Ion implantation in diamond using 30 keV Ga focused ion beam

Focused ion beam (FIB) technique is a well established technique for processing and modifying materials at micro- and nanoscale. FIB implantation with 30 keV Ga⁺ ions into a single crystal diamond has been studied via a combination of transmission electron microscopy (TEM) imaging and spectroscopy in the attempt to understand the damage formation in diamond. The damage formation has been studied as a function of implantation dose with eight different doses ranging from 6 × 10¹⁴ to 1 × 10¹⁶ ions/cm². The TEM studies have revealed different structure of low-dose and high-dose implanted regions. 3.5 nm diamond cap layer was observed in the low-dose implanted layer. TEM analysis has shown volume extension of around 50% in the amorphous region and up to 7% in diamond at the crystal-amorphous interface. The density of amorphous damage layer was measured to be 2.51 g/cm³ and 2.24 g/cm³ in the low-dose and high-dose implanted regions, respectively. The amorphisation threshold for ion implantation in diamond at room temperature was determined to be 5.2 × 10²² vacancies/cm³.     

Sharpening of CVD diamond coated tools by 0.5−10 keV Ar ion beam

CVD diamond coated tungsten carbide tools have been used for cutting and drilling of soft materials such as aluminum and copper alloys. However, it is very difficult to obtain a tool having a sharp tip of the order of sub-μm by mechanical abrasive polishing methods. Therefore, we applied ion beam processing for sharpening the cutting edge of diamond coated tungsten carbide tools. Result shows that it is possible to obtain a 20-80 nm order tip width of a CVD diamond coated knife processed by a 0.5-10 keV Ar⁺ ion beam, and the sharpening speed of a tip of the knife depends on the ion beam energy. Namely, a tip width of a knife processes by a 1.0 keV Ar⁺ ion beam at an ion dose of 2.7 × 10²⁰ ions/cm² becomes 20 nm, and a tip width of a knife processed by a 10 keV Ar⁺ ion beam at an ion dose of 5.4 × 10¹⁹ ions/cm² becomes 40 nm. However, a facet with an apex angle in the range of 60-100° was formed on the cutting edge of a knife with an initial apex angle of 55°, and we found that the facet angle can be controlled by choosing ion beam energy of 0.5-10 keV. Moreover, results show that the processed knife machined by a 0.5 keV Ar⁺ ion beam has very smooth rake and flank faces, and also has a small line edge roughness of the cutting edge compared to those of the sharpened knife by a 1.0-10 keV Ar⁺ ion beam.     

High-current electron emission characteristics of cathodes based on diamond films

Using different gas source, four types of diamond thin films were prepared on silicon substrate by microwave plasma chemical vapor deposition (MPCVD) technology, and characterized in detail through scanning electron microscopy (SEM), Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. High-current pulsed emission characteristics, tested with a 2 MeV line-inducing injector, showed that all of CVD diamond films had high emission current density (> 70 A/cm²) and [100] textured B-doped microcrystalline diamond film possessed the largest emission current density of 115.1 A/cm². No obvious bright light and luminescent zones from side view CCD images indicated a possible pure field-emission mechanism of these diamond cathodes. Simultaneously, large decrease in the electron emission capability, above 15%, could be observed after several pulsed measurements, but this decrease could be completely recovered through the treatment of surface re-hydrogenation for emitted diamond cathodes, suggesting that emission performance of CVD diamond cathodes was closely relevant to hydrogen coverage ratio. The present data indicated that as-deposited CVD diamond films could be a potential candidate as cold cathode for the application in high-current electron emission field.     

Comparative study of homoepitaxial single crystal diamond growth at continuous and pulsed mode of MPACVD reactor operation

Homoepitaxial growth of single crystal diamond by microwave plasma chemical vapor deposition in pulsed regime of a 2.45 GHz MPACVD reactor operation at pulse repetition rates of 150 and 250 Hz was investigated. The high quality CVD diamond layers were deposited in the H₂-CH₄ gas mixture containing 4% and 8% of methane, gas pressures of 250 and 260 Torr and substrate temperature of 900 °C without any nitrogen addition. The (100) HPHT single crystal diamond seeds 2.5 × 2.5 × 0.3 mm (type Ib) were used as substrates. At pulse repetition rate 150 Hz the high quality single crystal diamond was grown with growth rate of 22 μm/h. The comparison of the single crystal diamond growth rates in CW and pulsed wave regimes of MPACVD reactor operation at microwave power density 200 W/cm³ was made. It was found that at equal power density, the growth rate in pulsed wave regime was higher than in CW regime. Differences in single crystal diamond growth for two sets of experiments (with continuous and pulsed wave regimes) were explained.     

Focused Ion beam implantation of diamond

The interaction between diamond and a 30 kV Ga⁺ focused ion beam, has been studied. Electron backscattered diffraction identified the critical dose for amorphisation of the diamond surface at 2 × 10¹⁴ Ga⁺/cm². Scanning transmission electron microscopy identified a 35 nm amorphous carbon layer which, at higher doses, can swell up to 31% its original volume and accommodate a significant quantity of gallium. Electron energy loss and energy dispersive X-ray spectroscopy further characterised this layer and identified both excess hydrogen and oxygen contained within a stable amorphous carbon structure.     

Early stage of diamond growth at low temperature

We investigate the first stages of nanocrystalline diamond (NCD) thin film growth at low substrate temperature. NCD films were grown on silicon substrates by microwave plasma enhanced chemical vapor deposition (CVD) for 0–300 min at a temperature of 410 °C. Si substrates were ultrasonically seeded in suspension of detonation nanocrystalline diamond powder. The seeding density approached values up to 1 ⁎ 10¹² cm^− 2, which allows growth of ultra-thin fully closed layers. Stagnation of the AFM roughness indicates that the low temperature NCD growth is a) delayed due to the surface contamination of the used nanodiamond powder and b) possibly dominated by the growth in the lateral direction. XPS measurements showed that the measured surface exhibits changes from a multi-phase composite (seeding layer) to single-phase one (NCD layer).     

Influence of nucleation density on film quality,growth rate and morphology of thick CVD diamond films

The optimum growth parameters of our 5 kW microwave plasma CVD reactor were obtained using CH₄/H₂/O₂ plasma and high quality transparent films can be produced reproducibly. Among the films prepared in this system, the film of best quality has very smooth crystalline facets free of second nucleation and the full width at half maximum (FWHM) of the diamond Raman peak is 2.2 cm⁻¹, as narrow as that of IIa natural diamond. For this study, diamond films were grown on silicon substrates with low (10⁴–10⁵ cm⁻²) and high nucleation densities (>10¹⁰ cm⁻²), respectively. From the same growth run, a highly 〈110〉 textured 300 μm thick white diamond film with a growth rate of 2.4 μm/h was obtained from high nucleation densities (>10¹⁰ cm⁻²), and a white diamond film of 370 μm in thickness with a higher growth rate of 3 μm/h was obtained from low nucleation densities (5×10⁴–10⁵ cm⁻²) too. The effect of nucleation density on film quality, growth rate, texture and morphology was studied and the mechanism was discussed. Our results suggest that under suitable growth conditions, nucleation density has little effect on film quality and low nucleation density results in higher growth rate than high nucleation density due to less intense grain growth competition.     

The effects of 2 MeV Ag ion irradiation on multiwalled carbon nanotubes

Multiwalled carbon nanotubes were irradiated using 2 MeV Ag²⁺ ions with fluences 1 × 10¹⁵ and 1 × 10¹⁶ ions/cm². The samples were characterized by transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The graphene structure of the nanotube wall is found to be damaged upon irradiation. Although at a fluence of 1 × 10¹⁵ ions/cm² the outer morphology of the samples remain unchanged, samples irradiated with a fluence of 1 × 10¹⁶ ions/cm² show complete destruction of the graphene structure. The amorphous structure produced due to irradiation is found to show an increased number of sp³ hybridized carbon atoms.     

Schottky diode architectures on p-type diamond for fast switching, high forward current density and high breakdown field rectifiers

The electrical properties of Schottky contacts on the (100) surface of Boron doped diamond films epitaxially grown on Ib substrates are investigated in this work. The role of Boron doping concentration and extended defects detected by cathodoluminescence is correlated to current voltage characteristics, rectifying efficiency and high voltage performance of the diodes up to 1 kV and more. The influence of surface treatment prior to metal deposition is highlighted and the choice of metal for the Schottky contact is discussed. The paramount importance of using an oxidised diamond surface at the Schottky contacts and outside is demonstrated. Decreasing the series resistance of diodes is obtained with a stack of two layers, the upper one being lightly doped while the deeper one contains Boron concentrations close to the metallic conductivity threshold (4 × 10²⁰ B/cm³). Several architectures are studied. The ohmic contact directly laid on the heavily doped layer permits forward current densities of 66 A/cm² under 4 V at room temperature and switching times in the nanosecond range. This set of results shows that p-type diamond is an adequate semiconductor for implementing high speed, high power and high voltage electronic rectifiers.     

Electrical characterization of phosphorus-doped n-type homoepitaxial diamond layers

Electrical properties of phosphorus (P)-related donors have been investigated for P-doped homoepitaxial diamond layers grown by microwave plasma CVD. Temperature-dependent current–voltage (I–V), capacitance–voltage (C–V) measurements and frequency-dependent C–V measurements have been carried out with lateral dot-and-plane (with ring-shaped gap) Schottky barrier diodes. N-type Schottky junction properties were obtained. The ideality factor and the rectification ratio of the Schottky junction were obtained to be 1.9 and 1.7×10⁵ at ±10 V and 473 K, respectively. Frequency-dependent measurements on these Schottky barrier diodes have shown that the capacitance is reduced at high frequency, most likely due to the inability of deep centers to maintain an equilibrium ionization state under a high-frequency modulation. C–V measurements deduced that the net donor concentration was 6.2×10¹⁷ cm⁻³ and the corresponding built-in potential was 4.0 eV, when the P concentration was 8.3×10¹⁷ cm⁻³. Phosphorus electrical activity was 0.75 in the P-doped diamond layer. The carrier thermal activation energy (the donor level) was evaluated to be 0.6 eV from the relation between the net donor concentration and the carrier concentration.     

Studies of heteroepitaxial growth of diamond

Large-scale heteroepitaxial growth of diamond depends critically on the development of a suitable lattice-matched substrate system. Oxide substrates, notably MgO and SrTiO₃, on which thin epitaxial films of iridium serve as a nucleation layer for diamond have already shown considerable promise. We describe here improvements in the growth of single crystal diamond by low-pressure microwave plasma-enhanced CVD. Oxide substrates with flat, low-index surfaces form the initial basis for the process. Iridium was deposited on heated substrates in a UHV electron-beam evaporation system resulting in epitaxial films, typically 150–300 nm thick, with Ir (1 0 0) parallel to the surface of all substrates as confirmed by X-ray and electron backscattering diffraction. Following Ir deposition, the samples were transferred to a CVD reactor where a bias-enhanced nucleation step induced a dense condensate that completely covered the Ir surface. Uniform nucleation densities of order 10¹² cm⁻² were observed. Interrupted growth studies, carried out at intervals from seconds to minutes subsequent to terminating the nucleation step, revealed a rapid coalescence of grains. One hour of growth resulted in a smooth, nearly featureless, (0 0 1) diamond film. For extended growth runs, slabs of diamond were grown with thickness as great as 38 μm and lateral dimensions near 4 mm. The crystals were transparent in visible light and cleaved on (1 1 1) planes along 〈1 1 0〉 directions, similar to natural diamond. Of particular significance is the successful use of sapphire as an underlying substrate. Its high crystalline perfection results in epitaxial Ir films with X-ray linewidths comparable to those grown on SrTiO₃. However, Al₂O₃ possesses superior interfacial stability at high temperatures in vacuum or in a hydrogen plasma with a better thermal expansivity match to diamond. Since sapphire is available as relatively inexpensive large diameter substrates, these results suggest that wafer-scale growth of heteroepitaxial diamond should be feasible in the near future.     

Low resistivity p+ diamond (100) films fabricated by hot-filament chemical vapor deposition

By hot-filament (HF) chemical vapor deposition (CVD), heavily boron (B)-doped single-crystal diamond (100) films were fabricated and their structural and electrical properties were studied. We did not observe the soot formation, which is frequently observed and limits the performances in the case of microwave plasma (MWP) CVD. The B concentration was successfully controlled over the range from 10¹⁹ to 10²¹ cm^− 3. Hillock-free films were obtained, whose mean surface roughness measured by atomic force microscopy (AFM) was less than 0.1 nm. From the reciprocal space mapping (RSM) around 113 diamond reflection, it was revealed that the films possess the smaller lattice expansion than that expected from the Vegard's law. The room-temperature resistivity was decreased lower than 1 mΩ·cm for B concentration ~ 10²¹ cm^− 3. These results indicate that the HFCVD possesses large potential for fabricating the device-grade p⁺ diamond.     

Structure and electrochemical characterization of electrolytic Ni + Mo + Si composite coatings in an alkaline solution

Ni + Mo + Si coatings were obtained by nickel deposition from a bath containing suspension of molybdenum and silicon powders. These coatings were obtained in galvanostatic conditions, at the current density of j_dep = −0.100 A cm⁻². For determination of the influence of phase composition and surface morphology of obtained coatings on changes of corrosion resistance, these coatings were modified in argon atmosphere by thermal treatment at the temperature of 1100 °C during 1 h. A scanning electron microscope was used for surface morphology characterization of the coatings. Chemical composition of obtained coatings was determined by X-ray fluorescence spectroscopy method. Phase composition investigations were conducted by X-ray diffraction method. It was found that the obtained coatings are composed of three phase structures, i.e., nickel, molybdenum and silicon. Phase composition for the Ni + Mo + Si coatings after thermal treatment is markedly different. The main peaks corresponding to the Ni and Mo coexist with the new ones corresponding to new phases: Mo₅Si₃, NiSi, Mo₂Ni₃Si and Ni₆Mo₆C_1.06.Electrochemical corrosion resistance investigations were carried out in the 5 M KOH, using potentiodynamic and electrochemical impedance spectroscopy methods. On the basis of these investigations it was found that Ni + Mo + Si coatings after thermal treatment are more resistant in alkaline solution than Ni + Mo + Si as-deposited coatings. The reason of this is presence of silicides in the coatings.     

Performance of XSCoF (X = Ba, La and Sm) and LSCrX′ (X′ = Mn, Fe and Al) perovskite-structure materials on LSGM electrolyte for IT-SOFC

X_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (X = Ba, La and Sm) and La0.75Sr0.25Cr0.5X^′0.5O3−δ (X′ = Mn, Fe and Al) mixed ionic-electronic conducting perovskite-based oxides have been tested as SOFC electrode materials on La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSGM) electrolytes under different atmospheres (air, oxygen, argon and dry and wet 5% H₂/Ar) and the area-specific resistances (ASR) were compared. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCoF) possesses the lowest ASR values in air (0.04 Ω cm² at 1073 K) whilst La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCrM) possesses the lowest ASR values in wet 5% H₂/Ar (0.28 Ω cm² at 1073 K). In addition, fuel cell tests were carried out using wet 5% H₂/Ar as fuel and air as oxidant. The maximum power density (∼123 mW cm⁻²) at 1073 K was reached with the electrolyte-supported system BSCoF/LSGM/LSCrM (∼1.5 mm electrolyte thickness). Furthermore, LSCrX′ materials were used simultaneously as cathode and anode in fuel cell tests and the symmetric system LSCrM/LSGM/LSCrM (∼1.5 mm electrolyte thickness) reached a maximum power density of ∼54 mW cm⁻² at 1073 K.     

Characterization of nanocrystalline diamond films implanted with nitrogen ions

Nanocrystalline diamond films, prepared by a microwave plasma-enhanced CVD, were implanted using 110-keV nitrogen ions under fluence ranging from 10¹⁶–10¹⁷ ions cm⁻². AFM, XRD, XPS and Raman spectroscopy were used to analyze the changes in surface structure and chemical state of the films before and after implantation. Results show that high-fluence nitrogen ions implanted in the nanocrystalline diamond film cause a decline in diamond crystallinity and a swelling of the crystal lattice; the cubic-shaped diamond grains in the film transform into similar roundish-shaped grains due to the sputtering effect of implanted nitrogen ions. Nitrogen-ion implantation changes the surface chemical state of the nanocrystalline diamond film. After high-fluence implantation, the surface of the film is completely covered by a layer of oxygen-containing groups. This phenomenon plays an importance role in the reduction of the adhesive friction between an Al₂O₃ ball and the nanocrystalline diamond film.     

Characteristics of HPHT diamond grown at sub-lithosphere conditions (10-20 GPa)

We have conducted high pressure-high temperature (HPHT) diamond synthesis experiments at the conditions of growth of superdeep diamonds (10-20 GPa), equivalent to the transition zone, using MgCO₃ carbonate (oxidising) and FeNi (reducing) solvent catalysts. High rates of graphite-diamond transformation were observed in these short duration experiments (20 min). Transformation rates were higher using the metallic catalyst than in the carbonate system. High degrees of carbon supersaturation at conditions significantly above the graphite-diamond stability line, led to a high nucleation density. This resulted in the growth of aggregated masses of diamond outlined by polygonised diamond networks, resembling carbonado. Where individual crystals are visible, grown diamonds are octahedral in the lower pressure experiments (≤ 10 GPa in MgCO₃ and ≤ 15 GPa in FeNi) and, cubo-octahedral at higher pressure. All grown diamonds show a high degree of twinning. The diamonds lack planar deformation features such as laminations or slip planes, which are commonly associated with natural superdeep diamonds.     

Studies of interfacial layers between 4H-SiC (0 0 0 1) and graphene

The region between epitaxial graphene and the SiC substrate has been investigated. 4H-SiC (0 0 0 1) samples were annealed in a high temperature molecular beam epitaxy system at temperatures between 1100 and 1700 °C. The interfacial layers between the pristine SiC and the graphene layers were studied by X-ray photoelectron spectroscopy. Graphene was found to grow on the SiC surface at temperatures above 1200 °C. Below this temperature, however, sp³ bonded carbon layers were formed with a constant atomic Si concentration. C1s and Si2p core level spectra of the graphene samples suggest that the interface layer we observe has a high carbon concentration and its thickness increases during the graphitization process. A significant concentration of Si atoms is trapped in the interface layer and their concentration also increases during graphitization.     

Amorphization and graphitization of single-crystal diamond — A transmission electron microscopy study

The amorphization and graphitization of single-crystal diamond by ion implantation were explored using transmission electron microscopy (TEM). The effect of ion implantation and annealing on the microstructure was studied in (100) diamond substrates Si⁺ implanted at 1 MeV. At a dose of 1 × 10¹⁵ cm^− 2, implants done at 77 K showed a damage layer that evolves into amorphous pockets upon annealing at 1350 °C for 24 h whereas room temperature implants (303 K) recovered to the original defect free state upon annealing. Increasing the dose to 7 × 10¹⁵ Si⁺/cm² at 303 K created an amorphous-carbon layer 570 ± 20 nm thick. Using a buried marker layer, it was possible to determine that the swelling associated with the amorphization process was 150 nm. From this it was calculated that the layer while obviously less dense than crystalline diamond was still 15% more dense than graphite. Electron diffraction is consistent with the as-implanted structure consisting of amorphous carbon. Upon annealing, further swelling occurs, and full graphitization is achieved between 1 and 24 h at 1350 °C as determined by both the density and electron diffraction analysis. No solid phase epitaxial recrystallization of diamond is observed. The graphite showed a preferred crystal orientation with the (002)g//(022)d. Comparison with Monte Carlo simulations suggests the critical displacement threshold for amorphization of diamond is approximately 6 ± 2 × 10²² vacancies/cm³.     

Evaluation research of polishing methods for large area diamond films produced by chemical vapor deposition

The current study compared several polishing techniques of chemical vapor deposition (CVD) diamond films. Although research on various diamond polishing techniques has been carried for years, some issues still need to be examined in order to facilitate application on large areas in a cost-efficient manner. In the present work, microwave plasma enhanced chemical vapor deposition (CVD) was used to obtain diamond films with full width half magnitude (FWHM) less than 10 wavenumbers at 1332 cm^− 1 Raman peak. The diamond films were processed through mechanical polishing, chemical-assisted mechanical polishing, thermo-chemical polishing, excimer laser ablation, and catalytic reaction assisted grinding. A profilometer, an atomic force microscope, and a scanning electron microscope have been used to evaluate the surface morphology of diamond films before and after polishing. The results obtained by using the above mentioned techniques were analyzed and compared.     

Perovskite-type La2−xSrxNiO4 (0 ≤ x ≤ 1) as active anode materials for methanol oxidation in alkaline solutions

Perovskite-type ternary oxides with molecular formulae, La_2−xSr_xNiO₄ (0 ≤ x ≤ 1), were prepared by a modified citric acid sol-gel route at 600 °C for their possible use in a direct methanol fuel cell (DMFC). The study was conducted by cyclic voltammetry, chronoamperometry, impedance and anodic Tafel polarization techniques. The results showed that the electrocatalytic activity of the base oxide (x = 0) in 1 M KOH plus 1 M CH₃OH at 25 °C increases with x, the observed current densities being 23.6, 47.3, 43.2 and 50.9 mA cm⁻² at a scan rate of 10 mV s⁻¹ and E = 0.6 V versus Hg/HgO for oxides with x = 0, 0.25, 0.5 and 1.0, respectively. All the four perovskite anodes used in this study did not indicate any poisoning by the methanol oxidation intermediates/products. The methanol electro-oxidation reaction followed a Tafel slope of ∼2 × 2.303RT/3F (=40 mV decade⁻¹) on each oxide catalyst, regardless of Sr content.