高品级人造金刚石合成的晶体学规律及分析

文章从晶体生长的一般规律出发,阐述了石墨与金刚石晶体结构的不同,论述了在金刚石晶体生长过程中金属触媒的作用机理以及高温高压对碳原子活化速度、结合速度、排逸速度及金刚石晶体生长速度与高品级金刚石生长的关系。     

Epitaxial growth on nickel-plated diamond seeds at high pressure and high temperature

Epitaxial growth on nickel-plated diamond seeds at high pressure and high temperature (HPHT) was observed with graphite as carbon source. The thickness of the electroplating nickel film which acts as a catalyst/solvent ranges from 54.6 μm to 255.6 μm. The relationship between the Ni film thickness and diamond growth rate is investigated. When the nickel film thickness is from 90 μm to 129 μm, diamond crystals can nearly grow up to three times as large as the original seeds at ∼ 5.8 GPa and ∼ 1460 °C within 14 min. The mechanism of the crystal growth with nickel-plated diamond seeds under HPHT is discussed. The results and techniques might be useful for high quality saw-grade diamonds production and large diamond single crystal growth.     

Improving purity and size of single-crystal diamond plates produced by high-rate CVD growth and lift-off process using ion implantation

The lift-off process using ion implantation has recently been applied to produce large and thick single-crystal diamond plates by chemical vapor deposition (CVD). CVD growth conditions for undoped, as opposed to nitrogen-doped, diamond were investigated to improve the purity of plates produced by this technique. This utilized apparatus identical to that for high-rate growth with nitrogen addition under high-density plasma. By lowering the growth temperature to 900 °C, an undoped single-crystal CVD diamond plate with a maximum length of 9 mm and thickness of 0.47 mm was successfully produced without formation of non-epitaxial crystallites. The UV–Vis–NIR transmission spectrum of this plate was identical to high-pressure high-temperature (HPHT) synthetic IIa diamond, suggesting high purity of the plate. To increase the size of single-crystal CVD diamond plates, a process to enlarge the seed crystal by combining the lift-off process and a side-surface growth technique is proposed. By this process, a half-inch single-crystal CVD diamond seed crystal was successfully synthesized and half-inch freestanding single-crystal CVD diamond plates were produced from the seed.     

Synthesis of HPHT diamond containing high concentrations of nitrogen impurities using NaN3 as dopant in metal-carbon system

Diamond crystals have been synthesized from FeNi alloy — carbon system under high pressure and temperature of 5.0 to 5.8 GPa and 1500 to 1750 K. Concentrations of the nitrogen impurities of the crystals have been determined from FTIR spectra, and it is found that the concentrations of single substitutional nitrogen impurities exceeds 1000 ppm, when NaN₃ is added as dopant to the metal-carbon system. The crystals are 0.2–0.5 mm in diameter with cubo–octahedral morphology, although octahedron and cube were also found in the diamond products. They exhibit yellow green to deep green color.     

Production of single crystal diamond needles by a combination of CVD growth and thermal oxidation

Single crystal diamond needles were produced by combining chemical vapor deposition of a thin polycrystalline diamond film and its subsequent thermal oxidation. The deposition process has been carried out in a direct discharge activated hydrogen–methane gas mixture with parameters providing (100) textured diamond film growth. The oxidation has been performed by heating the deposited film in an air atmosphere with a temperature allowing the selective etching of the smallest fraction of the diamond crystallites in the film. As a result of this procedure, perfectly shaped micrometer sized single crystal diamond pyramids were obtained. The pyramids have a rectangular base plane with an apex tip curvature radius of about 2 to 10 nm. The atomic structure of the pyramids was established using high resolution transmission electron microscopy. We propose a model explaining the mechanism of the pyramids' formation.     

Investigation of the effect of the total pressure and methane concentration on the growth rate and quality of diamond thin films grown by MPCVD

The influence of total gas pressure (50–125 Torr) and methane concentration (0.75%–10%) on diamond growth by microwave plasma chemical vapor deposition (MPCVD) was investigated. Within the regimes studied, the growth rate was proportional to the methane concentration in the source gas while it exhibited a super-linear dependence on total pressure. For a fixed methane concentration, characterization by Raman spectroscopy, scanning electron microscopy and X-ray diffraction indicated there was a minimum pressure required for the growth of large grain diamond, and conversely, for a fixed pressure, there was a maximum methane concentration that yielded diamond deposition. Higher pressures and higher carbon concentrations yielded diamond growth rates more than 10 times higher than achieved by the conventional low pressure MPCVD process.     

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.     

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.     

Influence of phosphine on the diamond growth mechanism: a molecular beam mass spectrometric investigation

We have used a molecular beam mass spectrometer to investigate the effects of addition of phosphine on the growth behaviour of diamond films in a hot filament chemical vapour deposition (CVD) reactor. Films were grown using gas mixtures of 1% CH₄ with increasing amounts of PH₃ (1000–5000 ppm). Gas phase species prevalent during the growth process (e.g. CH₄, CH₃, C₂H₂, PH₃ and HCP) have been monitored quantitatively and compared with the corresponding growth rates, quality and properties of the resulting films. We find that addition of up to 2000 ppm PH₃ increases the film growth rate by a factor of 2–3, and changes the crystal morphology in favour of (100). At higher PH₃ concentrations (3000–5000 ppm) the growth rate decreases again, with predominantly (111) faceted crystals. These observations are discussed in terms of a model of the gas phase chemistry during the growth process.     

Impurities identification in a synthetic diamond by transmission electron microscopy

Three types of impurities, which were trapped in diamond single crystal during process of the diamond crystal growth under high temperature and high pressure in the presence of iron–nickel solvent catalyst, have been successfully and directly investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Both the chemical composition and the crystal structure of the impurities were identified and determined. The impurities may be derived from the starting materials and the medium (pyrophillite) for transmitting the pressure, they consisted of amorphous graphite, f.c.c. (FeNi)₂₃C₆ and orthorhombic FeSi₂.     

Analysis of trapping–detrapping defects in high quality single crystal diamond films grown by Chemical Vapor Deposition

The photoresponse of high quality single crystal diamond films homoepitaxially grown by Chemical Vapor Deposition (CVD) onto low cost diamond substrates has been studied. The time evolution electrical response to the excitation by 5 ns laser pulses at 215 nm closely reproduces the laser pulse shape. The single crystal diamond response is therefore much faster than the laser pulse duration. The output signal is also very stable and reproducible, without significant priming or memory effects. Single crystal diamond films can therefore be grown by CVD having enough high quality to be used as photodetectors.However, a minor slow component shows up in the charge-integrated sample response. A systematic speed up of this slow component when increasing the detector temperature from − 25 °C to + 50 °C demonstrates its thermally activated origin. The slow component is therefore attributed to detrapping effects from shallow trapping centres.A model of the charge transport mechanism in the presence of trapping–detrapping centres can be developed and the results can be compared to the experimental ones. The activation energy of the shallow defects is accordingly determined as E_a = 0.4 eV.     

High-quality homoepitaxial diamond (100) films grown under high-rate growth condition

We present advantages of high-power microwave plasma chemical vapor deposition (MPCVD) in homoepitaxial diamond film deposition. Diamond films grown at comparatively high growth rate of 3.5 μm/h showed intense free-exciton recombination emission at room temperature. The free-exciton decay time of the diamond film at room temperature, 22 ns, was much longer than that of type-IIa single crystal, indicating electronically high quality of the homoepitaxial films. Dislocation-related emissions were locally observed, a part of which created by mechanical polishing process was successfully removed by surface etching process using oxygen plasma. Another advantage of the high-power MPCVD is effective impurity doping; boron-doped diamond films with high carrier mobility and high carrier concentration were reproducibly deposited. An ultraviolet photodetector fabricated using the high-quality undoped diamond film showed lower noise equivalent power as well as higher photoresponsivity for ultraviolet light with better visible-blind property, compared to those of standard Si-based photodetectors. The high-power MPCVD is, thus, indispensable technique for depositing high quality diamond films for electronic devices.     

Conversion of carbon nanotubes to diamond by spark plasma sintering

The diamond phase has been converted directly from carbon nanotubes by spark plasma sintering (SPS), at 1500 °C under 80 MPa pressure, without any catalyst being involved. Well-crystallized diamond crystals, with particle sizes ranging from 300 nm to 10 μm were obtained. After sintering at 1200 °C, the tips of the carbon nanotubes were found to be open and the conversion from carbon nanotubes to diamond started. The mechanism for carbon nanotube to diamond conversion in SPS may be described as that from carbon nanotubes to an intermediate phase of carbon nano-onion, and then to diamond. It is believed that the plasmas generated by the low-voltage, vacuum spark, via a pulsed DC in the SPS process, played a critical role in the low pressure diamond formation. This SPS process provides an alternative approach to diamond synthesis.     

Synthesis of large single crystal diamond using combustion-flame method

A combustion flame method is used to synthesize large single crystal diamond in ambient atmosphere. The basic of this technique was originally described by Hirose and Kondo in 1988 [Hirose H, Komaki K. Eur Pat Appl 1988:EP324538]. The advantage of this method is the high growth rate of diamond films, which is about 60 μm/h [Alers P, Hanni W, Hintermann HE. A comparative study of laminar and turbulent oxygen-acetylene flames for diamond deposition. Diam Relat Mat 1992;2:393-6]. The diamond can grow on itself to achieve large single-crystal. Negative substrate-bias effects on diamond growth have been investigated. Diamonds films were characterized by scanning electron microscopy, Raman spectroscopy, and atomic force microscopy in tapping mode. For given conditions, diamond coatings with highly oriented {1 0 0} crystal facets were produced. Large singles crystals diamonds were obtained. The sizes of these crystals vary between 80 and 90 μm. These results are discussed with respect to the competing events occurring during the heteroepitaxial growth of diamond.     

The effect of nitrogen addition during high-rate homoepitaxial growth of diamond by microwave plasma CVD

The effect of nitrogen addition on growth rate, morphology and crystallinity during high-rate microwave plasma chemical vapor deposition (MPCVD) of diamond was investigated. Epitaxial diamond was grown on type Ib diamond (100) substrates using a 5-kW, 2.45-GHz microwave plasma CVD system with nitrogen addition in the methane and hydrogen source gases. In order to obtain high growth rates, we designed the substrate holders to generate high-density plasma. The growth rates ranged from 30 to 120 μm/h. The nitrogen addition enhanced the growth rate by a factor of 2 and was beneficial to create a macroscopic smooth (100) face avoiding the growth of hillocks. However, the (100) surfaces looked microscopically rough by bunched steps as the effect of nitrogen addition. The macroscopic smoothing during the growth enabled the long-term stable deposition required to obtain large crystals. The deposited diamond was characterized by optical microscope, Raman spectroscopy, cathodoluminescence spectroscopy and X-ray diffraction.     

Effects of the additive boron on diamond crystals synthesized in the system of Fe-based alloy and carbon at HPHT

In this paper, the boron-doped diamond crystals were synthesized at high pressure and high temperature (HPHT) by adding amorphous boron to the system of carbon and Fe-based solvent/catalyst. The effects of the additive boron on the properties of the synthesized diamond crystals, especially, on the growth process and the morphology of diamonds, have been extensively studied. We found that the integrity of diamond crystals synthesized at optimal growth condition and the growth region of particular morphology changed with increasing of the content of additive boron. It is found that the growth region of {111} face becomes wider, and that of {100} face becomes narrower and almost disappears with the increasing boron concentrations. The surface morphology of boron-doped diamonds were detected with scanning electron microscope (SEM), and we found a great deal of defects on {111} face when the content of additive boron increased to 0.25 wt.%. The results of X-ray photoelectron spectroscopy (XPS) on the surface of diamond show that boron lies on the surface of diamonds and bonds with C and O, respectively. We proposed a model of bald-point to explain these experimental observations.     

Diamond syntheses in multicomponent carbonate–carbon melts of natural chemistry: elementary processes and properties

It was found in experiments at pressures of 5.5–8.5 GPa that diamond effectively crystallizes in melt-solutions of the multicomponent carbonate–carbon system. By chemical mineralogy evidence, components of the system are among the compositions of the natural media for diamond formation. Carbon solutions oversaturated to diamond form in carbonate–carbon melts for two reasons: (1) the difference of solubilities of diamond and starting graphite (a thermodynamically instable phase at the diamond stability conditions) and (2) temperature gradient over the melts. The PT-boundary conditions for both spontaneous diamond crystallization and seeded diamond growth from, correspondingly, labile and metastable multicomponent carbonate–carbon melt-solutions oversaturated to diamond are experimentally determined. The processes of spontaneous re-crystallization of starting polycrystalline graphite into graphite single crystals (blocks, plates, spherules) in the metastable carbonate–carbon melt-solutions oversaturated to diamond is a phenomenal peculiarity of the diamond-forming systems. For the processes of diamond growth in the carbon solution-melts of natural chemistry, diffusion mass transfer mechanism reinforced by the convection one for the low-viscous carbon solution-melts is of main importance. Kinetics properties of carbon mass transfer as diamond crystallizes in the systems of natural chemistry depend on chemical composition of a solvent and carbon solubility therein, pressure and temperature of diamond growth processes. Growth rate is strongly variable within the range of several millimeters per minute to several micrometers per minute. In case of seeded diamond growth, this locates structurally the crystallization fronts and imposes morphological peculiarities of the layers overgrown: from polycentric and roughly blocked to smooth surfaced with nano-dimensional growth steps. For spontaneous crystallization, the estimated density of nucleation is not less than 3–5×10² (for individual octahedral crystals to 200 μm-sized, spinel-law and polysynthetic twins, aggregates) to 1×10⁵ nuclea of diamond phase in 1 mm³ (dense polycrystalline diamond aggregates of the natural diamondite type with microcrystals measuring from 10–20 nm to 50–100 μm).     

Low energy sputter yields for diamond, carbon–carbon composite, and molybdenum subject to xenon ion bombardment

Sputter yields have been measured for polycrystalline diamond, single crystal diamond, a carbon–carbon composite, and molybdenum subject to xenon ion bombardment. The tests were performed using a 3-cm Kaufman ion source to produce incident ions with energy in the range of 150–750 eV and a profilometry-based technique to measure the amount of sputtered material. The yields increased monotonically with energy with values ranging from 0.16 at 150 eV to 0.80 at 750 eV for the molybdenum and 0.06 to 0.14 for the carbon–carbon. At 150 eV the yield for both diamond samples was 0.07 and at 750 eV 0.19 and 0.17 for the CVD and single crystal diamond, respectively. A number of experimental factors affecting sputter yield measurements using this technique are described.     

Fullerenes as a co-catalyst for high pressure – high temperature synthesis of diamonds

The paper reports on the effect of a fullerene co-catalyst on high pressure – high temperature (HPHT) synthesis of diamond from graphite. It is shown that the co-catalyst increases the diamond yield by a factor of 1.30–1.45 at relatively low pressure (4.5 GPa) and temperature (about 1200 °C).     

Synthesis of diamond with high nitrogen concentration from powder catalyst-C-additive NaN3 by HPHT

Diamond crystals with high nitrogen concentration, 1000-2000 ppm, have been successfully synthesized from the system of powder catalyst (Fe-Ni)-C-additive NaN₃ in a cubic anvil high-pressure and high-temperature apparatus. The synthetic diamond crystals are cubo-octahedral or octahedral shape with a green or dark green color. The FTIR spectra of the diamond synthesized indicate that its nitrogen concentration increases with an increase of the NaN₃ additive. Furthermore, such additive increase also leads to an increase in the minimum temperature and pressure for graphite/diamond conversion. We found iron nitride in the sample synthesized with high content of NaN₃. We believe its presence is an indication that Fe content in the Fe-Ni alloy is reduced and the characteristics of catalyst are changed, leading to the increase of the minimum temperature and pressure for graphite/diamond conversion.