Exciton Region Research Articles

Multiple LO Phonon Scattering in Indium Bromide

Multiple LO phonon scattering lines with intensity alternation have been investigated in InBr under the excitation in the first (2.33 eV, E // c ) and the second (2.48 eV, E // b ) exciton regions between 2 K and 150 K. Every scattering line shows pronounced resonance enhancement at the first exciton energy, while at the second exciton energy only the 2LO line shows conspicuous resonance. Scattering lines are caused by the cascade relaxation of excitons due to LO phonon emission, which is confirmed by the thermal behavior of the spectra. Importance of the interband transition between p and s exciton states is pointed out to explain intensity alternation of scattering lines.

Ellipsometry and transient reflectivity near the excitonic resonance in CdSe.

We report two experiments. The first is an accurate measurement of the phase of the light reflected by a CdSe crystal in the excitonic region. The second is a direct measurement of the time delay experienced by a reflected wave packet in the same frequency range. We show that it is necessary to take into account the existence of an exciton-free surface layer to satisfactorily fit the experimental results. The very fast variation of phase near the longitudinal exciton frequency, larger than \ensuremath{\pi} over 0.3 meV, and the very large delay of 12 ps observed under the same conditions are attributed to multiple reflections inside the exciton-free layer in a frequency range where the layer acts as an antireflecting Fabry-P\'erot plate.

Excitonic effect in the optical spectrum of semiconductors.

The electron-hole interaction affects the optical spectrum of many semiconductors, not only in the bound exciton region in the energy gap, but also throughout the continuum. In particular, it controls the strength of the E1 and E2 peaks in group IV, III-V and II-VI semiconductors. By solving the Bethe-Salpeter equation in a multi-plane wave representation containing the relevant band structure features of this entire class of semiconductors, we account for the experimental E1 and E2 peaks. We discuss how the excitonic effect shifts the strength of the spectrum from the E2 to the E1 peak.

Resonant Raman scattering in CuC1 in the region of the edge excitons

The absolute Raman scattering efficiency of transverse optic phonons in CuC1 has been determined in the Z 12 and Z 3 exciton region (3.0 eV <ħω < 3.4 eV). The results are analyzed with a model that takes into account the exchange interaction of the exciton. The deformation potential d o obtained from the fit of the theory to the data is compared with the results of theoretical calculations.

Properties of CdTe films grown on InSb by molecular beam epitaxy

The optical and electrical properties of molecular beam epitaxial layers of CdTe grown on InSb(001) substrates at 200–225 °C and either doped with copper, arsenic, antimony or indium or undoped were investigated. Non-intentionally doped layers are found to be n type and to contain indium and antimony atoms out diffused from the substrates. For copper- and arsenic-doped layers, acceptor-bound exciton annihilation lines become dominant in the exciton region of the photoluminescence spectra at 6 K, whereas for antimony-doped layers the acceptor-bound exciton line remains weaker than the donor exciton line even at antimony concentrations of up to 1.2 × 10 18 cm -3. P-type conduction was observed only in heavily copper-doped samples.

The influence of impurities and additional illumination on the photoconductivity of semiinsulating GaAs in exciton region

Un pic aigu de la photoconductivite observe dans GaAs peu ou non dope avec Cr, 20 a 45 meV en dessous de la limite d'absorption fondamentale est attribue aux excitons lies aux accepteurs peu profonds ou aux paires donneur-accepteur. Ce pic peut etre attenue ou augmente par illumination additionnelle ou attenue par champ electrique

Magnetic polarons in exciton luminescence of Cd1-xMnxTe

Systematic investigations of the photoluminescence spectra of Cd1-xMnxTe mixed crystals were performed for 0<or=x<or=0.4 as a function of temperature and magnetic field. Two luminescence lines are distinguished in the excitonic region. The first line is caused by recombination of excitons bound to neutral acceptors. A detailed theoretical interpretation of this line is given in terms of the bound magnetic polaron. For the second line a structure on the curve of temperature dependence is observed in the vicinity of the spin-glass phase transition. That line is attributed to a magnetic localisation of excitons.

Effect of Electric Fields on Luminescence in GaAs

AbstractPhotoluminescence spectra near the exciton region are investigated under the application of external electric fields parallel and perpendicular to epitaxial GaAs surfaces. Quenching of the luminescence is observed and explained in terms of impact ionization of excitons. The emission peaks of the upper (UPB) and lower polariton branches (LPB) shift to higher and lower energy sides, respectively, with increasing the electric fields or the excitation intensity. The calculated peak of the LPB emission is located around 1.515 eV and shifts to higher energy side and dominates the UPB emission, which is contradictory to the present observation. New emission peak appears at 1.515 eV under an application of an electric field in pure samples with carrier concentrations less than 6×1013 cm−3.

Exciton and impurity electroabsorption in GaSe xTe 1- x〈Sn〉 single crystals

The exciton and impurity electroabsorption has been studied in GaSe x Te 1- x 〈Sn〉 (0.7 ⩽χ⩽1) exhibiting the properties of residual conductivity (RC). It has been shown that the broadening of the negative exciton peak, the nature of the field dependence of the electroabsorption signal, and its anisotropy in the exciton and impurity region are associated with the presence of internal chaotic fields E int ≈ 10 3 − 10 4 V cm −1 coming into play because of the fluctuating distribution of the tin impurity and the defects inherent in layer crystals. Based on investigations of the impurity electroabsorption (IEA), the depth of occurence ( hv = 1.18 eV) and the concentration ( N = 10 16 cm 3) of impurity centres have been determined.

The influence of ion implantation on the excitonic reflectance of ZnO

Implantation of Zn + ions with different energies and doses drastically changes reflectance spectra of ZnO crystals in the excitonic energy region. The experimental spectra can be explained by a model crystal consisting of the implanted layer with a complex dielectric constant and of the undisturbed bulk described by the dielectric function of excitons. A comparison of experimental and theoretical reflectance spectra offers the possibility of determining implantation parameters without sample destruction.

Influence of fluctuating strain on exciton reflection spectra

The influence of an internal distribution of strain on the exciton reflection spectra is investigated. The resulting fluctuating optical constants give rise to a fluctuating phase of reflectivity. The standard deviation $\ensuremath{\sigma}$ of these phase fluctuations is the quantity which can be observed, for example, between crossed polarizers or from ellipsometric measurements. Assuming the phase fluctuations to obey a Gaussian distribution, $\ensuremath{\sigma}$ can be expressed in a simple way in terms of the degree of polarization or the depolarization of the reflected light. $\ensuremath{\sigma}$ is then derived in terms of the standard deviations of the fluctuating strain components for wurtzite-structure crystals. To do this it is necessary to find the behavior of the six independent (complex) photoelastic coefficients in the excitonic resonance region. A simple approximation makes it possible to obtain these. Furthermore, it is necessary to derive the dependence of the phase of reflectivity on the direction of the fluctuating optical axis. The results obtained for $\ensuremath{\sigma}$ are compared with the experimental depolarization spectra of ZnO. The only fitting parameter is the common standard deviation of the strain components. It is found that the fluctuating strain can reproduce the positions and relative magnitudes of the resonant depolarization peaks, but fails to reproduce the background level of depolarization and the magnitude of the $A$ peak in one geometry.

Coherent interaction of picosecond frequency-tunable light pulses with excitons in a semiconductor

An investigation was made of the self-induced transparency effect in the semiconductor's exciton region using ultrashort frequency-tunable light pulses and an ultrafast streak camera.

Photoconductivity of CdS under high excitation - indicating no homogeneous Mott transition

Excitation spectra of photoconductivity in CdS crystal at He temperatures do not show the expected decrease of many order of magnitude in the excitonic region under intensities above 10 kW cm -2. Agreement with the rate equations permits us to estimate the exciton-exciton ionization coefficient as c = 2 × 10 -10cm 3s -1. No indication of a homogeneous Mott transition exists up to intensities at which emission spectra are electron-hole plasma (or liquid) like.

Spectroscopic studies of ZnSe grown by liquid phase epitaxy

The application of modern LPE techniques to ZnSe has recently resulted in material which shows a relatively small number of sharp lines in the bound exciton (BE) region and negligible deep level emission. BE transitions reveal the presence of Al and In donors and Li acceptors. Occasionally BE's related to Cl and Ga donors and Na acceptors are also observed. The sole unidentified line at 2.79 eV appears to be related to an isoelectronic center, on the basis of its moderate phonon coupling and lack of 2-electron transitions. These sharp narrow (<0.2 meV) BE lines indicate that the total shallow donor concentration is < 5 × 1016/cm3. No manifestation of vacancies or interstitials has been seen, and thus all the shallow donor or acceptor states in undoped material appear to be impurity related. In the donor-acceptor pair (DAP) region, the formerly observed Al-Li pair lines [1] are seen as doublets, suggesting a new shallow donor with a binding energy ~0.3 meV different from Al. Correlation with growth conditions suggests that the new donor is likely to be boron. N, P, and As-doped samples show DAP bands which indicate that they form shallow acceptors, but confirmation of their presence as simple substitutional acceptors has not yet been achieved. The activation energies, derived from DAP band positions, are 80, 85, and 100 meV, respectively, for N, P, and As.

Decay kinetics of excitons and the electron-hole plasma in GaP: N

The lifetime of excitons bound to nitrogen, ${\ensuremath{\tau}}_{e}$, and of the electron-hole plasma (EHP), ${\ensuremath{\tau}}_{p}$, in GaP: N was studied as a function of the following parameters: temperature (2-100 K), excitation intensity, and nitrogen-doping level. The photoluminescence was excited both selectively and above the gap by using a pulsed, tunable dye laser with photon flux varying in the range of ${10}^{14}$-${10}^{19}$ photons/${\mathrm{cm}}^{2}$ per pulse. The observed ${\ensuremath{\tau}}_{e}(T)$ for various excitation intensities was quantitatively fitted to a model which assumes the existence of saturable deep exciton and electron traps ("shunt paths"). An estimate of the concentration and capture cross section of these traps is obtained. In addition to the well-known exciton capture by the nitrogen, an additional free-electron---hole pair-capture process is observed. This occurs when the photoexcited electron-hole pair concentration is greater than that required for a Mott transition (so that no excitons are available). In crystals with nitrogen concentration greater than ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and under intense photoexcitation (resonantly with the $A$ line), the EHP is created and its lifetime is virtually temperature independent. In addition, ${\ensuremath{\tau}}_{e}\ensuremath{\ne}{\ensuremath{\tau}}_{p}$ for most of the temperature range studied. This indicates that particle exchange between EHP and exciton regions is small.

Resonance Raman scattering study on exciton and polaron anisotropies in InSe

The resonance Raman scattering by multiple-LO phonons has been studied in the E' 1 exciton region of the layer compound InSe. The Raman scattering efficiencies by 2LO phonons exhibit an anisotropic behavior with respect to the propagation direction. This behavior comes from the anisotropy in the translational effective mass of the exciton. The analysis in terms of the exciton-phonon cascade scattering process gives effective masses of electrons and holes. The nature of the electron-LO phonon interaction in InSe is discussed in terms of the polaron coupling constants.

Additional Waves and Polariton Dispersion in CdS Crystals

AbstractA study of additional waves in the exciton region of CdS crystals is reported. Transmittivity and reflectivity of thin crystals is measured showing a structure due to the Fabry‐Perot interference and the interference of additional waves. This structure is a direct reflection of the polariton dispersion law. Using the formulas for a spatially dispersive platelet and Pekar's additional boundary conditions the theoretical reflectivity curves are obtained numerically reproducing the essential experimental results. Values for all physical parameters defining dispersion and damping of the polaritons are found. Some evident deviations between the experimental curves and the present computer results for the reflectivity are pointed in connection with a discussion of the role of the surface exciton potential.

Determination of optical constants: the plane–parallel slab

Methods for determining the optical constants of a thin plane-parallel slab are developed without using information from other measurements. For the ideal case (negligible absorption and thickness inhomogeneity), a purely geometrical method even avoids the use of relative intensities. The more realistic situations (weak absorption or gain and finite thickness inhomogeneity) are also discussed. The methods are used to determine the refractive index of CdS platelets below the exciton region at room temperature.

On the possibility of the exciton-phonon bound states in GaSe

The experimental results of the optical absorption and the photoconductivity of GaSe layered crystals in the exciton region are presented. On the basis of the results obtained, we conclude the existence of the exciton-phonon quasibound states.

Electromodulation spectroscopy of excitons in simple cubic TlCl and TlBr

Transmission and electromodulated transmission spectra have been measured in the direct Wannier exciton region for TlCl and TlBr. The spectra were obtained at a sample temperature between 5 and 6 K for a range of appl\'{\i}ed electric fields. The data have been reduced to obtain the electric-field-induced changes in the dielectric function and compared in detail to the calculations of Blossey. The experimental results support the trends predicted by the calculations.