Epitaxial growth, intrinsic point defects and electronic transport optimization of MnTe films

Abstract

The NiAs-type MnTe compound is one of important and environmental friendly p-type thermoelectric materials for generating intermediate temperature powern. The low hole concentration in the pristine MnTe greatly restricts its thermoelectric performance. However, the systematic experimental studies of thermoelectric materials are still lacking so far. In this work, MnTe thin films are grown by molecular beam epitaxy (MBE) technique, and their intrinsic point defect structures are characterized by scanning tunneling microscope (STM). Through the regulation of the intrinsic point defects, the electrical transport performances of MnTe films are remarkably improved. The results show that Mn vacancies (V<sub>Mn</sub>) and Te vacancies (V<sub>Te</sub>) are the dominant intrinsic point defects in MnTe film. With the increase of the substrate temperature (<i>T</i><sub>sub</sub>) and the decrease of the Mn:Te flux ratio, the hole concentration in MnTe film increases greatly, reaching a maximum value of 21.5 × 10<sup>19</sup> cm<sup>–3</sup>, which is one order of magnitude higher than that of the intrinsic MnTe bulk. This is attributed to the significantly increased concentration of p-type V<sub>Mn</sub> in MnTe film, and thus leads the conductivity (<i>σ</i>) and power factor (<i>PF</i>) to increase remarkably. Finally, the MnTe film grown at <i>T</i><sub>sub</sub> = 280 ℃ and Mn∶Te = 1∶12 obtains the maximum <i>PF</i> of 1.3 μW·cm<sup>–1</sup>·K<sup>–2</sup> at 483 K in all grown films. This study clarifies the characteristics of intrinsic point defects and their relationship with the electrical transport properties of MnTe based compounds, which provides an importantguidance for further optimizing their thermoelectric performances.