Hirofumi Tomita

Si Quantum Dot Formation with Low-Pressure Chemical Vapor Deposition

We report a simple technique for fabricating a layer of isolated Si quantum dots. The procedure uses conventional low-pressure chemical vapor deposition (LPCVD) for an extremely short deposition time in the early stage of poly-Si film growth. The layer resulting from a deposition time of 60 s has isolated Si nanocrystals 5–20 nm in diameter and 2–10 nm in height. Optical absorption measurement shows that the Si-nanocrystal spectrum changes drastically and the onset of absorption shifts to higher energies compared to that of bulk Si. This shift can be explained by the energy gap widening caused by quantum size effects. Special attention is paid to the Brownian migration of Si nanocrystals for fabricating Si quantum dots.

Microstructure and optical absorption properties of Si nanocrystals fabricated with low‐pressure chemical‐vapor deposition

We report a simple technique for fabricating a layer of isolated Si quantum dots on SiO2 glass substrates. This technique uses conventional low‐pressure chemical‐vapor deposition for an extremely short deposition time in the early stage of poly‐Si film growth. The layer after a deposition time of 60 s has isolated Si nanocrystals 5–20 nm in diameter and 2–10 nm in height. The measurements of optical absorption coefficient α show that the absorption edge for Si nanocrystals shifts to higher energies compared to that of bulk Si, indicating a widening of the energy gap caused by quantum size effects. The linear relationship (αhν)1/2 against hν suggests that the Si nanocrystal, whose diameter is as small as 10 nm, basically maintains the properties of an indirect band‐gap semiconductor. Special attention must be paid to the Brownian migration of Si nanocrystals for fabricating Si quantum dots.

New diffractometer for thin‐film structure analysis under grazing incidence condition

We designed and constructed a new x‐ray diffractometer under grazing incidence condition. This diffractometer is mainly composed of two goniometers; (1) a goniometer for grazing incidence angle control and (2) a goniometer for a sample rotation. The axes of the goniometers are orthogonal to each other. Furthermore, a detector moves parallel and vertical to the sample surface. Therefore, lattice constants of vertical and parallel components can be identified. We will describe components and accuracy of our designed diffractometer for grazing incidence x‐ray diffraction and show applications for TiSi2 (several 10 nm thickness) and GaAs (20 monolayer) on Si.

Transmission Electron Microscopy Observation of CoSix Spikes in Si Substrates during Co-silicidation Process

Needle-like spikes penetrating into Si substrates have been detected by cross-sectional transmission electron microscopy (TEM) studies of Co silicide/Si interfaces. The spikes are crystalline CoSix and form at annealing temperatures of 400–425° C, when the transformations of Co→Co2Si and Co2Si→CoSi are both taking place. They sometimes extend to the p/n junction depth of 100 nm. During annealing at above 500° C, they become spherical and their density decreases. The temperature range in which CoSix spikes are formed and extend to near the junction depth corresponds well with that for the onset of junction leakage. Therefore, this supports the conclusion that Co silicide junction leakage is caused by such spike formation.

Suppression of the Phase Transition to C54 TiSi2 due to Epitaxial Growth of C49 TiSi2 on Si(001) Substrates in Silicidation Process

The epitaxial C49 TiSi2 formed after annealing of Ti films deposited on Si(001) substrates has been studied by grazing-incidence X-ray diffraction. The relations C49 TiSi2(10\bar1)?Si(001), C49 TiSi2(100)?Si(001), and C49 TiSi2(010)?Si(001) are observed on a highly BF2-implanted substrate, and the a or b face of C49 is sharply orientated toward the directions deviating ?5? from Si on a clean substrate in ultrahigh vacuum (UHV). These epitaxial C49 grains are thermally stable and difficult to transform into the C54 phase even after annealing at high temperature. In contrast, ramdomly distributed C49 grains have been observed on unimplanted and low-dosed substrates and for thick Ti films in UHV. These polycrystalline C49 grains are thermally unstable and easily transformed into the C54 phase. The influence of implantation and UHV process on the growth of the epitaxial C49 and the phase transition of C49 to C54 are discussed.

X-ray reflectivity from ZnSe/GaAs heterostructures

ZnSe/GaAs heterostructures have been studied using x-ray reflectivity. Two samples grown by molecular beam epitaxy (MBE) differed in initial growing conditions; the first was prepared by Se treatment of a GaAs substrate, and the second one was exposed to Zn before growth of the ZnSe film. The structure and morphology of the interface between the ZnSe film and GaAs substrate were investigated. The experimental x-ray reflectivity curves, measured at different wavelengths, were simulated using a distorted-wave Born approximation method. Fitting the experimental data indicated the presence of a Ga2Se3 transition layer between the ZnSe film and GaAs substrate for the Se-treated sample, confirming that Zn treatment during the MBE growing process improves the interface quality. Furthermore, the simulations indicated that the concentration of the Ga2Se3 was less than unity. From this, we propose that the transition layer is discontinuous, e.g., possesses an island-like morphology.

X-Ray Rocking Curve Study of the Strain Profile Formed by MeV Ion Implantation into (111) Silicon Wafers

Ni, Cu and Au ions were implanted into silicon (111) wafers at irradiation energies of 80–230 MeV with doses of 0.1–5.0×1014 atoms/cm2. X-ray rocking curves from the wafers were investigated using the dynamical diffraction theory. Strain profiles obtained by curve fitting revealed that the energy loss per unit length (energy deposition) due to a nuclear collision played an important role in the expansion of the lattice spacing. The height of the main peak of the strain profile was proportional to the dose of ion implantation. In the case of Au implantation, an additional distortion near the crystal surface and a uniform expansion of the lattice spacing in the distorted layer were observed.

Grazing Incidence X-Ray Diffraction Study on Effect of Implanted BF+2 and Linewidth on Titanium Silicidation

The C49 and C54 fractions in TiSi2 are examined quantitatively by the grazing incidence X-ray diffraction method in order to study the effect of BF+2 implanted in Si substrates and linewidth on TiSi2 silicidation and the phase transition from C49 to C54. BF+2 implantation suppresses both silicidation and the phase transition, and the phase transition is also markedly suppressed on narrow lines. As a result, all the C49 TiSi2 remains for 0.5 µ m lines with BF+2 dose of 2×1015 cm-2 after the 2nd rapid thermal annealing (800° C, 30 s). Furthermore, we have found that the grains of C49 TiSi2 are preferentially oriented to highly implanted substrates. This indicates that the phase of these grains is stable thermally and difficult to transform to C54 phase.

C49-TiSi2 epitaxial orientation dependence of C49-to-C54 phase transformation rate

Abstract We investigated the C49-TiSi2 epitaxial orientation dependence of the C49-to-C54 phase transformation rate for samples with different pre-amorphizing As implantation (PAI) conditions. The C49 epitaxial orientation to the Si(001) substrate is characterized on the basis of the (131) rocking curves obtained from grazing-incidence X-ray diffraction (GIXD) measurements. We found that the phase transformation rate increases with decreasing C49 epitaxial orientation, and this dependence is very sensitive for the samples with PAI. For comparison with the orientation dependence, the C49 grain size dependence was also examined. The C49 grain size distributions were obtained from transmission electron microscope (TEM) images. Although the phase transformation rate seems to roughly increase with increasing C49 grain size, it does not show a clear relationship. Therefore, we conclude that the C49-to-C54 phase transformation rate is largely determined by the C49 epitaxial orientation, although the effects of C49 grain size and the density of As-ions cannot be ignored for the PAI process.

Characterization of ZnSe/GaAs(001) Heteroepitaxial Interfaces by X-Ray Reflectivity Measurement

The interfacial structure of ZnSe/GaAs(001) epitaxial crystals is studied using X-ray reflectivity measurements. The samples are grown by molecular beam epitaxy (MBE) with Se- or Zn-treatment. To obtain clear oscillation profiles of reflectivity, the measurements are carried out using synchrotron radiation near the Se K absorption edge and at a distance from it. Comparing observed curves with curves calculated using Parratts theory, the thicknesses and the electron densities of the transition layers of ZnSe/GaAs are determined. The thicknesses and the electron densities compared with epitaxial ZnSe are one molecular layer (1 ML) and 82% for the Zn-treated sample, 3 MLs and 80% for the Se-treated sample, respectively.