A comparison of LaAlO3/SrTiO3 heterointerfaces grown by spin coating and pulsed laser deposition methods

https://doi.org/10.1016/j.jcrysgro.2020.125912Get rights and content

Highlights

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    The spin coating also can epitaxially grow LaAlO3 film consistent with (0 0 1) orientation of STO substrate.

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    All of the LAO/STO heterointerfaces exhibit a perfect metallic conductive behavior under the similar thickness.

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    The chemical method of spin coating can effectively avoid the ablation of high-energy plume.

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    The two dimensional electron gas prepared by the spin coating exhibits a smaller sheet resistance and higher mobility.

Abstract

The two dimensional electron gas (2DEG) exhibits a considerable potential in all-oxide electronics due to the emergence of novel physical phenomena at the typical LaAlO3/SrTiO3 (LAO/STO) heterointerfaces. However, the quality and property of 2DEG are distinctly influenced by the preparation method. Herein, we compare the pulsed laser deposition (PLD) and spin coating method by obtaining high-quality 2DEGs. The comparative studies show that all the LAO/STO heterointerfaces exhibit a perfect metallic conductive behavior under the similar thickness. Moreover, the results by SC exhibit smaller sheet resistance and higher mobility, which is attributed to fewer defect states at heterointerfaces because the chemical method can effectively avoid the ablation of high-energy plume in PLD process. Therefore, our works provide a promising fabrication method for oxide heterointerfaces with a convenient, gentle, large scale and low cost, expanding the way for the application of all-oxide electronic devices.

Introduction

Depending on the abundant characters of composition, crystal structure and stoichiometry et al., the heterointerfaces (e. g. LaAlO3/SrTiO3 (LAO/STO)) exhibit many fascinating physical properties and potential applications, which cannot be found in bulk materials, such as the two dimensional metallic conduction, ferromagnetic and/or superconductive properties [1], [2], [3], [4], [5]. Meanwhile, these exotic properties can also be modulated by electric field [6], [7], [8], [9], polarization [10], [11], [12], epitaxial strain [13], [14], [15], light illumination [16], [17], [18], [19], ionic liquid [20], [21], [22] and so on. So, many groups have carried out extensive researches to explore the fundamental physical mechanism and the possibility of novel oxide electronic devices. In addition, it is commonly recognized that the oxide-oxide heterointerfaces need an abrupt and atomic-scale smooth interface by the epitaxial growth under appropriate conditions. Therefore, the growth method of high-quality epitaxial heterointerfaces becomes a hot issue. Among these, pulsed laser deposition (PLD), which is one of the representative physical vapor deposition techniques, includes three processes. Firstly, the high-energy pulsed laser beam is focused on a target. Secondly, the plasma plume is then formed by melting materials. Thirdly, the plasma is deposited on the substrate. The entire process usually needs the condition of high temperature and high vacuum [23]. Owing to the advantages, operability and repeatability, the PLD technique has been widely applied to the preparation and exploration of oxide heterointerfaces by some groups [1], [21], [22], [24], [25], [26]. Different from the PLD methods, the chemical method of spin coating (SC) exhibits a great potential in the preparation of oxide two-dimensional electron gas (2DEG). For examples, Khan et al. [27], [28] obtained the high-mobility and spin-polarized 2DEG at LAO/STO interfaces by the SC method. Likewise, we have also fabricated Al2O3/STO and YAlO3/STO heterointerfaces by SC method, all revealing a two dimensional metallic conductive behavior [29], [30]. These results indicate that the SC technique is a mild chemical growth process and a convenient way to obtain the multicomponent oxide layers with different stoichiometric ratios. Meanwhile, the SC technique possesses a simple operation process and need no sophisticated equipment although this method can’t fabricate ultrathin film and accurately control the growth of film. Herein, we compare the oxide 2DEGs at LAO/STO heterointerface by SC and PLD methods, respectively, which can provide an effective candidate of 2DEG preparation methods in the future.

Section snippets

Samples growth

The (0 0 1) STO substrates with a dimension of 5 × 5 × 0.5 mm3 were cleaned in the ethanol and deionized water by the ultrasonic cleaner for 10 min, respectively. Subsequently, the smooth TiO2-terminated STOs were successively achieved by buffered HF acid etch (41 s) and high temperature annealing in air (970 °C, 2 h). Finally, the LAO layers were grown on the STO substrates by the PLD and SC method, respectively. The growth parameters of PLD method were as followed: laser energy of 150 mJ/pulse,

Results and discussions

The ideal TiO2-terminated STO substrates are essential to obtain high-quality 2DEG. We adopt the wet chemical method to pretreat the STO (0 0 1) substrates and their surface morphology is shown in Fig. 1. The AFM image shows that the surface of STO substrate favors a clear and uniform terrace-like topography. The linear scan of STO surface, which is the purple line in (a), is performed in the perpendicular direction of terrace. The step profile of line scan is shown in the Fig. 1(b), the result

Conclusion

In summary, we have fabricated the high-quality 2DEGs at LAO/STO heterointerfaces using PLD and SC methods. Both of the LAO films are smooth and uniform at atomic scale, and epitaxially grown on STO (0 0 1) substrates. Furthermore, the 2DEGs exhibit a perfect metallic conducting property in the range of 20–300 K. Under the similar thickness, the 2DEG prepared by the SC method exhibits a smaller sheet resistance and higher mobility, meaning that the SC method is a comparable technique to fabricate

CRediT authorship contribution statement

Ming Li: Conceptualization, Investigation. Ruishu Yang: Conceptualization. Yang Zhao: Conceptualization. Shuanhu Wang: Conceptualization. Kexin Jin: Conceptualization, Supervision, Project administration, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (Nos. 51572222, 61471301) and the Fundamental Research Funds for the Central Universities (Grant No. 3102017jc01001). We would like to thank the analytical & testing center of Northwestern Polytechnical University for SEM and XRD characterizations.

References (30)

  • C. Bell et al.

    Dominant mobility modulation by the electric field effect at the LaAlO3/SrTiO3 interface

    Phys. Rev. Let.

    (2009)
  • C.W. Bark et al.

    Switchable induced polarization in LaAlO3/SrTiO3 heterostructures

    Nano Lett.

    (2012)
  • J. Zhou et al.

    Spin responses and effective Hamiltonian for the two-dimensional electron gas at the oxide interface LaAlO3/SrTiO3

    Phys. Rev. B

    (2015)
  • K. Narayanapillai et al.

    Interfacial Rashba magnetoresistance of the two-dimensional electron gas at the LaAlO3/SrTiO3 interface

    Phys. Rev. B

    (2017)
  • S. Nazir et al.

    The role of uniaxial strain in tailoring the interfacial properties of LaAlO3/SrTiO3 heterostructure

    RSC Adv.

    (2015)
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