Showing papers on "Exciton published in 1990"

Electronic structure and photoexcited-carrier dynamics in nanometer-size CdSe clusters.

Abstract: We use transient optical hole burning and photoluminescence to investigate the static and dynamic electronic properties of 32-\AA{} CdSe quantum dots. We observe a number of discrete electronic transitions, resolve LO-phonon progressions, and obtain homogeneous linewidths and electron--LO-phonon couplings. We find that the band-gap luminescence is not from the exciton state, but from a surface trapped state. Rapid (\ensuremath{\sim}160 fs) trapping into these surface states results in long-lived (\ensuremath{\sim}10--100 ns) bleach and induced-absorption features in pump-probe experiments.

Temperature-dependent exciton linewidths in semiconductors

Abstract: The temperature-dependent linewidths of excitons in semiconductors due to the interaction of the exciton with both LO phonons and with acoustic phonons are studied with use of a Green's-function approach in which the exciton-phonon interaction is treated perturbatively. The interaction between the excitons and the LO phonons is taken to be of the Fr\"ohlich form, and the contribution to the linewidth is obtained in closed form. In this case it is found that scattering of the exciton to both bound and continuum states is important and that it is important to treat the continuum states fully as Coulomb scattering states. In describing optical-absorption processes, the fact that absorption occurs from polariton states, which are states composed of excitons coupled to light, is taken into account. The linewidths due to the exciton--LO-phonon interaction are evaluated for a series of II-VI and III-V compound semiconductors, and are shown to account for the existing experimental results for temperatures \ensuremath{\gtrsim}80 K. The contributions to the linewidth due to the interaction of excitons with acoustic phonons via both the deformation potential and the piezoelectric couplings are treated, and it is found that the deformation-potential coupling dominates for all of the materials considered. Because of the small velocity of sound, scattering to only intraband intermediate states, i.e., those in which the internal exciton quantum numbers do not change, is found to contribute to the linewidth. In the case of acoustic phonons, it is found to be important to treat optical absorption as originating from polariton states in order to evaluate properly the magnitude of this contribution to the linewidth. The acoustic-phonon contribution to the linewidths is compared with experiment for temperatures \ensuremath{\lesssim}80 K, for which it dominates the temperature dependence.

Dynamics of electron-hole pair recombination in semiconductor clusters

Abstract: The kinetics of radiative electron-hole pair recombination in CdS and Cd{sub 3}As{sub 2} clusters (where the radius of the cluster is smaller than the de Broglie wavelength of photogenerated excitons) were studied with picosecond photon counting luminescence decay measurements over wide temperature and energy ranges. The decay profiles were quantitatively examined with several models. The decays are composed of two distinct time regimes, each with very different temperature and emission energy dependence. The first (fast) regime is attributed to an unusually efficient thermal repopulation mechanism. The second (slow) component is well described by a distributed kinetic model. The kinetic behavior of wide (CdS) and narrow (Cd{sub 3}As{sub 2}) band gap materials was remarkably similar when composed of clusters in the quantum confined regime.

Organometallic Synthesis of GaAs Crystallites Exhibiting Quantum Confinement

Abstract: The optical spectra of semiconductor crystallites whose dimension is comparable to the bulk exciton diameter show quantum confinement effects. To date, experimental studies of nanometer-size crystallites have been restricted to II-VI and I-VII semiconductors, while III-V semiconductors, including one of the most important direct band gap semiconductors, GaAs, have not yet been studied in this form, because of the numerous difficulties encountered in their preparation. Compared to the I-VII and II-VI semiconductors, the III-Vs have a greater degree of covalent bonding, a less ionic lattice, and larger exciton diameters (the exciton diameter in GaAs is 190 {angstrom}, compared to 60 {angstrom} for CdS). For this reason, quantum size effects on the optical spectra have been predicted to be more pronounced in the III-V class of materials than in the II-VIs, and crystallites of GaAs are more likely to find application in optical devices than CdS clusters. In this paper the authors present the first documented preparation of relatively monodisperse, redissolvable, crystalline, nanometer-size particles of GaAs. The nanocrystals were prepared in polar organic solvents, using a reaction developed by Wells and co-workers. GaCl{sub 3} and As(SiMe{sub 3}){sub 3} react in hydrocarbons to give solid products, which on heating afford GaAs. Theirmore » X-ray diffraction experiments on the black GaAs powder prepared in this manner show that the domain size is 100 {angstrom}. They carried out the same reaction in quinoline.« less

Accurate theory of excitons in GaAs-Ga1-xAlxAs quantum wells.

Abstract: Binding energies of excitons in quantum wells are calculated including valence-band mixing and also other important effects, namely Coulomb coupling between excitons belonging to different subbands (which is predominantly with the exciton continuum), nonparabolicity of the bulk conduction band, and the difference in dielectric constants between well and barrier materials. All these effects are found to be of a comparable size, tend to increase the binding energies, and taken together result in very high binding energies, particularly in narrow GaAs/AlAs quantum wells. Binding energies can be even higher than the two-dimensional limit of four times the bulk Rydberg. Theoretical results agree within a few tenths of a milli-electron-volt with available photoluminescence excitation experiments. Valence-band mixing gives a finite oscillator strength to some excitons not in s states, but does not change the selection rules based on parity. Calculated oscillator strengths of the ground-state heavy- and light-hole excitons are found to be in good agreement with absorption and reflectivity experiments.

Theory of optically excited intrinsic semiconductor quantum dots

Abstract: The influence of the Coulomb interaction on one and two electron-hole-pair excitations in semiconductor quantum dots is analyzed. Using a numerical matrix-diagonalization scheme, the energy eigenvalues and the eigenfunctions of the relevant states are computed. Significant deviations from the strong-confinement approximation are observed. It is shown that the biexciton binding energy increases with decreasing dot size. This result is verified using third-order perturbation theory for small quantum dots. The optical properties of the quantum dots are computed, and it is shown that the Coulomb interaction significantly influences the allowed dipole transitions, causing increasing two-pair absorption on the high-energy side of the decreasing one-pair absorption. Surface-polarization effects are studied for quantum dots embedded in another dielectric medium.

Optical transient bleaching of quantum‐confined CdS clusters: The effects of surface‐trapped electron–hole pairs

Abstract: We studied the optical transient bleaching of ∼40 A, ammonia‐passivated CdS clusters in a polymer with nanosecond and picosecond pump‐probe techniques. The transient bleaching spectra behave differently in different time regimes. Within the 30‐ps pump laser pulse width, we tentatively attribute the bleaching to the exciton‐exciton interaction, and the magnitude can be enhanced by surface passivation. On time scales of tens of picoseconds and longer following the pump pulse, when only trapped electron‐hole pairs remain from the pump excitation, the bleaching is due to the interaction between such a trapped electron‐hole pair and a bound exciton produced by the probe light. Experimentally we determined that roughly one trapped electron‐hole pair can bleach the excitonic absorption of the whole CdS cluster. We developed a theoretical model which considers the effects of the trapped electron‐hole pair on the energy of the exciton transition and its oscillator strength. We found that, when a trapped electron and hole are present, the lowest exciton absorption is red‐shifted from the original exciton absorption, and this transition has a weak oscillator strength, which explains the observed efficient bleaching. The model also predicts that a trapped electron is more efficient than a trapped hole for bleaching the excitonic absorption of CdS clusters in the size regime considered here. This is confirmed by pulse radiolysis results. Finally, we discuss the possible effects of charged surface defects on the linear absorption spectra of semiconductor clusters.

Biexcitons in semiconductor quantum dots

Abstract: Theoretical and experimental results are reported which provide the first evidence for biexciton states in semiconductor quantum dots. The theory predicts an increasing biexciton binding energy with decreasing dot size. Unlike bulk semiconductors, quantum dots have excited biexciton states which are stable. These biexciton states are observed as pronounced induced absorption features on the high-energy side of the bleached exciton resonances in femtosecond and nanosecond pump-probe experiments of quantum dots in glass matrices.

Quasi-epitaxial growth of organic multiple quantum well structures by organic molecular beam deposition

Abstract: Multiple quantum well structures consisting of alternating layers of two crystalline organic semiconductors, namely, 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) and 3,4,7,8 naphthalenetetracarboxylic dianhydride (NTCDA), have been grown by organic molecular beam deposition. The individual layer thicknesses in the multilayer samples were varied from 10 to 200 A. X‐ray diffraction and birefringence data show that there is a strong structural ordering in all layers, as well as across large spatial distances along the sample surface. Thus, the growth is ‘‘quasi‐epitaxial’’ even though the PTCDA and NTCDA crystal structures are incommensurate. From the optical absorption spectra, it was found that the lowest energy PTCDA singlet exciton line shifts to higher energy with decreasing layer thickness. Comparison of these results with a quantum mechanical model based on exciton confinement in the PTCDA layers is proposed to describe the energy shift.

Resonant-photoemission study of SnO 2 : Cationic origin of the defect band-gap states

Abstract: The electronic properties of tin dioxide single-crystalline (110) surfaces have been studied in correlation with their structure by low-energy electron diffraction, angle-integrated and resonant photoemission using synchrotron radiation [ultraviolet photoemission spectroscopy (UPS)]. Energy distribution curves were measured from the Sn 4d core levels and from the valence band. The experimental valence band is compared with the theoretical density of states (DOS) from perfect and defective surfaces. UPS difference curves, normalized to the Sn 4d intensity, reflect mainly the increase in the oxygen partial DOS when the sample is annealed at increasing temperatures up to 1000 K after sputtering. Their comparison with simulated theoretical difference curves favors a bridging oxygen termination for annealing temperatures above 900 K. After argon-ion bombardment, band-gap defect states that are not predicted by the calculations are found at a maximum density 1.4 eV above the valence-band maximum (VBM). Various degrees of resonant enhancement occur throughout the valence band when the photon energy crosses the Sn 4d\ensuremath{\rightarrow}5p photoabsorption threshold, and these are used to establish the tin-derived character of the gap states, for which a tin 5s origin is proposed. Partial-yield spectra allow the localization of unoccupied Sn 5p states in the conduction band starting from 8 eV above the VBM with a maximum at 10 eV. The Sn 4d\ensuremath{\rightarrow}5p absorption threshold also shows possible core exciton formation for sputtered surfaces only.

Fine structure of excitons in type-II GaAs/AlAs quantum wells.

Abstract: Optically detected magnetic resonance in zero field as well as in a finite magnetic field has been used to study the excitons in type-II GaAs/AlAs quantum wells. The spectra are analyzed using the appropriate spin Hamiltonian for the quasi-two-dimensional indirect excitons. The electron-hole exchange interaction and the g factors for the electron and hole are obtained for several thicknesses of the GaAs and AlAs layers. Good agreement exists between the trend in the exchange interaction and the effective-mass theory of Rejaei Salmassi and Bauer. The anisotropy of the electron g factor is in accordance with a lifting of the threefold degeneracy of the AlAs X conduction-band minimum by the quantum-well potential giving the ${\mathit{X}}_{\mathit{z}}$ valley the lowest energy in the thin-layer quantum wells studied. The effective heavy-hole g value of \ensuremath{\sim}2.5 is much smaller than in the bulk and depends on the GaAs well thickness. This is probably a consequence of the valence-band mixing in quantum-well structures. Two classes of excitons are observed, each with a symmetry that is lower than the anticipated ${\mathit{D}}_{2\mathit{d}}$ point-group symmetry for excitons in quantum wells. The actual symmetry of the type-II excitons and the width of the exciton resonances are related to the microscopic structure of the GaAs/AlAs interface.

Relaxation dynamics of photoexcitations in polydiacetylenes and polythiophene

Abstract: The relaxation dynamics of photoexcitations in cast films of the polydiacetylenes, poly[4,6-decadiyne-1,10-diol bis([(n-butoxycarbonyl)methyl]urethane)] (PDA-3BCMU) and poly[5,7-dodecadiyne-1,12-diol bis([n-butoxycarbonyl)methyl]urethane)] (PDA-4BCMU), and in electrochemically prepared films of poly(3-methylthiophene) (P3MT) were investigated by femtosecond absorption and picosecond luminescence spectroscopies. 1Bu free excitons, generated by 100-fsec pump pulses, relaxed to the self-trapped (ST) state with time constants of 150 ± 50 fsec in PDA-3BCMU and 70 ± 50 fsec in P3MT. The difference was explained in terms of the relative sizes of the side chains. PDA-3BCMU has bulky side chains that are linked by hydrogen bonds, while P3MT has a repeating unit that contains a ring structure with a small methyl group. The lifetimes of the ST excitons at 10 K (290 K) were 2.0 ± 0.1 (1.5 ± 0.1) psec in PDA-3BCMU, 3.0 ± 0.3 (2.1 ± 0.2) psec in PDA-4BCMU, and 800 ± 100 (800 ± 100) fsec in P3MT. The decay kinetics of the ST exciton were explained by potential crossing and tunneling between two potential curves of the ST exciton and the ground state. The weak temperature dependence indicates that the activation process over the potential barrier between the ST exciton and the ground state is not dominant in the radiationless relaxation of the ST exciton. The relaxation dynamics of fluorescence from the PDA-4BCMU film cannot be represented by a single exponential decay; however, it can be described in terms of a random walk in the fractal dimension. By applying the fractal-dimension model, the spectral dimension was determined to be between 0.50 and 0.85. The spectral changes that are due to several nonlinear-optical processes, i.e., hole burning, Raman gain, and the dynamic Stark effect, were also observed in femtosecond-resolved spectra. The third-order susceptibility was determined for these nonlinear processes, such as absorption saturation at the exciton transition, Raman gain, and the resonant Kerr effect from the observed spectral change. The third-order susceptibility that corresponds to absorption saturation in PDA-3BCMU and in P3MT was obtained as Im[χ1111(3)(−ω;ω,ω,−ω)] equal to −2.6 × 10−9 and −3.5 × 10−10 esu, respectively, for ħω = 1.97 eV. The third-order susceptibility that corresponds to the Raman gain in PDA-3BCMU and in P3MT was determined to be Im[χ1111(3)(−ω2;ω2,ω1,−ω1)] equal to 5.8 × 10−10 and −1.5 × 10−10 esu, respectively, for ħω1 = 1.97 eV and ħω2 = 1.79 eV. From the resonant Kerr experiment in the P3MT film, |Δχ(3)|≡|χ1111(3)(−ω2;ω2,ω1,−ω1)−χ1122(3)(−ω2;ω2,ω1,−ω1)|=3.9×10−11 esu was determined for ħω1 = 1.97 eV and ħω2 = 1.88 eV.

Phase transition of an exciton system in GaAs coupled quantum wells

Abstract: We have observed a sharp reduction of the photoluminescence linewidth from a two-dimensional exciton system (coupled GaAs/AlGaAs quantum wells), at a certain critical temperature (${\mathit{T}}_{\mathit{c}}$) and under the influence of an electric field. We attribute this narrowing to a phase transition of the exciton system into an ordered state. The experiments suggest that a coherence length long enough to wash out the effect of the fluctuations, which cause the line broadening, is introduced below ${\mathit{T}}_{\mathit{c}}$.

Nitrogen pair luminescence in GaAs

Abstract: We report on the first observation of different nitrogen pair complexes in GaAs. These complexes, which have been searched for since the ’60s, are studied under the application of hydrostatic pressure. By carefully tuning the pressure, we make one after the other of the NNi pairs (1≤i≤10) appear in the band gap of GaAs and then become the major exciton recombination channel. We compare our results for nitrogen states in GaAs with the classical case of NNi excitons in GaP.

Luminescence and defect formation in undensified and densified amorphous SiO2.

Abstract: Luminescence and optical absorption induced by an electron pulse and by a subsequent laser pulse have been studied in densified and undensified amorphous Si${\mathrm{O}}_{2}$ at 77 K. We find that the decay of the optical-absorption change induced by an electron pulse consists of two components: one which decays following a power law within 1 ms and the other which decays much more slowly. The fast component exhibits the spectrum of the self-trapped excitons, while the spectrum of the slow component includes the ${E}^{\ensuremath{'}}$ band. The ${E}^{\ensuremath{'}}$ band thus generated is found to decay with a time constant of about 10 h at 77 K and to be annihilated almost completely by warming to room temperature. We also find that optical excitation of the slow component by a 222-nm laser pulse generates a luminescence band having a peak at 2.1 eV, which is identical to the luminescence band due to the self-trapped excitons. It is likely that the defect pair and the self-trapped exciton have the same composition and both are generated as the result of the relaxation of excitons.

Line shapes of intersubband and excitonic recombination in quantum wells: Influence of final-state interaction, statistical broadening, and momentum conservation

Abstract: A realistic and comprehensive theory of line shapes for spontaneous recombination of two-dimensional carriers in quantum-well (QW) structures is developed. Starting from the line shape for intersubband recombination, which takes into account the QW density of states and the thermal carrier distribution function, the impact of momentum (non)conservation on the luminescence line shape is considered. Then Lorentzian broadening due to the finite lifetime of the final states and Gaussian broadening due to statistical fluctuations of quantum-well eigenenergies characteristic, e.g., of interface roughness is incorporated. The effects of Coulomb interaction of the charge carriers at low densities are considered quantitatively, including both excitonic bound states and excitonic enhancement above the two-dimensional band gap. For a case study, GaAs QW luminescence line shapes are investigated. The line-shape evaluation definitely proves that recombination is excitonic at any temperature up to 300 K in contradiction to previous assumptions of some other authors. At low temperatures all lines consist, on a first view, of unresolved doublets which are very close in energy. The high-energy component is identified as the free-electron--heavy-hole exciton. Momentum is not found and does not need to be conserved in the free-exciton recombination process at low temperatures. At high temperatures, momentum conservationmore » is found to be reestablished: Momentum conservation is understood to depend on the relative amplitude of interface-roughness-induced lateral potential fluctuations as compared to the thermal energy of the excitons.« less

Polaronic satellites in x-ray-absorption spectra

Abstract: Satellite structure in the 2p absorption spectra of early 3d transition-metal compounds is reported. The spectral shape depends strongly on the local symmetry of the excited atom and gives evidence that in Ti oxides the excitation on the metal is screened by an exciton transition in the oxygen band and not by a charge-transfer or Mott-Hubbard transition as in the case of narrow-band metal compounds. The relative satellite intensities can be used to assess the anisotropy of the local electronic structure and the influence of the excited electron on the screening of the core-hole potential.

Temperature dependence of the electronic structure of oxides: MgO, MgAl2O4 and Al2O3

Abstract: We have studied the room temperature optical reflectivity of MgO, MgAl2O4, and α-Al2O3 from 5 to 40eV using a novel spectrophotometer with a laser plasma light source. Structure in the imaginary component of the dielectric response is analysed using critical point line shapes, and the origins of the major transitions in MgO and MgAl2O4 are determined using an ab initio pseudofunction band structure calculation of MgO. The exciton reflectivity has been studied in the three materials at temperatures between 300 and 1500 K, and exciton-phonon coupling appears to increase from MgO to α-Al2O3. The temperature dependence of the higher lying interband transitions in MgO has been determined to 1100 K, and we find that while the temperature dependence of the onset transitions at Γ and X are nearly identical (− 1.22meV/K at Γ), higher lying transitions have very different temperature dependences. Furthermore with increasing temperature the X point valence band separation increases at a rate of 0.38meV/K, while the conduction band separation at X decreases at −0.41meV/K.

Quantum beats of light hole and heavy hole excitons in quantum wells

Abstract: We report the first observation of quantum beats due to the interference of the polarization decay of heavy hole and light hole excitons in semiconductor quantum wells.

Image charges in semiconductor quantum wells: Effect on exciton binding energy

Abstract: Binding energies of excitons in a quantum-well structure are calculated including fully the effects of image charges, finite barriers, the z correlation of electrons and holes, and anisotropic hole masses. The influence of discontinuous masses and discontinuous dielectric constants across the interfaces is evaluated in detail: While the mass difference becomes important only when the excitonic wave function penetrates into the barrier, the image charges appreciably modify the Coulomb interaction and therefore influence the exciton binding energy even at well widths larger than the exciton Bohr radius. Results for technologically important, particular material systems are presented.

Near‐band‐gap photoluminescence of Si1−xGex alloys grown on Si(100) by molecular beam epitaxy

Abstract: Photoluminescence spectra in the near‐band‐gap region of Si1−xGex alloys (x=0.04 and 0.15) grown on Si(100) substrates by molecular beam epitaxy have been measured at 4.2 and 12 K. Radiative recombinations of free and bound excitons in thin layers of Si1−xGex alloys have been clearly observed for the first time. No‐phonon transitions and transverse‐optical (TO) phonon‐assisted transitions have been identified.The luminescence lines become broader with an increase in excitation intensity; the broadening is interpreted to be due to the generation of the bound multiexciton complexes (BMECs). The position of the band‐edge luminescence lines is determined by the strain in the epitaxial layer as well as the alloy composition. The defect‐related L band appears in the case of x=0.15.

Simple method for calculating exciton binding energies in quantum-confined semiconductor structures.

Abstract: We present a simple, general method for calculating the binding energies of excitons in quantum-confined structures. The binding energy is given by an integral (over the electron and hole coordinates perpendicular to the confining layers) of a prescribed function weighted by the squares of the electron and hole subband envelope functions. As a test of the method, we calculate the binding energies for heavy- and light-hole excitons in a rectangular GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As quantum well as functions of the well width. Very good agreement with previous results is obtained over a wide range of quantum-well widths. Also, we determine the binding energies for heavy-hole excitons as functions of electric field in a GaAs/${\mathrm{Al}}_{0.35}$${\mathrm{Ga}}_{0.65}$As asymmetric coupled-quantum-well structure. Our results compare favorably with those obtained in a treatment in which coupling of the electron subbands via the electron-hole Coulomb interaction is considered. Our method should be applicable to a variety of complex quantum-confined semiconductor structures for which more rigorous approaches require extensive numerical calculations.

Structural and optical properties of (100) InAs single-monolayer quantum wells in bulklike GaAs grown by molecular-beam epitaxy.

Abstract: A single (100) monolayer InAs in GaAs is grown by conventional molecular-beam epitaxy. The structural properties of these highly strained single-monolayer quantum-well samples are investigated with high-resolution double-crystal x-ray diffractometry, double-crystal x-ray reflection topography, and transmission electron microscopy. The calculation of the x-ray-diffraction patterns within the framework of the dynamical diffraction theory provides the precise determination of the strain state and the thickness of the layer even in the submonolayer region. This approach allows us to prove the superior crystalline quality of the samples. In particular, no misfit dislocations are generated at the highly strained InAs/GaAs heterointerface as observed by x-ray topography. These results are confirmed by the high-resolution lattice image of the heterointerface, indicating the excellent homogeneity and flatness of the InAs lattice plane.In addition, we present evidence for a strong impact of the strain on the incorporation of cations in the crystal lattice during epitaxial growth. This structural information on an atomic scale is necessary for a correct interpretation of photoluminescence (PL) and photoluminescence excitation (PLE) spectra. We correlate the results of our structural investigation with a simple analysis of the PL linewidth and the blue shift (Stokes shift) of the heavy-hole exciton resonance in the PLE spectrum with respect to the electron heavy-hole transition in the PL spectrum. Both the PL linewidth and the Stokes shift are explained in terms of a fluctuation of the well width of 1 monolayer and a lateral island size of about 10 nm, in excellent agreement with the lattice image of the structure. The important result of our investigation is that a single monolayer of InAs yields a pronounced redshift of the quantum-well luminescence as compared with that of bulklike GaAs. Additionally, the PLE spectrum provides direct experimental evidence for a weak confinement of the light-hole excitons in the InAs plane. This result clearly indicates a type-I band alignment for both heavy holes and light holes, as predicted by a simple envelope-function calculation. Finally, the samples exhibit a surprisingly high luminescence efficiency, resulting from a very efficient trapping of the photoexcited carriers into the InAs-monolayer quantum well with trapping times of the order of only 20 ps.

Ultrafast adiabatic following in semiconductors.

Abstract: The full semiconductor Bloch equations are solved to analyze the ultrafast exciton dynamics in semiconductors. Transient adiabatic following is obtained for large detunings and arbitrary intensities, leading to exciton Stark shift, bleaching, and recovery on the time scale of the pump pulse. The many-body Coulomb effects strongly influence the semiconductor response for resonant excitation conditions, almost doubling the effective Rabi frequency of the applied field

Creation of photostimulable centers in BaFBr:Eu2+ single crystals by vacuum ultraviolet radiation.

Abstract: Several experiments were performed in order to understand the creation and readout mechanisms of photostimulable (PS) centers in BaFBr:Eu 2+ single crystals. PS centers can be efficiently created starting from 6.7 eV, i.e., the minimal energy required to excite the first valence exciton. This exciton relaxes to an e-V k (Br 2 − ) pair. The relaxation of such a defect in the neighborhood of Eu 2+ yields a PS center, namely, an F-H(Br 2 − ) pair. An additional decay channel of the e-V k pair results in Eu 2+* emission due to the energy match between the emission of this pair and an absorption band of Eu 2+

Influence of the confinement potential on the electron-hole-pair states in semiconductor microcrystallites.

Abstract: The energies of the two energetically lowest dipole-allowed electron-hole-pair states in semiconductor microcrystallites are computed variationally. Details of the quantum confinement conditions, such as the finite value of the quantum confinement potential and the different effective electron-hole masses inside and outside the crystallites, are considered explicitly. Significant deviations from the infinite-potential approximation are obtained.