Magnetotransport and ARPES studies of large-area Sb2Te3 and Bi2Te3 topological insulators grown by MOCVD on Si

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.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.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.Following MR, samples were analysed by ex situ 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 in situ 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.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 (lφ) 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 lφ 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 lφ 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 lφ values are still lower than those previously reported for Sb2Te3 grown by MBE [6].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 EF is placed at 0 eV). ARPES showed that for both Sb2Te3 and Bi2Te3, the Dirac point is not exactly at EF, 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.Our results showed that the TI properties of Bi2Te3 and 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] **