Formation of low-resistance contact between titanium and lightly doped polycrystalline diamond using highly doped interlayer

A low-resistance ohmic contact between lightly doped polycrystalline diamond (poly-C) and metal was achieved for piezoresistive sensor applications using highly doped poly-C thin interlayer in the contact area for the first time for poly-C. Two Trimethylboron (TMB) doping concentrations were used during the growth of poly-C films using microwave plasma chemical vapor deposition (MPCVD), which yielded a 0.2 μm highly doped layer on top of 1.8 μm lightly doped layer. The resistivities of the highly and lightly doped poly-C layers are 0.022 and 151 Ω cm, respectively. The contacts were defined by partially etching the highly doped poly-C layer beyond the contact area. Kelvin bridges are fabricated to test the contact resistance. It is demonstrated that the contact resistivities are 0.0028 and 0.0083 Ω cm² for contacts with and without interlayer, respectively. This method reduced the contact resistance to one third of the original value and improved the performance of the piezoresistive sensor.     

Development of a TOF measurement system of charge carrier dynamics in diamond thin films using a UV pulsed laser

A fast TOF measurement system with 150 ps time resolution for transport behavior of free charge carriers in an intrinsic diamond film by using a UV pulsed laser was developed. The 213 nm UV laser light narrowed to approximately 80 μm widths could locally create hole–electron pairs in selected locations on a diamond film between two parallel electrodes on the surface. This system measured accurate charge transport characteristics in a diamond film, because created charge carriers moved in a part of the diamond film where they did not get any influence from the laser irradiation. Diamond samples used for verification of the TOF system were intrinsic CVD diamond films with thickness between 4 and 10 μm grown on HP/HT diamond substrates. Transit time of holes for one diamond film was 4.7 ns with a traverse distance of 250 μm. The local irradiation of laser made it possible to measure transport characteristics of electrons and holes separately. In addition, it substantially reduced the influence of photoelectron, because the laser beam did not irradiate electrodes. Through several examinations, excellent reliability of the TOF system was confirmed.     

Fabrication technology for single-material MEMS using polycrystalline diamond

Diamond, due to its large band gap of 5.5 eV, offers the possibility of making MEMS structures out of a single material by varying the doping level to achieve the semiconducting, metallic and insulating (undoped) properties needed in a typical MEMS structure. Polycrystalline diamond (poly-C) is inexpensive and retains many of the unique properties of single-crystal diamond. However, the development of diamond-based single-material MEMS (SMM) technology faces two major challenges; (a) producing highly-insulating and highly-conducting poly-C films in a multilayer structure and (b) developing dry-etching technology to produce multilayer structures made of poly-C. Furthermore, poly-C can be layered to perform a number of functions, whereas a complex stack of materials would otherwise be required. Consequently, due to poly-C's high selectivity as a masking material, the SMM fabrication process developed in the current work allows the reduction of the number of fabrication masks by a factor of 1.5–2 as compared to that used in a conventional MEMS process. A number of complex poly-C SMM structures were fabricated using SiO₂ as a sacrificial layer to address the SMM related issues in a single paper.     

Simple fabrication process of a screen-printed triode-CNT field emitter array

We introduced a simple fabrication process for field emission devices based on carbon nanotubes (CNTs) emitters. Instead of using the ITO material as a transparent electrode, a metal (Au) with thickness of 5–20 nm was used. Moreover, the ITO patterning process was eliminated by depositing metal layer, before the CNT printing process. In addition, the thin metal layer on a photoresist (PR) layer was used as UV block. We fabricated the CNT field emission arrays of triode structure with a simple process. And I–V characteristics of field emission arrays were measured. The maximum current density of 254 μA/cm² was achieved when the gate and the anode voltages were kept 150 and 3000 V, respectively. The distance between anode and cathode was kept constant.     

Production of single crystal diamond needles by a combination of CVD growth and thermal oxidation

Single crystal diamond needles were produced by combining chemical vapor deposition of a thin polycrystalline diamond film and its subsequent thermal oxidation. The deposition process has been carried out in a direct discharge activated hydrogen–methane gas mixture with parameters providing (100) textured diamond film growth. The oxidation has been performed by heating the deposited film in an air atmosphere with a temperature allowing the selective etching of the smallest fraction of the diamond crystallites in the film. As a result of this procedure, perfectly shaped micrometer sized single crystal diamond pyramids were obtained. The pyramids have a rectangular base plane with an apex tip curvature radius of about 2 to 10 nm. The atomic structure of the pyramids was established using high resolution transmission electron microscopy. We propose a model explaining the mechanism of the pyramids' formation.     

Machining behaviour of silicon nitride tools coated with micro-, submicro- and nanometric HFCVD diamond crystallite sizes

Very smooth CVD diamond films are used as direct coatings on Si₃N₄ tool substrates. By adjusting deposition parameters, namely Ar/H₂ and CH₄/H₂ gas ratios, and substrate temperature, nano- (27 nm) and submicrometric (43 nm) crystallite sized grades were produced in a hot filament reactor. Also, a conventional 5 and 12 μm micrometric grain size types were produced for comparison. Normalized coated inserts were tested for dry turning of WC–25 wt.% Co hardmetal. All the CVD diamond grades endured the hardmetal turning showing slight cratering, having the flank wear as the main wear mode. Their turning performance was distinct, as a consequence of morphology and surface roughness characteristics. Among all the tested tools, the more even surface and the submicrometric grade presented the best behaviour regarding cutting forces, tool wear and workpiece surface finishing. For this coating, the depth-of-cut force attained the lowest value, 150 N, the best combination of wear types (KM = 30 μm, KT = 2 μm and VB = 110 μm) and workpiece surface finishing (Ra = 0.2 μm).     

Growth rate improvements in the hot-filament CVD deposition of nanocrystalline diamond

The hot-filament CVD, a less used technique for NCD films growth using Ar/H₂/CH₄ gas mixtures, is optimized for the coating of silicon nitride ceramics. Parameters such as gas composition (Ar/H₂ and CH₄/H₂ ratios), total gas pressure, total mass flow and substrate and filament temperatures, are studied to assess their effect on NCD growth kinetics as well as on film quality and morphology. The smallest diamond crystallite sizes (8 nm) were recorded for the slowest growth rate of 0.1 μm h^− 1. A remarkable result is the very high growth rate of 1.6 μm h^− 1 of continuous NCD coatings with 28 nm of crystallite size, obtained in selected deposition conditions.     

Incorporation of hydrogen in diamond thin films

In this investigation, diamond thin films with grain size ranging from 50 nm to 1 µm deposited using hot filament chemical vapor deposition (HFCVD) have been analyzed by elastic recoil detection analysis (ERDA) for determining hydrogen concentration. Hydrogen concentration in diamond thin films increases with decreasing grain size. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) results showed that part of this hydrogen is bonded to carbon forming C–H bonding. Raman spectra also indicated the increase of non diamond phase with the decrease in crystallite size. Incorporation of hydrogen in the samples and increase of hydrogen content in nanocrystalline sample are discussed. Large separation between filament and substrate used for the synthesis of nanocrystalline film helped to understand the large incorporation of hydrogen in nanocrystalline diamond films during growth. The study addresses the hydrogen trapping in different samples and higher hydrogen concentration in nanocrystallites by considering the synthesis conditions, growth mechanisms for different grain sized diamond films and from the quality of CVD diamond films.     

Substrate pre-treatment by ultrasonication with diamond powder mixtures for nucleation enhancement in diamond film growth

Pre-treatment of silicon substrates by ultrasonic abrasion for nucleation enhancement in diamond film formation by hot-filament chemical vapour deposition is discussed. Scanning electron microscopy, atomic force microscopy and visible Raman spectroscopy were employed as analysis techniques. Ultrasonication was applied by suspensions of isopropanol with micro-or nanosized diamond powders, micro-sized metal and alumina particles and mixtures thereof. The root mean square roughness of the ultrasonically pre-treated samples varied from 0.2 to 12.0 nm depending on the applied powder mixture. All samples that were ultrasonically pre-treated had a larger diamond nucleation density than the untreated silicon wafer. As expected, for an effective increment of the diamond nucleation density by several orders of magnitude the application of diamond powder is necessary, since the generation of surface roughness alone is not sufficient to enhance the diamond nucleation kinetics satisfactorily. The simultaneous action of diamond powders and large alumina or titanium particles leads to an increase in diamond nucleation density up to a factor of 10⁶. When nano-diamond powder is used, the embedment of diamond fragments is best and in combination with titanium grains (50–75 µm) a diamond nucleation density of 8 × 10⁹ cm^− 2 is obtained. After 8 h of film growth, the diamond surface grains are significantly smaller for the samples that demonstrated higher nucleation densities, whereas the quality of the diamond layers is equal.     

The role of pressure on thin amorphous carbon films deposited using microwave surface wave plasma CVD

We report the effects of gas composition pressure (GCP) on the optical, structural and electrical properties of thin amorphous carbon (a-C) films grown on p-type silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition (MW SWP CVD). The films, deposited at various GCPs ranging from 50 to 110 Pa, were studied by UV/VIS/NIR spectroscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and current–voltage characteristics. The optical band gap of the a-C film was tailored to a relatively high range, 2.3–2.6 eV by manipulating GCPs from 50 to 110 Pa. Also, spin density strongly depended on the band gap of the a-C films. Raman spectra showed qualitative structured changes due to sp³/sp² carbon bonding network. The surfaces of the films are found to be very smooth and uniform (RMS roughness < 0.5 nm). The photovoltaic measurements under light illumination (AM 1.5, 100 mW/cm²) show that short-circuit current density, open-circuit voltage, fill factor and photo-conversion efficiency of the film deposited at 50 Pa were 6.4 μA/cm², 126 mV, 0.164 and 1.4 × 10^− 4% respectively.     

Self-aligned fabrication of single crystal diamond gated field emitter array

This paper describes a self-aligned fabrication process for diamond gated field emitter array (FEA). Utilizing the non-conformal coverage sputtering conditions of silicon oxide, an interesting “sphere on cone” structure is formed on diamond nano tip array, which is the key point of gate hole opening process. This structure causes shadowing at certain regions of side-wall during Ti / Au gate metal deposition. Removal of “sphere” by wet etching leads to the successful fabrication of a single crystalline diamond gated FEA. Scanning electron microscope observations reveal the fabrication of a uniform emitter array with tip radius of curvature (20 nm) and gate hole (1.4 μm). We also confirmed that no noticeable physical damage exists on tip. In field emission characteristics of the fabricated single crystal diamond gated FEA, gate voltage control of field emission current is realized.     

Homoepitaxial CVD diamond: Raman and time-resolved PL characterization

In this work, we report on the structural characterization of homoepitaxial Microwave Plasma Enhanced CVD diamond grown onto Ib diamond substrates by varying systematically the methane to hydrogen ratio in the gas mixture (1–7% CH₄). X-ray diffraction, Raman spectroscopy and photoluminescence (PL) have been used to characterize the diamond samples. Raman measurements pointed out the excellent crystalline quality and phase purity of the specimens. PL measurements in the 1.7–2.7 eV energy range have shown completely flat spectra, excluding the presence of nitrogen-related optical centers. Such results show that the homoepitaxial CVD diamond can be grown, at moderate microwave power (720 W), and at growth rates not too low ( 1 μm/h) preserving a good quality. Moreover, the homoepitaxial crystals exhibited a strong free-exciton recombination radiation at room temperature even at the highest methane concentration used (7%). Preliminary measurements of the lifetime of the free exciton at room temperature have been also performed. The excitation was produced by a 5 ns pulsed laser irradiation at energies above the diamond band gap. The results have been compared with the structural properties of the samples and correlated with the growth conditions.     

Improving purity and size of single-crystal diamond plates produced by high-rate CVD growth and lift-off process using ion implantation

The lift-off process using ion implantation has recently been applied to produce large and thick single-crystal diamond plates by chemical vapor deposition (CVD). CVD growth conditions for undoped, as opposed to nitrogen-doped, diamond were investigated to improve the purity of plates produced by this technique. This utilized apparatus identical to that for high-rate growth with nitrogen addition under high-density plasma. By lowering the growth temperature to 900 °C, an undoped single-crystal CVD diamond plate with a maximum length of 9 mm and thickness of 0.47 mm was successfully produced without formation of non-epitaxial crystallites. The UV–Vis–NIR transmission spectrum of this plate was identical to high-pressure high-temperature (HPHT) synthetic IIa diamond, suggesting high purity of the plate. To increase the size of single-crystal CVD diamond plates, a process to enlarge the seed crystal by combining the lift-off process and a side-surface growth technique is proposed. By this process, a half-inch single-crystal CVD diamond seed crystal was successfully synthesized and half-inch freestanding single-crystal CVD diamond plates were produced from the seed.     

Anisotropic dry etching of boron doped single crystal CVD diamond

Semiconducting boron doped single-crystal CVD diamond has been patterned using aluminum masks and an inductively coupled plasma (ICP) etch system. For comparison insulating HPHT diamond samples were also patterned using the same process. Diamond etch rates above 200 nm/min were obtained with an O₂/Ar discharge for a gas pressure of 2.5 mTorr using 600 W RF power. We have accomplished the fabrication of structures with a minimum feature size of 1 μm with vertical sidewalls in both CVD and HPHT diamond. The ICP etching produced smooth surfaces with a typical root-mean-square surface roughness of 3 nm. The dependence of etch rate on bias voltage was somewhat different for the two types of diamond. However, for all samples both the etch rate and anisotropy were found to improve with increasing bias voltage.     

Surfactant-modified bentonite as adsorbent for the removal of humic acid from wastewaters

The effectiveness of surfactant-modified bentonite (SMB) in removing humic acid (HA) from wastewaters was evaluated. Hexadecyl trimethylammonium (HDTMA) chloride was used to modify the surface of the clay mineral. The SMB exhibits very high adsorption potential for HA and at pH 3.0 more than 99% removal was achieved from an initial concentration of 25 μmol/L. The experimental kinetic data were analyzed using the pseudo-first-order kinetic model. Adsorption occurs through film diffusion at low as well as at higher concentrations and temperatures. The adsorption of HA using SMB was an exothermic process. HA adsorption was found to decrease with increase of ionic strength due to the formation of outer-sphere surface complexes on SMB. The equilibrium isotherms were determined and data were analysed using the Langmuir isotherm model. The maximum adsorption capacity, Q° was 73.52 μmol/g with binding constant, b = 0.155 L/μmol at 30 °C and pH 3.0. The adsorbent was suitable for repeated use (more than 3 cycles) without any noticeable loss of capacity.     

Characterization of high-quality polycrystalline diamond and its high FET performance

We characterized high-quality polycrystalline diamond with large grains and fabricated polycrystalline diamond field effect transistors (FETs). The polycrystalline diamond had (110) preferred orientation, and its typical grain size was  100 μm. Well-resolved free exciton related emissions were observed at room temperature in cathodoluminescence. The FETs showed extremely high DC and RF performance. The cut-off frequency for current gain (f_T) and power gain (f_max) were 45 and 120 GHz, respectively. The maximum drain current (I_DS) was 550 mA/mm. These values are the highest among diamond FETs, including those fabricated from single-crystal diamond. These results suggest that high-quality polycrystalline diamond, whose maximum size is 4 in., is very promising for diamond electronic devices.     

High-Q optomechanical circuits made from polished nanocrystalline diamond thin films

We demonstrate integrated optomechanical circuits with high mechanical quality factors prepared from nanocrystalline diamond thin films. Using chemomechanical polishing, the RMS surface roughness of as grown polycrystalline diamond films is reduced below 3 nm to allow for the fabrication of high-quality nanophotonic circuits. By integrating free-standing nanomechanical resonators into integrated optical devices, efficient read-out of the thermomechanical motion of diamond resonators is achieved with on-chip Mach–Zehnder interferometers. Mechanical quality factors up to 28,800 are measured for four-fold clamped optomechanical resonators coupled to the evanescent near-field of nanophotonic waveguides. Our platform holds promise for large-scale integration of optomechanical circuits for on-chip metrology and sensing applications.     

Change of cathodoluminescence spectra of diamond with irradiation of low energy electron beam followed by annealing

We have reported previously that cathodoluminescence (CL) spectra of type II diamond change with irradiation of 20 kV electron beam using a scanning electron microscope (SEM), although it is known that diamond has high resistance against radiation beams. Following the previous paper, the diamond irradiated with the electron beam was annealed at 500 °C, and the CL spectra were measured. It was found from the annealed sample that the CL spectra which changed with the irradiation are partially recovered, namely, the lines at 420 nm and 540 nm formed with the irradiation are annealed out, and the lines at 485 nm, 535 nm and 545 nm which disappeared with the irradiation are recovered. However the 420 nm broad band that appeared with the irradiation does not change with the heat treatment. The effect of the heat treatment suggests that the CL peaks which changed with the irradiation are related to ionization of defects.     

Effect of particle size on cesium exchange kinetics by K-depleted phlogopite

Cation exchange mechanism and rate of Cs⁺ exchange were investigated in < 2 μm and 20–2 μm particle size fractions of K-depleted phlogopite (Na-phlogopite). The K-depleted phlogopite was prepared from a natural phlogopite by a potassium removal method using sodium tetraphenylborate (NaTPB) at room temperature. X-ray diffraction (XRD) patterns revealed that interlayer K⁺ ions were completely replaced with sodium ions after the potassium removal treatment. Ion exchange isotherms and kinetics were determined for Na⁺ → Cs⁺ exchange with two particle size fractions. The isotherms indicated that both particle size fractions showed high selectivity for Cs⁺. Based on the isotherm tests, ΔG^o values of < 2 μm and 20–2 μm particle fractions were − 6.83 kJ/mol and − 7.08 kJ/mol, respectively. Kinetics of Cs exchange revealed that the 20–2 μm particle size fraction of the K-depleted phlogopite took up more Cs⁺ ions than the < 2 μm particle size fraction. Various kinetic models were applied to describe Na⁺ → Cs⁺ exchange process. Elovich model described the kinetic data of the < 2 μm particle size fraction well, while the modified first-order model or parabolic diffusion model described the data of the 20–2 μm particle size fraction well.