Table 4.1 Sample process parameters of different non-annealing Dy doped ZnO thin films ... 83 Table 4.2 Sample process parameters of different annealing Dy doped ZnO thin films ... 83 Table 4.3 Sample process parameters of different non-annealing Gd doped ZnO thin films ... 84 Table 5.1 Parameters of a Lorentizan fit for the Raman scattering spectrum of non-annealing different Dy doped ZnO thin films excited by a 532-nm laser line. All units are cm-1 ... 115 Table 5.2 Parameters of a Lorentizan fit for the Raman scattering spectrum of annealing different Dy doped ZnO thin films excited by a 532-nm laser line. All units are cm-1 ... 115 Table 5.3 Parameters of a Lorentizan fit for the Raman scattering spectrum of non-annealing different Gd doped ZnO thin films excited by a 532-nm laser line. All units are cm-1 ... 116 Table 5.4 The direct optical band gap energies of non-annealing different Dy doped ZnO thin films were determined by optical transmission measurements. All units are eV ... 117 Table 5.5 The direct optical band gap energies of non-annealing different Gd doped ZnO thin films
were determined by optical transmission measurement. All units are eV ... 117 Table 5.6 Parameters of a stacked layer model fit for non-annealing pure and different Dy doped ZnO thin films ... 117 Table 5.7 Parameters of a stacked layer model fit for annealing pure and different Dy doped ZnO thin films ... 118 Table 5.8 Parameters of a stacked layer model fit for non-annealing different Gd doped ZnO thin
films ... 118 Table 5.9 The direct optical band gap energies of non-annealing pure and different Dy doped ZnO
thin films determined by spectroscopic ellipsometry. All units are eV ... 119 Table 5.10 The direct optical band gap energies of annealing pure and different Dy doped ZnO thin
films determined by spectroscopic ellipsometry. All units are eV ... 119 Table 5.11 The direct optical band gap energies of non-annealing different Gd doped ZnO thin films determined by spectroscopic ellipsometry. All units are eV ... 119 Table 5.12 The exciton binding energies, and exciton broadening parameters of non-annealing pure
and different Dy doped ZnO thin films. All units are eV ... 120 Table 5.13 The exciton binding energies, and exciton broadening parameters of annealing pure and
different Dy doped ZnO thin films. All units are eV ... 120 Table 5.14 The exciton binding energies, and exciton broadening parameters of non-annealing pure
and different Gd doped ZnO thin films. All units are eV ... 120 Table 6.1 Parameters of a Lorentzian fit for the Raman scattering spectrum of a monolayer WS2 thin film excited by a 532-nm laser line. All units are cm-1 ... 171 Table 6.2 Parameters of lattice vibrations of WX2 (X = S, Se)... 172 Table 6.3 Correlation chart relating the irreducible representations of the site groups D3h and C3v
to those of the factor group D6h ... 173 Table 6.4 Parameters of a Lorentzian fit for the Raman scattering spectrum of a monolayer WSe2 thin film excited by a 532-nm laser line. All units are cm-1 ... 174 Table 6.5 Parameters of a stacked layer model fit for the monolayer WS2 and WSe2 thin films ... 174 Table 6.6 The exciton band-gap energies, and exciton binding energies, and exciton broadening
parameters of monolayer WS2 and WSe2 thin films. All units are in eV ... 175
Chapter 1
Introduction
In an era of advanced technology, semiconductors are playing an ever more important role. In
recent years, there has been a wealth of research into optoelectronic technology, especially the
optical and electrical properties of semiconductors [1,2]. Typical semiconductor devices include
integrated circuits, transistors, light emitting diodes, and so on. Operational theories on typical
semiconductor devices mainly focus on controlling the flow of carrier electrons through
semiconductors using electric fields. In which case, the key parameter is the charge on electrons or
holes. These devices, therefore, suffer in part from carrier velocity and energy-loss.
Spintronic devices include the magnetic data storage, and memory cells in nonvolatile and
magnetic random access memory (MRAM) systems, etc. In magnetic data storage, the key parameter
is electron spin, as this is the fundamental origin of magnetic moment [3]. The emerging field of
semiconductor spin transfer electronics (spintronics) seeks to exploit the spin of charge carriers in
semiconductors. There is exceptional potential in this technology if electronic functionality can be
exploited by controlling the injection, transfer, and detection of carrier spin at above room
temperature [4,5]. For this reason, spintronics has become a subject of great interest in recent years.
Research into diluted-magnetic semiconductor, materials possessing magnetic and semiconductor
properties, is a crucial aspect of spintronics [6].
The crystal structure and chemical bonding of diluted-magnetic semiconductor materials are
best viewed in the context of existing electronic-component semiconductor materials. Zener model calculations [7] predict that the Curie temperature (Tc) of group III, IV, and V (Fe [8], Mn [9])
compound doped wideband-gap semiconductor (ZnO) materials are higher than room temperature, a
feature important for room temperature operated spintronic devices. In the past 10 to 15 years, a lot
of studies on magnetic [10], electrical [11], optical [12], and photocatalytic [13] properties of
transition metal doped ZnO materials have been reported.
The crystal structure of ZnO is zinc-blende or hexagonal wurtzite where each anion is
surrounded by four zinc cations at the corners of a tetrahedron, and vice versa (Fig. 1.1) [14]. This
tetrahedral coordination is typical of sp3 covalent bonding. The physical properties of ZnO, such as
plastic deformation at relatively low loads (≧4 ~ 13 mN with an ~ 4.2 μm radius spherical indenter)
[15], hardness modulus (~ 5.0 GPa), strong Young's modulus (~ 111.2 GPa), thermal conductivity (~
0.87 W/cm-K) [16], and electron mobility (~ 300 cm2 V-1 s-1) [17], have been well documented.
Rare-earth doping in ZnO is considered an interesting alternative to 3d transition metals since it
has robust magnetic moments at room temperature due to its 4f states [18]. These 4f states of
rare-earth ions are responsible for improving hole conductivity as holes in 4f states are more active
than electrons [19]. Given this property, diluted-magnetic semiconductors are favored as future
candidates for next generation electronic, spintronic, and optoelectronic devices.
Among diluted-magnetic semiconductors, the optical properties of Dy and Gd doped ZnO thin
films have been little explored [20,21]. We are interested in the changes to the optical properties of
ZnO thin films doped with different concentrations of Dy and Gd. In this study, we plan to
investigate such products using Raman scattering spectroscopy, grating spectrometry, and
spectroscopic ellipsometry. These techniques can explore the effects of doping with Dy and Gd at
different concentrations on lattice phonons, transmission, and complex optical constants of ZnO thin
films. This study of Dy and Gd doped ZnO thin films is very important to our understanding of the
optical and electronic properties of devices based on thin film structures.
Two-dimensional materials have recently generated great interest because of their unique
physical properties and potential practical applications [22-24]. Among them, there has been
particular interest in graphene [25,26], transition metal dichalcogenides [27,28], transition metal
oxides including titania- and perovskite- based oxides [29,30], and boron nitride [31,32]. Transition
metal dichalcogenides are special in many respects. They exhibit a variety of surprising electronic,
optical, mechanical, chemical, and thermal properties [33-35]. Furthermore, they render potential
applications in catalysis [36], energy conversion [37,38],and optoelectronics [39,40].
The transition metal dichalcogenides have a common structural formula MX2, where M is a
transition metal element like Mo, W, Ti, etc. and X is a chalcogen (S, Se, Te) [41]. These materials
form layered structures of X-M-X covalently bonded hexagonal quasi-two-dimensional network
stacked by weak Van der Waals forces (Fig. 1.2) [41]. Based on strong surface effects, the properties
of the transition metal dichalcogenides change drastically with the number of layers in a sheet. The
band gap energy increases from multilayers to a monolayer, and transfers indirect band gap to direct
band gap [41-46]. These monolayer semiconductors have highly stable neutral and charged excitons
[47,48]. For monolayer transition metal dichalcogenides, confinement of electrons and holes to the
±K valleys gives rise to valley excitons and trions, formed at an energy-degenerate set of non-central
points in momentum space [49]. In principle, these valley excitons offer unprecedented opportunities
to dynamically manipulate a valley index using optical means, as has been done for optically driven
spintronics. These special properties make monolayer transition metal dichalcogenides complements
or substitutes for materials currently used in optoelectronic and energy harvesting applications.
Although the optical properties of transition metal dichalcogenides have been intensely studied
[50,51], no studies on the temperature dependence of complex optical constants have been reported
thus far. Therefore, in this study, we plan to investigate monolayer transition metal dichalcogenides
using Raman scattering spectroscopy and spectroscopic ellipsometry. These spectra can explore
lattice vibration phonons and the complex optical constants of monolayer transition metal
dichalcogenides (TMDs). Knowledge of the optical properties of TMDs is necessary when
conceiving of semiconductor device applications.
The organization of this thesis is as follows: Chapter 2 presents a literature review of previous
research into dysprosium and gadolinium doped zinc oxide and monolayer two-dimensional
transition metal dichalcogenides such as WS2 and WSe2; Chapter 3 describes the technical and
theoretical details of the experiment; Chapters 4 ~ 6 present the main experimental results; and
finally, Chapter 7 gives the thesis summary.
Fig. 1.1 Schematic representation of ZnO crystal structures: (a) hexagonal wurtzite structure; and (b) cubic zinc blende wurtzite structure. The yellow and white (grey) spheres denote Zn and O atoms, respectively [11].
Fig. 1.2 Schematic representation of (a) side view and (b) top view of monolayer MX2 structure, with the chalcogen atoms in yellow and the transition metal atoms in black [33].
(a) (b)
(a) (b) a
c b
z
y
x y
x