Thermodynamic approach to the conditions of chemical deposition of boron by contact with an inert substrate

The optimum conditions for the synthesis of boron by chemical vapour deposition (CVD) from BCl3-H2 or BBr3-H2 mixtures onto an inert substrate (boron or boronized metals) have been studied by a thermodynamic approach. This approach, which postulates that states close to equilibrium are reached in the vicinity of the hot substrate, is based on the minimization of the total Gibbs free energy of the system. Between 1200 and 1900 K and under a total pressure of 1 atm, the hydrogen reduction of BCl3 can lead to two types of by-products: BHCl2 at all temperatures, and BCl2 or BCl subhalides at high temperatures; BHCl2 is the main product of the reduction at the lowest temperatures. The hydrogen reduction of BCl3 is never complete for the conditions commonly used for the synthesis of boron. The amount of by-products and of BBr3 which must be recycled can be minimized by utilizing BCl3-H2 mixtures rich in hydrogen. The amount of boron deposited exhibits a maximum for a temperature close to 1700 K. Similar results have been obtained for BBr3. However, between 1000 and 1500 K and under a total pressure of 1 atm the amount of by-products (BHBr2 and BBr2) is smaller than in the case of BCl3. The boron yield from the reduction of BBr3 is higher than that from BCl3 and the percentage of boron halide which must be recycled is lower in the case of BBr3. Thus, BBr3 appears to be a better source than BCl3 for the CVD of boron. (Auth.)