Exciton Mobility Edge Research Articles

Distinguishing quantum dot-like localized states from quantum well-like extended states across the exciton emission line in a quantum well

We have closely examined the emission spectrum at the heavy-hole exciton resonance in a high-quality GaAs multi-quantum well sample using picosecond excitation-correlation photoluminescence (ECPL) spectroscopy. Dynamics of the ECPL signal at low and high energy sides of the excitonic photoluminescence (PL) peak show complementary behavior. The ECPL signal is positive (negative) below (above) the PL peak and it changes sign within a narrow band of energy lying between the excitonic absorption and emission peaks. The energy at which this sign change takes place is interpreted as the excitonic mobility edge as it separates localized excitons in quantum dot-like states from mobile excitons in quantum well-like states.

Excitonic mobility edge and ultra-short photoluminescence decay time in n-type GaAsN

We use time-resolved photoluminescence (PL) spectroscopy to study the recombination dynamics in Si-doped GaAsN semiconductor alloys with a nitrogen content up to 0.2%. The PL decay is predominantly monoexponential and exhibits a strong energy dispersion. We find ultra-short decay times on the high-energy side and long decay times on the low-energy side of the photoluminescence spectrum. This asymmetry can be explained by the existence of an additional non-radiative energy transfer channel and is consistent with previous studies on intrinsic GaAsN epilayers. However, the determined maximum decay times of GaAsN:Si are significantly reduced in comparison to undoped GaAsN. The determined excitonic mobility edge energy constantly decreases with an increase in the N content, in agreement with the two-level band anticrossing model.

Trion formation dynamics in monolayer transition metal dichalcogenides

We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides (TMDs), specifically molybdenum diselenide (MoSe2), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond timescale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by ~ 50%. This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in TMDs. The quasiparticle formation and conversion processes are important for interpreting photoluminescence and photoconductivity in TMDs.

Excitonic recombination near the mobility edge in CdSe/ZnSe nanostructures

The low-temperature photoluminescence and Raman scattering in CdSe/ZnSe nanostructures with individual CdSe inserts of 1.5 and 3.0 monolayers in nominal thickness were studied. The energy position of the photoluminescene band is governed by interdiffusion of Cd and Zn into the insert regions, whereas the shape of this band is controlled by strong interaction of localized excitons with optical phonons in the Zn1−xCdxSe solid-solution insert. Multiphonon processes of excitonic relaxation with involvement of acoustic phonons at the Brillouin zone edges are also important. The results obtained are interpreted in the context of a model for the effective excitonic mobility edge.

Dynamics of excitons near the mobility edge in CdSe/ZnSe superlattices

The dynamic properties of hot excitons near the effective excitonic mobility edge are investigated in a CdSe/ZnSe superlattice structure where thermalized luminescence of localized excitons is strongly suppressed due to tunneling towards the attached deeper ZnCdSe quantum well. Time-resolved selective-excitation luminescence measurements reveal a rather complicated relaxation behavior involving fast (20–30 ps) acoustical–phonon-assisted selective occupation of states located close to the mobility edge.

Exciton energy relaxation and exciton mobility edge in (CdMg)Te epilayers studied by time-resolved photoluminescence

Exciton localization is studied with time-resolved photoluminescence (PL) in Cd 1− x Mg x Te epilayers. The energy relaxation rate of excitons is found to be energy dependent and reduces drastically below an effective excitonic mobility edge. This particular property results in the saturation of the PL intensity rise time below the mobility edge and a two-exponential decay for the PL intensity and the energy relaxation of the excitons.

Asymmetric luminescence line shape and exciton energy relaxation in Zn1−x−yMgxCdySe epilayers

The asymmetric line shape of the transient photoluminescence (PL) spectra of high-quality Zn1−x−yMgxCdySe epilayers is deconvoluted to a narrow and a broad Gaussian peak at each time delay. The fitted energy difference between the two peaks corresponds to one longitudinal optical (LO) phonon energy. We assign the narrow peak to the recombination of free excitons (FXs) and the broad peak to the recombination of excitons localized in the broad band tail characteristic for this quaternary compound. The broad PL peak is found to arise from the energy relaxation of the FX at the exciton mobility edge to the localized states by LO phonon emission. Therefore, the population peak of the localized excitons is formed at one LO phonon energy below the FX for all times after the laser excitation.

Luminescence spectra and kinetics of disordered solid solutions

We have studied both theoretically and experimentally the luminescence spectra and kinetics of crystalline, disordered solid solutions after pulsed excitation. First, we present the model calculations of the steady-state luminescence band shape caused by recombination of excitons localized in the wells of random potential induced by disorder. Classification of optically active tail states of the main exciton band into two groups is proposed. The majority of the states responsible for the optical absorption corresponds to the group of extended states belonging to the percolation cluster, whereas only a relatively small group of ``radiative'' states forms the steady-state luminescence band. The continuum percolation theory is applied to distinguish the ``radiative'' localized states, which are isolated in space and have no ways for nonradiative transitions along the tail states. It is found that the analysis of the exciton-phonon interaction gives the information about the character of the localization of excitons. We have shown that the model used describes quite well the experimental cw spectra of ${\mathrm{CdS}}_{(1\ensuremath{-}c)}{\mathrm{Se}}_{c}$ and ${\mathrm{ZnSe}}_{(1\ensuremath{-}c)}{\mathrm{Te}}_{c}$ solid solutions. Further, the experimental results are presented for the temporal evolution of the luminescence band. It is shown that the changes of band shape with time come from the interplay of population dynamics of extended states and spatially isolated ``radiative'' states. Finally, the measurements of the decay of the spectrally integrated luminescence intensity at long delay times are presented. It is shown that the observed temporal behavior can be described in terms of relaxation of separated pairs followed by subsequent exciton formation and radiative recombination. Electron tunneling processes are supposed to be responsible for the luminescence in the long-time limit at excitation below the exciton mobility edge. At excitation by photons with higher energies the diffusion of electrons can account for the observed behavior of the luminescence.

Localized excitonic states in ZnS–ZnSe single quantum wells

We present low temperature photoluminescence spectra taken from an 11Å ZnSe quantum well in ZnS barriers. The samples are grown by the technique of photo-assisted vapour phase epitaxy (PAVPE) and the spectra show evidence for interface disorder. The observed dependences of the excitonic luminescence on excitation power and temperature are interpreted by a model involving excitonic localization below an exciton mobility edge. This mobility edge is measured for these samples to be 6 meV below the free exciton energy in the ideal quantum well.

Anisotropy of cubic semimagnetic Cd1−x MnxTe solid solutions and excitonic magnetic polaron energy from polarized-luminescence measurements

Polarized photoluminescence of Cd1−xMnxTe crystals in a weak magnetic field has been studied in Faraday and Voigt geometries. A simple method is proposed to determine the exciton mobility edge and excitonic magnetic-polaron energy. “Forbidden” polarization components of the recombination radiation have been experimentally detected. It has been established that the moments of magnetic polarons are oriented predominantly along the {111} axes.

Random Potential Fluctuations in Thin Quantum Wells Probed by Localized Excitons

GaAs/AlGaAs single quantum wells (QW) with different degrees of fluctuations of the exciton confinement potential are investigated by microphotoluminescence (μ-PL) as a function of the excitation energy and by conventional PL, PL excitation, as well as cathodoluminescence spectroscopy. The surface morphology has been studied by atomic force microscopy. For a small inhomogeneous broadening of the PL spectrum, the influence of resonant excitation of higher localized exciton states is only detected for energies below the center of the absorption line, which defines an effective exciton mobility edge Em. For broader PL spectra, resonant excitation of higher states can be observed far above the center of the absorption spectrum resulting in an effective blue-shift of Em. Furthermore, the position of Em can depend on the excitation location. In this case, in addition to nm-scale fluctuations, a variation of the random potential on the μm-scale was observed.

Effective exciton mobility edge in narrow quantum wells

An effective mobility edge for excitons in two-dimensional systems is shown to exist by comparing photoluminescence (PL) and PL excitation (PLE) with excitation-energy-dependent micro-PL spectra. The effective mobility edge of excitons shifts towards higher energies with increasing potential fluctuations. This blue shift is correlated with a sharp decrease of the PLE signal resulting in an extremely large energy separation between the PL and PLE spectra. Numerical simulations using a random potential confirm these results and predict a significant deviation of the PLE from the absorption spectrum.

Effect of Magnetic Polaron Formation on the Exciton Mobility Edge in Cd1-xMnxTe

Effect of Magnetic Polaron Formation on the Exciton Mobility Edge in Cd_1-xMn_xTe

Phase-conjugated wave and localized-delocalized transition of an exciton

An exciton in impure bulk crystals is expected to show a localized-delocalized transition with varying an excitation frequency. Around this mobility edge, phase-conjugated wave generation is theoretically shown to be singularly enhanced. It is proposed that the exciton mobility edge, which is insensitive to the linear optical response, will be clearly observed as a singular peak in this four-wave mixing spectroscopy.

Localized-delocalized transition of the exciton and scaling description of phase-conjugated wave generation.

Under nearly resonant pumping of Frenkel excitons, the third-order optical process generating phase-conjugated waves is theoretically investigated by taking full account of the effect due to static on-site disorder and finite sample size. The scaling theory of the Anderson localization allows us to show that the phase-conjugated signal has the spectral anomaly for pump and probe frequencies around the exciton mobility edge. We systematically evaluate the dependence of the phase-conjugated signal on various physical quantities, namely, randomness, the detuning of the pump and probe frequencies from the exciton mobility edge, the misalignment of two pump beams, and the system size.

Localized-delocalized transition of an exciton studied by generation of phase-conjugated waves

Under nearly resonant pumping of Frenkel exciton, the disorder effect on third-order optical processes of generating phase-conjugated waves is theoretically investigated by use of the scaling theory of Anderson localization. The phase-conjugated signal is expected to have the spectral anomaly for pump- and probe-frequencies around the exciton mobility edge.

Intrinsic exciton states in indirect-band-gap GaAsxP1-x: Composition dependence and effects of impurities.

The low-temperature (T=2 K) absorption and photoluminescence spectra of ${\mathrm{GaAs}}_{\mathrm{x}}$${\mathrm{P}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ (x\ensuremath{\le}0.46) are reported. The absorption spectrum for x\ensuremath{\ge}0.15 can be separated into the ${\mathrm{N}}_{\mathrm{X}}$ band (excitons bound to isoelectronic nitrogen impurities) and the intrinsic exciton band. The latter can be fitted to a sum of transitions into extended exciton states for which \ensuremath{\alpha}(E)\ensuremath{\propto}(E-${E}_{\mathrm{fx}}$${)}^{1/2}$ and a tail due to localized states which can be approximated by \ensuremath{\alpha}(E)\ensuremath{\propto}exp[(E-${E}_{s}$)/${E}_{0}$] for E\ensuremath{\le}${E}_{s}$. For x0.15, the (${\mathrm{N}}_{\mathrm{X}}$+LO)-phonon sideband overlaps the intrinsic absorption spectrum and the two contributions cannot be accurately separated. The luminescence spectra show the impurity-bound exciton bands and for xg0.15, the ${M}_{0}^{X}$ band which is due to intrinsic excitons localized by potential fluctuations. The ${M}_{0}^{X}$ band shape is analyzed in terms of exciton tunneling by acoustic-phonon-assisted transitions into terminal states. The high-energy cutoff of the ${M}_{0}^{X}$ band is identified as the excitonic mobility edge (ME). For x0.15, the ME is determined by monitoring the emission intensity under selective excitation into the intrinsic exciton tail. The composition dependence of the ME energy, ${E}_{\mathrm{ME}(\mathrm{x})}$, is analyzed in terms of a simplified Lifshitz model. It assumes that excitons which are localized by potential fluctuations have spatially overlapping wave functions. For a critical value of the density of exciton states, the wave function extends over the whole crystal. The calculated values of ${E}_{\mathrm{ME}(\mathrm{x})}$ which are based on this model fit well the experimental data in the composition range x\ensuremath{\ge}0.10 and follow the expression ${\mathit{E}}_{\mathrm{ME}}$(x)=1.963x+2.327(1-x)-0.194x(1-x) eV.

Exciton tunneling in GaAs xP 1-x:N, a weakly disordered semiconductor

The luminescence spectra of GaAs xP 1-x:N (0<×<0.4) are studied at T=2K, by selective excitation into the nitrogen-bound exciton band. The main spectral features are analyzed in terms of acoustic phonon-assisted exciton transfer between localized states. The energy of the excitonic mobility edge is determined spectroscopically and is found to vary as x(1-x).