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.     

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.     

Non-equilibrium crystallization-aggregation mechanism of a-C:H layer in a-C:H/a-Se complex films

Also representing: CCAST (World Laboratory), Beijing 100080, People's Republic of China and the Structure Research Laboratory, University of Sciences and Technology of China, Hefei 230026, People's Republic of China.     

Properties of nitrogen diluted hydrogenated amorphous carbon (n-type a-C:H) films and their realization in n-type a-C:H/p-type crystalline silicon heterojunction diodes

Nitrogen incorporated hydrogenated amorphous carbon (a-C:N:H) films were grown in an asymmetric rf PECVD system using C₂H₂ and N₂ gaseous mixture. Deposition rate, stress, hardness, optical bandgap, refractive index, and electrical characteristics have been studied as a function of self bias. Microstructures of these films were also studied using LASER Raman technique. Finally nitrogen diluted a-C:H films were realized as n-type semiconductor in n-type a-C:H/p-type crystalline silicon hetrojunction diodes. Current-voltage (I-V) and capacitance-voltage (C-V) characteristics have also been studied as a function of self bias on these heterojunction diodes.     

致密非晶碳氢薄膜的结构

利用完全抑制网络结构（ＦＣＮ）模拟计算氢化非晶碳（ａ－Ｃ：Ｈ）膜的组成。形成致密ａ－Ｃ：Ｈ膜的条件是Ｈ、ｓｐ＾２Ｃ和ｓｐ＾３Ｃ应在其三元相图中的一个三角形区域内。实验数据表明,模拟结果与实验结果吻合得很好。在ａ－Ｃ：Ｈ中存在氢、四面体碳、乙烯、苯环和双苯等结构,它们的相对含量在三角形区域内变化。该理论模拟对薄膜光学间隙的实验分析给出了很好的解释。     

Tribological behaviour a-C and a-C:H films doped with Ti in biological solutions

The coatings were deposited in a multi magnetron Teer sputtering device with two C targets and a Ti target in Ar atmosphere, on steel and Si substrates. To deposit hydrogenated coatings methane was introduced in the discharge atmosphere. The films were deposited in a range of 7-14 at.% Ti content. X-ray diffraction patterns revealed the presence of a nanocrystalline TiC phase. The hardness varied from 8 to 9 GPa. The tribological tests were performed under dry sliding and lubricated (0.9% NaCl water solution, physiological solution, PS, and 10% fetal bovine serum dissolved in Ringer’s saline solution, FBS) conditions, using a 100Cr6 steel balls with a diameter of 6 mm, in a pin-on-disc wear test apparatus.     

Hydrogen evolution from a-C:H films

Diamond-like amorphous carbon films with high hardness, high electrical resistivity, good optical transparency into the infra-red, and good adherence to their substrates may be produced by many methods, most of which result in the incorporation of 20–40 at% hydrogen. These a-C:H films are similar to a-Si:H films and the present work contributes additional evidence for this similarity. a-C:H films were deposited on glass or NaCl-coated glass in a capacitively coupled rf discharge in CH₄. The nature of the films depends upon the specific power density (SPD): polymeric for SPD < 10W cm^?2 torr^?1, diamond-like for SPD between 10 and 20, and graphitic for SPD>25. The amount of chemisorbed hydrogen in the films, as determined by differential scanned calorimetry (DSC), also depends on the SPD (and concomitant negative self-bias of the substrate), decreasing by a factor of six as the SPD increases from 10 to 20 and the negative self-bias increases form 400–650V. Additionally, the exothermic DSC peak sometimes consists of two or more partially resolved overlapping peaks.     

a—C：H（N）薄膜的慢正电子分析

采用Ｃ２Ｈ２和Ｎ２的混合气体在单晶Ｓｉ衬底上用射频－直流等离子化学气相沉积法制备ａ－Ｃ：Ｈ（Ｎ）薄膜,用Ｆｏｕｒｉｅｒ红外谱研究了ａ－Ｃ：Ｈ（Ｎ）薄膜的结构。用慢正电子湮没技术研究了类金刚石薄膜中缺陷的深度分布以及Ｎ的百分含量对薄膜中缺陷浓度和缺陷类型的影响。Ｆｏｕｒｉｅｒ红外测试结果表明,ａ－Ｃ：Ｈ（Ｎ）薄膜中氮含量随混合气体中Ｎ２百分含量的升高而增大,薄膜中碳氮原子形成Ｃ≡Ｎ键。慢正电子分析表     

Nanoindentation experiments on some amorphous hydrogenated carbon (a-C:H) thin films on silicon

Ultra-low-load indentation (nanoindentation) experiments have been used to investigate the load-displacement characteristics of two types of hydrogenated carbon films (a hard and a soft version 230 and 210 nm thick, respectively) deposited from methane on to (1 0 0) single-crystal silicon wafers by a radio frequency plasma-assisted chemical vapour deposition process. Further, the technique was used to explore the changes in the properties of the films with heat treatment in vacuum at temperatures of up to 650°C. In all cases, the elastic and plastic properties of the films were compared at indentation loads in the range 0–60 mN, the higher loads causing indentor displacements greater than the thickness of the films. For the harder, stiffer coating, penetration resistance was found to decrease with increasing indenter displacement, reflecting the greater load-carrying role taken by the softer silicon with increasing applied load. However, for the softer coating, penetration resistance generally increased with displacement, perhaps reflecting progressive compaction of the coating in addition to the increasing role of the silicon. In both cases, heat treatment severely degraded the mechanical properties of the films due to thermally induced chemical changes and, in the case of the hard coating, relaxation of residual stresses. Scanning electron microscopy of both nanoindentations and low-load microhardness indentations clearly reveals the deformation mechanisms associated with contact stresses to include flow and fracture of the film and interfacial decohesion.     

Laser-induced transformation of a-C:H thin films

In this work, we report the laser irradiation effects on the properties of various types of amorphous hydrogenated carbon (a-C:H) films. The influence of the initial carbon film (hydrogen concentration, sp³/sp² ratio, and sp² clustering) is studied. The results show that a loss of hydrogen and an increase of the sp² phase are the main processes in the laser power range between 1.8 and 5 MW/cm². Only these processes are stronger for “more polymer-like” and “graphite-like” films than for “more diamond-like” films.     

沉积参数对含氢非晶碳膜结构及性能的影响

本文利用射频等离子体增强化学气相沉积（RF-PECVD）技术,以CH4、H2为气源,Ar-为稀释气体,通过增加过渡层和改变输入功率和气体成分比例（CH／H2）,在不锈钢基底上制备了含氢非晶碳膜（a-C：H）。利用拉曼光谱（Raman）、X射线光电子能谱（XPS）、纳米显微硬度计和摩擦磨损试验机等研究手段对含氢非晶碳膜的形貌、结构、显微硬度和耐磨性进行了表征。Ramas光谱和XPS分析表明,薄膜是由sp^2和sp^3杂化组成的非晶碳膜。显微硬度和摩擦学测试表明,在较低射频功率和富氢等离子体中沉积的a—C：H膜表面光滑、结构致密,薄膜的硬度、摩擦系数、耐磨性和结合力等性能较好。     

Microstructure and optical properties of chromium containing amorphous hydrogenated carbon thin films (a-C:H/Cr)

Chromium containing amorphous hydrogenated carbon thin films was deposited using a dc sputter deposition technique under various mixtures of methane and Ar. The microstructure, composition, and optical properties of the resulting films were investigated. We show that a-C:H/Cr thin films exhibiting absorptance in certain wavelengths are greater than 95% and the average absorptance was 86% in the 0.3 to 2.5 µm wavelength can be obtained by using appropriate methane/Ar ratios and deposition times.     

Deposition of polymeric nitrogenated amorphous carbon films (a-C:H:N) using electron cyclotron resonance CVD

In this paper the deposition of polymeric nitrogenated amorphous carbon (a-C:H:N) films from a mixture of hydrogen, methane and nitrogen, using the electron cyclotron resonance chemical vapor deposition (ECR-CVD) system is reported. Rutherford backscattering spectroscopy (RBS) and elastic recoil detection analysis (ERDA) measurements were used to determine the actual amount of nitrogen, carbon and hydrogen in the films. The results showed that there is insignificant change in the atomic concentration of C but a slight decrease in the atomic concentration of H with increasing N incorporation. A slight decrease in the amount of bonded H and an increase in the C=N bond was also observed as deduced from infrared (IR) absorption measurements. The optical gap was found to decrease and the Urbach band tail width increase at larger N₂ flow ratio. The conductivity increased by three orders of magnitude compared to that of films prepared in the absence of N₂. The results suggest that the observed C=N double bond effectively bridges the aromatic rings resulting in a delocalization of carriers.     

Metal-containing amorphous carbon (a-C:Ag) and AlN (AlN:Ag) metallo-dielectric nanocomposites

In this work we study the structure of Pulsed Laser Deposited silver-containing amorphous carbon a-C:Ag and AlN:Ag composites. The films have the form of a dielectric matrix (a-C or AlN) incorporating metal nanoclusters of 3-10 nm diameters. The composition of the films and the crystal structure of the inclusions have been determined by in-situ Auger Electron Spectroscopy and transmission electron microscopy, respectively. We show that the incorporation of the metal nanoparticles may severely alter the structure and properties of the matrix and a simple rule of mixture does not apply. The interactions of the possible matrix-nanoparticle interfaces were investigated using ab-initio calculations.     

Mechanical and Tribological Properties of a-C:H:F Thin Films

a-C:H films were grown by plasma-enhanced chemical vapor deposition in atmospheres composed by 30 % of acetylene and 70 % of argon. Radiofrequency signal (RF) was supplied to the sample holder to generate the depositing plasmas. Deposition time and pressure were chosen 300 s and 9.5 Pa, respectively, while the excitation power changed from 5 to 125 W. The films were exposed to a post-deposition treatment during 300 s in RF-plasmas (13.56 MHz, 70 W) excited from 13.33 Pa of SF₆. Raman and X-ray photoelectron spectroscopy were used to evaluate the microstructure and chemical composition of the films. The thickness was measured by perfilometry. Hardness and friction coefficient were determined from nanoindentation and risk tests, respectively. With increasing power, the film thickness reduced, but a further shrinkage occurred upon the fluorination process. After that, the molecular structure was observed to vary with deposition power. Fluorine was detected in all samples replacing H atoms. Consistently with the elevation in the proportion of C atoms with sp³ hybridization, hardness increased from 2 to 18 GPa. Friction coefficient also increased with power due to the generation of dangling bonds during the fluorination process.     

Mechanical properties of a-C:H and a-C:H/SiOx nanocomposite thin films prepared by ion-assisted plasma-enhanced chemical vapor deposition

a-C:H and a-C:H/SiO_x nanocomposite thin films were deposited on silicon, aluminum and polyimide substrates at 25 °C in an asymmetric 13.56 MHz r.f.-driven plasma reactor under heavy ion bombardment. Fourier transform infrared spectra of the films indicate that the nanocomposite filmsappears to consist of an atomic scale random network of a-C:H and SiO_x. Raman spectroscopy revealed that the sp² carbon fraction in the nanocomposite film was reduced compared with the a-C:H film. The intrinsic stress of both films increased with increasing negative bias voltage (−V_dc) at the substrate. However, the nanocomposite films exhibited lower intrinsic stress compared w with a-C:H-only films. Especially, a thin SiO_x-rich interlayer was very effective in reducing the film stress and enhancing the bonding strength at the interface. The interlayer allowed deposition of thick films of up to 5 μm. Also, the nanocomposite films were stable in 0.1 M NaOH solution and showed good microhardness.     

非晶碳膜的表面润湿性对生物相容性的影响

采用等离子体注入沉积方法(PⅢ-D),混合通入C2H2、Ar或N2,制备了具有不同表面润湿性的非晶碳薄膜a-CH和掺N非晶碳薄膜a-CNH.采用Raman及XPS方法对薄膜的结构进行了分析,采用血小板黏附实验评价薄膜的抗凝血性能,采用材料表面的内皮细胞培养,对材料的细胞毒性以及生物相容性进行评价.结果表明非晶碳膜的生物相容性的提高与其表面的润湿性的变化密切相关,可通过掺杂特定的元素,增加a-CH薄膜表面张力的极性倾向,提高非晶碳膜的亲水性,来进一步改进a-CH薄膜的生物相容性,特别是血液相容性.     

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.     

Mechanical and tribological properties of gradient a-C:H/Ti coatings

The unusual combination of high hardness and very low friction coefficient are the most attractive tribological parameters of DLC (diamond-like carbon) layers. However, their usability is strongly restricted by the limited thickness due to high residual stress. The main goal of the presented work was to obtain thick, wear resistant and well adherent DLC layers while keeping their perfect friction parameters. As a proposed solution a Ti-Ti _x C _y gradient layer was manufactured as the adhesion improving interlayer followed by a thick diamond-like carbon film. This kind of combination seems to be very promising for many applications, where dry friction conditions for highly loaded elements can be observed. Both layers were obtained in one process using a hybrid deposition system combining PVD and CVD techniques in one reaction chamber. The investigation was performed on nitrided samples made from X53CrMnNiN21-9 valve steel. Structural features, surface topography, tribological and mechanical properties of manufactured layers were evaluated. The results of the investigation confirmed that the presented deposition technique makes it possible to manufacture thick and well adherent carbon layers with high hardness and very good tribological parameters. Preliminary investigation results prove the possibility of application of presented technology in automotive industry.