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Phonon diffraction and interference patterns are observed at the atomic scale, using molecular dynamics simulations in systems containing crystalline silicon and nanometric obstacles, such as voids or amorphous inclusions. The diffraction patterns due to these nano-architectured systems of the same scale as the phonon wavelengths are similar to the ones predicted by the simple Fresnel–Kirchhoff integral. The few differences between the two approaches are attributed to the nature of the interface and the anisotropy of crystalline silicon. Based on the wave description of phonons, these findings can provide insights into the interaction of phonons with nano-objects and can have applications in smart thermal energy management. [ABSTRACT FROM AUTHOR]

We developed an advanced 2ω method for thermal conductivity (κ) measurements that is also applicable to samples with a wide range of thicknesses, to which the flash method cannot be applied. The conventional 2ω method, which features a simple setup combined with thermoreflectance, is a κ measurement method for thin films on substrates. However, it is difficult to apply this method to bulk substrate samples without films because of the interfacial thermal resistance between the transducer metal film and the substrate. In the advanced 2ω method, the interfacial thermal resistance becomes negligible due to the coating of a bulk sample with a high-thermal-resistance film. We measured the κ values of various bulk samples (Si, Ge, fused quartz, and TiO2). Moreover, the κ value of thin (0.3-mm) bulk Si substrates, which is difficult to measure using the conventional flash method for bulk materials because of the high thermal diffusivity, could be measured. This study demonstrates that the range of sample thickness for the 2ω method is extended from thin films to bulk materials. [ABSTRACT FROM AUTHOR]

A method is developed to form epitaxial [Ge.sub.1-x][Sn.sub.x] quantum dots (QDs) on Si(111) substrates by codeposition of Ge and Sn on ultrathin Si[O.sub.2] films with predeposited Ge nuclei. High-resolution transmission electron microscopy has shown that the main formed [Ge.sub.1-x][Sn.sub.x] QDs have less strain and no misfit dislocations.

A scheme of photoabsorption spectroscopy (PAS) based on scanning tunneling microscopy (STM) is devised to reduce the difficulties inherent in STM-PAS measurements. The devised Fourier transform (FT) scheme has provided a quick assessment of photoabsorption spectrum with a spatial resolution of a few nanometers.

Research describing the optical and electronic properties of germanium-oxide deposited on a substrate and germanium-oxide clusters is presented. In particular the possibility that oxide layers contribute visible photoluminescence is investigated.

This work reports the study of ion dynamics produced by ablation of Al, Cu, Ag, Au, and Bi targets using nanosecond laser pulses at 193 nm as a function of the laser fluence from threshold up to 15 J cm-2. An electrical (Langmuir) probe has been used for determining the ion yield as well as kinetic energy distributions. The results clearly evidence that ablation of Al shows unique features when compared to other metals. The ion yield both at threshold (except for Al, which shows a two-threshold-like behavior) and for a fixed fluence above threshold scale approximately with melting temperature of the metal. Comparison of the magnitude of the yield reported in literature using other wavelengths allows us to conclude its dependence with wavelength is not significant. The evolution of the ion yield with fluence becomes slower for fluences above 4-5 J cm-2 with no indication of saturation suggesting that ionization processes in the plasma are still active up to 15 J cm-2 and production of multiple-charged ions are promoted. This dependence is mirrored in the proportion of ions with kinetic energies higher than 200 eV. This proportion is not significant around threshold fluence for all metals except for Al, which is already 20%. The unique features of Al are discussed in terms of the energy of laser photons (6.4 eV) that is enough to induce direct photoionization from the ground state only in the case of this metal. [ABSTRACT FROM AUTHOR]

We observed strong 1.5 μm photoluminescence (PL) from Si-capped Ge1-xSnx nanodots on Si (001) substrates formed using ultrathin SiO2 films. The lack of the quantum size effect in the PL revealed that the PL came from Si capping layers with radiative defects and not from the nanodots. The PL intensities were strongly enhanced when the Sn composition x increased in Ge1-xSnx nanodots or the nanodot size decreased. These results indicate that the formation of the radiative defects in Si is strongly correlated with the nanodot structures such as its geometry and strain states. This study demonstrates a new formation technique for light-emitting Si structures with high controllability and compatibility with Si technology. [ABSTRACT FROM AUTHOR]

Electronic structures of individual β-FeSi2 nanodots were investigated using electric field modulation spectroscopy combined with scanning tunneling microscopy (STM-EFMS) by optimizing experimental conditions to enhance the inherently small signal due to a small number of photocarriers generated in a single nanodot. The obtained STM-EFMS spectra in different schemes for electric field modulation exhibited common features around 0.73 eV, which agreed well with the indirect absorption edge of β-FeSi2 bulk crystal. Maps of the STM-EFMS signal prove that the 0.73 eV signal arises from individual nanodots with a spatial resolution of a few nanometers. Those results strongly suggest that the present β-FeSi2 nanodots are indirect semiconductors. [ABSTRACT FROM AUTHOR]

We found that photoabsorption spectra (PASs) of a n-type Si exhibit a pronounced sub-bandgap peak when the sample was probed by a scanning tunneling microscope tip in a recently introduced scheme of Fourier transform (FT) spectroscopy. The peak, not observable in the conventional scheme of lock-in detection, emerged when, simulating the multiplex light illumination in the FT scheme, the sample was illuminated with a suprabandgap light in addition to the spectroscopic subgap lights. We concluded that this peculiar phenomenon is due to the optical neutralization of ionized donors by the subgap light which reduces, rather than increases, the surface photovoltage induced by the subsidiary suprabandgap lights. The use of such a subsidiary light in addition to spectroscopic lights will serve to enhance the sensitivity for tip-probing PAS detection of sub-bandgap centers. [ABSTRACT FROM AUTHOR]

A method to form epitaxial Ge1-xSnx quantum dots (QDs) on Si (111) substrates has been developed by codeposition of Ge and Sn on ultrathin SiO2 films with predeposited Ge nuclei. Hemispherical Ge1-xSnx QDs with an ultrahigh density (∼1012 cm-2) were epitaxially grown in the nanometer-size range. The QD size was controlled by changing the GeSn deposition amount. High-resolution transmission electron microscopy observations revealed that the main formed Ge1-xSnx QDs had less strain and no misfit dislocations. [ABSTRACT FROM AUTHOR]

We studied the formation of β-FeSi2 nanodots by codeposition at disilicide stoichiometric deposition rates of Fe and Si on Si (111) substrates covered with ultrathin SiO2 films. Hemispherical β-FeSi2 nanodots with an ultrahigh density (>1012 cm-2) and with a narrow size distribution at the average size of ∼5 nm diameter were epitaxially grown by the codeposition at 500 °C on ultrathin SiO2 films with predeposited Si. High-resolution transmission electron microscope observations showed that the formed nanodots were strained with a main configuration of β-FeSi2 (110)/Si (111). We propose the formation mechanism that a reaction between the SiO2 films and deposited Si atoms formed voids in the SiO2 films to work as nucleation sites, followed by direct β-FeSi2 formation from Fe and Si without passing through other iron silicide phases. [ABSTRACT FROM AUTHOR]

Ge nanodots with a typical size of ∼4 nm and ultrahigh density (>1012 cm-2) were grown on ultrathin SixGe1-x oxide films made by oxidizing Ge wetting layers grown on Si (111)–(7×7) surfaces. The density of the nanodots was independent of the Ge deposition rate, indicating that the chemical reaction between Ge atoms and the oxide films determined the nucleation of Ge nanodots. The size and density of Ge nanodots grown on the ultrathin SixGe1-x oxide films depended on the growth temperature and deposition amount. These dependences indicate that we can form the Ge nanodots with ultrahigh density controllably by using ultrathin SixGe1-x oxide films. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Strong 1.5 [micro]m photoluminescence (PL) from Si-capped [Ge.sub..1-x][Sn.sub.x] nanodots on Si(001) substrates formed by using ultrathin Si[O.sub.2] films is observed. A new formation technique is developed for light-emitting Si structures with high controllability with Si technology and the results have shown that the formation of the radiative defects in Si is correlated with the nanodot structures.

A method is described for forming epitaxial GaSb quantum dots (QDs) on Si substrates by using ultrathin Si[O.sub.2] films that contain epitaxial Ge nuclei. The QDs with a type-I structure have displayed the quantum-confinement effect in photoluminescence (PL).

The electronic structures of individual [beta]-Fe[Si.sub.2] nanodots are examined by using electric field modulation spectroscopy and scanning tunneling microscopy (STM-EFMS). The results have shown that the present [beta]-Fe[Si.sub.2] nanodots are indirect semiconductors.

The formation of beta-Fe[Si.sub.2] nanodots by codeposition at discipline stoichiometric deposition rates of Fe and Si on Si(111) substrates covered with ultrathin Si[O.sub.2] films is analyzed. The formation mechanism is proposed that deposited Si atoms reaction with Si[O.sub.2] films to form voids, at which beta-Fe[Si.sub.2] is directly formed from one Fe and two Si atoms.

The authors investigated the annealing effects on atomic structures and elemental compositions in a stacking structure, Si capping layer on Ge1-xSnx nanodots on Si substrate covered with ultrathin SiO2 film, to clarify the origin of intense photoluminescence at ∼0.8 eV from the structure, using transmission electron microscopy. After the annealing, it was found that decay of Ge1-xSnx nanodots, formation of SiOx precipitates embedded in Si-rich Si1-xGex layer at the Si cap/Si substrate interface, formation of SnO2 nanoparticles on the oxidized surface of the Si capping layer, and morphological change of dislocations in the Si capping layer occur. Reaction products that appear as a result of the movement of dislocations can be related to the origin of intense photoluminescence. [ABSTRACT FROM AUTHOR]