Kazuyuki Hirose

Aluminum surface freshly shaven with a steel knife showed significant changes in work functions from 3.25 to 3.60 eV within one day. Successive changes of the work functions were measured in air using the open counter. XPS spectra showed no change fundamentally when the fresh Al surface was exposed to air for 1 min and 24 h. Molecular orbital calculations by use of the DV-X alpha method predicted an appearance of an additional local density of state near the Fermi edge when an Al-Al distance becomes large. Based on the experimental results and calculations, we proposed formation of an interface between an Al matrix and a surface oxide or oxyhydroxide, which has a stress induced density of state. This state appears at the beginning of air exposure and disappears after the lapse of time because such stress is released after rearrangement of surface and interface structures. (C) 1998 Elsevier Science B.V.

We report the significant improvement of the spectral properties of a cadmium telluride (CdTe) detector. With the use of a high quality CdTe crystal, we formed a high Schottky barrier for the holes on a CdTe surface by using a low work-function metal, indium. With a 2x2 mm(2) detector at a thickness of 0.5 mm, the leakage current is measured to be 0.7 nA at room temperature (20 degrees C) and below 1 pA at 70 degrees C for 400 V bias voltage. The low leakage current of the detector allows us to operate the detector at a higher bias voltage than the previous CdTe detector. The energy resolution we achieved at room temperature is 1.1-2.5 keV FWHM from the energy range of 2 keV to 150 keV at 20 degrees C without any charge-loss correction electronics. At -70 degrees C. we obtained an energy resolution of 1.0 keV FWHM at 122 keV and 2.1 keV FWHM at 662 keV.

Detailed changes in valence band spectra of ultra-thin SiO2 at initial stages of oxidation are revealed by high resolution X-ray photoelectron spectroscopy. Comparisons of the experimental data with molecular orbital calculations deduce the transition structure near the SiO2/Si interfaces. (C) 1998 Elsevier Science B.V.

We report the significant improvement of the spectral properties of a cadmium telluride (CdTe) detector. With the use of a high quality CdTe crystal, we formed a high Schottky barrier for the holes on a CdTe surface by using a low work-function metal, indium. With a 2x2 mm(2) detector at a thickness of 0.5 mm, the leakage current is measured to be 0.7 nA at room temperature (20 degrees C) and below 1 pA at 70 degrees C for 400 V bias voltage. The low leakage current of the detector allows us to operate the detector at a higher bias voltage than the previous CdTe detector. The energy resolution we achieved at room temperature is 1.1-2.5 keV FWHM from the energy range of 2 keV to 150 keV at 20 degrees C without any charge-loss correction electronics. At -70 degrees C. we obtained an energy resolution of 1.0 keV FWHM at 122 keV and 2.1 keV FWHM at 662 keV.

Detailed changes in valence band spectra of ultra-thin SiO2 at initial stages of oxidation are revealed by high resolution X-ray photoelectron spectroscopy. Comparisons of the experimental data with molecular orbital calculations deduce the transition structure near the SiO2/Si interfaces. (C) 1998 Elsevier Science B.V.

Thermally induced reactions at SiO2/Al interfaces are investigated by cross-sectional transmission electron microscopy. For the SiO2/Al interfaces fabricated by chemical vapor deposition of SiO2 on polycrystalline Al films, precipitates of reduced Si as the reaction products are observed in the Al films and along the SiO2/Al interfaces, after annealing at 500 degrees C for more than 2 h. On the other hand, the interfaces fabricated on the single-Al(111) films show no precipitation even after 4 h of annealing. The differences in tbe interfacial reactivities are discussed in relation to the existence of grain boundaries and Al film orientations. (C) 1996 American Institute of Physics.

We have measured microscopic distribution of Schottky barrier heights (SBHs) at epitaxial-Al/n-Si(111) interfaces by using scanning internal-photoemission microscopy. The SBH distribution at the interface was homogeneous in the as-deposited state. After annealing, the inhomogeneity of SBH arose and it caused the discrepancy between two SBHs determined by I-V and C-V methods. The thickness distribution of recrystallized p(+)-Si layer, formed during annealing process, was responsible for the distribution of SBH.

Thermally induced reactions at SiO2/Al interfaces are investigated by cross-sectional transmission electron microscopy. For the SiO2/Al interfaces fabricated by chemical vapor deposition of SiO2 on polycrystalline Al films, precipitates of reduced Si as the reaction products are observed in the Al films and along the SiO2/Al interfaces, after annealing at 500 degrees C for more than 2 h. On the other hand, the interfaces fabricated on the single-Al(111) films show no precipitation even after 4 h of annealing. The differences in tbe interfacial reactivities are discussed in relation to the existence of grain boundaries and Al film orientations. (C) 1996 American Institute of Physics.

A schematic reaction mechanism is proposed to explain the Ti/TiN chemical vapor deposition (CVD) process using the TiCl4 molecule as the source species. The scheme can be considered as two successive steps: (I) TiCl4 molecules are adsorbed onto surface reactive sites through concerted electron delocalizations, (II) ambient reducing agents such as H-2 or NH3 reactively eliminate Cl atoms from Ti atoms in the form of HCl. Preliminary molecular orbital (MO) calculations indicate that step (I) is an energetically easy process. Conversely, step (II) is expected to be the rate-determining process with a considerable activation energy. As a whole, the characteristics of resultant CVD films (e.g., the conformality) are probably controlled not by the TiCl4-adsorption step (I) but by the Cl elimination step (II).

A schematic reaction mechanism is proposed to explain the Ti/TiN chemical vapor deposition (CVD) process using the TiCl4 molecule as the source species. The scheme can be considered as two successive steps: (I) TiCl4 molecules are adsorbed onto surface reactive sites through concerted electron delocalizations, (II) ambient reducing agents such as H-2 or NH3 reactively eliminate Cl atoms from Ti atoms in the form of HCl. Preliminary molecular orbital (MO) calculations indicate that step (I) is an energetically easy process. Conversely, step (II) is expected to be the rate-determining process with a considerable activation energy. As a whole, the characteristics of resultant CVD films (e.g., the conformality) are probably controlled not by the TiCl4-adsorption step (I) but by the Cl elimination step (II).

Schottky-barrier height (SBH) for intimate Al/Si contacts is investigated in relation to the interfacial crystallographic alignment, which is observed by transmission-electron microscopy. Epitaxial and rotational Al films are obtained by low-temperature molecular-beam epitaxy on both Si(lll) and (100) surfaces, by controlling the surface structures. The SBH measurements for n- and p-type samples reveal that the absence of epitaxial alignment at the interfaces significantly lowers the Fermi-level pinning position for the contacts on both (111) and (100) surfaces.

The spatial distribution of Schottky barrier height (SBH) at epitaxial Al contacts on Si(111) substrates was investigated by using the scanning internal-photoemission microscopy. We show that low-SBH regions aligning in a certain direction appeared at the annealed Al/Si(111) interface.

Al films were formed by low temperature molecular beam epitaxy on Si(111) surfaces. The substrates were pretreated in a NH4F solution to obtain a nearly atomically flat surface by anisotropic etching. Planview transmission electron microscopy observation demonstrates that single-crystal Al films are successfully grown on the 7 X 7 surface structure. Such single-crystal growth is arrested on a disordered or hydrogen-terminated surface.

Epitaxial Al contacts on Si(111) are fabricated by electron beam evaporation at various substrate temperatures around 250-degrees-C. They are observed by high-resolution transmission electron microscopy. Schottky barrier heights (SBHs) of the contacts are measured using current-voltage and capacitance-voltage methods. In the case of single-crystalline Al film, the SBH does not change and its spatial distribution remains homogeneous up to an annealing temperature of 550-degrees-C. In contrast with this, for an epitaxial Al film containing grain boundaries, the spatial distribution of the SBH becomes inhomogeneous above 400-degrees-C. This is attributed to Si diffusion along the grain boundaries in the Al film.

The energy distributions of the occupied surface states of GaAs(001) surfaces with several kinds of surface superstructure are measured using photoemission yield spectroscopy. The surfaces are prepared by molecular beam epitaxy. The surface states are found to change in both distribution and density depending on the surface superstructure and the surface preparation conditions. It is concluded that not all observed surface states originate from ordered surface atomic structures.

Surface superstructures (reconstructed structures) have been observed by many techniques. However, it has not been easy to confirm that a superstructure does exist at an interface between two solid layers. The article reports superstructures at semiconductor interfaces studied by a grazing incidence X-ray diffraction technique with use of synchrotron radiation. At metal-semiconductor interfaces, different superstructures give different Schottky-barrier-height values. Superstructures are also found to exist at insulator-semiconductor interfaces fabricated by the metalorgamc chemical vapor deposition (MOCVD) method. In addition, a boron-induced superstructure is found to exist even at the interface between an epitaxial Si layer and a Si (111) substrate.