Hirokuni Watabe

Role of hydrogen atoms in anodized porous silicon

Abstract Hydrogen atoms chemisorbed in anodized porous silicon (PS) during anodization in a HF solution have been investigated by using both experimental techniques and a semi-empirical calculation method. The results show important roles of chemisorbed H atoms in PS on anodization mechanisms and a slight expansion of Si-Si bond length, as in the case of the structural change and low-temperature oxidation process of PS films previously reported. Fine structures observed in the infrared absorption band of the Si-H stretching vibrations can be related to charge redistributions of H-chemisorbed Si atoms which were calculated for various clusters with SinHm using the AM1 method. The calculation results on the Si-Si bond length in the clusters are also consistently explained in relation to slight increases in the lattice constant of PS: the origin comes from Si charges attracted by chemisorbed H atoms on the pore walls.

Structural Change of Crystalline Porous Silicon with Chemisorption

Anodized porous silicon (PS) has been investigated in relation to its structural change at elevated temperatures of 1100°C in a vacuum by means of infrared spectroscopy, scanning electron microscopy, X-ray diffraction and gas adsorption techniques. The structural change observed is strongly dependent on foreign atoms chemisorbed or bonded to Si atoms on the pores in PS; in the case of chemisorption of H atoms, a significant change in the pore structure is induced, while in the case of O, the formation of thin oxide walls (1–2 nm) prevents Si atoms from moving about. These results are compared with molecular orbital calculations (AM1 method) for Si14H20 and its related clusters. The importance of the surface energy is consistently indicated for the structural change observed.

Construction of a Soft X-Ray Emission Spectroscopy (SXES) Apparatus and Its Application for Study of Electronic and Atomic Structures of a Multilayer System

A soft X-ray emission spectroscopy (SXES) apparatus was constructed using a grating monochromator. The resolution was sufficient to show differences in valence electronic structures of Si compounds including pure Si crystal. A nondestructive analysis of a Ni/Si(111) specimen with heat treatment was carried out using either a clear difference in Si L2,3 SXES spectra of Ni silicide and Si single crystals or the fact that the soft X-ray production depth increases in a solid with increasing energy of a primary electron, Ep. The electronic and atomic structures of the surface and interface of specimens adopted were clarified with Ep varying between 1.5 and 10 keV.

Nondestructive depth profiling using soft X-ray emission spectroscopy by incident angle variation method

We constructed a sample reclining stage for soft X-ray emission spectroscopy (SXES), where we intended to carry out nondestructive depth profiling using an incident angle variation (IAV) method. The penetration depth of an incident electron decreases with a reclining sample stage. One of the characteristics of the IAV method is that the incident electron beam energy is constant; thus, it is easy to obtain information related to a quantitative analysis of the integrated intensity of soft X-ray emission spectra. This method has been applied for a specimen with NiSi2/Si(111) structure. We have analyzed the change in shape of the Si L2,3 emission band spectrum in a region of the Si substrate to the NiSi2 layer, and have shown that we can determine the distribution of the Si element from the integrated intensity of Si L2,3 emission spectra.

Soft X-ray emission spectroscopy study of CaF2(film)/Si(111) : non-destructive buried interface analysis

Abstract A soft X-ray emission spectroscopy (SXES) study under an energetic electron irradiation is first applied to a non-destructive buried interface analysis of a CaF 2 (film ∼ 40 nm)/Si(111) contact system, where the energy of primary electrons, E p , is ≤ 5 keV. The present work has explored the usefulness of the application of the SXES method to the interface study to give rise to the following findings: the CaF 2 /Si(111) interface shows rather sharp transition from the top CaF 2 to the substrate Si, there certainly is a Ca-silicide layer at the CaF 2 /Si(111) interface, the thickness of the silicide layer is estimated to be less than several nm, and the e-beam excited SXES non-destructive study is very powerful to analyze a specimen with rather thick top film (> 40 nm) and thin interface layer (

Ni-silicide formation : dependence on crystallographic orientation of Si substrates

We have analyzed the influence of the crystalline orientation of Si substrates on Ni-silicide formation. Ni-silicide/Si(111) and (100) samples formed through solid-phase reaction (SPR) were examined using soft X-ray emission spectroscopy (SXES), transmission electron microscopy (TEM) and grazing incidence X-ray diffraction. The formation of δ-Ni2Si and NiSi on Si(100) substrate occurs at a lower temperature than that on Si(111). However, the NiSi2 region is found to be formed only on the Si(111) substrate at a lower temperature (Ta=500°C) than previously reported. The NiSi2 regions are located at the interface region of NiSi/Si(111) and have a small island structure. On Si(100) substrates, only the NiSi layer is stably formed in the heat-treatment temperature range of 250-700°C.

Valence Band Density of States of the Iron Silicides Studied by Soft X-Ray Emission Spectroscopy.

Electronic states of manganese silicides with the composition of MnSi or MnSi 1.7 were investigated by soft X-ray emission spectroscopy. The formation of MnSi and MnSi 1.7 with single phase was identified by X-ray diffraction. Si-K β and Si-L 2,3 emission-band spectra were measured, where the Si-K β reflects the valence-band density of states with p-symmetry and the latter reflects the one with s- and/or d-symmetry. The spectrum of Si-L 2,3 for MnSi 1.7 was different from that of MnSi. The origin of this difference is discussed in comparison with those of other silicides.

Soft X-Ray Spectroscopic Analysis of Ni-Silicides

The valence-band electronic structure of Ni-silicide system, NiSi 2 , NiSi and Ni 2 Si, has been studied by measuring soft X-ray Si L 2,3 emission spectra. It is found that the Si L 2,3 spectra show very different spectral features among these three silicides. The spectra of silicides are compared with the theoretical density of states and discussion is given on the electronic structure of these silicides. It is suggested that Si s electronic state contributes significantly to the upper part of valence-band of the three silicide systems.

Study of iron silicide formation on Si(111) by soft x-ray emission spectroscopy

Abstract In this study, we identify the SiL 2,3 soft X-ray emission spectra of iron silicides which are formed on a Si substrate by solid-phase reaction. The investigation of Fe-silicide thin film formation on Si(111) using soft X-ray emission spectroscopy (SXES) gives information on the valence band density of states and also on the phase composition of silicides. From the SXES characterization, we confirm the phase mixing of α-FeSi 2 and β-FeSi 2 in the samples annealed at high temperatures.

Effect of crystallographic orientation of Si substrates on SPE NiSi2 formation

Abstract In this study, we have focused our attention on the influence of Si substrates, i.e., Si(111) and (100), on epitaxial growth of a NiSi2 layer. Ni-silicide/Si(substrate) samples heat-treated at 750°C have given information of kinetics of epitaxial NiSi2 growth transformed from a nonepitaxial NiSi layer. Particularly, the surface morphologies of Ni-silicide layers, which have been examined by SEM, have reflected the structural differences in Ni-silicide layers grown on Si(111) and (100) substrates. Using additional experimental results of SXES and cross-sectional TEM, we have suggested a model for the formation of a Ni-silicide layer. It could be explained by considering differences in the surface energy of different Ni-silicides and Si planes. From these investigations, growth kinetics of epitaxial NiSi2 on Si has been clarified in connection with the influence of the Si substrates.