Nanocrystalline Diamond Films Research Articles

Corrosion resistance of diamond films with different grain sizes

Titanium alloys are widely used in the aerospace industry due to their excellent comprehensive properties. However, as the application becomes more and more extensive, higher requirements are put forward for the corrosion resistance of titanium alloys. Due to its natural chemical stability, diamond has become a good corrosion-resistant material. Diamond films with three different grain sizes were prepared on the surface of titanium alloy by hot-filament chemical vapor deposition (HFCVD). The protective efficiency of the three kinds of films to the substrate is above 90 %. The corrosion resistance of the nanocrystalline diamond (NCD) films is obviously better than that of the microcrystalline diamond (MCD) films. For NCD films, as the diamond grain size decreases, a cluster structure of particle stacking is formed, which hinders the arrival of corrosive substances to the substrate and further improves the corrosion resistance.

Morphology evolution and non-uniformity removal mechanism analysis of diamond nanocone arrays by reactive ion etching

By etching diamond films, microstructures can be fabricated on the surfaces, giving films with extra functionalities. During the etching process, the evolution of diamond surface morphology will affect the electric field near the microstructure. These changes in the electric field will alter the trajectory of ions, thereby affecting the number of ion bombardments on the surface regions of the microstructure. Consequently, this will affect the removal rate of the material on the microstructure surface and consequently affects the evolution of microstructure morphology. In order to understand the morphology evolution process of diamond during etching. H2 and Ar were used as the reactive gases to etch the nanocrystalline diamond (NCD) films with masks. The electric field near the microstructure and the trajectories of reactive ions was stimulated. The number of H+ and Ar+ bombardments at various regions on the diamond microstructure surface has been recorded and analyzed. By observing the cross-sectional morphology of the microstructure at different periods, the etching process of nanocones was analyzed. The study found that the microstructure of the diamond surface would gradually be etched into convex shaped and waist drum shaped and cone. By stimulating the electric field near the microstructure, it was found that the electric field distorted near the microstructure, causing the electric field lines initially perpendicular to the diamond surface to be diverted. As a result, the trajectory of the particles originally arrived perpendicularly to the diamond surface was distorted and hence they bombarded against the sidewall of the diamond microstructures. The angle between the ion trajectory and the electric field line also increased gradually.

Reduced temperature in lateral (AlxGa1−x)2O3/Ga2O3 heterojunction field effect transistor capped with nanocrystalline diamond

The low thermal conductivity of β-Ga2O3 is a significant concern for maximizing the potential of this ultra-wide bandgap semiconductor as a power switching device technology. Here, we report on the use of nanocrystalline diamond (NCD) deposited via microwave plasma enhanced chemical vapor deposition (MP-CVD) as a top-side, device-level thermal management solution on a lateral β-Ga2O3 transistor. NCD was grown via MP-CVD on β-(AlxGa1−x)2O3/β-Ga2O3 heterostructures prior to the gate formation of the field-effect transistor. A reduced growth temperature of 400 °C and a SiNx barrier layer were used to protect the oxide semiconductors from etching in the MP-CVD H2 plasma environment. Raman spectroscopy showed a highly sp3-bonded NCD film was obtained at 400 °C, with grain size of about 50–100 nm imaged via atomic force microscopy. The incorporation of the NCD heat-spreading layer resulted in a β-(AlxGa1−x)2O3/β-Ga2O3 heterostructure field-effect transistor showing a decrease in the total thermal resistance at the gate by 42%. The fabrication process, including the NCD etch in the gate region, will need to be improved to minimize the impact of these processes on important device characteristics (i.e., drain current, threshold voltage, and leakage current).

Disappearance of cobalt particles above nanocrystalline diamond films by HF-CVD on cemented tungsten carbide

Abstract In this study, the possibility of the disappearance of Co particles on the surface of nanocrystalline diamond (NCD) films with large thicknesses was examined. It is shown that the number of Co particles decreased clearly with larger thicknesses. In the thickest case, the Co particles disappeared almost completely. Investigation with energy-dispersive spectroscopy for the cross section of the NCD films suggested that the Co particle was embedded during the growth of the NCD film. Our observations will contribute to depositing smooth NCD films on tools without the embrittlement of substrates by Co etching.

High performance field emission cathode based on the diamond nanowires prepared by nanocrystalline diamond films annealed in air

This paper reports the field emission (FE) characteristics of a diamond nanowires (DNWs) array. The nanocrystalline diamond (NCD) film was deposited on silicon by microwave plasma chemical vapor deposition (MPCVD) and then annealed in air forming DNWs and hydrogenated at last. A high-field flat-plate emission test structure with a 1.03 μm gap between anode and cathode was prepared and the electrical properties proved it feasible. The FE performance of DNWs array was measured in a vacuum test system and that of NCDs film as a comparison. Finally, their FE parameters were analyzed and extracted based on the Fowler–Nordheim (F-N) theory. The results show that transforming NCDs film into DNWs array can improve the FE characteristics greatly. The turn-on field is as low as 1.36 V μm−1 dropping by one order of magnitude, while the field enhancement factor and FE current density are up to 156.68 and 484.75 mA cm−2 respectively rising both by two orders of magnitude. This excellent FE performance stems from the characteristics of large aspect ratio, very small tip radius and high density of DNWs.

Micromechanical properties of micro- and nanocrystalline CVD diamond thin films with gradient microstructures and stresses

Chemical vapor deposition produced diamonds attract considerable scientific and industrial interest due to their exceptional mechanical and functional properties. Here, hot filament (HF) chemical vapor deposition was used to synthesize two diamond thin films with different cross-sectional microstructure and residual stresses. Structural characterization by scanning electron microscopy, Raman spectroscopy, and cross-sectional synchrotron x-ray nanodiffraction revealed different diamond morphologies. While the microcrystalline diamond film exhibits pronounced microstructural gradients expressed by gradually increasing (i) intensities of the 111 Debye-Scherrer rings, (ii) ⟨110⟩ fiber texture sharpness, (iii) grain size, and (iv) slightly oscillating residual stress at the level of −0.5 GPa, the nanocrystalline diamond film showed no pronounced cross-sectional variation of microstructure above the nucleation zone of ∼0.5 μm and a steady stress level of 0.25 GPa. In situ micromechanical cantilever bending tests revealed highly different mechanical properties of the two films. In detail, Young's modulus of 830 ± 53 and 459 ± 53 GPa, fracture stress of 12.4 ± 0.8 and 7.8 ± 1.0 GPa, and fracture toughness values of 6.9 ± 0.4 and 3.6 ± 0.3 MPa⋅m½ were evaluated for the micro- and nanocrystalline diamond films, respectively. In summary, this study provides valuable insights into the microstructure-residual stress correlation in micro- and nanocrystalline diamond films, especially illuminating their influence on micromechanical properties.

Effect of substrate roughness on the nucleation and growth behaviour of microwave plasma enhanced CVD diamond films – a case study

The influence of substrate surface roughness on the nucleation and growth of diamond films by chemical vapour deposition (CVD) is investigated. Silicon substrates were grinded with six different grit sizes of abrasive papers with a rotating wheel. Si was also etched by Ar+ ions to produce average surface roughness Ra = 11.29 nm on the mirror polished side (Ra = 1.17 nm). A comparison of the results of the effect of substrate roughness, on the growth behaviour of nanocrystalline diamond (NCD) films, by using both the resonant cavity and the linear antenna CVD systems, are presented here. Scanning electron microscopy (SEM) images and Raman spectroscopy reveal that under both the linear antenna and the resonant cavity microwave plasma CVD conditions, grown films are NCD. The diamond nanocrystals sizes vary from 80 to 180 nm, grown by both the reactors after few hours of deposition, irrespective of the substrate roughness, whereas their quality (defined by the relative percentage ratios of the Raman sp3 peak intensity to the non-sp3 peak intensity) varies from 33% to 45%, depending on the substrate surface roughness. Such nanocrystals grew into plate-like flat 1–6 μm size diamond grains after prolonged hours (64–69 h) of CVD growth. It is found specifically that the roughness created by the argon plasma treatment of the silicon substrate surfaces effectively enhances the nucleation and growth behaviour of the diamond films.

Investigation of the Effect of Hydrogen Content on the Conductivity of Nanocrystalline Diamond Films

Investigation of the Effect of Hydrogen Content on the Conductivity of Nanocrystalline Diamond Films

The effect of microstructure and film composition on the mechanical properties of linear antenna CVD diamond thin films

This study reports the impact of film microstructure and composition on the Young’s modulus and residual stress in nanocrystalline diamond (NCD) thin films (≈250nm thick) grown on silicon substrates using a linear antenna microwave plasma-enhanced chemical vapor deposition (CVD) system. Combining laser acoustic wave spectroscopy to determine the elastic properties with simple wafer curvature measurements, a straightforward method to determine the intrinsic stress in NCD films is presented. Two deposition parameters are varied: (1) the substrate temperature from 400°C to 900°C, and (2) the [P]/[C] ratio from 0ppm to 8090ppm in the H2/CH4/CO2/PH3 diamond CVD plasma. The introduction of PH3 induces a transition in the morphology of the diamond film, shifting from NCD with larger grains to ultra-NCD with a smaller grain size, concurrently resulting in a decrease in Young’s modulus. Results show that the highest Young’s modulus of (1130±50)GPa for the undoped NCD deposited at 800°C is comparable to single crystal diamond, indicating that NCD with excellent mechanical properties is achievable with our process for thin diamond films. Based on the film stress results, we propose the origins of tensile intrinsic stress in the diamond films. In NCD, the tensile intrinsic stress is attributed to larger grain size, while in ultra-NCD films the tensile intrinsic stress is due to grain boundaries and impurities.

Deposition and Characterisation of a Diamond/Ti/Diamond Multilayer Structure

In this work, a diamond/Ti/diamond multilayer structure has been fabricated by successively following thin-film CVD and PVD routes. It has been found that a combined pre-treatment of the silicon base substrate, via argon plasma etching for creating surface roughness and, thereafter, detonation nanodiamond (DND) seeding, helps in the nucleation and growth of well-adherent CVD diamond films with a well-defined Raman signal at 1332 cm−1, showing the crystalline nature of the film. Ti sputtering on such a CVD-grown diamond surface leads to an imprinted bead-like microstructure of the titanium film, generated from the underlying diamond layer. The cross-sectional thickness of the titanium layer can be found to vary by as much as 0.5 µm across the length of the surface, which was caused by a subsequent hydrogen plasma etching process step of the composite film conducted after Ti sputtering. The hydrogen plasma etching of the Ti–diamond composite film was found to be essential for smoothening the uneven as-grown texture of the films, which was developed due to the unequal growth of the microcrystalline diamond columns. Such hydrogen plasma surface treatment helped further the nucleation and growth of a nanocrystalline diamond film as the top layer, which was deposited following a similar CVD route to that used in depositing the bottom diamond layer, albeit with different process parameters. For the latter, a hydrogen gas diluted with PH3 precursor recipe produced smaller nanocrystalline diamond crystals for the top layer. The titanium layer in between the two diamond layers possesses a very-fine-grained microstructure. Transmission electron microscopy (TEM) results show evidence of intermixing between the titanium and diamond layers at their respective interfaces. The thin films in the composite multilayer follow the contour of the plasma-etched silicon substrate and are thus useful in producing continuous protective coatings on 3D objects—a requirement for many engineering applications.

Kelvin Probe Characterization of Nanocrystalline Diamond Films with SiV Centers as Function of Thickness

Optically active color centers in diamonds have been intensively studied due to their potential in photonics, energy harvesting, biosensing, and quantum computing. Silicon vacancy (SiV) center offers an advantage of suitable emission wavelength and narrow zero‐phonon line at room temperature. Measurement of surface potential and photovoltage can provide better understanding of the physics and control of SiV light emission, such as charge states and charging effects. Herein, optoelectronic properties of nanocrystalline diamond films with SiV centers at different layer thicknesses (10–200 nm, controlled by the growth time) under ambient conditions are studied. Time‐dependent measurements are performed in the light–dark–light cycle. Positive photovoltage arises on samples with SiV layer thicknesses below 55 nm on both H‐ and O‐terminated surfaces. Above 55 nm the photovoltage switches to negative. This layer thickness thus represents a halfway boundary between surface‐controllable and bulk SiV centers dominant contribution. A band diagram scheme explaining the photovoltage switching mechanism is provided.

A new route to fabricate high-quality double-side-polished freestanding diamond film

Freestanding diamond films are widely applied, but available fabrication techniques present obvious flaws. A new route to fabricate high-quality double-side-polished freestanding diamond film is proposed in this study. Typical MCD (Micro-crystalline diamond) or NCD (Nano-crystalline diamond) film is firstly deposited on pretreated SiC substrate by HFCVD technique. As-grown surfaces are smoothened by mechanical polishing. Diamond films are then separated from substrates by laser cutting. Finally, bottom surfaces, which are known as nucleated surfaces, are also polished. Compared with the Mo substrate used in the conventional route based on the natural peeling off of the diamond film, the thermal deformation and residual stress of the diamond film on the SiC substrate are smaller, mainly due to the reduced difference between thermal expansion coefficients of the diamond film and substrate. This is beneficial for avoiding the fracture of freestanding films in the cooling stage, and also beneficial for the subsequent polishing. Consequently, the interruptible repeated growth strategy with constant growth rates (1.0–1.2 μm/h for MCD and 1.3–1.5 μm/h for NCD) is feasible, and the long-duration operation of HFCVD device is dispensable. Compared with the Si substrate used in the route based on the Si etching, the SiC substrate is much more reusable in the present route.

Direct femtosecond laser writing of nanochannels by carbon allotrope transformation

Existing approaches for creating durable nanochannels are complex, expensive, and time-consuming. Here, a novel approach is presented in which a femtosecond laser is used to directly write optically accessible nanochannels with arbitrary lengths. Their cross-sections resemble slits and have dimensions that differ by two orders of magnitude. These nanochannels form between an ultra-thin nanocrystalline diamond film and a glass substrate, and their cross-sectional dimensions can be tuned via laser pulse energy. Microscopic investigations show that the laser writing process converts a portion of the sample into a nanostrip, and dedicated experiments indicate that this originates from laser light absorption in the NCD film. The nanostrip is flanked by two nanochannels formed through the delamination of the film. Within the nanostrip, there is non-diamond carbon that plays a vital role in supporting the delaminated portions of the film. This non-diamond carbon is produced when diamond changes into a different carbon allotrope. Besides investigating the mechanism underlying nanostrip formation in depth, film patterning through laser writing is also presented. To demonstrate the applicability of the laser-written nanochannels, a nanofluidic device is fabricated. The nanochannels of the device fill with water via capillary action, as supported by reflectance measurements and simulations. By bypassing complex fabrication techniques and utilizing a durable material system, this work opens doors to manufacturing affordable devices reliant on durable nanochannels, thereby offering promising prospects for future applications.

Study on tribology and cutting performance of boron doped diamond composite coated tool

Microcrystalline diamond (MCD), nanocrystalline diamond (NCD), boron doped microcrystalline diamond (BDMCD), and boron doped microcrystalline-nanocrystalline diamond (BDM-NCD) films were prepared by hot filament chemical vapor deposition (HFCVD) technique. Scanning electron microscopy (SEM), Raman spectroscopy and Rockwell indention testes were used to characterize the microstructure and properties of the coatings, and friction experiments were carried out to study the tribological properties of diamond films and GCr15 steel balls under different lubrication conditions. At the same time, the cutting performance of different diamond coated tools was studied by cutting 30% SiCp/Al composites. The results show that the residual compressive stress of the NCD film is the largest, which is −1.88 GPa. Due to the mitigation effect of the underlying BDMCD film, the BDM-NCD film has the smallest residual stress, which is −0.37 GPa. It can be seen from the friction test that the average coefficient of friction of NCD film is lower than that of MCD film due to the smooth surface. The average friction coefficient of BDM-NCD film is the lowest, which is 0.326. At the same time, the wear resistance of BDM-NCD film is the best, and the value is 0.53×10−6 mm3/Nmm. From the wear morphology, it can be seen that the amount of metastatic material adhered to the wear surface of the diamond films is less under water lubrication than under dry friction condition. Compared with dry friction, the wear scar depth of BDM-NCD film under water lubrication is smaller, showing the better tribological properties. Cutting experiments show that BDM-NCD film has the best wear resistance and the best cutting performance.

Microplasma Illumination Enhancement in N+P‐Co‐Ion‐Implanted Nanocrystalline Diamond Films

N‐ and P‐co‐ion implantation enhances the electrical conductivity of nanocrystalline diamond films to 6.9 s cm−1 and improves the microplasma illumination (MI) characteristics of the films to a low breakdown voltage of 340 V, large plasma current density of 6.3 mA cm−2 (@510 V) with plasma life‐time stability of 10 h. N ions induce nanographitic phases in the films and P ions lower the resistance at the diamond‐to‐Si interface together promoting the conducting channels for effective electron transport, consequently attaining the improved MI properties of the films.

Low temperature growth of nanocrystalline diamond: Insight thermal property

One of the limitations of materials for high-power devices and structural coatings applications is heat dissipation. Diamond is a suitable material for heat distribution due to its high thermal conductivity. Nevertheless, it is usually grown at high temperature (800–1200 °C), which limits its use as a coating for substrates vulnerable to degradation at high temperatures. In this work, it is studied the effect of the distance between the plasma source and substrate on the growth of nanocrystalline diamond layers on silicon substrates at low temperature (<450 °C) by microwave linear antenna plasma enhanced chemical vapour deposition (MW-LA-PECVD) in pulse mode. The nanocrystalline diamond films have been analysed by scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy. Finally, the superficial thermal conductivity of the diamond layers was determined by scanning thermal microscopy-AFM (SThM-AFM).

Understanding of the adhesive strength enhancement mechanisms of bilayer diamond film at nanoscale

In virtue of its outstanding comprehensive performances, the development of multilayer diamond holds significant potential for expanding the range of applications for diamond. However, there still is a major challenge to synthesize multilayer diamond with high adhesive strength. In this work, a bilayer diamond film with enhanced adhesive strength was successfully prepared through depositing nanocrystalline diamond film on plasma-etched polycrystalline diamond substrate using chemical vapor deposition. The adhesion strength was evaluated by the critical load when film failed during scratch test. The nanocrystalline diamond film deposited on plasma-etched substrate exhibited a critical load of 121.6 N, which is higher by 57 % than that deposited on substrate without plasma etching. TEM observation demonstrated that the nanocrystalline diamond has a preference for nucleating at the bottom of pits on the substrate produced by plasma etching, constructing a unique interface with a mortise and tenon joint structure. Moreover, Raman results verified the removal of the graphite phase in substrate surface during plasma etching. Our findings reveal the mechanisms responsible for the enhanced adhesive strength of the developed bilayer diamond film at nanoscale. The improved adhesive strength can be attributed to the combination of the mechanical interlocking interface and the removal of the graphite phase. In addition, the method presented in this work can also be employed on chemical vapor deposition of other multilayer materials beyond diamond, offering the potential to enhance their adhesive strength.

Influence of Surface Roughness on Nanocrystalline Diamond Films Deposited by Distributed Antenna Array Microwave System on TA6V Substrates

In this study, the characteristics of nanocrystalline diamond films synthesized at low surface temperature on Ti-6Al-4V (TA6V) substrates using a distributed antenna array microwave reactor aiming at biomedical applications were investigated. The surface roughness of the TA6V substrates is varied by scratching with emery paper of 1200, 2400, 4000 polishing grit. Nanocrystalline diamond (NCD) coatings with morphology, purity, and microstructure comparable to those obtained on silicon substrates usually employed in the same reactor and growth conditions are successfully achieved whatever the polishing protocol. However, the latter has a significant effect on the roughness parameters and hardness of the NCD films. The use of the finest polishing grit thus permits us to enhance the hardness value, which can be related to the work-hardening phenomenon arising from the polishing process.

Enhanced gas sensing capabilities of diamond layers using Au nanoparticles

The nanocrystalline diamond (NCD) film reveals a unique combination of physical, chemical, and optoelectronic properties, which makes it a promising material for various sensing applications. To improve a gas sensor's response, selectivity, or reproducibility, its surface is often modified with specific terminations, functional groups or (bio)molecules, thin films, etc. In this work, the NCD surface modification was achieved by a) layer morphology variation using two different types of chemical vapor deposition (CVD) systems, b) top surface termination (H-NCD and O-NCD), and c) Au nanoparticles (Au NPs). The properties of each structure are measured, compared and subsequently evaluated. The electrical properties (resistance changes) are measured for two types of active gas (oxidizing gas NO2 and reducing gas NH3) in a temperature range from 22 °C to 125 °C. Neutral synthetic air (80 % nitrogen and 20 % oxygen) was applied for flushing and resetting the sensors. Thin film fabrication, surface analysis (scanning electron microscopy and Raman spectroscopy), and measurement of electrical properties are described. Surface morphology greatly influences gas response because a large active surface area (higher roughness or 3D-like surface) enhances interaction with gas molecules. While the termination of the NCD with hydrogen is essential for the functionality of the gas sensor, the Au NPs further enhanced the dynamic response of the sensor and magnitude.

Investigation of phosphorus-doped nanocrystalline diamond films for photocathode application

This work is devoted to an experimental study of electron photoemission from phosphorus-doped nanocrystalline diamond (NCD) films under the effect of laser radiation with a wavelength of 266 nm and a pulse duration of 15 ns. Homogeneous NCD films with a thickness of 50–1200 nm were grown on conductive silicon substrates using the CVD method. The phosphorus content in the films was changed by varying the phosphine content in the hydrogen–methane gas mixture and the substrate temperature. A relation has been established between the growth conditions, the thickness of NCD films, and the measured quantum efficiency of diamond photocathodes. It was shown that 50 nm thick heavily doped NCD films with H-terminated surface demonstrated the maximum quantum efficiency of 3⋅10−5.