Daisei Shoji

Kinetics of oxidation on hydrogen‐terminated Si(100) and (111) surfaces stored in air

We have investigated the oxidation of hydrogen‐terminated Si(111) and (100) surfaces stored in air, using synchrotron radiation photoemission spectroscopy and infrared absorption spectroscopy in the multiple internal reflection geometry. We demonstrate that water present in air is predominantly involved in the oxidation of surface Si–H bonds, and that native oxide starts to grow when the surface hydrogen coverage is decreased. In order to explain the latter phenomenon, we propose a kinetic model of oxidation which considers the manner in which native oxide formation preferentially occurs on those portions of the surface where the Si–H bonds are oxidized. We suggest that the oxidation of surface Si–H bonds, the rate of which is strongly dependent on the humidity of air, is a rate‐limiting step in the native oxide formation on hydrogen‐terminated Si surfaces.

Oxidation processes on the H2O-chemisorbed Si(100) surface studied by in-situ infrared spectroscopy

Abstract The oxidation processes on the Si(100)-(2×1) surface during exposure to H 2 O and subsequent thermal annealing were investigated using in-situ infrared absorption spectroscopy in the multiple internal reflection geometry. Exposure to H 2 O results in the oxidation of Si–Si bonds (dimer bond and backbonds) even at room temperature. Upon annealing up to 500°C the dimer bond is cleaved to produce dihydride Si (Si–H 2 ), and the backbonds are attacked by atomic oxygen released from the surface Si–OH species to produce intermediate oxidation species such as SiH 2 (SiO). We demonstrate that most of the hydride species are driven out from the surface by annealing up to 500°C, but an intermediate oxidation species (SiH(O 3 )) persists up to 600°C. A model of wet oxidation is presented in which dissociation of water molecules, attack of the Si–Si bonds by atomic oxygen and hydrogen and hydrogen desorption are involved in the oxidation.

Hydrogen adsorption and desorption processes on Si(100)

The hydrogen adsorption and desorption processes on the Si(100)(2×1) surface were investigated using in-situ infrared absorption spectroscopy in the multiple internal reflection geometry. It is demonstrated that the distribution of hydride species (SiH, SiH2, and SiH3) significantly changes during adsorption of atomic hydrogen and desorption of molecular hydrogen. At the initial stages of hydrogen adsorption, the monohydride Si (Si–H) and dihydride Si (Si–H2) are populated, with Si–H being dominant. For higher hydrogen exposures the dihydride and trihydride Si are formed. Thermal annealing causes hydrogen to desorb from the hydride species. For annealing temperature up to approximately 400°C, the trihydride Si is etched away, producing a H-terminated surface which consists of monohydride (SiH) and dihydride (SiH2) species. We demonstrate that the conversion from the monohydride to the dihydride phase occurs during thermal annealing.

Initial stages of oxidation of hydrogen-terminated Si surface stored in air

We have investigated the oxidation of hydrogen-terminated Si(111) and (100) surfaces stored in air, using synchrotron radiation photoemission spectroscopy and infrared absorption spectroscopy in the multiple internal reflection geometry. We demonstrate that water present in air is predominantly involved in the oxidation of the topmost layer of the hydrogen-terminated surface. We find that native oxide starts to grow when the surface hydrogen coverage diminishes. This trend is interpreted in terms of a kinetic model of oxidation in which it is assumed that native oxide formation preferentially takes place on the portion of the surface where surface Si atoms having Si-H bonds are oxidized.

Single-stripe tunable laser with Chirped Sampled Gratings fabricated by nanoimprint lithography

Fabrication of diffraction gratings of Chirped Sampled Grating Distributed Reflector (CSG-DR) laser by nanoimprint lithography (NIL) has been demonstrated. The diffraction gratings with highly uniform linewidth and period have been successfully fabricated by the combination of the reverse-tone NIL and precise etching techniques. The fabricated CSG-DR laser using NIL shows sufficiently wide tuning range of 40 nm as we designed.

Effects of surface chemical treatment on the formation of metal GaAs interfaces

We have used synchrotron radiation photoemission spectroscopy to investigate the chemical interactions at metal/GaAs interfaces during deposition of Au and In onto GaAs(100) surfaces that are chemically treated in etching solutions. We determine that there exists a thin native oxide layer on the surface that is treated in H2SO4 solution. Au reacts with the oxide overlayer to generate AuGa alloy, but In does not interact appreciably with the oxide overlayer leading to the island growth of the In overlayer at high In coverages. We confirmed (NH4)2Sx treatment leads to a GaAs surface that is terminated with sulfur. For the Au deposition onto this surface, alloy formation and segregates both at interfacial regions and on metallic overlayers are significantly suppressed, leading to the island growth of the Au overlayer. On the other hand, at initial stages of In deposition, In strongly interacted with the (NH4)2Sx-treated surface to generate a thin layer of InxGa1−xAs alloy on which the In overlayer formed in ...

Si 2p Spectra of Initial Thermal Oxides on Si(100) Oxidized by H2O

Initial stage of wet thermal oxidation of Si(100)2×1 surfaces using water has been investigated by high-resolution Si 2p core-level measurements with synchrotron radiation at substrate temperatures of room temperature (RT) - 800 °C. Water was found to dissociatively adsorb at RT, forming Si-H and Si-OH bonds. At elevated temperatures, the four oxide components that are identical with those in dry oxidation also appeared in the spectrum in addition to the Si-H component. The Si-H component showed a peak shift toward higher binding energies with temperature, which suggests transformation from the monohydride to the dihydride at higher temperatures.

Band-dispersion-originated photoelectron intensity oscillations during Si epitaxial growth on Si(100)

Abstract The photoelectron intensities from the surface states on Si(100) periodically oscillate during Si growth and that oscillation is associated with the alternation between the 2x1 and the 1x2 surface reconstructions [Y.Enta et al., Surf. Sci. 313(1994)L797]. For clarifying the origin of the oscillation in more detail, angle-resolved-ultraviolet-photoelectron-spectroscopy measurements for both Si(100)2x1 and 1x2 clean surfaces hav been performed. As a result, it was found that the photoelectron intensity oscillations on Si(100) arise from the difference in the surface band dispersions between the 2x1 and the 1x2 clean surfaces.

Photoemission study of metal deposition on sulfur-treated GaAs(100)

Abstract The chemical properties of metal/GaAs interfaces formed by the deposition of metals, Au and Ag, onto the GaAs(100) surfaces treated with H2SO4 and (NH4)2Sx solutions, have been investigated using synchrotron-radiation photoemission spectroscopy. Photoemission data demonstrate that As atoms are segregated on the metal overlayer during Au and Ag deposition on GaAs surfaces with native oxide, while the As segregation is suppressed on the (NH4)2Sx-treated GaAs surface which has no native oxide. In the case of Au deposition, we demonstrate that for low coverages of Au the formation of AuGa alloy is suppressed on the (NH4)2Sx-treated surface. The island growth of Ag film and the absence of AgGa alloy during Ag deposition are observed, an indication that no reaction occurs between the Ag and the (NH4)2Sx-treated GaAs substrate. We suggest that the removal of native oxide and sulfur-termination are crucial to the passivation of metal/GaAs interfaces.

Low-temperature epitaxial growth of CaF2 on (NH4)2Sx-treated GaAs(100) surface

Abstract We have investigated the method of forming a high-quality CaF2 film onto a GaAs(100) surface. We used X-ray diffractometry and Rutherford backscattering spectrometry to characterize the film quality. We demonstrate that pretreatment of GaAs wafer surfaces by (NH4)2Sx solution leads to the epitaxial growth of CaF2 on the GaAs(100) surface at a substrate temperature of around 200°C. We suggest that both oxide removal by (NH4)2Sx etching and sulfur passivation of the surface are crucial to the low-temperature epitaxial growth of CaF2. At a substrate temperature of 300°C, the epitaxy quality of CaF2 is deteriorated.