Excitonic Line Shapes Research Articles

Photoluminescence in n-doped In0.1Ga0.9N/In0.01Ga0.99N multiple quantum wells

In0.1Ga0.9N/In0.01Ga0.99N multiple quantum wells (MQWs) with heavily Si-doped barriers, grown with Metal Organic Vapor Phase Epitaxy (MOVPE) at about 8000C, have been studied in detail with optical spectroscopy. Such structures are shown to be very sensitive to a near surface depletion field, and if no additional layer is grown on top of the MQW structure the optical spectra from the individual QWs are expected to be drastically different. For a sample with 3 near surface QWs and Si-doped barriers, only the QW most distant from the surface is observed in photoluminescence (PL). The strong surface depletion field is suggested to explain these results, so that the QWs closer to the surface cannot hold the photo-excited carriers. A similar effect of the strong depletion field is found in an LED structure where the MQW is positioned at the highly doped n-side of the pn-junction. The internal polarization induced electric field in the QWs is also rather strong, and incompletely screened by carriers transferred from the doped barriers. The observed PL emission for this QW is of localized exciton character, consistent with the temperature dependence of peak position and PL decay time. The excitonic lineshape of 35-40 meV in the QW PL is explained as caused by a combination of random alloy fluctuations and interface roughness; the corresponding localization potentials are also responsible for the localization of the excitons in the low temperature range (<150 K). These samples show no evidence of localization due to nanoscale In fluctuations, these commonly observed problems are concluded to be not present in our samples. A second PL feature at lower energy, observed at low temperatures, is shown to be related to an electron pocket at the interface to the underlying n-GaN buffer layer in these samples.

Cyclic molecular aggregates: influence of quasi-static and dynamic disorder on their excitonic line shapes

For Frenkel excitons moving on molecular rings modeling in part bacteria photosynthetic antenna systems we investigate the influence of static and dynamic disorders on their optical line shape. The static disorder, describing inhomogeneities and slow motions of the protein backbone and the dynamic disorder, simulating the influence of vibrational degrees of freedom, are taken into account by fluctuations of the local excitation energies of the BChI molecules. The fluctuations are represented by dichotomic Markov processes with colored noise with different amplitudes and decay constants of the correlation functions. We obtain a closed set of equations of motion for the correlation functions determining the optical line shape which is solved numerically. We display the optical line shapes of the circular aggregates with 9 BChls for different regimes of the exciton transfer under the influence of the disorders and discuss them for various sets of the model parameters describing static and dynamic disorders.

Modeling excitonic line shapes of photosynthetic light harvesting systems: influence of quasi-static and dynamic disorder

The modeling of optical line shape of molecular ring aggregates with 9 and 18 molecular units of photosynthetic bacteria light-harvesting systems is investigated theoretically. The molecular units are described by two-level systems, with an average excitation energy ε, which interact with each other through nearest-neighbor interaction. A possible dimerization is taken into account by allowing for alternating strong ( J S) and weak ( J W) transfer integrals. Furthermore, quasi-static and dynamic fluctuations of the local excitation energies are taken into account simulating slow motions of the protein backbone and fast molecular vibrations, respectively. The fluctuations are described by colored dichotomic Markov processes with exponentially decaying correlation functions with small ( λ s) and large ( λ) decay constants, respectively. The optical line shape is discussed with respect to the model parameters.

Field inhomogeneity effects on photoreflectance spectra of ZnSe/GaAs

Photoreflectance measurements from ZnSe epilayers of different thicknesses were carried out. In photoreflectance spectra, the exciton line shape “rotation” is observed to be much slower than that in the reflectance spectra by increasing the epilayer thicknesses. To analyze the exciton line shape rotation quantitatively, the photoreflectance spectra were calculated considering the built-in electric field inhomogeneity effects near the interface as well as the interference effects. Calculated line shapes of the photoreflectance spectra show a good agreement with the observations. Our results imply that inhomogeneity effects of the interface built-in electric field plays an important role in the spectral rotation in photoreflectance.

Photoreflectance study in the E1 and E1+1 transition regions of ZnTe

Photoreflectance spectra have been measured to determine the E1 and E1+1 energies in ZnTe at temperatures T between 77 and 300 K using an Ar+-ion laser as a modulation light source. The measured photoreflectance spectra can be successfully explained by an excitonic line shape. The temperature dependence of the E1 and E1+1 critical-point parameters (energy, amplitude and broadening parameter) have been analysed using the Varshni equation and an empirical expression of the Bose-Einstein type.A comment correcting this article has been published: Molina, Espinosa-Luna and Adachi 2004 J. Phys. D: Appl. Phys. 37 2047

Photoreflectance of a High Quality CuInSe2 Single Crystal

A melt growth CuInSe2 bulk single crystal was shown to be very high quality from its sharp and strong photoluminescence (PL) emissions. Photoreflectance (PR) measurement on a selected sample were performed from 12 K to room temperature. The PR line shape depends on surface treatment of the sample. Sharp features of PR spectra were obtained from bromine-methanol etched surface. PR spectra measured from 12 K to 140 K were analyzed by using both the well known Aspnes' relation and the confluent hypergeometric function suitable for exciton with Gaussian line shape. Result of the line-shape analysis shows that transitions involved are derived from two excitons with Gaussian line shape. By comparison with excitonic emission from its PL result, the two excitons are identified as the A- and the B-exciton which result from the splitting of the uppermost of the valence band. The energy difference of the two excitons computed from the PR spectrum at 12 K is 3.7 meV which agrees fairly well with 3.2 meV from PL result. The Gaussian exciton line shape at low temperature also indicates that the exciton-phonon coupling in a high quality CuInSe2 bulk single crystal is strong. PR spectra measured from an as-polished surface are weak and exhibit broader spectral lines which are well fitted by two excitons with the Lorentzian line shape. In addition, by carefully etching and polishing, the PR spectrum at 40 K exhibits a spectral line which can be interpreted as a superposition of the two spectra of different line shapes. The spectrum is well fitted by two Gaussian excitons and two Lorentzian excitons with two sets of transition energies. The presence of Lorentzian excitons indicates that excitons near the as-polished surface are weakly coupled to phonon which may be a result of residual strain left over by mechanical polishing.

Simulation of excitonic optical line shapes of cyclic molecular aggregates with 9 and 18 units: influence of quasi-static and dynamic disorder

Abstract For Frenkel excitons moving on molecular rings modeling in part bacteria photosynthetic antenna systems we investigate the influence of dynamic and static disorder on their optical line shape. The dynamic disorder, describing the influence of vibrational degrees of freedom, and static disorder are taken into account by fluctuations of the local excitation energies of the BChl molecules. The fluctuations are represented by dichotomic Markov processes with colored noise with different amplitudes and decay constants of the correlation functions. We obtain a closed set of equations of motion for the correlation functions determining the optical line shape which is solved numerically. We display the optical line shapes of the circular aggregates with 9 and 18 BChls for different regimes of the exciton transfer under the influence of the disorders. The line shapes are discussed for various sets of the model parameters describing static and dynamic disorders.

Photoreflectance study of hexagonal CdSe

Photoreflectance (PR) spectra have been measured to determine the fundamental-absorption-edge (E0) structure of hexagonal CdSe single crystal for both E c and E|| c polarizations at temperatures T between 77 and 300 K. The measured PR spectra are found to be successfully explained by an excitonic line shape for temperatures T below ~200 K and by a three-dimensional one-electron line shape for T>200 K. No clear evidence of the exciton interference effect has been observed within our experimental temperature range T = 77-300 K. The temperature dependence of the E0-exciton parameters (exciton energy, amplitude and broadening parameter) has been determined and analysed using the Varshni equation and an empirical expression of Bose-Einstein type.

Interplay of Excitonic Radiative Recombination and Ionization in Photocurrent Spectra of Thick Barrier GaAs/AlAs Multiple Quantum Wells

We have studied photocurrent (PC) spectroscopic features of a relatively thick barrier GaAs/AlAs multiple quantum well p-i-n diode at 18 K. It is found that the PC spectra do not reflect the photoabsorption spectral lineshape under the low field condition and show negative PC peaks at the exciton resonance wavelengths where the absorption coefficient is maximum. These PC dips are qualitatively explained by considering the distribution of photogenerated carriers and competition between the carrier transport and recombination. A tunneling model calculation is carried out to successfully explain the PC spectral features as well as the field dependence of PC intensity by taking into account the photogenerated carrier distribution and the excitonic absorption lineshape variations with the field.

Simulation of excitonic optical line shapes of cyclic molecular aggregates with 9 and 18 units: influence of quasi-static and dynamic disorder

For Frenkel excitons moving on molecular rings modeling in part bacteria photosynthetic antenna systems we investigate the influence of dynamic and static disorder on their optical line shape. The dynamic disorder, describing the influence of vibrational degrees of freedom, and static disorder are taken into account by fluctuations of the local excitation energies of the BChl molecules. The fluctuations are represented by dichotomic Markov processes with colored noise with different amplitudes and decay constants of the correlation functions. We obtain a closed set of equations of motion for the correlation functions determining the optical line shape which is solved numerically. We display the optical line shapes of the circular aggregates with 9 and 18 BChls for different regimes of the exciton transfer under the influence of the disorders. The line shapes are discussed for various sets of the model parameters describing static and dynamic disorders.

Excitonic optical line shapes of trimers and tetramers: influence of dichotomous coloured transfer integral fluctuations

The influence of static and dynamic disorder on the optical line shape of Frenkel excitations in trimers and tetramers is investigated. The disorder is taken into account by dichotomous stochastic processes with exponentially decaying correlation functions. For the case of fluctuating transfer integrals a closed set of equations of motion is derived for the correlation functions determining the optical line shape. The line shape is investigated for various sets of the parameters of the stochastic processes, for various arrangements of the molecules and their transition dipole moments.

Excitonic optical line shapes of trimers and tetramers: influence of dichotomous coloured transfer integral fluctuations

The influence of static and dynamic disorder on the optical line shape of Frenkel excitations in trimers and tetramers is investigated. The disorder is taken into account by dichotomous stochastic processes with exponentially decaying correlation functions. For the case of fluctuating transfer integrals a closed set of equations of motion is derived for the correlation functions determining the optical line shape. The line shape is investigated for various sets of the parameters of the stochastic processes, for various arrangements of the molecules and their transition dipole moments.

Interference effects in reflectance line shapes from [formula omitted] epilayers

The reflectance spectroscopy is performed at 77 K on ZnSe epilayers of different thicknesses grown on GaAs substrates. The exciton line shapes of the reflectance spectra change dramatically with the epilayer thickness. This observation suggests that the interference effects between the reflected waves from the surface and from the interface play an important role on the exciton line shape of the reflectance spectra. To analyze the change in the exciton line shapes quantitatively, the reflectance spectra were calculated using a simple oscillator model for a dielectric function and considering a multiple reflection. Calculated line shapes of the reflectance spectra show good agreement with the observations.

Photoluminescence properties of ZnS epilayers

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 exciton was described by a one-step quenching process with an activation energy of 14.4 meV. The self-activation (SA) center was also observed at 2.846 eV with a linewidth of 410 meV. The temperature dependence of the SA emission was well described by the configuration coordinate model. From the thermal broadening of the SA emission, an average phonon energy of 47.5 meV was determined, in good agreement with the LO phonon energy.

Exciton Lineshapes in Weakly Disordered Systems

AbstractExciton absorptison lines are calculated for weakly disordered tight‐binding systems, using the coherent‐potential approximation (CPA). The scaled lineshapes depend only on the dimension of the system. The CPA results are compared with known exact results for chains, and numerical results for finite two‐and three‐dimensional systems.

Exciton and trion spectral line shape in the presence of an electron gas in GaAs/AlAs quantum wells.

We studied the photoluminescence of (e1:hh1)1S excitons (X) and negatively charged excitons (trions, ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$) in quantum wells (QW's) having a low-density (${\mathit{n}}_{\mathit{e}}$5\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) two-dimensional electron gas (2DEG) at T\ensuremath{\leqslant}12 K. Mixed type-I--type-II GaAs/AlAs quantum wells are studied in which the 2DEG is photogenerated in the type-I QW's and ${\mathit{n}}_{\mathit{e}}$ is determined by the excitation intensity. At a given temperature and for every excitation intensity the photoluminescence spectrum is decomposed into a Lorentzian-shaped ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ line and a convoluted Lorentzian-Gaussian X line. Their intensity ratio is analyzed by assuming a thermal equilibrium distribution of X and ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ that is determined by the chemical potential of the 2DEG. The ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ linewidth dependence on ${\mathit{n}}_{\mathit{e}}$ is analyzed as originating from an increased ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ dephasing rate that is caused by trion-electron (${\mathit{X}}^{\mathrm{\ensuremath{-}}}$-e) scattering. We present a model of the elastic (${\mathit{X}}^{\mathrm{\ensuremath{-}}}$-e) scattering and calculate its rate as a function of ${\mathit{n}}_{\mathit{e}}$ assuming the 2DEG screening wave vector (${\mathit{q}}_{\mathit{s}}$) to be an adjustable parameter. Although of the same order of magnitude, the fitted ${\mathit{q}}_{\mathit{s}}$ values differ from those calculated for the ideal gas model using the Thomas-Fermi approximation. Since, to our knowledge, there is no model for calculating ${\mathit{q}}_{\mathit{s}}$ in the low 2DEG density range studied here and T>0, our spectroscopically extracted ${\mathit{q}}_{\mathit{s}}$(${\mathit{n}}_{\mathit{e}}$) values might serve as guidelines for the required theory. \textcopyright{} 1996 The American Physical Society.

Computer simulation of excitonic optical line shapes: transfer integral fluctuations with dichotomic coloured noise

For Frenkel excitons moving on cyclic and linear molecular chains (modeling in part photosynthetic antenna systems) we investigate the influence of dynamic and static disorder on their optical line shapes. The dynamic disorder describes the influence of vibrational degrees of freedom. In our recent papers we have taken into account fluctuations of the local excitation energy. Fluctuations of the transfer matrix element between neighbouring molecules will be treated in this paper. The fluctuations are represented by dichotomic Markov processes with coloured noise. We obtain a closed set of equations of motion for the correlation functions determining the optical line shape which is solved exactly. The line shapes are discussed for various sets of the model parameters.

Free exciton emission in GaN.

We present a detailed study of the free exciton emission in GaN. Photoluminescence and photoluminescence excitation techniques are employed to show the dominant role of exciton-phonon interaction in the creation of the free exciton states and the free exciton emission cascade in GaN. Up to six longitudinal optical (LO) phonon replicas are observed in both photoluminescence and photoluminescence excitation spectra. From an analysis of the free exciton line shape we are able to conclude that free exciton emission and the interaction of free excitons with LO phonons have to be described within the framework of the momentum conservation law. The exciton kinetic energy distribution is found to be a Maxwellian one having the temperature of the lattice. This suggests an enhanced rate of free exciton scattering on the acoustic phonons, in contrast to other wurzite-type semiconductors. \textcopyright{} 1996 The American Physical Society.

Polaron Problem in Quantum Wells

The problem of the polaron ground state energy and of effective mass renormalization in quantum wells has been widely discussed in literature [1, 2]. Considerably less attention has been paid to the quantum well polaron lifetime [3], or more generally, to the spectral density function which is closely related to the excitonic absorption lineshape function. We employ the cumulant expansion method which has been proved to be very accurate in the case of three-dimensional po larons [4]. The main idea of this method is to expand the correction to the phase of the time dependent Green's function in powers of the coupling constant. Such an expansion is much faster convergent than a standard perturbation series for the Green's function. In practice, already the lowest order term provides a very reasonable approximation.

Theory of excitonic optical line shapes of cyclic and linear hexamers: Influence of dichotomic colored noise

For Frenkel excitons moving on cyclic and linear molecular hexamers we investigate the influence of dyhamic and static disorder on their optical line shapes. The dynamic disorder describes the influence of vibrational degrees of freedom and is taken into account by fluctuations of the local excitation energy and of the transfer matrix demerit between neighbouring molecules. The fluctuations are represented by dichotomic Markov processes with colored noise. We obtain a closed set of equations of motion for the correlation functions determining the optical line shape which is solved exactly. The line shapes are discussed for various sets of model parameters as well as various arrangements of molecules and their transition dipole moments.