3d Density Of States Research Articles

TCAD simulations of FDSOI devices down to deep cryogenic temperature

In this paper we demonstrate the feasibility of FDSOI device TCAD simulations down to deep cryogenic temperatures. To this purpose, Maxwell-Boltzmann carrier statistics is replaced by an analytical approximation for the Fermi-Dirac integral with 3D density of states. The device electrostatic is investigated by solving the Poisson equation in 2D, whereas the transport is simulated using the Drift-Diffusion model. We explore the impact of temperature on device performances in linear and saturation regions as well as the impact of short channel effects, accounted for various gate and spacer lengths, at room and deep cryogenic temperatures. Finally, the obtained results are compared to some experimental data, highlighting the role of TCAD simulations in providing insights in device physics and performances.

Efficient Band-to-Trap Tunneling Model Including Heterojunction Band Offset

It is well-known that band-to-trap tunneling processes in semiconductor devices cause a significant increase in terminal currents under high electric fields [1-3]. Standard band-to-trap tunneling models such as those by Schenk [1-2] and Hurkx [3] are widely incorporated in TCAD device simulation tools [4]. However, these models were developed for silicon devices, so they did not include the effect of band offsets in heterojunction devices, though they are routinely used in simulating the latter. As shown recently by Myers [5], the band offsets at the emitter-base (E-B) junction in heterojunction bipolar transistors (HBTs) can greatly enhance the band-to-trap tunneling near the E-B junction and consequently exacerbate the degradation of gain due to radiation damage. In this paper, we propose an improved band-to-trap tunneling model based on Schenk’s approach, which includes heterojunction band offsets and can be easily implemented into device simulators. Within Schenk’s approach, the band-to-trap tunneling is modeled [1] as the Shockley-Read-Hall recombination with the field-dependent carrier lifetime τ(T, F) given by Fig. 1(a), where T and F denote temperature and electric field respectively, Et is the trap binding energy measured from the band edge, and I(z) denotes the modified Bessel function. Characteristics of band profile, including the effect of electric field, affect the field enhancement factor g(T, F) through the 3D tunneling density of states (DOS) ρF(E). Schenk used the constant field DOS expression in his original model, which is a reasonable approximation for homojunction devices. However, detailed calculations in Ref. [5] show that the E-B junction band offsets in HBTs can significantly increase the DOS near the junction. Myers [5] computes the DOS by numerically solving the Schrodinger equation for the actual potential profile in a NPN HBT. Different from Ref. [5], we approximate the actual potential (e.g., black curve in Fig. 1b) by a linear potential with band offset (red curve in Fig. 1b), and develop an analytic DOS model that includes the band offset, for efficiency and easy implementation in device simulation. The new DOS model is obtained through the following steps: (i) separate the plane parallel to the heterojunction from the direction perpendicular to the junction and use the zero-field 2D DOS for the parallel plane; (ii) approximate the 1D potential along the perpendicular direction using a piece-wise linear profile with band offsets and analytically solve for the wave function in each region with open boundaries at the two outmost ends; (iii) normalize the 1D wave functions according to the Dirac Delta normalization condition [6]; (iv) compute the 3D tunneling DOS that goes into the Schenk model from the zero-field 2D DOS and the normalized 1D wave functions. Figure 1(c) shows a comparison of the 3D tunneling DOS versus carrier energies, computed by several methods, for the red potential in Fig. 1(b). The constant field DOS expression depends only on the field strength, not on the spatial location, as shown by the black solid curve. The color solid curves were obtained using the Fortran code provided by Myers [5] that numerically solves the Schrodinger equation and computes the DOS for the red potential. The color dashed curves were computed using the analytic DOS model proposed in this work. As expected, our analytic model produces the same DOS as those of the numerical method for the given potential. It is clearly seen that, when taking account of the heterojunction band offset, the DOS can be much higher than that of the constant field approximation. In particular, the closer the trap is located to the heterojunction, the higher the DOS is than the constant field value. For example, at 5 nm from the junction, the computed DOS using our model is more than ten orders of magnitude higher than that of the constant field expression for energies of 0.25 eV below the band edge. The substantially increased DOS near the heterojunction is expected to significantly enhance the band-to-trap tunneling in the vicinity of the junction. It turns out that the proposed DOS model can be generalized to arbitrary piece-wise linear 1D potentials. The band-to-trap tunneling model with the new DOS is being implemented in CHARON, an open-source multi-dimensional parallel TCAD code developed at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. Figure 1

A potential descriptor for the CO2 hydrogenation to CH4 over Al2O3 supported Ni and Ni-based alloy catalysts

Supported Ni, Cu, Fe, and Ni-M (M=Cu or Fe) catalysts of the same total metal loading and different Ni to M ratios are examined for the CO2 hydrogenation to CH4. Substituting Ni with Cu or Fe in a Ni/Al2O3 catalyst leads to smaller average particle size, sharper particle size distribution and alloy formation, which were determined from transmission electron microscopy (TEM) and X-ray diffraction (XRD). Surface properties of the Ni/Al2O3 and Ni-M/Al2O3 catalysts are found to be different as revealed by hydrogen temperature programmed desorption (H2-TPD). Furthermore, the surface of the supported Ni-Cu catalyst was enriched with Cu. The catalytic activity increases for 75Ni25Fe/Al2O3 relative to Ni/Al2O3, but decreases for 75Ni25Cu/Al2O3. In both these catalysts, the Ni to M ratio was 3:1. The supported Ni and the two Ni3M alloy catalysts are modelled successfully as the shift in 2θ between experimental and simulated XRD pattern were in accordance. Moreover, the trend in shifts in experimental and simulated optimized lattice constants relative to Ni was similar. Density functional theory (DFT) calculations revealed changes in the electronic properties. The electronic properties calculated based on the d-density of states (d-DOS) of surface Ni atoms are the d-band center (εd) and number of d-DOS at Fermi level (NEF). An excellent correlation between TOFCH4and NEFexists, whereas εd did not satisfactorily capture the trends inTOFCH4. Thus, the NEFbest describes the catalytic activity trend of the supported Ni and two Ni-based alloy catalysts.

Formula omitted] hybridization anomaly in [formula omitted] alloys at quantum critical point

First-principles density functional theory computations of electronic structure and local magnetic properties of the non-fluctuating ground state of NixPd1−x alloy system around its quantum critical point xc=0.026 have been performed. The density of states at the Fermi energy and certain other parameters characterizing the Ni3d−Pd4d hybridization apparently follow power-laws with x similar to that obeyed by the reported ferromagnetic to paramagnetic transition temperature. The width of Pd 4d density of states (DOS) and centroid of Ni 3d DOS show peak-like anomalies in the neighbourhood of xc, and so indicate a possible scenario of the existence of a definite relation between the orbital hybridization and the emergence of quantum fluctuations in the system.

A Transition from Localized to Strongly Correlated Electron Behavior and Mixed Valence Driven by Physical or Chemical Pressure in ACo2As2 (A = Eu and Ca).

We demonstrate that the action of physical pressure, chemical compression, or aliovalent substitution in ACo2As2 (A = Eu and Ca) has a general consequence of causing these antiferromagnetic materials to become ferromagnets. In all cases, the mixed valence triggered at the electropositive A site results in the increase of the Co 3d density of states at the Fermi level. Remarkably, the dramatic alteration of magnetic behavior results from the very minor (<0.15 electron) change in the population of the 3d orbitals. The mixed valence state of Eu observed in the high-pressure (HP) form of EuCo2As2 exhibits a remarkable stability, achieving the average oxidation state of +2.25 at 12.6 GPa. In the case of CaCo2As2, substituting even 10% of Eu or La into the Ca site causes ferromagnetic ordering of Co moments. Similar to HP-EuCo2As2, the itinerant 3d ferromagnetism emerges from electronic doping into the Co layer because of chemical compression of Eu sites in Ca0.9Eu0.1Co1.91As2 or direct electron doping in Ca0.85La0.15Co1.89As2. The results reported herein demonstrate the general possibility of amplifying minor localized electronic effects to achieve major changes in material's properties via involvement of strongly correlated electrons.

Electronic structures of Pd30 Pt17.5 Cu32.5 P20 bulk metallic glass

Abstractauthoren The incident-photon energy, , dependence of the valence-band electronic structure was measured by photoemission spectroscopy (PES) on PdPtCuP (PPCP) bulk metallic glass, having a critical-cooling-rate similar to that of the PdNiCuP (PNCP) excellent glass-former. The density of states (DOS) near the Fermi energy is mainly composed of the Pd 4d and Pt 5d electrons. The partial Pd 4d, Pt 5d, and Cu 3d DOSs in the valence band were estimated by the dependence of the photo-ionization cross-sections, and a localized tendency was clarified for the Pd 4d electrons. Core-level PES measurements were also carried out at the Pd 3, Pt 4, Cu 3, and P 2p levels. The Pd 3 core level largely shifts toward the deeper electron energies by replacing Ni with Pt atoms from PNCP, indicating a charge transfer from the P and Pd atoms. The width of the peak becomes wider in PPCP than in PNCP for the Pd 3 core level, suggesting that local structures around the Pd atoms become heterogeneous or less uniform.

Magnetic Properties of Liquid 3d Transition Metal–Ge Alloys

The magnetic susceptibilities (χ) of liquid Ti–Ge and V–Ge alloys show a weak and positive temperature dependence in restricted regions of up to 30 at. % Ti and 40 at. % V, respectively. This suggests that the Ti and V ions in these liquid alloys are in a nonmagnetic state. The χ values of liquid TM–Ge (TM = Fe, Co, Ni) alloys on the Ge-rich side exhibit a similar temperature dependence. The data in the nonmagnetic state were analyzed using the Anderson model. In the nonmagnetic state, we found smooth variations in which both the effective intra-atomic d–d interaction and the density of 3d states at the Fermi level EF decrease with increasing EF of liquid TM0.1M0.9 (M = Sn → Ge → Si) alloys. Liquid Cr1−cGec with c ≥ 0.7 and Mn1−cGec with c ≥ 0.3 obeyed the Curie–Weiss law with regard to the temperature dependence of their χ. The composition dependences of the χ of liquid Cr–Ge and Mn–Ge alloys show maxima at compositions of 50 at. % Cr and 70 at. % Mn, respectively. The magnetic susceptibilities of liquid...

XANES and XPS study of electronic structure of Ti-enriched Nd–Fe–B ribbons

Electronic structure of rapidly quenched ribbons of nominal composition NdyFe(86−y−x)B14Tix (x = 0, 2, 4; y = 7, 8 at.%) was studied by X-ray absorption (Fe and Ti K-edge) and X-ray photoelectron spectroscopy. It was found by XANES that Ti addition promotes modifications of the electronic structure of Fe sub-band. From the analysis of differences in the pre-edges structure areas, coming from variations in the 3d density of states near the Fermi level, the evidences of acceptor nature of Ti and Fe atoms in NdFeB alloys are present. Using photoelectron spectroscopy, we found that neodymium, iron and titanium are in both, metallic and oxidized, states on the surface of the ribbons. A higher Ti0 content (at high vacuum) is characteristic of the annealed samples. The presence of oxidized titanium states was corroborated by XANES.

Effect of the doping level on the radiative life time in ZnO nanowires

The emission properties of ZnO nanowires grown by metal organic vapor phase epitaxy on sapphire and p-type GaN are compared using temperature dependent time resolved photoluminescence. The temperature dependence of the radiative decay time of nanowires grown on sapphire is well understood considering an exciton thermalization in a 3D density of states. In the case of growth on GaN, a great increase of the radiative decay time is observed compared to nanowires grown on sapphire. This increase of the radiative decay time could be due to a band bending effect that separates the electron and the hole. This band bending effect depends on the residual doping level and is not seen in the case of nanowires grown on sapphire probably because of a very high residual doping level that can be estimated from the radiative decay time temperature dependence.

Possible influence of electrostatics in molecular bonding at supported metal nanoparticles

A mechanism is proposed for interaction between gas molecules and supported metal nanoparticles. Hot electrons that are formed via electron–hole pair excitations caused by exothermic reactions on the particles, enter into the insulating support, thereby leaving a positive charge that is distributed over the volume of the nanoparticle. The lifetime of the charge depends on the size of the metal particle and on the electronic and dielectric properties of the support, and may be too short for equilibrium to be established between the particle and its surroundings on the time scale of the chemical reaction. The electrostatic potential of the particle depends on its size, and therefore the position of the d-density of states of the particle, and thus the interaction with adsorbed molecules may be tuned by changing particle size. Thus, catalytic activity and selectivity may be affected for chemical reactions.

Complementarity between high-energy photoelectron and L-edge spectroscopy for probing the electronic structure of 5d transition metal catalysts

We demonstrate the successful use of hard X-ray photoelectron spectroscopy (HAXPES) for selectively probing the platinum partial d-density of states (DOS) in a Pt-Cu nanoparticle catalyst which shows activity superior to pure Pt towards the oxygen-reduction reaction (ORR). The information about occupied Pt d-band states was complemented by Pt L(2)-edge X-ray absorption near-edge spectroscopy (XANES), which probes unoccupied valence states. We found a significant electronic perturbation of the Pt projected d-DOS which was narrowed and shifted to higher binding energy compared to pure platinum. The effect of this electronic structure perturbation on the chemical properties of the nanoparticle surface is discussed in terms of the d-band model. We have thereby demonstrated that the combination of L-edge spectroscopy and HAXPES allows for an experimental derivation of the valence electronic structure in an element-specific way for 5d metal catalysts.

Increased Tunneling Magnetoresistance Using Normally bcc CoFe Alloy Electrodes Made Amorphous without Glass Forming Additives

Using cross-section transmission electron microscopy we show that films of CoFe alloys, sandwiched between two conventional amorphous materials, are amorphous when less than approximately 25-30 A thick. When these amorphous layers are integrated into magnetic tunnel junctions with amorphous alumina tunnel barriers, significantly higher tunneling magnetoresistance is found compared to when these layers are made crystalline (e.g., by heating or by thickening them). We postulate that this is likely due to changes in interfacial bonding at the alumina-CoFe interface. Indeed, x-ray emission spectroscopy shows a significant increase in the Fe, but not the Co, 3d density of states at the Fermi energy for thin amorphous CoFe layers.

Electronic structure studies of Ni–X (X: B, S, P) alloys using x-ray photoelectronspectroscopy, x-ray induced Auger electron spectroscopy and density functional theorycalculations

The electronic structure of Ni–X (X = B, S, P) alloys was studied using x-ray photoelectron spectroscopy, x-ray induced Augerelectron spectroscopy and density functional theory. The spectroscopic data in the form ofthe Ni 2p shake-up satellite and the Ni 2p LMM, P 2p KLL and S 2p KLL Augerparameters combined with density of states (DOS) and charge difference plots suggestan overall charge transfer from the Ni sites towards the alloying addition sites.However, this is masked, with intra-atomic charge redistribution leading to anincreased occupancy of the Ni 3d states in the alloys. The Ni 3d DOS showsstrong similarity to that of Pt which is the best catalyst for hydrogen evolution.

Electronic structure of transition metal fluorides and oxides determined by resonant X-ray absorption and X-ray emission spectroscopies

Experimental data for x-ray absorption at the transition metal L 2, 3 edge for FeF2 and CoF2 are presented. They are compared with the results of calculations that include the intra-atomic effects due to the ion atomic multiplet and also of the ligand field splitting of the 3d orbitals. Good overall agreement between experiment and theory is found. We also show resonant x-ray emission data for these compounds obtained at selected excitation energies and compare them with the results of the ligand field atomic multiplet calculation. Very good agreement between the experimental and the calculated energy splittings is found. We also found good agreement for the relative intensities of the emission peaks. This indicates that, as in the case of manganese compounds, the low energy loss peaks are due to decay into d-excited states of the ground configuration. A comparison of the emission spectra of LaMnO3, MnF2, FeF2, and CoF2 obtained at excitation energies at the top of the corresponding L 2 absorption peak is presented. This provides data for the projected d-density of states of d 4, d 5, d 6, and d 7 compounds, which are in good agreement with the results of ligand field atomic multiplet calculations.

Simulation of atomic and electronic structure of liquid caesium by method of the first-principal molecular dynamics

Atomic and electronic structures of liquid caesium were investigated by means of the first-principle molecular dynamics method for a range of temperatures. The calculated atomic pair correlation function agrees well with experimental diffraction data. An analysis of the atomic configurations is carried out using the Voronoi polyhedron method. We present results for the atomic properties (pair correlation function and self diffusion coefficient) and electronic properties (s-, p-, d-density of states, including DOS at the Fermi level). An estimation of degree of electron localization for the entire temperature range is carried out. The obtained results are compared with those of classical molecular dynamics and reverse Monte Carlo methods, and the TB-LMTO method.

Magnetic properties of liquid 3d transition metal–Si alloys

The magnetic susceptibility has been measured for liquid transition metal–Si alloys (TM1−cSic, TM=Ni, Co, Fe, Mn) as a function of temperature. The magnetic susceptibilities of liquid Ni1−cSic with c⩾0.3, liquid Co1−cSic with c⩾0.4 and liquid Fe1−cSic with c⩾0.6 were found to be almost independent of temperature, which suggests that the Ni, Co and Fe ions on the Si side are in the non-magnetic state. Liquid Ni1−cSic with c⩽0.2, liquid Co1−cSic with c⩽0.3 and liquid Fe1−cSic with c⩽0.5 show the Curie–Weiss behavior with a reasonable value of effective Bohr magnetic number. Liquid Mn1−cSic with c⩽0.2 and c⩾0.6 were found to be in their non-magnetic state. However, the Curie–Weiss behavior was observed for liquid Mn1−cSic with c=0.3, 0.4 and 0.5. The magnetic susceptibility of their liquid alloys in the non-magnetic state has been studied by using Anderson model. The density of 3d states at the Fermi level can be estimated from the data of magnetic susceptibility.

Ab InitioStudy ofL-edge X-ray Resonant Scattering from Chromium in Spin Density Wave State

We propose X-ray diffraction analyses exploiting the L -edge resonant scattering in the spin density wave (SDW) phase of Cr metal. We predict that strong signals could be detected on satellite spots corresponding to the charge density wave (CDW), by an ab initio band structure calculation. The sinusoidal modulation of spin moment makes the SDW exchange potential different from site to site, and accordingly the 3 d density of states (DOS) locally varies. The signal detects this modulation in the unoccupied part of the local 3 d DOS. This new mechanism predicts the signal detectable even in the absence of CDW ordering. Analyzing the energy profile, we demonstrate that the L -edge RXS is a sensitive probe for studying unoccupied 3 d states.

Investigation of the metamagnetic transition in Y2Co7H6

The effects of hydrogen absorption on the crystalline and magnetic properties of the γ-phase Y2Co7H6 were examined by bulk magnetic measurements, neutron diffraction and X-ray absorption spectroscopy. An orthorhombic distortion of the unit cell occurs upon hydrogen insertion, accompanied by a reduction of the magnetic moment on Co and a decrease of the ordering temperature. Magnetic measurements indicate a metamagnetic transition occurring in the γ-phase hydride. The transition shows a broad hysteresis and the magnitude of the critical field is found to decrease with increasing temperature. The electronic structure of Co in Y2Co7 and Y2Co7H6 has been investigated by X-ray absorption spectroscopy. The observed decrease of the pre-peak intensities in the Co-K edge XANES spectra are related to the changes of the 4p–3d density of states upon hydrogen insertion. A comparative X-ray magnetic circular dichroism investigation at the Co-K edge has been carried out for Y2Co7 and Y2Co7H6. The observed XMCD signal enables us to propose that ferromagnetic ordering occurs in the γ-hydride.

Effect of Cu deficiency on the optical properties and electronic structure of CuInSe2 and CuIn0.8Ga0.2Se2 determined by spectroscopic ellipsometry

Spectroscopic ellipsometry measurements of CuInSe2 (CIS) and CuIn0.8Ga0.2Se2 (CIGS) reveal that there are important differences in electronic properties between stoichiometric CIS (CIGS) and Cu-poor CIS (CIGS). We find a reduction in the absorption strength in the spectral region of 1–3eV. This reduction can be explained in terms of the Cu 3d density of states. Cu-poor CIS (CIGS) materials show an increase in band gap due to the reduction in repulsion between Cu 3d and Se 4p states. The experimental results have important implications for the function of polycrystalline optoelectronic devices.

Mechanism of enhanced ferromagnetism in delta-doped (Ga,Mn)As studied by ab initio electronic structure calculation

Abstract A mechanism of the enhanced ferromagnetic Curie temperature ( T C ) is proposed for (Ga,Mn)As digital ferromagnetic heterostructures (DFH) obtained by incorporating Mn delta-doping layer into GaAs based on the first-principles calculations. Our calculation shows a strong enhancement of T C for the DFH compared with that of random alloy with the same Mn concentration. In order to verify this enhancement mechanism of T C , we confirm that higher 3d density of states at the Fermi level upon delta-doping is responsible for the enhancement of T C due to the ferromagnetic double exchange interaction.