Impurity Modes Research Articles

Impurity modes in the optical stop bands of doped colloidal crystals.

~Received 12 June 1996! Colloidal suspensions of charged polystyrene microspheres can self-organize into colloidal crystals, providing a three-dimensional periodic dielectric medium suitable for the study of photonic band structure effects in the infrared, near-infrared, and visible wavelength regimes. Light propagation along any particular direction in such a system displays bands of frequencies for which the propagation of light is strongly inhibited. Substitutional doping of these colloidal crystals with scatterers of slightly different size or different dielectric strength introduces optical disorder into these crystals which is manifested by the appearance of impurity modes within the optical stop bands. These defect modes can be tuned by appropriate selection of the defect volume and the dielectric constants of the dopants. Widening of optical stop bands with increasing impurity peak heights was observed in the doped crystals. @S0163-1829~96!02644-6#

Far Infrared Spectroscopy of Semiconductors at Large Hydrostatic Pressures

AbstractThe combination of high resolution far infrared spectroscopy and large hydrostatic pressures produced by diamond anvil cells (DAC) offers a unique approach to the study of dopants, defects, and bandstructure related properties of semiconductors. The problem of small optical throughputs typical of DACs has been solved. Using this unique tool, we have obtained several new results, including the discovery of a local vibrational mode of the DX center in GaAs:Si and the demonstration that the DX center is a negative‐U defect. We have also discovered the first evidence of DX centers in InP and studied the ground to bound excited state transitions of shallow impurities in GaAs at pressures at which GaAs is an indirect bandgap semiconductor with its conduction band minimum near the X symmetry point.

Host-isotope fine structure of local and gap modes of substitutional impurities in zinc-blende and wurtzite II-VI semiconductors.

The frequency of a local vibrational mode (LVM) of a light impurity substituting for either the group II or the group VI host atom in a II-VI semiconductor (e.g., ${\mathrm{Mg}}^{2+}$ replacing ${\mathrm{Zn}}^{2+}$ in ZnTe) exhibits two types of isotope shifts: in one, the shift is caused by the specific isotope of the impurity, whereas in the other a shift occurs in association with the isotopes of the nearest neighbor (NN) atoms of the impurity, Te in the above case. Depending on the number of isotopes of the host atoms, a variety of isotopic configurations in the surrounding ``cage'' of NN's can occur. The striking similarity of this host-isotope fine structure for the same type of NN's (e.g., Te in ZnTe and CdTe), irrespective of the impurity (X) and its isotopes, justifies the LVM to be associated with the high-frequency, infrared active, triply degererate (${\mathrm{\ensuremath{\Gamma}}}_{5}$) mode of an ${\mathit{XY}}_{4}$ molecule, the four Y's being the NN's. A numerical calculation of the normal modes of such an ${\mathit{XY}}_{4}$ molecule, using stretching and bending force constants, reproduces the entire host-isotope fine structure of an LVM with remarkable fidelity when all the isotopic NN combinations are appropriately taken into account. When the impurity has a mass such that in addition to a local mode a gap mode results, the numerical calculations correctly predict the absence of observable isotopic shifts in the gap mode ascribed to the lower-frequency ${\mathrm{\ensuremath{\Gamma}}}_{5}$ mode. When the substitutional impurity replaces the lighter host atom (while being heavier), the local mode is absent and only a gap mode can occur and can be associated with the high-frequency ${\mathrm{\ensuremath{\Gamma}}}_{5}$. If only a gap mode is to be expected as for ${\mathrm{Se}}^{2\mathrm{\ensuremath{-}}}$ in ZnS, both computer calculation as well as experimental observation display striking isotopic shifts associated with both the impurity and the host atoms. The model also satisfactorily describes the anisotropic effects for an LVM in a wurtzite host (e.g., ${\mathrm{Mg}}^{2+}$ in CdSe). \textcopyright{} 1996 The American Physical Society.

Green's-function theory for row and periodic defect arrays in photonic band structures.

The defect modes associated with a row or a periodic array of defects in a two-dimensional photonic band structure are studied using exact Green's-function methods. Specifically, we consider the above-described defect problems created in a pure photonic band-structure system that models an experimental system recently investigated by McCall et al. This is a square lattice array of cylindrical rods formed of linear dielectric material with \ensuremath{\epsilon}=9 surrounded by vacuum and of filling fraction f=0.4488. In one study a row of rods is replaced in the photonic band structure by a row of impurity rods that may be of either linear or nonlinear dielectric material, and conditions are determined for the existence of impurity modes, associated with the impurity array, in the photonic band gap. The electric fields of the impurity modes are computed for impurity modes in the gaps. In a second study, a two-dimensionally periodic array of defects is introduced into the photonic band structure such that the periodicity of the defect array may or may not coincide with the periodicity of the original photonic band structure. The defect rods are treated in the cases for which the impurities are formed from both linear and nonlinear dielectric materials, and the conditions necessary for defect modes to exist in the photonic band gaps are determined. \textcopyright{} 1996 The American Physical Society.

Using far-infrared two-photon excitation to measure the resonant-polaron effect in the Reststrahlen band of GaAs:Si

We demonstrate that we can use far-infrared two-photon excitation with picosecond pulses to measure the resonant polaron effect of the 1s-3d +2 silicon impurity transition in the middle of the Reststrahlen band of GaAs:Si. Contrary to single-photon measurements, our two-photon measurements do not suffer from the problems of strong reflection and small penetration depth of light in the Reststrahlen band of the GaAs host lattice. As a result, we are able to measure a significant fraction of the magnetic-field dependent anticrossing of the 1s-3d +2 transition with the LO-phonon.

GENERATION OF MID-INFRARED RADIATION BY IMPACT IONIZATION OF DEEP-CENTERS IN SEMICONDUCTOR STRUCTURES

Narrow-band mid-infrared electroluminescence associated with internal transitions of metal impurities embedded in semiconductors following the impact ionization of the latter is reported. We have demonstrated this phenomenon for Fe2+ deep acceptors in InP. At 10 K the spectrum is dominated by a set of sharp lines at 3.5 µm, corresponding to the symmetry allowed (5T2→5E) d-shell transitions of Fe2+. Optical power up to ≈40 nW has been measured.

Nonlinear Quantum Dynamics of Strong Vibration: Relaxation Jumps and Phonon Bursts*

We examine quantum decay of localized vibrations in anharmonic crystal lattice. The theory which describes two-phonon anharmonic relaxation can be applied both to local modes associated with substitutional impurity and to intrinsic local modes (ILM) in perfect lattices. It is found that for sufficiently high initial excitations relaxation of vibrations is non-exponential, it demonstrates explosion-like behavior at specific stages of evolution. The course of the relaxation is determined by the initial value of energy, temperature, direction of vibrations. As an example we present the results of calculations of the relaxation of an odd local (impurity) mode in a simple cubic lattice and discuss the influence of quantum fluctuations on the stability of the ILM in one-dimensional monatomic chain.

Morphotropic Phase Boundary in the Nanocrystalline PbZrxTi1-xO3 System

AbstractMicro Raman spectra for PbZrxTi1-xO3 (PZT) with a grain size of 60 nm have been recorded. The results show that the lowest E(TO) phonon mode of PbTiO3 (soft mode) displays a decrease in frequency and increase in linewidth with increasing Zr concentration. A discontinuous behavior of the phonon energy of the soft mode occurs at x=0.4 and it can be attributed to a phase transformation from the tetragonal ferroelectric to rhombohedral ferroelectric phase in PZT. The coupling between the soft mode and an additional impurity mode, has been observed, which plays an important role in the phase transformation. A lowest phonon mode at 10 cm−1 is detected and it is found to exhibit an increase in intensity for x above 0.2. The grain size dependence of the mode seems to be associated with the effect of grain size on the microstructure in PZT. The mechanism of grain size - induced new morphotropic phase boundary, which is lower than that of the corresponding bulk materials, is discussed.

Breatherlike impurity modes in discrete nonlinear lattices.

We investigate the properties of a disordered generalized discrete nonlinear Schr\"odinger equation, containing both diagonal and nondiagonal nonlinear terms. The equation models a linear host lattice doped with nonlinear impurities. We find different types of impurity states that form itinerant breather states in the vicinity of the impurities. We analyze the properties of these breathers analytically and numerically.

Transition-metal impurities in Fe: Magnetic- and hyperfine-field properties.

A description of the local properties, hyperfine field, and local magnetic moment of dilute transition impurities in Fe, including the next-neighbor perturbation is presented. The Hartree-Fock approximation for the electron-electron interaction is adopted, as in the usual description of these alloys. The use of only a few parameters and a simple self-consistent procedure is enough to account for the observed experimental trends in the hyperfine fields of these systems.

Studies of impurity mode and ion temperature gradient mode in toroidal plasmas

The impurity mode and ηi mode driven by impurity ions with outwardly peaked density profiles, near the boundary of tokamak plasmas, and the ion temperature gradient, respectively, are studied in high-temperature toroidal plasmas. The gyrokinetic theory is applied and finite Larmor radius effects of both hydrogenic and impurity ions are included. It is found that the impurity mode is enhanced by the ion temperature gradient. In addition, the impurity ions with outwardly peaked density profiles are demonstrated to have destabilizing effects on the ηi mode. These two modes are strongly coupled to each other so that it is impossible to distinguish between them when both the driving mechanisms are strong enough to drive the corresponding mode unstable independently. The correlation of the results with nonlinear simulations and the experimental observations are discussed.

Compositional dependence of cation impurity gettering in Hg1−xCdxTe

Cation impurity gettering in Hg1−xCdxTe is described in the context of process models which include the interactions of the impurities and the dominant native point defects. Experimental results are presented using secondary ion mass spectroscopy (SIMS) profiles of Au redistribution in Hg1−xCdxTe (x = 0.2,0.3,0.4) following Hg anneals and ion mills, which are processes known to inject excess Hg interstitials. In either process, the IB impurity distributes preferentially to high vacancy regions. The junction depth of the low to high impurity transition is determined by SIMS. For Hg-rich anneals of Au-doped high vacancy concentration material, the impurity junction behavior with respect to anneal time and temperature is compared to that expected for type converted electrical junctions in vacancy-only material. For milled Au-doped Hg0.7Cd0.3Te with a high vacancy concentration, the impurity junction depths are approximately proportional to the amount of material removed, as was the case with x = 0.2 material. Hg anneal type-conversion rates are found to have a strong compositional dependence which compares favorably with the strong self-diffusion coefficient dependence on x-value. In contrast, the mill conversion rate has a weak x-value dependence. Effects of trace vs dominant Au levels compared to the background vacancy concentration are quantified. True decoration of intrinsic defect processes requires Au <<[Cation Vacancies].

Magnetic excitations in praseodymium alloys

We have studied the effect of alloying on the magnetic excitations in Pr with the PRISMA spectrometer at ISIS. The dispersion of the excitations in single crystals of Pr 0.9Tm 0.1 and Pr 0.9Ce 0.1 along the a *- and c *-directions has been compared with that in pure Pr. We find that alloying Pr with Tm changes the excitation spectrum very significantly, due to strong interactions with the Tm impurity mode, whereas alloying with Ce produces more subtle changes.

Mode assignment for magnetic excitations associated with Co2+ impurities in antiferromagnetic FeF2.

We have performed far-infrared (FIR) absorption and Raman-scattering measurements on the Co-${\mathrm{FeF}}_{2}$ system for Co concentrations 0.05\ensuremath{\le}x\ensuremath{\le}5%, in magnetic fields 0\ensuremath{\le}H\ensuremath{\le}30 kG applied parallel to the crystal c axis. In addition to a careful study of the H dependence of the previously reported impurity associated mode at 85 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, we have observed an impurity-induced shift in the host two-magnon FIR absorption, and an impurity-pair excitation in Raman scattering at a frequency of 223 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Taken together, these results are shown to be inconsistent with the previous interpretation of the 85 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ mode as being localized on the impurity spin (i.e., an ${\mathit{s}}_{0}$ mode). Instead, from a mean-field cluster model, we show that the new observations are only consistent with an assignment of the original resonance to be that of a shell mode (${\mathit{s}}_{1}$). All of the data agrees with this assignment within the framework of the impurity Green's-function theory as well. The 85 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ resonance is, to our knowledge, the first ${\mathit{s}}_{1}$ impurity mode to have been observed, a not surprising result when one considers the rather stringent conditions on localization of an ${\mathit{s}}_{1}$ mode contained in Tonegawa's Green's-function theory.

Raman scattering spectra of indium-doped PbTe

The non-polarized Raman scattering spectra of indium-doped PbTe single crystals were measured in the temperature range between 10 and 300 K. Well resolved peaks at about 68, 126 and 143 cm-1 were observed for all temperatures. An additional mode appears at about 115 cm-1 for temperatures below 100 K. The intensity of this mode increases sharply when the temperature is lowered below 25 K, the temperature where a persistent photoconductivity effect in PbTe(In) appears. This mode is assigned as a local In impurity mode and represents a population of metastable states due to the transfer of electrons from two-electron to one-electron metastable impurity states.

Characterization of high purity GaAs far-infrared photoconductors

In this paper we report the results of an extensive study on the far-infrared photoconductivity of high purityn-type GaAs. The crystal, which was grown at Max-Plank-Institute for Solid State Physics using liquid-phase epitaxy, exhibited the fine structures of the excited state transitions of the residual shallow level impurities. The major peak in the spectral response belongs to the 1s-2p transition, with its responsivity about thirty five times higher than the continuum. At 3.4K detector temperature, 625 mV bias, and 100 Hz chopping frequency the detector responsivity at 35.4 cm−1 (279 µm) was measured to be 0.017 A/W. Under these same conditions, the NEP was 5.9×10−14 W/√Hz. The (DC) dark current at 25 mV bias was 5.6×10−14 A.

ITG and DTE mode driven transport in a contaminated toroidal plasma

The electrostatic quasi-linear particle and thermal diffusivities, due to the toroidal ion temperature gradient (ITG) driven modes and the dissipative trapped electron (DTE) mode, are considered in the dissipative limit, ω << vef/ϵ. Impurities are included by assuming a plasma with three particle species (electrons, main ions and impurity ions). The diffusivities are studied by computing interpretative radial transport profiles based on a TEXTOR background variation, using a saturation level approximation. The transport is found to be driven also by impurity modes, in particular when a light impurity species, such as helium, is assumed. The existence of inward fluxes of particles and heat is discussed

Asymmetric impurity modes in nonlinear lattices.

We analyze spatially localized large-amplitude oscillations in the vicinity of an isotopic impurity. We show that, in addition to a symmetric nonlinear impurity mode found earlier [see, e.g., Yu. S. Kivshar, Phys. Lett. A 161, 80 (1991)], there exists an asymmetric impurity mode which appears as a result of a bifurcation from the symmetric mode. The structure of this asymmetric impurity mode is calculated analytically in the three-particle approximation and its stability is demonstrated by direct numerical simulations.

Effects of an external magnetic field on shallow donor levels in semiconductors

The recent progress on investigation of the effects of external magnetic field on shallow impurity centers in semiconductors is discussed. As an example, the shallow donor levels of phosphorus in ultra pure silicon are investigated under magnetic fields and by use of photothermal ionization spectroscopy(PTIS). By selecting the configuration of B∥∥ kββ 〈100〉, the linear and quadratic Zeeman splittings of phosphorus donor states are observed. By selecting the configuration of B∥∥k∥∥ 〈111〉, the anticrossing between hybridized Zeeman states and the intensity evolution of Zeeman transitions near the anticrossing region are observed and measured quantitatively, which, in turn, gives the measurement of wavefunction composition and their evolutions with magnetic fields fox the hybridized Zeeman states of bounded electrons of impurities. An extension of Faulkner's method is made and a variational calculation in the framework of the effective mass approach(EMA) is performed to estimate theoretically the effects of magnetic fields on shallow donor levels and impurity transitions, especially to estimate the wave function compositions of hybridized states. A comparison has been made between the theoretical and experimental results.

Impurity modes from impurity clusters in photonic band structures

A Green function calculation for the impurity modes of single impurities and clusters of impurities in a photonic band structure is given. Specifically, a truncated two-dimensional periodic dielectric medium composed from a square lattice array of dielectric rods placed perpendicularly between two parallel perfectly conducting plates is studied. Impurities are introduced into this system substitutionally by replacing one (single impurity case) or a number (cluster impurity case) of the rods in the periodic system by impurity rods. Exact expressions for the impurity mode frequencies are obtained in terms of the Green functions of the pure system. These expressions are then evaluated for the case of impurities with small cross-sectional areas using numerically determined Green functions. Linear frequency-independent, linear frequency-dependent and non-linear impurity dielectric media are treated.