Patterning of superconducting two-dimensional electron gases based on AlO xKTaO 3111 interfaces Hugo Witt Srijani Mallik Luis Moreno Vicente-Arche Gerbold M enard Guilhem Sa z Daniela Stor-

2025-05-02 0 0 612.08KB 7 页 10玖币

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Patterning of superconducting two-dimensional electron gases based

on AlOx/KTaO3(111) interfaces

Hugo Witt Srijani Mallik Luis Moreno Vicente-Arche Gerbold M´enard Guilhem Sa¨ız Daniela Stor-

naiuolo Maria D’Antuono Isabella Boventer Nicolas Bergeal Manuel Bibes*

H. Witt, Dr. S. Mallik, Dr. L. M. Vicente-Arche, Dr. I. Boventer, Dr. M. Bibes

Unit´e Mixte de Physique, CNRS, Thales, Universit´e Paris-Saclay, 1 Avenue Augustin Fresnel, 91767

Palaiseau, France

Email Address: manuel.bibes@cnrs-thales.fr

H. Witt, Dr. G. M´enard, Dr G. Sa¨ız, Dr. N. Bergeal

Laboratoire de Physique et d’Etude des Mat´eriaux, ESPCI Paris, Universit´e PSL, CNRS, Sorbonne Uni-

versit´e, Paris, France

Dr. D. Stornaiuolo, Dr. M. D’Antuono

CNR-SPIN, Napoli, Italy

Dipartimento di Fisica, Universit`a di Napoli “Federico II”, Napoli, Italy

Keywords: oxide interfaces, superconductivity, two-dimensional electron gas, KTaO3, ﬁeld-eﬀect device

The versatility of properties displayed by two-dimensional electron gases (2DEGs) at oxide interfaces has fostered intense research

in hope of achieving exotic electromagnetic eﬀects in conﬁned systems. Of particular interest is the recently discovered superconducting

state appearing in (111)-oriented KTaO3interfaces, with a critical temperature Tc≈2 K, almost ten times higher than that of SrTiO3-

based 2DEGs. Just as in SrTiO3-based 2DEGs, fabricating devices in this new system is a technical challenge due to the fragility of the

2DEG and the propensity of bulk KTaO3to become conducting outside the devices upon adventitious oxygen vacancy doping. Here, we

present three diﬀerent techniques for patterning Hall bars in AlOx/KTaO3(111) heterostructures. The devices show superconducting

transitions ranging from 1.3 K to 1.78 K, with limited degradation from the unpatterned thin ﬁlm, and enable an eﬃcient tuning of

the carrier density by electric ﬁeld eﬀect. The array of techniques allows for the deﬁnition of channels with a large range of dimensions

for the design of various kinds of devices to explore the properties of this system down to the nanoscale.

The study of heterostructures based on KTaO3gained signiﬁcant interest with the discovery of super-

conductivity in two-dimensional electron gases (2DEG) at interfaces between KTaO3and various overlayers

[1, 2, 3]. Taking inspiration from the research on SrTiO3interfaces [4], KTaO3-based 2DEGs could serve

as the main building block for devices with applications in spintronics [5], orbitronics [6, 7, 8], and topo-

logical quantum computing [9]. Indeed, with a superconducting critical temperature (Tc) and a Rashba

splitting of an order of magnitude higher in KTaO3(111) [1, 8] compared to SrTiO3[10, 11, 12], the func-

tional perspectives of this material look promising. However, the discovery and control of the electronic

properties of KTaO3-based interfaces, and all the more so of KTaO3(111), are hampered by diﬃculties in

device fabrication. Indeed, on the material side, the KTaO3surface is sensitive, with volatile K cations

and O anions that can be lost by annealing, etching or polishing. Regarding processing, the use of classic

ultraviolet (UV) lithography polymer resist during growth is excluded by high temperature steps that are

integral parts to the 2DEG formation process [13]. The tunability of transport properties is also limited

by the lower dielectric constant of KTaO3than that of SrTiO3[14].

Hitherto, the majority of the studies of KTaO3that show superconductivity have been performed on

unpatterned thin ﬁlm samples [1, 2, 3, 15, 16, 17]. Nonetheless, a variety of techniques have been proposed

to design Hall bars: wet HCl etching on EuO/KTaO3(111) [18, 19], etching on YAlO3/KTaO3(111) [20],

scratching on Al2O3/KTaO3(111) [21], conducting AFM charging [22] and ultra-low voltage electron beam

lithography [22] as well as using a thick amorphous Al2O3hard mask deposited by pulsed laser deposition

[2] on LaAlO3/KTaO3(111) and LaAlO3/KTaO3(110), or resorting to ionic liquid gating on KTaO3(111)

[23].

arXiv:2210.14591v1 [cond-mat.mtrl-sci] 26 Oct 2022

In this study, we report the fabrication of Hall bars in superconducting KTaO3(111) 2DEGs patterned

employing three methods: (i) stainless steel shadow masking, (ii) Ar+beam milling at cryogenic temper-

ature, and (iii) using an insulating Al2O3hard mask deposited by sputtering. Combined with a simple Al

metal deposition method to create the 2DEG [3], these patterning recipes may be easily implemented using

classic lithography capabilities. We demonstrate a ﬁne tuning control of the carrier density by electric ﬁeld

eﬀect, modulated by the width of the channel.

For all samples the 2DEG was formed by growing a 1.8 nm Al ﬁlm at 500°C by DC magnetron sputtering

system (PLASSYS MP450S), as described in Ref. [3], with substrates supplied by MTI corporation. The

characterisation was conducted by optical and atomic force microscopy, preliminary transport measure-

ments were done in a Quantum Design Dynacool system and low temperature transport measurements in

a dilution refrigerator from 30 mK to 300 K.

Figure 1: Shadow mask method (a) Sketch of the fabrication principle. The stainless steel mask is brought as close as

possible to the substrate to minimize the shadow eﬀect. (b) Optical microscopic view of the 250 µm Hall bar. The horizontal

black line is a scratch on the substrate back side used to keep track of the crystallographic orientation of the substrate. (c)

Low-temperature dependency of the longitudinal resistivity showing the superconducting transition at 1.78 K.

The ﬁrst method we employed to pattern a Hall bar on KTaO3(111) substrates was shadow masking

with a stainless steel mask. The advantage of this technique is the possibility to minimize the chemical

impact on the surface of the as-received substrate. However, the dimensions of the features are limited by

the resolution of the steel cutting technique.

We used a 150 µm thick laser cut stainless steel mask in a 4 mm x 4 mm design of a 250 µm wide and

2 mm long Hall bar, mounted ex situ on the substrate plate without prior treatment to the substrate. A

thin layer of Al was then sputtered on the KTaO3substrate, as described in Figure 1 (a) and Ref. [3].

The shadow eﬀect is negligible compared with the dimensions of the device and channels are well deﬁned

(Figure 1 (b)). Atomic force microscopy (not shown) revealed a smooth surface on the Hall bar, comparable

to thin ﬁlm samples. X-ray photoelectron spectroscopy showed no contamination by the mask.

Figure 1 (c) displays the low-temperature dependence of the sheet resistance. We observe a metallic

behavior characteristic of a 2DEG, with a superconducting transition at Tc≈1.78 K, replicating the be-

haviour of an unpatterned thin ﬁlm [3].

The second method yielding a superconducting 2DEG channel at the AlOx/KTaO3(111) interface is

liquid N2-cooled argon ion beam etching. Ion beam etching is seldom used to pattern 2DEG-hosting

interfaces due to the formation of oxygen vacancies at the substrate’s surface upon ion irradiation [24, 25].

The vacancies dope the substrate with electrons, shunting the 2DEG in transport. KTaO3is anologous

to SrTiO3in this regard [26]. To avoid this eﬀect on the LaAlO3/SrTiO3system, a low energy ion beam

can be used to suppress conductivity by turning the LaAlO3amorphous without physically etching it,

avoiding SrTiO3irradiation [27]. Another approach consists in reducing the kinetics of the oxygen vacancy

formation and diﬀusion out of the sample by cooling it with liquid nitrogen during the ion beam etching

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Patterningofsuperconductingtwo-dimensionalelectrongasesbasedonAlOx/KTaO3(111)interfacesHugoWittSrijaniMallikLuisMorenoVicente-ArcheGerboldMenardGuilhemSazDanielaStor-naiuoloMariaD'AntuonoIsabellaBoventerNicolasBergealManuelBibes*H.Witt,Dr.S.Mallik,Dr.L.M.Vicente-Arche,Dr.I.Boventer,Dr.M.BibesUnit...

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Patterning of superconducting two-dimensional electron gases based on AlO xKTaO 3111 interfaces Hugo Witt Srijani Mallik Luis Moreno Vicente-Arche Gerbold M enard Guilhem Sa z Daniela Stor-

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