Influence of vacuum degree on growth of Bi₂Te₃ single crystal

Bi₂Te₃ single crystals were prepared by the solid-state reaction method. The effect of the vacuum on the growth of Bi₂Te₃ single crystals was studied with varying the oxygen content by controlling the air pressure in the silica tube. High quality Bi₂Te₃ single crystals have been obtained and there is no influence on the growth by an extremely small amount of oxygen in a high vacuum at 1.0 × 10⁻³ Pa. As the air pressure is increased at 1.0 × 10⁻² Pa, oxygen only mainly impacts on the growth of the surface for the prepared samples. Micron-sized rod-like structure and flower-like clusters are observed on the surface. For the samples prepared at 1.0 × 10⁻¹ Pa, x-ray diffraction data show that the yellow part on the surface is Bi₂TeO₅, while the Bi₂Te₃ single crystal is still the major phase as the inside part. More interestingly, various crystal morphologies are observed by scanning electron microscope for Bi₂Te₃ near the boundary between Bi₂Te₃ and Bi₂TeO₅. Possible growth mechanisms for Bi₂Te₃ with different morphologies are discussed in detail. (paper)