Density Of States Effective Mass Research Articles

Structural, optical, and electrical properties of strained La-doped SrTiO3 films

The structural, optical, and room-temperature electrical properties of strained La-doped SrTiO3 epitaxial thin films are investigated. Conductive La-doped SrTiO3 thin films with concentration varying from 5 to 25% are grown by molecular beam epitaxy on four different substrates: LaAlO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, SrTiO3, and DyScO3, which result in lattice mismatch strain ranging from −2.9% to +1.1%. We compare the effect of La concentration and strain on the structural and optical properties, and measure their effect on the electrical resistivity and mobility at room temperature. Room temperature resistivities ranging from ∼10−2 to 10−5 Ω cm are obtained depending on strain and La concentration. The room temperature mobility decreases with increasing strain regardless of the sign of the strain. The observed Drude peak and Burstein-Moss shift from spectroscopic ellipsometry clearly confirm that the La addition creates a high density of free carriers in SrTiO3. First principles calculations were performed to help understand the effect of La-doping on the density of states effective mass as well as the conductivity and DC relaxation time.

Optimum Carrier Concentration in n‐Type PbTe Thermoelectrics

Taking La‐ and I‐doped PbTe as an example, the current work shows the effects of optimizing the thermoelectric figure of merit, zT, by controlling the doping level. The high doping effectiveness allows the carrier concentration to be precisely designed and prepared to control the Fermi level. In addition to the Fermi energy tuning, La‐doping modifies the conduction band, leading to an increase in the density of states effective mass that is confirmed by transport, infrared reflectance and hard X‐ray photoelectron spectroscopy measurements. Taking such a band structure modification effect into account, the electrical transport properties can then be well‐described by a self‐consistent single non‐parabolic Kane band model that yields an approximate (m*T)1.5 dependence of the optimal carrier concentration for a peak power factor in both doping cases. Such a simple temperature dependence also provides an effective approximation of carrier concentration for a peak zT and helps to explain, the effects of other strategies such as lowering the lattice thermal conductivity by nanostructuring or alloying in n‐PbTe, which demonstrates a practical guide for fully optimizing thermoelectric materials in the entire temperature range. The principles used here should be equally applicable to other thermoelectric materials.

Importance of relativistic effects in electronic structure and thermopower calculations forMg2Si,Mg2Ge, andMg2Sn

We present a theoretical study of the influence of the relativistic effects on electronic band structure and thermopower of Mg2X(X= Si, Ge, Sn) semiconductors. The full potential Korringa-Kohn-Rostoker (KKR) method is used, and the detailed comparison between the fully relativistic and semi-relativistic electronic structure features is done. We show that the spin-orbit (S-O) interaction splits the valence band structure at Gamma point in good agreement with the experimental data, and this effect strongly depends on X atom. The S-O modifications of the topology of the Gamma-centered hole-like Fermi surface pockets lead to a change in electron transport properties, which are investigated using the Boltzmann approach. In addition, the simple and efficient method is presented for the calculation of density of states effective mass m*, and then used to examine the impact of relativistic effects on m*. It is found that S-O coupling of the valence bands reduces effective mass and therefore significantly lowers the thermopower, primarily in Mg2Sn, but also in Mg2Ge. A detrimental influence of the S-O interaction on thermoelectric performance of p-type Mg2X is analyzed in function of temperature (10-900 K) and carrier concentration (10^18-10^22 cm-3). Interestingly, similar calculations in n-type Mg2X, show negligible effect of the S-O interaction on lowest conduction bands and consequently also on the Seebeck coefficient.

Effects of Cu addition on band gap energy, density of state effective mass and charge transport properties in Bi2Te3 composites

The effects of Cu addition on band gap energy, effective mass, and charge transport properties in n-type CuxBi2Te3 composites are presented.

Transport properties and valence band feature of high-performance (GeTe)85(AgSbTe2)15 thermoelectric materials

This paper aims at elucidating the origin of the high thermoelectric power factor of p-type (AgxSbTex/2+1.5)15(GeTe)85 (TAGS) thermoelectric materials with 0.4 ⩽ x ⩽ 1.2. All samples exhibit good thermoelectric figures of merit (zT) which reach 1.5 at 700 K for x = 0.6. Thermoelectric and thermomagnetic transport properties (electrical resistivity, Seebeck, Hall and transverse Nernst–Ettinghausen coefficients) are measured and used to calculate the scattering factor, the Fermi energy, the density-of-states (DOS) effective mass and hole mean free path (mfp). The DOS effective mass is very high due to the large band mass of the primary valence band and the high degeneracy of pockets in the Fermi surface from the second valence band. The highly degenerate Fermi surface increased the total DOS without decreasing mobility, which is more desirable than the high DOS that comes from a single carrier pocket. The high-temperature hole mfp approaches the Ioffe–Regel limit for band-type conduction, which validates our discussion based on band transport and is also important for TAGS alloys having high zT with heavy bands. The present results show that multiple degenerate Fermi surface pockets provide an effective way of substantially increasing the power factor of thermoelectric materials with low thermal conductivity.

Density of state effective mass and related charge transport properties in K-doped BiCuOSe

We report the enhanced p-type conduction properties in BiCuOSe by doping of monovalent ions (K+). As compared with undoped BiCuOSe, simultaneous increase in both the carrier concentration and the Hall mobility was achieved in the K-doped BiCuOSe. The origin of the enhancement was discussed in terms of the two-band structure in the valence band of the BiCuOSe, and the density of state effective masses of the heavy (∼1.1 me) and light hole (∼0.18 me) were estimated by using Pisarenko relation.

Electron and phonon transport in Co-doped FeV0.6Nb0.4Sb half-Heusler thermoelectric materials

The electron and phonon transport characteristics of n-type Fe1−xCoxV0.6Nb0.4Sb half-Heusler thermoelectric compounds is analyzed. The acoustic phonon scattering is dominant in the carrier transport. The deformation potential of Edef = 14.1 eV and the density of state effective mass m* ≈ 2.0 me are derived under a single parabolic band assumption. The band gap is calculated to be ∼0.3 eV. Electron and phonon mean free paths are estimated based on the low and high temperature measurements. The electron mean free path is higher than the phonon one above room temperature, which is consistent with the experimental result that the electron mobility decreases more than the lattice thermal conductivity by grain refinement to enhance boundary scattering. A maximum ZT value of ∼0.33 is obtained at 650 K for x = 0.015, an increase by ∼60% compared with FeVSb. The optimal doping level is found to be ∼3.0 × 1020 cm−3 at 600 K.

Metal nanoparticle decorated n-type Bi2Te3-based materials with enhanced thermoelectric performances

In this study, n-type Cu and Zn metal nanoparticle decorated Bi2(Te0.9Se0.1)3 ingots were prepared by a large-scale zone melting technique, with the concept of ‘nanoparticle-in-alloy’ to separately tune the electrical and thermal transport properties. Cu and Zn additions play multiple but different roles in the materials, whereas both of them form metal nanoinclusions embedded in van der Waals gaps or grain boundaries, exerting influences on thermoelectric properties. Cu addition, accommodated in the tetrahedral vacancies formed by four Te(1) atoms, effectively adjusts the electron concentration by donating its valence electron, and appreciably optimizes the power factor. Coupled with the significant frustration of heat-carrying phonons by Cu nanoinclusions, a highest ZT of 1.15 can be achieved for the 1 at.% Cu sample, which is an ∼20% improvement compared with that of commercial halogen-doped ingots. Zn addition, however, acting as weak donor, noticeably increases the density of state effective mass and Seebeck coefficient, and gives rise to a high ZT of 1.1. In particular, the kilogram-grade production technique coupled with the high ZT makes metal nanoparticle decorated n-type materials very promising for commercial applications.

Electronic conductivity effective masses along arbitrary directional channel in uniaxial strained Si(001)

Electronic conductivity effective mass is one of the key parameters studing electron mobility enhancement in unixial strained Si material. Its in-depth study has the significant theoretical and practical values. In this paper, we first establish the E-k relation for conduction band in a unixial strained Si material. And the model of electronic conductivity effective mass along an arbitrary directional channel in the uniaxial strained Si (001) is obtained. Our concluding results are described as follows. 1) Tensile stress should be used to enhance electron mobility for unixial trained Si. 2) In the case of tensile stress application, both [110]/(001) and [100]/(001) directions are the desirable ones from the evaluation of electronic conductivity effective mass. And [110]/(001) direction should be preferable when the density of state effective mass is taken into consideration. 3) If [100] direction becomes the channel direction under [110]/(001) uniaxial strain, the further electron mobility enhancement will occur. The results above can provide valuable reference for the conduction channel design related to stress and orientation in the strained Si nMOS device.

Feasibility of enhancing the thermoelectric power factor in GaNxAs1−x

This study was motivated by the possibility of using N resonant levels interacting with the GaAs conduction band, in GaNxAs1−x (0 < x < 2.5%), to enhance the density of states effective mass (md) and consequently the thermoelectric power factor (S2σ )—where S is the Seebeck coefficient and σ is the electrical conductivity. However, it was observed that, compared with GaAs, the power factor was reduced in spite of a small increase in the md. The influences of carrier passivation and dopant type, as well as the changes in the carrier scattering mechanism, which degrades the carrier mobility, are discussed.

The realization of a high thermoelectric figure of merit in Ge-substituted β-Zn4Sb3 through band structure modification

In this study, we demonstrate a realization of a favorable modification of band structures and an apparent increase in the density of state effective mass in β-Zn4Sb3 compound by introduction of a slight amount of Ge at the Zn site, in a manner of adding a shape peak below the valence band edge and giving rise to a significant enhancement in the power factor which is similar to the case of Tl-doped PbTe. As a consequence, the high power factor exceeding 1.4 mW m−1 K−2, coupled with the intrinsic very low thermal conductivity originated from complex crystal structures and a high degree of disorder, results in a maximum figure of merit of ∼1.35 at 680 K for the 0.25 at% Ge-substituted sample, which is ∼20% improvement as compared with that of the unsubstituted sample in this study. What is most important is the average ZT between 300 and 680 K reaches ∼1.0, which is ∼35% enhancement in comparison with the unsubstituted sample and superior to most of p-type materials in this temperature range. Furthermore, the combination of high thermoelectric performance and improvement in the thermodynamic properties makes this natural-abundant, “non-toxic” and cheap Ge-substituted β-Zn4Sb3 compound a very promising candidate for thermoelectric energy applications.

Valence band structure and density of states effective mass model of biaxial tensile strained silicon based on k · p theory

After constructing a stress and strain model, the valence bands of in-plane biaxial tensile strained Si is calculated by k · p method. In the paper we calculate the accurate anisotropy valance bands and the splitting energy between light and heavy hole bands. The results show that the valance bands are highly distorted, and the anisotropy is more obvious. To obtain the density of states (DOS) effective mass, which is a very important parameter for device modeling, a DOS effective mass model of biaxial tensile strained Si is constructed based on the valance band calculation. This model can be directly used in the device model of metal—oxide semiconductor field effect transistor (MOSFET). It also a provides valuable reference for biaxial tensile strained silicon MOSFET design.

Electron Mobility Model of Strained Si&lt;sub&gt;1-x&lt;/sub&gt;Ge&lt;sub&gt;x&lt;/sub&gt;(001)

Solving the Schrödinger equation with strain Hamiltonian and combining with KP theory, we obtained the conductivity effective mass and density of states effective mass of strained Si1-xGex(001) in this paper. On the basis of conductivity effective mass and density of states effective mass, considered of Fermi golden rule and Boltzman collision term approximation theory, scattering rate model was established in strained Si1-xGex(001). Based on the conductivity effective mass and scattering rate models we discussed the dependence of electron mobility on stress and doping concentration in strained Si1-xGex(001), it shows that electron mobility decrease with the increasing of stress and doping concentration. This result can provide valuable references to the research of electron mobility of strained Si1-xGexmaterials and the design of devices.

The influence of dopant type and carrier concentration on the effective mass and Seebeck coefficient of GaNxAs1−x thin films

We probe whether an enhancement in the Seebeck coefficient (S) could be obtained in GaNxAs1−x through interactions between the N resonant states and the GaAs conduction band. Through experimental investigations, we then determined that an insufficient increase in the density of states effective mass (md) precludes such an enhancement. The relative influences of Group IV/VI dopants and the carrier concentration along with N passivation are discussed.

Model of DOS near the Top of Valence Band in Strained Si&lt;sub&gt;1-x&lt;/sub&gt;Ge&lt;sub&gt;x&lt;/sub&gt;/(001)Si

Strained Si1-xGex technology has been widely adopted to enhance hole mobility. One of the most important physical parameters is density of state near the top of valence band in strained Si1-xGex materials. In this paper, we first obtained the hole effective mass along arbitrarily k wavevector directions, the hole isotropic effective masses and density of state effective mass of hole in strained Si1-xGex/(001)Si with the framework of K.P theory. And then, model of density of state near the top of valence band in strained Si1-xGex/(001)Si materials was established, which can provide valuable references to the understanding on its material physics and theoretical basis on the other important physical parameters.

Subband structure and effective mass of relaxed and strained Ge (1 1 0) PMOSFETs

Effective mass and mobility of strained Ge (1 1 0) inversion layer in PMOSFET are studied theoretically in this paper. The strain condition considered in the calculations is the intrinsic strain resulting from growing the Ge layer on the (1 1 0) Si substrate. The quantum confinement effect resulting from the vertical effective electric field is incorporated into the k · p calculation. Various effective masses, such as quantization effective mass, m z , density of states effective mass, m DOS , and conductivity mass, m C , as well as the hole mobility of strained Ge (1 1 0) inversion layer for PMOS under substrate strain and various effective electric field strengths are all investigated.

Modeling of enhancement factor of hole mobility for strained silicon under low stress intensity

In order to quantitatively characterize the enhancement of hole mobility of strained silicon under different stress intensity conditions, changes of hole effective mass should be studied. In the paper, strained silicon under in-plane biaxially tensile strain based on (0 0 1) substrate and longitudinal uniaxially compressive strain along 〈1 1 0〉 are investigated thoroughly. By solving the Hamiltonian of valence band using K·P model, we can obtain the relationship of density of state effective mass ( m DOS), conductivity effective mass ( m C) and splitting energy in valence band energy with stress intensity for both biaxially tensile strain and uniaxially compressive strain. For the stress intensity less than 1 GPa, the paper presents the models of enhancement factor of hole mobility under the biaxially tensile strain and uniaxially compressive strain. The results show that biaxially tensile strain of silicon cannot enhance hole mobility under low stress intensity, while uniaxially compressive stress of silicon can enhance hole mobility greatly.

Thermoelectric Properties of SrTiO3: Bulks, Superlattices, and Transistors

Here I review the thermoelectric properties of electron doped SrTiO3. Since density of states effective mass of electron carrier in SrTiO3 crystal is rather large, heavily electron doped SrTiO3 crystal exhibits rather large Seebeck coefficient (S). The reliable thermoelectric figure of merit, ZT is ∼0.08 at room temperature and ∼0.3 at 1,000 K, which is highest among metallic oxides. Recently, we found that high-density (∼1021 cm-3) two-dimensional (2D) electron layers in SrTiO3 crystal exhibits giant S, which is ∼5 times larger than that of simple bulk, when the layer thickness is as thin as one unit cell (0.3905 nm). The 2D electron layer is realized by superlattices of SrTiO3/SrTi0.8Nb0.2O3 or TiO2/SrTiO3 heterointerface. Very recently, we fabricated high performance field effect transistors (FETs) on SrTiO3 single crystal to modulate the Seebeck coefficient by applying the gate voltage.

Effective mass and subband structure of strained Si in a PMOS inversion layer with external stress

This study theoretically examines the hole subband structures in inversion layers of strained Silicon (Si) transistors under different strain conditions. Two strain conditions of Si P-channel metal-oxide-semiconductor (PMOS), including intrinsic strain resulting from growing the Si on the (001) Silicon–Germanium (SiGe) substrate and mechanical stress applied externally, were considered in this work. Various effective masses of hole inversion layers used in technology computer-aided design (TCAD) simulations were investigated, such as the quantization effective mass, m z, the density of states effective mass, m DOS, and the conductivity mass, m σ. It was then demonstrated that the external stress applied parallel to the channel direction for holes in the inversion layer with intrinsic biaxial strain is an optimal method of lowering m σ and increasing m DOS for the top subband.

Density of states effective mass of SnBi4Se7 deduced from the temperature dependence of electrical conductivity in the activation regime

Current–voltage (I–V) measurements on polycrystalline samples of Bi2Se3 and stoichiometric ternary compound in the quasi-binary system of SnSe–Bi2Se3 at different temperatures in the vicinity of room temperature have been performed. Also, temperature dependence of electrical conductivity has been measured. From the analysis of the temperature dependence of electron concentration in the activation regime above room temperature, the density of states effective mass, m*, has been determined. Some intrinsic and contact properties such as barrier heights, ideality factors, and carriers concentrations have been investigated using I–V characteristics. It has been found that all samples exhibit ohmic and space charge limited conduction at low and high fields, respectively.