Bi2Te3 Crystals Heavily Doped with Germanium Atoms

Elemental Bi, Te, and Ge of 5N purity were used to prepare Bi₂Te₃(GeTe) single crystals with germanium content varying from 0 up to 2.3 × 10²¹ cm⁻³. The samples were characterized by reflectivity measurements in the plasma resonance frequency range and by measurements of the electrical conductivity. Germanium content in the samples was determined by means of energy dispersive analysis. The reflectivity spectra were interpreted on the basis of the Drude-Zener theory in the aim to obtain information on the concentration of the free carriers in the samples. It was found that Ge atoms in the Be₂Te₃ crystal lattice behave as acceptors. A comparison of the hole concentration with the amount of germanium built into the crystal lattice revealed that only about 1/100 of the total number of Ge atoms act as acceptors. This effect is explained by two different ways of incorporation of Ge atoms into the Bi₂Te₃ lattice, viz: the formation of substitutional Ge'_Bi defects acting as acceptors and the formation of seven-layer-lamellae of the Te Bi Te Ge Te Bi Te composition, which corresponds to the structure of GeBi₂Te₄.