Frequency Plasma-enhanced Chemical Vapor Deposition Research Articles

RF-PECVD synthesis of carbon nanowalls and their field emission properties

Carbon nanowalls (CNWs) were successfully fabricated on various substrates by radio frequency plasma-enhanced chemical vapor deposition using gas mixtures of acetylene, argon and hydrogen without aid of any catalyst or substrate pretreatment. The influence of synthesis parameters on the field emission behaviors of CNWs was investigated in depth. The results showed that the morphology and microstructure of CNWs could be adjusted by growth parameters (temperature, pressure and gas flow rate), and CNWs with sharp edges displayed good field emission properties. Especially, the sample prepared under the pressure of 300Pa and the temperature of 650°C with H2 flow rate of 120sccm exhibited the best field emission performance (the turn-on and threshold fields were 4.7 and 6.0V/μm, respectively). In addition, the field emission characteristics of CNWs after stability test had no obvious deterioration; however, the morphology and microstructure of CNWs changed.

High efficiency a-Si:H/a-SiGe:H tandem solar cells fabricated with the combination of V- and U-shaped band gap profiling techniques

Hydrogenated amorphous silicon germanium (a-SiGe:H) films prepared by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a mixture of SiH4, H2, and GeH4 were investigated for their use as the bottom cell of amorphous silicon/amorphous silicon germanium (a-Si:H/a-SiGe:H) tandem solar cell structures. Narrow optical band gaps (Eopt) in the range of 1.5 to 1.6 eV were obtained by varying the GeH4/(SiH4 + GeH4) gas flow rate ratio in low-temperature deposition. The a-SiGe:H films deposited with various GeH4/(SiH4 + GeH4) gas flow rate ratios were used as intrinsic layers for the a-Si:H/a-SiGe:H tandem solar cells with different graded band gaps: V-, VU-, and U-shapes. It was found that using the VU-shape improves the solar cell efficiency owing to a higher Jsc when compared with using V-shape. The VU-shape’s Voc and FF are also improved when compared with the U-shape’s Voc and FF. As a result, a high efficiency of 11.0% (Voc = 1.74 V, Jsc = 9.07 mA/cm2, and FF = 0.70) was successfully achieved with the solar cells fabricated using the VU-shape graded band gap technique.

High-efficiency microcrystalline silicon solar cells on honeycomb textured substrates grown with high-rate VHF plasma-enhanced chemical vapor deposition

The potential of high-rate growth of high-quality microcrystalline silicon (µc-Si:H) films for solar cell applications is investigated by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) under a high-pressure SiH4 depletion condition. It is found that the morphology of textured substrates plays an important role in not only light trapping but also µc-Si:H film growth. A high conversion efficiency of 11.1% is attained in a substrate-type µc-Si:H cell on a substrate with honeycomb textures, which has rounded concaves in a honeycomb arrangement with an appropriate period. It is also clarified that ZnO:B films grown by metal organic chemical vapor deposition (MOCVD) are beneficial in terms of carrier collection compared with the standard In2O3:Sn (ITO) film grown by sputtering. On the basis of these findings, a new world-record µc-Si:H cell with a certified conversion efficiency of 11.8% is developed with a relatively high deposition rate of 1 nm/s.

Free standing 3D graphene nano-mesh synthesis by RF plasma CVD using non-synthetic precursor

Three dimensional graphene nano-mesh (3D-GNM) has been synthesized by radio frequency plasma-enhanced chemical vapor deposition. The 3D-GNM with various shapes is obtained on nickel foam substrate without using any nano-network as a pattern mask. Moreover, non-synthetic precursor Melaleuca alternifolia oil, with environmentally-friendly properties, is used as the carbon source for synthesis of 3D-GNM. This synthesis route with low energy expenses and little environmental contamination provides a simple and cost effective fabrication of 3D-GNM for utilization in future fundamental studies related to graphene.

Surface morphology and grain analysis of successively industrially grown amorphous hydrogenated carbon films (a-C:H) on silicon

Silicon (100) has been gradually covered by amorphous hydrogenated carbon (a-C:H) films via an industrial process. Two types of these diamond-like carbon (DLC) coatings, one more flexible (f-DLC) and one more robust (r-DLC), have been investigated. Both types have been grown by a radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) technique with acetylene plasma. Surface morphologies have been studied in detail by atomic force microscopy (AFM) and Raman spectroscopy has been used to investigate the DLC structure. Both types appeared to have very similar morphology and sp2 carbon arrangement. The average height and area for single grains have been analyzed for all depositions. A random distribution of grain heights was found for both types. The individual grain structures between the f- and r-type revealed differences: the shape for the f-DLC grains is steeper than for the r-DLC grains. By correlating the average grain heights to the average grain areas for all depositions a limited region is identified, suggesting a certain regularity during the DLC deposition mechanisms that confines both values. A growth of the sp2 carbon entities for high r-DLC depositions is revealed and connected to a structural rearrangement of carbon atom hybridizations and hydrogen content in the DLC structure.

Improved photovoltaic properties of amorphous silicon thin-film solar cells with an un-doped silicon oxide layer

This paper proposes the use of undoped hydrogenated microcrystalline silicon oxide (μc-SiOx:H) deposited on the n-μc-Si:H layer of amorphous silicon single-junction superstrate configuration thin-film solar cells produced through 40MHz very high frequency plasma-enhanced chemical vapor deposition. Raman spectroscopy and optoelectronic analyses of the undoped μc-SiOx:H thin film revealed that adding a small amount of oxygen into a μc-network results in a low optical absorption, wide band gap, high optical band gap E04, high refractive index, reasonable conductivity, and crystalline volume fraction, which are advantageous properties in solar cells. Compared with a standard cell, the current density–voltage (J–V) characteristics of the cell with an undoped μc-SiOx:H/n-μc-Si:H structure showed an enhancement in short-circuit current density Jsc from 13.32 to 13.60mA/cm2, and in conversion efficiency from 8.53% to 8.61%. The increased Jsc mechanism can be attributed to an improved light-trapping capability in the long wavelength range between 510 and 660nm, as demonstrated by the external quantum efficiency.

Investigation of structural disorder using electron temperature in VHF-PECVD on hydrogenated amorphous silicon films for thin film solar cell applications.

Electrode distances and gas flow ratios are important parameters for fabricating intrinsic (i-type) layers of hydrogenated amorphous silicon (a-Si:H) films using a very high frequency plasma-enhanced chemical-vapor deposition (VHF-PECVD) system. In this work, we investigated the relationship between the electrode distances and gas flow ratios on the properties of i-type a-Si:H films. The electrical, chemical and structural properties are improved with decreasing electrode distances (20-40 mm) at a hydrogen ratio [R (H2/SiH4) = 4], due to the low electron temperature and heating effect. A low electron temperature generates silane-related-reactive species (SiH3) and decreases structural disorder resulting in high quality i-type a-Si:H films. The electrical, chemical and structural properties of the a-Si:H films are confirmed using Al coplanar electrodes, FTIR, Raman spectroscopy, and spectroscopy ellipsometry (SE). When a solar cell is fabricated using the a-Si:H film, J(sc) of 13.2-14.8 mA/cm2, photoconductivity of 1.5 x 10(-5)-8.6 x 10(-6) S/cm, Si--H2 content of 0-1.24 at.%, and hydrogen content of about 10 at.% are obtained. These results together with a model of the plasma chemistry indicate that H atoms and SiH3 radicals play an important role in the deposition process.

Fast Response, Vertically Oriented Graphene Nanosheet Electric Double Layer Capacitors Synthesized from C2H2

The growth and electrical characteristics of vertically oriented graphene nanosheets grown by radio frequency plasma-enhanced chemical vapor deposition from C2H2 feedstock on nickel substrates and used as electrodes in symmetric electric double layer capacitors (EDLC) are presented. The nanosheets exhibited 2.7 times faster growth rate and much greater specific capacitance for a given growth time than CH4 synthesized films. Raman spectra showed that the intensity ratio of the D band to G band versus temperature initially decreased to a minimum value of 0.45 at a growth temperature of 750 °C, but increased rapidly with further temperature increase (1.15 at 850 °C). The AC specific capacitance at 120 Hz of these EDLC devices increased in a linear fashion with growth temperature, up to 265 μF/cm(2) (2 μm high film, 850 °C with 10 min growth). These devices exhibited ultrafast frequency response: the frequency response at -45° phase angle reached over 20 kHz. Consistent with the increase in D band to G band ratio, the morphology of the films became less vertical, less crystalline, and disordered at substrate temperatures of 800 °C and above. This deterioration in morphology resulted in an increase in graphene surface area and defect density, which, in turn, contributed to the increased capacitance, as well as a slight decrease in frequency response. The low equivalent series resistance varied from 0.07 to 0.08 Ω and was attributed to the significant carbon incorporation into the Ni substrate.

Effects of germane flow rate in electrical properties of a-SiGe:H films for ambipolar thin-film transistors

In this work, the study of germane flow rate in electrical properties of a-SiGe:H films is presented. The a-SiGe:H films deposited by low frequency plasma-enhanced chemical vapor deposition at 300°C were characterized by Fourier transform infrared spectroscopy, measurements of temperature dependence of conductivity and UV–visible spectroscopic ellipsometry. After finding the optimum germane flow rate conditions, a-SiGe:H films were deposited at 200°C and analyzed. The use of a-SiGe:H films at 200°C as active layer of low-temperature ambipolar thin-film transistors (TFTs) was demonstrated. The inverted staggered a-SiGe:H TFTs with Spin-On Glass as gate insulator were fabricated. These results suggest that there is an optimal Ge content in the a-SiGe:H films that improves its electrical properties.

Synthesis of N-doped Ethylcyclohexane Plasma Polymer Thin Films with Controlled Ammonia Flow Rate by PECVD Method

In this study, we investigated the basic properties of N-doped ethylcyclohexene plasma polymer thin films that deposited by radio frequency (13.56 MHz) plasma-enhanced chemical vapor deposition (PECVD) method with controlled ammonia flow rate. Ethylcyclohexene was used as organic precursor with hydrogen gas as the precursor bubbler gas. Additionally, ammonia (NH₃) gas was used as nitrogen dopant. The as-grown polymerized thin films were analyzed using ellipsometry, Fourier-transform infrared [FT-IR] spectroscopy, UV-Visible spectroscopy, and water contact angle measurement. We found that with increasing plasma power, film thickness is gradually increased while optical transmittance is drastically decreased. However, under the same plasma condition, water contact angle is decreased with increasing NH₃ flow rate. The FT-IR spectra showed that the N-doped ethylcyclohexene plasma polymer films were completely fragmented and polymerized from ethylcyclohexane.

Influences of electrode separation on structural properties of μc-Si1-xGex：H thin films

Hydrogenated microcrystalline silicon germanium (μc-Si1-xGex:H) thin films have been prepared by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) using a mixture of SiH4 and GeH4 as the reactive gases. Effects of electrode separation on the structural properties of μc-Si1-xGex:H thin films have been investigated. Results show that reduction of the electrode separation can increase the Ge content in the films. Moreover, μc-Si1-xGex:H thin film deposited at a lower electrode separation of 7 mm possesses not only a stronger (220) orientation and a larger grain size, but also a lower microstructural factor. Then, the decomposition characteristics of the reactive gases are analyzed according to the variation of the structural properties of the μc-Si1-xGex:H thin films. It is found that the increase of the Ge content is due to the decrease of the SiH4 decomposition rate in the plasma. While the better film quality obtained at the lower electrode separation is attributed to the enhancement of the diffusibility of the Ge precursors caused by improving the proportion of GeH3 radicals

Sensitivity of thin film nanocrystalline silicon properties to radio frequency plasma-enhanced chemical vapor deposition (rf PECVD) parameters

Hydrogenated nanocrystalline silicon (nc-Si:H) films for efficient nc-Si:H solar cells are made at the transition to the nanocrystalline regime during radio frequency plasma-enhanced chemical vapor deposition (rf PECVD). This transition occurs within a sensitive process window and is affected by various deposition parameters. This paper reports an investigation into the material properties of nc-Si:H films as affected by the following rf PECVD parameters: deposition power, deposition pressure, substrate temperature and silane concentration. The aim was to verify the sensitivity of nc-Si:H material properties to rf PECVD parameters. Our results showed that i-layer deposition pressure and the silane concentration mainly affected the blue response of nc-Si:H solar cells. The a-Si:H/nc-Si:H transition can be attained by manipulating any of deposition pressure, deposition power and silane concentration while keeping all other deposition conditions constant. Keywords: Nanocrystalline, vapor deposition, thin film, parameters.

Optimization of an i-a-SiOx:H absorber layer for thin film silicon solar cell applications

Wide gap, high quality hydrogenated intrinsic amorphous silicon (i-a-Si:H) alloy films are essentially required for multi-junction solar cells fabrication to enhance the open-circuit voltage (Voc) of the top cell. In this paper, intrinsic hydrogenated amorphous silicon oxide films (i-a-SiOx:H) have been prepared using a very high frequency plasma-enhanced chemical vapor deposition technique for use as an absorber layer of the top cell for double-junction amorphous silicon thin film solar cells. We studied the effects of the carbon dioxide (CO2) to silane (SiH4) ratio on the properties of the i-a-SiOx:H films and also optimized these films for application in a-SiOx:H/a-Si:H double-junction solar cells. The Voc of the a-SiOx:H/a-Si:H double-junction solar cells was higher than that of conventional a-Si:H/a-Si:H double-junction solar cells. Using an optimized deposition condition, a-SiOx:H/a-Si:H double-junction solar cell with an initial cell efficiency (η) of 10.2% was produced.

Modeling and experiments of microcrystalline silicon film deposited via VHF-PECVD

A 1-D plasma model coupled with a well-mixed reactor model was used to simulate the growth properties of hydrogenated microcrystalline silicon film deposited by very high frequency plasma-enhanced chemical vapor deposition from SiH4 and H2 gas mixtures. Plasma parameters of the former, such as electron density and electron temperature, were determined and used as input values for the latter, in which concentrations of gas phase species, crystalline orientation, hydrogen content, surface fraction of dangling bonds, and deposition rate were calculated. Simultaneously, a series of in situ optical emission spectroscopy measurements and film depositions were carried out to investigate the correlation between the model and the experiments. Desired agreements between both were achieved.

Influence of duty ratio of pulsed bias on structure and properties of silicon-doped diamond-like carbon films by plasma deposition

We have investigated the influence of the duty ratio of pulsed substrate bias on the structure and properties of Si-doped diamond-like carbon (Si-DLC) films deposited by radio frequency plasma-enhanced chemical vapor deposition using CH4, Ar, and monomethylsilane (CH3SiH3) as the Si source. The Si/(Si+C) ratios in the Si-DLC films deposited using pulsed bias were higher than that of the dc-biased Si-DLC film, and the Si fraction increased with decreasing pulse duty ratio. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses revealed that Si-C, Si-Hn, and C-Hn bonds in the Si-DLC films increased with decreasing duty ratio. The internal stress decreased as the duty ratio decreased, which is probably due to the increase in Si-C, Si-Hn, and C-Hn bonds in the films. The Si-DLC films deposited using pulsed bias had higher adhesion strength than the dc-biased Si-DLC film because of the further reduction of internal stress. At higher duty ratios, although the Si fractions of the pulse-biased Si-DLC films were higher than that of the dc-biased Si-DLC film, the wear rates of the former were less than that of the latter.

Synthesis of large area carbon nanosheets for energy storage applications

We report the large area growth of highly conductive carbon nanosheets (CNS) composed of few layer graphene on 200mm diameter Si substrates using conventional radio frequency plasma-enhanced chemical vapour deposition. Raman spectroscopy is used to characterise the evolution of the CNS nucleation and growth with time in conjunction with TEM revealing the nano-sized graphene-like nature of these films and the intimate contact to the substrate. An individual sheet can have edges as thin as 3 graphene layers. The influence of the growth support layer is also discussed as film growth is compared on titanium nitride (TiN) and directly on Si. Electrochemical cyclic voltammogram (CV) measurements reveal these layers to form an excellent electrical contact to the underlying substrate with excellent stability towards oxidation whilst having a large electrochemical surface area. The resistance of a 150nm film was measured to be as low as 20μohmcm. The high percentage of narrow few layer graphene edge sites exposed allows for faster electrochemical reaction rates compared to carbon nanotubes (CNTs) and other electrode materials (glassy carbon and Pt).

The study of the substrate temperature depended growth properties of microcrystalline silicon films deposited by VHF-PECVD method

In this paper, we have measured the temperature depended growth properties of hydrogenated microcrystalline silicon (μc-Si:H) films, prepared by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) from SiH4 and H2 gas mixtures. And, a 1D plasma model coupled with a well-mixed reactor model is used to simulate the growth process, in which concentrations of gas phase species, the crystalline orientation, the hydrogen content and the deposition rate are calculated. It suggests that the increasing surface fraction of the dangling bonds with the increase of substrate temperatures is responsible for the increase in the grain sizes. At the same time, the observed variations of the X-ray-diffraction intensities and the deposition rates of the films with temperature result from the differences in the growth rates of the facets.

Effects of the Concentration of MgO in the Catalyst on the Growth of Carbon Nanotubes

Carbon nanotubes (CNTs) were prepared on Si substrate by radio frequency plasma-enhanced chemical vapor deposition with Ni/MgO as catalyst. The morphology and microstructure of CNTs were characterized by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. The results show that with increasing MgO concentration in the catalyst the diameter of CNTs becomes smaller and more uniform and their purity and crystallinity are improved. Open or close carbon nano-onion structure is formed at the top of the CNTs, and the catalyst particles are found inside the tubes. With increasing MgO concentration, nanoparticles tend to nanowires longer than 30 nm for MgO concentration of 75% in the catalyst compund. These nanowires are made by Ni. Besides, when MgO concentration is 50%, the graphite layer of the bending tube wall extends inside of the tube, dividing the cavity into several irregularly shaped spaces, which forms a new carbon nanostructure.

Superparamagnetic FeCo and FeNi Nanocomposites Dispersed in Submicrometer-Sized C Spheres

Magnetic spheres with average diameter 100 nm, consisting of FeCo and FeNi nanomagnets (with average diameters 5.9 ± 1 and 4.6 ± 0.3 nm, respectively) dispersed in spherical carbon colloid have been synthesized using the radio frequency plasma-enhanced chemical vapor deposition technique. The nanoparticles are tightly embedded in the C matrix and well protected from oxidation. Superparamagnetic features such as typical Langevin-like magnetization curves are observed for the nanocomposites. A comprehensive analysis of the presence of dipolar interactions has been carried out within the framework of the mean-field model. The size of the nanoparticles constituting the carbon-spheres is near to the optimal value where Néel losses are maximal and considerable Neel-type specific absorption rates are observed. The material may hence provide a new material-based approach to realize biocompatibly protected superparamagnetic beads for application in biological environments, in particular, for magnetic hyperthermia.

Microcrystalline Silicon Carbide p-Layer with Wide-Bandgap and Its Application to Single- and Triple-Junction Silicon Thin-Film Solar Cells

Wide-bandgap, high-quality p-type microcrystalline silicon carbide (p-µc-SiC) films have been prepared by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) technique for use as window layers for single- and triple-junction thin-film silicon solar cells. We have found that the p-µc-SiC films have wider optical bandgaps and lower absorption spectra in the short-wavelength region than the conventional p-µc-Si films. The new p-type doping layer was applied as a window layer of a microcrystalline silicon (µc-Si:H) single-junction cell, and the thin-film solar cell with the new window layer showed higher open circuit voltage (Voc) and conversion efficiency. In addition, the insertion of an optimized p/i buffer layer was essential for reducing atomic damage at the p/i interface and obtaining a higher conversion efficiency. The optimized p-µc-SiC layer and p/i buffer layer were adopted successfully as a new window layer for the bottom cell within the triple-junction cell structure.