Yong Dae Choi

Effects of synthesis temperature on particle size/shape and photoluminescence characteristics of ZnS:Cu nanocrystals

Abstract ZnS:Cu nanocrystals are prepared by solution synthesis technique, and the optimum range of synthesis temperature and the particle size/shape for the high photoluminescence (PL) properties of ZnS:Cu nanocrystals are investigated in this study. With an increase of synthesis temperature, the crystal structure is not changed. However, the particle size increases slightly. PL intensity is maximum for the sample prepared at 85 °C for the temperature range used in this study (70–95 °C). This is the first observation for the ZnS:Cu nanocrystals prepared by solution synthesis technique. The X-ray photoelectron spectroscopy (Cu2p3/2), performed to explain this phenomenon, demonstrates that with lower synthesis temperature ( 90 °C), however, Cu is transformed into CuO, and CuO acts as the nonradiative recombination center, resulting in the substantial decrease of PL intensity observed in this study.

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.

Resonant Raman scattering measurements of strains in ZnS epilayers grown on GaP

The relaxation of strain in the ZnS epilayers grown on (100) GaP was investigated with resonant Raman scattering measurement. The single LO phonon resonant Raman shift and the intensity increased but the full width at half maximum decreased with the increasing ZnS epilayer thickness. These were attributed to the relaxation of the biaxial tensile strain with the generating misfit dislocations. Finally, the critical thickness of ZnS/GaP epilayer was found to be around 35 nm.

Effect of Mn concentration on photoluminescence characteristics of Zn1–xMnxTe epilayers

Abstract Zn1–xMnxTe (0≤x≤1) epilayers were grown on the GaAs (100) substrates by hot-wall epitaxy, and their lattice constant and photoluminescence (PL) characteristics were investigated. It was found that the relation between Mn composition and the lattice constant satisfied the Vegard law. Without an external magnetic field, the excitation power and the temperature-dependence of PL were measured. The excitonic emission peak was affirmed with the Mn composition and the temperature-dependence of PL, and the donor–acceptor pair emission peak was identified with the excitation power and the temperature-dependence of PL. As the Mn composition increased, the PL intensity due to the intra-Mn+2 transition increased and the PL peak energy increase was relatively small in comparison with the near edge emission energy.

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.

Structural and optical properties of Zn1−xMnxTe epilayers as diluted magnetic II–VI semiconductors

Zn 1 x Mn x Te epilayers have been grown on GaAs (1 0 0) substrates by hot-wall epitaxy. The structure of the epilayers was found to be zincblende over the whole range of the composition. The lattice parameters and the compositions were determined with double crystal rocking curves. Exciton-related near-edge emissions were observed in the 10 K photoluminescence spectrum without an external magnetic field. The peak energy of the edge emission increased linearly with increasing Mn composition up to 60%. The intra-ion transition emissions were observed in high Mn composition epilayers and the transition energy did not depend on the Mn composition.

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.