Deposition of luminescent thin films for solar energy applications

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Deposition of luminescent thin films for solar

energy applications.

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft

op gezag van de Rector Magnificus Prof. Ir. K.C.A.M.Luyben, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op 22 juni 2015 om 10:00 uur

Door Michiel DE JONG

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Prof. Dr. P. Dorenbos

Copromotor: Dr. E van der Kolk

Samenstelling promotiecommissie:

Rector Magnificus, Voorzitter

Prof. dr. P. Dorenbos, Technische Universiteit Delft, promotor Dr. E. van der Kolk, Technische Universiteit Delft, copromotor Prof. dr. E.H. Brück, Technische Universiteit Delft

Prof. dr. B. Dam TNW, Technische Universiteit Delft Prof. dr. A Meijerinck, Universiteit Utrecht

Prof. dr. R.E.I. Schropp, Technische Universiteit Eindhoven Dr. J.C. Goldschmidt, Fraunhofer Institute of Technology Prof. dr. C. Pappas, Technische Universiteit Delft, reservelid

Financial support is acknowledged for this research from ADEM, A green Deal in Energy Materials of the Ministry of Economic Affairs of The Netherlands.

Printed & Lay Out by: Proefschriftmaken.nl || Uitgeverij BOXpress Published by: Uitgeverij BOXPress, ‘s-Hertogenbosch

Cover: Photograph of combinatorial sputtering process of NaCl and Tm.

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Introduction

1.1. Applications of luminescent thin films

Thin film technology plays an important role in device fabrication of a wide variety of technological applications, including microelectronic devices, magnetic storage media and protective surface coatings. An important class of thin film technology is based on luminescent materials, which has found application in thin film electronic devices (TFEL), white light emitting diodes (LED), and field emission displays (FED).1,2,3,4_{A luminescent} material applied in these types of devices is typically excited by either UV light (photoluminescence), high energy electrons (cathodoluminescence) or is excited by applying an electric field (electroluminescence), after which visible or near-infrared light is emitted.

More recent applications of luminescent thin films are related to efficiency enhancement of solar cells and luminescent solar concentration, which are the topics of this thesis. Such thin films are generally referred to as spectral conversion materials. More specifically spectral shifting materials, in case they simply absorb a high energy photon and emit a lower energy photon. These materials are distinguished from down-conversion (or quantum-cutting) materials that emit two lower energy photons for each absorbed high energy photon.

There is an extensive amount of reports available on the deposition of luminescent thin films,5,6,7_{as is also discussed in chapter 2 of this thesis. However, knowledge on} luminescent thin film deposition of materials that are optimal for the photovoltaic applications is limited.

Thin film deposition is a concept that based on either chemical or physical processes. With chemical deposition, a non-volatile solid film is formed on a surface through chemical reactions between liquid or gas-phase precursors that contain the required constituents. The main advantages of chemical deposition methods include high growth rates and good reproducibility of deposited thin films. One of the main disadvantages is the high temperature that is typically required to dissociate the reactants, especially with chemical vapor deposition (CVD).

Physical deposition techniques apply mechanical or thermodynamic processes to deposit a non-volatile thin film from solid precursors. Most physical deposition methods operate at low pressures and atoms are released from the surface of the solid precursor that condensate on another surface. This process is more accurately called physical vapor

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deposition. Among the advantages of physical vapor deposition processes are the hardness, high temperature resistance, high impact strength, high abrasion resistance and durability of deposited films.

Sputtering is an example of a physical vapor deposition technique that is used in a variety of fields, including integrated circuitry and production of anti-reflection coatings for windows. In this work, the applicability of sputtering is studied for luminescent thin film deposition from multiple reactants in a reactive atmosphere. A comprehensive description of the concept of sputtering is found in section 1.4.

1.2. Spectral conversion thin films for solar cells

There are several processes that limit the efficiency of conversion of solar energy into electrical energy in a solar cell. When considering a single junction solar cell device, energy is lost due to inefficient coupling of the sunlight (i), inefficient absorption of in-coupled light (ii) and inefficient conversion of absorbed light into electrical energy (iii). When sunlight illuminates the surface of a solar cell, a fraction of the sunlight is reflected off the surface. This is loss process (i), which is due to the typically high refractive index of solar cells. Of the light that does enter the cell, only photons with an energy that is equal to or higher than the band gap can be absorbed. Light with an energy lower than the band gap is not absorbed but transmitted through the cell, which is loss process (ii). Photons with an energy that is higher than the band gap are absorbed, creating free charge carriers. The energy of the photon that is in excess of the bandgap is lost after absorption as phonon emission, a process called thermalization. In addition, energy is lost because a fraction of the resulting free charge carriers that are created by photon absorption recombine before they are separated, resulting in re-emission of a photon that exits the solar cell. This occurs primarily at the front surface of the solar cell, where a large fraction of the UV and blue light is absorbed because the absorption strength of light in this range is high. Thermalization and recombination make up loss process (iii). In order to improve the efficiency of solar cells, improvement of the external quantum efficiency (EQE) is required. The external quantum efficiency is defined as the number of charge carriers that are collected by the solar cell, compared to the number of photons that the cell is irradiated with. The EQE can be expressed in a simple equation:

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9 ܧܳܧ ൌ ܫܳܧ ή ሺͳ െ ܴ െ ܶሻ

Where is the fraction of light that is reflected off the cell surface, and is the light that is transmitted through the cell without being absorbed. In the above equation the internal quantum efficiency (IQE) is included. The IQE is defined as the number of charge carriers that are collected by the cell after incident photons are actually absorbed and is determined by recombination and other loss factors like competitive absorption processes in other parts of the solar cell.

In principle, when using a low band gap semiconductor as solar cell, the non-absorption losses (R and T) will be low, resulting in a high quantum efficiency. However, the thermalization loss of such a cell is very high. Although this does not influence the quantum efficiency, a significant amount of solar energy is still lost. In contrast, when using a high band gap semiconductor, the thermalization loss is much lower, but the non-absorption loss is high. This trade-off results in a theoretical maximum efficiency that a single junction solar cell with a specific band gap could obtain.

By assuming the cell absorbs all light with an energy higher than its band gap and no light with an energy lower than the band gap, the theoretical maximum efficiency can be calculated. This maximum is called the detailed balance limit (or Shockley-Queisser limit).8_{It is determined from this limit that if the sun is treated as a black body with a} temperature of 6000 K, the ideal band gap for sunlight conversion would be 1.34 eV. The theoretical maximum efficiency of a single junction cell with such a band gap would be 33.7%.

The spectral irradiance of a solar cell by the sun is usually determined using the standardized AM 1.5 spectrum. The spectral range spans from UV-light (λ > 300 nm, Eph = 4.13 eV) to the visible part and the infrared (λ < 2500 nm, Eph = 0.50 eV). In practice, the efficiency of a solar cell is most accurately calculated by using the AM 1.5 spectrum. By performing the same detailed balance limit calculations with this solar spectrum, the ideal band gap for a solar cell is found at 1.15 eV, resulting in a theoretical maximum efficiency of 33.5% (considering a refractive index of 1 for the solar cell).12_{In Fig. 1.1 the} detailed balance limit is plotted as a function of band gap and the theoretical maximum efficiency is indicated for a number of semiconductors that are used in various types of solar cells.

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Fig. 1.1: Detailed balance limit of solar cells with refractive index of 1 under AM 1.5 solar irradiation as a function of the band gap of the cell. The limits of a number of semiconductors that are used in various types of solar cells are indicated.

In order to cross the detailed balance limit of solar cells, solutions for the fundamental loss principles are required. A proposed solution to overcoming thermalization losses in solar cells has been proposed in the form of applying a down-conversion material to the solar cell.9,10,11_{In such a material single photons are absorbed and multiple lower energy} photons are emitted. With this down-conversion principle the internal quantum efficiency could be doubled for photons with an energy that is at least twice the band gap. For example, with c-Si solar cells around 21% of the photons in the solar spectrum range have an energy that is at least twice the band gap.

A possible way of applying a down-conversion material is by placing it on top of the solar cell, as a thin semi-transparent layer. Sunlight entering this layer should either be absorbed and converted in the layer, or propagate through the layer into the solar cell. In a simple configuration, the conversion layer is sandwiched in between the solar cell and an encapsulation layer. The encapsulation layer is typically glass or a plastic material that protects the device from the outside environment.

Theoretical calculations have been performed on the resulting efficiency after applying a thin layer of such a material on the surface of different types of solar cells.12_{The results} of these calculations are found in Table I, which also contains the detailed balance limit ηSQ for each solar cell. Actual measured IQE curves of each of the solar cells were used and losses due surface reflection were taken into account to calculate the resulting efficiencies that are found in Table I.

The efficiency of the different solar cells can be calculated after applying the conversion layer with a theoretical optimum refractive index (ideally n = 2.0-2.4 for the solar cells considered here) and placing the encapsulation layer with refractive index n = 1.5 on top of that. If the absorption efficiency of the conversion layer is 0%, it serves as a transparent anti-reflection coating resulting in the efficiency ηtransp.

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11 Now the efficiency improvement of the various solar cells can be calculated by considering an optimal conversion by the layer. In this case the layer serves as a spectral shifter and the absorption edge is found at the edge of the spectral range where the IQE is not optimal and the layer emits light at a wavelength where the maximum IQE of the cell is found. If a luminescence quantum efficiency of 100% is assumed, the resulting theoretical efficiency is found in Table I as ηshift.

If the layer is capable of down-conversion, a luminescence quantum efficiency of 200% is possible. In this case the edge of absorption of the conversion layer is shifted to twice the energy of the band gap of the solar cells, resulting in the efficiency ηDC. Note that in this case the efficiencies of a-Si and CGS are omitted. Due to the large band gap of both materials, the fraction of sunlight with an energy that is twice that of the band gap is too small. A down-conversion layer would therefore not be a significant improvement over a transparent layer.

TABLE I: Efficiencies of various different solar cells with band gap Eg. ηSQ indicates the detailed balance

limit for various solar cells with band gap Eg with refractive index of 1 under AM 1.5 solar radiation. By placing a transparent layer with an optimal refractive index on top of the cell and the encapsulation layer on top of that, ηtransp is obtained. ηshift and ηDC are the resulting efficiencies if the conversion layer is an

optimal shifting or down-conversion layer.

Eg (eV) ηSQ (%) ηtransp (%) ηshift (%) ηQC (%)

c-Si 1.12 33.2 25.3 25.7 28.2 pc-Si 1.12 33.2 22.4 23.6 25.1 a-Si 1.7 28.9 20.3 21.2 - CdTe 1.44 32.8 19.6 24.2 27.4 GaAs 1.4 33.2 25.6 26.3 27.4 CIS 1.0 31.5 23.7 24.6 29.6 CIGS 1.15 33.4 25.6 27.2 29.6 CGS 1.7 28.9 19.9 22.3 - GaSb 0.7 23.9 13.9 17.4 25.1 Ge 0.67 22.5 14.8 16.5 23.6

Unfortunately, down-conversion materials with a high luminescence quantum efficiency have not been reported. Fortunately, spectral shifting layers also yield efficiency improvements as is observed by comparing ηtranspto ηshift in Table I. The efficiency improvement of spectral shifting layers is especially high in CdTe and CIGS solar cells. In

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these solar cells, competitive absorption processes of UV/blue light of a CdS buffer layer drastically decrease the amount of UV/blue light that is absorbed by the cell itself.13_By applying a spectral shifting layer on top of these cells, this loss is prevented and the resulting efficiency improvement is much higher compared to cells where only recombination losses are overcome.

The refractive index of the spectral shifting layer needs to be optimal in order to successfully incorporate it into a solar cell device. The material to use as conversion layer is therefore of great importance to the cell efficiency, since it also serves as an anti-reflection coating when placing it on top of the cell. Materials that are typically applied as anti-reflection coatings to solar cells are SiNx type materials, because of their favorable refractive index. Apart from that, SiNx layers show high transparency and thermal stability.14,15_{If a spectral shifting function is integrated into the SiN}

x layer, a layer is formed that serves as an anti-reflection coating as well.

Luminescence in inorganic materials can be achieved by doping the material with a low concentration of rare-earth ions. Inorganic materials doped with Eu in its divalent state typically emit visible light under UV/blue light excitation.16_{Since Si is stable in a} tetravalent state in compounds like SiNx, doping this material by substituting Si for Eu2+ requires a double charge compensation. This makes efficient doping of Eu2+_{in SiN}

x by substitution of Si with Eu unlikely. As an alternative, in MxSiyNz (M=Ca, Sr, Ba) materials the divalent alkaline metal ions can be substituted by divalent Eu ions effectively. For example, powder phosphor studies on M2Si5N8:Eu2+ (M=Ca, Sr, Ba) has shown high quantum efficiencies with UV/blue absorption followed by orange/red emission.17_By applying this material as the conversion layer, it can serve as a spectral shifting anti-reflection coating. An artist impression of the effect of placing such a layer on top of a solar cell is shown in Fig. 1.2.

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Fig. 1.2: Artistic impression by Eric Verdult of a spectral shifting layer placed on top of a c-Si solar cell that transmits visible and NIR light and absorbs UV and blue light. The absorbed light is re-emitted as red light unto the solar cell.

1.3. Luminescent solar concentrators

An important condition for working towards a more sustainable society based on generating energy with photovoltaic devices is the possibility of large scale integration of such devices into the environment. Photovoltaic power stations are being constructed outside of the built environment to generate power on an utility level. These photovoltaic power stations consist of large arrays of solar panels that are usually placed in agricultural areas. Examples of these “solar farms” are found in the United States (Topaz Solar Farm, CA being the largest, nominal power 550 MWp), Asia (Longyangxia Dam Solar park, China 320 MWp), Europe (Solarpark Meuro, Germany 166 MWp) and Afrika (Jasper Solar Energy Project, South Afrika 98 MWp).

Integration of the typical solar panel in the built environment is mostly limited to building rooftops. In order to generate large amounts of solar energy on a local level, highly efficient solar panels are required. Alternative methods of sunlight harvesting are proposed to utilize other surfaces than rooftops for solar energy generation. The most

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promising method is solar concentration. With this approach, sunlight is focused from a large surface and onto a very small surface. The simplest example of this technique is by using a lens to focus sunlight onto a small solar cell, as is demonstrated in Fig. 1.3.

Fig. 1.3: Schematic of a solar concentrator where a lens is used to focus sunlight onto a small solar cell.

Lens-based solar concentrators are not necessarily an improvement over the classical solar panel for integration in the built environment. Large support structures are required to hold the lens in place and the lens should track the position of the sun. A simpler device based on solar concentration by a flat glass plate was first proposed in the 70’s by Weber and Lambe.18,19,20_{They described a planar solar collector called a luminescent solar} concentrator (LSC).

The LSC is a plate of luminescent material that absorbs sunlight and re-emits it isotropically within the plate, as is shown schematically in Fig. 1.4. Due to the higher refractive index of the plate compared to air, a large fraction of the emitted light is trapped within the plate due to total internal reflection. The light is waveguided through the plate to the edges, where it exits the plate. If thin film photovoltaic cells would be placed on the edges of the plate, light would be collected and converted into electrical energy. Light incident on the plate is thus concentrated on a small area, greatly reducing the required surface area for photovoltaic cells to collect the light.

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Fig. 1.4: Schematic where the concept of the luminescent solar concentrator is demonstrated.

As an alternative to fabricating luminescent plates, thin layers of luminescent material can be coated on top of glass surfaces. This design makes the LSC of particular interest for integration in buildings as a power generating window. Since windows are omnipresent in cities, the possibility of utilizing the surface of windows for solar energy generation would be an enormous advance in building-integrated photovoltaics (BIPV).21,22

Unfortunately, development of an efficient LSC has not been successful to date, since it has proven difficult to find a luminescent material that combines absorption of a broad range of sunlight with emission of light with a very high quantum efficiency. The luminescence characteristics additionally need to be as such that spectral overlap between absorption and emission is very small, to prevent reabsorption of emitted light by the luminescent material. With every reabsorption a chance of emission at an angle within the escape cone is possible, resulting in emission at the front or the back of the plate. Emission within the escape cone and a lower than unity quantum efficiency of the luminescent material drastically decrease the efficiency of an LSC.

Organic luminescent dyes are attractive candidates to apply as luminescent coating due to their high abundance, low cost and simple processing into an LSC. However, because of the significant overlap between the absorption and emission bands of these materials, the amount of light that actually reaches the edge of the plate is decreased by a factor of 3 due to self-absorption alone.23_{Apart from that, the top available luminescent dyes like} Rhodamine 6G24_{and RED 305}25_{absorb sunlight with a wavelength of up to 600 nm, a} spectral range that only contains 30% of the power of the solar spectrum (a-Si solar panels typically absorb around 50%). The result is a brightly coloured LSC that is limited in power efficiency, which is unfit for large scale integration.

A more recent alternative to the organic dyes are the colloidal quantum dots. These quantum dots are versatile structures that can be tuned to absorb a large part of the solar spectrum. In addition, these small structures can be coated on the glass surface as a scatter-free layer. Luminescent quantum dots also suffer from self-absorption, but recent work resulted in the development of type-II core-shell colloidal quantum dots,26 Stokes-shifted engineered CdSe/CdS quantum dots and nanorods with a thick shell (giant

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quantum dots)27,28,29_{and Mn}2+_{doped ZnSe/ZnS quantum dots,}30_{all of which show a small} overlap between absorption and emission bands, resulting in low self-absorption rates. Apart from self-absorption, the challenges with quantum dots are related to their lifetime, limited absorption range and luminescence quantum efficiency. However, they have the potential to result in high efficiency LSC’s.

In general the limited absorption range and the effect of self-absorption with organic dyes and quantum dots result in limited LSC power efficiencies for windows with a surface larger than 1 m2_{to 2-3%.}22_{In chapter 7 of this thesis, NaCl doped with divalent} thulium (Tm2+_{) is presented as a material with the potential to overcome the limitations} mentioned for organic dyes and quantum dots. This luminescent material is capable of absorbing up to 55% of the power of sunlight, due to absorption bands that reach up to 750 nm, with virtually no self-absorption.31_{An LSC with a power efficiency of up to 11%} is possible when this phosphor is used, with the additional advantage that the absorption over the entire visible spectrum gives the possibility to obtain LSCs with no coloration. This new material might therefore lead to the development of an efficient LSC that is applicable in the built environment. An artistic impression of the resulting device is shown in Fig. 1.5.

Fig. 1.5: Illustration of the concept of an LSC coated with an inorganic Tm2+_{doped thin film.}

Total internal reflection of infrared Tm2+ luminescence Luminescence coating Argon protecting atmosphere Photovoltaic conversion

Band gap tuned CIGS strip solar cell

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1.4. Magnetron sputtering for thin film deposition

Sputtering is a versatile deposition technique that can be adapted to deposit thin films from sources of all classes of materials. A summary of the principles of sputtering is presented here, with each section representing a concept for depositing a different class of material. Examples of thin films that are deposited using these different concepts are found chapter 2, where a comprehensive review on luminescent thin film deposition by sputtering is presented. For a more detailed description of the sputter process, the reader is referred to Westwood.32

1.4.1. DC sputtering of metals and metal alloys.

Sputtering is a deposition technique that involves eroding a solid surface by bombardment with charged particles. Particles that are dislodged from this surface flow to a different surface where they condensate to form a layer. The charged particles are supplied by creating a plasma in the reaction chamber. Prior to initiation, the chamber is put at a high vacuum (typically 1 x 10-10_{bar). The source material (target) is connected to} a negative voltage supply. When a direct current (DC) is applied to the target, an electric field is created. The target serves as a cathode and a grounded surface opposite to the target serves as the anode. When a sputter gas is added to the chamber, free electrons in the gas accelerate towards the anode and collide with gas phase atoms, ionizing them. Usually an inert gas like argon is used as sputter gas. The simple ionization reaction is described as:

ܣݎ ൅ ݁ି_{ൌ ݁}ି_{൅ ܣݎ}ା_{൅ ݁}ି

The additional free electrons that are generated by these ionization reactions in turn ionize other atoms, a process called the Townsend avalanche. Apart from ionization, recombination also occurs between ions and free electrons, where the excess energy generated from recombination is emitted as photons. This results in a stable plasma. The positively charged plasma ions in the plasma are accelerated towards the negatively charged target. Upon collision with the target surface, the Ar+_{ion transfers its kinetic} energy to the crystal lattice of the target material and a succession of atom collisions occurs within the target. Eventually a target atom is ejected from the surface, moving in opposite direction to the impeding Ar+_{ions, as is shown in Fig. 1.6. This typically occurs} when the energy of the impeding Ar+_{ion is in the range 20-300 eV, which is called the} single knock-on energy regime. Ar+_{ions with an energy above 300 eV sputter in the linear} cascade regime. The ions transfer sufficient energy to a single target atom to sputter

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multiple other target atoms. In this regime bombardment can also result in sputtering of molecules or clusters of atoms.

Fig. 1.6: dislodging of a target atom (brown circle) by bombardment with an Ar+_ion.

With succeeding bombardments, a small fraction of electrons is also ejected from the target. The rate of electron ejection from a metal targets is typically in the order of 0.1 per Ar+_{ion collision. These electrons are also accelerated towards the anode and in turn} ionize Ar atoms by collision. In order to maintain the plasma, ejected electrons need to ionize a sufficient amount of electrons. This is achieved by maintaining a pressure in the order of 1 x 10-4_bar.

The efficiency of the sputter process is described by three parameters; the sputter yield, the sputter rate and the deposition rate. The sputter yield is defined as the average number of ejected atoms from the target per incident ion. A number of semi-empirical equations have been proposed for determining the sputter yield33,34,35_{. In practice, the} efficiency of the deposition process is determined experimentally by measuring the deposition rate of sputtered materials. This is commonly done by performing thickness measurements on deposited films and determining the deposited mass per time unit on the substrate. From these measured values the sputter rate could be determined, which is defined as the power delivered to the target surface per time unit. A detailed model for the determination of deposition rates has been developed by Fowlkes.36

Sputtered atoms pass through the plasma between the target and the substrate that contains Ar atoms and Ar+_{ions, as well as other sputtered atoms. A sputtered atom can} collide with these particles in its path. If the mass of the atom is much higher than that of the other particle, the path of the atom will be altered slightly. In contrast, sputtered atoms that are much lighter than the plasma atoms are deflected at a large angle. This collision interaction also influences the angular dependence of the deposition rate. With every collision, the sputtered atom transfers some of its kinetic energy to the atom with which it collides. If the atom makes enough collisions with plasma atoms, its energy will be reduced to the thermal energy of the Ar atoms in the chamber. If an atom is

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19 thermalized, it will only propagate through the chamber by diffusion. This is an undesired effect, since the direction of the diffusion cannot be controlled and the rate of atoms reaching the substrate through diffusion is very low. The thermalization distance of sputtered atoms at a pressure of 1 x 10-4_{bar is typically 1-5 cm.}37

1.4.2. Sputtering semiconducting and insulating materials.

When sputtering high band gap semiconducting or insulating materials, a DC power supply does not suffice for creating a stable plasma. The potential at the surface of the insulating target is much closer to ground potential than the actual potential that is applied to the electrode. The Ar+_{ions do not sputter the surface of the insulating target,} but instead accumulate on the substrate surface. Any sputtered secondary electrons will recombine with the Ar+_{ions, creating a shortage of generated secondary electrons to} ionize Ar atoms.

The solution for sputtering insulating targets is connecting a radio frequency (RF) generator to the electrode. This causes the potential at the surface of the target to alternate between positive and negative charge. The path of electrons in the plasma is influenced by the alternating electric field and start moving back and forth through the plasma. This increases their kinetic energy to above the threshold required for Ar ionization. The electrons that are created with succeeding ionizations sustain the plasma. RF sputtering therefore does not rely on generation of secondary electrons by sputtering. In the period of positive charge, electrons are attracted to the target and accumulate on the target surface. To prevent arcing on the target surface as a consequence of the charge build-up, it is important that the potential is negative for almost the entire RF period. The frequency of the potential oscillation should be above 1 MHz, but is typically at 13.56 MHz. In this frequency range, the potential change only influences the path of the electrons. The much heavier Ar+_{ions respond to the time-averaged negative potential V}

T on the target surface. That way, only a small fraction of the electron in the plasma accumulate on the surface during the period of positive potential. The resulting oscillating potential is shown schematically in Fig. 1.7, where VP is the potential of the plasma (about +20 V).

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Fig. 1.7: Schematic plot of the potential of an RF target over time. Here Vp is the potential of the plasma

and VT the time-averaged potential of the target.

Note that conducting targets can also be sputtered with an RF supply. In practice, this is done by placing a capacitor in between the target and the RF source, to prevent the flow of DC current that transports electrons away from the target surface. However, it is more common to use the simpler DC system when sputtering conductive targets, because RF supplies are more complex and more expensive. Furthermore, operating an RF driven electric field requires much higher voltages to operate, in the order of 1012_V.

1.4.3. Optimizing the deposition rate with magnetrons.

There are several approaches to optimizing the deposition rate of the sputter process, but the most effective and flexible approach is through the use of magnetrons. In a magnetron sputter setup a magnetic field is applied parallel to the target surface that traps electrons close to the target. When secondary electrons are emitted from the target during the sputtering process, the electrons are accelerated away from the target due to the negative potential of the target. But the Lorentz force applied by the magnetic field forces the emitted electrons into a semi-circular orbit over the target. The electron is initially accelerated by both the electric and the magnetic forces, but the Lorentz force will bend the electron path back towards the target. The electron is forced back to the negatively charged target, decreasing its net kinetic energy. If the electron collides with a plasma atom in its path, ionization can occur. Otherwise, the electron reaches the target surface and it is repulsed again and performs another semi-circular motion. This electron motion under influence of the electric field and the magnetic field is shown schematically in Fig. 1.8. For a target voltage of -500 V, it is estimated that a single electron trapped in the magnetic field produces ten ions and electrons through ionization collisions.38

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Fig. 1.8: Electron motion under influence of the electric field and the magnetic field close to the target (depicted as the opaque disc on top of the magnets, which are shown as N and S). The grey lines represent magnetic field lines and the black lines represent the electron path.

In order to initiate the ionization process controlled by the magnetic field, a high density of Ar atoms is necessary to create enough new electrons through ionization collisions. Once the plasma is established, the pressure can be reduced without extinguishing it. As was previously mentioned, simple DC sputtering requires an operating pressure of 1 x 10 -4_{bar. With magnetron sputtering, after ignition the working pressure can be decreased} to 1 x 10-6_{bar. In this pressure range, the thermalization distance of sputtered atoms} increases to 50-100 cm.37_{This allows for designing a sputter setup with multiple sputter} sources. In this case, multiple materials can be sputtered at the same time, a process that is called co-sputtering. A schematic of a multi-target magnetron sputtering system is shown in Fig. 1.9.

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1.4.4. Co-sputtering on a (non)rotating substrate.

The geometry of the sputter guns also influences the path and the resulting distribution of sputtered atoms. The influence of the geometry on the distribution of sputtered atoms will be handled in more detail in chapter 3. However at this point it should be addressed that there is a difference in distribution on a rotating and a non-rotating substrate. In a multi-target sputter setup, sputter targets are typically oriented at a tilted angle between 0-45o_{from the normal of the substrate as is shown in Fig. 1.9. When two} oppositely aligned sputter guns that are oriented identically to the normal the substrate plane are sputtered simultaneously on a substrate that rotates with 30 periods per minute, the two sputtered materials are optimally mixed and a film with a homogeneous particle distribution is formed. If the substrate is not rotated during sputtering, the rate of deposited atoms from both of the sputter guns differs with position on the substrate. Instead a composition gradient will form, a process called combinatorial sputtering. This is also the main sputtering approach that is used for the thin film deposition described in this thesis.

1.4.5. Reactive sputtering.

Sputter deposition of a compound target with Ar as sputter gas can yield a thin film with a deficiency of the lighter element in the compound. Sputtered atoms of lighter elements typically reach the substrate at a lower rate, because they are deflected more strongly from their path than the heavier atoms by collisions with gas particles in the chamber. This effect is typically observed with metal oxides or nitrides. When mixing the Ar gas with O2 or N2, additional oxidation or nitridation reactions occur on the surface of the substrate and the sputter target. This process is called reactive sputtering.

Depending on the partial pressure of the reactive gas in the chamber, it is possible to form a thin film with the same stoichiometry as the target material, or even with an abundance of the lighter element. In addition, reactive sputtering can be applied to deposit a compound thin film from a metal target. This method is applied in chapters 4-6, where Ca, Si and Eu targets are sputtered in an Ar/N2 atmosphere, resulting in a thin film consisting of CaNx, SiNx and EuNx.

1.5. Outline of this thesis

A number of applications of luminescent thin films in photovoltaic devices have been discussed in the introduction of this thesis. A range of rare earth doped materials have been introduced as candidates for luminescent thin films, all of which have been reported as phosphors in powder form. The aim of the work presented here is to develop a thin

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23 film combinatorial sputter deposition technique, for which the composition and doping concentration varies with position in the film.

In Chapter 2, a comprehensive review of luminescent thin film materials and deposition techniques using magnetron sputtering is presented. The review includes the method of sputtering, the type of host crystal and the type of dopant, post-deposition treatment and the main luminescence characteristics. The final part of the chapter serves as an introduction to the combinatorial sputter deposition technique applied in further chapters.

Chapter 3 presents a theoretical model for predicting the atom ratio in thin films of elements that are sputtered from different targets in a reactive sputtering processes. Predicting deposition rates of targets in a reactive sputtering experiment is problematic. The mass deposition rates of different target materials are calibrated using EDS measurements. The atomic ratio of the elements in the film is calculated using an approach that can be widely applied reactive sputter deposition of any combination of materials.

In chapters 4 and 5, the deposition method for spectral shifting thin films of Eu2+_silicates and silicon oxynitrides is demonstrated. By using a slow anneal treatment in a ceramic furnace, luminescent oxide compounds are formed (chapter 4). If instead an anneal treatment with a very high ramp rate is performed, oxynitride compounds form (chapter 5).

In chapter 6 the effect of rapid thermal processing on the formation of luminescent thin films is studied in more detail. Due to the non-homogeneous heating rate of thin films deposited on c-Si substrates by the rapid thermal processing technique, an as-deposited amorphous nitride thin film oxidizes into a luminescent thin film consisting of different crystalline oxide and oxynitride compounds. These compounds are characterized and a mechanism for the crystallization and the oxidation reaction of the thin film is proposed. Chapter 7 discusses combinatorial magnetron sputtering as a thin film deposition technique for depositing NaCl thin films doped with divalent Tm on glass for LSC applications. Broad band visible light absorption and infrared line emission is observed from the film. By performing combinatorial sputtering of NaCl and Tm, a Tm-Na concentration gradient is formed that is used to determine an atomic ratio of Tm to Na that results in an optimal luminescence efficiency.

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24

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12_{O. M. ten Kate, M. de Jong, H. T. Hintzen, and E. van der Kolk, Efficiency enhancement} calculations of state-of-the-art solar cells by luminescent layers with spectral shifting, quantum cutting, and quantum tripling function. J. Appl. Phys. 114 (2013) 084502. 13_{W. Witte, S. Spiering, D. Hariskos, Substitution of the CdS buffer layer in CIGS thin-film} solar cells. Vakuum in Forschung und Praxis 26, (2014) 23-27.

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25 15_{R. Hezel, R. Schorner, Plasma Si nitride-A promising dielectric to achieve high-quality} silicon MIS/IL solar cells. J. Appl. Phys. 52 (1981) 3076.

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17_{Y.Q. Li, J.E.J. van Steen, J.W.H. van Krevel, G. Botty, A.C.A. Delsing, F.J. DiSalvo, G. de} With, H.T. Hintzen, Luminescence properties of red-emitting M2Si5N8:Eu2+ (M = Ca, Sr, Ba) LED conversion phosphors. J. All. Comp. 417 (2006) 273.

18_{W.H. Weber and J. Lambe, Luminescent greenhouse collector for solar radiation. Appl.} Opt. 15 (1976) 2299-2300.

19_{A. Goetzberger, W. Greube, Solar energy conversion with fluorescent collectors. Appl.} Phys. A: Mater. Sci. Process. 14 (1977) 123-139.

20_{J. S. Batchelder, A. H. Zewail, T. Cole, Luminescent solar concentrators. 1: Theory of} operation and techniques for performance evaluation. Appl. Opt. 18 (1979) 3090-3110. 21_{B. Norton, P. C. Eames, T. K. Mallick, M. J. Huang, S. J. McCormack, J. D. Mondol, Y. G.} Yohanis, Enhancing the performance of building integrated photovoltaics. Sol. Energy 85 (2011) 1629.

22_{M. G. Debije, P. C. Verbunt, Thirty years of luminescent solar concentrator research:} Solar energy for the built environment, Adv. Energ. Mater. 2 (2012) 12-35.

23_{O. M. ten Kate, K. M. Hooning, E. van der Kolk, Quantifying self-absorption losses in} luminescent solar concentrators. Appl. Opt. 52 (2014) 5238-5245.

24_{R. W. Olson, R. F. Loring, M. D. Fayer, Luminescent solar concentrators and the} reabsorption problem. Appl. Opt. 20 (1981) 2934-2940.

25_{T. Dienel, C. Bauer, I. Dolamic, D. Brühwiler, Spectral based analysis of thin film} luminescent solar concentrators. Sol. Energy 84 (2010) 1366-1369.

26_{Z. Krumer, S. J. Pera, R.J.A. van Dijk-Moes, Y. Zhao, A. F. P. de Brouwer, E. Groeneveld,} W. G. J. H. M. van Sark, R. E. I. Schropp, C. de Mello Donegá, Tackling self-absorption in luminescent solar concentrators with type-II colloidal quantum dots. Sol. Energy Mater. Sol. Cells 111 (2013) 57-65.

27_{F. Meinardi, A. Colombo, K. A. Velizhanin, R. Simonutti, M. Lorenzon, L. Beverina, R.} Viswanatha, V. I. Klimov, S. Brovelli, Large-area luminescent solar concentrators based on ‘Stokes-shift-engineered’ nanocrystals in a mass-polymerized PMMA-matrix. Nature Photonics 8 (2014) 392-399.

28_{N.D. Bronstein, L. Li, L. Xu, Y. Yao, V.E. Ferry, A.P. Alivisatos, R.G. Nuzzo, Luminescent} solar concentration with semiconductor nanorods and transfer-printed micro-silicon solar cells. ACS Nano 8 (2014) 44-53.

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29_{I. Coropceanu, M.G. Bawendi, Core/shell quantum dot based luminescent solar} concentrators with reduced reabsorption and enhanced efficiency. Nano Lett. 14 (2014) 4097-4101.

30_{C.S. Erickson, L.R. Bradshaw, S. McDowall, J.D. Gilbertson, D.R. Gamelin, D.L. Patrick,} Zero re-absorption doped-nanocrystal luminescent solar concentrators. ACS Nano 8 (2014) 3461-3467.

31_{O. M. ten Kate, K. W. Krämer, and E. van der Kolk, Self-Absorption Free, Ultra}

Broad-Band Absorbing, Tm2+_{Doped Halides for Highly Efficient Luminescent Solar}

Concentrators. under review, Sol. Energy Mater. Sol. Cells, unpublished results. 32_{W. D. Westwood, Sputter Deposition, AVS, 2003, ISBN: 0-7354-0105-5.}

33_{N. Matsunami, Y. Yamamura, Y. Itikawa, N. Itoh, Y. Kazumata, S. Miyagawa, K. Morita,} R. Shimizu, H. Tawara, Energy dependence of the ion-induced sputtering yields of monatomic solids, At. Data Nucl. Data Tables, 31 (1984), 1–80.

34_{J. Bohdansky, A universal relation for the sputtering yield of monatomic solids at normal} ion incidence, Nucl. Instr. Meth. B, 2 (1984) 587–591.

35_{M.P. Seah, An accurate semi-empirical equation for sputtering yields, II: for neon, argon} and xenon ions, Nucl. Instr. Meth. B, 229 (2005) 348–358.

36_{J.D. Fowlkes, J.M. Fitz-Gerald, P.D. Rack,}_{Ultraviolet emitting (Y}

1−xGdx)2O3−δ thin films deposited by radio frequency magnetron sputtering: Combinatorial modeling, synthesis, and rapid characterization. Thin Solid Films 510 (2006) 68-76.

37_{W.D. Westwood, Calculation of depositon rates in diode sputtering systems, J. Vac. Sci.} Technol. 15 (1978).

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27

DC/RF magnetron sputtering for deposition of

luminescent thin films

A review of deposition of luminescent thin films obtained by magnetron sputtering is presented. Various aluminate, carbide, nitride, oxide, oxynitride, phosphate, selenide, silicate, sulphide vanadate thin films are presented. The reported luminescence emission originates from rare-earth metal-ions and transition metal-ions, as well as C, Si or Au nanoclusters, band-to-band recombinations, film defects, vacancies and interstitials. The direct sputtering, co-sputtering, combinatorial sputtering and reactive sputtering processes are listed, as well as post-deposition treatments. The reported luminescence excitation wavelength is listed as well as the maximum of the emission (band or line emission), resulting from photo-, electro-, or cathodoluminescence. The resulting review gives an understanding of the wide range of possibilities that magnetron sputter deposition provides.

2.1. Introduction

Luminescent materials have a wide range of possible applications, ranging from radiation detectors and medical scanners to field emission displays, light emitting diodes and photovoltaic devices. Among these are devices where the luminescent material is coated as a thin layer on a surface. This is of particular interest for application into devices like flat panel displays (FPDs), thin film electroluminescent devices (TFELs) and photovoltaic (PV) devices.

Thin film deposition can be performed using different chemical and physical processes. Chemical deposition techniques are commonly based on melting or evaporating source materials or precursors, which are flowed into a reaction chamber, where they react on the surface of a substrate. Opposed to chemical deposition techniques, physical deposition techniques are used for lower temperature deposition of materials with high melting and evaporation points and high binding energies. Such depositions can advantageously be carried out at relatively low temperatures compared to other (chemical) deposition techniques. Sputter deposition is one of such techniques that involves kinetic dislodging of the target material from a source onto a surface. The ability to control the rate of sputtered atoms from the target allows for a good control over the thickness of the

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28

resulting film. The use of magnetrons and radio-frequency alternating currents allow for higher sputter rates and sputtering of insulating materials, respectively.

In this chapter a comprehensive review is presented on luminescent materials that have been deposited as a thin film using magnetron sputter deposition. A table is presented that is ordered by material class and lists the source of the luminescence emission, details of the deposition, post deposition anneal treatment, luminescence excitation wavelengths and emission maxima wavelengths. In the discussion section an overview is presented of the various kinds of sputter depositions, ranging from straightforward direct sputtering of a single target on a surface to combinatorial reactive sputtering of multiple targets. The review is intended to provide the reader with a scope of the possibilities sputter deposition provides in this field.

2.2. Tabulation of luminescent thin films

The reports that were compiled for this review consist of inorganic compounds which are listed in the first column of the table. The luminescent center of each of the compounds is found in the second column. The luminescence originates from rare earth metal-ions and transition block metal-ions (Mn2+_{, Cr}3+_{, Ti}4+_{, Mo}5+_{), as well as C,} Si or Au nanoclusters, film defects, vacancies, interstitials and band-to-band recombination.

The targets that are used in the sputter deposition and the deposition method are listed in the third column. The fourth column lists the post-deposition treatment that is performed on the as-deposited film, if any was performed.

The luminescence that is observed in the resulting thin films is listed in the next two columns. The fifth column contains the excitation wavelengths that were used in the emission measurements. The sixth column lists the maxima of the different emission bands that are reported. The type of emission (band or line emission) is also indicated for each reported maximum. The outer right column lists the reference of the table entry.

Compiling the aforementioned data for a range of results in Table I that is ordered by material class, i.e. aluminates, nitrides, oxides, oxynitrides, phosphates, silicates, sulphides, vanadates and others. It is acknowledged that there are multiple reports on certain materials by the same authors, but multiple entries are omitted unless a variation on the sputter deposition was presented. This review only includes publications in which luminescence emission properties are presented.

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29 Magnetron sputter deposition is a diverse deposition technique due to the variety of approaches that are available to deposit a thin film from one or more target materials. By direct sputtering from a single material target, a thin film is deposited with a composition that is close or identical to the composition of the target. Compound films can be sputtered by co-sputtering multiple targets onto a single surface. When a film is desired that is an oxidized of nitridated phase of the target material, reactive sputtering is applied with O2 or N2 as a gas phase reactant. When depositing thin films, a homogeneous film composition is often desired. By performing a sputter deposition of multiple targets in which the deposition rate of different targets varies with position, a composition gradient is created. This combinatorial sputtering approach is often applied to deposit a single film where the composition of the host differs with position, or when a gradient in the luminescent dopant ion is desired. The details of each approach are discussed in this section and serve as an introduction to chapters 5-7, where co-sputtering, reactive co-sputtering and reactive combinatorial co-sputtering are used to deposit different materials.

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30

Host Lattice Luminescent center

Sputter targets and methods Post-deposition annealing treatment Excitation peak wavelength (nm) Emission peak wavelength (nm) Ref Aluminates

Al2O3 CeCl3 Al target with CeCl3 wafers glued to the

surface.

Reactive RF sputtered in Ar/O2

None 255 325 370 (band) 400 (band) 1 Er3+ _{Al and Er targets.}

Reactive DC/RF co-sputtered in Ar/O2.

None 660 1540 2

CaAl2O4 Eu2+ Pressed target from sintered mix of

CaCO3, Al2O3 and Eu2O3 powders.

700-1100o_{C for 3-6 hours in air and in N} 2

with 5% H2.

320 450 (band) 3

Eu2+ _{Pressed target from sintered mix of}

CaCO3, Al2O3 and Eu2O3 powders.

Reactive RF sputtered in Ar/O2.

900o_{C for 3 hours in N}₂_{with 5% H}₂_. ₃₂₅ _{445 (band)} 4

Eu2+ _{Pressed targets from sintered mix of}

CaO/BaCo3, Al2O3 and Eu2O3 powders.

Reactive combinatorial and direct RF sputtered in Ar/O2, Ar and Ar/H2.

500-1000o_{C for 30 minutes in Ar with 5%}

H2. 260 330 450 (band) 495 (band) 535 (band) 5

CuAlO2 n/a Pressed target from sintered mix of

Cu2O and Al2O3.

None 210 339 (band)

343 (band) 348 (band)

6

Sr4Al14O25 Eu2+, Dy3+ Pressed target from sintered mix of

SrCO3, Al(OH)3, Eu2O3 and Dy2O3

powders.

Direct RF sputtered in Ar.

800o_{C and 1200}o_{C for 2 hours in reducing}

atmosphere. 300, 350, 389 485 (band) 515 (band) 7

YAG Ce3+ _{Pressed target from YAG:Ce powder.}

1200o_{C for 5 hours in N}

2 atm. 450 550 (band) 8

Ce3+ _{Ceramic YAG:Ce target.}

800-1200o_{C for 10 hours in N}

2 atm. 450 550 (band) 9

Gd Y, Al and Gd targets.

Reactive combinatorial RF sputtering in Ar/O2.

1000o_{C for 10 hours in air atmosphere} _n/a _{312 (line)} 10

Carbides

SiC C-clusters

Film defects

SiC target. RF sputtered in Ar.

700o_{C, 900}o_{C and 1000}o_{C for 30 minutes}

in N2 atmosphere. 230 406, 560 (line) 588 (line) 11 SiC/ZnO Si nanocrystals ZnO defects

SiC and ZnO targets. RF co-sputtered in Ar

600-1000o_{C for 1 hour in N}₂_atmosphere. ₂₅₀ _{370, 380 (line)}

395, 412 (line)

12

Nitrides

AlN Er3+ _{Al target with small pieces of Er on top.}

Reactive RF sputtered in Ar/N2

None 325 1540 (line) 13 Er3+ _{Al and Er targets.} Reactive RF co-sputtered in N2. None 210 479, 538 (line) 557 (line) 14

Tb3+ _{Al Target with pressed Tb metal slug.}

Reactive RF sputtered in O2/N2

None 492 543 (line) 15

Cr3+ _{Al-Cr bimetal target.}

Reactive RF sputtered in N2.

None 532 696 (line) 16

Sm3+ _{Al Target with pressed Tb metal slug.}

None 488 598 (line)

707 (line)

17

Eu3+ _{Arc melted Al/Eu(1%) target.}

None 325 619 (line) 18

Tb3+ _TbF₃_{and Al targets.}

Reactive RF co-sputtering in Ar/N2.

None 236 490, 545 (line)

590, 625 (line)

19

Tb3+ _{Arc melted Al/Tb(1%) target.}

None 325

488

490, 550 (line) 590, 625 (line)

20

Tm3+_{, Sm}3+ _{Al Target with pressed Tm or Sm metal}

slug.

Reactive RF sputtered in N2

None n/a 371, 467 (line)

480, 802 (line) 564, 600 (line) 648, 707 (line)

21

Pr3+ _{Al Target with pressed Pr metal slug.}

Reactive RF sputtered in N2

None n/a 648, 526 (line)

488, 505 (line)

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31 AlN (continued) Tm3+_{, Tb}3+_, Dy3+_{, Eu}3+_, Sm3+_{, Yb}3+

Al target covered with 1% RE on top in. Reactive RF sputtered in N2 300-1100o_{C for 30 minutes in N} 2 atmosphere 488 575, 615 (line) 660, 720 (line) 23

BN Crystal defects Target not mentioned.

None n/a 233 (line)

281, 323 (line) 335 (band)

24

GaN Eu3+ _{Mix target of GaN and EuN.}

1000o_{C for 1 hour in NH}₃_atmosphere. ₃₂₅ ₆₂₂ 25

Mn2+ _{Ga target.}

None none (CL) 450 (band)

690 (band)

26

Tb3+ _{Ga target covered with thin Tb sheet.}

None 325 497, 552 (line)

594, 628 (line)

27

Tb3+ _{Tb target and sintered Ga}₂_O₃_target.

Combinatorial RF sputtered in Ar.

950o_{C for 15 minutes in NH}₃_atmosphere. _n/a _{369 (band)} 28

SiCN c-clusters band-to-band recombination

Sintered SiC target. Reactive RF sputtered in Ar/N2.

600o_{C, 800}o_{C and 1100}o_{C for 5 minutes in}

N2 atmosphere.

266 400 (band)

440 (band)

29

SiNx/SiO2 Recombi-

nation in SiO2.

Si target.

Reactive RF sputtered in Ar/N2/H2.

650-1200o_{C for 20 minutes in N}₂

atmosphere.

514.5 610 30

a-SiN Er3+_{, Sm}3+ _{Si target covered with Er/Sm platelets.}

Reactive RF sputtered in N2. None 488 520, 550 (line) 660 (line) 560, 600 (line) 650 (line) 31

Nd3+ _{pc-Si target covered randomly with Nd}

metal pieces. Reactive RF sputtered in N2.

200, 400, 600 800 and 1000o_{C in 15}

minute periods in Ar atmosphere.

514,5 920 (line)

1100 (line) 1400 (line)

32

Er3+ _{Si target covered with metallic Er}

platelets. RF sputtered in Ar. None 488 514.5 1000 (line) 1150 (line) 1540 (line) 33 Oxides BaTiO3 Defects O vacancies

Pressed target of sintered BaTiO3

powder. RF sputtered in Ar

500, 700, 800 and 900o_{C for 2 hours in O} 2 atmosphere. 335 373 (band) 450 (band) 475, 525 (line) 34

CaTiO3 Pr3+ Sintered powder target.

RF sputtered in Ar.

700-900o_{C for 6 hours in Ar atm.} ₃₅₀ _{615 (line)} 35

Pr3+ _{Sintered powder target.}

700o_{C for 3 hours in vacuum.} ₃₅₀ _{612 (line)} 36

Cu2O Crystal defects Cu target.

Reactive RF sputtered in Ar/N2/O2.

None. 325 680 (line) 37 Er2O3 Er3+ Er2O3 target. RF sputtered in Ar. 800-1200o_{C for 30s or 1 hour in O} 2 atmosphere. 488 1540 38 Gd3Ga5O12 SiO2 nanocrystals

SiO2, Gd2O3, Ga2O3, CeO2, EuF3 Tb4O7,

Ag, TiO2, Mn3O4, ZnO and Y2O3 targets.

DC/RF combinatorial sputtered in Ar.

200°C for >150 hours in air, 400°C for 24 hours in air, 600°C for 12 hours in air and 1000°C for 2 hours in Ar atmosphere.

254 440-500 39

(Gd2-xZnx)O3-δ Eu3+ GdF3,La2O3,Y2O3,Ta2O5, Zr, WO3, Mo,

ZnO, Al2O3, MgO, SrCO3,TmF3, EuF3,

TbF3 and CeF3 targets.

Combinatorial RF sputtering in Ar.

1100-1400o_{C for up to 4 hours in O} 2, H2, He and Ar. 250 270 610 (line) 620 (line) 40

HfO2 Crystal defects

Ce3+

Ceramic HfO2 and CeO2 targets

Reactive RF co-sputtered in Ar/O2

1100o_{C for 1 hour in air or vacuum.} ₃₂₅ _{385 (band)}

450 (band)

41

La2O3 Bi3+,

RE (Dy, Er, Eu, Tb or Tm)

Pressed target from sintered La2O3,

Bi2O3 and RE2O3 powders.

RF sputtered in Ar.

1050o_{C for 1 hour in Ar atmosphere.} ₂₀₀

300 320 450 (band) 480 (band) 525, 550 (line) 575, 620 (line) 42 La2O3-Gd2O3 -Y2O3

Bi3+ _{Pressed target from sintered La}₂_O₃_,

Gd2O3 and Y2O3 powders.

RF sputtered in Ar.

800-1050o_{C for 1 hour in Ar atmosphere} ₃₃₀

372 420 (band) 450 (band) 500 (band) 43 LaNbO4 GdNbO4 YNbO4

Bi3+ _{Pressed target from sintered}

La2O3/Gd2O3/Y2O3, Nb2O5 and Bi2O3

powders.

Reactive combinatorial RF sputtered in Ar or Ar/H2.

700-1000o_{C for 1 hour in air or Ar}

atmosphere. 250 300 420 (band) 450 (band) 500 (band) 44

MgWO4 Crystal defects Pressed target from sintered MgO and

WO3 powders.

650, 750 or 850o_{C for 2 hours.} _n/a _{475 (band)} 45

MoO3 Mo5+ Pressed MoO3 powder target.

RF sputtered in Ar.

200, 300 and 400o_{C for 1 hour in air.} ₂₅₀ _{350 (band)}

470 (line) 46 SiOx TiO2 ZnO Au nanoparticles

SiO2, Ti, Zn and Au targets.

Reactive RF co-sputtered in Ar/O2.

950-1050o_{C or 600-700}o_{C for 1 second to} 2 hours. 514 600 (band) 630 (band) 660 (band) 47

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32

SnO2 Sb3+, Sb5+ Pressed target from sintered SnO2 and

Sb2O3 powders.

400o_{C in air} ₃₂₅ _{390 (band)} 430 (band) 520 (band) 48 Nanocrystals and defects Pc-Sn target.

None n/a 400 (band) 49

Defects O vacancies

Ti target.

Reactive DC sputtered in Ar/O2

None. 325 400 (band)

434 (band) 505 (band)

50

Ta2O5 Er3+ pressed target from Er:Ta2O5 powder

mixed with 1% Er2O3.

450, 500, 550 and 600o_C. ₉₈₀ _{1540 (line)} 51

Er3+ _Ta₂_O₅_{target covered with Er}₂_O₃_tablets.

RF sputtered in Ar.

600-1100o_{C for 10-40 minutes in air} ₃₂₅ _{550 (line)}

670 (line)

52

Ta2Zn3O8 Mn2+ Ta and ZnO targets.

Rf co-sputtered in Ar.

1100o_{C in N}₂_atmosphere ₂₂₀ _{385 (band)}

425 (band) 500 (band)

53

TiO2 Eu3+ Metallic Ti-Eu mosaic target

Reactive RF sputtered in O2.

200o_{C, 400}o_{C and 800}o_{C for 2 hours in air}

atmosphere.

302 620 (line) 54

Eu3+_{, Tb}3+_,

Nd3+

metallic targets Ti-Eu, Ti-Tb and Ti-Nd. Reactive RF sputtered in O2.

200o_{C and 600}o_C ₃₀₂ _{545 (line)}

620 (line) 900 (line)

55

O-vacancies Ceramic TiO2 target

RF sputtered in Ar

600, 700, 800 or 1000o_{C for 2 hours in air.} ₃₂₀ _{370 (line)}

450 (band)

56

WO3 O-defects W target.

None. 375 410 (band) 460 (band) 57 Y2Ge2O7 Y2GeO5 Y4GeO8

Mn2+ _{Pressed target of sintered Y}

2O3, GeO2

and MnO2 powders.

RF sputtered in Ar

970-1070o_{C for 1 hour in Ar} ₃₃₃ _{575 (band)} 58

Y2O3-Ge2O3 Mn2+ Pressed target of sintered Y2O3, Ge2O3

and MnO2 powders.

RF sputtered in Ar.

900-1100o_{C for 1 hour in Ar atmosphere} _n/a _{500 (band)} 59

(Y1-xGdx)2O3-δ Gd Gd foil, Y metal targets.

Reactive combinatorial RF sputtering in Ar/O2

850o_{C for 12 hours.} _n/a _{312 (line)} 60

ZnGa2O4 Mn2+ Sintered ZnGa2O4 and ZnGa2O4:Mn

ceramic targets. Reactive RF sputtered in Ar/O2

700o_{C for 3 hours in N}₂_. ₃₅₀ _{510 (band)} 61

Mn2+ _{Pressed target from sintered mix of}

Ga2O3, ZnO and MnO powders.

600, 700, 800 and 900o_{C for 30 minutes in}

N2/H2(5%) atmosphere.

300 510 (band) 62

Mn2+ _{Pressed target from sintered mix of}

Ga2O3, ZnO and MnO powders.

700o_{C for 3 hours in N}₂_{, air or vacuum.} ₃₅₀ _{510 (band)} 63

Mn2+ _{Pressed target from ZnGa}

2O4 powder

covered with (CH3COO)2Mn pellet.

RF sputtered in Ar.

None 290 510 (band) 64

Dy3+ _{Pressed target from sintered Ga}₂_O₃_,

ZnO and Dy2O3 powders.

RF sputtered in Ar. None 260 494 (band) 583 (band) 676 (band) 65 Ce, Mn or Tb and Cr or Eu

ZnGa2O4 powder target.

RF sputtered in Ar. 800-1100o_{C for 1-5 hours in Ar} atmosphere. n/a 420 (band) 500, 540 (line) 620, 700 (line) 66

Zn2GeO4 Mn2+ Zn2GeO4:Mn target.

700o_{C for 1 hour in air} ₂₅₆

296

537 (band) 67

ZnO Eu3+ _{metallic Zn target with Eu parts on top.}

None. 365 620 (line) 68

Er3+ _{Pressed target from sintered ZnO and}

Er2O3 powders.

700o_{C for 3 hours in air and 5% H}₂

atmosphere

385 465 (line)

525 (line)

69

Cu+ _{Zn target with Cu chips on top.}

Reactive DC sputtered in Ar/O2

None. 325 380 (line)

580 (band)

Deposition of luminescent thin films for solar energy applications