States Effective Mass Research Articles

Electron Mobility Model of Strained Si<sub>1-x</sub>Ge<sub>x</sub>(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<sub>1-x</sub>Ge<sub>x</sub>/(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.

High Transport Si/SiGe Heterostructures for CMOS Transistors with Orientation and Strain Enhanced Mobility

We have demonstrated high mobility MOS transistors on high quality epitaxial SiGe films selectively grown on Si (100) substrates. The hole mobility enhancement afforded intrinsically by the SiGe channel (60%) is further increased by an optimized Si cap (40%) process, resulting in a combined ∼100% enhancement over Si channels. Surface orientation, channel direction, and uniaxial strain technologies for SiGe channels CMOS further enhance transistor performances. On a (110) surface, the hole mobility of SiGe pMOS is greater on a (110) surface than on a (100) surface. Both electron and hole mobility on SiGe (110) surfaces are further enhanced in a channel direction with appropriate uniaxial channel strain. We finally address low drive current issue of Ge-based nMOSFET. The poor electron transport property is primarily attributed to the intrinsically low density of state and high conductivity effective masses. Results are supported by interface trap density (Dit) and specific contact resistivity (ρc).

High sensitivity of palladium on porous silicon MSM photodetector

In this work, the nanocrystalline porous silicon (PS) is prepared through the simple electrochemical etching of n-type Si (1 0 0) under the illumination of a 100 W incandescent white light. SEM, AFM, Raman and PL have been used to characterize the morphological and optical properties of the PS. SEM shows uniformed circular pores with estimated sizes, which range between 100 and 500 nm. AFM shows an increase in its surface roughness (about 6 times compared to c-Si). Raman spectra of the PS show a stronger peak with FWHM=4.3 cm −1 and slight blueshift of 0.5 cm −1 compared to Si. The room temperature photoluminescence (PL) peak corresponding to red emission is observed at 639.5 nm, which is due to the nano-scaled size of silicon through the quantum confinement effect. The size of the Si nanostructures is estimated to be around 7.8 nm from a quantized state effective mass theory. Thermally untreated palladium (Pd) finger contact was deposited on the PS to form MSM photodetector. Pd/PS MSM photodetector shows lower dark (two orders of magnitude) and higher photocurrent compared to a conventional Si device. Interestingly, Pd/PS MSM photodetector exhibits 158 times higher gain compared to the conventional Si device at 2.5 V.

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.

Polaronic Correction to the Ground State Energy and Effective Mass in a Two- and Three-Dimensional Quantum Dot

A Landau-Pekar variational theory is employed to obtain an analytical expression for the polaronic correction to the ground state energy and the effective mass of an electron confined in a symmetric quantum dot potential in polar semiconductor in both two and three dimensions. It is found that polaronic correction is more pronounced in two dimension than that in three dimensional one and increases with the decrease in size of the quantum dot.

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.

Thermoelectric properties of electron doped SrO(SrTiO3)n (n=1,2) ceramics

Crystal structure and thermoelectric properties of Nb5+- and Ln3+-(rare earth: La3+, Nd3+, Sm3+, and Gd3+) doped SrO(SrTiO3)n (n=1,2) ceramics, which were fabricated by conventional hot-pressing, were measured to clarify the effects of Ti4+- and Sr2+-site substitution on the thermoelectric properties. The thermal conductivities are very close between the n=1 and 2 phases either doped with Nb5+ or Ln3+ and decreased by ∼60% at room temperature and ∼30% at 1000 K as compared to that of SrTiO3, which is likely due to an enhanced phonon scattering at the SrO/(SrTiO3)n (n=1,2) interfaces. The density of states effective mass md∗ (1.8–2.4 m0) and consequently the Seebeck coefficient |S| in Nb5+-doped samples are fairly smaller than those reported for SrTiO3, which probably resulted from a deterioration of DOS due to the formation of the singly degenerate a1g (Ti 3dxy) orbital as the conduction band bottom, which should be induced by the distortion of TiO6 octahedra in (SrTiO3)n layers. However, in the Ln3+-doped SrO(SrTiO3)2, the TiO6 octahedra were found to be restored, in contrast to the Nb5+-doped, with a gradually increasing O–Ti–O bond angle in the (100) plane at high temperatures, which would lead to the formation of triply degenerate Ti 3d-t2g (dxy, dyz, and dxz) orbitals to cause a significant enhancement in md∗ (∼7.5 m0 at 1000 K) and consequently in |S|. Accordingly, the maximum dimensionless figure of merit ZT∼0.24 obtained in 5%-Gd3+-doped SrO(SrTiO3)2 at 1000 K is about 70% larger than that of Nb-doped SrO(SrTiO3)2 (ZT1000 K∼0.14).

Impurity band in SnBi4Se7: thermoelectric power and electrical resistivity measurements

Thermoelectric power and electrical resistivity measurements on polycrystalline samples of Bi2Se3 and stoichiometric ternary compound in the quasi-binary system SnSe–Bi2Se3 in the temperature range of 90–420 K are presented and explained assuming the existence of an impurity band. The variation of the electron concentration with temperature above 300 K is explained in terms of the thermal activation of a shallow donor, by using a single conduction band model. The density of states effective mass m*=0.15m0 of the electrons, the activation energy of the donors, their concentration, and the compensation ratio are estimated. The temperature dependence of the electron mobility in conduction band is analyzed by taking into account the scattering of the charge carriers by acoustic phonon, optical phonon, and polar optical phonon as well as by alloy and ionized impurity modes. On the other hand, by considering the two-band model with electrons in both the conduction and impurity bands, the change in the electrical resistivity with temperature between 420 and 90 K is explained.

Enhancement of thermoelectric performance in rare earth-doped Sr3Ti2O7 by symmetry restoration of TiO6 octahedra

Aiming at the realization of the enhancement of thermoelectric performance through structural modification, the present work has clarified the significant effects of rare earth (RE=Gd, Sm, Nd, and La) doping at Sr-sites in Sr3Ti2O7, both on the structural restoration of distorted TiO6 octahedra and on the Seebeck coefficient, especially at high temperatures. The preferential substitution of RE 3+ at the nine-coordinate Sr-sites can facilitate the degeneration of the conduction band (Ti 3d-t 2g ) orbital, owing to its special capability in restoring TiO6 octahedra to a higher state of symmetry and thus enhance the density of states (DOS) effective mass of the carriers, which gives rise to a rather large increase in the Seebeck coefficient. The present findings have affirmed the effectiveness of structural restoration in enhancing the Seebeck coefficient by Sr-site-doping, which will help establish a useful solution for Ti-based thermoelectric oxides with inherently distorted TiO6 octahedra to achieve high thermoelectric performance.

Enhancement of Seebeck coefficient for SrO(SrTiO3)2 by Sm substitution: Crystal symmetry restoration of distorted TiO6 octahedra

We found that Sm3+ substitution in SrO(SrTiO3)2 is effective in improving the Seebeck coefficient (S). The ∣S∣ value increases notably with temperature, benefiting from an enhancement of the density of states (DOS) effective mass md* from ∼3m0 (300K)to∼7.5m0 (1000K), due to an improvement of the local symmetry of TiO6 octahedra, enhancing the degeneracy in the Ti 3d orbitals, which form the conduction band (CB), and also to an accompanying lattice expansion, which gives rise to a higher DOS at the bottom of the CB and, consequently, a larger md*.

The effect of Eu substitution on thermoelectric properties of SrTi0.8Nb0.2O3

Herein we demonstrate that Eu2+ substitution of SrTi0.8Nb0.2O3 is effective in reducing the thermal conductivity, with no reduction in the thermoelectric power factor (S2σ; S, Seebeck coefficient; σ, electrical conductivity). We prepared cubic (Sr,Eu)Ti0.8Nb0.2O3 crystals having the same lattice parameter (a=0.395 nm) over the whole composition range. Although no dependence upon the Eu content was observed for the electron transport properties, including the density of states effective mass and carrier relaxation time for (Sr,Eu)Ti0.8Nb0.2O3, the mean free path of the phonon was reduced ∼12% at room temperature when half of the Sr2+ was substituted by Eu2+.

Transport and magnetic properties of dilute rare-earth–PbSe alloys

Polycrystalline alloys of PbSe with rare-earth elements (Ce, Pr, Nd, Eu, Gd, and Yb) have been prepared and their magnetic susceptibility (from 4 to 120 K), galvanomagnetic and thermomagnetic transport (from 80 to 380 K) properties have been measured. Most samples are paramagnetic, and the concentration of rare-earth atoms in the PbSe lattice is deduced from fitting a Curie-Weiss law. The electrical conductivity, Hall, Seebeck, and transverse Nernst-Ettingshausen effects are interpreted in terms of the carrier density and mobility, the density of states effective mass, and the scattering exponent. In summary, Pb1−xEuxSe is a semiconductor with a wider gap than PbSe, but the carrier density is unaffected by the presence of Eu. The other rare earths, which are essentially trivalent atoms, act as donors, with a doping efficiency close to unity in the case of Ce and Nd, but much less for Gd and Yb. The mechanisms that govern the observed decrease in mobility are also discussed.

Electrical properties of CuGaSe2 single crystals and polycrystalline coevaporated thin films

Abstract CuGaSe2 single crystals were grown by iodine vapour transport method, whereas polycrystalline thin films were obtained by coevaporation technique from three sources. The variation of the hole concentration with temperature of CuGaSe2 single crystal in the high temperature region is interpreted assuming thermal activation in the valence band. The density of states effective mass of the holes is deduced. Acceptor and donor concentrations as well as their compensation ratio are reported. The temperature dependence of the hole mobility in valence band is analysed by taking into account several scattering mechanisms of the charge carriers. To account for the temperature dependant conductivity of polycrystalline CuGaSe2 thin film, grain boundary conduction process was suggested. Quantitative values of the mean potential barrier height at grain boundaries and the standard deviation are reported. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermoelectric phase diagram in a CaTiO3–SrTiO3–BaTiO3 system

Herein the authors propose a compositional thermoelectric phase diagram, which includes the density of states effective mass (md*) carrier relaxation time (τ), and thermoelectric power factor (S2σ, where S and σ are Seebeck coefficient and electrical conductivity) for high-quality epitaxial films composed of 20%-Nb-doped CaTiO3–SrTiO3–BaTiO3 solid solutions [(Ca,Sr,Ba)(Ti0.8Nb0.2)O3]. The md* values almost proportionally increase with the in-plane lattice parameter of (Ca,Sr,Ba)(Ti0.8Nb0.2)O3 epitaxial film, while the τ values drastically decrease when Ca and/or Ba are substituted for Sr in Sr(Ti0.8Nb0.2)O3, indicating that the Sr-site substitution negatively affects the thermoelectric performance of Nb-doped SrTiO3.

Electronic transport properties ofPbTeandAgPbmSbTe2+msystems

Transport calculations using the Boltzmann equation within energy-dependent relaxation time approximations were performed for $\mathrm{PbTe}$ and ${\mathrm{AgPb}}_{m}{\mathrm{SbTe}}_{2+m}$ (LAST-m) systems. We have used both the nonparabolic Kane model for the energy dispersion and the energy dispersion given by ab initio electronic structure calculations. For $\mathrm{PbTe}$ we find that the temperature dependence of the density of states effective mass ${m}_{d}$ is very important in order to have good agreement with experiment for electrical conductivity $\ensuremath{\sigma}$ and thermopower $S$. Transport calculations in $n$-type $\mathrm{PbTe}$ using the energy dispersion given by the ab initio electronic structure results in overestimation of $\ensuremath{\sigma}$ and underestimation of $S$ because the temperature dependence of ${m}_{d}$ cannot be incorporated in the calculation of the chemical potential. Transport calculations in $n$-type LAST-m systems using the nonparabolic Kane model for the energy dispersion show a small enhancement of the power factor $(\ensuremath{\sigma}{S}^{2})$ in $0--500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ temperature range relative to $\mathrm{PbTe}$. The observed large ZT values of the LAST-12 and LAST-18 systems are a combination of a small enhancement of the power factor and a strong reduction in the thermal conductivity due to the formation of $\mathrm{Ag}\text{\ensuremath{-}}\mathrm{Sb}$ microstructures.