Bulk Density Of States Research Articles

Silicon carbide photodiodes: Schottky and PINIP structures

This work deals with the study of the role of intra-gap density of states and films composition on the colour selection of the collection spectrum of glass/ITO/a-Si x :C 1− x :H/Al Schottky photodiodes produced in a conventional plasma-enhanced chemical vapour deposition (PECVD) system using as gas sources silane and a controlled mixtures of silane and methane. To do so, properties of the films were investigated, especially the one concerning the determination of the valence controllability of the films produced and the density of bulk states. Besides that, a PINIP device was also produced, using the a-Si x :C 1− x :layer that lead to the best Schottky diode performances.

Method for the determination of bulk and interface density of states in thin-film transistors

In this article we present a method for the accurate determination of interface and bulk density of states (DOS) in thin-film transistors (TFTs), based on the combined analysis of transfer (ID–VGS) and capacitance–voltage characteristics. This analysis has achieved a number of results, eliminating sources of inaccuracies that are known to be present in other methods. A procedure for the determination of the electron and hole flatband conductances and bulk Fermi energy is demonstrated. A recursive procedure is employed to extract the bulk DOS directly from Poisson’s equation. The advantages of this method are the greater immunity to noise from the original data, the use of the complete Fermi function (no 0 K approximation), and the applicability to thin active layers. This method yields the interface state density spectrum as well as the bulk DOS. This information is very important for device design, process characterization, and modeling of TFTs.

Structural, electronic, and effective-mass properties of silicon and zinc-blende group-III nitride semiconductor compounds

The electronic band structures of silicon and the zinc-blende-type III-N semiconductor compounds BN, AlN, GaN, and InN are calculated by using the self-consistent full potential linear augmented plane wave method within the local-density functional approximation. Lattice constant, bulk modulus, and cohesive energy are obtained from full relativistic total-energy calculations for Si and for the nitrides. Band structures and total density of states (DOS) are presented. The role played by relativistic effects on the bulk band structures and DOS is discussed. In order to provide important band structure-derived properties, such as effective masses and Luttinger parameters, the ab initio band structure results are linked with effective-mass theory. Electron, heavy-, light-, and split-off-hole effective masses, as well as spin-orbit splitting energies are extracted from the band-structure calculations. By using the Luttinger-Kohn $6\ifmmode\times\else\texttimes\fi{}6$ effective-mass Hamiltonian we derive the corresponding Luttinger parameters for the materials. A comparison with other available theoretical results and experimental data is made.

Nanostructured silicon films produced by PECVD

ABSTRACTThis paper presents the process conditions that lead to the production of nanostructured silicon films grown by plasma enhanced chemical vapour deposition close to the so-called gamma regime (powder formation), highly dense and with low density of bulk states. Thus, the powder management is one important issue to be addressed in this paper. As a general rule we observed that high quality films (low density of states and high μτ products) are obtained when films are grown under low ion bombardment at high hydrogen dilution and deposition pressure conditions, to allow the proper surface passivation and surface activation.

Electronic structure of F, -center in MgO

The embedded-cluster numerical variational method has been developed to calculate the electronic structure of perfect MgO, F and F+-centers in MgO. The energy band, bulk density of states has been calculated by cluster Mg14O13, Mg14O12F+ and Mg14O12F. The calculated absorption energy for F+ and F centers is in good agreement with experimental data. In our calculated defect energy levels, that the first excited state of F+-center is at CB-3.46 eV, indicates the necessity of a large photoelectron yielding energy. We also calculate the radius of color center electron, and plot the map of charge-density distribution of valence electrons in which the structure of the color center is shown directly.

Analysis of bias stress on unpassivated hydrogenated amorphous silicon thin-film transistors

Both the subthreshold slope and the threshold voltage in inverted-staggered amorphous silicon thin-film transistors (a-Si:H TFTs) are vulnerable to metastable changes in the density of states (DOS) due to Fermi level displacement. In previous work, we have used passivated and unpassivated TFTs to distinguish between the effects of bulk states and interface states at the top passivating nitride interface. Here we report the results of experimental measurements and two-dimensional (2-D) simulations on unpassivated TFTs. Since there are no top interface states, all the observed changes are due solely to the bulk DOS. The subthreshold current activation energies in a-Si:H TFTs are compared for n-channel nonpassivated TFTs before and after bias stress. The experimental results agree well with the 2-D simulations, confirming that the dependence of subthreshold current activation energy on gate bias reveals the distribution of the DOS in energy but cannot resolve the magnitude of features in the DOS. This type of analysis is not accurate for TFTs with a top passivating nitride, since the activation energies in such devices are affected by the interfere states.

Tunneling spectroscopy of mesoscopic charge density wave systems

The quasi-particle transport of normal metal/insulator/charge density wave (N/I/C) junctions with density modulation normal to the interface is investigated theoretically. In contrast to superconductors or ferromagnets, the tunnel current depends strongly on the macroscopic phase of the charge density wave condensate and does not approach the bulk density of states for strong barrier potentials. Characteristic sample-to-sample conductance fluctuations due to random phases in mesoscopic systems and quantum size effects in the tunneling spectra of a thin charge density wave film are predicted.

Surface magnetism studied within the mean-field approximation of the Hubbard model.

Collective surface magnetism is studied within the mean-field approximation of the semi-infinite Hubbard model for the (100) surface of a simple cubic lattice. We investigate the changes of electronic and related magnetic properties at the surface with respect to the bulk that are introduced due to the reduction of the coordination number of atoms at the surface. Using a standard recursion method, the equation of motion for the one-electron Green function is solved numerically yielding the spin- and layer-dependent local density of states (LDOS). From this the layer-dependent charge density and magnetization are derived self-consistently. The LDOS of the first few layers below the surface clearly deviates from the bulk density of states which results in strong changes of the layer magnetizations while charge-transfer effects may be rather small. We systematically investigate the dependence of the layer magnetization on the Coulomb interaction, the band filling, and the temperature for the ferromagnetic as well as for different antiferromagnetic structures. While for the antiferromagnetic solutions the (sublattice) magnetization of the topmost layer is always enhanced, a higher magnetization of the topmost layer is only found for not too strong Coulomb interaction in the case of the ferromagnet. In the fully polarized ferromagnetic state the magnetization is dominated by the charge transfer between the bulk and the surface. First-order transitions from the ferromagnetic to a layerwise antiferromagnetic phase are studied in detail. Metastable ferromagnetic solutions are found beyond the critical point (Coulomb interaction, temperature). The actual transition of the metastable ferromagnetic into the stable antiferromagnetic phase starts from the very surface, which acts as an immanent perturbation of the infinitely periodic lattice. The antiferromagnetic order quickly extends into the bulk layer by layer.

Electronic structure of γ-FeSi 2: angle resolved valence band photoemission and Si 2p photoemission

We have investigated the electronic properties of γ-FeSi 2 thin films using angle resolved valence band photoemission and Si2p core level photoemission. We find two features in the valence band data which are not present in the calculated density of states (DOS). One of these has a constant final state energy and we identify it as being due to a final state effect. Another feature, found near —1.8 eV binding energy, is located in an energy gap of the bulk DOS and is identified as a surface state. We have measured the energy dispersion of this surface state and of two bulk-like states. The surface state has a total energy shift of 0.4 eV and exhibits band extrema near positions of the (2×2) reconstructed surface Brillouin zone boundaries. High resolution Si 2p photoemission spectra on samples of varying thickness show that the Si 2p binding energy is higher in the suicide than in bulk silicon by 0.49 eV. We find no evidence of silicon adatoms in our Si 2p photoemission results.

THE ELECTRONIC STRUCTURE OF MoSi2(001)SURFACE

The electronic stucture of three kinds of MoSi2(001)surface with C11b crystal structure were studied by using LMTO一ASA method, their surface total electronic densities of states and local densities of states of surface layer as well as partial densities of states, were provided and compared with the correlated atomic bulk densities of states. As to the stability of the surfaces, the densities of states on Fermi level's were discussed, the Si monolayer surface is the most stable one among them, which support the experimental results obtained by Kemoda et al.

Field-effect conductance in amorphous silicon thin-film transistors with a defect pool density of states

A new computer program to analyze field-effect conductance measurements has been developed. In this program a defect pool model, where the equilibrium density of state is determined by the Fermi level, has been incorporated. Transistors with finite band bending, due to fixed charge in the insulator, will therefore have a density of states that is spatially inhomogeneous. The inhomogeneous density of states means that the subthreshold slope of a device is not always controlled by the density of states near the interface, but can become dominated by the bulk density of states, contrary to simpler models. Both electron and hole branches are modeled simultaneously and self-consistently with no assumptions made about the flatband voltage. Indeed, it is demonstrated that there is no flatband voltage in a transistor with an inhomogeneous density of state; however, a true flatband voltage can be achieved by a process of thermal bias annealing. Finite thickness effects and defect correlation energies are taken into account. The program is used to model the characteristics of thin-film transistors both before and after thermal bias annealing. The model fits the experimental results well, even though there are few free parameters. A key result is that the defect pool parameter Δ is determined, which is the energy separation of the D0/− transition observed in n-type material and the D+/0 transition in p-type material. It is found that Δ=0.44 eV for all devices fitted, which suggests this a fundamental parameter of a-Si:H. This value of Δ implies that there are about four times as many charged as neutral devices in bulk intrinsic amorphous silicon.

Feasibility study on the application of the method of the resolvent matrix to complex systems.

A local approach to the study of complex systems with low symmetry is examined. The approach leads to the direct calculation of the local density of states (DOS) via the local Green's function. The feasibility and reliability of the method are established by subjecting the method to a severe test, using it to calculate the bulk DOS of the crystalline GaP in a layer-by-layer approach. In this case, the calculation of the Green's function only involves a series of inversions of matrices whose dimensions are given by the number of orbitals per layer unit cell.

Level-spacing distribution in the tight-binding model of fcc clusters.

A lattice-gas Monte Carlo method is used to simulate metallic fcc clusters at finite temperatures. A tight-binding model including s and p electrons has been derived for reproducing the free-electron-like energy band for the bulk metal and this model is used for calculating the electronic structures of the fcc cluster. The resulting level-spacing distribution at the Fermi energy is a Wigner distribution. The width of the distribution in small clusters is smaller than that calculated from the bulk density of states. In the lattice gas clusters the energy gaps related to the electronic magic numbers do not show up at the Fermi level. The energy between the last occupied and the first unoccupied levels is, on average, larger than other level spacings near the Fermi level.

Systematic study of light-induced effects in hydrogenated amorphous silicon.

A systematic investigation of the origin of the photoinduced changes in the defect density of states in the pseudogap of undoped hydrogenated amorphous silicon (a-Si:H) has been carried out on films deposited by glow-discharge decomposition of silane under different conditions. We used several complementary techniques, such as junction capacitance versus temperature, electrical conductivity, electron-spin resonance, and photothermal deflection spectroscopy, to determine the shifts in the Fermi-level (${\mathit{E}}_{\mathit{F}}$) position as well as the variation of the defect density of states, within the forbidden gap, after annealing (state A) and light-soaking (state B) treatments. The total hydrogen concentration and the hydrogen bonding configurations of each sample were analyzed by elastic-recoil-detection analysis (ERDA) and infrared (ir) transmission measurements, respectively, both for state A and state B. The experimental results show in all cases an increase in the bulk density of states accompanied by a shift of ${\mathit{E}}_{\mathit{F}}$ towards the valence band in the light-soaked state B with respect to the annealed state A, but these changes depend considerably on the deposition conditions of the films. On the other hand, no change is detected either in the ERDA or in the ir spectra. Moreover, it is found that when the saturated light-soaked samples are partially annealed at temperatures ranging from 110 \ifmmode^\circ\else\textdegree\fi{}C to 130 \ifmmode^\circ\else\textdegree\fi{}C and for times less than 15 min, and then left in the dark at room temperature for 24 h and more, a long-time process, characterized by a variation of the density of states as well as by a motion of ${\mathit{E}}_{\mathit{F}}$ within the gap, occurs in these films. The observed behavior is completely different from one sample to another. From the analysis of the results, it is suggested that in some samples the photodegradation is mainly due to a trapping of the excess free carriers at charged spinless defects, which must be present in the sample at equilibrium in addition to the neutral dangling bonds.

Near-Surface Density of States in a-Si:H By Photoelectron Yield Spectroscopies.

ABSTRACTThe density of occupied states was investigated in a number of intrinsic a-Si:H samples by means of photoelectron yield spectroscopy operated in the constant final state mode. Valence band edge, exponential tail and deep state distribution were analyzed and compared with the bulk density of states as determined by spectral dependence of photoconductivity. By evaporating Cs atoms on the clean surface the Fermi level position was progressively shifted toward the conduction band and the evolution of localized states was investigated. On the assumption that these near-surface states are paramagnetic centers with positive correlation energy, the results agree with the prediction of the “defect pool” model.

Surface and Bulk GAP States Distributions in Amorphous Silicon Films as Obtained by Optical Methods

ABSTRACTThe aim of this work is to achieve detailed information about surface and bulk density of states in device quality a-Si:H films by means of deconvolution of PDS spectra. Unambiguous deconvolution of the spectra has been performed by calculating the first derivative vs. energy of the absorption coefficient of the samples. The results are compared with those obtained by means of the integration rule. Calculations of the density of states carried out on samples having different thickness suggest that the defect distribution differs from surface to bulk. The surface defect peaks are found to lie closer to the corresponding mobility edges, with respect to the bulk defect peaks: these results agree well with those obtained by means of other techniques, like total yield spectroscopy and can be explained in terms of a defect pool model.The present deconvolution procedure when applied to samples having different thickness provides new results about the film's homogeneity in terms of deep defect and valence tail states.

On the Correspondence of the Experimental EEL Spectra with the Electron Structure of Tungsten

In the previous paper [7] we have directly compared the experimental Electron Energy Loss (EEL) spectra with both the theoretical bulk density-of-states (BDOS) and surface density-of-states (SDOS) functions for tungsten. On the base of the ILEED intensity calculations we have estimated to what extend an EEL spectrum measured at some value of the primary electron energy EP , should correspond either to the SDOS or to the BDOS. Also, the angular acceptance of a CMA has been used to separate the EEL spectra measured as a function of Ep which corresponded either to the [001] direction or to the [011] in parallel to the studied (001) surface. In the present paper we propose a procedure which combines the Electron Energy Loss Spectra (EELS) simulation method based on Bauer's theory [1, 2] and the inelastic low energy diffraction (ILEED). If the standard simulation procedure is applied for tungsten an ambiguity would arise because the DOS function is very anisotropic and varies as a function of both the direction along the surface and the depth considered within several atomic layers. The application of ILEED has supplemented the simulation procedure with the method which identifies both the correspondence of a reciprocal-lattice vector to a measured EEL spectrum and the localization of a studied electron structure with regard to the distance from the surface. PACS numbers: 79.20.Fv, 61.80.Mk, 82.65.-i

An ILEED study of the density-of-states function of tungsten

Electron energy loss spectroscopy (EELS) and inelastic low-energy electron diffraction (ILEED) have been combined to study the density-of-states (DOS) function in tungsten. The EELS spectra measured as a function of the primary electron energy E p ( E p < 100 eV) by means of a CMA spectrometer are interpreted in terms of the angular-resolved EELS taking into account the electron diffraction phenomena. As a result, we could correlate the EELS spectrum measured at a particular value of E p with the one-dimensional DOS function. In addition, the LEED intensity theory, which we used in this work, has permitted us to estimate whether the correlation should be made with the surface-localized DOS (i.e. SDOS) or the bulk DOS (i.e. BDOS). Our experimental ILEED results are in good agreement with the theoretical DOS function published by Christensen and Feuerbacher [Phys. Rev. B 10 (1974) 2349]. In particular the energy gap in the theoretical electron structure along the [110] direction has been confirmed experimentally.

Surface and bulk density of states of a-Si:H determined by CPM and total-yield photoelectron spectroscopy

The density of states (DOS) of a-Si:H and the influence of light soaking were studied with the constant photocurrent method (CPM) and total-yield photoelectron spectroscopy for films prepared at substrate temperatures between 100°C and 400°C. While by the first method information about the bulk is obtained the other yields the DOS in the surface-near region. At the surface we observed a relative to the bulk enhanced density of states as well as an enhanced influence of light soaking. In addition changes of the DOS of fresh films occur upon annealing at 190°C which can be significantly larger than the light induced effects.

The influence of spatially non-uniform density-of-state distributions on the characteristics of a-Si: H thin-film transistors

Abstract The potential profile in the semiconductor of a thin-film transistor is calculated on the basis of model distributions for the bulk density of states in a-Si: H. In addition a spatially non-uniform defect density distribution close to the semiconductor-insulator interface and a model for the density of interface states is considered. From this potential the source rain current of the thin-film transistor is obtained. These data are then analysed using the conventional scheme in which a homogeneous density-of-states distribution in the bulk and no interface states are assumed. The resulting effective density-of-states function reflects the non-uniformities in a characteristic way. A simple method to determine interface state distributions from experimental field-effect data is also presented and experimental data are analysed in terms of interface states and non-uniform defect distributions.