characterizing luminescent properties of thin films by farisch hanoeman

Characterizing Luminescent Properties of Thulium Based Thin Films

F. Hanoeman

Delft

Uni

vers

ity o

f Tec

hnol

ogy

Characterizing Luminescent Properties of Thulium Based Thin Films

Hanoeman, F.V.

Delft University of Technology

October 22, 2015

Supervisor: dr. E. van der Kolk Co-reader: prof. dr. P. Dorenbos

FundamentalAspectsofMaterialsandEnergy(FAME)RadiationScienceandTechnologydepartment

FacultyofAppliedSciences

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Abstract

This research focuses on the development of an efficient Luminescent Solar Concentrator (LSC) to harness energy from sunlight. The LSC's used in this research are sputtered thin films of NaCl, with varying concentrations Tm2+, on a substrate of glass. The main focus of this research was finding the optimal sputter configuration and doping of Tm2+ that corresponded with maximum efficiency. To address this issue, several films were sputtered with Tm2+ and NaCl using a magnetron sputter system. The luminescent properties of these films were analysed using a Helium Neon laser at 632 nm, a USB Spectrometer and a brushless servo controller to position the films at 2500 different predefined X and Y points. These results were analysed using MATLAB and led to conclusions about the optimal sputter settings and ratio of NaCl and Tm2+. It was found that when the NaCl sputter gun is set to 30W and the Tm2+ sputter gun is set to 15W and a 2% mask is being used on top of the Tm2+ sputter gun, optimal results are found.

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Content Abstract 3 Introduction 6 Theory 7

2.1 Luminescence 7 2.1.1 Atomic Structure 7 2.1.2 Luminescence of Thulium 10 2.1.2 Non-radiative relaxation 11

2.2 Sputtering 13 2.2.1 Theory of Sputtering 13 2.2.2 Magnetron Sputtering 16

2.3 Optical Efficiency 17 2.3.1 Incoupling Efficiency 19 2.3.2 Light Harvesting Efficiency 20 2.3.3 Photoluminescent Quantum Yield 20 2.3.4 Trapping Efficiency 21 2.3.5 Stokes Efficiency 22 2.3.1 Waveguide Efficiency and Self-Absorption 22

Experimental Approach 25 3.1 Sputtering 25

3.1.1 Sputter Processes 26 3.1.2 Sputter Configurations 27 3.1.3 Masks 28

3.2 Luminescence Measurements 29 3.3 Excitation and Emission Measurements 30

Results 31 4.1 Introduction 31

4.1.1 Overview 32 4.2 Sample A - Test Sample 33 4.3 Sample B - 180o 36 4.4 Sample C - 90o 39

4.4.1 Sample C1 - 90o 40 4.4.2 Sample C2 - 90o 41

4.5 Sample D - Rotation 43 4.5.1 Sample D1 - Rotation 44 4.5.2 Sample D2 - Rotation 45 4.5.3 Sample D3 - Rotation 46 4.5.4 Sample D4 - Rotation 47 4.5.5 Sample D5 - Rotation 47 4.5.5 Conclusions Regarding the D-Series. 47

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Conclusion and Recommendations 49 Photos of Samples 51

Sample A - Test Sample 51 Sample B - 180o 51 Sample C1 - 90o 52 Sample C2 - 90o 52 Sample D1 - Rotation 53 Sample D2 - Rotation 53 Sample D3 - Rotation 54 Sample D4 - Rotation 54 Sample D5 - Rotation 55

MATLAB Code 56 References 59

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Introduction

In 2014, the total human consumption of energy was approximately 8.5·1019 J [1]. The sun delivers about 3.9·1024 J annually [2], which means that 0.02% of the energy provided by the sun could potentially power the whole world with clean energy. This perspective is the main reason that a lot of research on solar energy is being done. At this moment the most promising solar converting applications are solar panels, like the well known crystalline silicon solar panels on our houses. Next to these panels, other devices have been proposed, with the Luminescent Solar Concentrator (LSC) being one of them. The concept of the LSC was first proposed by Weber and Lambe in 1976 [3]. They described a device consisting of a luminescent medium that absorbs a portion of the solar spectrum and re-emits at longer wavelengths. If the refractive index of this medium is higher than its surroundings, part of the light will be trapped by total internal reflection and will propagate to the edges of the medium where it can be converted to energy by a strip of solar cells attached to those edges. In short, a typical LSC device consists of a flat polymer or glass plate, containing fluorescent dyes or rare earth doping, with one or more solar cells connected along the sides. Some advantages of an LSC device compared to regular solar cells would be less solar cell material, the ability to have various shapes other than flat and depending on the type of LSC, the ability to be used as a transparent window. Since the first description of the LSC, a lot of research has been done on which this thesis builds on. Mainly the conclusions of the theses of Willem Kesteloo [4] and Ferdinand Grapperhaus [5] led to the main research question of finding the optimal sputter configuration and sputter power ratio of NaCl and Tm2+. The conclusions and findings of this thesis will be used to provide more insights in manufacturing a working LSC device. The following chapter will start with giving an overview of the theory needed to understand luminescence, the sputtering process and how to calculate the optical efficiency. The Experimental Approach section will give some practical information on the measuring devices that are used in the experiments. In the Results section all the films that were analysed will be discussed. The final chapters give conclusions based on the Results section and will also give recommendations for further research.

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Theory

This chapter covers the most relevant physical concepts that play a role in the materials of this thesis. It starts with explaining the theory behind luminescence and what theory predicts about the luminescence of Thulium. It continues with a method used to produce thin films, magnetron sputtering, and will conclude with the theory of calculating the optical efficiency.

2.1 Luminescence

Luminescence is a form of cold body radiation, i.e. radiation that is not a result from heat. There are different types of luminescence that can be distinguished: radioluminescence, thermoluminescence, electroluminescence, chemiluminescence and photoluminescence, depending on what energy source excites the luminescence. This research will focus only on the subject of photoluminescence - hereafter referred to as luminescence- in which the excitation source is light.

2.1.1 Atomic Structure

To understand luminescence, an explanation is needed about the basic atomic structure. Each atom consists of a positively charged nucleus, which is surrounded by a negatively charged electron cloud. The electromagnetic force keeps the electron cloud bound to the nucleus. The electrons in the electron cloud have both particle-like as wave-like properties. To examine these properties, the electron cloud can be considered to exist inside a three-dimensional potential well, where the cloud forms a three-dimensional standing wave function. This wave function can be described by the Schrödinger equation. The solutions to this equation give a probability density function of the position of the electron. These solutions exist only in a discrete set described by so-called quantum numbers, which uniquely describe the wave function. The four quantum numbers are: The principle quantum number (n), the azimuthal quantum number (l), the magnetic quantum number (ml) and the spin projection quantum number (ms). An increase of the principle quantum number (n), correspondents to an increase of the radius, which means that the electron wave function spends more time further away from the nucleus. Larger

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orbital correspondents to a higher potential energy, therefore the principle quantum number represents the energy level of the electron wave function. The azimuthal quantum number (l), describes the orbital shape of the wave function and gives the magnitude of the orbital angular momentum. The magnetic quantum number (ml), describes the projection of the orbital angular momentum along a specified axis. The spin projection quantum number (ms) describes the spin of the electron in the orbital and gives the projection of the spin angular momentum, S, along the specified axis.

Each combination of quantum numbers is called a quantum state. The most

important rule concerning these states is the Pauli exclusion principle, which says that no two electrons can be in the same state. The quantum numbers follow specific rules related to the state they can occupy. The principle quantum number n, can only take positive integer numbers. Based on this principle quantum number, the electrons form certain shells, which are labelled K, L, M, N, ... . The quantum number l, the azimuthal quantum number, can only take values ranging from l= 0, 1, 2, .., (n-1). The letters used to represent the values for l are commonly referred to as: s(harp), p(rincipal), d(iffuse), f(undamental) followed by the alphabetic letters following f. The value of the azimuthal quantum number describes the orbital shape of the electron wave function. The last two quantum numbers are the magnetic quantum numbers ml and ms. The value of ml can take on values ranging from ml= -l, (-l+1), …, 0, .., (l-1), l. The spin quantum number for electrons ms can be either spin up, which is denoted as ms=1/2 or spin down, ms=-1/2. Following these described rules, each atom has its own electron configuration, which can be formulated systematically following the notation associated with the quantum numbers. The first electron shell, n = 1, has room for two electrons with l = 0, so this shell is named the 1s subshell. When this shell is completely filled, its configuration is notated as 1s2. The Lithium atom has two electrons in the first shell and one electron in the second, so its configuration is noted as 1s22s1.

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Each combination of quantum numbers corresponds to an energy level. The combination of quantum numbers that corresponds to the lowest energy is called the ground state configuration. A combination of quantum numbers with energy higher than the ground state is called an excited state. When the system absorbs a photon with energy equal to the difference in energy between the ground state and an excited state, the electron can be excited to an excited state. Electrons in the excited state tend to fall back to the ground state. In this process, which is called relaxation, a photon or a phonon is released. If in this process a photon is emitted, we speak of luminescence. This process is depicted schematically in figure 2.2.

Figure 2.2. In this figure the process of photoluminescence in a single electron system is depicted. The left figure shows a system in equilibrium. The figure in the middle shows the process of photon absorbance, whereby the electron gets excited. The figure on the right shows the process of photoluminescence.

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2.1.2 Luminescence of Thulium

In this research the focus lays on the use of NaCl samples doped with the thulium ion Tm2+. The reason for choosing Tm2+ is that the transitions that can take place are in the visible or infrared region of the spectrum. By doping Tm2+ in NaCl, strong absorption bands can be observed, which can be assigned to the 4f13 → 4f125d1 electronic transition. The 4f125d1 configuration is split due to a combination of several interactions: the NaCl crystal field splits the 5d orbitals; the Coulomb repulsion and spin-orbit coupling within the 4f12 electronic configuration result in an additional splitting in several 2s+1LJ terms and a third splitting is due to Coulomb repulsion of the 4f and 5d electrons, resulting in low-spin and high-spin states. Thulium has its most outer electrons in the 4f orbitals, which are only partially filled. Therefore the emission due to excitation will be regarding 4f - 4f transitions. The 4f orbitals are shielded by the filled outer shells, the 5s2 and 5p2 orbitals. This is the reason why they do not participate in bonding. The most stable Thulium ion is Tm3+. However, in a few compounds, the most stable Thulium ion is Tm2+. In this unusual 2+ valence state, it has a 4f13 electron configuration with two separate energy states giving rise to a single possible narrow 4f - 4f absorption line around 1140 nm, due to 2F7/2 → 2F5/2 transition [8].

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2.1.2 Non-radiative relaxation

The previous section mainly describes situations where the atom is isolated from any external interaction. However, to have a complete physical description, one should also consider the atom being part of a lattice, where it is influenced by neighbouring atoms. To physically describe these interactions, the configurational model is used. In this model the assumption is made that only neighbouring atoms are of influence on the luminescent particle and that atoms located further away from the luminescent particle are not of significant influence. The model describes the relation between the atoms by making use of Hooke’s law and gives the potential energy of the luminescent atom as a function of the configuration coordinate. An example of the potential curves of the ground state and the first excited state is given in figure 2.2. This figure gives a simplified view on how an excited electron can fall back into its ground state by releasing phonons (i.e. non-radiative relaxation) and photons. The potentials of the ground state Ug and the excited state Ue given in this figure can be described by

(2.1)

(2.2)

, where Kg and Ke are the spring constants, Q is the configurational coordinate and Q0 is the interatomic distance with energy U0 at that distance. The arrows A to B and C to D represent the absorption and emission of a photon. When the electron is excited, the wave function and thus its spatial distribution, changes. This results in a change in electron wave function overlap with neighbouring ions, which causes a shift in equilibrium position and net forces. This shift in equilibrium position causes the energy associated with the emission to be smaller than the energy associated with the absorption. This difference in energy is called the Stokes shift. Another situation occurs when thermal energy in the system is abundant. When this energy is high enough, in the figure depicted with ∆U, the system can reach a point where the potentials start to overlap, depicted with point E. At this point the system can relax back to point A solely by releasing phonons. This effect is called thermal quenching and causes a reduction in efficiency of luminescence [7].

212g gU K Q=

( )20 02e

eKU Q Q U= - +

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Figure 2.2. In this figure the process of absorption and emission inside a lattice is depicted. The system in rest at point A is being excited by absorption of a photon to point B. At point B the system relaxes to point C by releasing phonons. At point C the system emits a photon and relaxes to point D. The difference in energy of the photon that is absorbed and the photon that is emitted is called the Stokes shift. If the system has sufficient thermal energy for it to reach point E, the system can relax back to point A solely by releasing phonons. This effect is called thermal quenching [7].

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2.2 Sputtering

In this research, thin film depositions will be made using a form of sputtering called magnetron sputtering, a form of Physical Vapor Deposition (PVD). PVD entails a variety of methods with which the deposition of thin films in a vacuum is described by condensating a vaporized form of the desired target material on a selected substrate. PVD involves physical processes only, such as the bombardment with an inert gas in a plasma environment or the high-temperature vacuum evaporation with subsequent condensation. Other forms of physical vapor deposition are for example evaporative deposition, electron beam physical vapor deposition, pulsed laser deposition and cathodic arc deposition. The method used in this research, magnetron sputtering, allows multiple plasmas to be formed inside a vacuum using magnets, making it possible to deposit multiple materials in one thin film deposition. This method proves to be advantageous over other methods, because of the relative low temperature used in experiments and the fact that the film is sputtered atom by atom, which allows for very accurate control over the process. 2.2.1 Theory of Sputtering

The process of sputtering, the removal of atoms from objects by energy transfer in collisions of energetic atomic projectiles, needs to take place needs inside a high vacuum chamber. The reason behind this is because the air particles would interfere the sputtering process by colliding with the energetic particles and ejected atoms. This vacuum is also necessary for creating a plasma environment. Plasma is one of the four fundamental states of matter and can be described by a gaseous, high-density mixture of equal amounts of positively charged ions and negatively charged electrons in an atmosphere of neutral particles. In the high vacuum chamber, between a negatively charged cathode and a grounded anode, the plasma becomes a glow discharging plasma as depicted in figure 2.3. The anode used in the sputter chamber is the substrate where the thin film is sputtered on. Between the cathode and the anode, an electric field is created as a result of the voltage difference between the cathode and the plasma glow. Positively charged ions in this region are accelerated towards the negatively charged cathode and electrons in this region are accelerated towards the plasma glow. Neutral particles in this region move in a

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random direction. In the plasma glow itself, there is no electric charge, as there are equal amounts of positively and negatively charged particles on average and all particles are moving in a random manner. Figure 2.3 also shows the different spaces and glows that appear between the electrodes. They are the result of the glow discharge in a low-pressured environment with inert gas and an electric potential applied between the cathode and anode. This chamber is also filled with the inert gas Argon at pressure in the range of a few tens of mTorr (1mTorr ≈ 0.0013 mbar). The desire to use an inert gas is because these ions will not react with the atoms they are bombarded on. When Argon ions collide on the target, atoms can be ejected from the target. This happens only when the energy, with which the Argon ion collides, is greater than the surface binding energy of the atoms in the target [9]. The average number of atoms ejected from the target per incident ion is called the sputter yield and depends on the ion’s incident angle, the energy of the ion, the masses of the ion and target atoms and the surface binding energy of atoms in the target. When ions directly strike the cathode and the neutral gas atom strikes the surface of the target material, it's possible to eject atoms from the target. When the collision causes electrons to eject, the electric potential accelerates the electrons towards the anode. If the electron energy is larger than the ionization energy of the atoms, ionization will occur. The production of these charged particles balances out the loss of earlier ’lost’ electrons. Only a part of the atoms that are traveling in the plasma will reach the anode. The ones that do reach the substrate are building up a layer. This layer constructs the thin film that is desired. This process is displayed inside figure 2.3.

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Figure 2.3. This figure depicts the various regions between the cathode and the anode. The dark spaces do not emit light, whereas the glow spaces do emit light. It also depicts the collisions on the target, the ion-electron interactions inside the plasma and the way the substrate is build up. The voltage as a function of position is depicted at the bottom of the figure.

The source of the setup can be either direct current (DC) or alternating current (AC). The source uses a radio frequency (RF) type source, when using alternating current. When using a DC source for insulating materials, a positive charge will build up at the surface of the target, thus repelling the positively charged Argon ions and hereby stopping to sputtering process. To prevent this from happening, the RF type source should be used when using insulating target materials.

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2.2.2 Magnetron Sputtering

To further increase the efficiency of the sputtering process, one could try to confine the electrons in a desired trajectory. This can be done using magnetic fields that form circular tunnels above the target's surface. This principle is called magnetron sputtering. The efficiency can increase by a factor seven when compared to regular DC sputtering processes. Magnetron sputtering also makes it possible to simultaneously apply strong magnetic and electric fields to different plasmas in the same chamber. This results in the option to combine different materials or even dope materials with a second material. The setup used in this experiment is schematically depicted in figure 2.4. At the bottom two sputter guns are depicted. The targets take place in the sputter gun and a chimney forms the body that confines the plasma. The chimneys of the sputter guns are oriented with a variable angle towards the substrate. The variable angle results in the ability to locate the highest intensity of the plasma at a desired location on the substrate. The substrate is depicted at the top in the figure, attached to a rotatable substrate holder. At the right side a valve to a load lock is depicted through which a substrate can be transported with a loading arm. The fact that the chimneys encapsulate the magnetic fields helps the enhancement of the ionization efficiency and aims the sputtered atoms towards the substrate [9].

Figure 2.4. This figure depicts the various compartments of a magnetron sputter chamber. At the bottom, there are two sputter guns, where the magnetic field lines are drawn. At the top, there is a substrate where the material is deposited on [7].

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2.3 Optical Efficiency

To give a measure of the effectiveness of a luminescent solar concentrator (LSC), one should compare the power gained from a photovoltaic cell directly exposed to sunlight to a photovoltaic cell attached to a luminescent solar concentrator that has the same surface area as the photovoltaic cell. To measure this, the photon flux gain (PFG) is introduced by

φPFG =

AtopAedge

⋅ηopt ⋅ηPV λ( )

ηPV solar( ) (2.3)

, where is the top surface of the luminescent solar concentrator, where the incoming

light is collected and is the edge surface that is connected to the photovoltaic cell.

The optical efficiency of the luminescent concentrator is given by . The efficiencies of

the photovoltaic cell at emission wavelength of the LSC and under exposure of the full solar spectrum are respectively given by and ηPV solar( ) . This research focuses

only on the optical efficiency , which shall be explained more in depth in the following

paragraphs. The optical efficiency can be divided in the product of several efficiencies, following formula (2.4)

Each of these efficiencies has a value between zero and one, depending on different loss mechanisms. Figure 2.5 helps to explain these different loss mechanisms that give rise to the different efficiencies. The incoupling efficiency, , depicted in figure 2.5 with

number 1, represents the amount of light that is transmitted upon incidence on the surface of the waveguide. The light harvesting efficiency, , depicted in figure 2.5 with

number 2, represents the fraction of photons that is absorbed when transmitted trough the front surface.

Atop

Aedge

ηopt

ηPV λ( )ηopt

ηopt =ηinc ⋅ηLHE ⋅ηPLQY ⋅ηtrap ⋅ηSA ⋅ηStokes ⋅ηWG

ηinc

ηLHE

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Figure 2.5. This figure depicts the different LSC loss mechanisms. The different numbers represent (1) reflection on the incoupling surface of the LSC, (2) no photon absorption, (3) non-radiative relaxation, i.e. non 100% luminescent quantum efficiency, (4) emission in escape cone, (5) re-absorption and subsequent luminescent quantum efficiency loss or escape cone loss, (6) parasitic waveguide absorption, (7) internal waveguide scattering, (8) scattering due to waveguide roughness [11].

The photoluminescent quantum yield is the number of photons emitted per photon

absorbed. represents the trapping efficiency, depicted in figure 2.5 with number 4,

described by the ratio of absorbed photons that is not lost by emission inside the escape-cone. The self-absorption efficiency, , is caused by an overlap in emission and

absorption spectrum, where the overlapping part of the spectrum causes the photon to be reabsorbed. After reabsorption, the photon can be reemitted again. This process is depicted in figure 2.5 with number 5. The Stokes efficiency, , earlier described in

section 2.1.2, gives the ratio in energy between all absorbed and emitted photons. The waveguide efficiency, , accounts for all photon transport losses due to waveguide

imperfections such as scattering and parasitic absorption. This is represented in Figure 2.5 by numbers 6, 7 and 8, which are respectively: parasitic waveguide absorption, internal waveguide scattering and scattering due to waveguide roughness.

ηPLQY

ηtrap

ηSA

ηStokes

ηWG

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2.3.1 Incoupling Efficiency

The first step in the process of converting solar light in to electrical energy, is the incoupling of solar light from outside air in to the LSC medium. If light reaches the top of the device, it can either be reflected back in the air or transmitted into the LSC medium. The factors on which this process depends are the angles of incoming radiation with respect to the surface of the device, the index of refraction of air and the index of refraction of the top layer of the LSC. The part of light that is transmitted is given, using the Fresnel equations, by

τ =2 ⋅

n2

n1

⎛⎝⎜

⎞⎠⎟

2

⋅cos θ( ) ⋅ n2

n1

⎛⎝⎜

⎞⎠⎟

2

− sin2 θ( )

n2

n1

⎛⎝⎜

⎞⎠⎟

2

⋅cos θ( ) + n2

n1

⎛⎝⎜

⎞⎠⎟

2

− sin2 θ( )⎡

⎣

⎢⎢

⎤

⎦

⎥⎥

2 +2 ⋅cos θ( ) ⋅ n2

n1

⎛⎝⎜

⎞⎠⎟

2

− sin2 θ( )

cos θ( ) + n2

n1

⎛⎝⎜

⎞⎠⎟

2

− sin2 θ( )⎡

⎣

⎢⎢

⎤

⎦

⎥⎥

2

(2.5)

, where is the angle of the incoming radiation, is the refractive index of the first

medium, in this case air and n2 is the refractive index of the second medium, in this case

the top layer of the LSC. Using this relation and an angular distribution of the incoming radiation, the total fraction of light that is transmitted can be found when being integrated over all possible angles of incidence. This fraction represents the incoupling efficiency. When setting 𝜃 equal to 0, it is possible to simplify the equation to

τ =4 n2

n1⎛⎝⎜

⎞⎠⎟

3

n2n1

⎛⎝⎜

⎞⎠⎟

2

+ n2n1

⎛⎝⎜

⎞⎠⎟

⎛

⎝⎜

⎞

⎠⎟

2 (2.6)

Typical values can be found when using the refraction index for glass, n1=1 and air, n2=1.5, which gives an incoupling efficiency of 0.96.

θ n1

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2.3.2 Light Harvesting Efficiency

The harvesting efficiency is defined as the fraction of the incoupled solar energy that is absorbed by the LSC. This efficiency is a function of the wavelength dependent absorption strength of the LSC and depends on the light source’s emission spectrum, S, and the absorbance of the LSC, A.

( ) ( )

( )0

0

1 10 A

LHE

S d

S d

ll lh

l l

¥-

¥

é ù× - ×ë û=

×

ò

ò (2.6)

The most straightforward method to obtain the absorbance is by subtracting the reflection and transmission spectrum of the LSC. Another method is by using Lambert-Beer’s law

( ) ( )A C xl e l= × × (2.7)

, where ( )e l is the extinction coefficient, C is the concentration of luminescent particles

and x is the maximum path length of the incident photons in the waveguide. Typical values for the light harvesting efficiency range between 0.7 and 0.8 [10][12]. 2.3.3 Photoluminescent Quantum Yield

As explained in section 2.1.2, not all absorbed photons lead to an emitted photon. The ratio of photons emitted to photons absorbed defines the photoluminescent quantum yield, PLQYh . This ratio will be less than one when other pathways exist, via which an

excited electron can relax to the ground state. One of these pathways is due to thermal quenching, i.e. the non-radiative relaxation of the electron to its ground state as a result of an intersecting excited state and ground state parabola in the configurational model. Another parasitic pathway is the transfer of the energy of one excited luminescent center to another nearby luminescent center, which subsequently relaxes non-radiatively. The probability of this process depends on the distance between the luminescent centers and consequently depends on the rare-earth doping concentration. A typical value for the photoluminescent quantum yield for NaCl doped with Tm2+ is 0.019 [4].

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2.3.4 Trapping Efficiency

The higher refractive index of the LSC, compared to the surrounding medium results in lights being internally reflected back into the LSC. This effect only holds for angles smaller than the critical angle . The critical angle , can be found using Snell's

Law, (2.8)

, where n1 and n2 are the refractive indices of the two media the light travels through and and are the angles of the incoming and transmitted light relative to the

normal of the boundary. If the angle of the transmitted light reaches 90˚, and thus = 1, no light is transmitted and the critical angle , for total internal

reflection can be found by,

(2.9)

In three dimensions, the critical angle defines a cone, hereafter called the escape

cone. To calculate the efficiency, we use the fact that a photon is emitted in random direction and integrate over the escape cone volume. One minus this integrated value gives the efficiency. A typical value with n1=1 and n2=1.5 give a trapping efficiency of 0.745.

(2.10)

θc θc

n1 ⋅sin θ1( ) = n2 ⋅sin θ2( )

θ1 θ2

sin θ2( ) θc

θc = arcsin

n2

n1

⎛⎝⎜

⎞⎠⎟

( )2

1arcsin 2 2

0 02

2 sin1

4

nn

trap

d R d

R

pq q f

hp

æ ö×ç ÷

è ø× × × ×= -

× ×ò ò

( )cos cq=

2

2

1

1 nn

æ ö= - ç ÷

è ø

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2.3.5 Stokes Efficiency

The Stokes Efficiency describes the loss in energy when photons with high energy are absorbed and photons with low energy are emitted. The cause of this energy loss, described earlier in section 2.1.2, is the energy loss due to lattice vibrations. The energy of all emitted photons divided by the energy of all absorbed photons defines the Stokes Efficiency. This can be described by using formula

ηStokes =Φem λ( )

0

∞

∫ ⋅dλ

Φabs λ( )0

∞

∫ ⋅dλ (2.11)

, where is the emission spectrum and is the absorption spectrum. The value for

the Stokes efficiency of NaCl doped with Tm2+ is 0.47 [12]. 2.3.1 Waveguide Efficiency and Self-Absorption

When a photon is emitted outside the escape cone, there are still several other processes that can contribute to loss of a photon. These processes can results from the waveguide inefficiencies or from self-absorption. Self-absorption occurs as a consequence of overlapping absorption and emission spectra. In this process, a luminescent photon can again be absorbed by another luminescent particle in the waveguide. The loss processes that are due to waveguide inefficiencies are parasitic waveguide absorption, light scattering by imperfections in the waveguide material and light scattering from the surface of caused by surface roughness. To give a measure of the efficiency, the material's attenuation length 𝜇 is used to describe the exponential decay of a beam of light travelling through the waveguide as

I r( ) = I0 ⋅e− rµ (2.12)

, where r is the travelled distance of the beam in the waveguide. It should be noted that the self-absorption losses also contribute to the attenuation length. Therefore, the given efficiency is the product of both the self-absorption efficiency as the waveguide efficiency

Φem Φabs

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ηSA ⋅ηWG =ηSAWG (2.13)

In order to calculate this combined efficiency, assume a LSC of size W x H with power impinging upon the perimeter section Δω, from an infinitesimally small surface element of the LSC ΔxΔy as shown in figure 2.6.

Figure 2.6. Schematic drawing of a LSC of size W x H. This figure considers the luminescence from surface element ΔxΔy that radiates to section Δω over an angle Δβ [12].

When the surface element ΔxΔy radiates with an intensity of 𝜌, the power

impinging upon the perimeter section Δ𝜔ωcan be found using

, 2

r

x yP x y e µw

brp

-

D D D

D@ D D

×. (2.14)

Integrating the power radiated by the surface element over an entire edge results into

( )arctan sin2,

arctan2x y

W xyW x

x W W xH y

x yP e dp

µ br bpD D

ì ü-ï ïé ù -í ý+ê ú ×ï ï- î þë û= é ù-

ê ú-ë û

D D=

× ò . (2.15)

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Integrating over all surface elements to find the total power upon one edge and summing over all of the four contributing sides of the LSC results into the following equations: For a rectangular waveguide of size W x H,

And for a square waveguide when H = W,

( )arctan sin22 0 0 arctan

2W yx

W W W ySAWG W y

W x

e d dxdyW

pµ bh b

p

ì ü-ï ïé ù -+ í ýê ú ×- ï ïî þë û-é ù

ê ú-ë û

æ ö= ç ÷è ø ò ò ò

(2.17)

To obtain more insight, the efficiency is plotted against the dimensionless attenuation length 𝐿𝑎 = 𝜇/𝑊 [13].

Figure 2.7. Transport efficiency of LSCs as a function of the dimensionless attenuation length, La.

( ) ( )arctan arctansin sin2 2SAWG 0 0 0 0arctan arctan

1 1H y W xx y

H W W HH y W xH y W xW x H y

e d dxdy e d dydxWH WH

p pµ b µ bh b b

p p

ì ü ì ü- -ï ï ï ïé ù é ù- -+ í ý í ý+ê ú ê ú× ×- ï ï ï ï-î þ î þë û ë û-é ù é ù-

ê ú ê ú-ë û -ë û

æ ö æ ö= +ç ÷ ç ÷è ø è øò ò ò ò ò ò

25

Experimental Approach

This chapter is divided in three sections. The first section describes the sputter process that produces the thin films that are used in this thesis. The second and third section describe the methods that are used to characterize these thin films. First, the XY-scanner setup is described and second the excitation and emission-measuring device is described.

3.1 Sputtering

The depositions of the thin-films are performed with an AJA Orion 5 magnetron sputtering system. The magnetron sputter chamber used for these experiments is able to deposit up to four different target materials on one thin-film. Each target material is located in a sputter gun. Each of these guns is located at the bottom of the chamber, equally spaced from each other in a circular manner, all slightly tilted inward, aiming at the substrate holder that is located at the top of the chamber in the middle. A schematic drawing of the setup is shown in figure 3.1.

The guns in the sputter chamber are powered by either a DC source or a RF

source. These sources determine the rate at which the sputter process takes place. Each of the guns has an individual source, which makes it possible to set the desired sputter rate of each target material. In addition, it is also possible to set the guns angle with respect to the horizontal axis.

The load lock, depicted in figure 3.1 with number 4, is a separate chamber where

the substrates can be inserted in to the sputter chamber. The load lock has its own vacuum pomp, which ensures that several films can be sputtered without having to open the main chamber. Heating wires surround the main chamber and are used to vaporize moist. Two gas inlets control the Ar sputter gas flow in and out of the main chamber. To measure the sputter deposition rate, a Quartz Crystal Microbalance (QCM) is used. By applying a voltage over the QCM, it starts to vibrate at its resonance frequency. This resonance frequency changes during the sputter process, because of the depositions on the crystal, which increases its mass. The measured frequency change gives a measure of the

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Figure 3.1. This figure depicts a schematic setup of the magnetron sputter chamber. (1) Is the substrate holder, (2) points toward two of the four sputter guns, (3) is the substrate, (4) is the position of the load lock and (5) is the control which lowers and turns the substrate holder.

deposition rate per second. By combining this rate with the density of the deposited material, the thickness per second can be calculated. 3.1.1 Sputter Processes

The sputter process consists of several subprocesses, which have to be defined before the sputter process can start. In these subprocesses, several parameters can be defined. The main parameters that will be varied are rotation, temperature, duration of the subprocess and power of the sources. The first two subprocesses in the forthcoming experiments will always be "RF ignition" and "NaCl ramp". The subprocess "RF ignition" makes sure that the guns that will be used are properly initialized. The subprocess "NaCl ramp" is necessary because the sputter rate of NaCl is not stable in the first few minutes of the process. During this subprocess, a thin layer of sodium chloride with a thickness ranging between 200 nm and 200𝜇m is created. After these two subprocesses, the actual process of sputtering Tm2+ and NaCl can start. The conditions under which these two materials are sputtered in the final subprocess will vary, in hope to find the optimal conditions, under which the luminescence is maximized.

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3.1.2 Sputter Configurations

The availability of four different sputter guns gives the option to use different configurations when sputtering NaCl and Tm2+. The configurations that will be used in the forthcoming experiments are depicted in figure 3.2 and figure 3.3. The configuration in figure 3.2 will be referred to as the 180o configuration and the configuration in figure 3.3 will be referred to as the 90o configuration. Both names are referring to the angle between the active sputter guns. The 180o configuration does not account for the increase in thickness at the NaCl side of the sample. If the 90o configuration can be sputtered correctly, it would not only tell how the ratio of NaCl and Tm2+ influences the intensity of the luminescence, but it will also tell how the thickness influences the luminescence.

Figure 3.2. This figure shows a schematic drawing of the 180o configuration. In this drawing, the blue slab represents the sputtered NaCl layer and the red slab represents the sputtered Tm2+ layer. The sputter guns are directed opposite to each other.

Figure 3.3. This figure shows a schematic drawing of the 90o configuration. In this drawing, the blue slab represents the sputtered NaCl layer and the red slab represents the sputtered Tm2+ layer. The sputter guns are aligned at an angle of 90o relative to each other.

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3.1.3 Masks

Previous research showed that low concentrations of Tm2+, relative to the thickness of NaCl, improved luminescence [5]. The ways of obtaining a low Tm2+ concentration are by lowering the power of the source of the Tm2+ sputter gun or by applying a metal disk with holes, called a mask. Lowering the power of the source is not an option when the power reaches below 15W, because this will result in an unstable sputter rate. By combining multiple disks, even lower concentrations of Tm2+ can be achieved. The way that the disks are combined can be seen in figure 3.4. Applying one disk, leads to a decrease in surface area of 87%, leaving a sputter rate of 13%. Applying two disks, leads to a decrease in surface area of 98%, leaving a sputter rate of 2%.

Figure 3.4. This photo shows the metal disks that are used as masks to obtain lower sputter rates of Tm2+. The combination of disks portrayed in this photo leads to a decrease in sputter rate of 98%.

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3.2 Luminescence Measurements

Measurements of luminescence are done with a spectroscopic technique called emission spectroscopy. In these measurements, the films are excited by a Helium-Neon laser at 632.8 nm. The emission is usually measured at around 2500 different predefined X and Y points, where total number of X and Y values usually vary around 50. The integration time is usually set to one second. The apparatus, which executes and controls the measurements, is a THORLABS apt - brushless servo controller. This device is commonly known as, and will be referred to as, the XY-scanner. A schematic overview of the setup can be viewed in figure 3.5. The Ocean Optics (Near) Infrared Scientific-grade USB Spectrometer is used to measure the resulting emission spectra in the range of 900 nm - 1800 nm for each position, and a program written in LABVIEW stores all the incoming data.

The read-out of the emission-measurements is done using a program written in

MATLAB, which can be found in the appendix. This code gives a mapping of all important parameters of the measured emission spectra, like the relative intensity or peak wavelength, as a function of position. The range in which we are interested is located in between 1100 nm and 1200 nm.

Figure 3.5. This figure depicts a schematic overview of the used setup, which uses a 632 nm laser and optical fiber to a spectrometer, to perform the luminescence measurements.

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3.3 Excitation and Emission Measurements

To measure the luminescent properties of the thin films, an excitation and emission-measuring device, commonly known as the VUV setup, was used. In later sections this device will be referred to as the VUV setup. This section will explain the basic principles of this device. Figure 3.6 gives a schematic overview of the VUV. The VUV uses a 100W Xenon lamp as excitation source. The light then goes trough a Czeny-Turner double monochromator, which only allows a specific wavelength to pass. The excitation beam then reaches the sample, which is placed at 45o angle relative to the Xenon lamp. This sample absorbs the beam and emits light with its own specific emission spectrum. The signal is then filtered to contain only emission spectrum and then reaches a second monochromator. Ultimately a photomultiplier tube counts the number of photons per wavelength. To perform emission measurements, which register the intensity of the emission as a function of the wavelength, the excitation wavelength has to be kept constant. To perform an excitation measurement, which measurements the excitation as a function of the wavelength, the emission has to be kept constant, while the excitation wavelength is being changed. After measurement, a correction is required to account for the wavelength dependent intensity with which the sample is excited.

Figure 3.6. This figure gives a schematic overview of the Excitation and Emission Measuring setup.

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Results

4.1 Introduction

In this thesis several samples will be sputtered using the magnetron sputter system, described in section 3.1, and luminescent measurements will be performed using the XY-scanner described in section 3.2. Section 4.1.1 gives an overview of all the sputtered samples and explains why this set of configurations is chosen. Sections 4.2 to section 4.5 each show the results of the characterizations and discuss how these results can be interpreted.

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4.1.1 Overview

As described in section 3.1, in the final and main sputter process several parameters are varied, which provides the ability to fabricate several different thin films. Besides these parameters, all other controllable parameters are being kept constant. The substrate material for every film fabricated in these experiments will always be object glass. The first samples that will be characterized, are samples that are made in previous experiments. These measurements serve as a practice for testing the XY-scanner setup. The sample name of this practice sample will be Sample A - Test Sample. The second sample that will be characterized is Sample B - 180o, with the 180o- tag referring to the sputter configuration that is used to sputter this sample. This sample was characterized in an effort to reproduce earlier found results. The third sample, Sample C1 - 90o, also takes in account the effects that the thickness has on the amount of luminescence. Because this third sample failed to give proper results, a fourth sample was sputtered, named Sample C2 - 90o. The final batch of samples, the Sample D - Rotation series, will be without a gradient so that they can be used to have their overall efficiency measured. The details of these measurements and interpretations will be elaborated further in the following sections.

Name Sputter Time [s] Power NaCl [W] Power Tm2+ [W] Rotation Heat Mask Configuration

Sample A - Test Sample 4800 30 13 Off Off 13% 180o

Sample B - 180o 4800 20 30 Off Off 13% 180o Sample C1 - 90o 4800 30 15 Off Off 13% 90o Sample C2 - 90o 4800 30 15 Off Off 2% 90o

Sample D1 - Rotation 4800 30 15 On Off 13% None Sample D2 - Rotation 4800 30 25 On Off 13% None Sample D3 - Rotation 4800 30 35 On Off 13% None Sample D4 - Rotation 4800 30 15 On On 13% None Sample D5 - Rotation 4800 30 15 On Off 2% None

Table 4.1. This table gives an overview of al the sputtered samples. The samples are divided in four different categories, ranging from A to D.

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4.2 Sample A - Test Sample

The first samples that were measured were part of the thesis of, and sputtered by, Willem Kesteloo [4]. The name of the sample that was used is Film 3.2, but will be referred to in this thesis as Sample A. The excitation and emission spectrum that is typical for NaCl/Tm2+ films is depicted in figure 4.1. This figure shows an emission spectrum at 1140 nm by an excitation wavelength of 420 nm. It must be noted that in the forthcoming experiments, the results are obtained with a laser emitting at 632.8 nm.

Figure 4.1. This figure depicts the typical excitation and emission spectrum of NaCl/Tm2+ films. The black line represents the excitation spectrum obtained with an emission wavelength of 1140 nm and the red line represents the emission spectrum obtained with an excitation wavelength of 420 nm [4].

Before measuring the sample with the XY-scanner setup, a photo was made of the sample, which can be viewed in appendix, in figure A.1. In this photo a gradient in NaCl thickness can be seen by analyzing the opacity of the film by eye. The locations with low opacity have a high NaCl thickness and low Tm2+ concentration and the locations with high opacity have a low NaCl thickness and high Tm2+ concentration. A strip of tape was put on the sample during the sputter process and was taken off afterwards, which caused the transparent strip on the sample.

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Figure 4.2. This figure shows the different measurement outputs in MATLAB of Sample A. Starting from the upper left graph, the graphs represent; the integrated intensities over the whole measured spectrum from 900 nm to 1711 nm, the integrated intensities over the luminescent area from 1132 nm to 1168 nm, the wavelength of the peak in the luminescent area (1132 nm to 1168 nm), the intensity at that peak, the integrated intensities of the laser light at 1236 nm to 1287 nm and the spectrum of a chosen point, (8,7), in the first graph.

Figure 4.2 gives the output of the MATLAB script performed on the data output of the XY-scanner. The clean strip can be clearly observed in all the graphs around Y=17 to Y=25. The plot in the middle, in the top row, shows that locations with a high NaCl thickness and low Tm2+ concentration give the most luminescence. A typical spectrum at a location of high NaCl thickness and low Tm2+ concentration is given in the bottom right plot. An interesting result can be found in the upper right plot, where the wavelength of the peak in the luminescent area is plotted. This plot says that there is a shift in maximum peak wavelength, going from 1140 nm to 1138.5 nm when thickness of NaCl increases and when Tm2+ concentration decreases. It should be noted however that the resolution of the measured signal is 1.6 nm, so the error in this statement is 0.8 nm. To obtain more insight on these differences in peak, two typical plots with different maxima are plotted in figure 4.3.

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Figure 4.3. This figure shows two different spectra at different locations of Sample A. The red graph represents the spectrum with a peak at 1140 nm and the blue graph has a peak at 1138.5 nm. The red graph is taken from a sample position with a higher Tm2+ and lower NaCl thickness than the blue graphs position.

The spectra in figure 4.3 show that although the shape of both the spectra at the

luminescent area is roughly the same, the intensity is higher when the spectrum has a peak at 1138.5 nm. This figure also shows that in the area of the laser light reflection, 1200 nm to 1320 nm, the blue graph has a higher intensity, which corresponds to a location on the sample with a higher NaCl thickness and lower Tm2+ concentration. In figure A.1 it can be seen that this location also has a lower opacity, which can explain the more intense reflection of the laser light.

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4.3 Sample B - 180o

The second sample that was analyzed was sputtered again with a NaCl/Tm2+ gradient in opposite directions (180o configuration). A drawing of the sputtered layers and concentration gradient can be viewed in figure 4.4. In this drawing, the blue slab represents the sputtered NaCl layer and the red slab represents the sputtered Tm2+ layer. Figure A.2 shows a photo made of the sample, after measurement with the XY-scanner. In contrast to Sample A, Sample B does not show concentration gradients clearly just by analyzing the opacity by eye. When measuring luminescence with the XY-scanner, the fiber scratched the surface of the sample as can be observed in figure A.2.

Figure 4.4. This figure shows a schematic drawing of the sputter gradient of NaCl and Tm2+ in Sample B. In this drawing, the blue slab represents the sputtered NaCl layer and the red slab represents the sputtered Tm2+ layer. The sputter guns are directed opposite to each other.

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Figure 4.5. This figure shows the different measurement outputs in MATLAB of Sample B. Starting from the upper left graph, the graphs represent; the integrated intensities over the whole measured spectrum from 900 nm to 1711 nm, the integrated intensities over the luminescent area from 1132 nm to 1168 nm, the wavelength of the peak in the luminescent area (1132 nm to 1168 nm), the intensity at that peak, the integrated intensities of the laser light at 1236 nm to 1287 nm and the spectrum of a chosen point, (11,34), in the first graph.

Figure 4.5 gives the output of the MATLAB script, performed on the data output

of the XY-scanner. These results show much resemblance to earlier found results obtained from Sample A. As before, the intensity of the luminescent area increases when the NaCl thickness increases and the Tm2+ concentration decreases. Some differences can be found in the upper left graph of figure 4.5, which shows that there are some wavelike structures in the signal. These structures are not observed when analyzing the luminescent area, but can be found when analyzing the laser reflection in the area ranging from 1236 nm to 1287 nm. How this wavelike structure with a wavelength in the order of 10-2 m emerges is still unknown.

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A more detailed image of the intensity of the luminescent area, the area ranging from 1132 nm to 1168 nm, is given in figure 4.6. In this figure there is also a linear increasing gradient plotted as a transparent grid. This grid can be used to observe if there are any deviations in the expected luminescence, when assumed that the luminescence increases linearly with increasing NaCl thickness and decreasing Tm2+ concentration. The figure shows that the increase of luminescence is not linear and that there is also a gradient in luminescence in the direction of constant NaCl/Tm2+ ratio. This gradient can only be explained by deviations in alignment of the sputter guns, which should be considered carefully in further experiments.

Figure 4.6. This figure shows the integrated intensity of the luminescent area, in particular, the area ranging from 1132 nm to 1168 nm. This figure also contains a linear increasing grid, which can be used to check if there are any deviations in the expected values.

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4.4 Sample C - 90o

Analyzing figure 4.2 and figure 4.5, it can be concluded that a larger amount of NaCl and lower Tm2+ concentration leads to higher intensity in the luminescent area. Both Sample A as Sample B were sputtered in the configuration depicted in figure 4.4. This configuration does not account for the increase in thickness at the NaCl side of the sample. To account for the variation in film thickness, a different configuration is suggested, which is depicted in figure 4.7. In this configuration, two gradients are sputtered with a 90o angle relative to each other. This makes it possible to section the sample in parts where the amount of NaCl does not vary, but the Tm2+ concentration does, or vice versa. If this configuration can be sputtered correctly, it would not only tell how the relative amount of NaCl and Tm2+ concentration influences the intensity of the luminescence, but it will also tell how the thickness influences the luminescence.

Figure 4.7. This figure shows a schematic drawing of the sputter gradient of NaCl and Tm2+ of the sample series C. In this drawing, the blue slab represents the sputtered NaCl layer and the red slab represents the sputtered Tm2+ layer. The sputter guns are aligned at an angle of 90o relative to each other.

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4.4.1 Sample C1 - 90o

A photo of the first attempt of sputtering this configuration is shown in figure A.3. The left side of the sample has a high NaCl thickness and the topside of the sample has a high Tm2+ concentration. It can be seen that the part of the sample that has a high NaCl thickness also has a dark tint, which wasn't present in Sample A and Sample B. Another difference is that the previous samples were less transparent than Sample C1. When measuring this sample with the XY-scanner, no signal was found in the luminescent area. To analyze if there was any luminescent material present in the film at all, an emission measurement was done with the VUV. The excitation wavelength that was used in this measurement was 420 nm. The results of this measurement can be found in figure 4.8. It can be seen that Sample C1 does not show a distinct signal in the expected luminescent area around 1140 nm.

Figure 4.8. This figure shows the results of the VUV measurement on Sample C1. This measurement used an excitation wavelength of 420 nm to measure the emission spectra.

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4.4.2 Sample C2 - 90o

Although experiment C1 probably failed because of a fault in the NaCl target (which was replaced afterwards), the experiment was repeated with different settings. The power of the targets was kept the same, but the 13% mask was replaced with the 2% mask. This replacement led to improved results, as can be seen in the photo of the sample, in figure A.4, and in the output of the measurements, in figure 4.9. However, the results in figure 4.9 show very rugged graphs with outliers, which make the results uneasy to analyze. To obtain better results, a circular averaging filter was applied on the raw data results, which resulted in the results portrayed in figure 4.10.

Figure 4.9. This figure shows the different measurement outputs in MATLAB of Sample C2. Starting from the upper left graph, the graphs represent; the integrated intensities over the whole measured spectrum from 900 nm to 1711 nm, the integrated intensities over the luminescent area from 1132 nm to 1168 nm, the wavelength of the peak in the luminescent area (1132 nm to 1168 nm), the intensity at that peak, the integrated intensities of the laser light at 1236 nm to 1287 nm and the spectrum of a chosen point, (17,44), in the second graph.

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Figure 4.10. This figure shows the filtered results of the different measurement outputs in MATLAB of Sample C2. The blue arrow shows the sputter direction of the NaCl target and the red arrow shows the sputter direction of the Tm2+ target. Starting from the upper left graph, the graphs represent; the integrated intensities over the whole measured spectrum from 900 nm to 1711 nm, the integrated intensities over the luminescent area from 1132 nm to 1168 nm, the wavelength of the peak in the luminescent area (1132 nm to 1168 nm), the intensity at that peak, the integrated intensities of the laser light at 1236 nm to 1287 nm and the spectrum of a chosen point, (17,43), in the second graph.

Figure 4.10 shows interesting results. Not only does the sixth graph in figure 4.10 show that the luminescence has a distinct peak at 1138.5 nm, the second graph shows a surface function that rises towards a maximum point. This surface function describes how the luminescence varies on different locations on the sample. It is interesting to note that this point isn't located nearby the Tm2+ sputter gun, but further away from it. This observation can lead to conclusions that even lower Tm2+ concentrations that are used in this experimental setup may lead to an increase in luminescence. The second graph also says that an increase of NaCl may not lead to a further increase of luminescence

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4.5 Sample D - Rotation

The D series of this thesis were all sputtered with rotation, i.e. without a gradient. This was done in an effort to fabricate a useable sample that could function as a demonstration sample for the company PowerWindow. The first three samples differ by the amount of power of the Tm2+ sputter gun. Sample D1 starts with 15W, sample D2 uses 25W and sample D3 uses 35W. The fourth sample, sample D4 was sputtered with the heater on at 400 degrees Celsius, using the Tm2+ sputter gun at 15W. Samples D1 to D4 all used the NaCl sputter gun at 30W and used the 13% mask on the Tm2+ sputter gun. The final sample, sample D5 uses the Tm2+ sputter gun at 15W with a 2% mask and the NaCl sputter gun at 30W. Unlike all previous results, these results of the D series are plotted as a 2D intensity plot. This was done, because when rotation is turned on during the sputter process, the expected result is a circular symmetrically distributed amount of luminescence. When the distribution of luminescence deviates from this expectation, or even forms unexpected patterns, the 2D intensity plot can give a good overview of this.

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4.5.1 Sample D1 - Rotation

Figure A.5 shows a photo of sample D1 shortly after the sputter process finished. In this photo, it can be seen that flakes of the sputtered film came of, which caused transparent spots. The results that were obtained from the XY-scanner can be viewed in figure 4.11. In these plots there is no sign of any circular symmetry. The white areas in the third graph of figure 4.11 were due to the transparent spots on the sample.

Figure 4.11. This figure shows the different measurement outputs in MATLAB of Sample D1. Starting from the upper left graph, the graphs represent; the integrated intensities over the whole measured spectrum from 900 nm to 1711 nm, the integrated intensities over the luminescent area from 1132 nm to 1168 nm, the wavelength of the peak in the luminescent area (1132 nm to 1168 nm), where the red area's represents a wavelength of 1140 and the blue area's a wavelength of 1138.5 nm, the intensity at that peak, the integrated intensities of the laser light at 1236 nm to 1287 nm and the spectrum of a chosen point, (8,5), in the second graph.

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Figure A.6 shows a photo of sample D2 shortly after the sputter process finished. The results that were obtained from the XY-scanner can be viewed in figure 4.12. The plots show a strange circular pattern of circles. This pattern is due to the holes in the plateau on which the sample laid on during the XY-scanning process. These holes caused a reduction in the intensity of the reflection, which can be observed in the plots. The plotted spectrum, which can be viewed in the sixth graph, shows that this sample shows a reduced intensity in the luminescent area, relative to sample D1. This is also viewable in the third graph, where a peak is only plotted if it has a distinct maximum in the luminescent area.

Figure 4.12. This figure shows the different measurement outputs in MATLAB of Sample D2. Starting from the upper left graph, the graphs represent; the integrated intensities over the whole measured spectrum from 900 nm to 1711 nm, the integrated intensities over the luminescent area from 1132 nm to 1168 nm, the wavelength of the peak in the luminescent area (1132 nm to 1168 nm), the intensity at that peak, the integrated intensities of the laser light at 1236 nm to 1287 nm and the spectrum of a chosen point, (15,28), in the second graph.

46


Figure A.7 shows a photo of sample D3 shortly after the sputter process finished. The results that were obtained from the XY-scanner can be viewed in figure 4.13. The graphs in this figure show that a lot of locations have a blank space. This was due to a piece of tape, which was accidentally stuck on the sample, before the sputter process started. The plotted spectrum, which can be viewed in the sixth graph, shows that this sample shows an increase in luminescence relative to sample D2, but a decrease in luminescence relative to sample D1.


47


Sample D4 failed mainly because of faults in the preparation of the sample, before the sputter process. This sample was sputtered with the heater on 400 degrees Celsius. In the preparation, tape was used to attach the sample to the sample holder. This caused the sample to break, which can be seen in figure A.8. The sample also showed no signs of luminescence when it was measured with the spectrometer. 4.5.5 Sample D5 - Rotation

Figure A.9 shows a photo of sample D5 shortly after the sputter process finished. The results that were obtained from the XY-scanner can be viewed in figure 4.14. The sixth plot in this figure shows that this sample has high luminescence, surpassing all other configurations in the D-series. This high luminescence can only be explained by the use of low Tm2+ sputter rate due to minimum power supply and the application of the 2% mask. The other plots in figure 4.14 show that there are certain regions where the reflected signal and luminescence is stronger than other regions. When comparing figure A.9 with figure 4.14, it can be seen that the regions with a rougher surface and with lower opacity correspond with the regions with a higher reflected signal and stronger luminescence. 4.5.5 Conclusions Regarding the D-Series.

Consistent with conclusions regarding Sample A to Sample C, samples with the lowest amount of Tm2+ show the highest amount of luminescence. In Sample D5, the sputter rate was further lowered by applying a second mask, resulting in a lower deposition rate, while remaining a steady sputter rate. The fact that all the samples in the D-series were sputtered with rotation could lead to the hypothesis that the samples could show a sign of circular symmetry. This hypothesis is not supported by the data, which shows no signs of circular symmetry and also show no sings of regularity, which could predict patterns.

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49

Conclusion and Recommendations

The goal of this research was finding the optimal sputter configuration and ratio of NaCl and Tm2+ corresponding to optimal luminescent properties of sputtered films. This was done by examining nine different sputtered samples that were all sputtered with different sputter settings and alignment configurations. The settings varied the sputter rate during the sputter process and alignment varied from a 90o configuration to an 180o

configuration. The settings with which these samples were sputtered by varied in sputter rate, temperature during the sputter process and configuration of the sputter guns. The results consistently showed that a higher NaCl thickness and lower Tm2+ concentration lead to optimal luminescence. The first part of the results, which examined films with a gradient, consistently showed that regions with a high NaCl thickness and low Tm2+ concentration have optimal luminescent properties. Sample C2, which used a 90o configuration, even shows that an optimum in NaCl thickness can be found. What this optimum is in terms of sputter rate could be part of a follow up research. This could be done by combining these results with the sputter model that was used in the thesis of Willem Kesteloo [4]. The second series of sputtered films, which were sputtered without a gradient, showed similar results. In this series, the films that were sputtered with a minimal Tm2+

sputter rate, showed the highest amount of luminescence. Even more interesting, is that further lowering the sputter rate, by applying a 2% mask, showed improvement. This could mean that an optimum could be found by further lowering the Tm2+ sputter rate. When combining the obtained results with the theory presented in chapter 2.3, it can be hypothesized that the lower amount of luminescence, when using a higher amount of Tm2+, is caused by reabsorption of photons and possible non-radiative relaxation. The results also show that some samples had a more rugged surface than other samples, which causes a decrease in waveguide efficiency.

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In this thesis, only the Tm2+ sputter rate was varied. It still remains a question what will happen when the NaCl sputter rate is varied. The outcomes of Sample C2 showed indications that an optimal rate could be found by varying the NaCl sputter rate. Another follow up research could be in quantifying the actual power output of the samples, when these are measured with strips of solar cells at the edge of the samples. This could also be done using an integrating sphere.

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Photos of Samples

Part A of the appendix contains all the photos of the samples, which are taken after the sputter process was finished.

Sample A - Test Sample

Figure A.1. This figure shows a photo of Sample A. The gradient in NaCl thickness can be viewed in this photo by analyzing the opacity. The locations with low opacity have a high NaCl thickness and low Tm2+ concentration and vice versa.

Sample B - 180o

Figure A.2. This figure shows a photo of Sample B. The bottom of the sample has a high NaCl thickness and low Tm2+ concentration, while the top has vice versa. Several scratches made by the optical fiber can be observed.

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Sample C1 - 90o

Figure A.3. This figure shows a photo of Sample C1. The left side of the sample has a high NaCl thickness, while the topside has a high Tm2+ concentration. A schematic image of the sputter configuration can be viewed in figure 4.7.

Sample C2 - 90o

Figure A.4. This figure shows a photo of Sample C2. In this picture, the topside has a high NaCl thickness, while the left side has a high Tm2+ concentration. A schematic, but rotated, image of the sputter configuration can be viewed in figure 4.7.

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Sample D1 - Rotation

Figure A.5. This figure shows a photo of Sample D1. The film did not attach properly to the substrate, which caused the film to release from the substrate at certain locations. This can be observed in the photo.


Figure A.6. This figure shows a photo of Sample D2.

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Figure A.7. This figure shows a photo of Sample D3. A piece of tape can be seen on this sample, which was attached to sample by accident.


Figure A.8. This figure shows a photo of Sample D4. This sample was sputtered with the heater on 400 degrees Celsius. In the preparation, tape was used to attach the sample to the sample holder. This caused the sample to break and the tape to melt.

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Figure A.9. This figure shows a photo of Sample D5. The upper part of the sample shows a slightly rougher surface than the bottom part of the sample. This rougher part corresponds in the results with a higher luminescence.

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MATLAB Code

This part of the appendix shows the MATLAB code that was used to analyze the output of the XY-Scanner device. clear all close all clc %This function locates the files that will be used. myFolder = strcat('H:\Desktop\BEP FINAL\WORKMAP\19152'); cd (myFolder); %This function reads the data from the files. X=0:46; Y=0:47; for i=1:max(Y) for j=1:max(X) filename =sprintf('X0%02d_Y0%02d.txt',j,i); fid = fopen(filename,'rt'); str = fread(fid,'*char')'; fclose(fid); str = strrep(str,',','.'); [a,b] = strread(str,'%f %f','delimiter',';'); clear str M=[a,b] ; coor=find((b(1:187))==max([b(1:187)])); wl(i,j)=a(coor(1)); Imax(i,j)=b(coor(1)); punt1(i,j) = sum (M(:,2)); punt2(i,j) = sum(M(139:168,2)); punt3(i,j) = sum(M(210:242,2)); end; end; fig=figure; wl2 = wl.*(wl==wl(1,2)) + wl.*(wl==wl(21,24)); Z = punt1; L = punt3; %This part of the code normalizes and selects data. Wv = wl; regionOfInterest = Wv > 1120 & Wv <1144; Wv(~regionOfInterest) = NaN; Znorm = (1/max(Z(:))).*Z; Lnorm = (1/max(L(:))).*L; Imaxnorm = (1/max(Imax(:))).*Imax; punt2norm = (1/max(punt2(:))).*punt2;

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Wvnorm = (1/max(Wv(:))).*Wv; % This part of the code is only used for Sample B. % surf(punt2norm) % hold on % surf(NaCl,'FaceAlpha',.2) % axis tight % title('Integrated Intensities over Luminescent Area') % xlabel('No concentration gradient') % ylabel('Increasing NaCl concentration and decreasing Tm2+ concentration') subplot(2,3,1) surf(Znorm) axis tight title('Integrated intensities over whole range') subplot(2,3,2) surf(punt2norm) colorbar axis tight title('Integrated intensities over 1132nm - 1168nm') subplot(2,3,3) surf(Wv) colorbar axis tight title('Peak') subplot(2,3,4) surf(Imaxnorm) axis tight title('Intensity at peak') subplot(2,3,5) surf(Lnorm) axis tight title('Integrated second order laser reflection 1236nm - 1287nm') figure surf(punt2norm) colorbar title('Integrated intensities over 1132nm - 1168nm') % Disk filter over punt2norm; The Integrated Intensities. figure GG = fspecial('disk',10); MotionBlur = imfilter(punt2norm,GG,'replicate'); surf(MotionBlur) hold on GGG = fspecial('disk',5); MotionBlur1 = imfilter(punt2norm,GGG,'replicate'); surf(MotionBlur1)

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%This part of the code introduces the cursor mode. h=datacursormode(fig); datacursormode on info.Target=0; info.Position=[0 0 0]; info2.Target=0; info2.Position=[0 0 0]; while ~isempty(findall(0,'type','figure')) info=getCursorInfo(h); pause(0.2) if class(info)=='struct' info=getCursorInfo(h); if info2.Position(1)~=info.Position(1) || info2.Position(2)~=info.Position(2) if length(findall(0,'type','figure'))>=2 close(2:length(findall(0,'type','figure'))) end B=info.Position(1); A=info.Position(2); filename =sprintf('X0%02d_Y0%02d.txt',B,A); fid = fopen(filename,'rt'); str = fread(fid,'*char')'; fclose(fid); str = strrep(str,',','.'); [y,z] = strread(str,'%f %f','delimiter',';'); clear str M0=[y,z] ; M2=M0(50:250,:); Luminescence = M2(:,2); Luminescencenorm = mat2gray(M2(:,2)); subplot(2,3,6) plot(M2(:,1),Luminescence); axis([1000 1300 0 6.4e4]) title('Intensity'); xlabel('Wavelength (nm)'); ylabel('Counts'); plot(M2(:,1),Luminescence); axis([1000 1300 0 6.4e4]) title('Intensity'); xlabel('Wavelength (nm)'); ylabel('Counts'); end info2=info; end end

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References

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