films by hydrogen dilution. Finally, combining the FTIR data with the SAXS experiment suggests that the Si---H bonds in μc-Si:H and in polycrystalline Si thin films are located at the grain boundaries.     

Highly (100)-oriented CeO2 films prepared on amorphous substrates by laser chemical vapor deposition

CeO₂ films were prepared on amorphous silica substrates by laser chemical vapor deposition using cerium dipivaloylmethanate precursor and a semiconductor InGaAlAs (808 nm in wavelength) laser system. The laser spot size was about 20 mm, which was sufficient to cover the whole substrate. Highly (100)-oriented CeO₂ films were obtained at extraordinary high deposition rates ranging from 60 to 132 μm/h. Films exhibited a columnar feather-like structure with a large number of nano-sized voids, and a surface morphology consisting of either nearly flat or pyramidal top-ending columns depending on the laser power. Nearly flat top-ending columns could be fairly (100)-oriented at the top and (111)-oriented laterally.     

LiF enhanced nucleation of the low temperature microcrystalline silicon prepared by plasma enhanced chemical vapour deposition

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.     

Aluminum-induced crystallization of amorphous silicon films deposited by hot wire chemical vapor deposition on glass substrates

Aluminum-induced crystallization of amorphous silicon films is discussed. Amorphous Si films were deposited by hot wire chemical vapor deposition onto Al coated glass substrates at 430 °C. Complete crystallization of a-Si films was achieved during a-Si deposition by controlling Al and Si layer thicknesses. The grain structure of the poly-Si films formed on glass substrate was evaluated by optical and electron microscopy. Continuous poly-Si films were obtained using Al layers with a thickness of 500 nm or less. The average grain size was found to be 10-15 μm, corresponding to a grain size/thickness ratio greater than 20.     

Structural properties of microcrystalline Si films prepared by hot-wire/catalytic chemical vapor deposition under conditions close to the transition from amorphous to microcrystalline growth

The structural properties of microcrystalline Si films prepared by hot-wire/catalytic chemical vapor deposition, with various dilution ratios of silane in hydrogen, were investigated as regards to the role of hydrogen. A large surface roughness correlated with a low crystalline nuclei density was observed for microcrystalline Si films deposited near the transition from amorphous to microcrystalline growth. Investigations of hydrogen-related properties suggest the presence of molecular hydrogen in these films. We tentatively propose that the diffusion of atomic hydrogen into the subsurface layer of growing films, which leads to the relaxation of amorphous Si network and to the generation of molecular hydrogen, plays an important role for determining the film properties, besides top surface reactions.     

Silicon nanostructures fabricated by Au and SiH4 co-deposition technique using hot-wire chemical vapor deposition

In this study, the fabrication of Si nanostructures by Au and SiH₄ co-deposition technique using hot-wire chemical vapor deposition was demonstrated. A high deposition rate of 2.7 nm/s and a high density of silicon nanostructures with a diameter of about 140 nm were obtained at T_s of 250 °C. An increase in T_s led to a significant reduction in the size of the nanostructures. However, coalescence on the nanostructures was observed at T_s of 400 °C. The Si nanostructures exhibited a highly crystalline structure, which was induced by Au crystallites. The crystallite size and crystallinity of the Si nanostructures amplified with the increase in T_s. The presence of nanostructures enhanced the surface roughness of the samples and clearly reduced the reflection, especially in the visible region.     

On the photoluminescence of multilayer arrays of silicon rich oxide with high silicon content prepared by low pressure chemical vapor deposition

The photoluminescence emission of multilayer structures composed of layers of silicon rich oxide with high silicon content and layers of silicon rich oxide with low silicon content obtained by low pressure chemical vapor deposition is here presented. Different parameters for the preparation of the multilayers have been varied such as the Si concentration and the thicknesses of the layers. Additionally, the samples were oxidized at different temperatures. For all samples the photoluminescence seems to have the same origin: defects in the oxide matrix and defects at the interfaces between the Si nanocrystals. The structural and compositional properties of the multilayer structures are discussed.     

The incorporation of preformed metal nanoparticles in zinc oxide thin films using aerosol assisted chemical vapour deposition

The feasibility of Aerosol Assisted Chemical Vapour Deposition (AA-CVD) has been investigated for the growth of zinc oxide (ZnO) films containing preformed metal nanoparticles. The deposition parameters were first established for ZnO thin films, by varying the heating configuration, substrate temperature and deposition time. Films were characterised using Scanning Electron Microscopy and X-Ray Diffraction. As-deposited films, grown at 250 °C, were mostly amorphous and transformed to highly crystalline Wurtzite ZnO at higher substrate temperatures (400-450 °C). A change in the preferential orientation of the films was observed upon changing (i), the substrate temperature or (ii), the heating configuration. Following this, the applicability of the AA-CVD process for the incorporation of preformed nanoparticles (platinum and gold) in ZnO thin films was investigated. It was found that surface agglomeration occurred, such that the ZnO films were capped with an inhomogeneous coverage of the metal. These layers were characterised using Transmission Electron Microscopy and Electron Diffraction. A possible mechanism for the formation of these metal surface clusters is presented.     

Intrinsic microcrystalline silicon prepared by hot-wire chemical vapour deposition for thin film solar cells

Microcrystalline silicon (μc-Si:H) prepared by hot-wire chemical vapour deposition (HWCVD) at low substrate temperature T_S and low deposition pressure exhibits excellent material quality and performance in solar cells. Prepared at T_S below 250 °C, μc-Si:H has very low spin densities, low optical absorption below the band gap, high photosensitivities, high hydrogen content and a compact structure, as evidenced by the low oxygen content and the weak 2100 cm⁻¹ IR absorption mode. Similar to PECVD material, solar cells prepared with HWCVD i-layers show increasing open circuit voltages V_oc with increasing silane concentration. The best performance is achieved near the transition to amorphous growth, and such solar cells exhibit very high V_oc up to 600 mV. The structural analysis by Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows considerable amorphous volume fractions in the cells with high V_oc. Raman spectra show a continuously increasing amorphous peak with increasing V_oc. Crystalline fractions X_C ranging from 50% for the highest V_oc to 95% for the lowest V_oc were obtained by XRD. XRD-measurements with different incident beam angles, TEM images and electron diffraction patterns indicate a homogeneous distribution of the amorphous material across the i-layer. Nearly no light induced degradation was observed in the cell with the highest X_C, but solar cells with high amorphous volume fractions exhibit up to 10% degradation of the cell efficiency.     

Piezoresistive properties of nanocrystalline silicon thin films deposited on plastic substrates by hot-wire chemical vapor deposition

The piezoresistive property of n-type and p-type nanocrystalline silicon thin films deposited on plastic (PEN) at a substrate temperature of 150 °C by hot-wire chemical vapor deposition, is studied. The crystalline fraction decreased from 80% to 65% in p-type and from 84% to 62% in n-type films, as the dopant gas-to-silane flow rate ratio was increased from 0.18% to 3-3.5%. N-type films have negative gauge factor (− 11 to − 16) and p-type films have positive gauge factor (9 to 25). In n-type films the higher gauge factors (in absolute value) were obtained by increasing the doping level whereas in p-type films higher gauge factors were obtained by increasing the crystalline fraction.     

Ultrafast deposition of silicon nitride and semiconductor silicon thin films by hot wire chemical vapor deposition

The technology of Hot Wire Chemical Vapor Deposition (HWCVD) or Catalytic Chemical Vapor Deposition (Cat-CVD) has made great progress during the last couple of years. This review discusses examples of significant progress. Specifically, silicon nitride deposition by HWCVD (HW-SiN_x) is highlighted, as well as thin film silicon single junction and multijunction junction solar cells. The application of HW-SiN_x at a deposition rate of 3 nm/s to polycrystalline Si wafer solar cells has led to cells with 15.7% efficiency and preliminary tests of our transparent and dense material obtained at record high deposition rates of 7.3 nm/s yielded 14.9% efficiency. We also present recent progress on Hot-Wire deposited thin film solar cells. The cell efficiency reached for (nanocrystalline) nc-Si:H n-i-p solar cells on textured Ag/ZnO presently is 8.6%. Such cells, used in triple junction cells together with Hot-Wire deposited proto-Si:H and plasma-deposited SiGe:H, have reached 10.9% efficiency. Further, in our research on utilizing the HWCVD technology for roll-to-roll production of flexible thin film solar cells we recently achieved experimental laboratory scale tandem modules with HWCVD active layers with initial efficiencies of 7.4% at an aperture area of 25 cm².     

Epitaxial film growth of chromium dioxide by low pressure chemical vapor deposition using chromium carbonyl

Epitaxial chromium dioxide (CrO₂) thin films have been deposited by low pressure chemical vapor deposition (LPCVD) on (100) TiO₂ substrates using the precursor chromium hexacarbonyl (Cr(CO)₆) within a narrow temperature window of 380-400 °C. Normal θ-2θ Bragg x-ray diffraction results show that the predominant phase is CrO₂ with only a small amount of Cr₂O₃ present, mostly at the film surface. The LPCVD films have a reasonably smooth surface morphology with a root mean square roughness of 4 nm on a scale of 5 μm. Raman spectroscopy confirms the existence of rutile CrO₂ in the deposited films, while transmission electron microscopy confirms the single-crystalline nature of the films. The LPCVD films showing a dominant CrO₂ phase exhibit clear uniaxial magnetic anisotropy with the easy axis oriented along the c direction.     

Carbonized tantalum catalysts for catalytic chemical vapor deposition of silicon films

Catalytic chemical vapor deposition (Cat-CVD) has been demonstrated as a promising way to prepare device-quality silicon films. However, catalyst ageing due to Si contamination is an urgency to be solved for the practical application of the technique. In this study, the effect of carbonization of tantalum catalyst on its structure and performance was investigated. The carbonized Ta catalyst has a TaC surface layer which is preserved over the temperature range between 1450 and 1750 °C and no Si contamination occurs on the catalyst after long-term use. Si film prepared using the carbonized Ta catalyst has a similar crystal structure to that prepared by uncarbonized Ta catalyst. Formation of the TaC surface layer can alleviate the ageing problem of the catalyst, which shows great potential as a stable catalyst for Cat-CVD of Si films.     

A novel approach to the formation of amorphous carbon nitride film on silicon by ECR-CVD

Amorphous carbon nitride films have been synthesized on silicon by using an ECR-CVD system equipped with a DC bias and a mixture of C₂H₂, N₂ and Ar. Excess argon together with the application of DC bias can increase the ratio of nitrogen to carbon in the film up to 41% as determined by XPS. FTIR spectrum shows an absorption band between 1000 and 1700 cm⁻¹ which proves the incorporation of nitrogen atoms into the amorphous network of carbon. The plasma chemistry of the system was also analyzed by OES to investigate the active chemical species that were involved in the formation of carbon nitride. The result indicated that the addition of excess argon (four times more than nitrogen) can effectively enrich the excited-state CN radicals which subsequently promotes the concentration of nitrogen in the amorphous carbon nitride film. This observation is likely due to the lower ionization energy of argon (15.8 eV), argon's larger cross-section area for collision and its massive weight in comparison with the indispensable hydrogen gas as employed in the synthesis of other related materials.     

Hot wire CVD deposition of nanocrystalline silicon solar cells on rough substrates

In silicon thin film solar cell technology, frequently rough or textured substrates are used to scatter the light and enhance its absorption. The important issue of the influence of substrate roughness on silicon nanocrystal growth has been investigated through a series of nc-Si:H single junction p-i-n solar cells containing i-layers deposited with Hot-wire CVD. It is shown that silicon grown on the surface of an unoptimized rough substrate contains structural defects, which deteriorate solar cell performance. By introducing parameter v, voids/substrate area ratio, we could define a criterion for the morphology of light trapping substrates for thin film silicon solar cells: a preferred substrate should have a v value of less than around 1 × 10^- 6, correlated to a substrate surface rms value of lower than around 50 nm. Our Ag/ZnO substrates with rms roughness less than this value typically do not contain microvalleys with opening angles smaller than ~ 110°, resulting in solar cells with improved output performance. We suggest a void-formation model based on selective etching of strained Si-Si atoms due to the collision of growing silicon film surface near the valleys of the substrate.     

Effect of radio frequency power and total mass-flow rate on the properties of microcrystalline silicon films prepared by helium-diluted-silane glow discharge

Hydrogenated microcrystalline silicon thin films have been prepared by plasma-enhanced chemical vapor deposition at relatively low deposition temperatures (180 °C). Helium dilution of silane, instead of the more commonly approach of hydrogen dilution, has been used to promote microcrystalline growth. The effect of the applied radio frequency power (RFP) and the total gas flow on the structural, optical and electrical characteristics have been studied. As observed from the structural measurements, microcrystalline growth is favored as the applied RFP is increased and/or the total gas flow is decreased. Increasing the RFP however, brings associated an increase in the defect density in the amorphous tissue surrounding the crystalline grains and/or an increase in intra-grain defects as deduced from the structural, optical and electrical measurements. Microcrystalline growth and defect formation is rationalize in terms of the He* deexcitation process and high energy He⁺ ions bombardment.     

Characterization of thin TiO2 films prepared by plasma enhanced chemical vapour deposition for optical and photocatalytic applications

Thin titanium oxide films were deposited using a radio frequency (RF) plasma enhanced chemical vapour deposition method. Their optical properties and thickness were determined by means of ultraviolet-visible absorption spectrophotometry. Films of the optical parameters very close to those of titanium dioxide have been obtained at the high RF power input. Their optical quality is high enough to allow for their use in a construction of stack interference optical filters. At the same time, these materials exhibit strong photocatalytic effects. The results of structural analysis, carried out by Raman Shift Spectroscopy, show that the coatings posses amorphous structure. However, Raman spectra of the same films subjected to thermal annealing at 450 °C disclose an appearance of a crystalline form, namely that of anatase. Surface morphology of the films has also been characterized by Atomic Force Microscopy revealing granular, broccoli-like topography of the films.     

Electrical characterization of amorphous silicon carbide thin films deposited via polymeric source chemical vapor deposition

Polymeric source chemical vapor deposition (PS-CVD) was used to synthesize amorphous silicon carbide (a-SiC) thin films. The PS-CVD process was conducted at temperatures between 750 and 1000 °C. The substrates used were silicon single crystal wafers of p-type and n-type, and thermally grown silicon dioxide substrates. The chemical and electrical properties of the films were studied by various techniques, including Fourier transform infrared spectroscopy, elastic recoil detection (ERD), and capacitance-voltage technique. A correlation was observed between the average concentration of oxygen in the films and the deposition temperature, linking a low oxygen concentration to a high deposition temperature. However, the concentration of oxygen in the films deposited at the same temperature is independent of the substrate. The thin films deposited at low temperature showed insulating behaviour, while the semiconducting behaviour is obtained at high deposition temperatures. Ohmic contacts were obtained on the deposited semiconductor thin film by evaporating nickel contacts, followed by annealing of the sample at 800 °C for 2 min.     

Growth and microstructures of carbon nanotube films prepared by microwave plasma enhanced chemical vapor deposition process

Well-aligned carbon nanotubes (CNTs) were grown on iron coated silicon substrates by microwave plasma enhanced chemical vapor deposition. Effect of plasma composition on the growth and microstructures of CNTs were investigated by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and optical emission spectroscopy. Morphology and microstructure of nanotubes were found to be strongly dependent on the plasma composition. Aligned bamboo-shaped nanotubes consisting of regular cone shaped compartments were observed for C₂H₂/NH₃/N₂ and C₂H₂/NH₃/H₂ gas mixtures. Randomly oriented or no nanotubes growth were observed in C₂H₂/H₂ and C₂H₂/N₂ gas mixtures respectively. CNTs grown in nitrogen rich plasma had more frequent short compartments while compartment length increased with decreasing nitrogen concentration in the plasma. Raman spectroscopy of CNTs samples revealed that CNTs prepared in nitrogen rich plasma had higher degree of disorder than those in low nitrogen or nitrogen free plasma. In-situ optical emission spectroscopy investigations showed that CN and H radicals play very important role in both the growth and microstructure of CNTs. Microstructure of CNTs has been correlated as a function of CN radical concentration in the plasma. It is suggested that presence of nitrogen in the plasma enhances the bulk diffusion of carbon through the iron catalyst particles which causes compartment formation. Based on our experimental observations, growth model of nanotubes under different plasma composition has been suggested using base growth mechanism.     

Hydrogenated amorphous carbon and carbon nitride films deposited at low pressure by plasma enhanced chemical vapor deposition

Hydrogenated amorphous carbon (a-C:H) films were deposited by plasma enhanced chemical vapor deposition from methane, argon diluted methane, and nitrogen diluted methane at 26.7 Pa with a 13.56 MHz RF power supply. In this pressure regime, multiple-scattering of carbon species within the plasma phase is expected during the transport to the substrates placed on both the driven and the earthed electrodes. These films were analyzed using UV-VIS optical transmittance, monochromatic ellipsometry, Raman spectroscopy and current-voltage measurements. From these results, the effect of the plasma conditions and the effective flux of the carbon species controlled by the input power through the negative self bias are found to be important in the deposition process. The growth conditions at the higher pressure regime are important to synthesize a-C:H films from low energetic carbon species, since it reduces the defect density and improves the quality of the films. Furthermore, the effect of nitrogen on the growth conditions of a-C:H:N films is observed.