Free-standing Diamond Films Research Articles

Ultrathick, low-stress nanostructured diamond films

We describe a hot-filament chemical vapor deposition process for growing freestanding nanostructured diamond films, ∼80μm thick, with residual tensile stress levels ≲90MPa. We characterize the film microstructure, mechanical properties, chemical bond distribution, and elemental composition. Results show that our films are nanostructured with columnar grain diameters of ≲150nm and a highly variable grain length along the growth direction of ∼50–1500nm. These films have a rms surface roughness of ≲200nm for a 300×400μm2 scan, which is about one order of magnitude lower than the roughness of typical microcrystalline diamond films of comparable thickness. Soft x-ray absorption near-edge structure (XANES) spectroscopy indicates a large percentage of sp3 bonding in the films, consistent with a high hardness of 66GPa. Nanoindentation and XANES results are also consistent with a high phase and elemental purity of the films, directly measured by x-ray and electron diffraction, Rutherford backscattering spectrometry, and elastic recoil detection analysis. Cross-sectional transmission electron microscopy reveals a large density of planar defects within the grains, suggesting a high rate of secondary nucleation during film growth. These films represent a new class of smooth, ultrathick nanostructured diamond.

Electrical properties of [1 0 0]-oriented CVD diamond film

We reported the growth and its electrical properties of [100]-oriented diamond film using alcohol and hydrogen by HFCVD. SEM and Raman measurements indicated that high quality polycrystalline diamond films with [100]-faced structure were obtained. Dark current–voltage (I–V), capacitance–frequency (C–F), capacitance–voltage (C–V) and photocurrent under steady-state 55Fe 5.9keV X-ray excitation of freestanding diamond film were investigated at room temperature. Results indicated that after post-annealing for [100]-oriented diamond films dark current was in the order of 10−10A and the photocurrent was of in the order 10−8A by X-ray irradiation with the applied voltage of 40V, capacitance and dielectric loss were very small with the value of 2pF and 2×10−3 with the bias voltage of 0.05V, respectively, and almost had no variation with the change of frequency in high frequencies from 100kHz to 10MHz.

Optical Characterization of Diamond Synthesis Using CH3OH-H2 Gas Mixtures

Diamond films with high infrared transmittance have been successfully deposited using CH3OH-H2 gas mixtures through microwave plasma enhanced chemical vapor deposition (MWCVD). The primary purpose of this study is to determine the effect of the deposition conditions on the optical properties of MWCVD diamond films using CH3OH-H2 gas mixtures. Room temperature optical properties of freestanding diamond films were studied by Fourier transform IR spectroscopy. Experimental results indicated that under appropriate deposition temperature (620 °C) and methanol concentration (5.7%), the refractive index of CVD diamond films (2.33) was comparable with that of natural diamond (2.417). The average infrared transmittance was above 65% in the middle infrared region (500 cm-1-4000 cm-1), approaching to the theoretical value of diamond (71.4%). The mechanism of growing high IR transmittance diamond films by utilizing CH3OH-H2 gas system is that the high methanol concentration used in this study makes the surface roughness of diamond films decreased by increasing the secondary nucleation density and the high O/C ratio in CH3OH-H2 gas system, improved the quality of diamond films and therefore decreased the absorption of non-diamond carbon in the films.

Analysis of Interface between Free-Standing Diamond Films and Mo Substrates

Interfaces between Mo substrate and free-standing diamond films prepared by DC arc plasma jet operated at gas recycling mode were investigated, including for the first time used and multi-time used substrate. The morphology, phase composition and bonding state of elements in the interface between substrates and diamond films were examined by optical microscopy, XRD and XPS. The profiles of carbon concentration of Mo substrates were measured by GDOES. It showed that Mo2C and MoC were formed on the first time used Mo substrate, and MoC was found on diamond films nucleation side after detachment. It suggested that MoC was peeled off from Mo substrate. The stable Mo2C on Mo substrate was formed after multi-time use of Mo substrate. However, MoC has not been found on it. The thickness of carburizing layer on the first time used Mo substrate is up to 30µm, and the carburizing layer on the multi-time used substrate is much thicker than that on the first used. The amorphous carbon in the surface of the substrate and nucleation side of diamond films was found by XPS, including for the first time used and multi-time used substrate.

The Field Emission Properties of Sub-Micrometer Diamond Tube Arrays Fabricated by Focused Ion Beam Pattern

Sub-micrometer diamond tube arrays are formed on freestanding diamond film via focused ion beam pattern technology and chemical etching method. First, the sub-micrometer holes are fabricated on Si substrate by using FIB milling method. Then, diamond film is grown by hot filament chemical vapor deposition method on patterned Si substrate. By controlling the deposition parameters, the diamond can be grown along the wall of holes and the diamond tubes in sub-micrometer scale are formed. Finally, Si substrate is etched by chemical etching method and the diamond tube arrays are fabricated on a freestanding diamond film. Scanning electron microscopy and Micro-Raman spectroscopy measurements are performed to characterize the structure and phase purity of diamond tubes. The electron emission properties from the diamond tube arrays are studied, the result presents that the enhanced emission property can be obtained from diamond tube arrays.

Fabrication of Structure-Designed Free-Standing Diamond Film

Structure-designed free-standing diamond films have been fabricated by DC plasma jet method. The different dominant crystalline surface distributions were obtained under different deposition conditions. The as-grown films were polished by thermal chemical method. For the same crystalline structure, the removal rate was strongly affected by polishing parameters, such as polishing temperature, polishing time and applied pressure. The experimental results also showed that the dominant surface distribution in the films affected the polishing removal rate very much.

Photoconductive and photovoltaic properties of CVD diamond films

Free-standing undoped polished MPCVD diamond films (200–300 μm thick) have been used to fabricate photoconductive and photodiode structures for UV light detection. Multifinger planar Au/Cr electrodes of the photoconductive structures were realized on the diamond surface by lithographic technique. The sandwich semitransparent Ni electrodes of the photodiode structures were deposited by a magnetron sputtering on both sides of the diamond films. The films showed conductivity less then 10 −15 S/cm at room temperatures with activation energy close to 1 eV and low concentration of the electrically active defects according to charge-based deep-level transient spectroscopy (Q-DLTS). The distinct trap levels with activation energy of 0.5, 0.7 and ∼1.3 eV were evaluated. Planar photoconductive detectors with electrodes on the growth side showed the dark current below 1 pA at 10 V bias voltage with spectral discrimination ratio higher than 5×10 5 (at 50 V bias) in the 210–270 nm wavelength range. Two orders of magnitude lower responsivity were found on the nucleation side. The sandwich photodiodes showed a sharp cut-off in photoresponse at 220 nm and photovoltage as high as 2.3 V in open circuit regime.

The field emission properties of high aspect ratio diamond nanocone arrays fabricated by focused ion beam milling

High aspect ratio diamond nanocone arrays are formed on freestanding diamond film by means of focused ion beam (FIB) milling technology and hot-filament chemical vapor deposition (HFCVD) method. The structure and phase purity of an individual diamond nanocone are characterized by scanning electron microscopy (SEM) and micro-Raman spectroscopy. The result indicatesthat the diamond cones with high aspect ratio and small tip apex radius can be obtained by optimizing the parameters ofFIB milling and diamond growth. The diamond nanocone arrays were also used to study the electron field emission propertiesand electric field shielding effect, finding high emission current density, low threshold and weak shielding effect, all attributable to the high field enhancement factor and suitable cone density of the diamond nanocone emitter.

Interface characterization between large area freestanding diamond films and molybdenum substrates

A study of an interfacial layers between freestanding diamond films and substrate diamond film growth was performed. Polycrystalline diamond films were grown on molybdenum substrates by dc arc jet plasma methods. A molybdenum carbide layer epitaxially was formed on the substrates used several times later, and its structure and composition were analyzed by different characterization techniques. The concentration of carbon decreased with an increase of distance from the substrate surface. Thicker and better-defined interfacial layers were found in diamond films grown on substrates used several times. This interface efficiently avoided carbon and hydrogen diffusion and also improved the chemical bond between diamond film and substrate during diamond growth. Characterization by X-ray diffraction, Raman spectroscopy and SEM analysis were also carried out.

Microstructure and fracture strength of different grades of freestanding diamond films deposited by a DC Arc Plasma Jet process

The mechanical properties of chemically vapor deposited diamond films can be characterized in terms of fracture strength. In this paper, different grades of freestanding diamond films were deposited by DC Arc Plasma Jet chemical vapor deposition (CVD) under a variety of deposition conditions. The microstructure and fracture strength of different grades of diamond films were investigated by SEM, X-ray diffraction, Raman spectroscopy and three-point bending, respectively. The columnar structure has great influence on fracture strength; if a columnar structure exists in diamond films, we have found that the fracture strength may not higher than 250 MPa. There is a trend that dominant orientations of optical and mechanical grade diamond films with higher fracture strength are all {220}. As for the fracture mode, optical grade diamond film is mainly intergranular fracture at the lower temperature; at the higher deposition temperature, optical grade diamond film show transcrystalline fracture mode. Fracture mode of mechanical grade diamond film is mainly transcrystalline fracture.

Argon-to-hydrogen ratio in plasma jet diamond chemical vapour deposition

Computer modelling was used to predict the mole fraction of C, C 2H 2, C 2H and C 2 radicals along the substrate radial distance at three different argon-to-hydrogen ratio (AHR) values. The three AHR values used in this investigation were 0.6, 0.8 and 1.0. In addition to computer modelling, freestanding diamond films were deposited onto molybdenum (Mo) substrates using the DC arc Plasma Jet Chemical Vapour Deposition (PJCVD) technique. The principal objectives of this study were to (i) investigate how such key diamond-growth plasma species varied in mole fraction along the radial distance at different AHRs and (ii) establish whether these plasma species had a significant effect on the growth rate, crystallite size and film quality at different AHRs. The modelling results revealed that the mole fractions of the plasma species varied significantly with radial distance at all three AHRs investigated. The growth rates gradually decreased with AHR but remained constant at any one AHR along the radial distance. The average diamond grain size increased with AHR but remained constant along the radial distance at a constant AHR. The two principal factors, which govern film properties such as those investigated in this study, are (i) plasma temperature and (ii) methane (CH 4) concentration. The plasma temperature decreased along the Mo radial distance and increased with AHR, whereas the CH 4 concentration decreased with increasing AHR.

Dielectric property of thick freestanding diamond films by high power arcjet operating at gas recycling mode

Dielectric property of thick freestanding diamond films prepared by high power arcjet operating at gas recycling mode was measured by the high voltage electric bridge method at low frequencies (r.f.) and the wave guide resonance method at high frequencies (microwave). It was found that, with increasing frequencies, dielectric loss of freestanding diamond films increased at low frequencies, but decreased at high frequencies, with a maximum located at approximately 3 MHz. Measurements of dielectric loss of the freestanding diamond films at a microwave frequency of 5.2 GHz showed a strong dependence on the growth parameters such as substrate temperature and methane concentration. It was found that dielectric loss decreased with increasing substrate temperature, and increased with an increasing methane concentration in the feed gases. It is suggested that dielectric loss is closely related with the quality level of freestanding diamond film samples, as demonstrated by the results from Raman and SEM observations. Non-diamond carbon in the diamond films was found responsible for the increase in dielectric loss. Nitrogen was intentionally introduced into the Ar–H 2–CH 4 gas stream for diamond deposition to investigate the effect of impurities. It was shown that nitrogen addition to the feed gases seriously deteriorated the dielectric property of the resultant diamond films. This is again in agreement with our experimental observations by Raman and SEM, in that the addition of nitrogen also seriously deteriorated the quality of the diamond film. Mechanisms for the dielectric behaviour of the diamond films were discussed in detail.

Deposition of nanocrystalline diamond and titanium oxide coatings onto pyrolytic carbon using CVD and sol–gel techniques

In this paper, we report the deposition of freestanding nanocrystalline diamond (NCD) films and dense nanocrystalline titanium oxide (nc-TiO 2) coatings onto pyrolytic carbon (PyC) using advanced chemical vapour deposition (CVD) and sol–gel techniques, respectively. The micro-structural properties, such as surface morphology and crystallinity, of PyC, NCD and nc-TiO 2 were characterised using XRD, Raman spectroscopy and SEM analysis. Freestanding NCD films were prepared using our newly introduced time-modulated CVD (TMCVD) process. Furthermore, conventional sol–gel technique was used to uniformly coat PyC with dense nc-TiO 2 coatings. The as-grown nc-TiO 2 coatings were found to display smooth surface profiles and the average crystallite size was in the range approximately 30–50 nm.

Oxidation behaviour of high quality freestanding diamond films by high power arcjet operating at gas recycling mode

Oxidation may cause degradation of mechanical, thermal and optical properties of freestanding CVD diamond films at elevated temperatures, and thus may impose severe technical limitations for applications of freestanding diamond films. In the present investigation oxidation behaviour of high quality freestanding diamond films prepared by high power d.c. arc plasma jet operating at gas recycling mode has been studied. It was found by thermogravimetry that the diamond films started to oxidize at approximately 650 °C, whilst the rate of oxidation increased substantially with increasing temperatures. Experimental observations confirmed that grain boundaries were the most preferred site for oxidation damage. Detailed studies were made on the influence of high temperature oxidation on the optical, thermal, and mechanical properties of high quality freestanding diamond films. Our results demonstrated that the high quality freestanding films prepared by high power d.c. arcjet can be safely used below 800 °C for a short time period of 180 s, which is more than enough for certain important IR applications.

Surface acoustic wave properties of freestanding diamond films

"Ideal" diamond has the highest acoustic velocity of any material known, and is of great interest as a substrate material for high frequency surface acoustic wave (SAW) device structures. However, little is known of the acoustic wave propagation properties of polycrystalline diamond grown by chemical vapour deposition (CVD) techniques, the commercially accessible form of this material. We report on propagation of laser-generated SAW on three forms of freestanding CVD diamond samples, "white" polycrystalline, "black" polycrystalline, and "highly oriented" diamond. Despite differing sample nature, SAW waves propagating along the smooth (nucleation) side of the diamond showed similar velocities in the range 10600-11900 ms(-1). These results are discussed in terms of the potential of each form of CVD diamond for SAW device fabrication.

AC conductance and impedance spectroscopy of polycrystalline diamond films

Impedance measurements were performed in the frequency and temperature domain to infer the bulk and grain boundary transport of freestanding polycrystalline diamond films. Analyzed samples appear like insulators up to a threshold temperature where leaking becomes evident. Depending on deposition conditions and thickness, frequency independent conduction channels add to AC ones. The real component of conductivity, measured at different temperatures, can be reduced to a master curve according to a single hopping transport process. Therefore, also the real and imaginary component of impedance, measured at different temperatures in the 2–10 7 Hz frequency range, can be reduced to a master curve and plotted vs. a reduced angular frequency showing a circuit structure more complex than a simple Voigt element. The Nyquist plot is formed by two different lobes that have been interpreted as related to grain and grain boundary dielectric contributions. However, only a single dielectric loss peak is achieved by data elaboration. Dissimilarities among materials deposited by using different techniques are also evidenced and discussed.

High Quality MPCVD Epitaxial Diamond Film for Power Device Application

ABSTRACTAs a wide bandgap (5.47 eV) semiconductor material, single crystal diamond has high electron mobility (reportedly between 2000 and 4400 cm2V-1s-1), high electron saturation velocity (2×107 cms-1), high breakdown voltage (>107 Vcm-1), and high thermal conductivity (>21 Wcm-1K-1). Diamond-based semiconductor devices offer the potential of operation at high voltages, power levels, temperatures and under extreme radiation conditions. In this work, we present our effort to grow high quality homo-epitaxial diamond films on (100)-single crystal diamond substrates by microwave plasma chemical vapor deposition (MPCVD). The growth rate can vary from 0.01 to 100 micrometers per hour, depending on growth conditions, doping, and quality; and using a “lift-off” process, free-standing homo-epi films with remarkably low p-type doping (1×1014–1×1017 cm-3) and exceptionally low compensation ∼ 1×1013 cm-3 have been made. Vertical and lateral structure high voltage diamond Schottky rectifiers have been built for frequency dependent capacitance-voltage (C-V), and current-voltage (I-V) measurements. A breakdown voltage of 8 kV at 100 μm distance and 12.4 kV at 300 μm distance is recorded for lateral structure devices without Ohmic contact (back to back Schottky contacts), while an un-optimized vertical device with an back-side Ohmic contact has demonstrated a forward voltage drop of 7 V at 18 A/cm2 in a device that can only block 600 V. New test results show 3.7 kV blocking voltage vertical devices on 20 μm freestanding MPCVD diamond film. This data shows that the quality of diamond film extremely affect the electrical properties of the built devices.

Effects of nitrogen addition on morphology and mechanical property of DC arc plasma jet CVD diamond films

Nitrogen-doped diamond films have been synthesized by 100 KW DC arc plasma jet chemical vapor deposition using a CH 4/Ar/H 2 gas mixture. The effect of nitrogen addition into the feed gases on the growth and surface morphology and mechanical property of diamond film was investigated. The reactant gas composition was determined by the gas flow rates. At a constant flow rate of hydrogen (5000 sccm) and methane (100 sccm), the nitrogen to carbon ratio (N/C) were varied from 0.06 to 0.68. The films were grown under a constant pressure (4 KPa) and a constant substrate temperature (1073 K). The deposited films were characterized by scanning electron microscopy, Raman spectroscopy and X-ray diffraction. The fracture strength of diamond films was tested by three point bending method. The results have shown that nitrogen addition to CH 4/H 2/Ar mixtures had led to a significant change of film morphology, growth rate, crystalline orientation, nucleation density and fracture strength for free-standing diamond films prepared by DC arc plasma jet.

The use of CVD diamond to produce carbon K ultrasoft x-rays

Carbon K ultrasoft x-rays (278 eV) interact with biological material producing random, isolated tracks of single electrons with a range <7 nm (cf width of DNA helix ∼2 nm). The electron tracks produced by these ultrasoft x-rays are similar to the numerous secondary electrons and ‘track ends’ produced by essentially all ionizing radiations. They therefore provide a unique tool in the study of mechanisms of radiation action in the cell. Conventional carbon targets used with the Medical Research Councils cold-cathode discharge tubes have a limited lifetime of 40–50 min. We have investigated the use of thin, free-standing diamond films produced using chemical vapour deposition techniques as a new transmission target for a cold-cathode tube. We present here a novel use of diamond to produce CK ultrasoft x-rays for irradiation purposes. The system described produces an entrance absorbed dose rate to attached cells of ∼0.2 Gy min−1 which due to the long target lifetime is usable for low dose-rate CK x-ray studies.

Further investigations on the use of polycrystalline CVD diamond as a UV detector

Metal-semiconductor-metal planar structures were fabricated on free-standing diamond films. The devices were found to operate successfully as photodetectors for deep ultraviolet light with their response being dependent on the post-growth treatment. However, evidences of trapping were found both in the spectral photoresponse and temporal response to monochromatic light transients.