Homoepitaxial growth on fine columns of single crystal diamond for a field emitter

We have fabricated a pyramidal emitter array on single crystal diamond using an etching technique and a homoepitaxial growth technique. We carried out homoepitaxial growth on fine columns of a single crystal diamond in an NIRIM-type microwave-plasma-assisted chemical vapor deposition reactor in the undoped condition. It was found that temperature and methane concentration had a great influence on the oriented growth. We have found that there is a substantial tendency for a lower methane concentration to result in 〈111〉-oriented growth and for a higher methane concentration to give 〈100〉-oriented growth for a polycrystalline diamond film on Si. By controlling the growth conditions, we have obtained various shapes of diamond particle (cubic, cuboctahedral, octahedral) and have also fabricated a pyramidal sharp tip on single crystal diamond (100) and (110) surfaces. For a (100) substrate, we found that a fine pyramid tip was surrounded by not only four {111} planes but also four slightly slopes faces at the base. We surmise that the crystalline face agreed with the slightly sloped plane and grew selectively because it has the highest lateral growth rate among the other oriented crystalline faces. For a (110) substrate, a fine pyramidal tip was surrounded by two {100} planes and two {111} planes and the top of the tip had a ridge between two {111} planes because of the less than optimum conditions. The field emission from the (100) and (110) substrates was also measured. The emission current of the (110) substrate was comparatively large.     

Growth of homoepitaxial diamond doped with nitrogen for electron emitter

Although we had reported the remarkable low threshold emission from polycrystalline diamond heavily doped with nitrogen (N) [Nature 381 (1996) 140], the problems caused by polycrystallinity still remain for understanding the electron emission mechanism. This paper describes the growth of N-doped homoepitaxial diamond film {100}, {111} and {110}, and their electron emission properties. N-doped homoepitaxial diamond is grown on synthetic diamond by hot filament chemical vapor deposition. Urea [(NH₂)₂CO] is used as a dopant for N. Atomic force microscope (AFM) observations indicate that the relatively smooth surface morphologies are obtained for all the films. The epitaxial growth of all the film is confirmed using reflective high energy electron diffraction (RHEED) patterns. Reflective electron energy loss spectra (REELS) indicate that the very surfaces of {100} and {111} are diamond while {110} is graphite rather than diamond. Raman spectra suggest that the bulk of the obtained films are diamond. The resistivities of the films are found to be much higher than the detection limit of the system. The relatively low threshold emission was observed even from the smooth surface and the threshold voltage is confirmed to depend on the crystal orientation. It is speculated from the film characterizations and the electron emission properties that the low threshold emission is due to high resistance rather than rough surface and/or grain boundaries.     

Arrowhead-like diamond crystals formed by hot-filament chemical vapor deposition

Arrowhead-like diamond crystals have been formed by using a simple method of hot-filament-assisted chemical vapor deposition. These are contact twins with {111} as the twin plane, of which each individual is composed of {100}, {110} and {111} faces. These twins flatten along {110} face and elongate parallel to {111} contact plane. The flattened {110} face consists of many {110} terraces sided by 〈110〉 and 〈112〉 steps. So the twinned crystal looks like an arrowhead. These twins are formed just underneath the uppermost substrate temperature for diamond growth.     

Growth of {111}-oriented diamond on Pt/Ir/Pt substrate deposited on sapphire

Highly 〈111〉-oriented diamond films with azimuthal alignment were successfully deposited on platinum{111}/iridium{111}/platinum{111} formed on sapphire{0001} by microwave enhanced chemical vapor deposition. With oriented nucleation density of approximately 1×10⁸ cm⁻², the heteroepitaxial {111}-oriented diamond films were grown over a 10×10 mm² area without crack or delamination from the substrate. X-Ray diffraction rocking curve of diamond{111} has a full-width at half maximum value of 1.1°, which endorses a high crystal quality of the diamond film. The high density of oriented nucleation and improved adhesion of the diamond can be attributed to the Ir film inserted between the two Pt layers, which hinders diffusion of carbon through the Pt and graphite formation at the Pt/sapphire interface.     

Diamond films grown by a 60-kW microwave plasma chemical vapor deposition system

In order to use chemical vapor deposition (CVD) diamond films for electronic devices, it is necessary to establish technologies for producing diamond wafers with controlled quality. Most of existing diamond CVD systems are, however, designed primarily for laboratory use. To cross the technological gap between the commercial production and the laboratory experiments, the current CVD technologies of diamond must be scaled up and upgraded. Development of large-scale diamond deposition processes was undertaken by using a microwave plasma CVD system, equipped with a 915-MHz, 60-kW generator for generating a large-size plasma. Polycrystalline diamond films were deposited from a hydrogen/methane gas mixture with typical gas pressures and substrate temperatures of 80–120 torr and 800–1050°C, respectively. It was found that depending on the growth conditions, the deposited films have various surface morphologies. Some of the samples have well-defined {111} and {100} facets of up to tens of micrometers in size. The Raman spectra had an intense main peak due to diamond at 1333 cm⁻¹ without a trace of non-diamond carbon. The film quality in terms of Raman spectra was relatively uniform across the samples of 100 mm in diameter. Both 〈111〉 and 〈001〉 textured diamond films were obtained by selected growth conditions.     

Investigation of heterostructure between diamond and iridium on sapphire

Diamond thin film has outstanding physical and chemical properties. Diamond-on-iridium configurations have been prepared by several methods, such as microwave enhanced plasma CVD, direct currency plasma CVD, and hot filament CVD. In this study, an Ir interlayer was deposited on single crystal sapphires (Al₂O₃) with A-planes {1120} by an RF magnetron sputtering method after annealing samples. In addition, a diamond thin film was deposited by a microwave enhanced plasma chemical vapor deposition (MPCVD) method using a mixture of hydrogen and methane gases after a bias enhanced nucleation (BEN) procedure.Ir (001) was grown on the A-plane of sapphire by X-ray pole figure measurement. Diamond thin films were synthesized on each Ir/sapphire substrate and characterized by SEM, Raman spectroscopy. D {100} faces were exhibited in substantial areas of diamond films, and a flat D {100} plane was partially obtained. It is considered that diamond thin films on Ir {100} were mainly grown towards the <100> direction and were epitaxially grown in part.     

Cross-sectional TEM study of unepitaxial crystallites in a homoepitaxial diamond film

We investigated the structure of unepitaxial crystallites (UC; non-epitaxial crystallites) in homoepitaxial diamond films on Ib (001) diamond substrate grown by the chemical vapor deposition (CVD), employing field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HRTEM). The UC was classified into two types depending on the orientation relationship to the homoepitaxial film; one rotates by 70.5° around the common 〈110〉 axis, corresponding to Σ3 coincidence site lattice (CSL) relation. The other type does not have any particular angular relationship. It was found that the growth of the former type is closely related to a lattice dislocation on the substrate surface as well as the homoepitaxial film. On the other hand, there was hardly any lattice dislocation observed at the bottom of the latter type. A nanometer-sized crystalline diamond particle was observed at the nucleation site of the latter one.     

Characterization of substrate off-angle effects for high-quality homoepitaxial CVD diamond films

We have studied the substrate off-angle effects for the crystalline quality of the homoepitaxial diamond films mainly by using steady-state cathodoluminescence (CL) and time-resolved photoluminescence (PL) measurements. By means of the microwave plasma chemical vapor deposition method under high-power microwave power with high methane concentrations, the homoepitaxial diamond films were grown on the high-pressure/high-temperature-synthesized (HPHT) Ib (001) substrates inclined along either <110> or <100> direction by different off-angles ranging from 2° to 5°. In spite of high growth rates, we have succeeded in improving crystalline quality by employing the HPHT substrates with considerably large off-angles. Both steady-state CL and time-resolved PL measurements clearly indicate that larger off-angles lead to better crystalline quality of the homoepitaxial film, suggesting that further improvements in crystalline quality can be expected when using substrates having even larger off-angles.     

Nitrogen incorporation in CVD diamond

Photoluminescence, optical absorption and electron spin resonance results are reported for diamond films homoepitaxially grown by chemical vapor deposition on 〈100〉-oriented natural IIa diamond. Measured dependencies of the corresponding signals on the growth temperature in the range 950–2100°C suggest that, while most of nitrogen is present in our films as single atoms, a small part (approx. 10⁻³ of all nitrogen) could be incorporated as N₂ molecules.     

Improvements of crystallinity of single crystal diamond plates produced by lift-off process using ion implantation

A single crystal diamond substrate cut from a 9 mm thick ingot which was grown by chemical vapor deposition (CVD) was used to produce freestanding single crystal CVD diamond plates with improved crystallinity by the lift-off process using ion implantation. To reduce dislocations on the substrate surface, the ingot was sliced along the {100} plane parallel to the growth direction. In addition, the repeated lift-off processes reduced the surface damage on the substrate. These treatments were shown to improve the crystallinity of the CVD diamond plates produced by polarized light microscopy (PLM) and high-resolution X-ray.     

Substrate crystal recovery for homoepitaxial diamond synthesis

Homoepitaxial chemical vapor deposition (CVD) of diamond requires high quality substrate crystals. This paper describes the process of diamond substrate crystal recovery so that the original substrate can be reused for multiple synthesis processes. A three-stage treatment is applied after homoepitaxial CVD growth. First the original substrate is separated by laser cutting, then the cut surface is mechanically polished, and finally polycrystalline material at the edges of the recovered seed plate is laser trimmed. This recovery process yields reusable diamond substrates that do not differ appreciably from their original state in terms of stresses and impurity concentrations. While the recovery process was demonstrated using HPHT seed substrates the process can also be applied to the as-grown CVD diamond plates. Infrared absorption spectral analysis, surface profilometry, birefringence imaging and Raman spectroscopy are performed after each processing step to monitor crystal quality. The nitrogen concentration in the substrate crystal remains constant throughout CVD and recovery processes. When using HPHT type Ib substrates the detected nitrogen concentration is 110–180 ppm. The nitrogen is mainly incorporated in form of C center defects and no transformation to other forms of defect centers occurs during the CVD process. Birefringence imaging showed a low level of internal stress within the HPHT crystals. No change is observed during CVD growth and recovery processes. It is shown that the polycrystalline rim removal is essential for repeatable CVD deposition on the same seed substrate. Substrate crystal recovery allows growth of up to 20 crystals from one original seed.     

Further improvement in high crystalline quality of homoepitaxial CVD diamond

High-quality homoepitaxial diamond (001) films with macroscopically flat surfaces have been successfully grown using a high-power microwave-plasma chemical-vapor-deposition (MWPCVD) method. In this study, further optimization of the homoepitaxial growth condition has been accomplished mainly by controlling off-angles to 5° along the <110> or <100> direction of high-pressure/high-temperature-synthesized Ib diamond (001) substrates. We have found that the homoepitaxial films deposited at reasonably high growth rates under the optimized growth condition including the off-angle of 3°–4° along the <110> direction have macroscopically flat surfaces, accompany very low or almost negligible densities of growth hillocks and yield strong free-exciton emissions in both steady-state cathodoluminescence and time-resolved photoluminescence spectra measured at room temperature. These indicate that apparent lateral growths suitable for high-quality homoepitaxial layers in the case of the high-power MWPCVD method, which are similar to those previously reported in the case of MWPCVD processes with low methane concentrations, rather quickly occur from step edges on (001) terraces and that they can be achieved more preferentially on the vicinal substrates at high temperatures and high methane concentrations.     

Effect of substrate facets on homoepitaxial growth of diamonds during plasma-assisted hot filament chemical vapor deposition

In this report, the effect of substrate facets has been investigated during homoepitaxial growths of diamonds on polyhedral diamond grains in a plasma-assisted hot filament chemical vapor deposition (HFCVD) system. Homoepitaxial diamonds grown on the (100) planes present smooth surfaces, whereas textured surfaces form on the (111) facets, which is attributed to the different growth modes corresponding to the single-crystal substrate facets. With the accretion of the methane concentration in the gas supply, a few crystallites appear on the smooth growing surfaces of the (100) facets, and a change from (111) to (100) textured surface takes place on the (111) facets, showing that the variation of plasma vapor chemistry further significantly adjusts the homoepitaxy of diamonds. Photoluminescence spectroscopy investigations reveal that the 575-nm N–V peaks of the homoepitaxial grown layers on the (100) facets are much weaker than those of the (111) facets, demonstrating that there are less vacancies in the diamonds homoepitaxially grown from the (100) facets than the (111) ones.     

Morphology of chemical vapor deposition diamond particles grown at high temperature

{113} facets and irregular shapes of chemical vapor deposition (CVD) diamond particles are observed at high deposition temperature of 1200°C on Co base substrate. Microwave plasma CVD is used for the diamond deposition with 2 percent of methane in hydrogen under 1.2×10⁴ Pa (90 Torr). {113} facets form between {100} and {111} facets of diamond particles grown on heterogeneous substrate. This verifies that the condition for the stable {113} facet exists in CVD diamond growth. New small angle boundaries evolve on a flat surface of a growing single crystal diamond particle. Solid segments surrounded by the boundaries look like new grains, whose lattice orientations are misoriented to each other. The misorientation between the solid segments is small initially, but increases as the growth proceeds. Such evolution appears only on the upper particle surface which is parallel to the substrate surface irrespective of the facet index. The formation mechanism of the solid segments is discussed in terms of the lattice misfit within the diamond particle.     

Internal stresses in {111} homoepitaxial CVD diamond

Undoped and phosphorus-doped diamond thin films grown by microwave plasma-enhanced chemical vapour deposition (MPCVD) on Ib {111}-oriented diamond substrates have been studied by confocal micro-Raman spectroscopy. Thanks to the confocal optics, the Raman signal arising from the epilayer could be discriminated from that arising from the substrate. In this study, a distinct Raman peak, broader and approximately 6 cm⁻¹ lower than the optical phonon line of the substrate, was systematically detected and showed that the homoepitaxial layers were under an intense tensile stress. The magnitude of this stress increased with deposited thickness up to a few GPa. In the thicker films, a high compressive stress was also detected at the substrate near surface. Finally, it was observed that a network of oriented cracks could relieve the internal stress.     

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.     

The influence of recess depth and crystallographic orientation of seed sides on homoepitaxial growth of CVD single crystal diamonds

Microwave plasma chemical vapor deposition (MPCVD) has gained increasing attention as a feasible and effective way to produce large, high-quality, single-crystal diamonds. However, the growth of polycrystalline diamond on the periphery of the seed crystal and the cracking generated by the internal stress during the growing process lead to significant decline of the quality and integrity of the CVD diamond, thus increasing the difficulty of synthesizing large diamond layers. Although optimized growth parameters and refined substrate holders have been employed by some researchers to improve the periphery quality of CVD diamond layers, more research needs to be done in this area. In this study, we used a specially designed substrate holder with a circular recess, in which the seed crystal was placed. By designing substrate holders with different recess depths and a seed crystal with different side-surface crystallographic orientations, we aimed to determine the influence of the recess depths and the crystallographic orientation of seed sides on the growth quality according to polarizing microscope, laser Raman spectroscopy, UV fluorescence imaging, and photoluminescence (PL) mapping measurements. The results demonstrate that as the recess depth increases, the amount of polycrystalline diamonds and the internal stress on the periphery are controlled effectively. The crystalline quality is improved, and the growth rate is decreased. In addition, compared to the periphery with {100} seed sides, the periphery with {110} seed sides displays better crystalline quality, lower internal stress, and fewer polycrystalline diamonds after growth, which is probably due to the intrinsic nature of the growth steps propagating on the {100} diamond surface and the effect of nitrogen atoms on the growing process in the diamond lattice.     

p-type doping by B ion implantation into diamond at elevated temperatures

We have studied B ion implantation at 400 °C into undoped homoepitaxial chemical vapor deposition diamond films and high-pressure and high-temperature (HPHT) synthetic IIa substrates. The highest Hall mobility at room temperature is 268 cm²/Vs among B implanted homoepitaxial films, while it is 38 cm²/Vs for the B implanted HPHT synthetic IIa substrate. The present result reveals that the quality of a doped layer is strongly dependent upon that of a diamond substrate employed for ion implantation.     

Chemical and structural transformations of silicon submitted to H2 or H2/CH4 microwave plasmas

Silicon substrates are often used to synthesize polycrystalline diamond films by microwave plasma assisted chemical vapour deposition technique (MPCVD). In the case of highly oriented diamond films, several steps are employed to carefully prepare the silicon surface (pre-treatment steps), to nucleate diamond crystals (nucleation step) and to thick the film (growth step). In this study, we characterize {100} silicon substrates and diamond released from its silicon substrate by electronic microscopies (TEM and SEM), by Atomic Force Microscopy (AFM) and by X-ray photoelectron spectroscopy (XPS), to follow the substrate transformations after each step, particularly the formation and the evolution of the silicon carbide and to characterise the diamond films grown on the carburised silicon. We show that according to the experimental conditions and the level of surface/gas contamination by carbon and silicon species, isolated islands or continuous β-SiC compound are formed over the silicon surface and can generate defects such as voids or strip structures that influence the subsequent diamond nucleation and growth.     

Effect of Substrate Orientation on Interfacial and Bulk Character of Chemically Vapor Deposited Monocrystalline Silicon Carbide Thin Films

The high breakdown electric field, saturated electron drift velocity, and melting (decomposition) point of SiC have given continual impetus to research concerned with the development of thin films having minimum concentrations of line and planar defects and electronic devices for severe environments. To this end, epitaxial growth via chemical vapor deposition of monocrystalline films of β-SiC on Si (100) and 6 H -SiC {0001} substrates and 6 H -SiC on vicinal 6 H -SiC {0001} substrates have been conducted. High concentrations of stacking faults, microtwins, and inversion domain boundaries were produced in films grown directly on Si (100) as a result of a lattice parameter difference of ∼ 20% and the presence of single (or odd number) atomic steps on the substrate surface. Growth on Si (100) oriented 3° to 4° toward [011] completely eliminated the IDBs (but not the other defects) due to the preferential formation of double steps with dimerization axes on the upper terraces parallel to the step edges. Growth of β-SiC films on 6 H {0001} lowered the density of all defects but resulted in the formation of a new defect, namely, double positioning boundaries. The latter were eliminated by using 6 H {0001} oriented 3° toward [1120]. The defect density in these last films, relative to those grown on on-axis Si (100), was reduced substantially (to ∼10⁵ cm/cm³). However, the resulting film was 6 H -SiC. Significant improvements in electrical properties of simple devices were obtained as the defect density was progressively decreased.