Electronic and optical properties of thin-film superconductors and superconductor - semiconductor interfaces

Recently, the interest in the properties of superconductor-normal metal and especially superconductor-semiconductor interfaces increased in the basic research area. The superconducting proximity effect, known for about four decades, still poses many unanswered questions. Use of a semiconductor instead of a metal broadens the possibilities: superconductor-semiconductor interfaces are tunable, and their parameters can be varied in a wide range. This type of interfaces may also be used in future superconducting electronic device applications. Our studies of the interfaces between high-quality thin film Nb and NbN superconductors, and (100) n+-InAs (10^19cm^-3 ) are done by transport and optical measurements. For the first time, the proximity effect in superconductor-semiconductor interfaces is observed using Raman scattering. The InAs longitudinal optical phonon LO mode (237cm-1) and the plasmon-phonon coupled modes, L- (221cm-1) and L+ (position is carrier concentration dependent), are studied. The intensity ratio of the LO mode (associated with the near-surface charge accumulation region, CAR in InAs) to that of the L- mode (associated with bulk InAs) is observed to increase by up to 40%, when the temperature is decreased below the superconducting transition temperature Tc. Further experimental and theoretical studies are required to find the underlying mechanism. Effects of surface damage and surface passivation on the electronic properties of InAs are studied. Damage, produced by Ar-ion etching, reduces the surface band bending. Passivation of the InAs surface by application of alkanethiols is found to reduce the surface band bending and to preserve the surface of InAs from oxidation. In close collaboration with A. Shnirlnan, a theoretical model for the Andreev reflection contributions to the current at the SN interfaces is developed using the tunneling Hamiltonian method. Both, the interference due to multiple Andreev reflection, and the electron-electron interaction in the normal metal, contribute to the zero-bias conductance peak (ZBCP). The shape of the ZBCP is calculated as a function of temperature, strength of the interaction and other parameters of the normal metal. Application of an external ac field results in steps in the current-voltage characteristics at Vn = nhω/2e. Experimental parameters of a highly-doped n+-InAs are used to obtain estimates for future experiments to be conducted to test the predictions of the model.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Year dc:date

2012

Author dc:creator

• Roshchin, Igor Vladimirovich

Contributors dc:contributor

• Greene, Laura H.

• thin films
• superconductors
• superconductor-semiconductor interfaces
• superconducting proximity effect

Statement dc:rights

• ©2000 Roshchin

Language dc:language

en

Identifier

4285156

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/31299