Impurity Modes Research Articles

Assessment of microscopic lattice structures in dilute (AlGaIn)NAs laser materials by local mode spectroscopy and numerical simulations

In the framework of a rigid-ion model we report the results of a comprehensive Green’s function analyses of the Fourier transform infrared (FTIR) absorption and Raman scattering data on localized vibrational modes (LVMs) to investigate the microscopic lattice structures related with the nitrogen incorporation in (AlGaIn)NAs alloys. Contrary to the outcome of a recent FTIR report of LVMs on GaInNAs multiple-quantum well structures, our group-theoretical analyses of impurity vibrations support the earlier IR and Raman scattering experiments providing corroboration to the fact that upon annealing and/or by increasing In(Al) composition in GaIn(Al)NAs there occur structural changes causing N environment to transform from NAsGa4 to NAsIn(Al)Ga3 and/or NAsIn2(Al2)Ga2 microstructures. From the force variation correlation with bond covalency for the closest mass acceptor CAs(a−) and isoelectronic NAs(i) defects in GaAs we have obtained the corrected value of u for Al–N bond predicting NAs local mode in AlAs (∼510cm−1) at a higher frequency than that of GaAs:N (471cm−1). Theoretical results of impurity modes for Al–N complexes in GaAlNAs are compared and discussed with the existing Raman data.

Nonlinear Terahertz Response ofn-Type GaAs

Excitation of an n-type GaAs layer by intense ultrashort terahertz pulses causes coherent emission at 2 THz. Phase-resolved nonlinear propagation experiments show a picosecond decay of the emitted field, despite the ultrafast carrier-carrier scattering at a sample temperature of 300 K. While the linear THz response is in agreement with the Drude response of free electrons, the nonlinear response is dominated by the super-radiant decay of optically inverted impurity transitions. A quantum mechanical discrete state model using the potential of the disordered impurities accounts for all experimental observations.

Raman scattering on Mn or Co doped ZnO crystals

AbstractSingle crystals of bulk ZnO containing up to about 3% of Mn or Co were grown by chemical vapour transport method and characterized by Raman scattering. By comparison with Raman data taken on pure ZnO samples, a few new structures were observed in the spectra of the doped crystals. The determined frequencies of the new modes were compared with existing Raman data, corresponding to ZnO crystals containing various dopants, taken from the literature. A possible attribution of the observed modes to local impurity modes is discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Tailoring planar defect in three-dimensional colloidal crystals

We have fabricated three-dimensional (3D) colloidal crystals containing a two-dimensional (2D) defect as the middle layer by the Langmuir–Blodgett (LB) technique. Either a homogeneous sheet made of 200 layers of behenic acid or a single monolayer of silica colloidal particles of various sizes was inserted between two opal films of silica spheres. The presence of the extrinsic defect led to an impurity mode within the photonic stop band, which was observed as a pass band in the near-infrared (NIR) spectra. The position of this defect mode was found to vary periodically with the value of the ratio of the thickness of the defect layer to the diameter of the colloids of the upper and lower opals.

Harmonic vibrational excitations in graded elastic networks: Transition from phonons to gradons

We have identified a different type of transition from extended to localized vibrational states in one-dimensional graded elastic chains of coupled harmonic oscillators, in which the vibrating masses or nearest-coupling force constants vary linearly along the chain. We found that the delocalization transition occurs at the maximum frequency of the corresponding homogeneous chain, which is in a continuous single band. Although each state in the localized phase, called gradon, can be regarded as an impurity localized mode, the localization profile is clearly distinct from usual impurity modes or the Anderson localized modes. We also argue how gradons may affect the macroscopic properties of graded systems. Our results can provide insights into many analogous systems with graded characters.

Magnetopolaron interactions inn-type indium phosphide

Broadband far-infrared absorption spectroscopy is used to investigate $n$-type indium phosphide in magnetic fields of up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The large energy range $(\ensuremath{\simeq}2--14\phantom{\rule{0.3em}{0ex}}\mathrm{Ry})$ and the large magnetic field range $(0\ensuremath{\leqslant}\ensuremath{\gamma}<3)$ employed permit the observation of a rich variety of magnetopolaron interactions. We report on the magnetopolaron effect for bound states in InP, beginning with the coupling of the $2{p}_{+}$ state with the $1s+\mathrm{LO}$ phonon state. We further observe the magnetopolaron effect associated with the metastable state (210). In addition we report (i) the re-emergence of the (210) impurity state transition beyond the LO phonon manifold, (ii) the possible coupling of an impurity transition with the $1s+2\mathrm{LO}$ phonon state, and (iii) both low- and high-energy one-phonon transitions of impressive richness and detail.

Phonons in multicomponent alloys

The simple model for the calculation of phonons in multicomponent alloys is presented in this paper. This model represents an extension of the models for three-component mixed crystals based on the modified random element isodisplacement (MREI) model. The starting parameters of this model are the lattice constants, the dielectric constants and the phonon modes of boundary binary compounds as well as the corresponding impurity modes. We applied this model on the quaternary alloys Hg 1 − x Mn x Te 1 − y Se y , Zn 1 − x Mg x Se 1 − y Te y , Al 1 − x − y Ga x In y P and Cd 1 − x − y Zn x Mn y Te. Obtained results for the whole range of composition are shown in the 3D form. Our results are in good agreement with the available experimental data from literature.

Impurity modes and wave scattering in discrete chains with nonlinear defect states

We study the properties of localized impurity modes and the transmission of plane waves across a general nonlinear impurity embedded in an infinite one-dimensional linear chain. Using the formalism of the lattice Green function, we obtain nonlinear equations for the bound state energy and the transmission coefficient, for a wide class of nonlinear impurities. We specialize to a saturable nonlinear impurity, obtaining closed form expressions for the bound state energy and transmission coefficient.

Shallow impurity states and transition energies in cylindricalGaAs–Ga0.6Al0.4As quantum well wires under applied magnetic fields

In this work, using the effective-mass approximation within a variational approach, we have studiedthe behaviour of the binding and transition energies of a donor shallow impurity in a cylindricalGaAs–Ga0.6Al0.4As quantum well wire (QWW) as a function of the wire radius, the impurityposition and the applied magnetic field. The QWW is of infinite lengthwith a finite radial confining potential and the magnetic field is appliedparallel to the wire axis. In our calculations we have considered the 1s-,2p ± - and3p ± -like impurity states. We have found that for the 1s-like state the impurity binding energyincreases with the magnetic field for impurity positions close to the centre of the wire,but diminishes for on-edge impurities, highlighting the competition between thegeometrical and magnetic confinement. Also, we have observed that the energy of the2p ± - and3p ± -like excited states is greater than the energy of the electron ground state without thepresence of the impurity for small radius of the QWW, a result which is more pronouncedfor higher magnetic fields. Our results are in good agreement with previous theoreticalreports, with lower binding and transition energies than those which use infiniteconfinement potential, as expected.

Impurity mode techniques applied to the study of light sources

The dispersion relation, density of states, wave-functions and single site impurity bound and resonant modes of a photonic crystal slab are computed. The photonic crystal slab is formed by embedding a hexagonal lattice array of dielectric cylinders in a background dielectric. Outside the photonic crystal slab is a uniform, isotropic dielectric medium. The solution of Maxwell's equations for the modes of a free-standing photonic crystal slab are obtained by treating a self-consistent matrix eigenvalue problem that is solved iteratively using a fast computer routine. The properties of single site impurities in photonic crystal slabs are determined using both Green's function methods and effective mass equations based on a Wannier function representation. The impurity dielectric constant supporting an impurity bound state is shown to be a multiple valued function of the frequency of the impurity mode. Asymptotic forms are obtained for the behaviour of impurity bound states at frequencies in the neighbourhood of the band edges. Resonant modes at pass band frequencies are computed for the impurities formed by replacing a single dielectric cylinder of the photonic crystal slab with background dielectric media. Comparison of the theoretical results is made with experiments on an electrically injected photonic crystal microcavity light source.

Unintentionally doped n-type Al0.67Ga0.33N epilayers

Unintentionally doped Al0.67Ga0.33N epilayers were grown on AlN∕sapphire templates by metalorganic chemical vapor deposition. Optimized undoped Al0.67Ga0.33N epilayers exhibited an n-type conductivity as confirmed by Hall-effect measurement with a room-temperature resistivity of about 85Ωcm. Variable temperature Hall-effect measurement revealed the existence of a shallow donor level with activation energy of about 90meV. The photoluminescence (PL) spectra exhibited an emission peak at 4.13eV (4.06eV) related to an impurity transition at 10K (300K). Temperature dependent PL measurement also confirmed the existence of a shallow donor with comparable activation energy as that obtained by Hall-effect measurement. Isolated oxygen impurities are believed to be a strong candidate of the donor that remains as a shallow state in AlxGa1−xN up to x∼0.7. Compensating defects and the nature of the O donor in Al0.67Ga0.33N epilayers are also discussed.

Terahertz lasers based on germanium and silicon

Recent experimental and theoretical results of impurity doped germanium and silicon terahertz lasers are reviewed. Three different laser mechanisms exist in p-type germanium. Depending on the operating conditions and the properties of the crystal, laser transitions can occur between light- and heavy-hole subbands, between particular light-hole Landau levels or between impurity states. Electric and magnetic fields are required for laser operation. In n-type silicon lasing originates solely from impurity transitions of group-V donors, which are optically excited. The properties of these lasers depend upon the chemical nature of the impurity centre and the properties of the host material. The principles of operation are discussed in terms of their basic physical concepts. The state-of-the-art performance of these lasers is summarized.

Deep impurity transitions involving cation vacancies and complexes in AlGaN alloys

Deep ultraviolet (UV) photoluminescence (PL) spectroscopy has been employed to study deep impurity transitions in AlxGa1−xN (0⩽x⩽1) epilayers. Two groups of deep impurity transitions were observed, which are assigned to the recombination between shallow donors and two different deep level acceptors involving cation vacancies (Vcation) and Vcation complexes in AlxGa1−xN alloys. These acceptor levels are pinned to two different energy levels common to AlxGa1−xN alloys (0⩽x⩽1). The deep impurity transitions related with Vcation complexes were observed in AlxGa1−xN alloys between x=0 and 1, while those related with Vcation were only observed in AlxGa1−xN alloys between x=0.58 and 1. This points out to the fact that the formation of Vcation is more favorable in Al-rich AlGaN alloys, while Vcation complexes can be formed in the whole range of x between 0 and 1. The implications of our findings to the UV optoelectronic devices using AlGaN alloys are also discussed.

Manganese Impurity in Boron Nitride and Gallium Nitride

We carried out a theoretical investigation on the properties of manganese impurity centers in cubic boron and gallium nitrides. The calculations were performed using the all electron spin-polarized full-potential linearized augmented plane wave methodology. Our results indicate that manganese in boron nitride, in a neutral charge state, is energetically more favorable in a divacancy site as compared to a substitutional cation site. We present the results on stability, spin states, impurity magnetic moment, hyperfine parameters, and formation and transition energies of manganese at the divacancy site in several charge states.

Spectroscopic parameters of the absorption bands related to the local vibrational modes of carbon and oxygen impurities in silicon enriched with 28Si, 29Si, and 30Si isotopes

The IR spectra of all three Si isotopes in the form of bulk single crystals (28Si with an enrichment of more than 99.9%, and 29Si and 30Si with an enrichment of more than 90%) have been studied at T = 300, 17, and 5 K in the spectral range 550–1200 cm−1. The IR active local vibrational modes (LVM) of Si-12C centered at 605 cm−1 and Si-16O-Si quasi-molecules in the region of 1136 cm−1 for all the Si isotopes, in comparison with Si possessing a natural isotopic composition, as well as their isotopic shift at 300 and 17 K, have been determined. The dependence of the shape of the antisymmetric stretching vibration band for 28Si-16O-28Si in the spectrum of 28Si on spectral resolution has been studied. The possibility of generalizing the IR spectroscopy method for the determination of carbon and oxygen impurities in Si possessing a natural isotopic composition to monoisotopic Si have been discussed.

Silent Raman modes in zinc oxide and related nitrides

Anomalous Raman modes have been reported in several recent papers dealing with doped- and undoped-ZnO layers grown by different methods. Most of these anomalous Raman modes have been attributed to local vibrational modes of impurities or defects. However, we will show that most of the observed modes correspond to wurtzite-ZnO silent modes allowed by the breakdown of the translational crystal symmetry induced by defects and impurities.

Spectroscopy of excitons and shallow impurities in isotopically enriched silicon—electronic properties beyond the virtual crystal approximation

Recent high resolution spectroscopic studies of excitonic and impurity transitions in high-quality samples of isotopically enriched Si have dramatically expanded our understanding of the effects of isotopic composition on the electronic properties of semiconductors. Prior to these studies on Si, the results for other semiconductors, focusing mainly on the isotopic dependence of the electronic band gap energy E G in the T→0 limit, could all be explained within the virtual crystal approximation (VCA), in which only the average isotopic mass was relevant. Remarkably, not only were the effects of isotopic randomness observable in natural Si (when compared to enriched 28Si), but the random isotopic distribution present in natural Si was found to be the true source of what had been thought of as ‘fundamental’ spectroscopic limits in Si, including the linewidths of bound exciton emission lines and impurity absorption transitions, and the ‘intrinsic’ acceptor ground state splitting. Many of these transitions are far narrower in highly enriched 28Si than in the most perfect natural Si, challenging existing spectroscopic methods, and opening up new possibilities for precision measurements, and for the observation of new physics.

Morphological alteration upon phase transition and effects of oxygen impurities of chromium nanoparticles

The morphological alteration from δ - to α -Cr nanoparticles and the phase transition temperature were directly observed by in situ experiment using a transmission electron microscope. The icositetrahedral nanoparticle of δ -Cr transformed into a rhombic dodecahedral nanoparticle of α -Cr upon heating at 550 ∘ C . It was demonstrated that the most stable shape of the α -Cr nanoparticle is the rhombic dodecahedron. Upon heating the α -Cr nanoparticle at 650 ∘ C , the (1 1 3) layer of Cr 2 O 3 grew topotactically in the direction of [ 1 0 0 ] of α -Cr nanoparticle. We concluded that the α -Cr nanoparticle includes about 20–30 at% oxygen, as estimated from the volume of the Cr 2 O 3 surface layer.

Engineered Three-dimensional Colloidal Crystals Containing a Planar Defect

AbstractWe have fabricated three-dimensional (3D) colloidal crystals containing a two-dimensional (2D) defect as the middle layer by the Langmuir-Blodgett (LB) technique. Either a homogeneous sheet made of hundreds layers of behenic acid or a single monolayer of silica colloidal particles of various sizes was inserted between two opal films of silica spheres. The presence of the extrinsic defect led to an impurity mode within the photonic stop band, which was observed as a pass band in the near-infrared (NIR) spectra. The position of this defect mode was found to vary periodically with the value of the ratio of the thickness of the defect layer to the diameter of the colloids of the upper and lower opals.

Impurity quantum phase transitions

Recent work is reviewed on continuous quantum phase transitions in impurity models, both with fermionic and bosonic baths – these transitions are interesting realizations of boundary critical phenomena at zero temperature. The models with fermion bath are generalizations of the standard Kondo model, with the common feature that Kondo screening of the localized spin can be suppressed due to competing processes. The models with boson bath are related to the spin–boson model of dissipative two-level systems, where the interplay between tunneling and friction results in multiple phases. The competition inherent to all models can generate unstable fixed points associated with quantum phase transitions, where the impurity properties undergo qualitative changes. Interestingly, certain impurity transitions feature both lower-critical and upper-critical “dimensions” and allow for epsilon-type expansions. Results are presented for a number of observables, obtained by both analytical and numerical renormalization group techniques, and connections made to experiments.