​Department of Materials Science

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Abstract: Chalcogenide thin films have become of interest in energy conversion, as thermoelectric materials, and for spintronic applications. Amongst them, antimony telluride (Sb₂Te₃) and bismuth telluride (Bi₂Te₃) have gained attention as Topological Insulators (TI) [1,2]. In order to make a step toward technology transfer, it is of major importance to achieve epitaxial quality-TI on large area, Si-based substrates.<br/>We have recently developed Metal Organic Chemical Vapor Deposition (MOCVD) processes to grow Bi₂Te₃ and Sb₂Te₃ thin films on top of 4” Si(111) substrates [1,3]. In this contribution we report clear evidence of the existence of topologically-protected surface states (TSS) in both ~90 nm-thick Bi₂Te₃ and ~30 nm-thick Sb₂Te₃ films by making use of magnetotransport (MR) and angle resolved photoemission spectroscopy (ARPES) studies.<br/>MR measurements were performed in the Van der Pauw configuration on ~1 x 1 cm² samples without any processing or capping layers, in the 5-295 K temperature range.<br/>Following MR, samples were analysed by <i>ex situ</i> ARPES. In order to make this possible, prior to ARPES, the samples were cleaned under vacuum by 1.5 keV Ar ion sputtering for about 15 sec at 10^-5 mbar. The sputtering cycles were repeated as many times as necessary to obtain a clean surface free of C and O contaminants, as verified by <i>in situ</i> X-ray photoelectron spectroscopy. As a final step, annealing under vacuum was performed at 290 ^°C for ~10 min, in order to recover the damage induced by Ar⁺ sputtering. Finally, flat and well-ordered surfaces were obtained, as checked by streaky reflection high-energy electron diffraction patterns. ARPES spectra were acquired at room temperature with a 100 mm hemispherical electron analyzer equipped with a 2D CCD detector (SPECS). The He I (21.22 eV) resonant line was used to excite photoelectrons and the energy resolution of the system was greater than 40 meV.<br/>Both Sb₂Te₃ and Bi₂Te₃ films exhibited a metallic behaviour, reaching a resistivity of 0.83 mΩ cm and 1.4 mΩ cm at 5 K, respectively. From Hall measurements, we identified the carrier type, being holes in Sb₂Te₃ and electrons in Bi₂Te₃, as expected. The evolution of the carrier density with the lowering of the temperature turned out to be different for the two samples: an increasing of the hole density for the Sb₂Te₃ and a decreasing of the electron density for the Bi₂Te₃ were observed. The corresponding mobilities displayed a maximum at 5 K, suggesting a suppression of bulk conduction at low temperature with a potentially higher contribution from the TSS. Quite interestingly, at 5 K, we detected a 430% increase of electron mobility in Bi₂Te₃, to be compared with a marginal 6% increase of hole mobility in the case of Sb₂Te₃. In both Sb₂Te₃ and Bi₂Te₃, MR measurements highlighted the presence of clear weak antilocalization (WAL) at low temperature, as shown in figures 1 and 2. WAL was interpreted in the framework of the Hikami-Larkin-Nagaoka (HLN) model as a first proof of the existence of 2D-conduction channels connected to TSS [4]. The two HLN parameters α (being connected to the number of conducting channels) and the coherence length (l_φ) were extracted by fitting the magnetoconductance values (MC). The α values were 0.3 and 0.8 for Sb₂Te₃ and Bi₂Te₃ respectively, meaning that in Bi₂Te₃ the 2D-conduction is highly dominating when compared to Sb₂Te₃, in agreement with the corresponding temperature behaviour of electron and hole mobilities. At 5.5 K, the l_φ reached the value of 74 nm in Bi₂Te₃ and 55 nm in Sb₂Te₃, again indicating a more favourable TSS-connected transport in Bi₂Te₃ than in Sb₂Te₃. Comparing the obtained α and l_φ values with those reported in the literature for Bi₂Te₃ grown by MBE [5], we observe a very similar value for α and a slightly lower coherence length for our material. For what concerns Sb₂Te₃, the obtained α and l_φ values are still lower than those previously reported for Sb₂Te₃grown by MBE [6].<br/>As clearly shown in the insets of figures 1 and 2, ARPES measurements evidenced the typical Dirac-like band structure represented by a linear dispersion relation in both Sb₂Te₃ and Bi₂Te₃ (Fermi level E_F is placed at 0 eV). ARPES showed that for both Sb₂Te₃ and Bi₂Te₃, the Dirac point is not exactly at E_F, cutting the valence band in Sb₂Te₃ and the conduction band in Bi₂Te₃, in accordance with Hall measurements. ARPES data were in nice agreement with the partial overlap between TSS and bulk conduction observed at low T in transport measurements. This is most likely the explanation why, for both materials, we did not reach the ideal α=1 value expected for a pure TSS conduction.<br/>Our results showed that the TI properties of Bi₂Te₃and Sb₂Te₃ grown by MOCVD on large areas Si substrates, are approaching those obtained by state-of-the-art methods, such as MBE, thus making a fundamental step toward potential technology transfer of TI. On the other hand, to enhance the TSS contribution in the MOCVD-grown TIs and, therefore, their functionalities, the Fermi level must be moved in the bulk band gap, closer to the Dirac point. [7] References: [1] R. Cecchini, R. Mantovan,.. and M. Longo, Phys. Status Solidi PRL, Vol 12(8), 1800155 (2018)<br/>[2] Y. L. Chen, J. G. Analytis,.. and Z. X. Shen, Science 325, 178-181 (2009)<br/>[3] M. Rimoldi, R. Cecchini,.. and R. Mantovan, RSC Advances, Vol 10(34), 19936-19942 (2020)<br/>[4] S. Hikami, A. I. Larkin, Y. Nagaoka, Prog. Theor. Phys., Vol 63(2), 707-710 (1980)<br/>[5] A. Roy, S. Guchhait, .. and S. K. Banerjee, Apl, Phys. Lett., 102(16) 163118, (2013)<br/>[6] Y. Takagaki, A. Giussani, & R. Calarco, J. Phys.: Condens. Matter 25(34), 345801 (2013)<br/>[7] C. Chang, P. Tang,.. & Q. Xue, Phys. Rev. Lett.115(13) 136801 (2015) Images: https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/944d6ea6f697f1478d5927dfced2b881.jpg 1) HLN fit of the magnetoconductance and ARPES analysis of a Sb₂Te₃ film. https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/dc881fa1b2c83587acbe82b0640f1377.jpg 2) HLN fit of the magnetoconductance and ARPES analysis of aBi₂Te₃ film.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

This dataset provides the raw data associated with the publication "Halide Perovskites as Disposable Epitaxial Templates for the Phase-Selective Synthesis of Lead Sulfochloride Nanocrystals".It contains: A readme file meant to help the user navigate the database The raw data associated with all the plots and charts found in the Main Text and in the Supplementary information. The raw data collected during the 3D electron diffraction experiments on Pb3S2Cl2 Nanocrystals. The CIF files of all the crystal structures refined in the work An atomistic model of the Pb4S3Cl2/CsPbCl3 interface, which can be visualized with the freeware software Vesta.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

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Related Article: Irene Bassanetti, Corrado Atzeri, Dario Alberto Tinonin, Luciano Marchiò|2016|Cryst.Growth Des.|16|3543|doi:10.1021/acs.cgd.6b00506

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