OPTIMIZATION OF PLASMA ASSISTED MOLECULAR BEAM EPITAXY GROWN NbxTi1-xN FOR EPITAXIAL JOSEPHSON JUNCTIONS

Abstract

This thesis is an investigation into the growth and characterization of NbNx and TiN transition metal nitrides, along with the alloy NbxTi1-xN. These materials are commonly used in many applications ranging from superconducting quantum computing, superconducting conventional computing, high kinetic inductance devices such as single photon detectors, and hard coatings for industrial applications. This thesis will begin with an overview of superconducting quantum computing and superconducting materials, then review the fabrication of Josephson junctions and highlight the need for material improvement. The goal of this work is to grow a superconducting nitride material which can be engineered to lattice match with AlN, the barrier layer in a hypothetical all-nitride, epitaxially grown superconducting quantum computing structure. The alloy NbxTi1-xN is chosen as the superconducting alloy of choice due to the range of lattice constants available, the high critical temperature of these nitrides, and the high quality of material able to be grown using PAMBE. The first aim of this thesis studies the binary transition metal nitrides NbNx and TiN to generate endpoints for various properties of the alloy NbxTi1-xN. This thesis is one of the first investigations of multi-phase growth of ε-NbN and γ-Nb4N3, and demonstrates control over the phase, crystal orientation, superconducting properties, and surface morphology by changing PAMBE growth parameters. The second aim of this thesis demonstrates the growth of NbxTi1-xN and is the first investigation of tunable material properties for this alloy by adjusting the composition. The last aim of this work is the development of a novel annealing scheme used to prepare NbxTi1-xN thin films for Josephson junction integration. The novel annealing scheme ensures excellent surface roughness of NbxTi1-xN thin films, increases the superconducting critical temperature of this alloy from approximately 14 K to 16.8 K, and improves the crystal quality by way of nitrogen incorporation and improvement of the crystal quality. The results from this work will be crucial in developing NbxTi1-xN / AlN / NbxTi1-xN Josephson junctions with smooth, uniform interfaces and low-loss, defect free nitride materials. Additionally, this thesis represents an investigation into the relationship between phases of NbNx and TiN, the role of nitrogen incorporation caused by in-situ annealing, and a useful record of control over this material using PAMBE growth conditions and alloy composition.