Correlated metals as transparent conductors

Restricted (Penn State Only)
Author:
Zhang, Lei
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 12, 2017
Committee Members:
  • Roman Engel-Herbert, Dissertation Advisor
  • Roman Engel-Herbert, Committee Chair
  • Venkatraman Gopalan, Committee Member
  • Clive A Randall, Committee Member
  • Srinivas A Tadigadapa, Outside Member
  • Joan Marie Redwing, Committee Member
Keywords:
  • Transparent Conductors
  • Perovskite Oxide
  • Complex Oxide
  • Molecular Beam Epitaxy
Abstract:
The first section of this dissertation work discusses a new designing paradigm for transparent conductors which are widely used in energy and information technology. Most of previous efforts of developing high performance transparent conductors were given to the heavily-doped wide-bandgap semiconductors, which strike a delicate balance between electrical conductivity and optical transparency. However, the ideal conductors, metals, are much less considered and studied due to their nature of high reflection and high absorption in the visible spectrum (1.75~3.25 eV), making them unsuitable for the application as transparent conductors. Here a sizeable electron-electron correlation effect and a favorable band structure of perovskite oxide-based correlated metal give arise to a much higher transparency and largely maintain the electrical conductivity. The test bed materials in this thesis are SrVO3 and CaVO3. Both materials are made of earth-abundant elements and have electron mean free paths that are less than ~5 nm, which enables the possibility of transferring to a scalable process with the polycrystalline form. High quality materials are successfully grown by hybrid molecular beam epitaxy (MBE) using an adsorption-controlled growth mode. By combining a DFT calculation of magnitude of interband transitions and a combined experimental approach of high-resolution x-ray diffraction (XRD), atomic force microscopy (AFM), spectroscopic ellipsometry and electrical transport measurements, the analysis of the materials is given in term of transparent conductor figure-of-merit. Both materials demonstrate a figure-of-merit that is on par with epitaxial ITO, the most widely used material in industry. Beyond intrinsic correlated metals, a new strategy utilizes the aliovalent substitution of divalent element Sr with a trivalent element La in SrVO3¬ is discussed. By carefully controlling the substitution level of La (x), a blue-shift of absorption peak at UV-range was observed and an increase of electrical conductivity was found. The optimal substitution level for the highest figure-of-merit (~20% higher than that of SrVO3) and the highest optical transparency are found, indicating that correlated metals are quite robust with extrinsic charged substitution. The second section of this dissertation work discusses the monolithic integration of epitaxial complex oxide with Si using hybrid MBE. The monolithic integration of epitaxial complex oxide with Si could augment various functions that inherent to oxide, such as ferroelectricity, ferromagnetism, colossal magnetoresistance, superconductivity and multiferroic with industry work-horse Si. Hybrid MBE has advantages over thermal MBE in growing a thicker oxide template on Si, such as a higher growth rate and a better control of cation stoichiometry. The preparation of a high-quality SrTiO3 buffer layer is the key that enables hybrid MBE growth of SrTiO3 on Si substrate. A five-monolayer SrTiO3 buffer layer was deposited on Si without oxygen at low temperature and subsequently annealed in vacuum at slightly high temperature. The temperature for the buffer layer growth must neither be too high to expedite the kinetic process of Si oxidization which would terminate the epitaxial growth or too low to prevent the decomposition of metal-organic precursor into TiO2. A two-step thermal anneal was then introduced. A high temperature anneal was employed to further reduce the rocking curve FWHM down to 0.2° while preserving a smooth film surface morphology. The sheet resistance of as-grown and post-annealed samples was increased by five orders of magnitude exceeding 107 Ohm/sq using a lower temperature anneal. This two-step annealing method provides an easy and effective way to improve the crystalline quality of SrTiO3 thin films on Si, providing a path towards the development of electrically insulating virtual perovskite substrates.