Sung Han Lee

Superconductivity of the Ba-DOPED Bi2PbSr2Ca2Cu3Oy system

Abstract The Bi 2 Pb(Sr 2− x Ba x )Ca 2 Cu 3 O y samples with x = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 were prepared by the solid-state reaction and analyzed by DSC, ICP, EPMA, XRD and IR, and their resistivity and a.c. susceptibility were also measured. The Ba 2+ ion is substituted successfully for the Sr 2+ ion in the 2223 phase with the solubility of Ba 2+ /Sr 2+ = 0.15 in the BiPbSrCaCuO system. In this range, the c -axis and the unit cell volume are increased and the distance of BiO(Sr)Cu bond is increased with the increased of Ba-content. The transition temperature is enhanced a little and the transition width is narrowed with increasing in Ba-content within the solubility range. From the above results, it is inferred that the SrO layer in the structure is not involved directly in the superconducting mechanism and that the role of SrO layer is to confine the superconducting electrons in two-dimension.

Interference effects in reflectance line shapes from ZnSe/GaAs epilayers

The reflectance spectroscopy is performed at 77 K on ZnSe epilayers of different thicknesses grown on GaAs substrates. The exciton line shapes of the reflectance spectra change dramatically with the epilayer thickness. This observation suggests that the interference effects between the reflected waves from the surface and from the interface play an important role on the exciton line shape of the reflectance spectra. To analyze the change in the exciton line shapes quantitatively, the reflectance spectra were calculated using a simple oscillator model for a dielectric function and considering a multiple reflection. Calculated line shapes of the reflectance spectra show good agreement with the observations.

Tunneling effects in Cd0.24Zn0.76Te/ZnTe asymmetric double quantum wells

Abstract Photoluminescence (PL) measurements have been performed to investigate the tunneling effects in Cd0.24Zn0.76Te/ZnTe asymmetric double quantum wells (DQWs). The Cd0.24Zn0.76Te/ZnTe DQWs were separated by a ZnTe step potential barrier width between 30 and 200 A. The results for the PL data at 20 K showed clearly that the excitonic transition behavior from coupling to uncoupling depended on the embedded ZnTe barrier thickness. The electronic subband energies and the wavefunctions in the quantum wells were calculated by an envelope-function approximation which took into account the strain effects, and the calculated interband transition values were in reasonable agreement with those obtained from the PL measurements. The coupling behavior between the two Cd0.24Zn0.76Te wells which were separated by a ZnTe barrier in the asymmetric DQWs was investigated theoretically, and the calculated results were in qualitative agreement with those determined from the experimental data. These results indicate that Cd0.24Zn0.76Te/ZnTe asymmetric coupled DQWs hold promise for potential applications such as new types of optical bistability switching devices in the blue-green region of the spectrum.

Thermodynamic study of the effects of Pb-addition on the formation of the 2223 phase in the Bi-based superconductor system

Abstract Bi2Sr2Ca2Cu3Oy, Bi3Sr2Ca2Cu3Oy and Bi2PbSr2Ca2Cu3Oy were prepared by conventional solid-state reaction method. Based on the results of thermogravimetry, inductively coupled plasma emission spectroscopy, and power X-ray diffraction, the thermo-dynamic factors were discussed qualitatively for the effect of lead addition on the formation of the 2223 phase during reaction. Partial substitution of bismuth by lead is favorable to the development of the 2223 phase, but the 2223 phase is thermodynamically less stable than the 2212 phase. Compared with the case of the unleaded samples, when lead is added the contents of lead and bismuth in the bulk are greatly reduced during sintering. The partial evaporation of lead and bismuth increases the entropy change of the system, which may induce the enhancement of the formation of the 2223 phase.

The relaxation of hot excitons by longitudinal optical-phonons in Cd xZn 1−xTe/ZnTe asymmetric coupled double quantum wells

Abstract Photoluminescence (PL) and photoluminescence excitation (PLE) measurements on Cd x Zn 1−x Te/ZnTe asymmetric coupled double quantum wells were performed to investigate the hot-exciton luminescence phenomena. The PLE spectra indicated the creation of hot excitons and subsequent relaxation by longitudinal optical-phonon emission resulting from the localization of excitons due to fluctuation of the Cd x Zn 1− x Te alloy well.

ZnSe:N Epilayers Grown by Low-Pressure Metalorganic Chemical Vapor Deposition Using a Radio Frequency Plasma Source

Using nitrogen radio-frequency (RF) plasma operated in low-pressure metal-organic chemical vapor deposition (LP-MOCVD), nitrogen-doped ZnSe epilayers were grown and their optical properties were investigated. In the low-temperature photoluminescence (PL) of nitrogen-doped ZnSe epilayers, two emissions were observed, attributed to a nitrogen acceptor-bound exciton (I 1 ) at 2.792 eV and a donor-acceptor pair (DAP) at 2.70 eV. The characteristics of nitrogen acceptor-bound excitons and DAP emissions under excitation power dependent PL spectra were discussed. From theses results, we found that nitrogen was successfully incorporated into selenium sites and then acted as an acceptor. In addition, we obtained the net acceptor concentration 1.15 x 10 18 cm -3 by capacitance-voltage profiling.