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In temperature dependent studies, In his crystal ball, Patel sees great diffraction (PED) holography, and it was possible to determine conditions advantages for higher intensity x-ray photoelectron microscopy in a threeunder which nucleation sites are cor- sources of the future. Experiments on part presentation. related so that high-quality, single-crystal not-so-perfect crystals, experiments In ultra-ESCA, Tonner pointed out epitaxial layers will result. GaAs, InP, nearer 90° Bragg angles, use of higher that intrinsic line widths for low-Z and the As/InP growth surface have monochromator resolution, and studies elements are as low as 10 meV, which been studied by Fuoss and collabo- of low-Z adsorbates will all become can be observed using very high resolurators.
possible and offer exciting scientific tion X-ray excitation instead of the Toshio Takahashi of the Institute opportunities.
customary electron beam excitation. for Solid State Physics, University of An excellent presentation of x-ray PED holography, usable on metal, Tokyo, reported studies of the growth absorption fine-structure (XAFS) insulator, or semiconductor surfaces, of several metallic systems on Si(111), studies of organic adsorbents on Ni(100) has the capability of imaging the surusing similar techniques. Detailed struc- was presented by Toshiaki Ohta, cur- face with a resolution of 0.05 to 0.5 Å, tures were reported for partial mono- rently of Hiroshima University but soon which is competitive with scanning layers, full monolayers, and multiple to take the professorship of the retiring tunneling microscopy. The technique monolayers of B, Al, In, Sn, Ag, Sb, and Haruo Kuroda at Tokyo University. detects the angular dependence of the Bi. By studying the angular symmetry Ohta used both the x-ray absorption photoelectrons emitted from the surof the x-ray scattering about the Bragg near-edge spectra (often referred to as face as a hologram of the surface. Holoangle, the vertical distance of the XANES or NEXAFS) and the extended graphic reconstruction of the surface overlayer above the substrate is obtained, XAFS (EXAFS), along with tempera- produces a high-resolution, threeand the phase change of the x ray upon ture variation from cryogenic to 1,000 K, dimensional image. This image is Bragg reflection is determined. A highly in his studies of thiophenol, thiophene, compared with computed holographic detailed structure of the metal over- and CS, adsorbates on Ni(100). images to obtain quantitative atomic layer is obtained, but the experiment These experiments were carried out positions and to eliminate artifacts. requires 12 days of data taking! The near the K absorption edge of sulphur As examples of PED holography, large increase in intensity which will near 25 keV, a soft x-ray spectral region Tonner showed results of studies of the be available at SPring-8 will be welcome, that has been difficult to access in the Cu(111), Ir(111), and Cu/Ir(111) surindeed, if many such studies are to be past, due to the need for large d-spacing faces. These results indicate the prommade.
monochromator crystals and a vacuum ise of x-ray-generated holographic techAnother method for studying atomic environment. Ohta's results illustrate niques and illustrated the importance location, and also obtaining the thermal- the danger in extrapolation of behav- of increased x-ray flux if these techvibration amplitudes of surface atoms, ior of one molecular adsorbate to niques are to be used extensively in was presented by Jamshed Patel of another, even similar adsorbate. The surface science. AT&T Bell Labs. Patel uses the X-ray surface structure, effective coordina- Both photoemission microscopy and standing wave, as obtained from the x- tion, dissociation, and desorption of x-ray absorption microscopy were disray fluorescence that occurs concur- thiophene and thiophenol on the Ni cussed by Tonner. In the former case, rently with XRD. As the sample is rocked surface behave quite differently as a Tonner showed studies of a patterned slightly through the Bragg condition or function of temperature.
layer of AI, 50 nm thick, on GaAs. As the x-ray wavelength is scanned through Science is far from an adequate an example ofx-ray absorption microsthe Bragg condition, the XRD and the experimental or theoretical understand- copy, Tonner showed results of studies x-ray fluorescence are monitored. The ing of molecule-surface interactions of yttrium-barium-copper oxide using X-ray standing wave within the crystal and experiments such as Ohta's need to the Ba 4d x-ray absorption edge. The surface layers moves in phase, and the be conducted on many molecule-surface ability to select x-ray wavelengths above fluorescence yield varies according to systems of importance in technology. or below core-absorption edges, or at which layer of atoms lies at the peak of Brian Tonner, University peaks of near-edge x-ray absorption the exciting standing wave. Thermal of Wisconsin-Milwaukee and spectra, and to microscopically image vibration of the atoms determines the Synchrotron-Radiation Center, that data will help answer many detailed width of the angular or wavelength range Stoughton, followed the theme of chemical and physical questions about of fluorescence excitation. The results X-ray based analysis of surfaces and surfaces of importance technologically. are analyzed using the Debye model, interfaces. He presented results of Masaharu Oshima, self-styled which shows quite low Debye temper- “Ultra” ESCA (electron spectroscopy “Samurai Spectroscopist” of the NTT atures (<100 K) for surface atoms. for chemical analysis), photoelectron Interdisciplinary Research Laboratory, Tokyo, showed his application of photo- at the low temperature of 200 °C, fol- The final paper of the conference emission spectroscopy to three inter- lowed by the In layer before heating to was presented by Yoshitada Murata, facial systems of technological impor- the deposition temperature of InAs. Institute of Solid State Physics, Unitance: CaF/GaAs, metals on sulphur- The SrF, interlayer prevented the for- versity of Tokyo. He summarized the passivated GaAs, and InAs/SF_/ mation of InO prior to the higher study of surface and interface science. EuBa Cu, O, Using photoemission temperature InAs deposition. Up to the present, problems undernear the Ca(3p) absorption edge at In the future, a higher photon flux taken have been oriented toward the 114 eV, Oshima studied the surface from SPring-8 will allow combination study of interface physical structure, construction versus growth tempera- measurements, real-time analysis of electronic structure, and interface ture during epitaxial growth of CaF on growth situations, imaging of growing absorption spectra. A prime example is GaAs(111)A & B, and GaAs(160).
structures, and spatial resolution of the Si(111) 7x7 surface, including the Ultimately, Oshima obtained an MS characterization measurements. second layer studies by STM. (metal-semiconductor) structure that Studies of surface and thin-film Murata projects that in the future was free of pinning on the GaAs(111)B magnetism were discussed by Jürgen problems undertaken will utilize anglesurface.
Kirschner of the Freie Universität Berlin. resolved and time-resolved synchrotronAdditional studies of the noise Kirschner used polarized and unpolar- radiation techniques such as ARUPS performance of this MS field effect ized VUV synchrotron radiation to (angle-resolved ultraviolet photoemistransistor (FET) were made using X-ray excite spin-polarized photoelectrons sion spectroscopy), SXAFS (surface standing wave excited fluorescence from thin films of Fe(100), Cu(100), X-ray absorption spectroscopy), spinspectra of the S(K) absorption edge. Co/Cu(100), and multiple layers of Cu polarized (SP) UPS, or SPARUPS for The locations of the S atoms at the and Co. These studies show that a few the study of complex structures such as interface were determined for monolayers of Co on Cu induce polar- O/Ni(100) or for the production of new GaAs(100), GaAs(111)A, and ization of the Cu(3d) electrons and the materials utilizing surface properties. GaAs(111)B surfaces. Binding-energy Cu s-p bands. Co films nine mono- Dynamical processes, phase transitions, shifts of the S-Ga bonds were obtained layers thick show square domains polar- surface reconstruction dynamics, and and correlated with 1/f noise associated ized along the <110> directions. absorption-induced restructuring will with the interfacial region.
In the Fe films, the Fe(2p) levels at be important areas for research effort. The effects of sulphur passivation 680 and 695 eV are split by spin-orbit Absorption, diffusion, desorption, on aluminum layers on GaAs were interaction into spin-up and spin-down and reaction will be studied from the studied using similar techniques. It was components. However, spin splitting quantum-mechanical viewpoint. Nearly found that metallic Ga is formed, along of S-core states is very small. Kirschner elementary processes, thermal reactions, with Al-S bonds. These surfaces appear reported that the spin polarization of and nonthermal processes will be importo have considerable promise in GaAs s- and p-levels is not well understood tant to study. electronic devices. theoretically
Photodesorption and photonFinally, Oshima discussed his excit- The question of a possible spin- stimulated desorption will be pursued ing studies of superconducting transis- polarized scanning tunneling micro- in order to understand high-reactiontor structures: GaAs/EBCO/InAs, where scope (STM) was brought up by a rate processes with high selectivity. the GaAs will serve as the emitter, questioner. Kirschner indicated that, Murata suggested a strong need for EBCO (EuBa Cų, 0,) as the base, and although he has thought about it, no exclusive SR beamline facilities or for InAs as the collector. First, GaAs was ideas have occurred yet. He reported a compact, individual-laboratory SR oxidized and EBCO deposited. Then rumor from Basel, Switzerland, that sources for VUV experiments, espeSIF, was deposited, and finally InAs someone is working on such a device, cially for complex systems involving was deposited. In depositing the InAs, but emphasized that the rumor has not laser and SR beams and the like. only three monolayers were deposited been verified.
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