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Magneto-optical studies on self-organized quantum dots (QDs) of Cd 1− x Mn x Te were reported. Cd 1− x Mn x Te QDs with Mn composition less than 10% were grown successfully on the ZnTe(1 0 0) surface by atomic layer epitaxy. A typical dot size of 20 nm in diameter and 2 nm in height and density of the order of 10 11 cm −2 were confirmed by atomic force microscope (AFM) measurement. In photoluminescence (PL) measurements on capped QDs, the excitonic emission from Cd 1− x Mn x Te dots was observed but the spectra were composed of two lines with a separation of 20–26 meV at low temperatures. In the dependence on the temperature, the excitation intensity and the magnetic field, the PL line at the lower-energy side exhibited peculiar behaviors, compared to that in the higher-energy side — rapid quenching with an increase of temperature up to 20 K, much smaller Zeeman shift under magnetic fields, and the saturation of the circular polarization at low magnetic fields. These features of the lower-energy line were discussed in relation to the excitonic magnetic polaron in QDs. Preliminary results of time-resolved PL and spatial-resolved PL using the far-field microscope are also shown.

We have studied the effect of oxygen incorporation into AlGaInP on carrier recombination in GaInP/AlGaInP double heterostructures (DH) grown by MBE using a solid phosphorus valved cell. It was found that photoluminescence efficiency and interfacial recombination velocity of the DH were determined by residual oxygen concentration in the AlGaInP layers. The non-radiative recombination rate in AlGaInP was proportional to the oxygen concentration. We achieved AlGaInP with oxygen concentrations below 4 × 10 16 cm -3 (the detection limit of the SIMS measurement), nonetheless decay of the carriers in the DH was still governed by interfacial recombination. There was a discrepancy in interfacial recombination velocity as a function of oxygen concentration between samples previously grown by MOCVD and our samples currently grown by MBE.

We have studied time-resolved photoluminescence (PL) of type-II GaP/AlP/GaP quantum wells with different well widths and acquired detailed information concerning the mechanisms of the radiative recombination. At low temperatures, the PL spectra consist mainly of a no-phonon line. The decay of the no-phonon line was slow and nonexponential. The decay curves could be fitted to a model which assumes that the radiative recombination occurs as the result of both incoherent and coherent Γ-X scattering due to disorder at the interface. The fit revealed that the dominant Γ-X mixing mechanism was random scattering caused by fluctuations in the potential at the interface. Based on simple perturbation theory, the observed increase in the radiative decay rate with decreasing well width was explained by the increase of overlap of the electron envelope wave functions as well as the decrease of Γ-X energy separation. The model suggests that the magnitude of the potential responsible for the Γ-X mixing is considerably greater than those observed in the type II GaAs AlAs heterostructures.

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.

We have studied the electronic structure of strained-layer ZnSe/ZnS superlattices by using two kinds of two-photon spectroscopy: two-photon absorption spectroscopy and resonant second-harmonic generation. The exciton binding energy is found to exhibit a blue shift as compared to the bulk value due to the exciton confinement effect. Based on the observed subband energies, we derive a conduction-band offset of ∼ 70 meV and a valence-band offset of ∼ 800 meV.

Two-photon absorption spectroscopy is used as a novel tool to study residual strain in ZnSe epitaxial layers grown on GaAs substrates. Taking advantage of the polarization selection rules, we find that ZnSe layers thicker than 2 μm suffer in-plane tensile strain of the order of 10 -4 , while the epilayer thinner than 1 μm undergoes biaxial compression. The measured thickness dependence of misfit strains is compared to that obtained from X-ray diffraction.

Strong signals of both hyper-Raman scattering by optic phonons and hyper-Rayleigh scattering (SHG), the latter of which is forbidden by the usual selection rules, were found to emerge in two-photon resonance with excitation energy of the 2P excitons in a thin film sample of ZnSe on GaAs substrate. Both excitation spectra, which are found to be basically similar to each other, indicate an energy-splitting of the 2P excitons. As a result, it is found that the sample of 5 μm thickness suffer in-plane strains due to lattice misfit between ZnSe and GaAs, an order of which has been estimated thereby as e ≈ 5 × 10-4. A similar resonant hyper-Raman scattering was also found in a bulk sample of CdS, and is utilized for gaining into insight the microscopic origin of the phenomenon. On the basis of the results, it is discussed that the present phenomena, found for the first time in solids, would serve as novel spectroscopic methods for characterizing sample quality of II–VI compounds recently developed by state-of-the-art methods, through the strain-induced structure of the 2P excitons free from polariton effects.