Abstract Matthijn Dekkers

Summary

This thesis describes the research on thin films of transparent conducting oxides (TCOs) on polymeric substrates manufactured by pulsed laser deposition (PLD). TCOs are an indispensable part in optoelectronic applications such as displays, solar cells, light-emitting diodes, etc. At present, in many of these applications there is an increasing need for the use of flexible, cheap and light-weight substrates. Such polymer substrates however, limit the deposition temperature of thin films which results in deteriorated properties of the TCO. A profound understanding of the fundamental aspects of transparent conductors is required in order to improve either the properties of existing materials, or design new types of TCOs. These insights are of great scientific importance for the realization of high performance TCOs on polymer substrates.

This research focuses on thin film growth by PLD. This technique is a powerful tool for thin film research. A large freedom of choice in independently controllable deposition parameters allows one to quickly obtain results on the exploration and optimization of existing and new materials. The ablated species can be tuned over a large energetic range, enabling optimum conditions at lower substrate temperatures normally used for high performance TCO materials. This makes the deposition of TCOs on heat resistive substrates possible. The substrates utilized in this research are all commercial available and commonly used materials such as translucent polyethylene terephthalate (PET). Analysis of the PLD grown films is done by a variety of tools to obtain information on the electrical, optical and structural properties as well as the thin film composition.

Different TCO materials for deposition on polymers are investigated: SnO₂‑doped In₂O₃ (ITO), pure and Al₂O₃-doped ZnO (AZO) and the In₂O₃‑ZnO compound (IZO). These materials are all n-type wide bandgap semiconductors (E_g>3 eV). Their electrical and optical performance is in a large extent determined by fundamental properties as structure and composition, which can be tuned and manipulated in the deposition process.

ITO is a commonly used TCO in optoelectronic applications. In this work the deposition process of ITO on polymeric substrates is optimized. By careful control of the PLD parameters, films with high optical transmittance (>85%) and low resistivity (~5 x10^-4 Ωcm) are grown at room temperature on PET substrates. The influence of Sn-doping in In₂O₃ thin films on conductance, transmission, and granular structure is studied. It is found that the Sn-dopant is not thermally activated and does not contribute to enhanced conductivity in room temperature deposited films. The electrical properties in these ITO films are therefore governed by the oxygen vacancies, resulting in a strong dependence on the oxygen deposition pressure. Only in a narrow deposition pressure regime (0.012-0.017 mbar) high quality ITO thin films on polymer substrates can be grown. Somewhat higher pressure, in the order of only a few thousandth of a millibar compared to the optimum, results in decreased amount of oxygen vacancies, hence lower carrier density. The resistivity increases and the bandgap narrows due to the Burstein-Moss shift. On the other hand, a lower pressure results in higher carrier density. However, the increased ionized impurity scattering lowers the free carrier mobility, also causing a higher resistivity.

The resistivity of room temperature grown thin films ablated from Sn-doped targets is higher compared to samples of pure In₂O₃. A relation between the structural nature of the thin film and the amount of Sn doping is observed. The size of the formed grains during growth decreases at higher doping content. The obtained grain sizes are in the order of the mean free path of the charge carriers. The grain boundaries of these nanocrystalline films influence their electrical properties. Due to increased scattering, the charge carrier mobility is lowered. Sn-doped films with a higher grain boundary density compared to undoped samples are therefore more resistive.

TCOs composed of (doped) ZnO and Zn containing compounds are of major interest, since these materials are regarded as replacement for ITO. In this work undoped ZnO is deposited onto PET substrates. In order to grow smooth films of ZnO by PLD on polymers, the process parameters are different to those used for ITO deposition. The pressure is increased (0.050 mbar), whereas the fluency is lowered (1.5 J/cm²). This prevents excessive substrate heating by impingement of high kinetic energy of ablated Zn-species. Otherwise, it is this heating and the large difference in expansion coefficient between film and substrate that causes cracking of the thin film.

Similar to ITO, the properties of ZnO also showed to be strongly dependant on the oxygen partial deposition pressure. Optimized films show excellent optical properties (T~90%). The electrical resistivity is still a factor of ten higher compared to ITO thin films on PET. A small amount of Al₂O₃ doping (AZO) enhances the electrical properties of the room temperature grown films. However, analogous to SnO₂ in In₂O₃, addition of Al₂O₃ in ZnO decreases the grain size. This increases the grain boundary scattering in AZO films with higher doping content. The optimum Al₂O₃ doping content in room temperature deposited films on PET is therefore found at 1 %wt.

The benefits of both ZnO and In₂O₃ are combined in one compound system. The properties of this material are dependant on the In₂O₃ to ZnO ratio. The optical transmission of films from the ZnO-In₂O₃ compound system on PET substrates is high and constant over the whole composition range from pure In₂O₃ to pure ZnO. The electrical resistivity can be tuned by the ZnO content as it increases one order of magnitude on going from In₂O₃ to ZnO.

In general n-type TCOs are used as electrodes, but in combination with p‑type TCOs active components can be realized. After all, the p-n junction is an essential building block in all semiconducting electronics. The lack of appropriate p‑type wide bandgap semiconductors however, hampers the development of new optoelectronic devices based on polymeric substrates. Since the use of polymer substrates requires low temperature deposition, this research focuses on materials that are less dependent on the crystalline structure compared to existing p-type TCOs. These experiments resulted in the synthesis of a new p-type TCO; ZnIr₂O₄. This material is also suitable for deposition at room temperature on polymeric substrates.

The closed d¹⁰-shell in existing p-type TCOs as cuprates, avoid coloration of the material. However, transparency can also be achieved by using d⁶-transition-metal cations in octahedral geometry surrounded by oxygen ligands. A “quasi-closed” shell is induced since the ligands cause a splitting of the d-bands in a completely filled t_2g⁶ and empty e_g⁰ level. The bandgap of the material is determined by the energy split in between these levels. This phenomenon is observed in spinel ZnM₂O₄ thin films, where element M is a d⁶- transition metal.

ZnM₂O₄ thin films, where M=Co, Rh and Ir, are pulsed laser deposited on quartz and crystalline Al₂O₃ (0001) substrates at temperatures between 773 and 973 K. The material is ablated from targets obtained by solid state synthesis from binary oxide powders. The crystal structure is found to be spinel of both polycrystalline films on quartz and epitaxial films on Al₂O₃. The electronic band structure of the grown materials reveals that the valence band maximum is composed of occupied t_2g⁶ states. The observed bandgap is increasing for higher quantum number of element M, being as large as ~3 eV for ZnIr₂O₄. This increasing bandgap is expected from ligand field theory and scales with the theoretical predictions. P-type conductivity is confirmed in all compounds by positive Seebeck coefficients and the position of the Fermi level with respect to the valence level. The conductivity of ZnIr₂O₄ is well above 2 Scm^-1, whereas the optical transmission is around 60%.

Room temperature deposition of ZnIr₂O₄ on PET substrates results in amorphous films. A special deposition method referred to as “eclipse PLD” is used to avoid droplet formation on the film surface. The material cannot be identified as single phase spinel, but merely as a non-stoichiometric ZnO:x·IrO₂ compound, where 1.2<x<1.7. However, the film is hydrated and the majority of iridium in the film appears in a trivalent valency. Nanocrystals of about a few nanometers containing trivalent iridium in octahedral surrounding are dispersed in the Zn²⁺‑matrix. The network of octahedrons forms a conduction path for the positive holes generated by excess oxygen and cationic vacancies. The p-type conductivity of these films deposited on PET substrates is as large as 2 Scm^-1. The film is transparent up to 70% in the visible, and a 50% transparency is observed in the UV regime.

The film is used as p-layer in a junction device based on silicon. Diode rectifying behavior is observed and is in accordance with the different bandgap values of the separate p- and n-layer. A device deposited on a polymer substrate using ZnO as n-layer is also tested. Although the reproducibility of this device is low due to the critical manufacturing process, diode rectifying behavior is also observed in these junctions. This demonstrates the applicability of this material in devices containing all-amorphous wide bandgap semiconductors on polymeric substrates.

Depositions at low temperature for applications on polymers involve nanocrystalline and amorphous materials. The research topics in this thesis demonstrate that the understanding of the fundamental properties of wide bandgap semiconductors with such structures is essential. This knowledge is employed for the optimization of the deposition process, leading to the improvement of polymer-TCO systems. Comprehension of the fundamental aspects down to the atomic level resulted in design and synthesis of a new p-type TCO. The performance of this TCO is high in crystalline as well as the amorphous phase. The obtained results can contribute to the realization of transparent electronics on heat resistive substrates in the near future.

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Home News Events Strategic Orientations research groups Publications PhD students Education MESA+ NanoLab Starting entrepreneurs Tech Park Industrial Partners Vacancies Links Sitemap Search	Abstract Matthijn Dekkers Summary This thesis describes the research on thin films of transparent conducting oxides (TCOs) on polymeric substrates manufactured by pulsed laser deposition (PLD). TCOs are an indispensable part in optoelectronic applications such as displays, solar cells, light-emitting diodes, etc. At present, in many of these applications there is an increasing need for the use of flexible, cheap and light-weight substrates. Such polymer substrates however, limit the deposition temperature of thin films which results in deteriorated properties of the TCO. A profound understanding of the fundamental aspects of transparent conductors is required in order to improve either the properties of existing materials, or design new types of TCOs. These insights are of great scientific importance for the realization of high performance TCOs on polymer substrates. This research focuses on thin film growth by PLD. This technique is a powerful tool for thin film research. A large freedom of choice in independently controllable deposition parameters allows one to quickly obtain results on the exploration and optimization of existing and new materials. The ablated species can be tuned over a large energetic range, enabling optimum conditions at lower substrate temperatures normally used for high performance TCO materials. This makes the deposition of TCOs on heat resistive substrates possible. The substrates utilized in this research are all commercial available and commonly used materials such as translucent polyethylene terephthalate (PET). Analysis of the PLD grown films is done by a variety of tools to obtain information on the electrical, optical and structural properties as well as the thin film composition. Different TCO materials for deposition on polymers are investigated: SnO₂‑doped In₂O₃ (ITO), pure and Al₂O₃-doped ZnO (AZO) and the In₂O₃‑ZnO compound (IZO). These materials are all n-type wide bandgap semiconductors (E_g>3 eV). Their electrical and optical performance is in a large extent determined by fundamental properties as structure and composition, which can be tuned and manipulated in the deposition process. ITO is a commonly used TCO in optoelectronic applications. In this work the deposition process of ITO on polymeric substrates is optimized. By careful control of the PLD parameters, films with high optical transmittance (>85%) and low resistivity (~5 x10^-4 Ωcm) are grown at room temperature on PET substrates. The influence of Sn-doping in In₂O₃ thin films on conductance, transmission, and granular structure is studied. It is found that the Sn-dopant is not thermally activated and does not contribute to enhanced conductivity in room temperature deposited films. The electrical properties in these ITO films are therefore governed by the oxygen vacancies, resulting in a strong dependence on the oxygen deposition pressure. Only in a narrow deposition pressure regime (0.012-0.017 mbar) high quality ITO thin films on polymer substrates can be grown. Somewhat higher pressure, in the order of only a few thousandth of a millibar compared to the optimum, results in decreased amount of oxygen vacancies, hence lower carrier density. The resistivity increases and the bandgap narrows due to the Burstein-Moss shift. On the other hand, a lower pressure results in higher carrier density. However, the increased ionized impurity scattering lowers the free carrier mobility, also causing a higher resistivity. The resistivity of room temperature grown thin films ablated from Sn-doped targets is higher compared to samples of pure In₂O₃. A relation between the structural nature of the thin film and the amount of Sn doping is observed. The size of the formed grains during growth decreases at higher doping content. The obtained grain sizes are in the order of the mean free path of the charge carriers. The grain boundaries of these nanocrystalline films influence their electrical properties. Due to increased scattering, the charge carrier mobility is lowered. Sn-doped films with a higher grain boundary density compared to undoped samples are therefore more resistive. TCOs composed of (doped) ZnO and Zn containing compounds are of major interest, since these materials are regarded as replacement for ITO. In this work undoped ZnO is deposited onto PET substrates. In order to grow smooth films of ZnO by PLD on polymers, the process parameters are different to those used for ITO deposition. The pressure is increased (0.050 mbar), whereas the fluency is lowered (1.5 J/cm²). This prevents excessive substrate heating by impingement of high kinetic energy of ablated Zn-species. Otherwise, it is this heating and the large difference in expansion coefficient between film and substrate that causes cracking of the thin film. Similar to ITO, the properties of ZnO also showed to be strongly dependant on the oxygen partial deposition pressure. Optimized films show excellent optical properties (T~90%). The electrical resistivity is still a factor of ten higher compared to ITO thin films on PET. A small amount of Al₂O₃ doping (AZO) enhances the electrical properties of the room temperature grown films. However, analogous to SnO₂ in In₂O₃, addition of Al₂O₃ in ZnO decreases the grain size. This increases the grain boundary scattering in AZO films with higher doping content. The optimum Al₂O₃ doping content in room temperature deposited films on PET is therefore found at 1 %wt. The benefits of both ZnO and In₂O₃ are combined in one compound system. The properties of this material are dependant on the In₂O₃ to ZnO ratio. The optical transmission of films from the ZnO-In₂O₃ compound system on PET substrates is high and constant over the whole composition range from pure In₂O₃ to pure ZnO. The electrical resistivity can be tuned by the ZnO content as it increases one order of magnitude on going from In₂O₃ to ZnO. In general n-type TCOs are used as electrodes, but in combination with p‑type TCOs active components can be realized. After all, the p-n junction is an essential building block in all semiconducting electronics. The lack of appropriate p‑type wide bandgap semiconductors however, hampers the development of new optoelectronic devices based on polymeric substrates. Since the use of polymer substrates requires low temperature deposition, this research focuses on materials that are less dependent on the crystalline structure compared to existing p-type TCOs. These experiments resulted in the synthesis of a new p-type TCO; ZnIr₂O₄. This material is also suitable for deposition at room temperature on polymeric substrates. The closed d¹⁰-shell in existing p-type TCOs as cuprates, avoid coloration of the material. However, transparency can also be achieved by using d⁶-transition-metal cations in octahedral geometry surrounded by oxygen ligands. A “quasi-closed” shell is induced since the ligands cause a splitting of the d-bands in a completely filled t_2g⁶ and empty e_g⁰ level. The bandgap of the material is determined by the energy split in between these levels. This phenomenon is observed in spinel ZnM₂O₄ thin films, where element M is a d⁶- transition metal. ZnM₂O₄ thin films, where M=Co, Rh and Ir, are pulsed laser deposited on quartz and crystalline Al₂O₃ (0001) substrates at temperatures between 773 and 973 K. The material is ablated from targets obtained by solid state synthesis from binary oxide powders. The crystal structure is found to be spinel of both polycrystalline films on quartz and epitaxial films on Al₂O₃. The electronic band structure of the grown materials reveals that the valence band maximum is composed of occupied t_2g⁶ states. The observed bandgap is increasing for higher quantum number of element M, being as large as ~3 eV for ZnIr₂O₄. This increasing bandgap is expected from ligand field theory and scales with the theoretical predictions. P-type conductivity is confirmed in all compounds by positive Seebeck coefficients and the position of the Fermi level with respect to the valence level. The conductivity of ZnIr₂O₄ is well above 2 Scm^-1, whereas the optical transmission is around 60%. Room temperature deposition of ZnIr₂O₄ on PET substrates results in amorphous films. A special deposition method referred to as “eclipse PLD” is used to avoid droplet formation on the film surface. The material cannot be identified as single phase spinel, but merely as a non-stoichiometric ZnO:x·IrO₂ compound, where 1.2<x<1.7. However, the film is hydrated and the majority of iridium in the film appears in a trivalent valency. Nanocrystals of about a few nanometers containing trivalent iridium in octahedral surrounding are dispersed in the Zn²⁺‑matrix. The network of octahedrons forms a conduction path for the positive holes generated by excess oxygen and cationic vacancies. The p-type conductivity of these films deposited on PET substrates is as large as 2 Scm^-1. The film is transparent up to 70% in the visible, and a 50% transparency is observed in the UV regime. The film is used as p-layer in a junction device based on silicon. Diode rectifying behavior is observed and is in accordance with the different bandgap values of the separate p- and n-layer. A device deposited on a polymer substrate using ZnO as n-layer is also tested. Although the reproducibility of this device is low due to the critical manufacturing process, diode rectifying behavior is also observed in these junctions. This demonstrates the applicability of this material in devices containing all-amorphous wide bandgap semiconductors on polymeric substrates. Depositions at low temperature for applications on polymers involve nanocrystalline and amorphous materials. The research topics in this thesis demonstrate that the understanding of the fundamental properties of wide bandgap semiconductors with such structures is essential. This knowledge is employed for the optimization of the deposition process, leading to the improvement of polymer-TCO systems. Comprehension of the fundamental aspects down to the atomic level resulted in design and synthesis of a new p-type TCO. The performance of this TCO is high in crystalline as well as the amorphous phase. The obtained results can contribute to the realization of transparent electronics on heat resistive substrates in the near future.