Transparent ultrananocrystalline diamond films on quartz substrate

Highly transparent ultrananocrystalline diamond (UNCD) films were deposited on quartz substrates using microwave plasma enhanced chemical vapor deposition (MPECVD) method. Low temperature growth of high quality transparent UNCD films was achieved by without heating the substrates prior to the deposition. Additionally, a new method to grow NCD and microcrystalline diamond (MCD) films on quartz substrates has been proposed. Field emission scanning electron microscopy (FESEM) and Raman spectroscopy were used to analyze the surface and structural properties of the films. The surface morphology of UNCD film shows very smooth surface characteristics. The transparent property studies of UNCD film on quartz substrate showed 90% transmittance in the near IR region. The transparent and dielectric properties of UNCD, NCD, and MCD films on quartz substrates were compared and reported.     

Low temperature CVD diamond deposition using halogenated precursors — deposition on low melting materials: Al,Zn and glass

Using HFCVD and MWCVD processes with 0.5–2.0% C₂H₅Cl+H₂, deposition was carried out at substrate temperatures between 650 and 1073 K onto Si, Zn, Al and glass (DESAG type D263) substrates. A coolable substrate holder allowed control of the substrate temperature independently from the CVD process. The temperature was measured using thermocouples. The melting point of Zn (692 K) and the unchanged shape of the substrate after deposition experiments are further proof of the correct temperature measurements. The deposited films were analyzed using Raman spectroscopy and SEM. On Si substrates, the adhesion of diamond deposited at low temperatures (650–700 K) is rather poor. In contrast to this result, we observed quite good adhesion on Al (melting point T_m 933 K) for films deposited at 760 K. The Raman spectrum shows the presence of diamond, with a peak located at 1332 cm⁻¹ with a broad luminescence background. The deposition onto Zn (T_m 692 K) proved to be much more difficult than on Al or glass D 263.     

High optical quality nanocrystalline diamond with reduced non-diamond contamination

It is important for optical applications of nanocrystalline diamond to achieve low optical absorption as well as optical scattering. We discuss the optical and photocurrent spectra measured by the transmittance and reflectance, photothermal deflection spectroscopy, laser calorimetry and dual beam photocurrent spectroscopy of low non-diamond content nanocrystalline diamond films grown by microwave plasma enhanced chemical vapor deposition on fused silica glass substrates.     

Characterization of boron doped diamond epilayers grown in a NIRIM type reactor

Boron doped diamond layers have been grown on (100) single crystal substrates in a wide range of boron concentration. The boron doped layers have been electrically and optically characterized. Boron doped layers with Hall mobility closes to natural diamond holes mobility have been obtained. The films morphology has been observed by scanning electron microscopy and their purity has been assessed by cathodoluminescence. Fourier Transform Photocurrent spectroscopy results show the evolution of the photo-ionization onset and the excited states as boron concentration in the films increases.     

Diamond film synthesis at low temperature

Sulfur-assisted hot-filament chemical vapor deposition was successfully employed to synthesize diamond films at relatively low substrate temperatures, sub 500 °C, on Mo and glass substrates. This result is ascribed to sulfur-containing species that create an additional carbon transport channel to the substrate, which explicitly requires a large temperature gradient to effectively operate. The grain size and roughness were found to decrease when the Mo substrate temperature was decreased, while the quality factor only showed a slight decrease. Moreover, the diamond quality, grain size, and roughness were found to increase on glass substrates in comparison to those on Mo substrates.     

Selective deposition of diamond films on insulators by selective seeding with a double-layer mask

Polycrystalline diamond films have been patterned on Si₃N₄/Si and SiO₂/Si substrates by selective seeding with a double-layer mask via hot-filament chemical vapor deposition. High quality in the patterned diamond films and high selectivity were obtained by the process. The diamond films deposited on the insulators at different CH₄/H₂ concentrations were studied by scanning electron microscopy and Raman spectroscopy. The process proved to be far less damaging to the substrates, and yet effective in developing patterns of diamond films on a large and different substrate.     

Nucleation and Growth of Diamond Films on Ni-Cemented Tungsten Carbide: Effects of Substrate Pretreatments

The nucleation and growth of diamond films on Nicemented carbide is investigated. Substrates made of WC with 6 wt% of Ni were submitted to grinding, and then to different pretreatments (scratching, etching, and/or decarburization) before diamond deposition. Diamond synthesis was carried out by hot-filament chemical vapor deposition (HFCVD) using a mixture of CH₄ (1% v/v) and H₂. Depositions were performed for different lengths of time with the substrates at various temperatures. The specimens were analyzed before and after deposition by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffractometry (XRD). Raman spectra showed that the phase purity of the diamond films was not affected by the presence of nickel on the substrate surface. After wet etching pretreatments, the nucleation of diamond was enhanced, mainly at the WC grain boundaries. Continuous films were obtained on scratched and etched substrates. The decarburizing treatment led to the formation of metallic tungsten and of brittle nicke–tungsten carbide phases. These phases reacted in the early stages of diamond film formation with gaseous carbon species with a parallel process which competes with stable diamond nucleus formation. The diamond film formed after long-term deposition on these samples was not continuous.     

Fabrication of aluminium nitride/diamond and gallium nitride/diamond SAW devices

Surface acoustic wave (SAW) devices have been fabricated from thin films of gallium nitride and aluminium nitride deposited on a range of chemical vapour deposition (CVD) diamond substrates. The growth of aluminium nitride and gallium nitride layers on diamond by chemical beam epitaxy (CBE) is reported for the first time. Triethyl gallium and ethyldimethylamine alane precursors were used in conjunction with nitrogen from an RF atom source to deposit the gallium nitride and aluminium nitride layers at substrate temperatures in the range 540 to 575 °C. These layers have been characterised by Raman spectroscopy and atomic force microscopy. The SAW structures were completed by the deposition of gold or aluminium interdigitated electrode structures on the as-deposited nitride surfaces. Preliminary testing indicates that these devices operate as bandpass filters with characteristics consistent with the propagation of acoustic waves at very high phase velocities within the nitride–diamond multi-layer substrate.     

Nucleation and growth of diamond films on aluminum nitride by hot filament chemical vapor deposition

Nucleation and growth of diamond films on aluminum nitride (ALN) coatings were investigated by scanning electron microscopy, Raman spectroscopy and scratch test. ALN films were grown in a magnetron sputtering deposition. The substrates were Si(111) and tungsten carbide (WC). Chemical vapor deposition (CVD) diamond films were deposited on ALN films by hot filament CVD. The nucleation density of diamond on ALN films was found to be approximately 10⁵ cm⁻², whereas over 10¹⁰ cm⁻² after negative bias pre-treatment for 35 min was −320 V, and 250 mA. The experimental studies have shown that the stresses were greatly minimized between diamond overlay and ALN films as compared with WC substrate. The results obtained have also confirmed that the ALN, as buffer layers, can notably enhance the adhesion force of diamond films on the WC.     

Interface and interlayer barrier effects on photo-induced electron emission from low work function diamond films

Nitrogen-doped diamond has been under investigation for its low effective work function, which is due to the negative electron affinity (NEA) produced after surface hydrogen termination. Diamond films grown by chemical vapor deposition (CVD) have been reported to exhibit visible light induced electron emission and low temperature thermionic emission. The physical mechanism and material-related properties that enable this combination of electron emission are the focus of this research. In this work the electron emission spectra of nitrogen-doped, hydrogen-terminated diamond films are measured, at elevated temperatures, with wavelength selected illumination from 340 nm to 450 nm. Through analysis of the spectroscopy results, we argue that for nitrogen-doped diamond films on metallic substrates, photo-induced electron generation at visible wavelengths involves both the ultra-nanocrystalline diamond and the interface between the diamond film and metal substrate. Moreover, the results suggest that the quality of the metal–diamond interface can substantially impact the threshold of the sub-bandgap photo-induced emission.     

Comparative study of diamond films grown on silicon substrate using microwave plasma chemical vapor deposition and hot-filament chemical vapor deposition technique

Diamond films on the p-type Si(111) and p-type(100) substrates were prepared by microwave plasma chemical vapor deposition (MWCVD) and hot-filament chemical vapor deposition (HFCVD) by using a mixture of methane CH₄ and hydrogen H₂ as gas feed. The structure and composition of the films have been investigated by X-ray Diffraction, Raman Spectroscopy and Scanning Electron Microscopy methods. A high quality diamond crystalline structure of the obtained films by using HFCVD method was confirmed by clear XRD-pattern. SEM images show that the prepared films are poly crystalline diamond films consisting of diamond single crystallites (111)-orientation perpendicular to the substrate. Diamond films grown on silicon substrates by using HFCVD show good quality diamond and fewer non-diamond components.     

Diamond growth by hollow cathode arc chemical vapor deposition at low pressure range of 0.02–2 mbar

An attempt was made to synthesize diamond films on (001) silicon substrates by means of a graphite or tungsten hollow cathode arc chemical vapor deposition at a lower pressure range of 0.02–2 mbar. The hollow cathode arc provides the advantage of the generation of a large area, high-flux electron beam, a very high-density plasma, and the high kinetic reaction species due to relatively low pressure operation. Diamond films have been characterized by scanning electron microscopy and Raman spectroscopy. The quality of diamond films deposited using the graphite hollow cathode was better than that using the tungsten hollow cathode at 2 mbar pressure. With further decreasing the deposition pressure, the evaporation and sputtering of the graphite hollow cathode are increased and the film quality was deteriorated. The growth rate of diamond films decreased and the nucleation density increased with decreasing deposition pressure.     

Microstructure of c-BN thin films deposited on diamond films

Diamond films were used as substrates for cubic boron nitride (c-BN) thin film deposition. The c-BN films were deposited by ion beam assisted deposition (IBAD) using a mixture of nitrogen and argon ions on diamond films. The diamond films exhibiting different values of surface roughness ranging from 16 to 200 nm (in R_rms) were deposited on Si substrates by plasma enhanced chemical vapor deposition. The microstructure of these c-BN films has been studied using in situ reflexion electron energy loss spectroscopy analyses at different primary energy values, Fourier transform infrared spectroscopy and high resolution transmission microscopy. The fraction of cubic phase in the c-BN films was depending on the roughness of the diamond surface. It was optimized in the case of the smooth surface presenting no particular geometrical effect for the incoming energetic nitrogen and argon ions during the deposition. The films showed a nanocrystalline cubic structure with columnar grains while the near surface region was sp² bonded. The films exhibit the commonly observed layered structure of c-BN films, that is, a well textured c-BN volume lying on a h-BN basal layer with the (00.2) planes perpendicular to the substrate. The formation mechanism of c-BN films by IBAD, still involving a h-BN basal sublayer, does not depend on the substrate nature.     

Using fluorine and chlorine in the diamond CVD process

We deposited diamond films at low substrate temperatures T_sub using the halogen containing precursor gases CHF₃ and C₂H₅Cl with an abundance of hydrogen. Diamond film growth was possible down to T_sub=370 °C using a hot-filament chemical vapor deposition process. The possibility of low temperature growth of diamond could be correlated with an increase of radical density in the gas phase caused by the halogen addition. These radicals, especially atomic hydrogen, fluorine and chlorine are responsible for the creation of active surface sites on the diamond surface. Chlorine especially is able to break surface bonds even at low substrate temperatures. Recent secondary ion mass spectroscopy measurements revealed that halogens are involved in surface reactions and that fluorine and chlorine were incorporated in the deposited films especially at low T_sub. Along with the creation of active surface sites the surface diffusion is important for the diamond growth, which is strongly limited by reduction of the substrate temperature. We succeeded in good quality diamond growth on glasses and aluminium substrates at T_sub=490 °C. A further decrease of T_sub leads to a decrease of diamond film quality and a poor adhesion of the diamond films to the substrate.     

Effect of substrate temperature on the selective deposition of diamond films

Selective diamond films on roughened Si(100) substrates with patternings have been achieved by microwave plasma chemical vapor deposition (MP-CVD). The films have been characterized by scanning electron microscopy (SEM) and Raman spectra. The influence of substrate temperature on the selective deposition of diamond films has been discussed in detail: the diamond nucleation density on the SiO₂ mask increased with substrate temperature while the effect of the selective deposition of diamond films deteriorated; the optimized deposition temperature conditions have been concluded.     

Cubic boron nitride films for industrial applications

The effects of kinetic energy, chemical nature of substrates and temperature on the synthesis of cBN films are explored to obtain cBN films with industrial quality. Carbon including amorphous carbon, nanocrystalline and polycrystalline diamond enables deposition of stable, thick and adherent cBN films with characteristic Raman signature. Although temperature has been designated as an unimportant parameter, the deposition at higher temperatures yields higher quality of cBN films. The higher temperature (800 °C) was also employed at cBN deposition on diamond coated tungsten carbide (WC) cutting inserts using plasma enhanced chemical vapor deposition (PECVD). The quality of cBN films grown by PECVD significantly overcomes that prepared by physical vapor deposition (PVD) which is affected in large extent by the lower kinetic energies of particles used in PECVD. The low kinetic energy of particles induces surface growth mechanism which differs from the growth models previously proposed.     

Thermal oxidation of CVD diamond

A thermal oxidation process of diamond films grown by chemical vapor deposition (CVD) has been studied. The oxidation was realized via heating of the CVD films in air. Pristine and oxidized CVD diamond films were analyzed with Raman spectroscopy and scanning electron microscopy (SEM) techniques. Raman spectroscopy revealed substantial changes in the polycrystalline diamond film composition induced by oxidation. A selective oxidation of disordered carbon and small size diamond crystallites was obtained at appropriate temperatures. A model explaining the formation and oxidation of the CVD diamond films containing the micrometer single diamond cores surrounded by the nanocrystalline diamond and disordered carbon has been proposed on the basis of the obtained results.     

Evolution and properties of adherent diamond films with ultra high nucleation density deposited onto alumina

In this work, we report on adherent diamond films with thickness of up to 4.5 μm grown on polycrystalline alumina substrates. Prior to deposition, alumina substrates were ultrasonically abraded with mixed poly-disperse slurry that allows high nucleation density of values up to ∼5×10¹⁰ particles/cm². It was estimated that the minimal film thickness achieved for continuous films was ∼320 nm, obtained after a deposition time of 15 min with diamond particles density (DPD) of ∼4×10⁹ particles/cm². Continuous adherent diamond films with high DPD (∼10⁹ particles/cm²) were obtained also on sapphire surface after abrasion with mixed slurry and 15 min of deposition. However, after longer deposition time, diamond films peeled off from the substrates during cooling.The poor adhesion between the diamond and sapphire is attributed to the weak interface interaction between the film and the substrate and to difference in coefficient of thermal expansion. On the other hand, it is suggested that the reason for good adhesion between diamond film and alumina substrate is that high carbon diffusivity onto alumina grain boundaries allows strong touch-points at the grooves of alumina grains, and this prevents the delamination of diamond film. This adhesion mechanism, promoted by sub-micron diamond grain-size, is allowed by initial high nucleation density.The surface properties, phase composition and microstructure of the diamond films deposited onto alumina were examined by electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and high-resolution scanning electron microscopy (HR-SEM). The residual stress in the diamond films was evaluated by diamond Raman peak position and compared to a theoretical model with good agreement. Due to the sub-micron grain-size, the intrinsic tensile stress is high enough to partially compensate the thermal compressive stress, especially in diamond films with thickness lower than 1 μm.     

Effect of deposition temperature and quality of free-standing diamond substrates on the properties of RF sputtering ZnO films

Highly c-axis oriented ZnO film is often deposited on diamond substrates by RF magnetron sputtering and widely used for high frequency surface acoustic wave (SAW) devices. Deposition temperature is a key factor affecting the quality of the ZnO film. Different quality polished free-standing diamond films prepared by DC Arc Plasma Jet were used as the substrates to deposit ZnO films at different temperatures. Effect of the deposition temperature and the quality of the diamond films on the properties of the ZnO films were investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that highly c-axis oriented ZnO films can be much easier deposited on the optical-grade diamond films with < 111> preferred orientation than the tool-grade diamond films with < 220> preferred orientation. The optimal deposition temperature is 200 °C for highly c-axis oriented and lower roughness ZnO films. Acoustic phase velocity of more than 10,000 m/s for the SAW devices based on the ZnO/optical-grade free-standing diamond films was obtained.     

On the reduction of the non-diamond phase in nanocrystalline CVD diamond films

We present photocurrent spectra of nominally undoped nanocrystalline diamond (NCD) films grown on glass substrates by hot filament (HF) and microwave (MW) plasma enhanced chemical vapor deposition (CVD). The spectra were measured in a broad optical range (200–2000 nm) by dual-beam photocurrent spectroscopy (DBP) and Fourier-transform photocurrent spectroscopy (FTPS) in amplitude modulated step scan mode. The NCD films with carefully oxidized surface show photosensitivity and high dark resistivity. Unlike single crystal type IIa diamond with the photonization threshold at 5.5 eV, the photocurrent spectra of NCD films are dominated by the “non-diamond phase” with the photo-ionization threshold at about 0.8 eV. Some HF CVD samples have lower sub-band gap absorption (non-diamond phase contamination). The non-diamond phase content increases after annealing at elevated temperature. The non-diamond phase content can be reduced by exposing NCD to hydrogen plasma at temperature below 350 °C.