Growth of High-Mobility Bi₂Te₂Se Nanoplatelets on hBN Sheets by van der Waals Epitaxy

Abstract

The electrical detection of the surface states of topological insulators is strongly impeded by the interference of bulk conduction, which commonly arises due to pronounced doping associated with the formation of lattice vacancies. As exemplified by the topological insulator Bi 2 Te 2 Se, we show that via van der Waals epitaxial growth on thin BN substrates, the structural quality of such nanoplatelets can be substantially improved. The surface state carrier mobility of nanoplatelets on hBN is increased by a factor of about 3 compared to platelets on conventional Si/SiO x substrates, which enables the observation of well-developed Shubnikov-de Haas In contrast to the quantum Hall state, these boundary states are the result of strong spin-orbit coupling without any external magnetic field, such that time reversal symmetry is conserved and accordingly backscattering is forbidden. The unique combination of dissipation-less charge transport channels with intrinsic spin polarization renders TIs of strong interest for spintronic applications, including spin injection 4,5 or spin transfer torques 6 . In addition, they hold promise for novel and exciting fundamental effects, like the formation of magnetic monopoles 7 or Majorana fermions 8 .While theory predicts TIs to be perfect bulk insulators, in practice their charge transport properties are dominated by defects. In fact, a sizeable amount of point defects (vacancies or antisite defects) is commonly introduced during the crystal growth in case of the chalcogenides Bi 2 Se 3 , Bi 2 Te 3 , Sb 2 Te 3 and their alloys, which represent the currently most intensively studied three-dimensional (3D) TIs. The presence of such defects leads to strong bulk doping in these compounds 3,9,10 . Moreover, chemical reactions at the crystal surface have been documented to impart notable doping 11,12 . The bulk doping creates a quasi-metallic conduction channel parallel to the surface channels, rendering it difficult to unequivocally assign charge transport features to the topological protected surface states. Strategies that have been pursued to minimize the contribution of the bulk transport include compensation doping 13,14 , alloying of differently doped TIs 15,16 , or increasing the surface to bulk ratio by using ultra thin samples 17 . However, doping or alloying typically results in a concomitant drastic decrease in carrier mobility due to the introduction of defects. One possibility to avoid such decrease in the structural quality of the

High-quality Bi2Te3 thin films grown on mica substrates for potential optoelectronic applications

“…The mean-free path, l, and the surface quantum mobility, l s , can be further calculated as (1.79 6 0.02) Â 10 À7 m and 0.48 6 0.03 m 2 V À1 s À1 , respectively. Remarkably, the surface mobility obtained here is comparable to that of the Bi 2 Te 2 Se thin films grown on BN substrates 24 and the Bi 2 Te 3 single crystals, 22 again assuring the high quality of our thin films.…”

Characterization of bulk hexagonal boron nitride single crystals grown by the metal flux technique

“…Chan et al [4] reported a tenfold increase in the electron mobility of thin MoS 2 layers on hBN compared to SiO 2 substrates. Similarly, Gehring et al [5] saw the surface carrier mobility of the topological insulator BiTe 2 Se increase by a factor of three on hBN compared to SiO 2 . hBN has also been proposed as a substrate for atomically thin layers of silicon (silicene) [6] and germanium (germanene) [7].…”

Termination of single-crystal Bi2Se3 surfaces prepared by various methods

“…Se vacancies are often present in the as-grown materials and are generally thought to be the reason for the natural n-type doping of Bi 2 Se 3 [18,20,30,31].…”