skip to main content

Explore Research Products in the NSF-PAR

Title: Stoichiometry-dependent surface electronic structure of SrTiO 3 films grown by hybrid molecular beam epitaxy
We investigate the surface electronic structure of SrTiO 3 (STO) films grown by a hybrid molecular beam epitaxy that are both stoichiometric and nonstoichiometric by means of x-ray photoelectron spectroscopy and electron energy loss spectroscopy. Increasing the fraction of the surface that is terminated with an SrO layer is correlated with a decrease in the chemical potential whereby the valence band maximum moves closer to the Fermi level, but without a significant change in the bandgap. Inasmuch as SrO-terminated STO (001) has previously been shown to act as an electron scavenger in which carriers from the bulk are trapped, we argue that the high fraction of SrO in the terminal layer is what lowers the chemical potential in Sr-rich STO. Our experimental results provide important insights into various physical phenomena that can occur on STO (001) surfaces and their effect on bulk electronic properties.  more » « less
Award ID(s):
2011401
NSF-PAR ID:
10411275
Author(s) / Creator(s):
; ; ; ;
Date Published:
Journal Name:
Applied Physics Letters
Volume:
120
Issue:
12
ISSN:
0003-6951
Page Range / eLocation ID:
121604
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; ; ; ; ; ( , RSC Advances)
    null (Ed.)
    Controlling the growth of complex relaxor ferroelectric thin films and understanding the relationship between biaxial strain–structural domain characteristics are desirable for designing materials with a high electromechanical response. For this purpose, epitaxial thin films free of extended defects and secondary phases are urgently needed. Here, we used optimized growth parameters and target compositions to obtain epitaxial (40–45 nm) 0.67Pb(Mg 1/3 Nb 2/3 )O 3 –0.33PbTiO 3 /(20 nm) SrRuO 3 (PMN–33PT/SRO) heterostructures using pulsed-laser deposition (PLD) on singly terminated SrTiO 3 (STO) and ReScO 3 (RSO) substrates with Re = Dy, Tb, Gd, Sm, and Nd. In situ reflection high-energy electron diffraction (RHEED) and high-resolution X-ray diffraction (HR-XRD) analysis confirmed high-quality and single-phase thin films with smooth 2D surfaces. High-resolution scanning transmission electron microscopy (HR-STEM) revealed sharp interfaces and homogeneous strain further confirming the epitaxial cube-on-cube growth mode of the PMN–33PT/SRO heterostructures. The combined XRD reciprocal space maps (RSMs) and piezoresponse force microscopy (PFM) analysis revealed that the domain structure of the PMN–33PT heterostructures is sensitive to the applied compressive strain. From the RSM patterns, an evolution from a butterfly-shaped diffraction pattern for mildly strained PMN–33PT layers, which is evidence of stabilization of relaxor domains, to disc-shaped diffraction patterns for high compressive strains with a highly distorted tetragonal structure, is observed. The PFM amplitude and phase of the PMN–33PT thin films confirmed the relaxor-like for a strain state below ∼1.13%, while for higher compressive strain (∼1.9%) the irregularly shaped and poled ferroelectric domains were observed. Interestingly, the PFM phase hysteresis loops of the PMN–33PT heterostructures grown on the SSO substrates (strain state of ∼0.8%) exhibited an enhanced coercive field which is about two times larger than that of the thin films grown on GSO and NSO substrates. The obtained results show that epitaxial strain engineering could serve as an effective approach for tailoring and enhancing the functional properties in relaxor ferroelectrics. 
    more » « less
  2. ; ; ; ; ; ; ; ; ( , Advanced Materials Interfaces)
    Abstract

    The behavior of polar LaMnO3(LMO) thin films deposited epitaxially on nonpolar SrTiO3(001) (STO) is dictated by both the LMO/STO band alignment and the chemistry of the Mn cation. Using in situ X‐ray photoelectron spectroscopy, the valence band offset (VBO) of LMO/STO heterojunctions is directly measured as a function of thickness, and found that the VBO is 2.5 eV for thicker (≥3 u.c.) films. No evidence of a built‐in electric field in LMO films of any thickness is found. Measurements of the Mn valence by MnL‐edge X‐ray absorption spectroscopy and by spatially resolved electron energy loss spectra in scanning transmission electron microscopy images reveal that Mn2+is present at the LMO surface, but not at the LMO/STO interface. These results are corroborated by density functional theory simulations that confirm a VBO of ≈2.5 eV for both ideal and intermixed interfaces. A model is proposed for the behavior of polar/nonpolar LMO/STO heterojunctions in which the polar catastrophe is alleviated by the formation of oxygen vacancies at the LMO surface.

     
    more » « less
  3. ; ; ; ; ; ; ( , Advanced Electronic Materials)
    Abstract

    The emergence of a 2D electron gas (2DEG) on the (001) surface of oxygen‐deficient strontium titanate (SrTiO3−δ) is investigated. Using in situ soft X‐ray spectroscopy and effective mass modeling, a series of quantitative band diagrams are developed to describe the evolution of near‐surface and bulk carrier concentrations, downward band bending, and Fermi level along a lateral gradient of oxygen vacancies formed on SrTiO3−δby direct‐current resistive heating under ultrahigh vacuum conditions. Electrons are accumulated over a 3 nm region near the surface, confined within a potential well with saturated 300 meV downward band bending. The relation between Fermi levels and carrier concentrations near the surface suggests the density of states near the surface is much lower than the bulk density of states, which is consistent with the quantum‐confined subbands of a 2DEG. The quantitative relationship between the surface and bulk electronic structures developed in this work provides a guide for precise engineering of the oxygen‐vacancy‐induced 2DEG in SrTiO3.

     
    more » « less
  4. ; ; ; ; ; ; ; ; ; ; et al ( , APL Materials)
    We combine state-of-the-art oxide epitaxial growth by hybrid molecular beam epitaxy with transport, x-ray photoemission, and surface diffraction, along with classical and first-principles quantum mechanical modeling to investigate the nuances of insulating layer formation in otherwise high-mobility homoepitaxial n-SrTiO 3 (001) films. Our analysis points to charge immobilization at the buried n-SrTiO 3 /undoped SrTiO 3 (001) interface as well as within the surface contamination layer resulting from air exposure as the drivers of electronic dead-layer formation. As Fermi level equilibration occurs at the surface and the buried interface, charge trapping reduces the sheet carrier density ( n 2 D ) and renders the n-STO film insulating if n 2 D falls below the critical value for the metal-to-insulator transition. 
    more » « less
  5. ; ; ; ; ; ( , Advanced Electronic Materials)
    Abstract

    The electronic structure of the two‐dimensional electron system (2DES) found at the Al/SrTiO3(Al/STO) and LaAlO3/SrTiO3(LAO/STO) interfaces is measured by means of laser angle resolved photoemission spectroscopy, taking advantage of the large photoelectron escape depth at low photon energy to probe these buried interfaces. The possibility of tuning the electronic density in Al/STO by varying the Al layer thickness is demonstrated, and it is shown that the electronic structure evolution is well described by self‐consistent tight binding supercell calculations, but differs qualitatively from a rigid band shift model. It is shown that both 2DES are strongly coupled to longitudinal optical phonons, in agreement with previous reports of a polaronic ground state in similar STO based 2DESs. Tuning the electronic density in Al/STO to match that of LAO/STO and comparing both systems, it is estimated that the intrinsic LAO/STO 2DES has a bare band width of ≈60 meV and a carrier density of ≈6 × 1013cm−2.

     
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email