Exciton Pair Research Articles

Ion photodesorption from argon multilayers

Ion photodesorption from argon condensed as mul- tilayers on a platinum substrate (from 3 to 200 monolayers) has been studied for photon energies varying from 20 to 100 eV. A large variety of excitation processes have been in- vestigated throughout this energy range. These include the formation of exciton pairs, satellite states, and doubly and triply charged ions, and the simultaneous multiple excita- tion of two and three atoms. Kinetic energy measurements of mass-selected desorbed ions are also presented. This allows us to discuss in detail the ion-desorption dynamics.

Electron Beam Induced Impurity Electro-Migration in Unintentionally Doped GaN

AbstractElectron beam induced electromigration of ON+ and H+ impurities in unintentionally n-doped GaN was investigated using cathodoluminescence (CL) kinetics profiling, CL imaging of regions pre-irradiated with a stationary electron beam, and wavelength dispersive x-ray spectrometry (WDS). The presented results (i) illustrate induced impurity diffusion in wide bandgap semiconductors, (ii) provide experimental evidence for the (VGa-ON)2- model of yellow luminescence in GaN with low Si content (ii) confirm the roles of 0 in frequently reported bound exciton and donor-acceptor pair emissions and (iv) suggest the involvement of ON+ and hydrogenated gallium vacancies in a blue emission in autodoped GaN.

Erratum: Theory of exciton pair states and their nonlinear optical properties in semiconductor quantum dots [Phys. Rev. B55, 5153 (1997)

Erratum: Theory of exciton pair states and their nonlinear optical properties in semiconductor quantum dots [Phys. Rev. B<b>55</b>, 5153 (1997)

Biexciton and Triexciton States in Quantum Dots in the Weak Confinement Regime

Biexciton and triexciton states in CuCl quantum dots were studied by means of time-resolved size-selective pump-and-probe technique. A clear induced absorption band is observed on the high-energy side of the excitation photon energy. The new induced absorption is assigned to the transition from the exciton ground state to one of the weakly correlated exciton pair states which were theoretically predicted to exist and to play an important role in nonlinear optical processes. Its pump energy dependence and temporal evolution strongly support this assignment. Under high-density or two-color excitation condition, a triexciton state in quantum dots is observed for the first time.

Evidence for triplet interchain polaron pairs and their transformations in polyphenylenevinylene

Photoluminescence of films of poly-$(p$-phenylenevinylene) (PPV), and changes of its intensity under conditions of electron spin resonance as a function of temperature and light intensity were studied. The experimental technique was based on the modulation of the spin state of paramagnetic species by resonant microwave transitions between Zeeman sublevels of dynamically spin-polarized pairs. Three types of resonant signals were found in the magnetic resonance spectrum: (i) a narrow (1.7 mT width at the half height), (ii) a broad (140 mT) enhancement signal at $g$=2, and (iii) the signal at $g$=4. The results permitted one to conclude that Coulomb bound polaron pairs are produced in PPV with high yield under 488-nm photoexcitation. The narrow signal is assumed to appear due to microwave-induced resonant transitions in triplet polaron pairs. This implies that the resonant transitions change the rate of geminate recombination of the pairs that leads to the formation of triplet intrachain excitons. Those excitons annihilate in the second-order reaction and show themselves in the intensity of the photoluminescence. The annihilation rate was found to be influenced by resonant transitions in triplet exciton pairs as well and resulted in broad and $g$=4 signals. The lifetime of triplet intrachain excitons was estimated from microwave modulation frequency dependence of resonant signal intensities. The results showed that the energy level of the lowest polaron pair state situated below that of singlet intrachain exciton can act as a sink of the excitation energy influencing the quantum yields of the photoluminescence, electroluminescence, and photoconductivity.

The near band edge photoluminescence of cubic GaN epilayers

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.

Theory of exciton pair states and their nonlinear optical properties in semiconductor quantum dots

The exciton and two-exciton states in semiconductor quantum dots much larger in size than the exciton Bohr radius are investigated, and the energies and oscillator strengths of several exciton and biexciton states are calculated. The presence of weakly correlated exciton-pair states are identified and these have a large oscillator strength increasing proportional to the volume of the quantum dot. These states are shown to play a crucial role in determining the nonlinear optical response of large quantum dots. The weakly correlated exciton-pair states are found to cause a cancellation effect in the third-order nonlinear optical susceptibility at the exciton resonance, providing a consistent understanding of the experimentally observed saturation of the mesoscopic enhancement of the excitonic optical nonlinearity. The excited-state absorption in quantum dots is also studied and the excitation of the weakly correlated exciton-pair states is found to dominate the spectrum. The spectral features in the pump-probe spectroscopy are predicted in detail. The biexciton binding energy and oscillator strength are obtained in good agreement with experimental results on CuCl quantum dots. Also, the good correspondence of the excited-state absorption spectra between the theory and experiments provides convincing evidence for the presence of the weakly correlated exciton-pair states.

Influence of the [formula omitted] molar ratio on the structural and electronic properties of MOVPE grown GaN

Low pressure MOVPE has been used to grow a series of GaN epilayers, deposited onto (0001) sapphire substrates. All the growth parameters have been kept constant, except for the V III molar ratio, which has been changed from sample to sample by changing the ammonia flow rate. The growth rate has been measured vs the V III molar ratio at both low temperature (550°C, growth of the buffer layer) and normal process temperature. A strong dependence of the growth rate vs the V III molar ratio is found at both temperatures. Systematic photoluminescence and reflectivity experiments have been performed at 2 K on all samples. The photoluminescence is dominated by the donor bound exciton and donor acceptor pair are visible on samples grown under the lowest V III molar ratio, indicating higher impurity incorporation. An optimum V III molar ratio of 5000 is proposed, which corresponds to the highest growth rate observed and to the lowest full width at half maximum for the photoluminescence line.

PHOTOLUMINESCENCE OF Sb-DOPED Hg1-xCdxTe

The infrared photoluminescence spectroscopy was performed for Sb-doped Hg1-xCdxTe(x≈0.38)from 3.9K to 115K. The band to band transition,localized exciton and donor acceptor pair(D0A0)related luminescence peaks were observed. The acceptor level which is related to Sb-doping and about 30 meV above the valence band was observed in photoluminescence experiment in Hg1-xCdxTe(x≈0.38).

Microscopic Aspects of the Staebler-Wronski Effect

ABSTRACTThe microscopic origin and the creation mechanisms of metastable, light-induced defects in hydrogenated amorphous silicon are reviewed. Based on excitonic electron-hole pair recombination, a consistent quantitative description of defect creation kinetics can be obtained, including the experimentally observed differences between continuous wave and pulsed illumination as well as the effect of competing recombination pathways in compensated material. High resolution spin resonance spectra obtained by low-field spin-dependent transport are used to examine the interaction of metastable defects with hydrogen.

Evidence for triplet inter-chain polaron pairs in poly(phenylene vinylene) and their transformations

Photoluminescence of films of poly(phenylene vinylene) (PPV), and changes of its intensity under conditions of electron spin-resonance as a function of temperature and light intensity were studied. Three types of resonant signals were found in magnetic resonance spectrum:(i) a narrow (17mT), (ii) a broad (130mT) enhancement signals at g=2, and (iii) the signal at g=4. The results permitted to conclude that Coulomb bound polaron pairs are produced in PPV under photoexcitation. The narrow signal was shown, to appear due to a microwave induced resonant transition in triplet polaron pairs. The resonant transitions changed the rate of geminate recombination of the pairs that was revealed to lead to formation of triplet intra-chain excitons. Those excitons annihilated in the second order reactions showing themselves as delayed fluorescence. The annihilation rate was found to be influenced by resonant transitions in triplet exciton pairs as well. The lifetime of triplet intra-chain excitons was estimated from microwave modulation frequency dependence. The results showed that energy level of the lowest polaron pair state situated below that of singlet intra-chain exciton can act as a sink of the excitation energy influencing the quantum yields of the photoluminescence, electroluminescence and photoconductivity

Weakly correlated exciton pair states in large quantum dots.

We present a calculation of the two-exciton states in semiconductor quantum dots much larger in size than the exciton Bohr radius, and identify a weakly correlated exciton pair state that has a large oscillator strength, increasing proportionately to the volume of the quantum dot. This state is shown to be responsible for the saturation of the size dependence of the resonant excitonic optical nonlinearity. It also provides a satisfactory understanding of the blueshift of the excited-state absorption in quantum dots. These results and the biexciton binding energy and oscillator strength are in good agreement with reported experimental results on CuCl.

Properties Of Homoepitaxially Mbe-Grown Gan

AbstractBulk single crystals of GaN were used for epitaxial growth of GaN films by molecular beam epitaxy. Low temperature photoluminescence yields much higher intensity emission in the near bandedge region for epitaxial films with respect to the situation in bulk crystals. Character of this luminescence changes also. Dominant band-to-band transitions in the bulk crystals are exchanged by bound exciton and/or donor-acceptor pair transitions observed in the epitaxial layers. We will compare the obtained results with the available data on the homoepitaxial samples grown by metalorganic chemical vapor deposition method and discuss the importance of establishing the basic information on energetic positions of excitonic transitions in stress free samples.

Biexcitons in quantum wells: A quantum Monte Carlo study

The exact binding energies of biexcitons in GaAs Ga 1−x Al x As quantum wells in the effective mass model are evaluated by diffusion quantum Monte Carlo simulations. The results support the experimental interpretation that biexcitons can exist in quantum confined structures such as the quantum wells of GaAs Ga 1−x Al x As . The electron correlation effects and quantum confinement are shown to enhance the binding of the exciton pairs. The calculated results for various quantum wells provide a guide for further experiments in accurate determination of the binding energies of biexcitons in quantum wells.

Yields of triplet exciton pairs in polydiacetylene films

Triplet-triplet-induced absorption has been measured in pump-probe experiments on polydiacetylene DCH films. The excitation spectrum of the induced absorption has been obtained for pump photon energies in the range 1.95–3.40 eV. Above 2.15 eV the induced absorption depends linearly on the pump intensity. Below this energy a quadratic law is valid showing that triplet excitons are produced in a two-step process for the lower photon energies.

Structure of the self-trapped exciton and nascent Frenkel pair in alkali halides: An ab initio study.

Adiabatic-potential-energy surfaces of the self-trapped exciton (STE) of both the spin-singlet and -triplet states in NaF, NaCl, and NaBr are calculated with use of ab initio Hartree-Fock cluster methods. In all cases, there is clear evidence of adiabatic instability in the on-center geometry. The STE undergoes a spontaneous symmetry-breaking relaxation, resulting in a structure which is equivalent to an F-H center pair (a Frenkel defect pair in the anion sublattice). The potential energy is remarkably flat, to within about 0.2 eV, for further separation of the H center from the F center up to a distance of about 3 \AA{}, the largest distance attained in this study. In view of the flat nature of the excited-state energy surface, and given the difficulties inherent in an ab initio computation, the calculated recombination energies are found to be within an acceptable range of values when compared with experiment for both the \ensuremath{\sigma} and \ensuremath{\pi} bands. From this work, the initial state of the \ensuremath{\sigma} emission band is attributed to the singlet STE of the lowest orbital state. On the basis of the adiabatic potential energies obtained in this work, a mechanism of \ensuremath{\pi}-band quenching is proposed.

Enhanced exciton mobilities in GaAs/AlGaAs and InGaAs/InP quantum wells

The authors have studied lateral exciton and electron-hole pair transport by space- and time-resolved spectroscopy under conditions where enhanced mobilities and diffusivities are observed. A transport/transfer model, including relaxation, lateral diffusion, transfer between the islands and recombination, is used to evaluate the authors measurements of growth-interrupted quantum wells. For 40 K<T<180 K the variation of the diffusivities with temperature can be described by an isothermal diffusion of excitons considering various scattering mechanisms. For growth-interrupted quantum wells they found considerably higher mobilities and diffusivities at low temperatures compared to continuously grown samples. This is attributed to a partial suppression of interface roughness scattering due to island formation on growth-interrupted interfaces. Unexpectedly high diffusivities were observed for 10 K<T<40 K which may be caused by non-equilibrium effects in the carrier and phonon system. The unexpected increase of the diffusivities with rising temperature observed in thin quantum wells for 180 K<T<230 K is attributed to the growing thermal dissociation of excitons into free carrier pairs.

One-photon two-exciton excitations in solid argon probed by photon stimulated desorption of Ar + and Ar 2+

Excitation of exciton pairs in the bulk and on the surface of solid Ar by single photon absorption are investigated via photon stimulated desorption of ions. For films of various thicknesses, line shapes and branching ratios into Ar + and Ar 2 + emission are studied. For the photon energy range from 23 to 29 eV, four bands can be discriminated which exhibit different contributions from bulk and surface excitations, respectively.

Exciton coherence-size and phonon-mediated optical nonlinearities in restricted geometries

The magnitude of optical nonlinearities of molecular nanostructures is determined by a characteristic coherence-length which controls the cooperativity of the optical response. Equations of motion describing the evolution of the optical polarization coupled to two-exciton variables are derived, and used to calculate the third order optical response (χ(3)) of a one-dimensional molecular crystal or a polymer. We show that the coherence-length is determined by the interplay between intramolecular and intermolecular (nonlocal) nonlinearities and explore the limitations of the local-field approximation. Phonon-mediated exciton–exciton attractive interaction may result in the formation of bound exciton pairs (biexcitons). We show how unbound (two-exciton) as well as biexciton states could readily be observed as resonances in two-photon absorption and third harmonic generation.

Formation and dynamics of exciton pairs in solid argon probed by electron-stimulated ion desorption.

Electron-stimulated desorption of ${\mathrm{Ar}}^{+}$, ${\mathrm{Ar}}_{2}^{+}$, ${\mathrm{Ar}}^{2+}$, and ${\mathrm{Kr}}^{+}$ ions from solid Ar and Kr was studied for the electron energy range from 10 to 120 eV. For ${\mathrm{Ar}}^{+}$ and ${\mathrm{Ar}}_{2}^{+}$ excitation resonances with onsets at 24.2, 25.4, 34, and 50 eV have been found. The two lowest thresholds cannot be explained by excitations of single Ar atoms. They are interpreted as pairs of interacting surface and bulk excitions, respectively, on neighboring atoms. The bulk species can diffuse over more than 100 layers. Novel conclusions about the desorption mechanism can be derived.