Scheme 25.

The pyrolysis of polysilazane II in a NH3 atmosphere produces colorless Si3N4 in high yield (82%). The decomposition of polysilazanes involves different reaction products such as CH4 and H2, which determine the ceramic yield. The highly branched polysilazanes with low carbon content exhibit higher ceramic yield. The split-off of oligosilazanes or loss of volatile silazanes by thermolytic retroversion and transamination should be suppressed to increase the ceramic yield.

Boron nitride has attracted a growing interest for technological applications because of its excellent characteristics, namely high hardness, chemical inertness, and dielectric behavior. Hexagonal boron nitride (h-BN) with a layered and sp2-bonded structure is similar to graphite and is the stable ordered phase under ambient conditions. On the other hand, cubic boron nitride (c-BN) has a zinc blende structure with sp3-bonding resembling that of diamond. In fact, c-BN is better suited for electronic device applications as a high-temperature semiconductor because of the wide direct energy bandgap (6.4 eV). Moreover, it can easily be doped both n-type and p-type with Si and Be, respectively. Significant progress has been made in the synthesis of c-BN by various physical and chemical deposition methods [528]. However, the major disadvantage of c-BN films is that they are much more difficult to synthesize than diamond because of their narrow phase stability region, high compression stress, and problems in controlling boron and nitrogen sources [529, 530]. To overcome these problems, Boo et al. have used isopropyl amine and ferf-butylamine complexes of triethylborane ((Pr)H2NBEt3, ('Bu)H2NBEt3) as SSPs with boron and nitrogen in a 1:1 ratio to grow a crack-free ^-BN film on Si(100) at 850 °C [99].

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