Chemical bonding in amorphous semiconductors

Existing theoretical approaches to describing tetrahedrally-bound (TB) elemental a-semiconductors (a-SCs) are reviewed. Attention is given to local deformations in TB polymorphs, with consideration of the progressive changes occurrring at increasing pressures. It is noted that phase transitions involving distortion of the TB are associated with a change in the chemical bond, with losses in directional characteristics reaching 90%, the bond length, 4.2%, the density, 10.7%, and the rms angular distortion, 15.7 deg. An examination of the structure of Ge III crystals reveals that molecular orbitals are required for successful modeling of the hybridization of chemical bonds, and can be effected through a prescribed relaxation iterative procedure. Finally, the peculiarities of fluorine and hydrogen alloyed a-Si are discussed.