Oxidation Effects in Rare Earth Doped Topological Insulator Thin Films

Abstract

The breaking of time-reversal symmetry (TRS) in topological insulators is a prerequisite for unlocking their exotic properties and for observing the quantum anomalous Hall effect (QAHE). The incorporation of dopants which exhibit magnetic long-range order is the most promising approach for TRS-breaking. REBiTe3, wherein 50% of the Bi is substitutionally replaced by a RE atom (RE = Gd, Dy, and Ho), is a predicted QAHE system. Despite the low solubility of REs in bulk crystals of a few %, highly doped thin films have been demonstrated, which are free of secondary phases and of high crystalline quality. Here we study the effects of exposure to atmosphere of rare earth-doped Bi2(Se, Te)3 thin films using x-ray absorption spectroscopy. We demonstrate that these RE dopants are all trivalent and effectively substitute for Bi3+ in the Bi2(Se, Te)3 matrix. We find an unexpected high degree of sample oxidation for the most highly doped samples, which is not restricted to the surface of the films. In the low-doping limit, the RE-doped films mostly show surface oxidation, which can be prevented by surface passivation, encapsulation, or in-situ cleaving to recover the topological surface state.

Rare Earth Doping of Topological Insulators: A Brief Review of Thin Film and Heterostructure Systems

“…The value for Ho, on the other hand, is much reduced from the Hund's rules value of ≈10.6 µ B /(for Ho 3+ ) . Possible reasons for this difference in moments could be oxidation of the Ho ions, tµhe effects of crystal fields, as well as (partial) antiferromagnetic coupling between the Ho ions …”

Magnetic ordering in Ho‐doped Bi2Te3 topological insulator

“…In general, however, doping with transition metals has the intrinsic disadvantage that a divalent ion replaces the trivalent Bi due to hybridization, as it was shown for Cr doping of Bi 2 Se 3 . Doping with rare‐earth elements, on the other hand, leads to an isoelectronic and substitutional doping on the Bi site . Among them, GdBiTe 3 is particularly promising, as ab‐initio calculations predicted ferromagnetic order and the existence of the QAHE .…”

Experimental and density functional study of Mn doped Bi2Te3 topological insulator

“…Oxidation effects might also play a role in the structural disorder, as observed for Fe dopants in Bi 2 Se 3 12 and rare-earth dopants in Bi 2 Te 3 . 24 However, the Figure S4 and Table S4 of the supplementary material). These results point to Mn dopants incorporating heterogeneously, both as a substitutional and interstitial dopant in the van der Waals gap within the structure.…”

“…The presence of transition-metal (TM) and rare-earth dopants in the grown film, as well as their chemical state, can be studied by XANES, 24 hence we employed this technique to reveal the Mn chemical environment in the grown films. Figure 2(a) shows the normalized XANES spectra at the Mn K edge of the doped Bi 2 Te 3 thin film.…”