Microstructure in Nanophase and Amorphous Boron-Based Thin Films

Extract

Boron-based thin film materials have numerous uses ranging from application as hard and refractory protective coatings to potential employment in wide-band gap semiconductor electronics. Of particular interest are the boron carbides, nitrides, oxides, and phosphides. These compounds exhibit a broad range of structural and bonding variations. The crystalline form of elemental boron is based on an arrangement of 12-atom boron icosahedra positioned at the vertices of a rhombohedral lattice. Related materials, such as B₄C, and B₁₂P, also possess structures closely related to this icosahedrally coordinated prototype. However, the structural coordination of the boron carbides and phosphides will vary with composition. For instance, BP possesses a tetrahedrally coordinated zinc-blende structure, and the carbon-rich boron carbides will form in a graphitic structure. Boron nitride has the added complication that for the same composition (BN) multiple bonding and polytypic variations are possible: both soft, graphitic sp²-rich (e.g., hexagonal and rhombohedral BN) and hard, diamond-like sp³-rich phases (e.g., cubic and wurtzitic BN) can be formed.