AVS1997 Session SS-WeA: Electronic Structure: Modified Surfaces and Interfaces

Wednesday, October 22, 1997 2:00 PM in Room A7/8

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		SS-WeA-1 Resonant Photoelectron Spectroscopy of K-intercalated MoS₂ J.R. Lince, S.V. Didziulis (The Aerospace Corporation); J.A. Yarmoff (University of California, Riverside); L.J. Terminello (Lawrence Livermore National Laboratory); D.K. Shuh (Lawrence Berkeley Laboratory) The deposition of small amounts of the alkali metal K onto the basal plane (0001) surface of MoS₂ results in intercalation of the K between the basal planes.¹ In similar studies of alkali metals adsorbed on the surfaces of other transition metal dichalcogenides (TMDs), the alkali metal is shown to donate electron density to the conduction band of the TMD, with little changes to the band structure of the TMD (i.e., rigid band model).². We have conducted a detailed soft X-ray photoelectron spectroscopy (SXPS) study of the deposition of K onto the MoS₂ (0001) surface. SXPS of the valence band, and of the Mo 3d, S 2p, and K 3p core levels, confirmed that low K dosages resulted in intercalation in the MoS₂ (0001) surface region, with the resultant donation of electrons to the MoS₂ conduction band. We obtained resonant photoelectron spectroscopy (RPES) data of K/MoS₂ (0001) by varying the photon energy from 30 to 70 eV, spanning the Mo 4p-4d transition region. Previous RPES studies have shown its usefulness in elucidating the electronic structure of the MoS₂ (0001) surface, especially the predominantly Mo 4d regions in the valence band.³ We concentrated on the lowest unoccupied state in the conduction band in pure MoS₂, which becomes occupied after K-intercalation. A constant initial state intensity plot of this conduction band region exhibits a strong Fano-like resonance at hν= 42 eV. This strong resonance indicates that the bottom of the MoS₂ conduction band contains considerable Mo 4d character, in agreement with inverse photoelectron spectroscopy of pure MoS₂.⁴ Larger K dosages resulted in decomposition of the MoS₂ (0001) surface. Various theories explaining this decomposition will be discussed, including reaction of K with MoS₂ to produce metallic Mo, and the intercalation-induced phase transition of MoS₂ from the 2H to 1T structure. ¹M. Kamaratos, D. Vlachos, and C.A. Papgeorgopoulos, J. Phys.: Condens Matter 5 (1993) 535. ²C.A. Papgeorgopoulos and W. Jaegermann, Surf. Sci. 338(1-3) (1995) 83. ³J. R. Lince, S. V. Didziulis, and J. A. Yarmoff, Physical Review B 43 (1991) 4641. ⁴V. Langlais, H. Belkhir, J.-M. Themlin, J.-M Debever, et. al., Phys. Rev. B 52(16) (1995) 12095.
2:20 PM		SS-WeA-2 The Cs/MoS₂(0002) Electron Donor-Acceptor Complex: Element-, Orbital- and Site Specific Information Obtained by High Resolution X-ray Photoemission Spectroscopy K. Park, J. Hess, K. Klier (Lehigh University) The nature of surface bonding and electron transfer is probed by angle resolved valence band X-ray photoemission spectroscopy (AR-VBXPS) and photoelectron diffraction (PD), combined with the periodic FLAPW-DFT calculations of the band structure. Complementary to ARUPS, the AR-VBXPS provides the Brillioun zone-averaged photoemission data at a given energy and emphasizes thus diffraction phenomena. In the case of partially occupied crystal orbitals, it is shown that the initial state matrix element contribution to the photoelectron intensity strongly influences the shape of the angular background. These features are exemplified in the Cs/MoS₂(0002) system where the angle dependence of VB emissions have been referenced to that of selected core-level emissions to assess the atomic orbital contribution at specific binding energies within the VB. Further comparison with theory shows that the background intensity anisotropy near the top of the VB is influenced by the partially occupied, nonbonding Mo^IV(4d_z² + 4d_x²-y² + 4d_xy) initial atomic orbital characters more than other contributions. Deposition of sub-monolayer equivalent Cs onto the basal plane of MoS₂ introduces a density of states 1.25 eV above the top of the VB maximum. Theoretical and experimental intensity anisotropy of this Cs-induced VB peak indicates the Cs6s electron transfer into the bottom of the conduction band of the Cs/MoS₂ heterostructure to give Cs^delta+, forming an electron donor-acceptor complex. Finally, capture of the valence electrons through subsequent reaction with 20L of Cl₂, 40L of O₂, and 2x10⁷L of H₂O is also examined by high resolution VB XPS.
2:40 PM	Invited	SS-WeA-3 Theory of Silica Surfaces and Processes D.R. Hamann (Bell Laboratories, Lucent Technologies) SiO₂ is one of the most important materials in modern electronics and communications technology. Standard surface electronic structure techniques are challenged by its highly localized orbitals, structural complexity, and open structure. A new adaptive coordinate formulation, combined with ab-initio molecular dynamics and generalized-gradient exchange and correlation functionals, permits efficient and accurate calculations of its properties. This method will be described, along with tests of its efficacy, and its accuracy in the difficult case of reactions which change the coordination of Si. OH groups play an important role in SiO₂ surface chemistry, and further tests establish our method's ability to accurately describe hydrogen bonding. Calculations have been carried out to examine a number of issues. Strained ring structures form highly reactive centers on SiO₂ surfaces, and new estimates of the structure and strain energies of such "defects" will be discussed. A surface structure which can terminate SiO₂ with no broken bonds has been studied, and found to have a relatively low surface energy dominated by 3-member-ring strain. A simulated AFM image shows features by which to identify such a structure. The reaction of α-quartz and H₂O to form a "hydroxylated" surface is calculated to be slightly exothermic, with the reaction energy dominated by hydrogen bonding. Vibrational frequencies of a split pair of OH stretching modes have been calculated to provide a signature for this surface. Finally, O and O₂ diffusion barriers in SiO₂ have been calculated, as a step in understanding the process of SiO₂ growth on Si.
3:20 PM		SS-WeA-5 New Method for the Observation of Interface States: XPS Measurements under Biases H. Kobayashi (PRESTO, Japan Science and Technology Corporation and Osaka University, Japan); A. Asano, Y. Yamashita, Y. Nakato (Osaka University, Japan); Y. Nishioka (Texas Instruments Tsukuba Research & Development Center, Japan) The energy distribution of interface states in the Si band-gap for MOS devices with an ultrathin oxide layer of 2~3.5 nm thickness has been obtained using a new method developed by us, i.e., measurements of XPS spectra under biases between the metal overlayer and the Si substrate.^1,2 The Si 2p peak shifted toward the higher (or lower) binding energy upon applying a positive bias to the Si with respect to the metal layer. By analyzing the energy shifts measured as a function of the bias voltage, the energy distribution of interface states is obtained. The interface states have discrete energy levels, indicating that they are due to defects such as Si dangling bonds. The interface state spectra have one peak near the midgap, two peaks, one above and the other below the midgap, or three peaks near, below, and above the midgap, depending on the oxide formation methods. On the basis of calculations using a density function theory method, the interface state peak near the midgap is attributed to isolated Si dangling bonds, while that below (or above) the midgap is attributed to Si dangling bonds with which an oxygen or Si atom with an unpaired electron (or lone electron-pair) in the oxide layer interacts weakly. The variation of the interface state spectra is attributed to the different atomic density of the oxide layers. A new method to decrease the interface state density is developed, i.e., cyanide treatment in which Si is immersed in a KCN aqueous solution and rinsed in boiling water. ¹H. Kobayashi et al. Phys. Rev. B. 52, 5781 (1995). ²H. Kobayashi et al. Appl. Phys. Lett. 69, 2276 (1996).
3:40 PM		SS-WeA-6 Surface Metallic Behaviour in Icosahedral Quasicrystals G. Neuhold, S.R. Barman, K. Horn (Fritz-Haber-Institut der MPG, Germany); Ph. Ebert, K. Urban (Forschungszentrum Jülich, Germany) The electronic structure of icosahedral quasicrystals is of great current interest since in view of the unusual structure of these materials, conventional concepts in solid state physics have to be adapted in order to provide a meaningful description of their properties. Among the striking features of quasicrystals are their high electrical resistivity and the low electronic contribution to the specific heat compared to that of their metallic constituents, observations which suggest a low density of states (DOS) at E_F. Here we report on core and valence level photoemission experiments, using synchrotron radiation, from icosahedral AlPdMn quasicrystals. The surfaces were prepared by cleaving under ultra-high vacuum conditions, thus avoiding all complications due to surface preparation procedures. In order to characterize the electronic states into the region beyond the Fermi level E _F, we have recorded valence level spectra at elevated temperatures, thus populating the electronic states above E _F. These spectra demonstrate that a shallow pseudogap exists at about 90 meV above E _F, but a clear metallic Fermi level is also observed, in agreement with earlier reports. Al 2p core level photoemission data show that the core level line shape asymmetry, which is indicative of metallic behaviour, decreases with increasing photoelectron escape depth. This implies a decreasing metallicity with increasing distance from the surface, resolving the apparent contradiction between the valence level photoemission data and the low density of states at E _F suggested from measurements of the bulk properties.
4:00 PM		SS-WeA-7 Thickness Dependence of Work Function for Pd/Cu(111) and Au/Cu(111) Studied by STM Y. Hasegawa, J.-F. Jia, K. Inoue, Z.-Q. Li, Y. Kawazoe (Tohoku University, Japan); A. Sakai (Kyoto University, Japan); T. Sakurai (Tohoku University, Japan) Using STM, we measured local work function on Pd and Au overlayers deposited on Cu(111) substrate and studied its thickness dependence. Work function (or apparent barrier height) can be measured locally with a nano-meter scale resolution by applying modulation to the gap distance and detecting its response in tunneling current. We took a work function image simultaneously with a topographic STM image, and compare them to derive information on how work function varies spatially and how it is correlated with a thickness of the overlayers. It was found that thickness dependence of work function for Au and Pd layers is significantly different. In the case of Au/Cu(111), 1st Au layer shows an intermediate value between those of Au(111) and Cu(111) planes. This is consistent with the work done by Fauster et al. by two-photon photoemission spectroscopy. On the other hand, work function measured on 1st Pd layer reaches already the value equal to that of Pd(111) surface and it overshoots beyond the bulk value on 2nd and 3rd Pd layers, while the 4th layer shows some decrease. Theoretical study using a first-principle LDA method also supports the experimental results. We temporarily attribute the overshooting behavior observed on Pd/Cu(111) system to quantum size effect, which was originally proposed by Shulte. We also observed troughs of lower work function along the step edges in the images. Since distribution of electron density is smeared around the step edges (Smoluchowski effect), dipole moment perpendicular to the surface is formed along the steps. We found that simulated variation of local work function due to the dipole moment fits well with the experimental observation.
4:20 PM		SS-WeA-8 Fermi Surface and Electronic Structure of a Surface Charge-Density-Wave: Pb/Ge(111) A. Mascaraque (Universidad Autonoma de Madrid, Spain); J. Avila, M.C. Asensio (CSIC and LURE, Spain); E.G. Michel (Universidad Autonoma de Madrid, Spain) The Fermi surface and the electronic band structure of the room temperature phase α-√3x√3R30-Pb/Ge(111) and of the low temperature phase 3x3-Pb/Ge(111) have been probed using angle-resolved photoemission. The α-√3 phase is metallic. It presents a dispersing surface state which crosses the Fermi energy at 1/2ΓK. This value of the Fermi momentum fits with a 3x3 nesting vector, and supports the stabilization of a surface charge density wave at low temperature. The Fermi surface of this phase exhibits a fluted shape in agreement with theoretical calculations for this system ¹. The 3x3 phase is semiconducting. Concomitantly with the phase transition, we detect the opening of a band gap in the valence band. In addition to this, the low temperature phase exhibits striking changes in the band structure. These include band folding effects, and the appearance of a prominent, non-dispersing peak in the vicinity of the Fermi level, supporting that electron localization in the 3x3 is the reson for its semiconducting character. These results also support the formation of a surface charge-density-wave at low temperature, explain the electronic nature of the 3x3 phase, and evidence the importance of electron correlation effects in its stabilization. ¹J. Carpinelli et al. Nature 381, 398 (1996).
4:40 PM		SS-WeA-9 Novel Electronic and Magnetic Properties of Ultra-thin Chromium Oxide Films Grown on Pt(111) P.S. Robbert, H. Geisler, C.A. Ventrice, Jr. (University of New Orleans); J. van Ek, M. Kuhn, U. Diebold (Tulane University); S. Chaturvedi, J.A. Rodriguez (Brookhaven National Laboratory) The growth of epitaxial metal-oxide films on lattice-mismatched metal substrates often results in the formation of unique overlayer structures which do not exist in the bulk phase of the metal-oxide. In particular, epitaxial chromium oxide films grown on Pt(111) exhibit a p(2x2) symmetry through the first few monolayers of growth which has been attributed to the formation of a Cr₃O₄ spinel phase, which is not a stable bulk phase of chromium oxide ¹. Growth beyond a few monolayers results in the formation of a √3x√3 overlayer structure which corresponds to the formation of Cr₂O₃ (a stable bulk phase of chromium oxide). In order to determine the electronic and magnetic properties of Cr₃O₄, both electronic structure calculations and synchrotron-based ultraviolet photoelectron spectroscopy (UPS) measurements of these ultra-thin films have been performed. The band structure calculations predict ferrimagnetic ordering for Cr₃O₄ (similar to its iron analog Fe₃O₄) instead of the insulating, antiferromagnetic ordering of Cr₂O₃. UPS measurements show a shifting of the Cr valence emission from the Fermi level to ~2 eV below the Fermi level as the chromium oxide phase changes from Cr₃O₄ to Cr₂O₃. This behavior is supported by the theoretical results that predict Cr₃O₄ to be metallic. ¹L. Zhang, M. Kuhn, and U. Diebold, Surf. Sci. 375, 1 (1997).
5:00 PM		SS-WeA-10 Molecular Adsorption and Defect Creation on Thin Metal Oxide Films Studied with MIES and UPS(HeI) D. Ochs, P. Stracke, V. Kempter (Technische Universität Clausthal, Germany); S.C. Street, J. Günster, D.W. Goodman (Texas A&M University) Metastable Impact Electron Spectroscopy (MIES) in combination with UPS(HeI) has been used to study the interaction of various adsorbates (CO, CO₂, N₂O) with MgO and CaO surfaces. In MIES the kinetic energies of the electrons ejected during the interaction of slow metastable He^* atoms with surfaces are analyzed. This spectroscopy is only sensitive to the electronic structure of the outermost surface layer including adsorbates. MIES of differently prepared surfaces will be discussed and interpreted using ab initio theoretical calculations (CRYSTAL-code). MIES from MgO exposed to CO₂ show only weak CO₃^2- features which can be explained as CO₃^2- formation at defect sites only. The more intense features observed for CaO suggest that chemisorption in the form of CO₃^2-, occurs on majority sites. Results regarding temperature and coverage dependent defect formation on thin metal oxide films, obtained using MIES, will also be discussed.