Conductivity Effective Mass Research Articles

Piezoresistance effect of strained and unstrained fully-depleted silicon-on-insulator MOSFETs integrating a HfO 2/TiN gate stack

By applying an external uniaxial mechanical stress, we have extracted, for the first time, the piezoresistance coefficients of biaxially strained and unstrained fully-depleted silicon-on-insulator (FD-SOI) nMOS transistors that integrate a HfO 2/TiN gate stack. We have shown that when a uniaxial stress is added to a biaxial stress, the resulting piezoresistance effect is not a linear superposition of the individual influences of these stresses. Indeed, the measured piezoresistance coefficients were dependent on the initial level of biaxial stress. This contrasts with previous results showing for instance linear behaviour in the case of pure uniaxial stress along a 〈1 1 0〉 crystallographic direction. In addition, we have demonstrated that electron mobility can still be enhanced on biaxially strained FD-SOI devices, even for SOI films as thin as 10 nm, provided that channel stress includes a shear component. These results were explained by the effect of shear stress on the electron conduction effective mass. Such a shear stress can be generated for instance by applying uniaxial tensile (compressive) stress parallel (perpendicular) to the channel along a 〈1 1 0〉 crystallographic direction.

Mobility Modeling in Advanced MOSFETs with Ultra-Thin Silicon Body under Stress

Mobility in advanced MOSFETs with strained ultra-thin silicon body is investigated. We use a two-band k·p model to describe the subband structure in strained silicon thin films. The model provides the dependence of the conductivity effective mass on strain and film thickness. The conductivity mass decreases along tensile stress in [110] direction applied to a (001) silicon film. This conductivity mass decrease ensures the mobility enhancement in MOSFETs even with extremely thin silicon films. The two-band k·p model also describes the non-parabolicity dependence on film thickness and on strain. Dependence of the non-parabolicity parameter on both film thickness and strain reduces the mobility enhancement due to the conductivity mass modification, especially at higher strain values.

Giant Seebeck coefficient decrease in polycrystalline materials with highly anisotropic band structures: Implications in seeking high-quality thermoelectric materials

Present physical laws do not deny the existence of high-quality thermoelectric (TE) materials (with Z T > 3 – 4 or even more). But despite a fifty-year study, most of the known matters show ultra low heat-electricity conversion abilities (with Z T lower than 1). In this work, we take a canceling out of a Seebeck coefficient (thermopower, S ) of inner grains in some polycrystalline materials with highly anisotropic band structures, as one of the clues to the above contradiction. We show that an S could be drastically decreased about several hundred times (or even more) in some polycrystalline materials, compared to their single crystalline counterparts. These polycrystals can thus show low-estimated Z T (proportional to S 2 ), and should be re-examined in their single crystalline forms. Moreover, we also point out that the absence of a conductivity effective mass database at present means that there are blind spots in understanding and predicting some semiconductors’ orientation-dependent transport properties.

Monte Carlo simulation of low-field mobility in strained double gate SOI transistors

Electron transport in strained double gate silicon on insulator transistors has been studied by Monte Carlo method. Poisson and Schroedinger equations have been self-consistently solved in these devices for different silicon layer thicknesses both for unstrained and strained silicon channels. The results show that the strain of the silicon layer leads to a larger population of the no-primed subbands, thus decreasing the average conduction effective mass. However, strain also contributes to a larger confinement of the charge close to the two Si/SiO2 interfaces, thus weakening the volume inversion effect, and limiting the potential increase of the phonon limited mobility.

Hole mobility in silicon inversion layers: Stress and surface orientation

Hole transport in the p-type metal-oxide-semiconductor field-effect-transistor (p-MOSFET) inversion layer under arbitrary stress, surface, and channel orientation is investigated by employing a six-band k∙p model and finite difference formalism. The piezoresistance coefficients are calculated and measured at stresses up to 300MPa via wafer-bending experiments for stresses of technological importance: uniaxial and biaxial stresses on (001) and (110) surface oriented p-MOSFETs with ⟨110⟩ and ⟨111⟩ channels. With good agreement in the measured and calculated small stress piezoresistance coefficients, k∙p calculations are used to give physical insights into hole mobility enhancement at large stress (∼3GPa). The results show that the maximum hole mobility is similar for (001)∕⟨110⟩, (110)∕⟨110⟩, and (110)∕⟨111⟩ p-MOSFETs under uniaxial stress, although the enhancement factor is different. Strong quantum confinement and a low density of states cause less stress-induced mobility enhancement for (110) p-MOSFETs. For (001) p-MOSFETs, the dominant factor for the improved hole mobility is reduced conductivity effective mass at small stress and lower phonon scattering rate at large stress.

Uniaxial-process-induced strained-Si: extending the CMOS roadmap

This paper reviews the history of strained-silicon and the adoption of uniaxial-process-induced strain in nearly all high-performance 90-, 65-, and 45-nm logic technologies to date. A more complete data set of n- and p-channel MOSFET piezoresistance and strain-altered gate tunneling is presented along with new insight into the physical mechanisms responsible for hole mobility enhancement. Strained-Si hole mobility data are analyzed using six band k/spl middot/p calculations for stresses of technological importance: uniaxial longitudinal compressive and biaxial stress on [001] and [110] wafers. The calculations and experimental data show that low in-plane and large out-of-plane conductivity effective masses and a high density of states in the top band are all important for large hole mobility enhancement. This work suggests longitudinal compressive stress on [001] or [110] wafers and channel direction offers the most favorable band structure for holes. The maximum Si inversion-layer hole mobility enhancement is estimated to be /spl sim/ 4 times higher for uniaxial stress on (100) wafer and /spl sim/ 2 times higher for biaxial stress on (100) wafer and for uniaxial stress on a [110] wafer.

High electron mobility in well ordered and lattice-strained hydrogenated nanocrystallinesilicon

We report on the realization of high electron mobility (over103 cm2 V−1 s−1) in structure-ordered and lattice-strained hydrogenated nanocrystalline silicon (nc-Si:H)due to the decrease of conduction effective mass and phonon scattering. The nc-Si:H thinfilms were grown on crystalline silicon substrates by plasma-enhanced chemical vapourdeposition through the radio-frequency power to properly control the chemical reactions ofH atoms with the Si–Si network. The electron mobility and concentration in the nc-Si:Hhave been extracted with the aid of magnetic-field-dependent Hall effect measurements.X-ray diffraction, Raman, and infrared transmission experiments have been employed toyield information about the lattice strain and structural order in the Si nanocrystals. Theroom-temperature experimental mobility has been well explained by a generalized Drudetransport model unifying both the diffusive and ballistic transport mechanisms.

FIRST-PRINCIPLES INVESTIGATION OF SnO2 AT HIGH PRESSURE

The ground state properties and the structural phase transformation of tin dioxide ( SnO 2) have been investigated using first principle full potential-linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). We used local density approximation (LDA) and the generalized gradient approximation (GGA), which are based on exchange-correlation energy optimization, to optimize the internal parameters by relaxing the atomic positions in the force directions and to calculate the total energy. For band structure calculations, we utilized both the Engel-Vosko's generalized gradient approximation (EVGGA), which optimizes the exchange-correlation potential, and also GGA. From the obtained band structures, the electron (hole) valance and conduction effective masses are deduced. For compressed volumes SnO 2 is shown to undergo two structural phase transitions with increasing pressure from the rutile- to the CaCl 2-type phase at 12.4 GPa and to a cubic phase, space group [Formula: see text] at 22.1 GPa. The calculated total energy allowed us to investigate several structural properties, in particular, the equilibrium lattice constants, bulk modulus, cohesive energy, interatomic distances and the angles between different atomic bonds. In addition, we discuss the bonding parameter in term of charge density, which show the localization of charge around the anion side.

Structural and electronic properties of the wide-gapZn1−xMgxS,Zn1−xMgxSe and Zn1−xMgxTe ternary alloys

Zn1−xMgxS,Zn1−xMgxSe and Zn1−xMgxTe ternary wide-gap semiconductor alloys were investigated using the full potential–linearizedaugmented plane wave (FP-LAPW) method. We have studied the effect ofcomposition on structural properties such as lattice constants, bulk modulusand bond ionicity. The bandgap and the microscopic origins of compositionaldisorder have also been explained in detail. In addition, from the obtained bandstructures, the electron (hole) conduction and valence effective masses are deduced.These parameters were found to depend non-linearly on alloy compositionx, except the latticeparameter for Zn1−xMgxS, which follows Vegard’s law. The calculated band structuresfor all three alloys show a direct bandgap in the whole range ofx composition. We have paid special attention to the disorder parameter (gap bowing). Usingthe approach of Zunger and co-workers, we have concluded that the total bandgap energybowing was mainly caused by the charge exchange effect for the alloys of interest.

Ground state properties and structural phase transition of beryllium chalcogenides

The ground state properties and the structural phase transformation of beryllium chalcogenides (BeS, BeSe, and BeTe) have been investigated using first principle full potential-linearized augmented plane wave method (FP-LAPW) within density functional theory. We used local density approximation with and without generalized gradient correction as well as the Engel Vosko’s GGA formalism to find band gap. From the obtained band structures, the electron (hole) valence and conduction effective masses are deduced. We have determined the full set of first-order elastic constant, which have not been established experimentally. We have also calculated the energy–volume relations for these compounds in the zinc-blende (B3) and the NiAs (B8) phases. Hence we have obtained the lattice parameters, bulk modulus, pressure derivative of bulk modulus and cohesive energy as well as structural transition pressure. The calculated ionicity parameter which expected from the charge density behavior compared well with the Phillips’ ionicity scale.

In-plane magnetodrag in dilute bilayer two-dimensional systems: A Fermi-liquid theory

Motivated by recent experimental results reporting anomalous drag resistance behavior in dilute bilayer two-dimensional (2D) hole systems in the presence of a magnetic field parallel to the 2D plane, we have carried out a many-body Fermi liquid theory calculation of bilayer magnetodrag comparing it to the corresponding single layer magnetoresistance. In qualitative agreement with experiment we find relatively similar behavior in our calculated magnetodrag and magnetoresistance arising from the physical effects of screening being similarly modified ("suppressed") by carrier spin polarization (at "low" field) and the conductivity effective mass being similarly modified ("enhanced") by strong magneto-orbital correction (at "high" fields) in both cases. We critically discuss agreement and disagreement between our theory and the experimental results, concluding that the magnetodrag data are qualitatively consistent with the Fermi liquid theory.

Velocity-field characteristics in the multivalley model of gallium arsenide

The drift-velocity response to an applied electric field is investigated in a model based on an anisotropic distribution function of electrons. The randomly oriented velocity vectors in equilibrium transform to streamlined velocity vectors in a very high electric field, thereby yielding a saturation drift velocity that is comparable to the thermal velocity in the nondegenerate approximation. A clear distinction is made between the density-of-states effective mass and the conductivity effective mass for ellipsoidal valleys in gallium arsenide (GaAs). The effective mean free path, and hence the effective mobility, degrades as the electric field is increased. It is shown that as the applied electric field increases, electrons tend to follow the velocity-field profiles of those valleys with a higher Ohmic mean free path. The results are shown to be in excellent agreement with experimental data and with empirical relations obtained from the Monte Carlo simulations.

Numerical Foundation of Hot-Electron Diffraction Experiment Based on Ballistic Electron Emission Microscope

We provide the numerical foundation for a previously proposed experiment in hot-electron diffraction observation in solids based on the ballistic electron emission microscope (BEEM). It is found that the hot-electron wave function in a semiconductor becomes a quasi-spherical wave because of a large refraction effect at the base metal/semiconductor interface due to the large discontinuities between the conduction band bottoms and effective masses of the materials. Thus, it has been clarified that the quantum reciprocity, our measurement principle, can be applied to our proposed experimental configuration by using BEEM. We have also carried out numerical simulations for our experiment with a π-phase shifter structure as a wave front modulator by using the finite difference time domain (FDTD) method. The results showed diffraction patterns with high contrast, and thus our approach to hot-electron diffraction observation was affirmed.

Monte Carlo Simulation of Electron Velocity Overshoot in DGSOI MOSFETs

A comprehensive study of velocity overshoot and the dependences that might be established with technological parameters in symmetrical DGSOI transistors has been carried out. A one-electron Monte Carlo simulator was used to perform this work. It has been proved that velocity overshoot is dominated by the average conduction effective mass. Thus, when the silicon thickness decreases below 5 nm velocity overshoot peak increases showing the opposite behavior to electron mobility.

High Phonon-Drag Thermoelectric Efficiency of SiC at Low Temperatures

Results confirming the high values of Seebeck coefficient phonon-drag component Sph in SiC are discussed and the relevant consequences to the thermoelectric figure-of-merit ZT=T S 2  / are studied. The fact that Sph increases at low temperatures as T 2.4 similarly for low-doped p-6H- SiC, Si and Ge is demonstrated. Materials comparison criterion Sph  / meff, based on heat conductivity, lattice mobility and carrier effective mass is offered and Sph strength proportions 0.1 : 1 : 5 : 10 for Ge, Si, SiC and C (diamond) are proposed. Related to that high T500K phonon- drag domination limit is obtained for SiC. It is shown that the proportionality Sph , yielding ZT , could reverse some concepts accepted in thermoelectric material development. The best-case calculations predict good (>0.1) and excellent (>1) ZT values with steep temperature-dependence T 3…4 for low temperatures 50200K if sufficiently high electrical conductivity  could be ensured. The last condition may be feasible for n-SiC with relatively shallow donor levels but difficult to achieve in the case of p-SiC with relatively deep acceptor levels.

Double gate silicon on insulator transistors. A Monte Carlo study

Electron transport properties in double-gate-silicon-on-insulator (DGSOI) transistors are comprehensively studied. Quantum effects are analyzed by self-consistently solving the 1 D Poisson and Schroedinger equations. Once the electron distribution is known, the Bolztmann transport equation is solved by the Monte Carlo method, and the role of volume inversion is analyzed both at room and at lower temperatures. A comparison between symmetrical-gate and asymmetrical-gate configurations is also provided, showing the superior performance of symmetric devices. Finally, velocity overshoot is also studied. Monte Carlo simulations were performed to clarify the dependence of velocity overshoot effects on the low-field mobility, channel inversion charge and silicon layer thickness. We show that electron mobility is mainly determined by the increase in the phonon scattering rate as the silicon thickness is reduced, i.e., the lower the silicon thickness the lower the electron mobility, while velocity overshoot effects for ultrathin DGSOI inversion layers are dominated by the reduction of the average conduction effective mass, i.e., the lower the silicon thickness the higher the velocity overshoot peak.

Velocity overshoot in ultrathin double-gate silicon-on-insulator transistors

Concerning electron velocity overshoot in ultrathin double-gate silicon-on-insulator transistors, as a function of the silicon thickness and the electron sheet density, it is proved that for very small silicon thicknesses (smaller than 5 nm), velocity overshoot behavior is dominated by the average conduction effective mass; that is, the lower the average conduction effective mass, the higher the velocity overshoot peak. Thus, the velocity overshoot peak increases as the silicon thickness decreases, unlike low-field electron mobility, which diminishes abruptly at silicon thicknesses below 5 nm. This fact enables further reduction in the device channel length, in contrast to what might be supposed from the low-field mobility behavior.

A comprehensive study of velocity overshoot effects in double gate silicon on insulator transistors

A comprehensive study of velocity overshoot in double gate silicon on insulator (DGSOI) transistors has been undertaken. Monte Carlo simulations were performed to clarify the dependence of velocity overshoot effects on the low field mobility, channel inversion charge and silicon layer thickness. The relationships and dependences between the energy- and momentum-relaxation times were also investigated. In general, two opposite effects influence electron transport in DGSOI inversion layers as the silicon thickness is reduced: a reduction in the conduction effective mass and a phonon scattering increase. However, we show that electron mobility is mainly determined by the increase in the phonon scattering rate as the silicon thickness is reduced, i.e., the lower the silicon thickness the lower the electron mobility, while the velocity overshoot effects for ultrathin DGSOI inversion layers are dominated by the reduction of the average conduction effective mass, i.e., the lower the silicon thickness the higher the velocity overshoot peak.

Electron mobility in double gate silicon on insulator transistors: Symmetric-gate versus asymmetric-gate configuration

We have studied electron mobility behavior in asymmetric double-gate silicon on insulator (DGSOI) inversion layers, and compared it to the mobility in symmetric double-gate silicon on insulator devices, where volume inversion has previously been shown to play a very important role, being responsible for the enhancement of the electron mobility. Poisson’s and Schroedinger’s equations have been self-consistently solved in these structures to study and compare the distribution of the electrons. We show that the lack of symmetry in the asymmetric DGSOI structure produces the loss of the volume inversion effect. In addition, we show that as the silicon thickness is reduced the conduction effective mass of electrons in asymmetric devices is lower than that in the symmetric case, but that the greater confinement of electrons in the former case produces a stronger increase in the phonon scattering rate, and in the surface roughness scattering rate. We have solved the Boltzmann transport equation by the Monte Carlo method, and have evaluated the electron mobility. The electron mobility curves in asymmetric DGSOI devices are shown to be considerably below the mobility curves corresponding to symmetric devices, in the whole range of silicon thicknesses. The difference is greater in the range 5–25 nm, where electron mobility in symmetric DGSOI inversion layers is greatly improved by the volume inversion effect. We show that mobility in symmetric devices could be 2.5 times greater than that for their asymmetric counterparts.

Energy and temperature dependence of electron effective masses in silicon

A k⋅p model is used to theoretically investigate the energy and lattice temperature dependence of both transverse and longitudinal “curvature” electron effective masses in silicon. The temperature dependence of the carrier concentration conduction effective masses in the range of 10–550 K is also examined. Our results highlight the energy dependence of the longitudinal effective mass, usually considered to be equal to the band-edge effective mass, which varies from 0.917 to 1.6m0 when the carrier energy ranges from the bottom of the conduction band up to 1.5 eV. This energy dependence should have a significant impact on electronic transport simulations using drift–diffusion, hydrodynamic, or Monte Carlo methods, particularly for hot-carrier phenomena in microelectronic devices.