A. Hoffman

Possibility of carbon nitride formation by low‐energy nitrogen implantation into graphite: In situ electron spectroscopy studies

The possibility of carbon nitride formation by low‐energy nitrogen ion irradiation of graphite was investigated by in situ x‐ray photoelectron spectroscopy. Room‐temperature and hot 500‐eV N+2 implantations were performed with saturation doses for which a constant nitrogen concentration was obtained. Analysis of the N(1s) core level line indicates the existence of three different carbon–nitrogen bonding states in the room‐temperature implanted layer. Annealing experiments up to 500 °C revealed a slight, gradual decrease of nitrogen concentration in the implanted layer accompanied by a partial redistribution of the nitrogen bonding states. Hot nitrogen implantations at 300 and 500 °C resulted in a predominant population of the more covalent, with higher N(1s) binding energy, nitrogen bonding state. Such a distribution of carbon–nitrogen chemical bonds could not have been obtained by annealing of the room‐temperature implanted layer. These results may be of importance in finding a way to produce the elusive...

Adsorption kinetics and isotopic equilibration of oxygen adsorbed on the Pd(111) surface

Oxygen adsorption on the Pd(111) surface has been studied at 100 and 300 K by temperature programmed desorption (TPD) and isotopic mixing experiments. At 100 K, the sticking coefficient is determined to be 1 up to the coverage of 0.3 O/Pd. The saturation coverage is 0.62 O/Pd, 27% of which dissociates during thermal desorption. Three molecular desorption processes are observed with the activation energy of 7.6, 9.1, and 12.3 kcal/mol, respectively. At 300 K, the sticking coefficient increases with coverage from ∼0.14 at zero coverage to 0.87 at θ≊0.05 O/Pd, then decreases to zero at a saturation coverage of 0.25 O/Pd. The desorption activation energy of 53 kcal/mol is calculated for the associative desorption process with a lateral repulsive interaction of 0.7 kcal/mol. Based on the isotopic mixing results and previous high resolution electron energy loss spectroscopy (HREELS) data, a more complete picture concerning adsorption, conversion, equilibration, desorption, and dissociation processes is suggested.

Deposition of continuous and well adhering diamond films on steel

The present work reports a successful solution to the problem of diamond deposition onto steel. This was achieved through the use of an intermediate layer between the substrate and the deposited film. The intermediate layer consisted of a 10 μm thick nitridized chromium film. The chromium film was produced by electrochemical deposition and the nitridation was performed in an ammonia flow. This nitridation process results in the formation of mixed CrN and Cr2N crystalline phases, with the latter in contact with the steel substrate. During diamond deposition, partial carbidization of the chromium nitride interlayer took place resulting in the formation of a layer composed mainly of carbides and carbon phases onto which continuous diamond films were deposited. Beneath the carburized region a chromium nitride‐rich phase, consisting predominantly of Cr2N, was observed. The diamond films were deposited using a hot filament system at a rate of 1 μm per hour and a substrate temperature of 800 °C. The toughness of...

Nanometer rough, sub-micrometer-thick and continuous diamond chemical vapor deposition film promoted by a synergetic ultrasonic effect

Abstract In this paper we report on a surface treatment to seed substrates for the promotion of diamond nucleation. This surface treatment consists of an ultrasonic abrasion process using poly-disperse slurry composed of a mixture of small diamond particles ( 3 μm) which may consist of diamond, alumina, titanium, etc. Whereas ultrasonic abrasion with a mono-disperse diamond slurry results in a diamond nucleation density of ∼2–3×108 particles/cm2, treatment with poly-disperse slurries results in diamond nucleation density of values up to ∼5×1010 particles/cm2. This effect was found to display a similar effectiveness on a variety of substrates such as silicon, sapphire, quartz, etc. The enhancement in diamond nucleation is interpreted by a ‘hammering’ effect whereby the larger particles insert very small diamond debris onto the treated surface, thus increasing the density of nuclei onto which diamond growth takes place during the chemical vapor deposition process. By increasing the nucleation density to values of ∼5×1010 particles/cm2, continuous diamond films of thickness of less than ∼100 nm were grown after only 5 min of deposition. The roughness of continuous diamond films grown on substrates treated at optimum conditions obtains values of 15–20 nm. The effect of ultrasonic treatment on silicon substrates and the deposited films was investigated by atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.

Optical and photoemission studies of DLC films prepared with a systematic variation of the sp3:sp2 composition

Abstract The optical and photoemission properties of hydrogen-free DLC films prepared under conditions which result in different types of carbon bonding (previously determined from electron energy loss spectroscopy) are reported. The films were prepared using a mass selected ion beam deposition system covering the C+ energy range 10 eV–2 keV, mostly on Si held at room temperature. Previous measurements have shown that the sp3 content of these films varied from 0 to >80%. The optical constants (n, k, absorption coefficient (α)) of these films were measured by ellipsometry. The energy gaps were derived from Tauc plots (Eg) and from α = 104 cm−1 values (E04). The energy gaps were found to vary with the sp3 content from E

Absolute quantum photoyield of diamond thin films: Dependence on surface preparation and stability under ambient conditions

Absolute quantum photoyield (QPY) measurements (140–210 nm) of chemical vapor deposited (CVD) diamond films are reported. The dependence of the QPY on hydrogenation by exposure to a hydrogen microwave (MW) plasma and oxidation by a mixture of acids or on exposure to air under ambient conditions have been studied. Films deposited by MWCVD display a higher QPY than those grown by hot filament (HF) CVD. The QPY values are found to depend on the state of the surface. Hydrogen-terminated films exhibit values above 12% at 140 nm, whereas even small amounts of oxygen strongly degrade the QPY. B-doping, at the level of 1500 ppm, has no apparent effect on the photoemission properties. Exposure of the hydrogenated films to ambient conditions results in oxygen adsorption, leading to degradation of the photoemission properties. Analysis of the data within the three-step model of photoemission clearly shows that the state of the surface is a dominant factor determining the QPY.

Hydrogen concentration and bonding configuration in polycrystalline diamond films: From micro-to nanometric grain size

The present work studies the incorporation of hydrogen and its bonding configuration in diamond films composed of diamond grains of varying size which were deposited by three different methods: hot filament (HF), microwave (MW), and direct current glow discharge (dc GD) chemical vapor deposition (CVD). The size of diamond grains which constitute the films varies in the following way: hundreds of nanometers in the case of HF CVD (“submicron size,” ∼300nm), tens of nanometers in the case of MW CVD (3–30nm), and a few nanometers in the case of dc GD CVD (“ultrananocrystalline diamond,” ∼5nm). Raman spectroscopy, secondary ion mass spectroscopy, and high resolution electron energy loss spectroscopy (HR-EELS) were applied to investigate the hydrogen trapping in the films. The hydrogen retention of the diamond films increases with decreasing grain size, indicating that most likely, hydrogen is bonded and trapped in grain boundaries as well as on the internal grain surfaces. Raman and HR-EELS analyses show that ...

Carbon diffusion in uncoated and titanium nitride coated iron substrates during microwave plasma assisted chemical vapor deposition of diamond

Auger electron spectroscopy has been employed to investigate the effectiveness of thin films of TiN as barriers to carbon diffusion during chemical vapor deposition (CVD) of diamond onto Fe substrates. Auger depth profiling was used to monitor the C concentration in the TiN layer, through the interface and into the substrate both before and after CVD diamond deposition. The results show that a layer of TiN only 250 A thick is sufficient to inhibit soot formation on the Fe surface and C diffusion into the Fe bulk.

Adhesion improvement of diamond films on steel subtrates using chromium nitride interlayers

Abstract Direct deposition of diamond on ferrous materials suffers from adhesion problems due to the build up of a graphitic layer at the interface. A successful solution to this problem was attained through the use of an intermediate layer consisting of 20 μm thick nitrided chromium film. The initial stage of diamond deposition resulted in the partial carburization of the chromium nitride interlayer onto which a continuous diamond film was deposited. The adhesion of the diamond film to the substrate was enhanced by complicated mechanical building and chemical bonding of the interfacial region and was measured by an indentation test. No delamination events occurred during indentation up to loads of 1000 N. Phase transformation into the interlayer, diamond particle density and quality of obtained films were studied for different times of deposition. The samples were examined by a number of complementary techniques: AES, XRD, Raman Spectroscopy, SEM and EDS. Residual stresses in deposited films were calculated from the shift of the diamond peak in the Raman spectrum and were compared to thermal stresses resulting from mismatch of the thermal expansion coefficients of diamond and the substrate. It is suggested that a chromium nitride interlayer could be employed in the deposition of diamond films on iron based alloys for industrial purposes.

ENHANCEMENT OF DIAMOND NUCLEATION BY ULTRASONIC SUBSTRATE ABRASION WITH A MIXTURE OF METAL AND DIAMOND PARTICLES

A method of surface treatment was found to enhance diamond chemical vapor deposition nucleation on nondiamond substrates such as Si, SiO2, and Al2O3. The nucleation density obtained by ultrasonic abrasion with diamond powder alone was found to be enhanced by a few orders of magnitude using a mixed slurry consisting of diamond and metal powders. No strong nucleation enhancement was observed using a metal slurry only for surface treatment. The metal powders used were W, Ta, Mo, Nb, Ti, Al, Fe, Ni, Cu, and Si. It is concluded that the enhanced nucleation is associated with a physico‐chemical modification of the substrate surface, attained through a cooperative effect of both the metal and diamond particles during the ultrasonic abrasion process.