Amorphous hydrogenated carbon films on semiconductors

The adhesion quality of amorphous hydrogenated carbon films (a-C:H) on semiconductor substrates depends to a large degree on the properties of the interface. The present work complements the photoemission results of the preceding paper with a detailed investigation of the atomic structure of the a-C:H/Si and a-C:H/GaAs interfaces. We show that the method of substrate cleaning and the deposition parameters affect the thickness of the interfacial layer and the interface roughness. The carbide compounds that form in the interfacial layer are found to be amorphous and we present evidence for the precipitation of metallic Ga at the a-C:H/GaAs interface. Finally, we have determined the extent of atomic intermixing in the interfacial region and compare our results with different mechanisms of adhesion.     

Transport and optical properties of amorphous carbon and hydrogenated amorphous carbon films

In this paper we report on the electrical and optical properties of amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C:H) films. Resistivity of both types of films decreases with increase in temperature. At lower temperatures (60-250 K) the electron transport is due to variable range hopping for the a-C films. At higher temperatures (300-430 K) it is thermally activated for both types of films. Analysis of the heterojunction between diamond-like carbon (DLC) and bulk silicon (Si) leads to the conclusion that our a-C films are of n-type and our a-C:H films are of p-type. The optical measurements with DLC revealed a Tauc bandgap of 0.6 eV for the a-C films and 1-1.2 eV for the a-C:H films. An Urbach energy around 170 meV could be determined for the a-C:H films. Strain versus resistance plots were measured resulting in piezoresistive gauge factors around 50 for the a-C films and in between 100 and 1200 for the a-C:H films.     

Intrinsic mechanical properties of ultra-thin amorphous carbon layers

In this work, we extracted the film's hardness (H_F) of ultra-thin diamond-like carbon layers by simultaneously taking into account the tip blunting and the substrate effect. As compared to previous approaches, which did not consider tip blunting, this resulted in marked differences (30-100%) for the H_F value of the thinner carbon coatings. We find that the nature of the substrate influences this intrinsic film parameter and hence the growth mechanisms. Moreover, the H_F values generally increase with film thickness. The 10 nm and 50 nm thick hydrogenated amorphous carbon (a-C:H) films deposited onto Si have H_F values of, respectively, ∼26 GPa and ∼31 GPa whereas the 10 nm and 50 nm thick tetrahedral amorphous carbon (t-aC) films deposited onto Si have H_F values of, respectively, ∼29 GPa and ∼38 GPa. Both the a-C:H and t-aC materials also show higher density and refractive index values for the thicker coatings, as measured, respectively by X-ray reflectometry and optical profilometry analysis. However, the Raman analysis of the a-C:H samples show bonding characteristics which are independent of the film thickness. This indicates that in these ultra-thin hydrogenated carbon films, the arrangement of sp² clusters does not relate directly to the hardness of the film.     

New type of regular carbon nanostructures: Nanocones on the surfaces of carbon-silicon (a-C:H) composite films

The results of a study of a new type of regular carbon nanostructures, namely, nanocones on the film surfaces of carbon-silicon composites of the (a-C:H):Si type are given in this paper. Nanocones appear under the action of the electric field of the probe of an air-operated scanning probe microscope (SPM) in the case where the values of the voltage amplitude and the exposure times exceed the threshold ones. The produced nanoobjects preserve their shapes at thermal annealing up to 700°C. It is assumed that the process of nanocone formation is related to the local transformation of the carbon-silicon composite structure from the amorphous to the nanocrystalline state in the electric field of the SPM probe.     

Thermal stability of ultrathin amorphous carbon films synthesized by plasma-enhanced chemical vapor deposition and filtered cathodic vacuum arc

Abstract

Ultrathin hydrogenated amorphous carbon (a-C:H) films deposited by plasma-enhanced chemical vapor deposition (PECVD) and hydrogen-free amorphous carbon (a-C) films of similar thickness deposited by filtered cathodic vacuum arc (FCVA) were subjected to rapid thermal annealing (RTA). Cross-sectional transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) were used to study the structural stability of the films. While RTA increased the thickness of the intermixing layer and decreased the sp³ content of the a-C:H films, it did not affect the thickness or the sp³ content of the a-C films. The superior structural stability of the FCVA a-C films compared with PECVD a-C:H films, demonstrated by the TEM and EELS results of this study, illustrates the high potential of these films as protective overcoats in applications where rapid heating is critical to the device functionality and performance, such as heat-assisted magnetic recording.     

Simultaneous interaction of methyl radicals and atomic hydrogen with amorphous hydrogenated carbon films, as investigated with optical in situ diagnostics

Methyl radicals (CH₃) and atomic hydrogen (H) are dominant radicals in low-temperature plasmas from methane. The surface reactions of these radicals are believed to be key steps leading to deposition of amorphous hydrogenated carbon (a-C:H) films or polycrystalline diamond in these discharges. The underlying growth mechanism is studied, by exposing an a-C:H film to quantified radical beams of H and CH₃. The deposition or etching rate is monitored via ellipsometry and the variation of the stoichiometry is monitored via isotope labeling and infrared spectroscopy. It was shown recently that, at 320 K, methyl radicals have a sticking coefficient of 10^-4 on a-C:H films, which rises to 10^-2 if an additional flux of atomic hydrogen is present. This represents a synergistic growth mechanism between H and CH₃. From the interpretation of the infrared data, a reaction scheme for this type of film growth is developed: atomic hydrogen creates dangling bonds by abstraction of bonded hydrogen within a surface layer corresponding to the range of H in a-C:H films. These dangling bonds serve at the physical surface as adsorption sites for incoming methyl radicals and beneath the surface as radicalic centers for polymerization reactions leading to carbon–carbon bonds and to the formation of a dense a-C:H film. Received: 18 July 2000 / Accepted: 12 December 2000 / Published online: 3 April 2001     

Field electron emission from amorphous carbon thin films grown by RF magnetron sputtering

Using a RF magnetron sputtering, amorphous carbon (a-C) and N-doped a-C (a-C:N) thin films were fabricated as field electron emitter. These thin films were deposited on Si(0 0 1) substrate at several temperatures. The field emission property was improved for a-C thin films grown at higher substrate temperatures. Furthermore, a-C:N film exhibits field emission property better than that of undoped a-C film. These results are explained in terms of the change in surface morphology and structural properties of a-C film.     

Comparison of the X-ray photoelectron and electron-energy-loss spectra of the nitrogen-doped hydrogenated amorphous carbon bond

The composition of nitrogen-doped hydrogenated amorphous carbon (a-C : H : N) films grown in a magnetically confined rf plasma-enhanced chemical vapour deposition system has been determined by X-ray photoelectron spectroscopy (XPS) and compared with that determined using a combination of elastic recoil detection analysis, Rutherford back-scattering and nuclear reaction analysis. The importance of nitrogen doping or 'incorporation' in hydrogenated amorphous carbon (a-C : H) films is discussed in relation to the significant variation in the sp 2 -to-sp 3 ratio that takes place. At 7 at.% N in the a-C : H matrix, a critical change in the microstructure is observed, which governs the resulting mechanical, optical and electronic properties. Finally, the correlation between the sp 2 and sp 3 fractions determined by a non-destructive method of obtaining the bond fractions (XPS) and by electron-energy-loss spectroscopy is discussed, with a view to evaluating accurately the sp 2 fraction in a-C : H : N films.     

Structural and tribological properties of nitrogen doped amorphous carbon thin films synthesized by CFUBM sputtering method for protective coatings

Nitrogen doped amorphous carbon (a-C:N) films are a material that may successfully compete with DLC coatings, which have high hardness, high wear resistance, and a low friction coefficient. The a-C:N films were prepared on silicon substrate by a closed-field unbalanced magnetron sputtering method with a graphite target and using the Ar/N₂ mixture gases. And, we investigated the effects of various DC bias voltages from 0 to −300 V on the structural and tribological properties of the a-C:N films. This study was focused on improving physical properties of the a-C:N film by controlling process parameters like negative substrate DC bias voltage. The maximum hardness of the a-C:N film was 23 GPa, the friction coefficient was 0.08, and the critical load was 25 N on a Si wafer. Consequently, the structural and tribological properties of the a-C:N film showed a clear dependence on the energy of ions bombardment and the density of the sputtering and the reaction gases during film growth.     

Hydrogenated amorphous carbon thin films deposited by plasma-assisted chemical vapor deposition enhanced by electrostatic confinement: structure,properties, and modeling

Hydrogenated amorphous carbon (a-C:H) is a state-of-the-art material with established properties such as high mechanical resistance, low friction, and chemical inertness. In this work, a-C:H thin films were deposited by plasma-assisted chemical vapor deposition. The deposition process was enhanced by electrostatic confinement that leads to decrease the working pressure achieving relative high deposition rates. The a-C:H thin films were characterized by elastic recoil detection analysis, Rutherford backscattering spectroscopy, scanning electron microscopy, Raman spectroscopy, and nanoindentation measurements. The hydrogen content and hardness of a-C:H thin films vary from 30 to 45 at% and from 5 to 15 GPa, respectively. The hardness of a-C:H thin films shows a maximum as a function of the working pressure and is linearly increased with the shifting of the G-peak position and I _D/I _G ratio. The structure of a-C:H thin films suffers a clustering process at low working pressures. A physical model is proposed to estimate the mean ion energy of carbonaceous species arriving at the surface of a-C:H thin films as a function of processing parameters as pressure and voltage and by considering fundamentals scattering events between ion species and neutral molecules and atoms.     

Structural and optical properties of Fe-doped hydrogenated amorphous carbon films prepared from trans-2-butene by plasma enhanced metal organic chemical vapor deposition

Fe-doped hydrogenated amorphous carbon (a-C:H:Fe) films were deposited from a gas mixture of trans-2-butene/ferrocene/H₂ by plasma enhanced metal organic chemical vapor deposition. X-ray photoelectron spectroscopy, Fourier transform infrared spectra and Raman spectra were used to characterize the composition and the bonding structure of the a-C:H:Fe and a-C:H films. Optical properties were investigated by the UV–visible spectroscopy and the photoluminescence (PL) spectra. The Fe-doped films contain more aromatic structures and C=C bonds than the undoped films. The sp ² carbon content and sp ² clustering of the films increase, and aromatic-like rings’ structures become richer after Fe-doping. The Tauc optical gap of the a-C:H:Fe films become narrower by 0.3 eV relative to the value of the a-C:H films. The PL peak shifts from 2.35 eV of the a-C:H films to 1.95 eV of the a-C:H:Fe films, and the PL intensity of the a-C:H:Fe films is greatly enhanced. A deep level emission peak around 2.04 eV of the a-C:H:Fe films is observed.     

Rare earth Ce-modified (Ti,Ce)/a-C:H carbon-based film on WC cemented carbide substrate

WC cemented carbide suffers severe wear in water environments. A novel carbon-based film could be a feasible way to overcome this drawback. In this study, a rare earth Ce-modified(Ti,Ce)/a-C:H carbon-based film is successfully prepared on WC cemented carbide using a DC reactive magnetron sputtering process. The microstructure, mechanical properties,and tribological behavior of the as-prepared carbon-based film are systematically investigated. The results show that the doping Ti forms Ti C nanocrystallites that are uniformly dispersed in the amorphous carbon matrix, whereas the doping Ce forms CeO_2 that exists with the amorphous phase in the co-doped(Ti,Ce)/a-C:H carbon-based film. The mechanical properties of this(Ti,Ce)/a-C:H film exhibit remarkable improvements, which could suggest higher hardness and elastic modulus as well as better adhesive strength compared to solitary Ti-doped Ti/a-C:H film. In particular, the as-prepared(Ti,Ce)/a-C:H film presents a relatively low friction coefficient and wear rate in both ambient air and deionized water,indicating that(Ti,Ce)/a-C:H film could feasibly improve the tribological performance of WC cemented carbide in a water environment.     

Structure and phase composition of thin a-C:H films modified by Ag and Ti

The structure and phase composition of thin a-C:H and a-C:H〈M〉 films (M = Ag, Ti, or Ag + Ti) have been studied by Raman and X-ray photoelectron spectroscopy. The a-C:H〈M〉 films were prepared by ion-plasma magnetron sputtering of a combined target of graphite and metal in an Ar–CH₄ gas mixture. The Raman spectra of these films indicate that their structure is amorphous. The a-C:H〈Ag + Ti〉 films have a more graphitized structure in comparison with pure a-C:H films and films containing only one metal. It is established that carbon in the a-C:H〈Ag + Ti〉 films is in the sp ², sp ³, and C=O states, which are characteristic of the a-C:H, a-C:H〈Ag〉, and a-C:H〈Ti〉 films. In addition, there are also ether (–C–O–C–) or epoxy (?C?O–) carbon groups in the a-C:H〈Ag + Ti〉 films. It has been revealed that silver atoms in the a-C:H〈Ag〉 and a-C:H〈Ag + Ti〉 films form no chemical bonds with carbon, oxygen, and titanium. Titanium in the a-C:H〈Ti〉 and a-C:H〈Ag + Ti〉 films exists in the form of titanium IV oxide (TiO₂).     

Electron paramagnetic resonance of amorphous hydrogenated carbon grown under different conditions of deposition

Results from electron paramagnetic resonance (EPR) studies of amorphous hydrogenated carbon (a-C:H) obtained on a quartz substrate under different conditions of deposition E/p (where E is the electric field intensity between the electrodes, and p is the pressure of the gas mix in the capacitive chamber) and temperature of the substrate are presented. Correlations between the EPR line amplitude, spin density, and g-factor, and the optical, electrical, and mechanical properties of a-C:H are found.     

First-principles studies of the vibrational properties of amorphous carbon nitrides

Raman spectra of amorphous carbon nitride films (a-C:N) resemble those of typical amorphous carbon (a-C), and no specific features in the spectra are shown due to N doping. The present work provides a correlation between the microstructure and vibrational properties of a-C:N films from first principles. The six periodic model structures of 64 atoms with various mass densities and nitrogen contents are generated by the liquid-quench method using Car-Parinello molecular dynamics. By using Raman coupling tensors calculated with the finite electric field method, Raman spectra are obtained. The calculated results show that the vibrations of C=N could directly contribute to the Raman spectrum. The similarity of the Raman line shapes of N-doped and N-free amorphous carbons is due to the overlapping of C=N and C=C vibration bands. In addition, the origin of characteristic Raman peaks is also given.     

类金刚石a-C:H膜的热释氢和红外吸收研究

本文利用热释氢和红外吸收研究了H在非晶态碳(a-C:H)膜中的含量和组态。实验结果表明,随着样品制备时衬底温度的增大:1)H在a-C:H膜中的组态从两相结构过渡为单相结构;2)H在a-C:H膜中的含量单调减少;3)a-C:H膜中sP³/sP²键合比例单调增大。 关键词：     

金刚石表面无定形碳氢薄膜生长的分子动力学模拟

利用分子动力学模拟方法研究了CH₂基团轰击金刚石(111)面所形成的无定形碳氢薄膜(a-C:H)的生长过程. 结构分析表明, 得到的无定形碳氢薄膜中碳原子的局域结构(如C–C第一近邻数)与其中氢原子的含量密切相关. CH₂ 基团入射能量的增加会导致得到的薄膜的氢含量降低, 从而改变薄膜中类sp³成键碳原子的比例.     

Effect of Chromium on Structure and Tribological Properties of Hydrogenated Cr/a-C:H Films Prepared via a Reactive Magnetron Sputtering System

Hydrogenated Cr-incorporated carbon films(Cr/a-C:H) are deposited successfully by using a dc reactive magnetron sputtering system.The structure and mechanical properties of the as-deposited Cr/a-C:H films are characterized systematically by field-emission scanning electron microscope,x-ray diffraction,Raman spectra,nanoindentation and scratch.It is shown that optimal Cr metal forms nanocrystalline carbide to improve the hardness,toughness and adhesion strength in the amorphous carbon matrix,which possesses relatively higher nano-hardness of 15.7GPa,elastic modulus of 126.8 GPa and best adhesion strength with critical load(L_c) of36 N for the Cr/a-C:H film deposited at CH_4 flow rate of 20 sccm.The friction and wear behaviors of as-deposited Cr/a-C:H films are evaluated under both the ambient air and deionized water conditions.The results reveal that it can achieve superior low friction and anti-wear performance for the Cr/a-C:H film deposited at CH_4 flow rate of 20 sccm under the ambient air condition,and the friction coefficient and wear rate tested in deionized water condition are relatively lower compared with those tested under the ambient air condition for each film.Superior combination of mechanical and tribological properties for the Cr/a-C:H film should be a good candidate for engineering applications.     

a-C:N:H纳米尖端荧光产生的机理

用CH4,H2和NH3为反应气体,利用等离子体增强热丝化学气相沉积在沉积有碳膜的Si衬底上制备了a-C:N:H纳米尖端,并用扫描电子显微镜和微区Raman光谱仪对碳膜和纳米尖端进行了表征。结果表明:Raman谱中含有与碳和氮相关的峰,且纳米尖端的Raman谱比碳膜的Raman谱有很强的荧光背景。Raman谱中的峰说明沉积的碳膜和纳米尖端是a-C:N:H薄膜和a-C:N:H尖端。a-C:N:H纳米尖端的Raman谱中强荧光背景的产生表明其在激发光源照射的过程中发射了强荧光,对a-C:N:H纳米尖端产生强荧光的机理进行了探讨。     

Surface energy and hybridization studies of amorphous carbon surfaces

Surface properties of a large number of amorphous carbon (a-C) films have been investigated using contact angle measurements and X-ray photoelectron spectroscopy (XPS). Dense a-C surfaces with variable sp³/(sp² + sp³) average hybridization were grown using sputtering or pulsed laser deposition (PLD) and were further chemically modified by thermal annealing, ion bombardment or covalent grafting of organic monolayers. The average carbon hybridization, impurity level and mass density, were deduced from XPS and photoelectron energy loss spectroscopy (PEELS). The depth sensitivity of the dispersive (Lifshitz–van der Waals) interaction, estimated at 1–2 nm from the dependence of γ^LW on the grafted perflorodecene molecule coverage, is much better than XPS which probes a 3–5 nm depth. The observation of a non-monotonic behavior in the correlation between surface hybridization and electron donor component of surface energy reveals that the average carbon hybridization alone does not describe the entire surface energy physics. The role of π bond clustering in the polar interactions is thus considered and some implications on surface reactivity and mutual interactions with molecular or biomolecular species are discussed.