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Transparent conducting oxides: a δ-doped superlattice approach.

Cooper VR, Seo SS, Lee S, Kim JS, Choi WS, Okamoto S, Lee HN - Sci Rep (2014)

Bottom Line: We experimentally observe that these metallic superlattices remain highly transparent to visible light; a direct consequence of the appropriately large gap between the O 2p and Ti 3d states.In superlattices with relatively thin STO layers, we predict that three-dimensional conduction would occur due to appreciable overlap of quantum mechanical wavefunctions between neighboring δ-doped layers.These results highlight the potential for using oxide heterostructures in optoelectronic devices by providing a unique route for creating novel transparent conducting oxides.

View Article: PubMed Central - PubMed

Affiliation: Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.

ABSTRACT
Metallic states appearing at interfaces between dissimilar insulating oxides exhibit intriguing phenomena such as superconductivity and magnetism. Despite tremendous progress in understanding their origins, very little is known about how to control the conduction pathways and the distribution of charge carriers. Using optical spectroscopic measurements and density-functional theory (DFT) simulations, we examine the effect of SrTiO3 (STO) spacer layer thickness on the optical transparency and carrier distribution in La δ-doped STO superlattices. We experimentally observe that these metallic superlattices remain highly transparent to visible light; a direct consequence of the appropriately large gap between the O 2p and Ti 3d states. In superlattices with relatively thin STO layers, we predict that three-dimensional conduction would occur due to appreciable overlap of quantum mechanical wavefunctions between neighboring δ-doped layers. These results highlight the potential for using oxide heterostructures in optoelectronic devices by providing a unique route for creating novel transparent conducting oxides.

No MeSH data available.


(a) Optical transmittance (T) of the [L1/S2], [L1/S4] and [L1/S6] superlattices (60 nm in total thickness) on STO substrates as a function of photon energy (ω). Optical spectra for an undoped bare STO substrate and an oxygen-vacancy-doped STO film (100 nm thick) on STO are also shown for comparison. Superlattice samples clearly show the Drude absorption and no noticeable absorption due to in-gap states. Inset: Photographs of a bare STO substrate, [L1/S4], and oxygen-vacancy-doped STO film. The oxygen-vacancy-doped STO absorbs a significant portion of visible light with clear evidence of in-gap defect states, as marked with arrows. (b) Extinction coefficient (k) and refractive index (n), and (c) optical conductivity (σ1) of the [L1/S2], [L1/S4] and [L1/S6] superlattices obtained from a Drude fit to the experimental transmittance data. The net carrier density and mobility for each sample are listed for convenience. For all three samples the carrier density is within experimental error bars of 0.5 e−/interface1314.
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f1: (a) Optical transmittance (T) of the [L1/S2], [L1/S4] and [L1/S6] superlattices (60 nm in total thickness) on STO substrates as a function of photon energy (ω). Optical spectra for an undoped bare STO substrate and an oxygen-vacancy-doped STO film (100 nm thick) on STO are also shown for comparison. Superlattice samples clearly show the Drude absorption and no noticeable absorption due to in-gap states. Inset: Photographs of a bare STO substrate, [L1/S4], and oxygen-vacancy-doped STO film. The oxygen-vacancy-doped STO absorbs a significant portion of visible light with clear evidence of in-gap defect states, as marked with arrows. (b) Extinction coefficient (k) and refractive index (n), and (c) optical conductivity (σ1) of the [L1/S2], [L1/S4] and [L1/S6] superlattices obtained from a Drude fit to the experimental transmittance data. The net carrier density and mobility for each sample are listed for convenience. For all three samples the carrier density is within experimental error bars of 0.5 e−/interface1314.

Mentions: Typically, doped conducting transition-metal oxides, for instance La-, Nb-, or oxygen vacancy-doped STO, have high light-absorption due to accompanying defects and/or deep-level impurities1920, resulting in significant opacity (e.g., see the inset of Figure 1a). As such transition-metal oxides with partially filled d-band electrons are generally considered to be poor candidates for transparent conducting oxides (TCOs). However, LTO/STO superlattices with electronically-reconstructed interfaces represent completely different band structures as compared to those of chemically-doped STOs, which typically create in-gap states21. Figure 1a depicts the optical transmittance (T) for [L1/S2], [L1/S4] and [L1/S6] superlattices grown by pulsed laser deposition (PLD) on TiO2-terminated (001) STO single crystal substrates (i.e. STO spacer thicknesses of 2, 4 and 6 layers, respectively). Similar to an undoped STO crystal, all of the La δ-doped STO superlattices are transparent in the visible range, while showing clear evidence of conducting carriers, i.e., the Drude absorption, in the low photon energy region (<1.5 eV). Using a simple Drude model fit, we have calculated the extinction coefficient (k) of the complex refractive index (ň = n + ik) as a function of photon energy. This fit confirms that the δ-doped superlattices remain transparent in the visible range (Figure 1b). Note that the higher interface density in the [L1/S2] superlattices results in lower transmittance as compared to the longer period superlattices (Figure 1). It is worth noting that both the measured transmittance and resistivity of the superlattices are comparable to those of the most widely used TCO, i.e. Sn-doped In2O3 (ITO) (T = 80%, ρ = 200 μΩ·cm)22, and the superlattices' room-temperature carrier mobility is also comparable to that of ITO as we reported elsewhere1314.


Transparent conducting oxides: a δ-doped superlattice approach.

Cooper VR, Seo SS, Lee S, Kim JS, Choi WS, Okamoto S, Lee HN - Sci Rep (2014)

(a) Optical transmittance (T) of the [L1/S2], [L1/S4] and [L1/S6] superlattices (60 nm in total thickness) on STO substrates as a function of photon energy (ω). Optical spectra for an undoped bare STO substrate and an oxygen-vacancy-doped STO film (100 nm thick) on STO are also shown for comparison. Superlattice samples clearly show the Drude absorption and no noticeable absorption due to in-gap states. Inset: Photographs of a bare STO substrate, [L1/S4], and oxygen-vacancy-doped STO film. The oxygen-vacancy-doped STO absorbs a significant portion of visible light with clear evidence of in-gap defect states, as marked with arrows. (b) Extinction coefficient (k) and refractive index (n), and (c) optical conductivity (σ1) of the [L1/S2], [L1/S4] and [L1/S6] superlattices obtained from a Drude fit to the experimental transmittance data. The net carrier density and mobility for each sample are listed for convenience. For all three samples the carrier density is within experimental error bars of 0.5 e−/interface1314.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4127498&req=5

f1: (a) Optical transmittance (T) of the [L1/S2], [L1/S4] and [L1/S6] superlattices (60 nm in total thickness) on STO substrates as a function of photon energy (ω). Optical spectra for an undoped bare STO substrate and an oxygen-vacancy-doped STO film (100 nm thick) on STO are also shown for comparison. Superlattice samples clearly show the Drude absorption and no noticeable absorption due to in-gap states. Inset: Photographs of a bare STO substrate, [L1/S4], and oxygen-vacancy-doped STO film. The oxygen-vacancy-doped STO absorbs a significant portion of visible light with clear evidence of in-gap defect states, as marked with arrows. (b) Extinction coefficient (k) and refractive index (n), and (c) optical conductivity (σ1) of the [L1/S2], [L1/S4] and [L1/S6] superlattices obtained from a Drude fit to the experimental transmittance data. The net carrier density and mobility for each sample are listed for convenience. For all three samples the carrier density is within experimental error bars of 0.5 e−/interface1314.
Mentions: Typically, doped conducting transition-metal oxides, for instance La-, Nb-, or oxygen vacancy-doped STO, have high light-absorption due to accompanying defects and/or deep-level impurities1920, resulting in significant opacity (e.g., see the inset of Figure 1a). As such transition-metal oxides with partially filled d-band electrons are generally considered to be poor candidates for transparent conducting oxides (TCOs). However, LTO/STO superlattices with electronically-reconstructed interfaces represent completely different band structures as compared to those of chemically-doped STOs, which typically create in-gap states21. Figure 1a depicts the optical transmittance (T) for [L1/S2], [L1/S4] and [L1/S6] superlattices grown by pulsed laser deposition (PLD) on TiO2-terminated (001) STO single crystal substrates (i.e. STO spacer thicknesses of 2, 4 and 6 layers, respectively). Similar to an undoped STO crystal, all of the La δ-doped STO superlattices are transparent in the visible range, while showing clear evidence of conducting carriers, i.e., the Drude absorption, in the low photon energy region (<1.5 eV). Using a simple Drude model fit, we have calculated the extinction coefficient (k) of the complex refractive index (ň = n + ik) as a function of photon energy. This fit confirms that the δ-doped superlattices remain transparent in the visible range (Figure 1b). Note that the higher interface density in the [L1/S2] superlattices results in lower transmittance as compared to the longer period superlattices (Figure 1). It is worth noting that both the measured transmittance and resistivity of the superlattices are comparable to those of the most widely used TCO, i.e. Sn-doped In2O3 (ITO) (T = 80%, ρ = 200 μΩ·cm)22, and the superlattices' room-temperature carrier mobility is also comparable to that of ITO as we reported elsewhere1314.

Bottom Line: We experimentally observe that these metallic superlattices remain highly transparent to visible light; a direct consequence of the appropriately large gap between the O 2p and Ti 3d states.In superlattices with relatively thin STO layers, we predict that three-dimensional conduction would occur due to appreciable overlap of quantum mechanical wavefunctions between neighboring δ-doped layers.These results highlight the potential for using oxide heterostructures in optoelectronic devices by providing a unique route for creating novel transparent conducting oxides.

View Article: PubMed Central - PubMed

Affiliation: Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.

ABSTRACT
Metallic states appearing at interfaces between dissimilar insulating oxides exhibit intriguing phenomena such as superconductivity and magnetism. Despite tremendous progress in understanding their origins, very little is known about how to control the conduction pathways and the distribution of charge carriers. Using optical spectroscopic measurements and density-functional theory (DFT) simulations, we examine the effect of SrTiO3 (STO) spacer layer thickness on the optical transparency and carrier distribution in La δ-doped STO superlattices. We experimentally observe that these metallic superlattices remain highly transparent to visible light; a direct consequence of the appropriately large gap between the O 2p and Ti 3d states. In superlattices with relatively thin STO layers, we predict that three-dimensional conduction would occur due to appreciable overlap of quantum mechanical wavefunctions between neighboring δ-doped layers. These results highlight the potential for using oxide heterostructures in optoelectronic devices by providing a unique route for creating novel transparent conducting oxides.

No MeSH data available.