Planar Hall effect and magnetoresistance of Sb2Te3 epitaxial films

The measurements of anisotropic magnetoresistance (AMR), planar Hall effect (PHE) and temperature-dependent conductivity in materials with strong spin-orbit coupling yield valuable information about charge carrier scattering processes, localization effects, and band topology. Although electronic structure calculations establish the sesqui-chalcogenide $\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{3}$ a topological insulator (TI), detailed measurements of AMR and PHE are valuable to address the manifestations of the band topology on charge carrier transport in this system. Here, we report on measurements of the longitudinal and Hall resistivity, ${\ensuremath{\rho}}_{xx}$ and ${\ensuremath{\rho}}_{xy}$, respectively, of the $\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{3}$ films of varied crystallinity over a wide phase space of temperature ($T$), magnetic field (B), and the angle between B and charge current density (J). The films exhibit semiconducting or metallic behavior depending on their crystallinity. The epitaxial films on (0001) sapphire grown at $150{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ are metallic with a hole carrier density $({n}_{h})$ and mobility $({\ensuremath{\mu}}_{h})$ of $\ensuremath{\sim}{10}^{19}\phantom{\rule{4pt}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ and $\ensuremath{\sim}{10}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, respectively, at room temperature. The conduction in the semiconducting film exhibits a Shklovskii-Efros (SE)-type variable range hopping (VRH) at very low temperature, with a transition to the Mott type VRH at $T\ensuremath{\ge}30$ K. The SE-type VRH is characterized by a Coulomb gap of 0.3 meV and a localization length of \ensuremath{\approx}12 nm, which matches with the average crystallite size in these disordered films. While signatures of weak antilocalization are seen in the magnetoresistance (MR) of epitaxial films at $T\ensuremath{\le}20$ K, the MR at $T$ > 20 K agrees with Kohler's rule when corrected for the temperature variation of carrier density. The epitaxial films are characterized by a negative AMR and PHE which varies quadratically with magnetic field, but the orbital plots of ${\ensuremath{\rho}}_{xy}$ vs ${\ensuremath{\rho}}_{xx}$ negate the presence of a chiral anomaly in transport. The amplitude of MR anisotropy for $100{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ grown $\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{3}$ film is $\ensuremath{\approx}102\mathrm{n}\mathrm{\ensuremath{\Omega}}\mathrm{m}$ at 300 K, which is an order of magnitude larger than in $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{3}$ and potentially important for the development of AMR-based sensors.