Exciton Resonance Energy Research Articles

Valence Band Physics in Wurtzite GaN

ABSTRACTWe present a summary of recent progress towards the understanding of the valence-band physics in wurtzite GaN. Systematic studies have been performed on the strain dependence of the free exciton resonance energies by photoreflectance measurements using well-characterized samples. Analyzing the experimental data with the Hamiltonian appropriate for the valence bands, the values have been determined of the crystal field splitting, the spin-orbit splitting, the shear deformation potential constants, and the energy gap in the unstrained crystal. Discussions are given on the strain dependence of the energy gaps, of the effective masses, and of the binding energies for the free exciton ground states as well as on the valence band parameters. Using the obtained values and the generalized Elliott formula, the fundamental optical absorption spectra obtained experimentally were analyzed. The values of the elastic stiffness constants, which play a crucial role to determine the shear deformation potential constants, are also given.

Preparation and characterization of CuAlxGa1−xSe2 alloy layers grown by low-pressure metalorganic vapor phase epitaxy

CuAlxGa1−xSe2 alloy layers were successfully grown on GaAs(001) by low-pressure metalorganic vapor phase epitaxy. The distribution coefficient of Al was unity. All alloy layers had their c-axis normal to the substrate plane. Exciton resonance energies were determined as a function of x by means of photoreflectance measurements. A quadratic dependence of exciton energies on x was confirmed. The spin-orbit splittings of the epilayers were approximately the same as that of bulk crystals. The magnitudes of crystal-field splittings were larger than that of bulk crystals, and this was explained in terms of residual tensile biaxial strain in the epilayers. The color of the low-temperature photoluminescence (PL) changed from red to crimson, orange, yellow, green, and bluish-purple with increasing x. A peak due to a free-to-acceptor transition was dominant in the PL spectra of the alloy layers. The acceptor ionization energy increased with increasing x, and the result may reflect an increase of the hole effective mass.

Effects of biaxial strain on exciton resonance energies of hexagonal GaN heteroepitaxial layers

Exciton resonance energies of hexagonal (h-) GaN(0001) epilayers were determined by a combination of high-resolution modulated photoreflectance methods. The results were analyzed thoretically using the Luttinger-Kohn type Hamiltonian for the valence bands under the in-plain biaxial stress, and we obtained the shear deformation potential constants and energy gap in unstrained crystal. Occurrence of the anticrossing of B and C valence bands in tensile biaxially strained h-GaN was suggested.

Exciton-induced enhancement of optical waveguide confinement in (Zn,Cd)(Se,S) quantum well laser heterostructures

Room-temperature waveguide properties have been studied experimentally in ZnSe-based multiple-quantum-well laser structures. The spectra of waveguide transmission show distinct spectral features associated with heavy- and light-hole quantum well excitons. Refractive index dispersion in the multiple-quantum-well region is derived from interference spectra measured in short waveguide samples. The refractive index increases with approaching the exciton resonances, resulting in perfect confinement and low losses of the fundamental TE waveguide mode in a narrow spectral region below the heavy-hole exciton resonance energy. The spectra of laser generation have been measured under the conditions of optical pumping, showing the position of the lasing line to correlate with the spectral region of the waveguide transparency.

Frenkel Excitons in Ordered PbI2 Clusters Incorporated into Zeolite

Abstract Diffuse reflection spectra are measured for (PbI2)5 clusters incorporated into supercages of zeolite LTA crystals, where the Frenkel exciton of cluster crystal is expected to be realized. The reflectivity at the exciton resonance energy drastically increases as the loading density nears the saturation level. This result can not be interpreted by the increase of the cluster density, but by the cluster ordering. We propose the order-induced decrease in the damping energy of the Frenkel exciton, because the motional narrowing of the exciton is expected in the ordered (PbI2)5 clusters. Electron diffraction pattern shows that clusters are arrayed in twice period of the zeolite cages.

Optical characteristics of excitons in In1-xGaxAsyP1-y/InP quantum wells.

This paper presents a systematic study on the optical characteristics of ground-state electron--heavy-hole excitons in ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{As}}_{\mathit{y}}$${\mathrm{P}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$/InP quantum wells, specifically focusing on the resonance energy, spatial energy inhomogeneity, and optical-absorption strength. The exciton resonance energy can be predicted by conventional effective-mass equations. The low-temperature broadening of the photoluminescence spectrum was studied in relation to the composition of ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{As}}_{\mathit{y}}$${\mathrm{P}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$ and the excitation power density. We show the composition of ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{As}}_{\mathit{y}}$${\mathrm{P}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$ fluctuates not only statistically within the exciton volume but also macroscopically, which is the primary cause for the inhomogeneous broadening of the exciton resonance in ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{As}}_{\mathit{y}}$${\mathrm{P}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$/InP quantum wells. The exciton migration over macroscopically fluctuating ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{As}}_{\mathit{y}}$${\mathrm{P}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$ and its preferential recombination in the lower-energy regions is discussed based on our calculation of the exciton lifetime. The integrated intensity of the optical-absorption spectrum of exciton resonance was found to increase as the well band gap increased. Our theoretical formula indicates that this band-gap dependence is quantitatively correlated with the two-dimensional-exciton radius. We show that both the exciton radius and the magnitudes of composition fluctuations primarily determine the exciton optical-absorption strength in ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{As}}_{\mathit{y}}$${\mathrm{P}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$/InP quantum wells.

The temperature dependence (4.2 to 293 K) of the resonance energies of excitonic transitions in II–VI compounds

AbstractExcitonic reflectivity spectra R(hw) are measured by a digital technique for a number of II–VI compounds within a wide temperature range from 4.2 up to 293 K. The exact position of exciton resonance energy within the experimental R(hw) profiles is determined by comparison with reflectivity curves calculated in the model of bulk excitonic oscillator with classical dispersion and exciton‐free surface layer. The temperature dependences of exciton resonance energies thus obtained are found to satisfy the known Varshni formula which includes two empirical parameters. These are determined for each compound under study, so that reference data become available for the position of exciton resonance at any temperature ranging from 4.2 up to 293 K.

Optical Dephasing of Excitonic Polaritons in CuCl Studied by Time-Resolved, Nondegenerate Four-Wave Mixing

Optical dephasing of excitonic polaritons in CuCl is directly measured for the first time in the picosecond time domain by means of time-resolved, nondegenerate four-wave mixing. Dephasing of the ${\ensuremath{\omega}}_{1}$ polariton pulse is probed by an interrogation ${\ensuremath{\omega}}_{2}$ polariton pulse whose group velocity is faster than that of the ${\ensuremath{\omega}}_{1}$ polariton pulse. It is found that the dephasing damping constant $\frac{\ensuremath{\Gamma}}{2}$ of excitonic polaritons is of order of 0.01 meV and increases as $\ensuremath{\hbar}{\ensuremath{\omega}}_{1}$ approaches the exciton resonance energy from below.

Urbach's Rule and Phase Fluctuations of the Transmitted Light

AbstractIt is shown that both Urbach's rule (exponentially increasing absorption edge) and the large fluctuations of the phase of transmitted light observed in depolarization experiments can be derived from the same phenomenon, namely the fluctuating exciton resonance energy caused by the fluctuations in the phonon number in thermal equilibrium. A simple layer model is applied in order to derive quantitative results for absorption and phase fluctuation. The comparison of these results with experimental values for ZnO yields a good agreement. A coherence length of the LO phonons interacting with excitons is also obtained.

Polarization Properties of Resonance Raman Scattering in CdS

The polarization properties of longitudinal optical (LO) phonon overtones were investigated for resonance Raman scattering (RRS) in CdS. The experimental results indicate that the polarization of LO phonon overtones is strongly correlated with the intermediate electronic states which resonate with the scattered light energy. Temperature dependent polarization properties of Raman lines were observed and these results lead to a conclusion that RRS should be considered as inelastic scattering of polaritons by LO phonons. When the laser light excited the energy region a little lower the A -exciton resonance energy E T plus an integer times LO phonon energy \(n\hbar\omega_{\text{LO}}\), zero-phonon A -exciton emission, the Raman line and the hot A -exciton emission were simultaneously observed.

Tridimensional character of GaSe excitons in the Faraday effect

AbstractFaraday measurements on GaSe were performed :with magnetic field of intermediate intensity (≈︁ 50 kG). Four exciton lines were clearly resolved. A simple analytical model was used to fit the experimental data successfully, obtaining the values of some relevant parameters such as the exciton resonance energies, the damping factor, and the relative oscillator strength. An analysis of the data with the Wheeler and Dimmock model for anisotropic exciton gives α ≈︁ 1, for the anisotropic parameter, and μ‖/μ⊥ = 1.3 for the reduced exciton masses ratio. These data show that in GaSe crystals excitons are isotropic. This agrees with recent experimental results supporting the tridimensional character of the bands responsible for excitons. A new and extremely interesting problem now arises: the different behaviour of GaSe, tridimensional in respect to optical and transport phenomena, bidimensional in respect to mechanical and crystallographic properties.

Influence of Resonance Transfer and Exciton Diffusion on Luminescence Decay

AbstractTheoretical equations are formulated to describe the luminescence decay of donors and acceptors including both the effects of exciton diffusion and resonance energy transfer. Closed form expressions for the population and decay of excited donors and acceptors are developed using the Padé approximant method with the diffusion coefficient as an expansion parameter. Possible experiments are suggested.