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.     

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.     

Investigation of nitrogen addition on hydrogen incorporation in CVD diamond films from polycrystalline to nanocrystalline

The effect of nitrogen addition in the gas phase on hydrogen impurity incorporation into CVD diamond films was investigated. A series of thick diamond films of different morphology and quality ranging from large-grained polycrystalline to fine-grained nanocrystalline were deposited on silicon wafers using a 5 kW microwave plasma assisted CVD system. They were obtained only by changing the small amount of oxygen and nitrogen addition while keeping all other input parameters the same. Bonded hydrogen impurity in these diamond films was studied by using Fourier-transform infrared spectroscopy. It was found that with increasing the amount of nitrogen addition in the gas phase, the produced diamond films from large-grained polycrystalline gradually shift to fine-grained nanocrystalline and their crystalline quality is drastically degraded, while the amount of incorporated hydrogen impurity in the diamond films increases sharply. The role of nitrogen additive on diamond growth and hydrogen incorporation is discussed. These results shed light into the growth mechanism of CVD diamond films ranging from polycrystalline to nanocrystalline, and the incorporation mechanism of hydrogen impurity in CVD diamonds.     

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.     

Characterization of phosphorus doped CVD diamond films by cathodoluminescence spectroscopy and topography

A phosphorus doped homoepitaxial diamond film made by a CVD method was investigated by cathodoluminescence spectroscopy and topography. An exciton peak bound to phosphorus at 239 nm was observed from {001} as well as {111}, but the wavelength and width of the peak varied depending on location of the sample. This may be due to the difference of crystal perfection. It was found in the spectra that the slope with an array of weak peaks has a maximum of approximately 270 nm. This broad band may be an indicator of incorporation of phosphorus, although all the phosphorus doped diamonds do not exhibit the band. No other peak expected to be related to phosphorus was found, but several peaks commonly observed from CVD diamond were seen instead. These were also dependent on growth surface. Peaks at 415, 482, 500, 514 and 532 nm were strong on {111} surfaces of the non-epitaxial crystallites, whereas the 575-nm peak and another system of the 532-nm peaks were stronger in {001} epitaxial layers, as reported earlier.     

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.     

CVD金刚石薄膜技术发展现状及展望(下)

简要描述了CVD金刚石薄膜技术的发展历程。介绍了纳米特别是超纳米金刚石膜、CVD金刚石大单晶的技术特点及其应用。超纳米金刚石膜在MEMS(微机电系统)、电化学和生物医学上的应用和CVD金刚石大单晶是当前的研究热点。简言之,金刚石的发展向着更大或者更小的方向深入进行,即"非大即小"。     

Characterisation of CVD grown diamond and its residual stress state

One of the most important quality factors in the judgement of thin diamond layers is the adhesion between the substrate and the layer which is limited by the residual stress state. The main reason for residual stress in coatings is the misfit in various properties of the layer and also the substrate, e.g. thermal expansion and crystal lattice type. The most common method of residual stress determination, based on X-ray diffraction, has to date found little application in the study of diamond films. In this paper, the method is applied to determine the residual stress in a CVD diamond film grown on a polycrystalline Al substrate. The results are interpreted with regard to the crystal structure and orientation of the layer, determined by X-ray diffraction and scanning electron microscopy.     

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.     

X-ray topography studies of dislocations in single crystal CVD diamond

X-ray topography has been used to study single crystal diamond samples homoepitaxially grown by microwave plasma-assisted chemical vapour deposition (CVD) on high pressure high temperature (HPHT) and CVD synthetic diamond substrates. Clusters of dislocations in the CVD diamond layers emanated from points at or near the interface with the substrate. The Burgers vectors of observed dislocations have been determined from sets of {111} projection topographs. Dislocations have line directions close to the [001] growth direction and are either edge or 45° mixed dislocations. Where groups of dislocations originated at isolated points they tended to be of the edge variety. Where the substrate surface was deliberately damaged before growth, two sets of dislocations were observed to have propagated from each line of damage and there was a tendency for dislocations to be of the 45° mixed variety with a component of their Burgers vector parallel to the polishing direction. It is demonstrated that X-ray topography can be used to deduce the growth history of CVD synthetic diamond samples produced in multiple growth stages.     

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.     

A geometric model of growth for cubic crystals: Diamond

A mathematical and software implementation of a geometrical model of the morphology of growth in a cubic crystal system, such as diamond, is presented based on the relative growth velocities of four low index crystal planes: {100}, {110}, {111}, and {113}. The model starts from a seed crystal of arbitrary shape bounded by {100}, {110}, {111} and/or {113} planes, or a vicinal (off axis) surface of any of these planes. The model allows for adjustable growth rates, times, and seed crystal sizes. A second implementation of the model nucleates a twinned crystal on a {100} surface and follows the evolution of its morphology. New conditions for the stability of penetration twins on {100} and {111} surfaces in terms of the alpha, beta, and gamma growth parameters are presented.     

High quality,large surface area,homoepitaxial MPACVD diamond growth

The use of CVD diamond in electronics has very stringent requirements. For a CVD diamond industry to become viable it is mandatory to obtain very large growth rates (> 5 µm/h), all the while maintaining extremely high purity, a crystalline defect density as low as possible, and large usable surface areas. At the same time, one must keep the stress level within the growing crystal below acceptable limits to avoid crack formation and preserve the crystal structural integrity. These imperatives imply to work to improve both the plasma deposition process and the CVD diamond crystal growth. In this paper, we propose a three-pronged approach: (i) We use detailed plasma models to establish the influence of process parameters (in particular deposition pressure) on plasma chemistry in order to optimize film growth rate and diamond quality; (ii) We emphasize the need for careful substrate pre-treatment and selection (including choosing a single-sector face) to minimize defects in the growing films; (iii) We employ a 3D geometrical model to predict the crystal shape under given growth conditions, and exploit this knowledge to devise a growth strategy maximizing the usable film surface area while minimizing stresses inside the films.     

Analyses of single crystal diamond substrates grown in a pocket substrate holder via MPACVD

High quality single crystal diamond (SCD) substrates are required for several different important current applications. Microwave Plasma Assisted Chemical Vapor Deposition (MPACVD) is a convenient deposition method for high quality substrates. It is hence imperative to synthesize and analyze substrates grown via different CVD techniques. This paper describes the quality of single crystal diamond substrates which have been grown via one such deposition strategy, which is in a “pocket substrate holder” design. The growth process in such a holder helps in depositing substrates which have almost no polycrystalline diamond (PCD) rim growth. The exact pocket holder growth process at high pressures (240 Torr) and high microwave power densities (~ 500 W/cm³) has been discussed in a previous publication [1]. The SCD CVD substrates were analyzed with different characterization techniques. By synthesizing diamond substrates in a pocket holder, the lack of any/almost any PCD rim helped in reducing the amount of stress in the crystals. To study the electronic quality of the substrates, etching experiments were conducted to determine the etch pit density. Nomarski images confirm that the number of etch pits at the edges is higher than at the center of the etched surface thereby implying the feasibility of this simpler method of reducing the etch pit density. The pocket holder process thus not only reduces the PCD rim but also reduces the substrate etch pit density and hence shows good promise of delivering high quality substrates.     

Comparison of GaN and AlN nucleation layers for the oriented growth of GaN on diamond substrates

In this study, {0001} oriented GaN crystals have been grown on freestanding, polycrystalline diamond substrates using AlN and GaN nucleation layers (NLs). XRD measurements and SEM analysis showed that the application of a thin AlN NL gives the best structural results, because AlN has a thermal expansion coefficient in between GaN and diamond and thus delocalizes the stress to two interfaces. The optical quality of the layers, investigated with Raman microscopy and photoluminescence spectroscopy, is similar. Although no lateral epitaxy is obtained, new insight is gained on the nucleation of GaN on diamond substrates facilitating the growth of GaN epilayers on polycrystalline diamond substrates.     

Model of morphology evolution in the growth of polycrystalline β-SiC films

The growth of β-SiC films via chemical vapor deposition (CVD) has been under intensive investigation because this is viewed to be an enabling material for a variety of new semiconductor devices in areas where silicon cannot effectively compete. However, the difficulty in achieving single-crystal or highly textured surface morphology in films with low bulk defect density has limited the use of β-SiC films in electronic devices. Although several researchers have reported results relating the morphology of β-SiC films to deposition parameters, including substrate temperature and gas composition, detailed knowledge of the effects of deposition parameters on film morphology and crystallographic texture is still lacking. If these relationships between deposition parameters and film morphology can be quantified, then it may be possible to obtain optimal β-SiC film morphologies via CVD for specific applications such as high-power electronic devices.The purpose of this study is to predict the dependence of the surface morphology of β-SiC films grown by CVD on substrate temperature and inlet atom ratio of Si:C, and to model the morphological evolution of the growing polycrystalline film. The Si:C ratio is determined by the composition of the reactant gases, propane (C₃H₈) and silane (SiH₄). A two-dimensional numerical model based on growth rate parameters has been developed to predict the evolution of the surface morphology. The model calculates the texture, surface roughness, and grain size of continuous polycrystalline β-SiC films resulting from growth competition between nucleated seed crystals of known orientation. Crystals with the fastest growth direction perpendicular to the substrate surface are allowed to overgrow all other crystal orientations. When a continuous polycrystalline film is formed, the facet orientations of crystals are represented on the surface. In the model, the growth parameter α_2D, the ratio of the growth rates of the {10} and {11} faces, determines the crystal shapes and, thus, the facet orientations of crystals. The growth rate parameter α_2D used in the model has been derived empirically from the textures of continuous β-SiC films reported in the literature.     

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.     

Effects of the additive boron on diamond crystals synthesized in the system of Fe-based alloy and carbon at HPHT

In this paper, the boron-doped diamond crystals were synthesized at high pressure and high temperature (HPHT) by adding amorphous boron to the system of carbon and Fe-based solvent/catalyst. The effects of the additive boron on the properties of the synthesized diamond crystals, especially, on the growth process and the morphology of diamonds, have been extensively studied. We found that the integrity of diamond crystals synthesized at optimal growth condition and the growth region of particular morphology changed with increasing of the content of additive boron. It is found that the growth region of {111} face becomes wider, and that of {100} face becomes narrower and almost disappears with the increasing boron concentrations. The surface morphology of boron-doped diamonds were detected with scanning electron microscope (SEM), and we found a great deal of defects on {111} face when the content of additive boron increased to 0.25 wt.%. The results of X-ray photoelectron spectroscopy (XPS) on the surface of diamond show that boron lies on the surface of diamonds and bonds with C and O, respectively. We proposed a model of bald-point to explain these experimental observations.     

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.     

Effect of nitrogen and oxygen addition on morphology and texture of diamond films (from polycrystalline to nanocrystalline)

We investigate the effect of simultaneous nitrogen and oxygen addition into conventional methane–hydrogen plasma on morphology and texture of diamond films produced in a high power high pressure 5 kW microwave plasma chemical vapor deposition (MPCVD) reactor. Diverse diamond films ranging from large-grained polycrystalline to nanocrystalline can be achieved by simply changing the amount of nitrogen and oxygen addition, while keeping other parameters the same. This opens a new operating window for tailoring the growth of diamond for different applications. Our study shows that: (1) a small amount of oxygen addition leads to a {111}facet dominated large-grained polycrystalline diamond film of mixed <211> and <110> texture; (2) a small amount of nitrogen addition favours the formation of a <110> textured nanocrystalline diamond film; whereas (3) the coupled effect of simultaneous nitrogen and oxygen addition is in between them. The versatility of nitrogen and oxygen addition on tailoring diamond growth ranging from polycrystalline to nanocrystalline diamonds with different microstructures, morphologies and textures is clearly demonstrated.