Excited Impurity Research Articles

Long-lived excited impurity states in diamond-like semiconductors

The lifetime of charge carriers in the lowest excited states of some impurities of groups III and V in diamond, silicon, and germanium can be several (four to six) orders of magnitude longer that the lifetime of free carriers. Accumulation of carriers in these long-lived states may give rise to several new effects, such as hopping photoconductivity via long-lived excited states of impurities in dc and microwave electric fields, slow relaxation of induced absorption, and infrared absorption at energies lower than the impurity ionization energy.

Acoustophosphorescence double resonance in impurity C70 fullerene crystal

An acoustophosphorescent double resonance is investigated in terms of a low-temperature spin-phonon interaction mechanism determined by a coupling of translational and rotational molecular motions. Numerical values are obtained for the transition times of direct single-phonon processes in high, as well as zero, magnetic fields for a triplet excited isotopic impurity in crystalline C70 fullerene. The dependence of the transition rate on the propagation direction of the acoustic wave is analyzed in detail.

Second harmonic generation induced by charge-transfer complexes in a centrosymmetric medium

We consider the second harmonic generation (SHG) caused by impurities embedded in a nonlinear ( χ (3)) optical medium which itself does not display χ (2) optical response. In the case when the impurity molecules (charge-transfer (CT) complexes) possess an optical transition accompanied by charge transfer, we demonstrate that the excitation of impurities with a pump field may result in a contribution of the host medium to SHG. Depending on the presence or absence of the angular alignment of the excited static CT dipoles, there may be coherent SHG or weak incoherent SH scattering.

Binding energies of a hydrogenic impurity and of a Wannier exciton in an arbitrary corner structure

We calculate variationally the binding energies of a hydrogenic impurity and a Wannier exciton in an arbitrary corner of well material surrounded by the barrier material. The results show that the binding energy of ground impurity states in the corner can be comparable with that of highly excited impurity states in the bulk when the corner structure becomes small. However, the behaviour of the exciton in the corner is not the same as that of impurity; the binding energy of the exciton varies with the corner structure insensitively. The dependences of the impurity and exciton binding energies on the dielectric mismatch between the well material and barrier material are also discussed.

Hydrogen-Like Excitations of 3d and 4f Isoelectronic Impurities in Semiconductors

In the paper the energy states of structured isoelectronic impurities of transition metals and rare earth elements, donor and acceptor excitons of 3d and 4f impurities, tle role of donor and acceptor excitons of 3d and 4f impurities in energy transfer from the matrix to impuritles are discussed. It is shown that structured impurities may be classified as and isoelectronic impurities. The number of electrons in the 3d or 4f shells is changed at Ι. G Eg for open impuritles and is not changed for closed impurities. The structure of energy levels generated hy 3d and 4f impurities in a forbidden gap, the properties of donor and acceptor excitons of those impurities and energy transfer mechanisms from the matrix to an impurity (a capture of carriers into 3d or 4 f shells for open impurities and the Auger process for closed impurities) are essentially distmguished for two kinds of structured isoelectrnic impurities. For closed impurities a model is discussed in the frame of which the spectra of electroabsorption, photoluminescence, cathodoluminescence are described. PACS numbers: 71.35.+Ζ, 78.20.Jq, 78.20.Wc, 78.50.Ce

Barrier photo-emf studies of electronic transitions in α-RbAg 4I 5

A study has been made of the photo-emf of α-RbAg 4I 5 superionic crystals exposed to light from the impurity excitation region. The effects of local light irradiation and silver saturation on the photo-emf excitation spectrum were investigated. It was found that irradiation with light having energies 2.78 eV and 2.92 eV causes color centers formation in the crystals. Illumination intensity and time dependences of the photo-emf for the main excitation bands were measured at room temperature. The estimation of the photon capture cross sections for the electronic transitions gives values of the order of magnitude of 10 −17 cm 2. A mechanism for the origin of the photo-emf is proposed.

Capture Process by Shallow Donors in Silicon at Low Temperatures

Thermal capture by shallow attractive impurities has been studied by the Monte Carlo method at low temperatures. In order to model this process, the perturbation produced by the highest excited impurity states associated to the ideal Coulombic potential and the multiphonon mechanism were combined in the same simulation procedure. These excited states are considered to produce an energy quasicontinuum or impurity band where the carrier can move assisted by lattice phonons. To characterize the ground-state capture, the transition rates of multiphonon transitions from the impurity band and from the conduction band have been included. Electron capture cross sections have been calculated as a function of the temperature and compared compared with experimental data from the P + , As + and Sb + shallow donors in Si, achieving a good fitting with the Huang-Rhys factor, S, as the only free parameter.

Narrow-band electroluminescence at 3.5 μm from impact excitation and ionization of Fe2+ ions in InP

Electroluminescence associated with impact excitation and ionization of deep metallic impurities, enhanced by the presence of high electric field domains in semiconductors, is reported. A large and abrupt increase of midinfrared (λ≊3.5 μm) electroluminescence is observed at cryogenic temperatures above the onset for the transferred-electron effect in n-type InP doped with Fe. Optical powers up to 40 nW have been measured. At 5 K the spectrum is dominated by a set of sharp lines corresponding to the symmetry allowed d-shell transitions of Fe2+ ions.

Knudsen effect in gas of impuritons of3 He−4 He superfluid mixtures

Knudsen effect in the gas of impurity excitations (impuritons) of3He−4He superfluid mixtures was studied by theory with account of the interaction between quasiparticles and deviation of those dispersion law from quadratic one. The conditions under which the Knudsen effect in typical experimental device can be observed are discussed. Such a device consists of two vessels filled with superfluid mixtures at different temperatures and concentrations and connected by the set of narrow capillaries. Using the solution of kinetic equations the relations between temperatures and concentrations were found at stationary conditions. The influence of geometry and size of capillaries on Knudsen effect are investigated.

Excitation spectrum of a3He impurity in superfluid4He

We present a variational Monte Carlo calculation of the excitation spectrum of one3He impurity in superfluid4He over the full momentum rang. It includes backflow effects to high order. This is achieved with a shadow wave function in which the phase factors of the excitation are introduced in the subsidiary variables. Energy, effective mass and strength of the single excitation peak of the impurity are computed. Aboveq≃0.5A−1 the effecive mass increases significantly withq in agreement with experiment. No roton like minimum is found and the impurity excitation branch crosses the phonon-roton spectrum atq≃1.6A−1.

Magneto-optical effects on shallow donor states in 6H-SiC in high magnetic fields.

Impurity excitation spectra in 6H-SiC doped with nitrogen donors are studied by infrared absorption measurements in the frequency range 200--1400 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ under the influence of high magnetic fields up to 20 T. No line splitting is observed for any orientation of the magnetic field. However, four transitions show a diamagnetic shift if the magnetic field is oriented along the crystal c axis. This diamagnetic shift is found to be anisotropic, depending on the orientation of the viewing direction towards c. From these results we conclude that the effective mass ellipsoids describing the conduction-band minima must be oriented along lines of the Brillouin zone, which are parallel to the crystal c axis, and that the effective mass tensor has three independent components. Using the symmetry oproperties of the Brillouin zone it is shown that the conduction-band minimum must be located somewhere along the line M-L in the Brillouin zone. From the experimental results at B=20 T it is estimated that the average effective mass ${\mathit{m}}^{\mathrm{*}}$=(${\mathit{m}}_{\mathit{x}}$${\mathit{m}}_{\mathit{y}}$${\mathit{m}}_{\mathit{z}}$${)}^{1/3}$ must be ${\mathit{m}}^{\mathrm{*}}$>0.3${\mathit{m}}_{0}$.

Hot-electron impact cross section of impurities in semiconductors

This paper is devoted to the discussion of inelastic scattering of a high-energy conduction electron incident on an impurity in a semiconductor. On the one hand the rate of inelastic collisions of a hot electron samples the high-energy tail of the electron distribution, and thus may serve as a test of high-field transport theories. On the other hand the excited impurity may release its energy radiatively, and this is used in solid-state displays. We adopt a description of the impurity which emphasizes the structure of the excitable shell in a solid-state environment. Then we describe the direct and exchange paths between the initial and the final state, which we relate to spin selection rules. The scattering amplitudes are calculated to first order, with emphasis on the functional structure of each term. A crude computation of impact cross sections near the excitation threshold is performed, giving a rough magnitude of 10-18 cm2 which may be enhanced by exchange effects for a valence shell containing many electrons. More importantly, taking three examples we qualitatively discuss the factors affecting the cross section according to the structure of the impurity valence shell, and its location in the host lattice. The conclusions are of direct interest to device designers wanting to optimize the impact yield of hot-electron electroluminescence devices.

On the theory of multiphonon relaxation rates in solids

A theory of multiphonon relaxation for electronically or vibrationally excited impurities in crystals is developed. Two alternative approaches within the static-coupling scheme are presented and their relative importance is discussed. For each approach a closed-form analytical expression for the relaxation rate is given as a function of energy gap and temperature. We consider relaxation either by optical or acoustic phonons. From the analytical expressions we derive approximate energy gap laws for the zero-temperature rates, and discuss the origin and applicability of a popular phenomenological assumption for the rate’s temperature dependence. The predictions of the theory are compared with energy gap- and temperature-dependent experimental data for various electronic transitions of rare-earth impurities in YAlO3.

Photo-magneto-electric effect in semi-insulating GaAs: carrier lifetimes and influence of the defect structure

We report the investigation of the Photo-Magneto-Electric effect (PME) in semi-insulating Liquid-Encapsulated (LEC-) grown GaAs crystals, using both intrinsic and impurity excitation. The role of the majority and minority carriers on the conductivity phenomena was evaluated and the lifetimes of electrons and holes were determined depending on excitation conditions. Anomalously high PME voltages, reaching in some cases some volts, were measured, which demonstrate a sharp drop in the temperature region 320–360K. The observed changes are discussed supposing that the influence of the non-homogeneous defect structure of the samples is essential.

Excitation of metal impurities in a glow discharge in helium at atmospheric pressure

Excitation of metal impurities in a glow discharge in helium at atmospheric pressure

Luminescence and electron paramagnetic resonance studies of white-light emitting SrS: Pr, F thin film electroluminescent devices

The fluorescence emission and excitation spectra of white-light emitting SrS: Pr, F thin film electroluminescent devices have been investigated. It was determined from the results obtained that the dominant electroluminescence mechanism was that the ionization of Pr3+ centers occurs first, then subsequently recombination with electrons occurs, and finally Pr3+ center transitions give rise to luminescence. The emission mechanism of SrS: Pr, F seems to be the same as that of a SrS: Pr, K electroluminescent device, except for the appearance of strong peaks around 610–670 nm. The impurity excitation peak in the lower excitation energy, longer-wavelength region in the FL spectrum may be an important factor for the selection of an effective white-light emitting EL material. The electron paramagnetic resonance experiment of SrS: Pr, F was performed on powder and thin film specimens. The hyperfine structure of an isolated Mn2+ ion was observed in this SrS: Pr, F thin film. This Mn center which was substituted for Sr, seems to contribute to the strong red emission in the white EL spectrum.

Optical non‐linearities and excitation dynamics in II–VI bulk and epitaxial materials

AbstractExcitonic processes in the band edge regime determine the optical properties of II–VI bulk and novel epitaxial materials. The interdependences of excitonic systems (i.e. free and bound excitons), high‐density systems (e.g. biexcitons) and the transition into the electron‐hole plasma state are of high complexity and depend sensitively on sample quality and purity, dopant concentration, excitation condition and density, etc. Resonant excitation spectroscopy and time‐resolved analysis of creation and decay processes provide valuable experimental access to clarification of the above‐mentioned mutual interactions. In this paper recent results obtained using these methods are surveyed.In the first part the development of luminescence and resonant excitation of bound exciton systems is treated under various excitation densities, for high excitation levels accompanied by biexciton formation and exciton–exciton collision processes. The specific properties observable when using heteroepitaxial structures instead of conventional bulk samples are discussed.In the second part the time characteristics of excitonic transitions are evaluated for various impurities, dopants and dopant concentrations, excitation via particular resonant excitation channels, and various excitation densities. Relaxation and conversion channels between excitonic systems are analysed, in particular in strained heteroepitaxial systems which show splitting effects of the bands from which the carriers stem. Time‐resolved analysis is demonstrated to be extremely helpful for the analysis of unknown excitonic systems and transitions. Methods of varying the characteristic time constants are discussed, in particular with regard to intentional changes in impurity contents and excitation densities which are interesting for any application.

Magnetopolaron effect on shallow donors in GaAs.

Low-temperature experimental measurements and variational calculations of the transition energies of shallow donor (Si) impurities in bulk GaAs as a function of magnetic field throughout the resonant po- laron region are reported. Far-infrared photoconductivity spectra in magnetic fields up to 23.5 T show several anti-level-crossing processes at energies above the LO-phonon energy, clearly demonstrating the resonant interactions between GaAs LO phonons and impurity-bound electrons involving several excited impurity states. Very good agreement is obtained between experiment and calculated transition energies throughout the resonant region with the accepted value of the Fr\"ohlich coupling constant (\ensuremath{\alpha}=0.068). The effects of uncertainties in the measured values of the dielectric constants (thus the value of \ensuremath{\alpha}) are studied in the calculation, and the implication of these results for similar studies in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells is discussed.

Influence of impurities on broadband p-type-Ge laser spectra under uniaxial stress.

We have investigated the stimulated emission of a broadband p-type-Ge laser (light- to heavy-hole laser) with uniaxial stress up to 500 bars. The emission spectra can be divided into two ranges, the low- and the high-frequency ranges. Between these ranges a region without emission occurs (emission gap) which is explained by impurity absorption. This emission gap vanishes for stress values larger than 400 bars. The structure of the stimulated emission in the low-frequency range is explained by impurity absorption lines. The spectral features of the stimulated emission in the high-frequency range with applied stress suggest that the emission of the p-type-Ge laser is not caused by transitions between the light- and the heavy-hole subbands. Rather, the emission is generated by optical transitions between Landau levels of the light holes and excited impurity states.

The mechanisms of electronic excitation of rare earth impurities in semiconductors

Impact and Auger excitation of threefold ionized rare earth ions by carriers are calculated for silicon and III-V semiconductors. The interaction between strongly localized f-electrons and local vibrations, which makes it possible to achieve energy balance in Auger processes, has been taken into account. It is shown that the probability of Auger excitation increases if there are additional localized electron states in the forbidden gap, which correspond to defects containing rare earth ions.