⇦Chinese Physics Letters, 2020, Vol. 37, No. 8, Article code 087401Express Letter Zirconium Aided Epitaxial Growth of In$_{x}$Se$_{y}$ on InP(111) Substrates Cheng Zheng (郑澄)1, Dapeng Zhao (赵大鹏)1,2, Xinqiang Cai (蔡新强)1, Wantong Huang (黄万通)1, Fanqi Meng (孟繁琦)3, Qinghua Zhang (张庆华)3, Lin Tang (唐林)1, Xiaopeng Hu (胡小鹏)1, Lin Gu (谷林)3, Shuai-Hua Ji (季帅华)1,4*, Xi Chen (陈曦)1* Affiliations 1State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China 2Beijing Academy of Quantum Information Sciences, Beijing 100193, China 3Laboratory for Advanced Materials & Electron Microscopy, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 4RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan Received 20 June 2020; accepted 30 June 2020; published online 02 July 2020 Supported by the National Natural Science Foundation of China (Grant No. 11874233).
*Corresponding authors. Email: shji@mail.tsinghua.edu.cn; xc@mail.tsinghua.edu.cn Citation Text: Zheng C, Zhao D P, Cai X Q, Huang W T and Meng F Q et al. 2020 Chin. Phys. Lett. 37 087401    Abstract Layered material indium selenide (In$_{x}$Se$_{y}$) is a promising candidate for building next-generation electronic and photonic devices. We report a zirconium aided MBE growth of this van der Waals material. When co-depositing zirconium and selenium onto an indium phosphide substrate with a substrate temperature of 400℃ at a constant zirconium flux rate of 0.01 ML/min, the polymorphic In$_{x}$Se$_{y}$ layer emerges on top of the insulating ZrSe$_{2}$ layer. Different archetypes, such as InSe, $\alpha$-In$_{2}$Se$_{3}$ and $\beta$-In$_{2}$Se$_{3}$, are found in the In$_{x}$Se$_{y}$ layers. A negative magnetoresistance of 40% at 2 K under 9 T magnetic field is observed. Such an In$_{x}$Se$_{y}$/ZrSe$_{2}$ heterostructure with good lattice-matching may serve as a candidate for device applications. DOI:10.1088/0256-307X/37/8/087401 PACS:74.70.Xa, 81.15.Hi, 74.78.Fk © 2020 Chinese Physics Society Article Text The past two decades have witnessed much progress of pursuing novel materials as building blocks for next-generation electronic and photonic devices.[1–5] Two-dimensional van der Waals (vdW) materials, such as graphene,[6] black phosphorus[7] and various kinds of transition-metal dichalcogenides[8–12] (TMD), all serve as promising candidates because of their appealing electronic, optical, thermal and even mechanical properties. In the family of vdW materials, extensive studies were performed in recent years on the layered-structure III–VI group metal chalcogenides,[13–16] among which indium selenide (In$_{x}$Se$_{y}$) has stimulated great interest for its novel properties and the potential of fabricating high-performance devices.[17–26] Until now, most progresses were made by using exfoliated samples. Tamalampudi et al. reported that the photo-responsivity of broadband few-layer InSe photodetectors could be greatly enhanced from 6.9 to 157 A$\cdot$W$^{-1}$ by applying 70 V back-gate voltage.[18] Bandurin et al. found that the electron density in an atomically thin film of InSe could be controlled by gate voltage in a wide range, and therefore discovered in this thin film a high Hall mobility of $\sim $$10^{3}$ cm$^{2}\cdot$V$^{-1}$s$^{-1}$ at room temperature and $\sim $$10^{4}$ cm$^{2}\cdot$V$^{-1}$s$^{-1}$ at liquid helium temperature.[20] Si et al. created a new type of field-effect transistors (FETs) in which $\alpha$-In$_{2}$Se$_{3}$, a ferroelectric semiconductor, was employed as the channel material.[26] Such FETs could be useful in future non-volatile memory technology. Generally, it is highly desirable to build heterostructures made of In$_{x}$Se$_{y}$ together with insulating layers for gating. Layered In$_{x}$Se$_{y}$ single crystals can exist at ambient pressure in several different stoichiometries, such as InSe and In$_{2}$Se$_{3}$, each having various polymorphs.[27–32] InSe is formed by the stacking of Se–In–In–Se atomic layers. $\beta$, $\gamma$ and $\varepsilon$ phases of InSe differ in the stacking sequences from each other. However, the layered $\alpha$-In$_{2}$Se$_{3}$ and $\beta$-In$_{2}$Se$_{3}$ are determined by the atomic arrangements inside a single Se–In–Se–In–Se building block, while different stacking sequences are denoted by 2H and 3R. Despite the success of obtaining exfoliated flakes of In$_{x}$Se$_{y}$ crystals grown by the Bridgman method,[33] the size of such flakes limits its potential for further investigations. Therefore, it is worthwhile to explore the approaches to grow In$_{x}$Se$_{y}$ thin films. The growth of a highly oriented two-dimensional vdW crystal usually relies on a substrate of small lattice mismatch, even though the lacking of dangling bonds makes the vdW materials to be easily grown on various substrates with a sharp interface and low in-plain strain. To date, the attempts to epitaxially grow different phases of In$_{x}$Se$_{y}$, whether by chemical vapor deposition (CVT), physical vapor transport (PVT) or molecular beam epitaxy (MBE), have been made on various substrates such as graphene,[24,34] GaAs,[35,36] MoS$_{2}$,[37] GaSe,[38,39] SiO$_{2}$/Si,[25,40–42] mica[23,43,44] and Corning glass,[45] most of which are of large lattice mismatching. For device applications, it is still essential to find insulating substrate materials with good lattice-matching with In$_{x}$Se$_{y}$ to build heterostructures. In this study, we establish an approach to preparing heterostructure of In$_{x}$Se$_{y}$ and insulating ZrSe$_{2}$ by MBE on an InP(111) substrate. The flatness of the ZrSe$_{2}$ layers in large scale and various structures of monolayer (ML) InSe, $\alpha$-In$_{2}$Se$_{3}$ and $\beta$-In$_{2}$Se$_{3}$ are revealed by scanning transmission electron microscopy (STEM). A large negative magnetoresistance up to 40% at 2 K under 9 T out-of-plane magnetic field is consistently observed in the structures. The experiments were performed on a homemade MBE system with a base pressure lower than $2\times 10^{-10}$ torr. The semi-insulating InP(111)B substrates were ultrasonically cleaned by acetic acid and acetone successively for 20 min each before being transferred into the MBE chamber. The substrates were degassed at 380℃ by indirect heating for 5 h and then annealed at 430℃ for 20 min to eliminate oxides and obtain clean and flat surfaces. High-purity Se was deposited from a standard Knudsen cell. Atomic Zr flux was supplied by a high-purity Zr rod of 2 mm diameter in an e-beam evaporator. Due to the low vapor pressure of Zr even at its melting point, this material was deposited at a low flux rate down to 0.01 ML/min with a heating power of 24 W. During growth, the temperature of substrates was fixed at 400℃, at which the deposited atoms react with indium in the substrate to form ZrSe$_{2}$ and In$_{x}$Se$_{y}$, respectively. The residue P atoms were then desorbed into vacuum. The MBE growth was monitored by in situ reflection high energy electron diffraction (RHEED). The lattices of InP and ZrSe$_{2}$ can be clearly resolved by RHEED (Fig. 1(a)). The (111) oriented surface of the zincblende InP has an in-plane lattice constant of 4.15 Å.[46] Bulk ZrSe$_{2}$ is a CdI$_{2}$-type vdW material with lattice constants $a = b = 3.77$ Å and $c = 6.13$ Å.[47] However, the layered hexagonal InSe, $\alpha$-In$_{2}$Se$_{3}$ and $\beta$-In$_{2}$Se$_{3}$ have similar in-plane lattice constants of $\sim $4.0 Å.[27–32] Thus, identifying different phases of In$_{x}$Se$_{y}$ from RHEED patterns is difficult. Two distinct growth modes exist depending on the flux ratio between Se and Zr. It is noted that no In$_{x}$Se$_{y}$ emerges without the aid of Zr. If Se:Zr $ < 15\!:\!1$, only the RHEED pattern of ZrSe$_{2}$ will appear. If Se:Zr $> 30\!:\!1$, ZrSe$_{2}$ will be briefly visible in the RHEED pattern at the beginning of growth. Then the RHEED pattern switches to that of In$_{x}$Se$_{y}$. Such behavior indicates the film structure as shown in Figs. 1(b) and 1(c). As the growth continues, both Zr and In atoms diffuse through the film,[48] but in the opposite directions. As a result, ZrSe$_{2}$ always forms adjacent to the substrate and In$_{x}$Se$_{y}$ floats on top of ZrSe$_{2}$. It is suggested that the surface will be passivated by Se without the involvement of Zr. Zr atoms help to break the In–Se bonds, making it possible for the In atoms to persistently diffuse to the top. Given the substrate temperature and the Zr flux rate, the growth modes can be simply controlled by the Se flux rate during the growth. Figure 1(d) shows a typical morphology characterization of the second mode by atomic force microscopy (AFM), which indicates a layer-plus-island growth pattern of both modes.
cpl-37-8-087401-fig1.png
Fig. 1. (a) RHEED patterns of InP, ZrSe$_{2}$ and In$_{x}$Se$_{y}$ in the epitaxial growth process. (b) An illustration of the MBE growth. (c) Lattice structures of InP, ZrSe$_{2}$, $\beta$-InSe, $\alpha$-In$_{2}$Se$_{3}$ and $\beta$-In$_{2}$Se$_{3}$. (d) A typical morphology characterization of the sample with In$_{x}$Se$_{y}$ top layers by AFM.
The In$_{x}$Se$_{y}$/ZrSe$_{2}$ structure is confirmed by STEM. Samples prepared with large Se:Zr ratio and capped with amorphous Se are investigated by STEM measurements. Figure 2 shows the typical atomically resolved high-angle annular dark-field (HAADF) images of the heterostructure viewed along the $\langle 100\rangle$ direction. It demonstrates that 1 T-ZrSe$_{2}$ layers are sandwiched between the InP substrate and In$_{x}$Se$_{y}$ layers. The ZrSe$_{2}$ layers are flat in a large scale with the same in-plane lattice constant as that of the bulk. However, the translational symmetry along the $c$-axis is broken and the out-of-plane lattice constant is enlarged to $\sim $6.30 Å, giving rise to even more two-dimensional characteristics. The In$_{x}$Se$_{y}$ layers can be distinguished from ZrSe$_{2}$ by evidently larger in-plane lattice constants. Compared with the existing structure models, different archetypes can be identified in the polymorphic In$_{x}$Se$_{y}$, including InSe (Se–In–In–Se), $\alpha$-In$_{2}$Se$_{3}$ and $\beta$-In$_{2}$Se$_{3}$ (Se–In–Se–In–Se). All of these species are able to interface with ZrSe$_{2}$. Interestingly, if the flux ratio of Se and Zr is decreased after the emergence of In$_{x}$Se$_{y}$ during growth, the ZrSe$_{2}$ layers will reappear on top of In$_{x}$Se$_{y}$ as shown in Fig. 2(c).
cpl-37-8-087401-fig2.png
Fig. 2. HAADF-STEM images of In$_{x}$Se$_{y}$/ZrSe$_{2}$/InP(111) heterostructures viewed along the $\langle 100\rangle$ direction. (a) InSe at the interface of In$_{x}$Se$_{y}$/ZrSe$_{2}$. (b) $\beta$-In$_{2}$Se$_{3}$ at the interface of In$_{x}$Se$_{y}$/ZrSe$_{2}$. (c) In$_{x}$Se$_{y}$ sandwiched by ZrSe$_{2}$. (d) The interface between the ZrSe$_{2}$ layer and the InP substrate.
Figure 3(a) exhibits the atomic-resolved scanning tunneling microscopy (STM) image of the ZrSe$_{2}$ layer. The hexagonal lattice structure and the in-plane lattice constant of 3.8 Å are consistent with the STEM and RHEED results. The scanning tunneling spectroscopy (STS) measurement on ZrSe$_{2}$ (Fig. 3(b)) shows a bandgap of 1.1 eV, which is larger than that of bulk ZrSe$_{2}$ ($\sim $0.9 eV) and close to that of monolayer ZrSe$_{2}$ ($\sim $1.2 eV) determined by angle resolved photoemission spectroscopy (ARPES).[49] The surface of In$_{x}$Se$_{y}$ (Fig. 3(c)) has a $\sqrt 3 \times \sqrt 3$ reconstruction with the lattice constant of 6.93 Å ($\sqrt 3 \times 4.0$ Å). The energy bandgap (Fig. 3(d)) is 1.4 eV, located between the levels of the bulk and the monolayer InSe.[21] The insulator behavior of ZrSe$_{2}$ layers is confirmed by ex situ transport measurement using van der Pauw geometry.[50] The resistivity of the ZrSe$_{2}$ sample prepared with a flux ratio of Se:Zr $\sim $$15\!:\!1$ is too large to be measured even at room temperature. The film can become more conducting by reducing the flux ratio Se:Zr to be less than $5\!:\!1$ in order to generate enough Se vacancies. A positive magnetoresistance of $\sim $10% under 9 T is found at 2 K in these heavily doped ZrSe$_{2}$ films as shown in Fig. 4(a).
cpl-37-8-087401-fig3.png
Fig. 3. (a) and (b) Atomic-resolved STM image of a 10-layer ZrSe$_{2}$ and STS. (c) and (d) Atomic-resolved STM image of In$_{x}$Se$_{y}$ monolayer and STS.
The In$_{x}$Se$_{y}$/ZrSe$_{2}$ heterostructure behaves as a semiconductor (Fig. 4(b)). The sign of Hall resistance (Fig. 4(c)) reveals that the carriers in the samples are of n-type, indicating Se vacancies as the main doping source. The $R_{xy}$–$B$ curve (Fig. 4(c)) shows good linearity in the range of 0–9 T below 200 K. The sheet carrier density (Fig. 4(d)) calculated from the Hall resistance increases gradually with decreasing temperature, peaked at 90 K ($\sim $$1.2\times 10^{13}$ cm$^{-2}$), and decreases quickly to the minimum at 2 K ($\sim $$5.2\times 10^{12}$ cm$^{-2}$). On the other hand, the Hall mobility (Fig. 4(e)) drops monotonically with decreasing temperature. The mobility at room temperature ($\sim $160 cm$^{2}\cdot$V$^{-1}$s$^{-1}$) is larger than that of the In$_{x}$Se$_{y}$ epitaxially grown on other lattice-mismatching substrates,[41,42] but still not comparable with the record of 1055 cm$^{2}\cdot$V$^{-1}$s$^{-1}$ by using exfoliated samples.[17] Figure 4(f) is the typical magnetoresistance for the In$_{x}$Se$_{y}$/ZrSe$_{2}$ heterostructure. Under an out-of-plane magnetic field up to 9 T, all the films show similar negative magnetoresistance effect at low temperature with no hysteresis. At 2 K, the sheet resistivity decreases rapidly with increasing field and tends to saturate at $\sim $40% level under 9 T regardless of the different zero-field resistivity and film thickness of ZrSe$_{2}$ underneath. The negative magnetoresistance effect becomes weaker when the temperature arises, but can still be visible at 150 K. The observed negative magnetoresistance cannot be explained by the weak localization theory[51] since the conductivity ($\sim $1 µS) at 2 K of most samples is much less than the conductance quantum ($e^{2}/h \sim 39$ µS). It could be related to other effects, such as electron-electron or electron-phonon interactions. Among the three species of In$_{x}$Se$_{y}$, we are still not able to identify which one contributes to the observed negative magnetoresistance. Both InSe and In$_{2}$Se$_{3}$ have been reported to show negative magnetoresistance[52,53] and we expect them to play roles in the current case.
cpl-37-8-087401-fig4.png
Fig. 4. (a) Typical positive magnetoresistance of the heavily doped ZrSe$_{2}$ layers grown on the InP(111) substrate at 2 K. (b)–(e) Transport measurement of a sample consisting of two layers of InSe and two layers of In$_{2}$Se$_{3}$ sandwiched between 12 layers of ZrSe$_{2}$ at the bottom and 4 layers of ZrSe$_{2}$ on the top. The magnetic field dependence of the Hall resistance $R_{xy}$ shows good linearity under 200 K. The sheet carrier density $n_{\rm s}$ peaks at 90 K when $n_{\rm s} = 1.2\times 10^{13}$ cm$^{-2}$. The Hall mobility $\mu$ is 160 cm$^{2}\cdot$V$^{-1}$s$^{-1}$ at 300 K, and decrease monotonically while cooling down to 2 K. (f) Typical magnetoresistance of In$_{x}$Se$_{y}$/ZrSe$_{2}$/InP(111) heterostructure at various temperatures ranging from 2 K to 300 K.
In conclusion, by co-depositing Zr and Se onto InP(111) substrates, we have successfully grown the polymorphic In$_{x}$Se$_{y}$ layers on top of the insulating ZrSe$_{2}$. It is found that selenium flux together with the indium atoms from InP substrates are not enough to grow In$_{x}$Se$_{y}$. Zirconium is needed to avoid surface passivation and help to form layers of In$_{x}$Se$_{y}$ persistently. The establishment of this MBE growth recipe of In$_{x}$Se$_{y}$/ZrSe$_{2}$ heterostructure with good lattice matching creates new opportunity to fabricate indium selenide devices. We thank Ding Zhang for valuable discussion. References Electronics based on two-dimensional materialsSemiconductors grown large and thinNanomaterials in transistors: From high-performance to thin-film applicationsProgress, Challenges, and Opportunities in Two-Dimensional Materials Beyond GrapheneLarge-scale chemical assembly of atomically thin transistors and circuitsElectric Field Effect in Atomically Thin Carbon FilmsBlack phosphorus field-effect transistorsElectronics and optoelectronics of two-dimensional transition metal dichalcogenidesThe chemistry of two-dimensional layered transition metal dichalcogenide nanosheetsEmerging Photoluminescence in Monolayer MoS 2Single-layer MoS2 transistorsEpitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interfaceSynthesis of Few-Layer GaSe Nanosheets for High Performance PhotodetectorsGaS and GaSe Ultrathin Layer TransistorsSynthesis and Photoresponse of Large GaSe Atomic Layers2D layered group IIIA metal chalcogenides: synthesis, properties and applications in electronics and optoelectronicsBack Gated Multilayer InSe Transistors with Enhanced Carrier Mobilities via the Suppression of Carrier Scattering from a Dielectric InterfaceHigh Performance and Bendable Few-Layered InSe Photodetectors with Broad Spectral ResponseExtraordinary Photoresponse in Two-Dimensional In 2 Se 3 NanosheetsHigh electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSeTuning the Bandgap of Exfoliated InSe Nanosheets by Quantum ConfinementElectrically Driven Reversible Phase Changes in Layered In 2 Se 3 Crystalline FilmIntercorrelated In-Plane and Out-of-Plane Ferroelectricity in Ultrathin Two-Dimensional Layered Semiconductor In 2 Se 3Molecular-Beam Epitaxy of Two-Dimensional In 2 Se 3 and Its Giant Electroresistance Switching in Ferroresistive Memory JunctionThickness-dependent transition of the valence band shape from parabolic to Mexican-hat-like in the MBE grown InSe ultrathin filmsA ferroelectric semiconductor field-effect transistorRevised and new crystal data for indium selenidesNew results on the phase transformations of In2Se3Crystal Structures of α-and β-Indium Selenide, In 2 Se 3Phase Transition of In 2 Se 3X-Ray Diffraction Measurement of Lattice Parameters of In2Se3Controlled Crystal Growth of Indium Selenide, In 2 Se 3 , and the Crystal Structures of α-In 2 Se 3Large InSe single crystals grown from stoichiometric and non-stoichiometric meltsControlled Growth of Atomically Thin In 2 Se 3 Flakes by van der Waals EpitaxyEpitaxial Growth of γ-In 2 Se 3 Films by Molecular Beam EpitaxyReflection high‐energy electron diffraction studies of InSe and GaSe layered compounds grown by molecular beam epitaxyInvestigation of the growth mechanism of an InSe epitaxial layer on a MoS2 substrateOptical and photovoltaic properties of indium selenide thin films prepared by van der Waals epitaxyEpitaxial growth of γ -InSe and α , β , and γ -In 2 Se 3 on ε -GaSeBand structure of an epitaxial thin film of InSe determined by angle-resolved ultraviolet photoelectron spectroscopyControlled Synthesis of High-Quality Monolayered α-In 2 Se 3 via Physical Vapor DepositionWafer-Scale Synthesis of High-Quality Semiconducting Two-Dimensional Layered InSe with Broadband PhotoresponsePatterning two-dimensional chalcogenide crystals of Bi2Se3 and In2Se3 and efficient photodetectorsThickness-Dependent Thermal Conductivity of Suspended Two-Dimensional Single-Crystal In 2 Se 3 Layers Grown by Chemical Vapor DepositionStructural control of In2Se3 polycrystalline thin films by molecular beam epitaxyStructural and electronic properties of narrow-band-gap semiconductors: InP, InAs, and InSbNon-stoichiometry in ZrS2 and ZrSe2Special aspects of diffusion in thin filmsHfSe 2 and ZrSe 2 : Two-dimensional semiconductors with native high-κ oxidesSpin-Orbit Interaction and Magnetoresistance in the Two Dimensional Random SystemTwo-Dimensional Character of Electron Gas in Layered InSe CrystalsNegative magnetoresistance in In2Se3 single crystals
[1] Fiori G, Bonaccorso F, Iannaccone G et al. 2014 Nat. Nanotechnol. 9 768
[2] Marks T J and Hersam M C 2015 Nature 520 631
[3] Franklin A D 2015 Science 349 aab2750
[4] Butler S Z, Hollen S M, Cao L et al. 2013 ACS Nano 7 2898
[5] Zhao M, Ye Y, Han Y et al. 2016 Nat. Nanotechnol. 11 954
[6] Novoselov K S, Geim A K, Morozov S V et al. 2004 Science 306 666
[7] Li L, Yu Y, Ye G J et al. 2014 Nat. Nanotechnol. 9 372
[8] Wang Q H, Kalantar-Zadeh K, Kis A et al. 2012 Nat. Nanotechnol. 7 699
[9] Chhowalla M, Shin H S, Eda G et al. 2013 Nat. Chem. 5 263
[10] Splendiani A, Sun L, Zhang Y et al. 2010 Nano Lett. 10 1271
[11] Radisavljevic B, Radenovic A, Brivio J et al. 2011 Nat. Nanotechnol. 6 147
[12] Li M Y, Shi Y, Cheng C C et al. 2015 Science 349 524
[13] Hu P, Wen Z, Wang L et al. 2012 ACS Nano 6 5988
[14] Late D J, Liu B T, Luo J et al. 2012 Adv. Mater. 24 3549
[15] Lei S, Ge L, Liu Z et al. 2013 Nano Lett. 13 2777
[16] Huang W, Gan L, Li H et al. 2016 CrystEngComm 18 3968
[17] Feng W, Zheng W, Cao W et al. 2014 Adv. Mater. 26 6587
[18] Tamalampudi S R, Lu Y, Kumar U R et al. 2014 Nano Lett. 14 2800
[19] Jacobsgedrim R B, Shanmugam M, Jain N et al. 2014 ACS Nano 8 514
[20] Bandurin D A, Tyurnina A V, Yu G et al. 2017 Nat. Nanotechnol. 12 223
[21] Mudd G W, Svatek S A, Ren T et al. 2013 Adv. Mater. 25 5714
[22] Choi M S, Cheong B, Ra C H et al. 2017 Adv. Mater. 29 1703568
[23] Cui C, Hu W, Yan X et al. 2018 Nano Lett. 18 1253
[24] Poh S M, Tan S J, Wang H et al. 2018 Nano Lett. 18 6340
[25] Kibirev I A, Matetskiy A V, Zotov A V et al. 2018 Appl. Phys. Lett. 112 191602
[26] Si M W, Saha A K, Gao S J et al. 2019 Nat. Electron. 2 580
[27] Popović S, Tonejc A, Gržeta-Plenković B et al. 1979 J. Appl. Crystallogr. 12 416
[28] Manolikas C 1988 J. Solid State Chem. 74 319
[29] Osamura K, Murakami Y and Tomile Y 1966 J. Phys. Soc. Jpn. 21 1848
[30] Miyazawa H and Sugaike S 1957 J. Phys. Soc. Jpn. 12 312
[31] Čelustka B and Bidjin D 1971 Phys. Status Solidi A 6 301
[32] Kupers M, Konze P M, Meledin A et al. 2018 Inorg. Chem. 57 11775
[33] De Blasi C, Micocci G, Mongelli S et al. 1982 J. Cryst. Growth 57 482
[34] Lin M, Wu D, Zhou Y et al. 2013 J. Am. Chem. Soc. 135 13274
[35] Ohtsuka T, Nakanishi K, Okamoto T et al. 2001 Jpn. J. Appl. Phys. 40 509
[36] Emery J Y, Brahimostmane L, Hirlimann C et al. 1992 J. Appl. Phys. 71 3256
[37] Hayashi T, Ueno K, Saiki K et al. 2000 J. Cryst. Growth 219 115
[38] Sanchez-Royo J F, Segura A, Lang O et al. 2001 J. Appl. Phys. 90 2818
[39] Balakrishnan N, Steer E D, Smith E F et al. 2018 2D Mater. 5 035026
[40] Amokrane A, Proix F, Monkad S E et al. 1999 J. Phys.: Condens. Matter 11 4303
[41] Zhou J D, Zeng Q S, Lv D H et al. 2015 Nano Lett. 15 6400
[42] Yang Z B, Jie W J, Mak C H et al. 2017 ACS Nano 11 4225
[43] Zheng W, Xie T, Zhou Y et al. 2015 Nat. Commun. 6 6972
[44] Zhou S, Tao X, Gu Y et al. 2016 J. Phys. Chem. C 120 4753
[45] Okamoto T, Nakada Y, Aoki T et al. 2006 Phys. Status Solidi C 3 2796
[46] Massidda S, Continenza A, Freeman A J et al. 1990 Phys. Rev. B 41 12079
[47] Whitehouse C R and Balchin A A 1978 Phys. Status Solidi A 47 K173
[48] Balluffi R W and Bkakely J M 1975 Thin Solid Films 25 363
[49] Mleczko M J, Zhang C, Lee H R et al. 2017 Sci. Adv. 3 e1700481
[50]Pauw L J 1958 Philips Res. Rep. 13 1
[51] Hikami S, Larkin A I and Nagaoka Y 1980 Prog. Theor. Phys. 63 707
[52] Dmitriev A I, Kovalyuk Z D, Lazorenko V I et al. 1990 Phys. Status Solidi B 162 213
[53] Romeo N 1974 Phys. Status Solidi A 26 K187