Thermoelectric properties of $p$-type Bi$_2$Te$_3$/Sb$_2$Te$_3$ and $n$-type Bi$_2$Te$_3$/Bi$_2$Te$_{3-x}$Se$_x$ superlattices*

Thermoelectric superlattices are good candidates for obtaining high figure of merit (ZT) values. Indeed, the highest ZT of 2.4 at room temperature in $p$-type Bi$_2$Te$_3$/Sb$_2$Te$_3$ superlattices and the high ZT of 1.4 in $n$-type Bi$_2$Te$_3$/Bi$_2$Te$_{2.83} $Se$_{0.17}$ superlattices are found. \footnote{R.Venkatasubramanian, E.Siivola, T.Colpitts, and B. O'Quinn, Nature {\bf 413}, 597 (2001).} While it is well known that phonon-blocking and electron- transmission is a possible mechanism for the highest ZT in superlattices, the electron-transmission near the interface has not been studied much at the microscopic level. By first-principles calculations with the highly precise full- potential linearized augmented plane wave (FLAPW) method, \footnote{Wimmer, Krakauer, Weinert, Freeman, Phys.Rev.B, {\bf 24}, 864 (1981).} the electronic structures and thermoelectric properties of bulk Bi$_2$Te$_3$, Sb$_2$Te$_3$ and Bi$_2$Te$_{3-x}$Se$_x$ and of their superlattices Bi$_2$Te$_3$/Sb$_2$Te$_3$ and Bi$_2$Te$_3$/Bi$_2$Te$_{3-x}$Se$_x$ are investigated and will be reported.