Heteroepitaxial Nucleation Of Diamond Research Articles

Comparison of heteroepitaxial diamond nucleation and growth on roughened and flat Ir/SrTiO3 substrates

Heteroepitaxial diamond deposition on iridium substrates allows production of single crystal diamond layers on large area that is difficult to achieve with homoepitaxy. Critically important is the first stage of the process when diamond nucleates on Ir upon biasing (a voltage applied to the substrate). Here, we compared the heteroepitaxial nucleation and successive growth of diamond thin films in a microwave plasma CVD reactor on Ir/SrTiO3 substrates with different surfaces modified by bias voltage. The roughened surface consists of furrows and ridges while the flat surface is a little wrinkled. The film morphology, crystal quality and stress are characterized with SEM, AFM, TEM, X-ray diffraction and Raman spectroscopy. It is found that the film on the roughened Ir surface has a lower roughness, smaller mosaic spread, but a larger residual stress than the film deposited on the flat Ir surface, which mainly originates from the difference in the domain density. By comparing diamond (111) lattice spacings of different positions from the Ir-diamond interface, it is inferred that diamond nucleation could occur at the furrows and then overgrow the ridges for Ir-roughened sample while diamond may nucleate at the wrinkle and flat regions for Ir-flat sample. These finding are helpful in understanding of details of diamond film heteroepitaxy, and in searching the ways to improve their crystallinity.

Formation of icosahedron twins during initial stages of heteroepitaxial diamond nucleation and growth

Heteroepitaxy of diamond on single-crystalline iridium via bias enhanced nucleation remains the most successful, and simultaneously, the most unexplored crystallization process developed for fabrication of wafer-scale thin films and free-standing substrates. Unique conditions provided by nucleation and growth from “soft-implanted” carbon clusters cause the formation of three-dimensional crystallographic irregularities, which take the form of various polyhedrons, also known as multiply twinned particles (MTPs). It is shown here that the diamond MTPs retain a thermodynamically stable structure on the micrometer-scale and can be controllably reproduced using specific nucleation and growth conditions. Particular crystallization mechanisms leading to the formation of icosahedron twins and completed icosahedrons in thin diamond epilayers are discussed, and the experimental findings are supported by a phenomenological model based on thermodynamic and kinetic concepts.

Ion bombardment induced buried lateral growth: the key mechanism for the synthesis of single crystal diamond wafers

A detailed mechanism for heteroepitaxial diamond nucleation under ion bombardment in a microwave plasma enhanced chemical vapour deposition setup on the single crystal surface of iridium is presented. The novel mechanism of Ion Bombardment Induced Buried Lateral Growth (IBI-BLG) is based on the ion bombardment induced formation and lateral spread of epitaxial diamond within a ~1 nm thick carbon layer. Starting from one single primary nucleation event the buried epitaxial island can expand laterally over distances of several microns. During this epitaxial lateral growth typically thousands of isolated secondary nuclei are generated continuously. The unique process is so far only observed on iridium surfaces. It is shown that a diamond single crystal with a diameter of ~90 mm and a weight of 155 carat can be grown from such a carbon film which initially consisted of 2 · 1013 individual grains.

Heteroepitaxial growth of diamond films on 3C-SiC/Si substrates with utilization of antenna-edge microwave plasma CVD for nucleation

Antenna-edge microwave plasma chemical vapor deposition was utilized for heteroepitaxial nucleation of diamond on 3C-SiC(001)/Si(001) substrates. A diamond grain density of 3 × 109 cm−2 was obtained after 2 h of growth under the 〈001〉 preferential growth conditions. A two-step growth process led to a highly-oriented diamond films by suppressing the growth of non-aligned grains. Sharply-defined reflection high energy electron diffraction patterns of diamond were observed. The crystalline quality of the diamond films were investigated by X-ray diffraction pole figure analysis and micro-Raman spectroscopy. After 30 h of growth, heteroepitaxial diamond films were obtained with a full width at half-maximum of 3.8° in the X-ray diffraction polar angle and 7 cm−1 in the diamond Raman peak. The surface of diamond films was observed by atomic force microscopy. Flat domain surfaces with a root mean square roughness of 0.45 nm in 1 × 1 µm2 were obtained.

A narrow biasing window for high density diamond nucleation on Ir/YSZ/Si(100) using microwave plasma chemical vapor deposition

We present a comprehensive study of the parameter space for biasing the Ir(100) surface to determine the optimal conditions for high density heteroepitaxial diamond nucleation. The nucleation density was determined in a bias voltage range from 0 to 225V, and methane concentration range from 0 to 10% using scanning electron microscopy imaging. These data show that high density nucleation exceeding 1011cm−2 occurs only in a narrow bias voltage range from 125 to 175V and a narrow methane concentration range from 1.5 to 3%. At bias voltages and methane concentrations outside of these windows epitaxial diamond nucleation densities fall abruptly to near zero. Using the conditions for high density epitaxial diamond nucleation 80nm thick featureless continuous diamond films grow already after 20min. In several microns thick films the misfit dislocations are confined within a narrow band of 1μm near the diamond–Ir interface showing low residual stress according to x-ray diffraction measurements.

Preparation of 4-inch Ir/YSZ/Si(001) substrates for the large-area deposition of single-crystal diamond

Diamond/Ir/YSZ/Si(001) is currently the most promising multilayer structure for the future realisation of large-area diamond single crystals. A decisive key is the preparation of the iridium layers on silicon. It is shown in this work that high quality iridium films with mosaic spread below 0.2° can be grown on oxide buffer layers with a mosaic spread higher than 1°. An averaging process during the coalescence of the iridium islands provides a plausible mechanism for this phenomenon. The oxide buffer and the iridium overlayers can be grown homogeneously on 4-inch wafers in a similar quality as for 1 × 1 cm 2 samples. Bias enhanced nucleation followed by 40 h growth on the large-area Ir/YSZ/Si(001) wafers yields diamond films with a mosaicity of 0.16° (tilt) and 0.34° (twist). For a further increase of the area of heteroepitaxial diamond nucleation the homogeneity of the plasma discharge has to be improved.

A kinetic model of diamond nucleation and silicon carbide interlayer formation during chemical vapor deposition

The presence of thin silicon carbide intermediate layers on silicon substrates during nucleation and the early stages of diamond deposition have been frequently reported. It is generally accepted that the intermediate layer is formed by the bulk diffusion of carbon atoms into the silicon carbide layer and the morphology and orientation of the diamond film subsequently grown on the intermediate layer are strongly affected by that layer. While there have been considerable attempts to explain the mechanism for intermediate layer formation, limited quantitative data are available for the layer formation under the operating conditions conducive to diamond nucleation. This study employs a kinetic model to predict the time evolution of a β-SiC intermediate layer under the operating conditions typical of diamond nucleation in hot filament chemical vapor deposition reactors. The evolution of the layer is calculated by accounting for gas-phase and surface reactions, surface and bulk diffusions, the mechanism for intermediate layer formation, and heterogeneous diamond nucleation kinetics and of its dependence on the operating conditions such as substrate temperature and inlet gas composition. A comparison between the time scales for intermediate layer growth and diamond nuclei growth is also performed. Discrepancies in published adsorption energies of gaseous hydrocarbon precursors on the intermediate layer—ranging from 1.43 to 4.61 eV—are examined to determine the most reasonable value of the adsorption energy consistent with observed saturated thicknesses, 1–10 nm, of the intermediate layer reported in the literature. The operating conditions that lead to intermediate layer growth followed by diamond deposition vs. those that yield heteroepitaxial diamond nucleation without intermediate layer formation are discerned quantitatively. The calculations show that higher adsorption energies, 3.45 and 4.61 eV, lead to larger surface number densities of carbon atoms, lower saturated nucleation densities, and larger intermediate layer thicknesses. The observed saturated thicknesses of the intermediate layer may be reproduced if the true adsorption energy is in the range of 3.7–4.5 eV. The intermediate layer thickness increases by increasing substrate temperature and inlet hydrocarbon concentration and the dependence of the thickness on substrate temperature is especially significant. Heteroepitaxial diamond nucleation without intermediate layer formation reported in experimental results can be readily explained by the significant decrease of the intermediate layer thickness at lower substrate temperatures and at higher diamond nucleation densities. Further, the present model results indicate that the intermediate layer thickness becomes saturated when growing diamond nuclei cover a very small surface area of that layer.

Heteroepitaxial diamond nucleation and growth on silicon by microwave plasma-enhanced chemical vapor deposition synthesis

Heteroepitaxial diamond films were successfully nucleated and deposited on 1-inch diameter Si(001) substrates by microwave plasma-enhanced chemical vapor deposition (MPECVD). The precursor gases for the synthesis were methane and hydrogen. Before the application of a negative d.c. bias to the substrate, an in-situ carburization pre-treatment on the silicon was found to be an indispensable step towards the heteroepitaxial diamond on the silicon. Morphologies of the films were characterized by scanning electron microscopy (SEM). Interface observations based on the cross-sectional HRTEM directly reveal the heteroepitaxial diamond nucleation phenomena in detail. No interlayers of silicon carbide and/or amorphous carbon phases were observed. Tilt and azimuthal misorientation angles between the heteroepitaxial diamond crystals and the substrate were determined by combining the Ewald sphere construction in the reciprocal lattice space and the selected area diffraction (SAD) patterns taken across the interface.

Surface melting in the heteroepitaxial nucleation of diamond on Ni

Surface melting associated with the heteroepitaxial nucleation of diamond on Ni was investigated. Scanning electron microscopy of quenched samples revealed flow patterns and a recrystallized surface morphology. A combination of techniques including in situ optical monitoring, differential thermal analysis, Auger depth profile analysis, and cross-section transmission electron microscopy (TEM) analysis were performed to identify the nature of the molten layer. Data obtained from different experiments were in good mutual agreement. All experimental results strongly indicated that a molten Ni–C–H surface layer was involved in the nucleation process. The presence of both carbon and atomic hydrogen played an important role in the depression of the melting point which was measured to be >300°C less than the melting point of pure Ni.

The effects of a negative bias on the nucleation of oriented diamond on Si

Abstract The highly oriented diamond film was deposited on Si(001) substrate using the bias enhanced nucleation (BEN) proceeded at −250 V for 10 min in 3%CH4/H2 plasma at substrate temperature 750°C. The {111} X-ray pole figure of the oriented diamond film with a thickness of ca 5 μm is clearly dominated by the four {111} peaks of diamond. The full width at half maximum of the peaks in azimuth rotation and tilt are below 9 and 10°, respectively. Negative bias effects on heteroepitaxial nucleation of diamond have been investigated. It is found that the heteroepitaxial SiC was formed on the Si substrate and the high dose (ca 4 × 1016per cm2s−1) of positive ions bombarded the substrate during the bias treatment. In previous work, it was also found that the concentrations of activated atomic hydrogen and carbonaceous radical in the dense plasma were increased by negative bias. From the results, we try to combine the information based on surface and bulk analytical data and that based on the plasma analysis during the BEN process in order to explain the oriented nucleation of diamond on the Si substrate. It is suggested that the heteroepitaxial diamond can be obtained by the combination of forming heteroepitaxial SiC on the Si substrate, selective etching of Si in the SiC network, filling the vacant Si sites by carbonaceous ions, adding hydrocarbon radicals to the oriented sp3 bond carbon clusters and etching non-diamond carbon components and misoriented sp3 bond carbon clusters during the BEN process.

Limitations of the process window for the bias enhanced nucleation of heteroepitaxial diamond films on silicon in the time domain

The parameter space for the heteroepitaxial nucleation of diamond on Si(001) using the bias process was studied by x-ray diffraction texture measurements. It was found that heteroepitaxial orientation can be achieved over a wide range of different parameters provided that the bias time is within a definite time interval. It was observed that twidth, the width of the time window, and topt , the bias time for optimal azimuthal alignment, strongly decrease with the absolute value of the bias voltage. For high bias voltages an extremely low value of topt (20 s at −300 V) was found. Applying the bias conditions longer than topt resulted in a strong decrease of the pole density maxima of the heteroepitaxial grains accompanied by a significant broadening of their azimuthal distribution that is interpreted in terms of two different routes for the loss of epitaxy. The different time constants characterizing the process window for a fixed bias voltage are traced back to feedback of the growing film on the plasma and on the electrical field distribution above the substrate.

Heteroepitaxial nucleation of diamond by bias pretreatment in an electron cyclotron resonance microwave chemical vapor deposition system

Bias-enhanced nucleation (BEN) pretreatment was used for the nucleation of diamond particles on β-silicon carbide substrates in an electron cyclotron resonance (ECR) microwave chemical vapor deposition (CVD) system. Consequently, experimental conditions enabled the pretreatment of large-area substrates. After BEN and diamond growth, the density of diamond particles was strongly dependent on the BEN pretreatment parameters, such as the negative bias voltage, the total pressure, the duration of pretreatment and the methane concentration. Moreover, under appropriate BEN conditions, heteroepitaxial nucleation of the diamond particles was obtained and the proportion of particles oriented with the single-crystal substrates was also determined as a function of the BEN conditions. The maximum rate of oriented diamond particles was around 30% and typical particle densities were in the range 5 × 10 7 cm −2 to 5 × 10 8 cm −2. Heteroepitaxial nucleation was also obtained on substrates over a 40 mm length. For pressures increasing from 13 Pa (0.3 Torr) to 270 Pa (2.0 Torr), the maximum rate of oriented particles was obtained with an optimum bias voltage increasing from −30 V to −90 V. Epitaxial nucleation of diamond could then be strongly dependent on the energy distribution of positive ions involved in the BEN process. Although particle densities and rates of oriented particles deteriorated with the decrease in the substrate temperature from 900 °C to 500 °C, temperatures around 400 °C were suitable for obtaining particle densities close to those obtained at high temperatures.

AFM study on the non-monotonic texture evolution of heteroepitaxially nucleated diamond films

After the heteroepitaxial nucleation of diamond on Si(001) the texture of the film depends strongly on the subsequent growth process. According to the van der Drift mechanism of evolutionary selection, a growth parameter α slightly below 3 should result in a splitting of the central pole density maximum at χ = 0 ° in the {400} pole figure into four maxima for high film thicknesses. In contrast, a strong narrowing of this maximum for the studied films was observed experimentally. Based on simple geometric considerations a two-dimensional simulation is presented which shows that in the growth competition between two selected grains for α < 3 the preferred crystallite which will finally survive may occupy considerably less volume and surface area in an intermediate stage of film growth. Atomic force microscopy (AFM) has been used in order to test whether this non-monotonic behaviour has an influence on the macroscopic film texture resulting in a transient narrowing of the pole figure. The measured correlation between the tilt angles and the mean areas of the top {001} facets supports the assumption that the effect described by the simulation contributes to the observed texture evolution.

Recent Developments in Heteroepitaxial Nucleation and Growth of Diamond on Silicon

After the successful controlled heteroepitaxial nucleation of diamond on 3C-SiC epitaxial films and on bare silicon wafers, respectively, obtained by research groups in the USA and Germany in 1992, considerable technical progress and scientifical understanding in the area of heteroepitaxy of CVD diamond have been achieved. The decisive role of ion bombardment for the bias-enhanced epitaxial diamond nucleation is clearly demonstrated. Cross-sectional high resolution electron microscopic (XREM) investigations show the direct epitaxy of diamond on silicon with a periodic 3-to-2 registry of {111} atom planes of the epitaxial diamond—silicon interface. Although the epitaxial growth is of Volmer-Weber type with slight misorientation, i.e., polycrystalline with epitaxial orientation of individual crystallites, the crystallographical and physical properties of these films represent a significant improvement over the usual randomly oriented films, owing to the reduction of the grain boundary angle distribution width to a few degrees. This paper summarizes results obtained recently in our group in the area of heteroepitaxy of diamond on silicon and discusses problems still to be resolved.

Study of the initial growth phase of chemical vapor deposited diamond on silicon(001) by synchrotron radiation

Very thin diamond films (thickness ∼0.1 μm) have been investigated by x-ray diffraction pole figure measurements using synchrotron radiation in order to understand the mechanism of heteroepitaxial diamond nucleation and the first steps of film growth on silicon(001) substrates. The diamond layers consist of an epitaxially aligned component with a crystallographic orientation identical to the substrate. The initial orientational spread of the grains around the perfect epitaxial orientation prior to any modification by a subsequent textured growth step has been determined. In the studied temperature range for the nucleation step the misalignment decreases slightly with increasing temperature. Besides the epitaxial crystallites their corresponding twins of first and second order have been found. The intensity distribution of the pole figures indicates that the process of twinning plays a dominating role in the initial growth stage which shows a tendency to become even more pronounced for higher substrate temperatures.

Heteroepitaxial nucleation of diamond on Si(001) in hot filament chemical vapor deposition

Heteroepitaxial nucleation of diamond on mirror-polished Si(001) substrates was achieved in a conventional hot filament reactor. The key to this achievement was the development of a bias-enhanced nucleation process step analogous to the procedure for heteroepitaxial nucleation in microwave plasma chemical vapor deposition. The nucleation and subsequent growth of diamond were characterized by means of scanning electron microscopy, scanning tunneling microscopy, and Raman spectroscopy. Well-developed diamond (001) crystal faces are observed even for very thin (∼0.3 μm) diamond films. High-resolution scanning electron microscopy investigations of the early stage of growth revealed oriented crystallites on the substrate surface with a nucleation density of 109 cm−2.