We report values of pseudodielectric functions $〈\ensuremath{\epsilon}〉=〈{\ensuremath{\epsilon}}_{1}〉+i〈{\ensuremath{\epsilon}}_{2}〉$ measured by spectroscopic ellipsometry and refractive indices $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}=n+ik$, reflectivities $R$, and absorption coefficients $\ensuremath{\alpha}$ calculated from these data. Rather than correct ellipsometric results for the presence of overlayers, we have removed these layers as far as possible using the real-time capability of the spectroscopic ellipsometer to assess surface quality during cleaning. Our results are compared with previous data. In general, there is good agreement among optical parameters measured on smooth, clean, and undamaged samples maintained in an inert atmosphere regardless of the technique used to obtain the data. Differences among our data and previous results can generally be understood in terms of inadequate sample preparation, although results obtained by Kramers-Kronig analysis of reflectance measurements often show effects due to improper extrapolations. The present results illustrate the importance of proper sample preparation and of the capability of separately determining both ${\ensuremath{\epsilon}}_{1}$ and ${\ensuremath{\epsilon}}_{2}$ in optical measurements.

Dielectric functions and optical parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV

The Al x Ga1−x As/GaAs heterostructure system is potentially useful material for high‐speed digital, high‐frequency microwave, and electro‐optic device applications Even though the basic Al x Ga1−x As/GaAs heterostructure concepts are understood at this time, some practical device parameters in this system have been hampered by a lack of definite knowledge of many material parameters Recently, Blakemore has presented numerical and graphical information about many of the physical and electronic properties of GaAs [J S Blakemore, J Appl Phys 5 3, R123 (1982)] The purpose of this review is (i) to obtain and clarify all the various material parameters of Al x Ga1−x As alloy from a systematic point of view, and (ii) to present key properties of the material parameters for a variety of research works and device applications A complete set of material parameters are considered in this review for GaAs, AlAs, and Al x Ga1−x As alloys The model used is based on an interpolation scheme and, therefore, necessitates known values of the parameters for the related binaries (GaAs and AlAs) The material parameters and properties considered in the present review can be classified into sixteen groups: (1) lattice constant and crystal density, (2) melting point, (3) thermal expansion coefficient, (4) lattice dynamic properties, (5) lattice thermal properties, (6) electronic‐band structure, (7) external perturbation effects on the band‐gap energy, (8) effective mass, (9) deformation potential, (10) static and high‐frequency dielectric constants, (11) magnetic susceptibility, (12) piezoelectric constant, (13) Frohlich coupling parameter, (14) electron transport properties, (15) optical properties, and (16) photoelastic properties Of particular interest is the deviation of material parameters from linearity with respect to the AlAs mole fraction x Some material parameters, such as lattice constant, crystal density, thermal expansion coefficient, dielectric constant, and elastic constant, obey Vegard’s rule well Other parameters, eg, electronic‐band energy, lattice vibration (phonon) energy, Debye temperature, and impurity ionization energy, exhibit quadratic dependence upon the AlAs mole fraction However, some kinds of the material parameters, eg, lattice thermal conductivity, exhibit very strong nonlinearity with respect to x, which arises from the effects of alloy disorder It is found that the present model provides generally acceptable parameters in good agreement with the existing experimental data A detailed discussion is also given of the acceptability of such interpolated parameters from an aspect of solid‐state physics Key properties of the material parameters for use in research work and a variety of Al x Ga1−x As/GaAs device applications are also discussed in detail

GaAs, AlAs, and AlxGa1−xAs: Material parameters for use in research and device applications

Modern nanotechnology allows one to scale down various important devices (sensors, chips, fibers, etc.) and thus opens up new horizons for their applications. The efficiency of most of them is based on fundamental physical phenomena, such as transport of wave excitations and resonances. Short propagation distances make phase-coherent processes of waves important. Often the scattering of waves involves propagation along different paths and, as a consequence, results in interference phenomena, where constructive interference corresponds to resonant enhancement and destructive interference to resonant suppression of the transmission. Recently, a variety of experimental and theoretical work has revealed such patterns in different physical settings. The purpose of this review is to relate resonant scattering to Fano resonances, known from atomic physics. One of the main features of the Fano resonance is its asymmetric line profile. The asymmetry originates from a close coexistence of resonant transmission and resonant reflection and can be reduced to the interaction of a discrete (localized) state with a continuum of propagation modes. The basic concepts of Fano resonances are introduced, their geometrical and/or dynamical origin are explained, and theoretical and experimental studies of light propagation in photonic devices, charge transport through quantum dots, plasmon scattering in Josephson-junction networks, and matter-wave scattering in ultracold atom systems, among others are reviewed.

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Fano resonances in nanoscale structures

Elastic constants for zinc-blende and wurtzite AlN, GaN, and InN are obtained from density-functional-theory calculations utilizing ab initio pseudopotentials and plane-wave expansions Detailed comparisons are made with the available measured values and with results obtained in previous theoretical studies These comparisons reveal clear discrepancies between the different sets of elastic constants which are further highlighted by examining derived quantities such as the perpendicular strain in a lattice-mismatched epitaxial film and the change in the wurtzite c/a ratio under hydrostatic pressure Trends among results for the three compounds are also examined as well as differences between results for the zinc-blende and wurtzite phases

Elastic properties of zinc-blende and wurtzite AlN, GaN, and InN

We review two strategies for growing ZnO nanowires from zinc salts in aqueous and organic solvents. Wire arrays with diameters in the nanoscale regime can be grown in an aqueous solution of zinc nitrate and hexamethylenetetramine. With the addition of poly(ethylenimine), the lengths of the wires have been increased to 25 μm with aspect ratios over 125. Additionally, these arrays were made vertical by nucleating the wires from oriented ZnO nanocrystals. ZnO nanowire bundles have been produced by decomposing zinc acetate in trioctylamine. By the addition of a metal salt to the solution, the ZnO wires can be doped with a range of transition metals. Specifically, ZnO nanowires were homogeneously doped with cobalt and showed a marked deviation from paramagnetic behavior. We conclude by highlighting the use of these solution-grown nanowire arrays in dye-sensitized solar cells. The nanowire cells showed an improvement in the charge collection efficiency over traditional nanoparticle cells.

Solution-grown zinc oxide nanowires.

Radiative transitions due to the recombination of excitons localized at neutral donor and acceptor impurities were first recognized by Haynes in the low temperature near band-gap lum-inescence of lightly doped silicon.(1) Shortly afterwards, similar transitions were identified in doped germanium(2) and subsequently in many other materials. Both no-phonon and phonon-assisted bound exciton recombinations were seen. The identified phonons are those which conserve momentum (M. C. phonons) in the indirect inter-band transitions, as is shown from a comparison between the intrinsic absorption and lumines- cence spectra. The impurity center remains in its ground electronic state during these transitions. Additional, weaker, impurity-induced luminescence lines were observed in the early work, particularly in germanium by Benoit a la Guillaume and Parodi(2) (hereafter B.P.). The transitions responsible for these bands, which were inadequately explained in the early work, are the subject of the present paper. It will be shown that they all involve recombinations in which the impurity center is left in an excited state.

New Radiative Recombination Processes Involving Neutral Donors and Acceptors in Silicon and Germanium

The optical absorption edge has been measured at many temperatures between 1.6 and 300\ifmmode^\circ\else\textdegree\fi{}K in exceptionally perfect single crystals of gallium phosphide. The corresponding transitions are of the allowed indirect type and involve the creation of free excitons and electron-hole pairs. Absorption components associated with four different phonon energies have been resolved, one of which is apparently due to a two-phonon process. The energies of the three single phonons are 12.8\ifmmode\pm\else\textpm\fi{}0.5, 31.3\ifmmode\pm\else\textpm\fi{}0.5, and 46.5\ifmmode\pm\else\textpm\fi{}1.0 meV. These are thought to be the respective energies of the transverse-acoustic, longitudinal-acoustic, and transverse-optic branches of the lattice dispersion curves at the $〈100〉$-type boundaries of the reduced zone, although the longitudinal-acoustic energy is \ensuremath{\sim}10 meV larger than previous estimates for this phonon. Indirect absorption associated with the longitudinal optic phonon has not been resolved, apparently because these transitions are forbidden via the energetically favorable conduction-band minimum ${\ensuremath{\Gamma}}_{1}$. The internal binding energy of the indirect exciton is 10.0\ifmmode\pm\else\textpm\fi{}1.0 meV. Fine structure, probably associated with an excited state of the indirect exciton and also with anomalies in the combined densities of states for the exciton and the acoustical phonons, has been resolved in the low-temperature spectra. The indirect energy gap is 2.339\ifmmode\pm\else\textpm\fi{}0.002 eV at 1.6\ifmmode^\circ\else\textdegree\fi{}K and 2.259\ifmmode\pm\else\textpm\fi{}0.003 eV at 300\ifmmode^\circ\else\textdegree\fi{}K. These values are considerably larger than previous estimates, especially at 300\ifmmode^\circ\else\textdegree\fi{}K. The consequences of the larger energy gap in the quantitative analysis of donor-acceptor-pair fluorescence spectra and of the spectra of bound exitons in gallium phosphide are considered.

Intrinsic Absorption-Edge Spectrum of Gallium Phosphide

Optical studies of the phonons and electrons in gallium nitride

We have observed a large number of sharp lines in photoluminescence spectra of ZnSe at 4.2\ifmmode^\circ\else\textdegree\fi{}K, leading to a broad band at lower energy. The peak energy of this band increases from \ensuremath{\sim}2.70 to \ensuremath{\sim}2.72 eV with large increase in excitation intensity. The energies and intensities of the sharp lines are consistent with recombinations between distant donor-acceptor pairs. This is the first report of such a spectrum from a II-VI compound or any direct gap semiconductor. Two high-energy pair spectra (HEPS) occur with similar patterns of sharp lines mutually displaced by 2.0\ifmmode\pm\else\textpm\fi{}0.2 meV. A lower-energy pair spectrum (LEPS), with peak near 2.69 eV, also often occurs in ZnSe. Sharp lines have not been resolved in the LEPS. A high-energy series of broad bands, whose properties suggest free-to-bound recombinations, is readily seen in association with the LEPS, but not with the HEPS. The thermal-quenching rate is greater for the LEPS than for the HEPS. This difference in behavior can be understood if the donor and acceptor in the HEPS have comparable ionization energies, while these energies differ considerably in the LEPS. The identities of the donors and acceptors are unknown.

Pair Spectra and "Edge Emission" in Zinc Selenide

Absorption and photoluminescence spectra of excitons weakly bound to sulphur, selenium, and tellurium donors in gallium phosphide have been studied at 25\ifmmode^\circ\else\textdegree\fi{}K and below. Relatively weak satellite photoluminescence lines have been discovered. These photoluminescence satellites are phonon replicas, and some of the corresponding absorption satellites have been observed. Comparison with the intrinsic absorption-edge spectrum shows that some of these are the momentum-conserving (MC) phonons in the indirect transition. The more accurate estimates of these phonon energies provided from the bound exciton spectra are, (TA) 13.1\ifmmode\pm\else\textpm\fi{}0.1 meV, (LA) 31.5\ifmmode\pm\else\textpm\fi{}0.1 meV, (TO) 45.3\ifmmode\pm\else\textpm\fi{}0.1 meV (sulphur spectrum). Replicas associated with the zone-center optical phonons, of energy 45.4\ifmmode\pm\else\textpm\fi{}0.1 meV (TO) and 50.1\ifmmode\pm\else\textpm\fi{}0.1 meV (LO), are also prominent. Luminescence spectra associated with the relatively heavy donors selenium and tellurium also contain prominent 23- and 47-meV phonon replicas which do not appear in absorption. These bands are apparently associated with "in-band resonance" local modes occurring at possible regions of low density of thelattice modes. The relative intensity of adjacent Mc (LA) replicas is increased when the local modes are prominent. Apart from this apparent interference with the local modes, the intensities of the optical and acoustical (MC) phonon replicas vary together between spectra involving different donors, as do the intensities of the no-phonon line and the zone-center replicas. The binding energy of the exciton to the donor does not vary as expected with the ionization energy of the donor or with the strength of the coupling to the momentum-conserving phonons. An absorption satellite 40.6\ifmmode\pm\else\textpm\fi{}0.2 meV above the no-phonon lines for all three donors may also be a phonon replica, but is anomalously strong in the absorption spectra. Additional absorption satellites are apparently associated with excited states of the exciton-neutral-donor complex. The intensities of the satellites relative to the principal no-phonon line vary more than the relative transition energies between these three group-VI donors. The spectral positions of these excited states and the activation energy for thermal quenching of the luminescence intensity (29\ifmmode\pm\else\textpm\fi{}1.5 meV) suggest the liberation of free electrons and holes rather than free excitons from these complexes.

P. J. Dean

Papers

New Radiative Recombination Processes Involving Neutral Donors and Acceptors in Silicon and Germanium

Intrinsic Absorption-Edge Spectrum of Gallium Phosphide

Optical studies of the phonons and electrons in gallium nitride

Pair Spectra and "Edge Emission" in Zinc Selenide

Absorption and Luminescence of Excitons at Neutral Donors in Gallium Phosphide