RF PE CVD deposition of amorphous a-SixNy:H layers for application in solar cells

The most promising technologies of silicon nitride hydrogenated layers for application in solar cells are based on plasma-chemical methods. In this paper, results concerning radio-frequency plasma-enhanced chemical vapour deposition (RF PE CVD) method applied in the technology of a-Si_xN_y:H layers on glass and polycrystalline silicon are presented. For the purpose of this research the series of the samples have been deposited and subjected to optical and structural studies. The layers have been grown from silane and ammonia used as gaseous precursors. A special attention has been paid to the choice of a gas flow ratio [NH₃]/[SiH₄]. The results show that it has a significant influence on the atomic-level structure, chemical composition and optical properties of the layers.     

Multilayer amorphous hydrogenated carbon (a-C:H) and SiOx doped a-C:H films for optical applications

In this study SiO_x doped amorphous hydrogenated carbon (a-C:H) films were formed from hexamethyldisiloxane (with hydrogen transport gas) by closed drift ion beam deposition applying variable ion beam energy (300-800 eV). The band gap dependence on the deposition energy was determined and used in production of SiO_x doped a-C:H and a-C:H (formed from acetylene gas) multilayer (two and four layers) stack. Optical properties of the multilayer structures as well as individual layers were analysed in the UV-VIS-NIR range (200-1000 nm). It was shown that employing double or four layer systems, the reflectivity of the multilayer structure-crystalline silicon can be tuned to almost 0% at specific wavelength range (550-950 nm), important in solar cell applications.     

Properties of composite a-C:H/metal layers deposited by combined RF PECVD/magnetron sputtering techniques

A sequential deposition method for synthesis of composite materials is presented. It consists of the periodical exposing the substrate to separated plasma assisted chemical vapor deposition and magnetron sputtering deposition sources. The plasma sources are operated independently. Using this deposition method, a-C:H/W composite layers were deposited on silicon substrates. The material morphology, structure and composition were investigated using specific techniques (AFM, SEM, EDX, SIMS and XRD). The material composition, as example the W concentration in the range of 10-50 at.%, can be tuned in a-C:H/W composites by proper setting the exposure time of the substrate to each of the plasma sources. The process limit that makes the difference between deposition of composites and multilayers is discussed.     

On the chemistry at the Si,Ti-doped a-C:H/TiC interface

Silicon-titanium-doped a-C:H, deposited via plasma assisted chemical vapor deposition and its chemistry at the titanium carbide interface has been studied via X-ray photoelectron spectroscopy. In the a-C:H film, as well as at the interface, the carbide species TiSiC is formed. Thermal treatment of Si,Ti-a-C:H films on TiC causes an increase in TiSiC at the interface leading to a better adhesion performance.     

Ellipsometric detection of gases with the surface plasmon resonance effect on gold top-coated with sensitive layers

An ellipsometric gas sensor based on the surface Plasmon resonance (SPR) effect of ~ 43 nm thick gold layers was investigated. To protect the gold layer from contamination and to improve the detection limits, the gold layers were top-coated with 5-6 nm thick layers of organic a-C:H or with inorganic metal oxides TiO₂ or ZrO₂. The additional layers increased the long-term stability, whereas the metal oxide layers protect better than a-C:H. Furthermore, the additional layers decreased the detection limits by one order of magnitude in case of a-C:H and two orders of magnitude in case of the oxides. It could be shown that the detection limits also depend on the kind of preparation (sol-gel process or physical vapour deposition) of the additional layers.     

Catalytic CVD growth and properties of a-C:H and a-C:N

Hydrogenated amorphous carbon (a-C:H) and nitrided amorphous carbon (a-C:N) films have been synthesized on quartz substrates at a substrate temperature of 700 °C using a catalytic chemical vapor deposition (Cat-CVD) method. Raman spectra of a-C:H films showed two principal bands, the G-band at 1600 cm⁻¹ and the D-band at 1350 cm⁻¹. Those of a-C:N films showed similar spectra, with a G′ band at 1640 cm⁻¹, the peak energy of which is higher than that of the G-band in a-C:H. The intensity ratio /I_D, which is a measure of the degree of order in a-C:H, decreased for a-C:H with increasing CH₄/H₂ gas-flow ratio. On the contrary, the /I_D ratio increased with increasing CH₄/H₂ gas-flow ratio.     

XPS study of the a-C:H/Ti and a-C:H/a-Si interfaces

In this study ultrathin hydrogenated amorphous carbon (a-C:H) films have been grown onto the titanium and amorphous silicon (a-Si) overlayers by direct ion beam deposition using acetylene gas as a hydrocarbon source. X-ray photoelectron spectroscopy (XPS) was used for study of the DLC-Ti and DLC-Si interfaces. It was revealed that a-Si is a good interlayer for improvement of adhesion in the case of diamond-like carbon film deposition onto the steel substrate at room temperature. a-C:H film growth without substantial intermixing occurred on the a-Si. On the other hand, adhesion between the Ti interlayer and the diamond like carbon film was very sensitive to the deposition conditions (presence of the pump oil) as well as structure and stress level of the Ti film. It was explained by strong intermixing between the growing carbon film and Ti. Bad adhesion between the growing DLC film and Ti interlayer was observed despite formation of the TiC. At the same time, formation of the TiO_x was not an obstacle for good adhesion. It is shown that composition of the used hydrocarbon gas, structure of the Ti thin film and mechanical stress in it had greater influence on adhesion with a-C:H film than elemental composition of the Ti interlayer surface.     

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.     

Effects of annealing temperature on the optical, bonding, structural and electrical properties of nitrogenated amorphous carbon thin films grown by surface wave microwave plasma chemical vapor deposition

We have studied the effects of annealing temperature (AT) on the properties of nitrogenated amorphous carbon (a-C:N) films grown at room temperature (RT) on quartz substrates by surface wave microwave plasma chemical vapor deposition (SWMP-CVD) using camphor alcohol gas as carbon plasma sources. The thickness, optical, bonding, structural and electrical properties of the as-grown (RT) and anneal-treated in range from 100 to 500°C of a-C:N films were measured and compared. The film thickness is decreased rapidly with increasing AT above 350°C. The wide range of optical absorption characteristics is observed depending on the AT. The optical band gap of as-grown a-C:N films is approximately 2.8 eV, gradually decreased to 2.5 eV for the films anneal-treated at 300°C and beyond that it decreased rapidly up to 0.9 eV at 500°C . Visible-Raman Spectroscopy (Raman) revealed the amorphous structure of as-grown a-C:N films and, the growth of nanocrystallinity of a-C:N films upon increase of AT. Raman and Fourier transform infrared spectroscopy (FTIR) analyses respectively shown the structural and composition of the films can be tuned by optimizing the AT. The change of optical, bonding, structural and electrical properties of SWMP-CVD grown a-C:N films with higher AT was attributed due to the fundamental changes in the bonding and band structure of the a-C:N films.     

Adherent amorphous hydrogenated carbon films on metals deposited by plasma enhanced chemical vapor deposition

This paper reports the findings of a study of the structural, mechanical, and tribological properties of amorphous hydrogenated carbon (a-C:H) coatings for industrial applications. These thin films have proven quite advantageous in many tribological applications, but for others, thicker films are required. In this study, in order to overcome the high residual stress and low adherence of a-C:H films on metal substrates, a thin amorphous silicon interlayer was deposited as an interface. Amorphous silicon and a-C:H films were grown by using a radio frequency plasma enhanced chemical vapor deposition system at 13.56 MHz in silane and methane atmospheres, respectively. The X-ray photoelectron spectroscopy technique was employed to analyze the chemical bonding within the interfaces. The chemical composition and atomic density of the a-C:H films were determined by ion beam analysis. The film microstructure was studied by means of Raman scattering spectroscopy. The total stress was determined through the measurement of the substrate curvature, using a profilometer, while micro-indentation experiments helped determine the films' hardness. The friction coefficient and critical load were evaluated by using a tribometer. The results showed that the use of the amorphous silicon interlayer improved the a-C:H film deposition onto metal substrates, producing good adhesion, low compressive stress, and a high degree of hardness. SiC was observed in the interface between the amorphous silicon and a-C:H films. The composition, the microstructure, the mechanical and tribological properties of the films were strongly dependent on the self-bias voltages. The tests confirmed the importance of the intensity of ion bombardment during film growth on the mechanical and tribological properties of the films.     

Preparation and comparison of a-C:H coatings using reactive sputter techniques

Amorphous hydrogenated carbon (a-C:H) coatings are widely used in several industrial applications. These coatings commonly will be prepared by plasma activated chemical vapor deposition (PACVD). The main method used to prepare a-C:H coating in industrial scale is based on a glow discharge in a hydrocarbon gas like acetylene or methane using a substrate electrode powered with medium frequency (m.f. — some 10 to 300 kHz). Some aims of further development are adhesion improvement, increase of hardness and high coating quality on complex geometries. A relatively new and promising technique to fulfil these requirements is the deposition of a-C:H coatings by a reactive d.c. magnetron sputter deposition from a graphite target with acetylene as reactive gas. An advancement of this technique is the deposition in a pulsed magnetron sputter process. Using these three mentioned techniques a-C:H coatings were prepared in the same deposition machine. For adhesion improvement different interlayer systems were applied. The effect of different substrate bias voltages (d.c. and d.c. pulse) was investigated. By applying the magnetron sputter technique in the d.c. pulse mode, plastic hardness values up to 40 GPa could be reached. Besides hardness other mechanical properties like resistance against abrasive wear were measured and compared. Cross sectional SEM images showed the growth structure of the coatings.     

镍基合金基体上离子镀TiN薄膜的制备及性能

在镍基合金Inconel 740H基底上通过多弧离子镀制备Ti N薄膜.控制温度、气体流量、过渡层成分等重要参数,研究其对Ti N薄膜的表面形貌、力学性能以及耐腐蚀性的影响.多弧离子镀沉积过程中,沉积温度分别为200、250、300℃;过渡层成分分别为Al、Cr、Ti;气体流量分别为Ar 5 Sccm∶N240 Sccm,Ar 6 Sccm∶N248 Sccm,Ar 8 Sccm∶N264 Sccm.实验结果表明:在本实验的温度范围内,Ti N薄膜的致密度、结合力以及表面硬度均随着沉积温度的提高而提高;Cr作为过渡层的效果优于Al和Ti,薄膜成分均匀、表面致密,硬度更高,且耐腐蚀性能优异;在Ar、N2流量比一定的情况下,气体流量对Ti N薄膜的表面形貌和力学性能影响不大.本实验的最佳参数是:沉积温度300℃,过渡层成分为Cr,气体流量为Ar 6 Sccm、N248 Sccm.     

Mass spectrometric studies of the mechanism of film inhibition in hydrogen/methane plasmas in the presence of nitrogen

Complete suppression of amorphous C:H film deposition has been reported for N₂/CH₄ ratios ∼1, in low-pressure technical plasmas. This finding has been recently used by the authors for the proposal of a possible technique for the inhibition of re-deposited T-containing carbon films at the divertor region of fusion devices. Although several works aiming to the understanding of the underlying physicochemical processes have been published, the complexity of the system is far from being properly described by the proposed models. In the present work, experiments in DC glow discharges at low pressure of H₂/CH₄/N₂ admixtures (90:0-5:0-5) are described. Mass spectrometry of neutral species as well as plasma mass spectrometry for ion detection have been used as the main diagnostics. Several plasma conditions (plasma current, gas composition, etc.) as well as isotopic exchange (H/D) have been investigated. Also, the effect of progressive film growing on the metal walls of the reactor in the composition of gas-phase species has been investigated. It is concluded that wall carbonisation is required to trigger the inhibition process. Ethylene and acetylene are found as the main reaction products.     

Microstructure, mechanical and tribological properties of Ti(C, N)/a-C gradient composite films

Ti(C, N)/a-C composite films with compositional gradient from Ti-TiN-Ti(C, N) to Ti-containing a-C layers have been prepared by closed-field unbalanced magnetron sputtering. Within the composite films, the carbon contents gradually increase and achieve maximum in the a-C layer by increasing the power applied to the graphite targets, the nitrogen contents gradually decrease to zero from Ti(C, N) layer of the interface to a-C layer of the films. In order to achieve a good combination of the mechanical and tribological properties in the composite films, a designed experimental parameter basing on various substrate rotation speeds is also selected. Results show that the compositional gradient result in the microstructure change of composite films where the Ti(C, N) layers consist of fine nanocolumnar Ti(C, N) grains and the a-C layers consist of 2-7 nm TiC nanocrystallites embedded in an amorphous C matrix. The Ti(C, N) layers also exhibit clear multilayer structure where the period thickness gradually decreases as substrate rotation speed increases. Under higher rotation speed, disappearance of the multilayer structure is accompanied with simultaneous increase in the crystallinity of Ti(C, N) layer and also the Ti(C, N) grain size. In the a-C layer, the TiC nanocrystallites embedded in the a-C matrix is produced by the high rotation speeds. The Ti(C, N)/a-C gradient composite films exhibit high microhardness values (~5000 H_V) and low friction coefficient (~ 0.15), which is related to the hard Ti(C, N) layer and self-lubricate a-C layer, respectively. The combination of the Ti(C, N) layer with a-C layer increases the load and the wear resistance capacity of the composite films, which gives satisfactory friction performance in the pin-on-disk tests with a wear rate of 3.7 × 10^− 17 m³/mN.     

Study of the ability to field emission from diamond-like carbon layers and carbon nanotubes

The aim of this work was to study the relationship between parameters of the electron field emission and the film deposition method. In this study two methods were applied: classical radio frequency plasma-assisted chemical vapor deposition (RF PACVD) to produce diamond-like carbon (DLC) layers and chemical vapor deposition (CVD) to produce carbon nanotubes (CNT). DLC layers were grown on n-type silicon substrates and CNT were grown on n-type and p-type silicon substrates.Atomic force microscopy (AFM) and Raman spectroscopy were used to investigate the physical and chemical parameters of DLC films after deposition process. The electrical parameters of capacitors with the DLC layer as an insulator were extracted from the capacitance-voltage (C-V) and current-voltage (I-V) characteristics. Measurements of the field emission were performed after characterization of the layer properties.     

DC substrate bias effects on the physical properties of hydrogenated amorphous carbon films grown by plasma-assisted chemical vapour deposition

Hydrogenated amorphous carbon (a-C:H) films have been grown from argon/methane gas mixtures by electron cyclotron resonance chemical vapour deposition (ECR-CVD) on silicon substrates. The effects of the application of a DC substrate bias on the structural, morphological and mechanical properties of the films have been explored by multiple analysis techniques such as infrared and micro-Raman spectroscopy, atomic force microscopy, nanoindentation and pin-on-disk wear testing. In general, within the range of applied substrate bias (i.e. from −300 up to +100 V) we have observed a strong correlation between all measured properties of the a-C:H films and the ion energy. This work shows that the properties can differ greatly and indicates a threshold energy in the order of 90 eV. For the production of hard, low-friction coatings energies above this value are required.     

Microcrystalline silicon carbide alloys prepared with HWCVD as highly transparent and conductive window layers for thin film solar cells

Crystalline silicon carbide alloys have a very high potential as transparent conductive window layers in thin-film solar cells provided they can be prepared in thin-film form and at compatible deposition temperatures. The low-temperature deposition of such material in microcrystalline form (µc-Si:C:H) was realized by use of monomethylsilane precursor gas diluted in hydrogen with the Hot-Wire Chemical Vapor Deposition process. A wide range of deposition parameters has been investigated and the structural, electronic and optical properties of the µc-SiC:H thin films have been studied. The material, which is strongly n-type from unintentional doping, has been used as window layer in n-side illuminated microcrystalline silicon solar cells. High short-circuit current densities are obtained due to the high transparency of the material resulting in a maximum solar cell conversion efficiency of 9.2%.     

Properties of tin/plasma polymer nanocomposites

Thin composite layers (tin in plasma polymer matrix) were prepared in a stainless steel vacuum chamber. An RF powered magnetron with tin target was used to excite the discharge and to activate the monomer species (n-hexane). The gas mixture introduced comprised Ar and n-hexane vapours. The properties of the films and chemical composition were characterized by AFM (surface morphology), TEM and Electron tomography (bulk structure characterization), XPS and FTIR spectroscopy (chemical composition analyses). Current-voltage characteristics were measured to examine the electrical properties of the layers and their dependence on the deposition parameters.     

Amorphous hydrogenated silicon-nitride films for applications in solar cells

The application of anti-reflective coatings (ARC) is a good method to improve the solar cell construction. The authors developed the RF plasma enhanced chemical vapour deposition method for preparation of amorphous silicon-nitrogen (a-Si:N:H) films for potential optoelectronic applications. The films have been obtained on borosilicate glass and monocrystalline silicon (1 0 0) (Cz-Si) in a process with optimised technological parameters such as a content of gaseous mixture of silane (SiH₄) and ammonia (NH₃). The properties of samples have been investigated by optical spectroscopy (PERKIN-ELMER Lambda 19) and scanning electron microscopy (SEM). The correlation between film properties and process parameters has been found. The results of optical investigations show that these materials are characterised by a variable optical gap dependent on the nitrogen content. After deposition of a-Si:N:H, a decrease in the total reflectivity, as compared to that of monocrystalline Si, was observed. The simulation of multicrystalline silicon solar cells performance with and without the ARC was done with the use of PC1D programme. The influence of the ARC on solar cell efficiency was observed. The obtained results indicate that a-Si:N:H films are suitable for application as antireflective and protective coatings for solar cells.     

Synthesis and physicochemical characterization of silicon oxynitride thin films prepared by rf magnetron sputtering

SiO(x) N(y) thin films deposited by rf magnetron sputtering to realize low-loss optical multilayers have been studied. We have analyzed the variations of the optical and physicochemical properties of oxynitride layers according to the deposition parameters: the gas partial pressures, the rf power, and the target composition. A linear variation of the layer refractive index as a function of the oxygen partial pressure was observed as well as a strict substitution of O atoms by N atoms. Thanks to IR spectrophotometric analyses, a model of the oxynitride amorphous structure was proposed and confirmed by Bruggeman and Gained approximation methods. Finally, the absorption level of the oxynitride layers was studied by photothermal deflection spectroscopy.