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Audrey Grockowiak

Superconductive semiconductors

Published on 22 November 2012


Thesis presented November 22, 2012

Abstract:
This experimental PhD thesis explore the superconductivity of heavily boron doped epilayers as well as some unusual properties observed at high temperatures. The superconductivity of Si :B is observed below 1K and triggered by boron content exceeding the solubility limit of boron into silicon. For such high boron contents, the silicon layers are metallic. In a first part, we develop the various experimental techniques used. We explain the principles of the out-of-equilibrium doping techniques required to doped beyond the solubility limit. We develop also on the characterization techniques used to control the quality of the samples, as well as the low temperatures measurement techniques. In a second part, we show the results obtained on the superconductivity of Si :B, obtained first by varying the boron content at a given layer thickness, and then as a function of the layer thickness. We show that the evolution of Tc with the electron-phonon coupling constant lambda doesn't follow the classical McMillan law, but rather a power of law as it was reported for superconducting diamond. We show that this result can be explained by a double layer model with dislocations resulting in two different lambda values for each sublayer. The study of the temperature and angular dependency of the Hc2 also show that Si :B is an intrinsic type I superconductor turned into type II with defect effects, and that the superconductivity is bidimensionnal. In a third part, we present the anomalous high temperature behavior of some Si :B epilayers, starting from 50K and observed at least up to 400K. We present magnetotransport, Hall effect and thermoelectric measurements that all show a highly non linear behavior, unusual for a metal. The origin of these anomalies is still an open question. We finally present some future perspectives, including the first measurements on a Si :B SQUID-like geometry.

Keywords:
Doped semiconductors, Cryogenics, Condensed matter physics, Superconductivity

On-line thesis.