Ikurou Umezu

Emergence of Very Broad Infrared Absorption Band by Hyperdoping of Silicon with Chalcogens

We report the near through mid-infrared (MIR) optical absorption spectra, over the range 0.05–1.3u2009eV, of monocrystalline silicon layers hyperdoped with chalcogen atoms synthesized by ion implantation followed by pulsed laser melting. A broad mid-infrared optical absorption band emerges, peaking near 0.5u2009eV for sulfur and selenium and 0.3u2009eV for tellurium hyperdoped samples. Its strength and width increase with impurity concentration. Its strength decreases markedly with subsequent thermal annealing. The emergence of a broad MIR absorption band is consistent with the formation of an impurity band from isolated deep donor levels as the concentration of chalcogen atoms in metastable local configurations increases.

Effects of thermal processes on photoluminescence of silicon nanocrystallites prepared by pulsed laser ablation

The temperature dependence of the photoluminescence (PL) spectra of Si nanocrystallites prepared using an inert-gas-ambient pulsed laser ablation technique was characterized. Although the PL intensity of as-deposited Si nanocrystallites was very weak at room temperature, it increased with annealing in N2 or O2 gas. The PL intensity of the sample annealed in O2 gas was the largest at room temperature since the reduction in intensity with increased temperature was small. The nonradiative recombination process is discussed in terms of the temperature dependence of the PL intensity. Our results suggest that annealing of Si nanocrystallites in O2 gas reduces the nonradiative recombination of electron–hole pairs.

Effect of structure on radiative recombination processes in amorphous silicon suboxide prepared by rf sputtering

We studied the relationship between the structure and photoluminescence (PL) mechanism of amorphous silicon suboxide (a-SiOx) thin films prepared by the cosputtering method. The microscopic structure of the film was estimated by x-ray photoemission spectroscopy and infrared absorption spectroscopy. Electronic states were investigated by optical absorption and photothermal deflection spectroscopy. The results indicate that the film is separated into two types of regions: Si-rich cluster regions and amorphous SiO2-rich regions. The size of the Si-rich cluster is estimated, according to the quantum confinement model in which no other effects are assumed to exist, to be less than several nm, when the oxygen fraction x is larger than 1.0. The PL peak energy increased monotonically with the x value, whereas it showed different temperature dependences between the larger x value samples and the smaller ones. The PL characteristics observed can consistently be explained by assuming that there are two origins for P...

Photoluminescence from Excited Energy Bands in Au25 Nanoclusters

The photoluminescence of Au25 nanoclusters was studied to elucidate its mechanism. We observed luminescence from the excited band at 2.8 eV in addition to the well-known 1.8 eV emission. The excited band luminescence showed rapid decay described by a stretched exponential function with the mean relaxation time of 2.2 ns. The 1.8 eV luminescence indicated double exponential function decay with the relaxation times of 1.9 and 0.7 µs. We compared the results with a model and found that the 1.8 eV emission corresponds to the transition between quantum levels of 6sp electronic states, while the excited band luminescence corresponds to that between the 6sp and 5d states.

Deposition of silicon nitride films by pulsed laser ablation of the Si target in nitrogen gas

We prepared SiNx film by pulsed laser ablation from Si target and nitrogen gas. We found that control of gas pressure is important to prepare stable and near stoichiometric film. The nitrogen content in the film increased with increasing nitrogen pressure up to 10 Pa. On the other hand, the film prepared at above 15 Pa easily oxidized in the atmosphere. The nitrogen molecule was decomposed to radicals by plume and reactions between Si and nitrogen take place up to 10 Pa. At higher pressure, formation of SiNx cluster in the plume prevent the production of high quality SiNx film.

Correlation between Natural Oxidation Process and Photoluminescence Properties of Hydrogenated Si Nanocrystallites Prepared by Pulsed Laser Ablation

Natural oxidation processes of hydrogenated Si nanocrystallites were investigated to clarify effects of surface oxidation on photoluminescence wavelength. Hydrogenated Si nanocrystallites were prepared by pulsed laser ablation in hydrogen gas ambient. The Si–H bonds on the surface of the nanocrystallites enable us to estimate the local configuration of Si–O bonds using infrared frequency shifts. The natural oxidation process was investigated by measuring the density and local configuration of Si–O bonds. The oxidation process can be classified into first and second stages. The first stage is due to the diffusion of oxygen molecules in the nanocrystallites through voids in the porous structure, and the second stage is due to the oxidation of each nanocrystallite from the top surface to the sub-surface. The configurations of Si–O bonds in the first and second stages are silicon-rich and oxygen-rich compositions, respectively. The photoluminescence wavelength was blue-shifted with increasing Si–O bond density. This PL peak shift was not continuous, but three PL peak regions at 800, 600–700, and 400–500 nm were observed. This result indicates that the origin of this PL peak shift is not due to quantum confinement because of decreased diameter of Si nanocrystallites, but is due to the existence of surface oxide. A photoluminescence peak at 800 nm was observed in fresh specimens, and those at 600–700 and 400–500 nm were observed from the first and second stages of oxidation, respectively.

A comparative study of the photoluminescence properties of a-SiOx:H film and silicon nanocrystallites

Photoluminescence (PL) properties in a-SiOx and silicon crystallites with nanometer dimensions were compared to investigate recombination processes and to reveal the effect of a-SiOx on silicon crystallites. The PL spectra and temperature dependence of PL intensity of a-SiOx (where x>1) and silicon crystallites surrounded by SiOx layer shared some similarities. These results indicate that the PL peak found near 1.6 eV has its origin in the SiOx layer at the surface of the silicon core. Recombination processes are discussed by temperature dependence and lifetime of PL spectra. The PL intensity of silicon crystallites at room temperature originates from reduction of the non-radiative recombination probability rather than increase in the radiative one.

Crossover from Efros–Shklovskii variable range hopping to nearest-neighbor hopping in silicon nanocrystal random network

We investigate how random structure affects the electrical transport of a silicon nanocrystal network. The temperature dependence of conductivity follows G ~ exp[−(T0/T)1/2] between 70 and 160 K. By using T0 = 5765 K obtained by data fitting, the electron localization length is estimated to be 4.1 nm, which corresponds to the mean diameter of silicon nanocrystals. Above 160 K, G follows Arrhenius-like behavior. These temperature dependences are well described by Efros–Shklovskii variable range hopping (ES VRH) with a Coulomb gap and nearest-neighbor hopping (NNH). A crossover between ES VRH and NNH is observed at 160 K.

Inter-dot electron transport in coupled InAs quantum dots under a magnetic field

We studied electrical transport properties of coupled InAs quantum dots (QDs) embedded in GaAs. A resonance peak in the current–voltage characteristics was observed in the low temperature region. When the magnetic field was applied, a linear shift of the resonance voltage was observed. As a result of the g-factor estimation, the resonance is attributed to the current corresponding to the electron transport through coupled InAs QDs.

Numerical Analysis of Behavior on Opposing Unsteady Supersonic Jets in a Flow Field with Shields

Collision dynamics of opposing unsteady supersonic jets injected in background gas with shock waves were calculated to simulate double pulsed laser ablation. Since the jets are deflected by collision and the motion of debris is ballistic. This characteristic can be used to reduce the number of debris when shields are mounted in front of substrate. The flow of jets through installed shields is complicated by the interaction between shields and jets, and between shields and shock waves. We investigate influence of shield position on the shock waves and the jets by numerical calculations. Axisymmetric two-dimensional compressible Euler equations were solved using the finite volume method by using ANSYS Fluent 14.0.0 code. The shields with slit was mounted parallel to the direction of initially injected jets. In order to investigate the influence of shield position on the shock waves and the jets, the shield position and background gas pressure were adopted as parameters. The jets and shock wave are deflected by collision and they can pass through the slit of shields. The passed shock wave reflects at the substrate mounted behind the slits and it forces back the jet to decrease the jet velocity. The shield position governs the velocity and amount of the jet that reach the substrate.