Analyses of single crystal diamond substrates grown in a pocket substrate holder via MPACVD

High quality single crystal diamond (SCD) substrates are required for several different important current applications. Microwave Plasma Assisted Chemical Vapor Deposition (MPACVD) is a convenient deposition method for high quality substrates. It is hence imperative to synthesize and analyze substrates grown via different CVD techniques. This paper describes the quality of single crystal diamond substrates which have been grown via one such deposition strategy, which is in a “pocket substrate holder” design. The growth process in such a holder helps in depositing substrates which have almost no polycrystalline diamond (PCD) rim growth. The exact pocket holder growth process at high pressures (240 Torr) and high microwave power densities (~ 500 W/cm³) has been discussed in a previous publication [1]. The SCD CVD substrates were analyzed with different characterization techniques. By synthesizing diamond substrates in a pocket holder, the lack of any/almost any PCD rim helped in reducing the amount of stress in the crystals. To study the electronic quality of the substrates, etching experiments were conducted to determine the etch pit density. Nomarski images confirm that the number of etch pits at the edges is higher than at the center of the etched surface thereby implying the feasibility of this simpler method of reducing the etch pit density. The pocket holder process thus not only reduces the PCD rim but also reduces the substrate etch pit density and hence shows good promise of delivering high quality substrates.     

Substrate crystal recovery for homoepitaxial diamond synthesis

Homoepitaxial chemical vapor deposition (CVD) of diamond requires high quality substrate crystals. This paper describes the process of diamond substrate crystal recovery so that the original substrate can be reused for multiple synthesis processes. A three-stage treatment is applied after homoepitaxial CVD growth. First the original substrate is separated by laser cutting, then the cut surface is mechanically polished, and finally polycrystalline material at the edges of the recovered seed plate is laser trimmed. This recovery process yields reusable diamond substrates that do not differ appreciably from their original state in terms of stresses and impurity concentrations. While the recovery process was demonstrated using HPHT seed substrates the process can also be applied to the as-grown CVD diamond plates. Infrared absorption spectral analysis, surface profilometry, birefringence imaging and Raman spectroscopy are performed after each processing step to monitor crystal quality. The nitrogen concentration in the substrate crystal remains constant throughout CVD and recovery processes. When using HPHT type Ib substrates the detected nitrogen concentration is 110–180 ppm. The nitrogen is mainly incorporated in form of C center defects and no transformation to other forms of defect centers occurs during the CVD process. Birefringence imaging showed a low level of internal stress within the HPHT crystals. No change is observed during CVD growth and recovery processes. It is shown that the polycrystalline rim removal is essential for repeatable CVD deposition on the same seed substrate. Substrate crystal recovery allows growth of up to 20 crystals from one original seed.     

Reduction of dislocation densities in single crystal CVD diamond by using self-assembled metallic masks

The development of diamond-based electronic devices designed to operate at high power is strongly hampered by the lack of low dislocation single crystal material. Dislocations in Chemically Vapor Deposited (CVD) diamond are indeed generally responsible for leakage current, seriously deteriorating the performance of the devices. They can be due to defects such as polishing damage or contamination found at the substrate's surface, or they can directly originate from existing bulk defects that extend into the homoepitaxial layer. Although significant improvements have been achieved by using adapted surface treatments, dislocations found in CVD diamond grown on standard quality single crystal substrates are still typically in the range 10⁵–10⁶ cm^− 2. In this work, we report on a new growth strategy aiming at preventing threading dislocations from propagating into CVD diamond layers. It is based on the selective masking of existing defects revealed at the surface of the substrates by Pt nanoparticles. The interaction of dislocations with such embedded particles has been assessed and critical remarks are given as to the use of this technique in order to reduce dislocation densities in synthetic diamond.     

Improvements of crystallinity of single crystal diamond plates produced by lift-off process using ion implantation

A single crystal diamond substrate cut from a 9 mm thick ingot which was grown by chemical vapor deposition (CVD) was used to produce freestanding single crystal CVD diamond plates with improved crystallinity by the lift-off process using ion implantation. To reduce dislocations on the substrate surface, the ingot was sliced along the {100} plane parallel to the growth direction. In addition, the repeated lift-off processes reduced the surface damage on the substrate. These treatments were shown to improve the crystallinity of the CVD diamond plates produced by polarized light microscopy (PLM) and high-resolution X-ray.     

Control of surface and bulk crystalline quality in single crystal diamond grown by chemical vapour deposition

In order to improve the performance of existing technologies based on single crystal diamond grown by chemical vapour deposition (CVD), and to open up new technologies in fields such as quantum computing or solid state and semiconductor disc lasers, control over surface and bulk crystalline quality is of great importance. Inductively coupled plasma (ICP) etching using an Ar/Cl gas mixture is demonstrated to remove sub-surface damage of mechanically processed surfaces, whilst maintaining macroscopic planarity and low roughness on a microscopic scale. Dislocations in high quality single crystal CVD diamond are shown to be reduced by using substrates with a combination of low surface damage and low densities of extended defects. Substrates engineered such that only a minority of defects intersect the epitaxial surface are also shown to lead to a reduction in dislocation density. Anisotropy in the birefringence of single crystal CVD diamond due to the preferential direction of dislocation propagation is reported. Ultra low birefringence plates (< 10^? 5) are now available for intra-cavity heat spreaders in solid state disc lasers, and the application is no longer limited by depolarisation losses. Birefringence of less than 5 × 10^? 7 along a direction perpendicular to the CVD growth direction has been demonstrated in exceptionally high quality samples.     

Effect of crystal structure on the electrochemical behaviour of synthetic semiconductor diamond: Comparison of growth and a nucleation surfaces of a coarse-grained polycrystalline film

Comparative studies of the electrochemical behaviour of the growth and nucleation surfaces of a free-standing boron-doped polycrystalline diamond film grown in a microwave plasma CVD reactor are performed. The uncompensated acceptor concentration in diamond is determined from the electrochemical impedance (Mott–Schottky plots), uncompensated boron acceptor concentration from infrared absorption measurements, and the total boron concentration, by the SIMS method. In the diamond bulk adjacent to the nucleation surface, constituted from submicrometre-sized crystallites, both the boron concentration and the total acceptor concentration are found to be significantly higher than near the growth surface, where the film crystallinity is more perfect. This difference is tentatively attributed to the increased concentration of crystal lattice defects near the nucleation surface. These defects, in addition to boron atoms, play the role of acceptors in diamond.     

X-ray topography studies of dislocations in single crystal CVD diamond

X-ray topography has been used to study single crystal diamond samples homoepitaxially grown by microwave plasma-assisted chemical vapour deposition (CVD) on high pressure high temperature (HPHT) and CVD synthetic diamond substrates. Clusters of dislocations in the CVD diamond layers emanated from points at or near the interface with the substrate. The Burgers vectors of observed dislocations have been determined from sets of {111} projection topographs. Dislocations have line directions close to the [001] growth direction and are either edge or 45° mixed dislocations. Where groups of dislocations originated at isolated points they tended to be of the edge variety. Where the substrate surface was deliberately damaged before growth, two sets of dislocations were observed to have propagated from each line of damage and there was a tendency for dislocations to be of the 45° mixed variety with a component of their Burgers vector parallel to the polishing direction. It is demonstrated that X-ray topography can be used to deduce the growth history of CVD synthetic diamond samples produced in multiple growth stages.     

Observations of nanotube and ‘celery’ structures following diamond CVD on single crystal diamond substrates

Diamond films have been grown onto polished single crystal high-pressure high-temperature diamond substrates using microwave plasma chemical vapour deposition. After deposition, unusual whiskers were observed on the surface of the film originating from growth step edges. Some of these whiskers curled up lengthwise to resemble ‘celery’-like structures of length ∼20 μm and diameter <1 μm. Electron microscope analyses of these structures revealed the whiskers to be largely amorphous, although some smaller whiskers contained crystalline material embedded within them. A suggestion for their mechanism of formation involving metal-catalysed growth at step edges is presented.     

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.     

Diamond deposition on Ni3Ge single- and polycrystalline substrates

In order to find new materials for heteroepitaxial diamond growth Ni₃Ge single- and polycrystalline wafers were produced and used as substrates for diamond deposition in a microwave plasma system.The cubic phase Ni₃Ge substrate revealed to be an interesting and potential material for heteroepitaxial diamond chemical vapour deposition due to its: (1) lattice parameter matching within <1% the lattice parameter of diamond; and (2) coexistence with carbon up to its (congruent) melting point. Thus centimetre-size crystal boules were pulled from the melt using the Czochralski crystal growth method. These boules were sectioned into wafers and polished.Low-pressure diamond was grown on the Ni₃Ge wafers under various deposition conditions. The orientation of isolated diamond single crystals grown on the Ni₃Ge substrate surface show that heteroepitaxial nucleation occurred. Diamond nucleation was low, as seeding methods to enhance nucleation were not used.     

Initial thickness measurements and insights into crystal growth of methane hydrate film

The initial thickness of methane hydrate film was directly measured by suspending a single methane bubble in water at 274.0, 276.0, and 278.0 K. The results show that the initial hydrate film thickness decreases from tens of micrometers to about 10 µm with the subcooling increased from 0.5 K to about 3 K. When subcooling is higher than 1.0 K, all initial film thickness data measured under different temperatures vary inversely with the subcooling. Notable three‐dimensional growths of hydrate crystals of different sizes and shapes at film front and emergence of new crystal were clearly observed at lower subcooling that resulting in the rougher surface of hydrate film and uncertainty of initial thickness measurement under lower subcooling. The hydrate film growth was dominated by film growth in thickness, not by lateral growth at low subcooling. The growth in thickness of hydrate shell covering one whole bubble surface was also investigated. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2145–2154, 2013     

Higher coverage of carboxylic acid groups on oxidized single crystal diamond (001)

The effects of crystal orientation to the surface chemistry of single crystal diamond (001) and (111) were investigated after wet chemical oxidation. Direct carboxylation has been successfully achieved via wet chemical oxidation on native diamond (001) and (111) surface with distinguished portions of carboxylic acid groups (-COOH). High resolution X-ray photoelectron spectroscopy (XPS) analysis revealed that various kinds of chemical groups including both single and double oxygen-related components were covalently functionalized onto the single crystal diamond. The percentages of -COOH are approximately 9.2% and 4.7% on (001) and (111) surface respectively, showing evidently that the density of -COOH groups on (001) surface is surprisingly higher than that of (111) surface. Comprehensive comparison revealed that oxygen-related groups is higher on (001) compared with that of (111) surface. The conversion mechanism was supposed to explain the evolution from hydrogenated to oxygenated functionalizations on diamond with differently oriented crystal facets, and the crystal orientation was the significant factor in controlling the surface reactivity and hence the oxidization process.     

Polymer semicrystalline texture made by interplay of crystal growth

Chain-folded lamellar crystal growth of polymers typically makes a semicrystalline texture, which exhibits a layer-by-layer, alternating assembly of crystalline and amorphous phases. We performed dynamic Monte Carlo simulations of lamellar crystal growth in a row structure. We found that parallel growth of lamellar crystals appears staggered at high temperatures, and those loops and cilia on the fold-end crystal surfaces interfere with the in-between lagging growth of the third crystal. This mechanism produces an intrinsic amorphous layer between two crystals, with its thickness comparable with coil sizes of single polymers. The results may facilitate our better understanding about the low crystallinity of high-molecular-weight polyethylenes, and the scaling law of their amorphous thickness on chain lengths.     

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.     

High mobility single crystal diamond detectors for dosimetry: Application to radiotherapy

Diamond exhibits properties of interest for applications in the medical field. It is a very attractive material for detector fabrication due to its intrinsic properties and particularly its soft-tissue equivalence (Z = 6 compared to Z = 7.42 for human tissue), mechanical robustness and radiation hardness. Detectors fabricated from natural diamonds are used in several hospitals as dosimetric tools for the dose measurement received by the patient during radiotherapy and for beam calibration. Natural diamond based devices are expensive and long delivery times are common. The use of synthetic single crystal diamond is a promising issue for point dosimeter. Here we report on the growth of synthetic diamond using the CVD technique to fabricate free standing single crystals. Samples were characterized from their optical and electronic properties (Raman, TOF) and mounted as solid ionisation chambers with blocking contacts, for the evaluation of their dosimetric properties. Clinical tests were conducted in a medical facility at the Institute Gustave Roussy (IGR) in France specialised in the medical treatment of tumours. The results obtained demonstrate that our single crystal diamond detectors comply with the required specifications for radiotherapy applications.     

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.     

The effect of substrate temperature and growth rate on the doping efficiency of single crystal boron doped diamond

The substrate growth temperature dependence of the plasma gas-phase to solid-phase doping efficiency in single crystal, boron doped diamond (BDD) deposition is investigated. Single crystal diamond (SCD) is grown by microwave plasma assisted chemical vapor deposition (MPACVD) on high pressure, high temperature (HPHT) type Ib substrates. Samples are grown at substrate temperatures of 850–950 °C for each of five doping concentration levels, to determine the effect of the growth temperature on the doping efficiency and defect morphology. The substrate temperature during growth is shown to have a significant effect on the grown sample defect morphology, and a temperature dependence of the doping efficiency is also shown. The effect of the growth rate on the doping efficiency is discussed, and the ratio of the boron concentration in the gas phase to the flux of carbon incorporated into the solid diamond phase is shown to be a more predictive measure of the resulting boron concentration than the gas phase boron to carbon ratio that is more commonly reported.     

Recent advances in high-growth rate single-crystal CVD diamond

There have been important advances in microwave plasma chemical vapor deposition (MPCVD) of large single-crystal CVD diamond at high growth rates and applications of this diamond. The types of gas chemistry and growth conditions, including microwave power, pressure, and substrate surface temperatures, have been varied to optimize diamond quality and growth rates. The diamond has been characterized by a variety of spectroscopic and diffraction techniques. We have grown single-crystal CVD diamond over ten carats and above 1 cm in thickness at growth rates of 50–100 μm/h. Colorless and near colorless single crystals up to two carats have been produced by further optimizing the process. The nominal Vickers fracture toughness of this high-growth rate diamond can be tuned to exceed 20 MPa m^1/2 in comparison to 5–10 MPa m^1/2 for conventional natural and CVD diamond. Post-growth high-pressure/high-temperature (HPHT) and low-pressure/high-temperature (LPHT) annealing have been carried out to alter the optical, mechanical, and electronic properties. Most recently, single-crystal CVD diamond has been successfully annealed by LPHT methods without graphitization up to 2200 °C and < 300 Torr for periods of time ranging from a fraction of minute to a few hours. Significant changes observed in UV, visible, infrared, and photoluminescence spectra are attributed to changes in various vacancy centers and extended defects.     

Numerical analysis for transient point defect behavior in Czochralski silicon crystal growth

To study the transient point defect distribution in Czochralski-grown silicon single crystals, a continuum model of point defect dynamics to predict the concentration of interstitial and vacancy is established by estimating expressions for the thermo-physical properties of point defects and the point defect distribution in silicon crystals. It is well known that the concentration of intrinsic point defects in growing silicon crystals is a function of the crystal pull rate (V) and the temperature gradient (G) at the solidification interface inside the crystal, and steady state predictions from point defect dynamics are well agreed with experiment. In this study, finite element simulations have been performed for the growth halt experiment with 150 mm silicon single crystals to study the transient behavior of intrinsic point defects. It has been demonstrated that predicted point defect distributions are in good agreement with experimental results.     

A mechanistic growth model for inorganic crystals: Growth mechanism

Inorganic crystals grown from solution find wide application. A mechanistic growth model based on the spiral growth mechanism that operates at low supersaturation on inorganic crystal surfaces is presented. The long‐range electrostatic interactions on inorganic crystal surfaces are captured by methods developed in our previous article (Dandekar and Doherty, AIChE J., in press). The interactions of kink site growth units with the solvent molecules partially determine the growth kinetics. Relevant experimental parameters are systematically accounted for in the expression for the kink incorporation rate along step edges on the crystal surfaces. The growth model accurately predicts the asymmetric growth spirals on the surface of calcite crystals. The effect of supersaturation and ionic activity ratio on the step velocities of the acute and obtuse spiral edges is also correctly captured. This model can be used to predict the shapes of solution grown inorganic crystals and to engineer the growth process to design inorganic solids with functionally desirable shapes. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3720–3731, 2014