Codoping ofSb2Te3thin films with V and Cr

Abstract

Magnetically doped topological insulators (TIs) are key to realizing the quantum anomalous Hall (QAH) effect, with the prospect of enabling dissipationless electronic devices in the future. Doping of the well-established three-dimensional TIs of the (Bi,Sb)2(Se,Te)3 family with the transition metals Cr and V is now an established approach for observing the QAH state at very low temperatures. While the magnetic transition temperatures of these materials is on the order of 10's of K, full quantization of the QAH state is achieved below ∼100 mK, governed by the size of the magnetic gap and thus the out-of-plane magnetic moment. In an attempt to raise the size of the magnetic moment and transition temperature, we carried out a structural and magnetic investigation of codoped (V,Cr):Sb2Te3 thin films. Starting from singly doped Cr:Sb2Te3 films, free of secondary phases and with a transition temperature of ∼72 K, we introduced increasing fractions of V and found a doubling of the transition temperature, while the magnetic moment decreases. In order to separate the properties and contributions of the two transition metals in the complex doping scenario independently, we employed spectroscopic x-ray techniques. Surprisingly, already small amounts of V lead to the formation of the secondary phase Cr2Te3. No V was detectable in the Sb2Te3 matrix. Instead, it acts as a surfactant and can be found in the near-surface layers at the end of the growth. Our study highlights the importance of x-ray-based studies for the doping of van der Waals systems, for which the optimization of magnetic moment or transition temperature alone is not necessarily a good strategy.

Magnetic-impurity-induced modifications to ultrafast carrier dynamics in the ferromagnetic topological insulators Sb2−xVxTe3

“…The observed Sb 5p magnetic moment at the non-magnetic element is most likely induced by the V magnetic dopant. This result is in good agreement with the previous XMCD studies for Cr-or V-doped compounds [29,33,35,36,38]. More importantly, very small but clear XMCD signals are observed at the Te M 4,5 absorption edges.…”

“…The microscopic origin of the ferromagnetism and the QAH state has been extensively investigated by theoretical and experimental approaches [26][27][28][29][30][31][32][33][34][35][36][37][38]. Element-specific x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) experiments revealed that the long-range ferromagnetic order in the magnetic TIs is caused by a magnetic coupling between p-d electrons of the host and magnetic dopants because the clear XMCD signals were observed not only at V (Cr) sites but also at Sb sites [29,33,35,36,38]. In addition, first principles calculations, scanning tunneling microscopy/spectroscopy (STM/STS) and resonant photoemission spectroscopy identified the impurity states of vanadium dopants at or near the Dirac node [26,30,32,36].…”

Cr2Te3 Thin Films for Integration in Magnetic Topological Insulator Heterostructures

“…The Cr state for the covalent compound Cr 2 Te 3 (54% d 3 and 46% d 4 character in the wave function) resembles that of (V,Cr) x Sb 2− x Te 3 (ref. 26 ), but strongly differs from that of Cr x Sb 2− x Te 3 (ref. 17 ) and Cr x Bi 2− x Se 3 (refs 15,27 ).…”

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

“…In an attempt to increase the useful QAHE temperature range, a number of more unconventional doping approaches have been successfully explored, such as co‐doping, modulation doping to separate the dopant from the charge‐carrying surface state layer, and doping with higher magnetic moment rare earth ions, to name a few. Note that co‐doping with different transition metals can also lead to undesired phase separations in this materials class, while modulation doping with Cr promises greater inherent benefits owing to the nature of the doping mechanism, which have, for example, led to the first successful observation of the axion insulator state . Modulation doping of heterostructures, that is, non‐uniform doping along the growth direction, is a well‐known technique for achieving ultrahigh electron mobilities in III–V semiconductors .…”