MAGNESIUM DIBORIDE (MGB2) THIN FILMS ON COPPER AND SILICON FOR RADIOFREQUENCY CAVITY AND ELECTRONIC APPLICATIONS

Abstract

Magnesium diboride is a known material since the 1950s. However, superconductivity in MgB2 was discovered in 2001. Soon after the discovery of superconductivity in MgB2, there was a rush to understand its complex nature of superconductivity and other properties. However, current research in MgB2 is mainly focused on applications. MgB2 possesses excellent superconducting properties such as a high transition temperature (Tc) of 39 K, a high critical current density (Jc) of ~107 A·cm-2, a high thermodynamic critical field (Hc), absence of weak links at grain boundaries, etc. Because of these properties, it is considered one of the candidate materials for applications such as superconducting wires, superconducting radiofrequency (SRF) cavities, superconducting electronic devices, etc. SRF cavities play an important role in modern particle accelerators. The main objective of an SRF cavity is to accelerate charged particle beams. SRF cavities are characterized by two figures of merit: the quality factor (Q) and the accelerating gradient (Eacc). Q characterizes the energy efficiency of an RF cavity and Eacc is the average accelerating field of an RF cavity. The state-of-the-art SRF technology is based on niobium. It is a well-matured technology and it is reaching the theoretical limits on both Q and Eacc. Additionally, Nb cavities operate at 2 K, which requires large-scale liquid helium refrigeration and distribution systems. This adds substantial capital and operational costs for large particle accelerators such as HL-LHC and proposed ILC. Because of these reasons, new SRF materials with higher Q, higher Eacc, and higher operational temperatures are desired. Currently, few superconducting materials such as Nb3Sn and MgB2 are in the research and development process. Nb3Sn has a Tc of 18 K, which is significantly lower than the Tc of MgB2. MgB2-coated cavities are theoretically predicted to have higher Q and Eacc compared to Nb cavities. In addition, owing to its high Tc, MgB2-coated cavities are expected to operate above 4.2 K (20-25 K). Operation at around 20–25 K will allow the use of hydrogen- or neon-based cryocooler technology, eliminating the use of helium. This will substantially reduce the capital and operational cost of a MgB2-based accelerator. However, this will not be possible with Nb3Sn-based SRF cavities due to the low Tc of Nb3Sn. The main goal of the research presented in this thesis is to develop MgB2-coated copper superconducting radiofrequency cavities utilizing hybrid physical-chemical vapor deposition (HPCVD) technique. MgB2-coated Cu SRF cavities will have an added advantage due to the high thermal conductivity of the Cu. The excellent thermal conductivity of Cu enhances the heat transfer between the superconducting MgB2 layer and the cavity body, thus providing better resistance to thermal breakdown. RF characterization of MgB2–coated Cu is a crucial requirement because it is the first step toward the MgB2 –coated Cu SRF cavities. For these characterizations, small-sized samples (e.g., 2-inch diameter) are usually required. Among several MgB2 growth techniques, the HPCVD process produces the best quality MgB2 thin films. However, the growth of MgB2 films on Cu using the HPCVD technique is challenging as Mg, and Cu readily react to form several Mg-Cu alloys. Therefore, a new MgB2 growth process on Cu was developed by modifying the existing HPCVD process and in the new process, the deposition takes place at ~470 °C. With this new process, high-quality MgB2 thin films were successfully deposited on 2-inch diameter Cu discs, and these samples were characterized in terms of structural and superconducting properties. Surface morphology showed well-connected crystallites with no pinholes on the coating, and the cross-sectional studies showed conformal growth of MgB2 on Cu. The Tc of these samples were ~37 K and the ~107 A·cm-2 zero field Jc was observed. Most importantly, RF characterizations at 11.4 GHz showed Q close to 2 x 107 at 4 K, which was comparable to the Q of Nb. After successful RF testing of MgB2-coated Cu discs, this process was scaled up to coat 3 GHz Cu RF cavities. As the first step, a MgB2 thin film was synthesized on the inner wall of Cu tubes with dimensions (~1.5-inch inner diameter and 8-inch length), similar to a beam tube of a 3 GHz RF cavity. The MgB2 film on the Cu tubes showed conformal coating with Tc ~37 K. Next, the coating of the 3 GHz Cu test cavity was carried out. Cu test cavities were assembled using two half-cells pressed at Thomas Jefferson National Accelerator Facility (JLab) and two beam tubes machined at Temple University. The MgB2 film was successfully synthesized on the inner wall of 3 GHz test cavities and the MgB2 coating on the two half-cells showed uniform growth with Tc distributed around 35 K. However, slight damages to the cavity wall were observed and these damages were mainly due to the deformation of the Cu surface, caused by the formation of Mg-Cu alloy liquid. Current research is focused on developing damage-free MgB2-coated Cu RF cavities. In addition to MgB2 growth on Cu for SRF cavity applications, development of high-quality MgB2 thin film on Si substrates was carried out. This will be used in electronic device applications such as fabrication of hot-electron bolometers (HEB). An issue similar to the Mg-Cu reaction was observed with Si; Si and Mg react at elevated temperatures, forming Mg2Si, and this was observed at around 550°C. This reaction prevents the use of the HPCVD technique directly on Si. Previous attempts at synthesizing MgB2 films on Al2O3-buffered Si substrates at low temperatures (500–600°C) were reported. However, these films have shown extremely rough surfaces with poor superconducting properties. In this work, a ~220 nm-thick boron buffer layer was used to prevent the Mg-Si reaction, and it was observed that the boron was effective even above 700°C. High-quality MgB2 thin films were synthesized on boron-buffered Si substrates using the standard HPCVD technique. However, the resultant films showed enhanced roughness due to the polycrystalline growth. Ar ion milling at an ultra-low angle (1°) was used to smooth the MgB2 films, and the resultant films showed roughness comparable to epitaxial films grown on SiC substrates with a slight degrade in superconducting properties. Finally, Al ion implantation in the MgB2 thin film was studied and this project was carried out to synthesize MgB2 films with modified superconducting properties. In this study, 80 nm-thick MgB2 films were irradiated with a 75 keV Al ion beam. A 30 nm Au buffer layer was used on top of the MgB2 films in order to position the projected range of Al ions near the center of the MgB2 films. Al ion doses were kept between 2×1011–1×1016 atoms·cm-2. Superconducting properties and the structural properties of these Al ion irradiated films showed systematic change with the Al dose. Superconducting transition temperature decreased with increasing Al dose. Also, for the Al ion dose at or above 2 × 1014 atoms·cm-2, the irradiated samples did not show any superconducting transition. Al ion irradiated films showed an increase in the c-axis lattice parameter of MgB2 with increasing ion dose. These observed changes in the superconducting properties and structural properties of Al ion irradiated films can be attributed to the ion damage.