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

The structure and composition of Bi2Te3-delta 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 delta 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 delta >= 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-delta 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.