Quantum size effects and transport phenomena in thin Bi layers

For thin Bi films with thicknesses d=10–60 nm the dependences of the Hall coefficient, Seebeck coefficient, electrical conductivity, and Hall carrier mobility on d have been obtained at room temperature. Distinct oscillations of the transport properties with period Δd=(5±1) nm have been observed in the thickness range d=25–60 nm and attributed to quantization of the energy spectrum of holes. It has been suggested that a deep minimum observed in the thickness dependences of the kinetic coefficients at d～25 nm is connected with the manifestation of the electronic spectrum quantization and/or manifestation of a semimetal–semiconductor transition. The experimental data are in good agreement with the results of theoretical calculations.     

Effect of Ce-Doping on Thermoelectric Properties in PbTe Alloys Prepared by Spark Plasma Sintering

Ce-doped Pb_1−x Ce _x Te alloys with x = 0, 0.005, 0.01, 0.015, 0.03, and 0.05 were prepared by induction melting, ball milling, and spark plasma sintering techniques. The structure and thermoelectric properties of the samples were investigated. X-ray diffraction (XRD) analysis indicated that the samples were of single phase with NaCl-type structure for x less than 0.03. The lattice parameter a increases with increasing Ce content. The lower Ce-doped samples (x = 0.005 and 0.01) showed p-type conduction, whereas the pure PbTe and the higher doped samples (x = 0, 0.015, 0.03, and 0.05) showed n-type conduction. The lower Ce-doped samples exhibited a much higher absolute Seebeck coefficient, but the higher electrical resistivity and higher thermal conductivity compared with pure PbTe resulted in a lower figure of merit ZT. In contrast, the higher Ce-doped samples exhibited a lower electrical resistivity, together with a lower absolute Seebeck coefficient and comparable thermal conductivity, leading to ZT comparable to that of PbTe. The lowest thermal conductivity (range from 0.99 W m⁻¹ K⁻¹ at 300 K to 0.696 W m⁻¹ K⁻¹ at 473 K) was found in the alloy Pb_0.95Ce_0.05Te due to the presence of the secondary phases, leading to a ZT higher than that of pure PbTe above 500 K. The maximum figure of merit ZT, in the alloy Pb_0.95Ce_0.05Te, was 0.88 at 673 K.     

Solution-Chemical Syntheses of Nano-Structured Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> and PbTe Thermoelectric Materials

In this work, nano-structured Bi₂Te₃ and PbTe thermoelectric materials were synthesized separately via solvothermal, hydrothermal and low-temperature aqueous chemical routes. X-ray diffraction (XRD), field-emission scanning-electron microscopy (FESEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS) were used to analyze the powder products. Results showed that the as-prepared Bi₂Te₃ samples were all single-phased and consisted of irregular spherical granules with diameters of ∼30 nm whereas the PbTe samples were mainly composed of well-crystallized cubic crystals with average size of approximately 100 nm. Some nanotubes and nanorods were found in Bi₂Te₃ and PbTe samples, respectively; these were identified as Bi₂Te₃ nanotubes and PbTe nanorods by EDS analysis. Possible reaction mechanisms for these syntheses are discussed in detail herein.     

Coherent Sb/CuTe Core/Shell Nanostructure with Large Strain Contrast Boosting the Thermoelectric Performance of n-Type PbTe

The exploration of n-type PbTe as thermoelectric materials always falls behind its p-type counterpart, mainly due to their quite different electronic band structure. In this work, elemental Sb and Cu₂Te are introduced into an n-type base material (PbTe)₈₁-Sb₂Te₃. The introduction of extra Sb can effectively tune the concentration of electrons; meanwhile, Sb precipitates can also scatter low-energy electrons (negatively contribute to the Seebeck coefficient) thus enhance the overall Seebeck coefficient. The added Cu₂Te is found to always co-precipitate with Sb, forming an interesting Sb/CuTe core/shell structure; moreover, the interface between core/shell precipitates and PbTe matrix simultaneously shows coherent lattice and strong strain contrast, beneficial for electron transport but adverse to phonon transport. Eventually, a peak figure of merit ZT_max  ≈  1.6 @ 823K and simultaneously an average ZT  ≈  1.0 (323–823 K) are realized in the (PbTe)₈₁Sb₂Te₃-0.6Sb-2Cu₂Te sample, representing the state of the art for n-type PbTe-based thermoelectric materials. Moreover, for the first time the three existing forms of Cu atoms in Cu₂Te alloyed PbTe are unambiguously clarified with aberration-corrected scanning transmission electron microscopy (C_s-STEM).     

Thermoelectric Properties of (Pb,Sn,Ge)Te-Based Alloys

The search for alternative energy sources is presently at the forefront of␣applied research. In this context, thermoelectricity for direct energy conversion from thermal to electrical energy plays an important role. This␣paper is␣concerned with the development of highly efficient p-type [(PbTe)(SnTe)(Bi₂Te₃)] _x (GeTe)_1−x alloys for thermoelectric applications using spark plasma sintering (SPS). Varying the carrier concentration of GeTe was achieved by alloying of PbTe, SnTe, and/or Bi₂Te₃. The rhombohedral to cubic phase transition temperature, T _c, was found to be sensitive to the degree of alloying. Highest power factor values (P ≤ 33 μW/cm K²) were obtained for (GeTe)_0.95(Bi₂Te₃)_0.05 composition.     

Thermomagnetic Effects in High-Mobility PbTe Films

The narrow-gap compound semiconductor PbTe has high Hall mobility. The Fermi surface at the L-point in the Brillouin zone has large anisotropy. In this work, we measured thermomagnetic effects in PbTe thin films to confirm anisotropy of the Nernst coefficient A _Ne and show Nernst mobility from the ratio of A _Ne and the Seebeck coefficient S: μ _Ne = A _Ne/S. Angular dependences of the Nernst voltage show that A _Ne is independent of the angle between the temperature gradient and the magnetic field, because of the high L-point symmetry. The calculated Nernst mobility was compared with the Hall mobility. Because the former is smaller, the Mott equation cannot explain the Seebeck coefficient at room temperature.     

<Emphasis Type="Italic">p</Emphasis>-Type PbTe Thermoelectric Bulk Materials with Nanograins Fabricated by Attrition Milling and Spark Plasma Sintering

This work reports a manufacturing process that combines attrition milling and spark plasma sintering (SPS) in the preparation of PbTe bulk materials with nanosize grains. The process involves milling raw PbTe ingots into nanocrystalline powders and subsequent compacting of these powders into dense bulk materials by spark plasma sintering. Sintered samples with relative densities of over 95% and grain sizes as small as 80 nm to 1 μm were obtained through this process. The thermoelectric properties of the samples were measured and compared with those of raw ingots at temperatures from 300 K to 400 K to demonstrate the influence of grain size on thermoelectric properties. The results reveal that reducing the grain size improved thermoelectric performance.     

Terahertz Generation and Optical Properties of Lithium Ternary Chalcogenide Crystals

We have investigated the generation of THz radiation in lithium ternary compounds LiInSe₂, LiGaSe₂, LiInS₂, LiGaS₂ and characterized these materials by THz time-domain spectroscopy. Using 800 nm femtosecond excitation pulse, all crystals produce THz radiation due to an optical rectification corresponding to the nonlinear optical coefficient d ₃₃. We have measured refractive indices along the x-axis and the z-axis for all crystals in the range 150–700 μm and fitted them by using Sellmeier equation. With respect to the obtained results, velocity-matching between the incident laser pulse and the generated THz wave cannot be achieved at 800 nm, but for shorter wavelengths. Hence, an enhanced THz generation in Lithium ternary compounds may be observed by using a laser emitting below 800 nm.     

Optical Study of Filled Tetrahedral Compounds Li<Subscript>3</Subscript>AlN<Subscript>2</Subscript> and Li<Subscript>3</Subscript>GaN<Subscript>2</Subscript>

A detailed analysis of the optical properties of filled tetrahedral semiconductors Li₃AlN₂ and Li₃GaN₂ has been performed, using the full potential linearized augmented plane wave method within the density functional theory. The real and imaginary parts of the dielectric function ε(ω), the optical absorption coefficient I(ω), the reflectivity R(ω), and the electron energy loss function are calculated within the random phase approximation. The interband transitions responsible for the structures in the spectra are specified. Looking at optical matrix element, we note that the major peaks are dominated by transition from metal s, N 2p states to N 2p, Ga 3d states. The theoretical calculated optical properties and electron energy loss spectrum yield a static dielectric constant of 5.34 and a plasmon energy of 19.47 eV for Li₃GaN₂. In the Li₃AlN₂ compound, the static dielectric constant decreases to 4.75 and yields a plasmon energy of 18.5 eV. The effect of spin–orbit coupling on the optical properties is also investigated and found to be quite small, especially in the low-energy region. In order to check the reliability of our calculations, analogous results obtained for Be₃N₂ in the same structure [space group Ia3(206)] are included in this work.     

Bulk Nanostructured Thermoelectric Materials: Preparation,Structure and Properties

Bulk nanostructured materials have recently emerged as a new paradigm for improving the performance of existing thermoelectric materials. Here, we fabricated two kinds of bulk nanostructured thermoelectric materials by a bottom-up strategy and an in situ precipitation method, respectively. Binary PbTe was fabricated by a combination of chemical synthesis and hot pressing. The grain sizes of the hot pressed bulk samples varied from 200 nm to 400 nm, which significantly contributed to the reduction of thermal conductivity due to the enhanced boundary phonon scattering. The highest figure of merit ZT of the binary PbTe sample reached 0.8 at 580 K. Mg₂(Si,Sn) solid solutions have shown great promise for thermoelectric application, due to good thermoelectric properties, non-toxicity, and abundantly available constituent elements. The nanoscale microstructure observation of the compounds showed the existence of nanophases formed in situ, which is believed to be related to the relatively low lattice thermal conductivity in this material system. The highest ZT of Sb-doped Mg₂(Si,Sn) samples reached 1.1 at 770 K.     

Intrinsic defect states in PbTe single-crystal films grown by laser-modulated epitaxy

The electrical properties of PbTe/KCl(KBr) layers grown by infrared-laser-modulated epitaxy and their dependences on conditions of fabrication (power density of the laser beam W, substrate temperature T _s ) have been investigated. It is established that the R _H (T) dependences can be explained within the framework of a two-level model in which one of the levels (E _d1) is in the conduction band and the second level E _d2 is in the band gap. The positions of the energy levels and their density of states depend on the conditions of growth. Fiz. Tekh. Poluprovodn. 32, 47–49 (January 1998)     

Thermoelectric Properties of NdGd<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript> Prepared by CS<Subscript>2</Subscript> Sulfurization

Ternary rare-earth sulfides NdGd_1+x S₃, where 0 ≤ x ≤ 0.08, were prepared by sulfurizing Ln₂O₃ (Ln = Nd, Gd) with CS₂ gas, followed by reaction sintering. The sintered samples have full density and homogeneous compositions. The Seebeck coefficient, electrical resistivity, and thermal conductivity were measured over the temperature range of 300 K to 950 K. All the sintered samples exhibit a negative Seebeck coefficient. The magnitude of the Seebeck coefficient and the electrical resistivity decrease systematically with increasing Gd content. The thermal conductivity of all the sintered samples is less than 1.9 W K⁻¹ m⁻¹. The highest figure of merit ZT of 0.51 was found in NdGd_1.02S₃ at 950 K.     

Properties of Bi-doped PbTe epitaxial layers and pn junction diodes grown by TDM-CVP

Effects of Bi-doping in PbTe liquid-phase epitaxial layers grown by the TDM-CVP have been investigated. For Bi concentration in the solution, x_Bi, lower than 0.2 at.%, Hall mobility is low. In contrast, for x_Bi>0.2 at.%, Hall mobility is high, while carrier concentration is in the range 10¹⁷ cm⁻³. However, ICP emission analysis shows that, for x_Bi=1.0 at.%, Bi concentration in epitaxial layer is N_Bi=2.3–2.7×10¹⁹ cm⁻³.These results indicate that Bi behaves not only as a donor but also as an acceptor; the nearest neighbor or very near DA pairs are formed. Carrier concentration for Bi-doped layers takes a minimum value at a Te vapor pressure of 2.2×10⁻⁵ Torr for growth temperature 470°C, which is coincident with that of the undoped PbTe. And broad contact pn junctions with highly Bi-doped layers easily cause laser emission compared to undoped junctions. The result suggests that the nearest lattice site Bi–Bi DA pairs behave as strong radiative centers in PbTe.     

Highly Efficient Ge-Rich Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te Thermoelectric Alloys

The search for alternative energy sources is presently at the forefront of applied research. In this context, thermoelectricity for direct energy conversion from thermal to electrical energy plays an important role. This paper is concerned with the development of highly efficient p-type Ge _x Pb_1−x Te alloys for thermoelectric applications, using spark plasma sintering. The carrier concentration of GeTe was varied by alloying of PbTe and/or by Bi₂Te₃ doping. Very high ZT values up to ~1.8 at 500°C were obtained by doping Pb_0.13Ge_0.87Te with 3 mol% Bi₂Te₃.     

Aluminum‐doped Zn1 − xMgxO as transparent conductive oxide of Cu(In,Ga)(S,Se)2‐based solar cell for minimizing surface carrier recombination

A 19.5%‐efficient Cu(In,Ga)(S,Se)₂ (CIGSSe)‐based solar cell is obtained by replacing traditional CdS/ZnO buffer layers with Cd_0.75Zn_0.25S/Zn_0.79Mg_0.21O buffer layers for increasing short‐circuit current density because band‐gap energies of Cd_0.75Zn_0.25S and Zn_0.79Mg_0.21O are wider than those of CdS and ZnO, respectively. This yields the increase in external quantum efficiency in a short wavelength range of approximately 320 to 550 nm. Moreover, difference of conduction band minimum (E _C) between Zn_1 − x Mg_x O:Al (transparent conductive oxide, TCO) layer and CIGSSe absorber is optimized by varying [Mg]/([Mg] + [Zn]), x . It is revealed that Zn_1 − x Mg_x O:Al films with [Mg]/([Mg] + [Zn]) in a range of 0.10 to 0.12, enhancing E _g from 3.72 to 3.76 eV, are appropriate as TCO because of their enhanced mobility and decreased carrier density. Addition of 12% Mg into ZnO:Al to form Zn_0.88Mg_0.12O:Al as TCO layer effectively decreases surface carrier recombination and improves photovoltaic parameters, especially open‐circuit voltage and fill factor. This is the first experimental proof of the concept for optimizing E _C difference between TCO and absorber to minimize surface carrier recombination. Ultimately, conversion efficiency (η ) of CIGSSe solar cell with alternative Cd_0.75Zn_0.25S/Zn_0.79Mg_0.21O/Zn_0.88Mg_0.12O:Al (TCO) layers is enhanced to 20.6%, owing to control of total E _C alignment, which is higher η up to 12.6% relative as compared with the solar cell with traditional CdS/ZnO/ZnO:Al layers.     

Rutherford Backscattering Spectrometry Analysis of Self-Formed Ti-Rich Interface Layer Growth in Cu(Ti)/Low-<Emphasis Type="Italic">k</Emphasis> Samples

A new fabrication technique to prepare ultrathin barrier layers for nanoscale Cu wires was proposed in our previous studies. Ti-rich layers formed at Cu(Ti)/dielectric layer interfaces consisted of crystalline TiC or TiSi and amorphous Ti oxides. The primary control factor for the Ti-rich interface layer composition was C concentration in the dielectric layers rather than the formation enthalpy of the Ti compounds. To investigate Ti-rich interface layer growth in Cu(Ti)/dielectric layer samples annealed in ultrahigh vacuum, Rutherford backscattering spectrometry (RBS) was employed in the present study. Ti peaks were obtained only at the interfaces for all samples. Molar amounts of Ti atoms segregated to the interfaces (n) were estimated from Ti peak areas. Log n values were proportional to log t values. Slopes were similar for all samples, suggesting similar growth mechanisms. The activation energy (E) for Ti atoms reacting with the dielectric layers containing carbon (except SiO₂) tended to decrease with decreasing C concentration (decreasing k), while those for the SiO₂ layers were much higher. Reaction rate coefficients [Z · exp(−E/RT)] were insensitive to C concentration in the dielectric layers. These factors lead to the conclusion that growth of the Ti-rich interface layers is controlled by chemical reactions, represented by the Z and E values, of the Ti atoms with the dielectric layers, although there are a few diffusion processes possible.     

Epitaxial Lead Chalcogenides on Si for Mid-IR Detectors and Emitters Including Cavities

Lead chalcogenide (IV–VI narrow-gap semiconductor) layers on Si or BaF₂(111) substrates are employed to realize two mid-infrared optoelectronic devices for the first time. A tunable resonant cavity enhanced detector is realized by employing a movable mirror. Tuning is across the 4 μm to 5.5 μm wavelength range, and linewidth is <0.1 μm. Due to the thin (0.3 μm) PbTe photodiode inside the cavity, a higher sensitivity at higher operating temperatures was achieved as compared to conventional thick photodiodes. The second device is an optically pumped vertical external-cavity surface-emitting laser with PbTe-based gain layers. It emits at ∼5 μm wavelength and with output power up to 50 mW pulsed, or 3 mW continuous wave at 100 K.     

Thermoelectric Properties of PbTe,SnTe, and GeTe at High Pressure: an <Emphasis Type="Italic">Ab Initio</Emphasis> Study

In this work we present an ab initio study of the transport properties of PbTe, SnTe, and GeTe crystals in the B1 structure under zero and high pressure and analyze the possibility of pressure-induced thermoelectric performance enhancement. GeTe displays higher thermoelectric coefficients in both the p- and n-doping cases at zero pressure, but with applied pressure they drop quickly. n-Type SnTe has a higher Seebeck coefficient and figure of merit (ZT) than p-type SnTe at ambient conditions. With increased pressure its thermoelectric performance is improved initially and degrades later. The highest ZT appears at about 5 GPa. p-Type PbTe possesses attractive thermoelectric properties at zero pressure. With pressure applied, the ZT of this material undergoes a decline–climb–decline variation, and the optimal ZT occurs at 8 GPa to 10 GPa. Thermoelectric properties of n-type PbTe degrade slightly with increasing pressure and improve later; the improvement can be observed for pressures up to 20 GPa. These results suggest possible enhancement of thermoelectric properties for SnTe under intermediate pressure and PbTe under high pressure.     

Properties of Bi-doped PbTe layers grown by liquid phase epitaxy under controlled Te vapor pressure

Effects of Bi doping in PbTe liquid-phase epitaxial layers grown by the temperature difference method under controlled vapor pressure (TDM-CVP) are investigated. For Bi concentrations in the solution, x_Bi, lower than 0.2 at.%, an excess deep-donor level (activation energy E_d≈0.03–0.04 eV) appears, and Hall mobility is low. In contrast, for x_Bi>0.2 at.%, Hall mobility becomes very high, while carrier concentration is in the range of 10¹⁷ cm⁻³. Inductive coupled plasma (ICP) emission analysis shows that, for x_Bi=1 at.%, Bi concentration in the epitaxial layer is as high as N_Bi=2.3–2.7 × 10¹⁹ cm⁻³. These results indicate that Bi behaves not only as a donor but also as an acceptor, and the nearest neighbor or very near donor-acceptor (D-A) pairs are formed, so that strong self-compensation of Bi takes place. Carrier concentration for highly Bi-doped layers shows a minimum at a Te vapor pressure of 2.2 × 10⁻⁵ torr for growth temperature 470°C, which is coincident with that of the undoped PbTe.     

Electron-phonon interaction and electron mobility in quantum-well type-II PbTe/PbS structures

Electron mobilities in PbTe layers were calculated, taking into account electron scattering by longitudinal polar optical phonons, for low-dimensional structures — multiple PbTe/PbS quantum wells, which are type-II structures. Comparison with the electron mobilities obtained from Hall coefficient and magnetoresistance investigations in undoped multiple PbTe/PbS quantum wells versus the magnetic field intensity showed good agreement between the computed and experimental results for these structures. Fiz. Tekh. Poluprovodn. 32, 739–742 (June 1998)