Showing papers on "Exciton published in 1974"

Quantum States of Confined Carriers in Very Thin Al x Ga 1 − x As -GaAs- Al x Ga 1 − x As Heterostructures

Abstract: Quantum levels associated with the confinement of carriers in very thin, molecular-beam—grown AlxGa1−xAs−GaAs-Alx Ga1−xAs heterostructures result in pronounced structure in the GaAs optical absorption spectrum Up to eight resolved exciton transitions, associated with different bound-electron and bound-hole states, have been observed The heterostructure behaves as a simple rectangular potential well with a depth of ≈088ΔE g , for confining electrons and ≈012ΔE g for confining holes, where ΔE g is the difference in the semiconductor energy gaps

Luminescence in epitaxial GaN : Cd

Abstract: Investigations on luminescent properties of good‐quality GaN single‐crystalline epitaxial layers at temperatures in the range 1.6–300 K are reported. The high‐energy region (3.27–3.48 eV) at 1.6 K is dominated by different bound exciton transitions. Peaks are observed at 3.469, 3.454, 3.447, 3.400, 3.377, 3.355, and 3.287 eV at 1.6 K in this region; their possible origin is discussed. At higher temperatures (T > 30 K) the free A‐exciton emission is also clearly observed, and thus its position EAx=3.475±0.0005 eV at 1.6 K could be established (neglecting polariton effects) together with a value of 6.4±0.4 meV for the binding energy of an exciton to a neutral donor. The energy region 3.0–3.3 eV is dominated by the previously studied donor‐acceptor pair spectra, which peak at 3.263±0.003 in all our good‐quality samples (undoped and doped). From the assignment of a subsidiary high‐energy peak as free‐to‐bound transitions, a value of 29±6 meV for the donor binding energy is experimentally determined, in good a...

Temperature dependence of the radiative recombination coefficient in silicon

Abstract: The temperature dependence of the intrinsic radiative recombination coefficient B in silicon was measured from 100 to 400 °K. In contrast to previous calculations, B decreases with temperature. For interpretation of the measurements, a unique theory of the indirect radiative band-to-band and free exciton recombination is given. The decrease of B with increasing T is due to the decrease to the exciton concentration and of the Coulomb-enhancement of the band-to-band recombination rate. The latter however is smaller than the exciton recombination part even at room temperature. The constants of the exciton and band-to-band recombination are determined.

Excited-state absorption spectroscopy of self-trapped excitons in alkali halides

Abstract: Optical transitions between the lowest metastable triplet state and higher states of the self-trapped exciton are reported for nine alkali halides, in the energy range from 0.6 to 5.5 eV. Measurements were made on pure crystals using techniques of time-resolved spectroscopy and electron-pulse irradiation. Variation of the transition energies with lattice constant provides additional evidence for the classification of the spectra in terms of two basic categories: hole transitions localized on the $X_{2}^{}{}_{}{}^{\ensuremath{-}}$ core, and electron transitions largely decoupled from the influence of the core ions. Optical binding energies of the self-trapped exciton are estimated from the spectra, and are found to be 2-4 times greater than the binding energies of corresponding unrelaxed excitons. The relaxation of the lattice around the exciton is discussed in terms of a two-dimensional configuration-coordinate model. Hole self-trapping and the Stokes shift of emission occur principally through the axial mode, as commonly accepted. The increase of exciton binding energy upon relaxation to the equilibrium configuration is attributed to a nonaxial mode of relaxation which may be viewed as analogous to the breathing mode that broadens and shifts $M$-center transitions.

Exciton Polarons of Molecular Crystal Model. I. –Dynamical CPA–

Abstract: Energy spectrum of exciton polaron is studied with the dynamical CPA (coherent potential approximation), which is introduced for inelastic scattering by Einstein phonons at every lattice site. The coherent potential at energy E is determined by the potentials at energies apart from E by integral times the phonon energy. We determine applicability ranges of the concepts used in the limiting cases; (I) the nearly free exciton for weak coupling, (II) multiple bands of the vibronic excitons for small excitation transfer, and (III) the self-trapped exciton for strong coupling. An ambiguous boundary separating (II) and (III) lies about S ≃6, with S representing the ratio of the energy gain of exciton localization to the phonon energy. The change with increasing excitation transfer for a fixed S is characterized by the sharp crossing of (III) with (I) for \(S{\gtrsim}6\), while by the gradual merging of (II) into (I) for \(S{\lesssim}6\).

Nature of the dead layer in CdS and its effect on exciton reflectance spectra

Abstract: Experimental spectra of exciton reflectance in CdS at \ensuremath{\sim} 2\ifmmode^\circ\else\textdegree\fi{}K are measured for different conditions of the surface The results are explained in terms of optical interference across a "dead layer" which can be of either intrinsic or extrinsic origin In the former case, a one-to-one correspondence between the dead-layer depth and the orbital size of each hydrogenic line is established The Hopfield and Thomas additional boundary conditions with spatial dispersion are shown to be quite adequate at explaining the observed features The general significance of the surface interference effect in the reflectance behavior of Wannier excitons is demonstrated by consideration of results known for other semiconductors

Impurity quenching of molcular excitons. III. Partially coherent excitons in linear chains

Abstract: We have extended a model of exciton quenching, previously developed for pure Frenkel excitons, to include the exciton‐phonon interaction. We introduce a mathematically tractable exciton density matrix equation that describes the motion of excitons on a finite chain quenched at both ends. From this equation we find that the excitation on the chain as a function of time exhibits several types of kinetic behavior that result from the interplay of coherent, incoherent, and quenching terms. This behavior changes as a continuous function of the strength of the incoherent terms in such a way as to define five kinetic regions, ranging from that of pure Frenkel excitons to that of completely incoherent (hopping) excitons. In each of these five regions the average excitation decay rate constant is proportional to cp, p = 0, 1, 2, or 3, where c is the quencher concentration.

Coherent energy migration in solids i. band-trap equilibria at boltzmann and non-boltzmann temperatures

Abstract: A model is presented which relates the dynamics of energy migration in crystals to the mechanism by which thermal equilibrium between delocalized band states and localized trap states is achieved. Central to this model is the requirement that coherent energy migration must be the dominant mode of migration at low temperatures in order to achieve Boltzmann equilibrium between band and trap states within the lifetime of the excited electronic state. Second, a stochastic model for detrapping is developed which is based on an irreversible radiationless relaxation process of a phonon-trap intermediate into the density of delocalized band states. Explicit account is taken of phonon-trap interactions in the formation of the excited trap intermediate. Further, the relation between detrapping and the ability of a crystal to achieve thermal equilibrium within the excited-state lifetime is developed and applied to one-dimensional crystals. Experimental results on molecular crystals representing examples of one-dimensional exciton bands are also presented. Specifically, the temperature dependence of phosphorescence from excited triplet trap states is interpreted in terms of the above considerations. From these experiments one can obtain both the sign of the intermolecular interaction and the dispersion of the first excited triplet band in addition to an estimate of the coherence length associated with exciton migration in the Frenkel limit. Finally, some new and unique methods for studying energy migration are presented which utilize optically detected magnetic-resonance techniques in zero field. They include experiments based on the measurement of electron-spin coherence in the rotating frame and the relationship of the spin coherence to the various rate processes important in trap-exciton interactions.

On the origin of bound exciton lines in indium phosphide and gallium arsenide

Abstract: The authors show from a comparison of optical absorption and emission spectra of high-quality single crystals that the characteristic narrow doublet in the edge luminescence is produced by the decay of excitons bound to neutral acceptors from the J=5/2 and J=3/2 exciton states, with the former lying lowest. The further J=1/2 bound exciton state expected on a j-j coupling model is now firmly identified with a weak higher-energy line previously reported for both materials. These assignments are consistent with the more positive of the conclusions obtained from Zeeman analysis of these transitions, although some less certain features conflict.

Preliminary Studies of Charge Carrier Transport in Mercuric Iodide Radiation Detectors

Abstract: Mercuric iodide single crystals have been grown by static and dynamic sublimation methods. Characteristics of contacts and detector capacitance have been studied by photon excitation methods. Gamma and X-ray spectrometry has been carried out with completed detectors showing resolutions comparable to the best results published to date. A measurement of hole trapping length has been made from the spectral shapes observed and has been found to be approximately 0.3 mm. Transient waveform analysis with alpha-particle excitation shows hole mobilities of approximately 3 cm2/V-sec for a highly purified crystal and 0.05 for an expected less pure crystal. Electron mobilities of 120 cm2/V-sec are observed. An attempt is made to explain the observed transient waveforms in terms of a single dominant trap model, with only partial success. Due to the strongly excitonic character of the material, it is proposed that the unfamiliar observations made regarding transport properties with the HgI2 detectors studied may be due to exciton dissociation under high electric fields, to long exciton lifetimes and to interactions between excitons and trapping centers in the material.

Optical Absorption Edge in GaSe under Hydrostatic Pressure

Abstract: A study of the optical absorption spectrum of GaSe at the energy gap shows that the strength of the exciton line decreases as hydrostatic pressure is increased. It is proposed that this effect comes from increasing interference of the discrete exciton line with a continuum of structure due to indirect interband electronic transitions. The sign and magnitude of the pressure coefficients of the gaps in gallium chalcogenides are found to be consistent with recent band-structure studies.

Photoluminescence Study of Carbon Doped Gallium Arsenide

Abstract: The photoluminescence studies of the carbon doped epitaxial gallium arsenide revealed that a pair of emission bands at 1.493 eV (B-A) and 1.490 eV (D-A) were attributed to the carbon acceptor on arsenic site. The sharp doublet emission lines at 1.5127 eV and 1.4937 eV were identified with the exciton recombination bound to a neutral carbon acceptor and the two-hole transition in which the neutral carbon acceptor is left in an excited state, respectively. From the observation of these emissions, the 1S3/2 ground and 2S3/2 excited state energies of carbon acceptor were found to be 26.5 meV and 7.5 meV above the valence band edge, respectively. The results were supported by the effective mass arguments and by the dependence of the binding energy on the atomic number of impurities.

Optical Properties and Energy Band Structure of Zn3P2and Cd3P2 Crystals

Abstract: Edge absorption, photoconductivity, and reflectivity spectra are investigated of Zn3P2 crystals in the range from 1 to 12.5 eV and reflectivity spectra of Cd3P2 crystals in the range from 1 to 12.5 eV. The Eg value of Zn3P2 has been experimentally determined at 293 and 77 OK. Its nature has been found to be due to direct transitions. Three peaks of exciton and interband character have been observed in the photoconductivity spectrum of Zn3P2 single crystals. Complex structures (about 12 peaks) have been observed in the reflectivity spectra of both compounds. Most of these structures may be accounted for in terms of the well-known theoretical calculations of their bands. Several peaks have been observed in the most thoroughly studied spectrum of Zn3P2. These peaks cannot be explained by the theoretical band scheme and are probably caused by the peculiarities of the bands of real crystals of the Zn3P2 type, which have been lost in the well-known theoretical model because of the accepted simplifications of the crystal lattice. [Russian Language Ignored].

The Effect of High Density Excitons on the Exciton Bands in CuCl

Abstract: The effect of the strong laser irradiation on the exciton bands of CuCl has been studied. The Z 3 and Z 1,2 exciton bands are found to shift to high energy side linearly with the laser intensity. The fact is explained in terms of Hanamura's theory which describes the increase of the exciton formation energy due to the interaction between excitons. The ratio of the peak shift of the Z 1,2 band to that of the Z 3 band is 0.89. From the ratio and the reduced mass of the Z 3 exciton, the effective masses of the electron and hold are deduced to be 0.44 m 0 and 3.6 m 0 , respectively.

Photoelectrical properties, energy level spectra, and photogeneration mechanisms of pentacene

Abstract: The main optical, dark and photoconductivity, photo-emf, and photoemission characteristics are studied for thin-layers of pentacene (Pc) crystals. It is shown that in the h = = 2.2 to 4.0 eV spectral region intrinsic photogeneration is dominant in Pc crystals, presumably proceeding via an autoionization mechanisms with a threshold value Ec = = (2.20 ± 0.05) eV. Whereas in the hvv = 1.4 to 2.2 eV spectral region an exciton photogeneration mechanism, namely, generation of excess charge carriers through triplet exciton-trapped hole interaction, seems most probably. The energy level spectra of neutral and ionized states of the Pc crystal are determined. The Pc crystal ionization energy Ic equals to (5.07 ± 0.05) eV; the forbidden energy gap ΔE0 = (2.20 ± 0.05) eV; the intrinsic dark conductivity activation energy E = ΔE0/2 = (1.15 ± 0.15) eV. The electron affinity Ac of the Pc crystal, determined by three independent methods equals to (2.87 ± 0.10) eV, (2.70 ± 0.15) eV, or (2.73 ± 0.2) eV, respectively. The characteristic energies of the quasi-continuous trap distribution for vacuum evaporated thin Pc crystals layers are kTc = 0.12 eV in an exponential approximation and ± = 0.28 eV in a Gaussian one as determined from SCLC measurements. [Russian Text Ignored].

Energy and Charge Transfer in Organic Semiconductors

Abstract: Opening Address.- Charge Carriers.- Voltage Dependence of Unipolar Excess Bulk Charge Density in Organic Insulators.- Geminate Charge-Pair Recombination in Molecular Crystals.- Various Detrapping Processes in Anthraquinone-Doped Anthracene Crystal.- UV Excitation of Anthracene.- Photoconduction and Emission of Phthalocyanine in the Near Infrared.- Optical and Electron Paramagnetic Properties of Radicals in Naphthalene and Anthracene Crystals.- Contribution of Free Radicals to Electrical Conductivity.- Theory.- Green Function of an Exciton Coupled with Phonons in Molecular Crystals.- Theory of Frenkel Excitons Using a Two-Level-Atom Model.- Crystallization.- Glass-Forming Photoconductive Organic Compounds. I. Phase Change and Single Crystal Growth of 1,3-Diphenyl-5-(p-Chlorophenyl)-Pyrazoline.- Semi conductive Coordination Polymers: Dithiooxamides Copper Compounds.- Growth and Purification of Phthalocyanine Polymorphs.- Transport in Amorphous State and Polymers.- Mobility Measurements in Polymers by Pulsed Electron Beams.- Photoconductivity of Poly-N-Vinylcarbazole.- Sensitized Photocurrent in Dye-Deposited Poly-(N-Vinylcarbazole).- Charge Injection and Transport in Disordered Organic Solids.- Charge Transport in TNF:PVK and TNF: Polyester Films and in Liquid, Amorphous and Crystalline TNF.- Superconductivity.- The Problem of Superconductivity in Organic and Organo-Metallic Compounds.- Comments on the Metallic Conductivity in (TTF)(TCNQ) Complex.- Design and Study of 1-Dimensional Organic Conductors I: The Role of Structural Disorder.- Design and Study of 1-Dimensional Organic Conductors II: TTF-TCNQ and Other Organic Semimetals.- Solid Molecular Complexes.- The Structures and Properties of Solid Molecular Complexes and Radical Salts: A Brief Review of Recent Studies in Japan.- Microwave Dielectric Constants of Solid Molecular Complexes.- Electrical and Optical Properties of the Phenothiazine-Iodine and Related Complexes.- Author Index.

Recombination processes in highly excited CdS

Abstract: Experimental results on the spontaneous and stimulated emission of CdS are presented in the temperature range from liquid‐helium temperature to room temperature under electron‐beam excitation. At temperatures below 100 K, the dominating laser process results from the exciton‐exciton interaction. In addition, lasing on the first and second phonon replica of the A1 exciton has been observed. At temperatures above 100 K, the laser emission is found to shift to lower energies much stronger than that of the band gap. This shift can quantitatively be accounted for by the exciton‐electron interaction process.

Indirect-Forbidden Transitions in TlCl and TlBr

Abstract: The optical absorption spectra of purified TlCl and TlBr have been investigated near the band edge. It was found that the absorption rises as a three half power of the energy near the edge and as a cube of the energy in the high energy region. It is concluded that the edge is due to the creation of the is indirect-forbidden exctions of X 6 + ×R 6 - and the lowest minimum in the conduction band is at the R-point. Theroretical calculation of an indirect-forbidden exciton transition is made. The result is differnt from what has been proposed by the previous works, but it describes well the expermental results. The binding energy of the indirect-forbidden exction in TlBr is determined as 23±5 meV.

Anisotropic exciton polaritons

Abstract: Some aspects of exciton polariton behaviour in anisotropic crystals are investigated for a specific model of the response function of an exciton split by a uniaxial field The extraordinary-polariton dispersion scheme shows an additional branch for propagation in a nonprincipal direction appearing in the anomalous dispersion gap of the cubic case The reflectivity spectrum obtained for this case is in good agreement with measurements performed on the basal plane of the uniaxial crystal PbI2 under oblique incidence For a consideration of spatial-dispersion effects (finite exciton mass) the theory of Hopfield and Thomas is extended to uniaxial crystals It is found that three simultaneous waves exist for every frequency and that a sharp extra reflection peak appears for every long-wavelength polariton mode of nonzero frequency Reflection curves are calculated for different values of the exciton massm and the thicknesst of the Hopfield layer The experimental normal-incidence «extraordinary” spectrum obtained by Biellmannet al on the\((10\bar 11)\) of PbI2 which shows two additional sharp peaks can be reproduced by appropriate choice of the parametersm andt

Interband transitions in ZnO observed in low energy electron spectroscopy

Abstract: Using a high resolution two-axis-spectrometer energy loss spectra of electrons specularly reflected from (1¯100) ZnO-surfaces have been investigated. The electron impact energy was varied between 25 and 60 eV. The spectra show a strong dependence on the orientation of thec-axis with respect to the plane of incidence. The dependence of the loss intensities on the angle of incidence and the shape of the spectra can be qualitatively described by the “Dielectric Theory”. Contrary to optical reflection measurements the excitation of excitons could not be observed in agreement with the existence of a surface barrier for excitons.