Growth Of Polycrystalline Diamond Research Articles

Effect of hydrogen flow rate on the plasma state and quality of diamond crystals grown by MPCVD

The electronic application of diamond is still immature because of the relatively high density of dislocation. Hydrogen flow rate is one of the key parameters that determine the crystalline quality of diamond, but most of the previous works focus on the growth of polycrystalline diamond. Herein, the effects of hydrogen flow rate on the growth rate and crystalline quality of single crystal diamond have been evaluated while keeping the CH4/H2 ratio. It demonstrates that a medium H2 flow rate can balance the C2 concentration and nitrogen leakage in the chamber, resulting in the maximum of growth rate. Similarly, the dislocation density decreases firstly and then increases with a minimum value of 4 × 104/cm2 under a medium H2 flow rate. The variation of growth rate and dislocation density versus hydrogen flow rate can be double-confirmed from both high pressure and high temperature and chemical vapor deposition samples. The incline angle of step riser relative to the terrace is smaller than 20°, which helps to switch the propagation direction of dislocation towards the raiser region. A relatively wider step width under a medium H2 flow rate means a lower density of step riser region to display the dislocation. Therefore, we conclude that a medium hydrogen flow rate can effectively balance the crystalline quality and growth rate of the single crystal diamond.

Diamond-SiC composite substrates: A novel strategy as efficient heat sinks for GaN-based devices

Combining the structural advantages of SiC with super thermal conductivity of diamond to construct an efficient heat dissipation substrate for GaN-based transistors is a promising strategy to solve the self-heating problem. Herein, a scheme for direct growth of polycrystalline diamond on 4H–SiC substrate, followed by thinning of the 4H–SiC substrate to an optimum thickness, is designed and implemented. Diamond-SiC composite substrates consisting of 200 μm 4H–SiC and 200 μm diamond are fabricated. The simulation of GaN devices is carried out to provide theoretical support for device fabrication. Then, compatible with established epitaxial and device manufacturing technology on SiC substrate, GaN devices are fabricated on diamond-SiC composite substrates. A 52.5 °C reduction in surface temperature and about 41 % reduction in thermal resistance of the improved heat dissipation structures are exhibited, by contrast with GaN-on-SiC technology at base temperature of Tb = 25 °C and dissipation power of Pdiss = 7.2 W mm−1. The stable output characteristics and reliability of GaN-on-diamond/SiC structures are confirmed at high temperatures. This work is anticipated to afford a promising heat dissipation strategy for devices with high power density and illuminate a new perspective on exploring and constructing novel composite substrates with diamond.

Formation mechanism and regulation of silicon vacancy centers in polycrystalline diamond films

Diamond silicon vacancy centers (SiV centers) have important application prospects in quantum information technology and biomarkers. In this work, the formation mechanism and regulation method of SiV center during the growth of polycrystalline diamond on silicon substrate are studied. By changing the ratio of nitrogen content to oxygen content in the growing atmosphere of diamond, the photoluminescence intensity of SiV center can be controlled effectively, and polycrystalline diamond samples with the ratios of SiV center photoluminescence peak to diamond intrinsic peak as high as 334.46 and as low as 1.48 are prepared. It is found that nitrogen promotes the formation of SiV center in the growth process, and the inhibition of oxygen. The surface morphology and photoluminescence spectrum for each of these samples show that the photoluminescence peak intensity of SiV center is positively correlated with the grain size of diamond, and the SiV center’s photoluminescence peak in the diamond film with obvious preferred orientation of crystal plane is higher. The distribution of Si centers and SiV centers on the surface of polycrystalline diamond are further characterized and analyzed by photoluminescence, Raman surface scanning and depth scanning spectroscopy. It is found that during the growth of polycrystalline diamond, the substrate silicon diffuses first into the diamond grain and then into the crystal structure to form the SiV center. This paper provides a theoretical basis for the development and application of SiV centers in diamond.

In-situ monitoring of microwave plasma-enhanced chemical vapour deposition diamond growth on silicon using spectroscopic ellipsometry

The quality of polycrystalline diamond films is heavily dependent on the nucleation and early stages of growth, making the ability to monitor these early stages highly desirable. Spectroscopic ellipsometry (SE) allows for real-time monitoring of the thickness, composition, and morphology of films with sub-nanometre precision. In this work, ex-situ SE spectra were used to develop an optical model for film characterisation, which was then applied to in-situ data. The coalescence of individual crystallites into a single film was observed through a parabolic decrease in void content followed by peaks in sp2 content and surface roughness. These observations were validated using ex-situ Raman spectra and AFM images of samples grown for durations between 5 and 30 min. The model was also used to investigate the impact of varying the methane concentration, finding that a higher methane fraction resulted in earlier coalescence and a higher peak in sp2 content. This work demonstrates that SE is a powerful tool for monitoring and optimisation of the critical early stages of polycrystalline diamond growth.

Highly phosphorus-doped polycrystalline diamond growth and properties

In this work, phosphorus-doped polycrystalline diamond layers were grown using a new gas control process to increase the incorporation of phosphorus in diamond. Topographical characteristics and crystalline quality of the phosphorus-doped polycrystalline diamond layers grown on Si substrates were analyzed using Scanning Electron Microscopy (SEM) and Raman spectroscopy. The phosphorus concentration was determined using Glow-Discharge Optical Emission Spectroscopy (GDOES). Polycrystalline diamond layers have a good crystalline quality with a sp3/sp2 carbon ration over 75%. The growth rate reaches up to 440 nm∙h−1, and the phosphorus concentration is well above 1020 cm−3. Novelty statementThis work reports on a new method for the production of phosphorus-doped polycrystalline diamond layers based on the pulsed injection of methane during the growth by microwave plasma enhanced chemical vapor deposition.

Microwave plasma modelling in clamshell chemical vapour deposition diamond reactors

A microwave plasma model of a chemical vapour deposition (CVD) reactor is presented for understanding spatial heteroepitaxial growth of polycrystalline diamond on Si. This work is based on the TM0(n>1) clamshell style reactor (Seki Diamond/ASTEX SDS 6K, Carat CTS6U, ARDIS-100 style) whereby a simplified H2 plasma model is used to show the radial variation in growth rate over small samples with different sample holders. The model uses several steps: an electromagnetic (EM) eigenfrequency solution, a frequency-transient EM/plasma fluid solution and a transient heat transfer solution at low and high microwave power densities. Experimental growths provide model validation with characterisation using Raman spectroscopy and scanning electron microscopy. This work demonstrates that shallow holders result in non-uniform diamond films, with a radial variation akin to the electron density, atomic H density and temperature distribution at the wafer surface. For the same process conditions, greater homogeneity is observed for taller holders, however, if the height is too extreme, the diamond quality reduces. From a modelling perspective, EM solutions are limited but useful for examining electric field focusing at the sample edges, resulting in accelerated diamond growth. For better accuracy, plasma fluid and heat transfer solutions are imperative for modelling spatial growth variation.

GaN-based heterostructures with CVD diamond heat sinks: A new fabrication approach towards efficient electronic devices

A new approach to the fabrication of efficient heat sinks for GaN-based transistors is demonstrated. A key feature of this work is the growth of polycrystalline diamond coating on the functional silicon layer of SOI wafers followed by etching of a thick silicon substrate and a thin thermal oxide. As a result, composite epi–ready substrates consisting of a thin (410 nm) monocrystalline silicon functional layer on top of the 150 µm-thick polycrystalline diamond heat sink were fabricated. GaN heterostructures were grown on top of the silicon layer, which resulted in an effective thermal contact between CVD diamond and GaN structure. The packaged ungated transistors were made to analyze the efficiency of the developed heat sink. Improved heat removal structures showed the decrease in surface temperature by more than 50 °C at base temperature of Tb=85 °C and dissipation power of Pdiss=6.9 W/mm compared to conventional GaN-on-SiC technology and by more than 20 °C at Tb=25 °C, Pdiss=6.9 W/mm compared to up-to-date GaN-on-Diamond equivalent transistors reported by other groups. New substrate fabrication technology positively impacts GaN-based device output characteristics and reliability, which is important in improving communication systems, radars, and secondary power supply systems.

Record-Low Thermal Boundary Resistance between Diamond and GaN-on-SiC for Enabling Radiofrequency Device Cooling.

The implementation of 5G-and-beyond networks requires faster, high-performance, and power-efficient semiconductor devices, which are only possible with materials that can support higher frequencies. Gallium nitride (GaN) power amplifiers are essential for 5G-and-beyond technologies since they provide the desired combination of high frequency and high power. These applications along with terrestrial hub and backhaul communications at high power output can present severe heat removal challenges. The cooling of GaN devices with diamond as the heat spreader has gained significant momentum since device self-heating limits GaN's performance. However, one of the significant challenges in integrating polycrystalline diamond on GaN devices is maintaining the device performance while achieving a low diamond/GaN channel thermal boundary resistance. In this study, we achieved a record-low thermal boundary resistance of around 3.1 ± 0.7 m2 K/GW at the diamond/Si3N4/GaN interface, which is the closest to theoretical prediction to date. The diamond was integrated within ∼1 nm of the GaN channel layer without degrading the channel's electrical behavior. Furthermore, we successfully minimized the residual stress in the diamond layer, enabling more isotropic polycrystalline diamond growth on GaN with thicknesses >2 μm and a ∼1.9 μm lateral grain size. More isotropic grains can spread the heat in both vertical and lateral directions efficiently. Using transient thermoreflectance, the thermal conductivity of the grains was measured to be 638 ± 48 W/m K, which when combined with the record-low thermal boundary resistance makes it a leading-edge achievement.

Effect of GaN-on-diamond integration technology on its thermal properties

Two different integration technologies of GaN-on-diamond are explored, direct growth of polycrystalline diamond (PCD) on GaN (sample A) and deposition of GaN on PCD (sample B). The reversed evolution of PCD grain structure is assumed for each technology, resulting in an increasing/decreasing tendency of thermal conductivity with growth thickness. Simulation of AlGaN/GaN high electron mobility transistors on PCD substrate with capped diamond is made considering anisotropic and inhomogeneous thermal conductivities of GaN and PCD, different combinations of thermal boundary resistance (TBR) values at the top diamond/GaN and bottom GaN/PCD substrate are investigated. The results show that the effect of top TBR is limited and increasing the bottom TBR from 6.5 to 100 m2K GW−1 results in a temperature rise of 80.8 °C for sample A and 85.3 °C for sample B at power dissipation of 10 W mm−1. Enlarging the thickness of capped diamond helps to reduce the junction temperature, which is more effective at small top TBR under constant bottom TBR and at large bottom TBR with fixed top TBR. In addition, increasing gate pitch is a promising solution to reduce junction temperature and a weak dependence of junction temperature on gate width is revealed.

Deposition of Boron-Doped Thin CVD Diamond Films from Methane-Triethyl Borate-Hydrogen Gas Mixture

Boron-doped diamond is a promising semiconductor material that can be used as a sensor and in power electronics. Currently, researchers have obtained thin boron-doped diamond layers due to low film growth rates (2–10 μm/h), with polycrystalline diamond growth on the front and edge planes of thicker crystals, inhomogeneous properties in the growing crystal’s volume, and the presence of different structural defects. One way to reduce structural imperfection is the specification of optimal synthesis conditions, as well as surface etching, to remove diamond polycrystals. Etching can be carried out using various gas compositions, but this operation is conducted with the interruption of the diamond deposition process; therefore, inhomogeneity in the diamond structure appears. The solution to this problem is etching in the process of diamond deposition. To realize this in the present work, we used triethyl borate as a boron-containing substance in the process of boron-doped diamond chemical vapor deposition. Due to the oxygen atoms in the triethyl borate molecule, it became possible to carry out an experiment on simultaneous boron-doped diamond deposition and growing surface etching without the requirement of process interruption for other operations. As a result of the experiments, we obtain highly boron-doped monocrystalline diamond layers with a thickness of about 8 μm and a boron content of 2.9%. Defects in the form of diamond polycrystals were not detected on the surface and around the periphery of the plate.

Fabrication of diamond diffractive optics for powerful CO2 lasers via replication of laser microstructures on silicon template

New approach to fabricate diamond diffractive optical elements (DOEs) with continuous relief for powerful CO2 lasers is proposed and tested. It involves short-pulse laser microstructuring of a silicon wafer, which further is used as a substrate for polycrystalline diamond growth in a microwave plasma-assisted CVD process. After fine mechanical polishing of the growth side of the diamond film, the silicon substrate is removed via chemical etching. Two different DOEs providing close to 100% diffraction efficiency were fabricated with this technique: cylindrical Fresnel lens with kinoform surface profile and three-beam splitter with continuous microrelief. Optimization of the laser processing conditions has made possible to reduce the final roughness of the structured diamond surface to 200–400 nm depending on the local relief depth (0–7 μm). Both DOEs tested with a CO2 laser have demonstrated high transparency and diffraction efficiency, as well as low radiation scattering of the IR radiation at the surface irregularities.

Method to increase the thickness and quality of diamond layers using plasma chemical vapor deposition under (H, C, N, O) system

A method to increase the thickness of diamond layers grown by plasma chemical vapor deposition (CVD) is presented. Undesired polycrystalline diamond growth that typically occurs during long time periods of deposition was suppressed by introducing oxygen into the source gas mixture. To maintain a stable growth environment, a movable stage was introduced to control the distance from the top surface of the substrate to the core of the discharge region. This made it possible to sustain uninterrupted growth for over 200 h until the thickness of the CVD layer has reached 6 mm. The use of this continuous deposition process prevents the formation of growth interfaces, which are believed to be an origin of dislocations, in the CVD-grown layer.

A study on the nucleation and MPCVD growth of thin, dense, and contiguous nanocrystalline diamond films on bare and Si3N4-coated N-polar GaN

We report upon the microwave plasma chemical vapor deposition (MPCVD) growth of polycrystalline diamond on varying crystallographic orientations of GaN (Ga-polar and N-polar), as well as on Si3N4-coated N-polar GaN, as applied to top-side heat spreading layers for III-N-based high electron mobility transistors. Integration of diamond with GaN in this configuration is subject to differing process constraints from previous research focusing on backside heat sinks. A critical requirement is to prevent hydrogen-related etching of the III-N surface in the H+ plasma ambient, as well as to avoid plasma damage to the GaN crystal or HEMT channel region near the surface. Thus we developed the seeding and MPCVD techniques under a low power density plasma with low sample temperatures during deposition. We systematically investigated samples seeded via polymer-assisted dip seeding in a nanoparticle suspension, which were then subjected to MPCVD growth under identical conditions. We evaluated the quality of the films via scanning electron microscopy and Raman spectroscopy to understand the relationship between grain size, interface abruptness, propensity for surface etching, as well as film uniformity. Finally, we were able to identify a process window for both N-polar GaN and Si3N4-coated N-polar GaN which yields thin, fully coalesced, nanocrystalline diamond films with an abrupt interface to the underlying substrate. These diamond films exhibit microscopic uniformity in crystal grain size, and macroscopic uniformity in thickness and interface abruptness via a process which is scalable to large wafer sizes.

Group growth of polycrystalline diamond coating by MPCVD technique on a hard alloy tool with a thin blade

A coating technology for application of hardening of diamond coatings on carbide tools for treatment of mineral fiberglass based on quartz texture with chromate-phosphate cohesive (FQTCC) is proposed. The purpose of this development is to increase the edge strength of the tool. The use of the technology described above allowed, in comparison with the cutter without diamond coating, to increase the productivity of processing by 11.5 times, by cutting speed by 2.3 times, and by the minute supply by 5.0 times. The approach is based on changing the configuration of the microwave field when applying a diamond coating on hard alloy substrates in the microwave plasma of the ARDIS-100 reactor by using specially designed plateholders. This method allows for the group coating on several substrates in a single process. A simplified mathematical model for calculating the temperature of the top of the cutter during the growth of a CVD diamond film, taking into account heat transfer through thermal conductivity and radiation, has been constructed. Temperature distribution on the surface of a tungsten carbide cutter in the temperature range optimal for diamond growth was constructed. The difference between the average (experimental) and maximum (calculated) temperature in such a cutter was from 80 to 138°C.

A Study on the Growth Window of Polycrystalline Diamond on Si3N4-coated N-Polar GaN

Diamond has the most desirable thermal properties for applications in electronics. In principle, diamond is the best candidate for integration with other materials for thermal management due to its high thermal conductivity. Therefore, if low thermal boundary resistance can be developed between diamond and the semiconductor material, it would most effectively channel the heat away from areas of high power dissipation. Recent advancement of N-polar GaN in high power RF and conventional power electronics motivated us to study the diamond/Si3N4/GaN interface to understand how effectively the heat can be transferred from the GaN channel to diamond heat-sink. Prior studies showed that there are challenges in incorporating diamond with GaN while still maintaining the high crystalline quality necessary to observe the desirable thermal properties of the material. Therefore, in this study we investigated the influence of methane concentration (0.5–6%), gas pressure (40–90 Torr), sample surface temperature (600–850 °C), and growth duration (1~5 h) on polycrystalline diamond growth. The diamond/Si3N4/GaN interface looks abrupt with no signs of etching of the GaN for the samples with methane concentration above 2%, pressures up to 90 Torr, and temperatures < 850 °C, allowing for incorporation of diamond close to the active region of the device. This approach contrasts with most prior research, which require surface roughening and thick growth on the backside.

Enhanced deposition rate of polycrystalline CVD diamond at high microwave power densities

We report on growth of polycrystalline diamond (PCD) by chemical vapor deposition (CVD) in a microwave plasma reactor under high absorbed microwave power density (MWPD) and high pressures, in conditions more typical for single crystal diamond epitaxy. The growth rates as high as 30–36 μm/h have been achieved in CH4-H2 mixtures at pressure of 320 Torr and MWPD of ~700 W/cm3 even at relatively low (3%) CH4 concentration, using PCD substrates with diameter of 5–20 mm and different textures. The structure and quality of the produced PCD thick films were assessed with SEM, Raman and X-ray diffraction. The plasma shapes and spatial profiles of species (excited atomic hydrogen and C2 dimer) were characterized with spatially resolved optical emission spectroscopy (OES) at moderate and high MWPD. The maximum rotational gas temperature Tg significantly enhanced to 3700 K in the latter case (675 W/cm3) compared with Tg ≈ 3100 K for lower pressure and MWPD (100 Torr, 160 W/cm3), being in agreement with the obtained high growth rates at high pressures. Analysis of numerous literature data on PCD growth rate vs substrate size in MPCVD reactors reveals a clear trend of enhanced growth rate with diminishing of the substrate dimension, our findings further confirming this tendency indirectly related to a change in absorbed power density.

Thermal characteristics of GaN-on-diamond HEMTs: Impact of anisotropic and inhomogeneous thermal conductivity of polycrystalline diamond

The CVD growth of polycrystalline diamond (PCD) on GaN is a novel and effective approach to improve heat dissipation for high power GaN-based transistors. However, the growth-induced spatial variation in PCD's thermal conductivity makes it difficult to assess the thermal characteristics of GaN-on-diamond devices using bulk diamond properties. By including the depth-dependent anisotropic thermal conductivity of PCD, a finite element thermal simulation is used to study the heat spreading process within a device. The effective thermal conductivity (κeff) of the PCD substrate as “seen” by the device is found to be lower than the average thermal conductivity (κavg) of PCD, suggesting that the low thermal conductivity diamond film during the initial growth has a significant impact on the device thermal resistance. Moreover, κeff is investigated as a function of the thermal boundary resistance between GaN and diamond, the gate pitch of the device as well as the grain boundary conductance and the grain evolution rate of diamond polycrystals. The results provide reliable thermal assessment for the PCD substrate as used in a GaN-on-diamond device and have important implications for thermally optimizing the device layout and engineering the thermal conductivity of PCD heat spreaders for devices.

Selective Area Deposition of Hot Filament CVD Diamond on 100 mm MOCVD Grown AlGaN/GaN Wafers

A new technique is reported for selective growth of polycrystalline diamond by hot filament chemical vapor deposition (HFCVD) on AlGaN/GaN-on-Si (111) wafers without degradation of the underlying layers. Selective diamond seeding is accomplished by dispersing nanodiamond seeds in photoresist and patterned lithographically prior to HFCVD growth. A thin layer of plasma enhanced CVD SiNx, deposited prior to seeding and diamond deposition, was found to be essential to protect the AlGaN/GaN wafer. A methane concentration of 3.0% was used to achieve an increased diamond growth rate and faster surface coverage. Excellent selectivity and minimal AlGaN surface damage were achieved with increased methane concentration. Damage mitigation was confirmed by comparison of atomic force microscopy, X-ray diffraction, and Raman spectroscopy, each conducted before and after diamond deposition, and by SEM images of the final structures.

The influence of recess depth and crystallographic orientation of seed sides on homoepitaxial growth of CVD single crystal diamonds

Microwave plasma chemical vapor deposition (MPCVD) has gained increasing attention as a feasible and effective way to produce large, high-quality, single-crystal diamonds. However, the growth of polycrystalline diamond on the periphery of the seed crystal and the cracking generated by the internal stress during the growing process lead to significant decline of the quality and integrity of the CVD diamond, thus increasing the difficulty of synthesizing large diamond layers. Although optimized growth parameters and refined substrate holders have been employed by some researchers to improve the periphery quality of CVD diamond layers, more research needs to be done in this area. In this study, we used a specially designed substrate holder with a circular recess, in which the seed crystal was placed. By designing substrate holders with different recess depths and a seed crystal with different side-surface crystallographic orientations, we aimed to determine the influence of the recess depths and the crystallographic orientation of seed sides on the growth quality according to polarizing microscope, laser Raman spectroscopy, UV fluorescence imaging, and photoluminescence (PL) mapping measurements. The results demonstrate that as the recess depth increases, the amount of polycrystalline diamonds and the internal stress on the periphery are controlled effectively. The crystalline quality is improved, and the growth rate is decreased. In addition, compared to the periphery with {100} seed sides, the periphery with {110} seed sides displays better crystalline quality, lower internal stress, and fewer polycrystalline diamonds after growth, which is probably due to the intrinsic nature of the growth steps propagating on the {100} diamond surface and the effect of nitrogen atoms on the growing process in the diamond lattice.

Nucleation and Chemical Vapor Deposition Growth of Polycrystalline Diamond on Aluminum Nitride: Role of Surface Termination and Polarity

We have investigated the growth and atomic interface structures of diamond on aluminum nitride (AlN). The two-step chemical vapor deposition technique is used to control diamond nucleation density, crystal size, and AlN surface orientation and polarity. Highly uniform diamond layers with a nucleation density in the range of 105–1011 cm–2 and a grain size of 0.1–5 μm are fabricated. Crystallographically abrupt interfaces between polycrystalline diamond and single-crystal AlN(0001) layers have been observed via high-resolution transmission electron microscopy and electron energy-loss spectroscopy. A majority of the diamond crystals have been found to have the diamond(111)/AlN(0001) interface relationship. Atomistic models of the bonding mechanism at the heterointerface are used to elucidate experimental observations and the role of hydrogen plasma on the growth of diamond on AlN. Nonpolar and semipolar AlN surfaces have been found to have higher resistance to process plasma and led to better crystallinity o...