Theodora Kyratsi

Scanning tunneling microscopy of defect states in the semiconductor Bi2Se3

Scanning tunneling spectroscopy images of Bi2Se3 doped with excess Bi reveal electronic defect states with a striking shape resembling clover leaves. With a simple tight-binding model, we show that the geometry of the defect states in Bi2Se3 can be directly related to the position of the originating impurities. Only the Bi defects at the Se sites five atomic layers below the surface are experimentally observed. We show that this effect can be explained by the interplay of defect and surface electronic structure. Understanding the electronic properties of defects and the ability to control them will be crucial for the performance of the future microelectronic devices. 1 Scanning tunneling microscopy ~STM! represents a unique tool for the studies of defects as it combines atomic scale resolution with local spectroscopic capability. However, STM observation and analysis of defect states in semiconductors are complicated by surface effects such as in-gap surface states and reconstruction. These effects are avoided at the ~110! surfaces of a number of III-V semiconducting systems, 2 attracting extensive research. 3‐ 8 A number of point defect types have been observed. However, the positions of these defects with respect to the surface plane could be inferred only from indirect observations. The interpretation of such observations is complicated by the drastic effect the surface proximity may have on the defect states. 9 Modeling STM measurements of defects in semiconductors is not straightforward: Approximation of the STM images by maps of the local surface electronic density of states 10 is justified only if the charge relaxation rates of defect states significantly exceed the tunneling rate of electrons between the tip and the sample. 11 Tip-induced effects also need to be taken into account. These may include both local band bending, 3 and charging of the defect states by the tunneling current, resulting in bias voltage-dependent lattice relaxation in the vicinity of the defect atoms. 8 Careful analysis is necessary to clearly separate these effects from the intrinsic defect properties, and the bulk features of the observed defect states from the surface effects.

Highly anisotropic crystal growth and thermoelectric properties of K2Bi8−xSbxSe13 solid solutions: Band gap anomaly at low x

The thermoelectric properties of solid solutions of the type β-K2Bi8−xSbxSe13 (0<x<8) were studied with respect to thermal behavior, band gap variation, and charge transport properties as a function of x. At x values between 0 and 1.5, the energy band gap is observed to decrease (anomalous) before it widens with increasing x values as would be expected. For selected members of the solid solutions, the Bridgman technique was applied to obtain well-grown oriented ingots that were used to measure the thermal conductivity and charge transport properties in different growth directions. The measurements showed a strong anisotropy in thermoelectric properties with the largest anisotropy observed in the electrical conductivity. Lattice thermal conductivities of the selected solid solutions were observed to decrease when the x value increases. Preliminary doping studies on the x=1.6 member were carried out and it was shown that it is possible to significantly increase the power factor.

Bi/Sb distribution and its consequences in solid solution members of the thermoelectric materials K2Bi8-xSbxSe13

Abstract In an effort to understand the nature of mass fluctuations introduced when β-K 2 Bi 8 Se 13 is alloyed with K 2 Sb 8 Se 13 we determined the detailed crystal structures of two solid solutions of the type K 2 Bi 8− x Sb x Se 13 with x =4.0 and x =2.4. The structure is that of β-K 2 Bi 8 Se 13 which is a promising thermoelectric material with low thermal conductivity. The single crystal structures of K 1.49 Bi 4.72 Sb 3.79 Se 13 and K 1.49 Bi 5.55 Sb 2.97 Se 13 were determined and the Bi/Sb distribution in the structures was examined.

Structure inhomogeneities, shallow defects, and charge transport in the series of thermoelectric materials K2Bi8−xSbxSe13

The charge transport properties of the low-dimensional thermoelectric materials K2Bi8−xSbxSe13 (0<x⩽8) were studied as a function of temperature and composition. The Seebeck coefficient shows an evolution from n- to p-type character with increasing incorporation of Sb, and at the same time the temperature dependence of the electrical conductivity changes from that of a degenerate semiconductor to that of an intrinsic or compensated semiconductor. These changes, however, are not monotonic with composition due to the nonuniform substitution of Sb atoms at the Bi sites of the structure. Three separate composition regions can be assigned depending on x each with different charge transport characters. Electronic transport in K2Bi8−xSbxSe13 was analyzed on the basis of the classical semiconductor theory and discussed in the context of recent band calculations. The results suggest that the K2Bi8−xSbxSe13 materials possess coexisting domains with semimetallic and semiconducting characters whose ratio is influence...

Lattice thermal conductivity of K2(Bi1−zSbz)8Se13 solid solutions

The family of solid solutions of the type β-K2(Bi1−zSbz)8Se13 (0<z⩽1) was studied with respect to thermal conductivity as a function of temperature and stoichiometry. At low temperature, the variation of lattice thermal conductivity with composition shows a transition from a typical crystalline to glasslike behavior. Analysis of the high-temperature data shows a contribution due to the mixed occupation of Bi/Sb crystallographic sites as well as an additional contribution due to point defects.

THERMOELECTRIC MODULE FOR LOW TEMPERATURE APPLICATIONS

The possibility of a prototype thermoelectric cooling device for operation near liquid nitrogen temperatures has been explored. In these devices, the figure of merit involves a combination of the properties of the two branches of the module. Here, we investigate the fabrication of a module with a new low temperature material, CsBi 4 Te 6 ( p -type), and the best known low temperature n -type materials Bi 85 Sb 15 . Transport measurements for each of these materials show high performance at low temperatures. Known values for the figure of merit Z max of CsBi 4 Te 6 is 3.5 × 10 −3 K −1 at 225K and for Bi 85 Sb 15 is 6.5 × 10 −3 K −1 at 77K. At 100K these values drop to 2.0×10 −3 K −1 for CsBi 4 Te 6 and 6.0×10 −3 K −1 for Bi 85 Sb 15 . Theoretical simulations based on these data show a cooling of δT = 12K at 100K, which is almost three times the efficiency of a Bi 2 Te 3 module at that temperature. We present transport measurements of elements used in the fabrication of a low temperature thermoelectric module and properties of the resulting module.

IR Reflectivity Studies of Mechanically Alloyed PbTe Nanocrystals

Nano-crystalline lead telluride powder was synthesized by mechanical alloying using a high-energy planetary ball mill. The broadening of the X-ray diffraction peaks vs ball milling time, indicates small crystalline size of the order of 30nm. IR spectroscopy results are discussed and compared to the material prepared from melt.

On the optical properties of thermoelectric alkali metal chalcogenide compounds

Solid solutions of K 2 Bi 8−X Sb X Se 13 is an interesting series of complex bismuth-chalcogenide compounds, which are attractive for thermoelectric investigations. The highly anisotropic structure in these compounds results in needle-like morphology along the b- crystallographic axis, leaving structural tunnels where K+ ions reside. The complex electronic structure that arises from a large and low symmetry unit cell and the weakly bound K+ ions, lead to high Seebeck coefficient, highly anisotropic electrical properties and very low thermal conductivity. Reflectivity spectra in the FIR region are presented for several members in the Sb-rich (x≥6) side. Optical investigations are carried out on crystalline and pelletized samples. The received spectra are analyzed, examined comparatively and discussed. Results indicate that upon Bi/Sb substitution in the Sb-rich region, phonons develop a mixed-mode behavior in the FIR spectral region.

Thermoelectric Materials and Applications on the Recovery of Waste Heat Energy

Today, thermoelectric (TE) technology offers a solution regarding large amounts of energy that are lost as waste heat from various applications. In this paper, the need for better thermoelectric materials is addressed and our results on chalcogenides compounds are presented. Thermoelectric nanocomposite materials were developed and showed increased Seebeck coefficient as well as low thermal conductivity.

Structural Characterization of Nano-Crystalline Mg2Si Prepared by Ball Milling

In this work, nano-crystalline Mg2Si powder was prepared by ball milling and structural studies vs ball milling time are presented. The identification of the phases of the materials and the evaluation of their purity were performed using Powder X-ray diffraction (PXRD). Crystallite size evolution during ball milling was followed by PXRD and single line analysis, based on Scherrer equation. Transmission Electron microscopy (TEM) observations and IR Reflectivity measurements were used for the investigation of nano-features and confirmation of the PXRD results.