Dynamics of Defects in van der Waals Epitaxy of Bismuth Telluride Topological Insulators

Abstract

Potential applications in spintronics and quantum information processing have motivated much recent research in epitaxial films of bismuth telluride. This system is also an example of van der Waals (vdW) epitaxy, where the interface coherence between the film and substrate is based on vdW bonds instead of strong ionic or covalent bonds. Because of the weakness of the vdW bonds, the overall quality of the epitaxial films is difficult to control and structural defects are easily introduced with a significant impact on the electronic phase diagram of the epitaxial films. To elucidate the evolution of defects as a function of the growth parameters, we combine nondestructive methods for electrical and structural analysis, as well as to establish intercorrelations between structural features and density of free charge carriers. It clearly shows that point defects and twinned domains favor p-type of charge carriers. Passivation of points defects by formation of metallic bismuth bilayers (BLs) drastically changes the whole film properties. By replacing vdW bonds with weak covalent bonds, the presence of BLs increases the film stiffness, leading to a smaller lattice misfit and a larger lateral lattice coherence length. Charge carriers are flipped to n-type. A few percent of BLs can be a strategy to achieve films with enhanced performance for device applications.

X-ray diffraction tools for structural modeling of epitaxic films of an intrinsic antiferromagnetic topological insulator

“…For samples S60 and S62, these lengths are close to the nominal thickness of 20 nm. But for sample S57, the coherence length is smaller than the nominal thickness of 40 nm, probably indicating to structural defects unaccounted for in the X-ray diffrac- Although vdW epitaxy can take place on substrates with relatively large lateral lattice mismatch [11,[45][46][47], it has been demonstrated in Bi 2 Te 3 (001) films on BaF 2 (111) that even mismatch as small as 0.02% can drastically impact the lateral lattice coherence length, or the lateral size of crystalline domains [12]. In MnBi 2 Te 4 (001) films on BaF 2 (111) the lattice mismatch |∆a/a| = |a A −a s |/a S is much bigger, of about 1.1%.…”

“…A key point in vdW epitaxy is the weakness of interlayer forces. It favours, in principle, flexibilization of the lateral lattice matching requirements [32,33] at the same time that makes challenging the control of film composition and other lattice defects [12,[34][35][36]. Modeling disordered heterostructures is a necessary step towards general procedures for structural analysis of materials based on vdW epitaxy.…”

“…However, the weakness of vdW interlayer forces lead in general to systems undergoing drastic changes as a function of subtle variation in growth conditions. Finding controllable fabrication processes of such systems has proven challenging [12,13]. On top of this, the recently discovered intrinsic magnetic topological insulator MnBi 2 Te 4 has added a new chapter to the phenomenological exploration of combining non-trivial electronic band topology and magnetism.…”

Two‐Dimensional Metal Telluride Atomic Crystals: Preparation, Physical Properties, and Applications

“…[ 260 ] Morelhao et al. [ 261 ] reported a TI of 2D Bi 2 Te 3 epitaxial film with a vdW structure for potential spintronic applications. The Sb 2 Te 3 TI is also considered as an ideal tunnel barrier for the realization of the spin‐polarized carrier injection and transport in the ferromagnet/semiconductor devices.…”

Simulation of X-ray diffraction in Mn$_x$Bi$_2$Te$_{3+x}$ epitaxic films

“…[13][14][15][16][17] However, the misfit between film and substrate in-plane lattice parameters impacts the lateral lattice coherence length, that is the size of perfect crystallographic domains. 18 Small uncorrelated domains lead to thickness fluctuation that can be inferred by the absence of thickness fringes in the specular reflectivity curves. At grazing angles there is the problem that severe thickness inhomogeneity can compromise data analysis.…”