P. Lošt’ák

High room-temperature figure of merit of thin layers prepared by laser ablation from Bi2Te3 target

The figure of merit ZT is measured by a Harman method on simple devices prepared on single thermoelectric layers of different thicknesses. The thermoelectric layers are prepared at different conditions by laser ablation from Bi2Te3 target. The best measured figure of merit ZT is for our devices ZT=2.65. This result is comparable with the results obtained on superlattices. ZT oscillated with the thickness of the layers. On some devices the Seebeck coefficient is measured and using conductivity measurements along the thermoelectric layers the thermal conductivity is estimated from ZT. The low thermal conductivity of samples is explained by the quantum size effect and by existence of few phases of type Bi2(m+n)Te3n in the thermoelectric layers.

Defect structure of Pb-doped Bi2Te3 single crystals

Pb-doped Bi2Te3 single crystals with a Pb concentration of c Pb = 0−9.4 ×  1025 m−3 were prepared from the elements Bi, Pb and Te of 99.999% purity by means of the Bridgman method. Samples of these crystals were characterized by measurements of the Hall coefficient R H(B∥c), electrical conductivity σ⊥c and Seebeck coefficient α(c⊥ΔT) over the temperature range 90–420 K. It was found that Pb impurities in Bi2Te3 behave as acceptors; some (40%) of the incorporated Pb atoms are electrically inactive. This effect is explained by the model of crystal lattice defects based on the assumption that the Pb atoms interact with native defects in the crystal lattice of undoped Bi2Te3, that is with antisite defects and vacancies in the Te sublattice.

n-type to p-type crossover in quaternary BixSbyPbzSe3 single crystals

We report on the preparation and some physical properties of a quaternary system based on Bi2Se3 codoped with Sb and Pb. Single-crystal samples were prepared using the Bridgman technique and were characterized by measurements of the lattice parameters, electrical resistivity, Hall coefficient, Seebeck coefficient, and thermal conductivity. Atomic emission spectroscopy was used to find the concentration profiles of Sb and Pb along the single-crystalline ingots. Progressive codoping of the Bi2Se3 crystal lattice with Sb and Pb leads to a crossover of the initially n-type conduction to that of the p type. It is assumed that both Sb and Pb enter the Bi sublattice. Physical properties as well as the change in the dominant carrier type are discussed.

Magnetic and transport properties of the V2–VI3 diluted magnetic semiconductor Sb2−xMnxTe3

We have measured electrical and magnetic properties of single crystals of Sb2−xMnxTe3 with x=0–0.045 at temperatures of 2 K to 300 K. Hall effect measurements indicate that each manganese atom donates approximately one hole to the valence band. The magnetic susceptibility is paramagnetic down to 2 K, and both Curie–Weiss and Brillouin analyses show that manganese substitutes for Sb and takes the Mn2+ state with S=5/2. Contrary to the case of III–V host matrices, manganese does not stimulate ferromagnetic order in the family of bulk layered V2–VI3 diluted magnetic semiconductors, at least in the range of magnetic impurity and carrier concentrations studied here.

Defects in Bi2Te3−x Se x single crystals

Single crystals of a ternary system based on Bi2Te3−x Se x (nominally x=0.0–0.2) were prepared using the Bridgman technique. Samples with varying content of Se were characterized by the measurement of lattice parameters, electrical conductivity σ⊥c and Hall coefficient R H(B‖ c). The actual concentration of selenium c Se in the samples was determined using atomic emission spectroscopy. While a small selenium concentration enhances the free hole concentration P after passing a maximum, the hole concentration decreases at higher selenium concentrations. The extreme-like dependence P=f(c Se) is explained in terms of a change of the native defect concentration due to the substitution of selenium atoms by tellurium ones.

Defect structure of Sb2−xCrxTe3 single crystals

Single crystals of Sb2Te3 doped with Cr (cCr=0–6×1020 cm−3) were prepared by the Bridgman method. The measurements of the Hall coefficient reveal a nonmonotonous dependence of hole concentrations on the Cr content in the crystal. The hole concentration decreases at low content of Cr, while at higher content of Cr it increases again. However, according to magnetic measurements, Cr atoms enter the structure and form uncharged substitutional defects CrSb×, which cannot affect the free carrier concentration directly. The observed dependence can be elucidated by means of a point defect model. The model is based on an assumption that defect structure of Sb2Te3 can be treated as hybrid Schottky and antisite defect disorder. Thus, we assume an interaction of CrSb× with the most populated native defects in the structure—antisite defects SbTe−1 and vacancies in the Te sublattice VTe+2.

Magnetic and transport properties of the V[sub 2]–VI[sub 3] diluted magnetic semiconductor Sb[sub 2−x]Mn[sub x]Te[sub 3]

We have measured electrical and magnetic properties of single crystals of Sb2−xMnxTe3 with x=0–0.045 at temperatures of 2 K to 300 K. Hall effect measurements indicate that each manganese atom donates approximately one hole to the valence band. The magnetic susceptibility is paramagnetic down to 2 K, and both Curie–Weiss and Brillouin analyses show that manganese substitutes for Sb and takes the Mn2+ state with S=5/2. Contrary to the case of III–V host matrices, manganese does not stimulate ferromagnetic order in the family of bulk layered V2–VI3 diluted magnetic semiconductors, at least in the range of magnetic impurity and carrier concentrations studied here.

nn-type to pp-type crossover in quaternary BixSbyPbzSe3BixSbyPbzSe3 single crystals

We report on the preparation and some physical properties of a quaternary system based on Bi2Se3 codoped with Sb and Pb. Single-crystal samples were prepared using the Bridgman technique and were characterized by measurements of the lattice parameters, electrical resistivity, Hall coefficient, Seebeck coefficient, and thermal conductivity. Atomic emission spectroscopy was used to find the concentration profiles of Sb and Pb along the single-crystalline ingots. Progressive codoping of the Bi2Se3 crystal lattice with Sb and Pb leads to a crossover of the initially n-type conduction to that of the p type. It is assumed that both Sb and Pb enter the Bi sublattice. Physical properties as well as the change in the dominant carrier type are discussed.