Electronic and optical properties of thin-film superconductors and superconductor - semiconductor interfaces
Roshchin, Igor Vladimirovich
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https://hdl.handle.net/2142/31299
Description
Title
Electronic and optical properties of thin-film superconductors and superconductor - semiconductor interfaces
Author(s)
Roshchin, Igor Vladimirovich
Issue Date
2000
Doctoral Committee Chair(s)
Greene, Laura H.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
thin films
superconductors
superconductor-semiconductor interfaces
superconducting proximity effect
Language
en
Abstract
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.
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