Density Of States Profile Research Articles

Capacitance–Voltage Measurement With Photon Probe to Quantify the Trap Density of States in Amorphous Thin-Film Transistors

We report on a characterization method to quantitatively estimate the interfacial trap density of states (DOS) in thin-film transistors (TFTs): photon-probe capacitance-voltage ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CV</i> ) measurement. The photo- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CV</i> method was compared to photoexcited charge-collection spectroscopy, which is another photon-probe method using current-voltage ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> ) measurement to meet the same purpose. Here, we directly characterized the DOS of amorphous-Si- and amorphous-InGaZnO-based TFTs using the photon-probe <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CV</i> measurement, and the results turned out to mainly focus the trap DOS at channel/dielectric interface. On the one hand, the DOS profile by photon-probe <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> method appeared to give additional trap information from the transistor back channel.

Classification of Oxide Compounds through Data‐Mining Density of States Spectra

Pattern recognition techniques were used to extract features from the density of states (DOS) curves derived from density functional theory calculations of over a dozen related oxide systems. Features in the DOS profiles that were associated with crystal structure, chemistry, and stoichiometry were identified. Classification maps identifying trends in the electronic structure with respect to crystal chemistry were created using multivariate analysis methods. It was found that crystal structure appeared to have a more significant impact on the DOS than chemistry for the systems studied. The differences in the electronic structure of HfO2 and ZrO2 were captured as a function of crystal structure, and the features of the DOS curve, which represent these differences, were identified. Correlations between crystal symmetry and metal/oxygen ratio were also uncovered from this type of analysis.

Temperature dependence of the charge carrier mobility in disordered organic semiconductors at large carrier concentrations

Temperature-activated charge transport in disordered organic semiconductors at large carrier concentrations, especially relevant in organic field-effect transistors (OFETs), has been thoroughly considered using a recently developed analytical formalism assuming a Gaussian density-of-states (DOS) distribution and Miller-Abrahams jump rates. We demonstrate that the apparent Meyer-Neldel compensation rule (MNR) is recovered regarding the temperature dependences of the charge carrier mobility upon varying the carrier concentration but not regarding varying the width of the DOS. We show that establishment of the MNR is a characteristic signature of hopping transport in a random system with variable carrier concentration. The polaron formation was not involved to rationalize this phenomenon. The MNR effect has been studied in a OFET based on ${\text{C}}_{60}$ films, a material with negligible electron-phonon coupling, and successfully described by the present model. We show that this phenomenon is entirely due to the evolution of the occupational DOS profile upon increasing carrier concentration and this mechanism is specific to materials with Gaussian-shaped DOS. The suggested model provides compact analytical relations which can be readily used for the evaluation of important material parameters from experimentally accessible data on temperature dependence of the mobility in organic electronic devices. Experimental results on temperature-dependent charge mobility reported before for organic semiconductors by other authors can be well interpreted by using the model presented in this paper. In addition, the presented analytical formalism predicts a transition to a Mott-type charge carrier hopping regime at very low temperatures, which also manifests a MNR effect.

Terahertz laser-induced 1D–0D crossover in the density of states for electrons in a cylindrical semiconductor quantum wire

The effects of a linearly polarized, terahertz laser field on the density of states (DOS) for carriers confined in a cylindrical semiconductor quantum wire are investigated here within a nonperturbative scheme, based upon a Green’s function approach. In our model, the functional dependence of the DOS on energy is significantly modified in two ways: (i) an uniform blueshift with respect to the laser-free DOS; and (ii) a strong suppression, with the appearance of Franz–Keldysh-like oscillations. Interestingly, with the increase (decrease) of the laser intensity (frequency) the DOS profile evolves from the usual shape for one-dimensional (1D) systems to a discrete set of sharp peaks that resembles the DOS for quasi-zero-dimensional (quasi-0D) electrons confined in quantum dots.

Effect of a terahertz laser field on the electron-DOS in a GaAs/AlGaAs cylindrical quantum wire: finite well model

The influence of a linearly polarized terahertz laser field on the density of states (DOS) for carriers confined in a cylindrical semiconductor quantum wire is investigated here within a nonperturbative scheme, based on the Green's function approach introduced by Xu (1997 Semicond. Sci. Technol. 12 1559). In our finite potential well model, the functional dependence of the DOS on energy is significantly modified by a monochromatic, nonresonant, intense laser radiation polarized along the wire axis. Apart from a uniform blueshift with respect to the laser-free DOS, which is proportional to the laser intensity and to the inverse of the square of the frequency, a remarkable reduction of the DOS, followed by pronounced Franz–Keldysh-like oscillations above the bottom of each 1D sub-band, is obtained. The period of these oscillations is seen to depend more on the laser frequency than on the intensity. Interestingly, with an increase of the laser intensity and/or a reduction of the laser frequency, the DOS profile changes from the usual shape found in 1D electronic systems to a series of discrete peaks similar to that found in quasi-0D systems, such as quantum dots.

Direct observation of the non-supported metal nanoparticle electron density of states by X-ray photoelectron spectroscopy

Synchrotron-based X-ray photoelectron spectroscopy on copper and silver cluster beams created by a magnetron-based gas-aggregation source has allowed mapping the electron density of states (DOS) of free metallic nanoparticles. The cluster DOS profiles obtained in the experiments strongly resemble the infinite solid DOS shapes, but the extracted cluster work-functions are lower than those for the bulk metal. The latter observation is explained by the initial negative charge on most of the clusters, created by the source.

First-principle study of ferroelectricity in PbTiO3/SrTiO3 superlattices

We performed the first-principles calculation to investigate the electronic structure and polarization behaviors in PbTiO3/SrTiO3 (PST) superlattices. The DOS (density of state) profiles show that there are strong hybridizations of atom Ti–O and Pb–O which play very important roles on ferroelectricity of the PbTiO3/SrTiO3 superlattices. Comparing to the corresponding paraelectric phase, we find the electrons of the PT (PbTiO3) layers occupy lower energy states and electrons of the ST (SrTiO3) layer occupy higher energy states. It is shown that the polarizations of the superlattices decrease with proportion of SrTiO3 increasing. The constant polarization of local layer indicates that PST superlattices with small modulation lengthen can be approximately considered as a single ferroelectric material. Furthermore, according to electrostatic model, we find that directions of internal electric fields in PT and ST layers are opposite. In PST superlattices, internal electric field in PT layer leads to the loss of polarization of this layer, but the polarization of ST layer is induced by internal electric field of this layer. Compared to the value of the polarization in bulk PbTiO3, polarization of PST is smaller.

Numerical modeling of thermally-stimulated currents for the density-of-states determination in thin-film semiconductors

The paper reports on thermally stimulated conductivity studies used for characterization of the density of states profile in thin film semiconductors, by numerically solving the non-linear time-dependent rate equations for free and trapped charge. We explore the derivation of energy and density scales from temperature and conductivity data. We examine the distinction between ‘strong’ and ‘weak’ re-trapping and the use of low ‘effective’ values of attempt-to-escape frequencies in establishing an energy scale, and the ad hoc inclusion of a temperature-dependent lifetime. It is confirmed for several illustrative model systems that the technique can afford surprisingly good fidelity in recovery of the density of states under a range of conditions.

Electronic structure calculations of an oxygen vacancy inKH2PO4

We present first-principles total-energy density-functional theory electronic structure calculations for the neutral and charge states of an oxygen vacancy in KH{sub 2}PO{sub 4} (KDP). Even though the overall DOS profiles for the defective KDP are quite similar to those of the perfect KDP, the oxygen vacancy in the neutral and +1 charge states induces defect states in the band gap. For the neutral oxygen vacancy, the gap states are occupied by two electrons. The difference between the integral of the total density of states (DOS) and the sum of the DOS projected on the atoms of 0.98 |e|, indicates that one of the two electrons resulting from the removal of the oxygen atom is trapped in the vacancy, while the other tends to delocalize in the neighboring atoms. For the +1 charge oxygen vacancy, the addition of the hole reduces the occupation of the filled gap-states in the neutral case from two to one electron and produces new empty states in the gap. The new empty gap states are very close to the highest occupied states, leading to a dramatic decrease of the band gap. The difference between the integral of the total DOS and the sum of the DOS projected on the atoms is 0.56 |e|, which implies that more than 56% of the redundant electron is trapped in the oxygen vacancy, and 44% spreads over the neighboring atoms. In sharp contrast, no defect states appear in the energy gap for the +2 charge O vacancy. Thus, the addition of the two holes completely compensates the two redundant electrons, and removes in turn the occupied gap states in the neutral case.

Properties of conduction-band dilute-magnetic-semiconductor quantum wells in an in-plane magnetic field: A density of states profile that is not steplike

Leibniz Institute for Neurobiology, Special Lab for Non-Invasive Brain Imaging, Brenneckestrasse 6, D-39118 Magdeburg, Germanyand Physics Department, Solid State Section, University of Athens, Panepistimiopolis, Zografos, GR-15784 Athens, Greece~Received 6 September 2003; published 12 March 2004!We examine how an in-plane magnetic fieldB modifies the density of states ~DOS! in narrow-to-wide,conduction-band dilute-magnetic semiconductor quantum wells. We demonstrate that the DOS diverges sig-nificantly from the ideal steplike two-dimensional electron gas form and this causes severe changes to thephysical properties, e.g., to the spin-subband populations, the internal and free energy, the Shannon entropy,and the in-plane magnetizationM. We predict a considerable fluctuation of M in cases of vigorous competitionbetween spatial and magnetic confinement.DOI: 10.1103/PhysRevB.69.113302 PACS number~s!: 73.20.2r, 85.75.2d, 75.75.1a, 65.40.Gr

Protein Dynamics in a Family of Laboratory Evolved Thermophilic Enzymes

Molecular dynamics simulations were employed to study how protein solution structure and dynamics are affected by adaptation to high temperature. Simulations were carried out on a para-nitrobenzyl esterase (484 residues) and two thermostable variants that were generated by laboratory evolution. Although these variants display much higher melting temperatures than wild-type (up to 18°C higher) they are both >97% identical in sequence to the wild-type. In simulations at 300K the thermostable variants remain closer to their crystal structures than wild-type. However, they also display increased fluctuations about their time-averaged structures. Additionally, both variants show a small but significant increase in radius of gyration relative to wild-type. The vibrational density of states was calculated for each of the esterases. While the density of states profiles are similar overall, both thermostable mutants show increased populations of the very lowest frequency modes (<10cm−1), with the more stable mutant showing the larger increase. This indicates that the thermally stable variants experience increased concerted motions relative to wild-type. Taken together, these data suggest that adaptation for high temperature stability has resulted in a restriction of large deviations from the native state and a corresponding increase in smaller scale fluctuations about the native state. These fluctuations contribute to entropy and hence to the stability of the native state. The largest changes in localized dynamics occur in surface loops, while other regions, particularly the active site residues, remain essentially unchanged. Several mutations, most notably L313F and H322Y in variant 8G8, are in the region showing the largest increase in fluctuations, suggesting that these mutations confer more flexibility to the loops. As a validation of our simulations, the fluctuations of Trp102 were examined in detail, and compared with Trp102 phosphorescence lifetimes that were previously measured. Consistent with expectations from the theory of phosphorescence, an inverse correlation between out-of-plane fluctuations on the picosecond time scale and phosphorescence lifetime was observed.

Photoconductive properties of hydrogenated amorphous silicon in the light of the positive dangling bond as the main recombination centre – consequences from the defect pool model

We discuss the photoconductive properties of hydrogenated amorphous silicon in the presence of comparable numbers of neutral and charged dangling bond defects and in particular for the defect distribution according to the defect pool model. We numerically model transient and modulated photoconductivity experiments. Analysis of time-of-flight data identifies the deep-trapping time with the capture time into the positive dangling bond. Testing numerically the equivalence of secondary transient and modulated photoconductivity employed for spectroscopic purposes, we reproduce the experimental optical bias effect in the modulated photocurrent spectra and the related change in the reconstructed density of states. We suggest performing these spectroscopic experiments with a series of slightly p-type doped samples for which the numerical modelling reveals two differently shaped reconstructed density of state profiles for either the defect pool or a single defect band distribution.

Electronic structure of polymeric KC 60 - a crystal orbital analysis

The band structure of orthorhombic KC 60 is investigated by a crystal approach based on an intermediate neglect of differential orbital Hamiltonian. The title compound crystallizes in the space group Pnnm with covalent intermolecular carboncarbon bonds. Polymeric KC 60 is a metal with a low electronic density of states (DOS) at the Fermi energy ϵ F. This metallic behaviour differs from the electronic ground state calculated for isotropic fcc KC 60 with potassium occupying the octahedral intersitial site. The reduced width of the conduction band in the fcc structure favours an insulating Mott state relative to the metallic configuration. The dimensionality of the title compound is discussed on the basis of intercell energies, the DOS profile and dispersion curves. The theoretical results are compared with experimental observations. Wiberg bond-indices are employed to describe the chemical bonding within the distorted C 60 soccerball.

A model for the electrical and magnetic properties of amorphous carbons based on fullerene-type structures†

The discovery of the closed-cage fullerene and related structures raises numerous questions in relation to the properties, both electrical and magnetic, of amorphous carbons prepared by the thermal decomposition of organic materials in an inert atmosphere. It is suggested that non-planar structures containing pentagonal rings, which are closely related to fullerenes, can arise in amorphous carbons, and free radical material, in particular, probably odd-alternate n-radicals, may be trapped by the cage environment provided by non-planar carbon networks. Such an occurrence leads to a density of states profile by which the electrical behaviour of carbons can be rationalized. A half-filled band of localized mid-gap states is predicted to give rise to a metal-insulator transition at a spin concentration and minimum metallic conductivity of the same magnitude as is measured experimentally.

Electronic density of states in highly tetrahedral amorphous carbon

Highly tetrahedral amorphous carbon (ta-C) films and its hydrogenated form (ta-C:H) have been deposited using a filtered cathodic arc. In this paper, the optical and electronic band-gaps of amorphous diamond and its hydrogenated form ta-C:H, are shown to be in the range 2 eV. The H content of ta-C:H is found to be about 3 at.% using elastic recoil detection analysis (ERDA) and electron energy loss spectroscopy (EELS) is used to show that ta-C:H does keep its tetrahedral structure. Both materials are found to be stable at temperatures up to 1000 K. The room temperature conductivity of 10 −7−10 −8 Ω −1 cm −1 is shown to be thermally activated, with activation energies in the range 0.2–0.3 eV. A detailed study made from the current-voltage characteristics of ta-C/ p-Si heterojunctions shows that current flow is space-charge limited (SCLC) and influenced by bulk traps. The density of states profile, N( fE), in the region of the quasi-Fermi level is calculated using the differential method and found to be of the order 10 18 cm −13 eV −1. Optical absorption data is coupled with the electrical results to confirm that the valence band density of states is of the order 10 21 cm −13 eV −1. It is also shown that there is a reduction of nearly one order of magnitude in the N( E) at the quasi-Fermi level upon hydrogenation. SCLC measurements are complemented with surface admittance measurements (SAM) which yield a value of 10 11 cm −2 eV −1 for the interfacial density of states at the ta-C/ p-Si junction. A decrease in N ss is also observed at the ta-C:H/ p-Si interface in agreement with the SCLC results.

Molecular orbital study of germanium, germanium-gallium and germanium-arsenic clusters

The quasirelativistic INDO/1 method has been used to generate molecular orbitals for some {Gex}, doped {GexGay} and {GexAsy} clusters. These one-electron energy levels predefine the density of states (DOS) and/or hole functions. The effect of the cluster size (x=24, 56, 92) and that of dopants on the DOS profiles are discussed. The calculations are compared with those generated by periodic crystal orbitals of the EHT quality.

Simulation of modulated photoconductivity in sandwich configuration

We report on the simulation of modulated photoconductivity (MPC) in sandwich configuration, employing a numerical method that has been used for the simulation of time-of-flight (TOF) transport measurements in amorphous semiconductors with arbitrary density of states (DOS). This particular type of the experimental MPC-method allows the determination of spatial in addition to energetic DOS profiles. We demonstrate by computer modelling that the sensitivity function S ( x ) introduced for the purpose above accurately describes the spatial contribution of an inhomogenous distribution of localized states to the phase shift of the modulated photocurrent. We discuss the shape of S ( x ) for light with different absorption depths and for inhomogenous field profiles within the sample, including also the effect of diffusion of free carriers.

Sensitivity analysis of the modulated photocurrent method

This study presents a simulation that is used to determine the sensitivity of the phase shift analysis of the modulated photocurrent method to the differences in the fine scale structures in the density of states (DOS) distributions. Four DOS distributions are considered and the expected data are obtained. The results show that the modulated photocurrent method is very sensitive to such fine features in the DOS distributions. A comparison is also made with the sensitivity of other techniques commonly used in the determination of the DOS profiles.

The transport and isolation properties of polycrystalline GaAs selectively grown by molecular beam epitaxy

Selective-area polycrystalline GaAs using SiO 2 masking is planarly grown by molecular beam epitaxy (MBE). The electric properties of the polycrystalline GaAs are investigated because this technology is very promising for device isolation in GaAs integrated circuit and electro-optic integration. Compared with the isolation characteristics of semi-insulating GaAs, polycrystalline GaAs has similar low-field resistivity, higher high-field leakage current, and no well-defined trap-fill-limited voltage. The grain boundary (GB) states of polycrystalline GaAs trap negative charge that builds up a potential barrier to hinder electron current. The GB density of states profile estimated from the IV characteristics shows a peak value 5 × 1012cm-2.eV1and a wide energy distribution, 0.33 eV above the equilibrium Fermi energy.

Structural phase transitions and superconductivity in hexagonal tungsten bronzes

The anomalous behaviour of the superconducting transition temperatureTc as a function ofx in MxWO3 (hexagonal tungsten bronzes), where M = K, Rb or Cs and 016 <x < 0.33, is explained in terms of structural transitions involving WO6 octahedral chains which are subject to Jahn-Teller (J-T) effect. Above a critical concentration (xc ∼ 0.24) there is cooperative J-T effect resulting in a structural transition. Below this, there will exist only local distortions. The effect of these is to modify the density of states and lead to shifting and splitting of the relevant peaks. It is shown that the density of states profile at the Fermi level controls the superconducting transition temperatures as a function ofx.