Diamond single-crystal homoepitaxial

Figure 3. SEM micrographs of (a) aggregate after 12 h hydrothermal treatment of nanocrystalline diamond at 740°C and 300 MPa, (b) plate-like erystals on the surface of a diamond seed crystal after 150h hydrothermal treatment of graphite and diamond single crystals at 800°C and 300 MPa, (c) liquid-phase hydrothermal homoepitaxy of diamond on diamond seed at 170 MPa and 400°C in a specially prepared water solution. Reproduced from [15] with permission from A. Szymanski.

In the last part of this chapter, our attention will be focused on the electrochemical properties of individual crystal faces of HTHP diamond single crystals, as well as single-crystal (homoepitaxial) CVD diamond films. Our preliminary studies showed that the HTHP single crystals, on the whole, are similar to the CVD polycrystalline films in terms of their electrode behavior. In particular, both the polycrystalline thin-film electrodes and the HTHP single crystal electrodes are equally characterized by the special type of frequency-dependent capacitance described by the CPE. 

In spite of the importance of single-crystal diamond electrodes, the number of reports on their electrochemistry [1-7] is still small, much smaller than that for polycrystalline diamond electrodes. The main reason for this situation is the limited availability of the single-crystal diamond samples. Generally, in the case of CVD film preparation, the area of the polycrystalline diamond thin film depends on the capability of the deposition apparatus (e.g., power), because large silicon wafers can be used as substrates. In contrast, in the case of single-crystal homoepitaxial diamond thin films, the... 

Differences in the surface conductivity with surface termination of diamond can be applied to the nanolithographic modification of diamond surfaces by use of atomic force microscopy (AFM) techniques [50-52]. Modification can be carried out by applying an electrical bias to the sample surface via a conductive cantilever tip, e.g., Au coated Si (Fig. 8.8). Surface modification using such an AFM technique is relatively general, and has been achieved for semiconductor materials such as Si [53], GaAs [54] and metals such as Ti [55]. Recently, Tachiki et al. and Kondo et al. have applied this technique to single-crystal homoepitaxial diamond thin films, undoped and boron-doped, respectively. In this section, we discuss the properties of diamond surfaces modified via AFM techniques and possible applications. 

Figure 8.1. STM images of a homoepitaxial diamond layer surface deposited on single crystal diamond (100) surface [138].

The electrical properties of single crystal diamond will be useful to study those of heteroepitaxial diamond films. As a reference. Figures 13.1 (a)-(c) show the resistivity, mobility, and carrier density of single crystal diamond as a function of temperature [107]. Figures 13.2 (a) and (b) are the resistivity and mobility as a function of the carrier density. A more thorough study on B-doped homoepitaxial diamond is presented in Ref. [416], where AFM observation of the layer surface. Hall measurements at different temperatures, and other data are presented. 

The carrier concentrations and hole mobilities in HOD films were measured in Ref. [294]. In this case, B-doped diamond layers of 1.7-pm thickness were simultaneously deposited on a polycrystalline diamond film, an undoped HOD film of roughly 30- im thickness, and a type Ila single crystal (100) using a NIRIM-type reactor. The measured carrier concentrations and the mobilities of the polycrystalline diamond film, the HOD film, and the homoepitaxial layer are shown in Figures 13.3 (a) and (b), respectively. The B concentration determined by SIMS was 4 X 10 cm [294]. Interestingly, the surface of the B-doped layer (deposited on the HOD film) was much smoother than that of the basal HOD film, presumably... 

Small amounts of well crystallized diamond were found after hydrothermal treatment of P-SiC powder at 700-750°C in the presence of diamond seed [55]. After removing silica and nondiamond carbon, small (<3pm) carbon particles of predominantly octahedral shape, thus being probably diamond, were found. They were attached to the surfaces of the single crystal seed but could be removed by intense ultrasonic treatment. Tetrahedral hillocks of etch pits appeared on the seed diamond surfaces. The hillocks always showed a faceted structure and sometimes common orientation, as expected for homoepitaxial diamond growth. 

The best surfaces are obtained, however, when single crystal diamond is overgrown by homoepitaxy. This yields surfaces with well-developed terraces and monoatomic steps [3,55]. A comprehensive review of methods to prepare atomically clean and smooth diamond surfaces can be found in Ref. [56]. 

We now turn to the homoepitaxial CVD-diamond films deposited onto dielectric single crystal diamond substrates. The three crystal faces, that is, (ill)-, (llO)-, and (lOO)-orientated, were studied by differential capacitance measurements [28], They differ in their capacitance markedly. The Mott—Schottky plots (see, e.g.. 

Fig. 9.2. XPS survey spectra for DNPH modified single crystal diamond surfaces- (A) (lOO) and (B) (ill) homoepitaxial boron doped diamond films.

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