Showing papers on "Exciton published in 1997"
TL;DR: In this article, structural analysis was performed on a purple laser diode composed of In0.20Ga0.80N (3 nm)/In0.05Ga 0.95N (6 nm) multiple quantum wells.
Abstract: Structural analysis was performed on a purple laser diode composed of In0.20Ga0.80N (3 nm)/ In0.05Ga0.95N (6 nm) multiple quantum wells, by employing transmission electron microscopy and energy-dispersive x-ray microanalysis, both of which are assessed from the cross-sectional direction. It was found that the contrast of light and shade in the well layers corresponds to the difference in In composition. The main radiative recombination was attributed to excitons localized at deep traps which probably originate from the In-rich region in the wells acting as quantum dots. Photopumped lasing was observed at the high energy side of the main spontaneous emission bands.
850 citations
TL;DR: In this article, the electronic structure of spherical PbS and PbSe quantum dots is calculated with a four-band envelope-function formalism, accounting for both exciton energies and wave functions with the correct symmetry of materials.
Abstract: The electronic structure of spherical PbS and PbSe quantum dots is calculated with a four-band envelope-function formalism. This calculation accounts for both exciton energies and wave functions with the correct symmetry of the materials. The selection rules and the strength of the dipole transitions of lead-salt quantum dots are derived accounting for the symmetry of the band-edge Bloch functions of the lead salts. The calculated energies of the optically allowed exciton states are found to be in good agreement with experimental data. The effects of many-body perturbations, such as Coulomb interactions and intervalley scattering, are also discussed.
692 citations
TL;DR: In this paper, the absorption coefficient for a 0.4-μm-thick GaN layer grown on a polished sapphire substrate was determined from transmission measurements at room temperature.
Abstract: The absorption coefficient for a 0.4-μm-thick GaN layer grown on a polished sapphire substrate was determined from transmission measurements at room temperature. A strong, well defined exciton peak for the A and B excitons was obtained. The A, B, and C excitonic features are clearly defined at 77 K. At room temperature, an energy gap Eg=3.452±0.001 eV and an exciton binding energy ExA,B=20.4±0.5 meV for the A and B excitons and ExC=23.5±0.5 meV for the C exciton were determined by analysis of the absorption coefficient. From this measured absorption coefficient, together with the detailed balance approach of van Roosbroek and Shockley, the radiative constant B=1.1×10−8 cm3/s was obtained.
665 citations
TL;DR: In this article, the authors studied the luminescence of surface modified CdSe nanocrystallites with a variety of organic and inorganic ligands, and found good agreement between experiment and theory for the nonresonant and resonant luminecence.
Abstract: We study the luminescence of surface modified CdSe nanocrystallites There has been much speculation as to the origin of the band edge emission in these quantum confined structures Because of their large surface to volume ratios it has been suggested that the emission originates from surface-related states However, recent theory suggests that the band edge luminescence arises from an optically inactive fine structure state or “dark” exciton To address this issue we modify the surface of CdSe nanocrystallites with a variety of organic and inorganic ligands We then monitor the effect changing the surface has on the energetics of the band edge luminescence through photoluminescence and fluorescence line narrowing experiments Our results are compared with theoretical predictions for the nonresonant and resonant luminescence We find good agreement between experiment and theory for CdSe nanocrystallites passivated with trioctylphosphine oxide, ZnS, 4-picoline, 4-(trifluoromethyl)thiophenol, and tris(2-eth
655 citations
TL;DR: In this article, the Coulomb interaction gives rise to several exciton bound states as well as an increase of the energy gap in carbon nanotubes (CN) and the exciton energy is shifted to higher energy side than the unperturbed band gap because the effect on the band gap is larger.
Abstract: Exciton energy levels and corresponding optical spectra are calculated in carbon nanotubes (CN) in the conventional screened Hartree-Fock approximation within a k · p scheme. The Coulomb interaction gives rise to several exciton bound states as well as the increase of the energy gap. The exciton energy is shifted to higher energy side than the unperturbed band gap because the effect on the band gap is larger. The considerable amount of the optical intensity is transferred to exciton bound states because of the one-dimensional nature of CN's.
569 citations
TL;DR: In this paper, the authors demonstrate inelastic phonon scattering to be the dominant intradot carrier-relaxation mechanism in self-organized InAs/GaAs quantum dots.
Abstract: Carrier relaxation and recombination in self-organized InAs/GaAs quantum dots (QD's) is investigated by photoluminescence (PL), PL excitation (PLE), and time-resolved PL spectroscopy We demonstrate inelastic phonon scattering to be the dominant intradot carrier-relaxation mechanism Multiphonon processes involving up to four LO phonons from either the InAs QD's, the InAs wetting layer, or the GaAs barrier are resolved The observation of multiphonon resonances in the PLE spectra of the QD's is discussed in analogy to hot exciton relaxation in higher-dimensional semiconductor systems and proposed to be intricately bound to the inhomogeneity of the QD ensemble in conjunction with a competing nonradiative recombination channel observed for the excited hole states Carrier capture is found to be a cascade process with the initial capture into excited states taking less than a few picoseconds and the multiphonon (involving three LO phonons) relaxation time of the first excited hole state being 40 ps The |001〉 hole state presents a relaxation bottleneck that determines the ground-state population time after nonresonant excitation For the small self-organized InAs/GaAs QD's the intradot carrier relaxation is shown to be faster than radiative (g1 ns) and nonradiative (\ensuremath{\approx}100 ps) recombination explaining the absence of a ``phonon bottleneck'' effect in the PL spectra
408 citations
TL;DR: In this paper, a real-space pseudopotential method was used to calculate quasiparticle gaps, self-energy corrections, exciton Coulomb energies, and optical gaps in Si quantum dots.
Abstract: Quasiparticle gaps, self-energy corrections, exciton Coulomb energies, and optical gaps in Si quantum dots are calculated from first principles using a real-space pseudopotential method. The calculations are performed on hydrogen-passivated spherical Si clusters with diameters up to 27.2 \AA{} ( $\ensuremath{\sim}800\mathrm{Si}$ and H atoms). It is shown that (i) the self-energy correction in quantum dots is enhanced substantially compared to bulk, and is not size independent as implicitly assumed in all semiempirical calculations, and (ii) quantum confinement and reduced electronic screening result in appreciable excitonic Coulomb energies. Calculated optical gaps are in very good agreement with absorption data.
332 citations
TL;DR: In this article, the authors used spectroscopic ellipsometry (SE) together with the optical transmission method to determine the refractive index n and absorption coefficient α of undoped gallium nitride film over the spectral range of 0.78-4.77 eV of photon energy.
Abstract: Spectroscopic ellipsometry (SE) together with the optical transmission method is successfully used to determine the refractive index n and absorption coefficient α of undoped gallium nitride film over the spectral range of 0.78–4.77 eV of photon energy. The SE measurement is carried out at angle of incidence of 60° over the 1.5–4.77 eV energy range and optical transmission measurement over the 0.78–3.55 eV energy range. The refractive index n and absorption coefficient α obtained by both methods show unique results in the overlap wavelength region. Refractive index n is found to follow the Sellmeir dispersion relationship n2(λ)=2.272+304.72/(λ2−294.02) below the fundamental band edge. A free excitonic structure at the band is clearly observed at room temperature, with the transmission energy of free exciton at 3.44 eV, which is in reasonable agreement with the reported results.
270 citations
TL;DR: In this paper, a pure ceramic ZnO target was ablated by the KrF laser pulses (248 nm, 10 Hz, 1 J/cm2) in an ultra high vacuum to deposit ZnOs on sapphire (0001) substrate.
Abstract: High quality ZnO thin film was grown by Laser MBE. A pure ceramic ZnO target was ablated by the KrF laser pulses (248 nm, 10 Hz, 1 J/cm2) in an ultra high vacuum to deposit ZnO film on sapphire (0001) substrate. The lateral grain size was about 50 nm for the sample with thickness of 55 nm. At room temperature, the peak of the exciton absorption and the photoluminescence have the same energy. Under high density excitation (355 nm, 35 ps, 10 Hz), an exciton–exciton collision process was observed as P2 and P lines where 2S exciton and ionized exciton remain. From the edge of the sample, a very rapid increase of the P line was observed with the increase of the excitation power. A fine structure that comes from the cavity mode was also observed. These facts suggest that the lasing of the exciton was observed at room temperature.
269 citations
TL;DR: In this article, an exact correspondence between Coulomb blockade in the device's vertical transport properties and Pauli blocking in the transmission spectra is established, and substantial shifts, up to 20 meV, in the energies of higher excitations on occupation of the electron ground state are observed.
Abstract: Interband excitations of an ensemble of InAs self-assembled quantum dots have been directly observed in transmission experiments. The dots are embedded in a field-effect structure allowing us to load the dots electrically. We establish an exact correspondence between Coulomb blockade in the device's vertical transport properties and Pauli blocking in the transmission spectra. We observe substantial shifts, up to 20 meV, in the energies of the higher excitations on occupation of the electron ground state. We argue that this is a consequence of an exciton-electron interaction.
254 citations
TL;DR: In this paper, the growth modes of ZnO epilayers were investigated by reflection high-energy electron diffraction, and a transition from two-dimensional nucleation to three-dimensional (3D) nucleation is found at the initial growth stage.
TL;DR: In this article, the exciton-phonon coupling strength of PbS nanocrystals that strongly confine both charge carriers was investigated using resonant Raman scattering, and the strength of this coupling was four orders of magnitude greater than that predicted theoretically for the intrinsic states of the nanocrystal.
Abstract: The exciton-phonon coupling strength of PbS nanocrystals that strongly confine both charge carriers is investigated using resonant Raman scattering. The strength of this coupling (Huang-Rhys parameter S\ensuremath{\sim}0.7) is four orders of magnitude greater than that predicted theoretically for the intrinsic states of the nanocrystal, but is the same order of magnitude as that measured for Cd(S,Se) nanocrystals. The large exciton-phonon coupling is consistent with one charge carrier being localized at the surface of the nanocrystal.
TL;DR: In this paper, coherent acoustic phonons were observed in PbS quantum dots using femtosecond optical techniques, and the phonons are generated through the deformationpotential coupling to the quantum-dot exciton.
Abstract: Coherent acoustic phonons in PbS quantum dots are observed using femtosecond optical techniques. This is the first observation of coherent acoustic phonons in a semiconductor quantum dot; the phonons are generated through the deformation-potential coupling to the quantum-dot exciton. The acoustic modes are weakly damped, and we also find extremely weak coupling ( $S\ensuremath{\sim}0.01$) to the optical modes. These conclusions have important consequences for the vibronic nature of the exciton transition in the quantum dot and its dephasing.
TL;DR: In this paper, the authors systematically studied the strain dependence of the free-exciton resonance energies in wurtzite GaN by photoreflectance measurements using well-characterized samples.
Abstract: We have systematically studied the strain dependence of the free-exciton resonance energies in wurtzite GaN by photoreflectance measurements using well-characterized samples. The experimental data have been analyzed using the appropriate Hamiltonian for the valence bands in wurtzite GaN and determined the values of the crystal field splitting, the spin–orbit splitting, the shear deformation potential constants, and the energy gap in the unstrained crystal. Discussions are given on the strain dependence of the energy gaps, of the effective masses, and of the binding energies for the free-exciton ground states as well as on the valence-band parameters.
TL;DR: In this paper, the effects of electron-hole interaction on the exciton energy of semiconductor quantum dots are calculated using pseudopotential wave functions, and a comparison with the widely used, but never tested, effective mass approximation (EMA) is made.
Abstract: The effects of electron-hole interaction on the exciton energy of semiconductor quantum dots are calculated using pseudopotential wave functions. A comparison with the widely used, but never tested, effective-mass approximation (EMA) shows that the electron-hole Coulomb energy is significantly ( $\ensuremath{\sim}40%$) overestimated by the EMA, and that the scaling with the dot size $R$ is sublinear in $1/R$. The exchange splitting is much smaller than the Coulomb energy, and in the case of CdSe quantum dots shows significant deviations from the ${1/R}^{3}$ scaling predicted by the EMA.
TL;DR: In this paper, femtosecond pump-probe experiments are combined with computer simulations using the recently established crystal structure of these systems to assess the nature of excitation motion.
Abstract: We have studied excitation energy transfer in the photosynthetic antenna systems LH1 and LH2 of purple bacteria. Femtosecond pump−probe experiments are combined with computer simulations using the recently established crystal structure of these systems to assess the nature of excitation motion. We have measured the transient absorption kinetics and spectra of the LH1 and LH2 complexes in the temperature range from 4.2 to 296 K with femtosecond time resolution. The calculations based on the Pauli master equation disagreed with experimentally measured population and anisotropy kinetics, suggesting that the simple model of excitation hopping between bacteriochlorophyll a molecules is not a proper description for energy transport in LH1 and LH2. As a next step we have used the exciton theory to reproduce the transient absorption spectra of LH2, and we found that the coherence length of the exciton in B850 of LH2 1.5 ps after excitation of B800 is 4 ± 1.
TL;DR: In this article, a comprehensive study of the photoluminescence properties of ZnS thin films between 1.6 and 320 K grown by metalorganic molecular beam epitaxy and chemical beam epitaxys on GaAs substrates was reported.
Abstract: A comprehensive study is reported of the photoluminescence properties of ZnS thin films between 1.6 and 320 K grown by metalorganic molecular beam epitaxy and chemical beam epitaxy on GaAs substrates. Both heavy- and light-hole free excitons were observed at low temperatures with linewidths of 7.0 and 5.3 meV, respectively, as well as donor- and acceptor-bound excitons and free-to-bound recombination along with their longitudinal optical (LO) phonon replicas. The free exciton emission was observed up to 320 K, and enabled the room temperature band gap of ZnS to be unambiguously determined as 3.723 eV. The temperature dependence of the peak position, intensity, and linewidth was well described by the conventional empirical relations and by Toyozawa’s exciton line shape theory. The bound exciton peak positions were found to follow the temperature dependence of the band gap whereas the free-to-bound recombination feature was displaced by (1/2)kT above the band gap energy. Thermal quenching of the donor-bound...
TL;DR: In this paper, the lifetime of excess charge carriers in GaN epitaxial layers grown on sapphire at temperatures up to 300 K was measured by using picosecond time-resolved photoluminescence.
Abstract: By using picosecond time-resolved photoluminescence we have measured the lifetime of excess charge carriers in GaN epitaxial layers grown on sapphire at temperatures up to 300 K. The decay time turns out to be dominated by trapping processes at low excitation levels. The radiative lifetime derived from our data is dominated by free excitons at temperatures below 150 K, but also clearly shows the gradual thermal dissociation of excitons at higher temperatures. From our data, we are able to determine the free exciton binding energy and the free carrier radiative recombination coefficient. By combining these data with optical absorption data, we find the interband momentum matrix element and an estimate for the hole effective mass, which is much larger than previously thought.
TL;DR: In this article, fullpotential linearized LMTO calculations within the local density approximation were used to determine the conduction and valence-band effective mass tensors and the related Rashba-Sheka-Pikus and Kohn-Luttinger Hamiltonian parameters for wurtzite and zinc-blende GaN and AlN.
Abstract: Full-potential (FP) linearized muffin-tin orbital (LMTO) calculations within the local density approximation are used to determine the conduction and valence-band effective mass tensors and the related Rashba-Sheka-Pikus and Kohn-Luttinger Hamiltonian parameters for wurtzite and zinc-blende GaN and AlN. Spin-orbit coupling effects are obtained from atomic-sphere approximation (ASA) LMTO calculations. The $c$-axis uniaxial strain dependence of the crystal field splitting is determined and the combined results are used to interpret experimental data on the strain dependence of exciton splittings in GaN. Corrections beyond the local density approximation on the effective masses and crystal-field splittings as well as the accuracy of the quasicubic model are discussed.
01 Mar 1997
TL;DR: The mean-field theory of Keldysh and Kopaev, exact in both the high density and the low density limits, is solved numerically and the electron-hole pairing gap and the excitation spectrum of the exciton condensate are obtained.
Abstract: We propose that the exciton condensate may form in a well-controlled way in appropriately arranged semiconductor quantum well structures. The mean-field theory of Keldysh and Kopaev, exact in both the high density and the low density limits, is solved numerically to illustrate our proposal. The electron-hole pairing gap and the excitation spectrum of the exciton condensate are obtained. The energy scales of the condensate are substantial at higher densities. We discuss how such densities could be achieved experimentally by generating an effective pressure.
TL;DR: In this paper, a real-space formulation of time-resolved fluorescence of molecular aggregates is developed using the one-exciton density matrix ρ(t) of the optically driven system.
Abstract: A real-space formulation of time-resolved fluorescence of molecular aggregates is developed using the one-exciton density matrix ρ(t) of the optically driven system. A direct relationship is established between the superradiance enhancement factor Ls and the exciton coherence size Lρ associated with the off-diagonal density matrix elements in the molecular representation. Various factors which affect the latter, including finite temperature, energetic disorder, coupling with phonons, and polaron formation are explored. The theory is applied for the interpretation of recent measurements in the B850 system of the LH2 photosynthetic complexes.
TL;DR: In this paper, a microscopic definition of the exciton mean free path (Lf) and localization size (Lρ) is given in terms of the off-diagonal elements of the green function and density matrix, respectively.
Abstract: The pump−probe signal from the light-harvesting antenna LH2 of purple bacteria is analyzed using a Green function expression derived by solving the nonlinear exciton-oscillator equations of motion (NEE). A microscopic definition of the exciton mean free path (Lf) and localization size (Lρ) is given in terms of the off-diagonal elements of the exciton Green function and density matrix, respectively. Using phonon-induced (homogeneous) and disorder-induced (inhomogeneous) line widths compatible with superradiane measurements, we find that at 4.2 K the localization size is Lρ = 15 and that the shift ΔΩ between the positive and negative peaks in the differential absorption is determined by a different effective size Lf/2 = 5.6 associated with the exciton mean free path. Our model further predicts the recently observed superradiance coherence size determined by Lρ.
TL;DR: In this article, the photoinduced ultrafast dynamics of singlet excitons in light-harvesting antennae using multilevel Redfield theory is investigated, where the authors focus on the influence of dynamic exciton-vibrational coupling and static diagonal disorder on transient absorption spectra of peripheral antennae in photosynthetic purple bacteria.
Abstract: The photoinduced ultrafast dynamics of singlet excitons in light-harvesting antennae is investigated using multilevel Redfield theory. Formulating the equations of motion for the reduced exciton density operator in terms of one- and two-exciton eigenstates we focus attention on the influence of dynamic exciton-vibrational coupling and static diagonal disorder on transient absorption spectra of peripheral antennae in photosynthetic purple bacteria. The simulations are discussed in view of recent experimental results obtained for the B850 absorption band of Rhodobacter sphaeroides. Further, we suggest a new way of estimating the size of the exciton coherence domain in these systems which puts emphasis on the dynamic character of exciton localization. For the B850 pigment pool we find that at room temperature the pump–pulse initially prepares a coherent superposition of one-exciton eigenstates which can be delocalized over the whole aggregate. With increasing delay time the exciton coherence domain shrinks t...
TL;DR: The distance over which singlet energy is transferred in polycrystalline films of perylene bis(phenethylimide), PPEI, was measured by a surface quenching technique in films ranging in thickness from 004 to 23 μm as discussed by the authors.
Abstract: The distance over which singlet energy is transferred in polycrystalline films of perylene bis(phenethylimide), PPEI, was measured by a surface quenching technique in films ranging in thickness from 004 to 23 μm Radiative energy transfer was not observed Accurate values of the exciton transfer length could be obtained only with quenchers exhibiting rapid surface quenching velocities (>105 cm/s), such as poly(3-methylthiophene) The measured singlet exciton transfer length of 25 ± 05 μm is apparently the longest yet reported Its approximate value can be inferred directly from the experimental data and is therefore essentially independent of the assumed theoretical model Our measurements contain no direct information about the mechanism of the exciton motion; however, if it is assumed to be diffusional, the calculated intermolecular exciton hopping time, τh ≪ 100 fs, is unusually fast This suggests that excitons, in fact, may be delocalized over a number of molecules and that coherent energy transf
TL;DR: In this article, the authors investigate polariton states in a cavity containing organic and inorganic quantum wells (QW) when the energies of 2D Frenkel exciton in the organic QW and of a semiconductor Wannier exciton are near the energy of the cavity optical mode.
TL;DR: In this article, the near band edge photoluminescence (PL) of cubic GaN epilayers grown by radio frequency (rf) plasma-assisted molecular beam epitaxy on (100) GaAs is measured.
Abstract: The near band edge photoluminescence (PL) of cubic GaN epilayers grown by radio frequency (rf) plasma-assisted molecular beam epitaxy on (100) GaAs is measured. Since the PL is excited with an unfocused laser beam it resembles the layer properties rather than the properties of micron-size inclusions or micro crystals. The low temperature PL spectra show well separated lines at 3.26 and 3.15 eV which are due to excitonic and donor-acceptor pair transitions (donor binding energy 25 meV, acceptor binding energy 130 meV). No emission above the band gap of the cubic phase is detected. PL results are confirmed by x-ray diffraction and atomic force microscopy which reveal only negligible contributions from hexagonal inclusions and micron size single crystals. The room temperature PL consists of an emission band at about 3.21 eV with a full width at half maximum of 117 meV.
TL;DR: In this article, a forward reconvolution fit procedure based on a microscopic incoherent transport model, including diagonal disorder, dipolar intersite coupling, and a density-of-states (DOS) of molecular site excitations, is presented.
Abstract: Exciton relaxation in poly(phenylenevinylene), PPV, has been probed by femtosecond luminescence-up-conversion. We report on excitonic luminescence profiles that depend on the spectral position of the detection window (e=2.7,2.6,2.5, and 2.4 eV, respectively). In an attempt to reveal the transient steps implied in fs relaxation, we present a quantitative forward reconvolution fit procedure that is based on a microscopic incoherent transport model, including diagonal disorder, dipolar intersite coupling, and a density-of-states (DOS) of molecular site excitations. Special emphasis has been placed (i) on the analysis of luminescence lifetime distributions 〈φ(τ;e,τ0)〉 which directly map out the spectra of hopping modes of energy-cascading neutral excitations, and (ii) on the rigorous evaluation of (radiationless) transfer population from high-energy subensembles to low-energy tail states of the DOS. We quantitatively show that the absence of significant rise terms in the S0ν=0←S1ν=0 luminescence transition ...
TL;DR: The perovskite quantum-well films showed strong exciton absorption at around 2.4 eV and sharp exciton emission even at room temperature, and the X ray diffraction measurement on the vacuum-deposited films demonstrated that the layer structure was oriented parallel to the film plane as discussed by the authors.
Abstract: Oriented thin films of layered perovskite compounds (RNH3)2PbI4, which possess a quantum well structure where a two-dimensional semiconductor layer of PbI4 and an organic ammonium layer of RNH3 are alternately piled up, were found to grow in a self-organizing manner on fused quartz substrates through the simple dual-source vapor deposition of organic ammonium iodide RNH3I and lead iodide PbI2. The perovskite quantum-well films showed strong exciton absorption at around 2.4 eV and sharp exciton emission even at room temperature. Further, the X ray diffraction measurement on the vacuum-deposited films demonstrated that the layer structure of the vacuum-deposited film was oriented parallel to the film plane.
TL;DR: In this article, the Coulomb interaction parameters for poly(para-phenylenevinylene) have been investigated in a model Hamiltonian with variable on-site and long-range Coulomb interactions between the electrons.
Abstract: Within a model Hamiltonian with variable on-site and long-range Coulomb interactions between the \ensuremath{\pi} electrons for poly(para-phenylenevinylene), we conduct a thorough search in the parameter space to determine the magnitudes of the effective Coulomb interaction parameters necessary to fit all four absorption bands that are seen in the experimental absorption spectra of this material. We find best agreement between the calculated and experimental absorption spectra with Coulomb interactions that are slightly smaller than the standard Pariser-Parr-Pople parameters. For these values of the Coulomb parameters, the primary photoexcitation in poly(para-phenylenevinylene) is to an exciton with binding energy close to 0.9\ifmmode\pm\else\textpm\fi{}0.2 eV. This result, obtained from fitting the linear absorption, is in agreement with nonlinear absorption studies, viz. electroabsorption, two-photon absorption, and picosecond photoinduced absorption, within our model. We have also calculated the energies of the lowest triplet state, and the final state to which triplet absorption occurs. The excited triplet state is an exciton. We show that the latter result, taken together with the known experimental triplet absorption energy, indicates that estimates of 0.2 eV or less for the binding energy are incorrect. We briefly discuss the possibility that the binding energy has an intermediate magnitude.
TL;DR: In this article, a model of the exciton bound to stacking faults (SF) in GaN is suggested, and it is shown that SFs are potential wells for electrons and potential barriers for holes.
Abstract: A model of the exciton bound to stacking faults (SF) in GaN is suggested. It is shown that SFs are potential wells (depth ≈120 meV and width ≈10 A) for electrons and potential barriers (height ≈60 meV and width ≈10 A) for holes. The binding energy of excitons at SF found from variational calculation is 45 meV. The 364 nm line in GaN photoluminescence at 4 K is attributed to excitons bound to SFs.