High Quality Diamond Films Research Articles

Electric Characteristics of Diamond Film Synthesized by Combustion Flame

The purpose of this research was to clarify the electric characteristics of diamond film synthesized by combustion flame and to synthesize a suitable diamond film for use in electroric device applications. When the film contains amorphous carbon, the dielectric loss increases, i.e., the electric characteristics of the diamond film become worse. To decrease the amorphous carbon in the diamond film we utilized three methods, the detection of the optimal equivalence ratio, the addition of hydrogen into synthesizing gas and heat treatment (annealing) of the diamond film. All of these methods can decrease the amount of amorphous carbon effectively, thus systematically changing the factors influencing the optimal synthesis for improving the film's electric characteristics. We have successfully developed a method for making high-quality diamond film with excellent electric characteristics, which has no dielectric loss in the wide frequency rage (102-108 Hz).

Trapping-detrapping defects in single crystal diamond films grown by chemical vapor deposition

High-quality single-crystal diamond films were homoepitaxially grown by chemical vapor deposition onto low cost high-pressure high-temperature diamond substrates. The transport properties of the obtained samples were studied by photoresponse characterization. Fast ultraviolet (5 ns) laser pulses at 215 nm were used as a probe. The time evolution of the photoinduced current was observed to closely reproduce the laser pulse shape, thus indicating a time response lower than the adopted laser pulse duration. Very stable and reproducible response was measured, so that neither priming nor memory effects are observed. However, a minor slow component shows up in the charge-integrated sample response, whose temperature dependence was investigated in a −25–+50°C range. A systematic speed up of this slow component of the sample signal is observed, indicating the presence of shallow centers producing trapping-detrapping effects. The experimental results are discussed in the framework of a trapping-detrapping model affecting the charge transport mechanism and an activation energy of Ea=0.4eV was derived for the shallow trapping centers.

Aspects of scaling CVD diamond reactors

Among the several techniques available for the synthesis of diamond films the hot filament CVD approach offers some unique advantages in terms of scalability for the deposition of diamond over large area substrates. In this paper some key data that supports the contention that the hot filament CVD process can be readily scaled to deposit high quality diamond films on 300 mm silicon wafers will be presented. The uniformity of wafer temperature has been characterized utilizing an instrumented 300 mm wafer with 12 individual thermocouples attached to the wafer. Temperature variations of < 20 °C across the wafer at a wafer temperature of 895 °C have been achieved. The impact of this is very uniform film quality which has been characterized by Raman spectroscopy. Control over the intrinsic stress in diamond films has been achieved to enable achieving either compressive or tensile stresses in the diamond. Modulating the process variables enables achievement of diamond coated wafers that meet the flatness requirements for the fabrication of advanced semiconductor products utilizing these wafers. This tool and process were developed to deposit thick diamond films, up to ∼40 μm, on 200 and 300 mm silicon wafers.

Modeling of microwave discharges of H2 admixed with CH4 for diamond deposition

Microwave discharges of H2 admixed with CH4 in a moderate-pressure quartz bell jar reactor used for diamond deposition are studied numerically. Special attention was devoted to high-power densities which provide the most effective way for producing high-quality diamond films. First, a one-dimensional radial model describing the coupled phenomena of chemistry, energy transfer, as well as species and energy transport along the reactor’s radial coordinate was developed. Species densities predicted with the model were compared with measurements with infrared tunable diode laser spectroscopy, resulting in validation of the model. Second, a one-dimensional axial model was used to describe the plasma flow along the reactor axis in a region between the reactor end wall and the substrate surface. This model was particularly useful for studying the plasma behavior in the vicinity of the substrate surface, where thermal and composition gradients are large. Both the radial and axial transport models are based on the same discharge model in which the plasma is described as a thermochemically nonequilibrium flow with different energy distributions for heavy species and electrons. The chemistry was described with a model containing 28 species and 131 reactions. The electron temperature, the gas temperature, and the species concentration were determined by solving a coupled set of equations. A wide range of experimental conditions used for diamond deposition was simulated, from low microwave power density (9Wcm−3, i.e., 600W, 2500Pa, and Tg∼2200K) to high-power density (30Wcm−3, i.e., 2kW, 12000Pa, and Tg∼3200K). The main chemical paths were identified, and the major species, transport effects, and reaction pathways that govern diamond deposition plasmas are discussed.

Magnesium interlayered diamond coating on silicon

Diamond thin films have been deposited using hot filament chemical vapour deposition technique on manually scratched p-Si(1 0 0) substrate, with and without magnesium interlayer. In spite of magnesium melting point being lower ( T m = 649 °C) than the growth temperature of the substrate ( T s ∼ 750 °C) used in these experiments, it was found that high quality diamond films could be grown on Mg covered substrate. A liquid substrate is probably generated during the diamond film growth. Raman spectroscopy analysis exhibited only the triply degenerate, zone centre optical phonon peak at 1333 cm −1 indicating that nearly stress free crystallites were present. Broadening of the Raman peak (∼11.76 cm −1) indicates that some small crystallites also are present. Scanning electron and atomic force microscopy accompanied by X-ray diffraction analysis where used to compare the details of diamond film growth directly on scratched Si(1 0 0) and Mg interlayered scratched Si(1 0 0) substrates.

Chemical vapor deposition of uniform and high-quality diamond films by bias-enhanced nucleation method

Diamond films on 1 × 1 cm 2 Si (100) substrates were synthesized by microwave plasma chemical vapor deposition (MWCVD) using mixture of methane and hydrogen gases. Bias-enhanced nucleation method was used to avoid any mechanical pretreatments. Distribution of deposited diamond crystallites in terms of density, size, and morphology has been significantly improved over all the Si substrate surface area by using a novel designed Mo anode. Films were characterized from the center to the edges of substrates using scanning electron microscopy, transmission electron microscope, and Raman analysis. The results also show that uniform diamond films can be obtained by short bias nucleation period using a dome-shaped Mo anode. The diamond crystallites were directly deposited on Si substrate. Using 2% CH 4 in the growth stage, high quality diamond films in the <100> texture can be obtained with relatively smooth surface.

Synthesis of polycrystalline diamond at low temperature on temperature sensitive materials of industrial interest

Diamond film deposition at low temperature is of considerable interest in electronics. We report on the growth of high-quality microcrystalline and nanocrystalline diamond films by sulfur-assisted hot filament chemical vapor deposition at substrate temperature ∼440 °C. The effects of substrate temperature in the presence of sulfur on diamond growth and structure were investigated. The deposition rate was found to increase as the substrate temperature is reduced, accompanied by a reduction in grain size and roughness, while the quality factor decreased only slightly. When glass is used as a substrate instead of molybdenum, the diamond quality, grain size, and roughness increase. The deposition of diamond at this relatively low temperature is interpreted in terms of the profound changes induced by the presence of sulfur in the gas phase chemistry.

High-quality homoepitaxial diamond (100) films grown under high-rate growth condition

We present advantages of high-power microwave plasma chemical vapor deposition (MPCVD) in homoepitaxial diamond film deposition. Diamond films grown at comparatively high growth rate of 3.5 μm/h showed intense free-exciton recombination emission at room temperature. The free-exciton decay time of the diamond film at room temperature, 22 ns, was much longer than that of type-IIa single crystal, indicating electronically high quality of the homoepitaxial films. Dislocation-related emissions were locally observed, a part of which created by mechanical polishing process was successfully removed by surface etching process using oxygen plasma. Another advantage of the high-power MPCVD is effective impurity doping; boron-doped diamond films with high carrier mobility and high carrier concentration were reproducibly deposited. An ultraviolet photodetector fabricated using the high-quality undoped diamond film showed lower noise equivalent power as well as higher photoresponsivity for ultraviolet light with better visible-blind property, compared to those of standard Si-based photodetectors. The high-power MPCVD is, thus, indispensable technique for depositing high quality diamond films for electronic devices.

Monte Carlo Simulation of Electron Velocity Distribution and Gas Phase Process in Electron-Assisted Chemical Vapor Deposition

The gas phase process of diamond film deposition from CH4/H2 gas mixture by electron-assisted chemical vapor deposition is simulated by the Monte-Carlo method. The electron velocity distribution under different E/P (the ratio of the electric field to gas pressure) is obtained, and the velocity profile is asymmetric. The variation of the number density of CH3 and H with different CH4 concentrations and gas pressure is investigated, and the optimal experimental parameters are obtained: The gas pressure is in the range of 2.5 kPa-15 kPa and the CH4 concentration is in the range of 0.5%-1%. The energy carried by the fragment CH3 as the function of the experiment parameters is investigated to explain the diamond growth at low temperature. These results will be helpful to the selection of optimum experimental conditions for high quality diamond films deposition in EACVD and the modeling of plasma chemical vapor deposition.

Extreme ultraviolet single-crystal diamond detectors by chemical vapor deposition

High-quality single-crystal diamond films, homoepitaxially grown by microwave chemical vapor deposition, have been used to produce diamond-based photodetectors. Such devices were tested over a very wide spectral range, from the extreme ultraviolet (UV) (20 nm) up to the near IR region (2400 nm). An optical parametric oscillator tunable laser was used to investigate the 210–2400 nm spectral range in pulse mode. In this region, the spectral response shows a UV to visible contrast of about 6 orders of magnitude. A time response shorter than 5 ns, i.e., the laser pulse duration, was observed. By integrating the pulse shape, a minor slow component was evidenced, which can be explained in terms of trapping–detrapping effects. Extreme UV gas sources and a toroidal grating vacuum monochromator were used to measure the device response down to 20 nm in continuous mode. In particular, the extreme UV He spectrum was measured and the He II m, 30.4 nmand He I 58.4 nm emission lines were clearly detected. The measured time response of 0.2 s is totally due to the instrumental readout time constants. In both experimental setups an extremely good stability and reproducibility of the device response were obtained, whereas no persistent photoconductivity nor undesirable pumping effects were observed.

Plasma-enhanced Deposition of Nano-Structured Carbon Films

By pre-treating substrate with different methods and patterning the catalyst, selective and patterned growth of diamond and graphitic nano-structured carbon films have been realized through DC Plasma-Enhanced Hot Filament Chemical Vapor Deposition (PE-HFCVD). Through two-step processing in an HFCVD reactor, novel nano-structured composite diamond films containing a nanocrystalline diamond layer on the top of a nanocone diamond layer have been synthesized. Well-aligned carbon nanotubes, diamond and graphitic carbon nanocones with controllable alignment orientations have been synthesized by using PE-HFCVD. The orientation of the nanostructures can be controlled by adjusting the working pressure. In a Microwave Plasma Enhanced Chemical Vapor Deposition (MW-PECVD) reactor, high-quality diamond films have been synthesized at low temperatures (310°C-550°C) without adding oxygen or halogen gas in a newly developed processing technique. In this process, carbon source originates from graphite etching, instead of hydrocarbon. The lowest growth temperature for the growth of nanocrystalline diamond films with a reasonable growth rate without addition of oxygen or halogen is 260°C.

High rate growth and electrical/optical properties of high-quality homoepitaxial diamond (100) films

Undoped and boron doped homoepitaxial diamond films were grown by high-power microwave plasma chemical vapor deposition (MPCVD). Growth rate was 2.5 μm/h under optimized growth conditions. The grown undoped film thus grown showed large diffusion length of 10.2 μm while short lifetime of 15 ns for electron. The growth rate increased up to 25 μm/h with increasing methane concentration, while free-exciton recombination emission was observed at room temperature. We found the surface became smooth and growth rate increased with increasing substrate temperature. Hall measurement for boron-doped sample revealed high hole mobility of 1500 cm 2/Vs at 290 K for relatively high acceptor density of 1.4×10 18 cm –3. The high-power MPCVD is, thus, indispensable for high-rate growth of diamond films for electronic devices.

106 電子デバイス応用のための高品質ダイヤモンド膜の合成(FM-1 技術移転を目指す熱・燃焼工学最前線,研究発表講演)

106 電子デバイス応用のための高品質ダイヤモンド膜の合成(FM-1 技術移転を目指す熱・燃焼工学最前線,研究発表講演)

Synthesis of diamond films and nanotips through graphite etching

A hot filament CVD process based on hydrogen etching of graphite has been developed to synthesize diamond films and nanotips. The graphite sheet was placed close to the substrate and only hydrogen was supplied during deposition. No hydrocarbon feed gases are required for this process. High quality diamond films were synthesized with high growth rate on P-type (1 0 0)-oriented silicon wafers without discharge or bias. The diamond growth rate is approximately five times higher than that through conventional hot filament chemical vapor deposition using a gas mixture of methane and hydrogen (1 vol.% methane) under similar deposition conditions. The diamond films synthesized in this process exhibit smaller crystallites and contain smaller amount of non-diamond carbon phases. Synthesis of well-aligned diamond nanotips with various orientation angles was achieved on the CVD diamond-coated Si substrate when the substrate holder was negatively biased in a DC glow discharge. The nanotips grown at locations far enough from the sample edges are aligned vertically, while those around the sample edges are tilted and point away from the sample center. The alignment orientation of the nanotips appears to be determined by the direction of the local electric field lines on the sample surfaces.

Transport properties of electron-beam and photo excited carriers in high-quality single-crystalline chemical-vapor-deposition diamond films

We have investigated transport properties of carriers excited in high-quality homoepitaxial diamond (100) films by 5.6eV photons or 15keV electrons. The high-quality single-crystalline diamond films were homoepitaxially grown on type-Ib diamond substrates at a rate of 2.5μm∕h by high-power microwave-plasma chemical-vapor-deposition (MPCVD). In cathodoluminescence (CL) measurements, strong free-exciton recombination emissions were observed at room temperature from the almost whole specimen surface, indicating the grown films have substantially high quality. It is found through an analysis of the visible emission band originating from the type-Ib substrate that decay constants estimated for excited carriers were ∼5μm in the depth direction. This is consistent with the fact that the intensity of spotlike CL images varied with an exponential function of the lateral length. From transient photocurrent measurements using ultrashort pulse laser excitations, decay times τ for the present high-quality diamond were estimated to be 15 and 100ns for electrons and holes, respectively. Charge collection distances at an electric field E of 830V∕cm were deduced to be ∼190μm and over 1.2mm for electrons and holes, respectively. The former may give a high electron drift mobility of μ∼1600cm2∕Vs while the diffusion coefficients estimated for electrons are 55±14cm2∕Vs, which is comparable with or even higher than those of Si. These physical quantities demonstrate high quality of the diamond films grown by means of the high-power MPCVD method.

Electron behaviour in CH 4 /H 2 gas mixture in electron-assisted chemical vapour deposition

The behaviour of electrons in CH4/H2 gas mixture in electron-assisted chemical vapour deposition of diamond is investigated using Monte Carlo simulation. The electron drift velocity in gas mixture is obtained over a wide range of E/P (the ratio of the electric field to gas pressure) from 1500 to 300000 (V/m kPa−1). The electron energy distribution and average energy under different gas pressure (0.1–20kPa) and CH4 concentration (0.5%–10.0%) are calculated. Their effects on the diamond growth are also discussed. It is believed that these results will be helpful to the selection of optimum experimental conditions for high quality diamond film deposition.

A route to diamond wafers by epitaxial deposition on silicon via iridium/yttria-stabilized zirconia buffer layers

A multilayer structure is presented which allows the deposition of high-quality heteroepitaxial diamond films on silicon. After pulsed-laser deposition of a thin yttria-stabilized zirconia (YSZ) layer on silicon, iridium was deposited by e-beam evaporation. Subsequently, diamond nucleation and growth was performed in a chemical vapor deposition setup. The epitaxial orientation relationship measured by x-ray diffraction is diamond(001)[110]∥Ir(001)[110]∥YSZ(001) [110]∥Si(001)[110]. The mosaicity of the diamond films is about an order of magnitude lower than for deposition directly on silicon without buffer layers and nearly reaches the values reported for single-crystal diamond on Ir/SrTiO3. In the effort towards single-crystal diamond wafers, the present solution offers advantages over alternative growth substrates like large-area oxide single crystals due to the low thermal expansion mismatch.

Use of ultrafine-dispersed nanodiamond for selective deposition of boron-doped diamond films

A suspension of ultrafine-dispersed nanodiamond was used for introducing (in particular, selectively) high-density centers of diamond nucleation on various substrates. High-quality doped diamond films to be used as electrochemistry electrodes were deposited from the gas phase in a microwave discharge on certain substrates treated using ultrafine-dispersed nanodiamond. A uniform distribution of nucleation centers with concentrations greater than 1010 cm-2 on silicon substrates was obtained. Electrochemical current-potential curves were measured for continuous films. Diamond meshes of different transparency were grown using selective nucleation. Successful production of high-quality doped diamond meshes gives grounds to consider them the most promising electrodes for use in electrochemistry.

Highly sensitive UV photodetectors fabricated using high-quality single-crystalline CVD diamond films

We have characterized the diamond-based photodetectors (DBPs) fabricated using high-quality single crystalline diamond film. The diamond film was homoepitaxially grown at high growth rate of ∼2 μm/h by high-power microwave plasma chemical vapor deposition (CVD). The grown film had a substantially flat surface without non-epitaxial crystallites and gave strong free-exciton recombination emission from almost whole homoepitaxial region. Planar-type DBPs with interdigitated electrodes were fabricated using the diamond film with 20-μm thickness by standard photolithograph techniques. The DBP with 43-μm electrode spacing revealed small noise equivalent power (NEP), <1 pW, for 220-nm ultraviolet (UV) light. The NEP was four orders smaller than that for UV-sensitive Si photodetector (SP), that means high performance to faint UV light detection. Photo responsivity of the DBP for 400-nm visible light was at least four orders smaller than that for 220-nm UV light. The low NEP for UV light and high spectral selectivity of the present DBPs reflects high crystalline quality of the diamond film.

Two-dimensional simulations of temperature fields of the reactor wall during hot-filament CVD diamond film growth over a large area

In order to study the growth of diamond films over a large area in a traditional hot-filament chemical vapour deposition (HFCVD) reactor, two-dimensional mathematical models were first developed to investigate the temperature fields of the reactor walls, which made significant contributions to thermal round-flow of the reactant gases under different energy transfer systems. The set of partial differential equations involved in the thermal conduction system was solved with different boundary conditions by the finite control volume method. Numerical simulations showed that the temperature space distributions were heterogeneous when thermal radiation was assumed to be the only mechanism in heat transfer from the filaments to the reactor walls. However, taking into account the effects of thermal conduction under adiabatic and different isothermal temperature boundary conditions, the temperature uniformities improved greatly. In addition, thermal convection did not affect the temperature distributions but only increased the total temperature of the reactor wall. These results not only give insight into the dominant reasons resulting in low nucleation density and low growth rate of diamond films, but also provide a basis for the design of industrial HFCVD reactors to obtain high-quality diamond films over a large area.