Density Of States Model Research Articles

Universal features of point defect spectrum in graphene

We investigate the electronic structure of localized point defects in two-dimensional graphene. We note that this has been studied in the recent past. Our work is distinguished by its focus on universal features of the point defect spectra. We show that the defect levels converge to three “fixed points” at large defect potentials. In other words the defect levels are “pinned”. We investigate the “pinning”, its robustness and show that it is a consequence of density of states of the host system. We study the localization or otherwise of these “pinned” levels and also of other levels. A critical defect potential is identified. These and other features are studied both using model density of states as well as realistic nearest neighbour and next nearest neighbour tight binding Hamiltonians. We consider both orthogonal as well as non-orthogonal cases. Finally, we discuss future programs of study.

Laser irradiation-induced structural, microstructural and optical properties change in Bi-doped As40Se60 thin films

ABSTRACTThe laser irradiation with band gap light of thermally evaporated As40Se55Bi5 and As40Se45Bi15 thin films was found to be accompanied by structural changes which in turn changed the optical constant. The thin films under investigation were characterized by the X-ray diffraction method to study the structural change and the surface morphology was checked by field emission scanning electron microscope. The composition of the films was verified from the energy dispersive X-ray analysis. The changes in optical properties due to the influence of laser irradiation were calculated from the transmission spectrum recorded by UV–Visible spectrometer in the wavelength region 400–1200 nm. The decrease in optical band gap (photodarkening) is explained on the basis of density of state model proposed by Mott–Davis. The Raman analysis showed the Raman shift and symmetric stretching vibration of As–Se.

On the adsorptive properties of silicon carbide: gas molecule adsorption

On the bases of two different approaches (solid-state and quantum-chemical) adsorption of nitrogen atom and ammonia molecules on SiC substrate is considered. In the solid-state approach with the use of the Haldane- Anderson density of state model for the 4 H- and 6 H-SiC polytypes, it is shown that the binding energies for N and N2 are 6 and 3 eV correspondingly. In the quantum-chemical approach with the use of the surface molecule model for N the binding energy values are 6 eV for adsorption on C-face and 4 eV for adsorption on Si-face. It is demonstrated also that the charge transfer between adsorbate and substrate for all the cases considered are negligible.

Simulation and Compact Modeling of Organic Thin Film Transistors (OTFTs) for Circuit Simulation

In this paper we present TCAD simulation and compact modeling of OTFTs. Finite element method (FEM) based numerical simulations have been performed using density of state model and field dependent mobility model to simulate the electrical behavior of the OTFT devices. Further Universal Organic Thin Film Transistor (UOTFT) compact model has been applied for simulating the DC I-V characteristics of the device using device modeling software UTMOST-IV. TCAD simulated characteristics and compact model based simulated characteristics were compared and contrasted. Device model parameters were extracted using UOTFT model. Extracted model is applied in designing the OTFT based PMOS-like inverter and hybrid operational amplifier.

Effect of Pb Addition on Optical and Spectral Properties of Se–Ge Thin Films

The present paper reports the optical and spectral properties of amorphous Se58Ge42 – xPbx (x = 6, 12, 18, 20) thin films deposited onto pre-cleaned glass substrate under a vacuum of 10–5 Torr. The amorphous samples have been prepared using melt quenching method. The structural characterization of the samples has been done using XRD and EDXA. The optical absorption and transmission spectra were recorded using UV-Vis spectrophotometer in the wavelength range 400–2500 nm. The study of absorption spectra shows that sample exhibits indirect optical band gap which decreases on increase of Pb content in Se–Ge system. This decrease in band gap has been explained in terms of chemical bond approach and density of state model. Transmission spectra have been utilized to obtain refractive index, dielectric constant and thickness of films. It is found that refractive index increases with increase in Pb content in Se–Ge system.

Extraction of ZnO thin film parameters for modeling a ZnO/Si solar cell

Zinc oxide (ZnO) is a semiconductor with promising electrical and optical properties. Pure ZnO is an n-type degenerate semiconductor. It is almost entirely transparent in the optical region of the electromagnetic spectrum. In this work sol–gel spin-coating method was used to grow a ZnO thin film from Zinc acetate decomposition. The optical properties of the thin film were extracted and investigated. A ZnO/p-Si heterostructure solar cell was realized using this film and its performance (efficiency) was found to be quite poor. A numerical procedure is used to elucidate this poor performance. A first step is to use the extracted optical parameters in the simulation. However, these parameters did not lead to the reproduction of measured current-voltage and figures of merit of the solar cell by simulation when other parameters of ZnO are taken from literature. It was therefore considered that ZnO is similar to an amorphous semiconductor with a continuous distribution of states in its band gap. The density of states model used is composed of tail bands and Gaussian distribution deep level bands. Eventually, and by adjusting the constituents of the band gap states and by increasing the surface recombination velocity in ZnO/p-Si interface, it was possible to obtain a good agreement between simulated and measured J-V characteristics of this solar cell.

Photoluminescence Study of Amorphous InGaZnO Thin-Film Transistors

The defects within optical gap of the amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) before and after bias temperature stress (BTS) are studied using in situ photoluminescence (PL) spectroscopy. After applying positive BTS, the TFT threshold voltage (Vth) increased, and intensity of deep-level PL emission peak at 1.82 eV became stronger. The exact opposite is observed when negative BTS is applied. Both the 250-nm bulk a-IGZO film and 50-nm channel region TFT show the same PL peak locations of 3.35 and 1.82 eV. From the measured results and density of state model of a-IGZO, the radiative recombination between electrons trapped in oxygen-related acceptor-like states and holes in the valence band is responsible for the deep-level PL emission. This method of measuring the PL response within the TFT channel region could be used as a tool to evaluate the quality of a-IGZO TFT manufacturing processes.

Kinetics of degradation-induced polymer luminescence: A polyphenylene sulfide/elastomer blend under dry heat exposure

The thermally induced changes of the steady-state photoluminescence of a polyphenylene sulfide/elastomer blend under dry heat exposure are investigated. Two signals, a phenylene fluorescence signal and a degradation-induced luminescence signal are identified and found to exhibit different spectral and kinetic properties. A detailed kinetic analysis of the thermally induced changes reveals, that the phenylene fluorescence signal can consistently be interpreted in terms of a cold crystallization process limited by the viscous flow of the polymeric material with an apparent activation energy of 33 ± 12 kJ/mol. On the other hand, the changes of the degradation-induced steady-state luminescence signal can be described by a power law kinetics with an apparent activation energy of 113 ± 25 kJ/mol. The obtained apparent activation energies of the thermally induced material changes and the underlying molecular mechanisms are discussed and related to data available from the literature. In addition to the kinetic analysis of the changes of the degradation-induced steady-state photoluminescence, an equivalent exposure time and temperature can be derived that describe the exposure history of the sample. Furthermore, a relationship between exposure at room temperature and exposure above the glass transition temperature is suggested that, in combination with the density of states model as proposed before, can be used to predict the development of the steady-state luminescence changes over many years of exposure at room temperature.

Electrical properties of the LaLiyCo1 – yO3 – δ (0 ≤ y ≤ 0.10) oxides

The effect of the Li ion concentration on the phase composition, the electrical conductivity, and the thermoelectric power of the LaLi y Co1–yO3–δ (0 ≤ y ≤ 0.1) oxides synthesized by cocrystallization has been studied. It is found that the region of the perovskite-like solid solution LaLi y Co1–yO3–δ is no higher than y = 0.037. In the temperature range 300–1020 K, lithium alloying leads to an increase in the electrical conductivity and a decrease in the positive thermoelectric power of the single-phase samples compared to LaCoO3–δ. The results are discussed using the density of states model proposed by Senarus Rodriguez and Goodenough for LaCoO3–δ and La1–xSr x CoO3–δ and using the Mott theory of noncrystalline substances.

Optical band gap tuning of Sb-Se thin films for xerographic based applications

In the present paper we have studied the effect of Sb addition on the optical band gap tuning of thermally evaporated SbxSe100-x (x = 0, 5, 20, 50 and 60) thin films. The structural investigations revealed that all thin films were amorphous in nature. Transmission spectrum was taken in the range 400–2500 nm shows that all films are highly transparent in the near infrared region. The fundamental absorption edge shifts towards longer wavelength with Sb incorporation. The optical band gap decreases with addition of antimony in a-Se thin films. A good correlation has been drawn between experimentally estimated and theoretically calculated optical band gap. The decrease in optical band gap of thin films has been explained using chemical bond approach and density of states model. Decrease in optical band gap with Sb addition increases the concentration of electron deep traps which increases the X-ray sensitivity of Sb-Se thin films. Thus by tuning the optical band gap of Sb-Se alloy, it could be utilized for xerographic based applications.

Enhancement in threshold voltage with thickness in memory switch fabricated using GeSe1.5S0.5 thin films

Investigations on the electrical switching, structural, optical and photoacoustic analysis have been undertaken on chalcogenide GeSe1.5S0.5 thin films of various thicknesses prepared by vacuum evaporation technique. The decrease of band gap energy with increase in film thickness has been explained using the ‘density of states model’. The structural units of the films are characterized using Raman spectroscopy and the deconvoluted Raman peaks obtained from Gaussian fit around 188cm−1, 204cm−1 and 214cm−1 favors Ge–chalcogen tetrahedral units forming corner and edge sharing tetrahedra. All the thin films samples have been exhibited memory-type electrical switching behavior. An enhancement in the threshold voltages of GeSe1.5S0.5 thin films have been observed with increase in film thickness. The thickness dependence of switching voltages provide an insight into the switching mechanism and it is explained by the Joule heating effect.

Thickness dependence of electron transport in pure a-Se photoconductive films

Electron transport in vacuum-deposited a-Se films with thicknesses varying from 13 to 501 μm has been investigated by conventional time-of-flight (TOF) and interrupted field TOF experiments. To separate the influences of electric field and the thickness, all TOF experiments were performed at a constant electric field. It has been found that the electron mobility is relatively constant in thick films (L > 50 μm) and increases in thinner films (L < 50 μm) with decreasing thickness. On the other hand, the electron lifetime is relatively thickness independent in films with thickness L > 50 μm, but drops sharply in thin films when L < 50 μm. These observations can be explained based on the density of states model that includes three types of traps forming Gaussian-like distributions within the mobility gap as reported in Koughia et al. (J. Appl. Phys. 97, 033706 (2005)).

Adsorption of hydrogen atom on graphene

The adsorption of atomic hydrogen on graphene monolayer is considered using the model density of states (M model). Analytical expressions for the density of states of H adatom and its occupation number na are obtained. The main (local) contribution nl to the total occupation number na is due to a local level of the adatom, which occurs below the edge of the continuous spectrum. A variant of the surface molecule approximation in the case of nl ≫ na − nl is proposed.

Uniform existence of the integrated density of states for models on $${\mathbb{Z}}^d$$

We provide an ergodic theorem for certain Banach-space valued functions on structures over $${\mathbb{Z}}^d$$ , which allow for existence of frequencies of finite patterns. As an application we obtain existence of the integrated density of states for associated discrete finite-range operators in the sense of convergence of the distributions with respect to the supremum norm. These results apply to various examples including periodic operators, percolation models and nearest-neighbour hopping on the set of visible points. Our method gives explicit bounds on the speed of convergence in terms of the speed of convergence of the underlying frequencies. It uses neither von Neumann algebras nor a framework of random operators on a probability space.

Limitation of Mean Field Approximation for Carrier States and Ferromagnetism in Diluted Magnetic Semiconductors

In spite of a great deal of recent experimental and theoretical activities, there has not yet been a consensus on the fundamental mechanism leading to the ferromagnetism of Ga1 xMnxAs. 1) In diluted magnetic semiconductors (DMSs), there are two types of disorder: substitutional disorder and the thermal fluctuation of localized spins. Neglecting disorder effects, the mean-field Zener model (the phenomenological model) predicts the possibility of a high Curie temperature for some materials. Properly taking the disorder effects into consideration, however, is indispensable in the calculation of the Curie temperature TC of DMSs, as clarified in this study. To show the importance of the disorder effects, we calculate TC by applying three approximations to a simple model: the coherent potential approximation (CPA), the virtual crystal approximation (VCA), and the mean-field approximation (MFA). In the present model, Ga1 xMnxAs is regarded as a semiconducting alloy in which a mole fraction x of Ga ions (symbol: A) in GaAs are replaced at random by Mn ions (symbol:M). A single carrier (a p hole) moving in Ga1 xMnxAs is affected by the local potential EA or EM I Sm, depending on whether it is on a Ga site or a Mn site. Here, EA and EM denote the spinindependent potentials on the Ga ion and Mn ion, respectively; I Sm represents the p–d exchange interaction between the carrier (p hole) and the localized spin Sm (d spin) of Mn located on the m site. Setting EA 1⁄4 0, we employ the model density of states (DOS) of semicircular form:

Effect of substrate temperature on the properties of vacuum evaporated indium selenide thin films

Thin films of InSe were obtained by thermal evaporation techniques on glass substrates maintained at various temperatures (Tsb = 30°, 400°C). X-ray diffraction analysis showed the occurrence of amorphous to polycrystalline transformation in the films deposited at higher substrate temperature (400°C). The polycrystalline films were found to have a hexagonal lattice. Compositions of these films have been characterized by EDAX and the surface analysis by scanning electron microscopy. Optical properties of the films, investigated by using spectrophotometer transmittance spectra in the wavelength range (300 – 1100 nm), were explained in terms of substrate temperatures. Films formed at room temperature showed an optical band gap (Egopt) 1.56 eV; where as the films formed at 400°C were found to have a Egopt of 1.92 eV. The increase in the value of Egopt with Tsb treatment is interpreted in terms of the density of states model as proposed by Mott and Davis. The analysis of current -Voltage characteristics, based on space charge limited currents (SCLC) measurements, confirms the exponential decrease of density of states from the conduction band edge towards the Fermi level for both the amorphous and polycrystalline films. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A quantitative understanding of pressure dependent conductivity of FeSi 1− xGe x

A new density of states model, referred to as the Gaussian density of states, is proposed for the quantitative understanding of the electrical conductivity behaviour of FeSi Kondo insulating system. The effects of electron correlation and disorder, responsible for the physical properties of this system, are judiciously incorporated in this model. Within the framework of this model, a detailed quantitative analysis of the temperature and pressure dependent electrical conductivity data of FeSi 1− x Ge x ( x=0.0, 0.05 and 0.20) reported by Awadhesh Mani et al. [Phys. Rev. B 63 (2001) 115103] has been carried out. From these analyses the complicated pressure dependence of energy gap seen experimentally in these samples could be satisfactorily rationalized.

Sawtoothlike de Haas–van Alphen oscillations of a two-dimensional electron system

We present studies on the low-temperature magnetization of a two-dimensional electron system (2DES) in the integer quantum Hall-effect regime. The 2DES was formed in a modulation-doped AlGaAs/GaAs heterojunction. Using molecular-beam epitaxy it has been integrated into a highly sensitive micromechanical cantilever magnetometer. We observe de Haas–van Alphen oscillations at even filling factors up to ν=40 which for ν< 20 are almost perfectly sawtoothlike. Oscillations at odd filling factors ν=3,5,7, and 9 are due to the spin splitting of Landau levels. For a quantitative analysis of our data, calculations of the magnetization based on a model density of states (DOS) have been performed. We describe the DOS by narrow Gaussian broadened Landau levels with an energy-independent background. In particular we find that this background DOS increases linearly with ν. This behavior is qualitatively explained using the concept of edge channels. From our data we evaluate the level broadening and, at odd filling factors, the exchange-energy contribution to the spin splitting of Landau levels.

Evolution of the Kondo insulating gap inFe1−xRuxSi

Electrical resistivity measurements are reported in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x}\mathrm{Si} (x=0.0$ to 0.30) as a function of temperature (4.2 K to 300 K) and as a function of temperature and pressure (0--5 GPa) for $x=0.04$ and 0.1. The temperature-dependent resistivity data have been analyzed based on a model density of states (DOS) taking into account both electron correlations and disorder. Using this model DOS and temperature-dependent mobility, the conductivity as a function of temperature is calculated and fitted to the experimental data for several external pressures and Ru concentrations. The parameters of the fit as a function of $x,$ show extremal values at the Ru composition $x=0.06.$ An effective band gap $({\ensuremath{\Delta}}_{\mathrm{eff}})$ obtained from these parameters is seen to decrease with Ru substitution up to $x=0.06$ beyond which it increases again. Band-structure calculations performed for several stoichiometric compositions of x in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x}\mathrm{Si},$ indicate a broadening of the bands and an overall increase in the gap from 0.13 eV for $x=0.0$ to 0.3 eV for $x=1.0.$ The existence of localized states within the gap leads to the description of the transport behavior at low temperature (5--25 K) in terms of variable range hopping (VRH) mechanism. The VRH parameter ${T}_{0}$ decreases by more than three orders of magnitude for x up to 0.06, beyond which it increases again. It is surmised that with Ru substitution, (FeRu)Si goes over from a Kondo insulator to a conventional band semiconductor via an intermediate metallic phase. The results of temperature-dependent resistivity under high pressure in the samples with compositions $x=0.04$ and 0.1 are also reported and analyzed based on the model.

Elastic Scattering of Polaritons in High Quality Microcavities and Weak Localization

We theoretically study the elastic scattering of lower cavity polaritons on the static disorder in high quality semiconductor microcavities in the strong coupling regime. We consider the dominant contribution of the resonant scattering on localized exciton levels and calculate for a model density of states the corresponding elastic mean free path and inhomogeneous broadening. Our analytical results compare well with available linewidth data and large scale numerical simulations. They also provide a justification for a widely used phenomenological model assuming the polariton wavevector conservation. Finally, we discuss the possibility of weak localization in state of the art microcavities.