Aishi Yamamoto

Biexciton emission from high-quality ZnO films grown on epitaxial GaN by plasma-assisted molecular-beam epitaxy

We have investigated the optical and structural properties of high-quality ZnO films grown on epitaxial GaN (epi-GaN) by plasma-assisted molecular-beam epitaxy employing low-temperature buffer layers. High-resolution x-ray diffraction for both symmetric and asymmetric reflexes shows that crystalline defects in ZnO films have a similarity to epi-GaN used as a substrate. The quality of ZnO epilayers grown on epi-GaN is basically determined by epi-GaN. The photoluminescence (PL) spectrum at 10 K exhibits very sharp exciton emission with a linewidth of 1.5 meV, while deep-level emission is negligible, indicative of small residual strain. At 77 K, PL is dominated by a free-exciton emission line in the low-excitation regime, while it is overtaken by a new emission band due to biexcitons at its low-energy side as the excitation intensity increases. This biexciton emission band emerges even under the intermediate excitation regime of 100 W/cm2, which is 100 times smaller than the previously reported threshold for...

Dynamics of photoexcited carriers in ZnO epitaxial thin films

Optical gain spectra of ZnO epitaxial thin films have been measured by using a pump–probe technique. The optical gain is thought to be due to electron–hole plasma. In the differential absorption spectra, we observed saturation of the exciton absorption, band-gap renormalization, as well as the optical gain. From the temporal changes of these structures, the dynamical properties of the photoexcited carriers are discussed.

Biexciton luminescence in high-quality ZnO epitaxial thin films

A photoluminescence (PL) band, the M band, was observed in photoluminescence spectra for various excitation densities in high-quality ZnO epitaxial thin films. The M band intensity increased superlinearly with an increase in the excitation intensity, suggesting that the observed PL band is due to a biexciton state. In order to prove this, the photoluminescence excitation (PLE) spectrum of the M band and time dependence of the PL intensity were measured. A shoulder that originates from two-photon absorption of the biexciton state appeared in the PLE spectrum. The biexciton binding energy was estimated to be 15 meV. The temporal behavior of the PL intensities of the M and free-exciton bands can be explained by modified rate equations assuming that the M band is caused by radiative annihilation of a biexciton leaving a free exciton and that the rate of creation of biexcitons is proportional to the 1.5th power of the exciton density. This power dependence is consistent with experimental results showing that t...

Nanocrystalline Zn2SiO4:Mn2+ grown in oxidized porous silicon

Zn2SiO4:Mn2+ nanocrystals were grown in an oxidized porous silicon layer using a chemical impregnation method. Apparently two classes of samples have been obtained. One is characterized by the formation of α-phase zinc silicate crystalline particles, which show green luminescence, and the other one is characterized by β-phase particles, showing yellow luminescence. It was found that in general prolonged annealing, as well as a high degree of impregnation leads to the formation of green-luminescent samples. The decay time of both yellow and green luminescence decreases with the concentration of Mn activator, while generally the decay time of yellow luminescence is considerably larger than that of green luminescence.

Growth of luminescent Zn2SiO4:Mn2+ particles inside oxidized porous silicon: emergence of yellow luminescence

Porous silicon layers were activated by the addition cof Zn 2 SiO 4 :Mn 2+ which resulted in green or yellow luminescent samples depending on the treatment conditions. The yellow peak occurs at 575 nm and has a considerably longer decay time than the green peak, which occurs at 525 nm. It is demonstrated that the yellow luminescence collies from small particles, typically 30 nm in size, which have been crystallized in the β-phase while the green luminescence comes from the usual α-phase, which occurs with larger particles. β-Phase appears to be the dominant phase for small particles and any condition that results in the formation of small particles gives rise to yellow emission.

Biaxial splitting of optical phonon modes in ZnSe-ZnS strained-layer superlattices

Raman scattering studies were performed on ZnSe‐ZnS strained‐layer superlattices with the incident light parallel as well as perpendicular to the interface plane. We found for the first time that the optical phonon modes split into two types, that is a singlet and a doublet, by the built‐in biaxial stress. A new method to characterize the directional stress is demonstrated.

Activation of porous silicon layers using Zn2SiO4:Mn2+ phosphor particles

A new method is proposed to activate porous silicon layers with luminescent phosphor particles. It is based on chemical impregnation of porous silicon followed by controlled heat treatment. Porous silicon is used as one of the starting materials and as a template for the synthesis of silicate phosphor particles. In this paper, we present the results for the case of Zn2SiO4 :M n 2+ into mesoporous silicon layers. Samples show strong photoluminescence peaking at about 525 nm. The resulting layer is a porous SiO2 body in which Zn2SiO4 :M n 2+ particles with typical size of 150 nm are incorporated. This method offers wide flexibility, in terms of the morphology of the matrix and the possibility to synthesize various silicates with different activators to cover the whole range of visible spectrum. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.55.Mb; 82.33.Ln; 81.07.b; 81.40.Tv

Photoluminescence Dynamics of Mn2+-Doped CdS/ZnS Core/Shell Nanocrystals: Mn2+ Concentration Dependence

The controlled doping of impurities into semiconductor nanocrystals is essential for producing and enhancing optical and magnetic functionalities. We fabricated Mn 2+ -doped CdS nanocrystals overcoated with a ZnS shell layer (CdS:Mn/ZnS core/shell nanocrystals) and studied photoluminescence (PL) properties of CdS:Mn/ZnS core/shell nanocrystals with different Mn 2+ concentrations. The intensity and decay profile of the Mn 2+ intra-3d transition PL are determined by the Mn 2+ concentration. Our findings suggest that the Mn 2+ concentration dependence of the PL dynamics is caused by exchange-coupled Mn pairs in nanocrystals.

Photocarrier generation in metal‐free phthalocyanines: Effect of the stacking habit of molecules on the photogeneration efficiency

The photocarrier generation mechanism in metal‐free phthalocyanines (H2Pc) was studied by time‐of‐flight photoconductivity measurements. The quantum efficiencies of photogeneration of holes depend on the crystal structure of H2Pc. The photogeneration efficiencies in X‐ and τ‐H2Pc are high compared with those of α‐ and β‐H2Pc. The lifetimes of the precursor state of free carriers in X‐ and τ‐H2Pc are between those of α‐H2Pc and β‐H2Pc. Moreover, infrared absorption and Raman spectra show that X‐ and τ‐H2Pc are intermediate states between α‐H2Pc and β‐H2Pc. It is pointed out that the photogeneration dynamics of H2Pc is controlled by the intermolecular overlap of the π‐electron wavefunctions and the stacking habit of molecules.

A comparative study of photoluminescence of Zn-polar and O-polar faces in single crystal ZnO using moment analysis

We report on differences in photoluminescence (PL) spectra between Zn-polar and O-polar faces in single crystal ZnO. The PL intensity ratios of one phonon to two phonon replicas of free-excitons in the two polar faces were found to be the same. This result clearly indicates that exciton-phonon coupling strengths in both faces are the same. From moment analysis, however, the relative PL intensity of the zero-phonon free-excitons in the O-polar face was larger than that in the Zn-polar face. We propose that the opposite band bending at the two polar faces causes the difference in the PL properties.