exciton-LO Phonon Interaction Research Articles

Phonon-assisted laser-induced third-order optical susceptibility for interband transition in a group II–VI asymmetric quantum well

Exciton–LO phonon interaction of the Fröhlich type is studied in the CdS/CdSe/CdS asymmetric double quantum well under an applied laser field. The intensity of laser effect on the exciton binding energy, oscillator strength, optical transition energies, and third-order harmonic generation are investigated by changing the right well sizes and barrier sizes. The laser-associated confined energies and their corresponding binding energies are brought out with the variation of well sizes. The position-dependent potential is altered by the laser-associated parameter. The laser-field-dependent third-order optical susceptibilities due to interband transitions in the asymmetric well are investigated. The effective Coulomb potential, using the Aldrich and Bajaj polaronic effect, is included in the Hamiltonian. The results show that the laser field considerably affects the confining potential of the quantum well and causes a drastic deformation of the confining potential profile. The obtained optical nonlinearities suffer a redshift in the asymmetric quantum well; also, their peak values are determined by the dipole matrix elements.

Reduced carrier trapping in CdSe/ZnS/CdSe heterostructure quantum dots inferred from temperature dependent spectral studies

We report temperature dependent photoluminescence (TDPL) characteristics of CdSe/ZnS core-shell and coupled 0D-2D CdSe/ZnS/CdSe heterostructure quantum dots (HQDs). Anomalous, behavior of temperature dependent PL linewidth seen in core-shell HQDs is ascribed to the relaxation of carriers into interfacial strain related local energy minimum. TDPL characteristics show largest exciton-acoustic phonon and exciton-LO phonon coupling in core-shell HQDs compared to other QDs. These are caused by enhanced deformation potential interaction and enhanced Fröhlich interaction due to the interfacial strain field. Further, relatively lower strength of exciton-acoustic phonon coupling in 0D-2D HQDs is due to relaxation of interfacial strain. Temperature dependence of spectrally integrated PL intensity shows nonradiative relaxation below 150 K due to thermal activation of carriers in surface tarp states while at temperatures above, it is dominated by carrier thermal escape mediated by exciton-LO phonon interaction involving lesser number of phonons in 0D-2D HQDs.

Photoluminescence studies of ZnO films fabricated by using a combination of a hydrothermal method and plasma-assisted molecular beam epitaxy regrowth

ZnO films were deposited on Si (100) substrates by using a two-step growth process. In the first step, ZnO nanorods were grown by using the hydrothermal method at 140 °C for 5 min. In the second step, a ZnO amorphous layer was deposited on the ZnO nanorods by spin-coating. After completion of the growth process, the films were annealed at 800 °C for 10 min, and a ZnO active layer was deposited on top of the amorphous layer by using plasma-assisted molecular beam epitaxy. Further, temperature-dependent photoluminescence (PL) measurement were conducted to study the optical properties of the prepared films. In the low-temperature PL spectra, emission peaks in the near-band-edge region were observed at 3.370, 3.362, 3.347, 3.329, 3.317, 3.288, 3.263, 3.219, 3.191, and 3.116 eV; these peaks were attributed to free excitons, neutral donor bound excitons, neutral acceptor donor excitons, two electron satellites, and donor acceptor pairs, respectively. These peaks were red-shifted, and their intensity decreased with increasing temperature. The binding energy of the donor was calculated as 43.1 meV by using the Haynes rule. Further, the value α and β, factors in the equation for the energy of localized excitons of donors and acceptors were obtained as 0.73 meV and 750 K, respectively, by fitting the free exciton (FX) peak according to Varshni’s equation. The full width at half-maximum of PL for the films was about 95.1 meV at room temperature; moreover, the following values were obtained for the films by using theoretical equations: the background impurity broadening, Γ0 = 62 meV, the parameter describing exciton-LO phonon interaction, Γ LO = 80 meV, LO phonon energy, ħω LO = 72 meV, and, the coupling strength of an exciton-acoustic phonon interaction, γ ph = 0.087 meV/K Furthermore, the activation energy was about 60.1 meV.

Excitonic properties of a spherical semiconductor quantum dot: The role of phonons

The present work investigates the exciton–LO phonon interaction in a semiconductor spherical quantum dot. Through a variation method, a fully detailed analysis of contributions is given. The exciton–LO phonon interaction is studied in two different cases: (i) with a finite confining potential surrounding the quantum dot and (ii) with a infinite confining potential. The calculation for different material such as CdSe/ZnSe and GaAs/AlxGa1−x As, shows that the interaction energy is increasing with increasing the quantum dot radius. Also it has been observed that the interaction energy for QDs with finite confining potential is higher than these with infinite confining potential, furthermore because of confinement potential variation with Al molar fraction in GaAs/AlxGa1−x As QDs a different behavior for interaction energy has been achieved at different Al molar fraction.

Effect of Exciton-Longitudinal Optical Phonon Interaction on Exciton Binding Energies in CdxZn1-xS/ZnS Single Quantum Wells

We study the effect of the exciton-longitudinal optical (LO) phonon interaction on the exciton binding energies in CdxZn1-xS/ZnS single quantum wells (SQWs) for the Cd alloy content (x) range 0.1—0.3. The heavy- and light-hole exciton binding energies increase with the exciton-LO phonon interaction. The increase in the maximum heavy-hole (light-hole) exciton binding energy for x = 0.3 is 9.1 meV (4.9 meV). In narrow CdxZn1-xS/ZnS SQWs, the heavy-hole exciton binding energy calculated by taking into account the exciton-LO phonon interaction for values of x in the range 0.1—0.3 exceed the LO phonon energy of CdxZn1-xS. [DOI: 10.1380/ejssnt.2010.340]

Porosity-induced blueshift of photoluminescence in CdSe

Porous CdSe layers have been fabricated by anodic etching of n-type single crystalline substrates with different values of conductivity. The morphology and porosity of the layers thus produced were found to be controlled by the conductivity of the material, anodization voltage, and conditions of in situ UV illumination. The porosity-induced changes in the photoluminescence spectra were studied. The decrease of the skeleton size down to 10–20nm was found to result in a blueshift of the excitonic emission lines by 10meV which was attributed to quantum-size effects in the nanocrystalline CdSe porous skeleton. An increase of the exciton–LO-phonon interaction by a factor of 1.5 in a weak-to-intermediate confinement regime was deduced from the analysis of temperature dependence of the free exciton luminescence line.

Resonant spectroscopy of II-VI self-assembled quantum dots: Excited states and exciton–longitudinal optical phonon coupling

Using resonantly excited photoluminescence (PL) along with photoluminescence excitation (PLE) spectroscopies, we study the carrier excitation processes in $\mathrm{CdTe}∕\mathrm{ZnTe}$ and $\mathrm{CdSe}∕\mathrm{ZnSe}$ self-assembled quantum dots (QD's). PLE spectra of single $\mathrm{CdTe}$ QD's reflect two major mechanisms for carrier excitation: The first, associated with the presence of sharp and intense lines in the spectrum, is a direct excited-state--ground-state transition. The second, associated with the appearance of up to four much broader excitation lines, is a longitudinal optical (LO) phonon-assisted absorption directly into the QD ground states. LO phonons with energies of both QD's and $\mathrm{ZnTe}$ barrier material are identified in the PLE spectra. Resonantly excited PL measurements for the QD ensemble as a function of excitation energy makes it possible to separate the contributions of these two mechanisms. We find that for $\mathrm{CdTe}$ QD's the distribution of excited states coupled to the ground states reflects the energy distribution of the QD emission, but shifted up in energy by $100\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. This large splitting between excited and ground states in $\mathrm{CdTe}$ QD's suggests strong spatial confinement. In contrast, the LO phonon-assisted absorption shows significant size selectivity. In the case of $\mathrm{CdTe}$ dots the exciton-LO phonon coupling is strongly enhanced for smaller-sized dots which have higher emission energies. In contrast, for $\mathrm{CdSe}$ QD's the strength of exciton-LO phonon coupling is nearly uniform over the whole ensemble---that is, the dot energy distribution determines the intensities of LO phonon replicas. We show that for $\mathrm{CdTe}$ QD's after annealing, that is, after an increase in the average dot size, the exciton-LO phonon interaction reflects the dot energy distribution, as observed for $\mathrm{CdSe}$ QD's.

Exciton‐LO phonon interaction in II–VI self‐assembled quantum dots

We study the exciton–LO phonon interaction in CdTe/ZnTe and CdSe/ZnSe self-assembled quantum dots (QDs) by means of resonantly excited PL spectroscopy. We find that in the case of large (∼8–10 nm in diameter) CdSe QDs the strength of the exciton–LO phonon coupling is nearly independent of the dot size (e.g. emission energy). In contrast, for CdTe dots (significantly smaller than the CdSe dots) the exciton–LO phonon coupling increases strongly for QDs with higher emission energy (smaller size). Finally, we show for annealed CdTe QDs that, the exciton–LO phonon interaction again becomes independent of emission energy, consistent with the larger dot sizes created by the annealing. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

ANOMALOUS ANISOTROPY OF RESONANT RAMAN SCATTERING IN OPEN ZnCdSe/ZnSe NANOWIRES

Anomalous strong linear anisotropy of resonant Raman scattering in open ZnCdSe / ZnSe nanowires has been found. The anisotropy value is more than twice as that due to the anisotropy of exciton–photon interaction which is the characteristic for open nanowires. It is suggested that the anomalous anisotropy is connected with different probability of Frohlich exciton–LO phonon interaction at scattering via transverse and longitudinal exciton states.

Field and size dependence of exciton–LO-phonon interaction in a semiconductor quantum dot

We theoretically study the exciton--LO-phonon interaction in a semiconductor quantum dot in the presence of an electric field. The confinement potential of the dot in the direction of the field is assumed to be a quantum well with various barrier heights while that in the direction transverse to the field is modeled by a parabolic form. The adiabatic approximation is employed to treat the effect of LO phonon. A trial wave function with a parameter describing the electron-hole correlation is used in the calculation of the ground-state energy of the exciton by the variational method. The magnitude of the exciton--LO-phonon interaction energy $|{E}_{\mathrm{e}\mathrm{x}\ensuremath{-}\mathrm{p}\mathrm{h}}|$ and the corresponding Huang-Rhys factor S are evaluated as functions of field strength and the size of the quantum dot for different barrier heights. With the field strength, the separation between electron and hole is enhanced and thus $|{E}_{\mathrm{e}\mathrm{x}\ensuremath{-}\mathrm{p}\mathrm{h}}|$ increases. Moreover, lowering of the barrier height further increases the separation, resulting in greater values of $|{E}_{\mathrm{e}\mathrm{x}\ensuremath{-}\mathrm{p}\mathrm{h}}|.$ With a fixed strength of field, the increase of size of the quantum dot leads to both delocalization of wave packets for electrons and holes and an increase of their separation simultaneously. Consequently the size dependence of $|{E}_{\mathrm{e}\mathrm{x}\ensuremath{-}\mathrm{p}\mathrm{h}}|$ is complicated. For comparison, an additional calculation is performed for a quantum dot with the shape of a cylindrical disk and barrier of infinite height in all directions and the contrast feature is discussed.

Chemomechanical Polishing Effects on the Room Temperature Photoluminescence of Bulk ZnO: Exciton–LO Phonon Interaction

A study of the effects of chemomechanical polishing on the room temperature photoluminescence of both polar faces of ZnO single crystals is presented. A distinct peak which we associate with second order exciton–phonon (FX–2LO) emission and which dominates the LO emission for both (0001) and () etched surfaces is observed. Chemomechanical polishing results in reduction of the distinct FX–2LO peak, and an increase in the relative importance of first order exciton–phonon (FX–1LO) emission. We attribute this to disorder introduced by the polishing process.

Determination of exciton transition energy and bowing parameter of AlGaN alloys in AlGaN/GaN heterostructure by means of reflectance measurement

The normal-incidence reflectance measurement was employed to obtain the free exciton transition energy (EFX) of AlGaN alloys in AlxGa1−xN/GaN/sapphire heterostructure grown by metalorganic chemical vapor deposition. It was found that the thickness variation of the AlGaN layer may cause a noticeable change in the line shape of reflectance spectrum and impede the identification of the desired excitonic position. By using a reflection model of two absorbing layers with a transparent substrate, the experimental reflectance spectra were theoretically simulated and utilized to explain the reflection mechanism in AlxGa1−xN/GaN heterostructures. On the basis of the above analysis, the feasibility of the reflectance measurement for such heterostructures is confirmed. At room temperature, the EFXs obtained from the fitting showed an excellent agreement with the corresponding peak energies in the photoluminescence spectra. Furthermore, at the optical energy position about 100 meV above the EFX, the spectral feature of exciton-LO phonon interaction was observed in the reflectance spectrum record for low Al composition (x⩽0.16). Using the Al mole fraction derived from x-ray diffraction measurement, the bowing parameter of the epitaxial AlGaN layer was determined. In the range of 0⩽x<0.3, the resulting bowing parameter shows a downward value of 0.53 eV.

Phonon-assisted polar exciton–transitions in self-organized InAs/GaAs quantum dots

Phonon-assisted exciton transitions are investigated for self-organized InAs/GaAs quantum dots (QDs) using selectively excited photoluminescence (PL) and PL excitation spectroscopy. The results unambiguously demonstrate intrinsic recombination in the coherent InAs/GaAs QDs and the absence of a Stokes shift between ground state absorption and emission. Phonon-sidebands corresponding to a phonon energy of ∼34 meV are resolved and Huang–Rhys parameters of ∼0.015 and ∼0.08 are found for phonon-assisted emission and absorption, respectively, which are about one order of magnitude larger than in bulk InAs. Calculations of the exciton–LO–phonon interaction based on an adiabatic approximation and realistic wave functions for ideal pyramidal InAs/GaAs QDs show this enhanced polar coupling to result from the particular confinement and the strain-induced piezoelectric potential in such strained low-symmetry QDs.

Exciton–LO phonon interaction in a quantum dot

Exciton–LO phonon interaction in a spherical quantum dot embedded in non-polar matrix is studied using the dielectric continuum model and the intermediate-coupling-type theory. The interaction contribution can be divided into the self-energy-type and the screening-type terms. Both terms depend on the size of the quantum dot and tend to cancel especially much in the smaller dot. In the limit of the smaller dot both terms approach to zero and then effects of the exciton–LO phonon interaction vanish. These properties reflect the nature of the exciton–LO phonon interaction in the quantum dot.

Exciton–LO-phonon interaction in zinc-compound quantum wells

The properties of optical phonons and excitons in four zinc-compound quantum wells, in which the well layers are made of ternary mixed crystals, are investigated theoretically. The optical phonon frequencies and the electron--LO-phonon coupling constants of the well materials are calculated by using a mixed crystal theory. The interface optical phonon energies and dispersion relations, the exciton binding energies and the exciton--LO-phonon interaction energies are calculated as functions of the well width and the concentration x. The effects of the mixed crystal on the exciton--optical-phonon interaction in the quantum wells are discussed.

Properties of Ga1−xInxN mixed crystals and Ga1−xInxN/GaN quantum wells

The fundamental properties of Ga1−xInxN mixed crystals and Ga1−xInxN/GaN quantum wells are investigated theoretically. The mode number and the energies of optical phonons, the high frequency and static dielectric constants of Ga1−xInxN crystals, are obtained by using an improved mixed crystal theory. The carrier-longitudinal optical (LO)-phonon interaction in Ga1−xInxN mixed crystals is studied by a perturbational polaron theory. The feature of optical phonons in Ga1−xInxN/GaN quantum wells is studied based on the dielectric continuum approach. The frequencies and the dispersion relation of the interface phonons are obtained. The 1s and 2s exciton binding energies and the exciton–LO-phonon interaction energies in Ga1−xInxN/GaN quantum wells are calculated numerically. Based on the present theoretical results, the general properties of Ga1−xInxN mixed crystals and Ga1−xInxN/GaN quantum wells are discussed.

Quantum kinetic exciton–LO-phonon interaction in CdSe

Oscillations with a period of $\ensuremath{\sim}150$ fs are observed in the four-wave mixing (FWM) signal of bulk CdSe and interpreted in terms of non-Markovian exciton--LO-phonon scattering. The experiments show evidence of phonon quantum kinetics in semiconductors of strong polar coupling strength and high exciton binding energy. By comparison of the spectral and temporal response of the FWM signal in bulk CdSe and CdSe quantum dots, we demonstrate the influence of continuum states on the interference of electron-hole pair polarizations coupled via an LO phonon.

Coherent Exciton–LO-Phonon Interaction in the Optical Response of Semiconductors

We present a numerical study of coherent exciton–LO-phonon interaction in two-band semiconductor models. The exciton–LO-phonon coupling is treated in the self-consistent Born approximation. In the extreme low density excitation regime, the linear optical susceptibility is calculated with Lanczos iteration techniques for a 1. quasi-one-dimensional extended Hubbard semiconductor model and 2. the two-band jellium model in the s-symmetry approximation. The exciton–LO-phonon resonance in the continuum absorption of semiconductors strongly depends on the spatial confinement of the center-of-mass motion of the excitons, which is discussed for the one-dimensional Hubbard model.

Exciton–LO-phonon interaction in CuCl spherical quantum dots studied by resonant hyper-Raman spectroscopy

Resonant hyper-Raman-scattering spectroscopy was applied for studies of the exciton--LO-phonon interaction in spherical CuCl nanocrystals of different sizes embedded in a glass matrix. Both the LO and 2LO phonon bands have shown a prominent resonance with the lowest energy $1S$ confined exciton state. The ratio of the integral intensities of the LO and 2LO bands \ensuremath{\rho} was found to increase with increase of the incident photon energy, or decrease of the nanocrystal radius. The Huang-Rhys factor $S$ has been calculated as a function of the nanocrystal size from the experimentally measured values of \ensuremath{\rho} on the offset harmonic-oscillator model of electron-vibrational coupling. It has been found that $S$ increases monotonically from 0.22 to 0.7, with the nanocrystal radius decreasing from 3.6 to 1.6 nm.

Resonant hyper-Raman scattering in semiconductor quantum dots

Resonant hyper-Raman scattering spectra by LO phonon were successfully observed for both CdS and CuBr quantum dots embedded separately in a glass matrix. The respective radii for the former and latter are 2.0 and 3.2 nm, thereby categorized as the strong and weak confinement regime, respectively. It is found that for the CdS dot, the observed excitation spectrum differs from that of resonant Raman scattering, and that the signal arises from interband and intraband Frohlich interaction for CdS and CuBr, respectively. For the latter, the exciton-LO phonon interaction is so strong due to the confinement effect, that up to 5 multiple phonon scattering signals can be observed. Moreover, new electronic excitation states inherent to the respective regimes are also found.