Structure and composition of bismuth telluride topological insulators grown by molecular beam epitaxy

Abstract

The structure and composition of Bi2Te3−δ topological insulator layers grown by molecular beam epitaxy is studied as a function of beam flux composition. It is demonstrated that, depending on the Te/Bi2Te3 flux ratio, different layer compositions are obtained corresponding to a Te deficit δ varying between 0 and 1. On the basis of X‐ray diffraction analysis and a theoretical description using a random stacking model, it is shown that for δ≥ 0 the structure of the epilayers is described well by a random stacking of Te–Bi–Te–Bi–Te quintuple layers and Bi–Bi bilayers sharing the same basic hexagonal lattice structure. The random stacking model accounts for the observed surface step structure of the layers and compares very well with the measured X‐ray data, from which the lattice parameters a and c as a function of the chemical composition were deduced. In particular, the in‐plane lattice parameter a is found to continuously increase and the average distance of the (0001) hexagonal lattice planes is found to decrease from the Bi2Te3 to the BiTe phase. Moreover, the lattice plane distances agree well with the linear interpolation between the Bi2Te3 and BiTe values taking the strain in the epilayers into account. Thus, the chemical composition Bi2Te3−δ can be directly determined by X‐ray diffraction. From analysis of the X‐ray diffraction data, quantitative information on the randomness of the stacking sequence of the Bi and Te layers is obtained. According to these findings, the layers represent random one‐dimensional alloys of Te–Bi–Te–Bi–Te quintuple and Bi–Bi bilayers rather than a homologous series of ordered compounds.

Topological Insulator Film Growth by Molecular Beam Epitaxy: A Review

“…Fulop et al [81] found that a beam equivalent pressure ratio below 17 favored the growth of Bi 4 Te 3 , though this growth was not conducted using a cracking source. When Steiner et al [85] studied Bi 2 Te 3-δ (where δ is the Te deficit) they found that δ close to 0 produced the most uniform quintuple layer steps despite good streaky reflection high energy electron diffraction (RHEED) being present for δ from 1 to 0, as shown in Figure 8. Therefore, RHEED may not be a reliable metric for achieving the correct composition.…”

Structural and electronic properties of manganese-doped Bi2Te3epitaxial layers

“…Indeed, recent compositional analysis of bulk crystals has revealed that Mn may also be incorporated interstitially as in Mn x Bi 2 Te 3 [14], but no precise information on the actual incorporation sites was reported. It has also been suggested that Mn may incorporate in excess Bi-Bi double layers [12,15] easily formed in bismuth telluride when grown under tellurium deficient conditions [12,18,19]. Thus, the mechanism of Mn incorporation has been rather unclear.…”

High-resolution x-ray diffraction of epitaxial bismuth chalcogenide topological insulator layers

“…The growth temperature was set constant to 350 °C for all samples, the layer thicknesses were between 350 and 500 nm. The details of the growth can be found in [15].…”

“…In the intermediate cases, only some diffraction maxima survive. In order to describe this effect we developed a statistical model of a Bi m X n lattice assuming that the number of QLs between two following Bi-Bi DLs is random with the mean value N. In reference [15] we presented the details of the model and a formula for the diffracted intensity averaged over all random sequences of QLs and DLs. The mean result of the model connects the full-widths at half maxima (FWHMs) δQ of the modulation satellites with the root-mean square (rms) deviation σ N of the number of QLs between two DLs according to …”

“…However, little is known about the real structure and crystal quality of Bi m X n thin films grown by molecular beam epitaxy (MBE). In our previous papers [14][15][16] we investigated the crystal structure of MBE grown epitaxial Bi 2 Te 3 and Mn-doped Bi 2 Te 3 layers using highresolution x-ray diffraction (HRXRD) and x-ray absorption spectroscopy (EXAFS) and angle-resolved photoelectron spectroscopy (ARPES). We found that the chemical composition, crystal structure as well as the crystal quality substantially depend on the excess flux of Te during the MBE growth.…”