廣瀬 和之

The chemical and electronic structures of SiO2/Si(1 1 1) and SiO2/Si(1 0 0) interfacial transition layers are reviewed. It will be shown, considering the penetration of electronic states from the Si substrate into SiO2 in the analysis of the thickness dependence of the energy loss of O 1s photoelectrons measured with a probing depth of 0.41 nm, that energy barriers for valence electrons, which are almost comparable to those found in SiO2, were formed when the oxide layer formed on Si(1 1 1) and Si(1 0 0) was thicker than 0.61 and 0.51 nm, respectively. In other words, the SiO2/Si(1 1 1) and SiO2/Si(1 0 0) interface defined with electronic band structure exists, respectively, in the oxide located effectively 0.61 and 0.51 nm away from the nominal interface defined with the atomic structure. In other words, incorporation of one atomic layer of oxygen at the interface does not form an electronic barrier. The extreme uniformity of the oxide layer formed on Si(1 0 0) was verified using synchrotron radiation excited high resolution XPS. (C) 2003 Elsevier Science B.V. All rights reserved.

The dielectric constant epsilon of ultrathin (0.55-7.96 nm) SiO2 films formed on a Si(001) substrate was characterized in terms of the modified Auger parameter of Si atoms, alpha(Si)('). The alpha(Si)(') was found to be as much as 0.7 eV higher for an ultrathin (0.68 nm) SiO2 film than for thick SiO2 films. From the observed oxide thickness dependence of alpha(Si)('), the epsilon of ultrathin SiO2 films was estimated by calculating the change in the polarization energy and the change in the electrostatic screening energy originating from dielectric discontinuity at the SiO2/Si interface. The epsilon of ultrathin (0.68-2.13 nm) SiO2 films was identical to that of bulk SiO2 within +/-1%.

We demonstrate that CoSi2 grows epitaxially on H-terminated Si(001) and present the growth mechanism. It was found that direct reaction of Co with Si is suppressed on H-terminated Si below 400 degreesC. Thus, the hydrogen at the Co/Si interface hinders the formation of Co2Si and CoSi. Upon thermal desorption of hydrogen at around 400-550 degreesC, CoSi2, which is closely lattice-matched to Si(001), grows on Si(001) and thus, thin epitaxial CoSi2 films are formed on Si(001). The {111}-faceting was completely suppressed in the epitaxial CoSi2/Si(001), leading to the atomically flat interface. (C) 2003 American Institute of Physics.

We demonstrate that CoSi<Sub>2</Sub> directly grows epitaxially on H-terminated Si(001) and the interface is atomically flat. The hydrogen present on the Si surface seems to suppress the direct reaction of Co with Si up to &sim;400<Sup>o</Sup>C. Thus, the hydrogen at the Co/Si interface hinders the formation of low temperature (metal-rich) phases such as Co<Sub>2</Sub>Si and CoSi. Upon thermal desorption of hydrogen at around &sim;460<Sup>o</Sup>C, the direct epitaxial growth of CoSi<Sub>2</Sub> on Si(001) occurs. However, with the increase of initial Co film thickness, cracks are formed partially due to a strong tensile stress. More detailed study is needed concerning the effects of such defects for the practical applications to VLSI.

We fabricate 128-Kbit SRAMs using a rad-hard circuit design based on a mixed-mode three-dimensional simulation in a commercial silicon-on-insulator foundry with 0.2-mum design rules. Appropriate design increases the critical linear energy transfer of single-event upset over 164.4 MeV/(mg/cm(2)).

We report the results of the X-ray photoelectron spectroscopy (XPS) time-dependent measurement of expanded Xray irradiation times ranging from 1 to 10, 000 min. It was Found that the Si 2p peak binding energy of a Si Substrate covered with an ultrathin SiO2 film first increases, then decreases, and again increases during X-ray irradiation. A shift toward a higher binding energy indicates that the amount of positive charge in the SiO2 film is increasing, and a shift toward a lower binding energy indicates the amount of negative charge in the SiO2 film is increasing. The complex hole/electron trap phenomena in ultrathin SiO2 film can be explained by a trap generation model of the SiO2 film during Xray irradiation. (C) 2002 Elsevier Science B.V. All rights reserved.

We studied the affection of thin (i.e., 0.2-0.8 nm) Ni films on hydrogen-terminated Si(111) substrate surface by using strain-sensitive X-ray diffraction. It was reported that Ni deposition onto hydrogen-terminated Si surface apparently does not cause film growth, but rather diffuses into the Si crystal, creating an "Ni diffusion layer" up to Ni deposition 0.8 nm thick. Measured rocking curves of the Si 113 reflection and integrated intensities of the rocking curves for the substrate provide information about the evolution of the strain field introduced near the substrate surface during Ni diffusion into the substrate. Comparing the measured and calculated rocking curves indicates that compression of the {111} spacing of the Si occurs gradually up to an Ni thickness of 0.6 nm, and that above this thickness, strain relaxation occurs. We found that the slope of the integrated intensity of the rocking curve versus X-ray wavelength correlates to the strain field near the surface, in the same way that the shape of the rocking curves correlate to the strain field near the surface. Dynamical diffraction calculations indicate that measurement of the slope of the integrated intensity of the rocking curve versus X-ray wavelength is useful for strain analysis, because the dependence is not only sensitive to strain fields, but is also insensitive to the effect of absorption by the overlayer, which otherwise would cause deformation of the shape of the rocking curve. (C) 2002 Elsevier Science B.V. All rights reserved.

The depth profiling of O 1s energy loss in silicon oxide near the SiO2/Si interface was performed using extremely small probing depth. As a result, the energy loss of O 1s photoelectrons with threshold energy of 3.5 eV was found. This value of 3.5 eV is much smaller than the SiO2 bandgap of 9.0 eV, but quite close to direct interband transition at Gamma point in energy band structure of silicon. This can be explained by considering the penetration of electronic states from silicon substrate into silicon oxide up to 0.6 nm from the interface. In addition, the penetrating depth is larger than the thickness of the compositional transition layer. (C) 2002 Elsevier Science B.V. All rights reserved.

By decreasing probing depth down to 0.41 nm, the energy loss of O 1s photoelectrons with threshold energy of 3.5 eV. which was equivalent to excitation energy required for direct interband transition at F point in energy band structure of Si. was successfully observed even through 1.12-nm-thick oxide film. It was found, considering penetration of electronic states from Si substrate into SiO2 in the analysis of thickness dependence of energy loss of O 1s photoelectron caused by the direct interband transition, that SiO2/Si interface exists in the oxide located effectively 0.61 nm away from nominal interface defined with atomic structure.

Si(111)面上に形成した極薄シリコン酸化膜からのO ls光電子のエネルギー損失スペクトルを0.41nmの検出深さで測定した。その結果、酸化膜中へ沁み出したシリコン基板の電子状態間のΓ点における直接遷移に起因するO ls光電子のエネルギー損失が、検出深さよりも厚い膜厚1.12nmのシリコン酸化膜から観測された。このO ls光電子のエネルギー損失の酸化膜厚依存性を解析した結果、サブオキサイド層の価電子帯上端はSiの価電子帯上端とほぼ等価であるいう知見を得た。換言すれば、電子状態から決まる界面は化学結合状態から決まる界面より0.61nmだけ酸化膜側に移動する。

To study the interaction between the Martian upper atmosphere and the solar wind, the mars orbiter, NOZOMI was launched in July 1998. In NOZOMI, 15 Ah Ni metal hydride (NiMH) cells were used for the first time in the world to meet the requirement of lightweight of the orbiter. Two onboard batteries, each of which consists of 16 NiMH cells connected in series, have been normally operated in a cruise orbit up to the mars for about 3.3 years by means of a management method like charge or reconditioning operations. For future spacecraft, improvement in an increase in the internal pressure of the NiMH cells has been recently made so that charge operations of their onboard batteries may be made removable. This paper describes a development review of the NiMH cells aboard NOZOMI and their on-orbit operations.

We have investigated the effect of homogenous uniaxial and biaxial strains on the thermal oxidation processes of Si with newly developed techniques. X-ray photoelectron spectroscopy (XPS) and ellipsometry measurements have revealed that the biaxial compressive strain suppresses the oxidation rate at temperatures lower than 800 degreesC, while the uniaxial strain does not affect the oxidation rate. It has been also found that stress during the thermal oxidation does not cause any change in the electronic states of the oxide which are characterized by XPS and x-ray absorption near edge structure. (C) 2001 American Institute of Physics.

X-ray photoelectron spectroscopic evaluation of the SiO2/Si(111) interface reveals that the valence-band offset differs by about 0.19 eV between two types of atomic structures that contain Si atoms in different intermediate-oxidation states. The difference in the valence-band offset is reproduced by a first-principles molecular orbital calculation for model clusters (Si17O4H36 and Si17O3H36) of the two interface structures. It is concluded that the valence-band offset at the interface with more Si atoms in the 3+ oxidation state than in the 1+ oxidation state is larger than the valence-band offset at the interface with more Si atoms in the 1+ oxidation state. This is thought to be a result of the depolarization effect making the interface dipole smaller at the interface with more Si atoms in the 3+ oxidation state.

We measured total-dose and single-event-upset (SEU) resistance in advanced 128-Kbit SRAMs fabricated on SOI using 0.2 mum design rules. Our results indicate that the 128-Kbit SRAMs can be used in specific space technologies.

We have characterized the carrier-trapping phenomena in ultrathin (1.3-3.5 nm) SiO2 films (practical used thermal oxide and oxynitride) by using x-ray photoelectron spectroscopy time-dependent measurements. It was found that the net amount of hole traps in the ultrathin oxynitride is smaller than that in the ultrathin thermal oxide. This result is consistent with the previously reported results for the thick thermal oxide and oxynitride using conventional electrical measurements. We consider what is responsible for the contribution to the formation of hole traps. (C) 2000 American Institute of Physics. [S0003-6951(00)03552-X].

The valence-band and O 2s core-level spectra of ultrathin (about 1 nm) SiO2 layers formed in the initial stages of the oxidation of Si(100) substrates were measured by high-resolution X-ray photoelectron spectroscopy (XPS), and the energy difference between the bonding states in the valence-band and the core-level was found to be larger than the corresponding difference for the bulk SiO2. The energy difference between the top of the valence-band and the core-level was also found to be larger than that for the bulk SiO2. From the first-principle molecular orbital (MO) calculations for SiO2 model clusters, (Si5O16H12), it was concluded that the atomic structure of SiO2 at the SiO2/Si interfaces is characterized by a narrow intertetrahedral bond angle, about 135°.

The effects of oxidation on the photoelectron diffraction pattern originating from a silicon substrate were measured and simulated using the Monte Carlo method for calculating the path of scattered electrons in silicon oxide formed on Si(100). The results of these simulations revealed a total elastic scattering cross-section of 1.5 X 10(-20) m(2) and an inelastic scattering cross-section of 1.6 X 10(-20) m(2). In only particular photoemission directions, the effect of elastic scattering of electrons in silicon oxide is shown to be minimized on the basis of the results of these simulations, and the concept of escape depth can be used to determine the oxide film thickness. (C) 2000 Elsevier Science B.V. All rights reserved.

The formation of the Ni/Si interface was investigated by using molecular beam epitaxy to deposit Ni on hydrogen-terminated Si(lll) surfaces at room temperature. Reflection high-energy electron diffraction observations showed that a sample on which 0.8-nm-thick Ni had been deposited shows the same 1 x 1 streak pattern that the original hydrogen-terminated Si surface did. And X-ray photoelectron spectroscopy measurements of the Si 2 p core-level showed that all the hydrogen atoms terminating the original Si surface are still there after the Ni deposition. This indicated that the Ni atoms diffused beneath the Si surface without breaking the surface Si-Si bonds. It thus seems that hydrogen atoms terminating the Si surface dangling bonds suppress the silicide reaction at room temperature. (C) 2000 Elsevier Science B.V. All rights reserved.

We report a significant improvement of the spectral properties of cadmium telluride (CdTe) detectors, fabricated in the form of a Schottky CdTe diode. With the use of high quality CdTe wafer, we formed a Schottky junction by evaporating indium on the Te-face and operated the detector as a diode. This allows us to apply much higher bias voltage than was possible with the previous CdTe detectors. A 2 mm x 2 mm detector of thickness 0.5 mm, when operated at a temperature of 5 degrees C, shows leakage current of only 0.2 and 0.4 nA for an operating voltage of 400 and 800 V, respectively. We found that, at a high-electric field of several kV cm(-1), the Schottky CdTe diode has very good energy resolution and stability, suitable for astronomical applications. The broad low-energy tail, often observed in CdTe detectors due to the low mobility and short lifetime of holes, was significantly reduced by the application of a higher bias voltage which improves the charge collection efficiency. We achieved very good FWHM energy resolution of 1.1% and 0.8% at energies 122 and 511 keV, respectively, without any rise time discrimination or pulse height correction electronics. For the detection of hard X-rays and gamma-rays above 100 keV, we have improved the detection efficiency by stacking a number of thin CdTe diodes. Using individual readout electronics for each layer, we obtained high detection efficiency without sacrificing the energy resolution. In this paper, we report the performance of the new CdTe diode and discuss its proposed applications in future hard X-ray and gamma-ray astronomy missions. (C) 1999 Elsevier Science B.V. All rights reserved.

Al/Si(111)-H Schottky interfaces are studied by X-ray photoelectron spectroscopy and atomic force microscopy. It is shown that all hydrogen atoms that terminated the original clean Si(111) surface still exist at the buried Al/Si(111)-H interface. It is also shown that the existence of hydrogen at the Al/Si(111)-H Schottky interface suppresses agglomeration of the Al film at 923 K. (C) 1999 Elsevier Science B.V, All rights reserved.

We report a technique to characterize carrier-trapping phenomena in SiO2 by measuring the Si 2p core-level energy of Si substrates covered with thin SiO2 layers as a function of x-ray irradiation time. It is found that the Si 2p peak energy, which corresponds to the band bending at the SiO2/Si interface, changes as the x-ray irradiation time increases. We attribute this to carrier-trapping phenomena in SiO2. By using this technique, it is found that the carrier-trapping phenomena differ remarkably among several chemical oxides. We also discuss the atomic structure of the traps that cause the trapping phenomena. (C) 1999 American Institute of Physics. [S0003-6951(99)04014-0].

The cross-sections for elastic and inelastic scattering of Si 2p photoelectrons in silicon oxide were uniquely determined from the analysis of oxidation-induced changes in Si 2p photoelectron diffraction patterns arising from single crystalline Si substrate under the assumption that the structure of hydrogen-terminated Si substrate does not change by the oxidation. It was found from the Monte Carlo calculation of path of elastically and inelastically scattered Si 2p photoelectrons for the simulation of oxidation-induced changes in photoelectron diffraction patterns that the total elastic scattering cross-section and the inelastic scattering cross-section are 1.54 X 10(-20) and 1.26 X 10(-20) m(2), respectively. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

Metal-semiconductor (MES) field-effect transistors (FETs) and metal oxide-semiconductor (MOS) FETs are fabricated using p-type conductive layers on hydrogen-terminated diamond surfaces. The FETs exhibit complete channel pinch-off and drain-current saturation. Both enhancement-mode and depletion-mode MESFETs are realized, the threshold voltage of which is controlled by changing the electronegativity of the gate metal. The MOSFETs, using evaporated SiOx as gate insulators, operate in depletion mode. The best transconductance of each type of FET exceeds 10 mS mm(-1) with a gate length of 3-7 mu m. The DC performance of the diamond FETs is evaluated by two-dimensional device simulations, varying the distribution depth of the acceptors. In the simulations, a distribution depth of less than 1 nm or the two-dimensional acceptor distribution on the surface reproduces well the actual DC characteristics. In this case, the hole concentration at a depth of 10 nm is decreased by three orders of magnitude as compared to that at the surface. This thin surface channel realizes enhancement-mode operation in MESFETs. Hydrogen-terminated diamond surfaces can already be equipped with FETs with shallow junction depths of less than 10 nm, which is necessary for short gate lengths such as 50 nm. Microfabrication technology on hydrogen-terminated diamond surfaces may give rise to a new field of nanoscale devices. (C) 1999 Elsevier Science S.A. All rights reserved.