Photoresponse of ferroelectric BaTiO? thin films /
- Autores: Fanmao Liu
- Directores de la Tesis: Josep Fontcuberta i Griñó (dir. tes.), Ignasi Fina Martínez (codir. tes.), Jordi Sort Viñas (tut. tes.)
- Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2017
- Idioma: español
- Tribunal Calificador de la Tesis: Brahim Dkhil (presid.), Mariano Campoy Quiles (secret.), Ingrid Laura Cañero Infante (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia de Materiales por la Universidad Autónoma de Barcelona
- Materias:
- Enlaces
- Resumen
BaTiO3 is a lead-free ferroelectric oxide material. Its unique electric, optic, mechanic, acoustic, thermodynamic properties and the coupling between them have been widely studied in the past half-century. Research on BaTiO3 thin films becomes an important area since 1980s due to the development of thin films’ preparation methodologies, making possible to integrate them in semiconductor technology and to achieve micro/nano scaled non-volatile memories. Nowadays, the scientific community has renewed the interest on BaTiO3 by focusing the attention on its interaction with light, because it holds promising characteristics for rapidly developing areas such as photovoltaics, photoelectric sensing and photocatalysis.
The main objective of this thesis is to understand photoelectric effects on nanometric BaTiO3 thin film and give light on relevant mechanisms that control them, aiming to build a more comprehensive understanding of its properties and to guide potential applications. Photoelectric effects studied in this thesis mainly refer to short-circuit photocurrents (Jsc) and the effect of light on polarization (P), focusing on the following aspects: 1. Influence of imprint Eimp and depolarization Edep fields on the photoresponse in BaTiO3 thin films. Eimp is an internal electric field that commonly exists in ferroelectric thin films owing to strain gradient, pinned ferroelectric domains, differences on metallic properties of top and bottom electrodes, etc. Edep is an internal electric field originated from imperfect screening of polarization. Obviously photocarriers generated by a suitable illumination will be sensitive to both Eimp and Edep and they can modify P or be driven towards an external charge circuit/reservoir (Jsc). We have aimed here to disentangle the relative role of Eimp and Edep on P and Jsc. It is shown that by changing the electrode configuration, namely its symmetry, relative contribution of Eimp and Edep can be modified. This allows revealing how the Jsc depends on Eimp and Edep, and how the polarization switching of these nano ferroelectric capacitors operates.
2. Influence of microstructure and surface adsorbates on the photoresponse in BaTiO3 thin films. Polarization screening in ferroelectric thin films can be achieved either by internal (internal screening) or external charges (electrodes and/or adsorbates). Therefore, the microstructure of ferroelectric films, epitaxy-induced strains and strain gradients, point defects distribution and their nature, surface morphology and surrounding atmosphere, should have a critical role in polarization screening. Disentangling these coexisting contributions is challenging. Here we have focused on the role of strain acting on the films and of the ubiquitous water adsorbates. We show that the latter (water adsorbates) can be used to trigger enhanced changes of the switchable polarization under suitable illumination. On the other hand, it has been found that strain relaxation leads to coexistence of BaTiO3 phases with different strain states and different ferroelectric Curie temperatures, which its effects on photocurrent have not been disclosed.
3. Attempt to narrow the band-gap of BaTiO3 thin films. Although significant photoresponses have been observed and used in previous sections, this part requires using light of the most energetic band of the solar spectrum. It results from the fact that BaTiO3 has a rather large band-gap thus most of the solar radiation cannot generate photocarriers. Therefore, either inducing band-gap states or directly reducing the band-gap are two possible strategies to enhance the absorbance of BaTiO3 to solar light and its efficiency. N substitution at oxygen sites appears a promising strategy due to the smaller electronegativity of N compared to O. Therefore, partial substitution of oxygen by forming some kind of oxynitride BaTiO3−xNx could be advantageous. Insertion of N cannot be easily done during the thin film growth but annealing films in NH3 could be an alternative way. Some preliminary steps in this direction have been undertaken. Although much work remains to be done, the available results show that the photoresponse of BaTiO3 films can be enlarged by suitable ammonia annealing and ferroelectricity is preserved. This opens new opportunities for BaTiO3 ferroelectric thin films in photoresponsive applications.
