The electrical and optical anisotropy of rhenium-doped WSe2 single crystals

Single crystals of rhenium-doped WSe2 with large edge plane have been grown by the chemical vapour transport method using bromine as the transporting agent. From the x-ray diffraction patterns, both the doped and undoped crystals are found to crystallize in the 2H structure. The role of rhenium in affecting both the electrical and optical properties of the WSe2 crystals is examined. The thicker Re-doped WSe2 samples enable easier exploration of the electrical and optical anisotropies of the materials both parallel and perpendicular to the crystal c-axis. The samples are n-type in nature from Hall measurements and the conductivity anisotropy decreases drastically as a result of doping. Photovoltage measurements revealed that both undoped and Re-doped WSe2 are indirect semiconductors. The indirect band gap of the doped sample showed considerable red shift compared with the undoped sample and also exhibited anisotropy along and perpendicular to the c-axis. The anisotropy in the band gap is attributed to crystal anisotropy, which was also studied via the polarization-dependent electrolyte electroreflectance measurements taken along the van der Waals plane (E perpendicular to c; k parallel to c) and the as-grown edge plane (E perpendicular to c, E parallel to c; k perpendicular to c). The strong dependence of excitonic transitions A and B on polarization compared with that of the undoped samples.