T. Kushida

Optical characterization of CdS nanocrystals in Al2O3 matrices fabricated by ion-beam synthesis

We have studied optical properties of CdS nanocrystals formed by sequential Cd+ and S+ ion implantation into Al2O3 matrices. Two bands related to free excitons in the wurtzite CdS are clearly observed in the absorption spectrum at low temperatures. Efficient photoluminescence (PL) appears near the absorption edge. At high temperatures, the band edge PL band consists of two components. One is the free-exciton emission with a short lifetime (several hundreds of picoseconds), while the other is the bound exciton emission at shallow localized states with a long lifetime (several nanoseconds). The temperature dependence of the band gap energy has been determined for wurtzite CdS nanocrystals. Spectroscopic analysis shows that high-quality compound semiconductor nanocrystals are fabricated by the ion-beam synthesis technique.

Decay dynamics of visible luminescence in amorphous silicon nanoparticles

We have studied the recombination dynamics of carriers in amorphous silicon (a-Si) nanoparticles prepared by electrochemical anodization of hydrogenated a-Si films. The photoluminescence (PL) lifetime of the fast-decay component in a-Si nanoparticles is shorter than that in crystalline silicon (c-Si) nanoparticles. The energy dependence of the PL lifetime in a-Si nanoparticles is different from that in c-Si nanoparticles. The PL decay dynamics of a-Si nanoparticles is determined by recombination processes of carriers localized in the band-tail state. The origin of the blueshift of the PL spectrum in a-Si nanoparticles will be discussed.

Photoluminescence dynamics of amorphous Si/SiO2 quantum wells

We have studied photoluminescence (PL) spectrum and dynamics of amorphous silicon (a-Si) based quantum wells at low temperatures. In a-Si/SiO2 quantum-well samples with thin a-Si layers, the PL spectra appear in the visible spectral region. With a decrease of the a-Si well thickness, the PL peak energy shifts to higher energy and the PL lifetime becomes shorter. The well-thickness and temperature dependence of the PL lifetime show that the nonradiative recombination of carriers occurs at the a-Si/SiO2 interface and the lifetime is determined by the energy relaxation process in the a-Si well and the carrier diffusion to the interface. The quantum confinement and localization of carriers in a-Si/SiO2 quantum-well structures will be discussed.

Sharp photoluminescence of CdS nanocrystals in Al2O3 matrices formed by sequential ion implantation

We report on photoluminescence (PL) experiments in CdS nanocrystals fabricated by sequential ion implantation in Al2O3 matrices. The PL spectrum and the spatial image of the PL intensity have been studied at 8 K using a scanning near-field optical microscope. The PL spectrum at each bright spot has been found to consist of narrow lines of various energies, although the spectrum measured by conventional optics shows a single and broad band locating below the free-exciton absorption energy. The origin of the sharp PL lines in CdS nanocrystals will be discussed.

Size effects on the luminescence spectrum in amorphous Si/SiO2 multilayer structures

We have studied size effects on the photoluminescence (PL) spectrum in amorphous silicon (a-Si)-based multilayer structures. At low temperatures, the PL spectrum and the PL lifetime are sensitive to the a-Si well thickness in a-Si/SiO2 multilayers. With a decrease of the a-Si well thickness, the PL peak energy and the mobility edge shift to higher energy. The temperature dependence of the PL spectrum shows that the size-dependent visible luminescence is ascribed to radiative recombination of carriers localized in the band-tail state, rather than the band-edge emission in quantum-confined states. The quantum confinement and localization of carriers in the band-tail states will be discussed.

Free excitons in cubic CdS films

Cubic CdS layers have been epitaxially grown on (001) GaAs substrates by a low-pressure metalorganic chemical vapor deposition technique, and their free exciton states have been studied by means of modulated reflectance spectroscopy. Light-hole and heavy-hole excitons are split by biaxial compressive strains in the 2 μm CdS films. It has been found that a sharp photoluminescence (PL) appears at low temperatures and the PL energy coincides with the light-hole exciton energy. Two peaks in the PL excitation spectrum agree well with the light-hole and heavy-hole exciton energies. The free exciton energy has been determined for cubic CdS films on GaAs substrates.

Fabrication and Optical Characterization of Trivalent-Rare-Earth Doped CdS Nanocrystals

CdS nanocrystals doped with trivalent rare earth ions have been fabricated, and their photoluminescence (PL) properties have been studied at 10 K using excitation light chopped at 750 Hz. In Eu 3+ -doped CdS nanocrystals, sharp intra-4f emissions of Eu 3+ are superposed on a broad defect-related PL band of CdS nanocrystals in the PL spectrum in phase with the excitation light. In the out-of-phase PL spectrum, on the other hand, only the intra-4f emissions of Eu 3+ are observed. However, the excitation spectrum for the out-of-phase 5 D 0 - 7 F 2 PL of Eu 3+ shows a peak due to CdS nanocrystals in addition to the peaks due to the intra-4f transitions of Eu 3+ . Further, PL properties of Eu 3+ -doped CdS nanocrystals are much different from those of Eu(CH 3 COO) 3 , a raw material of the sample. These observations indicate that Eu 3+ ions are effectively doped into CdS nanocrystals and the energy transfer occurs from CdS nanocrystals to Eu 3+ ions.

A near-infrared photoluminescence study of GaAs nanocrystals in SiO2 films formed by sequential ion implantation

We have studied photoluminescence (PL) properties of Ga+ and As+ implanted SiO2 films on Si substrate. After thermal annealing, zinc blende GaAs nanocrystals are formed in SiO2 films and several PL bands appear in the red and near-infrared spectral region. Defects and impurities in GaAs nanocrystals and SiO2 cause weak luminescence in the near-infrared spectral region at low temperatures. After low-energy deuterium implantation, the defect PL intensity decreases and the red PL from GaAs nanocrystals is clearly observed. It is demonstrated that GaAs/SiO2 nanocompostites with low defect density are fabricated by sequential ion implantation followed by thermal annealing and hydrogen passivation.

Visible luminescence from Si/SiO2 quantum wells and dots: confinement and localization of excitons

Abstract We have studied luminescence properties of crystalline silicon (c-Si) and amorphous silicon (a-Si)-based quantum wells and quantum dots. The PL peak energies of excitons of c-Si and a-Si quantum structures are blueshifted from those of bulk c-Si and a-Si. In c-Si/SiO 2 and a-Si/SiO 2 quantum dots, excitons are localized at the interface between the Si interior and the surface SiO 2 layer. In c-Si/SiO 2 quantum wells, TO-phonon-related fine structures are observed in resonantly excited luminescence spectra. In two-dimensional (2D) wells, 2D excitons and localized excitons at the c-Si/SiO 2 interface contribute to a visible luminescence at low temperatures. The confinement and localization of excitons in Si quantum structures is discussed.

Near-infrared photoluminescence from Ge nanocrystals in SiO2 matrices

We have fabricated light-emitting Ge nanocrystals by means of Ge + ion implantation into SiO 2 followed by thermal annealing. Ge nanocrystals with the 5-nm average diameter are formed uniformly in SiO 2 films. A photoluminescence band with fine structures is observed near 0.9-1.0 eV. We will discuss the mechanism of the near-infrared luminescence from Ge nanocrystals.