Density Of States Measurements Research Articles

A-Si:H transport parameters from experiments based on photoconductivity

In this paper we review some of the techniques based on the photoconductivity property of hydrogenated amorphous silicon (a-Si:H) from which it is possible to extract transport parameters as well as density of states (DOS) spectroscopies. We also present a new experiment based on the steady state photocarrier grating technique. We show that combined with simple steady state photoconductivity it gives information on the DOS. The comparison of these results with those of other techniques used for DOS measurements theoretically allows determination of transport parameters in a-Si:H.

Alternative phenomenological coupling parameter for finite-size analysis of numerical data at criticality

We introduce a parameter $W(\beta,L)= (\pi\,\langle |m| \rangle^2/\langle m^2 \rangle - 2)/(\pi-2)$ which like the kurtosis (Binder cumulant) is a phenomenological coupling characteristic of the shape of the distribution $p(m)$ of the order parameter $m$. To demonstrate the use of the parameter we analyze extensive numerical data obtained from density of states measurements on the canonical simple cubic spin-$1/2$ Ising ferromagnet, for sizes $L=4$ to $L=256$. Using the $W$-parameter accurate estimates are obtained for the critical inverse temperature $\beta_c = 0.2216541(2)$, and for the thermal exponent $\nu = 0.6308(4)$. In this system at least, corrections to finite size scaling are significantly weaker for the $W$-parameter than for the Binder cumulant.

Orlicz–Sobolev extensions and measure density condition

We study the extension properties of Orlicz–Sobolev functions both in Euclidean spaces and in metric measure spaces equipped with a doubling measure. We show that a set E ⊂ R satisfying a measure density condition admits a bounded linear extension operator from the trace space W 1 , Ψ ( R n ) | E to W 1 , Ψ ( R n ) . Then we show that a domain, in which the Sobolev embedding theorem or a Poincaré-type inequality holds, satisfies the measure density condition. It follows that the existence of a bounded, possibly non-linear extension operator or even the surjectivity of the trace operator implies the measure density condition and hence the existence of a bounded linear extension operator.

Electronic state modification in laser deposited amorphous carbon films by the inclusion of nitrogen

In this study, we investigate the effect of the inclusion of nitrogen in amorphous carbon thin films deposited by pulsed laser deposition, which results in stress induced modifications to the band structure and the concomitant changes to the electronic transport properties. The microstructural changes due to nitrogen incorporation were examined using electron energy-loss spectroscopy and Raman scattering. The band structure was investigated using spectroscopic ellipsometry data in the range of 1.5–5 eV, which was fitted to the Tauc Lorentz model parametrization and optical transmittance measurements. The dielectric constant evaluated using optical techniques was compared to that obtained with electrical measurements, assuming a Poole-Frenkel type conduction process based on the best fits to data. The electrical conduction mechanism is discussed for both low and high electric fields, in the context of the shape of the band density of states. By relating a wide range of measurement techniques, a detailed relationship between the microstructure, and the optical and the electrical structures of a-CNx films is obtained. From these measurements, it was found that, primarily, the change in density of the film, with increasing nitrogen pressure, affects the band structure of the amorphous carbon nitride. This is due to the fact that the density affects the stress in the film, which also impacts the localized states in the band gap. These results are supported by density of states measurements using scanning tunneling spectroscopy.

Measure density and extendability of Sobolev functions

We study necessary and sufficient conditions for a domain to be a Sobolev extension domain in the setting of metric measure spaces. In particular, we prove that extension domains must satisfy a measure density condition.

Sobolev embeddings, extensions and measure density condition

There are two main results in the paper. In the first one, Theorem 1, we prove that if the Sobolev embedding theorem holds in Ω, in any of all the possible cases, then Ω satisfies the measure density condition. The second main result, Theorem 5, provides several characterizations of the W m , p -extension domains for 1 < p < ∞ . As a corollary we prove that the property of being a W 1 , p -extension domain, 1 < p ⩽ ∞ , is invariant under bi-Lipschitz mappings, Theorem 8.

Magnetoconductance oscillations in quasiballistic multimode nanowires

We calculate the conductance of quasi-one-dimensional nanowires with electronic states confined to a surface-charge layer in the presence of a uniform magnetic field. Two-terminal magnetoconductance (MC) between two leads deposited on the nanowire via tunnel barriers is dominated by density-of-states (DOS) singularities when the leads are well apart. There is also a mesoscopic correction due to a higher-order coherent tunneling between the leads for small lead separation. The corresponding MC structure depends on the interference between electron propagation via different channels connecting the leads, which in the simplest case, for the magnetic field along the wire axis, can be crudely characterized by relative winding numbers of paths enclosing the magnetic flux. In general, the MC oscillations are aperiodic due to the Zeeman splitting, field misalignment with the wire axis, and a finite extent of electron distribution across the wire cross section, and are affected by spin-orbit coupling. The quantum-interference MC traces contain a wealth of information about the electronic structure of multichannel wires, which would be complimentary to the DOS measurements. We propose a four-terminal configuration to enhance the relative contribution of the higher-order tunneling processes and apply our results to realistic InAs nanowires carrying several quantum channels in the surface charge-accumulation layer.

Post-growth annealing effects in compensated μc-Si:H samples

In this work we study the effect of low temperature annealing on the transport properties of μc-Si:H. We performed post-growth annealing on a series of micro-doped μc-Si:H samples having different degrees of compensation, extending from n-type to p-type character. The samples were isothermally annealed at 423 K, 10 K below the deposition temperature. An increase of up to four orders of magnitude was observed in the dark conductivity ( σ dk) for the low temperature region. Simultaneously, the activation energy ( E a) of the dark conductivity changes depending on the doping level and the annealing time. Absorption coefficient and density of states measurements have been done for different annealing stages by using the constant photocurrent method and the modulated photocurrent technique, respectively. The density of defects in the midgap region increases during the annealing while the band tails remain unaffected. The σ dk and E a behavior can be interpreted in terms of a shift of the Fermi level towards the valence band. A model based in doping activation is proposed in order to explain this behavior.

Coulomb gap in a doped semiconductor near the metal-insulator transition: Tunneling experiment and scaling ansatz

Electron tunneling experiments are used to probe Coulomb correlation effects in the single-particle density-of-states (DOS) of boron-doped silicon crystals near the critical density of the metal-insulator transition (MIT). At low energies, a DOS measurement distinguishes between insulating and metallic samples with densities 10 to 15 % on either side of the MIT. However, at higher energies the DOS of both insulators and metals show a common behavior, increasing roughly as the square-root of energy. The observed characteristics of the DOS can be understood using a classical treatment of Coulomb interactions combined with a phenomenological scaling ansatz to describe the length-scale dependence of the dielectric constant as the MIT is approached from the insulating side.

Vortex core shapes measured by STM

We have studied vortex core shapes in superconducting NbSe2 by STM, as function of temperature and bias voltage. The experimentally measured tunnel current profiles are compared with the results of calculations using microscopic theory. We find that, at low temperatures (T/T c ≃ 0.25), the apparent vortex core radius strongly depends on the bias voltage, which demonstrates the energy dependence of the scale for spatial variation of the quasiparticle density of states. Good quantitative agreement between measured and calculated profiles is found by using the accepted value for the superconducting coherence length ΞS, without further adjustable parameters. This shows that the bias dependence is a useful extra tool in the interpretation of local density of states measurements.

Scanning tunneling microscopy of porous silicon surfaces

The scanning tunneling microscope (STM) was used to examine several porous silicon based surfaces. First are samples of photoluminescent porous silicon made from single crystal wafers of 13.5 Ω cm p− (100) and 0.002 Ω cm p+ (100) silicon. These were observed in an ambient He environment. The development of the porous structure in these wafers due to different etching stages is investigated with the STM. Measurements show that the vertical surface roughness increases with etching while the lateral dimensions of the structures present remain almost constant in agreement with known etching models. The porous silicon surface density of states (DOS) was also measured using scanning tunneling spectroscopy. DOS measurements of porous silicon formed on both p− and p+ substrates do not show the expected increase in band gap energy when compared with unetched crystalline silicon. However, these measurements do show that while the unetched silicon surface remains p type, porous silicon surfaces fabricated from p− Si behaves like an n-type material whereas those formed from p+ Si remain p type after etching.

Scanning probe microscopy and scanning tunneling spectroscopy of porous silicon

Atomic force microscope and scanning tunneling microscope images of photoluminescent anodically etched porous silicon are presented and the development of the porous structure due to different etching stages is investigated. These measurements show that the vertical surface roughness increases with etching while the lateral dimensions remain almost constant, in agreement with known etching models. In addition, the results give strong evidence for an amorphous surface layer that partially covers the porous structure. Tunneling spectroscopy of porous silicon and unetched silicon are also presented. The surface density of states (DOS) measurements of porous silicon are compared with its photoluminescence spectrum. The DOS measurements do not show a peak corresponding to the 1.8 eV photoluminescent peak energy nor do they show an increase in the band gap energy of porous silicon compared with crystalline silicon. However, these measurements do show that while the unetched silicon surface remains p type, the porous silicon surface behaves like an n-type material.

DOS investigation of 3d transition metals by ionization loss spectroscopy

Abstract The electron density of states (DOS) of solids is one of the most important properties which basically determines the behavior of materials. In addition, the DOS obtained by experimental methods can be used for the verification purpose of the theoretical model. A simple method has been developed in our laboratory, i.e. Ionization Loss Spectroscopy (ILS) for an unoccupied DOS measurement which can be done by existing Auger electron spectroscopy. The ILS spectra of 3d transition metals, Ti, Cr, Ni, Cu have been measured. The results of spin-orbit splitting and binding energy obtained by ILS are in good agreement with those of ESCA. The typical integral spectra exhibit a peak-like shape with different intensity, and width which reflects the width of the unoccupied DOS. Since there is no unoccupied DOS of the 3d band, the ILS spectrum of Cu measured is shown by a step-like structure resulting from the flat, narrow 4s band.