Enhanced persistent photoconductivity in $\ensuremath{\delta}$-doped LaAlO${}_{3}$/SrTiO${}_{3}$ heterostructures

Enhanced persistent photoconductivity in $δ$ -doped LaAlO $_{3}$ /SrTiO $_{3}$ heterostructures

A. Rastogi, J. J. Pulikkotil, and R. C. Budhani

Phys. Rev. B 89, 125127 – Published 31 March 2014

Abstract

We report the effect of $δ$ doping at the LaAlO $_{3}$ /SrTiO $_{3}$ interface with LaMnO $_{3}$ monolayers on the photoconducting (PC) state. The PC is realized by exposing the samples to broadband optical radiation of a quartz lamp and 325 and 441 nm lines of a He-Cd laser. Along with the significant modification in electrical transport which drives the pure LaAlO $_{3}$ /SrTiO $_{3}$ interface from metal-to-insulator with increasing LaMnO $_{3}$ sub-monolayer thickness, we also observe an enhancement in the photoresponse and relaxation time constant. A possible scenario for the PC based on defect clusters, random potential fluctuations, and large lattice relaxation models, along with the role of structural phase transition in SrTiO $_{3}$ , is discussed. For pure LaAlO $_{3}$ /SrTiO $_{3}$ , the photoconductivity appears to originate from interband transitions between Ti-derived $3 d$ bands which are $e_{g}$ in character and O 2 $p$ –Ti $t_{2 g}$ hybridized bands. The band structure changes significantly when fractional layers of LaMnO $_{3}$ are introduced. Here the Mn $e_{g}$ bands ( $\approx$ 1.5 eV above the Fermi energy) within the photoconducting gap lead to a reduction in the photoexcitation energy and a gain in overall photoconductivity.

Received 23 October 2013
Revised 15 March 2014

DOI:https://doi.org/10.1103/PhysRevB.89.125127

Authors & Affiliations

A. Rastogi¹, J. J. Pulikkotil², and R. C. Budhani^1,2,*

¹Condensed Matter Low Dimensional Systems Laboratory, Department of Physics, Indian Institute of Technology, Kanpur 208016, India
²Division of Quantum Phenomena and Applications, CSIR–National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India

^*rcb@nplindia.org

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Vol. 89, Iss. 12 — 15 March 2014

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Figure 1
The temperature dependence of the sheet resistance ( $R_{□}$ ) for the samples with and without LaMnO $_{3}$ ( $δ = 0$ and $0.6$ ML) at the LaAlO $_{3}$ /SrTiO $_{3}$ interface. The LaAlO $_{3}$ /SrTiO $_{3}$ sample shows a metallic behavior as the temperature is lowered to $≃$ 30 K followed by an upturn, while the sample with $0.6$ ML LaMnO $_{3}$ at the interface shows insulating behavior [40]. The resistance goes beyond the measurement limit below $≃$ 120 K. The inset shows the variation of resistance at 300 K with the LaMnO $_{3}$ layer thickness in two different units, with the left axis representing it in units of k $Ω$ while other axis is represented in the units of quantum resistance $R_{k} = h / e^{2}$ , where $h$ and $e$ are the Planck's constant and electronic charge.
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Figure 2
(a) The change in the channel resistance at 20 K as a function $δ$ -layer thickness. The main panels (b) and (c) respectively show the relaxation of the normalized resistance for different $δ$ doping at 20 and 300 K after switching off the illumination from a halogen lamp. The recovery dynamics follow a stretched exponential behavior which is represented as solid lines in (b). The inset of (b) shows the relative change in the resistance at 300 K upon radiating the samples with 325 and 441 nm lines of a He-Cd laser. A comparison of the photoresponse as a function of temperature for different samples is made in inset (c). The $0.5$ monolayer LaMnO $_{3}$ shows a threefold increase in the photoresponse in comparison with the $δ$ = 0 sample.
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Figure 3
Main panel shows the photoconductivity behavior of a bare TiO $_{2}$ -terminated STO and a LMO (1 uc)/STO. The sample resistance becomes measurable only after $\sim$ 10 s of light exposure. Inset is adapted from Ref. [15]. A complete recovery is seen here for a bare STO substrate. The recovery dynamics of bare STO also follow a stretched exponential behavior and extracted time constant ( $τ$ ) is nearly $\sim$ 27 sec. A much shorter recovery time compared to data in Fig. 2 is noteworthy.
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Figure 4
Comparison of the relaxation time constant $(τ)$ of three system as indicated by the legends. The solid curves are the fits to the Arrhenius equation. The relaxation time constant scales in proportion with increasing LaMnO $_{3}$ sub-monolayer thickness. For system corresponding to $0.5$ ML thick LaMnO $_{3}$ , the relaxation time constant was estimated to be higher.
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Figure 5
Main panel shows the effect of electric field in the PPC recovery state for $δ$ = $0.2$ ML sample at 20 K. Points A and B in the figure show the time at which the illumination is turned ON and OFF, respectively. This photoinduced recovery state is then subjected to the gate field ( $E_{g}$ ) at times t1, t2, t3, and t4. Inset shows the zero bias conductance [ $G_{V} (0)$ ] of all three samples in dark at 20 K in the range $\pm$ 4 kV/cm.
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Figure 6
Fat band representation of the Mn $3 d$ majority states (circle) of the $δ$ -doped LaMnO $_{3}$ in LaAlO $_{3}$ /SrTiO $_{3}$ heterostructures, calculated using the LDA+U method, with $U = 8$ eV. The conduction band around $E ≃ 1.5$ eV is derived from the Mn $3 d_{z^{2}}$ spin-up states, while that in the valence band in the range $1.7$ $<$ E (eV) $<$ $0.5$ corresponds to the Mn $3 d_{x y}$ spin-up states.
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Physical Review B

covering condensed matter and materials physics

Enhanced persistent photoconductivity in $δ$ -doped LaAlO $_{3}$ /SrTiO $_{3}$ heterostructures

A. Rastogi, J. J. Pulikkotil, and R. C. Budhani

Phys. Rev. B 89, 125127 – Published 31 March 2014

Abstract

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Physical Review B

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Enhanced persistent photoconductivity in δ-doped LaAlO3/SrTiO3 heterostructures

A. Rastogi, J. J. Pulikkotil, and R. C. Budhani

Phys. Rev. B 89, 125127 – Published 31 March 2014

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Enhanced persistent photoconductivity in $δ$ -doped LaAlO $_{3}$ /SrTiO $_{3}$ heterostructures