Optical and mechanical properties of diamond like carbon films deposited by microwave ECR plasma CVD

Diamond like carbon (DLC) films were deposited on Si (111) substrates by microwave electron cyclotron resonance (ECR) plasma chemical vapour deposition (CVD) process using plasma of argon and methane gases. During deposition, a d.c. self-bias was applied to the substrates by application of 13·56 MHz rf power. DLC films deposited at three different bias voltages (−60 V, −100 V and −150 V) were characterized by FTIR, Raman spectroscopy and spectroscopic ellipsometry to study the variation in the bonding and optical properties of the deposited coatings with process parameters. The mechanical properties such as hardness and elastic modulus were measured by load depth sensing indentation technique. The DLC film deposited at −100 V bias exhibit high hardness (∼ 19 GPa), high elastic modulus (∼ 160 GPa) and high refractive index (∼ 2·16–2·26) as compared to films deposited at −60 V and −150 V substrate bias. This study clearly shows the significance of substrate bias in controlling the optical and mechanical properties of DLC films.     

Structure of diamond and diamond like carbon thin films grown by hot-filament chemical vapour deposition technique

Micro-crystalline diamond (MCD) and diamond like carbon (DLC) thin films were deposited on silicon (100) substrates by hot-filament CVD process using a mixture of CH₄ and H₂ gases at substrate temperature between 400–800°C. The microstructure of the films were studied by X-ray diffraction and scanning electron microscopy. The low temperature deposited films were found to have a mixture of amorphous and crystalline phases. At high temperatures (> 750°C) only crystalline diamond phase was obtained. Scanning electron micrographs showed faceted microcrystals of sizes up to 2μm with fairly uniform size distribution. The structure of DLC films was studied by spectroscopic ellipsometry technique. An estimate of the amount of carbon bonds existing insp ² andsp ³ form was obtained by a specially developed modelling technique. The typical values ofsp ³/sp ² ratio in our films are between 1·88–8·02. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Evaluation of microstructures and mechanical properties of diamond like carbon films deposited by filtered cathodic arc plasma

Diamond-like carbon (DLC) films were deposited by a cathodic arc plasma evaporation (CAPD) process, using a mechanical shield filter combined with a magnetic filter with enhanced arc structure at substrate-bias voltage ranging from − 50 to − 300 V. The film characteristics were investigated using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM). The mechanical properties were investigated by using a nanoindentation tester, scratch test and ball on disc wear test. The Raman spectra of the films showed that the wavenumber ranging from 900 to 1800 cm^− 1 could be deconvoluted into 1140 cm^− 1, D band and G band. The bias caused a significant effect on the sp³ content which was increased with the decreasing of I_D/I_G ratio. The XPS spectra data of the films which were etched by H⁺ plasma indicated the sp³ content are higher than those of the as-deposited DLC films. This implied that there is a sp²-rich layer present on the surface of the as-deposited DLC films. The nanoindentation hardness increased as the maximum load increased. A 380 nm thick and well adhered DLC film was successfully deposited on WC-Co substrate above a Ti interlayer. The adhesion critical load of the DLC films was about 33 N. The results of the wear tests demonstrated that the friction coefficient of the DLC films was between 0.12 and 0.2.     

Deposition of diamond-like carbon films by using liquid phase electrodeposition technique and its electron emission properties

We have deposited diamond-like carbon (DLC) films by electrodeposition technique in methanol liquid. XPS showed the films mainly contain carbon. IR spectrum indicated that as-deposited films are hydrogenated carbon films, with the hydrogen mainly bonded to sp³ carbon. Raman measurement suggested that the films consisted of sp³ and sp² carbon. The field emission (FE) property of DLC coated on Si has been measured. The field emission of DLC films started at an applied voltage of 160 V, compared with silicon tip arrays at 600 V, and an emission current of DLC films up to 55 A at 360 V was achieved.     

XPS and IR study of carbon thin films prepared under negative substrate DC voltage bias

Hydrogenated amorphous carbon (a-C:H) thin films were prepared on glass substrates at different applied DC voltage bias by the HF-CVD method. Other factors of deposition were kept constant. The IR and XPS spectra of the films were obtained. By the deconvolution of the IR and XPS spectra sp³/sp² ratio calculated. The sp³/sp² ratio varies nonlinearly with bias voltage and it has a minimum and maximum in the 0–70 V range of the bias voltage.     

Ion beam energy effects on structure and properties of diamond like carbon films deposited by closed drift ion source

In the present study DLC films deposited from acetylene gas by a closed drift ion source were investigated. Ion beam energy effects on structure as well as optical and electrical properties of the synthesized films were studied. Non-monotonic dependence of structure of the DLC films on ion beam energy was observed. The highest sp³/sp² ratio as well as highest optical transparency was observed in the case of the films synthesized by 500 eV energy ion beam. However, the bandgap of the DLC films synthesized by 500 eV energy ion beam was the lowest between all investigated samples, while resistivity non-monotonically decreased with increase of the ion beam energy. These results were explained by changes of the sp³/sp² ratio, structure of sp² bonded clusters as well as hydrogen content in the film due to the competition between the increased (decreased) ion beam energy and decreased (increased) ion/neutral ratio.     

Comparing internal stress in diamond-like carbon films with different structure

Diamond-like carbon (DLC) films with different structures were deposited on Si (100) and stainless steel substrates in a hybrid deposition system with Ar and CH₄ as the feedstocks. The effects of the bias voltage, Ti-interlayer, Ti functional gradient layer and Ti-doping on the internal stress in DLC films were investigated. The results show that the internal stress in DLC films arises from both the intrinsic stress generated during the film growth and the thermal stress generated due to the mismatching of the thermal expansion coefficient between the DLC films and the substrate materials. The intrinsic stress can be released through doping titanium element at the expense of reducing the sp³/sp² ratio. The thermal stress in DLC films can be decreased through introducing Ti-interlayer or Ti functional gradient layer. Noticeably, DLC films with very low internal stress deposited on stainless steel can be obtained through the combination of Ti-doping and Ti functional gradient layer.     

Tunnelling through diamond-like carbon nanofilms deposited by electron-beam-induced deposition

The tunnelling properties in metal/diamond-like carbon (DLC)/semiconductor junctions and structural characteristics of thin DLC films produced using different electron beam conditions were studied. We show that under the same electron dose conditions, thicker DLC films were obtained using lower accelerating voltages (2 kV) than when using higher accelerating voltage (20 kV). However, under the settings used the thicker films showed worse insulating performance than the thinner films. We attribute this effect to the variation of tunnelling barrier height in DLC deposited using different accelerating voltages. DLC films with a tunnelling barrier height of up to 3.12 eV were obtained using a 20 kV electron-beam, while only 0.73 eV was achieved for 2 kV DLC films. The X-ray photoemission spectra of the C 1s core level in these films reveal components at 284.4 ± 0.1 eV and 285 ± 0.1 eV, which were identified as the sp² and sp³ hybrid forms of carbon. The sp³/sp² concentration ratio increased with increasing electron beam accelerating voltage. We show how this effect is responsible for the barrier height variation.     

Field emission properties of composite nano-Au/DLC films prepared by CVD technique

Nanocrystalline gold incorporated diamond-like carbon (nano-Au/DLC) films were deposited by capacitively coupled plasma (CCP) r.f. chemical vapour deposition (CVD) technique. Gold content in the DLC matrix was controlled by the amount of argon in the argon + methane mixture in the plasma. Field emission properties of these films were studied critically. Bonding environment (sp²/sp³ ratio) in these films was obtained from Raman measurements. Modification of the surface with the incorporation of gold nanocrystallites and associated modulation of sp²/sp³ ratio in the films culminated in improved field emission properties. Fowler-Nordheim model was used to ascertain the work function (?) which varied between 19 and 64 meV. The field factor (β) varied between 172 and 1050.     

Antireflection properties of diamond-like carbon films on Cd<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te (<Emphasis Type="Italic">x</Emphasis> ∼ 0.04) single crystals

The optical properties of diamond-like carbon (DLC) films obtained by plasmachemical deposition on Cd_{1 − x} Zn _x Te (x ∼ 0.04) single crystals have been studied by ellipsometry. The ellipsometric data have been interpreted within the framework of a three-layer model of the DLC film-semiconductor crystal refractory system with transition layers between the film and substrate. It is found that DLC films exhibit antireflection properties in this refractory system in the IR spectral range. It is established that the proposed antireflection film-substrate structure is stable with respect to thermal cycling and ultrasonic treatment.     

Influence of PECVD parameters on the properties of diamond-like carbon films

The properties of diamond-like carbon (DLC) are strongly affected by the amount of carbon atoms bonded in sp² and sp³ electronic hybridizations. Also the amount of incorporated hydrogen and oxygen plays an important role in the final properties of DLC films. Usually, the structure and chemical composition of thin DLC films can be changed by varying the deposition parameters. Therefore, the influence of PECVD process parameters on the properties of DLC films, grown on Si substrates, was investigated in this work.Thin DLC films were deposited in a CH₄/H₂ plasma by using Ar as a gas carrier. Different ratios of gas flows were used as a variable parameter of the PECVD process. The effect of cathodic ion bombardment was also investigated.The chemical composition of DLC specimens was studied by X-ray photoelectron spectroscopy (XPS). The ratio of carbon in sp² and sp³ hybridizations was determined by analyzing the first derivative of Auger C KLL spectra. These results were also confirmed by the measurements of electrical resistivity. The changes of surface morphology and microadhesion were analyzed by Atomic Force Microscopy (AFM).     

Influence of negative voltage on the structure and properties of DLC films deposited on NiTi alloys by PBII

Diamond-like carbon (DLC) films have been successfully deposited on Ti-50.8 at.%Ni using plasma based ion implantation (PBII) technique. The influences of the pulsed negative bias voltage applied to the substrate from 12 kV to 40 kV on the structure, nano-indentation hardness and Young’s modulus are investigated by the X-ray photoelectron spectroscopy (XPS) and nano-indentation technique. The results show that C 1s peak depends heavily on the bias voltage. With the increase of bias voltage, the ratio of sp²/sp³ first decreases, reaching a minimum value at 20 kV, and then increases. The DLC coating deposited at 20 kV shows the highest hardness and elastic modulus values as a result of lower sp²/sp³ ratio. The corrosion resistance of specimen deposited under 20 kV is superior to uncoated NiTi alloy and slightly better than those of the other samples deposited at 12 kV, 30 kV and 40 kV.     

Friction force microscopy study of annealed diamond-like carbon film

In this paper we introduce mechanical and structural characteristics of diamond-like carbon (DLC) films which were prepared on silicon substrates by radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD) method using methane (CH₄) and hydrogen (H₂) gas. The films were annealed at various temperatures ranging from 300 to 900 °C in steps of 200 °C using rapid thermal processor (RTP) in nitrogen ambient. Tribological properties of the DLC films were investigated by atomic force microscopy (AFM) in friction force microscopy (FFM) mode. The structural properties of the films were obtained by high resolution transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The wettability of the films was obtained using contact angle measurement. XPS analysis showed that the sp³ content is decreased from 75.2% to 24.1% while the sp² content is increased from 24.8% to 75.9% when the temperature is changed from 300 to 900 °C. The contact angles of DLC films were higher than 70°. The FFM measurement results show that the highest friction coefficient value was achieved at 900 °C annealing temperature.     

Chemical bond analysis of amorphous carbon films

Hydrogen-free carbon films with the various sp³ bond fractions between 83% and 40% were prepared by mass-separated ion beam deposition (MSIBD). These sp³ bond fractions were obtained by electron energy loss spectroscopy (EELS). Chemical bond analysis of these carbon films was performed by x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and Raman spectroscopy, and the comparison of these methods was examined. XPS C1s spectra of carbon films show two contributions at the energies of 284.5 and 285.5 eV, which are originated from sp²-bonds and sp³ bonds, respectively. The sp³ bond fractions obtained by XPS are in good agreement with the values given by EELS. The fine structure of AES spectra at the kinetic energy region between 245 and 265 eV reflects the sp³ bond fraction. AES spectra are changed from the diamond-like feature to the graphite-like one with decreasing the sp³ bond fraction. Raman spectra show two broad peaks of G band and D band, the ratio of two peak intensities is independent on the sp³ bond fraction of films. The shift of G peak position has a correlation with the sp³ bond fraction in the sp³ bond rich region.     

Relative fraction of sp bonding in boron incorporated amorphous carbon films determined by X-ray photoelectron spectroscopy

Boron incorporated amorphous carbon (a-C:B) films were deposited by a filtered cathodic vacuum arc system using various percentage of boron mixed graphite cathodes. X-ray photoelectron spectroscopy (XPS) was employed to determine the properties of the films as a function of boron concentration. Deconvolutions of the XPS C 1s core level spectra were carried out using four different components. The relative fraction of sp³ bonding was then evaluated from the area ratio of the peaks at 285.0, 284.1 eV which were individually attributed to sp³ C-C, sp² CC hybridizations. The results showed that the sp³ content of a-C:B film decreases from 73.8 to 58.6% for the films containing boron from 0.59 to 2.13 at.%, and then gradually reduced to 42.5% at a slower rate with boron concentration up to 6.04 at.%. Furthermore, a series of a-C:B films with fixed boron content (2.13 at.%) were prepared to identify the relationship between sp³ bonding and substrate bias. It was found that the fraction of sp³ bonding increased from 50.28% at the bias voltage of 0 V and reached a maximum value of 66.3% at −150 V. As the bias voltage increased up to −2000 V, the sp³ content decreased sharply to 43.9%.     

Metal-doped diamond-like carbon films synthesized by filter-arc deposition

Diamond-like carbon (DLC) thin films are extensively utilized in the semiconductor, electric and cutting machine industries owing to their high hardness, high elastic modulus, low friction coefficients and high chemical stability. DLC films are prepared by ion beam-assisted deposition (BAD), sputter deposition, plasma-enhanced chemical vapor deposition (PECVD), cathodic arc evaporation (CAE), and filter arc deposition (FAD). The major drawbacks of these methods are the degraded hardness associated with the low sp³/sp² bonding ratio, the rough surface and poor adhesion caused by the presence of particles. In this study, a self-developed filter arc deposition (FAD) system was employed to prepare metal-containing DLC films with a low particle density. The relationships between the DLC film properties, such as film structure, surface morphology and mechanical behavior, with variation of substrate bias and target current, are examined. Experimental results demonstrate that FAD-DLC films have a lower ratio, suggesting that FAD-DLC films have a greater sp³ bonding than the CAE-DLC films. FAD-DLC films also exhibit a low friction coefficient of 0.14 and half of the number of surface particles as in the CAE-DLC films. Introducing a CrN interfacial layer between the substrate and the DLC films enables the magnetic field strength of the filter to be controlled to improve the adhesion and effectively eliminate the contaminating particles. Accordingly, the FAD system improves the tribological properties of the DLC films.     

Electrochemical performance of diamond-like carbon thin films

Diamond-like carbon (DLC) thin films used in this study were intended for their electrochemical properties. The DLC films were deposited by a filtered cathodic vacuum arc (FCVA) process on p-type silicon (100) substrates biased at different pulse voltages (0-2000 V). The chemical bonding structures of the DLC films were characterized with micro-Raman spectroscopy and the electrochemical properties were evaluated by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The DLC films showed high impedance, high polarization resistance and high breakdown potential in a 0.5 M H₂SO₄ aqueous solution, which were attributed to the high sp³ content and uniformity of the films. The excellent chemical inertness of the DLC films made them promising corrosion resistant coating materials.     

Characterization of boron-doped diamond-like carbon prepared by radio frequency sputtering

Instead of the sophisticated deposition processes and boron sources reported in literature, the study used the radio frequency magnetron sputtering method to prepare boron-doped diamond-like carbon (DLC) films with p-type conduction. The adopted sputtering targets were composed of boron pellets buried in a graphite disc. The undoped DLC films prepared exhibited n-type conduction, based on the Hall-effect measurement. For boron content ≥ 2.51 at.%, the films showed semiconductor behavior converted from n-type to p-type conduction after annealing at 450 °C. B-DLC films with a boron content of 5.91 at.% showed a maximum carrier concentration of 1.2 × 10¹⁹ cm⁻³, a mobility of 0.4 cm²/V s, and an electrical resistivity of 1.8 Ω cm. The results of XPS and Raman spectra indicated that the motion of boron atoms was thermally activated during post-annealing, helping promote the formation of C-B bonds in the films. Moreover, the doping of boron in DLC films decreased sp³ bonding and facilitated carbon atoms to form sp² bonding and graphitization.     

Structure and electrochemical hydrogen storage properties of Pd/Mg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/Pd thin films prepared by pulsed laser deposition

Three-layered Pd/Mg_1−x Al _x /Pd (x = 0, 0.13, 0.21, 0.39) thin films were prepared by means of pulsed laser deposition. In the present Al concentration range, X-ray diffraction analyses showed that the Mg_1−x Al _x layer was constituted of a single phase Mg(Al) solid solution. The Mg(Al) grains are preferentially orientated along the c-axis and their size decreased (from 18.5 to 10.5 nm) as the Al content increased. Scanning electron microscopy and atomic force microscopy observations indicated that all the films exhibited a globular surface structure. However, the surface roughness of the films decreased as the Al concentration increased. Rutherford backscattering spectroscopy revealed that the Mg–Al layer density (porosity) was strongly dependent on the Al content. Successive hydriding charge/discharge cycles were performed on the different Pd/Mg_1−x Al_x/Pd films in alkaline media. The highest discharge capacity was obtained with the Pd/Mg_0.79Al_0.21/Pd film, namely ~85 μAh cm⁻² μm⁻¹ or 320 mAh g⁻¹, which corresponds to a H/M atomic ratio of ~0.48 in the Mg–Al layer.     

Characterizations of NbAlO thin films grown by atomic layer deposition

Niobium-aluminate (NbAlO) thin films have been prepared on silicon (100) with different Nb₂O₅:Al₂O₃ growth cycle ratio by atomic layer deposition (ALD) technology. The structural, chemical and optical properties of NbAlO thin films are investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE). The results show that all the obtained NbAlO films are amorphous and fully oxidized. It is also found that the proportion of components in the NbAlO film can be well-controlled by varying the ALD growth cycles of the independent oxides. Furthermore, the refraction index of the prepared films is observed to increase with an increase of the concentration of Nb in the mixtures.