Homoepitaxial single crystal diamond growth at different gas pressures and MPACVD reactor configurations

Homoepitaxial growth of single crystal diamond by microwave plasma chemical vapor deposition in a 2.45 GHz reactor was investigated at high microwave power density varied from 80 W/cm³ to 200 W/cm³. Two methods of achieving high microwave power densities were used (1) working at relatively high gas pressures without local increase of electric field and (2) using local increase of electric field by changing the reactor geometry (substrate holder configuration) at moderate gas pressures. The CVD diamond layers with thickness of 100–300µm were deposited in H₂–CH₄ gas mixture varying methane concentration, gas pressure and substrate temperature. The (100) HPHT single crystal diamond seeds 2.5 × 2.5 × 0.3 mm (type Ib) were used as substrates. The high microwave power density conditions allowed the achievement of the growth rate of high quality single crystal diamond up to 20 µm/h. Differences in single crystal diamond growth at the same microwave power density 200 W/cm³ for two process conditions—gas pressure 210 Torr (flat holder) and 145 Torr (trapezoid holder)—were studied. For understanding of growth process measurements of the gas temperature and the concentration of atomic hydrogen in plasma were made.     

The effect of CO2 on the high-rate homoepitaxial growth of CVD single crystal diamonds

In this paper, we report the effect of gaseous carbon dioxide (CO₂) introduced in the typical reaction atmosphere of CH₄/H₂/N₂ (60/500/1.8 in sccm) on the growth rate, morphology and optical properties of homoepitaxy single crystal diamonds (SCDs) by microwave plasma chemical vapor deposition. The additional carbonaceous sources supplied by CO₂ are favorable to increase the growth rate, and meanwhile, the oxygen related species generated would enhance the etching effect not only to eliminate the non-diamond phase of SCD but also to decrease the growth rate. The appropriate addition of CO₂ can increase the high growth rate, decrease the surface roughness, and reduce the concentration of N-incorporation. It is demonstrated that adding CO₂ strongly affects the contents of various reaction species in plasma, which would determine the growth features of CVD SCDs.     

Homoepitaxial single crystal diamond grown on natural diamond seeds (type IIa) with boron-implanted layer demonstrating the highest mobility of 1150 cm/V s at 300 K for ion-implanted diamond

The homoepitaxial single crystal diamond growth by microwave plasma assisted CVD at high microwave power density 200 W/cm³ in a 2.45 GHz MPACVD reactor using natural diamond seeds (type IIa) was investigated. The semiconductor CVD diamond of p-type was obtained by doping technique of ion implantation. Boron ions were implanted at the acceleration energy of 80 keV with two cases of dose: 5 · 10¹⁴ and 3 · 10¹⁵ cm^− 2. To recover the damage layer and activate dopants in CVD diamond the rapid annealing at nitrogen atmosphere at 1380° C was used. B-implanted diamond layer showing the mobility of 1150 cm²/V s at 300 K which is the highest for ion-implanted diamond was obtained.     

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.     

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.     

Preparation of low index single crystal diamond surfaces for surface science studies

In this review I summarise the procedures that have been developed to prepare well ordered, low index single crystalline diamond surfaces for surface science studies. Particular emphasis is placed on methods to smooth as polished surfaces with the aim to obtain well developed terraces with atomic order by different atomic hydrogen etching techniques.     

High-rate homoepitaxial growth of CVD single crystal diamond by dc arc plasma jet at blow-down (open cycle) mode

Growth and applications of large size high quality single crystal diamond is one of the most significant progresses in the field of CVD diamond film research ever made in the past 15 years. However, up to now most of the works were done by microwave plasma CVD operating at high pressures. In the present investigation it is demonstrated that relatively high quality single crystal diamond layer with the FWHM of the diamond Raman peak of less than 2 cm^− 1 and the FWHM of the diamond (400) reflection X-ray Rocking Curve of 0.028° can be obtained by the 20 kW dc arc plasma jet operating at blow down (open cycle) mode at a growth rate as high as 36 μm/h. The reason why dc arc plasma jet is also suitable for high quality epitaxial growth of single crystal diamond is that very high concentration of atomic hydrogen can be easily provided by the extremely high gas temperature due to the arc discharging. Detailed results on the effects of the ratio of H₂/Ar, the distance between the substrate to the anode nozzle of the arc plasma torch, the concentration of methane, and the substrate temperature on the growth of single crystal diamond are presented, and discussed comprehensively, and compared to that with the MWCVD, and with our previous work on the 30 kW dc arc plasma jet operating at arc root rotation and gas recycling mode.     

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.     

Effects of sintering aid and atmosphere powder on the growth of (K0.5Na0.5)NbO3 single crystals fabricated by solid-state crystal growth method

(K_0.5Na_0.5)NbO₃ (KNN) + x (= 1, 0.5, 0.05, and 0) wt%Co₃O₄ single crystals were fabricated by a solid-state crystal growth method with a KTaO₃ seed crystal and a KNN atmosphere powder, and the effects of the sintering aid content x and the addition of Co₃O₄ to the atmosphere powder on the growth of the single crystals were investigated. The formation of pores in the single crystals was suppressed by a decrease of x, which, however, decreased the crystal growth length. On the other hand, dense and large KNN single crystals could be fabricated by sintering with a KNN + 5 wt%Co₃O₄ atmosphere at x = 0. The dielectric, ferroelectric, and piezoelectric properties of the KNN single crystals were comparable to those of reported (K,Na)NbO₃ single crystals. These results would be useful for fabricating dense and large single crystals by the solid-state crystal growth method.     

Anisotropic growth kinetics and electric properties of PZT-5H single crystal by solid-state crystal growth method

The PZT-5H single crystal growth on [111]c-, [110]c-, and [001]c-oriented seed crystal by solid-state crystal growth (SSCG) method was investigated. The growth rate of PZT-5H single crystal strongly depends on seed crystal orientation and annealing time. The mean growth distance is 682, 620, and 93 μm for [111]c-, [110]c-, and [001]c-oriented PZT-5H single crystal grown at 1150°C for 8 h, respectively. The growth kinetics of SSCG-grown PZT-5H single crystal was discussed. It is found that the growth of single crystal is driven by the solubility difference between the matrix grains and single crystal growth front interface, arising from the local curvature and the crystallographic directions dependent solubility. The growth of [001]c-oriented PZT-5H single crystal was mainly contributed from the difference solubility arising from the local curvature of growth front interface, while the growth of [111]c- and [110]c-oriented PZT-5H single crystal was mainly contributed from the difference solubility between {111} and {110} plane of single crystal and matrix grains. The piezoelectric coefficient d₃₃ of up to 1028pC/N (about 50% larger than that of the same component ceramic) was obtained in a [110]c-oriented PZT-5H single crystal with a Curie temperature of about 230°C. The large field piezoelectric constants d₃₃* of up to 1160 pm/V (about 50% larger than that of the same component ceramic) at 15 kV/cm was also obtained in [110]c-oriented PZT-5H single crystal with a large strain of 0.18%. This work deepens our understanding on the growth kinetics of SSCG and pushes the way of growth of soft PZT single crystal by SSCG.     

Dislocations and impurities introduced from etch-pits at the epitaxial growth resumption of diamond

The fabrication of diamond-based electronic devices requires that several active layers with different doping concentrations are grown in different reactors. In this paper, we have investigated the effect of interrupting and resuming the epitaxial growth of a homoepitaxial diamond film using high-power plasma CVD. It was found that long lifetime blue phosphorescence which is localised on regions with a high dislocation density is generated. Such phosphorescence is related to a higher uptake of impurities at the interface between two subsequent films, due to an increased surface roughness from etching at the epitaxial growth resumption. Etching was found to occur preferentially on threading dislocations leaving typical etch-pits. Cathodoluminescence helps identify boron as the main background impurity. Besides, the formation of new dislocations was observed on the facets of these etch-pits. The continuation of epitaxy on a roughened surface thus comes with a substantial decrease in crystal quality.     

Effect of bias treatment in the CVD diamond growth on Ir(001)

The effect of bias treatment (BT) on direct-current plasma CVD diamond growth has been studied in situ by X-ray photoelectron diffraction (XPD) together with LEED and XPS. It was found that C 1s XPD patterns from the sample after BT are similar to those of diamond (001). Coverage of carbon after BT is several tens of ML when BT is very successful. However, LEED shows no diamond (001) spots for the sample after BT. These apparently contradictory findings are explained by the sizes of the diamond (001) crystallites, which, after BT, are large enough to produce C 1s XPD patterns of diamond, but too small to have coherent interference spots in LEED. It is concluded from this and other information that BT in a DC plasma creates hetero-epitaxial diamond crystallites a few nm or less. These diamond crystallites may be related to the atomically abrupt diamond/Ir interfaces of DC plasma CVD-grown samples revealed by TEM [A. Sawabe, H. Fukuda, T. Suzuki, Y. Ikuhara, T. Suzuki, Surf. Sci. 467 (2000) L845].     

Role of oxygen in surface kinetics of SiO2 growth on single crystal SiC at elevated temperatures

Understanding surface kinetics of SiO₂ growth on single crystal SiC at elevated temperatures is crucial to fabricate high-performance SiC-based devices. However, the role of oxygen in the evolution mechanism of SiC surface at atomic scale has not been comprehensively elaborated. Here, we reveal the manipulation effect of oxygen on the competitive growth of thermal oxidation SiO₂ (TO-SiO₂) and thermal chemical vapor deposition SiO₂ (TCVD-SiO₂) on the 4H-SiC substrate at 1500 °C. TO-SiO₂ is formed by the thermal oxidation of SiC, in which the substrate undergoes layer-by-layer oxidation, resulting in an atomically flat SiC/TO-SiO₂ interface. TCVD-SiO₂ growth includes the sublimation of Si atoms, the reaction between sublimated Si atoms and reactive oxygen, and the adsorption of gaseous Si_xO_y species. A relatively high sublimation rate of Si atoms at SiC atomic steps causes the transverse evolution of the nucleation sites, leading to the formation of nonuniform micron-sized pits at the SiC/TCVD-SiO₂ interface. The low oxygen concentration favors TCVD-SiO₂ growth, whose crystal quality is much better than that of TO-SiO₂ due to the high surface mobility in the thermal CVD process. We further achieve the epitaxial growth of graphene on 4H-SiC in an almost oxygen-free reaction atmosphere. Additionally, ReaxFF reactive molecular dynamic simulation results illustrate that the decrease in oxygen concentration can promote the growth kinetics of SiO₂ on single crystal SiC from being dominated by thermal oxidation to being dominated by thermal CVD.     

Simulations of crystal aggregation and growth: Towards correct crystal area

Simulating crystal growth and aggregation can provide insight into factors that control the final product properties. Classical models involve formation of a volume-equivalent single crystal upon aggregation. This approach does not preserve particle area, resulting in an inadequate model of supersaturation depletion. Alternatively, crystal area can be computed accurately by a Monte Carlo method where each primary particle of an aggregate is described in its full geometric complexity. However, the drawback of this method is its computational cost. Thus, a third method is introduced as a compromise, describing particles by their volume and area and preserving both upon aggregation. The so-obtained two-dimensional model requires growth rate expressions in volume and area. We provide a method for parametrizing these expressions so that total volume and area closely match the values obtained with the method based on full geometric information. The parameters depend on primary particle geometry and the amount of growth. © 2019 American Institute of Chemical Engineers AIChE J, 65: e16525, 2019     

Experimental study and simulation of the polymerization of methyl methacrylate at high temperature in a continuous reactor

The bulk polymerization of MMA at high temperature (120–180°C) in a continuous pilot‐plant reactor has been studied. The polymerization is initiated by diterbutyle peroxide and the chain transfer agent is 1‐butanethiol. A simulation program has been developed to predict the steady state behavior of the reactor. The particular features of the kinetic at above‐T_g temperature are included in the model, especially the thermal initiation of the reation and the attenuation of the autoacceleration effect. For the flow and mixing model, the actual vessel cannot be approximated to a single ideal reactor because of its design and of the moderate agitation imposed by the high viscosity of the reacting fluid. A tanks in series model with a recycle stream between tanks is proposed to evaluate the backmixing caused by the special design of the agitator. The parameters of the model are determined with the help of the experimental residence time distribution measured on the reactor. The data collected on the actual reactor, i.e., operation, conversion, molecular weight, temperature, are compared to the calculated one. The agreement is satisfactory but the tendencies are slightly underestimated. The program is a tool to evaluate the effect of modifications of the design of the reactor or changes on the operation parameters like input rate, temperature, and agitation on its behavior. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2038–2051, 2001     

Mechanistic study of ultrasound-assisted solvent leaching of sodium and potassium from an Indian coal using continuous and pulsed modes of operation

Unburnt coal in a boiler gets converted to fly ash and bottom ash. Fly ash particles travel along with flue gas and get deposited on heat transfer surfaces along the way. Fouling and slagging are phenomena associated with fly ash deposits which could potentially damage the heat transfer surface, if left unattended. Sodium and potassium are directly linked to the deposition of fly ash, as they form a molten salt film which aids in sticking of fly ash particles. Ultrasound is one of the mitigation techniques used to prevent ash deposition. This study aims at reducing the concentrations of alkali metals in coal as a pretreatment method, using ultrasound. Two modes of operation are chosen: continuous and pulsed. A solvent has been used to enhance the leaching process. In the pulsed mode, the sample is alternated between periods of exposure and nonexposure. A maximum leaching rate of 62% for potassium and 24.5% for sodium was observed in the continuous mode at 360?kHz of operation while pulsed mode of operation showed a maximum leaching rate of 91.3% and 54.4% for potassium and sodium operated at 360?kHz, respectively. The experimental data have been fit to the shrinking core model, and the rate-controlling step has been found to be surface diffusion of reactants into the core. The effect of various parameters is analyzed and compared for pulsed and continuous modes of operation. Order of the reaction, activation energy, and leaching kinetics are obtained by fitting the data into the shrinking core model.     

Growth and characterization of l-glutamic acid and sodium sulphate doped tri glycine sulphate single crystal

Single crystal of tri glycine sulphate has been grown in the presence and absence of l-glutamic acid and sodium sulphate by slow evaporation growth. The grown crystal was analyzed by single crystal X-ray diffraction. FTIR studies were used to confirm the presence of functional groups present in the grown crystals. The optical absorbance studies show the cut off wavelength to be 235 nm. The phase transition temperature was found to be 50 °C from DSC analysis. The dielectric behavior was identified. The AC conductance of the grown crystal was plotted. PE loop measurement is taken.     

Hydrodynamics and flow regime transition study of trickle bed reactor at elevated temperature and pressure

The hydrodynamics in a trickle bed reactor (TBR) in non-ambient conditions are studied for air-water and air-acetone (pure organic liquid of low surface tension) systems. A flow map experiments for air-water and air-acetone systems are performed in a pilot plant reactor of 0.05 m i.d. and 1.25 m height. It has been demonstrated from the experimental results that the pressure drop tends to increase with increasing superficial gas and liquid velocity and reactor pressure, while it tends to decrease with increasing bed temperature. The results also show that the dynamic liquid holdup increases with increasing liquid velocity and decreases with increasing superficial gas velocity, reactor pressure and bed temperature. The dynamic liquid holdup and pressure drop values are obviously higher than those measured for air-water system at the same fluid fluxes, reactor pressure and bed temperature due to the surface tension effects. For higher reactor pressure and temperature, the trickle to pulse transition boundary shifts towered higher superficial velocities of both gas and liquid.     

High performance lead-free piezoelectric 0.96(K0.48Na0.52)NbO3-0.03[Bi0.5(Na0.7K0.2Li0.1)0.5]ZrO3-0.01(Bi0.5Na0.5)TiO3 single crystals grown by solid state crystal growth and their phase relations

0.96(K_0.48Na_0.52)NbO₃-0.03[Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5]ZrO₃-0.01(Bi_0.5Na_0.5)TiO₃ single crystals were grown for the first time by the solid state crystal growth method, using [001] or [110]-oriented KTaO₃ seed crystals. The grown single crystal shows a dielectric constant of 2720 and polarization-electric field loops of a lossy normal ferroelectric, with P_r = 45 μC/cm² and E_c = 14.9 kV/cm, while the polycrystalline samples with a dielectric constant of 828 were too leaky for P-E measurement due to humidity effects. The single crystal has orthorhombic symmetry at room temperature. Dielectric permittivity peaks at 26 °C and 311 °C, respectively, are attributed to rhombohedral-orthorhombic and tetragonal–cubic phase transitions. Additionally, Raman scattering shows the presence of an orthorhombic-tetragonal phase transition at ∼150 °C, which is not indicated in the permittivity curves but by the loss tangent anomalies. A transition around 700 °C in the high temperature dc conductivity is suggested to be a ferroelastic-paraelastic transition.