Non-diamond Carbon Phases Research Articles

Study on the compound film of diamond for absorbing radiation

The solar radiation has significant impact on the earth climate and environment, so it’s important to detect it. High thermal conductivity and high absorptivity material for absorbing radiation is highly needed to improve the performance of the radiation detector. The compound diamond films were deposited using microwave plasma chemical vapor deposition （MW-PCVD） and hot cathode direct current plasma chemical vapor deposition （DC-PCVD） methods. The absorptivity of the compound diamond film is 99%—99.2%. With the increase of the thickness of black diamond layer, the thermal conductivity of the compound diamond film decreases a little, but the thermal conductivity is always larger than 16 W/K·cm when the thickness of black diamond layer is less than 15 μm and so it is still a high thermal conductivity material. The black diamond layer deposited on high purity diamond film by hot cathode DC-PCVD method has apparent wide Raman peaks at 1500—1600 cm-1and 1350 cm-1 which correspond to non-diamond carbon phase. With the increase of methane, this non-diamond carbon phase also increases. As the non-diamond carbon phase, like graphite, increases, the transmissivity of the compound diamond films decreases. The black diamond layer deposited on the high purity diamond acts as the heat sink and has high surface adhesion property, and high thermal conductivity.

Hydrogen plasma and atomic oxygen treatments of diamond: Chemical versus morphological effects

Chemical bonding and morphology of chemical vapor deposited diamond films were studied using high resolution electron energy loss spectroscopy and atomic force microscopy, following hydrogen plasma and atomic oxygen exposures. The hydrogen plasma exposure resulted in preferential etching of nondiamond carbon phases, selective etching of diamond facets, and termination of the diamond surfaces by sp3-C–H species. Exposure to atomic oxygen, on the other hand, produced significant chemical changes resulting in oxidized hydrocarbon ill defined top layer, while the morphology of the surface remained almost unchanged.

Thermal conductivity of sintered nanodiamonds and microdiamonds

We studied the sintering mechanisms and thermal conductivity of composites based on well purified detonation nanodiamonds (ND) prepared by Dr. Aleksenskii A.E. The thermal and electrical conductivities of composites from natural diamonds of 10–14 μm in size were also examined. Both types of composites were sintered at high pressures (5.0–7.0 GPa) and high temperatures (1200–2300 °C) for 10–25 s. It was found that the thermal conductivity of composites from natural diamonds increased as the sintering temperature approached the diamond–graphite equilibrium in the pressure range of 4.5–6.5 GPa because of the interdiffusion of the diamond particles. Above the phase equilibrium temperature, the thermal conductivity was observed to decrease due to the sample bulk graphitization. The maximum value of this parameter for the ND samples was observed at approximately 1900 °С. Higher temperatures caused sample damage at lowered pressures, which seems to be due to the ND transition to the nondiamond carbon phase possessing a lower density. When we added 5 wt.% of С 60 fullerene to the initial ND, the diamond transition to a nondiamond carbon-like state occurred at a temperature below 1400 °С and the thermal conductivity increased from 50 to 100 W/(m · K). Thermal conductivity was found to be about 50 W/(m · K) for the ND samples and about 500 W/(m · K) for the microdiamonds.

Low temperature growth of ultrananocrystalline diamond film and its field emission properties

Ultrananocrystalline diamond (UNCD) film is deposited at a substrate temperature lower than 500 °C. This film possesses diamond crystal of nanometer size embedded in a graphitic (or non-diamond carbon) phase. The presence of non-diamond carbon in the grain boundaries of diamond crystal plays a crucial role to the film properties and its corresponding application such as electron field emission. The present work reports the growth of UNCD films at different methane concentrations to alter the film properties that could make it suitable for higher electron field emission. The surface morphology of an as-grown film was examined with a field emission scanning electron microscope. Nucleation density in the range of 10 11–10 12/cm 2 is obtained in the as-grown films. The grain size of diamond increases from 5 nm to 25 nm with an increase in CH 4 concentration from 1% to 7.5% in the argon plasma. The presence of different carbon phases in the diamond films was investigated qualitatively by Raman studies. Near edge X-ray fine structure study ascertains that the as-grown films mainly possess diamond phase. A direct correlation of field emission properties with the CH 4 concentration during UNCD growth is obtained.

Microwave CVD Thick Diamond Film Synthesis Using CH4/H2/H2O Gas Mixtures

Thick diamond films with a thickness of up to 1.2 mm and a area of 20 cm2 have been grown in a homemade 5 kW microwave plasma chemical vapor deposition (MPCVD) reactor using CH4/H2/H2O gas mixtures. The growth rate, radial profiles of the film thickness, diamond morphology and quality were evaluated with a range of parameters such as the substrate temperature of 700 oC to 1100 oC, the fed gas composition CH4/H2 = 3.0%, H2O/H2 = 0.0%∼ 2.4%. They were characterized by scanning electron microscopy and Raman spectroscopy. Translucent diamond wafers have been produced without any sign of non-diamond carbon phases, Raman peak as narrow as 4.1 cm−1. An interesting type of diamond growth instability under certain deposition conditions was observed in a form of accelerated growth of selected diamond crystallites of a very big lateral size, about 1 mm, and of a better structure compared to the rest of the film.

Partially stabilized zirconia substrate for chemical vapor deposition of free-standing diamond films

In this work we investigated the use of partially stabilized zirconia (PSZ) as the substrate for deposition of CVD diamond films. The polycrystalline PSZ substrates were sintered at high temperatures and the results showed that this material has unique properties which are very appropriated for the growth of free-standing diamond films. The diamond nucleation density on PSZ is high, even without seeding, and the CVD diamond film was totally released from the substrate after the deposition process, without cracking. Micro-Raman analysis revealed that the free-standing diamond film had a good crystallinity on both surfaces with practically no stress in the structure. The same PSZ substrate can be reutilized for the deposition of a large number of diamond films. The average growth rate is about 5–6 μm/h in a microwave plasma reactor at 2.5 kW. The deposition process causes the reduction of ZrO 2, producing ZrC. The high mobility of oxygen in the zirconia matrix at high temperature would probably help to etch the interface region between the substrate surface and the diamond film, decreasing the adhesion strength and eliminating some defects in the film structure related to non-diamond carbon phases.

Synthesis of diamond films and nanotips through graphite etching

A hot filament CVD process based on hydrogen etching of graphite has been developed to synthesize diamond films and nanotips. The graphite sheet was placed close to the substrate and only hydrogen was supplied during deposition. No hydrocarbon feed gases are required for this process. High quality diamond films were synthesized with high growth rate on P-type (1 0 0)-oriented silicon wafers without discharge or bias. The diamond growth rate is approximately five times higher than that through conventional hot filament chemical vapor deposition using a gas mixture of methane and hydrogen (1 vol.% methane) under similar deposition conditions. The diamond films synthesized in this process exhibit smaller crystallites and contain smaller amount of non-diamond carbon phases. Synthesis of well-aligned diamond nanotips with various orientation angles was achieved on the CVD diamond-coated Si substrate when the substrate holder was negatively biased in a DC glow discharge. The nanotips grown at locations far enough from the sample edges are aligned vertically, while those around the sample edges are tilted and point away from the sample center. The alignment orientation of the nanotips appears to be determined by the direction of the local electric field lines on the sample surfaces.

Ultraviolet stimulated photocatalytic activity on polycrystalline diamond film surfaces

A photocatalytic reaction produced on polycrystalline diamond film surfaces by ultraviolet (UV) light was studied by micro-Raman spectroscopy. The diamond films were specially prepared for this study by the rf-assisted chemical vapor deposition technique. The photocatalytic activity was analyzed in terms of the sharpness of the typical diamond Raman band at 1332 cm−1 and by the weak nondiamond carbon (NDC) features around 1550 cm−1. The micro-Raman spectra show a decrease of the NDC content originally codeposited on diamond film surfaces during the synthesis. This decreasing is proportional to the UV irradiation time, suggesting that the photocatalytic reaction can be controlled by the UV irradiation dose. Since a selective reduction of the NDC phase was achieved on diamond surfaces. The UV irradiation was applied continuously during 1, 2, and 3 days in open atmosphere at room temperature. The role of oxygen radicals, such as ozone (O3) and atomic oxygen (O*) on the photocatalytic reaction is discussed, as well.

Vacuum-annealed undoped polycrystalline CVD diamond: a new electrode material

Vacuum-annealing imparts conductivity to initially insulating undoped polycrystalline chemical–vapor-deposited diamond, thus turning it to a possible electrode material. The diamond film annealed at 1775 K appeared to be practically not conducting. With further increase in the annealing temperature above 1825 K, the film effective resistivity decreased from initial value of 10 11 to 10 12 Ω cm down to less than 0.1 Ω cm; the differential capacitance increased from ∼10 −3 to ∼50 μF per 1 cm 2 of geometrical surface; the transfer coefficients for electrochemical reactions in the [Fe(CN) 6] 3−/4− redox solution increased from ∼0.2 to 0.5; and the degree of reversibility of the electrochemical reaction increased. The observed changes in the electrode properties are attributed to gradual change in the thickness and/or properties (first and foremost, conductivity) of the nondiamond carbon phase formed along the intercrystallite boundaries upon the annealing; the conducting phase is outcropping at the film surface as an array of microelectrodes (“active sites”).

Electrochemical properties of undoped CVD diamond films vacuum-annealed at 1775–1915 K

Electrochemical behavior of vacuum-annealed undoped polycrystalline diamond films is studied. The film annealed at 1775 K appeared to be practically not conducting. With further increase in the annealing temperature above 1825 K, the film effective resistivity decreased from initial value of 10 11–10 12 Ω cm down to less than 0.1 Ω cm; the differential capacitance increased from ∼10 -3 to ∼50 μF per 1 cm 2 of geometrical surface; the transfer coefficients for electrochemical reactions in the [Fe(CN) 6] 3−/4− redox solution increased from ∼0.2 to 0.5; and the degree of reversibility of the electrochemical reaction increased. The observed changes of the electrode properties can be attributed to gradual change of the thickness and/or properties (first and foremost, conductivity) of the non-diamond carbon phase formed along the intercrystallite boundaries upon the annealing and outcropping at the film surface as ‘active sites’.

High pressure annealing of CVD diamond films

CVD diamond films were annealed from 600 to 1900 °C at 7.7 GPa in a toroidal high pressure (HP) apparatus, always inside the diamond-phase stability region. The annealed films were analyzed by Raman and infrared (IR) spectroscopy and the results showed that the diamond grains remained stable while the non-diamond carbon phases and impurities, responsible for the intricate film structure, changed after processing. For the HP annealing from 600 to 1300 °C, there were no major changes in the Raman spectra of the film, however, the film became easily broken and the IR spectra indicated a high reactivity of carbon with chemical elements from the environment. After annealing at 1500 °C and 7.7 GPa, the formation of diamond-like (DLC) and graphitic structures in-between the diamond grains were observed, while the reaction with the environment elements decreased. For higher temperatures, the DLC and graphitic structures persisted up to 1700 °C and the film incorporated OH in large amounts. The results showed that the non-diamond carbon species are susceptible to the HP annealing, and structural modifications in between the diamond grains are significant for temperatures above 1300 °C at 7.7 GPa.

Cathodoluminescence characteristics of polycrystalline diamond films grown by cyclic deposition method

Polycrystalline diamond films were deposited using a cyclic deposition method where the H 2 plasma for etching ( t E) and the CH 4+H 2 plasma for growing ( t G) are alternately modulated with various modulation ratios ( t E/ t G). From the measurement of full width at half maximum and I D/ I G intensity ratio obtained from the Raman spectra, it was found that diamond defects and non-diamond carbon phases were reduced a little by adopting the cyclic deposition method. From the cathodoluminescence (CL) characteristics measured for deposited films, the nitrogen-related band (centered at approximately 590 nm) as well as the so-called band-A (centered at approximately 430 nm) were observed. As the cyclic ratio t E/ t G increased, the relative intensity ratio of band-A to nitrogen-related band ( I A/ I N) was found to monotonically decrease. In addition, analysis of X-ray diffraction spectra and scanning electron microscope morphologies showed that CL characteristics of deposited diamond films were closely related to their crystal orientations and morphologies.

Bias assisted growth on diamond single crystals: the defect formation due to ion bombardment studied by ion channelling, electron backscatter diffraction, and micro-Raman spectroscopy

The basic mechanisms of defect formation due to ion bombardment during bias assisted growth of diamond films were studied. Diamond films were deposited at bias voltages between −60 and –120 V for 2 h on Ib single crystals. In ion channelling experiments a steep increase in dechannelling was found: For the film grown at the lowest bias voltage of –60 V dechannelling increased by a factor of 2 as compared to the 0-V reference film. For the subsequent sample, deposited at –80 V, channelling was already completely suppressed. Analysis of backscatter Kikuchi patterns indicates that up to –85 V the decrease in pattern contrast is caused by defects in the crystal lattice equally distributed over the film rather than by variations of the crystal lattice orientation (mosaicity). According to the Raman spectra three different regimes can be distinguished: At low bias voltages up to –60 V, signal from non-diamond carbon phases is negligible. At high bias voltages of –100 V and above, the typical signature of amorphous and graphitic carbon phases is found in the spectra. In the third regime at approximately –80 V, these spectral features exhibit pronounced fine structure and a strong enhancement in intensity. Our experiments suggest that luminescence as well as resonance Raman effects contribute to the form of these spectra.

Rapid nucleation of diamond films by pulsed laser chemical vapor deposition

The nucleation of diamond films could be greatly enhanced on mirror-polished Si substrate by a pulsed Nd:YAG laser beam without any thermal- and plasma-assisted processes during a very short time. The nucleation density increased with decreasing laser power density from 1.38×10 10 to 1.17×10 9 W/cm 2 and deposition pressure from 1013 to 4 mbar. The pulsed laser beam made no contribution to enhance nucleation at substrate temperature as low as 650°C. X-ray diffraction measurements showed the (1 1 1) diffraction peak of diamond for the samples obtained using only pulsed laser during 40 min. The enhanced nucleation and growth of diamond crystallites were attributed to effective excitation of reactive gases and etching of non-diamond carbon phases by the pulsed laser beam.

Field- and photo-emission properties of CVD-diamond with different microcrystalline structure

Electron emission properties of diamond films with different microstructures have been investigated by field- and photo-emission measurements. Both the crystalline quality of diamond and the amount of non-diamond carbon phases appear to largely affect the energy threshold for electron photoemission, as well as its intensity and spectral distribution, suggesting different types of transition occurring below and above the diamond band gap energy. A similar dependence on the amount of non-diamond carbon phases was also found for electric field threshold in field-emission measurements, where electric field enhancement effects related to the change of film morphology were also detected. The experimental results are discussed in terms of excitation of electrons out of defect states related to the diamond surface.

Investigation of non-diamond carbon phases and optical centers in thermochemically polished polycrystalline CVD diamond films

Polycrystalline chemical vapor deposition (CVD) diamonds films grown on silicon substrates using the microwave-enhanced CVD technique were polished using the thermochemical polishing method. The surface morphology of the samples was determined by optical and scanning electron microscopes before and after polishing. The average surface roughness of the as-grown films determined by the stylus profilometer yielded 25 μm on the growth side and about 7 μm on the substrate side. These figures were almost uniform for all the samples investigated. Atom force microscopic measurements performed on the surface to determine the average surface roughness showed that thermochemical polishing at temperatures between 700 °C and 900 °C reduced the roughness to about 2.2 nm on both the substrate and growth sides of the films. Measurements done at intermittent stages of polishing using confocal micro-Raman spectroscopy showed that thermochemical polishing is accompanied by the establishment of non-diamond carbon phases at 1353 cm−1 and 1453 cm−1 at the initial stage of polishing and 1580 cm−1 at the intermediate stage of polishing. The non-diamond phases vanish after final fine polishing at moderate temperatures and pressures. Photoluminescence of defect centers determined by an Ar+ laser (λlexct= 514.532 nm) showed that nitrogen-related centers with two zero-phonon lines at 2.156 eV and 1.945 eV and a silicon-related center with a zero-phonon line at 1.681 eV are the only detectable defects in the samples.

The formation of a nondiamond carbon phase in a microwave gas discharge plasma under electron cyclotron resonance conditions

Special features in the nucleation and growth of a nondiamond carbon phase were studied in a microwave plasma of various acetylene-containing gas mixtures.

The Influence of Surface Interactions on the Reversibility of Ferri/Ferrocyanide at Boron‐Doped Diamond Thin‐Film Electrodes

The electrochemistry of four redox analytes and methyl viologen, was investigated at polycrystalline, boron‐doped diamond thin‐film electrodes before and after anodic polarization and hydrogen plasma treatment. The as‐deposited diamond surface is predominantly hydrogen terminated, and quasi‐reversible cyclic voltammograms were observed for all of these couples at 0.1 V/s. After anodic polarization in , the surface atomic O/C ratio, as determined by X‐ray photoelectron spectroscopy, increased from 0.02 to ca. 0.20. Concomitant with the increase in surface oxygen, the for increased to over 200 mV, while the values for the other redox systems remained relatively unchanged. After acid washing and rehydrogenating the surface in a hydrogen plasma (i.e., atomic hydrogen), the for returned to ca. 80 mV, while the values for the other three redox analytes remained close to the original values. The results demonstrate that electron transfer for ferri/ferrocyanide is very sensitive to the presence of surface carbon‐oxygen functionalities and that the electron transfer involves a site associated with the hydrogen‐terminated surface. The results also unequivocally rule out the influence of adventitious nondiamond carbon phases as the sole sites for the electron transfer. © 1999 The Electrochemical Society. All rights reserved.

Growth of high quality, large grain size, highly oriented diamond on Si (100)

Microwave and hot filament chemical vapor deposition processes were combined to deposit highly oriented and textured diamond films on Si (100). The sequential growth process yields closely packed crystallites with (100) surfaces that show no grain boundaries over areas of up to 60 000 μm2 by scanning electron microscopy. The diamond crystallites have the same orientation as the Si (100) substrate and their orientational order and surface quality as measured by low energy electron diffraction is comparable to that of single crystal diamond. Micro-Raman spectroscopy confirms the exceptional quality of the film surface by the complete absence of a luminescence background as well as the absence of spectral features attributed to nondiamond carbon phases.

Effect of nitrogen incorporation on electron emission from chemical vapor deposited diamond

Two different types of the nitrogen-doped chemical vapor deposited (CVD) diamond films were synthesized with N2 (nitrogen) and C3H6N6 (melamine) as doping sources. The samples were analyzed by scanning electron microscopy, Raman scattering, photoluminescence spectroscopy, and field-emission measurements. More effective substitutional nitrogen doping was achieved with C3H6N6 than with N2. The diamond film doped with N2 contained a significant amount of nondiamond carbon phases. The sample produced with N2 exhibited a lower field emission turn-on field than the sample produced with C3H6N6. It is believed that the presence of the graphitic phases (or amorphous sp2 carbon) at the grain boundaries of the diamond and/or the nanocrystallinity (or microcrystallinity) of the diamond play a significant role in lowering the turn-on field of the film produced using N2. It is speculated that substitutional nitrogen doping plays only a minor role in changing the field emission characteristics of CVD diamond films.