Nanocrystalline Diamond Thin Films Research Articles

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.

Strain-Relaxed Nanocrystalline Diamond Thin Films with Silicon Vacancy Centers Using Femtosecond Laser Irradiation for Photonic Applications

Nanocrystalline diamond thin (thickness of 100–500 nm) films with ensembles of negatively charged light-emitting silicon vacancy (SiV) centers might find an application in the field of photonics or biosensing due to their scalable and low-price fabrication process in comparison with monocrystalline diamond. The potential application of diamond thin films is, however, limited due to the presence of radiative and nonradiative defects. The former type is a source of an unwanted background in the photoluminescence spectra, the latter lowers the quantum efficiency of light emission, and both types of defects cause the reabsorption of SiV emission during its propagation within the layer. Here we show that femtosecond (fs) laser pulses focused via a microscope objective on a nanocrystalline diamond thin film can be employed to reduce background photoluminescence intensity by a factor of 5 with respect to the luminescence of SiV centers. Raman spectra show that this decrease is mainly caused by laser ablation of the sp2-related carbon phase present in-between the grains. The reduction of the sp2 carbon phase also leads to a local decrease in the optical absorption followed by an almost twofold increase in the SiV emission peak intensity. Moreover, the fs-irradiation also entails the release of strain inside such a layer as manifested by the observed shift of the Raman diamond peak toward the value measured in the monocrystalline diamond and by a blue-shift of the zero-phonon-line peak position of the SiV centers. Such a finding might enable the improvement of the optical quality of the nanocrystalline diamond-based photonic nanostructures or even might be employed over a larger area by defocusing the irradiation laser in order to create large-scale strain-relaxed nanocrystalline diamond thin films.

Thermionic performance of nanocrystalline diamond/silicon structures under concentrated solar radiation

Thin films of nanocrystalline diamond with thickness around 100 nm were deposited on highly doped p-type silicon substrates to evaluate the electron emission performance of these structures under illumination of concentrated sunlight in the temperature range 500–700 °C. By comparing the emitted current densities measured using a pure thermal source and a concentrated light source simulating the solar radiation spectrum (Xe lamp), an increase up to about 80 times at 600 °C was found using the concentrated light source, thus demonstrating the boost on the thermionic emission thanks to the sunlight absorption. At temperatures higher than 600 °C the action of the photon-enhanced thermionic emission (PETE) mechanism begins to vanish, starting the regime of pure thermionic emission. The opening of the quasi-Fermi levels reducing the barrier height down to 0.33 eV for electron emission is considered to explain the overall behavior of the diamond-silicon system in the PETE regime.

Detection of globular and fibrillar proteins by quartz crystal microbalance sensor coated with a functionalized diamond thin film

This study presents a sensor based on quartz crystal microbalance (QCM) coated with nanocrystalline diamond (NCD) thin film, functionalization method and novel application of such sensor. Diamond-coated QCMs (DQCMs) were superficially terminated by hydrogen and oxygen (H-NCD and O-NCD) to control its surface dipole/potential. Two protein solutions were tested: bovine serum albumin (BSA) and fibronectin (FN). We performed reference measurements of serial resonant frequency (SRF) of clean QCMs loaded with protein and compared them with SRF shifts DQCMs loaded with proteins. In order to investigate the influence of the deposited NCD thin film on QCM measuring capabilities, additional FEM analysis was performed. The simulation results showed that QCM sensors maintain the sensing capabilities with a rigid thin film of NCD on its surface. The shift of SRF was demonstrably caused by the weight of protein adhered to the diamond film's surface. We compared masses estimated from the Sauerbrey equation to characterize the adhesive properties of the studied proteins. Comparing bare QCM and DQCM, we discovered diamonds enhance the sensing performance for proteins. At the same time, it saturates quickly with phosphate buffer saline used as a diluent solution for proteins. Results showed a significant increase in protein adhesion confirmed by the increase of the mass for both oxygen and hydrogen-terminated DQCMs. Moreover, a different time-dependent behaviour (i.e. different adsorption rate, degrees of physisorption and/or preference of the diamond surface functionalization) of the O-NCD and H-NCD QCMs was observed for BSA and FN proteins. In this meaning, we propose aschematic model which describes the detection principle of BSA and FN proteins on H- and O-terminated DQCM sensors. Finally, a simple proof of concept for using the functionalized diamond-coated sensors with current stimulation and EQCM (Electrochemical Quartz Crystal Microbalance) is also proposed.

Simultaneous Evaluation of Heat Capacity and In-plane Thermal Conductivity of Nanocrystalline Diamond Thin Films

ABSTRACT As wide bandgap electronic devices have continued to advance in both size reduction and power handling capabilities, heat dissipation has become a significant concern. To mitigate this, chemical vapor deposited (CVD) diamond has been demonstrated as an effective solution for thermal management of these devices by directly growing onto the transistor substrate. A key aspect of power and radio frequency (RF) electronic devices involves transient switching behavior, which highlights the importance of understanding the temperature dependence of a material’s heat capacity and thermal conductivity when modeling and predicting a devices electro-thermal response. Due to the complicated microstructure near the interface between CVD diamond and electronic material, it is difficult to measure both properties simultaneously. In this work, we use time-domain thermoreflectance (TDTR) to simultaneously measure the in-plane thermal conductivity and heat capacity of a 1-µm-thick CVD diamond film via multi-frequency analysis. We obtain temperature dependent thermal properties by using the pump beam to heat the sample according to increasing power. This mitigates the need for a more complicated setup using a thermal stage but has limited upper temperature boundaries based on the sample geometry and thermal properties. The results show that the in-plane thermal conductivity varied slightly with an average of 103 W/m-K over a temperature range of 302–327 K, while the specific heat capacity has a strong temperature dependence over the same range and compares well with heat capacity data of natural diamond in literature.

Effect of pH variation and annealing on covalently assembled nanodiamond films

This work examines the effect of using low-ionic strength zwitterionic buffer and post-deposition thermal annealing on the microstructure and thermal conductivity of continuous nanocrystalline diamond films that are deposited via the directed covalent assembly of detonation nanodiamonds. Variation of the zwitterionic buffer pH was found to tune the surface coverage, film thickness, thermal conductivity, and film morphology as quantified by apparent porosity and pore size distribution. The sequential annealing of these films up to 400 °C, showed the anticipated severance of amide bonds holding the nanodiamonds together, followed by the detrimental aggregation of the nanodiamonds to segregated islands, losing film continuity and increasing apparent porosity. The change in thermal conductivity with annealing was subjective to buffer pH, suggesting that the morphology changes associated with annealing may have a complex relationship with thermal conductivity. The thermal conductivity data were further analyzed using a phonon hopping model to quantify the degree of phonon movement across boundaries. Overall, the results demonstrate a way to achieving porous, low-cost nanocrystalline diamond thin films with tunable film morphology and thermal conductivity.

Suspended nanocrystalline diamond ridge waveguides designed for the mid-infrared

A comprehensive study and design of air-clad suspended ridge diamond waveguides for operation across the 2.5–16 µm spectral range is presented, specifically targeting nanocrystalline diamond (NCD) thin films directly grown on silicon substrates. Three film thicknesses of 520, 1000 and 2000 nm are considered, to cover overlapping sub-bands of 2.5–5, 4–9 and 8–16 µm, respectively. Within each sub-band, the waveguide dimensions for single mode quasi-TE operation are found and the waveguide material losses and bending losses are estimated at each design point. In addition, in each case the minimum required undercut depth and etch hole placement for optical isolation of the waveguide mode from the silicon substrate is also quantified. We also estimate the losses associated with scattering from surface roughness, which is an unavoidable byproduct of the NCD thin film growth process. Our results indicate that despite the relatively low film thickness-to-wavelength ratio, mechanically stable waveguides with good optical confinement and low material and bending losses can be realised to cover the full 2.5–16 µm range. In addition, scattering loss estimations predict a drastic drop in roughness-induced scattering losses above 6 µm, even for relatively rough films. In addition to highlighting the utility of suspended NCD as a versatile platform for mid-infrared integrated photonics, the approaches and results presented here can be used to inform the design of suspended air-clad waveguides in other material platforms.

On the structure of (111) twist grain boundaries in diamond: atomistic simulations with Tersoff-type interatomic potentials

In nanocrystalline diamond thin films, the high grain boundary density and the associated grain boundary properties may dominate the overall material behavior. In the present work, we systematically investigate (111) twist grain boundaries in diamond carbon by using atomistic simulations with Tersoff-type potential. Our work reveals the relation between atomic scale grain boundary structure and bicrystallography, bond deformation, point defect population. A comparative study with diamond silicon highlights general trends in the grain boundary energy as a function of the misorientation. We predict a transition between glide-plane grain boundary position and shuffle-plane grain boundary with increasing misorientation in diamond cubic materials with a (111) twist grain boundary.

Steady-state methods for measuring in-plane thermal conductivity of thin films for heat spreading applications.

The development of high thermal conductivity thin film materials for the thermal management of electronics requires accurate and precise methods for characterizing heat spreading capability, namely, in-plane thermal conductivity. However, due to the complex nature of thin film thermal property measurements, resolving the in-plane thermal conductivity of high thermal conductivity anisotropic thin films with high accuracy is particularly challenging. Capable transient techniques exist; however, they usually measure thermal diffusivity and require heat capacity and density to deduce thermal conductivity. Here, we present an explicit uncertainty analysis framework for accurately resolving in-plane thermal conductivity via two independent steady-state thermometry techniques: particle-assisted Raman thermometry and electrical resistance thermometry. Additionally, we establish error-based criteria to determine the limiting experimental conditions that permit the simplifying assumption of one-dimensional thermal conduction to further reduce thermal analysis. We demonstrate the accuracy and precision (<5% uncertainty) of both steady-state techniques through in-plane thermal conductivity measurements of anisotropic nanocrystalline diamond thin films.

Perspectives from research on metal-semiconductor contacts: Examples from Ga2O3, SiC, (nano)diamond, and SnS

As part of a Special Issue in Honor of 30 years of the American Vacuum Society’s Nellie Yeoh Whetten Award, this Invited Perspective discusses results and trends from the authors’ and other published research on metal contacts to β-Ga2O3, (4H and 6H)-SiC, nanocrystalline diamond (NCD), and nanocrystalline thin films and single-crystalline nanoribbons of α-SnS. The paper is not a comprehensive review of research on contacts to each of these semiconductors; it is instead a perspective that focuses on Schottky barrier height (Φb) measurements and factors that affect Φb, such as metal work function (Φm) and crystallographic surface plane. Metals and the associated processing conditions that form ohmic or Schottky contacts to each of these semiconductors are also described. Estimates of the index of interface behavior, S, which measures the dependence of Φb on Φm, show large variations both among different semiconductors (e.g., S ∼ 0.3 for NCD and S ∼ 1.0 for SnS nanoribbons) and between different surface planes of the same semiconductor [e.g., (2¯01) vs (100) Ga2O3]. The results indicate that Φb is strongly affected by the nature of the semiconductor surface and near-surface region and suggest that the sharp distinction between covalent and ionic semiconductors as described in seminal theories can be adjustable.

Fabrication of electrolyte-gate nanocrystalline diamond- based field effect transistor (NCD-EGFET) for HIV-1 Tat protein detection

In this paper, we reported on the fabrication process of electrolyte-gate field effect transistor using nanocrystalline diamond as a sensing transducer. The fabrication procedure was begin with the growth of nanocrystalline diamond thin film on silicon/silicon dioxide (Si/SiO2) substrate using microwave plasma-enhanced chemical vapour deposition (CVD). Then the photolithography process was performed in order to design and pattern the field effect transistor device with the active gate channel of 60 µm length and 20 µm width. Each device consists of three active gate channel which connecting to three different pairs of source and drain contact. The surface morphology of fabricated NCD-EGFET was characterized using Scanning Electron Microscope to clarify the active gate channel of the device and the grain size of nanocrystaline diamond. The current-voltage (I-V) measurement of the device were carried out to study the electrical behaviour for HIV-1 Tat protein detection via RNA aptamer as sensing probe.

Low Temperature Growth of CVD Nanocrystalline Diamond Thin Film by Designed SWP-CVD with Various Working Pressures

Nanocrystalline diamond (NCD) is exceptionally useful for a variety of applications and is of significant interest to researchers in technological and scientific fields due to its excellent mechanical and chemical properties, such as its hardness and high thermal conductivity. We have modified a microwave plasma chemical vapor deposition (MPCVD; Astex Inc.) system with a slot antenna designed for surface wave plasma (SWP) and successfully fabricated high quality thin NCD film. This SWP-CVD process fabricates high quality diamond film at 300 °C, while a normal MPCVD process requires the temperature of the substrate to be above 800 °C. We studied the fabricated NCD samples in detail, measuring their surface morphology by field emission scanning electron microscopy (FESEM); their structural-chemical properties by Raman spectroscopy; and their surface roughness by atomic force microscopy (AFM).

Superconducting Diamond on Silicon Nitride for Device Applications

Chemical vapour deposition (CVD) grown nanocrystalline diamond is an attractive material for the fabrication of devices. For some device architectures, optimisation of its growth on silicon nitride is essential. Here, the effects of three pre-growth surface treatments, often employed as cleaning methods, were investigated. Such treatments provide control over the surface charge of the silicon nitride substrate through modification of the surface functionality, allowing for the optimisation of electrostatic diamond seeding densities. Zeta potential measurements and X-ray photoelectron spectroscopy (XPS) were used to analyse the silicon nitride surface following each treatment. Exposing silicon nitride to an oxygen plasma offered optimal surface conditions for the electrostatic self-assembly of a hydrogen-terminated diamond nanoparticle monolayer. The subsequent growth of boron-doped nanocrystalline diamond thin films on modified silicon nitride, under CVD conditions, produced coalesced films for oxygen plasma and solvent treatments, whilst pin-holing of the diamond film was observed following RCA-1 treatment. The sharpest superconducting transition was observed for diamond grown on oxygen plasma treated silicon nitride, demonstrating it to be of the least structural disorder. Modifications to the substrate surface optimise the seeding and growth processes for the fabrication of diamond on silicon nitride devices.

Antibacterial mechanisms of nanocrystalline diamond film and graphene sheet

Abstract In this study, we investigated the antibacterial activities of a nanocrystalline diamond (NCD) thin film and graphene sheet. In addition, we functionalized the surfaces of the NCD film and graphene sheet with fluorine by plasma treatments to increase their antibacterial activities. The antibacterial activities of the NCD film and graphene sheet were confirmed by both colony forming unit (CFU) counting and adenosine triphosphate (ATP) bioluminescence assay based on the drop-test method. The CFU counting showed that the ratios of surviving bacteria on the NCD film and graphene sheet were 27.5% and 13.9%, respectively. The ratios of surviving bacteria on the fluorinated NCD (F-NCD) film and fluorinated graphene (F-graphene) sheet were decreased to 15.2% and 9.3%, respectively, which indicated that the antibacterial activities of the F-NCD film and F-graphene sheet increased upon fluorine functionalization. In addition, an ATP bioluminescence assay was employed based on the concentration of ATP released from cultured Escherichia coli (E. coli), which confirmed that the fluorinated surface had high antibacterial activity. The ratios of metabolism-cultured E. coli on the F-NCD film and F-graphene sheet were 15.0% and 7.4%, respectively. Furthermore, we observed the morphologies of E. coli cultured on the NCD film and graphene sheet. The antibacterial activity was attributed to the destruction of the E. coli membranes. These results show that the antibacterial activities of the NCD film and graphene sheet depend on their surface functional groups and conductivities.

Air-clad suspended nanocrystalline diamond ridge waveguides.

A hybrid group IV ridge waveguide platform is demonstrated, with potential application across the optical spectrum from ultraviolet to the far infrared wavelengths. The waveguides are fabricated by partial etching of sub-micron ridges in a nanocrystalline diamond thin film grown on top of a silicon wafer. To create vertical confinement, the diamond film is locally undercut by exposing the chip to an isotropic fluorine plasma etch via etch holes surrounding the waveguides, resulting in a mechanically stable suspended air-clad waveguide platform. Optical characterization of the waveguides at 1550 nm yields an average optical loss of 4.67 ± 0.47 dB/mm. Further improvement to the fabrication process is expected to significantly reduce this waveguide loss.

Study of Ni-Catalyzed Graphitization Process of Diamond by in Situ X-ray Photoelectron Spectroscopy.

Graphene on diamond has been attracting considerable attention due to the unique and highly beneficial features of this heterostructure for a range of electronic applications. Here, ultrahigh-vacuum X-ray photoelectron spectroscopy is used for in situ analysis of the temperature dependence of the Ni-assisted thermally induced graphitization process of intrinsic nanocrystalline diamond thin films (65 nm thickness, 50–80 nm grain size) on silicon wafer substrates. Three major stages of diamond film transformation are determined from XPS during the thermal annealing in the temperature range from 300 °C to 800 °C. Heating from 300 °C causes removal of oxygen; formation of the disordered carbon phase is observed at 400 °C; the disordered carbon progressively transforms to graphitic phase whereas the diamond phase disappears from the surface from 500 °C. In the well-controllable temperature regime between 600 °C and 700 °C, the nanocrystalline diamond thin film is mainly preserved, while graphitic layers form on the surface as the predominant carbon phase. Moreover, the graphitization is facilitated by a disordered carbon interlayer that inherently forms between diamond and graphitic layers by Ni catalyst. Thus, the process results in formation of a multilayer heterostructure on silicon substrate.

Co‐implantation of Er and Yb ions into single‐crystalline and nano‐crystalline diamond

The paper reports on the results of Er‐Yb and Er‐Al co‐implantation into commercial single‐crystalline diamonds as well as into nano‐crystalline diamond (NCD) thin films and their photoluminescence properties. The NCD thin films were deposited on fused silica glass by linear‐antenna microwave plasma‐enhanced chemical vapour deposition (PE MWCVD). Er implantation was performed at an energy of 190 keV and fluences of 1 × 1015 and 5 × 1015 ions.cm−2 followed by Yb co‐implantation under the same conditions or Al co‐implantation at an energy of 40 keV and the same fluences. The concentration depth profiles of the elements and the degree of their crystal damage (structural ordering) were measured using Rutherford backscattering spectroscopy and Rutherford backscattering spectroscopy/channelling, respectively. Structural changes were studied by Raman spectroscopy. All Er‐Yb doped samples revealed measurable photoluminescence at approximately 1.5 μm. A comparison with Er‐implanted samples showed that the Er‐Yb co‐implantation was beneficial for the photoluminescence at 1.5 μm in both substrates. Er‐doped NCD films revealed higher intensity of luminescence than Er‐doped single‐crystal diamond. This observation is attributed to the specific incorporation of Er into the diamond and at the grain boundaries.

Linear antenna microwave chemical vapour deposition of diamond films on long-period fiber gratings for bio-sensing applications

The growth processes of nanocrystalline diamond (NCD) thin films on fused silica optical fibers with UV-induced long-period gratings (LPGs) were investigated with regard to biosensing applications. The films were deposited using a linear antenna microwave plasma enhanced chemical vapor deposition system, which allows for the growth of diamond at temperatures below 350&#x00B0;C. The films exhibited a high refractive index n = 2.32, as estimated at &#x03BB; = 550 nm. The biosensing applications of NCD-coated LPG were considered in relation to bovine serum albumin (BSA) as an external medium. In response to BSA binding and the subsequent formation of a thin bio-layer on the NCD surface, the LPG resonances slightly split and shifted towards lower wavelengths.

Effect of slurry composition on the chemical mechanical polishing of thin diamond films

Nanocrystalline diamond (NCD) thin films grown by chemical vapour deposition have an intrinsic surface roughness, which hinders the development and performance of the films’ various applications. Traditional methods of diamond polishing are not effective on NCD thin films. Films either shatter due to the combination of wafer bow and high mechanical pressures or produce uneven surfaces, which has led to the adaptation of the chemical mechanical polishing (CMP) technique for NCD films. This process is poorly understood and in need of optimisation. To compare the effect of slurry composition and pH upon polishing rates, a series of NCD thin films have been polished for three hours using a Logitech Ltd. Tribo CMP System in conjunction with a polyester/polyurethane polishing cloth and six different slurries. The reduction in surface roughness was measured hourly using an atomic force microscope. The final surface chemistry was examined using X-ray photoelectron spectroscopy and a scanning electron microscope. It was found that of all the various properties of the slurries, including pH and composition, the particle size was the determining factor for the polishing rate. The smaller particles polishing at a greater rate than the larger ones.

Osteoblast adhesion, migration, and proliferation variations on chemically patterned nanocrystalline diamond films evaluated by live-cell imaging.

Cell fate modulation by adapting the surface of a biocompatible material is nowadays a challenge in implantology, tissue engineering as well as in construction of biosensors. Nanocrystalline diamond (NCD) thin films are considered promising in these fields due to their extraordinary physical and chemical properties and diverse ways in which they can be modified structurally and chemically. The initial cell distribution, the rate of cell adhesion, distance of cell migration and also the cell proliferation are influenced by the NCD surface termination. Here, we use real-time live-cell imaging to investigate the above-mentioned processes on oxidized NCD (NCD-O) and hydrogenated NCD (NCD-H) to elucidate cell preference to the NCD-O especially on surfaces with microscopic surface termination patterns. Cells adhere more slowly and migrate farther on NCD-H than on NCD-O. Cells seeded with a fetal bovine serum (FBS) supplement in the medium move across the surface prior to adhesion. In the absence of FBS, the cells adhere immediately, but still exhibit different migration and proliferation on NCD-O/H regions. We discuss the impact of these effects on the formation of cell arrays on micropatterned NCD. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1469-1478, 2017.