Free-standing Diamond Films Research Articles

The 4-inch free-standing diamond film prepared by MPCVD at 10kW

The plasma emission spectrum and polycrystalline diamond growth within 4 inches was studied by using a microwave plasma chemical vapor deposition (MPCVD) equipment with a microwave frequency of 2.45 GHz and a power of 10 kW, a 4-inch free-standing diamond thick film was prepared, and the growth uniformity, crystal quality at different positions, room temperature thermal conductivity, ultraviolet-visible-near infrared spectral transmittance and dielectric properties of diamond film were characterized. The results show that the freestanding diamond films are prepared with good uniformity and crystallinity at 10 kW power with high concentration and chemical activity of carbon and hydrogen active particles within the plasma diameter of about 100 mm. The thermal conductivity at room temperature reaches more than 1894 W/m·K, the infrared transmittance reaches more than 70%, the dielectric constant is 5.5, and the dielectric loss is 5.9 × 10−5. This study provides a theoretical possibility for the preparation of large-area high-quality diamond films in low-power MPCVD equipment.

Structural evolution and self-destructive behavior of Mo/Ti transition layers during free-standing diamond-film preparation

Graphite-based metal transition-layer substrates hold great potential for preparing large-area diamond films and achieving crack-free release. In this study, a five-inch crack-free free-standing diamond film was prepared on a graphite substrate with a sacrificial Mo/Ti double-layer transition layer using direct-current arc plasma jet chemical vapor deposition (DC jet CVD). The interfacial bonding and diffusion behavior between the diamond and the transition layers were investigated. The effect of abundant metal particles and elemental diffusion between layers on film adhesion and stress release are discussed and analyzed. The surface of Ti layer prepared by the multi-arc ion-plating contains numerous metal particles that provide a template for the fine-crystalline Mo layer. Metal particles are encapsulated and embedded in the diamond and act as anchors that ensure the stability of the substrate–diamond bond during high-temperature deposition. After deposition, those metal particles act as weak points and play crucial stress-release roles. Mutual elemental diffusion occurs between the Ti, Mo, and diamond layers. Ti penetrates the Mo layer to form a continuous 140-nm-thick thin layer during the initial deposition period, facilitating diamond nucleation. Carbon from the plasma and diamond diffuses into the metal transition layer. Thermal stress generated between the diamond and the graphite substrate is released by destroying the metal transition layer with high thermal expansion coefficient and low elastic modulus during cooling from the deposition temperature. Cracks rapidly propagate along the interface between destroyable transition layers and graphite substrate, which results in the liftoff of a fully crack-free diamond film.

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.

Physical bending of heteroepitaxial diamond grown on an Ir/MgO substrate

With increasing demand for large-diameter heteroepitaxial diamond, understanding the origin of the thermally induced bending of heteroepitaxial diamond, which is caused by the difference in the coefficient of thermal expansion between diamond and the substrate during cooling after the growth process, has become an important task. We investigated the physical bending of heteroepitaxial diamond grown on Ir/MgO substrates, and the state of the grown diamond as a result of bending was summarized as a function the diamond film thickness. When the diamond thickness (dd) was less than ~2.5 μm, partial delamination of the diamond/Ir from the MgO substrates was observed. When dd was approximately 2.5–200 μm, diamond/Ir was delaminated and broken into small pieces. The small pieces of diamond/Ir films were highly convexified. When dd was greater than ~200 μm, the diamond/Ir films spontaneously separated from the MgO substrates without any breakage, resulting in freestanding diamond films with the same dimension as the MgO substrates. The critical thickness to obtain freestanding heteroepitaxial diamond was found to be ~200 μm. When dd was in the range 6–550 μm, the separated MgO substrates were convexly bent, meaning that the MgO was plastically deformed. Theoretical calculations of the substrate curvature after epitaxy were performed for the elastic bending of the diamond/Ir/substrate heteroepitaxial system. On the basis of the calculations, the maximum curvature by physical bending was expected to reach −22,800 km−1 at ~150 μm thickness and to change to reduction with increasing diamond thickness. The curvature of breakage-free freestanding diamond samples with 243–550 μm thickness ranged from −7200 to −4400 km−1, showing moderate matching with the theoretical calculations of physical bending for the diamond/Ir/MgO multilayered system. Freestanding diamond films and MgO continued to affect each other at least until the structure experienced plastic deformation.

Structural transformation of C+ implanted diamond and lift-off process

Lift-off is a promising method to prepare thin, high-finish, and freestanding large-area single crystal and polycrystalline diamonds. However, accurate control of the damage layer characteristic to achieve both stress release and lift-off process is difficult. In this work, high-energy C + was implanted in single crystal diamond and polycrystalline diamond to create subsurface damage. The defect behavior and structural transformation of the ion implantation and annealing process were investigated. The homoepitaxial growth was subsequently carried out and the epitaxial layer was lifted off from the substrate by selectively etching the damaged layer. The results show that the cap layer and the substrate (below the damage layer) keep the sp 3 carbon structure before and after annealing, which is confirmed by the atomic-scale electron energy loss spectroscopy (EELS). The high-resolution transmission electron microscopy (HRTEM) images show that after annealing at 1000 °C for 1 h, the damaged layer was transformed from amorphous carbon to a mixture of graphite and amorphous carbon, providing the damaged layer that could be removed by electrochemical solution. Meanwhile, the distorted diamond area was changed to a sharp interface, which ensures the low roughness of the substrate surface after the lift-off process. After etching for 30 h, a freestanding polycrystalline diamond film with a thickness of 100 μm and a surface roughness of 1.68 nm was obtained. The roughness of the lift-off substrate surface is 1.10 nm, indicating the epitaxial growth can be repeated directly without polishing. • The defect behavior and structural transformation of the high-energy C + implanted diamonds before and after annealing process were investigated. • The stress state of the diamonds before and after ion implantation and after lift-off process were evaluated. • A freestanding polycrystalline diamond film with a thickness of 100 μm and a surface roughness of 1.68 nm was successful obtained.

Preparation of nanowires on free-standing boron-doped diamond films for high performance micro-capacitors

A free-standing nanostructured boron-doped diamond (BDD) electrode was fabricated by reactive ion etching (RIE) with oxygen plasma. The free-standing architecture and nanowires endowed the BDD electrode with high specific capacitance and good stability. As a result, the BDD nanowires electrode etched for 60 min (BDD-NWs-60) featured the best capacitive performance with a specific capacitance of 24.9 mF cm−2 at a scan rate of 10 mV s−1 and capacitance retention of 90% after 6,000 cycles at a constant current density of 1.0 mA cm−2. Furthermore, a symmetric capacitor device was assembled using two BDD-NWs-60 electrodes, the device exhibited the highest energy and power densities of 0.41 mJ cm−2 and 0.79 mW cm−2, respectively. For a practical application, three BDD-NWs-60//BDD-NWs-60 symmetrical capacitors in series could light a red LED and rotate a homemade fan. Based on the above satisfactory behavior of the BDD electrode, the influence of the RIE process on the capacitive performance of the BDD electrode was proposed.

Fabrication of rutile TiO2 nanoarrays/free-standing diamond composite film and its field emission properties

Highly aligned TiO2 nanoarrays (TNAs) are fabricated on free-standing diamond (FSD) films and Si wafers (for comparison). Different anodizing times (e.g., 5, 15, 25, 45, and 60 min) are used to investigate the morphologies of TNAs and their effect on the electron field emission (EFE) properties. With the increase of the anodizing time, the features of TNAs grown on the FSD film undergo the transition from nanorods to nanopores and then to nanotubes. The XRD, Raman, and TEM results reveal that TNAs synthesized on the FSD film are preferentially oriented rutile phase, while those synthesized on the Si wafers show the anatase structure. Rutile TiO2 has a relatively low work function and is favorable to EFE performance. Importantly, the nanotubes on the FSD substrate with a 45 min anodizing time grow perpendicularly to each crystal face of the diamond grains and exhibit a superior EFE behavior, which can be turned on at a low field of 0.7 V/μm and attain a high current density of 0.6 mA/cm2 at an applied field of 1.6 V/μm. The enhanced EFE performance of TNAs/FSD composite film is ascribed to the synergetic effects among the electron acceleration layer played by the FSD film, more electron emission sites provided by the nanotubes with large aspect ratios, and lower work function of rutile TiO2. Moreover, the heterogeneity of the FSD substrate and TNAs also can decrease the interface barrier to make electrons transmit effectively.

Lapping performance of diamond cone array tool prepared by double bias assisted HFCVD

In this paper, the free-standing diamond (FSD) and microcrystalline diamond (MCD) film were etched by double-bias assisted hot filament chemical vapor deposition (HFCVD) to prepare two kinds of diamond cone array tools (DCATs) for precision processing. It is the first time to use free-standing diamond cone array tool (FSD-CAT) and microcrystalline diamond cone array tool (MCD-CAT) to perform the lapping processing experiment for monocrystalline silicon and to carry out the performance evaluation of DCAT. The surface roughness of the silicon lapped by FSD-CAT is 28.9 nm, and the surface roughness of the silicon lapped by MCD-CAT is 18 nm. By using the combination of both DCATs to lapped silicon, a smooth surface with a surface roughness of 5 nm is obtained. By comparing the surface roughness of monocrystalline silicon which lapped by diamond tools with and without etching, because of the smaller cross-section of the cones cut into the surface of monocrystalline silicon, DCAT can effectively reduce the Surface roughness of the silicon.

The 7-in. freestanding diamond thermal conductive film fabricated by DC arc Plasma Jet CVD with multi-stage magnetic fields

Diamond has attracted increasing attention because it has high thermal conductive. Here, a freestanding diamond thermal conductive film with a diameter of about 7 in. and an average thickness of 1.54 mm was prepared by DC arc plasma jet CVD with multi-stage magnetic fields. Specifically, the thickness uniformity was ensured by adding the magnetic field during the deposition of the freestanding diamond film. The morphology, structure and thermal conductivity of the diamond film were studied. The results showed that the expansion area of the DC arc plasma approached the surface of the substrate increasing duo to add the bottom magnetic field. Structural characterization revealed that the grain of the diamond film in the arc main were coarse, affected by the shape of DC arc plasma, while in the arc center and edge were fine. The main orientation of polycrystalline structure of the diamond film was the crystalline plane of (220). The Raman peaks of diamond film in different areas moved to a higher wavelength, indicating the existence of compressive stress inside the diamond film. The coarse columnar grain and a few grain boundaries were in the arc main. Therefore, the average value of the thermal conductivity (k⊥) of diamond film measured in the arc main was 1728.9 ± 4.9 W/(m·k). Meanwhile, the thermal conductivity of the arc center and edge area was lower than that of the arc main.

Nearly degenerate ground state of phosphorus donor in diamond

We investigate phosphorus (P) donors in P-doped diamond epitaxial films by means of electron paramagnetic resonance spectroscopy (EPR). In a diamond thin film on IIa diamond substrate, we could observe an EPR signal of the NIMS1 center, which has been assigned to a neutral P donor with a ${D}_{2d}$ symmetry. In contrast, we could not observe this center in another free-standing diamond film with the same P concentration $([\mathrm{P}]\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3})$. This striking contrast can be reasonably accounted for in a model where the P donor in diamond has a nearly degenerate ground state due to ${t}_{2}$ and $e$, in contrast to P donors in group-IV semiconductors like Si and Ge, where ${a}_{1}$ singlet ground state is present. We observed a strong uniaxial strain only in the former diamond thin film, which was confirmed by Raman microscopy and preferential orientations of NIMS1 EPR centers. This strain splits the nearly degenerate ground state of the P donor, resulting in the observation of the NIMS1 center. We point out that the electronic system of the substitutional P donor in diamond is similar to that of interstitial Li donor in Si.

Influence of nanosecond laser processed surface textures on the tribological characteristics of diamond films sliding against zirconia bioceramic

CVD Diamond films have shown much potential in bioimplantable applications. Aiming at enhancing the tribological properties of CVD diamond and bioceramic tribopair, two types of surface texture, namely groove surface and square surface, are fabricated on free-standing diamond films by nanosecond (ns) laser processing technology. Reciprocating friction and wear experiments against zirconia bioceramic (ZBC) balls in simulated body fluid (SBF) lubrication are performed to evaluate the influence of as-processed surface textures on the tribological behavior of diamond films. It is demonstrated that the counterpart balls sliding against surface-textured diamond films present obviously lower wear rate than that sliding against the untreated diamond films. This can be ascribed to the reduction of adhesive wear and abrasive wear by the surface texture. On the other hand, the heat-affected zone (HAZ), which is caused by the ns laser process, can greatly degrade the friction behavior and wear resistance of as-fabricated surface textures. In this work, after the HAZ is worn out, the groove surface exhibits lower friction coefficient than the untreated surface and square surface. And yet owing to more severe in-depth HAZ, the square surface shows higher friction coefficient and much more abrasion loss than the groove surface.

Fracture strength and toughness of chemical-vapor-deposited polycrystalline diamond films

The strength of diamond films prepared by chemical vapor deposition (CVD) is usually much lower than that of natural-type IIa single-crystal diamonds. In this work, the fracture strength of free-standing diamond films deposited by direct current arc plasma jet CVD has been examined by conducting three-point bending experiments. The results obtained for both the polished and as-grown samples were in good agreement with a well-known Hall–Petch equation describing the relationship between the fracture strength and grain size, indicating that grain refinement represented an effective way of improving the mechanical properties of CVD diamond films. Furthermore, the diversification of the crystalline texture of the films achieved by polishing apparently increased their fracture strength, which was inversely proportional to the film thickness. A theoretical method for estimating the fracture strength of free-standing CVD diamond films by approximating their intrinsic strength was proposed, whereas their fracture toughness was determined by conducting simplified four-point bending tests at room temperature, 25 °C, using a single-edge pre-cracked beam method.

Quantitative study on graphitization and optical absorption of CVD diamond films after rapid heating treatment

Slightly graphitization might be incurred on CVD diamond films when suffering instantaneous high temperature thermal shock. In this paper, phase transitions of CVD free-standing diamond films have been studied experimentally at the temperature range of 1500–1800 °C through a plasma rapid heating process. Raman spectra and X-ray photoelectron spectra (XPS) are compared and discussed for the purpose of measuring the percentage of graphitic carbon component in CVD diamond films after heat treatment at different temperatures. An obvious optical degradation took place in both visible and infrared bands with the increased treating temperature. The result indicates that graphitization is dominated along grain boundaries rather than in internal grains. In addition, the nitrogen related absorption peaks were not covered by the enhanced optical absorption background, which was caused by the opaque graphitic carbon formed at high temperature. Nitrogen concentrations have also been evaluated for these graphitized diamond films and there is no evidence to show the movement or aggregation of nitrogen atoms.

Self-separation of freestanding diamond films using graphite interlayers precipitated from C-dissolved Ni substrates

Freestanding diamond films were fabricated by a new self-separation method. Thick poly-crystalline diamond films were grown on poly-crystalline Ni substrates by microwave plasma-enhanced chemical vapor deposition after the substrates were saturated with carbon via a saturation process using a carbon solid solution. This saturation process suppressed the erosion of diamond nuclei on the Ni substrates. During the cooling process after diamond growth, the carbon atoms dissolved in the Ni substrates became supersaturated and precipitated as graphite interlayers at the interfaces between the diamond films and Ni substrates. The graphite interlayers caused the thick diamond films to spontaneously separate from the Ni substrates without cracking, allowing the Ni substrates to be reused. This self-separation method provides a low-cost way to fabricate freestanding diamond films.

Mechanism of graphitization and optical degradation of CVD diamond films by rapid heating treatment

Polycrystalline CVD free-standing diamond films have been heat treated through a rapid heating process using a direct current (DC) arc plasma jet system within the temperature range of 1500–2000°C. The optical transmission of the diamond film decreases when the treatment temperature exceeds 1500°C. When the diamond film was annealed to 1800°C for just 1min, the infrared (IR) transmittance at λ=10.6μm of the film was significantly decreased from 60.5% to 4.0%, due to the transformation from diamond to graphite. Further increase of the temperature to 2000°C leads to complete loss of optical transmittance in only 30s, due to the detrimental occurrence of graphitization. XRD patterns indicate that graphite ultimately grows preferentially along the (0002) plane by forming disordered or turbostratic graphite as intermediate product. Raman spectra and SEM images verify that at 1500–1850°C graphitic carbon transforms from the diamond surface in the form of protruding submicron crystallites, which were still containing a number of sp3 CC bonds. In addition to graphitization on the external surface, the diamond-graphite phase transition also occurs along the grain boundaries and the calculated activation energy of the grain boundary graphitization process is 276kJ·mol−1 for the CVD diamond.

Structural evolution of Ti destroyable interlayer in large-size diamond film deposition by DC arc plasma jet

The addition of titanium (Ti) interlayer was verified to reduce the residual stress of diamond films by self-fracturing and facilitate the harvest of a crack-free free-standing diamond film prepared by direct current (DC) arc plasma jet. In this study, the evolution of the Ti interlayer between large-area diamond film and substrate was studied and modeled in detail. The evolution of the interlayer was found to be relevant to the distribution of the DC arc plasma, which can be divided into three areas (arc center, arc main, and arc edge). The formation rate of titanium carbide (TiC) in the arc main was faster than in the other two areas and resulted in the preferred generation of crack in the diamond film in the arc main during cooling. Sandwich structures were formed along with the growth of TiC until the complete transformation of the Ti interlayer. The interlayer released stress via self-fracture. Avoiding uneven fragile regions that formed locally in the interlayer and achieving cooperatively released stress are crucial for the preparation of large crack-free diamond films.

Optical, electrical and microstructural properties of nanocomposite Ag/diamond by Ag ion implantation and subsequent annealing

An optical-grade, free-standing diamond (FSD) film is prepared by home-made microwave plasma chemical vapor deposition reactor at a condition of input power 8 kW and gas pressure18 kPa. Then, it is implanted with 80 keV Ag ions up to a fluence of 5.0 × 1016 ions/cm2. After implantation, a subsequent annealing in Ar ambient at 500 °C is carried out to fabricate Ag nanoparticles (NPs). Grazing incidence x-ray diffraction results clearly showed that Ag NPs with a (111) orientation are effectively formed after 500 °C annealing. Results from field emission scanning electron microscope and atomic force microscope observations indicated that Ag NPs could aggregate and grow on the surface of the diamond film during the annealing process. The Hall effects results showed that Ag-implanted FSD film exhibited an enhanced carrier mobility of 34.7 cm2 V−1 S−1 after 500 °C annealing. Moreover, Raman spectroscopy results show that the damages of the Ag-implanted FSD film are essentially a little and the crystallinity is almost recovered by annealing, it is implied that the enhancement in carrier mobility could be attributed to the conductive paths provided by Ag NPs.

Wafer bowing control of free-standing heteroepitaxial diamond (100) films grown on Ir(100) substrates via patterned nucleation growth

The potential of patterned nucleation growth (PNG) technique to control the wafer bowing of free-standing heteroepitaxial diamond films was investigated. The heteroepitaxial diamond (100) films were grown on an Ir(100) substrate via PNG technique with different patterns of nucleation regions (NRs), which were dot-arrays with 8 or 13μm pitch aligned to <100> or <110> direction of the Ir(100) substrate. The wafer bows and the local stress distributions of the free-standing films were measured using a confocal micro-Raman spectrometer. For each NR pattern, the stress evolutions within the early stage of diamond growth were also studied together with a scanning electron microscopic observation of the coalescing diamond particles. These investigations revealed that the NR pattern, in terms of pitch and direction of dot-array, strongly affects the compressive stress on the nucleation side of the diamond film and dominantly contributes to the elastic deformation of the free-standing film. This indicates that the PNG technique with an appropriate NR pattern is a promising solution to fabricate free-standing heteroepitaxial diamond films with extremely small bows.

Erosion difference of growth and nucleation sides of free-standing diamond films prepared by DC arc plasma jet CVD

Solid particle erosion behaviors of the nucleation and growth surface of free-standing diamond films as a function of erosion time were studied by a laboratory designed blast type erosion apparatus. The surface roughness and the transmission loss were used to evaluate the extent of erosion performance. Results showed that the erosion damage features generated by impact appeared after erosion for 5s and 30s for nucleation and growth surface of diamond films respectively, and primarily occurred at grain boundaries in the form of micro-cracks. Then the local chipping of small sections on the border of cracks was followed resulting in material removal. The threshold erosion time, to maintain the transmission at 10.6μm still above 60%, was 60s and 300s for nucleation and growth surface of diamond films. The growth surface of diamond films exhibited about 5 times higher erosion resistance for ending up with relatively lower surface roughness and transmission loss compared with the nucleation surface, which was in accordance with the ratio of grain sizes.

Effect of methane concentration in hydrogen plasma on hydrogen impurity incorporation in thick large-grained polycrystalline diamond films

We investigate the impact of methane concentration in hydrogen plasma on the growth of large-grained polycrystalline diamond (PCD) films and its hydrogen impurity incorporation. The diamond samples were produced using high CH4 concentration in H2 plasma and high power up to 4350W and high pressure (either 105 or 110Torr) in a microwave plasma chemical vapor deposition (MPCVD) system. The thickness of the free-standing diamond films varies from 165µm to 430µm. Scanning electron microscopy (SEM), micro-Raman spectroscopy and Fourier-transform infrared (FTIR) spectroscopy were used to characterize the morphology, crystalline and optical quality of the diamond samples, and bonded hydrogen impurity in the diamond films, respectively. Under the conditions employed here, when methane concentration in the gas phase increases from 3.75% to 7.5%, the growth rate of the PCD films rises from around 3.0µm/h up to 8.5µm/h, and the optical active bonded hydrogen impurity content also increases more than one times, especially the two CVD diamond specific H related infrared absorption peaks at 2818 and 2828cm−1 rise strongly; while the crystalline and optical quality of the MCD films decreases significantly, namely structural defects and non-diamond carbon phase content also increases a lot with increasing of methane concentration. Based on the results, the relationship between methane concentration and diamond growth rate and hydrogen impurity incorporation including the form of bonded infrared active hydrogen impurity in CVD diamonds was analyzed and discussed. The effect of substrate temperature on diamond growth was also briefly discussed. The experimental findings indicate that bonded hydrogen impurity in CVD diamond films mainly comes from methane rather than hydrogen in the gas source, and thus can provide experimental evidence for the theoretical study of the standard methyl species dominated growth mechanism of CVD diamonds grown with methane/hydrogen mixtures.