V.A. Kulbachinskii

Giant improvement of thermoelectric power factor of Bi2Te3 under pressure

The pressure (P) dependencies of both the thermopower (Seebeck effect) S and the electrical resistance (R) for p-type single crystals of Bi2Te3 and indium-doped bismuth telluride (InxBi2−xTe3,0.04≤x≤0.10) are reported on a pressure range of 0–8.5 GPa. The thermoelectric power factor (efficiency) (ae=S2/R) exhibits two maxima: the first one near ∼1 GPa and the second near ∼2.5–4.5 GPa. These features evidence a giant increase in the power factor by a factor of ∼10. Possible values of the dimensionless figure of merit under pressure are also estimated. The maxima are explained in terms of pressure-driven changes in an electron structure. The second feature may be also addressed to an intermediate high-pressure phase detected in x-ray diffraction studies.

The time-dependent process of oxidation of the surface of Bi2Te3 studied by x-ray photoelectron spectroscopy

The process of oxidation of the Bi2Te3 surface was investigated by x-ray photoelectron spectroscopy (XPS). The oxidized surface layer was found to have a definite thickness, with configurations where O is bonded with Bi and Te, and Bi and Te are bonded with three and four oxygens, respectively. The oxidation time dependence of the oxidized layer thickness d(t) estimated from the XPS behaved as (t-t0)1/2 when d(t) was smaller than the thickness of a single oxidized quintuple atomic layer in our oxide model and behaved as t-t1 when it was larger than that. Experimental data were compared to our oxidation process model for the layered structure with the van der Waals gap and very good agreement was found.

Thermoelectric Power and Scattering of Carriers in Bi2—xSnxTe3 with Layered Structure

Thermoelectric power, electrical resistivity, and Hall effect of p-type Bi2—xSnxTe3 (0 ≤ x ≤ 0.030) single crystals have been measured in the temperature range 4.2 to 300 K. By doping Sn atoms into the host Bi2Te3 lattice, enhancements in the thermopower and resistivity are observed in the intermediate temperature range 30 to 150 K, and activation type behaviors are found in the resistivity versus temperature curves with an activation energy of the order of 10 meV which corresponds to the Sn-induced impurity band located above the second lower valence band. For our experimental results, we have also given a qualitative discussion about the scattering mechanism. Thermospannung, elektrischer Widerstand und Halleffekt von p-Bi2—xSnxTe3 (0 ≤ x ≤ 0,030) Kristallen werden bei Temperaturen von 4,2 bis 300 K gemessen. Die Dotierung mit Sn-Atomen in den Bi-Schichten von Bi2Te3 erhoht die Thermospannung und den Widerstand in einem mittleren Temperaturgebiet von 30 bis 150 K, und die Temperaturabhangigkeit des Hallkoeffizienten oder des Widerstands ergibt eine Aktivierungsenergie in der Grose von 10 meV, die dem Sn-induzierten Fremdband, das oberhalb des zweitniedrigsten Valenzbands liegt, entspricht. Fur diese experimentellen Ergebnisse geben wir eine qualitative Diskussion uber das Bandmodel und den Einflus der Sn-Dotierung auf die elektronischen Eigenschaften.

Influence of Ti doping on galvanomagnetic properties and valence band energy spectrum of Sb2-xTixTe3 single crystals

The Shubnikov-de Haas (SdH) effect in Sb2-xTixTe3 single crystals was investigated as a function of Ti content for 0 0.001 we observed the second frequency of the SdH oscillations associated with the lower valence band. The second frequency decreased with increasing x. From the temperature dependence of the resistivity and the Hall effect in the temperature range 4.2<T<300 K, it was found that the increase of Ti content suppresses the initial hole concentration. Parameters of the crystal lattice and thermopower were measured at room temperature as a function of Ti content.

Influence of Sn on Galvanomagnetic Properties of Layered p‐(Bi1—xSbx)2Te3 Semiconductors

The temperature dependence of the resistivity and the Hall effect in the range 0.3-300 K, and the Shubnikov-de Haas effect have been investigated in Sn-doped p-(Bi 1-x Sb x ) 2 Te 3 (0 ≤ x ≤ 1.0) single crystals. Doping of (Bi 1-x Sb x ) 2 Te 3 with tin showed that Sn exhibits acceptor properties in all crystals. The anomalous temperature and magnetic field behavior of the Hall coefficient was explained quantitatively by a model, which involves the complicated two-valence band structure of p-(Bi 1-x Sb x ) 2 Te 3 . The quantization of the Hall resistivity Q H in the form of plateaus in the dependence of Q H on the magnetic field B is observed. The minima of the transverse magnetoresistivity Q correspond to the start of plateaus. The oscillation of Q H is due to the presence of a carrier reservoir. An impurity resonant band with a high density of states in Sb 2 Te 3 or the second lower valence band with a higher hole effective mass in (Bi 1-x Sb x ) 2 Te 3 serves as the reservoir. The valence band structure of (Bi 1-x Sb x ) 2 Te 3 is also discussed.

Pressure spectroscopy of impurity states and band structure of bismuth telluride

The pressure and temperature dependences of the resistivity, the Hall effect and the Shubnikov-de Haas effect were investigated in p-Bi2Te3, p-Bi2Te3(Ge) and p-InxBi2-xTe3 for pressures up to 12 kbar and temperatures in the range 1.5 0.1 an increase of the upper hole band concentration of carriers under pressure and an increase of the resistivity and of the Hall coefficient Rx at T<10 K were observed. The results were interpreted as pointing to the existence of impurity states in In-doped samples.

Effect of the built-in electric field on optical and electrical properties of AlGaAs/InGaAs/GaAs P-HEMT nanoheterostructures

This study is concerned with the photoluminescence spectra and electrical parameters of AlGaAs/InGaAs/GaAs pseudomorphic high-electron-mobility transistor (P-HEMT) structures with a quantum well grown at different depths Lb with respect to the surface. The samples are produced so as to make the concentration of electrons in the quantum well unchanged, as the barrier layer thickness Lb is reduced. It is established that the photoluminescence spectra of all of the samples exhibit peaks at the photon energies ħω = 1.28−1.30 and 1.35–1.38 eV. The ratio between the intensities of these peaks increases as Lb is decreased. Calculations of the band structure show that variations in the spectra are due to the fact that the built-in electric field increases as the quantum well is set closer to the surface.

Quantum oscillations of Hall resistance, magnetoresistance in a magnetic field up to 54 T and the energy spectrum of Sn doped layered semiconductors p-(Bi1−xSbx)2Te3

The Hall effect and the Shubnikov–de Haas (SdH) effect have been investigated in magnetic fields up to 54 T in p-(Bi1−xSbx)2Te3 (0 ≤ x ≤ 1.0) Sn doped single crystals. Doping of (Bi1−xSbx)2Te3 with tin has shown that Sn exhibits acceptor properties in all crystals. We discuss the valence band structure of (Bi1−xSbx)2Te3 with the upper valence band (light hole band (LHB)), the lower valence band (heavy hole band (HHB)) and Sn-induced impurity band (IB). The Hall resistivity ρH as a function of magnetic field shows quantization in the form of plateaus. The calculated Landau levels of the LHB with the best-fit parameters are in agreement with the experiment. The oscillation of ρH is due to the presence of the carrier reservoir. The impurity resonant band with a high density of states or the HHB with a higher hole effective mass serve as the reservoir.

Electron effective masses in an InGaAs quantum well with InAs and GaAs inserts

We have measured and calculated effective masses m* and the band structure of the In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum well on the InP substrate with one or two InAs inserts in the quantum well and GaAs inserts in heterointerface barriers. The effective mass m* was measured by the Shubnikov?de Haas effect. Double symmetric InAs inserts in a quantum well lead to decreasing of m* by about 10%?35% as compared with the uniform In0.53Ga0.47As lattice-matched quantum well.

Ferromagnetism in a new dilute magnetic semiconductor Sb2-xCrxTe3

Magnetic and galvanomagnetic properties of single crystals of a new dilute magnetic semiconductor p-Sb2−xCrxTe3 (x = 0, 0.0115, 0.0215) are investigated in a temperature range of 1.7–300 K. A ferromagnetic phase with a Curie temperature of TC ≈ 5.8 (x = 0.0215) and 2.0 K (x = 0.0115) is detected. The easy magnetization axis is parallel to the C3 crystallographic axis. Analysis of the Shubnikov-de Haas effect observed in these crystals in strong magnetic fields leads to the conclusion that the hole concentration decreases as a result of doping with Cr. Negative magnetoresistance and the anomalous Hall effect are observed in Cr-doped samples at liquid helium temperature.