Single Crystal Diamond Films Research Articles

Dielectric behavior and defects of nitrogen-containing single crystal diamond films

Single crystal diamond (SCD) film, as a new substrate material, has attracted growing attention because of its low dielectric loss. The nitrogen was usually introduced into microwave plasma chemical vapor deposition (MPCVD) process in order to enhance the growth rate of SCD films. So SCD films with nitrogen were prepared by MPCVD compared with the undoped film. The impurities and defects of diamond films were characterized by Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectroscopy (PL). The dielectric behavior of SCD films was measured using the LCR measurement in the frequency range of 1 kHz to 110 MHz. The results indicate that the primary dielectric loss in SCD films is conductivity loss and space charge polarization below 1 MHz, while the main dielectric loss is dipole orientation polarization and electronic relaxation polarization in the frequency range of 1 MHz to 110 MHz. Point defects like substitutional N+, NV− and SiV− exert a significant influence on polarization loss. This is essential to improve the growth of SCD with excellent dielectric properties.

Optimizing single crystal diamond mosaic growth: A study on seed thickness variation and pre-growth treatment

The limitations on the size of single-crystal diamond applications still need to be addressed for practical use. This study systematically investigates the impact of variations in seed crystal thickness and pre-growth treatments on step bonding, defect formation, and stress distribution in the mosaic growth of single-crystal diamond films. The findings suggest that variations in seed crystal thickness within 50 μm result in a smooth mosaic junction and high crystal quality, while a 100 μm variation results in noticeable defects and stress concentrations, hindering adequate bonding. Pre-growth treatment improves crystal quality by reducing polycrystalline formations and stress concentrations. Optical microscopy and Raman mapping confirm that pre-growth treated mosaic regions exhibit smooth steps and seamless bonding, with FWHM values between 3 and 5 cm−1, consistent with the quality of non-mosaic growth regions.

Controlled boron content in lightly B-doped single crystal diamond films by variation of methane concentration

Obtaining desirable electrical properties from B-doped single crystal diamond (SCD) films hinges on precise control of boron incorporation into the crystal lattice structure. In this study, the impact of methane concentration during plasma deposition on boron incorporation of lightly B-doped SCD films is investigated. SCD layers are grown successively by microwave plasma enhanced chemical vapor deposition (CVD) at different methane-to-hydrogen concentrations (1%, 2%, and 3%), with residual boron atoms present in the CVD reactor. An increase in methane concentration leads to surface defects such as unepitaxial crystallites and pyramidal hillocks. The charge carrier mobility, electrical conductivity, and boron content of samples are evaluated and discussed. The temperature-dependent mobility is analyzed through theoretical modeling, revealing dominant scattering mechanisms at different temperatures. At 300 K, the maximum hole mobility reached 1200 cm2/V·s for the 1% methane concentration sample, transitioning to hopping conduction at lower temperatures. An increase in boron-doping level with rising methane concentration is detected by Fourier transform infrared spectroscopy, cathodoluminescence spectroscopy, Hall effect, and X-ray photoelectron spectroscopy measurements. These findings highlight the potential of methane concentration in plasma feedgas to control boron concentration in CVD diamond and open avenues for crafting efficient high-power electronic applications using p-type SCD films.

Secondary electron emission of B-doped/undoped double-layer heteroepitaxial single crystal diamond film

A B-doped/undoped double-layer single crystal diamond structure was proposed to enhance secondary electron emission (SEE), and the double-layer single crystal diamond film was heteroepitaxially grown on MgO substrate by microwave plasma chemical vapor deposition. The crystal quality, surface morphology and conductivity of the film were studied. It is found that the film has high crystal quality, the full width at half maximum of X-ray diffraction rocking curve is 0.14°, and the surface roughness is small. For these reasons, the B-doped/undoped double-layer single crystal diamond film has better SEE performance than the B-doped/undoped double-layer polycrystalline diamond film. At the incident electron energy of 2 keV, the SEE yield of B-doped/undoped double-layer single crystal diamond reached 22.4. The physical model calculation results indicate that at a low B doping concentration, less sp2 carbon and smaller surface roughness are beneficial to improve the SEE yield.

High-rate growth of single-crystal diamond with an atomically flat surface by microwave plasma chemical vapor deposition

High quality single crystal diamond builds the foundation for the applications in high-power, high-frequency electronic devices. In this paper, high-quality (100) single-crystal diamond film grown by microwave plasma chemical vapor deposition (MPCVD) was studied. By optimizing the pre-etching process before growth, where the distance between the substrate and plasma plays an important role, high-quality chemical vapor deposition (CVD) diamond layer with a smooth surface can be acquired. The root-mean-square roughness of CVD diamond surface was reduced to 0.2 nm ∼ 0.5 nm within 5 μm × 5 μm area while maintaining the growth at a high rate of 3 μm/h ∼ 30 μm/h. The sheet resistance of the CVD diamond layer was 6.1 kΩ/sq by transmission line method, indicating a high potential to fabricate high-performance hydrogen-terminal diamond-based electronic devices.

Effects of Boron Doping on the Bulk and Surface Acoustic Phonons in Single-Crystal Diamond.

We report the results of the investigation of bulk and surface acoustic phonons in the undoped and boron-doped single-crystal diamond films using the Brillouin-Mandelstam light scattering spectroscopy. The evolution of the optical phonons in the same set of samples was monitored with Raman spectroscopy. It was found that the frequency and the group velocity of acoustic phonons decrease nonmonotonically with the increasing boron doping concentration, revealing pronounced phonon softening. The change in the velocity of the shear-horizontal and the high-frequency pseudo-longitudinal acoustic phonons in the degenerately doped diamond, as compared to that in the undoped diamond, was as large as ∼15% and ∼12%, respectively. As a result of boron doping, the velocity of the bulk longitudinal and transverse acoustic phonons decreased correspondingly. The frequency of the optical phonons was unaffected at low boron concentration but experienced a strong decrease at the high doping level. The density-functional-theory calculations of the phonon band structure for the pristine and highly doped samples confirm the phonon softening as a result of boron doping in diamond. The obtained results have important implications for thermal transport in heavily doped diamond, which is a promising material for ultra-wide-band-gap electronics.

An X-ray beam profile monitoring system at a beamline front-end combining a single-crystal diamond film and energy discrimination using droplet analysis.

This work has successfully demonstrated a system for monitoring pink-beam X-rays exiting from a beamline front-end, which has a specific spatial distribution based on each energy component. In this study, the X-rays scattered from a single-crystal chemical-vapor-deposited diamond film were converted into a cross-sectional image using pinhole optics, followed by digitization with a direct detection complementary metal-oxide-semiconductor 2D detector. By using single crystals instead of poly-crystals, good quality images were obtained with no diffraction bright spots. As a result of applying photon energy discrimination using the droplet analysis to the image information, the spatial distribution of each energy component of the undulator radiation was successfully visualized. The result was found to be in good agreement with the theoretically calculated result obtained using the synchrotron radiation calculation code SPECTRA. The new synchrotron radiation beam monitor proposed in this paper can serve as a powerful beam diagnostic tool for diffraction-limited rings that require strict light sourcestability.

Nitrogen-doping effect on single-crystal diamond synthesis by HFCVD

Nitrogen-doped homoepitaxial single-crystal diamond (SCD) films were grown on an SCD substrate by hot filament chemical vapor deposition (HFCVD) to confirm the potential that can yield high-quality single-crystal diamonds. The SCD films doped with 50 sccm nitrogen, grown by HFCVD, showed excellent surface properties based on optical microscopy, with a growth rate of 3.4 [Formula: see text]m/h, and a Raman peak around 1332 cm[Formula: see text] with a full width at maximum (FWHM) of 5.8 cm[Formula: see text], indicating high crystallinity. The FWHM of the (004) peak in the rocking curve was 392 arcsec for the nitrogen-doped SCD film, and that of the SCD substrate was 489 arcsec. Therefore, the crystalline properties and growth rates of the SCD films grown in the HFCVD system were improved, and the effect of nitrogen doping was verified.

Ohmic and Schottky contacts of hydrogenated and oxygenated boron-doped single-crystal diamond with hill-like polycrystalline grains* *Project supported by the Key-Area Research and Development Program of Guangdong Province (Grant No. 2020B0101690001) and the National Natural Science Foundation of China (NSFC) (Grant No. 51972135).

Hill-like polycrystalline diamond grains (HPDGs) randomly emerged on a heavy boron-doped p+ single-crystal diamond (SCD) film by prolonging the growth duration of the chemical vapor deposition process. The Raman spectral results confirm that a relatively higher boron concentration (∼ 1.1 × 1021 cm−3) is detected on the HPDG with respect to the SCD region (∼ 5.4 × 1020 cm−3). It demonstrates that the Au/SCD interface can be modulated from ohmic to Schottky contact by varying the surface from hydrogen to oxygen termination. The current–voltage curve between two HPDGs is nearly linear with either oxygen or hydrogen termination, which means that the HPDGs provide a leakage path to form an ohmic contact. There are obvious rectification characteristics between oxygen-terminated HPDGs and SCD based on the difference in boron doping levels in those regions. The results reveal that the highly boron-doped HPDGs grown in SCD can be adopted as ohmic electrodes for Hall measurement and electronic devices.

Coessential-connection by microwave plasma chemical vapor deposition: a common process towards wafer scale single crystal diamond

Large size single crystal diamond (SCD) wafer has been strongly desired for various of advanced applications, while two major potential approaches, including mosaic growth and heteroepitaxy based on chemical vapor deposition method, are both stuck with respective technical barriers. This paper reveals and summarizes the essential commonality of the two schemes, and denominates the concept of “coessential-connection” (CC) growth. Such generalized concept involved the nature of the single crystal and polycrystalline diamond film deposition with similar mechanism and processes. The principle of CC growth process with detailed classification was elaborated, and influence of nucleus size and orientation mismatch was clarified, which is regarded as the core problem of large area SCD film growth via coessential-connection process.

Annealing Temperature on Contact Properties between Nickel Film and Hydrogen-Terminated Single Crystal Diamond

Ohmic contact of nickel on hydrogen-terminated single-crystal diamond film was investigated with an annealing temperature ranging from room temperature to 750 °C in hydrogen atmosphere. Nickel film was deposited on a hydrogen-terminated single-crystal diamond surface with gold film in order to protect it from oxidation. Contact properties between nickel and hydrogen-terminated single crystal diamond were measured by a circular transmission line model. The lowest specific contact resistivity was 7.82 × 10−5 Ω cm2 at annealing temperature of 750 °C, indicating good ohmic contact, which reveals improved thermal stability by increasing temperature.

Comparison of α particle detectors based on single-crystal diamond films grown in two types of gas atmospheres by microwave plasma-assisted chemical vapor deposition

Chemical vapor deposition (CVD)-grown diamond films have been developed as irradiation-resistant materials to replace or upgrade current detectors for use in extreme radiation environments. However, their sensitivity in practical applications has been inhibited by space charge stability issues caused by defects and impurities in pure diamond crystal materials. In this study, two high-quality CVD-grown single-crystal diamond (SCD) detectors with low content of nitrogen impurities were fabricated and characterized. The intrinsic properties of the SCD samples were characterized using Raman spectroscopy, stereomicroscopy, and X-ray diffraction with the rocking curve mode, cathode luminescence (CL), and infrared and ultraviolet-visible-near infrared spectroscopies. After packaging the detectors, the dark current and energy resolution under α particle irradiation were investigated. Dark currents of less than 5 pA at 100 V were obtained after annealing the electrodes, which is comparable with the optimal value previously reported. The detector that uses a diamond film with higher nitrogen content showed poor energy resolution, whereas the detector with more dislocations showed poor charge collection efficiency (CCE). This demonstrates that the nitrogen content in diamond has a significant effect on the energy resolution of detectors, while the dislocations in diamond largely contribute to the poor CCE of detectors.

Temporally interleaved optical time-stretch imaging

Optical time-stretch imaging has shown potential in diverse fields for its capability of acquiring images at high speed and high resolution. However, its wide application is hindered by the stringent requirement on the instrumentation hardware caused by the high-speed serial data stream. Here we demonstrate temporally interleaved optical time-stretch imaging that lowers the requirement without sacrificing the frame rate or spatial resolution by interleaving the high-speed data stream into multiple channels in the time domain. Its performance is validated with both a United States Air Force (USAF)-1951 resolution chart and a single-crystal diamond film. We achieve a 101 Mfps 1D scanning rate and 3 µm spatial resolution with only a 2.5 GS/s sampling rate by using a two-channel-interleaved system.

Wettability and Surface Energy of Hydrogen- and Oxygen-Terminated Diamond Films**Supported by the National Natural Science Foundation of China under Grant Nos. 51672102 and 51972135.

The contact angle and surface energy values of diamond are systemically investigated in terms of surface treatments (hydrogen- and oxygen-terminations), structure feature (single crystal diamonds and polycrystalline diamond films), crystal orientation ((100), (111) and mixed (100)/(111) orientations), different fluids (probes of polar deionized water and nonpolar di-iodomethane). It is found that the hydrophobic/hydrophilic characteristic and surface energy values of diamond are mainly determined by the surface hydrogen/oxygen termination, and less related to the structural features and crystal orientation. Based on the contact angle values with polar water and nonpolar di-iodomethane, the surface energies of diamond are estimated to be about 43 mJ/m2 for hydrogen-termination and about 60 mJ/m2 for oxygen-termination. Furthermore, the varying surface roughness of diamond and fluids with different polarities examined determine the variation of contact angles as well as the surface energy values. These results would be helpful for a more detailed understanding of the surface properties of diamond films for further applications in a broad number of fields, such as optical and microwave windows, biosensors, and optoelectronic devices, etc.

Systematic study of the response of single crystal diamond neutron detectors at high temperature

For many years artificial diamond detectors have been studied for application in high temperature environments. An open question is whether the performance of such detectors is influenced by the electrical contacts. In the attempt to better understand this point, a systematic study of electrical contacts deposited with different techniques and materials on top of commercial artificial single crystal diamond (SCD) films was performed. In this paper we report on the results obtained using three types of electric contacts produced both using metal and not metal layers: a) double Schottky, b) double ohmic; c) Schottky-ohmic contacts. The detectors were studied under 14 MeV neutron irradiation, and were operated in pulse mode. The pulse height spectra (PHS) produced by the 14 MeV neutrons were recorded as a function of temperature and the detector behaviour was studied by analysing the variation of the peak produced in the PHS by the 12C(n,α)9Be reaction induced by 14 MeV neutrons in carbon. The maximum operational temperature limit of the tested detectors was found to be around 230̂C and it resulted to be slightly dependent, within ±10̂C, upon the type of the electrical contact. It turned out that, together with the type of the electrical contact, other parameters such as the intrinsic diamond properties and the film thickness have to be considered to understand the response of the detector at high temperature.

Synthesizing single-layer diamond

Thin Films The carbon allotropes of diamond and graphene have different types of bonding that lead to their exceptional properties. Bakharev et al. pull off the impressive trick of making a monolayer carbon film that is diamond-like in its bonding. The authors accomplish this by attaching fluorine atoms to the carbon film, creating “F-diamane.” Diamane is a long-sought-after, but challenging to make, material that should have useful properties. F-diamane may find use in a variety of applications, from microelectronics as a semiconductor to a seed material for growing single-crystal diamond films. Nat. Nanotechol. 15 , 59 (2020).

Genetic Role of Calcium Content in Olivine Crystals of Ultramafic and Mafic Rocks

The results of genetic systematization of olivine compositions formed both under experimental conditions and its natural differences from rocks of basic and ultramafic compositions of various depth facies and formed in various geodynamic settings are presented. Similar work [53], performed in the 70s of the last century and has been actively cited to this day. Generalization of the subsequent accumulated data showed the error of one of the main conclusions in this work – the dependence of the Cao content in olivine on the hydrostatic pressure during its formation from the melt. According to experimental studies conducted in recent years, up to a pressure of 29 GPA, the content of calcium in olivine, which has grown from the main-ultrabasic melts, does not depend on the pressure and is not lower than 0.1 wt.% CaO. Experiments in solidus conditions involving fluid and natural data have demonstrated that one of the leading factors affecting the calcium content in olivine are metasomatic processes that lead to the removal of calcium from olivine crystals. The role of metasomatic transformations of olivine in terms of calcium content, despite the apparent insignificance of secondary changes in it, is clearly visible in the examples of basalts and gabbro of the modern oceanic crust and the same facies differences in ophiolite complexes. The wide development of metasomatic transformations of igneous rocks of various facies and ages indicates the need to take into account the calcium content in olivine as an equilibrium criterion when calculating temperatures and pressures for paragenesis involving olivine. Olivines, which are part of ultrabasic xenoliths carried out by sub-alkaline magmas, including those carried out by kimberlites, are overwhelmingly represented by low-calcium differences. Low levels of calcium in olivines from these mantle fragments suggest that magmatic melts of the main-ultramafic compositions are not in equilibrium with the mantle substance to depths of about ~ 200 km, and possibly more. Among the compositions of olivines from fresh effusive rocks, its inclusions and microlites are mostly represented by calcium-containing differences. Only in kimberlites, almost all the differences in its crystals (inclusions, microlites) in the rock are represented by low-calcium differences. Olivines included in diamonds are also overwhelmingly represented by low calcium differences. This suggests that the composition of kimberlite olivine is associated with metasomatic transformations, and the growth of diamonds from kimberlites is due to the fluid. Inside the natural single crystals of diamond, there are polymetallic films buried in the body of crystals. Similar films were formed on the faces of diamond crystals formed in the Lav pores (that is, almost on the surface of the day) in 2012-13 of the Tolbachinsky Fissure eruption. These data allow us to create artificial "soft" conditions (CVD, solution, etc.) for the growth of single-crystal diamond films on similar polymetallic or single-element films. For these purposes, elements such as zirconium, dysprosium, erbium, and others can be used.

Growth time experiments highlight potential for single crystal diamond film optoelectronics

High pressure high temperature epitaxial growth experiments highlight the importance of pretreatment and point to silicon integration as a route for optoelectronic applications.

Evolution of surface morphology and optical transmittance of single crystal diamond film by epitaxial growth

The single crystal diamond (SCD) has great potential in the application of optical windows, photoelectric devices, semiconductors and other fields owing to its excellent performance in optics, mechanics, and thermotics. The SCD was homoepitaxially deposited on High Pressure and High Temperature (HPHT) seed substrate through microwave plasma chemical vapor deposition (MPCVD) method using CH4/H2 as the reaction gas. Hydrogen plasma treatment was proposed to pretreat the seed crystal. The top surface of the epitaxial layer of SCD has a creased morphology and no polycrystalline rim growth on the side. The results showed that the transmittance of the epitaxial SCD film was primarily affected by the surface roughness, which was mainly influenced by the growth time. The photoluminescence at 738 nm was attributed to the silicon color center in the grown SCD, suggesting the application in optoelectronic devices.

Simulation-Based Development of a New Cylindrical-Cavity Microwave-Plasma Reactor for Diamond-Film Synthesis

A 2.45 GHz microwave-plasma chemical-vapor deposition (MPCVD) reactor was designed and built in-house by collaborating with Guangdong TrueOne Semiconductor Technology Co., Ltd. A cylindrical cavity was designed as the deposition chamber and a circumferential coaxial-mode transformer located at the top of the cavity was adopted as the antenna. Two quartz-ring windows that were placed far away from the plasma and cooled by water-cooling cavity walls were used to affix the antenna to the cavity and act as a vacuum seal for the reactor, respectively. This design improved the sealing and protected the quartz windows. In addition, a numerical simulation was proposed to predict the electric-field and plasma-density distributions in the cavity. Based on the simulation results, a microwave-plasma reactor with TM021 mode was built. The leak rate of this new reactor was tested to be as low as 1 × 10−8 Pa·m3·s−1, and the maximal microwave power was as high as 10 kW. Then, single-crystal diamond films were grown with the morphology and crystalline quality characterized by an optical microscope, atomic force microscope (AFM), Raman spectrometer, photoluminescence (PL) spectrometer, and high-resolution X-ray diffractometer. It was shown that the newly developed MPCVD reactor can produce diamond films with high quality and purity.