Ultrafast carrier and phonon dynamics in Bi2Se3 crystals

in this material, 11 particularly the electron‐electron, electron‐phonon, and phonon‐phonon interactions. In this letter, we report the ultrafast time-resolved optical spectroscopy study of Bi2Se3 crystals in both the time domain and the energy domain. Our measurements reveal three underlying relaxation processes in the transient response of Bi2Se3, each associated with different physical mechanisms. It is also shown that the relative strength of these processes is sensitive to air exposure of the samples. The observed charge trapping and air doping effects are likely due to the presence of Se vacancies, a major issue material scientists working to use the properties of Bi2Se3 will face in the near term. The Bi2Se3 single crystals studied in this work were synthesized via the Bridgman method at Purdue University and Fudan University. During crystal growth, the mixture of high purity elements was first deoxidized and purified by multiple vacuum distillations, and then heated to 850‐900 °C for 15 h, followed by a slow cool down under a controlled pressure of Se to compensate for possible Se vacancies. Afterwards, the samples were zone refined at a speed of 0.5‐1.5 mm/hour with a linear temperature gradient set to 4‐5 °C /cm, until a temperature of 670 °C was reached. The as-grown Bi2Se3 crystals from both groups are naturally n-doped due to remnant Se vacancies. 4 Hall mea