BN phase

The c-BN phase was first obtained in 1957 [525] by exposing hexagonal boron nitride phase (h-BN) to high pressures and low temperatures. A pressure of more than 11 GPa is necessary to induce the hexagonal to cubic transformation, and these experimental conditions prevent any practical application for industrial purposes. Subsequently, it has been found that the transition pressure can be reduced to approximately 5 GPa at very high temperature (1300-1800°C) by using catalysts such as alkali metals, alkali metal nitrides, and Fe-Al or Ag-Cd alloys [526-528]. In addition, water, urea, and boric acid have been successfully used for synthesis of cubic boron nitride from hexagonal phase at 5-6 GPa and temperature above 800-1000°C [529]. It has been... 

Parts prepared of h-BN as well as c-BN are of great interest for industrial applications but also for materials science. The thermodynamic data for c-BN and the BN-phase diagrams found in literature are not in agreement. After the first high pressure experiments the B-N phase diagram was designed, and after some modifications c-BN was described as metastable phase at room temperature. Contrary to this opinion in 1988 it was reported that c-BN is the stable phase. Many experiments have confirmed this result, but exact thermodynamic data are still not available. 

In this section we give a short review of various BN phases and their properties. 

The rhombohedral (r-BN) structure is similar to the h-BN phase but the atomic layers sequence is ABC ABC. It was reported that r-BN is formed during conversion of c-BN into h-BN [22] (Fig. 4). 

The atomic layers from c-BN (ABCABC) have to rearrange into an ABAB stacking sequence of h-BN during the solid state phase conversion. A possible mechanism would be the intermediate formation of the rhombohedral BN phase (r-BN) with ABCABC stacking. The r-BN phase is structurally related to the hexagonal phase, but only differs in the d-values (h-BN d = 6.66 A r-BN d = 10.0 A) of the layers (Fig. 5b). Subsequently the rhombohedral phase is transformed into the hexagonal modification at the reaction temperatures [10]. 

The transformation of h-BN into c-BN (at 6.5 GPa) and the reverse transformation of c-BN to h-BN (from 0.6 to 2.1 GPa) were investigated in a Li3N-BN catalyst system [40], Synchrotron radiation was used to check the phases and to examine reactions between the BN-phases and the catalyst. 

Fig. 6. BN phase diagrams described by Bundy and Wentorf [19] and by Corrigan and Bundy [11]...

Due to the complexity of BN-structures and atomic bonding situations, the characterization of BN-phases by spectroscopic methods (e.g., IR and Raman) is difficult. It is not possible to identify BN phases using only one analytical method. For example, the X-ray diffraction peaks of c-BN correspond to those of Cu, Ni, and many other cubic phases. Elemental composition must be known or measured to be sure that no other phases are present. 

Table 4 summarizes spectroscopic data for BN phases prepared by different methods [36, 56-65]. 

A rather interesting discovery was made by Kobayashi et al. [174]. At 1470-1870 K and 4.2 GPa, c-BN was transformed into h-BN. According to the earlier BN phase diagrams this should not have happened, because these conditions are located in the region where c-BN should be stable. 

The deposition of the c-BN phase is the result of precursor substances that are created by the reaction with Ar+ ions. 

Fig. 4.4. Schematic diagram to illustrate the growth process of the ArBs layer in the A-AiBn reaction couple due to the counter diffusion of both components and the partial decomposition of the A/Bn phase at interface 3.

Further ultrahigh pressure structural modifications of zinc-blende BN are considered possible, with a rock salt structure preferred over a /i-Sn structured A metastable E-BN phase is obtained during electron-beam-assisted crystallization of BN. Other metastable phases of BN are reported in the condensation products upon sudden cooling in conditions of a pulsed plasma discharge Disordered structures are reported when mixed plasma/chemical processes are usedd Some compounds of type B N are listed in Table... 

Cubic boron nitride has high thermal conductivity, high dielectric constant, great hardness, and good chemical stability. The material can be doped n-type with Si and p-type with Be to form p-n junctions. While cubic boron nitride (c-BN) has been successfully doped p- and n-type to produce the first UV-LEDs, it is an indirect bandgap semiconductor which will ultimately limit emission efficiency. Relatively few studies have been performed on this material system. ECR-LPCVD techniques [23, 24] and LPCVD [25] have had the most success informing BN films. As with other specialty materials there is a lack of BN substrates. In order to produce the c-BN phase, high deposition temperatures often are combined with assisted techniques. 

As in the case for precursor-derived Si-C-N ceramics (see Sect. 5.3, Fig. 19) reaction (2) (see Sect. 5.3 and reaction Dg in Fig. 26) appears at 1757 K. Si3N4 reacts completely with graphite to form SiC and gas phase. Excess graphite remains. The BN phase does not take part in reaction (2 Dg) and its amount remains stable up to a temperature of 2586 K. From this result a thermal decomposition of the PHBS(p)-derived material with significant mass losses is expected. However, thermogravimetric analysis shows remarkable thermal... 

Reinke et al. [33] analyzed numerous deposition methods with respect to film structure (cBN or hBN) on the one hand and ion bombardment on the other hand. To describe the ion bombardment quantitatively they used the ion energy and the ratio of fluxes of impinging ions (/) and deposited boron atoms (a). Figure 1 shows the results for the ratio i/a against the ion energy. The total number of deposited atoms can be estimated taking into account the deposition rate and the mass densities of the BN phases. As can be seen from that presentation. 

Actually, the IR absorption features depend on the chemical short-range order of the lattice, and their appearance is only a necessary (not a sufficient) condition for the existence of the respective phases. There exist some more BN phases with either sp or sp short-range order which show essentially the same IR absorption as hBN and cBN, respectively. However, if used in combination with more direct methods such as electron diffraction, IR absorption is a fast and reliable method for determination of the phase content of BN thin films and has found widespread application. In Fig. 4 the IR absorption spectrum of a BN film is given as an example which shows a distinct absorption near 1050 cm and only weak absorption at about... 

Figure 10. Cubic BN phase content together with the deposition rate of BN films deposited at different substrate bias voltages during step 4.

Structural feature, the new BN phase can be expected to exhibit interesting hardness properties. 

To illustrate this point. Figures 2.12 and 2.13 compare the crystallinity as a function of processing temperature for two different specimens [25]. The first specimen (Figure 2.12), consisted of the borazine oligomer from the melt to which no external pressure has been applied. A turbostratic BN phase with an intermediate d(002) value of 3.41 A is formed only upon heating to 1200°C. Specimens like that shown in Figure 2.6c were also investigated. In these specimens, pressure was... 

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