Low temperature semiconductor-superconductor junctions & their optimisation

The research presented in this thesis focuses on the relatively young field of direct electron cooling via semiconductor-superconductor tunnel junctions. These devices utilise the superconducting energy gap to form a high pass electron filter, capable of lowering the effective electron temperature in the semiconductor electrode. The presented research focuses on optimizing the performance of such junctions i.e. the minimum achieved electron temperature, primarily by modifying the electrical properties of the semiconductor/superconductor and influencing the quality of the semiconductor-superconductor interface. In the semiconducting electrode, the effect of carrier type on the thermal coupling of the system is studied, as is the effect of mechanical strain, induced via lattice mismatch. A study is made of alternative superconductors, modifying their properties through the use of thin films for the superconducting contact. Finally, a comprehensive investigation is presented into the importance of fabrication procedures on device characteristics, leading to the development of the first semiconductor-insulator-superconductor junction.