Roughening of atomically flat diamond (111) surfaces by a hot HNO3/H2SO4 solution

A systematic investigation of the roughening of atomically flat diamond (111) surfaces by immersion in a hot mixture of HNO₃/H₂SO₄ acids was performed using atomic force microscopy. Before the immersion the diamond surfaces were atomically flat, with single bilayer steps and terraces; surface roughness increased after the immersion in the hot acid mixture. This result suggests that surface oxidation by the hot acid mixture etches the surface causing surface roughening.     

Scanning probe microscopy study of chemical vapor deposition grown graphene transferred to Au(111)

Graphene is grown by chemical vapor deposition (CVD) on copper films and transferred ex situ to atomically flat Au(111) films, after which the sample is annealed in ultra-high vacuum (UHV) prior to scanning tunneling microscopy (STM) investigation. STM imaging at 78 K reveals large, clean and defect-free atomically flat areas that are separated by graphene wrinkles and grain boundaries. In addition to the graphene atomic structure, the flat surface regions exhibit patterns with larger periodicity that can be interpreted as Moiré patterns formed by the atomic lattices of the graphene and the gold. Our findings show that the CVD growth and ex situ transfer of graphene (G) to atomically flat Au(111) surfaces allows obtaining clean and high-quality G/Au surfaces that are suitable for in situ deposition of, e.g., molecules and atoms, for UHV investigation purposes. This approach may offer a higher degree of freedom in preparing bare and doped graphene on atomically flat surfaces compared to a full in situ approach.     

Surface roughening of diamond (001) films during homoepitaxial growth in heavy boron doping

A systematic investigation of heavily boron-doped diamond (001) surfaces was performed by atomic force microscopy. Heavily boron-doped diamond surfaces produced by chemical vapor deposition with boron/carbon (B/C) ratios of more than 1600 ppm in the gas phase were rough, whereas an unintentionally doped diamond surface was atomically flat. These results show that the surface roughening is due to heavy doping by boron during homoepitaxial diamond growth.     

High quality homoepitaxial CVD diamond for electronic devices

Recent achievements in homoepitaxial CVD diamond films for electronic devices have been discussed. We have successfully synthesized high-quality homoepitaxial diamond films with atomically flat surface by the microwave plasma chemical vapor deposition (CVD) using a low CH₄ concentration of CH₄/H₂ gas system less than 0.15% CH₄/H₂ ratio and Ib (001) substrates with low-misorientation angle less than 1.5°. These films are atomically flat over an area as large as 4×4 mm² and have shown a strong excitonic emission of 5.27 eV line, even at room temperature, with no essential emission lines in the visible light region in the cathodoluminescence (CL) spectra. Furthermore, high-quality Schottky junctions between Al and P type high-conductivity layers near the surface of these films have been obtained. Based on this growth method, we have also successfully synthesized B-doped diamond films using trimethylboron [B(CH₃)₃,TMB] gas as a B-doping source, whose Hall mobility is 1840 cm²/Vs at 290 K. Schottky junction fabricated by the B-doped diamond also shows excellent performances, indicating that the homoepitaxial diamond films presented here have a high potentiality for electronic devices.     

Flattening of oxidized diamond (111) surfaces with H2SO4/H2O2 solutions

Changes in the topography of a diamond (111) surface with atomically flat and wide terraces, caused by immersion in HNO₃/H₂SO₄ and H₂SO₄/H₂O₂ solutions were investigated by atomic force microscopy. We observed surface roughening from the HNO₃/H₂SO₄ treatment, and flattening of the HNO₃/H₂SO₄ treated surface from the H₂SO₄/H₂O₂ treatment. This suggests that the H₂SO₄/H₂O₂ treatment is an effective wet-process for preparing atomically flat oxidized diamond (111) surfaces.     

Growth of atomically step-free surface on diamond {111} mesas

Diamond homoepitaxy by microwave plasma-enhanced chemical vapor deposition was investigated on {111} substrate. Growth at a low CH₄/H₂ ratio of 0.025% in a gas phase results in the formation of an atomically step-free surface over 10 × 10-µm² mesas of diamond {111} substrate, when there are no screw dislocations in the mesas. This was achieved through ideal lateral growth, in which two-dimensional terrace nucleation was completely suppressed. The application of the selective formation of the step-free surface and the lateral growth of diamond films will open the way for the realization of high-quality electronic devices using diamond.     

The role of methyl radicals and acetylene in [100] vs. [111] diamond growth

The interaction of mechanistic experiments and detailed models are greatly improving our understanding of the mechanism of diamond growth by chemical vapor deposition. Methyl-radical models typically predict growth rates on (111) planes that are much smaller than experiments, unless contributions from acetylene in nucleating new layers are included. These models predict rather different contributions of methyl radicals and acetylene to growth on (100) vs. (111) planes. On the other hand, other models predict rapid inter-conversion of adsorbed hydrocarbons and surface migration, and equivalence of the behavior of methyl radicals and acetylene (apart from a sticking coefficient) might be expected. We have nucleated and grown μm-sized diamond particles at 800°C in a flow-tube apparatus that permits growth from only methyl radicals or acetylene in atomic hydrogen, in contrast to the complex mixture of species found in a normal reactor. Growth from methyl radicals only produced cubo-octahedral crystals with an α value (√3× the ratio of growth rates in the [100] and [111] directions) near 1.8, indicating that the absence of acetylene is not a significant impediment in nucleating new (111) planes. Diamond growth from pure acetylene produced octahedra (α=3), indicating that (100) growth is much more facile than (111) growth in the absence of methyl radicals, and the (111) facets had a high concentration of contact twins. The implications of these results for the mechanism of diamond growth are discussed.     

Photochemical attachment of amine-layers on H-terminated undoped single crystalline CVD diamonds

The initial nucleation and growth characteristics of amine layers photochemically attached from 10-amino-dec-1-ene molecules protected with tri-fluoroacetic acid group to H-terminated undoped single crystal CVD diamond surfaces are characterized by tapping and contact mode atomic force microscopy (AFM) and scanning tunneling microscopy (STM) with nanometer resolution. The diamond is atomically flat which allows attributing variations of tunneling currents in STM to amine grafted surface areas. Island formation and growth of islands with increasing photochemical attachment time is revealed by these experiments. This results support the growth model, where hydrogen cleavage reactions from the diamond surface by amine molecules bonded to diamond is the major growth mechanism. These reactions might be initiated by UV light illumination of pyrolysis of organic peroxides in the presence of the adsorbate, which are proposed to break H–C bonds to create surface dangling bonds. It occurs at the periphery of islands which gives rise to island growth.     

Homoepitaxial diamond film with an atomically flat surface over a large area

Homoepitaxial diamond films with atomically flat surface were grown using the microwave plasma chemical vapor deposition method at a low CH₄ concentration of less than 0.05% in a CH₄ and H₂ mixed gas system. In Ib (001) diamond substrates having misorientation angles of 0.5°, atomic force microscope image on the surface of film grown at 0.025% CH₄ concentration showed that the films had atomically flat surface with mean roughness of 0.04 nm in area as large as 4×4 mm² (the whole region of the substrate).     

Synthesis of long group IV semiconductor nanowires by molecular beam epitaxy

We report the growth of Si and Ge nanowires (NWs) on a Si(111) surface by molecular beam epitaxy. While Si NWs grow perpendicular to the surface, two types of growth axes are found for the Ge NWs. Structural studies of both types of NWs performed with electron microscopies reveal a marked difference between the roughnesses of their respective sidewalls. As the investigation of their length dependence on their diameter indicates that the growth of the NWs predominantly proceeds through the diffusion of adatoms from the substrate up along the sidewalls, difference in the sidewall roughness qualitatively explains the length variation measured between both types of NWs. The formation of atomically flat {111} sidewalls on the <110>-oriented Ge NWs accounts for a larger diffusion length.     

Quantifying CVD diamond growth and morphology: a useful technique for studying growth kinetics

We present formulae for determining α and the average growth rate in the 〈100〉 and 〈111〉 directions (V₁₀₀ and V₁₁₁) from a micrograph of a single diamond crystal. The growth-rate equations provide a simple and powerful tool for studies into the kinetics of diamond chemical vapour deposition (CVD), as alternative measures of the growth rate are based on polycrystalline films. A large part of the film kinetics is confounded with issues such as nucleation, film evolution, and differences between the crystallographic faces. Our measure of growth rate avoids these issues. The α equation provides a simpler and more accurate method to determine its value than other methods described in literature. The derivation of the equations is based on the progressive truncation of cubic and octahedral crystals by removing prisms from the corners to leave cubo-octahedral crystals. A worked example of the application of the equations is presented.     

Effect of Magnesium Oxide Addition on Surface Roughening of Alumina Grains in Anorthite Liquid

Alumina sintered with 5 wt% anorthite at 1620°C for 48 h has grains with flat boundaries and a size distribution representing abnormal grain growth. TEM observations of the grain triple junctions show flat grain surfaces, some of which are the (0001), ([Onemacr]012), and (1[Onemacr]01) planes. HRTEM observations confirm these surfaces to be atomically flat and hence singular. When sintered further after embedding in MgO powder, the {0001} and { 01[Onemacr]2} planes remain flat, but curved surface segments also appear. These curved surface segments are confirmed to be atomically rough by HRTEM. They are connected to the flat segments with discontinuously changing slopes. Thus, when MgO is added, the singular and rough surface phases coexist.     

Multiple twinning and nitrogen defect center in chemical vapor deposited homoepitaxial diamond

Homoepitaxial diamond films were grown on polished {100} faces of single crystal type IIa diamond substrates using microwave plasma assisted chemical vapor deposition system. 14 homoepitaxial diamond films were grown under a variety of substrate temperatures (1000–2000°C), methane concentration (1–6% in hydrogen gas) and processing pressure (60–200 Torr). Electron paramagnetic resonance (EPR) studies demonstrate that nitrogen is incorporated as a singly substitutional impurity (P1-defect center) and the nitrogen concentration is in the range 10–100 parts per million (ppm). The substitutional nitrogen concentration in homoepitaxial diamond was observed to decrease with increasing substrate temperature. Multitwin percentages of all grown diamonds derived from EPR spectra are correlated with the growth parameter α, which is simply the growth velocity along the 〈100〉 direction divided by the growth velocity along the 〈111〉 direction. With the aid of multitwin morphology and multitwin percentages derived from EPR, we describe conditions under which a twin-free and low defect single crystal diamond can be grown from the vapor phase on the {100} oriented substrates.     

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.     

Process optimization in the low-pressure flat flame growth of diamond

The influence of oxy-acetylene gas mixture (O₂/C₂H₂ gas ratio — 0.95:1.06) and substrate temperature (ranging from 650 to 850°C) on diamond growth in the low-pressure flat flame is reported. Deconvolution of the Raman spectra was employed to qualitatively estimate the ratio of diamond-to-non-diamond carbon in the film deposits by an area comparison of these discriminate peaks. The diamond crystallinity was assessed quantitatively by a determination of the full-width-at-half-maximum of the 1332 cm⁻¹ Raman line representing sp³-bonded carbon. An optimum oxygen/acetylene molar ratio of ∼1.05 and substrate temperature of 650–750°C were observed for limiting both the non-diamond carbon content and a deterioration in the diamond crystallinity. The crystallite morphology was also evaluated as a function of this same parametric regime based on assignment of the parameter α describing growth rate competition between the {100} and {111} faces. The collective data indicates the process conditions required to produce the optimum in film quality according to a desired film morphology.     

Heteroepitaxy of Diamond on Cubic Boron Nitride

The epitaxial growth process of diamond from the gas phase on a cubic boron nitride (c-BN) {111} surface has been investigated. At the initial growth stage, carbon adsorption progressed on a boron-terminated surface of c-BN ({111}_B). The coordination of the carbon atoms was found to be the same as that observed in diamond, as confirmed by electron energy loss spectroscopy (EELS). The epitaxial growth of diamond particles has been observed after formation of the carbon layer. On the other hand, on the nitrogen-terminated surface ({111}_N), neither stable adsorption of carbon nor nucleation of diamond has been observed. The stability of adsorbed carbon atoms in the chemical vapor deposition (CVD) ambient, in which large amounts of atomic hydrogen are supplied to the substrate heated at high temperature, is quite important for the nucleation of diamond. Using cross-sectional transmission electron microscopy (TEM), numerous crystal defects were observed, both in c-BN and diamond. Formation of the epitaxial diamond particles has been observed especially at defect sites on c-BN. The misfit dislocation has been observed near the interface with the diamond particle. Even though there exist misfit dislocations that relieve the stress caused by the lattice mismatch between diamond and c-BN, the epitaxial film involved retains a tensile strain of about 0.29% for a film thickness of about 200 nm.     

AlN films grown on diamond and silicon carbide

Structural changes in surface layers of CVD AlN films were studied by the reflection high-energy electron diffraction technique. Film thickness was varied from 0.2 to 20 μm. Single crystals of diamond, silicon carbide and also bilayered structures of natural diamond/CVD diamond films were used as substrates. On the (111) surfaces of natural diamond, (111) CVD diamond films and (00.1) 6H-SiC at AlN thickness up to 1 μm one can observe the epitaxial correspondence between the growing film and the substrate: (00.1) [11.0] AlN//(111) [110] C_α or (00.1) [10.0] AlN//(00.1) [10.0] 6H-SiC. At larger film thickness the epitaxial growth was replaced gradually by the formation of one of axial textures for which the planes (00.1), (10.3) or (11.4) of wurtzite-like aluminum nitride were parallel to the substrate. During the epitaxial growth the twinning structures with the twinning planes of types (10.1) and (11.1) were observed.     

Abnormal Grain Growth in Alumina with Anorthite Liquid and the Effect of MgO Addition

Abnormal grain growth (AGG) in alumina with anorthite liquid has been observed with varying anorthite and MgO contents, at 1620°C. When only anorthite is added to form a liquid matrix, the grain–liquid interfaces have either flat or hill-and-valley shapes indicating atomically flat (singular) structures. The large grains grow at accelerated rates to produce AGG structures with large grains elongated along their basal planes. This is consistent with the slow growth at low driving forces and accelerated growth above a critical driving force predicted by the two-dimensional nucleation theory of surface steps. With increasing temperature, the AGG rate increases. The number density of the abnormally large grains increases with increasing anorthite content. The addition of MgO causes some grain–liquid interfaces to become curved and hence atomically rough. The grains also become nearly equiaxed. With increasing MgO content the number density of the abnormally large grains increases until the grain growth resembles normal growth. This result is qualitatively consistent with the decreasing surface step free energy associated with partial interface roughening transition.     

Temperature influence on the interlayer and surface morphology of diamond coating on 3D porous titanium substrates

Nanocrystalline (NCD) and/or microcrystalline (MCD) diamond films grown on three-dimensional porous titanium (Ti) substrate were obtained by hot filament chemical vapor deposition (HFCVD) technique. The morphology variation of diamond films grown on porous three-dimensional titanium substrate was studied at four different deposition temperatures to investigate their influence on nucleation density. Scanning electron microscopy images depicted the continuous change from microcrystalline diamond grains with a random crystallographic orientation, at 500 °C and 600 °C, to a cauliflower-like structure for deposits at 700 °C and 800 °C. Visible Raman spectroscopy confirmed the good quality of diamond films and revealed that the amount of amorphous carbon increased associated to the film morphology changes from MCD to NCD. X-ray diffraction analyses, performed both through θ–2θ scans and at grazing incidence angle, allowed the investigation of the crystallographic properties and structural evolution of the different film/substrate interface phases, such as TiC(111), TiC(200) and TiH₂. The results revealed that the temperature enhanced the nucleation sites for diamond growth.     

Crystallographic anisotropy of growth and etch rates of CVD diamond

The investigation of orientation dependent crystal growth and etch processes can provide deep insights into the underlying mechanisms and thus helps to validate theoretical models. Here, we report on homoepitaxial diamond growth and oxygen etch experiments on polished, polycrystalline CVD diamond wafers by use of electron backscatter diffraction (EBSD) and white-light interferometry (WLI). Atomic force microscopy (AFM) was applied to provide additional atomic scale surface morphology information. The main advantage of using polycrystalline diamond substrates with almost random grain orientation is that it allows determining the orientation dependent growth (etch) rate for different orientations within one experiment. Specifically, we studied the effect of methane concentration on the diamond growth rate, using a microwave plasma CVD process. At 1% methane concentration a maximum of the growth rate near <100> and a minimum near <111> is detected. Increasing the methane concentration up to 5% shifts the maximum towards <110> while the minimum stays at <111>. Etch rate measurements in a microwave powered oxygen plasma reveal a pronounced maximum at <111>. We also made a first attempt to interpret our experimental data in terms of local micro-faceting of high-indexed planes.