Crystal growth and investigations on the effects of hydrogen doping of VO2
Author
Yavuz, Koray
Advisor
Kasırga, Talip Serkan
Date
2019-03Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
Vanadium Dioxide(VO2) has been studied extensively for its interesting electronic
structure that allows it to go through Metal-Insulator Transition(MIT) at 65 C.
The nature of this phenomena is not entirely clear and more research is needed to
firmly establish the science behind it and to realize possible applications; such as
ultra-fast electrical and optical switching, sensor devices and Mott-Field Effect
Transistors. One of the important experiments to understand the electronic
structure of a material is Hall-effect measurements but due to acicular (needle
like) nature of VO2 crystals, this subject is only studied either on millimeter sized
samples which are not suitable for many device applications or on poly crystalline
thin films that are under non-uniform stress due to the substrate effects which
gives unsatisfactory results when performing experiments. This thesis suggest
a new method of chemical vapour deposition(CVD) growth for low aspect ratio
VO2 crystals that have lengths between 50-100 m and thicknesses between 40-
170 nm. These crystals can be mechanically removed from the substrate and
transferred to use in different applications such as Hall-effect measurements or
Transmission Electron Microscope(TEM) studies. Additionaly this work shows
some aspects of the surface chemistry of the widely used Silica, Si, quartz and
Sapphire substrates; relating with the control of oxygen saturation on the surface.
Another VO2 growth method for c-plane sapphire that leads to considerably more
crystal yield is shown.
Hydrogenation of the VO2 crystals suppresses the MIT so understanding
this phenomena might help us better understand the effects lying behind the
transition. To study this phenomena a crystal is doped only from half by blocking
the passage of hydrogen to other half so the interplay between the insulating phase
and hydrogenated conductive phase can be observed. As the analysis tool, TEM
is used on this sample. Using a two-terminal device of a VO2 crystal, the effects
of hydrogenation on the electronic properties have also been studied.
Overall this thesis introduces a new method for CVD growth of VO2 which is
used in various applications such as Hall-effect experiments, two terminal devices
and TEM studies. To control the growth process the interplay between oxygen
and surface chemistry of sapphire, silica, Si and quartz substrates have been
investigated. With these studies a better understanding of the mechanics of
growth is intended.
Keywords
Vanadium dioxideCrystal growth
Strongly correlated materials
Metal-insulator transition
Hall-effect