Metal-semiconductor transition materials [Elektronische Ressource] : FeS and VO_1tn2 thin films by RF reactive sputtering / von Ganhua Fu

Ghfu

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• Physik

Informations

• Publié par: justus-liebig-universitat_giessen
• Publié le: 01 janvier 2007
• Nombre de lectures: 8
• Langue: English
• Poids de l'ouvrage: 4 Mo

Extrait

Metal-Semiconductor Transition Materials:
FeS and VO Thin Films by RF Reactive Sputtering 2


Dissertation zur Erlangung des Doktorgrades
der Naturwissenschaften von


Ganhua Fu


Betreuer: Prof. Dr. B. K. Meyer

I. Physikalisches Institut
Justus-Liebig-Universität Gießen




Gießen, June 2007
Contents

1 Introduction...………………………….………………………………………………1
2 Preparation and characterization techniques……………………………..………...….4
2.1 Radio frequency reactive sputtering. ……………………..……………………...4
2.2 Characterization techniques..…………………………………………………...6
2.2.1 X-ray diffraction and reflectivity ……………..…………………………..6
2.2.2 Scanning electron microscopy …………………...……………………….9
2.2.3 Energy-dispersive X-ray spectroscopy ……………………….................10
2.2.4 Secondary ion mass spectrometry …………………………………….....12
2.2.5 Rutherford Backscattering Spectroscopy ………..………………………13
2.2.6 Elastic recoil detection analysis ………………………………................16
2.2.7 Optical transmittance...………………………………….….……………17
2.2.8 Electrical resistivity………………………………..………………….…17
3 FeS material: a brief introduction ……………………………...……………………..19
3.1 Crystal structure ………………..……………………........................................19
3.2 Electrical and magnetic properties….…………………………………………..21
4 Deposition, characterization and electrical properties of FeS thin films ……………22
4.1 Structure and morphology of FeS films……………………...............................22
4.1.1 Deposition of FeS films…………………………………………………..22
4.1.2 Influence of the substrate temperature…..26
4.1.3 Influence of the sputter power…………………………………….………27
4.1.4 Infle substrates ……………….31
4.2 Electrical properties.……………………………………...….………………....35
4.3 Influence of the annealing on MST of FeS films……………………………….41
4.3.1 FeS films on float glass………….………………………..41
4.3.2 FeS films on sapphire with (0001) orientation………………………….47
4.5 Influence of the aging on MST of FeS films…………………..49
4.6 Influence of the thickness of FeS films on MST ……………………………..53
iContents
4.7 O doping in FeS films…………………………………………………………...57
4.8 Summary………………………………………………………………………...59
5 VO material: a brief introduction................ ...............................................................61 2
5.1 Crystal structure ………………..…………………….................61
5.2 Electronic properties…….……….…………………………………….………..62
5.3 Optical properties……………………………….……………………….……...64
6 Li/H doping and thermal stability of VO thin films………….…………….….……..65 2
6.1 Li doping …………………………………………………………………….…66
6.1.1 Li doping by the V target with Li foil………………….…66
6.1.2 Li doping by VO2:Li O targets…………………………………………...68 2
6.1.2.1 Pure VO target……………….……………………………….….68 2
6.1.2.2 The VO : Li O target (2%)……….………………70 2 2
6.1.2.3 The VO : Li O target (5%)………………….……………………75 2 2
6.1.3 Conclusion………………………………………………………………..76
6.2 H doping ………………………………………………………………………..77
6.2.1 Metallic target…………….78
6.2.2 VO target………………….…………………………………..80 2
6.2.3 Conclusion………………………………………………………………..81
6.3 Thermal stability of VO films…………...……………………………………..81 2
6.3.1 Experimental details………………………………………………………81
6.3.2 Results and discussion……82
6.3.3 Conclusion………………………………………………………………..87
6.4 Summary ……………………………………………………………………….87
7 Summary ………………………..…….89
8 Zusammenfassung………..…………………………………………………….92
References............................................................................................................................95
Publications………………………..………………………………………………………99
Acknowledgements………………………………………….…………………………..100
iiChapter 1 Introduction

Metal-nonmetal transition in many transition-metal oxides and sulfides has been the
subject of considerable experimental and theoretical work for over sixty years. Many
reviews and books are published on this subject [1-6]. From the theoretical side, different
mechanisms, such as Anderson transition, Peierls transition and Mott-Hubbard transition
were proposed to clarify the metal-nonmetal transition. From the experimental side,
numerous systems are found to show this transition and the physical properties around the
transition are extensively investigated. A metal-nonmetal transition is accompanied by the
abrupt change in some physical properties of systems, such as the electrical conductance,
optical transmittance, and so on. By detecting the variation of these physical properties
around the transition, it is possible to make some switching devices triggered by heat,
pressure, etc. In this work, two Metal-Semiconductor Transition (MST) systems, VO and 2
FeS in the thin film state, were investigated.

Vanadium Oxide

oVanadium dioxide (VO ) exhibits a reversible MST at 68 C [7]. Below this transition 2
temperature, it is a narrow gap (0.65 eV) semiconductor with a monoclinic structure.
oAbove 68 C, it transforms into a tetragonal (TiO ) structure and exhibits metallic 2
properties. This transition is accompanied by the abrupt change in the electrical resistivity,
optical transmittance and reflectance. For example, the VO film in the infrared region has 2
a very low transmittance in the metallic phase but rather high transmittance in the
semiconducting phase. This has lead to many applications of the material in infrared light
(IR)-switching or bolometric devices [8, 9] or especially as intelligent energy conserving
window coating (smart window) [10].
The smart window plays an important role in future glazings [11]. Upon a change in
electrical field, light intensity or temperature, it exhibits a large change in optical
properties totally or partly over the visible and solar spectrum. The smart window can
control the flow of heat through a window and thus has a considerable energy advantage
over that of conventional double glazed windows. Applications include glazings in
1 Chapter 1 Introduction

buildings, vehicles, aircrafts, spacecrafts and ships. The smart window is classified into
two major types: non-electrically activated and electrically activated. The electrically
activated devices have the advantage of user or automatic control. The phase dispersed
liquid crystals, dispersed particle systems and electrochromics belong to this type. The
non-electrically activated type has the advantage of being self-regulating with local
control. This type includes some photochromic, thermochromic and thermotropic
materials.
VO is one of the most potential materials for the application as non-electrically 2
activated smart window. For an application as smart window coating, the transition
otemperature has to be lowered to about 25 C, the luminous transmittance T , the lum
transmittance of the semiconducting phase in the visibe region, should be as high as
thin film should be human comfortable, for example, possible and the color of the VO2
colorless or blue. The required reduction of the transition temperature can be achieved by
the substituting doping. Tungsten is the most effective dopant [12, 13], with an effect of ≈
–23 K/at.% W, up to concentrations of a few percent. Other dopants are, e.g. fluorine (-20
K/at.% F) [14], rhenium (-18 K/at.% Re) and molybdenum (-10 K/at.% Mo) [15]. The
4+reduction of the transition temperature by the replacement of V with higher-valence ions
was explained with a charge-transfer mechanism [16]. The luminous transmittance can be
enhanced by depositing a TiO , ZrO , or SiO layer on the VO layer as antireflection layer 2 2 2 2
[17-19]. M. S. R. Khan et al. claimed that the electrochemical lithiation of the VO thin 2
film not only changed its color to be blueish but also increased its luminous transmittance
[20]. This method, however, is not available for the large scale production. In this work, Li
and H were tried to dope into VO thin films to modify the switching behavior by reactive 2
sputtering. In addition, the thermal stability of VO thin films was investigated. 2

Iron sulfide

oIron sulfide (FeS) shows a MST at around 147 C [21]. It is associated with the structural
transition from the NiAs type structure at high temperatures to the closely related
superstructure at low temperatures. The transition temperature is sensitive to the
composition. With the decrease of the temperature through this transition temperature there
is an abrupt decrease by two orders of magnitude in the electrical conductivity and FeS
2Chapter 1 Introduction

transforms from a metal into a semiconductor. This is probably accompanied by the change
of the optical transmittance and reflectance, which has never been investigated. The
prerequisite for studying the optical properties is the successful preparation of FeS films
because the bulk material is too thick to measure the transmittance for some nontransparent
materials. However, FeS films have never been prepared although it is still important for
understanding of this material. Here we present the deposition and characterization of FeS
films by the reactive sputtering.
The plan of this thesis is as follows. Chapter 2 first introduces the principles and
instrumentation of the depos