The exciton luminescence in Zn(Cd)Se/ZnMgSSe quantum wells

We study exciton states in Zn(Cd)Se/ZnMgSSe quantum wells (QWs) with various degrees of diffusion blurring in the interfaces by the methods of optical spectroscopy. We show that at low temperatures the QW emission spectra are determined by free and neutral donor-bound excitons. Blurring of the heterointerfaces leads to the increase in the energy shift between the emission line maxima of free and bound excitons. We explain the nonlinear dependence of the steady-state photoluminescence intensity on the excitation-power density in terms of the neutralization of charged donors at the photoexcitation of heterostructures. We observed a complex long-time dynamics of the reflection coefficient, evoked by the charge-redistribution processes in the heterostructure, near the QW exciton resonances under the irradiation.     

Effect of heteroboundary spreading on the properties of exciton states in Zn(Cd)Se/ZnMgSSe quantum wells

Exciton states in Zn(Cd)Se/ZnMgSSe quantum wells with different diffusion spreading of interfaces are studied by optical spectroscopy methods. It is shown that the emission spectrum of quantum wells at low temperatures is determined by free excitons and bound excitons on neutral donors. The nonlinear dependence of the stationary photoluminescence intensity on the excitation power density and the biexponential luminescence decay are explained by the neutralization of charged defects upon photoexcitation of heterostructures. With the stationary illumination on, durable (about 40 min) reversible changes in the reflection coefficient near the exciton resonances of quantum wells are observed. It is shown that, along with the shift of exciton levels, the spreading of heteroboundaries leads to three effects: an increase in the excitonphonon interaction, an increase in the energy shift between the emission lines of free and bound excitons, and a decrease in the decay time of exciton luminescence in a broad temperature range. The main reasons for these effects are discussed.     

Features of the effect of heterointerface spreading on exciton states in Zn(Cd)Se/ZnMgSSe quantum wells

The exciton states in Zn(Cd)Se/ZnMgSSe quantum wells with various degrees of diffusive spreading of interfaces are studied by optical spectroscopy methods. Luminescence lines of free excitons are identified. It is shown that, in addition to exciton level shift, heterointerface spreading enhances exciton-phonon coupling and increases the energy shift between maxima of emission lines of free and bound excitons. The major causes of the observed phenomena are discussed.     

Experimental evidence of the exciton centre-of-mass quantization in Zn1−x Cd x Se/ZnSe multiple quantum wells

Il Nuovo Cimento D - We report the evidence of exciton-polariton centre-of-mass quantization in II–VI quantum wells consisting of ternary alloy. Absorption and photoluminescence studies on...     

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.     

Optically detected magnetic resonance of Mn-related excitations in (Cd,Mn)Te quantum wells

Optically detected magnetic resonance (ODMR) was applied to reveal the exchange interaction effects between Mn²⁺ ions and confined holes in (Cd,Mn)Te quantum wells with 2D hole gas. Two anisotropic ODMR signals with different angular variations were found and ascribed to isolated manganese ions and to exchange-coupled complexes consisting of manganese and holes. It is shown that calculations on the basis of spin Hamiltonian for these systems are in agreement with the experimental data.     

Optical properties and recombination processes in (Zn,Cd)Se Graded Index Separate Confinement Heterostructures

Graded Index Separate Confinement Heterostructures have been grown by MOVPE using (Zn,Cd)Se and ZnSe wide bandgap semiconductors. These structures are composed of a deep Zn_1-xCd_xSe (x<0.23%) central well embedded between two thick (Zn,Cd)Se graded layers within which the cadmium composition varies from 0 up to 10% on one side of the well, and from 10 down to 0% on the other side. Both carriers and photon confinement occur in such structures making them suitable for blue-green stimulated emission.Reflectance and photoreflectance data, taken at 2K on a series of samples of different designs allowed us to observe excitonic transitions up to the third quantum number. The structures exhibit a strong photoluminescence line due to the recombination of the carriers in the quantum well. The photoluminescence intensity is analyzed as a function of the temperature and displays a strong thermal quenching. This quenching is due to an increase of the non-radiative recombinations through defects at low temperature and to the thermal escape of the carriers above 40-60K. The value of the activation energy for thermal escape shows that this mechanism is unipolar and concerns the heavy holes.     

Ion implantation of Zn,Se and Cd in BP crystals

Ion implantation of 70 keV Zn, Se and Cd has been done in n-type BP as a function of dose and implantation temperature, and defects and lattice locations have been measured by means of channeling techniques. Crystals used were grown on Si substrates by vapor phase epitaxy. It was observed that epitaxial layers thicker than 6μm have no defects due to a lattice mismatch between Si and BP. High temperature implantation above 500°C gave high substitutional fraction of ～ 70% and low residual defects. Reordering of an amorphous layer formed by implantation below 400°C was not observed even after annealing at 900°C. The present results indicate that the effect of implantation temperature is the same with other III-V compounds such as GaAs and Gap.     

The quantum confinement effect observed in the multiple quantum wells Mg0.27Zn0.73O/ZnO

The multiple quantum wells (MQW) Mg_0.27Zn_0.73O/ZnO have been grown by pulsed laser deposition method with different well width L _w . The optical and structural characteristics of MQW Mg_0.27Zn_0.73O/ZnO have been investigated. The quantum confinement effect showing up in the blue shift of exciton peak in low temperature (8 K) photoluminescence spectra at well width reduction has been studied. It is established that intensity exciton peak I _ex and Einstein’s characteristic temperature Θ _E increase at reduction of well width L _w . It is revealed that the discontinuity ratio of conduction and a valence bands in heterostructure Mg_0.27Zn_0.73O/ZnO is 0.65/0.35 that corresponds to the literature.     

Polaritons in large-period multiple quantum wells

Il Nuovo Cimento D - Experimental studies of high-resolution reflectivity and a temperature dependence of the photoluminescence for periodic MQWs of CdTe/CdZnTe with periods equal to a one-half and...