Polycrystalline Diamond Thin Films Research Articles

Light emission dynamics of silicon vacancy centers in a polycrystalline diamond thin film.

Diamond thin films can be, at a relatively low-cost, prepared with a high-density of light-emitting negatively charged silicon vacancy (SiV) centers, which opens up the possibility of their application in photonics or sensing. The films are composed of diamond grains with both the SiV centers and sp2-carbon phase, the ratio of these two components being dependent on the preparation conditions. The grain surface and the sp2-related defects might act as traps for the carriers excited within the SiV centers, consequently decreasing their internal photoluminescence (PL) quantum efficiency. Here, we show that in a 300 nm thick polycrystalline diamond film on a quartz substrate, the SiV centers in the diamond grains possess similar temperature-dependent (13-300 K) PL decay dynamics as the SiV centers in monocrystalline diamond, which suggests that most of the SiV centers are not directly interconnected with the defects of the diamond thin films, i.e. that the carriers excited within the centers do not leak into the defects of the film. The activation energy ΔE = 54 meV and the attempt frequency α = 2.6 were extracted from the measured data. These values corresponded very well with the published values for SiV centers in monocrystalline diamond. We support this claim by measuring the transient absorption via a pump and probe technique, where we separated the nanosecond recombination dynamics of carriers in SiV centers from the picosecond decay dynamics of polycrystalline diamond defects. Our results show that PL emission properties of SiV centers in polycrystalline diamond thin films prepared via chemical vapor deposition are very similar to those in monocrystalline diamond thereby opening the door for their application in diamond photonics and sensing.

Structural and thermal analysis of polycrystalline diamond thin film grown on GaN-on-SiC with an interlayer of 20 nm PECVD-SiN

A 1.5-μm polycrystalline diamond was deposited on the AlGaN/GaN heterojunction on the SiC substrate with a 20-nm SiN dielectric. A 4.9% increase in 2DEG density after the diamond growth due to the increase in tensile strain of the GaN layer is confirmed by micro-Raman measurements. The interfacial analysis through the transmission electron microscopy and electron energy loss spectroscopy shows a thickness reduction in the SiN layer of ∼1.7 nm, which converts to a thin SiC layer at the diamond/SiN interface, and no carbon diffusion is found in the SiN layer after the diamond growth. Device simulation using the thermal properties extracted by time domain thermoreflectance predicts a temperature drop of 17.1 °C when the diamond only covers the device access region and reveals that the improvement of thermal boundary resistance is much more effective than that of the diamond thermal conductivity for the top-side heat spreading, which is mainly due to the limited thickness of the top diamond film.

Electrochemical reduction of carbon dioxide in an aqueous solution using phosphorus-doped polycrystalline diamond electrodes

Phosphorus-doped (P-doped) polycrystalline diamond thin films were deposited on a conductive Si substrate by microwave plasma-enhanced chemical vapor deposition (MWCVD), using a liquid source containing phosphorus atoms. The thin films were then used as electrodes for the electrochemical reduction of carbon dioxide (CO2) dissolved in aqueous solution. The P-doped diamond electrode exhibited approximately 1 V higher overpotential for the hydrogen evolution reaction than conventional boron-doped (B-doped) diamond electrodes in an aqueous electrolyte. A cathodic current attributed to the CO2 reduction reaction was observed at the P-doped diamond electrode in 0.2 M Na2SO4 bubbled with CO2. An analysis of the soluble products showed that the main reduction product was formic acid. We have experimentally demonstrated that P-doped diamond thin film is ideal as an electrode material for the electrochemical reduction of CO2 in aqueous solution, as it does not suffer from the interference of hydrogen evolution by water electrolysis.

X-ray Spectrum Reconstruction by Diamond Detectors with Linear Response to Dose Rate

The absorbers method is here applied by interposing filters of variable thickness between the X-ray source and a detector so to attenuate the radiation intensity by using the attenuation coefficient as a selective photon energy operator. The analysis of the signal provided by a polycrystalline diamond thin film detector exposed to the energy-selectively-attenuated X-ray beam was used for the reconstruction of the radiation spectrum. The 50 μm thick diamond detector achieves conditions of linear response to the dose rate of the incident radiation (linearity coefficient of 0.997 ± 0.003) for a bias voltage ≥90 V, corresponding to an electric field ≥1.8 × 104 V/cm. Once the absorbers method is applied, only the detector signal linearity to dose rate allows reconstructing the source X-ray bremsstrahlung spectrum with sufficiently high accuracy.

(Invited) Diamond Based Chemical/Biochemical Sensors: State-of-the-Art and Perspectives

Diamond is grown in some laboratories by either HPHT process or Plasma-Enhanced Chemical Vapor Deposition (MP-CVD) since a few decades. Single crystal diamond exhibits outstanding properties including a high optical transparency over a broad electromagnetic spectrum, high thermal conductivity approx. five times higher than copper, and acoustic wave velocity close to 19 000 m.s-1. It displays also remarkable mechanical properties with e.g. a Young’s modulus exceeding 1000 GPa along with high resistance to fracture, to name a few. Some of these properties remain also remarkable in its polycrystalline form when compare to most other materials. Furthermore, diamond can be doped with nitrogen or boron during growth, offering electrical properties from semiconducting to quasi-metallic regimes. When heavily doped with boron (~2.1021 cm-3), the so-called Boron Doped Diamond (BDD) electrodes become attractive electrodes featuring a high potential window > 3V in water and low double-layer capacitance. Moreover, diamond is extremely resilient to corrosion and more generally to chemical attacks. It is also biocompatible, which makes it very attractive for in-vivo sensing applications. Finally, the carbon nature of the diamond offers wide opportunities for surface grafting of chemical or biochemical functional groups through highly stable covalent carbon-carbon bonding. These properties can be exploited advantageously to enhance the analytical performances and stability of chemical/biochemical sensors and have motivated our research over the last 15 years. Our work focuses mainly on polycrystalline diamond thin films that can be grown typically on inches silicon substrates, thus offering access to some clean-room processes and potentially large-scale production.Several processes were elaborated to micro-pattern diamond layers in order to design chemical transducers such as gravimetric MEMS devices, electrodes, field effect transistors, etc. Diamond microstructures may also be transferred to flexible parylene or polyimide substrates, thus making them attractive e.g. for wearable sensors and implantable medical electrodes. Further techniques have also been developed to enhance the active surface area of diamond transducer surfaces at the nanoscale and thus increase drastically the sensitivity of the resulting sensors by multiplying the number of active sites. Here diamond is typically grown onto well-chosen high aspect ratio templates that can withstand the growth conditions of diamond in high-density hydrogen plasma. Most of these methods have been “standardized”. They involve clean-room processing including dry etching, photolithography, and so on and forth. As examples, heavily doped diamond electrodes were developed successfully both as macro- and micro-electrodes for biomedical, pharmaceutical or foodstuff analysis applications. These applications benefit both from the high analytical performances of diamond electrodes in particular due to their low background signals and high reactivity, and high stability and reliability. BDD electrodes may also be modified with transition metal nanoparticles to enhance their catalytic behavior. From this concept, diamond multi-electrode arrays were designed for chemical patterns identification, for instance for sensory analysis of coffee, or for the monitoring of environmental pollutants. BDD electrodes offer also significant advantages in electrochemiluminescence (ECL) techniques, which are being investigated for various applications ranging from foodstuff analysis to narcotics detection. A key benefit of BDD electrodes for all of the above applications is certainly that they can be electrochemically reactivated following fouling, sometimes directly in the analytical medium, to maintain high reactivity thus opening the way to reusable sensors and online monitoring. Besides, BDD microelectrode arrays have also been transferred to flexible substrates for neural stimulation and recording, along with in-vivo neuromodulators measurements. Finally, diamond based MEMS devices (microcantilevers, SAW sensors) take advantage both of the mechanical properties of diamond, along with steady carbon interface for convenient bio-functionalization. Our work here focused mainly on the detection of odorant molecules, using biomolecular receptors involved in olfaction in Nature as sensitive layers, including Odorant Binding Proteins (OBPs), Major Urinary Proteins (MUPs) and Olfactory Receptors (OR). Multisensor array instrumentations were developed around this concept, for applications ranging from breathe analysis to security applications.

Study of Atomic Hydrogen Concentration in Grain Boundaries of Polycrystalline Diamond Thin Films

This paper describes research focused on investigating the effect of hydrogen (H) atom insertion into the grain boundaries of polycrystalline diamond (PCD) films. This is required in order to understand the key morphological, chemical, physical, and electronic properties of the films. The PCD films were grown using the hot filament chemical vapor deposition (HFCVD) process, with flowing Ar gas mixed with CH4 and H2 gases to control film growth into microcrystalline diamond (MCD, 0.5–3 µm grain sizes), nanocrystalline diamond (NCD, 10–500 nm grain sizes), and ultrananocrystalline diamond (UNCD, 2–5 nm grain sizes) films depending on the Ar/CH4/H2 flow ratios. This study focused on measuring the H atom concentration of the PCD films to determine the effect on the properties indicated above. A simple model is presented, including a hypothesis that the two dangling bonds per unit cell of C atoms serve as the site of hydrogen incorporation. This correlates well with the observed concentration of H atoms in the films. Dangling bonds which are not passivated by hydrogen are postulated to form surface structures which include C double bonds. The Raman peak from these surface structures are the same as observed for transpolyacetyline (TPA). The data reveal that the concentration of H atoms at the grain boundaries is around 1.5 × 1015 atoms/cm2 regardless of grain size. Electrical current measurements, using a conductive atomic force microscopy (CAFM) technique, were performed using an MCD film, showing that the current is concentrated at the grain boundaries. Ultraviolet photo electron spectroscopy (UPS) confirmed that all the PCD films exhibited a metallic behavior. This is to be expected if the nature of grain boundaries is the same regardless of grain size.

Thin Diamond Film on Silicon Substrates for Pressure Sensor Fabrication.

Thin polycrystalline diamond films chemically vapor deposited on thinned silicon substrates were used as membranes for pressure sensor fabrication by means of selective chemical etching of silicon. The sensing element is based on a simple low-finesse Fabry–Pérot (FP) interferometer. The FP cavity is defined by the end-face of a single mode fiber and the diamond diaphragm surface. Hence, pressure is evaluated by measuring the cavity length by an optoelectronic system coupled to the single mode fiber. Exploiting the excellent properties of Chemical Vapor Deposition (CVD) diamond, in terms of high hardness, low thermal expansion, and ultra-high thermal conductivity, the realized sensors have been characterized up to 16.5 MPa at room temperature. Preliminary characterizations demonstrate the feasibility of such diamond-on-Si membrane structure for pressure transduction. The proposed sensing system represents a valid alternative to conventional solutions, overcoming the drawback related to electromagnetic interference on the acquired weak signals generated by standard piezoelectric sensors.

Nanosphere Lithography for Structuring Polycrystalline Diamond Films

This paper deals with the structuring of polycrystalline diamond thin films using the technique of nanosphere lithography. The presented multistep approaches relied on a spin-coated self-assembled monolayer of polystyrene spheres, which served as a lithographic mask for the further custom nanofabrication steps. Various arrays of diamond nanostructures—close-packed and non-close-packed monolayers over substrates with various levels of surface roughness, noble metal films over nanosphere arrays, ordered arrays of holes, and unordered pores—were created using reactive ion etching, chemical vapour deposition, metallization, and/or lift-off processes. The size and shape of the lithographic mask was altered using oxygen plasma etching. The periodicity of the final structure was defined by the initial diameter of the spheres. The surface morphology of the samples was characterized using scanning electron microscopy. The advantages and limitations of the fabrication technique are discussed. Finally, the potential applications (e.g., photonics, plasmonics) of the obtained nanostructures are reviewed.

Directed covalent assembly of nanodiamonds into thin films

The wide-spread use of polycrystalline diamond thin films is limited by the costs associated with chemical vapor deposition, high growth temperature, and poor substrate adhesion. Here, a solution-phase method to grow nanocrystalline diamond films is demonstrated through carbodiimide-mediated cyclic attachment of carboxylated nanodiamonds and a diamine linker on amine-functionalized substrates. XPS and Raman spectroscopy confirm formation of the desired amide bonds and incorporation of the linker to form the covalently-bonded nanoparticle network. The assembly process was demonstrated in solution phase as well as via spin coating with both methods resulting in similar film morphology as observed using SEM and AFM. The assembly process leaves the films amine-terminated, which is convenient for application-specific surface modifications. The thermal conductivity of the covalently assembled NDs as measured via the 3ω method ranged between 2 and 20 W m−1 K−1, which is comparable to those of CVD nanocrystalline diamond films of similar grain size. Future strategies to reduce porosity during the directed covalent assembly strategy hold strong potential for applications requiring thermal dissipation or mechanical and chemical stability.

N+ ion beam irradiation as a strategy to enhance the electrical conductivity of polycrystalline diamond thin films

100 keV 14N+ ion beam (at fluences of 1016 and 1017 ions/cm2) irradiation was used to alter the sp2 bonded carbon networks (as revealed by Raman scattering analysis) and incorporate N atoms into the surface and sub-surface regions (as revealed by Rutherford backscattering analysis) in the typical polycrystalline diamond films and as a consequence, the electrical conductivity of the diamond thin films was enhanced several orders in comparison to the as-deposited film. The enhancement in electrical conductivity was achieved with negligible morphological and structural changes in the diamond thin film. This work demonstrates an easy and effective way of altering the electrical conductivity of diamond thin films to a required sub-surface depth.

Hydrogen retention and nitrogen distribution in delta-doped diamond films

Nitrogen delta-doped diamond layers can be fabricated by exposing diamond substrates to radio frequency (RF) N2 plasma, followed by growth of a diamond capping layer. A recent report shows that preferential retention of hydrogen occurs within the volume where the nitrogen atoms are localized. Here we investigate the distribution and source of hydrogen in the delta layer as well as the influence of the grain boundaries on nitrogen distribution. Delta-doped diamond layers are fabricated onto polycrystalline diamond thin films having different grain boundary densities and roughness. The capping layer was grown using D2 by replacing H2 in the gas mixture to investigate the source of hydrogen in the delta layer. In-situ X-ray photoelectron spectroscopy was used to study the chemical states of nitrogen present on the RF N2 plasma treated diamond surfaces. Time-of-flight - secondary ion mass spectrometer was used to measure the nitrogen and hydrogen concentrations. The concentration of nitrogen is of the order of 1019 cm−3 in nitrogen delta-doped layers and the width of the delta layers is up to ∼33 nm. Our study shows that the source of hydrogen is the capping layer deposition process and the grain boundary density has no significant influence on the nitrogen retention in the delta layer.

Polycrystalline Diamond Thin-Film Waveguides for Mid-Infrared Evanescent Field Sensors.

Photonic design and optimization of thin-film polycrystalline diamond waveguides are shown, serving as advanced evanescent field transducers in the mid-infrared fingerprint regime (2000–909 cm–1; 5–11 μm). Design constraints inherent to optical/system considerations and the material were implemented in a finite element method (FEM)-based simulation method that allowed three-dimensional modeling of the overall structure. Thus, lateral mode confinement, attenuation in the direction of radiation propagation, and physical resilience were evaluated. In a final step, the designed structures were fabricated, and their utility in combination with a broadly tunable external cavity quantum cascade laser for chemical sensing of a liquid phase analyte was demonstrated.

Raman scattering of nitrogen incorporated diamond thin films grown by hot filament chemical vapor deposition

A detailed Raman scattering analysis of nitrogen incorporated polycrystalline diamond thin films grown using NH3/CH4/H2 gas mixture in hot filament chemical vapor deposition system is presented. To understand the nitrogen bonding configuration in these films, diamond films are grown by replacing H2 with D2 in the gas mixture. The Raman peak observed at ~1190 cm−1 showed an isotopic shift to ~830 cm−1 upon replacing H2 with D2 in the gas mixture. With the present Raman analysis, the peak at ~1190 cm−1 is assigned to CNH. Secondary ion mass spectroscopy revealed the abundance of nitrogen in the sub-surface region of the annealed diamond thin films.

Charge-based deep level transient spectroscopy of B-doped and undoped polycrystalline diamond films

The undoped and B-doped polycrystalline diamond thin film was synthesized by hot filament chemical vapor deposition and microwave plasma, respectively. The structural characterization was performed by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The electrical properties of synthesized diamond layer were characterized by dc-conductivity method and charge deep level transient spectroscopy. The B-doped diamond layers show higher sp2/sp3 ratios in comparison with that of undoped layers what can have an essential influence on the localized density of states associated with shallow hydrogen acceptor states what is reflected in the values of activation energies which reached the values of 38 meV for B-doped and 55 meV for undoped diamond layers, respectively. The existence of deep level traps, as, for example, associated with B-related acceptors, was not observed.

Improved adhesion of polycrystalline diamond films on copper/carbon composite surfaces due to in situ formation of mechanical gripping sites

Diamond coatings are investigated for thermal management, wear protection and corrosion resistance in harsh environments. In power electronic industries, copper (Cu), which shows high thermal conductivity, is considered as a promising substrate for diamond based heat-spread materials. However, the coefficient of thermal expansion (CTE) mismatch between diamond and Cu induces thermo-mechanical stresses that affect the integrity of the diamond-Cu assembly. In fact, diamond films deposited on Cu substrates tend to peel-off upon cooling due to the compressive stresses present at the diamond-Cu interface. This investigation is focused on the growth of polycrystalline diamond thin films onto Cu/carbon fibers (CFs) composite materials, using combustion flame chemical vapor deposition (CVD). It has been found that increased CF content in the Cu/CF materials leads to a reduced CTE improving, hence, the adhesion between the diamond film and the Cu/CF substrate and reduces Cu/CF-diamond interfacial residual thermal stresses. At a CF content of 40% in volume, the residual thermal stress of the diamond film deposited on the Cu/CF composite is lower than that on bare Cu and adapted with CVD diamond growth. Naturally engineered composite surfaces have enhanced the adhesion of the diamond film on the composite substrate via mechanical interlocking.

Thin polycrystalline diamond films protecting zirconium alloys surfaces: From technology to layer analysis and application in nuclear facilities

Zirconium alloys can be effectively protected against corrosion by polycrystalline diamond (PCD) layers grown in microwave plasma enhanced linear antenna chemical vapor deposition apparatus. Standard and hot steam oxidized PCD layers grown on Zircaloy2 surfaces were examined and the specific impact of polycrystalline Zr substrate surface on PCD layer properties was investigated. It was found that the presence of the PCD coating blocks hydrogen diffusion into the Zircaloy2 surface and protects Zircaloy2 material from degradation. PCD anticorrosion protection of Zircaloy2 can significantly prolong life of Zircaloy2 material in nuclear reactors even at temperatures above Zr phase transition temperatures.

Observation of the Ge-vacancy color center in microcrystalline diamond films

In germanium-doped thin polycrystalline diamond films grown in microwave plasma on Ge substrates, strong photoluminescence in the orange spectral region with a zero-phonon line (ZPL) at a wavelength of 602 nm is detected. The ZPL width is 4–5 nm at room temperature and 1.2 nm at T = 5K. It is assumed that the new optically active defect is a germanium–vacancy (Ge–V) complex similar to the known Si–V color center in diamond.

Local impedance imaging of boron-doped polycrystalline diamond thin films

Local impedance imaging (LII) was used to visualise surficial deviations of AC impedances in polycrystalline boron-doped diamond (BDD). The BDD thin film electrodes were deposited onto the highly doped silicon substrates via microwave plasma-enhanced CVD. The studied boron dopant concentrations, controlled by the [B]/[C] ratio in plasma, ranged from 1 × 1016 to 2 × 1021 atoms cm−3. The BDD films displayed microcrystalline structure, while the average size of crystallites decreased from 1 to 0.7 μm with increasing [B]/[C] ratios. The application of LII enabled a direct and high-resolution investigation of local distribution of impedance characteristics within the individual grains of BDD. Such an approach resulted in greater understanding of the microstructural control of properties at the grain level. We propose that the obtained surficial variation of impedance is correlated to the areas of high conductance which have been observed at the grain boundaries by using LII. We also postulate that the origin of high conductivity is due to either preferential boron accumulation, the presence of defects, or sp2 regions in the intragrain regions. The impedance modulus recorded by LII was in full agreement with the bulk impedance measurements. Both variables showed a decreasing trend with increasing [B]/[C] ratios, which is consistent with higher boron incorporation into BDD film.

Analysis of Fracture Behavior of Thin Polycrystalline Diamond Films

The effect of the substrate temperature and CH4 concentrations on the fracture behavior of thin polycrystalline diamond films was systematically investigated by X-ray diffraction and scanning electron microscopy. The results show that the fracture behavior of thin polycrystalline diamond films synthesized by direct current plasma jet chemical vapor deposition is closely related to the substrate temperature and CH4 concentrations. A high substrate temperature, due to difference in the thermal expansion coefficients of the substrate and the diamond film, causes thin polycrystalline diamond films to generate high residual stresses, which usually exceed fracture strength of thin diamond film and even that of diamond. The fracture toughness is found to drop with the increasing ratio of CH4 concentration. In case of high CH4 concentrations, various defects and impurities, such as cracks, microscopic holes, graphite, and amorphous carbon were observed in the films. Thus, the substrate temperature and CH4 concentrations should be strictly controlled within an appropriate range

Low propagation loss in a one-port SAW resonator fabricated on single-crystal diamond for super-high-frequency applications

Diamond has the highest known SAW phase velocity, sufficient for applications in the gigahertz range. However, although numerous studies have demonstrated SAW devices on polycrystalline diamond thin films, all have had much larger propagation loss than single-crystal materials such as LiNbO3. Hence, in this study, we fabricated and characterized one-port SAW resonators on single-crystal diamond substrates synthesized using a high-pressure and high-temperature method to identify and minimize sources of propagation loss. A series of one-port resonators were fabricated with the interdigital transducer/ AlN/diamond structure and their characteristics were measured. The device with the best performance exhibited a resonance frequency f of 5.3 GHz, and the equivalent circuit model gave a quality factor Q of 5509. Thus, a large fQ product of approximately 2.9 × 10(13) was obtained, and the propagation loss was found to be only 0.006 dB/wavelength. These excellent properties are attributed mainly to the reduction of scattering loss in a substrate using a single-crystal diamond, which originated from the grain boundary of diamond and the surface roughness of the AlN thin film and the diamond substrate. These results show that single-crystal diamond SAW resonators have great potential for use in low-noise super-high-frequency oscillators.