Radio-frequency Plasma-enhanced Chemical Vapor Deposition Research Articles

Adsorption of polar molecules on diamond-like carbon films with different trapped charge densities

This work investigates the effect of trapped charge densities present in diamond-like carbon films (DLC) on adsorption of polar molecules on the film surface. Hydrogen-free and hydrogenated DLC films with different hydrogen/carbon contents were produced using filtered-cathodic arc sputtering (FCA-sputtering) and radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD) techniques, respectively. After treating the as-prepared films in a dilute solution of dipropylene glycol monomethyl ether, adsorption of such molecules was found throughout the film surface. Capacitance–voltage (C–V) measurements have shown that the DLC films with lower hydrogen/carbon content inherently have larger trapped charge accumulations within their structures. A charge density of 1600nC/cm2 was measured in a DLC film with a lower hydrogen/carbon content (produced from C2H4 and Ar precursors), while a much smaller charge density of 64nC/cm2 was measured in a DLC film with a higher hydrogen/carbon content (produced from 10% CH4 and 90% H2 precursors). A correlation between charge densities and the degree of polar molecule adsorption was observed. Films with higher charge densities appeared to be more prone to polar molecule adsorption.

Influence of Deposition Time on Bonding and Morphology of Amorphous Carbon Nitride Thin Films

Amorphous carbon nitride (a-CNx) is a well-known material that can be used in various applications such as coating for hard disk, wear resistant, humidity sensor and others. In this research, a-CNx thin films have been deposited by using radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD) with the mixing of pure methane (CH4) and nitrogen (N2). The gas ratio of CH4/N2, electrode distance, pressure and temperature of deposition is kept constant while deposition time is allowed to vary from 30 to 150 minutes. Raman spectroscopy and field emission scanning electron microscopy (FESEM) have been used to study the bonding and morphology of these films respectively. An increase in deposition time resulted with thedecrease in sp2 content in the a-CNx thin films. Theincrease in the Id/Ig intensity ratio with the increase in deposition time can be explained by the reducing size of graphitic cluster. Long deposition time retarded the growth rate of a-CNx thin films due to etching effects. Longer exposure to the etching effect resultingin the creation of small graphitic cluster with the formation of spongy-like porous features.

Tuning the mechanical properties of vertical graphene sheets through atomic layer deposition

We report the fabrication and characterization of graphene nanostructures with mechanical properties that are tuned by conformal deposition of alumina. Vertical graphene (VG) sheets, also called carbon nanowalls (CNWs), were grown on copper foil substrates using a radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) technique and conformally coated with different thicknesses of alumina (Al2O3) using atomic layer deposition (ALD). Nanoindentation was used to characterize the mechanical properties of pristine and alumina-coated VG sheets. Results show a significant increase in the effective Young’s modulus of the VG sheets with increasing thickness of deposited alumina. Deposition of only a 5 nm thick alumina layer on the VG sheets nearly triples the effective Young’s modulus of the VG structures. Both energy absorption and strain recovery were lower in VG sheets coated with alumina than in pure VG sheets (for the same peak force). This may be attributed to the increase in bending stiffness of the VG sheets and the creation of connections between the sheets after ALD deposition. These results demonstrate that the mechanical properties of VG sheets can be tuned over a wide range through conformal atomic layer deposition, facilitating the use of VG sheets in applications where specific mechanical properties are needed.

Developments of c-Si1-xGex:H thin films as near-infrared absorber for thin film silicon solar cells

Hydrogenated microcrystalline silicon germanium (c-Si1-xGex:H) thin films have been developed as alternative bottom sub-cell absorbers for multi-junction thin film silicon solar cells due to their narrower band-gaps and higher absorption coefficients than conventional hydrogenated microcrystalline silicon (c-Si:H) thin films. However, since the structure complexity was increased a lot by Ge incorporation, the influences of c-Si1-xGex:H film properties on Ge composition have not been understood yet. In this work, c-Si1-xGex:H thin films with various Ge content and similar crystalline volume fraction are fabricated by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD). The evolutions of c-Si1-xGex:H material properties by Ge incorporation are characterized by X-ray fluorescence spectrometry, Raman spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, absorption coefficient spectrum, and conductivity measurement. The results show that the properties of c-Si1-xGex:H thin films are strongly determined by Ge content. With the increase of Ge content, the absorption coefficient, (111) grain size, microstructure factor, and dark conductivity of c-Si1-xGex:H thin films increase, while the H content, (220) grain size, and photosensitivity of c-Si1-xGex:H thin film decrease. Then, c-Si1-xGex:H is used as the intrinsic layer in the single junction solar cells. The performances of c-Si1-xGex:H solar cells with different Ge content and two types of transparent conductive oxide (SnO2 and ZnO) substrates are systematically studied. The results indicate that although c-Si1-xGex:H thin films become more defective and less compact with Ge incorporation, c-Si1-xGex:H solar cells exhibit a significant improvement in near-infrared response, especially under the circumstances of thin cell thickness and inefficient light trapping structure. Meanwhile, by using ZnO substrates, initial efficiencies of 7.15% (Jsc=22.6 mA/cm2, Voc=0.494 V, FF=64.0%) and 7.01% (Jsc=23.3 mA/cm2, Voc=0.482 V, FF=62.4%) are achieved by c-Si0.9Ge0.1:H solar cell and c-Si0.73Ge0.27:H solar cell, respectively. Furthermore, the c-Si0.73Ge0.27:H solar cell is used as the bottom sub-cell of the double-junction solar cell, and a Jsc.bottom of 12.30 mA/cm2 can be obtained with the bottom sub-cell thickness as thin as 800 nm, which is even higher than that of c-Si:H bottom sub-cell with 1700 nm thickness. Finally, an initial efficiency of 10.28% is achieved in an a-Si:H/c-Si0.73Ge0.27:H double junction cell structure. It is demonstrated that by using the c-Si1-xGex:H solar cell as the bottom sub-cell in multi-junction thin film silicon solar cells, a higher tandem cell performance can be achieved with a thin thickness, which has a great potential for cost-effective photovoltaics.

Characterization of Residual Stress in a-SiC:H Deposited by RF-PECVD for Manufacturing of Membranes for Cell Culture

ABSTRACTHydrogenated amorphous silicon carbide (a-SiC:H) was deposited by radiofrequency-plasma enhanced chemical vapor deposition (RF-PECVD) on monocrystalline silicon substrates with different process parameters in order to analyze the residual stress, and the roughness and uniformity of the films, which are the most important characteristics in the production of membranes for cell culture. The residual stress was calculated using Stoney's equation by measuring the thicknesses of the substrate and the deposited film, in addition to the radius of curvature of the substrate with and without deposited material. From the results it was observed that as power increases from 15 to 30 W, the residual stress increases from -180 to -400 MPa. Even at low power, the residual stress is high. However, an annealing process at 450 °C in N2 atmosphere significantly reduces the residual stress to 7 MPa. It was found that the film uniformity increases when the pressure rises in the process chamber from 450 to 900 mTorr. Finally, the RMS roughness (0.7 to 5.1 nm) can be controlled by the power and pressure, allowing us to obtain a material with excellent morphological characteristics for the adherence and growth of specific cells.

Gold nanoparticles embedded in carbon film: Micromorphology analysis

In this paper the sputtered gold nanoparticles (NPs) deposited by Radio Frequency-Plasma Enhanced Chemical Vapor Deposition (RF-PECVD) method on the glass substrates in trace of amorphous hydrogenated carbon (Au NPs @ a-C: H) were analyzed to study the three-dimensional (3-D) surface texture. The prepared Au NPs @ a-C: H films were used as research materials. The synthesis of samples was carried out in different powers from 80 to 120W while all other parameters were kept constant. The X-ray diffraction patterns and UV–vis spectra were applied to study the structure and Localized Surface Plasmon Resonance (LSPR), respectively. An atomic force microscope in a non-contact mode was used to record the sample surface images then the fractal geometry was studied. The images were numerically processed to calculate the Areal Autocorrelation Function (AACF), which was used to determine the anisotropy ratio Str, and to compute the Structure Function (SF). The log–log plots of the latter were used to calculate fractal properties of the studied surfaces, such as fractal dimension D, and pseudo-topothesy K. The analysis of 3-D surface texture was helpful in optimizing the functional performance of the thin film.

Optimization of a-Si:H absorber layer grown under a low pressure regime by plasma-enhanced chemical vapor deposition: Revisiting the significance of the p/i interface for solar cells

In this study, we revisited the significance of the p/i interface for hydrogenated amorphous silicon (a-Si:H) solar cells. Initially, intrinsic and extrinsic (p and n type) a-Si:H layers were grown in a low pressure regime (0.5–0.1Torr) using the conventional RF plasma-enhanced chemical vapor deposition process and their opto-electronic properties were optimized for the fabrication of p–i–n a-Si:H solar cells. Subsequently, we obtained new insights in terms of the activation energy and band gap at the p/i interface in these solar cells. The absorber layers deposited at pressures of 0.23Torr and 0.53Torr had the highest photosensitivity with a band absorption edge at ~700nm. Furthermore, the photosensitivity was shown to be correlated with the estimated diffusion length, which effectively defined the carrier transport within the solar cell layers. Moreover, the cell efficiency increased from 1.53% to 5.56% due to the improved p/i interface as well as the higher photosensitivity of the intrinsic/absorber layer.

The effect of hydrogen and acetylene mixing ratios on the surface, mechanical and biocompatible properties of diamond-like carbon films

In this study, diamond-like carbon (DLC) films were deposited on silicon substrates using different hydrogen and acetylene mixing ratios with the same total flow rate by RF plasma-enhanced chemical vapor deposition. By altering the ratios of ion and neutral radicals, different precursor combinations resulted in different film properties. To obtain the optimal precursor combinations, the morphological, structural, hydrophilic, mechanical, and biocompatibility properties were probed by AFM, Raman spectra, contact angle measurements, nanoindentation, and ECV-304 cell cultures, respectively. The results showed that, for DLC films with high hydrogen contents, the depleted carbon gas and high hydrogen etching resulted in a lower film thickness and smoother surface morphology. In terms of the film structure, the hydrogen-rich DLC films possessed a higher ID/IG ratio and an upward G-peak position shift due to an increase in the sp2 grain size and bond-angle order. The high hydrogen content also led to increase in the residual stress, hardness, and the Young's modulus. The primary mechanisms for this increase were determined to be due to extensive hydrogen bombardment and the presence of covalent bonds. In biocompatibility, the cell number for ECV-304 ascended if the hydrogen precursor ratio was increased. The mechanism for this increase was characterized by protein absorption associated with hydrophilicity. The results of this research demonstrated that the DLC deposited at moderate hydrogen levels might possess the best overall film properties.

Characteristics of industrially manufactured amorphous hydrogenated carbon (a-C:H) depositions on high-density polyethylene

Industrially high-density polyethylene (HDPE) was successively covered by two types of amorphous hydrogenated carbon (a-C:H) films, one more flexible (f-type) and the other more robust (r-type). The films have been grown by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) technique with acetylene plasma. The surface morphology of both types has been studied by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Contact angle measurements and Raman spectroscopy analysis were done to investigate the surface wettability and carbon chemical composition. Both types display similar morphology and grain growth pattern. Contact angle measurements revealed surface modifications especially at smaller r-depositions with a pronounced hydrophilic behavior. The f-depositions remain almost unchanged. For high r-depositions a rearrangement of the carbon composition is found and related to a growth of sp2 hybridized carbon species, despite of the hydrogen content. The values of the average grain heights are correlated to the values of the average grain areas and are localized in a limited area, indicating a given regularity during the different carbon depositions.

Annealing induced amorphous/crystalline silicon interface passivation by hydrogen atom diffusion

Post-annealing is an efficient method to improve passivation quality of the amorphous/crystalline silicon configuration and it’s been widely used in fabrication of amorphous/crystalline silicon heterojunction solar cells. In this study, hydrogenated amorphous silicon thin films are deposited on n type monocrystalline silicon, using a single chamber radio-frequency plasma-enhanced chemical vapor deposition system. After passivation the best result with effective minority carrier lifetime (τeff) exceeding 1600 μs is achieved. Fourier transform infrared spectrum is utilized to investigate the structure evolution after annealing. The result shows that, the improved passivation quality is attributed to interface dangling bonds saturated by H atoms diffusion during annealing, the reason of passivation quality affected by annealing and film thickness is also proposed.

Microstructure and Tribological Properties of FeNPs@a-C:H Films by Micromorphology Analysis and Fractal Geometry

This paper analyzes the three-dimensional (3D) surface texture of amorphous hydrogenated carbon films with sputtered iron nanoparticles (FeNPs@a-C:H) deposited by a radio-frequency plasma-enhanced chemical vapor deposition method on the quartz substrates. The prepared FeNPs@a-C:H films were used as research materials. The synthesized samples were deposited at four different pressures of 2.5, 3, 3.35, and 3.5 N/m2 in an acetylene gas atmosphere. The Fe and C contents of the thin films were obtained from X-ray photoelectron spectroscopy. The X-ray diffraction profile and electron diffraction pattern indicate that iron nanoparticles with body-centered-cubic crystalline structure are formed in these films. The localized surface plasmon resonance peak that is the signature of the existence of the iron core nanoparticles appears in visible spectra of these films. The sample surface images were recorded using an atomic force microscope operating in a noncontact mode and analyzed to reveal the statistical, fracta...

Pore morphology of low-k SiCxNy films prepared with a cyclic silazane precursor using plasma-enhanced chemical vapor deposition

Low-k SiCxNy films were prepared using radio-frequency plasma-enhanced chemical vapor deposition (PECVD), with only 1,3,5-trimethyl-1,3,5-trivinylcyclotrisilazane (VSZ) as the precursor; VSZ has cyclic Si–N–Si linkages and three pendent vinyl groups. At lower PECVD temperatures, SiCxNy films possess relatively low film densities, indicating the existence of a loose structure or voids within the cross-linked matrix structure. The pore morphology of SiCxNy films deposited at distinct temperatures were examined using grazing-incidence small-angle X-ray scattering, while the chemical bondings and structural information were analyzed using Fourier-transform infrared spectroscopy. At 100°C, SiCxNy films without porogen displaying 4.9nm pores and a mean pore spacing of 30.1nm generated low-density films because Si-(CH2)n-Si and/or Si-(CH2)n-CH3 could be incorporated free volume into the N–Si–C cross-linked structure under plasma deposition. At 300°C, the N–Si–C cross-linked structure and the some organic phase were disrupted and transformed into a denser structure, reducing pore size (3.5nm) and losing pore correlation. Thus, low deposition temperatures facilitate the formation of large pores and the ordering of the pores. Post annealing converted the 100 and 300°C as-deposited SiCxNy films into loose and dense structures, respectively, and maintained slightly reduced pore size and pore correlation in the annealed films.

Optical properties of silicon carbonitride films produced by plasma-induced decomposition of organic silicon compounds

Optical properties (refractive index; UV, visible, and IR transmission spectra; optical bandgap) of silicon carbonitride SiCxNy films, promising materials for silicon photonic devices and microelectronics, have been studied. The films have been synthesized by rf plasma-enhanced chemical vapor deposition with thermal activation from parent organosilicon compounds in mixtures with helium, ammonia, nitrogen, or oxygen at a reduced pressure and temperatures of 373–1023 K. The parent substances used are aminosilane- and silazane-series organosilicon precursors.

Electrical properties of hydrogenated microcrystalline silicon carbon alloys: effect of deposition parameters and light soaking

New materials with electrical properties similar to those of hydrogenated amorphous silicon (a-Si:H) or hydrogenated amorphous silicon germanium alloys, but that are stable under light soaking, could be a breakthrough for multi-junction thin-film silicon solar cell technology. With this aim, hydrogenated microcrystalline silicon-carbon alloy (c-:H) thin films seem like promising candidates as the variation of the carbon in the alloy composition can be used to tune the effective bandgap, keeping a high crystalline fraction. In this study, a highly hydrogen-diluted silane and methane gas mixture was used to deposit c-:H thin films by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). The effect of the independent variation of three deposition parameters (variation of methane flow rate, addition of a small amount of silicon tetrafluoride and variation of the RF-power density) on the electronic transport and defect properties was investigated. Selected samples were studied in their ‘as deposited’ state, after light soaking until saturation, and after a high temperature annealing. Particular care was paid to comparing the behaviour of this new material system to that of a reference a-Si:H sample, deposited by standard RF-PECVD.

3D micro- and nano-machining of hydrogenated amorphous silicon films on SiO2/Si and glass substrates

We report on the hydrogen-assisted deep reactive ion etching of hydrogenated amorphous silicon (a-Si:H) films deposited using radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD). High aspect-ratio vertical and 3D amorphous silicon features, with the desired control over the shaping of the sidewalls, in micro and nano scales, were fabricated in ordered arrays. The suitable adhesion of amorphous Si film to the underlayer allows one to apply deep micro- and nano-machining to these layers. By means of a second deposition of amorphous silicon on highly curved 3D structures and subsequent etching, the fabrication of amorphous silicon rings is feasible. In addition to photolithography, nanosphere colloidal lithography and electron beam lithography were exploited to realize ultra-small features of amorphous silicon. We have also investigated the optical properties of fabricated hexagonally patterned a-Si nanowire arrays on glass substrates and demonstrated their high potential as active layers for solar cells. This etching process presents an inexpensive method for the formation of highly featured arrays of vertical and 3D amorphous silicon rods on both glass and silicon substrates, suitable for large-area applications.

Characterisation of intrinsic silicon oxide absorber layers for use in silicon thin film solar cells

The use of a wide bandgap absorber layer in the top cell of a multi-junction silicon thin film solar cell is necessary to achieve a high-conversion efficiency. A higher bandgap of the absorber results in a higher open-circuit voltage (Voc) of the cell. In this work, intrinsic hydrogenated amorphous silicon oxide (i)a-SiO:H films have been prepared by using 13.56 MHz radio frequency plasma enhanced chemical vapour deposition (RF-PECVD) at a substrate temperature of 195 °C. The carbon dioxide (CO2) to silane (SiH4) ratio rc was varied and the influence of the ratio rc on the optoelectronic film properties was investigated. These thin films have been studied in detail in terms of their dark (σd) and photo (σph) conductivity and photoresponse PR (σph/σd). The defect density Nd and Urbach energy EU were determined by constant photocurrent method (CPM). The optical bandgap Eg,Tauc was derived from Tauc plots. Optical constants were determined by spectroscopic ellipsometry measurements in the range between 300 and 1000 nm. It was found that the increase in the CO2 to SiH4 ratio rc not only leads to a higher optical bandgap, but also higher defect density Nd and Urbach energy EU and on the other hand to a lower photoresponse PR. A suitable photoresponse PR = 8.64 × 105 at a high bandgap of Eg,Tauc = 1.91 eV and a defect density of Nd = 1.7 × 1016 cm−3 was achieved. The refraction index and extinction coefficient were lowered with increasing rc. The analysis of light-induced degradation in the (i)a-SiO:H layers showed a smaller increase of deep defects Nd and a higher increase of the Urbach energy EU in terms of the light soaking time with respect to the (i)a-Si:H reference layer.

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.

Topographic Characterization of Cu-Ni NPs @ a-C:H Films by AFM and Multifractal Analysis.

In the present work three-dimensional (3-D) surface topography of Cu-Ni nanoparticles in hydrogenated amorphous carbon (Cu-Ni NPs @ a-C:H) with constant thickness of Cu and three thicknesses of Ni prepared by RF-Plasma Enhanced Chemical Vapor Deposition (RF-PECVD) system were investigated. The thin films of Cu-Ni NPs @ a-C:H with constant thickness of Cu and three thicknesses of Ni deposited by radio frequency (RF)-sputtering and RF-PECVD systems, were characterized. To determine the mass thickness and atomic structure of the films, the Rutherford backscattering spectroscopy (RBS) spectra was applied. The absorption spectra were applied to study localized surface plasmon resonance (LSPR) peaks of Cu-Ni NPs (observed around 608 nm in visible spectra), which is widened and shifted to lower wavelengths as the thickness of Ni over layer increases, and their changes are also evaluated by the 3-D surface topography. These nanostructures were investigated over square areas of 1 μm × 1 μm using atomic force microscopy (AFM) and multifractal analysis. Topographic characterization of surface samples (in amplitude, spatial distribution, and pattern of surface characteristics) highlighted 3-D surfaces with multifractal features which can be quantitatively estimated by the multifractal measures. The 3-D surface topography Cu-Ni NPs @ a-C:H with constant thickness of Cu and three thicknesses of Ni prepared by RF-PECVD system can be characterized using the multifractal geometry in correlation with the surface statistical parameters.

A Comparison between Thin-Film Transistors Deposited by Hot-Wire Chemical Vapor Deposition and PECVD

The effect of new growth techniques on the mobility and stability of amorphous silicon (a-Si:H) thin film transistors (TFTs) has been studied. It was suggested that the key parameter controlling the field-effect mobility and stability is the intrinsic stress in the a-Si:H layer. Amorphous and microcrystalline silicon films were deposited by radiofrequency plasma enhanced chemical vapor deposition (RF-PECVD) and hot-wire chemical vapor deposition (HW-CVD) at 100 ºC and 25 ºC. Structural properties of these films were measured by Raman Spectroscopy. Electronic properties were measured by dark conductivity, σd, and photoconductivity, σph. For amorphous silicon films deposited by RF-PECVD on PET, photosensitivity's of >105 were obtained at both 100 º C and 25 ºC. For amorphous silicon films deposited by HW-CVD, a photosensitivity of > 105 was obtained at 100 ºC. Microcrystalline silicon films deposited by HW-CVD at 95% hydrogen dilution show σph~ 10-4 Ω-1cm-1, while maintaining a photosensitivity of ~102 at both 100 ºC and 25 ºC. Microcrystalline silicon films with a large crystalline fraction (> 50%) can be deposited by HW-CVD all the way down to room temperature.

Optimization of Intrinsic Silicon Passivation Layers in nc-Si:H/c-Si Silicon Heterojunction Solar Cells

A series of intrinsic silicon thin films were prepared using radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) at low temperature and low power density. We investigated the influence of silane concentration (CS) on the structural, optical, and electronic properties, and passivation quality of the intrinsic silicon films, and the performances of hydrogenated nanocrystalline silicon/crystalline silicon (nc-Si:H/ c-Si) silicon heterojunction (SHJ) solar cells. The results show that with decreasing silane concentration, substantial changes in the crystalline volume fraction, hydrogen concentration, structure factor, optical bandgap, and photosensitivity of the film take place in the transition zone. The passivation quality of intrinsic silicon thin films is decided by the hydrogen content and bonding structure of the film. Films close to the transition zone show good compactness and photosensitivities, high hydrogen content, and low state densities, and contain abundant SiH bonds. The films provide excellent passivation for c-Si surfaces and significantly enhance the open-circuit voltages of nc-Si:H/c-Si SHJ solar cells. However, the passivation quality deteriorates seriously when the film is too thin. In this work, the optimum silane concentration was found to be 6% (molar fraction).