Dual-heater reactor design for hybrid physical-chemical vapor deposition of MgB₂ thin films

A new reactor configuration for the hybrid physical-chemical vapor deposition (HPCVD) of MgB₂ thin films was developed. In this new design, the substrate is resistively heated and inversely mounted in the gas stream. The Mg supply is placed downstream of the substrate and is inductively heated. The dual-heater reactor is designed to enable the growth of MgB ₂ thin films at lower substrate temperatures, permit the growth of thick films and is amenable to scale-up for film deposition over larger substrate areas than the original HPCVD design. Computational fluid dynamics (CFD) based reactor modeling was used to simulate the gas flow, temperature and gas phase concentration profiles in the reactor in order to identify conditions suitable for film deposition. A study of boron thin film deposition from B ₂He in the dual-heater configuration was carried out and the experimental results were compared to CFD simulations which utilized a previously developed gas-phase and surface chemistry model to predict the growth rate. The boron film growth rate slightly decreased as the substrate temperature was increased from 650°C to 750°C in both the experiments and model simulation. The modeling results indicate that boron deposition is limited by the mass transfer of BH₃ to the substrate under these conditions and the temperature dependency arises due to the upstream depletion of the boron precursor. MgB₂ films were grown at a substrate temperature of 710°C and Mg temperature of 720°C using the dual-heater configuration. The film properties were consistent with previously published results of MgB₂ on sapphire in the original HPCVD configuration with a superconducting transition temperature, T_c, of 40.3 K and residual resistivity ratio (RRR) of 13.