Takenari Goto

High temperature excitonic stimulated emission from ZnO epitaxial layers

The emission spectrum of high quality ZnO epilayers is studied from room temperature up to 550 K. At room temperature and low excitation power a single emission peak is observed which may be identified with the free exciton from its peak energy and dependence on temperature. However, when excitation intensities exceed 400 kW cm−2 a sharp peak emerges at lower energy which we attribute to exciton-exciton scattering. At higher excitation intensities (>800 kW cm−2) a second stimulated emission peak emerges at even lower energies: we attribute this peak to be stimulated emission of an electron hole plasma. Similar features are observed for all temperatures up to 550 K.

Growth of ZnO single crystal thin films on c-plane (0 0 0 1) sapphire by plasma enhanced molecular beam epitaxy

Abstract ZnO single crystal thin films were grown by plasma enhanced molecular beam epitaxy on (0 0 0 1) sapphire. The growth modes of ZnO epilayers were investigated by reflection high-energy electron diffraction. A transition from two-dimensional nucleation to three-dimensional nucleation is found at the initial growth stage. Optical properties of the films, studied by photoluminescence spectroscopy, exhibit a dominant bound exciton emission at 3.361 eV at 4 K, and a deep level emission centered at 2.42 eV which is associated with either impurities or native defects. The deep level emission which is successfully suppressed to 1 500 of intensity of the excitonic emission. Fabrication of these high-quality ZnO epilayers had lead to observation of stimulated emission at room temperature.

Exciton state in two-dimensional perovskite semiconductor (C10H21NH3)2PbI4

Abstract Reflection, luminescence and absorption spectra are measured in an exciton region of (C 10 H 21 NH 3 ) 2 PbI 4 . Crystal structure is determined to be of layered perovskite type by X-ray diffraction. PbI 4 layers are sandwiched by barrier layers consisting of alkylammonium chains. The exciton state has a large binding energy of 370 meV and is very stable even at room temperature. Its oscillator strength is estimated to be about 0.7 per a formula unit. The binding energy is too large to be explained only by the two-dimensional character of the exciton and is ascribed to the small dielectric constant of the barrier layer, which makes Coulomb interaction between an electron and a hole stronger.

Formation and Radiative Recombination of Free Excitonic Molecule in CuCl by Ruby Laser Excitation

New emission bands called the M , N 1 , N 2 and N 3 bands have been found near the band edge in CuCl single crystal with excitation by the two-photon absorption of a Q -switched ruby laser. All emission intensities show non-linear dependences upon the excitation intensity. One of them, the M band located at λ3919 A at 4.2°K, is concluded as the radiative decay of a free excitonic molecule. The shape variation and peak shift to the low energy side of the M emission band with respect to the increase of the laser power are similar to those due to the raising of temperature. The line shape is an inverted replica of the Maxwell distribution. The system of the excitonic molecule has been concluded to have the effective temperature which is higher than the lattice temperature in the high density state of excitonic molecule. The mechanism of the radiative decomposition of the excitonic molecule has been discussed.

Exciton Features in 0-, 2-, and 3-Dimensional Networks of [PbI6]4- Octahedra

We have investigated reflection spectra at 4 K for single crystals of (CH 3 NH 3 ) 4 PbI 6 ·2H 2 O which consists of isolated [PbI 6 ] 4- octahedra, (C 10 H 21 NH 3 ) 2 PbI 4 and (CH 3 NH 3 )PbI 3 , which consist 2- and 3-dimensional networks of [PbI 6 ] 4- , respectively. The formation energy of the lowest exciton state increases from 1.633 eV to 3.4 eV with decreasing the dimensionality in the crystal structure, from the transfer energy between the adjacent octahedra is estimated to be about 0.5 eV. The on-site Coulomb energy in the octahedron is estimated to be 2.3 eV. We suggest that the large on-site Coulomb energy enhances the exciton binding energy, especially in the 2-dimensional system.

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.

Exciton Luminescence of CuCl, CuBr and CuI Single Crystals

Emission, absorption and reflection spectra have been studied on single crystals of cuprous halides. Three types of emission are observed: 1) resonance emission due to the annihilation of the lowest energy intrinsic exciton, 2) narrow lines due to bound excitons at defects or impurities, 3)broad bands and overlapping equidistant narrow bands. In the excitation spectrum for the resonance emission a structure of equal spacing, which corresponds to the longitudinal optical (LO) phonon energy, is found in the exciton absorption band region at low temperatures below 40°K. The resonance emission bands in CuBr and emission lines belonging to the type 2) in CuI are accompanied by the LO phonon assisted satellite bands. The LO phonon energies are obtained as 210±12, 166±7 and 151±8 cm -1 in CuCI, CuBr and CuI crystals, respectively.

Surface electronic structure of a single-domain Si(111)4×1-In surface : a synchrotron radiation photoemission study

The electronic structure of a Si(111)4 × 1-In surface has been studied by angle-resolved photoelectron spectroscopy (ARPES). Using a 1.1° off-axis Si(111) wafer as substrate, a single-domain Si(111)4 × 1-In surface has been prepared in order to determine the dispersion of surface state (SS) without the obscurity arising from multi-oriented 4 × 1 domains. Three SSs that cross the Fermi level have been found. Thus, the Si(111)4 × 1-In surface is concluded to be metallic. The dispersions of the metallic SS appeared to be almost one-dimensional, suggesting one-dimensional metallic bonds among In atoms. Completely occupied SSs have been also found. The characteristics of SSs are discussed in relation to the existing structural models for the Si(111)4 × 1-In surface.

Emission of Cuprous Halide Crystals at High Density Excitation

Super-linear luminescences of cuprous halide crystals under the high density excitation by an N 2 -laser have been studied at temperatures above 100 K, where excitons and free charge carriers are considered to co-exist. A new emission, which is called H band, is found in CuCl, CuBr and CuI crystals. The H band has an asymmetric shape tailing to the low energy side. The peak energy, measured from the exciton band, varies linearly with temperature. By assuming that the emission is due to the interaction of exciton with free electron, the properties of the H band are explained and the effective masses of the electron and hole are obtained as m e =0.43 m o , m h =4.2 m o in CuCl, m e =0.28 m o , m h =1.4 m o in CuBr and m e =0.33 m o , m h =14 m o in CuI crystals.

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...