Extended Band States Research Articles

Temperature-dependent study of GaAs1−x−yNxBiy alloys for band-gap engineering: photoreflectance and k · p modeling

Photoreflectance measurements were performed for GaAs1−x−yNxBiy layers in the temperature range of 20–300 K. For each sample a transition related to the band-gap was observed, which red-shifts with increasing nitrogen and bismuth content. The temperature dependencies of the band-gap were fitted by the Varshni and Bose–Einstein formulas and simulated within the band anticrossing model of the interaction between the extended band states of the GaAs and the localized states associated with nitrogen and bismuth atoms. The reduction of the band-gap was found to be ∼80–100 meV.

Scaling of the breakdown in the impurity band of doped GaAs/AlGaAs quantum well demonstrating virtual Anderson transition

We observed breakdown behavior for highly doped and low compensated p-type layers located within the central part of GaAs/AlGaAs quantum wells at rather small values of the applied bias. This behavior is attributed to the presence of the band of extended states (BES) within the impurity band. Namely, we believe that the breakdown is due to impact ionization of the carriers from the Fermi level to BES while the fact that the highly conducting state is supported at lower voltages than the threshold ones is explained by detrapping of the carriers captured from the BES to intermediate localized levels. A special attention was paid to the temperature behavior of the threshold voltages as functions of the current as well as of the voltages corresponding to highly conducting state. It appeared that these voltages are independent of the current and are controlled exclusively by the properties of the sample. To our opinion, it gives a strong evidence of the validity of our model.

Aqueous transition-metal cations as impurities in a wide gap oxide: the Cu(2+)/Cu(+) and Ag(2+)/Ag(+) redox couples revisited.

The interactions of the d electrons of transition-metal aqua ions with the solvent are usually divided in short-range electronic interactions with ligand water molecules and long-range electrostatic interactions with molecules beyond the first coordination shell. This is the rationale behind the cluster continuum and QM/MM methods developed for the computation of the redox potentials. In the density functional theory based molecular dynamics (DFTMD) method, the electronic states of the complex are also allowed to mix with the extended band states of the solvent. Returning to the Cu(+) and Ag(+) oxidation reaction, which has been the subject of DFTMD simulation before, we show that coupling to the valence band states of water is greatly enhanced by the band gap error in the density functional approximation commonly used in DFTMD (the generalized gradient approximation). This effect is analyzed by viewing the solvent as a wide gap oxide and the redox active ions as electronic defects. The errors can be reduced significantly by application of hybrid functionals containing a fraction of Hartree-Fock exchange. These calculations make use of recent progress in DFTMD technology, enabling us to include sp core polarization and Hartree-Fock exchange in condensed-phase model systems.

Effect of low nitrogen concentrations on the electronic properties ofInAs1−xNx

We report cyclotron resonance (CR), transverse magnetoresistance (MR), and Hall effect studies of a series of $n$-type ${\text{InAs}}_{1\ensuremath{-}x}{\text{N}}_{x}$ epilayers grown on GaAs with $x$ up to 1%. The well-resolved CR absorption lines, the classical linear MR, Shubnikov--de Haas magneto-oscillations, and negative MR revealed in our experiments provide a means of probing the effect of the N atoms on the electronic properties of this alloy system and reveal qualitative differences compared to the case of the wider gap III-N-V compounds, such as ${\text{GaAs}}_{1\ensuremath{-}x}{\text{N}}_{x}$. In ${\text{GaAs}}_{1\ensuremath{-}x}{\text{N}}_{x}$ electron localization by N levels that are resonant with the extended band states of the host crystal act to degrade the electrical conductivity at small $x$ $(\ensuremath{\sim}0.1%)$. These phenomena are significantly weaker in ${\text{InAs}}_{1\ensuremath{-}x}{\text{N}}_{x}$ due to the smaller energy gap and higher energy of the N levels relative to the conduction band minimum. In ${\text{InAs}}_{1\ensuremath{-}x}{\text{N}}_{x}$ the electrical conductivity retains the characteristic features of transport through extended states, with electron coherence lengths (${l}_{\ensuremath{\varphi}}\ensuremath{\sim}100\text{ }\text{nm}$ at 2 K) and electron mobilities ($\ensuremath{\mu}=6\ifmmode\times\else\texttimes\fi{}{10}^{3}\text{ }{\text{cm}}^{2}\text{ }{\text{V}}^{\ensuremath{-}1}\text{ }{\text{s}}^{\ensuremath{-}1}$ at 300 K) that remain relatively large even at $x=1%$.

Possible existence of a band of extended states induced by inter-Landau-band mixing in aquantum Hall system

The mixing of states with opposite chiralities in a quantum Hall system is shown to have adelocalization effect. It is possible that extended states may form bands because of thismixing, as is shown through a numerical calculation on a two-channel networkmodel. Based on this result, a new phase diagram with a narrow metallic phaseseparating two adjacent QH phases and/or separating a QH phase from the insulatingphase is proposed. The data from recent non-scaling experiments are reanalysedand it is shown that they seem to be consistent with the new phase diagram.However, due to finite-size effects, further study on large system size is still needed toconclude whether there are extended state bands in the thermodynamic limit.

Excitation transfer between extended band states and N-related localized states in [formula omitted] with x up to 1%

We present time-resolved photoluminescence (PL) results of a series of GaP 1 - x N x samples with x up to 0.01. The temperature dependence, the concentration dependence as well as the temporal behavior indicate that the PL is dominated by excitation transfer processes between the extended band states and localized N-related states (such as the isolated N-impurity, various N-pair states and higher N-clusters) as well as by excitation transfer between the various localized N-related states themselves. The excitation transfer processes in conjunction with the concentration-dependent statistics of the various N-related states alone are sufficient to explain the observed red-shift of the luminescence of GaP 1 - x N x with increasing x as well as the spectral dependence of the PL decay times. However, this implies that the PL data alone do not give any conclusive evidence that a hard transformation from an indirect to a direct-gap semiconductor takes place in Ga(N,P) with increasing N up to 2% as often stated.

Theory of current-induced breakdown of the quantum Hall effect

By studying the quantum Hall effect of stationary states with high values of injected current using a von Neumann lattice representation, we found that broadening of extended state bands due to a Hall electric field occurs and causes the breakdown of the quantum Hall effect. The Hall conductance agrees with a topological invariant that is quantized exactly below a critical field and is not quantized above a critical field. The critical field is proportional to $B^{3/2}$ and is enhanced substantially if the extended states occupy a small fraction of the system.

Hall electric field-dependent broadening of extended state bands in Landau levels and breakdown of the quantum Hall effect

Current dependence of activation energy has been measured in the diagonal resistance of two standard butterfly-type Hall bars and a butterfly-type Hall bar with a short and narrow channel in the central part made from GaAs/AlGaAs heterostructures. The activation energy E A in each sample decreases with increasing current and is expressed by E A= E A0− ael B F H where E A0=ℏ ω C/2 at the plateau center for i=2 and 4, a=38±8, l B is the radius of the ground Landau orbit and F H the Hall electric field. The result suggests that broadening of extended state bands in Landau levels due to the Hall electric field give rise to the magnetic field dependence of the critical Hall electric field, F cr∝ B 3/2, obtained in various butterfly-type Hall bars.

Photoexcited carrier relaxation inYBa2Cu3O7−δby picosecond resonant Raman spectroscopy

The temperature dependence of the energy relaxation of photoexcited (PE) carriers is used as a probe of the electronic structure of YBa_2Cu_3O_(7-d) in the insulating d~0.8 and metallic d~0.1 phases. The energy relaxation rate to phonons is obtained by measuring the non-equilibrium phonon occupation number, n_neq, with pulsed Raman Stokes/anti-Stokes spectroscopy using 1.5 and 70 ps long laser pulses. We can distinguish between relaxation via extended band states and localized states, since theoretically in the former, the relaxation is expected to be virtually T-independent, while in the latter it is strongly T-dependent. From the experiment - which shows strong temperature dependence of n_neq - we deduce that at least part of the PE carrier relaxation proceeds via hopping between localized states and we propose a simple theoretical model of the relaxation process. In addition, we compare the coupling of different vibrational modes to the carriers to find that the apical O vibrational mode is significantly more involved in the energy relaxation process that the in-plane 340 cm^(-1}) mode. This implies that the localized states are mainly (but not entirely) coupled to out-of plane vibrations.

Collapse of the quantum Hall state by floating-up of extended-state bands with increasing disorder

We measured the diagonal and Hall conductivities across the transition between the integer quantum Hall effect state and the insulating state in a two-dimensional electron system formed in Si-MOSFETs. Sample dependence of the transition indicates that the insulating state is caused by Anderson localization due to disorder, not by a formation of an electron solid.

Mechanism of carrier photogeneration in amorphous selenium: Fast transient photoconductivity.

Studies of fast transient photoconductivity in amorphous selenium reveal two distinct transport mechanisms. The first is a short-lived one due to carrier dynamics at extended band states and possibly states near the band edges where the carriers tunnel progressively into lower states. The second is a long-lived one due to phonon assisted multiple trapping at band tails. We find the quantum efficiency in this prototypic low-mobility system independent of temperature and electric field up to the maximum applied field of 5\ifmmode\times\else\texttimes\fi{}${10}^{5}$ V/cm, inconsistent with previous models of carrier photogeneration such as the Onsager geminate recombination model. In order to reconcile the controversial issue of the carrier photogeneration mechanism in this material, we examine in detail the processes underlying various transient photoconductivity measurements. Our analysis shows that in time-of-flight and xerographic-discharge measurements the external field may modify the extent of carrier recombination and thereby determines the carrier supply yield, whereas measurements utilizing the microstripline Auston switch configuration are better suited for investigating the intrinsic properties of the quantum efficiency. \textcopyright{} 1996 The American Physical Society.

Internal photoemission on a-Si:H Schottky barrier structures revisited

Direct measurements of mobility gaps in hydrogenated amorphous silicon (a-Si:H) using internal photoemission (IPE) of electrons and holes have been carried out. The analysis used in the measurement of metal/a-Si:H Schottky barrier heights in the previous studies was based on the classical Fowler theory. More detailed treatment of IPE in a-Si:H Schottky barriers, however, shows that the IPE yields Y(hν) α (hν − E T ) 2+c rather than (hν − E T ) 2 , where E T is the threshold energy. In the case of parabolic extended band states, c = 1 2 and the exponent 5 2 in place of 2. The extrapolations of yield versus hν result in barrier heights lower than those determined by the Fowler theory but can be fitted over a larger energy range. The effects of the new analysis of experimental results on different a-Si:H materials are presented and discussed.

Density-functional approach to the valence change in SmS: Normal and high-pressure phases

The electronic structure of SmS in dependence of the lattice constant is calculated by means of density-functional theory. The local-density approximation (LDA) to the exchange and correlation potential is employed. The 4f electrons, however, are treated in three different ways, viz., (i) as localized core states, (ii) as extended band states, and (iii) as self-interaction corrected (SIC) band states. While the experimentally observed Sm valency of normal state SmS (black phase) cannot be described by methods (i) and (ii) the SIC calculations are consistent with the measured value. For pressures above 30 kbar, method (ii) agrees well with the observed Sm valency, whereas methods (i) and (iii) fail. Therefore we conclude that the phase transition in SmS is very similar to the \ensuremath{\alpha}-\ensuremath{\gamma} transition in Ce metal. The SIC band structure shows the splitting between the occupied and empty 4f states due to the large on-site electron correlation. The Fermi level ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{F}}$ is situated in the gap between the S 3p states and the Sm band states mainly of 5d character. In the high-pressure phase the 4f states presumably become delocalized, resulting in vanishing self-interaction corrections. Therefore the uncorrected LDA results are appropriate for the high-pressure phase.

Theory of electron two-band self-trapping in atomic soft configurations: Hybridization of states, formation of negative-U centers, and anharmonic atomic dynamics.

A consistent theory of two-band self-trapping of electrons (holes) in semiconductors is presented, in which negative-U centers in the interband (mobility) gap are formed as singlet electron (hole) pairs self-trapped in atomic soft configurations. Both the configuration softness and hybridization of the bare single-electron state with extended band states are essential for formation of the self-trapped states and negative-U centers. The hybridization significantly changes appropriate soft configurations, introducing additional anharmonic features in the related atomic dynamics. Examples are mobility-gap states and negative-U centers and the related atomic dynamics in glassy semiconductors, the properties of which are discussed.

First-principles studies of the structural and optical properties of crystalline poly(para-phenylene).

We present first-principles local-density band structure calculations for one-dimensional and three-dimensional crystalline poly (para-phenylene) (PPP) using the full-potential linearized augmented-plane-wave and the pseudopotential methods. Optimized structural parameters for PPP chains and for orthorhombic crystalline phases with space groups Pbam and Pnnm are determined. A torsion angle of 27\ifmmode^\circ\else\textdegree\fi{} is predicted in PPP chains and 17\ifmmode^\circ\else\textdegree\fi{} in the crystals. The dielectric tensor and the absorption coefficient are calculated. We find very good agreement with experimental data, indicating that the excitations are extended band states. The interchain coupling leads to energy band splittings of the order of 0.5 eV. It is shown that the energy gap can be varied over a wide energy range by relatively small structural modifications.

New Type of Magnetic Polaron Lattice Formation in CeSb

A new model of the electron-hole pair magnetic polaron lattice formation in magnetic semimetals is proposed and the stability condition is studied on a simple model. It is shown that in Ce-monopnictides the conduction electrons form either neutral electron-hole pair magnetic polaron or extended band states loosely coupling with hole induces magnetic polaron. Various anomalous properties in CeSb under high pressure are explained well by this model.

Electronic transport properties of NiPS3.

The temperature dependence of conduction mechanisms in pure ${\mathrm{NiPS}}_{3}$ has been investigated by means of conductivity, photoconductivity, and thermopower measurements. Evidence for a p-type conduction has been found from the thermoelectric power data. The observed activated behaviors of thermopower, dark conductivities, and photoconductivities have allowed us to deduce more detailed information about the distribution of the electronic density of states in the energy-gap region. The results, fitted by assuming a single conduction path through extended-band states, have been interpreted in terms of an energy-band scheme based on a model regarding as insignificant the contribution of the 3d transition-metal states in bond.

Transient photocurrent study of the dangling bond centre in undoped amorphous silicon

Abstract The spectra of all four cross-sections for optical transitions associated with a correlated dangling bond centre in undoped amorphous silicon have been derived by combining transient and modulated photocurrent measurements. Analysis of the spectral shape of these cross-sections yields the energies of transitions between extended band states and the dangling bond centres with single and double occupancy, providing a unique energy placement of the dangling bond states. The results show that the singly and doubly occupied dangling bond states are located about 0.88 and 0.62 eV below the conduction-band edge, respectively, and the effective correlation energy is around 0.26 eV. Lattice relaxation energies are found to be between 0.09 and 0.13 eV. This implies a negligible role for lattice relaxation in the dangling bond transitions.

Thermoelectric power, photo and dark conductivity of doped amorphous silicon

Conductivity, photoconductivity and thermopower measurements have been made as functions of temperature on a series of p- and n-type a-Si: H films prepared by a reactive, evaporation method. The results have been fitted with a model including two conduction paths: in the high-temperature range through extended band states and in the low-temperature range as hopping in an impurity band, whose density of states increases with increasing dopant content. A shift of the Fermi level up to 0.4 eV towards the mobility edges for both types of dopants has been found. Moreover, phosphorus doping radiply increases the photoconductivity values by about two orders of magnitude, while boron incorporation causes considerable reduction.

Solitons, disorder and charge conduction in nearly metallic polyacetylene

Polyacetylene doped in the range one to five per cent has been shown to have low susceptibility yet high conductivity. A coordinated study of the structural, optical, magnetic and transport properties of polyacetylene doped with iodine to these nearly metallic levels shows that essentially all charges go into soliton-like states. The measurements are in quantitative agreement with charge transport via variable range hopping among soliton-like levels. A parallel conduction mechanism of thermal activation of charge carriers to ‘high mobility’ extended band states is proposed, and may dominate in less disordered samples.