Atomic and electronic structure of binary 3D semiconductors in the 2D limit

A.V. Kolobov

Institute of Physics, Herzen State Pedagogical University of Russia


When the thickness of a three-dimensional crystal is decreased to the limit of a few monolayers, surface effects start to play a dominant role. Surfaces of such semiconductor slabs possess a large number of dangling bonds. In addition, because of the different electronegativities of the constituent species, the opposite are polar. Polar surfaces are intrinsically unstable thanks to the divergence of the surface energy. Several mechanisms have been considered that may stabilize such surfaces, among which the formation of planar graphene-like structures was proposed as a stabilization mechanism for wurtzite structures, whereby the trigonal-planar configuration of the cations and anions minimizes the surface dipole moment. In this talk, we consider GaN and CdTe as two examples. Our calculations demonstrated that in a few monolayer limit the wurtzite phase of GaN becomes unstable and the materials transforms into a graphite-like flat and then into a 4|8 Haeckelite phase, in which the dipoles, existing in the wurtzite phase, disappear [1]. This phase is a direct-gap semiconductor, whose properties can be controlled by application of strain. A single monolayer becomes flat, also minimizing the surface dipoles. In CdTe, on the other hand, the electronegativity difference is much smaller and, while a monolayer partially flattens, it does not become fully flat. Consequently, the formation of a Haeckelite phase is not possible. At the same time, for a two monolayer thick slab, it becomes energetically favourable to swap atomic planes from the Cd-Te-Cd-Te stacking, to a symmetric Te-Cd-Cd-Te stacking, where the metal planes are sandwiched between two chalcogen planes [2]. The structure of this slab is very similar to a generic structure of layers in van der Waals solids, where typically one-to several atomic planes are encapsulated by chalcogen atomic planes (X-Mo-X, X-In-In-X, X-Sb-X-Sb-X being examples). Consequently, a van der Waals bulk CdTe crystal can be built, whose stability has been verified by calculations of phonon dispersion curves [3]. Implications from these results for engineering new materials are further discussed. This work was supported by the Russian Science Foundation (grant 22-19-00766). 1. Kolobov, A. V., Fons, P., Tominaga, J., Hyot, B., & Andre, B. (2016). Instability and spontaneous reconstruction of few-monolayer thick GaN graphitic structures. Nano Letters, 16(8), 4849-4856. 2. Kolobov, A. V., Kuznetsov, V. G., Fons, P., Saito, Y., Elets, D. I., & Hyot, B. (2021). Polymorphism of CdTe in the Few‐Monolayer Limit. Physica Status Solidi (RRL)–Rapid Research Letters, 15(11), 2100358. 3. Kuznetsov, V.G., Gavrikov A.A., & Kolobov A.V. (to be published) Bandgap Engineering in Ultimately Thin Slabs of CdTe with Different Layer Stackings