Article pubs.acs.org/crystal
Formation Mechanism of Superconducting Fe1+xTe/Bi2Te3 Bilayer Synthesized via Interfacial Chemical Reactions Gan Wang,†,‡ Qing Lin He,† Hong-Tao He,†,‡ Hong-Chao Liu,† Mingquan He,† Jian-Nong Wang,† Rolf Lortz,† George Ke Lun Wong,† and Iam Keong Sou*,† †
Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong S.A.R., China ‡ Department of Physics, South University of Science and Technology of China, 1088 Xueyuan Rd., Nanshan District, Shenzhen, Guangdong, China S Supporting Information *
ABSTRACT: This work focuses on the formation mechanism of a superconducting Fe1+xTe/Bi2Te3 bilayer fabricated through chemical reactions between a Bi2Te3 flux and an annealed Fe layer grown on a ZnSe(111)B buffer via the molecular beam epitaxy technique. The studies using energy-dispersive X-ray spectrometry and highresolution transmission electron microscopy performed on a number of samples fabricated with different desired schemes provide evidence that several interfacial chemical reactions taking place at the Fe/ZnSe interface through the edges of voids of the annealed Fe layer contributed to the formation of the superconducting bilayer. We have also revealed that the bonding strength of the involved Fe compounds at the bottom interface of the annealed Fe layer seems to play an essential role.
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INTRODUCTION Since the discovery of superconductivity in LaFeAsO1−xFx, tremendous efforts have been devoted to studying the Fe-based high-temperature superconductors with various compositions, such as the original 1111-type ReFeAsO (Re = rare earth), 111type AFeAs (A = alkali metal), and 11-type FeX (X = chalcogen).1−3 For 1111-type and 111-type Fe-based superconductors, the trilayers are separated by a “bridging layer” consisting of alkali, alkaline-earth, or rare earth atoms and oxygen/fluorine. For 11-type FeX chalcogenide superconductors, such as FeSe, FeSe1−xTex, and FeTe (only its thin film form shows superconductivity; see below), the trilayers are bonded to each other by van der Waals forces without any bridging atoms.3 Therefore, superconductivity is realized in them with the simplest lattice structure among all Fe-based superconductors. Compared with 1111- or 111-type Fe-based superconductors, even though 11-type iron chalcogenide superconductors possess a relatively low critical temperature, they still attract considerable attention due to two reasons: (1) their lower toxicity due to the absence of arsenic and (2) their higher application potential attributed to their higher upper critical field Hc2(0) and lower anisotropy.4 It is also worth mentioning that most of the 11-type iron chalcogenide superconductors display considerable enhancement of superconductivity once they are fabricated into thin films. For instance, FeTexSe1−x films have Tc0 = 18 K (defined by zero resistance) on LaAlO3 (LAO) and 16.15 K on CaF2, while the Tc0 of the bulk FeTexSe1−x is around 14 K.4 Remarkably, FeTe only exhibits superconductivity (Tc0 = 9 K) in thin film form under certain conditions, whereas pure bulk crystals are not © 2014 American Chemical Society
superconducting either under ambient pressure or under hydrostatic pressure.5 Recently, an interface-induced enhanced superconductivity (Tc0 ∼ 30 K) was observed in single unit-cell FeSe films on SrTiO3 substrates, which is a significant improvement compared to the superconductivity of bulk FeSe.6 So far, pulsed laser deposition (PLD) has been widely used as a feasible method for the fabrication of superconducting epitaxial thin films.7 In order to grow high-quality superconducting thin films via the PLD method, several growth conditions should be optimized, such as the deposition temperature, the laser frequency, energy and the total number of shots, and the distance between the substrate and the source target.7,8 Besides the growth conditions, the preparation of a stoichiometric target is also vital for the growth, and normally, this process costs a few days. Till now, there are very limited reports of superconducting iron chalcogenide thin films grown by other common thin film synthesis techniques, such as molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), sputtering, etc. For FeSe thin films, MBE-grown single unit-cell FeSe coupling with substrates already showed great enhancement of superconductivity.6 For FeTe thin films, only Han et al. reported the successful epitaxial growth of superconducting FeTe films on LAO, LSAT, and STO via the PLD method.5 Thus, it is of considerable interest to explore more applicable growth methods for the fabrication of iron-based superconducting thin films on other traditional Received: February 27, 2014 Revised: May 2, 2014 Published: May 15, 2014 3370
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semiconducting substrates, such as Si or GaAs, etc. Recently, our group has discovered two-dimensional superconductivity at the interface of an MBE-grown epitaxial Bi 2Te3/FeTe heterostructure.9 In this study, we report a new fabrication scheme for achieving a superconducting Fe1+xTe/Bi2Te3 (FB) bilayer on a GaAs(111)B substrate using the MBE technique. By varying the growth conditions and performing detailed microstructural analysis, the growth mechanism has been studied thoroughly, which is found to be governed by a unique interfacial chemical reaction scheme.
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EXPERIMENTAL SECTION
Figure 1. Cross-sectional TEM images of a type-1 sample; the image on the right is a high-resolution image of this FB bilayer. Inset shows the FFT pattern obtained from the Fe1+xTe layer.
The growth of the superconducting FB bilayers was performed on semi-insulating epi-ready GaAs(111)B substrates in a VG V80H MBE system. The GaAs(111)B substrate was first deoxided at 580 °C until a streaky reflection high-energy electron diffraction (RHEED) pattern representing a crystalline surface appeared. A ZnSe buffer layer with a thickness of ∼90 nm was deposited at 250 °C to smoothen the surface of the substrate, followed by the deposition of an Fe thin layer of a few nanometers at 150 °C. The substrate temperature was then raised to 250 °C, and a flux provided by a Knudsen effusion cell containing a high purity source of Bi2Te3 was applied until a desired nominal thickness of Bi2Te3 was reached, where the nominal thickness is calibrated from the growth of Bi2Te3 performed on a ZnSe buffer at 250 °C. After the flux supply from the Bi2Te3 cell was ended, a ZnS or Au layer was grown in situ at 100 °C for protecting the specimen from ex situ oxidation or contamination. In this report, the experimental results of three types of samples, named type-1, type-2, and type-3, are discussed. The growth conditions and whether it is superconducting for each type of sample are listed in Table 1. The nominal thicknesses
indicates that both the FeTe and the Bi2Te3 layers possess fine single crystalline structures. Lattice-structure analysis combined with a fast Fourier transform (FFT) analysis (inset of Figure 1) shows that the FeTe layer displays a typical P4/ nmm tetragonal structure oriented along the [001] direction, while the underneath Bi2Te3 layer has an R3̅m trigonal lattice stacking along the [111] direction (confirmed by highresolution X-ray diffraction (HRXRD) analysis as described in the Supporting Information). It is worth mentioning here that the interface between the FeTe and the Bi2Te3 layers is rather sharp, which can be attributed to the van der Waals bonding nature possessed by this interface. Systematic EDS analysis on the chemical composition of the FeTe layers in several type-1 samples indicates that the stoichiometric ratio in FeTe actually is between 1.1:1 and 1.2:1; thus, it is more accurate to label the bilayer as the Fe1+xTe/Bi2Te3 bilayer (x = 0.1−0.2). It is worth pointing out that the calculated amount of Fe within the Fe1+xTe layer per unit surface area matches quite well with that of a 3 nm Fe layer; however, the combined amount of Te per unit surface area contained in the bilayer largely exceeds that of a 6 nm Bi2Te3 layer. This can be explained by the fact that Fe is more reactive in grabbing Te than Bi under the same flux provided by the Bi2Te3 effusion cell.10,11 To investigate the transport behavior of the resultant bilayer structure, a four-probe van der Pauw device was fabricated on a type-1 sample for performing the resistance versus temperature (RT) measurement. Figure 2 shows the RT curve of the FB
Table 1. Growth Conditions of Three Types of FB Bilayers and Whether They Are Superconducting (SC) name
ZnSe thickness (nm)
Fe nominal thickness (nm)
Bi2Te3 nominal thickness (nm)
SC (Y/N)
type-1 type-2 type-3
∼90 ∼90 0
3 3 3
6 12 6
Y N N
of Bi2Te3 in type-1 and type-2 samples were adjusted to be different, and for type-3 samples, the fabrication of an FB bilayer was performed on a bare semi-insulating deoxided GaAs(111)B substrate without a ZnSe buffer layer, so as to study the underlying growth mechanism of the bilayer structure. Systematic chemical and microstructural analyses were carried out on a JEOL-2010F high-resolution transmission electron microscope (HRTEM) with built-in X-ray energy-dispersive spectroscopy (EDS). Four-probe devices with dimensions of ∼2 × 5 mm were fabricated for electrical transport measurements in a Quantum Design Physical Property Measurement System (PPMS). An external voltmeter and current sources were used to enhance its performance.
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RESULTS AND DISCUSSION Cross-sectional TEM imaging studies performed on type-1 samples reveal that, except for some submicrometer-sized nanocrystals with their roots sitting at the ZnSe buffer surface, a smooth multilayer structure is dominated. The left side of Figure 1 shows a cross-sectional TEM image of the resultant epitaxial layers of a type-1 sample. Selected-area EDS analysis shows that the epitaxial layers shown in the figure are ZnS, FeTe, Bi2Te3, and ZnSe, respectively. The uniform FeTe layer is around 10 nm, while the thin Bi2Te3 layer sandwiched between FeTe and ZnSe is around 4 nm. Figure 1 displays a high-magnification TEM image on the right side, which
Figure 2. RT curve of a type-1 sample; inset shows the superconducting transition on a finer scale. 3371
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bilayer displayed in Figure 1. The inset shows a sharp drop of resistance below an onset temperature Tonset = 13 K, which is similar to the resistance drop observed in the transport properties of tensile-stress induced superconducting Fe1.08Te thin films.5 As can be seen in Figure 2, an anomaly of falling appears at ∼80 K, which marks that the RT behavior becomes a metallic type at lower temperature. This kind of RT anomaly shown around 80 K is a landmark representing a broadened first-order structural and magnetic phase transition possessed by the superconducting Fe1.08Te thin film.5 Taking the similarity of the two striking features shared by both the PLD-grown superconducting Fe1.08Te thin film and our FB bilayer into consideration, the observed superconductivity in our FB bilayer can be unambiguously attributed to the Fe1+xTe layer. A systematic study on the transport and magnetic properties of type-1 samples shows that superconductivity and ferromagnetic properties are found to coexist in the iron-rich Fe1+xTe layer. The detailed work related to the study of transport and magnetic properties will be reported elsewhere. In this report, we mainly discuss the growth mechanism governing the fabrication of the SC FB bilayer. By reviewing the whole epitaxial growth procedure of type-1 samples, one may notice that the flux provided by the Bi2Te3 cell was applied on the annealed Fe layer during the growth. However, both the structural and the chemical analyses performed on this sample reveal a fact that an FB bilayer structure instead of Bi2Te3/Fe was formed on the ZnSe surface. At least two significant points are worth discussing here: (1) the formation mechanism of the superconducting Fe1+xTe layer and (2) the intercalation of the thin Bi2Te3 layer between the Fe1+xTe layer and the ZnSe buffer. The formation of the Fe1+xTe layer is most likely induced by a chemical reaction between the flux provided by the Bi2Te3 cell and the Fe layer, which can be considered to proceed with the following reaction
Figure 3. Cross-sectional TEM image of a type-2 sample.
As can be seen in Figures 1 and 3, both type-1 and type-2 samples contain a thin Bi2Te3 layer on top of the ZnSe buffer layer, and its thickness is ∼4−5 nm. Figure 4 shows a cross-
2Fe + Te2 → 2FeTe
where Te2 is the dominant tellurium cluster that resulted from the flux provided by the Bi2Te3 cell.12 However, the above reaction only provides a possible explanation for the formation of the Fe1+xTe layer and it also leads one to expect that surplus Bi2Te3 would appear above the Fe1+xTe layer, but not under it, as what was actually observed in the FB bilayer. In fact, reaching an understanding on the formation mechanism of the bottom thin layer of Bi2Te3 seems to be quite essential for revealing the formation mechanism of the entire FB bilayer as a whole. With the aim of achieving these understandings, type-2 samples were fabricated. As can be seen in Table 1, type-2 samples were grown via the same growth sequences as those of type-1 samples, except that the nominal Bi2Te3 thickness of type-2 samples is around 12 nm, which is twice that of type-1 samples. It was found that the resulting RT curves of type-2 samples do not show any superconductivity feature. Figure 3 shows the cross-sectional TEM image obtained from a type-2 sample. In contrast to type-1 samples, the resulting layers on the ZnSe buffer of the type-2 sample are found to be Bi2Te3 (∼11 nm), FeTey (y ∼ 2, ∼17 nm), and Bi2Te3 (∼4 nm), respectively, as revealed by structural and chemical analyses. The thicker Bi2Te3 layer above the FeTey/Bi2Te3 bilayer is obviously the surplus Bi2Te3 formed after the Fe contents are completely consumed in forming the FeTey layer of the FeTey/ Bi2Te3 bilayer, which is attributed to the fact that the nominal thickness of Bi2Te3 of type-2 samples is twice that of type-1 samples.
Figure 4. Cross-sectional TEM image taken near a Bi2Te3 nanocrystal of a type-1 sample; inset shows an FFT pattern taken at the edge region of the Bi2Te3 nanocrystal.
sectional TEM image taken in the neighborhood of a submicrometer-sized Bi2Te3 nanocrystal of a type-1 sample, which provides us some insightful information about the formation mechanism of the thin Bi2Te3 layer. As can be seen in Figure 4, the root of the Bi2Te3 nanocrystal directly connects with the thin Bi2Te3 layer of the FB bilayer. This observation leads us to propose that the thin Bi2Te3 layer might be intercalated into the Fe/ZnSe interface through the voids of the annealed Fe layer prior to the formation of the upper Fe1+xTe layer. This hypothesis was investigated by preparing a 3 nm Fe layer grown on a ZnSe buffer, which was then annealed at 250 °C for 2 min. The results of cross-sectional TEM imaging performed on this sample indeed reveal that some voids of submicrometer size within the resulting Fe layer formed during 3372
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the annealing process. On the basis of the findings described above, we propose, in the following, a phenomenological model for the formation of the FB bilayer structure observed in type-1 samples, in which the underlying mechanisms for the formation of the bottom thin Bi2Te3 layer and the top Fe1+xTe layer of the bilayer as well as the cause that stops the further growth of the thin Bi2Te3 layer while its thickness reaches ∼4−5 nm are addressed. Figure 5 displays a schematic drawing illustrating the interfacial growth mechanism of the FB bilayer of type-1
nanocrystals initiate their growth at the edges of the voids. After a nanocrystal grows to a certain size from the edges of a void, it will close up the void, and thus, it will block the source supply for the growth of the sliding-in Bi2Te3 thin film. That is why the latter terminates its growth when it reaches a thickness of ∼4− 5 nm. The edges of the Bi2Te3 nanocrystal being the (111) planes has another consequence that, even at the time when the height of the nanocrystals has reached the height of the Fe layer, the (111) edges of the nanocrystals could still act as sliding planes for the Te2 clusters to enter the Fe layer until basically all the Fe contents are consumed to form Fe1+xTe. The nominal thickness ratio of Fe over Bi2Te3, that is, 0.5, used for the fabrication of type-1 samples seems to be a right ratio for forming a superconducting FB bilayer. For type-2 samples, this ratio is reduced to 0.25, and it is worth mentioning that, at the end of the first-half period of supplying the flux from the Bi2Te3 cell, a bilayer structure exactly the same as that of type-1 samples should have been formed. The resulting FeTey layer with y ∼ 2 in type-2 samples, as shown in Figure 3, is believed to come from a conversion of the Fe1+xTe layer through the surplus Te flux during the second-half period of the flux supply, which is the underlying cause of why the type-2 samples are not superconducting. As it is revealed in Figure 3 as well, the remaining oversupply of Bi2Te3 flux results in a deposition of a multiphase Bi2Te3 on top of the FeTey layer, which was grown at a fast growth rate, making its resulting thickness to be even thicker than the nominal thickness of Bi2Te3 (which is calibrated from the high-quality expitaxial growth performed on a smooth ZnSe buffer). Regarding the above proposed formation mechanism of the FB bilayer structure, some straightforward questions may arise, such as whether the formation of the first Te-Fe-Te triatomic layer, which makes the growth of the sliding-in Bi2Te3 thin film become possible, requires a specific interfacial condition and whether the ZnSe buffer is needed to fulfill this requirement. To investigate these issues, a type-3 sample was fabricated, in which the growth schemes used for fabricating type-1 samples were repeated except that the growth and annealing of the Fe layer and the supply of Bi2Te3 flux were conducted directly on a semi-insulating epi-ready GaAs(111)B substrate without the growth of a ZnSe buffer layer. The measured RT curve of a type-3 sample shows the typical metallic behavior without any signature of superconductivity and the first-order structural and magnetic phase transition of an FeTe thin film as observed on type-1 samples. Figure 6 shows the cross-sectional TEM image of a type-3 sample, in which one can see that the resulting structure is a Bi2Te3/Fe bilayer with fine crystallinity. In particular, the annealed Fe layer (∼3 nm) on top of the bare GaAs(111)B substrate is relatively more continuous than that grown on top of a ZnSe buffer. These observations indeed lead us to seek the answers to the questions raised in the beginning of this paragraph and indicate that the ZnSe buffer plays an important role in the formation of the FB bilayer structure observed in type-1 samples. Below, we will address these issues based on the difference in the bonding strength of the different iron compounds involved in the interfaces of type-1 and type-3 samples. For (111)B oriented zinc-blende structures, the nonmetal elements are terminated at the top surface;14,15 thus, one should expect that a GaAs(111)B substrate has an Asterminated top surface, whereas a ZnSe(111)B buffer has a Seterminated top surface, which have been confirmed by conversion beam diffraction characterization performed on a ZnSe buffer grown on a GaAs(111)B substrate (see Figure 2 in
Figure 5. Illustration of the interfacial growth mechanism of the FB bilayer structure of type-1 samples.
samples. As shown, an Fe layer with voids is formed after the Fe layer deposited on a ZnSe buffer has gone through an annealing process at 250 °C. The flux provided by the heated Bi2Te3 effusion cell is likely composed of BiTe molecules and Te2 clusters.12 On the basis of the step edge growth mode usually taken during the epitaxial growth of Bi2Te3 thin film,13 the existence of the voids in the Fe layer makes the kinks at the edges of these islands become the most favorable sites for the arriving BiTe molecules and Te2 clusters. Because of the high reactivity between Fe and Te, Te2 clusters can enter the bottom Fe layer to form a Te-Fe-Te triatomic layer. Once this Te-FeTe triatomic layer is formed, a van de Waals gap arises at the interface next to the top of the ZnSe buffer layer. As this gap has a much weaker bonding nature, it allows the accumulated BiTe molecules and Te2 clusters at the kink edges to quickly slide in to form a thin Bi2Te3 layer under the Te-Fe-Te triatomic layer. At the same time, FeTe will become thicker and thicker by further consuming the Fe contents of the Fe layer through reacting with the incoming Te2 clusters from the edges, while the growth of the Bi2Te3 nanocrystals at the voids also proceeds as well. The inset of Figure 4 shows the FFT pattern taken at the edge region of the Bi2Te3 nanocrystal, which shows that the orientation of the Bi2Te3 nanocrystal is different from that of the sliding-in Bi2Te3 thin film; the former aligns itself with the [111] direction perpendicular to the edge of the void. Similar observations can also be found in the TEM images of a number of Bi2Te3 nanocrystals, and thus, we believe that these 3373
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Article
ASSOCIATED CONTENT
S Supporting Information *
High-resolution X-ray diffraction broad scan of a type-1 sample and a covergent beam diffraction pattern of a ZnSe buffer layer. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by the Research Grants Council of Hong Kong Grants SEG_HKUST03, SEG_CUHK06, 604910, 605011, 603010, and 605512. G.W. acknowledges the support of the National Natural Science Foundation of China under grant 11204182.
Figure 6. Cross-sectional TEM image of a type-3 sample. Top and bottom insets show the FFT patterns for Bi2Te3 and Fe layers, respectively.
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REFERENCES
(1) Kamihara, Y.; Watanabe, T.; Hirano, M.; Hosono, H. J. Am. Chem. Soc. 2008, 130 (11), 3296−3297. (2) Wang, X. C.; Liu, Q. Q.; Lv, Y. X.; Gao, W. B.; Yang, L. X.; Yu, R. C.; Li, F. Y.; Jin, C. Q. Solid State Commun. 2008, 148 (11−12), 538− 540. (3) Hsu, F. C.; Luo, J. Y.; Yeh, K. W.; Chen, T. K.; Huang, T. W.; Wu, P. M.; Lee, Y. C.; Huang, Y. L.; Chu, Y. Y.; Yan, D. C.; Wu, M. K. Proc. Natl. Acad. Sci. U.S.A. 2008, 105 (38), 14262−14264. (4) Paolo, M. Sci. Technol. Adv. Mater. 2012, 13 (5), 054301. (5) Han, Y.; Li, W. Y.; Cao, L. X.; Wang, X. Y.; Xu, B.; Zhao, B. R.; Guo, Y. Q.; Yang, J. L. Phys. Rev. Lett. 2010, 104 (1), 017003. (6) Wang, Q. Y.; Li, Z.; Zhang, W. H.; Zhang, Z. C.; Zhang, J. S.; Li, W.; Ding, H.; Ou, Y. B.; Deng, P.; Chang, K.; Wen, J.; Song, C. L.; He, K.; Jia, J. F.; Ji, S. H.; Wang, Y. Y.; Wang, L. L.; Chen, X.; Ma, X. C.; Xue, Q. K. Chin. Phys. Lett. 2012, 29 (3), 037402. (7) Yoshida, K.; Yoshida, Y.; Ichino, Y.; Takai, Y.; Ichinose, A.; Matsumoto, K.; Kiss, T.; Mukaida, M. Phys. C (Amsterdam, Neth.) 2011, 471 (21−22), 1185−1188. (8) Nie, Y. F.; Brahimi, E.; Budnick, J. I.; Hines, W. A.; Jain, M.; Wells, B. O. Appl. Phys. Lett. 2009, 94 (24), 242505. (9) He, Q. L.; Liu, H.; He, M.; Lai, Y. H.; He, H.; Wang, G.; Law, K. T.; Lortz, R.; Wang, J.; Sou, I. K. Condens. Matter Supercond. 2013, arXiv:1309.6040v3 [cond-mat.supr-con]. (10) Semenkovich, S. A.; Melekh, B. T. Thermodynamic Properties of Bi2Te3, Bi2Se and Sb2Te3, Sb2Se3; Defense Technical Information Center: Fort Belvoir, VA, 1972. (11) Westrum, E. F.; Chou, C.; Grlnvold, F. J. Chem. Phys. 1959, 30 (3), 761−764. (12) Rowe, D. M., Ed. Modules, Systems, and Applications in Thermoelectrics; Taylor & Francis: Boca Raton, FL, 2012. (13) Ernst, K. H.; Ludviksson, A.; Zhang, R.; Yoshihara, J.; Campbell, C. T. Phys. Rev. B 1993, 47 (20), 13782−13796. (14) Thornton, J. M. C.; Woolf, D. A.; Weightman, P. Appl. Surf. Sci. 1998, 123−124, 115−119. (15) Osherov, A.; Matmor, M.; Froumin, N.; Ashkenasy, N.; Golan, Y. J. Phys. Chem. C 2011, 115 (33), 16501−16508. (16) Miedema, A. R. J. Less-Common Metals 1976, 46 (1), 67−83. (17) Groenvold, F. Acta Chem. Scand. 1968, 22 (4), 1219−1240.
the Supporting Information). As a consequence, one expects that an Fe-As biatomic layer is formed at the bottom of the annealed Fe layer of type-3 samples while an Fe-Se biatomic layer is formed at the bottom of the annealed Fe layer of type-1 samples. It is well-known that the bonding strengths of FeAs, FeTe, and FeSe follow a decreasing order.11,16,17 Thus, for type1 samples, Te2 clusters at the kink edge of the voids are thermodynamically more favorable to enable a bond-breaking of the interfacial Fe-Se biatomic layer and a replacement with a Te-Fe-Te triatomic layer at the top surface of the ZnSe buffer layer, which then leads to the formation of the FB bilayer structure through the several steps as described in our proposed phenomenological model. However, for type-3 samples, the interfacial Fe-As biatomic layer enjoys a higher bonding strength than FeTe and FeSe. This explains why the annealed Fe layer is relatively more continuous with a lower density of voids, and Te2 clusters at the kink edges of the voids are not thermodynamically favorable to initiate the formation of the Te-Fe-Te triatomic layer. Thus, it explains why the supply of Bi2Te3 flux on the annealed Fe layer only results in a Bi2Te3/Fe bilayer instead.
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CONCLUSIONS In this study, the formation mechanism of an MBE-grown superconducting Fe1+xTe/Bi2Te3 bilayer fabricated on a ZnSe(111)B buffer using chemical reactions between the flux provided by a Bi2Te3 cell and an annealed Fe layer is discussed. The results of chemical and microstructural characterization performed on several samples grown with different desired schemes reveal that the formation of the superconducting bilayer structure involves several interfacial chemical reactions that occurred at the Fe/ZnSe interface through the edges of voids created during the annealing of the Fe layer. Our studies also indicate that the occurrence of interfacial chemical reactions strongly depends on the bonding strength of Fe compounds at the bottom interface of the Fe layer. 3374
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