Raman microscopy in 2D materials

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ZnO as TCE

One of the promising application for ZnO is as transparent conducting electrode. The ability to easily produce high quality thin-lms on a transparent substrate (glass or quartz for example) makes ZnO a very attractive material in this eld. If its transparency is high thanks to its large band-gap, on the other hand, like in the case of graphene, its conductivity needs to be modulated with doping in order to lower the resistivity of the lm. As mentioned before, Al doping is a commonly used technique for the fabrication of conductive ZnO for displays applications [68] or light emitting diodes [55].
However as said above Al doping has some drawbacks especially in terms of transparency of the doped thin-lm which is signicantly altered.

Transfer of CVD graphene

As mentioned in Chapter 1, chemical vapour deposition (CVD) is a widely used technique for the production of large surfaces of graphene. The graphene sheet is fabricated by heating a Cu foil at 1000C in the presence of carbon carrier gases [11]. The Cu foil acts as a catalyzer for carbon which is deposited on its surface and crystallizes into graphene. The principal advantage of using this technique is that the area of the graphene sample is only limited by the size of the copper foil. Furthermore, the quality of the graphene film produced is compatible for electronic applications, though it is still not comparable with the quality of anodic bonded graphene or graphene produced with the adhesive tape method.
As-fabricated CVD graphene cannot be used for electronics because it is grown on a conducting substrate (the Cu foil). A transfer process of graphene onto an insulating substrate is thus necessary. The polymer assisted transfer is the most widely used technique in research [81] and is a delicate step for the realization of electronic devices with CVD graphene. In the following, the transfer process of CVD graphene used in this thesis will be described in details.

Optical microscope

The optical microscope is a useful tool for a rst characterization of the graphene and ZnO samples. The images of the samples are recorded with a Leica DM2500 and a CCD camera with 5X, 10X, 50X and 100X objectives in the bright eld imaging mode. For the case of anodic bonded graphene, the optical microscope characterization is useful to determine the number of layers of the sample through the optical contrast while for CVD graphene and zinc oxide it is useful to determine the quality of the surface of the sample.

Atomic Force Microscopy

Atomic force microscopy (AFM) was invented in 1986 [84] as an evolution of the Scanning Tunneling Microscope (STM). It is a tool which is able to scan the surface of a sample to obtain a topography at extremely high resolution. Contrary to STM which measures the tunneling current between the sample and the scanning tip, AFM measures the forces at the atomic scale and thus it makes possible to scan conductive and insulating samples [85]. The basic principles is describedin the following.

Raman Spectroscopy

Raman spectroscopy is a powerful and non invasive tool for the characterization of materials. It is based on the Raman eect, a phenomenon of light scattering discovered in 1928 by C. V. Raman and K. S. Krishnan which led C. V. Raman to be awarded the Nobel prize in 1930 [86]. The origin of
this phenomenon comes from the interaction of an incident light beam with the phonons in a solid or from molecular vibrations in molecules, producing inelastic scattering of light. When light hits a material, the most part of the phonons experience elastic scattering, called Rayleigh scattering and the intensity of this portion of light is 10􀀀3 times the intensity of the incident light. The portion of light which undergoes the Raman scattering is even less, 10􀀀6 times the intensity of the incident light. In the following, the principles of the Raman scattering will be explained.

Van der Pauw method

The van der Pauw method for resistivity and Hall measurements was developed in 1958 by L. J. van der Pauw [90] and provides a useful measurement for the determination of the resistivity thin samples of arbitrary shape, provided that there are no holes in the sample. Also, it is possible to measure the Hall eect simply by selecting specic contacts for current injection and voltage measurement. In the practice, square sample with metal contacts at the corners are often used. The contacts must also be small compared to the dimension of the sample. The contact conguration for sheet resistance(RS) measurements is shown in Figure 2.10a. The current is injected from one contact and collected from one of the adjacent contact while the voltage is measured from the opposite pair of contacts. This is repeated for all possible pairs of contacts and it is possible to dene a set of resistances from the measured voltage from a pair of contacts divided by the current injected from the opposite pair of contacts. For example, the RAB;DC is calculated from the ratio VDC=IAB. Averageing between the various resistances we can nally dene a vertical and a horizontal resistance resulting from the average of the resistances calculated by owing the current in the device vertically or horizontally, respectively, denoted as RV and RH. From these two values, the RS is derived from: RS = ln(2) RV + RH 2 f.

Electronic transport measurements setup

Space charge doping, electronic transport and Hall measurements are all carried out under high vacuum (< 10􀀀6 mbar) in a custom made continuous He ow cryostat. It allows to control the temperature of the sample in the range 3 􀀀 420 K, thus allowing to perform the doping of the samples and the low temperature transport measurements in situ. The cryostat is held inside an electromagnet capable of reaching a magnetic eld of 2 T. The doping and resistivity measurements are controlled by a LabVIEW program which coordinates the measurement instruments and saves the data into a le.

The principles of space charge doping

To understand the working principles of space charge doping it is worth whileto take a closer look at the structure of glasses in general and of ionic transport inside glass.

Glass atomic structure

It is known that glasses are formed by an amorphous network of certain glassforming oxides which respect some rules [95]. Commercial glasses are formed by three-dimensional networks of SiO2, as in the case of soda-lime glass, with B2O3 also contributing in the borosilicate glasses. The building block of the glass atomic network is the oxygen tetrahedron which surrounds silicon atoms and each tetrahedron shares a corner with other tetrahedra. The angle between the different tetrahedra varies in an unpredictable way, and that is why the atomic structure of glass is considered amorphous. Network modifiers are added to the glass during the production in order to enhance certain properties. Sodium oxide (Na2O) and calcium oxide (CaO) are the main compounds used as modifiers. The introduction of these species breaks some Si-O-Si bridges creating non bridging oxygen units which serve as anions for the Na+ and Ca2+ cations which are incorporated in these sites. Oxygen atoms are linked covalently to the network and Ca2+ ions posses a very low mobility, making Na ions the most mobile species in glass. Figure 3.1 is a simplified and schematic representation of the structure of glass at the atomic level.

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Space charge doping applied to graphene

There is considerable litterature concerning the use of graphene as a transparent conducting electrode (TCE) [14, 96, 97]. In fact, graphene has been considered as a very promising material for this application thanks to its exceptional electronic transport properties (Section 1.3.1) and its transparency, being higher than 95% in the visible range of the light ( 97% at a wavelength of 550 nm). However, its resistivity is of the order of few k = at low doping, too high to be used as a TCE. Doping is thus necessary to lower graphene sheet resistance. Several techniques have been used to dope graphene (see Section 1.3.5), but several drawbacks exist including limits on the maximum reachable carrier concentration, longer-term instability or transparency issues [23, 43, 45]. We applied the space charge doping for the rst time to graphene in order to fully characterize our doping technique and in parallel investigate its applicability as a TCE. The study was performed on several graphene samples deposited on glass via polymer assisted transfer of CVD graphene or with the anodic bonding method. The experiments are carried out on microscopic samples (lateral size l 50 m), for limiting extrinsic eects and monitoring the sample area with micro-Raman spectroscopy before and after the doping, and macroscopic samples (l 1 cm) to validate the technique for large areas. The techniques used for the sample fabrication are described in Chapter 2 and in the following the experimental details of the sample fabrication and characterization will be given.

Fabrication of the graphene samples CVD graphene

CVD graphene we used in this study was purchased online (Graphene Supermarket, medium quality CVD graphene to perform our rst experiments on space charge doping. The manufacturer declares that the 20 m thick copper foil is entirely covered with a graphene layer on both sides with a small portion of bilayer islands (10 􀀀 30%). The samples were deposited on glass (soda-lime or borosilicate) following the PMMA assisted transfer procedure described in Section 2.1.2. At the end of the process the glass surface is almost entirely covered by the graphene sheet having an eective area of the order of 1 cm2. The surface of the graphene presents some impurities and little holes which arise from the transfer process, as shown in Figure 3.4. The impurities are residues of the PMMA used as mechanical support for the graphene during the transfer, while the holes can be attributed to mechanical stress during the whole process. The wrinkles which are slightly visible on the surface come from the quality of the CVD graphene we purchased.

Table of contents :

Acknowledgments
Abstract
1 2D materials and doping 
1.1 Introduction
1.2 Electrostatic doping
1.3 Graphene
1.3.1 Electronic structure
1.3.2 Synthesis
1.3.3 Graphene characterization
1.3.4 Electronic transport properties
Scattering mechanisms and mobility
Mobility
Magnetotransport
1.3.5 Doping of graphene
Electrostatic doping
Chemical and substitutional doping
1.3.6 Graphene as transparent conducting electrode
1.4 Zinc oxide
1.4.1 Crystal and electronic structure
1.4.2 Crystal growth
RF magnetron sputtering
Other deposition techniques
1.4.3 Characterization of ZnO
X-ray diraction spectrum of ZnO
AFM
1.4.4 Doping of ZnO
n-doping p-doping
1.4.5 Magneto-transport properties
1.4.6 ZnO as TCE
2 Experimental 
2.1 Samples fabrication
2.1.1 Glass substrates involved
2.1.2 Graphene
2.1.3 Zinc oxide
2.2 Characterization methods
2.2.1 Optical microscope
2.2.2 Atomic Force Microscopy
2.2.3 Raman Spectroscopy
The principles of Raman scattering
Raman microscopy in 2D materials
2.2.4 X-ray diraction
2.3 Device fabrication
2.3.1 Van der Pauw method
2.3.2 Contact deposition
2.3.3 Sample shaping
2.4 Electronic transport measurements setup
3 Space Charge Doping 
3.1 The principles of space charge doping
3.1.1 Glass atomic structure
3.1.2 Ionic drift
3.2 Space charge doping applied to graphene
3.2.1 Fabrication of the graphene samples
CVD graphene
Anodic Bonded graphene
Contact deposition and shaping
3.2.2 Results
Ambipolar doping
Fine doping and doping limit
Reversibility
Substrate surface quality
Transmittance
Quality of the doped samples
3.3 Comparison with other doping methods
3.4 Control measurement on quartz
3.5 Conclusions on space charge doping
4 Ultra-high doping of ZnO1􀀀x thin lms 
4.1 ZnO1􀀀x device fabrication
4.1.1 Zinc oxide deposition on glass
4.1.2 X-ray diraction, AFM and transmittance
4.1.3 Sample shaping and contact deposition
4.2 Space Charge Doping applied to the ZnO thin lm
4.2.1 Fine control of doping in the thin lm
4.2.2 Carrier scattering mechanism
Lattice phonon scatterimg
Grain boundary scattering
Ionized impurity scattering
4.3 Variable Range Hopping and mobility edge
4.3.1 Variable range hopping in ZnO thin lms
4.3.2 Mobility edge
4.3.3 2D nature of the doped lm
4.4 Conclusions on space charge doping of ZnO
5 Magneto-transport and spin orbit coupling 
5.1 Weak localization
5.1.1 Backscattering
5.1.2 Temperature dependence of the conductivity
5.2 Magneto-conductivity
5.2.1 Theoretical considerations
Eect of the weak localization
In uence of spin-orbit coupling on the magneto-conductivity
5.2.2 Magneto-conductivity of ZnO
WL to WAL transition as a function of temperature .
WL to WAL transition as a function of carrier concentration
Considerations on the characteristic transport lengths
5.3 Origin of the spin-orbit coupling
5.3.1 D’Yakonov-Perel’ mechanism
5.3.2 Rashba eect
5.3.3 Elliott-Yafet mechanism
5.4 Considerations on the metal-insulator transition
5.5 Conclusions of the magneto-transport in ZnO
6 Conclusions and perspectives 
List of abbreviations and symbols

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