Native defects in tetradymite Bi2(TexSe3−x) topological insulators

Abstract

Formation energies of native defects in Bi${}_{2}$(Te${}_{x}$Se${}_{3\ensuremath{-}x}$), with comparison to ideal Bi${}_{2}$Te${}_{2}$S, are calculated in density-functional theory to assess transport properties. Bi${}_{2}$Se${}_{3}$ is found to be $n$ type for both Bi- and Se-rich growth conditions, while Bi${}_{2}$Te${}_{3}$ changes from $n$ to $p$ type going from Te- to Bi-rich conditions, as observed. Bi${}_{2}$Te${}_{2}$Se and Bi${}_{2}$Te${}_{2}$S are generally $n$ type, explaining observed heavily doped $n$-type behavior in most samples. A (0/\ensuremath{-}) transition level at 16 meV above valence-band maximum for Bi on Te antisites in Bi${}_{2}$Te${}_{2}$Se is related to the observed thermally active transport gap causing a $p$-to-$n$ transition at low temperature. Bi${}_{2}$(Te${}_{x}$Se${}_{3\ensuremath{-}x}$) with $x>2$ are predicted to have high bulk resistivity due to effective carrier compensation when approaching the $n$-to-$p$ crossover. Predicted behaviors are confirmed from characterization of our grown single crystals.