Search

The local field ion densities of W(112) surface have been observed using a micro-probe hole field ion microscope combined with a pulse counting analysis. The mapped field ion densities clarified the anisotropic emission property within a single bright spot of a surface W atom site at a step edge. Based on the most relevant formula for tunneling barrier strength provided by B. Halpern and R. Gomer, we derived the local "across-atom" barrier field variations in the same step edge area. We found that the location of maximum barrier field strength is noticeably shifted towards the step side, rather than being on the top of the substrate surface atom site. The result indicates that the locus in which most highly promoted field ionization events take place is much smaller than previously estimated because of the extremely localized barrier field produced by the field adsorption of helium imaging gas atom.

We have studied photoreflectance spectra of GaAs/Al0.3Ga0.7As heterostructures from the viewpoint of the phase of Franz–Keldysh (FK) oscillations. The phase of the FK oscillations originating from the GaAs buffer layer shifts with the change of the Al0.3Ga0.7As-layer thickness, while the period does not vary. The FK-oscillation phase does not depend on the pump power, which suggests that the phase shift is not caused by a difference in the magnitude of modulation. We propose a calculation model for FK oscillations that includes the interference of probe light. Photoreflectance spectra calculated on the basis of this model are compared with the measured spectra. We conclude that mixing of the real and imaginary parts of a modulated dielectric function, which is caused by the probe-light interference, gives rise to the phase shift of the FK oscillations. We also derive a novel analysis method for a linear plot of the extremum positions of FK oscillations based on the above line-shape model. The present method remarkably reduces ambiguity in the estimation of band-gap energy that is considerable in a conventional analysis.

We investigated the optoelectronic properties of orientation-controlled lead phthalocyanine (PbPc) films on (0001) sapphire and (001) KCl substrates prepared by vacuum deposition. The results obtained from X-ray diffraction measurements indicate that the orientations of the molecular planes in the PbPc films on the sapphire and KCl substrates are perpendicular and parallel to the substrate surface, respectively. It is found from current–voltage characteristics under dark and illumination conditions that the in-plane conductivity of the PbPc film on the sapphire substrate is about 20 times as large as that on the KCl substrate. This finding indicates that the in-plane conductivity is dominated by the molecular-plane orientation, which is considered to be due to their one-dimensional conduction nature via Pb ions. Furthermore, we discuss the contribution of excitons to the photocarriers based on the results of the absorption and photocurrent spectra.

We have investigated self-narrowing and photoetching effects on the size distribution of CdS quantum dots (QDs). The QDs with a narrow size distribution are prepared by a reverse-micelle method and their mean radii are controlled by changing the water content in the reverse micelle. The mean radius and size-distribution width estimated from the line-shape analysis of absorption spectra agree well with those obtained from transmission electron microscope images, which indicates that the analysis is a convenient and reasonable method of estimating the size-distribution width. For the QDs whose initial size distribution is larger than ~10%, the size-distribution reduction due to a "self-narrowing process" is observed: The size-distribution width is spontaneously reduced with the time elapsed after the QD preparation. The further reduction of the size-distribution width is achieved by performing a photoetching treatment. The above results clearly show the possibility of preparing CdS QDs with the narrow size distribution by using the self-narrowing and photoetching effects.

We have investigated the photoluminescence properties of ZnO thin films grown by electrochemical deposition. By applying a thermal treatment at 500°C in O2 atmosphere, a free-exciton photoluminescence is observed. Furthermore, under high-density excitation conditions, a photoluminescence band appears on the low-energy side of the free exciton. The photoluminescence intensity exhibits an almost quadratic dependence on the excitation power, and the energy spacing between the photoluminescence band and the free exciton is almost equal to the exciton binding energy of ZnO. We conclude from the above results that the photoluminescence band under high-density excitation conditions originates from an inelastic scattering process of excitons, the so-called P emission.

We have investigated the scintillation mechanism of Na-doped CsI (CsI:Na) thin films grown by vacuum deposition from the aspects of their optical and structural properties. It is found from measurements of X-ray excitation luminescence that the Na-related band with the peak energy of 2.95 eV, which is conventionally used for scintillators, is markedly enhanced by heat treatment. The results of atomic force microscope measurements clearly indicate that the heat treatment causes the formation of nanoparticles. Considering the excitation spectrum of the Na-related luminescence, we conclude that the scintillation of CsI:Na originates from the self-trapped exciton in NaI nanoparticles, which presents a novel model of the scintillation mechanism.

We report on the coherent control of terahertz (THz) waves emitted from coherent longitudinal optical (LO) phonons in a GaAs/AlAs multiple-quantum-well structure at room temperature using two-pulse excitation. When the time difference between the first pump pulse and second weak control pulse is increased, the intensity of THz waves from the coherent phonons is cancelled and enhanced by the control pulse, and the phase of THz waves shows periodic changes accompanied by a phase jump. The time-difference dependences of the intensity and phase of THz waves are explained in terms of the superposition of two coherent oscillations.

We have investigated the terahertz electromagnetic waves from undoped GaAs/n-type GaAs (i-GaAs/n-GaAs) structures with various i-GaAs-layer thicknesses, focusing on the relation between the sub-picosecond-range carrier-transport processes and terahertz-wave frequency. It is observed that the intense monocycle oscillation induced by the surge current of photogenerated carriers is followed by the signal of the coherent GaAs longitudinal optical (LO) phonon. The Fourier power spectra of the terahertz waveforms reveal that an increase in the built-in electric field of the i-GaAs layer, which is controlled by the i-GaAs layer thickness, causes a high frequency shift of the band due to the surge current. Consequently, we conclude that the photogenerated carriers are accelerated by the built-in electric field in the sub-picosecond range without being affected by the intervalley scattering. This demonstrates that the frequency tunable terahertz emitters are realized. Furthermore, we find that terahertz band of the coherent LO phonon is relatively intense, compared with those from bulk crystals. This phenomenon is discussed from the viewpoint of the effects of the built-in electric field on the terahertz radiation mechanism.

We have investigated the characteristics of exciton polaritons in a CuCl microcavity with distributed Bragg reflectors (DBRs). Two sets of multilayers, PbBr2/PbF2 and HfO2/SiO2, were adopted as the DBRs in order to study the temporal stability of the CuCl microcavity. The thickness of the CuCl active layer was fixed to an effective 3λ/2 length. Angle-resolved reflectance spectra clearly demonstrate the formation of the cavity polaritons. From the phenomenological analysis with a 3×3 Hamiltonian for the cavity-polariton modes originating from the Z 3 exciton, Z 1,2 exciton, and cavity photon, the Rabi splitting energies are evaluated to be 97 and 162 meV for the Z 3 and Z 1,2 excitons, respectively, in the fresh CuCl microcavity with the PbBr2/PbF2 DBR. However, the Rabi splitting energies remarkably decrease within 6 days from the sample preparation, which is due to the degradation of the DBR resulting from alloying of PbBr2 and PbF2. On the other hand, in the CuCl microcavity with the HfO2/SiO2 DBR, the Rabi splitting energies of 105 and 168 meV for the Z 3 and Z 1,2 excitons, respectively, hardly change during 360 days from the sample preparation. This indicates that the stability of the oxide materials of HfO2 and SiO2 prevents the degradation of the DBR and CuCl active layer. Thus, a stable CuCl microcavity can be prepared by adopting the multilayer of HfO2/SiO2 as the DBR, which is a merit in applications.

We have investigated the transformation processes of the first (n=1) and second (n=2) minibands to the Wannier–Stark (WS) localization states in a GaAs (6.8 nm)/AlAs (0.9 nm) superlattice embedded in a p–i–n structure by electroreflectance spectroscopy. The high sensitivity of electroreflectance enabled us to observe the interband optical transitions associated with the n=2 minibands in addition to those associated with the n=1 minibands. The systematic results of the electroreflectance spectra as a function of electric field strength demonstrate that the electric field strength for the formation of the WS-localization state of the n=2 miniband is higher than that of the n=1 miniband, which reflects a difference in the energy widths of the n=1 and n=2 minibands. The experimental results are quantitatively discussed on the basis of the electric-field-strength dependence of the energies and envelope functions of the n=1 and n=2 minibands calculated using a transfer-matrix method. We find a clear correlation between the miniband widths and the critical electric field strengths for the formation of the WS-localization states.

We have prepared ZnS–CdS alloy (Zn1-x Cd x S) quantum dots (QDs) using chemical synthetic methods and investigated size-selective photoetching effects on the size distribution. The preparation of Zn1-x Cd x S QDs with an alloy composition from x=0 to x=1 is successful using either of the following two methods: a reverse-micelle method using AOT/heptane micellar solutions, and a standard colloidal method in aqueous polyvinyl pyrrolidone solution. Reduction of the size distribution of the colloidal alloy QD to ∼5% was achieved by a size-selective photoetching process. The present results indicate the potential of preparing Zn1-x Cd x S QDs with a narrow size distribution.

We have investigated the influence of Γ-X resonance on the photocurrent-voltage characteristics in GaAs/InAlAs type-I strained superlattices. To explain the experimental results, we have calculated the energies of the Γ and X electron states, which are confined in the GaAs and InAlAs layers, respectively, using an effective-mass approximation taking into account the strain effect. We conclude that the Γ1-X1 x y resonance causes peaky profiles in the photocurrent-voltage characteristics.

We have investigated the energy states above the barrier potential (above-barrier states) in a (GaAs)10/(Al0.3Ga0.7As)10 superlattice using photoreflectance and resonant-Raman scattering spectroscopies. The energies of the photoreflectance signals observed in the above-barrier region higher than the band-gap energy of Al0.3Ga0.7As just agree with the peak energies of the resonant-Raman profiles of the longitudinal-optical phonons. From the miniband energies calculated by an effective-mass approximation, we conclude that the photoreflectance signals and the resonant Raman profiles are caused by the transitions associated with the above-barrier states at the mini-Brillouin-zone edge.

The effect of hydrostatic pressure up to 1.44 GPa upon the luminescence bands associated with self–trapped excitons (STE's) and impurity (Tl+) in KI has been measured at various temperatures. A sapphire anvil cell with helium as a pressure medium is used. The STE luminescence bands show strong blue shift (0.43 eV/GPa for the σ band), which is much larger than the blue shift of the 1s exciton energy (0.16 eV/GPa). The intensities and half-widths of the STE luminescence bands decrease with pressure. These results are discussed on the basis of the exciton-lattice interaction strength depending on hydrostatic pressure.

We report the photoluminescence (PL) and absorption (ABS) properties of (InAs)1/(GaAs) m strained-layer superlattices with m=7, 10, 20, and 30 monolayers grown on a (001) GaAs substrate by molecular-beam epitaxy. We have clearly observed the ABS bands of the heavy-hole and light-hole excitons associated with the first subbands and the PL band of the heavy-hole exciton. It is found that the exciton energies increase as the GaAs-layer thickness increases. The GaAs-layer-thickness dependence of the Exciton energy is reasonably explained by the calculated results on the basis of an effective-mass approximation including strain effects. In addition, the band-offset energy of the GaAs/InAs heterojunction is discussed.

We have investigated photoreflectance and reflectance line shapes of heavy-hole and light-hole excitons with quantum number n=1 at 77 K in (GaAs) m /(AlAs) m superlattices (m=35 and 53 monolayers) grown by molecular-beam epitaxy. The photoreflectance and reflectance line shapes of the superlattice with m=53 are fully reversed to those of the superlattice with m=35. It is demonstrated that the difference of the photoreflectance line shapes is due to a Fabry-Perot-type interference effect causing the difference of the reflectance line shapes.

We have systematically investigated the photoluminescence (PL) properties of (GaAs)10/(AlAs) m and (GaAs) l /(AlAs) l superlattices with m=2–20 and l=8–18 monolayers in the wide temperature range from 5 to ∼300 K. Two types of PL bands appear in the superlattices with m≥6 and l≤12: the type-I PL between the Γ electrons and the Γ heavy holes of GaAs, and the type-II PL between the X electrons of AlAs and the Γ heavy holes of GaAs. The layer-thickness dependences of the type-I and type-II PL energies in both the (GaAs)10/(AlAs) m and (GaAs) l /(AlAs) l superlattices are explained by the results calculated on the basis of an effective-mass approximation with a valence-band-offset ratio of 0.34. We discuss the Γ-X crossover in the conduction band, the anisotropy of the X electron state, and the temperature dependences of the intensities and energies of the type-I and type-II PL bands.

The synthesis and X-ray diffraction study of superlattices grown by exactly alternate depositions of monolayers of GaAs and AlAs are reported. The mono- and bi-layer superlattices were synthesized by molecular beam epitaxy utilizing the intensity oscillations in the specularly reflected beam in the RHEED pattern during growth. The X-ray diffraction profile implies that the synthesized monolayer crystal has a simple cubic symmetry. The satellite diffractions for the synthesized bilayer superlattice were clearly detected at L=0.5, 1.0, 1.5, 2.5, 3.0 and 3.5.

Here, we report on the first evaluation of two-dimensional hole masses of GaAs layers in In x Al1-x As/GaAs strained-layer superlattices. The mass change in the valence band of the GaAs layers under a biaxial-layer strain has been determined by comparing the photoluminescence line shape with a theoretical calculation of the spectral dependence of the spontaneous emission rate. Reversals of the effective hole masses parallel to the heterointerfaces are important for evaluating optical transitions in strained-layer superlattices.

Raman spectroscopy has been used to study the zone-folding effects on both longitudinal acoustic (LA) and longitudinal optic (LO) phonons in (GaAs) n -(AlAs) n superlattices with integral n (2 ∼12) grown by molecular-beam epitaxy. All the observed frequencies of the zone-foled LA phonon modes up to the sixth order agree well with those calculated using the elastic model, while the observed frequencies of the zeroth- and first-order zone-folded LO phonon modes agree with those calculated using the linear-chain model. However, since long-range interactions are neglected in the linear-chain model, this model is not a good approximation for the higher-order modes. In contrast to the LA phonon modes, all the GaAs- and AlAs-like LO phonon modes are confined in the GaAs and AlAs layers, respectively.

The temperature dependence of the growth and evaporation rates in GaAs, In x Ga1-x As and In x Al1-x As grown by molecular-beam epitaxy (MBE) was measured by reflection high-energy electron-diffraction (RHEED) oscillations. The growth rate and the evaporation rate of GaAs were fitted using a thermodynamical approach. For ternary alloys the calculated results were in agreement with the observed growth rates.