Sandra B. Schujman

Semiconducting Ge clathrates: Promising candidates for thermoelectric applications

Transport properties of polycrystalline Ge clathrates with general composition Sr8Ga16Ge30 are reported in the temperature range 5u2002K⩽T⩽300u2002K. These compounds exhibit N-type semiconducting behavior with relatively high Seebeck coefficients and electrical conductivity, and room temperature carrier concentrations in the range of 1017–1018u2002cm−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>700u2002K, thus exceeding that of most known materials.

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.

Ultraviolet semiconductor laser diodes on bulk AlN

Current-injection ultraviolet lasers are demonstrated on low-dislocation-density bulk AlN substrates. The AlGaInN heterostructures were grown by metalorganic chemical vapor deposition. Requisite smooth surface morphologies were obtained by growing on near-c-plane AlN substrates, with a nominal off-axis orientation of less than 0.5°. Lasing was obtained from gain-guided laser diodes with uncoated facets and cavity lengths ranging from 200 to 1500u2002μm. Threshold current densities as low as 13u2002kA/cm2 were achieved for laser emission wavelengths as short as 368 nm, under pulsed operation. The maximum light output power was near 300 mW with a differential quantum efficiency of 6.7%. This (first) demonstration of nitride laser diodes on bulk AlN substrates suggests the feasibility of using such substrates to realize nitride laser diodes emitting from the near to deep ultraviolet spectral regions.

Electromechanical coupling coefficient for surface acoustic waves in single-crystal bulk aluminum nitride

The electromechanical coupling coefficient K2 for surface acoustic waves propagating on c and a surfaces of bulk AlN single crystals has been measured using the S11-parameter method in the frequency range of 160–360 MHz. The extracted values of K2 are 0.11% and 0.47% for the c and a surfaces, respectively. By fitting our experimental data to our numerical simulation results, we have estimated piezoelectric constants, which are in a reasonable agreement with literature data. Our results are consistent with the negative sign of the e15 constant.

Surface acoustic wave velocity in single-crystal AlN substrates

The surface acoustic wave velocity has been measured on a-plane (c-propagation) and c-plane oriented bulk aluminum nitride (AlN) single crystals using the S/sub 11/-parameter method in the frequency range 160-360 MHz. The SAW velocity is 5760 m/s for both orientations. From comparison of this value with the simulations using various elastic constants of AlN available in literature, we estimated the elastic constant C/sub 44/ to be 122 /spl plusmn/ 1 GPa.

Electron paramagnetic resonance of a donor in aluminum nitride crystals

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectra are obtained from a donor in aluminum nitride (AlN) crystals. Although observed in as-grown crystals, exposure to x rays significantly increases the concentration of this center. ENDOR identifies a strong hyperfine interaction with one aluminum neighbor along the c axis and weaker equivalent hyperfine interactions with three additional aluminum neighbors in the basal plane. These aluminum interactions indicate that the responsible center is a deep donor at a nitrogen site. The observed paramagnetic defect is either a neutral oxygen substituting for nitrogen (ON0) or a neutral nitrogen vacancy (VN0).

Structural analysis of Sr8Ga16Ge30 clathrate compound

Crystal structural characterization of the Sr8Ga16Ge30 compound was carried out with the Rietveld refinement method using x-ray powder diffraction data. The structure corresponds to that of the type-I clathrate hydrate, with the Sr atoms filling the “cages” formed by the Ga and Ge atoms. Anisotropic displacement parameters were refined. The limiting figure for rms values of the Sr(1) atoms enclosed in the dodecahedral cages is a sphere, while that for the Sr(2) atoms in the tetrakaidecahedral cages move is an oblate ellipsoid. A comparison of the sizes of the cages in this structure with the ones in other M8GaxGe46−x (M=K, Ba, or void) structures shows that once the structure is filled (i) there is very little variation in size of the tetrakaidecahedral cages and (ii) the dodecahedral cages expand only slightly (measurably) as a function of the filler size. This is an important result from the thermoelectric point of view because it means that once the structure is full, the cages will not “close over” th...

Electrical behavior of isolated multiwall carbon nanotubes characterized by scanning surface potential microscopy

We measured the surface electric potential distribution on individual, electrically contacted and biased, multiwall carbon nanotubes using scanning surface potential microscopy. The voltage varies linearly along the nanotube and the voltage drop is directly proportional to the bias applied between electrodes. Its resistance decreases four times when a 4.5 V bias is applied between the nanotube and the substrate. Under these conditions, we were able to resolve the voltage drop along the nanotube and at the contacts, providing a unique way of measuring contact resistance, which is observed in this case to be on the order of 50 kΩ.

Very low dislocation density AlN substrates for device applications

Native Aluminum Nitride (AlN) single-crystal substrates with ultra-low dislocation density are very promising for use in III-nitride epitaxial growth required for ultraviolet (UV) electro-optical applications and high power radio frequency (RF) devices. They offer a better lattice and thermal expansion match to AlGaN alloys, especially those with high Al content, than foreign substrates such as SiC or sapphire. An additional advantage of bulk substrates is the possibility of slicing and preparing surfaces with the desired orientation, such as non-polar and pre-determined, specific misorientations, which will permit the fabrication of devices with specific, special properties. In this paper we present chemical and electrical characterization of the AlN material. Secondary Ion Mass Spectroscopy (SIMS) measurements show that oxygen is the main impurity, with concentrations in the order of mid 1018 cm-3. The electrical resistivity of the AlN was measured, giving a lower limit of 1012Ω-cm at room temperature. The prepared surface of substrates with different orientations, as well as of homo-epitaxial and hetero-epitaxial layers of AlGaN with different Al:Ga ratios were measured by Atomic Force Microscopy. The observation of atomic steps in the bare substrates and step flow in the epilayers are an indication of the good surface preparation. The crystalline quality of the epilayers was assessed by measuring the full width at half maximum (FWHM) of both symmetric and asymmetric X-ray rocking curves.

Analysis of Antimony-Tin-Based Skutterudites

Since the proposal of skutterudites as possible Phonon-Glass, Electron-Crystal materials, a lot of work has been done trying to fill the structural voids with foreign “rattling” atoms. In order to keep the electronic count per unit cell constant (and thus, the semiconducting properties of most of the compounds under study) partial replacement of either the cation or the anion in the original formula by an appropriate neighbor in the periodic table is an option. In the case of antimonides, replacing part of the Sb with Ge or Sn in order to compensate the extra charge introduced by void fillers has proved useful for compounds based on rare-earth filled IrSb 3 . In the case of RhSb 3 , we found that large quantities of Sn can be incorporated into the skutterudite structure of RhSb 3 without either filling the voids or producing charge carriers. We have analyzed the stability of several cross-sections of the Rh-Sb-Sn ternary system and have found a wide range of compositions with the basic skutterudite structure as we vary the Sn content. In all the cases, the tin goes substitutionally into the antimony sites. The voids remain empty. Density measurements suggest the existence of metal vacancies, confirmed by Rietveld refinement of the powder X-ray diffraction patterns. The possibility of Sn-induced mixed-valence of Rh on the anion sites is being investigated.