Ki-Seon Lee

Photoluminescence characteristics of ZnTe epilayers

High quality ZnTe epilayers have been grown on semi-insulating GaAs(100) substrates by hot-wall epitaxy, and the photoluminescence characteristics were investigated. The free exciton binding energy is found to be 12.7 meV and the free exciton reduced mass to be 0.095m0 from the clearly resolved excitonic peaks. From the temperature dependence of the free exciton line, the room temperature energy gap of ZnTe epilayer is found to be 2.278 eV. The longitudinal optical phonon energy from the resonance Raman shift is determined as 26.2 meV. The binding energy of the exciton bound to the neutral acceptor is found to be 4.9 meV. The temperature dependence of the bound exciton peak intensity is explained with a two-step thermal quenching mechanism. Deep level emissions such as donor–acceptor pair emission, Y-band, and oxygen-bound emission are also studied.

Band gap energy and exciton peak of cubic CdS/GaAs epilayers

Cubic zinc blende CdS epilayers were grown on (100) GaAs substrates by hot-wall epitaxy. The lattice constant of cubic CdS was measured by x-ray diffraction and it was found that the compressive strain remained in the CdS films. Photoluminescence (PL) measurement showed the free exciton emission at the room temperature. Room temperature energy gap and exciton binding energy were determined by absorption and PL spectra.

Characterization and growth of high quality ZnTe epilayers by hot-wall epitaxy

ZnTe epilayers of high quality have been grown by hot-wall epitaxy and their characteristics have been investigated. By Rutherford backscattering, the atomic ratio of Zn : Te in the epilayers was found to be almost constant over the wide range of the growth temperature. It was found that the quality of the films depended on the pre-heating temperature. The best value of the FWHM of the double crystal rocking curve, 93 arcsec, was obtained at the pre-heating temperature of 590–610°C. The PL spectrum with the strong free exciton peaks and no oxygen-bound exciton peaks showed the high quality of the films. From the PL spectrum and the lattice constants, it could be concluded that a tensile strain remained in the ZnTeGaAs epilayers.

Growth and characterization of cubic CdS epilayers on GaAs substrates

Cubic CdS epilayers were grown on (100) GaAs substrate by hot-wall epitaxy. X-ray diffraction and photoluminescence measurements revealed that the hexagonal phase was dominant in the layers grown at low temperatures, and the cubic phase became dominant with increasing growth temperature. The photoluminescence emission lines in cubic CdS were identified.Cubic CdS epilayers were grown on (100) GaAs substrate by hot-wall epitaxy. X-ray diffraction and photoluminescence measurements revealed that the hexagonal phase was dominant in the layers grown at low temperatures, and the cubic phase became dominant with increasing growth temperature. The photoluminescence emission lines in cubic CdS were identified.

Thickness dependence of double crystal rocking curves and photoluminescence in ZnS epilayers grown on GaAs(100) and GaP(100)

Abstract High quality ZnS/GaAs and ZnS/GaP epilayers are grown by hot wall epitaxy. The full width at half maximum (FWHM) of the double crystal rocking curves (DCRC) and photoluminescence (PL) are measured to investigate the crystalline quality of ZnS/GaAs and ZnS/GaP epilayers. The best value of the FWHM of the DCRC for ZnS/GaAs epilayers at 3.8 μm thickness is 373 arcsec, and for ZnS/GaP epilayers at 4.7 μm thickness of 173 arcsec. The PL spectra of ZnS/GaAs and ZnS/GaP epilayers reveal very strong near-edge emission peaks, no deep level peaks and self-activated peaks. The band gap energy E g for ZnS epilayer is measured at 3.729 eV at the room temperature. The tensile strain due to the lattice mismatch and the thermal expansion coefficient difference between ZnS epilayer and GaAs (or GaP) substrate is existed in ZnS epilayers. In this study, the thickness dependence of DCRC and PL for ZnS epilayers is analyzed taking into account the tensile strain.

Strain effect and photoluminescence of ZnS epilayers grown on GaP(100) substrates by hot-wall epitaxy

ZnS epilayers were grown on GaP (100) substrates by hot-wall epitaxy and their properties were investigated. The films grown under the optimum conditions show high quality. The full width at half-maximum (FWHM) of the double-crystal rocking curves and the lattice parameter show that the epilayers grow coherently up to 0.035 μm. And it is also found that in ZnS epilayers thicker than 1.5 μm the lattice mismatch induced strain is almost relaxed. The strong near-band excitonic peaks and the hardly observable deep level emission in photoluminescence spectra indicate the high quality of films. The room temperature free exciton line was observed at 3.6682 eV with a FWHM of 40 meV.

Lattice relaxation in ZnS epilayers grown on GaP

The lattice relaxation of ZnS epilayers grown on GaP substrates by hot-wall epitaxy was investigated. The dependence of lattice constants and full widths at half-maximum of the double crystal rocking curves upon layer thickness was observed. The critical thickness for ZnS/GaP is found to be about 350 A. The epilayers thinner than the critical thickness have almost the same lattice constants. The strain due to the lattice mismatch is almost relaxed in epilayers thicker than 2.5 μm.

Compositional dependence of the Raman frequencies and line shapes of ZnS1−xTex alloys

The Raman spectra for ZnS1−xTex alloys were investigated for a wide range of Te composition x. The lattice vibration showed two-mode behavior. The asymmetric broadening of the line shape was found to be dependent on the Te composition x and could be explained by the “Spatial Correlation” model.

Optimum growth conditions and near band-edge photoluminescence of high quality ZnS/GaAs(100) epilayers

Abstract High quality ZnS epilayers were grown on GaAs(100) substrate by hot wall epitaxy. The optimum growth conditions for high quality ZnS epilayer were found. Near band-edge photoluminescence (PL) spectrum of high quality ZnS epilayers showed sharp and narrow exciton peaks and no self-activated peaks. The room temperature energy gap of ZnS/GaAs was found to be 3.729 eV from the experimentally observed free exciton PL peaks. The existence of the free exciton at room temperature reveals that the crystallinity of ZnS epilayers is excellent. The crystallinity became better with the increasing epilayer thickness according to the result of PL and the double crystal rocking curve. The temperature dependence of the PL linewidth broadening was analyzed in terms of exciton scattering process. From the splitting of the heavy hole and the light hole exciton peaks, the strain was identified.

Enhanced thermoelectric figure-of-merit in Bi-Sb-Te nanocomposites with homogenously dispersed oxide ceramic ZrO2 nanoparticles

In this research, p-type BiSbTe/ZrO2 nanocomposite powders were fabricated by high-energy ball milling. Different weight percentages of ZrO2 (2, 4, and 6 wt. %) nanoparticles were incorporated into the bulk (BiSbTe) matrix by consolidation of as-synthesized nanocomposites (NCs) powder by spark plasma sintering at 673 K. The phase and existence of ZrO2 nano-inclusions was confirmed by X-ray diffraction and transmission electron microscopy-selected area electron diffraction analysis. The Seebeck coefficient of the BiSbTe/ZrO2 NCs was significantly improved (∼36% for 4 wt. % added NCs) by a decrease in the carrier concentration and energy filtering effect, whereas the thermal conductivity was much reduced via strong scattering of carriers/phonons. The peak thermoelectric figure-of-merit (1.34 ± 0.06) was obtained for BiSbTe into which 2 wt. % ZrO2 was dispersed, which was approximately 20% greater than that of the undispersed sample. The hardness of the nanocomposites was significantly improved (∼27%) due to...