Layer Stacking-Induced\nTransition to a Kane-Band Semiconductor\nin Sb<sub>2</sub>Te<sub>3</sub>

We investigate the electronic properties of phases in\nthe Sb<sub>2</sub>Te<sub>3</sub> system based on the alternate AA\nstacking that\nwas recently experimentally identified in Sb<sub>2</sub>Te<sub>3</sub> nanowires. The conventional ABC stacking of these materials is the\nbasis of well-established thermoelectric materials related in part\nto their complex topological electronic structures. We find topological\nbehavior in AA-stacked Sb<sub>2</sub>Te<sub>3</sub>, as in the ABC-stacked\ncase, but the carrier pockets are distinctly different from those\nin the ABC-stacked case. While the electronic structure near the band\nedges remains three-dimensional with the AA-stacked Sb<sub>2</sub>Te<sub>3</sub>, it is less so than in the ABC case, particularly\nfor n-type. The band structure shows a Kane band shape in the in-plane\ndirection, with a small band gap and small effective masses. We find\nan unusual combination of low in-plane effective mass and relatively\nhigh optical absorption. This combination is a sought-after feature\nfor infrared detection. AA-stacked Sb<sub>2</sub>Te<sub>3</sub> has\na quasi-direct band gap (0.14 eV) and ultrasmall hole and electron\ntransport effect masses at low carrier concentrations (e.g., <i>m</i><sub>e/h(ab)</sub><sup>*</sup> ≈ 2 × 10<sup>–2</sup> at 10<sup>17</sup> cm<sup>–3</sup> and 2 × 10<sup>–3</sup> at 10<sup>16</sup> cm<sup>–3</sup>). The distinctly different electronic\nproperties of AA-stacked Sb<sub>2</sub>Te<sub>3</sub> as compared\nto the conventional ABC stacking yield different properties that may\npotentially be exploited in bulk form or in heterostructures. We additionally\nreport results for the hypothetical AA-stacked Bi<sub>2</sub>Te<sub>3</sub> sister compound.