Polymorphism in boron deposited by the H2 reduction of BCl3

Boron polymorphism is discussed from studies of the relationships between the mechanisms of chemical vapour deposition of boron and the resulting deposit structures. The deposition rate of boron is controlled either by mass transfer in the vapour phase (combined with thermodynamic equilibrium at the gas-solid interface or combined with surface kinetics) or by the surface kinetics alone. These mechanisms determine which of two situations exists at the interface: (1) near equilibrium between the vapour phase and solid boron; (2) significant supersaturation of the vapour phase. The structure of the boron deposit is related directly to the temperature, the amount of supersaturation and the deposition time. For longer deposition times boron remains amorphous more readily as the temperature decreases and as the supersaturation increases. β rhombohedral boron appears as the thermodynamically stable form at all temperatures. The deposition of α rhombohedral and of tetragonal boron is determined by kinetic factors; α rhombohedral and tetragonal boron appear as the metastable forms at low and high temperatures respectively. These conclusions are corroborated by solid state transformations. (Auth.)