Density Of States Effective Mass Research Articles

Band and Phonon Engineering for Thermoelectric Enhancements of Rhombohedral GeTe.

Rhombohedral GeTe can be approximated as the directional distortion of the cubic GeTe along [111]. Such a symmetry-breaking of the crystal structure results in an opposite arrangement in energy of the L and Σ valence bands, and a split of them into 3L+1Z and 6Σ+6η, respectively. This enables a manipulation of the overall band degeneracy for thermoelectric enhancements through a precise control of the degree of crystal structure deviating from a cubic structure for the alignment of the split bands. Here, we show the effect of AgBiSe2-alloying on the crystal structure as well as thermoelectric transport properties of rhombohedral GeTe. AgBiSe2-alloying is found to not only finely manipulate the crystal structure for band convergence and thereby an increased band degeneracy, but also flatten the valence band for an increased band effective mass. Both of them result in an increased density of state effective mass and therefore an enhanced Seebeck coefficient along with a decreased mobility. Moreover, a remarkably reduced lattice thermal conductivity of ∼0.4 W/m-K is obtained due to the introduced additional point defect phonon scattering and bond softening by the alloying. With the help of Bi-doping at the Ge site for further optimizing the carrier concentration, thermoelectric figure of merit, zT, of ∼1.7 and average zTave of ∼0.9 are achieved in 5% AgBiSe2-alloyed rhombohedral GeTe, which demonstrates this material as a promising candidate for low-temperature thermoelectric applications.

Enhanced thermoelectric performance of Bi85Sb15-graphene composite by modulation carrier transport and density of state effective mass

The Bi-Sb material is regarded as a potential thermoelectric (TE) material for low temperature cooling applications and doping is a commonly used way to modulate the thermoelectric performance of this material. In the present study, graphene (Gr) dopant was introduced into the Bi85Sb15 powder by ball milling and fully densified (Bi85Sb15)1-xGrx (x = 0.0, 0.02, 0.1, 0.5, 1.0 wt%) solid composites were prepared by spark plasma sintering. The scanning electron microscopy and transmission electron microscopy reveal that the inclusion of Gr nanosheets refined the structures, and distributed throughout the grain boundaries and the matrix of the solid Bi-Sb composites, which eventually created more coherent and new interfaces and structural deformities in the matrix of the Bi-Sb alloys. The electric conductivity simultaneously enhanced with Seebeck coefficient, attributed to the increase in carrier concentration and density of state effective mass, respectively. The thermal conductivity is surprisingly increased with graphene doping, especially at high temperature. This is in turning enhanced the TE properties of the (Bi85Sb15)1-xGrx composites, the highest achieved figure of merit, 0.39 (260 K), is obtained for the sample with 0.5 wt% Gr, which is two times larger than that of the pristine sample, 0.18 (280 K). The improvement in TE properties seems to be promising strategy to be followed up for the potential TE materials with Gr doping.

Mechanism and application method to analyze the carrier scattering factor by electrical conductivity ratio based on thermoelectric property measurement

Carrier scattering factor is one of the most important parameters for semiconductors. In this paper, we propose the mechanism and the application method to analyze the carrier scattering factor(s) by comparing the ratios of electrical conductivity σ(T)/σ(T0 = 300 K) vs. temperature T in the theoretical calculation and experimental results. It is demonstrated that σ(T)/σ(T0 = 300 K) is only related to the carrier scattering factor when the density of states effective mass, m*, is assumed to be constant in small temperature ranges. Therefore, the carrier scattering factor dependence of the ratios of σ(T)/σ(T0 = 300 K) can be used to pinpoint the carrier scattering mechanism. Taking Bi0.5Sb1.5Te2.7+xSe0.3 as an example, it is found that no matter what theoretical models for the Seebeck coefficient over a range of the reduced Fermi energy are used, the analysis results for the scattering mechanism are unique. The reason behind such an observation is that the ratio of σ(T)/σ(T0) is only dependent on the carrier scattering for a certain material. As such, we can neglect the effect of degeneracy on the carrier scattering mechanism, and select the simplest theoretical Seebeck coefficient model to estimate the scattering mechanism before the self-consistent η(T) (reduced Fermi level) is obtained. The effect of temperature dependence of the m*(T) on the σ(T)/σ(T0) is also discussed.

Modelling and Calculation of Silicon Conduction Band Structure and Parameters with Arbitrary Uniaxial Stress

Using the k·p theory, the coupling effect between the Δ1 and Δ2’ bands on the energy band structure of different energy valleys is studied. The analytical model of the energy-dispersion relationship applicable to uniaxial stress for arbitrary crystal plane and orientation as well as different energy valleys is established. For typical crystal orientations, the main parameters of energy band structure such as band edge level, splitting energy, density-of-state (DOS) effective mass and conductivity effective mass are calculated. The calculated results are in good agreement with the data reported in related literature. Finally, the relationship between the DOS effective mass, conductivity effective mass and the change of stress and orientation of different crystal planes is given. The proposed model and calculation results can provide a theoretical reference for the design of nano-electronic devices and TCAD simulation.

Characterization and analysis of thermoelectric transport using SPB model in nanostructured aluminum doped zinc tellurium

Low-temperature electronic and thermal transport measurements are carried out on nanostructured Zn1−xAlxTe (0 ⩽ x ⩽ 0.15) fabricated using hydrothermal synthesis followed by evacuated-and-encapsulated sintering. A single parabolic band with acoustic phonon scattering is used to analyze thermoelectric transport data. It is found that reduced Fermi energy gets closer to the valence band edge and density of states effective mass, effective density of states, and Hall factor decrease with increasing x in doped samples. The chemical carrier concentration, carrier density independent mobility, β, and theoretical zT values increase with increasing x in doped samples. The nanostructured Zn1−xAlxTe exhibits significant reduction of thermal conductivity at 300 K (1.82–3.71 W m−1 K−1) as compared to bulk ZnTe (18 W m−1 K−1). The point-defect scattering and phonon-grain scattering play an important role in reducing the lattice thermal conductivity. In addition, partial substitution of Al3+ for Zn2+ significantly improves both the power factor and zT values.

Band engineering and tuning thermoelectric transport properties of p-type Bi0.52Sb1.48Te3 by Pb doping for low-temperature power generation

Herein, we report the results of a systematic study on the effect of Pb doping on the thermoelectric transport properties of p-type BiSbTe alloys to validate its potential applications for low-temperature power generation. The maximum power factor (~4.4mWm−1K−2) at 300K was obtained using 0.31at.% Pb-doped Bi0.52Sb1.48Te3 and was found to originate from an enlarged density of states effective mass as a result of the band engineering effect. The maximum efficiency of thermoelectric power generation (ηmax) could be enhanced by 150% at ΔT=220K when the Pb concentration was optimized.

First-principle calculation of the electronic structure, DOS and effective mass TlInSe2

The electronic structure, density of states (DOS), effective mass are calculated for tetragonal TlInSe2 from first principle in the framework of density functional theory (DFT). The electronic structure of TlInSe2 has been investigated by Quantum Wise within GGA. The calculated band structure by Hartwigsen–Goedecker–Hutter (HGH) pseudopotentials (psp) shows both the valence band maximum and conduction band minimum located at the T point of the Brillouin zone. Valence band maximum at the T point and the surrounding parts originate mainly from 6s states of univalent Tl ions. Bottom of the conduction band is due to the contribution of 6p-states of Tl and 5s-states of In atoms. Calculated DOS effective mass for holes and electrons are [Formula: see text], [Formula: see text], respectively. Electron effective masses are fairly isotropic, while the hole effective masses show strong anisotropy. The calculated electronic structure, density of states and DOS effective masses of TlInSe2 are in good agreement with existing theoretical and experimental results.

Effect of Si content on the thermoelectric transport properties of Ge-doped higher manganese silicides

Polycrystalline bulks of Si-content tuned Ge-doped higher manganese silicides (HMSs) were fabricated to elucidate the effects of Si content on the phase formation behavior and thermoelectric properties. The phase formation and electronic transport characteristics of HMSs were significantly dependent on Si content. Improved power factor was obtained at higher Si contents because of an enhanced Seebeck coefficient due to the increase in density of states effective mass, maintaining electrical conductivity. Furthermore, the lattice thermal conductivity was reduced through Si-content tuning, which suppressed the formation of secondary phases. Thus, a maximum ZT of 0.61 at 823K was obtained in MnSi1.77Ge0.027.

The effect of charge carrier and doping site on thermoelectric properties of Mg2Sn0.75Ge0.25

Mg2Sn0.75Ge0.25 has been recently demonstrated to be a promising thermoelectric material for power generation in the temperature range from room temperature to 723 K because of the high power factor of ∼54 μW cm−1 K−2 upon Sb doping to the Sn site. The enhanced density of states effective mass and weak electron scattering from the alloying effect are believed to be the main reasons for the high power factor (PF) and hence high figure of merit (ZT). In this study, it is shown that the right choice of carrier donor also plays an important role in obtaining high power factor. The effect of carrier donors Y and La at Mg-site and Bi and P at Sn-site in Mg2Sn0.75Ge0.25 is systematically investigated. It is found that charge donors at the Sn-site are much more effective than at the Mg-site in enhancing PF and ZT. Bi doped Mg2Sn0.73Bi0.02Ge0.25 shows a peak ZT of ∼1.4 at 673 K, a peak PF of ∼54 μW cm−1 K−2 at 577 K, which resulted in an engineering figure of merit (ZT)eng of ∼0.76 and (PF)eng of ∼2.05 W m−1 K−1 for cold side fixed at 323 K and hot side at 723 K.

Effects of Cu incorporation as an acceptor on the thermoelectric transport properties of CuxBi2Te2.7Se0.3 compounds

We report the effects of Cu incorporation on the thermoelectric transport properties in n-type CuxBi2Te2.7Se0.3 (0 ≤ x ≤ 0.08) compounds. The acceptor roles of Cu in the compounds were elucidated by the charge and thermal transport properties. Considerable change in the density of state (DOS) effective mass depending on the Cu content was evidenced by the cross-over of Seebeck coefficients, and the effects of the DOS effective mass on the thermoelectric transport properties were interpreted by temperature-normalized Pisarenko plot. Significant enhancement of thermoelectric figure of merit and the control of optimum operating temperature could be achieved by the Cu incorporation, and their origins were discussed.

Composition-dependent charge transport and temperature-dependent density of state effective mass interpreted by temperature-normalized Pisarenko plot in Bi2−xSbxTe3 compounds

Abstract

Effects of K-Doping on Thermoelectric Properties of Bi1−x K x CuOTe

The effects of K-doping on the thermoelectric properties of Bi1−x K x CuOTe (x = 0 to 0.08) have been investigated. The compounds were synthesized by a one-step solid-state reaction method and consolidated by a spark plasma sintering process. As the amount of K-doping was increased, the electrical and thermal conductivities increased while the Seebeck coefficient decreased due to increasing hole concentration. A ZT value of 0.69 was obtained for the compound K0.01Bi0.99CuOTe at 700 K, to the best of our knowledge the highest value reported for this material system. The origin of this enhanced ZT is discussed in terms of the density of states effective mass estimated by a single parabolic band model and electronic structures calculated based on density functional theory.

Enhanced thermoelectric properties and their controllability in p-type (BiSb)2Te3 compounds through simultaneous adjustment of charge and thermal transports by Cu incorporation

We report the enhanced thermoelectric properties in CuxBi0.38Sb1.62Te3 compounds through simultaneous achievement of improved charge transport and suppressed thermal transport by Cu incorporation. Incorporated Cu acted as an acceptor rather than donor, resulting in the significant improvement of p-type electric conduction. Density of state effective mass was almost linearly proportional to the Cu content in the compounds, and it led to the enhancement of power factor in Cu-doped compounds regardless of the increase in hole concentration. Furthermore, reduction of bipolar and lattice thermal conductivities could also be achieved by the Cu incorporation, consequently leading to the improved thermoelectric properties of the compounds at relatively high temperatures. Effects of Cu on the thermal and charge transport properties are discussed in detail, and the controllability of the thermoelectric performance for optimum operating temperature is also reported.

Play the heavy: An effective mass study for α-Fe2O3 and corundum oxides.

Iron(iii) oxide (α-Fe2O3) is a known water splitting catalyst commonly used in photoelectrochemical cells. These cells are severely impaired by poor conductivity in α-Fe2O3, and resolving the conductivity issue is therefore crucial. One of the most intrinsic properties of matter, which governs conductivity, is the carrier effective masses. In this work, we investigate the carrier effective masses in α-Fe2O3 and other corundum oxides, including Al2O3, Cr2O3, Ga2O3, and In2O3 with different theoretical constructs: density functional theory (DFT), DFT+U, hybrid DFT, and G0W0. We find DFT sufficiently describes the carrier masses and a quasi-particle theory is only required for accuracies better than 30% for the conduction band effective mass. Additionally, we compare the density of states (DOS) and band effective mass approximations and conclude the DOS effective mass provides poor results whenever the band structure is anisotropic. We find that the charge carriers in Fe2O3 "play the heavy" since they have large effective masses that reduce conductivity and device efficiency. Finally, we conclude that the less heavy electron effective masses of other corundum oxides studied relative to Fe2O3 could contribute to efficiency improvements in Fe2O3 upon Al2O3, Ga2O3, and In2O3 coverage.

Carrier distribution in multi-band materials and its effect on thermoelectric properties

Band convergence is one of the most interesting topics in recent studies of thermoelectrics. However, its effect on thermoelectric properties is only simply stated as improving band degeneracy. In this paper, the enhanced thermoelectric performance due to band convergence is clarified from the viewpoint of distribution of carriers in the electronic bands. The n-type Mg2Sn0.75Ge0.25 is used as a case study, and the effect of band offset E on its thermoelectric properties is investigated based on the three-band model, i.e., one light conduction band, one heavy conduction band, and one valence band. The results show that E has a decisive effect on controlling the distribution of carriers in the two conduction bands, thus affecting the thermoelectric properties. Since the optimal carrier concentration nopt is related to the density of state effective mass m∗ at a given temperature, an appropriate distribution of carriers should be a higher carrier concentration in the heavy band (with larger m∗) and a lower carrier concentration in the light band (with smaller m∗). In order to achieve a proper distribution of carriers, E should be as small as possible at any temperature, which explains the reason why band convergence could lead to the enhanced thermoelectric performance.

Co-doping of Al and Bi to control the transport properties for improving thermoelectric performance of Mg2Si

We investigated the thermoelectric properties of Al and Bi co-doped Mg2Si polycrystalline bulks fabricated using a solid state reaction combined with the spark plasma sintering technique. Through controlled doping of Al and Bi, the power factor could be enhanced due to an increase in the Seebeck coefficient benefiting from an enhancement of the density of states effective mass. The lattice thermal conductivity was reduced due to intensified point-defect phonon scattering originating from the mass difference between the host Si atoms and Bi dopants. By these synergetic effects, the dimensionless figure of merit (ZT) of 1.02 was obtained at 873K.

Neutron diffraction and thermoelectric properties of indium filled InxCo4Sb12 (x = 0.05, 0.2) and indium cerium filled Ce0.05In0.1Co4Sb12 skutterudites

The thermoelectric properties on polycrystalline single (In) and double filled (Ce, In) skutterudites are characterized between 300 and 700 K. Powder neutron diffraction measurements of the skutterudite compositions InxCo4Sb12 (x = 0.05, 0.2) and Ce0.05In0.1Co4Sb12 as a function of temperature (12–300 K) were carried out, which gives more insight into the structural data of single and double‐filled skutterudites. Results show that due to the annealing treatment, a Sb deficiency is detectable and thus verifies defects at the Sb lattice site of the skutterudite. Furthermore, we show by electron microprobe analysis that a considerable amount of indium is lost during synthesis and post‐processing for the single indium filled samples, but not for the double cerium and indium skutterudite sample. In our experiments, the double‐filled skutterudite is superior to the single‐filled skutterudite composition due to a higher charge carrier density, a comparable lattice thermal resistivity, and a higher density of states effective mass. Furthermore, we obtained a significantly higher Einstein temperature for the double‐filled skutterudite composition in comparison to the single‐filled species, which reflects the high sensitivity due to filling of the void lattice position within the skutterudite crystal.

On Intensifying Carrier Impurity Scattering to Enhance Thermoelectric Performance in Cr‐Doped CeyCo4Sb12

The beneficial effect of impurity scattering on thermoelectric properties has long been disregarded even though possible improvements in power factor have been suggested by Ioffe more than a half century ago. Here it is theoretically and experimentally demonstrated that proper intensification of ionized impurity scattering to charge carriers can benefit the thermoelectric figure of merit (ZT) by increasing the Seebeck coefficient and decreasing the electronic thermal conductivity. The optimal strength of ionized impurity scattering for maximum ZT depends on the Fermi level and the density of states effective mass. Cr‐doping in CeyCo4Sb12 progressively increases the strength of ionized impurity scattering, and significantly improves the Seebeck coefficient, resulting in high power factors of 45 μW cm−1 K−2 with relatively low electrical conductivity. This effect, combined with the increased Ce‐filling fraction and thus decreased lattice thermal conductivity by charge compensation of Cr‐dopant, gives rise to a maximum ZT of 1.3 at 800 K and a large average ZT of 1.1 between 500 and 850 K, ≈30% and ≈20% enhancements as compared with those of Cr‐free sample, respectively. Furthermore, this study also reveals that carrier scattering parameter can be another fundamental degree of freedom to optimize electrical properties and improve thermal‐to‐electricity conversion efficiencies of thermoelectric materials.

III-V Tri-Gate Quantum-Well Mosfet for 10nm Technology and Beyond

With sub-10nm node technology approaching fast, counteracting short channel effects (SCEs), sub-threshold conduction, reducing transistor off-current and gate leakage current have become aspects of real concern. Integration of III-V semiconductors in multigate device architectures has become a topic of intense research and study for low power logic implementation [1]. Multigate device architectures like FinFETs, GAA FETs provide better control over channel carrier accumulation and improved short channel performance over planar structures. Recently, In0.53GA0.47As Quantum-Well (QW) Tri-Gate MOSFET with bi-layer high-k dielectrics of Al2O3/HfO2 has been reported with channel width and height scaled down to 30nm and 20nm respectively and gate length scaled down to 60nm [2]. In this work, we present a simulation study of inversion Capacitance-Voltage (C-V) and Threshold (VTH) characteristics of a III-V tri-gate device (Fig.1 inset) using self-consistent modeling. The device structure used for modeling in this study incorporates 2nm Al2O3 with In composition kept at 0.7 in the undoped InGaAs QW channel. The In0.52Al0.48As back barrier thickness is kept at 60nm. In this study TiN has been used as the gate metal. In all these studies, temperature is considered to be fixed at 300K. Finite Element Method (FEM) has been used to solve Schrodinger-Poisson equations in a coupled manner applying proper boundary conditions, using COMSOL Multiphysics and MATLAB, taking into account wave function penetration and other quantum mechanical effects. 2D Schrodinger equation is solved using effective mass approximation and open boundary condition to determine electron wave functions and eigen states in the quantum well. Using carrier wave functions in the tri-gate channel, inversion carrier concentration is determined by applying 1D density of states and Fermi-Dirac distribution function. The effect of fixed oxide charges, interface trap charges and oxide border trap charges is not taken into account. The developed simulator is also benchmarked with the simulation results obtained for a III-V GAA nanowire transistor [3]. A quantum definition of threshold voltage for multigate FETs is available in literature [4]. According to this definition, the threshold voltage can be presented as a combination of classical and quantum terms. Here, peak electron concentration npeak(x,y) and average electron concentration navg are used to define threshold voltage of the device. At threshold point, the profile of npeak(x,y)/navg would show a change in slope which results from shift in carrier accumulation as device operation moves from subthreshold to inversion mode [4]. In this study, same definition of threshold voltage has been used. The simulation reveals strong carrier accumulation at the corners of the oxide/semiconductor interface which is expected for multigate device structures (Fig. 1). The simulation also reveals strong subband quantization in the QW channel (Fig. 2). Occupied subbands below the Fermi level contribute to the carrier concentration. As the device dimension is shrunk, volume inversion effect becomes more and more significant and carriers begin to accumulate at the middle portion of the tri-gate channel. This phenomenon leads to an increase in carrier concentration in the middle portion of the QW fin as channel dimension is scaled (Fig. 3). This phenomenon shifts channel formation towards the middle portion of the tri-gate QW fin. Simulation also reveals higher inversion capacitance at lower In composition in the channel which may be attributed to higher density of states effective mass at lower In composition (Fig. 4). Although variation of top gate oxide thickness reveals effect on inversion capacitance, the threshold voltage remains mostly unchanged with top gate oxide thickness. Study of variable channel dimension while keeping WFin=HFin reveals lower carrier accumulation in the device cross section per unit channel length which may lead to lower inversion capacitance with lower channel dimension (Fig. 5). Lowering channel dimension results in stronger quantum confinement and subband splitting which eventually leads to an increase in threshold voltage (Fig. 6). Lowered In composition in the channel also increases threshold voltage of the device (Fig. 6). In this work, a simulation study of a III-V tri-gate quantum well device for 10nm technology and beyond is presented. The outcome of this work would be useful in the implementation of III-V multigate device structures for high speed and low power logic applications.

Low effective mass and carrier concentration optimization for high performance p-type Mg2(1-x)Li2xSi0.3Sn0.7 solid solutions.

Mg2Si1-xSnx solid solutions are promising thermoelectric materials for power generation applications in the 500-800 K range. Outstanding n-type forms of these solid solutions have been developed in the past few years with the thermoelectric figure of merit ZT as high as 1.4. Unfortunately, no comparable performance has been achieved so far with p-type forms of the structure. In this work, we use Li doping on Mg sites in an attempt to enhance and control the concentration of hole carriers. We show that Li as well as Ga is a far more effective p-type dopant in comparison to Na or K. With the increasing content of Li, the electrical conductivity rises rapidly on account of a significantly enhanced density of holes. While the Seebeck coefficient decreases concomitantly, the power factor retains robust values supported by a rather high mobility of holes. Theoretical calculations indicate that Mg2Si0.3Sn0.7 intrinsically possesses the almost convergent double valence band structure (the light and heavy band), and Li doping retains a low density of states (DOS) on the top of the valence band, contrary to the Ga doping at the sites of Si/Sn. Low temperature specific heat capacity studies attest to a low DOS effective mass in Li-doped samples and consequently their larger hole mobility. The overall effect is a large power factor of Li-doped solid solutions. Although the thermal conductivity increases as more Li is incorporated in the structure, the enhanced carrier density effectively shifts the onset of intrinsic excitations (bipolar effect) to higher temperatures, and the beneficial role of phonon Umklapp processes as the primary limiting factor to the lattice thermal conductivity is thus extended. The final outcome is the figure of merit ZT ∼ 0.5 at 750 K for x = 0.07. This represents a 30% improvement in the figure of merit of p-type Mg2Si1-xSnx solid solutions over the literature values. Hence, designing low DOS near Fermi level EF for given carrier pockets can serve as an effective approach to optimize the PF and thus ZT value.