Experimental and Modeling Studies of the Hybrid Physical-Chemical Vapor Deposition of Superconducting Magnesium Diboride Thin Films

Open Access
- Author:
- Lamborn, Daniel Ray
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 03, 2007
- Committee Members:
- Joan Marie Redwing, Committee Chair/Co-Chair
Kristen Ann Fichthorn, Committee Member
Antonios Armaou, Committee Member
Thomas Nelson Jackson, Committee Member - Keywords:
- magnesium diboride
superconductor
numerical modeling
reactor design
process development - Abstract:
- MgB2, with a Tc of 39 K, is a promising material for superconducting electronics and high field magnet applications. The development of deposition processes for MgB2 has been hampered by the unusually high Mg overpressure required for phase stability at elevated temperatures. Hybrid physical-chemical vapor deposition (HPCVD), a process developed at Penn State, combines thermal decomposition of B2H6 gas with an evaporative Mg flux to deposit MgB2 and is able to provide sufficient Mg overpressure for high temperature MgB2 growth. The HPCVD process does, however, have limitations arising from the original reactor configuration. The substrate and Mg supply are heated on the same inductively heated susceptor, which prevents independent temperature control and limits both the size of substrates and the amount of Mg available for growth. This in turn limits the useable range of deposition parameters such as substrate temperature and restricts the growth time which is problematic for thick films and coatings. The goals of this study were to develop an improved understanding of the HPCVD deposition process and design a new HPCVD reactor that addresses and improves upon the limitations of the original configuration. A combination of computational fluid dynamics simulations and growth experiments were used to study the HPCVD process in the original reactor. A transport and chemistry model for the growth of boron films from B2H6 was developed and used to evaluate new reactor configurations. The simple chemistry model consists of the gas-phase decomposition of B2H6 to BH3, the adsorption of BH3 onto an activated site to form a BH2-Site complex and the transformation of the complex into a boron film and the growth rates from this model were in quantitative agreement with experimental data. A vertical dual-heater reactor configuration which was interchangeable with the original configuration was then developed to provide independent temperature control of the substrate and Mg source and increased substrate size. MgB2 film growths in this configuration were achieved, with comparable film properties from the original configuration; however the process had poor reproducibility and low growth rates. Boron film growth experiments and numerical modeling, incorporating the previously developed chemistry model showed that there is substantial B2H6 depletion upstream of the substrate and indicated that this configuration was not an optimal design. An impinging jet reactor configuration was then designed to reduce parasitic deposition upstream of the substrate, yet maintain the independent temperature control of the substrate and Mg source. Numerical modeling was used to explore process parameters for boron thin film growth and provide a guide for growth experiments. Initial experiments showed that a balance needed to be established between the loss of B2H6 and the gas-phase nucleation of Mg particles from cold-finger effects induced by the inlet jet flow. Superconducting MgB2 films were successfully grown in the impinging jet reactor that a Tc of 39.8 K and critical current density greater than 2x106 A/cm2 at zero magnetic field. The impinging jet configuration introduced a large degree of flexibility in MgB2 deposition with thin axis oriented films deposited at low jet flow rates with improved uniformity and thick coatings with growth rates in excess of 100 m/hr at higher jet flow rates.