G. A. Slack

Semiconducting Ge clathrates: Promising candidates for thermoelectric applications

Transport properties of polycrystalline Ge clathrates with general composition Sr8Ga16Ge30 are reported in the temperature range 5 K⩽T⩽300 K. These compounds exhibit N-type semiconducting behavior with relatively high Seebeck coefficients and electrical conductivity, and room temperature carrier concentrations in the range of 1017–1018 cm−3. The thermal conductivity is more than an order of magnitude smaller than that of crystalline germanium and has a glasslike temperature dependence. The resulting thermoelectric figure of merit, ZT, at room temperature for the present samples is 14 that of Bi2Te3 alloys currently used in devices for thermoelectric cooling. Extrapolating our measurements to above room temperature, we estimate that ZT>1 at T>700 K, thus exceeding that of most known materials.

Glasslike Heat Conduction in High-Mobility Crystalline Semiconductors

The thermal conductivity of polycrystalline semiconductors with type-I clathrate hydrate crystal structure is reported. Ge clathrates (doped with Sr and /or Eu) exhibit lattice thermal conductivities typical of amorphous materials. Remarkably, this behavior occurs in spite of the well-defined crystalline structure and relatively high electron mobility ( ,100 cm 2 y Vs ). The dynamics of dopant ions and their interaction with the polyhedral cages of the structure are a likely source of the strong phonon scattering. [S0031-9007(98)08334-3]

The effect of rare‐earth filling on the lattice thermal conductivity of skutterudites

Polycrystalline samples of Ir4LaGe3Sb9, Ir4NdGe3Sb9, and Ir4SmGe3Sb9 have been made by hot isostatic pressing of powders. The lattice thermal conductivity of these filled skutterudites is markedly smaller than that of IrSb3; thus, void filling shows promise as a method for improving the thermoelectric properties of these materials. We present the lattice thermal conductivity of these filled skutterudites in an effort to quantify the impact of void filling in this structure. It is believed that the atoms ‘‘rattle’’ in the voids of the structure and therefore interact with a broad spectrum of lattice phonons, reducing their mean free paths substantially below that in the ‘‘unfilled’’ skutterudites. An additional phonon scattering mechanism is caused by phonon‐stimulated transitions between the low‐lying energy levels of the 4f electron configurations in the case of Nd3+ and Sm3+. Magnetic susceptibility and Hall‐effect measurements are also presented.

Some effects of oxygen impurities on AlN and GaN

Oxygen is a common substitutional impurity in both AlN and GaN crystals. In the wurtzite 2H phase it can be present in AlN up to concentrations of 1 x 10 21 /cm 3 while in GaN it can reach concentrations of 3 × 10 22 /cm 3 . These high concentrations of oxygen affect the luminescence, the optical absorption, the thermal conductivity, and the crystal perfection. The effects are somewhat similar in AlN and GaN. Representative experimental data will be presented to demonstrate the similarities, and to show how the oxygen content may be estimated from these property measurements.

Design Concepts for Improved Thermoelectric Materials

Some new guidelines are given that should be useful in the search for thermoelectric materials that are better than those currently available. In particular, clathrate and crypto-clathrate compounds with filler atoms in their cages offer the ability to substantially lower the lattice thermal conductivity.

Chapter 6 Semiconductor clathrates: A phonon glass electron crystal material with potential for thermoelectric applications

Publisher Summary This chapter describes semiconductor clathrates. The word “clathrate” derives from the Latin “clathratus” meaning “furnished with a lattice.” It is currently used in chemistry to describe a particular type of compound, usually a polyatomic compound, in which one component forms a cage structure imprisoning the other. The crystalline complexes of water, H 2 O, with simple molecules such as chlorine, Cl 2 , have been known to form clathrate compounds for more than a century. The chapter discusses the two common forms of ice clathrates, which are formed when the water is cooled and agitated in the presence of a sufficient concentration of the guest atoms. Type-I silicon (Si) and germanium (Ge) clathrates are metallic, whereas type-II clathrates maintain the semiconducting properties of Si and Ge for low alkaline metal concentrations. The semiconductivity diminishes as the metal concentration increases. A clathrate material is one in which the voids in the host lattice are present in the absence of a guest atom or molecule. These are called “true clathrates.” One good example in the field of thermoelectrics are the semiconducting skutterudites based on CoAs 3 . The chapter outlines several methods employed in forming Si, Ge, and tin (Sn) clathrates and in mixed-crystal clathrates containing two or more of these elements.

Low‐temperature transport properties of the filled and unfilled IrSb3 skutterudite system

We have measured the electrical resistivity, ρ, thermoelectric power, α, and thermal conductivity, κ, of the skutterudite material IrSb3 in a temperature range from 300 down to 4 K. It is found that the electrical resistivity and thermopower decrease monotonically as the temperature is reduced to 50–60 K. Below approximately 60 K the resistivity rises in a semiconducting manner. It appears the thermopower exhibits a large phonon drag peak at around 20 K and then falls towards zero. The thermal conductivity increases rapidly as the temperature is decreased with a maximum at around 20 K, corresponding to the peak in the thermopower. We will discuss these results and compare them to higher temperature data from G. A. Slack and V. G. Tsoukala [(IrSb3) J. Appl. Phys. 76, 1635 (1994)]. We have also measured some of the so‐called ‘‘filled skutterudites,’’ Ir4LaGe3Sb9, Ir4NdGe3Sb9 and Ir4SaGe3Sb9. The thermoelectric properties of these materials are considerably different than those of the unfilled sample. The thermopower is considerably lower and the resistivity is a factor of 2–4 times higher than the unfilled sample at room temperature. The thermal conductivity is markedly reduced by the filling, as much as a factor of 20 reduction for some of the systems.

High Lattice Thermal Conductivity Solids

The lattice thermal conductivity κ of various classes of crystalline solids is reviewed, with emphasis on materials with κ > 0.5Wcm−1K−1. A simple model for the magnitude of the lattice thermal conductivity at temperatures near the Debye temperature is presented and compared to experimental data on rocksalt, zincblende, diamond, and wurtzite structure compounds, graphite, silicon nitride and related materials, and icosahedral boron compounds. The thermal conductivity of wide-band-gap Group IV and Group III-V semiconductors is discussed, and the enhancement of lattice thermal conductivity by isotopic enrichment is considered.

RAMAN SCATTERING STUDY OF ANTIMONY-BASED SKUTTERUDITES

Raman spectra of single‐crystal CoSb3 and RhSb3 and of polycrystalline IrSb3 have been studied. We have also studied four different polycrystalline‐filled skutterudite samples, Ir4LaGe3Sb9, Ir4NdGe3Sb9, Ir4SmGe3Sb9, and Fe4CeSb12, where rare‐earth ions occupy the voids in the skutterudite structure. These void‐filling ions interact with some of the lattice vibrations in this structure and produce a shift and broadening of the observed lines. The most prominent lines in all of the samples are the Sb4 ring‐breathing modes.Raman spectra of single‐crystal CoSb3 and RhSb3 and of polycrystalline IrSb3 have been studied. We have also studied four different polycrystalline‐filled skutterudite samples, Ir4LaGe3Sb9, Ir4NdGe3Sb9, Ir4SmGe3Sb9, and Fe4CeSb12, where rare‐earth ions occupy the voids in the skutterudite structure. These void‐filling ions interact with some of the lattice vibrations in this structure and produce a shift and broadening of the observed lines. The most prominent lines in all of the samples are the Sb4 ring‐breathing modes.

Near-bandedge cathodoluminescence of an AlN homoepitaxial film

Cathodoluminescence experiments were performed on a high-quality AlN epitaxial film grown by organometallic vapor phase epitaxy on a large single crystal AlN substrate. The low-temperature near-bandedge spectra clearly show six very narrow lines. The thermal quenching behavior of these emission lines provides insight on how to assign them to free and bound exciton recombination processes. The binding energy for the free-exciton-A in AlN was found to be nearly twice that in GaN. The observation of the free-exciton-A first excited state permitted us to estimate its reduced effective mass and, by using recent reported values for the hole effective mass in Mg-doped AlN, the electron effective mass in AlN has been deduced.