Free Exciton States Research Articles

Coexistence in photoluminescence of free exciton and bound exciton in low nitrogen content GaInNAs layers

We study by time resolved photoluminescence (TRPL) low N and In content GaInNAs (GINA) alloy layers grown by molecular beam epitaxy on GaAs substrate. The TRPL experiments show the coexistence and the carrier exchanges between bound- and free-exciton states, in this kind of alloy. Temperature dependent experiments demonstrate the thermal ionization of bound excitons and the high temperature stability of free excitons. For a temperature higher than 60 K the PL spectra are totally dominated by the free exciton line. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Resonance exciton-phonon spectra in open ZnCdSe/ZnSe nanowires: Raman scattering and hot luminescence, extended and localized exciton states

Low-temperature (T ∼ 8 K) secondary emission spectra of open Zn0.87Cd0.13Se/ZnSe nanowires were studied comprehensively in the region of the ground excitonic state under tunable laser excitation. The spectra revealed a fine structure produced in the interaction of excitons with optical phonons. The observed resonance exciton-phonon (REP) lines are shown to be formed through two different mechanisms. The strongest component is due to Raman scattering via free excitonic states. The other REP lines are generated in the hot luminescence of localized excitons. The spectrum of the optical phonons involved in the formation of the REP lines in the biaxially strained Zn0.87Cd0.13Se/ZnSe structures was analyzed.

Resonant Raman scattering spectra in a ZnCdSe/ZnSe structure with a quantum well and open nanowires

Resonance Raman scattering (RS) spectra of a ZnCdSe/ZnSe sample containing a single quantum well and quantum well-based open nanowires were studied at T=300 K. The longitudinal optical (LO) phonons involved in the formation of the observed spectra of the quantum-well and nanowire regions differ noticeably in energy. The LO phonon energies in the structures under study were calculated taking into account the compositional effect (doping of Cd into ZnSe) and biaxial strain. When excited in the exciton resonance region, RS is shown to occur via free (extended) excitonic states with the involvement of LO phonons of the ZnCdSe strained layer with final wave vectors near the Brillouin zone center. When excited below the excitonic resonance in the ZnCdSe layer, resonance scattering via localized exciton states provides a noticeable contribution to the observed RS lines. Because of the finite size of a localized state, phonons with large wave vectors are involved in these scattering processes. The RS lines produced under excitation in the excitonic region of the thick barrier layers are due to scattering from the ZnSe barrier phonons.

Confinement-enhanced biexciton binding energy in ZnO/ZnMgO multiple quantum wells

By employing a nanosecond pump-probe method, biexciton formation process was investigated in ZnO/Zn1−xMgxO (x=0.26) multiple quantum wells (MQWs) grown on ScAlMgO4 substrate by laser molecular-beam epitaxy. Bleaching of absorption due to the saturation of excitonic states, and induced absorption related to the exciton–biexciton transition were observed in their spectra. It is demonstrated that the pump-probe method allows us to precisely determine binding energies of exciton complexes even applied to the semiconductor quantum structures where the localization effect are not negligible. This is because a transition from free-excitonic states to free-biexcitonic states is involved in the induced absorption process. The biexciton binding energy is a monotonically decreasing function of well width (Lw). For the MQWs with Lw smaller than 2.5 nm, the biexciton binding energy is larger than 25 meV, comparable to the thermal energy of room temperature.

Free excitons in cubic CdS films

Cubic CdS layers have been epitaxially grown on (001) GaAs substrates by a low-pressure metalorganic chemical vapor deposition technique, and their free exciton states have been studied by means of modulated reflectance spectroscopy. Light-hole and heavy-hole excitons are split by biaxial compressive strains in the 2 μm CdS films. It has been found that a sharp photoluminescence (PL) appears at low temperatures and the PL energy coincides with the light-hole exciton energy. Two peaks in the PL excitation spectrum agree well with the light-hole and heavy-hole exciton energies. The free exciton energy has been determined for cubic CdS films on GaAs substrates.

A Comparative Study of the Near Band Edge Luminescence of Epitaxial ZnSe:N Grown by MBE and MOVPE

A comparative study of the photoluminescence, reflection, and photoluminescence excitation spectra of MBE- and MOVPE-grown nitrogen-doped ZnSe samples has been carried out. MOVPE-grown samples demonstrate impurity recombination via shallow donor states while MBE-grown samples show mainly recombination via deep donor states, with significantly lower donor concentrations. Thus, the shallow donors are the main cause of compensation in MOVPE-grown samples. In MBE-grown samples, the excitation transfer to impurity states takes place mainly via free excitonic states and acceptor-bound excitons, while in MOVPE-grown ZnSe : N the excitation transfer occurs mainly via donor-bound excitons. A simultaneous presence of a broad 2.6 eV band and a narrower 2.7 eV band was observed in time-integrated spectra of MOVPE-grown ZnSe : N.

UlTRAFAST LATTICE RELAXATION DYNAMICS OF EXCITON IN A QUAISI-1-D METAL-HALOGEN COMPLEX

A quasi-one-dimensional halogen-bridged mixed-valence metal complex is studied by time-resolved pump and probe spectroscopy with sub-5 fs time resolution. Two kinds of oscillatory signals are observed, which are attributed to the wave packet motions both in an electronic ground state and in a self-trapped exciton (STE) state. The onset of the wave packet motion is found to be delayed by about 50 fs, comparing with the ideal wave packet in the electronic excited state. The delay reflects the thermalization process in a free exciton (FE) state and a lattice relaxation process from FE to STE states.

Wave packet dynamics in a quasi-one-dimensional metal-halogen complex studied by ultrafast time-resolved spectroscopy.

An ultrafast optical response is studied in a quasi-one-dimensional halogen-bridged mixed-valence metal complex [Pt(en)(2)] [Pt(en)2I2] (ClO4)(4) with ultrafast time resolution. Wave packet motions both in the ground and self-trapped exciton (STE) states are observed as oscillatory modulations in the time-resolved reflectivity. The wave packet motion on the STE potential surface begins after about 50 fs with respect to the photoexcitation. This delay is attributed to the lattice relaxation from the free exciton state to the STE state.

Excitonic States of Alkylammonium Lead-Iodide Layered Perovskite Semiconductors under Hydrostatic Pressure to 25 GPa

Optical absorption and photoluminescence (PL) spectra of the layered perovskite semiconductor (C8H17NH3)2PbI4 were measured under hydrostatic pressure to 25 GPa in order to investigate the effect of pressure-induced perturbations on the electronic structure and the excitonic states of the two-dimensional sublattice of corner-sharing [PbI6]4— octahedra. The predominant PL band, which originates from a free exciton state, shifts to lower energies sublinearly with pressure, and then disappears abruptly above PC ≅ 12 GPa. The exciton absorption band also disappears at PC, and a broad absorption tail appears at a higher energy above PC. It is found that the pressure dependence of the band gap energy above PC is significantly different from that of the exciton band energy below PC, indicating that the electronic band structure of the two-dimensional sublattice changes at PC. A large hysteresis on the recovery of the exciton band was observed upon decreasing pressure.

Temperature quenching mechanisms for photoluminescence of MBE-grown chlorine-doped ZnSe epilayers

We report on a detailed investigation on the temperature-dependent behavior of photoluminescence from molecular beam epitaxy (MBE)-grown chlorine-doped ZnSe epilayers. The overwhelming neutral donor bound exciton (Cl 0X) emission at 2.797 eV near the band edge with a full-width at half-maximum (FWHM) of ∼13 meV reveals the high crystalline quality of the samples used. In our experiments, the quick quenching of the Cl 0X line above 200 K is mainly due to the presence of a nonradiative center with a thermal activation energy of ∼90 meV. The same activation energy and similar quenching tendency of the Cl 0X line and the I 3 line at 2.713 eV indicate that they originate from the same physical mechanism. We demonstrate for the first time that the dominant decrease of the integrated intensity of the I 3 line is due to the thermal excitation of the “I 3 center”-bound excitons to its free exciton states, leaving the “I 3 centers” as efficient nonradiative centers. The optical performance of ZnSe materials is expected to be greatly improved if the density of the “I 3 center” can be controlled. The decrease in the luminescence intensity at moderately low temperature (30–200 K) of the Cl 0X line is due to the thermal activation of neutral-donor-bound excitons (Cl 0X) to free excitons.

Optical properties of high-quality CuInSe2 single crystals

CuInSe 2 single crystals have been studied employing photoluminescence (PL), optical reflection (OR), optical absorption (OA) and wavelength derivative reflection (WDR) techniques at temperatures from 4.2 to 300 K. Exciton-related peaks were observed in the near-band-edge region of the PL spectra: several narrow lines, with full width at half maximum (FWHM) of about 0.3 meV, and two wider peaks (FWHM about 0.7 meV) at 1.0414 (A) and 1.0449 eV (B). The A and B peaks were also observed in the OR and OA spectra and identified as A and B free excitonic states. The narrow lines were attributed to bound exciton recombination on intrinsic defects. A third exciton resonance (C) was observed in the WDR spectra at 1.2779 eV. The crystal-field and spin-orbit splittings were derived to be 5.3 and 234.7 meV, respectively.

Exciton–phonon interaction and conversion of excitons from free to self-trapped states in layered metal iodide crystals under hydrostatic pressure

Exciton–phonon interaction parameters in layered metal iodide crystals, BiI 3, SbI 3 and PbI 2, have been estimated from the measurements of the steepness constants σ 0 in the Urbach tails of absorption edge spectra under hydrostatic pressure. Luminescence from self-trapped exciton (STE) and free exciton (FE) states in these crystals has been examined under pressure. Simple but reasonable interpretation for the drastic change in the luminescence as well as in the interaction parameter values is given in terms of excitonic band anomaly.

Time-dependent variational approach to the dynamics of the exciton self-trapping process

We present a new theoretical method in the investigation of the exciton self-trapping dynamics in one-dimensional system based upon a time-dependent variational method. The time-resolved emission spectrum is calculated by this method. The effect of the exciton transfer allows a free exciton state to survive for some time just after photoexcitation, which is clearly reflected in the short-time behavior of the time-resolved emission spectrum.

A dynamic model for exciton self-trapping in conjugated polymers. II. Implementation

We examine the electronic and vibrational dynamics of a model conjugated polymer using a particle–hole treatment for electronic excitation described in Ref. . We observe the transition from a delocalized free exciton state to a self-trapped exciton, and compare the characteristics of the process of localization to those predicted by existing theories. We find that the reaction path to self-trapping involves a well-defined intermediate state, complicating the process of cooling for the self-trapped exciton. We also find that high-energy excitons do not couple strongly to the lattice, and therefore do not self-trap.

Optical Properties of Excitonic-Grade CuInSe2 Single Crystals

A number of CuInSe2 (CIS) single crystals have been studied employing photoluminescence (PL), optical reflection (OR), optical absorption (OA) and wavelength derivative reflection (WDR) techniques at temperatures from 4.2 to 300K. In the near-band-edge region the PL spectra, taken at 4.2K, had several very narrow, with full width at half maximum (FWHM) of about O.3meV, lines and two well resolved wider peaks (FWHM about O.7meV) at 1.0414eV (A) and 1.0449eV (B). These two peaks were also observed in the OR and OA spectra at the same energy. It is unambiguous evidence that these peaks are due to free excitonic states. A third exciton related peak (C) was observed in the WDR spectra at 1.2779eV. From the energy separation of the peaks A, B and C the crystal-field and spin-orbit splittings were estimated to be 5.3 and 234.7meV, respectively. The narrow lines were attributed to bound exciton radiative recombination on intrinsic defects.

Exciton Energy Structure in Wurtzite GaN

Results of theoretical and experimental magneto-optical studies of free excitons in wurtzite GaN are presented. The direct photoluminescence from free exciton states of high-quality GaN epilayers was studied by polarization-dependent magneto-optical measurements in magnetic fields up to 15 T. Zeeman splittings and diamagnetic shifts of the n = 1 and n = 2 states of the A exciton were observed for magnetic fields parallel and perpendicular to the hexagonal axis. The experimental data are described by a theory of the exciton energy structure in hexagonal semiconductors with wurtzite symmetry in the framework of the quasi-cubic approximation.

Luminescence of CsPbBr 3-like quantum dots in CsBr single crystals

Luminescence and decay kinetics of the Pb 2+ aggregates in CsBr host crystals were measured in the 4–300 K temperature interval and in 10 −10–10 −3 time scale. Their emission properties are similar to those of CsPbBr 3 bulk crystal showing a subnanosecond free exciton emission in the 520–540 nm spectral region and slower trapped exciton emission in the 530–580 nm spectral region. An efficient energy exchange between the free and trapped exciton states is shown by the temperature dependencies of emission spectra. The quantum size effect is demonstrated in the high energy shift and broadening of the absorption and emission spectra and an estimate of the size of the CsPbBr 3-like aggregates is provided. Independent evidence of the presence of the CsPbBr 3 and Cs 4PbBr 6 aggregated phases in the CsBr host was obtained by X-ray structural studies.

Reactive ion etch-induced effects on the near-band-edge luminescence in GaN

GaN grown on c-plane sapphire substrates has been reactive ion etched successfully in a SF6 plasma with an etch rate of 29 nm/min. The etch rate does not change with substrate temperatures between 10 and 50 °C. Optical transitions have not been destroyed after etching, instead, two additional lower energy transitions appear close to the band-edge luminescence. The two additional transitions are related to defect states that bind excitons. The defect-bound states exhibit different behavior compared to the free excitonic states in that their normalized intensities decrease more rapidly as temperature increases, the peaks exist only up to 80 K, and their line energies show no temperature dependence.

Spectral and relaxation features of excitons mixed with excimer quasi-molecular states

For some class of crystals, an excited atomic state gives rise, along with free excitons, to another related type of crystalline excitations also formed by resonance bindings, viz. an excimer two-atomic quasi-molecule. Such an exciton polaron, due to its intrinsic vibrational degree of freedom, rather moves fast through the lattice and mixes with free-exciton states. It is shown that this mixing results in a distortion of the lower part of the exciton band via splitting it into narrow polaronic subbands corresponding to vibrational levels of the excimer quasi-molecule. This entails essential physical consequences involving absorption spectrum and anomalous relaxation phenomena. The theory is in good qualitative agreement with experiment.

Optical properties of CuCl particles in fluorozirconate glass

Optical extinction spectra of CuCl particles in a fluorozirconate glass whose composition is close to that of ZBLAN20 (53% ZrF 4, 20% BaF 2, 4% LaF 3, 3% AlF 3, and 20% NaF) are reported. Spectra show a sharp but structured band at 375 nm at room temperature which is resolved into two lines at 378 and 386 nm at 15 K. These two lines are attributed to the free exciton states Z 1,2, Z 3 of CuCl. Spectra are compared with calculations of optical extinction efficiency of spherical CuCl particles, based on Mie scattering theory. Calculations show that the shape of extinction spectra depends markedly on particle radius for radii above about 20 nm, and that the shape may thus be used to estimate the mean particle size.