Antonio DiVenere

ANTISITE DEFECTS OF BI2TE3 THIN FILMS

We have successfully grown Bi2Te3 thin films on CdTe(111)B using molecular-beam epitaxy. Structural and transport properties have been investigated using in situ reflection high-energy electron diffraction, θ–2θ x-ray diffraction analysis, thermopower, and Hall measurements. Both the crystallinity and the transport are found to be strongly affected by nonstoichiometry. The most stoichiometric sample had a high crystallinity, high thermopower, and high electron mobility. However, Bi2Te3 films with excess Te had a reduced lattice constant, poorer crystallinity, reduced magnitude of the thermopower, and reduced mobility. All of these observations can be explained in terms of antisite defects in which excess Te occupies Bi lattice sites and behaves as a n-type dopant.

Structural and thermoelectric transport properties of Sb2Te3 thin films grown by molecular beam epitaxy

We have studied the structural and transport properties of Sb2Te3 thin films prepared by molecular beam epitaxy as a function of the Te/Sb flux ratio during deposition. Both the crystallinity and the transport properties are found to be strongly affected by nonstoichiometry. The most stoichiometric sample (prepared with a Te/Sb ratio of 3.6) had a high degree of crystallinity, high thermopower, and high carrier mobility. However, Sb2Te3 films with excess Sb or Te had poorer crystallinity, reduced magnitude of the thermopower, and reduced mobility as a result of the formation of antisite defects. These antisite defects were able to be reduced by controlling the relative flow rate ratio of Te to Sb during growth.

Thermoelectric transport properties of n-doped and p-doped Bi0.91Sb0.09 alloy thin films

In order to understand the doping behavior of extremely narrow band gap materials and to optimize their characteristics for use in a thermoelectric module, we performed n- and p-type doping experiments on semiconducting Bi0.91Sb0.09 alloy thin films using the group VI(IV) element Te(Sn) as donor (acceptor). Thermoelectric power (TEP), electrical resistivity, and Hall effect were studied in the range of temperatures 5–300 K. Increased Sn doping causes the TEP to change sign (from negative to positive) and the maximum in the TEP can be controlled with the dopant concentration. Increased Te doping causes the TEP to decrease. The maximum Te-doped electron concentration was about 5×1020 cm−3 and the highest Sn-doped hole concentration was about 1×1021 cm−3. Highly Sn- and Te-doped samples show degenerate behavior in the electrical resistivity, TEP and Hall measurements.

Thermoelectric power of MBE grown Bi thin films and BI/CDTE superlattices on CdTe substrates

Abstract We have measured the thermoelectric power (TEP) of epitaxial Bi thin films and Bi/CdTe superlattices grown on CdTe substrates as a function of temperature in the range 20–300 K. We have observed that the TEP of a 10 000ABi thin film is in good agreement with the bulk single crystal value and that the TEPs for superlattices with 400Aand 800ABi well thicknesses are enhanced over the bulk values. However, p -type doping effects in both thin films and superlattices lead to a positive TEP in samples with thinner quantum wells and a suppression of the magnitude in all cases. This suggests that the TEP may be enhanced further by altering the growth conditions to reduce the excess hole concentration.

Polarity inversion of CdTe(111) orientation grown on Bi (00.1) by molecular beam epitaxy

Using in situ reflection high‐energy electron diffraction analysis and chemical etching it is shown that CdTe grown on Bi layers deposited on CdTe (111)B terminated surfaces result in (111)A terminated surfaces. The Bi layers exhibit streaked diffraction patterns with clear Kikuchi lines; this is the first direct evidence for the layer by layer two‐dimensional growth of Bi on CdTe by molecular beam epitaxy.

Anisotropic Seebeck and magneto-Seebeck coefficients of Bi and Bi0.92Sb0.08 alloy thin films

We have grown Bi and BiSb alloy thin films on (211)CdTe substrates by molecular beam epitaxy. Growth proceeds with the Bi or BiSb (00.1) axis oriented along the CdTe[111] direction, which is tilted by 19° with respect to the substrate normal. Measurements of the Seebeck coefficient reveal a strong dependence on angle within the plane, due to the anisotropic electronic structure. The coefficient measured along the [111] axis, which includes a trigonal contribution from the Seebeck tensor, is considerably higher than the value along the [011] axis. The magneto-Seebeck coefficient was also studied, for magnetic fields of 0–0.7 T. We observe a strong dependence on both crystal-axis and magnetic field direction, the so-called “umkehr” effect.

Thermoelectric and magnetotransport properties of Bi1-xSbx thin films and Bi/CdTe superlattices

The magnetotransport properties of Bi1-xSbx thin films (x = 0, 0.09 and 0.16) and Bi/CdTe superlattices have been determined by applying the quantitative mobility spectrum analysis (QMSA) and multicarrier fitting to the magnetic-field-dependent resistivities and Hall coefficients, using algorithms which account for the strong anisotropy of the mobilities. The experimentally derived electron and hole densities and mobilities are then used as the input to a nonparabolic and anisotropic band model that derives the thermal occupations and relaxation times for L-valley electrons and holes as well as T-valley and H-valley holes. This allows a calculation of the thermoelectric properties for comparison with experimental results. While the data for a Bi thin film and Bi/CdTe superlattices are reproduced quite well without any adjustable parameters, for Bi1-xSbx thin films a non-uniform doping profile must be assumed.

Bi1−xSbx/Bi superlattice grown by molecular beam epitaxy

Superlattices of Bi1−xSbx/Bi have been grown by molecular beam epitaxy on CdTe(111) substrates. The typical multilayer, consisting of Bi1−xSbx (85 A with x=0.16) and Bi (75‐A) layers repeated 50 times, was grown at a substrate temperature of 150 °C. The samples were characterized by reflection high‐energy electron diffraction (RHEED), θ‐2θ x‐ray diffraction analysis, and high‐resolution transmission electron microscopy. The streaked RHEED patterns with clear Kikuchi lines and the x‐ray satellite peaks indicate a good epitaxial layer quality. The bright field transmission electron microscopy image of the superlattice film confirms that a composition modulation exists, even though the Bi1−xSbx and Bi layers have only a slight image contrast.

BI/SB SUPERLATTICES GROWN BY MOLECULAR BEAM EPITAXY

Epitaxial Bi/Sb superlattices have been grown by molecular beam epitaxy on CdTe(111)B substrates. The superlattice modulation wavelength was in the range of 20–200 A. Structural properties have been investigated using in situ reflection high-energy electron diffraction (RHEED), θ−2θ x-ray diffraction (XRD) analysis, and high-resolution transmission electron microscopy (TEM). The streaked RHEED patterns of Bi on Sb (or Sb on Bi) with clear Kikuchi lines indicate layer-by-layer growth with good epitaxial layer quality. The narrow XRD rocking curves for the central and the satellite peaks suggest that the interfaces are very sharp and that the superlattice periods do not fluctuate, which is demonstrated in this article by cross-sectional TEM.

Localization and interaction effects in CdTeBi superlattices

Abstract The transport properties of CdTeBi superlattice have been measured as a function of the modulation wavelength. A logarithmic dependence with temperature was observed for the resistance at low temperature. The resistivity varied from 0.1 to 3.9 mΩcm at 1.7 K as the modulation wavelength varied from 177 to 81 A. A logarithmic behavior with H was observed for the transverse (longitudinal) magnetoresistance for low (high) fields. Samples were studied at temperatures down to 1.7 K and in magnetic fields up to 5 Tesla. No negative magnetoresistance as predicted by weak localization theory was observed. Hall coefficient measurements showed a logarithmic dependence with temperature which is indicative of Coulomb correlation effects (interaction theory). All measurements indicate that the transport behavior of CdTeBi superlattices is dominated by spin-orbit and/or interaction effects while weak localization plays a minimal role.