Daniel Bilc

Resonant states in the electronic structure of the high performance thermoelectrics AgPbmSbTe2+m: The role of Ag-Sb microstructures

Ab initio electronic structure calculations based on gradient corrected density-functional theory were performed on a class of novel quaternary compounds AgPb(m)SbTe(2+m), which were found to be excellent high temperature thermoelctrics with a large figure of merit ZT approximately 2.2 at 800 K. We find that resonant states appear near the top of the valence and bottom of the conduction bands of bulk PbTe when Ag and Sb replace Pb. These states can be understood in terms of modified Te-Ag(Sb) bonds. The electronic structure near the gap depends sensitively on the microstructural arrangements of Ag-Sb atoms, suggesting that large ZT values may originate from the nature of these ordering arrangements.

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.

new ternary aluminides grown from aluminum flux

Abstract A series of ternary aluminide intermetallics with a new structure type were formed from the reaction of gold and rare-earth metals in aluminum flux. The REAu3Al7 structure was obtained with all rare earths RE with the exception of La and Eu. These materials crystallize in the rhombohedral space group R-3c, with unit cell parameters a=8.0922(6) A and c=21.066(2) A for PrAu3Al7 as an example. The variation in cell edges with rare-earth size is regular with the exception of the Yb analogue. The possible mixed valency indicated by this result was confirmed by magnetic susceptibility measurements. Density functional theory-based band structure calculations on YbAu3Al7 indicate the ytterbium f-orbitals are located just below the Fermi level, further supporting the mixed valence description of this material.

R3Au6+xAl26T (R = Ca, Sr, Eu, Yb; T = Early Transition Metal): a Large Family of Compounds with a Stuffed BaHg11 Structure Type Grown from Aluminum Flux

A collection of new quaternary intermetallic compounds with a cubic, stuffed BaHg(11) structure type has been synthesized by the combination of a divalent rare earth or alkaline earth metal R, an early transition metal T, and gold in an excess of molten aluminum. Structural characterization of these R(3)Au(6+x)Al(26)T compounds by powder and single crystal X-ray diffraction indicates that the unit cell varies with the radii of the early transition metal T and the rare earth/alkaline earth R as expected. The element T (where T = group 4, 5, 6, and 7 element) appears to be responsible for the stabilization of up to 43 different members of the R(3)Au(6+x)Al(26)T family of compounds. Varying amounts of disorder and trends in partial occupancies of the Au stuffed site--the site that is vacant in the parent compound BaHg(11)--are also indicated by the diffraction studies of this family of compounds. Magnetic susceptibility data reveals that the transition metal atoms in these materials do not possess local magnetic moments. For the magnetic rare earth containing materials, the europium compounds undergo a ferromagnetic transition at 10 K, and the ytterbium analogues show mixed valent behavior. Band structure calculations also support a mixed valent state for Yb in these compounds.

Electronic Structure of Complex Bismuth Chalcogenide Systems

Narrow band gap semiconductor Bi2Te3and its alloys with Sb and Se are bulk materials with one of the highest figure of merit Z (= α2σ/κ, where α is the thermopower, σ is the electrical conductivity, and κ is the thermal conductivity) in the range of room temperature.1Because of this, several attempts have been made to improve Z by making novel Bi-Te-Sb-Se based materials using new concepts such as quantum confinement2(QC) and phonon glass electron crystal3(PGEC). Within the QC picture, first proposed by Hicks et. al.2, model calculations predict an increase in Z for a two dimensional layer having thickness smaller than about 300 A (for example an idealized Bi2Te3-multiple quantum well (MQW) structure) due to enhanced power factor P = α2σ. Even larger power factor enhancement occurs in one-dimensional quantum wires. (See the article on bismuth nanowires by Dresselahaus et. al.4in this volume.) In contrast to the QC idea, within PGEC picture one uses superlattice (SL) structures consisting of two materials both having favorable α and σ (such that electronic properties are not affected by the SL structure), but reduces к by engineering the phonon band structure in suitably chosen transport direction.5A third approach where both the above ideas have been exploited to certain extent is to chemically synthesize new ternary and quaternary narrow band gap semiconductors containing Bi, Te, Se atoms with different arrangements of Bi-Te-Se blocks which we will refer to as different quantum architectures.6These compounds have low and promisingly high values of P.This has been the focus of our research program at Michigan State University. Some of these new compounds are BaBiTe3, CsBi4Te6, K2Bi8Se13, of which the last two show considerable promise.

V2Al5Ge5: first ternary intermetallic in the V–Al–Ge system accessible in liquid aluminium

The intermetallic compound V2Al5Ge5 grown from Al flux is reported. V2Al5Ge5: orthorhombic, Cmcm, a = 5.4072(10), b = 12.978(2), and c = 11.362(2) A, the structure features distorted pentagonal prismatic columns defined by Al and Ge atoms. Vanadium atoms occupy the central axis of columns forming a chain with long-short alternation of V-V bonds. This compound is resistant to air oxidation up to 500 degrees C.

Theoretical Study of Electronic Structures of Bi 2 Te 3 /Sb 2 Te 3 Superlattices

To understand thermoelectric properties of multiplayer Bi 2 Te 3 /Sb 2 Te 3 superlattices, especially their charge transport properties, electronic structure calculations were carried out using ab-initio gradient corrected density functional theory. The superlattice structures of (1,1) and (1,2) Bi 2 Te 3 /Sb 2 Te 3 multilayers were optimized and their band structures were compared with each other. Different lattice relaxation effects are observed for the two structures. The cross-plane and inplane effective masses for both these systems are found to be comparable, consistent with experimental mobility measurements.

Ab initio Studies of Electronic Structure of Defects in PbTe

Received 17 May 2006; revised manuscript received 13 July 2006; published 19 October 2006 Understanding the detailed electronic structure of deep defect states in narrow band-gap semiconductors has been a challenging problem. Recently, self-consistent ab initio calculations within density functional theory using supercell models have been successful in tackling this problem. In this paper, we carry out such calculations in PbTe, a well-known narrow band-gap semiconductor, for a large class of defects: cationic and anionic substitutional impurities of different valence, and cationic and anionic vacancies. For the cationic defects, we study the chemical trends in the position of defect levels by looking at series of compounds RPb2n�1Te2n, where R is vacancy or monovalent, divalent, or trivalent atom. Similarly, for anionic defects, we study compounds MPb2nTe2n�1, where M is vacancy, S, Se or I. We find that the density of states near the top of the valence band and the bottom of the conduction band get significantly modified for most of these defects. This suggests that the transport properties of PbTe in the presence of impurities may not always be interpreted by simple carrier doping from bound impurity states in the gap concepts, confirming such ideas developed from qualitative and semiquantitative arguments.

Effect of K/Bi ordering on the electronic structure of K2Bi8Se13

K 2 Bi 8 Se 13 belongs to a class of complex chalcogenides which show potential for superior thermoelectric performance. This compound forms in two distinct phases, α and β. The β-phase, which has several sites with mixed K/Bi occupancy is a better thermoelectric. To understand the origin of this difference we have carried out electronic structure calculations within ab initio density functional theory using full potential linearized augmented plane wave (FLAPW) method. The generalized gradient approximation was used to treat the exchange and correlation potential. Spin-orbit interaction (SOI) was incorporated using a second variational procedure. The α-phase is found to be a semiconductor with an indirect band gap of 0.47eV compared to 0.76eV for the observed direct optical gap. For the β-phase we have chosen two different ordered structures with full occupancies of K and Bi atoms at the “mixed sites”. The system is a semi-metal for both the ordered structures. To incorporate the effect of mixed occupancy we have chosen a 1x1x2 supercell with an alternative K/Bi occupancy at the “mixed sites”. The superlattice ordering gives a semiconductor with an indirect gap of 0.38eV. Mixed occupancy is crucial for the system to be a semiconductor because the Bi atoms at the “mixed sites” stabilize the p orbitals of the neighboring Se atoms by lowering their energy, and opening up a gap at the chemical potential.

Electronic structure of (AgSb)xPbn-2xTen

Complex quaternary chalcogenides (AgSb)xPbn-2xTen (0<x<n/2) are thought to be narrow band-gap semiconductors which are very good candidates for room and high temperature thermoelectric applications. These systems form in the rock-salt structure similar to the well known two component system PbTe (x=0). In these systems Ag and Sb occupy Pb sites randomly although there is some evidence of short-range order. To gain insights into the electronic structure of these compounds, we have performed electronic structure calculations in AgSbTe2 (x=n/2). These calculations were carried out within ab initio density functional theory (DFT) using full potential linearized augmented plane wave (LAPW) method. The generalized gradient approximation (GGA) was used to treat the exchange and correlation potential. Spinorbit interaction (SOI) was incorporated using a second variational procedure. Since it is difficult to treat disorder in ab initio calculations, we have used several ordered structures for AgSbTe2. All these structures show semimetallic behavior with a pseudogap near the Fermi energy. Te and Sb p orbitals, which are close in energy, hybridize rather strongly indicating a covalent interaction between Te and Sb atoms.