Exciton Region Research Articles

Exciton-phonon coupling in indirect gap AgBr nanocrystals

The resonance Raman scattering observed in the exciton region of indirect AgBr nanocrystals with mean particle radius 〈 R〉 ≈ 5 nm is analyzed. Considering the processes as absorption followed by emission and calculating the size dependent exciton energies by the donor-like exciton model of Ekimov et al. [1], the 1TO(L) and 2TO(L)&LO(Γ) resonance behavior can be well-reproduced. The analysis of the Raman spectral lineshape based on linear exciton-phonon coupling shows a substantial increase of coupling strength with excitation photon energy. It is attributed to enhanced acoustic and surface phonon interaction of higher exciton states.

Exciton region reflectance of homoepitaxial GaN layers

Reflection measurements in the exciton region of GaN homoepitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) on GaN single crystals are reported. At low temperature (4.2 K) three free exciton lines have been found with energies; EA=3.4776 eV, EB=3.4827 eV, and EC=3.502 eV. The spin-orbit parameter Δso=19.7±1.5 meV and the crystal field parameter Δcr=9.3±0.3 meV have been obtained. From temperature dependence of exciton spectra the energy gap dependence has been found: E(T)=E(0)−λ/[exp(β/T)−1] (λ=0.121 eV, β=316 K).

Polaritonic effects in superlattices.

We present a method to compute the optical functions of superlattices in the excitonic energy region including the effect of the coherence between the electron-hole pair and the electromagnetic field. The electron-hole screened Coulomb potential is adopted and the valence-band structure is taken into account in the cylindrical approximation, thus separating light- and heavy-hole motions. The calculated optical functions have poles in correspondence to the polariton eigenvalues for a multiplicity of excitonic states. We also calculate the amplitudes of higher polariton branches and the line shapes of the optical functions. Numerical examples appropriate to GaAs/${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As superlattices are given and the effect of coherence is displayed in the line shape of the excitation spectrum. \textcopyright{} 1996 The American Physical Society.

Magnetic circular dichroism study of KBr and RbBr excitons

The magnetic circular dichroism (MCD) spectra in the exciton region of KBr and RbBr were measured for the first time utilizing an ultrahigh-vacuum compatible superconducting magnet system. The obtained effective g values (g eff) at the fundamental exciton peaks are well explained as being a spin-orbit pair at the Γ point. Negative g eff values in the following higher energy regions suggest the involvement of a d-like character at related conduction bands.

Magnetic circular dichroism study of KBr and RbBr excitons

The magnetic circular dichroism (MCD) spectra in the exciton region of KBr and RbBr were measured for the first time utilizing an ultrahigh-vacuum compatible superconducting magnet system. The obtained effective g values (geff) at the fundamental exciton peaks are well explained as being a spin-orbit pair at the Γ point. Negative geff values in the following higher energy regions suggest the involvement of a d-like character at related conduction bands.

Luminescence and reflectivity in the exciton region of homoepitaxial GaN layers grown on GaN substrates

In this work we report results of photoluminescence (PL) and reflectivity measurements in the exciton region of GaN homoepitaxial layers grown by metalorganic chemical vapour deposition on GaN substrates. At low temperature (4.2K), very narrow (FWHM = 1.0meV) PL lines related to excitons bound to neutral acceptor (3.4666eV) and neutral donor (3.4719eV) were observed. The energies of free excitons from reflectivity and PL measurements were found to be: EA = 3.4780eV, EB = 3.4835eV and EC = 3.502eV.

Novel results on compensation processes in ZnSe:N

Compensation in ZnSe:N was investigated. The total nitrogen concentration in the samples was determined by two different techniques: secondary ion mass spectroscopy and nuclear reaction 15N(p, αγ)12C analysis. The net free hole concentration NA − ND was correlated with these results. A ratio of (NA − ND)[N] up to 50% was observed. Furthermore, we studied the excitonic region of the photoluminescence of ZnSe:N. Two different transitions correlated with nitrogen complexes could be observed. In lightly doped ZnSe:N a donor like complex was observed, while in highly doped an acceptor like complex dominates. The origin of these complexes is discussed.

Deep-center photoluminescence in nitrogen-doped ZnSe.

In this paper, we report optical studies of nitrogen-doped ZnSe epilayers grown by molecular-beam epitaxy. Photoluminescence spectra of the donor-acceptor pair region at different temperatures and different carrier concentrations show that two donors are present in the samples: residual shallow donors with activation energy 26 meV and deep donors with activation energy of 4 meV previously assigned to a ${\mathrm{V}}_{\mathrm{Se}}$-Zn-${\mathrm{N}}_{\mathrm{Se}}$ complex. In the exciton region, we observe an emission at 2.765 eV, which shows an increased intensity when the epilayer is compensated by the deep donor. We therefore propose that this transition may be related to a deep-donor bound exciton.

Photoluminescence of CdTe thin films containing a mixed crystal orientation

Photoluminescence spectroscopy was employed to study the effect of mixed crystal orientation on defect centers in CdTe thin films grown by metallo-organic chemical vapor deposition on a GaAs substrate. We show that some novel spectral features at 1.36 and 1.32 eV are observed when a substantial degree of both (211) and (331) CdTe are present and that these features disappear gradually if the sample is subjected to a series of low-temperature annealing steps under an inert atmosphere. Optimum annealing conditions, as indicated by the enhancement of the excitonic region, are established. Activation energies are estimated for the 1.59, 1.47, 1.36, and 1.32 eV bands and quantitative measurements are provided for the shift in transition energy due to temperature and annealing procedure.

Phonon‐Assisted Two‐Photon Absorption in the Reststrahl Region of Excitons in Alkali Halides

AbstractThe two‐photon spectrum of RbI is investigated in the energy region of the lowest IS exciton. A significant increase of the two‐photon signal is observed when crossing the transverse energy and scanning through the excitonic reststrahl region. This phenomenon is explained by a LO‐phononassisted absorption process based on the Fröhlich exciton‐phonon coupling. Evaluating the two‐photon excitation within a two‐band model allows a quantitative analysis, which shows good agreement with the experimental data. Besides a scaling factor, the only fit parameter is the exciton Bohr radius that is found in reasonable agreement with the Wannier exciton model.

Excitonic optical nonlinearities and transport in the layered compound semiconductor GaSe.

Dephasing and transient grating experiments in the direct excitonic absorption region of GaSe at low temperatures show that a fast relaxation within the one-dimensionally disordered excitonic band results in band filling being the dominant mechanism of the optical nonlinearity. Correspondingly, we observe a blueshift of the nonlinear signal with excitation density. The temperature dependence of the exciton diffusion constant measured in directions parallel to the GaSe layer planes indicates that temperature-independent scattering (trapping) and scattering by acoustic phonons determine the exciton mobility at low temperatures.

Structural, optical and related properties of some natural three- and lower-dimensional semiconductor systems

The room-temperature structural, optical and related properties, in the (low frequency) excitonic spectral region, of some single valence metal-halide compounds based on Pb, Sn and Ge are briefly reviewed and some new results are reported. Some similarities with the artificially prepared semiconducting systems were found.

Excitonic properties of Zn1-xCdxSe/ZnSe strained quantum wells.

We present a systematic investigation of the linear optical properties of ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Cd}}_{\mathit{x}}$Se/AnSe multiple quantum well structures in the excitonic region using absorption and photoluminescence measurements in the 10--300 K temperature range. Heavy- and light-hole excitonic transitions and heavy-hole exciton binding energies were measured for different well widths and compositions. A set of consistent parameters was determined within a two-band model, which accurately describes the excitonic properties of the strained ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Cd}}_{\mathit{x}}$Se quantum wells.

Defect processes induced by electronic excitation at halide crystal surfaces

Atomic force microscopy (AFM) provides a new topographic view of radiation effects near surfaces, complementing earlier electron microscopy as well as desorption and spectroscopic studies. Potassium iodide crystal surfaces have been imaged by AFM while undergoing exposure to ultraviolet light in the exciton absorption spectrum. Pronounced erosion and modification of the surface by ultraviolet radiation has been observed with resolution down to ∼ 50 nm. Prospects for extending such measurements to atomic resolution are considered. In related studies of KI under illumination in the exciton region, photoelectron spectra reveal photoinduced gap states which appear attributable to F centers and alkali clusters. In both of these studies, fourth harmonic radiation from a Ti:sapphire mode-locked laser has proven especially useful because of its tuneability through the strongly absorbing exciton region of alkali iodides, producing near-surface excitation densities comparable to electron beams.

Excitonic energy range dielectric function in GaAs/Ga0.7Al0.3As single quantum wells at room temperature

GaAs/Ga0.7Al0.3As single quantum-well heterostructures, grown by molecular beam epitaxy, are investigated at room temperature using reflectance spectroscopy in the energy range 1.2–3.6 eV. The dielectric function of GaAs in the wells is deduced in the excitonic transition region around the Γ point of the Brillouin zone. The optical constants are determined on the basis of reflectance data using a Kramers–Kronig analysis, followed by an iterative technique of extraction applied to a multilayer scheme for the samples. The optical transitions, accurately located at the peaks of the absorption spectra, are compared to the transition energies calculated using the finite-square-well model in the effective-mass approximation.

Influence of cleavage in liquid helium on the shape of the n=1 exciton reflection band in KI and RbI crystals

A noticeable change in the n=1 exciton reflection band of KI and RbI crystals exposed to air was revealed after cleavage in liquid helium. This change is explained by disturbance of the near-surface layer by charged surface dislocations and other defects, which appeared after cleavage at a low temperature. A moderate irradiation by photons in the exciton region and/or heating leads to the removal of the disturbance and, correspondingly, to relaxation of the crystal surface.

Resonant Brillouin scattering in biaxially strained ZnSe

Resonant Brillouin scattering experiments have been performed in the 1S exciton region of ZnSe layers grown on GaAs substrate. Information about the biaxial strain in the ZnSe epilayer, the exchange splitting δ, and the longitudinal-transverse splitting Δ LT can be obtained by comparing the experimental Stokes shifts with those calculated from the dispersion of the 1S exciton-polariton in the strained crystal. For these calculations we used a Hamiltonian which contains the kinetic energy of the exciton center-of-mass motion, the electron-hole exchange interaction and the strain-induced effects. The polariton dispersion is calculated using a multi-component oscillator model. We find a quantitative agreement between the calculated and experimental Stokes shifts for a biaxial strain of +0.03% and exchange parameters δ=0.4 meV and Δ LT=1.2 meV.

Study of electronic structure in strained ZnSe/GaAs thin films by nonlinear optical- and Brewster-angle reflection spectroscopies

Two kinds of novel spectroscopic methods of two-photon resonant second-harmonic generation (SHG) and Brewster-angle reflection were investigated for exploring the electronic structure in strained ZnSe/GaAs thin films. A line splitting into two, which is caused by in-plane tensile strain, in the excitation spectrum of the normally forbidden SHG signal was observed in resonance with the 2P exciton, whereas three lines in resonance were observed with the 1S exciton polaritons. In the Brewster-angle reflection spectrum, on the other hand, only one broad line with the width presumably broadened inhomogeneously, was observed in the 1S exciton region. Next, Brewster-angle reflection spectroscopy was also investigated in a few samples of ZnSe/ZnS quantum wells. For those, two split lines were distinctly observed in the 1S exciton region, as it should be. Comparison between those spectral lines and the one-photon excitation spectrum of luminescence indicates that they are consistent. Thus, it is concluded that both methods should serve as a simple and novel tool for characterizing II–VI thin films.

Deep centre photoluminescence in nitrogen doped ZnSe

In this paper, we report optical studies of nitrogen doped ZnSe epilayers grown by molecular beam epitaxy. Photoluminescence spectra of the donor-acceptor pair region at different temperatures and different carrier concentrations show that two donors are present in the samples, residual shallow donors with activation energy 26 meV and deep donors with activation energy of 46 meV previously assigned to a VSe-Zn-NSe complex. In the exciton region we observe a new emission at 2.765 eV and the intensity increases when the epilayer is compensated by the deep donor. We therefore propose that this transition is related to a deep donor bound exciton. Excitation power dependent photoluminescence measurements show the presence of two deep transitions separated by 23 meV at low excitation intensities which we propose are due to deep donor-acceptor pairs.

High‐density and optical non‐linearity effects in epitaxially grown ZnTe layers

AbstractWe investigated the high‐density luminescence and non‐linear transmission of ZnTe layers grown by metal‐organic vapour phase epitaxy (MOVPE). For the high‐density luminescence we compared ZnTe layers on (001) GaAs substrate with free‐standing ZnTe layers of equal thickness. At high excitation intensities a strong luminescence P band appears a few meV below the free exciton energy which is assigned to resonant exciton‐scattering processes. A second strong luminescence N band occurs only in free‐standing layers. It shows a remarkable red shift with increasing intensity and becomes the dominant emission for an excitation intensity Iexc > 1.5 MW cm−2. This lower‐energy band was interpreted as electron–hole plasma recombination.The optical non‐linearity of thin ZnTe layers in the excitonic region was investigated by pump‐and‐probe experiments at 2 K and room temperature (RT). For these experiments we used two different pump energies, one above (at 2 K and RT) and one below (at 2 K) the band gap energy. In all cases a red shift and a large non‐linear decrease in the excitonic absorption with increasing pump intensity was observed. The experimental results can be explained by many‐body effects of exciton screening.