Kinetics of indirect photoluminescence in GaAs/AlxGa1−x As double quantum wells in a random potential with a large amplitude

The kinetics of indirect photoluminescence of GaAs/Al_xGa_1−x As double quantum wells, characterized by a random potential with a large amplitude (the linewidth of the indirect photoluminescence is comparable to the binding energy of an indirect exciton) in magnetic fields B≤12 T at low temperatures T≥1.3 K is investigated. It is found that the indirect-recombination time increases with the magnetic field and decreases with increasing temperature. It is shown that the kinetics of indirect photoluminescence corresponds to single-exciton recombination in the presence of a random potential in the plane of the double quantum wells. The variation of the nonradiative recombination time is discussed in terms of the variation of the transport of indirect excitons to nonradiative recombination centers, and the variation of the radiative recombination time is discussed in terms of the variation of the population of optically active excitonic states and the localization radius of indirect excitons. The photoluminescence kinetics of indirect excitons, which is observed in the studied GaAs/Al_xGa_1−x As double quantum wells for which the random potential has a large amplitude, is qualitatively different from the photoluminescence kinetics of indirect excitons in AlAs/GaAs wells and GaAs/Al_xGa_1−x As double quantum wells with a random potential having a small amplitude. The temporal evolution of the photoluminescence spectra in the direct and indirect regimes is studied. It is shown that the evolution of the photoluminescence spectra corresponds to excitonic recombination in a random potential. Zh. éksp. Teor. Fiz. 115, 1890–1905 (May 1999)     

Dynamics of excitonic states in GaAs/AlGaAs quantum wells

The influence of photoexcited carriers on the dynamics of the absorption spectra of GaAs/Al_xGa_1−2x As multilayer quantum wells is investigated experimentally. It is found that at quasiparticle densities all the way up to 10¹¹ cm⁻² the saturation of the excitonic absorption is due to both a decrease of oscillator strength and broadening of the excitonic lines. It is shown that in the case of femtosecond resonance laser exci-tation the decrease of oscillator strength is due to free electron-hole pairs, while the broadening and energy shift of the excitonic lines are due to the exciton-exciton interaction. The lifetimes of free electron-hole pairs and excitons (≈65 ps and ≈410 ps, respectively) are determined from the exponential decrease of the change in the oscillator strength and in the width and energy position of the excitonic lines. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 139–144 (10 August 1997)     

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

Polarized photoluminescence of Cd_1−x Mn_xTe 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. Fiz. Tverd. Tela (St. Petersburg) 40, 894–896 (May 1998)     

Excitonic magnetic polaron dynamics of MBE grown CdTe/Cd1 − xMnxTe,Cd1 − xMnxTe/Cd1 − vMgyTe single quantum wells in magneti fields

Excitonic lifetimes in Cd_1 − xMn_Ue2Te, Cd_1 − xMg_xTe epilayers and CdTe/Cd_1 − xMn_xTe, Cd_1 − xMn_xTe/Cd_1 − vMg_yTe single quantum wells with different well widths and Mn, Mg compositions are investigated. The excitonic lifetimes are found to reduce drastically by applying external magnetic fields to samples with giant Zeeman splittings. The observed phenomenon is interpreted in terms of the PL decay time contribution from the long-life dark excitons which can convert to excitons for recombinations by a spin-flip process. We attribute the lifetime reduction to the depletion of dark excitons due to their crossing over the exciton energies for dipole allowed transitions in magnetic fields.     

Photosensitivity of surface-barrier and point structures on Cd<Subscript>1 − x</Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te single crystals

Two new types of photosensitive structures are proposed and fabricated for the first time on solid-solution single crystals of diamondlike Cd_{1 − x} Mn _x Te (x = 0−0.7) magnetic semiconductors. The photoelectric properties of surface-barrier (In/Cd_{1 − x} Mn _x Te) and welded (weld/Cd_{1 − x} Mn _x Te) structures are studied at T=300 K. The photosensitivity spectra of these structures are analyzed and compared; as a result, the character of the interband optical transitions and the energy gaps of the Cd_{1 − x} Mn _x Te crystals are determined. These structures can be applied in magnetic photoelectronic devices.     

Magnetic field effect on transitions between direct and indirect excitons in diluted magnetic semiconductor double quantum wells

Transitions between direct and indirect excitons with change of magnetic field in double quantum well heterostructure Cd_1−xMg_xTe/Cd_1−yMg_yTe/Cd_1−xMg_xTe/Cd_1−zMn_zTe/Cd_1−xMg_xTe in external magnetic field are studied. The structure contains diluted magnetic semiconductor (Cd,Mn)Te layer that forms magnetic quantum well with the depth depending on the magnetic field intensity. Above some magnetic field the indirect exciton becomes the lowest excited state of the system. The indirect exciton lifetime exceeds by several orders of magnitude of the direct exciton one. The range of quantum well widths for which the indirect exciton is the exciton lowest state was estimated for the proposed system.     

Orthorhombic symmetry of valence-band states in CdTe/Cd1−x MnxTe quantum wells

A study is reported of the anisotropy in magnetic-field-induced linear polarization in (001) CdTe/Cd_1−x Mn_xTe quantum wells. The observed limiting anisotropy is shown to be due to the low C _2v symmetry of the quantum well. The relations obtained for the C _2v point group are in a good agreement with experiment. Considered on the microscopic scale, the effect is associated with the heavy-hole g-factor anisotropy in the well plane. Fiz. Tverd. Tela (St. Petersburg) 41, 903–906 (May 1999)     

Optical properties and ferromagnetism of ternary Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te nanocrystals

Cd_1−x Mn _x Te (x = 0, 0.1, 0.2) nanocrystals have been synthesized by mechanical alloying (MA) Cd, Mn, and Se elemental powders. XRD patterns and HRTEM images confirmed the formation of cubic Cd_1−x Mn _x Te nanocrystals. All the diffraction peaks from elemental Cd, Mn, and Te powders disappeared completely in those XRD patterns of as-milled Cd_1−x Mn _x Te nanocrystals for more than 20 h. When the MA process was carried out for 40 h, typical zinc blende structure diffraction mode was exhibited in the XRD pattern. Subsequently, capping the surface of as-milled Cd_1−x Mn _x Te nanocrystals with long chain trioctylphosphine/trioctylphosphine oxide/nitric acid (TOP/TOPO/NA) molecules has achieved colorful dispersion solution, which shows similar optical properties to those CdTe nanocrystals prepared by wet chemical process. The grain size is within the range of 2–8 nm for the capped Cd_1−x Mn _x Te nanocrystals being ball milled for 40 h. The PL excitation peak red shifts to longer wavelength side with increasing Mn concentration. Pure CdTe nanocrystals show ferromagnetism behavior at room temperature, the saturation magnetization value and magnetic hysteresis loop increase with the content of substituting Mn ions within the Cd_1−x Mn _x Te nanocrystals.     

Influence of nonequilibrium phonons on exciton luminescence in CdTe/CdMnTe quantum wells

The luminescence method has been employed for the first time to detect nonequilibrium phonons in CdTe quantum wells with Cd_0.6Mn_0.4Te barriers. The method makes use of the giant Zeeman splitting of exciton states in CdTe/(Cd,Mn)Te quantum wells and is promising for application in high-sensitivity subterahertz phonon spectrometry. The method can also be useful in revealing the spin-phonon coupling mechanisms in diluted magnetic semiconductors. Fiz. Tverd. Tela (St. Petersburg) 40, 816–819 (May 1998)     

Influence of quantum dot density on excitonic transport and recombination in CdZnTe/ZnTe QD structures

We report on dynamics of excitons in Cd_xZn_1−xTe/ZnTe quantum dots (QDs) and present information of excitonic transport and recombination. Due to different growth methods, samples with different QD's densities were obtained. Time-resolved measurements reveal three decay mechanisms: (i) radiative recombination of excitons in the individual QDs; (ii) thermally activated escape of excitons and (iii) escape due to tunneling (hopping). In the high QD-density samples the hopping (r_HB=2700 ns⁻¹) is two orders of magnitude more efficient than in the low QD-density samples (r_HB=33 ns⁻¹). Radiative recombination rates are similar in both types of samples, r_R=1-1.3 ns⁻¹. Due to the good radiative to nonradiative recombination ratio, the low-density QDs can be a potential source of entangled photon pairs.     

Spatial dispersion phenomena in magneto-optics

An experimental study of nonreciprocal spatial-dispersion effects in para-(Cd_1−x Mn_xTe), ferro-(LiFe₅O₈), and antiferromagnetic (Cr₂O₃) crystals caused by an external magnetic field or magnetic order is reported. Fiz. Tverd. Tela (St. Petersburg) 40, 946–948 (May 1998)     

Manifestation of magnetically induced spatial dispersion in the cubic semiconductors ZnTe, CdTe, and GaAs

Nonreciprocal birefringence due to magnetically induced spatial dispersion was observed in the T _d-class cubic semiconductors ZnTe, CdTe, and GaAs near the fundamental absorption edge. The dispersion of the parameters A and g, describing the contributions from terms of the type B _ik_j to the diagonal and off-diagonal components of the permittivity tensor ε _ij(ω,B,k), is determined for ZnTe and CdTe. Analysis of the dispersion and anisotropy of the nonreciprocal birefringence shows that in ZnTe, CdTe, and GaAs, in contrast to magnetic semiconductors of the type Cd_1−x Mn_xTe, it is due excitonic mechanisms. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 7, 514–519 (10 April 1999)     

Intraband transitions in magnetoexcitons in coupled double quantum wells

A theory of far-infrared (FIR) magneto-optical intraband s→p ^± transitions of direct and indirect excitons in semiconductor coupled double quantum wells has been developed. The case of symmetric strained In_xGa_1−x As/GaAs quantum wells with nondegenerate valence band in the regime of both narrow and wide barriers has been analyzed. The energies and dipole matrix elements of transitions between the ground s and excited p ^± states in a quantizing magnetic field B>2 T and electric field ℰ perpendicular to the quantum well plane have been studied. The regimes of direct (in a weak electric field) and indirect (in a strong electric field) transitions, and the transition between the direct and indirect regimes, have been investigated. Zh. éksp. Teor. Fiz. 113, 1446–1459 (April 1998)     

Photomaganetization in diluted magnetic semiconductors

Starting from a many–body Hamiltonian for a system of photogenerated electrons and holes, spin-split by magnetic ions in diluted magnetic semiconductors, we derive, presumably for the first time, an expression for the photomagnetization as a function of the photon power, frequency, excitonic interaction and the magnetic ion concentration. Damping of nonequilibrium carriers and spin excitons is considered phenomenologically. Our results agree qualitatively with some of the systematics of the photomagnetization observed in Hg _1?x Mn _x Te.     

Effect of free electron-hole pairs on the saturation of excitonic absorption in GaAs/AlGaAs quantum wells

The pump-probe experimental method is used to investigate the effect of photoexcited carriers on the dynamics of the exciton absorption spectra of GaAs / Al_xGa_1–x As-multilayer quantum wells. Use of the method of moment analysis for processing the results makes it possible to identify the simultaneous contribution of changes in oscillator strength and width of the exciton lines in the saturation of exciton absorption. It was found that the oscillator strength recovers its initial value in the course of the first 100–130 ps, whereas broadening and energy-shift of the exciton lines is observed for 700–800 ps. These are the first experimental measurements of the excitation densities at which the oscillator strength of the excitonic state saturates when the latter is perturbed only by free-electron-hole pairs, and when it is perturbed only by other excitons. Fiz. Tverd. Tela (St. Petersburg) 40, 1130–1133 (June 1998)     

Interionic exchange interaction in A 1−x II MxBVI systems

The superexchange interaction integral J _NN is investigated in semimagnetic semiconductors of the type A _1−x ^II M_xB^VI systems, where the Mn, Fe, and Co ions are considered as the magnetic component M. Calculations are done in the framework of Racah’s many particle theory including the ground orbital states of the M ions in a cubic crystal. The results of these calculations are compared with published experimental data and with data from a study of the temperature dependence of Faraday rotation. The superexchange mechanism can be used to explain the correlation in the critical values x* at which the interionic interaction becomes important for the Cd_1−x Mn_xTe and Cd_1−x Fe_xTe systems. Fiz. Tverd. Tela (St. Petersburg) 39, 344–348 (February 1997)     

Excitonic polarons in quasi-two-dimensional structures

A theory of excitonic polarons in semiconductor quantum wells is presented. Using a unitary transformation, we have diagonalized the exciton-phonon interaction operator in a quasi-two-dimensional system partially and then calculated the ground-state energy of an excitonic polaron. We have numerically evaluated the energy gap shift and effective mass of an excitonic polaron. We have numerically evaluated the energy gap shift and effective mass of an excitonic polaron in GaAs-Al _x Ga_1–x As systems. The results obtained here indicate that the polaronic effect is significant in the case of the light hole excitons in quantum wells of small well widths.     

Dispersion of the Voigt effect in the magnetic semiconductors Cd1−x MnxTe

Spectral measurements of the Voigt birefringence Δn were performed for the cubic magnetic semiconductor Cd_1−x Mn _x Te (0≤x≤0.52) in order to investigate how the exchange interaction of Mn²⁺ ions with itinerant electrons depends on the electron wave vector. It was determined that Δn/x ² is independent of x and the magnetic field direction, i.e., the effect is due to the Mn²⁺ ions and is isotropic. Below the band gap edge the dispersion of the birefringence Δn can be described well in all samples by the unusual dependence Δn∼(E _g −ℏω) ^−3.5. This can be explained by a decrease of the exchange interaction of Mn²⁺ ions with itinerant electrons with increasing distance from the center of the Brillouin zone. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 569–573 (25 April 1998)     

Application of pseudopotential theory to magnetic semiconductor heterostructures

The results of empirical pseudopotential calculations for the semiconductor compound Cd_1 − xMn_xTe are presented. The effective electron and hole masses obtained from the pseudopotential calculations are then employed in an envelope function approximation, using two different effective mass Hamiltonians to evaluate the transition energies of the excitonic ground state in CdTe– Cd_1 − xMn_xTe quantum wells of variable width. It is shown that in non-magnetic systems it is not possible to utilize exciton energies alone to either distinguish between different model Hamiltonians or to quantify the interface roughness. However, it is shown that the latter can be quantified in magnetic systems via the resulting Zeeman effect.     

Optical absorption edge of Cd1−x Mn x Ga2Se4 crystals

Summary We report the absorption edge spectra of the new family of diluted magnetic semiconductors Cd_1−x Mn _x Ga₂Se₄ (0≤x≤1), grown from the vapour phase by chemical transport. Absorption bands observed under the gap of CdGa₂Se₄ are attributed to intra-Mn²⁺ transitions involving excited states of the 3d ⁵ electrons, split by the crystal field. The authors of this paper have agreed to not receive the proofs for correction.