Microstructure and mechanical properties of pulsed laser deposited boron nitride films

Boron nitride films were prepared by pulsed laser ablation from a boron nitride target using a KrF-excimer laser, where the growing films were deposited in nitrogen atmosphere or bombarded by a nitrogen/argon ion beam. Films deposited without or at weak ion bombardment (such films will be called l-BN in this paper) are hexagonal with amorphous to turbostratic microstructure (l-BN) and show high adhesive strength to silicon and stainless steel substrates. By using them as intermediate layers, the adhesion of pure cubic boron nitride films (c-BN) can significantly be improved. l-BN films and l-BN/h-BN/c-BN layer systems have been investigated by in-situ ellipsometry, infrared spectroscopy and cross-section and plan-view high-resolution transmission electron microscopy, including diffraction. The mechanical properties, i.e. stress and hardness, of these films and layer systems are presented. l-BN films deposited at higher laser energy densities have compressive stresses as high as 11.5 GPa. Films deposited at lower laser energy densities have stresses in the range of 4.7 to 1.3 GPa and a Vickers hardness in the range of 18.6 to 7.5 GPa depending on substrate temperature and ion bombardment. The compressive stresses of 400 nm thick adherent c-BN films were estimated to be 4.5 GPa.     

Temperature effect on the nitrogen insertion in carbon nitride films deposited by ECR

The impact of the temperature on the local structure of carbon nitride coating a-C_1-x N_x:H was investigated by spectroscopic analysis. A set of carbon nitride films were deposited at several substrate temperatures (77 K, 300 K, 673 K and 900 K) by electron cyclotron resonance (ECR) ion gun technique fed of CH₄/N₂ plasma.The films were in situ characterized by X-ray photoelectron spectroscopy (XPS). A drastic decrease of the nitrogen content was observed when increasing the deposition temperature from 77 K to 900 K. Qualitative structural and electronic changes were followed after air exposure by infrared (FTIR), near-edge X-ray absorption fine structure (NEXAFS) and Ultraviolet photoelectron (UPS) spectroscopy. Below 300 K, the films are hydrogenated with aliphatic structure and nitrogen is bonded to carbon in many kind of configuration. Between 300 K and 600 K, the nitrogen amount is reduced while both the aromatic and the aliphatic carbons increase. The local structure of the films radically changes at 900 K, whereas the nitrogen surrounding is the same at 673 K. In that case the hydrogen fraction into the films is reduced to zero. The increase of the sp³ carbon as well as the ratio π?/σ? on the nitrogen K edge can be observed. This behaviour may be explain by nitrogen substituted to sp² carbon which induces local changes in the distribution of the π? states.     

Optical properties of graphitic carbon nitride films prepared by evaporation

Graphitic carbon nitride (g-C₃N₄) consists of two-dimensional sheets of carbon and nitrogen atoms. Films of g-C₃N₄ were prepared by evaporating guanidine carbonate at four different substrate temperatures. The optical absorption band of the films appears at 3.3 eV and the optical energy gaps are calculated to be 2.83–2.90 eV. Band intensity increases with increasing substrate temperature, but the energetic band position does not shift. The photocurrent of g-C₃N₄ films can be observed by irradiation with monochromatic light. While the photosensitivity spectra are in almost complete correspondence with the optical absorption spectra, it is also found that the photocurrent is generated by irradiation at photon energies below the optical energy gap down to 2.5 eV.     

Piezoresistive,optical and electrical properties of diamond like carbon and carbon nitride films

In the present study diamond like carbon (DLC) and carbon nitride (a-CN_x:H) films were deposited by closed drift ion source from the acetylene and nitrogen gas mixture. The piezoresistive, electrical and optical properties of ion beam synthesized DLC films were investigated. Piezoresistive properties of the diamond like carbon and carbon nitride films were evaluated by four point bending test. The piezoresistors were fabricated on crystalline alumina substrates using Al-based interdigitated finger type electrodes. Effects of the nitrogen concentration on the piezoresistive gauge factor were investigated. The dependence of the resistance of the metal/a-CN_x:H/metal structures on temperature has been studied. Current–voltage (I–V) and capacitance–voltage characteristics were measured for a-CN_x:H/Si heterostructures. The main current transport mechanisms were analyzed. Optical parameters of the synthesized films such as optical bandgap and B parameter (slope of the linear part of the Tauc plot) were investigated to study possible correlation with the piezoresistive properties.     

The infrared characteristics investigation of carbon nitride films

Two series of a-C(N):H films, with diamond-like character and graphite-like character respectively, are prepared. Without N incorporation, the two kinds of films have very close IR bands in the range of 1000–1800 cm^− 1. However, the difference in IR activity of the two series films became dramatical as N was introduced into both kinds of carbon films, which is attributed to purely electronic effect, the electronegativity of N. The N incorporated in carbon films is able to induce bond dipole of CC bonds in sp² graphite cluster, leading to a degree of dipoles for all the aromatic sp² CC bonds, a permanent electric dipole effect arises and this could lead to the increase in IR activity of the sp² clusters. As the N content exceeds 20 at.% in the carbon films, strong conjugation of C≡N bonds with aromatic graphite rings can induce conjugated π bond dipole too.     

Effect of deposition voltage on the microstructure of electrochemically deposited hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films were deposited on Si substrates by electrolysis in a methanol solution at ambient pressure and a low temperature (50 °C), using various deposition voltages. The influence of deposition voltage on the microstructure of the resulting films was analyzed by visible Raman spectroscopy at 514.5 nm and X-ray photoelectron spectroscopy (XPS). The contents of sp³ bonded carbon in the various films were obtained by the curve fitting technique to the C1s peak in the XPS spectra. The hardness and Young’s modulus of the a-C:H films were determined using a nanoindenter. The Raman characteristics suggest an increase of the ratio of sp³/sp² bonded carbon with increasing deposition voltage. The percentage of sp³-bonded carbon is determined as 33–55% obtained from XPS. Corresponding to the increase of sp³/sp², the hardness and Young’s modulus of the films both increase as the deposition voltage increases from 800 V to 1600 V.     

Effects of normal load on nanotribological properties of sputtered carbon nitride films

The nanotribological properties of amorphous carbon nitride (CN_x) films of ∼380 nm thickness were investigated, in the normal (contact) load range of 2–20 mN, using a Berkovich diamond indenter. The amorphous CN_x films tested in this work were grown on Si(100) substrates by reactive sputtering and energetic ion bombardment during deposition (IBD). The dependence of the friction behavior of the CN_x films on normal load (NL) was investigated in terms of nanomechanical properties, deformation mode and Atomic Force Microscopy (AFM) images of scratched surfaces, and the intensity of IBD. In films sputtered without IBD, the increase of the normal load caused the coefficient of friction to decrease initially to a minimum value and, subsequently, to increase to a maximum value, after which, it remained constant. The dominant friction mechanism in the low-load range was adhesion, while both adhesion and ploughing mechanisms contributed to the friction behavior in the intermediate and high-load ranges. Elastic and plastic deformation (PD) and delamination of the amorphous CN_x films occurred, depending on the normal-load ranges. On the other hand, films sputtered with high-energy IBD showed a load-dependent transition in both the scratch and the friction responses. Nanoscratching below 5 mN showed mainly elastic behavior of the film, while above 10 mN, a mixed elastic–plastic behavior was identified. Testing under a normal load of 20 mN resulted in local grooving at the film surface; however, in situ profiling of the scratch trace and AFM images showed no evidence of film failure. The increased load-carrying capacity, higher hardness and elastic response obtained with films grown with high-energy IBD, and the dominant friction mechanism at each load range illustrate the normal load dependence of the nanotribological properties of the sputtered CN_x films.     

Carbon nitride films deposited from organic solutions by electrodeposition

An electrodeposition process was applied to synthesize and deposit CN_x thin films from organic solution. The depositions from acetonitrile (CH₃CN) and dicyandiamide–acetone (C₂H₄N₄–C₃H₆O) solution were reported. The processes were performed near to room temperature. The films deposited from acetonitrile liquid contained 25% nitrogen, the nitrogen content in the films obtained from dicyandiamide solution reached to 48%. X-ray photoelectron spectrometry (XPS) and Fourier transform infrared (FTIR) were used to characterize the structure of the films. It indicated that high nitrogen content films contained CN bonds besides the CN, and CN bonds which composed both films.     

Electrical and optical properties of diamond-like carbon films deposited by pulsed laser ablation

Pulsed laser ablation of a graphite target was carried out by ArF excimer laser deposition at a laser wavelength of 193 nm and fluences of 10 and 20 J/cm² to produce diamond-like carbon (DLC) films. DLC films were deposited on silicon and quartz substrates under 1 × 10^? 6 Torr pressure at different temperatures from room temperature to 250 °C. The effect of temperature on the electrical and optical properties of the DLC films was studied. Laser Raman Spectroscopy (LRS) showed that the DLC band showed a slight increase to higher frequency with increasing film deposition temperature. Spectroscopic ellipsometry (SE) and ultraviolet–visible absorption spectroscopy showed that the optical band gap of the DLC films was 0.8–2 eV and decreased with increasing substrate temperature. These results were consistent with the electrical resistivity results, which gave values for the films in the range 1.0 × 10⁴–2.8 × 10⁵ Ω cm and which also decreased with deposition temperature. We conclude that at higher substrate deposition temperatures, DLC films show increasing graphitic characteristics yielding lower electrical resistivity and a smaller optical band gap.     

Barrier properties of carbon films deposited on polymer-based devices in aggressive environments

The physical and chemical properties of diamond-like carbon (DLC) and carbyne-containing films on different substances, in particular on plastic materials, are advantageous for a great number of novel applications. These coatings, which possess the necessary mechanical characteristics of high adhesion to the plastic substrate material, chemical inertness and biocompatibility, have been proved to be hypoallergenic. In present paper, we report on the use of DLC and carbyne-containing films, formed by different ion-assisted methods, for strengthening and protecting of the surface of polymer-based medical products and for providing new functional properties. The medical products examined are: probes of different type, catheters, drainage tubes, polymer contact lenses and contact lens cases. It is shown that the DLC and carbyne-containing films are biocompatible, possess high chemical inertness to biological media of the human body, and provide the necessary rigidity to products. Antibacterial activity, a wide range of contact wetting angle changes, and a wide range of optical and colour characteristics open unlimited possibilities for the improvement of medical wares. Results on the mechanical properties of probes and contact lenses coated by carbon films, both from in vitro and clinical testing, are also presented.     

Electronic properties of hydrogenated amorphous carbon films deposited using ECR-RF plasma method

A combination of junction capacitance, electron spin resonance and electrical conductivity measurements are used to investigate the electronic properties of two different types of a-C:H films grown in a dual ECR-RF glow discharge system at substrate bias equal to −30 and −600 V, respectively. The analysis of the steady state admittance (both capacitance C and conductance G) as a function of frequency (ω=5 Hz–1 kHz) and temperature (20–350 K) allows an estimate of the density of states at the Fermi level of approximately 7×10¹⁵ and 9.7×10¹⁶ eV⁻¹ cm⁻³ for the −30 and −600 V deposited samples, respectively, values well below those deduced for the density of spins from the electron spin resonance experiments, of approximately 10¹⁹–10²⁰ cm⁻³. Concerning the conductance results, two transport processes operating, respectively, below and above 290 K are shown. The high temperature process is associated with an activation energy of 0.5 and 0.41 eV for the −30 and −600 V samples, respectively, in good agreement with the values obtained in the high temperature range (>300 K) for the activation energy of the electrical conductivity. Regarding the effect of the frequency and temperature on the conductance, we show that for temperatures below 290 K, a Variable Range Hopping mechanism is possible by facing our data to the Mott's model. Annealing at high temperature induces structural changes accompanied by an increase in the spin density in both types of samples with however, a different behaviour from one type to another.     

Lanthanum nickelate thin films deposited by spray pyrolysis: Crystallization,microstructure and electrochemical properties

Thin films from the La_n+1Ni_nO_3n+1 system exhibit favorable dielectric and electrochemical properties that may prove useful for a variety of devices ranging from ferroelectrics to low-temperature solid oxide fuel cells. The present work covers the compositional, microstructural and electrochemical characterization of thin lanthanum nickelate films deposited by spray pyrolysis. In accordance with the phase diagram, LaNiO_3?δ or La₄Ni₃O_10?δ films were obtained during annealing of spray deposited films at temperatures between 540 °C and 1100 °C. Whereas LaNiO_3?δ films exhibited a high metallic conductivity, La₄Ni₃O_10?δ films were semiconducting. Electrochemical impedance spectroscopy indicated an increase of the area specific oxygen reduction resistance with the annealing temperature. The study highlights how the phase and microstructure of thin films from the La_n+1Ni_nO_3n+1 system can be tailored by annealing of initially amorphous films. LaNiO_3?δ films have a high potential for application in electrochemical devices operating at low temperatures where high electrical conductivity is required.     

Application-relevant characterization of magnetron-sputtered carbon nitride films

Carbon nitride (CN_x) films were deposited onto Si substrates by radio frequency magnetron sputtering at nitrogen pressures up to 0.3 Pa (the resulting nitrogen concentration ranged from 32 to 41%). Their optical properties, structure and bonding were characterized by spectroscopic ellipsometry, X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy and scanning electron microscopy. The films exhibit a behavior typical for disordered systems, and no known C₃N₄ structure was identified. A pronounced effect of the nitrogen partial pressure is found for the low pressure region. The presence of various types of CN bond, as well as of hydrogen and oxygen, is revealed. The complementarity of information inferred from Raman and IR spectra is discussed.     

Characterization of magnetron sputtered carbon nitride films

A set of carbon nitride samples has been prepared by reactive magnetron sputtering. The only parameter varied was the nitrogen partial pressure p_N2. It turns out, however, that p_N2 has noticeable influence on the composition and the structure of the films only below 0.1 Pa. The composition of the bulk of the samples was investigated by elastic recoil detection (ERD), energy dispersive X-ray analysis (EDX), wavelength dispersive X-ray analysis (WDX) and Rutherford backscattering (RBS); although all of them give the same trend, some systematic differences were observed concerning the absolute values. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) measurements revealed that the surface is somewhat depleted in nitrogen. Sputter depth profiling could not be applied due to very strong preferential sputtering of nitrogen. X-ray diffraction (XRD) showed that the films are almost amorphous. Structural information was obtained from Fourier transform infrared spectroscopy (FTIR), EELS, and XPS; it turned out that the films are graphitic or paracyanogen-like with a density of approx. 2 g cm^-3. All of the characterization methods applied are discussed in view of the information they yield on carbon nitride films, and problems of their application to this special type of film.     

Thermal stability of the optical properties of plasma deposited diamond-like carbon thin films

In present paper we studied the optical constants of the diamond-like carbon (DLC) films and their changes with annealing. The multisample modification of combined variable angle spectroscopic ellipsometry and near normal spectroscopic reflectometry was used. The optical constants of the DLC films were simulated by our recently published six-parameter dispersion model employing a parameterization of the density of electronic states (DOS). Based on the dispersion model parameters the density of π and σ electrons were evaluated. We showed that from our model and the independently determined hydrogen atomic fraction of the films before and after annealing the ratio between momentum matrix elements of π → π* and σ → σ* transitions and the correct sp³-to-sp² carbon bonding configuration ratio can be calculated. It is worth to notice that the first quantity is usually assumed to be equal to unity but we showed that this assumption may cause a significant error in the determination of the sp³-to-sp² ratio. Therefore, our suggested method represents a novelty in this field.     

Stress and structural properties of diamond-like carbon films deposited by electron beam excited plasma CVD

Diamond-like carbon (DLC) films prepared using CH₄ or C₆H₆ with varying deposition parameters by an electron beam excited plasma CVD system were investigated for the internal stress, dynamic hardness and structural properties such as the film density, total, bonded and unbound hydrogen contents, sp³ ratio and graphite crystallite. From the correlations between internal stress and structural properties, the following conclusions were derived. The fraction of unbound hydrogen to total hydrogen content was the most influential factor for the compressive stress of the DLC films deposited from CH₄. It is suggested that unbound hydrogen may be trapped into the disordered microstructure of graphite crystallites embedded in the network of film. For the DLC films deposited from C₆H₆, it was shown that the compressive stress was correlated with not only the fraction of unbound hydrogen content but also the degree of cross-linking between graphite crystallites in the film.     

Preparation and microstructure properties of tetrahedral amorphous carbon films by pulsed laser deposition using camphoric carbon target

The properties of tetrahedral amorphous carbon (ta-C) films grown by pulsed laser deposition (PLD) using camphoric carbon (CC) target and their respective effects of diamond percentages by weight in the target (Dwt.%) are discussed. Scanning electron microscopy (SEM), atomic force microscopy (AFM), Visible-Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses indicated that the Dwt.% noticeably modified the sp³ bonds content and the morphology of the ta-C films. The optical gap (E_g) and electrical resistivity (ρ) increase with Dwt.% up to 1.6 eV and 5.63×10⁷ (Ω cm), respectively, for the ta-C films deposited using target with higher of 50 Dwt.%. We found that the Dwt.% has modified the surface morphological, structural, bonding and physical properties of the camphoric carbon films.     

Blood compatibilities of carbon nitride film deposited on biomedical NiTi alloy

Carbon nitride (CNx) film, diamond-like carbon (DLC) film, and titanium nitride (TiN) film were deposited on biomedical NiTi alloy substrates using direct current magnetron sputtering, respectively. In order to improve the adhesive strength between the deposited hard film and the NiTi alloy, a Ti transition layer was pre-deposited firstly. We emphatically evaluated the blood compatibilities of the NiTi alloy substrate and the deposited hard films by haemolysis test and platelet conglutination test. It was shown that the blood compatibilities of NiTi alloy can be improved effectively by the deposition of hard films. In comparison with TiN and DLC film, CNx film had the best surface modification effects covering the minimum haemolysis ratio and the best anticoagulation property.     

Effect of the post-heating temperatures on the microstructure,mechanical and electrical properties of silicon nitride thin films

Silicon nitride (SiN_x) thin film is a potential candidate for the fabrication of the insulating layer of thin-film thermocouples, which can be utilised to measure cutting temperatures, owing to its excellent insulation properties and hardness and a thermal expansion coefficient similar to that of carbide tool. Thus, it is necessary to investigate the stability of the SiN_x microstructure and its mechanical and electrical properties at high temperatures. In this study, SiN_x thin films were deposited using reactive magnetron sputtering, followed by post-heating in an air atmosphere at 200–600 °C. The microstructure, adhesion, and sheet resistance were investigated using atomic force microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, scratch tests, and four-probe resistance tests. The results showed that the SiN_x film had a typical amorphous structure. During the heating process, the grain size increased, as did the content of columnar structures. When the temperature was increased from room temperature to 200 °C, the SiN_x film was oxidised to SiN_xO_y. The oxidative product (SiO₂) and escaping nitrogen gas were not observed until the film was heated above 400 °C, revealing the different oxidation reactions and products induced by the elevated temperature. The adhesive strength of the SiN_x film increased monotonically with increasing temperature but was severely weakened when the film was heated to temperatures above 400 °C. Oxygen could not completely invade the deeper layers of the film until the temperature reached 600 °C. The sheet resistance of the SiN_x film improved at 200 °C, but reduced severely when the temperature exceeded 400 °C.     

Capacitance properties of electrochemically deposited polyazulene films

Polyazulene films formed by electrochemical oxidation of azulene have been studied as active components in electrochemical capacitors. The film shows reversible electrochemical behavior in the positive potential range and exhibits p-doping properties. The influence of film formation conditions on the films electrochemical properties has been investigated. A strong effect of solvent on the polyazulene deposition has been observed. The highest yield of film deposition was found for dichloromethane. Polyazulene films also exhibit stable voltammetric properties in aprotic solvents. The voltammetric response of the film is affected by the size of the anion of the supporting electrolyte. In solutions containing tetra(alkyl)ammonium perchlorates, tetrafluoroborates or hexafluorophosphates, reversible oxidation of polyazulene is obtained. In the presence of large tetra(phenyl)borate anions, polyazulene is irreversibly oxidized upon electrochemical oxidation. The capacitance properties of these materials have been investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The polyazulene film displays a relatively high specific capacitance close to 400 F g⁻¹. Such high value of C_s locates this material among very good polymeric redox pseudo-capacitors.