Diamond Peak Research Articles

Interfacial characterization and residual stress analysis in diamond films on LiNbO3

Diamond films were deposited via microwave plasma chemical vapor deposition on lithium niobate (LN) substrates. Characterization of the interfacial regions formed between diamond films and LN substrates was carried out by several techniques. Secondary ion mass spectroscopy (SIMS) was used to determine the depth profiles of carbon, lithium, niobium, and oxygen in the LN substrate covered with diamond nuclei and in the substrate without nucleation. Results indicate that the diamond nuclei promoted growth of diamond, and in addition, a reduced depth of the lithium deficient zone formed in the LN substrate was observed. Grazing incidence x-ray diffraction and transmission electron microscopy observations corroborated the results obtained by SIMS. Residual stresses determined experimentally by an x-ray method or by the shift in the characteristic diamond peak in Raman spectroscopy were much smaller than the calculated thermal residual stresses. The results further emphasize that the interfacial phases are responsible for relieving the thermal stresses generated during cooling of the layered structure. The thickness of the interfacial region was found to be well below for attenuation of the surface acoustic waves in the gigahertz frequency surface acoustic wave devices.

An investigation of structural and electrical properties of boron doped and undoped nanocrystalline diamond films

AbstractNanocrystalline and microcrystalline undoped and boron doped diamond films are investigated by means of SEM, Raman and photocurrent techniques. SEM measurements on nanocrystalline diamond show the typical diamond columnar structure whereas for microcrystalline diamond such structure is not observed. Raman investigation shows a clear diamond peak at 1332 cm–1 for all investigated films and a Fano resonance is observed for the microcrystalline sample prepared with 2000 ppm of boron in the gas mixture indicating a transition into the semi‐metallic state. Photocurrent measurements also show a presence of boron in some films. It is found that when temperature is decreased lower than 80 K, the boron photocurrent threshold in the case of NCD material behaves differently compared to bulk diamond. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polymerized phase and amorphous diamond synthesized from [formula omitted] fullerene by shock compression

C 60 fullerene films were shock compressed to 23 and 52 GPa. Both the recovered samples exhibit fracture into platelets and broad photoluminescence, and intensity of these increases with increasing pressure. At 23 GPa, a characteristic single broad band appears at 1560– 1570 cm - 1 , which is similar to the one found for three-dimensional (3D) polymerized C 60 fullerene under high-pressure–high-temperature treatment. At 52 GPa, on the other hand, the single broad band has disappeared and a diamond peak sometimes appears, depending on platelets.

Comparative study on nanocrystalline diamond growth from acetylene and methane

The amount and composition of amorphous non-diamond phases in nanocrystalline diamond (NCD) films deposited from microwave C 2H 2/H 2/Ar-rich and CH 4/H 2/Ar-rich plasmas have been compared to obtain a clue to phase-pure NCD films. The emission intensity of C 2 radicals for C 2H 2 was a few times higher than that for CH 4, indicating efficient production of C 2 as important growth precursors by using C 2H 2. The threshold Ar concentration for a transition from microcrystalline to nanocrystalline diamond was found to be around 70 and 90 vol.% for C 2H 2 and CH 4 systems, respectively. Visible Raman spectroscopy for NCD films with average crystallite sizes of 15–25 nm revealed that the amount of amorphous carbon corresponding to the D peak intensity was reduced and consequently the diamond peak was clearly observed when C 2H 2 was used. The reduction of amorphous carbon was likely accompanied by an increase in the fraction of polymer-like hydrogenated carbon including transpolyacetylene, suggesting the crucial role of hydrogen in phase control.

Effect of hydrogen plasma exposure on the amount of trans-polyacetylene in nanocrystalline diamond films

The interactions between hydrogen-rich plasmas and nondiamond phases contained in nanocrystalline diamond films have been examined to explore a possibility of reducing the amount of nondiamond phases. Diamond films with average grain sizes of 15–25nm were deposited on silicon in microwave Ar-rich∕H2∕CH4 plasmas. The 1140cm−1 peak in the visible Raman spectra for the films was used as a signature of trans-polyacetylene (TPA) segments. The amount of TPA relative to amorphous carbon increased with increasing H2∕CH4 ratio in the Ar-rich∕H2∕CH4 plasmas or simply by exposing the as-grown film to hydrogen plasmas after deposition. However, the Raman peak of diamond, which was superimposed by the D peak of amorphous carbon, was little enhanced by exposure to these hydrogen-rich plasmas. It was also found that an increase in the relative amount of TPA was not always accompanied by an increase in hydrogen content in the films. The results were interpreted along the possible mechanisms of TPA formation, which are correlated to the content of hydrogen in the gas phase as well as in the solid phase.

Effects of High D.C. Hias Voltage on the Properties of Diamond-Like Carbon Films

ABSTRACTDiamond-like carbon (DLC) films were synthesized using pulsed plasma sputtering deposition techniques. Microscope and Raman scattering techniques were used to study the effects of bias voltages on the properties of diamond-like carbon films. With d.c. bias voltage up to 1000V, the smooth and thick DLC films together with a few nanoparticles were obtained. An increase of the d.c. bias voltage up to 2000V yielded thicker DLC films but its surface became slightly rough and size of particles became large. The crystalline properties of these particles were studied. A tiny diamond peak from Raman spectrum was also observed.

Pressure measurements within optical cells using diamond sensors: accuracy of the method below 1 GPa

The accuracy of the diamond pressure gauge has been investigated in the moderate pressure range (0–1 GPa) between 210 and 310 K. Experiments have been conducted using a sapphire anvil cell coupled to a Raman spectrometer. The temperature shift of the first-order Raman peak of diamond is very small (−7.56×10−4 cm−1/K−1.5), which confirms previous results obtained on a wider temperature range. Such a trend implies a very small shift over the studied temperature range, of the same order than the dispersion due to external factors (isotopic composition, orientation). The pressure dependence is found equal to 3.20 (±0.02) cm−1/GPa. A statistical description of the data is provided, indicating that the diamond gauge provides pressure estimates below 1 GPa with an accuracy of±39 MPa. Application of these results to science fields involved in the moderate pressure range below 1 GPa is discussed.

Diamond synthesis under atmospheric pressure from ethanol-water solution using hot-filament chemical vapor deposition method assisted by electrospray

The effect of water on diamond synthesis was investigated in the hot-filament chemical vapor deposition process under atmospheric pressure. We adopted an ethanol-water-hydrogen system; an ethanol-water mixture was supplied by an electrospraying technique. With increasing the water content, the morphology of deposited diamond varied from mirror-ball-like to small-crystalline particles. Dense diamond films with distinct facets were obtained at 5.58mol% ethanol with 1.39mol% water and 5.27mol% ethanol with 2.26mol% water. In Raman spectroscopy, the full width at half maximum (FWHM) of the diamond peak at 1333cm−1 decreased with the water content and a broad peak of nondiamond carbon around 1550cm−1 was markedly reduced. The FWHM also decreased by lowering the ethanol concentration in an ethanol-hydrogen system. Only the ethanol-water-hydrogen system yielded the high-quality diamond with a relatively high deposition rate. The major roles of water were cleaning the filament and removing nondiamond carbon in the deposit with the water-gas reaction, H2O+C→H2+CO.

Growth of Free-Standing Diamond Films on Stainless Steel

Free-standing diamond films have been successfully deposited on stainless steel substrates using microwave plasma-assisted chemical vapor deposition. Although iron, which is the main element of stainless steel, is known to inhibit the nucleation of diamond and enhance the formation of graphite, we were able to grow relatively thick films (- 1.2 mm). The films were easily detachable from the substrates. The poor adhesion made it possible to obtain free-standing diamond films without chemical etching. Raman spectroscopy showed the 1332 cm-1 characteristic Raman peak of diamond and the 1580 cm-1, 1350 cm-1 bands of graphite on the growth surface and backside of the films, respectively. By energy dispersive x-ray spectroscopy it was only possible to detect iron on the back of the films, but not on the surface. The role of iron in the film growth is discussed.

NANOCRYSTALLINE DIAMOND FILMS ON HIGH SPEED STEEL BY MICROWAVE PLASMA CHEMICAL VAPOR DEPOSITION

Nanocrystalline diamond films were deposited on high speed steel substrates by microwave plasma chemical vapor deposition. The best hardness improvement was about 130% of the substrate hardness, using -150 V substrate bias deposition conditions. Raman peaks of nanocrystalline diamond at 1190 cm-1 and of graphite nanoclusters at 1522 cm-1 wereobserved in this sample. Scanning electron microscope images show crystallite grains of different sizes between 25–250 nm.

Magnetoplasmadynamic (MPD) accelerator assisted synthesis of diamond**This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA.

The use of a magnetoplasmadynamic (MPD) accelerator for diamond synthesis is investigated. The acceleration process in an MPD device is reviewed and results presented for a test in which a low power (15 kW) MPD accelerator was used as the gas activation source for diamond deposition. A hydrogen–argon mixture was used in the discharge with externally injected methane as the carbon precursor. Results, including SEM images, Raman and x-ray diffraction spectra are presented for a 3.3 µm thick film deposited over a 100 min period. The Raman spectra include a broad background and shift in the diamond peak, indicative of contamination and stresses in the deposited film. Potential scaling benefits of the MPD accelerator as well as some of the drawbacks associated with methane injection and sample contamination underscored by the experiments are identified and discussed.

Study of nanocrystalline diamond film growth in rf hybrid laser deposition systems in O2, H2 and H2+Ar ambients

Pulsed laser deposition (PLD) and hybrid pulsed laser deposition (HPLD) systems were used for nanocrystalline diamond (NCD) film growth on Si(100) and sapphire(0001) substrates. The PLD system was based on a KrF excimer laser (λ=248 nm,20 ns), which ablated a graphite target in pure oxygen or hydrogen ambient. The HPLD system was based on a combination of PLD and additional capacitively coupled radio-frequency discharges (13.56 MHz) in argon–hydrogen ambient. Radio-frequency power was applied to two plane parallel electrodes or directly to the substrate holder. The working atmosphere pressure was varied from 1 Pa to 220 Pa and the substrate temperature was varied from 20 °C to 660 °C. X-ray diffraction analysis, Raman spectroscopy and a profilometer were used to study the deposited film properties. A band around 1180 cm-1, which can be attributed to NCD, occurred in the Raman spectra. A characteristic diamond peak at 1332 cm-1 was not observed. Films prepared by HPLD showed better compositional homogeneity (from Raman analysis) than films created by PLD.

The effect of negative bias pulse on the bonding configurations and properties of DLC films prepared by PBII with acetylene

The effect of negative bias pulse applied to substrate on the bonding configurations and properties of diamond-like carbon (DLC) films prepared by plasma-based ion implantation (PBII) with acetylene were investigated. The research results show that as the plasma density is 10 9 cm −3 and the negative bias pulses applied to substrate decrease from 50 to 10 kV, the Raman spectra of the carbon films all possess the most dominant characterizations of typical a-C:H [J. Robertson, Mater. Sci. Eng., R 37 (2002) 129-281.], the positions and FWHM for G and D peaks vary no distinguished, but the ratio ID/IG decreases monotonically, as the negative bias pulse decreasing to 5 kV, the Raman spectrum possesses rather strong photoluminescence characterized the polymer-like phase. The variation of Raman spectra for plasma density 10 8 cm −3 is analogous to that of the plasma density 10 9 cm −3. The binding energies of XPS C1s peak decrease from the side of diamond peak to the side of graphite peak with the increasing of negative bias pulse from 10 to 50 kV monotonously, the sp 3 content in the films increases with the decreasing in the negative bias pulse. With the increasing of negative bias pulse from 0 to 50 kV, the surface electric resistance of the films decreases monotonously, but the surface nanohardness at first increases with the increasing of the negative bias pulse from 0 to 10 kV, then decreases with the increasing of the negative bias pulse from 10 to 50 kV monotonously. These properties of the films are corresponding to the bonding configurations of the films. The reason for the highest sp 3 fractions of the a-C:H films formed at higher effective ion energy per C atom in PBII is discussed in this paper.

Transport through a double quantum dot in the sequential tunneling and cotunneling regimes

We study transport through a double quantum dot, both in the sequential tunneling and cotunneling regimes. Using a master equation approach, we find that, in the sequential tunneling regime, the differential conductance $G$ as a function of the bias voltage $\Delta\mu$ has a number of satellite peaks with respect to the main peak of the Coulomb blockade diamond. The position of these peaks is related to the interdot tunnel splitting and the singlet-triplet splitting. We find satellite peaks with both {\em positive} and {\em negative} values of differential conductance for realistic parameter regimes. Relating our theory to a microscopic (Hund-Mulliken) model for the double dot, we find a temperature regime for which the Hubbard ratio (=tunnel coupling over on-site Coulomb repulsion) can be extracted from $G(\Delta\mu)$ in the cotunneling regime. In addition, we consider a combined effect of cotunneling and sequential tunneling, which leads to new peaks (dips) in $G(\Delta\mu)$ inside the Coulomb blockade diamond below some temperature scales, which we specify.

Structural investigation of nanocrystalline diamond/amorphous carbon composite films

Nanocrystalline diamond/amorphous carbon (NCD/a-C) composite films have been prepared by microwave plasma chemical vapor deposition (MWCVD) from methane/nitrogen mixtures. The complex nature of the coatings required the application of a variety of complementary analytical techniques in order to elucidate their structure. The crystallinity of the samples was studied by selected-area electron diffraction (SAED). The diffraction patterns revealed the presence of diamond crystallites within the films. From the images taken by transmission electron microscopy (TEM) the crystallite size was determined to be on the order of 3–5 nm. The results were confirmed by X-ray diffraction (XRD) measurements exhibiting broad (111) and (220) peaks of diamond from which the average size of the crystallites was calculated. The grain boundary width is 1–1.5 nm as observed by TEM images which corresponds to a matrix volume fraction of about 40–50%. This correlates very well with the crystalline phase content of about 50% in the films estimated from their density (2.75 g/cm 3 as determined by X-ray reflectivity). The bonding structure of the composite films was studied by electron energy loss spectroscopy (EELS) in the region of carbon core level. The spectra were dominated by a peak at 292 eV indicating the diamond nature of the investigated films. In addition, the spectra of NCD/a-C films possessed a shoulder at 284 eV due to the presence of a small sp 2 bonded fraction. This phase was identified also by X-ray photoelectron spectroscopy (XPS). The sp 2/sp 3 ratio was on the order of 10% as determined by deconvolution of the C1s XPS peak.

Prediction of a threshold density of atomic hydrogen for nanocrystalline diamond growth at low pressures

A lower-threshold density of atomic hydrogen (H) for nanocrystalline diamond growth in inductively coupled plasma has been examined. Vacuum-ultraviolet absorption spectroscopy was employed to measure the absolute H density in low-pressure conditions below 10.6 Pa, where nanocrystalline diamond films with grain sizes of ∼20 nm showing a distinct diamond peak in the Raman spectra were deposited. A mechanistic explanation for the growth limitation was made by comparing kinetic rates of incorporation and desorption of adsorbed methyl radicals as principal growth precursors. The results predict a threshold H density for net diamond growth of the order of 10 12 cm −3.

Characterization of 〈111〉 diamond thin films by micro-Raman spectroscopy

High-resolution confocal micro-Raman spectra show that for homoepitaxial growth on 〈111〉-oriented surfaces the zone-center phonon peak of the epilayers may occur a few cm −1 below the peak of the relaxed diamond. This indicates a strong tensile strain in the films, while close to the interface the substrate is under compression. This effect was observed in both phosphorus-doped and undoped epilayers, as well as on some facets of a polycrystalline sample. Results obtained on 〈111〉-oriented films grown by microwave plasma chemical vapor deposition in three different laboratories are compared, providing some hints as to how such undesirable strain effects can be avoided.

Non-destructive determination of the boron concentration of heavily doped metallic diamond thin films from Raman spectroscopy

Raman spectra of heavily boron doped polycrystalline diamond films prepared by MPCVD exhibit ‘500’ and ‘1220 cm −1’ structures on a wider boron range than the ‘Fano’ deformation of the ‘1332 cm −1’ peak of diamond induced by the metallic conductivity of these films. Therefore, these peaks were ascribed to the boron concentration rather than directly to the metallic conductivity of the films. The ‘500 cm −1’ wide peak down-shifts as the boron content in the film increases. It can be fitted satisfactorily by the sum of a Gaussian and a Lorentzian component. The wavenumber of the Lorentzian component exhibits a simple and accurate one-to-one relationship with the boron concentration. Therefore, analysis of the Raman spectra can be used as a non-destructive technique to deduce easily and without contact the boron concentration of diamond films in the range 2×10 20 to 10 22 cm −3.

Non-destructive determination of the boron concentration of heavily doped metallic diamond thin films from Raman spectroscopy

Raman spectra of heavily boron doped polycrystalline diamond films prepared by MPCVD exhibit ‘500’ and ‘1220 cm −1 ’ structures on a wider boron range than the ‘Fano’ deformation of the ‘1332 cm −1 ’ peak of diamond induced by the metallic conductivity of these films. Therefore, these peaks were ascribed to the boron concentration rather than directly to the metallic conductivity of the films. The ‘500 cm −1 ’ wide peak down-shifts as the boron content in the film increases. It can be fitted satisfactorily by the sum of a Gaussian and a Lorentzian component. The wavenumber of the Lorentzian component exhibits a simple and accurate one-to-one relationship with the boron concentration. Therefore, analysis of the Raman spectra can be used as a non-destructive technique to deduce easily and without contact the boron concentration of diamond films in the range 2×10 20 to 10 22 cm −3 .

Lonsdaleite diamond growth on reconstructed Si (100) by hot-filament chemical vapor deposition (HFCVD)

In this paper, the growth of Lonsdaleite diamond using hot-filament chemical vapor deposition (HFCVD) on flashed and reconstructed Si (100) is reported. Surface morphology studies using scanning electron microscopy (SEM) show that the film is composed of decahedron and icosahedron diamond particles. The X-ray diffraction (XRD) pattern has a strongest peak at 47° and a peak at 41°, which is indicative of Lonsdaleite nature of the grown diamond film. The Raman spectrum of the film shows a broadened diamond peak at wave number of 1,329 cm−1, which has shifted towards the peak position corresponding to Lonsdaleite nature of the diamond (1,326 cm−1).