N. Sanada

Charge transfer associated with iodine intercalation in Bi2Sr2Can−1CunOy, n=1−3

Abstract Intercalation of iodine into the Bi 2 Sr 2 Ca n -1 Cu n O y ( n =1,2,3) superconductors has been studied. A variety of measurements indicate charge transfer is associated with the intercalation process, both directly to the BiO layers and also to the superconducting CuO 2 sheets. Changes in T c on intercalation can be understood in terms of hole doping to the CuO 2 planes. Hall coefficient measurements on a single crystal n =2 sample supported an increase in hole concentration, and an XPS study clearly showed the ionic character for the intercalated iodine, accompanied by changes in the Fermi level. Investigation of the c -axis resistivity, also on n =2 single crystals, indicated a change from the semiconductive behaviour of the parent crystal to an apparently metallic character, though the c -axis resistivity remained high.

(NH4)2Sx‐treated InP(001) studied by high‐resolution x‐ray photoelectron spectroscopy

The chemical state of sulfur on (NH4)2Sx‐treated InP(001) surfaces has been studied by high‐resolution x‐ray photoelectron spectroscopy. We find three kinds of chemical states of sulfur (S2p3/2 lines at 161.2, 162.0, and 163.4 eV) on the sample treated at RT. It is suggested that they correspond to sulfur in phosphorus sites (in the second layer of the InP(001) surface), to sulfur bonded to indium on the first layer, and to elemental sulfur on sulfide, respectively. One (S2p3/2=162.0 eV) of them becomes predominant with long‐time exposure to atmosphere at RT. Upon annealing the sample at 400 °C, the 163.4 eV line disappears, while the lines at 162.0 and 161.2 eV remain on the surface. A model of the treated surface is presented.

Surface structures and electronic states of H2S‐treated InP(001)

We find two different surface structures, (1×2) and (1×1), for H2S‐treated InP(001). They depend upon exposure of H2S at about 350 °C. The coverage of sulfur is estimated to be about a half monolayer and one full monolayer for the (1×2) and (1×1) structures, respectively. The (1×1) structure is reconstructed to the (1×2) structure upon annealing at about 550 °C. It is suggested that sulfur is bonded to only In atoms and substitutes some of the phosphorus atoms below the first layer. Inverse photoemission spectra show strong reduction in intensity of 1.2 eV peak above the Fermi level for a clean InP(001)‐(4×2) surface upon adsorption of H2S. This reduction implies a decrease in unoccupied surface states due to dangling bonds of indium dimers on the clean surface. The result of adsorption of oxygen on the (1×2) and (1×1) surfaces indicates significant passivation to oxidation of the surfaces.

Clean GaP(001)‐(4×2) and H2S‐treated (1×2)S surface structures studied by scanning tunneling microscopy

Clean GaP(001)‐(4×2) and H2S‐treated (1×2) surfaces are studied by scanning tunneling microscopy (STM). We have observed a (4×2)/c(8×2) STM image for the cation‐stabilized GaP(001) surface. The result suggests that the unit cell of the (4×2) structure consists of two Ga dimers with two missing Ga dimers. For the (1×2)S surface, the previous model that sulfur atoms are adsorbed on the Ga dimer and that a missing row of sulfur is formed along the [110] direction is supported by the STM result.

Surface reconstruction of InP(001) upon adsorption of H2S studied by low-energy electron diffraction, scanning tunneling microscopy, high-resolution electron energy loss, and x-ray photoelectron spectroscopies

Reconstruction of an InP(001) surface structure upon H2S adsorption has been studied by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), high-resolution electron energy loss (HREELS), and x-ray photoelectron spectroscopies (XPS). The HREELS result indicates that H2S is dissociated on the surface even at RT, leading to evolution of hydrogen from the surface. LEED patterns show (2×4) and (2×1) structures for the surface with sulfur coverages, 0 and 0.5–1 ML, respectively. A complex pattern appears at 0.25 ML. We find in STM images that the (2×1) structure starts to form even at 0.1 ML sulfur coverage at 350 °C. The (2×1) structure is almost established at 0.5 ML although the protrusions of about two atoms in size exist on the surface. The new (2×1) structure begins to grow from protrusions above 0.5 ML on the (2×1) surface found at 0.5 ML and it is established at about 1 ML where the c(2×2) structure with small domain is found. XPS result shows one chemical state of sulfur at 0.5...

Nitridation of an InP(001) surface by nitrogen ion beams

Abstract Nitridation of an InP(001) surface with low-energy nitrogen ion beams has been studied by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The efficiency (N 1s intensity/total currents) of nitridation decreases monotonously as a function of the beam energy (0.1–3 keV). We find three kinds of chemical states of nitrogen on the irradiated surface. The XPS result suggests that they correspond to InN, InNP, and PN bondings. The ratio of the InN bonding for the surface is large at high beam energy. Upon annealing the sample irradiated at 1 keV at RT, the InNP bonding disappears and the ratio of the InN bonding became about 95%. The angle-dependent XPS In 3d5/2 spectra suggest that penetrated nitrogen atoms exhibit the Gaussian distribution in the bulk. The surface irradiated by neutral nitrogen is flatter than that irradiated by the ions.

Surface structures and electronic states of clean and (NH4)2Sx-treated InAs(111)A and (111)B

The surface structures, unoccupied and occupied electronic states, and chemical states of surface atoms for clean and (NH4)2Sx-treated InAs(111)A and (111)B surfaces have been studied using low-energy electron diffraction (LEED), inverse photoemission spectroscopy (IPES), and (x-ray and ultraviolet) photoemission spectroscopy (PES). Thermal stability of the treated surfaces upon annealing in an ultrahigh vacuum is also investigated. A diffuse (1×1) LEED pattern appears for the treated -(111)A and -(111)B surfaces annealed at 230 and 330 °C, respectively, Upon annealing the (111)B sample at 380 °C, the (1×1) structure remains and the LEED spots become clearer. For the (111)A annealed at 380 °C, the pattern changes to a clear (2×2) structure which is found for the first time for sulfurized (111) surfaces of III–V compounds. Sulfur is completely desorbed from both the (111)A and (111)B surfaces at 440 °C, exhibiting the (2×2) and (1×1) structures, respectively. IPES and PES results indicate that unoccupied a...

Spectroscopic evidence for reduction of unoccupied states in the band gap of GaP(001) by H2S passivation

Clean and H2S‐adsorbed GaP(001) surfaces have been studied by inverse and ultraviolet photoemission spectroscopy (IPES) and (UPS) and by high‐resolution electron energy loss spectroscopy. H2S is found to be dissociated on the surface, leaving only sulfur on it, which is consistent with UPS results. IPES spectra show strong reduction in intensity at 1.5 and 4.5 eV above the Fermi level upon sulfur adsorption. The reduction in the former indicates tremendous decrease of unoccupied states, which correspond to dangling bonds of surface gallium atoms, in the band gap by H2S passivation.

A (NH4)2Sx-treated InSb(001) surface studied by using x-ray photoelectron spectroscopy, low-energy electron diffraction, and inverse photoemission spectroscopy

A (NH4)2Sx-treated InSb(001) surface has been studied by using x-ray photoelectron spectroscopy, low-energy electron diffraction, and inverse photoemission spectroscopy (IPES). A thick sulfide layer is formed on the as-treated and annealed surfaces at less than about 400 °C. The thickness of the sulfide layer is estimated to be about 6–7 ML. Sulfur is bonded to both In and Sb in the as-treated surface layer although it is bonded only to indium in the layer annealed at more than 310 °C. A (2×1) structure appears for the treated surface annealed at 310 °C. The binding energy shift (−0.3 eV) of In 3d5/2 and Sb 3d3/2 is found for the (2×1) surface. The IPES spectra show that the density of states of unoccupied dangling bonds for surface indium is reduced by the (NH4)2Sx treatment. The binding energy shift and structure of the sulfide layer are discussed.

The structure of the InP(001)-(4 × 2) surface studied by scanning tunneling microscopy

Abstract The structure of an InP(001)-(4 × 2) surface, prepared by ion bombardment and annealing, has been studied by scanning tunneling microscopy. In the occupied states (sample bias V s = −1.5 V) image three protrusions arranged in a triangular shape are seen in the (4 × 2) surface unit cell. Three kinds of combination of the triangle, α, β and γ, are found in the [110] direction. Conversely, one protrusion in the unoccupied states ( V s = +1.5 V) image is seen in the unit cell. A structure model of the (4 × 2) surface is proposed.