Electron Trapping and Detrapping in an Oxide Two-Dimensional Electron Gas: The Role of Ferroelastic Twin Walls

Shashank Kumar Ojha, Sankalpa Hazra, Prithwijit Mandal, Ranjan Kumar Patel, Shivam Nigam, Siddharth Kumar, and S. Middey

Phys. Rev. Applied 15, 054008 – Published 5 May 2021

Abstract

The choice of electrostatic gating over the conventional chemical doping for phase engineering of quantum materials is attributed to the fact that the former can reversibly tune the carrier density without affecting the system’s level of disorder. However, this proposition seems to break down in field-effect transistors involving ${Sr Ti O}_{3}$ (STO)-based two-dimensional electron gases. Such peculiar behavior is associated with electron trapping under an external electric field. However, the microscopic nature of the trapping centers remains an open question. In this paper, we investigate electric-field-induced charge-trapping and charge-detrapping phenomena at the conducting interface between the band insulators $γ$ - ${Al}_{2} O_{3}$ and STO. Our transport measurements reveal that the charge trapping under a positive back-gate voltage ( $V_{g}$ ) above the tetragonal-to-cubic structural transition temperature ( $T_{c}$ ) of STO has a contribution from electric-field-assisted thermal escape of electrons from the quantum well, and from clustering of oxygen vacancies as well. We observe an additional source of trapping below $T_{c}$ , which arises from the trapping of free carriers at ferroelastic twin walls in the STO. Application of a negative $V_{g}$ results in charge detrapping, which vanishes above $T_{c}$ . This feature demonstrates the crucial role of structural domain walls in the electrical transport properties of STO-based heterostructures. The number of charges trapped (detrapped) at (from) a twin wall is controlled by the net polarity of the wall and is completely reversible with a sweep of $V_{g}$ .

Received 24 December 2020
Revised 6 April 2020
Accepted 13 April 2021

DOI:https://doi.org/10.1103/PhysRevApplied.15.054008

Physics Subject Headings (PhySH)

Domain walls Ferroelasticity Surface & interfacial phenomena

Field-effect transistors Heterostructures Oxides Two-dimensional electron gas

Transport techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Shashank Kumar Ojha ^*, Sankalpa Hazra , Prithwijit Mandal , Ranjan Kumar Patel , Shivam Nigam, Siddharth Kumar , and S. Middey ^†

Department of Physics, Indian Institute of Science, Bengaluru 560012, India

^*shashank@iisc.ac.in
^†smiddey@iisc.ac.in

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Vol. 15, Iss. 5 — May 2021

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Images

Figure 1
(a) XRD pattern of GAO/STO heterostructure around the STO (002) peak. The inset shows a RHEED image of the GAO film taken after cooling down to room temperature. (b) X-ray reflectivity pattern of GAO film. The inset shows an AFM image of the GAO film. (c) Temperature dependence of $R_{s}$ for the GAO/STO heterostructure. The inset shows the device geometry for measurement of the resistance of the 2DEG in a back-gating configuration. (d) Variation of $R_{s}$ at 80 K when $V_{g}$ is swept in the sequence 0 V $\to$ 20 V $\to$ 5 V $\to$ 30 V $\to$ 15 V $\to$ 40 V $\to$ 25 V $\to$ 50 V $\to$ 0 V. Total measurement time $t_{M}$ for this entire voltage sweep is 10 min, 90 min, and 240 min for left, middle, and right panel, respectively.
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Figure 2
(a) Back-gate-voltage sweeping protocol (lower panel) and corresponding variation of $R_{s}$ with time at 80 K (upper panel). (b)–(d) Magnified views of features B, C, and D along with fittings with a single exponential function. The insets of (b)–(d) show the respective functional forms of the formula used for fitting. $a$ and $b$ are constants. $t_{0}$ is the starting time of features B, C, and D. $τ_{B}$ , $τ_{C}$ , and $τ_{D}$ denote the time constants associated with features B, C, and D, respectively. (e) Comparison of time constants of exponential features B, C, and D and their variation with cycle number.
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Figure 3
(a) Relative percentage change in resistance [ $10^{2} (Δ R / R_{0})$ ] of feature A due to charge trapping ( $Δ R = R - R_{0}$ , where $R_{0}$ is the resistance just at the start of feature A). For comparison, the starting times of feature A for all the cycles have been manually shifted to zero. (b) Schematic illustration to show thermal escape of electrons under positive $V_{g}$ . Application of a positive back-gate voltage bends the conduction band and lowers the barrier height $Δ E_{barr}$ for thermal escape. Application of a negative $V_{g}$ would bend the band upwards in energy, which would increase $Δ E_{barr}$ for thermal escape. (c) Variation of $τ_{A}$ , $τ_{OV}$ , and $τ_{E}$ upon multiple cycling. (d) Variation of thermal-escape contribution with increasing cycle number.
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Figure 4
(a) Temperature dependence of feature B (first cycle). (b) Temperature dependence of feature A (first cycle) for $V_{g} = 50 V$ . (c) Temperature evolution of $τ_{A}$ and $| β |$ obtained from fitting of feature A (upper panel). The lower panel shows the temperature evolution of $τ_{B}$ obtained from fitting of feature B, and the magnitude of the maximum relative percentage change in resistance ( $10^{2} | Δ R / R_{0} |_{\max}$ ) due to detrapping in feature B. The left and right axes in upper and lower panels of (c) correspond to open and filled symbols, respectively.
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Figure 5
(a) A set of schematic illustrations to show electric-field-induced trapping and detrapping of charges at twin walls in STO. The first panel shows trapped electrons at twin walls in STO at zero back-gate voltage. Application of a 0 $\to$ positive step $V_{g}$ leads to transfer of electrons from the 2DEG to the twin walls (second panel). Switching off the field after the 0 $\to$ positive step $V_{g}$ leads to reversible transfer of electrons from the twin walls to the 2DEG (third panel). Application of a 0 $\to$ negative step $V_{g}$ leads to transfer of electrons from the twin walls to the 2DEG (fourth panel). Switching off the voltage after application of the 0 $\to$ negative step $V_{g}$ leads to reversible transfer of electrons from the 2DEG to the twin walls (fifth panel). For visual clarity, the STO substrate and the 2DEG are shown physically separated from each other. (b) Variation of twin-wall contribution to feature A in the first cycle at 80 K with $V_{g}^{\max}$ . (c) Variation of magnitude of maximum change in resistance due to the detrapping feature B in the first cycle at 80 K with $V_{g}^{\max}$ . (d) Variation of magnitude of maximum change in resistance due to feature C in the first cycle at 80 K with $V_{g}^{\min}$ . (e) Variation of magnitude of maximum change in resistance due to feature D in the first cycle at 80 K with $V_{g}^{\min}$ . (f) Schematic illustration depicting the different charge-trapping mechanisms present above and below $T_{c}$ of STO. The change in resistance due to each of the three processes at the end of feature A (i.e., 4 h after the application of a constant $V_{g} = 50$ V) is used to quantify their relative contributions. The dashed lines in (b)–(e) denote fittings with a linear function. The insets of (c)–(e) show the definitions of $Δ R_{\max}$ for the respective features.
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