Minimum State Density Research Articles

Al2O3/InGaAs interface passivation by fluorine-containing anodic layers

The morphology, chemical composition, and electronic properties of Al2O3/InGaAs interfaces with and without anodic oxide layers, formed in DC plasma (O2, Ar) with different contents of the fluorinating component (CF4), were studied. It is shown that thin fluorinated anodic oxide layers, in combination with annealing at 300 °C, reduce the density of interface states by a factor of 3–4 over the entire bandgap. The minimum state density values near the midgap determined by the Terman method are about 2 × 1012 eV−1 cm−2. However, it is demonstrated that, in contrast to the Al2O3/InGaAs interface, the interface with a fluorinated oxide is not stable and degrades when heated above 300 °C.

The effects of a Stone–Wales defect on the performance of a graphene-nanoribbon-based Schottky diode

The effects of a Stone–Wales defect on the performance of a graphene-nanoribbon-based Schottky diode are studied herein. To this end, the transmission, energy band structure, density of states, carrier concentration, and current density of the proposed device are modeled analytically in two cases, viz. a pristine and defective graphene nanoribbon, and the results are compared. The results reveal that the introduction of a Stone–Wales defect into the symmetric graphene nanoribbon system obviously changes some of the distinctive properties. After the introduction of a Stone–Wales defect, the slope of the energy levels in the graphene nanoribbon is reduced, leading to a decrease in the Fermi velocity. In this case, the band gap near the Dirac points in the energy band structure is increased. The minimum density of states of the defect-free graphene is almost zero, which can be explained by the shape of the energy band diagram at the Dirac point. Moreover, the minimum density of states in the presence of a Stone–Wales defect is higher than in the defect-free condition, owing to the presence of bands throughout the energy diagram. Finally, the effects of the temperature and channel width on the I–V characteristic of the proposed Schottky diode based on a defect-free or defective graphene nanoribbon are studied analytically, and the efficiency of the device is investigated.

Electronic-structure origin of the glass-forming ability and magnetic properties in Fe–RE–B–Nb bulk metallic glasses

(Fe0.71RE0.05B0.24)96Nb4 (RE=Gd, Tb, Ho, Er, Tm) bulk metallic glasses (BMGs) were found exhibiting excellent glass-forming ability (GFA) with critical diameters ranging from 3.5 to 6.5mm, and high compressive fracture strength larger than 4300MPa. Moreover, they displayed good soft-magnetic properties with saturation magnetic flux density of 0.71–0.87T, coercive force of 1.23–39.76A/m and effective permeability of 1500–12,740 at 1kHz. X-ray photoelectron spectroscopy was performed to clarify the origin of the excellent GFA from the viewpoint of electronic structure. It was found that the Tm doped alloy displayed unique electronic structure including the deepest core-level binding energy, the most numerous RE–B bonds and the minimum density of states near the Fermi level, making this alloy the best glass former. The various trends noticed in the magnetic properties were ascribed mainly to the differences in the magnetic anisotropy and magnetic moment of RE elements.

Effect of back-gate on contact resistance and on channel conductance in graphene-based field-effect transistors

We study the contact resistance and the transfer characteristics of back-gated field effect transistors of mono- and bi-layer graphene. We measure specific contact resistivity of ~7 k Ω μm2 and ~30k Ω μm2 for Ni and Ti, respectively. We show that the contact resistance is a significant contributor to the total source-to-drain resistance and it is modulated by the back-gate voltage. We measure transfer characteristics showing a double dip feature that we explain as the effect of doping due to charge transfer from the contacts causing minimum density of states for graphene under the contacts and in the channel at different gate voltage.

Energy of bonding an adsorbed atom with a single-layer graphene

The change in the local density of states δρg of a single-sheet graphene due to adsorption of a single atom has been calculated in the framework of the M model proposed earlier. The dependence of the local density of states δρg on the position of the adatom energy level ɛa with respect to the Dirac point and other parameters of the problem has been analyzed. It has been shown that the largest changes in the local density of states δρg are caused by adatoms with the ɛa levels lying in the vicinity of the Dirac point, so that the minimum density of states of graphene remains equal to zero. An analytical expression has been derived for the energy of bonding Wads of the adatom with graphene. The obtained estimates of the bonding energy Wads in the weak and strong adatom-substrate bonding regimes are presented. Atoms of alkali metals and halogens have been considered as specific adsorbates.

Formation and control of MBE-ZnSe/GaAs heterointerface and regrown homointerface

A combination of ex-situ HF-pretreatment and in-situ Se-pretreatment, ( HF + Se)-pretreatment, is successfully applied to GaAs surface to control MBE-ZnSe/GaAs (100) heterointerface. c–v analysis of the interface indicates that the minimum state density, N ss,min( E), is considerably reduced to 1 × 10 11 cm −2 eV −1, being one order of magnitude smaller than that for the standard chemically etched sample. The excess voltage drop across the n-ZnSe/n +-GaAs interface for current conduction is consistently reduced by the pretreatment, and the behavior of the i–v characteristics can be theoretically fitted by using a previously proposed model considering interface charge and obtained N ss( E) shapes. The quality of the ZnSe epilayer, which is assessed from X-ray diffraction and Raman scattering measurements, is also improved by the ( HF + Se)-pretreatment. Air-exposing and regrowth process is found to introduce a relatively high potential barrier at the ZnSe/ZnSe homointerface even when the regrowth is undertaken after a slight etching of the first grown epilayer.

The metal adatom on GaAs(110): A surface negative U center

Metallic atom adsorption on the GaAs(110) surface is studied in the tight‐binding approximation. By comparison with the experimental results and the lack of As localized states in the band gap, we first show that adsorption cannot occur on a completely unrelaxed surface. The other point concerns the Fermi level position which is located in a minimum density of states. We show that adatom relaxation is quite important and must be considered as it strongly modifies the energy to add or to remove an electron from the crystal. We show that a possible explanation of the observed adatom level energies is the existence of a negative U center.

Analysis of the kinetics of the thermal decomposition of pentafluoroethane

The thermal decomposition of pentafluoroethane has been studied experimentally over the temperature range 1180–1470 K under 3500 torr pressure by Tschuikow-Roux and co-workers (1). Although the principal process which occurred at around 1200 K was the elimination of hydrogen fluoride, the formation of radicals by carbon–carbon bond scission is suggested at around 1400 K. In this article the C2HF5 molecule has been submitted to a treatment in terms of the RRKM theory to test plausible activated complex models and to explain the formation of different products at different temperatures. The hydrogen fluoride elimination reaction is modeled after a four-membered ring for the activated complex whereas two different approaches (Gorin model and the criterion of minimum state density) have been used to explain the C—C bond rupture. In this work it is confirmed satisfactorily by calculation of the rate energy dependences for the different mechanisms that, at around 1200 K, the alkene (C2F4) is the dominant product; as the temperature approaches 1400 K carbon–carbon bond rupture becomes feasible. Keywords: RRKM, bond rupture, HF elimination, pyrolysis, rate coefficient.

The density of states of sputtered a-Si: H studied by the space-charge-limited current technique The influence of deposition parameters, light and keV-electron irradiation

Abstract The space-charge-limited current (SCLC) technique has been used to determine the density of states (DOS) near the Fermi level for a-Si: H films prepared by magnetron sputtering under various experimental conditions. From a study of the thickness dependence it can be concluded that surface or interface states might significantly contribute to the DOS. A minimum DOS of 5[sdot]5 × 1015 cm−3 eV−1 at E c — E = 0[sdot]75 eV was determined for well passivated films. The generation of metastable defects by light soaking and keV electron irradiation has been directly observed by the change in the DOS. For the same energy deposited, defect creation by electrons is a factor of 3300 higher than that by light. In addition, the generation process is found to be different. A model is suggested that explains the creation of defects by electrons via a direct electronic excitation process.

Plasma enhanced metal-organic chemical vapor deposition of aluminum oxide dielectric film for device applications

Low temperature (≤250 °C) deposition of aluminum oxide gate dielectric film using a new plasma enhanced metal-organic chemical vapor deposition technique is described. With this technique, the substrate was not directly exposed to the plasma which minimizes radiation damage to both the substrate and the film. A dc potential was used to generate the plasma and the deposition of the film was achieved at extremely low plasma power (≤2 W) using trimethylaluminum and nitrous oxide reactant sources. The dielectric films were found to have a resistivity of ∼1014 Ω cm, dielectric constant of 7.9±0.2, and breakdown field ≥106 V/cm. A linear dependence of the film growth rate upon TMAl concentration was observed, indicating that the growth process is mass transport limited. Metal–oxide–semiconductor device structures were fabricated both on n-Si and p-InP substrates and the interface properties were evaluated by capacitance–voltage and Auger measurements. Both the devices show sharp interface with a minimum density of states in the range of 8×1010–1011 cm−2 eV−1 for Si and 4×1011 cm−2 eV−1 for p-InP.

Electrical characteristics and memory behavior of Ge 3N 4GaAs MIS devices

MIS devices have been fabricated by the low temperature chemical vapor deposition of Ge 3N 4 on n-GaAs. From the current-voltage data an estimate of the Ge 3N 4 dielectric constant is made as 6.3 ± 0.2 and devices exhibit a breakdown field strength of ∼ 5 × 10 6 V/cm. Capacitance and conductance measurements have been performed to investigate the electrical characteristics of the Ge 3N 4GaAs interface. The interface properties of the devices are found to depend on the Ge 3N 4 deposition parameters. No major hysteresis is observed in the C-V plot and under large negative gate bias, the capacitance increases as the measurement frequency is lowered. Interface state distribution, evaluated from the conductance data, is found to have a minimum density of states of 2 × 10 11 cm −2 eV −1 with a distinct shoulder between 0.4 and 0.55 eV from the conduction band. This shoulder is assigned to an electron trap level and from thermally stimulated current measurements we obtained the density of traps as 3 × 10 17 cm −3. GaAs-MNOS devices have also been fabricated and their charge storage properties have been studied. Pulse voltages as large as 30–35 V are needed to write/erase the memory in the devices.

DLTS response due to localized states in hydrogenated amorphous silicon

Abstract The transient response of the localized states in a-Si : H involved in DLTS measurements is analyzed in the frame of a detailed model for the band bending. The model is based on an exponential distribution of the density of states, spatially uniform throughout a Schottky-barrier depletion layer. The thermally emitted charge is calculated and compared with the estimate of the charge from the simple crystalline model to assess the effect of nonparabolic band bending. The influence of spatially nonuniform space charge density and uncompensated shallow level concentration is analyzed by calculating the DLTS signal and retrieving the density of states by the modified crystalline model. Our results show that the simple modified crystalline model accurately determines the localized density of states in a-Si:H from DLTS data obtained with large reverse bias voltages. for lower reverse bias voltages the analysis of the DLTS spectra based on the crystalline model yields substantially lower values for the minimum density of states.

Interpretation of Characteristics in a-Si:H Solar Cells

Current versus voltage characteristics in an illuminated a-Si:H solar cell which shows conversion efficiency of 4.3% (cell area: 1.0 cm2) are interpreted by assuming a simplified junction model and a distribution of the density of states in the band gap. Since the width of the depletion region relates to this distribution, a depletion width (W) versus bias voltage (V) relation can be calculated from this distribution. Also, another W-V relation is obtained from light-generated current data on the basis of a few assumptions. These two different estimations of the W-V relation excellently agree when one assumes a hyperbolic distribution of the density of states in the band gap which is symmetric with respect to the middle of the energy band gap and has a density of mid-gap states of 8.3×1015 (cm3 eV)-1. This value is comparable to the minimum density of states in the a-Si:F:H reported by W. Czubatyj et al.

Criterion of minimum state density in the transition state theory of bimolecular reactions

We discuss two minimum-density-of-states criteria for the location of generalized transition states for chemical reactions. One is due to Bunker and Pattengill; the other is due to Wong and Marcus. We prove that both provide upper bounds on the exact classical equilibrium rate constant. In addition, we show that for several-dimensional systems both methods are exact at threshold, and in the limit of an infinite number of dimensions they agree with the variational theory of reactions of Wigner, Horiuti, and Keck. However, it is also shown that for a finite number of degrees of freedom both methods yield rate constants which are only as accurate as or less accurate than rate constants given by the variational theory of reactions. We note that, where tested by others for actual systems, the differences of the results obtained with the variational and Bunker–Pattengill criteria have been minor.

The criterion of minimum state density in unimolecular rate theory. An application to ethane dissociation

The application of the criterion of minimum state density is described and a model potential energy function is determined for ethane dissociation. Comparisons are made between thermal rate constants calculated using the RRKM and activated complex theories. Activated complex structures are chosen by a Boltzmann average of the minimum state density criterion.

Remarks on the Concept of Minimum State Density in the Activated Complex Theory

The concept of minimum state density in the activated complex theory and its relation to the transmission coefficient were discussed by formulating the average reaction cross-section.

Theoretical Critical Configuration for Ethane Decomposition and Methyl Radical Recombination

A theoretical critical configuration is described which yields calculated rate parameters in agreement with those measured for both the decomposition and recombination reactions in the ethane-methyl radical system. It is shown that if structural effects in the molecule are considered as the reaction co-ordinate is extended, this critical configuration is nearly identical to one determined by using the concept of minimum state density.

Concept of Minimum State Density in the Activated Complex Theory of Bimolecular Reactions

In some bimolecular reactions, for example, certain low potential-energy barrier reactions, the saddle-point choice for the position of the activated complex is vague and sometimes incorrect. In these and in some others a different choice is needed. The concept of minimum state density for this purpose, its relation to adiabatic transition state theory and to the maximum free energy criterion, are discussed. It is shown how it may be applied to bimolecular reactions using microcanonical activated complex theory for these reactions.