Higher mobility in bulk semiconductors by separating the dopants from the charge-conducting band – a case study of thermoelectric PbSe

In the rigid band approximation dopants in semiconductors only change the Fermi level and carrier concentration such that different dopants are thought equivalent when fully ionized. In this work we examine the small but significant difference in mobility due to the type of dopant in heavily doped PbSe by studying n-type samples doped with Br, In and Bi. We propose that cation and anion dopants lead to a difference in mobility at high concentrations. This can be understood considering the predominance of cation states to the conduction band and anion states to the valence band. For higher mobility and better performance for most applications of heavily doped semiconductors, dopants should be on the site that is of less influence on the charge-conducting band. This concept can be viewed as an analog of modulation doping on the atomic level. Its physical origin is the random potential due to disorder that perturbs carriers, which is also the origin of Anderson localization at low temperature, a well-studied topic in theoretical physics. In thermoelectric PbSe, the selection of dopant can lead to 10% difference in mobility and in zT

Thermoelectric properties and electronic structure of the Zintl phase Sr_5Al_2Sb_6

The Zintl phase Sr_5Al_2Sb_6 has a large, complex unit cell and is composed of relatively earth-abundant and non-toxic elements, making it an attractive candidate for thermoelectric applications. The structure of Sr_5Al_2Sb_6 is characterized by infinite oscillating chains of AlSb_4 tetrahedra. It is distinct from the structure type of the previously studied Ca_5M_2Sb_6 compounds (M = Al, Ga or In), all of which have been shown to have promising thermoelectric performance. The lattice thermal conductivity of Sr_5Al_2Sb_6 (∼0.55 W mK^(-1) at 1000 K) was found to be lower than that of the related Ca_5M_2Sb_6 compounds due to its larger unit cell (54 atoms per primitive cell). Density functional theory predicts a relatively large band gap in Sr_5Al_2Sb_6, in agreement with the experimentally determined band gap of E_g ∼ 0.5 eV. High temperature electronic transport measurements reveal high resistivity and high Seebeck coefficients in Sr_5Al_2Sb_6, consistent with the large band gap and valence-precise structure. Doping with Zn^(2+) on the Al^(3+) site was attempted, but did not lead to the expected increase in carrier concentration. The low lattice thermal conductivity and large band gap in Sr_5Al_2Sb_6 suggest that, if the carrier concentration can be increased, thermoelectric performance comparable to that of Ca_5Al_2Sb_6 could be achieved in this system

Thermoelectric properties of the Yb_9Mn_(4.2-x)Zn_xSb_9 solid solutions

Yb_9Mn_(4.2)Sb_9 has been shown to have extremely low thermal conductivity and a high thermoelectric figure of merit attributed to its complex crystal structure and disordered interstitial sites. Motivated by previous work which shows that isoelectronic substitution of Mn by Zn leads to higher mobility by reducing spin disorder scattering, this study investigates the thermoelectric properties of the solid solution, Yb_9Mn_(4.2−x)Zn_xSb_9 (x = 0, 1, 2, 3 and 4.2). Measurements of the Hall mobility at high temperatures (up to 1000 K) show that the mobility can be increased by more than a factor of 3 by substituting Zn into Mn sites. This increase is explained by the reduction of the valence band effective mass with increasing Zn, leading to a slightly improved thermoelectric quality factor relative to Yb_9Mn_(4.2)Sb_9. However, increasing the Zn-content also increases the p-type carrier concentration, leading to metallic behavior with low Seebeck coefficients and high electrical conductivity. Varying the filling of the interstitial site in Yb_9Zn_(4+y)Sb_9 (y = 0.2, 0.3, 0.4 and 0.5) was attempted, but the carrier concentration (~10^(21) cm^(−3) at 300 K) and Seebeck coefficients remained constant, suggesting that the phase width of Yb_9Zn_(4+y)Sb_9 is quite narrow

Intrinsic Josephson Effects in the Magnetic Superconductor RuSr2GdCu2O8

We have measured interlayer current transport in small sized RuSr2GdCu2O8 single crystals. We find a clear intrinsic Josephson effect showing that the material acts as a natural superconductor-insulator-ferromagnet-insulator-superconductor superlattice. So far, we detected no unconventional behavior due to the magnetism of the RuO2 layers.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let

Magnetotransport of lanthanum doped RuSr2GdCu2O8 - the role of gadolinium

Strongly underdoped RuSr_1.9La_0.1GdCu_2O_8 has been comprehensively studied by dc magnetization, microwave measurements, magnetoresistivity and Hall resistivity in fields up to 9 T and temperatures down to 1.75 K. Electron doping by La reduces the hole concentration in the CuO2 planes and completely suppresses superconductivity. Microwave absorption, dc resistivity and ordinary Hall effect data indicate that the carrier concentration is reduced and a semiconductor-like temperature dependence is observed. Two magnetic ordering transitions are observed. The ruthenium sublattice orders antiferromagnetically at 155 K for low applied magnetic field and the gadolinium sublattice antiferromagnetically orders at 2.8 K. The magnetoresistivity exhibits a complicated temperature dependence due to the combination of the two magnetic orderings and spin fluctuations. It is shown that the ruthenium magnetism influences the conductivity in the RuO2 layers while the gadolinium magnetism influences the conductivity in the CuO2 layers. The magnetoresistivity is isotropic above 4 K, but it becomes anisotropic when gadolinium orders antiferromagnetically.Comment: 7 pages, 9 figures, submitted to European Physical Journal

Magneto-superconductivity of 100-atm O2-annealed RuSr2Gd1.5Ce0.5Cu2O10

Studied 100-atm O2-annealed RuSr2Gd1.5Ce0.5Cu2O10 (Ru-1222) compound crystallized in a tetragonal I4/mmm space group crystal structure. Thermo-gravemetric (TG) analysis of the compound showed the release of oxygen and breaking to metallic constituents in two distinct steps at around 350 and 500 0C. The DC magnetization data (M vs. T) revealed magnetic transition at 100 K followed by superconducting transition at 40 K. Low field M vs. H hysteresis loop showed a lower critical field (Hc1) value of around 25 Oe. Ferromagnetic component is evidenced at 5, 10, 20 and 40 K. Near saturation field of above 5 Tesla is observed at 5 K. Zero-field returning moment (Mr) and zero-moment coercive field (Hc) values at 5 K are 0.35mB and 250 Oe. The resistance vs. temperature (R vs. T) behaviour of the sample confirmed superconductivity at around 43 K. Superconductivity transition (Tc) is broadened under magnetic field with strong granularity like steps.Comment: 16 pages including text and six figure

Dopants effect on the band structure of PbTe thermoelectric material

PbTe is a promising thermoelectric material and its dimensionless figure of merit, zT, can be enhanced by optimizing the band structure near the Fermi level via chemical doping. This letter describes the dopants effect on bandgap, E_g, and effective mass, m*, for disordered La- and I-doping, based on theoretical calculations. E_g increases with increasing La and decreases with increasing I concentration. While m* increases upon La-doping, I-doping does not change m* noticeably. The calculated results are qualitatively consistent with the experimental results and explain the higher zT, up to 1.4 at 800 K, observed in I-doping PbTe compared to La-doping

Validity of rigid band approximation of PbTe thermoelectric materials

The tuning of carrier concentration through chemical doping is very important for the optimization of thermoelectric materials. Traditionally, a rigid band model is used to understand and guide doping in such semiconductors, but it is not clear whether such an approximation is valid. This letter focuses on the changes in the electronic density of states (DOS) near the valence band maximum for different p-type dopants (Na, K, Tl, or vacancy on Pb site) maintaining the high symmetry of the NaCl structure. Na-and K-doped, and vacancy-introduced PbTe show a clear rigid-band like change in DOS unlike that concluded from supercell based calculations