Mueller matrix spectroscopic ellipsometry is applied to determine anisotropic optical properties for a set of single-crystal rhombohedral structure α-(Al x Ga 1− x ) 2 O 3 thin films (0 [Formula: see text] x [Formula: see text] 1). Samples are grown by plasma-assisted molecular beam epitaxy on m-plane sapphire. A critical-point model is used to render a spectroscopic model dielectric function tensor and to determine direct electronic band-to-band transition parameters, including the direction dependent two lowest-photon energy band-to-band transitions associated with the anisotropic bandgap. We obtain the composition dependence of the direction dependent two lowest band-to-band transitions with separate bandgap bowing parameters associated with the perpendicular ([Formula: see text] = 1.31 eV) and parallel ([Formula: see text] = 1.61 eV) electric field polarization to the lattice c direction. Our density functional theory calculations indicate a transition from indirect to direct characteristics between α-Ga 2 O 3 and α-Al 2 O 3 , respectively, and we identify a switch in band order where the lowest band-to-band transition occurs with polarization perpendicular to c in α-Ga 2 O 3 whereas for α-Al 2 O 3 the lowest transition occurs with polarization parallel to c. We estimate that the change in band order occurs at approximately 40% Al content. Additionally, the characteristic of the lowest energy critical point transition for polarization parallel to c changes from M 1 type in α-Ga 2 O 3 to M 0 type van Hove singularity in α-Al 2 O 3 .
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First-principles indicators of ferroic parameters in epitaxial BiFeO 3 and BiCrO 3
Density-functional theory is used to validate spin-resolved and orbital-resolved metrics of localized electronic states to anticipate ferroic and dielectric properties of [Formula: see text] and [Formula: see text] under epitaxial strain. Using previous investigations of epitaxial phase stability in these systems, trends in properties such as spontaneous polarization and bandgap are compared to trends in atomic orbital occupation derived from projected density of states. Based on first principles theories of ferroic and dielectric properties, such as the Modern Theory of Polarization for spontaneous polarization or Goodenough–Kanamori theory for magnetic interactions, this work validates the sufficiency of metrics of localized electronic states to predict trends in multiple ferroic and dielectric properties. Capabilities of these metrics include the anticipation of the transition from G-Type to C-Type antiferromagnetism in [Formula: see text] under 4.2% compressive epitaxial strain and the interval of C-Type antiferromagnetism from 3% to 7% tensile epitaxial strain in [Formula: see text]. The results of this work suggest a capability of localized electronic metrics to predict multiferroic characteristics in the Bi X[Formula: see text] systems under epitaxial strain, with single or mixed B-site occupation.
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- Award ID(s):
- 1555015
- PAR ID:
- 10340824
- Date Published:
- Journal Name:
- Journal of Applied Physics
- Volume:
- 132
- Issue:
- 2
- ISSN:
- 0021-8979
- Page Range / eLocation ID:
- 024102
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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