| dc.description.abstract | Absorption of optical light in superconducting electronics is a major limitation on the quality of circuit architectures that integrate optical components with superconducting components. Such absorption causes losses in the optics and quasiparticle generation in the superconductor, decreasing the performance of both [1]. However, integration of optical and superconducting components will be crucial for the development of electro-optical transducers for quantum networking [2], scalable readout of single photon detectors [3], and neuromorphic computing [4]. Ideally, we could fabricate the superconducting electronics in these systems out of a material that is transparent to the wavelengths used by the optical components.
Few conductive materials are transparent to optical wavelengths though, let alone superconducting materials. Typical metals have a high carrier concentration and no band gap, resulting in strong absorption for light below x-ray frequencies [5]. However, certain degenerately doped semiconductors known as transparent conductive oxides have ultraviolet band gap energies, high mobilities, and low carrier concentrations, thus allowing for both good conduction and optical transparency. Under the right conditions, these materials may superconduct as well. One such material, indium tin oxide (ITO), has been shown to superconduct with a maximum transition temperature of about 4 K when doped to carrier concentrations of about 1021cm−3 [6]. In particular, arbitrary samples of ITO can superconduct when sufficiently doped by electrochemical reduction [7].
In this thesis, we characterize the effects of electrochemical reduction on the electronic properties, structure, and composition of ITO and evaluate its suitability for superconducting electronics. First, in Chapter 1, we outline the theory of transparent superconductivity and review existing work on such materials. Then in Chapter 2 we describe the basic theory and design of our electrochemical cell and discuss the characterization techniques we will use to evaluate our films. In Chapter 3 we present our findings on the electronic properties, structure, and composition of ITO reduced to different total reduction charge densities. In Chapter 4 we quantify the optical properties of reduced ITO and compare it to niobium, a common material for superconducting electronics. In Chapter 5 we consider different methods for fabricating electronics on reduced ITO and evaluate the resulting microwires. Finally, in Chapter 6 we discuss the implications of our findings and future directions for work on transparent superconductors. | |
