Epitaxy and structural properties of (V,Bi,Sb) 2Te3 layers exhibiting the quantum anomalous Hall effect

Abstract

The influence of Sb content, substrate type and cap layers on the quantum anomalous Hall effect observed in V-doped (Bi,Sb)2Te3 magnetic topological insulators is investigated. Thin layers showing excellent quantization are reproducibly deposited by molecular beam epitaxy at growth conditions effecting a compromise between controlled layer properties and high crystalline quality. The Sb content can be reliably determined from the in-plane lattice constant measured by X-ray diffraction, even in thin layers. This is the main layer parameter to be optimized in order to approach charge neutrality. Within a narrow range at about 80% Sb content, the Hall resistivity shows a maximum of about 10 kΩ at 4 K and quantizes at mK temperatures. Under these conditions, thin layers grown on Si(111) or InP(111) and with or without a Te cap exhibit quantization. The quantization persists independently of the interfaces between cap, layer and substrate, the limited crystalline quality, and the degradation of the layer proving the robustness of the quantum anomalous Hall effect.A quantum anomalous Hall effect (QAHE), in which a Hall plateau with a resistance of h/e 2 can be observed even in the absence of a magnetic field, was predicted to occur in ferromagnetically doped topological insulators (TIs) [1][2][3][4]. The effect was first observed in 2013 in a 5 nm Cr 0.15 (Bi 0.1 Sb 0.9 ) 1.85 Te 3 layer without a cap layer, and grown on a SrTiO 3 substrate [5]. The QAHE has since been reproduced in such tetradymite-type layers with different layer thicknesses, layer compositions, magnetic dopants (Cr or V), with/without a cap layer and on various substrates [6][7][8][9][10][11]. Perfect quantization was observed in Cr-doped (Bi,Sb) 2 Te 3 layers capped by an AlO x layer on GaAs substrates, and V-doped (Bi,Sb) 2 Te 3 layers capped by Te on SrTiO 3 substrates and Si(111) substrates [6,[9][10][11]. Many authors point out that the layer properties have to be optimized to observe the QAHE, but details of the epitaxial growth conditions, the structural layer properties and the applied characterization techniques are sparse.An apparent requirement for observing the QAHE is the optimization of Sb content in order to approach charge neutrality by compensation of n-and p-type intrinsic point defects. Further, an applied gate bias is needed to tune the Fermi level into the bandgap of topological surface states (TSSs) caused by magnetization [5]. The layer thickness, ranging from about 4 to 10 nm in the literature, likely affects the hybridization of the TSSs and possibly also the mechanisms leading to the QAHE. Theoretical predictions and experiments both reveal a degradation of the surface states due to oxidation in Bi 2 Te 3 layers and shifts of the Fermi level in epitaxial tetradymite layers due to adsorbates [12][13][14][15]. The influence of the interfaces between the layer and the sub- * M. Winnerlein and S. Schreyeck contributed equally to this work.† Karl.Brunner@physik.uni-wuerzburg.de strate, as well as between the layer and either an...

Codoping ofSb2Te3thin films with V and Cr

“…Our study highlights the difficulties of doping thin films of the (Bi,Sb) 2 (Se,Te) 3 family of compounds, which is characterized by its layered crystal structure. Doping with a single element, such as Cr [10,57] or V [12,16,19,25], results in ferromagnetically ordered films with a sizable T c and decent crystalline properties, i.e., without the occurrence of secondary phases up to rather high doping concentrations. On the other hand, adding very small amounts of another dopant, in our case V added to Cr:Sb 2 Te 3 , seems to force the formation of the direct telluride Cr 2 Te 3 and prevent the substitutional incorporation of the dopants.…”

“…It has previously been demonstrated that MTIs can be realized by magnetically doping three-dimensional TI materials of the (Bi,Sb) 2 (Se,Te) 3 type with transition metals such as Mn [6][7][8] and Fe [9], and more recently Cr and V [10][11][12][13][14]. In both V-and Cr-doped (Bi,Sb) 2 Te 3 epitaxial thin films, the QAH effect has been successfully demonstrated [4,13,15,16] at extremely low temperatures (<100 mK). Doping Sb 2 Te 3 with V has been reported to exhibit a more stable long-range ferromagnetic order than that of Cr-doped films [17], while retain-ing high magnetic transition temperatures [18,19].…”

Coexistence of Surface and Bulk Ferromagnetism Mimics Skyrmion Hall Effect in a Topological Insulator

“…[31] The temperature read by the thermocouple during the growth of all of the layers was kept constant at 190 • C. The Bi/Sb ratio was determined by X-ray diffraction (XRD) measurements of the lateral lattice constant a. [31] It was determined to be 0.21/0.79 in a 9 nm thick film with uniform V doping of 1.5 % of all atoms (device D8 exhibiting the quantum anomalous Hall effect, see section 6). Selective magnetic doping was performed by the time controlled opening and closing of the vanadium cell shutter during the MBE growth, and the magnetic doping concentration was controlled by the vanadium cell temperature.…”

“…All investigations are performed on samples with a standard Hall bar geometry prepared by optical lithography, and are equipped with an electrostatic gate that allows tuning of the Fermi level in the layers. The topological insulator heterostructures are (Bi,Sb) 2 Te 3 based, with the magnetic layers doped by V. They are grown using molecular beam epitaxy (MBE) on insulating, hydrogen passivated Si (111) substrates, and are covered in-situ with 10 nm of Te to protect the surface from aging [12,31]. The Bi/Sb ratio is 0.79/0.21 for all samples.…”

Precision measurement of the quantized anomalous Hall resistance at zero magnetic field

“…The devices we used were made of 9 nm thick films of the ferromagnetic topological insulator V0.1(Bi0.21 Sb0.79)1.9Te3, grown by molecular beam epitaxy (MBE) on a hydrogen passivated Si(111) substrate and covered in-situ by a 10 nm thick Te protective cap [24]. The devices were patterned using standard optical lithography.…”