Diamond Growth Process Research Articles

Significant enhancement of electrical conductivity in boron-doped diamond through HPHT post-annealing treatment

Boron-doped diamond (BDD) is expected as a promising semiconductor candidate for potential application in next-generation high-power and high-voltage electronics. Unfortunately, a feasible strategy that focuses on the improvement of the semiconducting performance of BDD materials is still lacking and remains a fundamental challenge in material science. Thus, we aim to study the stability and adjustment behaviors of boron dopants in BDD single crystals through post-annealing treatment under high pressure and high temperature (HPHT) conditions. In this work, our study demonstrates that boron dopants show high structural stability, and no extra complex defects were observed during the HPHT annealing process, a superior advantage for the device application in extreme environments. Our work further reveals that the HPHT post-annealing process is effective to eliminate the intrinsic stress and local lattice strain that were inherently formed during the diamond growth process. This process is accompanied by the enhancement of ionization ratio of boron dopants. The carrier content of one 〈111〉-GS BDD increases by more than ten times after post-annealing at 1000 °C and 5.5 GPa, resulting in significant improvement of conductivity by a factor of 2. This result provides an effective strategy to improve the semiconducting performance of BDD materials through HPHT annealing approach and promote their practical device applications.

Integration of Carbon Nanotubes in an HFCVD Diamond Synthesis Process in a Methane-Rich H2/CH4 Gas Mixture.

In this work, we present experimental data on carbon nanotubes integration during diamond synthesis. Carbon nanotubes layers were preliminarily deposited on silicon and diamond substrates, after which the substrates were loaded into the HFCVD reactor for further growth of the diamond phase. The CVD process was held in an argon-free H2/CH4 working gas mixture without the use of a catalyst for carbon nanotubes growth. It is shown that in a wide range of studied working gas composition (CH4 concentration up to 28.6 vol.%) nanotubes etched from the substrate surface before the diamond growth process began.

The Etching Mechanisms of Diamond, Graphite, and Amorphous Carbon by Hydrogen Plasma: A Reactive Molecular Dynamics Study

AbstractUnderstanding how hydrogen plasma etches various potential products during the diamond growth process can contribute to improving the knowledge of diamond growth. However, due to the absence of an in situ characterization technique during the etching process, the complex chemical reactions involved in the process obscure the atomic‐scale etching mechanisms. In this paper, the etching mechanisms of diamond (001), graphite (0001), and amorphous carbon substrates irradiated by hydrogen plasmas are investigated and compared using molecular dynamics simulations based on ReaxFF. When the incident energy of H atoms is 1 eV, the rate of carbon loss from graphite and amorphous carbon are far higher than that from diamond. As the incident energy of H atoms increases, the etching rate of diamond shows a slow increase, while the etching rates of amorphous carbon and graphite exhibit more significant increases. It can be concluded that the etching rate of diamond is significantly lower than that of graphite and amorphous carbon under H plasma. In the Chemical Vapor Deposition (CVD) process of diamond growth, the generated graphite and amorphous carbon are rapidly etched, leaving only diamond. This offers a plausible explanation for the growth mechanism of diamond through CVD.

Effect of bias-enhanced nucleation on the microstructure and thermal boundary resistance of GaN/SiNx/diamond multilayer composites

The low effective thermal boundary resistance (TBReff) at the GaN/diamond interface is very important for the development of high-power, high-frequency and high-temperature GaN-on-diamond devices. The nucleation and growth of diamond are key processes for modulating the TBReff. This work proposed the bias enhanced nucleation technique to adjust the nucleation of GaN/SiNx/diamond multilayer composites in the MPCVD process at different bias voltages (400–700 V), thereby adjusting TBReff. Pulse bias is beneficial to a stable plasma environment and to obtain a complete GaN/diamond interface structure. The transient thermoreflectance characterization indicated that the GaN/SiNx/diamond multilayer composite prepared under 700 V bias nucleation condition had the lowest TBReff (26 ± 10 m2K/GW), whereas the GaN/SiNx/diamond multilayer composite at 600 V bias had the highest TBReff (83 ± 18 m2K/GW). The role of bias enhanced nucleation process in the TBReff was systematically analyzed by electron microscopies (TEM and SEM) and Raman spectroscopy. The GaN/SiNx/diamond multilayer composite prepared at 600 V showed a thick mixed transition layer containing multiphase structures and rough interfaces due to efficient subsurface ion implantation, resulting in high TBReff. In contrast, at 700 V, a thinner nucleation zone and smoother interface result in the lowest TBReff. This work demonstrated the potential of adjusting TBReff at the GaN/diamond interface by using bias enhanced nucleation technique to modulate the diamond nucleation and growth processes.

Temperature- and Pressure-Reducing Regimes in the Growth Cell of HPHT Diamonds, Optimal for Preserving Crystal Integrity after Growth Completion

With its exceptional strength characteristics, diamond has some mechanical drawbacks, significant brittleness being among them. In particular, some HPHT-grown diamonds crack when the extreme parameters inherent to the diamond growth process gradually decrease. The cracking is caused by excessive stress due to the poor plastic properties of the diamond growth catalytic medium at certain stages of reducing the pressure and the temperature. An insulating container with the growth cell and heating circuit fragment inside can also make a significant contribution to the probability of cracking. This paper considers the possibility of minimizing the mechanical stress in the growth cell and, consequently, in the diamond crystal by choosing the optimal trajectory for the decrease in the pressure and temperature from diamond growth conditions to normal conditions.

Influence of polycrystalline diamond on silicon-based GaN material

Self-heating has become a limited factor for the performance improvement of GaN electronics. Growing polycrystalline diamond directly on GaN material to solve the heating problem of GaN devices has become one of the research highlights. Polycrystalline diamond on Si-based GaN material has the advantages of being close to the channel region and high heat dissipation efficiency. However, there is a problem that the thermal expansion mismatch between polycrystalline diamond and GaN material leads to the deterioration of electrical characteristics of GaN. In this work, we adopt microwave plasma chemical vapor deposition (MPCVD) method to grow polycrystalline diamond on 2-inch Si-based GaN material. The test results show that the polycrystalline diamond is uniform as a whole. The average thickness is in the range of 9–81 μm. With the thickness of polycrystalline diamond increasing, the XRD (002) diffraction peak FWHM increment and mobility loss gradually increase for the Si-based GaN material. Through laser cutting and acid etching, the Si-based GaN material is successfully stripped from the polycrystalline diamond. It is found that during the process of diamond growth at high temperature, hydrogen atoms etch the defect positions of the silicon nitride epitaxial layer, forming a hole area in the GaN, and the etching depth can reach the intrinsic GaN layer. During the process of cooling, a crack area is formed around the hole area. Raman characteristic peaks, full widths at half maximum of XRD (002) diffraction peaks, and electrical properties of the stripped Si-based GaN materials are all returned to their intrinsic states. The above results show that the thermal expansion mismatch between polycrystalline diamond and Si-based GaN introduces stress into GaN, which leads to lattice distortion of GaN lattice and the degradation of electrical property of GaN material. The degradation of GaN material is recoverable, but not destructive.

Interaction mechanism of Ge, Ti, and N in diamond prepared by high pressure and high temperature conditions

In this paper, Ge-doped large single crystal diamond was synthesized under high pressure and high temperature (HPHT) conditions in NiMnCo-C and NiMnCo-Ti-C systems. The effects of Ge additives on the diamond morphology, nitrogen impurity content, and internal inclusions were investigated, and the interaction mechanism among germanium (Ge), nitrogen (N), and titanium (Ti) during the diamond growth process was discussed. The results show that the number of inclusions in diamond increase with increases in Ge addition in both systems. The N content of diamond grown in NiMnCo-C systems decreases with increases in Ge addition. However, Ge can more easily be doped into the diamond in a NiMnCo-Ti-C growth system. The experimental results show that the N can inhibit the amount of Ge entering the diamond lattice under HPHT conditions. When Ti was introduced into the system, it effectively removed N by combining with it, such that more Ge atoms could enter the diamond lattice.

Source of carbon as a key factor in diamond deposition

Diamond growth from ethylene/acetylene + hydrogen gas mixture has been simulated. Deposition of carbon-containing species is analyzed under different conditions. It is shown that variation of the carbon-hydride composition of the initial mixture in the experiments can clarify the role of ethane and acetylene contribution to the diamond growth process.

Origin of micrometer-sized impact diamonds in ureilites by catalytic growth involving Fe-Ni-silicide: The example of Kenna meteorite

The occurrence of shock-induced diamonds in ureilite meteorites is common and is used to constrain the history of the ureilite parent bodies. We have investigated a fragment of the Kenna ureilite by micro-X-ray diffraction, micro-Raman spectroscopy and scanning electron microscopy to characterize its carbon phases. In addition to olivine and pigeonite, within the carbon-bearing areas, we identified microdiamonds (up to about 10 μm in size), nanographite and magnetite. The shock features observed in the silicate minerals and the presence of microdiamonds and nanographite indicate that Kenna underwent a shock event with a peak pressure of at least 15 GPa. Temperatures estimated using a graphite geothermometer are close to 1180 °C. Thus, Kenna is a medium-shocked ureilite, yet it contains microdiamonds, which are typically found in highly shocked carbon-bearing meteorites, instead of the more common nanodiamonds. This can be explained by a relatively long shock event duration (in the order of 4–5 s) and/or by the catalytic effect of Fe-Ni alloys known to favour the crystallization of diamonds. For the first time in a ureilite, carletonmooreite with formula Ni3Si and grain size near 4–7 nm, was found. The presence of nanocrystalline carletonmooreite provides further evidence to support the hypothesis of the catalytic involvement of Fe-Ni bearing phases into the growth process of diamond from graphite during shock events in the ureilite parent body, enabling the formation of micrometer-sized diamond crystals.

Fabrication of low stress GaN-on-diamond structure via dual-sided diamond film deposition

The fabrication of GaN-on-diamond structure by the process of diamond growth on GaN technology is becoming more and more attractive for high-power GaN devices. However, due to the high-temperature steps employed during the GaN epitaxial layer transfer process, significant stress accumulates in the GaN-on-diamond structure and induces the formation of cracks or deformations in the GaN layer. To improve the stress characteristics of the GaN layer, in this work, dual-sided diamond film deposition was carried out to accomplish the GaN-on-diamond structure. The surface morphology, interface microstructure and crystal quality of diamond on GaN were analyzed by using scanning electron microscopy, atomic force microscopy, transmission electron microscopy and X-ray diffraction. The results showed that the stress accumulation originating from the GaN epitaxial layer transfer process was effectively alleviated as compared with other reporteds in the literature and that the crystal quality of the GaN was only slightly altered. The stress relaxation was attributed to the low-quality diamond used as the temporary carrier during the GaN transfer process. In addition, a thin SiC transition layer in the interface is also beneficial in alleviating the mismatch between the coefficients of thermal expansion for the GaN and the diamond.

Sulfur regulation of boron doping and growth behavior for high-quality diamond in microwave plasma chemical vapor deposition

In this work, sulfur addition has been employed on the boron-doped diamond growth process, and a significant regulation of the boron doping and the growth behavior has been realized by microwave plasma chemical vapor deposition. It is interesting to find that the sulfur incorporation will lead to an accordant evolution on the boron doping efficiency, hole mobility and concentration, crystal quality, surface morphology, and growth rate. In the presence of sulfur with appropriate dosage, for a boron-to-carbon ratio of only 2.5 ppm in gas phase during growth, a very high doping concentration of 1.2 × 1019 at/cm3 has been achieved, indicative of a very efficient boron doping. Besides, the hole mobility of the sample is 853 cm2/V s at 300 K, which is better than the state of the art for p-type doping in diamond. The regulation mechanism of the sulfur addition will be discussed from the point of view of sulfur-induced plasma change and possible B–S complex formation. This study may provide an effective way for high-quality p-type conductive diamond layer growth and further for the potential diamond-based opto-electronic device applications.

Research of n-type arsenic doped diamond: Theoretical analysis of electronic and mechanical properties

Substitutional defects of arsenic in diamond have been investigated in terms of the ionization energy, electronic property, and elastic constants for one and two arsenic doped configurations by density functional theory. Arsenic (As) particle doped diamond has been found to present attractive n-type doping properties. The ionization energy has been calculated based on different exchange-correlation functionals and competitive ionization energies are exhibited for one As atom doped and two As atoms doped (separated by one carbon atom) models. Besides, doping activities make the band gaps decrease for all configurations with the band structure of single As atom and separated site doped configurations remaining indirect, while the adjacent site of configuration with two As atoms has a direct band structure. Moreover, As doping makes the Young's module of diamond decrease and the elastic constants of adjacent site configuration present metastable state. Finally, one arsenic particle adsorption on diamond (001) surface has also been studied. It is identified that Bridge site is the most suitable adsorption site for arsenic doping during diamond growth process. As a conclusion, arsenic impurity is believed to be a competitive candidate of n-type doped diamond.

Optical investigation of as-grown NV centers in heavily nitrogen doped delta layers in CVD diamond

In this paper, we realize growth of the delta doped layer (3 nm thick) with high concentration of nitrogen atoms (about 1.4 × 1019 cm−3) during CVD diamond growth process. We experimentally investigate the distribution of the nitrogen inside the grown layer and formation of the NV centers during such growth. Using confocal microscopy, we analyze the spatial distribution of the NV centers and investigate the formation efficiency of the NV centers in delta doped layers. The spatial distribution is measured by two methods, using second-order correlation function and emission volume analysis, and NV center concentration is found to be 3.9 ± 0.6 and 2.7 ± 0.2 μm-2 consequently. The divergence between the methods is discussed. As-grown NV centers formation efficiency was found to be 30 times lower than in the case of uniform doped diamond growth. Nevertheless, coherence time of an electron spin for a single NV center inside the delta layer was found to be around 1 μs which is quite reasonable given the concentration of the nitrogen in the sample.

Comparison of continual and molecular modeling of gas flow for diamond deposition

For the purpose of simulation of diamond-growth processes, the comparison of reacting gas flows modelled in a heated cylindrical channel has been performed by two different approaches: DSMC method and solution of Navier-Stokes equations. The impact of the channel length on the degree of hydrogen and methane dissociation in the gas mixture has been analyzed. The obtained data were compared with the experimental data.

Thinning strategy of substrates for diamond growth with reduced PCD rim: Design and experiments

The appearance of polycrystalline diamond (PCD) at the edges of deposited layers is a major obstacle in MPCVD diamond growth process. In this work, diamond growth states with various substrate thicknesses were simulated to obtain the distribution of electric field intensity, electron density and temperature on the substrate surface. The simulation results indicate that the distribution of surface plasma can be controlled by the design of substrate thickness, thus affecting the growth of diamond. Three samples with different substrate thicknesses were grown and the variation of crystal quality from the edges to the centers was analyzed by Raman spectroscopy. The growth results showed that as the thickness of the substrate decreased, the formation of PCD rim was inhibited. One reason for this phenomenon is that the plasma density at the edges and corners decreases when a thinner substrate is used, which suppresses the probability of two-dimensional nucleation and defect formation.

Development of the Effective Reaction Model for Diamond Deposition Simulation within the Continuous Media Approach

Three models for diamond growth process by the chemical vapor deposition of methane are proposed. They differ in the degree of detail of the surface reaction description. The most complete model contains the reactions of deposition, etching and insertion. Gas-dynamic simulations have been performed for all those models. The species delivery to the substrate and the contribution from different species to the growth process is analysed. It is shown that different surface reaction models lead to different profiles of the species concentrations in the immediate vicinity of the substrate, thus, the experimental data on the growth rate may give information on the growth mechanism.

Quantum behavior of hydrogen-vacancy complexes in diamond

Hydrogen plays an essential role in the growth process of artificial diamond and can easily form complexes with lattice vacancies. Despite substantial efforts to resolve the electronic structure and the ground-state properties of the hydrogen-vacancy (HV) center, the final remarks are ambiguous, while the complexes of vacancy with two and more hydrogen atoms remain unexplored. In this paper, we used spin-polarized, hybrid density-functional theory method to investigate electronic structure and magneto-optical properties of various hydrogen-vacancy clusters in diamond. Our theoretical results indicate a very strong tendency toward the formation of ${\mathrm{H}}_{n}\mathrm{V}$ complexes up to four hydrogen atoms that are mostly electrically and optically active centers. One of the investigated defects introduce highly correlated electronic states that pose a challenge for density-functional theory and, therefore, require special treatment when charge- and spin-density-related properties are determined. We introduced an extended Hubbard model Hamiltonian with fully ab initio provided parameters to analyze the complex electronic structure of highly correlated ${\mathrm{H}}_{2}{\mathrm{V}}^{0}$ defects. The role of quantum tunneling of hydrogen in HV center and its impact on the hyperfine structure was discussed. We demonstrate that experimentally observed ${\mathrm{HV}}^{1\ensuremath{-}}$ center is similar to well-known ${\mathrm{NV}}^{1\ensuremath{-}}$, i.e., I) it possesses triplet $^{3}\mathrm{A}$ ground state and $^{3}\mathrm{E}$ excited state in ${C}_{3v}$ symmetry; II) the calculated zero-phonon line is 1.71 eV (1.945 eV for ${\mathrm{NV}}^{1\ensuremath{-}}$). A detailed experimental reinvestigation based on optically detected electron paramagnetic resonance spectroscopy is suggested to verify whether the ${\mathrm{HV}}^{1\ensuremath{-}}$ center has metastable singlet shelving states between the ground and excited state triplets and, as a result, whether it may exhibit a spin-selective decay to the ground state.

Compacts of Boron-Doped Synthetic Diamond: Lowering of Synthesis Temperature and Its Effect on the Doping Level and Electrochemical Behavior

Substituting of metal (Co, Ni) borides for boron carbide in the boron carbide–graphite growth system for the process of diamond growth in the region of diamond thermodynamic stability allowed lowering the synthesis temperature for the electrodes of this new electrode material―boron-doped diamond compacts―significantly (by ~1000°C) without any deterioration of their electrochemical properties. On the other hand, using of amorphous boron with finer grain as compared with the boron carbide, mixed with graphite, results in a marked increase of the electrodes’ electrochemical efficiency, due to increase in their roughness after the chemical removing of boron-containing inclusions from their surfaces. Thus obtained compact electrodes have wide potential window and low background current in supporting electrolytes, they are well reproducible. Special features of their electrochemical impedance spectroscopy are similar to those of the recently studied compacts synthesized on the basis of boron carbide.

Exploring constant substrate temperature and constant high pressure SCD growth using variable pocket holder depths

SCD substrates were successfully synthesized using an optimized pocket holder design. The pocket holder design creates an appropriate thermal environment to shield the diamond substrate from the intense microwave discharge. This substrate configuration allows for a polycrystalline diamond (PCD) rimless, uniform single crystal diamond (SCD) growth process. Square shaped pocket holders with a constant width but with varying depths were used. The SCD growth procedures were carried out via microwave plasma assisted chemical vapor deposition (MPACVD) in a 2.45GHz microwave cavity plasma reactor C at a constant pressure of 240Torr and high power density of ~500W/cm3. By continuously adjusting incident microwave power (Pinc), the substrate temperature (Ts) was held at 1020°C±5°C throughout the entire process cycle. Under these growth conditions, the crystal morphologies exhibited a smooth and flat surface. The average growth rate of the SCDs varied between 30 and 24μm/h as the pocket depth increased from 2.0 to 2.9mm. The PCD rim was eliminated for all pocket depths and the shape of the final, as grown, top SCD surface varied from a square to a circular shape as the pocket holder depth increased. When using the 2.0mm and 2.3mm pockets, the final top SCD surface area increased to almost twice the original HPHT diamond seed area and the lateral growth rate was slightly larger than the vertical growth rate. Birefringence imaging of the grown CVD diamond indicated low stress and SIMS analysis indicated nitrogen concentration levels in the freestanding, CVD diamond plates were in the range of 50ppb to 140ppb. Prime novelty statementSingle crystal diamond is grown by high power density, microwave plasma assisted chemical vapor deposition in a pocket holder held at a constant temperature of 1020°C, a constant pressure of 240Torr and with 5% CH4/H2. Experimental variables were pocket depth and deposition time. Thick, PCD rimless growth was achieved, and for shallow holders, top growth surface area gains of 1. 4–1.9 were demonstrated.

Optical emission spectroscopy for diagnosis of diamond growth and etching processes in microwave plasma

Optical emission spectroscopy for diagnosis of diamond growth and etching processes in microwave plasma