Diamond nuclei

The nucleation density of diamond on a pristine Si wafer surface is in the order of only 10 /cm. Since the average distance between diamond nuclei is 10 pm, it is virtually impossible to make a continuous film on this substrate. To increase the nucleation density, the Si surface is mechanically polished with diamond powder or paste. Alternatively, the Si substrate is immersed in alcohol with diamond powder suspension, and ultrasonicated. As a result, the Si surface is subject to mechanical... 

Finally, it should be mentioned that electronic state calculations of H-terminated cBN surfaces and diamond growth are studied in Refs. [163, 164]. Also, in a recent paper [165], diamond was deposited on large cBN crystals of 200-350 pm in size that were embedded in a Cu plate. It appeared that (i) diamond nuclei were cubo-octahedral crystallites with approximately 100nm in diameter on the (111) faces of cBN, (ii) in some cases, dense carbon tubes with a diameter of lOOnm and a few micrometer in length were grown, and (iii) diamond crystals grown on Cu had deep holes in the center of the (111) faces. This article also compiled past articles on diamond growth of cBN. 

Ni surface [171], Fullerene (Ceo) powder of 0-25 pm and graphite powder of 10-15 pm were also used [172]. In Refs. [173, 174], diamond powder of 0.5 pm in size, which was suspended in acetone, was apphed on the Ni surface. For heteroepitaxial growth of diamond on Ni, the seeding step is very important to make oriented diamond nuclei and suppress the graphitization of diamond simultaneously. 

Since the BEN technique was found by Yugo et al. [3], numerous works have been done on BEN. The establishment of the HOD film growth technique was one of the most important motivations for the BEN studies. Even so, there are still controversy over fundamental issues among researchers the first is how diamond nuclei are formed by BEN, and the second is whether an interfacial layer exists, or an interfacial layer is necessary to grow HOD films. 

In Ref. [261], a process optimization for oriented diamond nuclei was done using a software for statistical experimental design [262] (design for experimental method). The process parameters for BEN and the oriented growth in the three-step process are listed in Table H.3. The substrate used was Si(lOO), which had been carburized for 3h under the following conditions P = 20Torr, Pm=1000W, 7 s = 900°C, and... 

The nucleation process was monitored by LRI (see Section 11.10), and the application of bias voltage was terminated if diamond nucleation was detected by LRI. No appreciable diamond nucleation was observed in the AC-BEN treatment, if the voltage was less than 125 Vrms ( 175 V peak to peak). The optimum biasing time for the AC-BEN treatment was 45 min, much longer than the DC-BEN treatment that needed only 12 min. In terms of the number of oriented nuclei, it was more than 50% for the AC-BEN treatment, while it was only less than 10% for the DC-BEN treatment. It was hence inferred that concurrent processes of forming an epitaxial SiC layer and the diamond nucleation, induced by the AC-BEN treatment, were responsible for the increase in the number of oriented diamond nuclei. 

In this model, the SiC layers play a role of (i) a diffusion barrier of C and Si, (ii) an accumulation layer of C for supersaturation and forming diamond nuclei or... 

It is of most intrigue that unlike previous nucleation models, this model assumes a formation of diamond nuclei or nucleation sites inside the P-SiC layer, while the p-SiC layer concurrently plays a role of basal lattice for diamond epitaxial growth like in the precedent models. The second point of intrigue is the fact that the exposure and survival of the diamond nuclei or nucleation sites are made possible by a subtle balance of etching rates of Si, P-SiC, diamond, and other forms of carbon. This is consistent with the fact that HOD films can be formed only when the substrate was pretreated by proper BEN conditions. 

Figure 7. Schematic diagram showing the proposed nucleation mechanism diamond nuclei form on a DLC interlayer. (I) Formation of carbon clusters on substrate surface and change in bonding structure from sp to sp. (II) Conversion of sp sp bonding.

Figure 9. Schematic diagram showing the proposed nucleation mechanism diamond nuclei form on a carbide interlayer on a carbide-forming refractory metal substrateJ Initially, carburization consumes all available C to form a carbide surface layer. A minimum C surface concentration required for diamond nucleation cannot be reached on the substrate surface. With increasing carbide layer thickness, the C transport rate stows and the C surface concentration increases. When the C surface concentration reaches a critical level for diamond nucleation, or a surface C cluster attains a critical size, a diamond nucleus forms. (Reproduced with permission.)...

The primary difficulty inherent in this issue is the small niunber of materials with suitable crystal structures and lattice constants. Some transition metals and ceramics, such as Ni, Cu, Fe, and cBN (Table 5, Ch. 3), are the few isostructural materials with sufficiently similar lattice constants (mismatch <5%). In addition, the extremely high surface energies of diamond (ranging from 5.3 to 9.2 J m for the principle low index planes) and the existence of interfacial misfit and strain energies between diamond films and non-diamond substrates constitute the primary obstacles in forming oriented two-dimensional diamond nuclei. Earlier attempts to grow heteroepitaxial diamond on the transition metals were not successful. The reasons may be related to the high solubility/ mobility of C in/on the metals (for example, Fe, Co, or the... 

D diamond nuclei and single-crystal growth. The high solubility/mobility of C in/ on the metals (Fe, Co, Ni), or the formation of an intermediate layer (carbides or graphite) may inhibit the possible development of an orientational, epitaxial relationship between diamond films and the substrates. 

Yugo et al.P employed a negative bias and high CH4 concentrations during pretreatment to generate diamond nuclei on a Si mirror surface in PACVD. The several-minute pretreatment resulted in an enormous nucleation enhancement. Diamond nucleation densities as high as 10 cm were achieved. For the onset of diamond nucleation, a minimum voltage of -70 V and a minimum concentration of 5 vol.% CH4 in H2 were necessary. 

Surface mobility of the carbon may be enhanced by the bombardment during the biasing. Some of these clusters become stable and form diamond nuclei. The SiC layer is calculated to reach a maximum of 9 nm by 1 h and then to decrease to 5 nm by 2 h. 

As most of the carbide islands reach the critical thickness, more free carbon becomes available to form new diamond nuclei. 

As biasing continues, there are ongoing adsorption of carbon and etching of the surface, with SiC etched preferentially relative to the more stable diamond nuclei. 

Since Si is preferentially dq)leted from the carbide, carbon concentrations in those local regions are increased so that carbon clusters may actuaOy form on thinner r ons of the carbide, close to the Si substrate. The etching, cluster formation, and diamond nucleation continue until the surfrce is eventually covered with diamond nuclei. 

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