GROWTH AND FUNCTIONALITIES OF VANADATE THIN FILMS

Open Access
Author:
Zhang, Haitian
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 04, 2017
Committee Members:
  • Roman Engel-Herbert, Dissertation Advisor
  • Roman Engel-Herbert, Committee Chair
  • Venkatraman Gopalan, Committee Member
  • Long-Qing Chen, Committee Member
  • Chris Giebink , Outside Member
Keywords:
  • Molecular beam epitaxy
  • Thin film
  • Strongly correlated oxides
  • Transition metal oxides
  • Metal to insulator transition
  • VO2
  • LaVO3
Abstract:
Transition metal oxides have attracted tremendous research interest due to their fertile functional properties, including ferroelectricity, magnetism, high temperature superconductivity and metal to insulator transition (MIT). Over recent decades, one of the research focuses has been utilizing these functional features in device applications, which requires a deeper understanding of the material science and also advances in thin film deposition in order to tailor these properties. The binary vanadium dioxide has drawn much attention due to its orders of magnitude change in resistivity during the MIT near room temperature, opening up the possibilities to use this material as next generation transistors, memory devices and radio frequency switches in communication applications. However, to bridge the gap between the frontier of fundamental research in VO2 films and their realization in commercial products, wafer scale growth of the oxide thin films with ‘electronic grade’ is necessary. This task requires precise control over the valence state of normally multivalent transition metal cations, while the device performance will be largely derogated if the valence state is not well controlled. To deposit the VO2 with precise valence state control on wafer scale, a combinatorial approach was used to establish a valence state gradient of vanadium cation, from which the optimal condition for stoichiometric VO2 was extracted. Under the optimal growth condition, a high quality 30-nm thick VO2 film was grown on 3 inch sapphire wafer, showing the highest MIT resistivity ratio for ultrathin films on wafer scale, which is relevant for modern device applications. Besides the growth of high quality MIT thin films on wafer scale, a novel strategy to optically write and erase complex circuitry into VO2 thin films was also developed. We’ve successfully demonstrated the optically induced MIT in VO2 which is persistent after the light source is turned off. We use this method to optically imprint local conductive areas into an otherwise insulating VO2 film. In contrast to conventional thin film patterning techniques that require chemical etching of patterns defined through lithography steps, the optical imprint is performed by irradiating single crystalline VO2 thin films with focused ultraviolet light in a nitrogen atmosphere. A conductive pattern is sketched into the resistive VO2 matrix, resulting in a close to 4 orders of magnitude increase in electrical conductivity at room temperature. Significantly, the inscribed pattern, which is permanent, can be completely erased by a few minutes thermal annealing process at moderately elevated temperature. This development can potentially find its application in reconfigurable optical elements. It can also be harnessed as rewritable bottom electrode material for back-gating structures by inscribing complex contacting schemes using UV radiation. Beyond the binary vanadium oxides, Mott insulators such as LaVO3 have recently been suggested as promising solar cell materials with suitable band gap, high absorption coefficient, as well as the potential to beat the Shockley-Queisser limit owing to their unique strong electron-electron correlation effect that is not present in conventional semiconductors. However, the quality of strongly correlated oxides has been far inferior compared to conventional semiconductors. The high defect concentration of oxide thin films impedes the realization of Mott solar cells with competitive performance due to the lack of stoichiometry control. By taking advantage of the unique self-regulated growth mechanism available in hybrid molecular beam epitaxy, strongly correlated LaVO3 films were grown which revealed a record-low defect concentration. The optical and electrical properties of these films were studied as a function of stoichiometry and a more than two orders of magnitude improvement in defect-related properties compared to results reported in literature was demonstrated, showing that Mott insulators can indeed be synthesized with high perfection using hybrid molecular beam epitaxy.