Polymorphism of CdTe in the Few-Monolayer Limit

Decreasing the thickness of semiconductors to the few monolayer limit often results in structural relaxation and the appearance of new properties. Herein, the bistability of 2ML thick CdTe, where the zinc blende structure of the bulk phase is metastable and the stable (ground) state is represented by an inverted structure with Cd atoms sandwiched by Te planes, is demonstrated. The thermodynamic stability of both phases is demonstrated by the absence of imaginary modes in the phonon dispersion spectra of both phases fully relaxed at 0 K. Both phases are direct-gap semiconductors and the transformation from the zinc blende phase to the inverted phase is accompanied by a marked increase of the bandgap from 0.13 to 1.03 eV. In combination with the stable alpha-CdTe phase, results demonstrate the polymorphism of ultrathin CdTe. A pronounced property contrast between the phases suggests the possible use of few-monolayer CdTe for memory applications.