Impurity Modes Research Articles

Polarized Raman scattering of impurity modes in beryllium-doped cubic boron nitride single crystals

Impurity-induced modes at 212 and 535 cm−1 have been studied by using Raman scattering spectroscopy in Be-doped cubic boron nitride single crystals, which have been grown by the temperature-gradient method under high pressure. Those bands show prominent polarization properties of T2 and A1 modes for the 212 and 535 cm−1 bands, respectively. Based on the polarization properties and a simple calculation of a molecular model, the origin of the bands is explained by postulating resonance modes of substitutional Be with a weak force constant between the host atoms and the Be. Anomalies of a forbidden transverse optical mode and the T2 mode due to phonon–plasma coupling caused by photoexcited free carriers have been also observed in high excitation density.

Impurity Mode in Microstrip Line Photonic Crystal in Millimeter Wave Region

One-dimensional photonic crystals were fabricated by using a periodic microstrip line with a lattice constant of 16.3 mm. Wide band gaps around 5 GHz were observed by a network analyzer. Microstrip lines with an impurity line that disturbed the periodicity were also fabricated and a sharp transmission peak, which is due to the impurity mode, was observed in the band gap region. The measured transmission and reflection spectra were in very good accordance with those calculated by the method of moments. The Q factor of the impurity modes was strongly dependent on the number of unit cells of the periodic lines adjacent to the impurity line and also on the value of the dielectric loss of the substrate material. The spatial distribution of the electric field of the transmitting modes and the impurity modes of the electromagnetic wave were simulated using the method of moments, and localization of the impurity modes was clearly evident.

Instabilities and bifurcations of nonlinear impurity modes.

We study the structure and stability of nonlinear impurity modes in the discrete nonlinear Schrödinger equation with a single on-site nonlinear impurity emphasizing the effects of interplay between discreteness, nonlinearity, and disorder. We show how the interaction of a nonlinear localized mode (a discrete soliton or discrete breather) with a repulsive impurity generates a family of stationary states near the impurity site, as well as examine both theoretical and numerical criteria for the transition between different localized states via a cascade of bifurcations.

Characterisation of intrinsic and compensated defect microstructures in dilute III–V–N alloys

A comprehensive analysis of the Fourier transform infrared (FTIR) absorption and Raman scattering data on impurity modes is reported for the technologically important dilute ternary GaAs 1-x N x , and GaP 1-x N x (x x > 0.015), the formations of nonradiative complex microstructures involving N and/or intrinsic defects are energetically favourable. The impurity modes for such complex centres were predicted and the possibility of observing them by optical spectroscopy was evaluated.

Crystalline structure and dielectric properties of (Sr1-1.5xBix) TiO3 ceramics

The crystalline structure, microstructure and dielectric properties of the (Sr1−1.5xBi x )TiO3 (0 ≤ x ≤ 0.267) ceramics were studied. Cubic solid solutions were determined for x ≤ 0.2 at room temperature. However, lattice distortion was detected by Raman spectra. A dense microstructure with the grain sizes of 2–4 μm was obtained for (Sr1−1.5xBi x )TiO3 (0 ≤ x ≤ 0.2) ceramics. The Bi concentration was examined and found to be in agreement with the nominal composition and overall uniformly distributed in the sample. Different from the observations in the earlier literature for other doped quantum paraelectrics, where only an induced dielectric anomaly was reported, there are three Bi induced dielectric modes A, B, and C in the Bi doped SrTiO3 samples. The occurrence of the “impurity modes” and the “ferroelectric relaxor mode” and their evolution are demonstrated as a function of Bi concentration.

Improvement of solar cell efficiencies by impurity transitions

Abstract Transitions via impurities, in addition to transitions from the valence band to the conduction band, increase the generation rate of electron–hole pairs. By the presence of impurities, however, the recombination rate is also increased. In this study, the recombination process is assumed to be entirely radiative. The probability for photon absorption in generation and for photon emission in recombination depend on the occupation probability of the impurity states. The variation of the occupation resulting in a variation of the optical properties along a current–voltage characteristic is taken into account. With a single impurity state a maximal efficiency of 46% for the AM0 spectrum is found for a band gap of 2.3 eV and an impurity level at 0.88 eV. No improvement is found for band gaps less than 1.2 eV. For two different impurity levels, transitions between these levels must be excluded, otherwise the lower level is mostly occupied and the higher level is mostly empty leading to poor optical absorption for some of the possible impurity transitions.

Electrical characterization of acceptor levels in Be-implanted GaN

We have investigated electrically the acceptor levels that are present in Be-implanted GaN. Slight p-type conductivity was attained in undoped GaN films by Be implantation and subsequent annealing at 1050 °C with a SiO2 encapsulation layer. Capacitance-frequency measurements showed a typical dispersion effect characteristic of deep acceptors in fabricated Schottky diodes. Thermal admittance spectroscopy measurements revealed a discrete deep level located at ∼231 meV above the valence band. This energy level is in reasonable agreement with the frequency dependence of the capacitance in view of the impurity transition frequency. Therefore, this energy level can most probably be assigned to a Be-related deep acceptor.

Infrared absorption by pairs of group III and V impurities in silicon

Infrared absorption bands, shifted to regions of lower energies relative to narrow lines of transitions of impurities to excited states, are investigated in silicon doped with group III and V impurities in concentrations above 1016 cm−3. It is found that the band structure is peculiar to each of the investigated impurities but independent of their concentrations, and the absorption coefficients in the bands increase approximately quadratically with concentration. This leads one to infer that the bands are caused by the excitation of charge carriers bound on impurity pairs localized within several Bohr radii.

Magnetic moments of transition impurities in antiperovskites

Starting from a first principles electronic structure calculation for superconducting antiperovskite MgCNi 3 and using a simple Hartree–Fock approximation, we calculate the magnetic moment of transition impurities T in the dilute limit. The calculation shows that Co impurities are non-magnetic and for the remaining ones at least a small moment exists. Experimental studies of the change with concentration of the superconducting temperature T c may indicate, for non-magnetic impurities, a d-wave behavior of these superconductors.

E 0 photoreflectance spectra of GaAs: Identification of the features related to impurity transitions

In the room-temperature photoreflectance spectra of moderately doped crystalline GaAs wafers near the fundamental E0 critical point, components corresponding to impurity transitions were identified. This was done by phase analysis of the spectra, and their variation with the laser-excitation density was examined. The dependences of the impurity component magnitude and the retardation phase on the excitation density were determined experimentally. It was established that the impurity component is strongly affected by the condition of the sample surface.

Terahertz emission from silicon doped by shallow impurities

A new approach to generate stimulated emission in the terahertz (THz) range of the electromagnetic spectrum is presented. At low temperatures, population inversion between excited states of shallow impurity centers in silicon can occur due to peculiarities of electron–phonon interaction. The terahertz stimulated emission from impurity transitions was observed from bulk monocrystalline silicon doped by group V elements (P, Bi), excited by CO 2 laser radiation.

Interaction of sine-Gordon kinks with defects: phase space transport in a two-mode model

We study a model derived by Fei et al. [Phys. Rev. A 45 (1992) 6019] of a kink solution to the sine-Gordon equation interacting with an impurity mode. The model is a two degree of freedom Hamiltonian system. We investigate this model using the tools of dynamical systems, and show that it exhibits a variety of interesting behaviors including transverse heteroclinic orbits to degenerate equilibria at infinity, chaotic dynamics, and an extremely complex and delicate structure describing the interaction of the kink with the defect. We interpret this in terms of phase space transport theory.

Coupling of impurity modes in one-dimensional periodic systems.

One-dimensional periodic dielectric structures are known to exhibit band gaps because of their symmetry. Defect states can be found in the band gaps if an impurity layer is added to the lattice such that the symmetry of the structure is broken. In this paper, we consider the case where a second impurity layer is added and we discuss the existence of coupling between the two defects. We discuss the possibility of exploiting the coupling of impurity modes in the realization of tunable wavelength emitting devices and dual-wavelength vertical-cavity surface-emitting lasers.

Shallow-deep transitions of impurities in semiconductor nanostructures

We study the hydrogenic impurity in a quantum dot (QD). We employ the effective mass theory with realistic barrier and variable effective mass. The model is simple, but it predicts features not previously observed. We observe that the shallow hydrogenic impurity becomes deeper as the dot size (R) is reduced and with further reduction of the dot size it becomes shallow and at times resonant with the conduction band. Such a shallow-deep (SHADE) transition is investigated and a critical size in terms of the impurity Bohr radius (aI*) is identified. A relevant aspect of a QD is reduction in the dielectric constant, ε, as its size decreases. Employing a size dependent ε(R), we demonstrate that the impurity level gets exceptionally deep in systems for which aI* is small. Thus, carrier “freeze out” is a distinct possibility in a wide class of materials such as ZnS, CdS, etc. The behavior of the impurity level with dot size is understood on the basis of simple scaling arguments. Calculations are presented for III–V (AlGaAs) and II–VI (ZnS, CdS) QDs. We speculate that the deepening of the impurity level is related to the high luminescence efficiency of QDs. It is suggested that quantum dots offer an opportunity for defect engineering.

Nonlinearity and disorder: classification and stability of nonlinear impurity modes.

We study the effects produced by competition of two physical mechanisms of energy localization in inhomogeneous nonlinear systems. As an example, we analyze spatially localized modes supported by a nonlinear impurity in the generalized nonlinear Schrödinger equation and describe three types of nonlinear impurity modes, one- and two-hump symmetric localized modes and asymmetric localized modes, for both focusing and defocusing nonlinearity and two different (attractive or repulsive) types of impurity. We obtain an analytical stability criterion for the nonlinear localized modes and consider the case of a power-law nonlinearity in detail. We discuss several scenarios of the instability-induced dynamics of the nonlinear impurity modes, including the mode decay or switching to a new stable state, and collapse at the impurity site.

Competition between deep impurity and dopant behavior of Mg in GaN Schottky diodes

The effect of the deep acceptor Mg on the electrical characteristics of p-doped GaN Schottky diodes is analyzed. The theoretical study is based on the numerical resolution of the basic semiconductor equations, including the continuity equation for the Mg-related acceptor level. It gives the steady-state and small-signal analysis of p-doped GaN:Mg Schottky diodes, yielding as final result the frequency dependent capacitance and conductance of the structure. It is shown that the low-frequency characteristics are determined by the carrier exchange between the Mg related impurity level and the valence band, whereas above the impurity transition frequency, the hole modulation of the depletion layer edge governs the electrical response. Detailed results are shown on the effect of temperature, applied steady-state voltage and series resistance. The study of two back-to-back connected GaN Schottky diodes reveals the appearance of typical features in the electrical characteristics, depending on the respective Schottky barrier height of the two junctions.

Theoretical Study of s-p, Noble and Transition Impurities in 3d-Ferromagnetic Hosts: Magneticand Hyperfine Field Properties

A description of local properties, hyperfine fields and local magnetic moments, of dilute impurities in 3d-ferromagnetic hosts, including the next neighbor perturbation is presented. The cases of s-p and noble impurities are described within an extension of the Daniel–Friedel model. The case of nd (n=3,4,5) impurities is described within the Wolff–Clogston picture, the Hartree–Fock approximation for the electron–electron correlation being 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 available experimental data

Electric field broadening of arsenic donor states in strongly compensated n-type Ge:(As, Ga)

We discuss on the broadening of ground-state to bound excited-state transitions of shallow donors in strongly compensated n-type Ge in the presence of electric fields and their gradients arising from randomly distributed ionized impurities. Low-temperature ( T=3.2 K ) far-infrared absorption spectra of strongly compensated n-type Ge:(As,Ga) have been obtained for samples having ionized impurity concentration N I =2.2×10 13–2.6×10 14 cm −3 . Absorption peaks corresponding to 1s–2p ± transition of arsenic impurities are observed, and broadened linearly with the ionized impurity concentration due to interactions between electrons and the quadrupole moments of ionized donors. The slope of the peak line width plotted against N I changes at N I ≈8×10 13 cm −3 due to the transition of ionized impurity distribution from random ( N I <8×10 13 cm −3 ) to correlated ( N I >8×10 13 cm −3 ).

Impurity modes in a two-dimensional photonic crystal: coupling efficiency and Q factor

A finite two-dimensional photonic crystal with a triangular lattice of air columns in a dielectric background is designed with a waveguide and a resonant cavity. By a time-domain solution of Maxwell’s equations we observe the existence of impurity modes inside the photonic bandgap. An electromagnetic wave launched through the waveguide, with an appropriate frequency, can enhance the field inside the cavity and yield a peak in the transmission. Considering a manufactured structure, in which the columns are not perfectly equal, we repeat our calculations and analyze how the impurity modes are modified. For both cases, periodic and real, we measure the quality factor of the cavity.

Computational study of spin transitions in BaLiF3 containingparamagnetic impurities

The aim of this work is the computational study of the electronic andmagnetic properties of [FeO4]5- clusters diluted in a cubichost crystal: BaLiF3. Experimental results show that these systemsdisplay a bistable magnetic behaviour (intermediate spin S = 3/2-high spinS = 5/2 transition), which can be reversibly controlled by temperature orlight illumination. Our calculations, using density functional theory forboth molecular and crystalline models, on two different [FeO4]5- structures, one with D4h symmetry andanother one with C2v symmetry, lead to results that are ingood qualitative agreement with experimental results: the structure withD4h symmetry has a quartet ground state and the structurewith C2v symmetry has a sextet ground state. The observedspin transition may be understood as a consequence of oxygen jumpings intoneighbouring fluorine vacancies in the BaLiF3 matrix under lightor thermal excitation, which would allow transformation from one [FeO4]5- configuration into the other.