Measurement of charge carrier dynamics in diamond thin films using a fast TOF system with a UV pulsed laser

Charge carrier dynamics in CVD homoepitaxial diamond thin films were evaluated. Thicknesses of these thin diamond films were 4.5–14 μm. Measurement was carried out using a fast TOF system with time resolution of 150 ps. This system adopted the localized irradiation mechanism of a 213-nm UV pulsed laser. Induced current caused by charge carrier motion was simulated by a one-dimensional approximation. Then measured signals were analyzed using the physical model of induced current that was confirmed through the simulation. The best lifetimes, τ, of electrons and holes in evaluated diamond films were 1.8 ± 0.3 ns for both charge carriers. However, drift velocities and mean free paths of trapping were not determined because insufficient charge carriers arrived at the further side electrode without trapping.     

The application of polycrystalline diamond in a thin film packaging process for MEMS resonators

This paper reports the design, fabrication and testing of a polycrystalline diamond (poly-C) thin film packaging process for a MEMS cantilever type resonator using a 4-mask fabrication process, which integrates chemical vapor deposition (CVD) diamond thin film technology with an encapsulation packaging process. After poly-C cantilever beam resonators were fabricated using the first two masks, a sacrificial PECVD SiO₂ layer with a thickness in the range of 4–5 μm was deposited at 350 °C and patterned to create the package anchor. Then, a 4-μm-thick poly-C film was grown and patterned to create the thin film packaging structure containing fluidic access ports for the removal of the sacrificial layer. The fluidic access ports were finally sealed with an additional poly-C growth. To evaluate the efficacy of the poly-C encapsulation process, poly-C cantilever beam resonators were tested using a piezoelectric actuation and laser detection method before and after the poly-C packaging process. Resonance frequencies measured before and after are in the range of 240–320 kHz, which is consistent with predicted calculations. A modified fabrication process was designed to test the fluidic hermiticity of the thin film package.     

A lateral time-of-flight system for charge transport studies

A measurement system for lateral ToF charge carrier transport studies in intrinsic diamond is described. In the lateral ToF geometry, carriers travel close to the sample surface and the system is therefore particularly suited for studies of thin layers as well as the influence of different surface conditions on transport dynamics. A 213 nm pulsed UV laser is used to create electron–hole pairs along a line focus between two parallel metal electrodes on one surface. The use of reflective UV-optics with short focal length allows for a narrow focal line and also for imaging the sample in UV or visible light without any dispersion. A clear hole transit was observed in one homoepitaxial single crystalline diamond film for which the substrate was treated by a Ar/Cl plasma etch prior to deposition. The hole transit signal was sufficiently clear to measure the near-surface hole drift mobility of about 860 cm²/Vs across a contact spacing of 0.3 mm.     

Measurement of charge carrier's transportation in a large size self-standing CVD single crystal diamond film fabricated using lift-off method

Using lift-off method, we synthesized large self-standing plasma CVD diamond films on various substrates. Charge carrier transportation in diamond was measured using α particle measurements and TOF methods with a short-pulsed UV laser. The high-quality films were synthesized rapidly. We observed the maximum transit time of holes and electrons shorter than 5 ns. The lift-off method is useful to fabricate the high-quality diamond with excellent drift velocities of the charge carrier. The charge transport characteristics of our diamond films are comparable to those of a commercially available (Element Six Ltd.) electronics grade IIa diamond single crystal.     

Extreme UV single crystal diamond photodetectors by chemical vapor deposition

The detection properties of a UV photodetector realized on a 150 μm thick CVD single crystal diamond film, grown at Roma “Tor Vergata” University on a low cost HPHT diamond substrate, are reported. The device was tested in the 210–2400 nm spectral range using pulsed laser irradiation and in the 20–250 nm range in continuous mode by both a deuterium lamp and a helium DC gas source irradiation.The detector shows more than five orders of magnitude of visible/UV rejection ratio, a very sharp signal drop of about 10⁴ being observed in correspondence of the diamond energy gap. In the extreme UV range, the He II 25.6 and 30.4 nm as well as the He I 58.4 nm emission lines are clearly detected. The diamond time response is demonstrated to be considerably lower than 5 ns and 0.2 s in pulse and continuous mode, respectively.The extremely good signal to noise ratio, stability and reproducibility of the device response obtained, indicate that no persistent photoconductivity nor undesirable pumping effects are present, which represented so far the main problems preventing the use of diamond based detectors for UV applications.     

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).     

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.     

UV Schottky photodiode on boron-doped CVD diamond films

We report on experimental study of photosensitivity and Q-DLTS spectra of polycrystalline CVD diamond UV photodetectors. The measured characteristics of Schottky photodiode on boron-doped diamond films are compared with those obtained for planar photoconductive structures (photoresistor type) based on undoped CVD diamond. The Schottky photodiode exhibited a sharp cut-off in photoresponse with spectral discrimination ratio (between wavelengths of 190 nm and 700 nm) as high as 5 · 10⁵ at zero bias voltage (at zero dark current). The photodiode showed the maximum of photoresponse at wavelength < 190 nm, and a low density of trapping and recombination centers as evaluated with the Q-DLTS technique. The devices demonstrated the photoresponsivity at 190 nm from 0.03 to 0.1 A/W with quantum yield of 0.20 to 0.67 in closed circuit, while the photovoltage ≥ 1.6 V was measured in open circuit regime. Another type of UV detector, the planar photoconductive structures with interdigitizing ohmic electrodes fabricated on undoped diamond film and operated under a bias voltage, revealed a higher density of (surface) defect centers and the maximum photoresponse at  210 nm wavelength. A strong influence of UV light illumination on the Q-DLTS spectra of the planar photoconductive structures was observed. This effect can be used for development of new UV detectors and dosimeters based on the Q-DLTS signal measurements.     

Saturation drift velocity measurement for CVD diamond by combination of a charge distribution measurement and a TOF method using a UV pulsed laser

Drift velocities of charge carriers in polycrystalline diamonds were measured by a self-triggered time-of-flight (TOF) method with alpha particles based on a radiation measurement technique. Based on these measured results, a synthesis method for polycrystalline diamond was verified, and the electric properties of polycrystalline diamonds were improved; drift velocity was increased from 5 × 10² to 3 × 10⁴ cm/s. The mean free paths (MFPs) of capture of charge carriers in a CVD single crystal diamond was obtained by induced charge distribution measurements with alpha particles, and drift velocity was measured by another TOF method using a UV pulsed laser. MFPs of capture of electron and hole in a CVD single crystal diamond were determined to be 5.4 and 9.6 μm, respectively; the hole and electron drift velocities were 5 × 10⁵ cm/s and 3 × 10⁵ cm/s in an electric field of 24.4 kV/cm, respectively. For diamond, short transit times of several nanoseconds and short MFPs of capture in several micrometers were successfully obtained for the first time by combining of these methods.     

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.     

Analysis of trapping–detrapping defects in high quality single crystal diamond films grown by Chemical Vapor Deposition

The photoresponse of high quality single crystal diamond films homoepitaxially grown by Chemical Vapor Deposition (CVD) onto low cost diamond substrates has been studied. The time evolution electrical response to the excitation by 5 ns laser pulses at 215 nm closely reproduces the laser pulse shape. The single crystal diamond response is therefore much faster than the laser pulse duration. The output signal is also very stable and reproducible, without significant priming or memory effects. Single crystal diamond films can therefore be grown by CVD having enough high quality to be used as photodetectors.However, a minor slow component shows up in the charge-integrated sample response. A systematic speed up of this slow component when increasing the detector temperature from − 25 °C to + 50 °C demonstrates its thermally activated origin. The slow component is therefore attributed to detrapping effects from shallow trapping centres.A model of the charge transport mechanism in the presence of trapping–detrapping centres can be developed and the results can be compared to the experimental ones. The activation energy of the shallow defects is accordingly determined as E_a = 0.4 eV.     

Low temperature time of flight mobility measurements on synthetic single crystal diamond

We study the temperature dependence of low charge injection drift mobility in single crystal (SC) diamond using an alpha particle source. We present time of flight (ToF) mobility measurements to investigate the charge carrier scattering mechanisms in SC synthetic diamond in the temperature range 200 K–300 K. We have used a gold contacted pad detector, with a “sandwich” contact structure, fabricated using a SC chemical vapour deposition (CVD) diamond synthesised by Element Six Ltd. ToF analysis of alpha particle induced current pulses shows a strong increase in hole mobility at reduced temperatures, consistent with acoustic phonon scattering processes dominating the charge carrier transport. On the other hand, electron mobility values appear to remain relatively constant with lower temperatures suggesting different mechanisms than optical or acoustic phonon scattering limiting the charge transport.     

Alpha particle transient response of a polycrystalline diamond detector

We report the transient response of room temperature pulses generated from alpha particles from a chemical vapor deposition (CVD) polycrystalline diamond detector. For transient signals dominated by electron transport only prompt pulse shapes were observed with an average rise time of 160 ns limited by the preamplifier rise time. For transient signals dominated by hole transport significant slow components were observed in the majority of pulses due to thermal emission of charge from shallow hole traps. These slow pulses were observed from the device when in its as-grown state, without any previous ‘priming’. Two separate slow components were identified from the hole pulses, with average rise times of 600 ns and 1–10 μs, respectively. These data suggest that room temperature electron transport in polycrystalline CVD diamond is prompt, with no evidence for thermal de-trapping. In contrast hole transport in our sample at room temperature contains a significant delayed component due to thermal emission of holes from at least two bands of shallow defects.     

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.     

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.     

Charge transport in high mobility single crystal diamond

Time of Flight (TOF) measurements using conventional laser TOF and α-particle TOF setups have been carried out on high quality CVD diamond samples to study the electron and drift mobility and to compare them with the mobility data for IIA diamond. The measured mobilities for all samples investigated are in the range 2000–2250 cm²/Vs for holes and 2200–2750 cm²/Vs for electrons, thus close to the theoretical prediction as well as to IIa diamond mobility values. The charge transient profile measured in the laser TOF measurements is influenced by the electric field profile in the sample, which might be changed based on the charge trapping at low electric fields applied, depending on the surface atomic termination. The temperature dependence of the drift mobility indicates that at room temperature the scattering on acoustic phonons is the main dominant scattering mechanism and the contribution of other types of carrier scattering mechanism is negligible.     

Recent IBIC measurements on epitaxial CVD diamond

Frontal IBIC (Ion Beam Induced Current) mesurements have been carried out on new single crystal epitaxial CVD diamond. The sample consists of about 100 μm synthetic diamond grown by microwave CVD on a 300 μm thick, low cost, HPHT diamond substrate (see Balducci et al. – this conference). Both proton and alpha microbeams of energies 3 and 4.5 MeV have been used, with a beam diameter spot of about 1.5–3 μm. Scanned areas varied from 450 μm × 450 μm down to 150 μm × 150 μm and the homogeneity of charge collection efficiency (cce) was suitably monitored. At voltage bias of 80–100 V, the average cce was in the range 42–50%. Depending on the scanned surface area and on the beam type, energy resolutions FWHM from 1.3% to 4.1% FWHM have been obtained, even at counting rates as high as 700 cps.     

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.     

Enhancement of the field emission properties of low-temperature-growth multi-wall carbon nanotubes by KrF excimer laser irradiation post-treatment

Multi-wall carbon nanotube (CNT) films were fabricated by microwave plasma chemical vapor deposition at low temperatures ( 500 °C). The films when properly post-treated by laser irradiation exhibited a factor of 2–3 enhancement in the emission current, while the turn-on field (E_on) was reduced from 4.89–5.22 to 2.88–3.15 V/μm. The introduction of excessive oxygen during laser irradiation, however, degrades the performance of field emission properties drastically. Raman spectroscopy measurements revealed the intimate correlation between the parameter I_D/I_G (intensity ratio between the two representative Raman peaks seen in carbon nanotubes) and the field emission performance. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses showed that the irradiation-induced modification of the tube morphology and crystallinity might be responsible for the observations.     

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.