P. Lošťák

Inversion of conductivity type in Bi2Te3−xSx crystals

Abstract Transport parameters and optical properties of Bi 2 Te 3− x S x single-crystals with x=0–0.18 were studied. With increasing sulphur content the concentration of free current carriers decreases up to x =0.12, due to the interaction of S x Te defects with antisite defects Bi ′ Te , and then the p -type conductivity changes to the n -type. The optical gap of Bi 2 Te 3− x S x crystals increases with increasing S content. The obtained results led to the preparation of Bi 2 Te 3 -Bi 2 Te 3− x S x p – n junction by the heat treatment of p -type Bi 2 Te 3 in S vapours.

Structural defects in Cu-doped Bi2Te3 single crystals

The relation between the concentration of free charge carriers and the concentration of copper atoms in Bi2Te3 single crystals doped with copper over a wide range of concentrations has been investigated, with the aim of clarifying the existence of inactive Cu ions. Changes in the concentration of free charge carriers arising from Cu-doping of the melt with that induced by electrochemical intercalation of copper are compared. Models of possible defect structures are proposed for both doped and intercalated single crystals of Bi2Te3.

Figure of merit of quaternary (Sb0.75Bi0.25)2−xInxTe3 single crystals

Single crystals of a quaternary system based on (Sb0.75Bi0.25)2Te3 doped with In were prepared using the Bridgman technique. Samples with varying contents of In were characterized by the measurements of electrical conductivity σ⊥c, Hall coefficient RH(B∥c), Seebeck coefficient S(ΔT⊥c), and thermal conductivity κ(ΔT⊥c). The measurements indicate that by incorporating In in (Sb0.75Bi0.25)2Te3 one lowers the concentration of free holes. This effect is explained in terms of a point defect model in the crystal lattice. We also discuss the temperature dependence of the thermoelectric figure of merit Z=σS2∕κ of the samples. It is observed that low concentrations of In atoms in the (Sb0.75Bi0.25)2Te3 crystal lattice result in a substantial increase in the parameter Z in the temperature region 100–300K.

Electrical and optical properties of TL-doped Bi2Te3 crystals

Abstract Modified Bridgman method was used to grow Bi2Te3 single crystals doped with thallium in the concentration range of 1017−1018 atoms/cm3. Experimental values of the electrical conductivity and Hall constant indicate that the introduction of Tl atoms into the lattice of p-Bi2Te3 leads to a pronounced suppression of the concentration of holes. Also it follows from the decrease of the absorption coefficient in the IR region that the concentration of holes decreases with increasing content of Tl atoms built into the lattice. Analysis of possible point defects and comparison of the structure of Bi2Te3 and TlBiTe2 suggest a probable explanation that the mentioned effect could be due to the substitution of the (Te–Bi–Te–Tl–V**Te) + 2e′ fragment for the Te1–Bi–Te2–Bi–Te1 five-layer stack in the Bi2Te3 structure.

Thermoelectric properties and nonstoichiometry of GaGeTe

The polycrystalline samples of composition Ga1+xGe1-xTe (x = −0.03÷0.07) were synthesized from elements of 5N purity using solid state reaction. The products of synthesis were identified by X-ray diffraction. The samples for transport measurements were prepared using hot-pressing. They were characterized by the measurement of electric conductivity, Hall coefficient and Seebeck coefficient over a temperature range 80–450K and thermal conductivity over a temperature range 300-500 K. The samples show all p-type conductivity and we observe an increase in hole concentration with increasing x (content of Ga). We discuss the influence of Ga/Ge ratio on the phase purity of the samples and free carrier concentration. The investigation of thermoelectric properties shows that ZT parameter of these samples is too low at room temperature but increase distinctly with temperature.The polycrystalline samples of composition Ga1+xGe1-xTe (x = −0.03÷0.07) were synthesized from elements of 5N purity using solid state reaction. The products of synthesis were identified by X-ray diffraction. The samples for transport measurements were prepared using hot-pressing. They were characterized by the measurement of electric conductivity, Hall coefficient and Seebeck coefficient over a temperature range 80–450K and thermal conductivity over a temperature range 300-500 K. The samples show all p-type conductivity and we observe an increase in hole concentration with increasing x (content of Ga). We discuss the influence of Ga/Ge ratio on the phase purity of the samples and free carrier concentration. The investigation of thermoelectric properties shows that ZT parameter of these samples is too low at room temperature but increase distinctly with temperature.