Investigation of growth, structural and electronic properties of V_1tn2O_1tn3 thin films on selected substrates [Elektronische Ressource] / vorgelegt von Alexei Nateprov

Natepral

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• Physik

Informations

• Publié par: universitat_augsburg
• Publié le: 01 janvier 2008
• Nombre de lectures: 13
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

Investigation of growth, structural and
electronic properties of V O thin films on 2 3
selected substrates




Dissertation zur Erlangung des Doktorgrades der
Mathematisch-Naturwissenschaftlichen Fakultät der
Universität Augsburg




Vorgelegt von

Alexei Nateprov




August 2006






























Erstgutachter: Prof. Dr. S. Horn
Zweitgutachter Priv.-Doz. Dr. Helmut Karl


Tag der Einreichung: 1 December 2006

Contents


1 Introduction..................................................................................... 1


2 Basics................................................................................................ 3

2.1 Theoretical Approaches.......................................................... 3
2.2 The V O System................................................................... 9 2 3
2.2.1 Phase Diagram ............................................................... 9
2.2.2 Crystal Structure .......................................................... 12
2.2.3 Electronic Structure ..................................................... 14
2.3 Thin Films Growth Modes.................................................... 17
2.4 Surface Acoustic Waves (SAW) .......................................... 21
3 Experiments .................................................................................. 27

3.1 Thin Film Growth................................................................. 27
3.1.1 Experimental Setup...................................................... 27
3.1.2 Target Preparation ....................................................... 31
3.1.3 Substrate Preparation................................................... 34
3.1.4 Deposition and Annealing ........................................... 36
3.2 Characterization Techniques ................................................ 39
3.2.1 Dynamic Secondary Ion Mass Spectrometry (SIMS).. 39
3.2.2 Rutherford Backscattering Spectrometry (RBS) ......... 40
3.2.3 X-ray Diffraction (XRD) ............................................. 41
3.2.4 Atomic Force Microscopy (AFM)............................... 44
3.2.5 Resistivity Measurements............................................ 46
3.2.6 Surface Acoustic Wave (SAW) Experiments.............. 47




i 4 Results and Discussion ...............................................................51

4.1 V O Thin Films Grown on Diamond Substrates....................51 2 3
4.1.1 Resistivity Measurements...............................................51
4.1.2 Atomic Force Microscopy (AFM)..................................58
4.1.3 Dynamic Secondary Ion Mass Spectrometry (SIMS) ....60
4.1.4 X-ray Diffraction (XRD) Measurements........................61
4.1.5 Rutherford Backscattering Spectrometry (RBS)............65
4.1.6 Conclusions ....................................................................67
4.2 V O Thin Films Grown on LiNbO Substrates ......................69 2 3 3
4.2.1 X-ray Diffraction (XRD) Measurements........................69
4.2.2 Atomic Force Microscopy (AFM)..................................70
4.2.3 Resistivity Measurements...............................................71
4.2.4 Surface Acoustic Waves (SAW) Investigations.............72
4.2.5 Conclusions ....................................................................77

5 Summary..........................................................................................79


6 Bibliography ...................................................................................81



















Introduction
Transition metal oxides are of continuous interest in modern solid state physics
due to their exceptional electronic, magnetic and optical properties
[Goodenough1971]. Many transition metal oxides show phase transitions from
metallic to insulating or semiconducting states. The electronic structure of these
materials is strongly influenced by the partially filled d-orbitals and related strong
Coulomb interaction [Mott1968].
Vanadium oxides are one the most typical examples of transition metal oxides.
In particular, vanadium sesquioxide (V O ) is often referred to as a prototype of a 2 3
strongly correlated electron system. As firstly reported by M. Foex in 1946
[Foex1946], this compound shows a sharp metal to insulator (MI) transition at
T ~160-170 K, quantified by a six orders of magnitude increase of the resistance. MI
Since that time a lot of experimental and theoretical studies were done to
understand the origin of this transition. However, a consistent model, accounting
for the features of the MIT as well as of the related magnetic and structural phase
transitions, is absent till now, thus making these phenomena and the material itself
still highly actual nowadays.
The present work is devoted to the experimental study of the MI transition in
V O thin films, grown on different substrates. Why actually thin films? Firstly, 2 3
this is because the thin film technology is mostly appropriated for any device
applications. Secondly, the study of the MIT transition in bulk V O is an 2 3
extremely delicate issue, because of an practically unavoidable crash of the
sample during the cooling and heating cycles due to large volume change across
the metal to insulator transition. Thin film technologies make it possible to
accommodate the above stress, thus providing stable samples for experimental
studies as well as planar device structures for applications. Very interesting for
applications seems to be V O films grown on diamond substrates. Due to high 2 3
value of the energy gap in diamond, optical filtering, based on the change of
1 Introduction
optical constants in V O thin films across the MI transition looks promising. 2 3
Other applications include infra-red mirrors, resistive transducers, etc.
The main goal of this work was to develop a technology of growth of V O 2 3
thin films on substrates with different electrical and structural properties (diamond
and LiNbO ), designed for specific applications. Because of the large lattice 3
mismatch between V O and diamond, the film growth presents a rather 2 3
complicated problem, which was not solved before the present work started.
Another important problem of growth technologies is the temperature of
substrate during the deposition. Especially, because of high mobility of atoms of
light elements (like Li) at higher temperatures, the diffusion of these elements into
the growing film is enhanced, yielding a deterioration of the properties of V O . 2 3
Thus, the conventional UHV techniques, which usually involve an in situ growth
at high substrate temperatures, T =600 °C, cannot be applied in the case of Sub
diamond and LiNbO substrates. 3
We present here a new growth strategy for V O thin films, based also on the 2 3
UHV technique, which successfully solves the above mentioned difficulties.
Using this technology, thin films of V O with reproducible parameters were 2 3
grown on diamond and LiNbO substrates for the first time. We have 3
characterized in detail the structural and electrical properties of the obtained films
with a special focus on their potential applications.
The present work is organized as follows. The “Basics” (Chapter 2) gives an
introduction to the main properties of the V O system, including the phase 2 3
diagram, the crystal and electronic structure. In this part also the modes of thin
film growth as well as the theory of surface acoustic waves are described.
The “Experimental” part (Chapter 3) includes the description of the film
preparation and characterization techniques.
The structural properties, studied by dynamic secondary ion mass
spectrometry (SIMS), X-ray powder diffraction (XRD), Rutherford backscattering
spectrometry (RBS), atomic force microscopy (AFM), as well as electronic
properties of the films are described in Chapter 4. In this chapter the results of the
surface acoustic waves study are also presented.
2 2. Basics
2.1 Theoretical Approaches
The electronic properties of solid state systems can be described by the following
ab initio Hamiltonian [Fazekas1999]
2∧  h3 +ˆ ˆH = d rΨ (r,σ ) − Δ +V (r) Ψ(r,σ )∑  ion ∫ 2mσ  e  (2.1.1)
1 3 3 + +ˆ ˆ ˆ ˆ′ ′ ′ ′ ′ ′+ d rd r Ψ (r,σ )Ψ (r ,σ )V (r − r )Ψ(r ,σ )Ψ(r,σ ).∑ ee∫2 σσ ′
+ˆ ˆHere, Ψ is field operator that creates an electron, Ψ is the field operator that
annihilates an electron at position r with spin σ, Δ is the Laplace operator, m is e
the electron mass, e is the electron charge. V denotes the one-particle ionic ion
potential of all ions with charge eZ at given positions R via i i
Z2 i *V (r) = −e (2.1.2) ion ∑
r − Ri i
and