SURFACE AND INTERFACE ANALYSIS, zyxwvutsrqpo VOL. 12, 293-296 (1988) The Interaction of Group IB Metals with van der Waals Faces of Semiconducting Metal Dichalcogenides W. Jaegermann,* F. S. Ohuchi and B. A. Parkinson E. I. du Pont de Nemours zyxwvutsrqp & Co, CR&D, Experimental Station, Wilmington, DE 19898, USA The interaction of UHV cleaved van der Waals surfaces of the layered semiconductors zyxw SnS, , zy SnSe, , ZrS, , MoS, and WS, with deposited Cu, Ag and Au has heen studied with XPS and LEED. It was observed for Cu on SnS, and for Cu and Ag on SnSe, that a bulk reaction occurs leading to the formation of a new phase containing reduced Sn and oxidized Cu or Ag. Even at high coverages a metallic overlayer was not formed. An interfacial reaction at low coverages and subsequently a metallic layer was obtained for Cu on ZrS, , Ag on SnS, and Au on SnSe, . Cu, Ag and Au on MoS, or WS, form a metallic overlayer with an atomically abrupt semiconductor/metal interface. Epitaxial growth of a Au(ll1) layer was observed on WS,(OOOl). The differences in reactivity are related to differences in electrochemical standard potentials of the involved redox processes, structural features and to kinetic factors, e.g. ion transport. INTRODUCTION Owing to its importance in the electronics industry, the reactivity of semiconductor surfaces with metals has been the subject of intense study.lS2 UHV surface inves- tigations contribute to a microscopic understanding of the processes which are the basis for the control of Schottky barrier and ohmic contact formation. Metal atoms adsorbed on surfaces may form either metal layers, clusters or react with the substrate. Factors determining the occurrence of a specific process are of growing interest especially for the more complex com- pound semiconductors. Deposition of Cu, Ag and Au on van der Waals (OOO1) faces of a number of semiconducting layered dichalcogenides (MoS, , WS, , ZrS, , SnS, , SnSe,) has been investigated. Potential applications of these materials include intercalation battery systems3 and photoelectrochemica14 and solid state (Schottky barrier)5 solar cells. Their special feature is pronounced structural anisotropy resulting from strong chemical bonds in two dimensions. In the crystals sheets of X-M-X layers are bound to each other via weak van der Waals interactions. The van der Waals surfaces consist of close packed chalcogenide atoms with satu- rated chemical bonding below the surface plane which is considered to be the reason for the chemical inertness of the surfaces. This inertness and the lack of dangling bonds, which may act as surface recombination centers for charge carriers, may result in special advantages for use in semiconductor devices. The two-dimensionality of the substrate and the systematically variable chemical properties by substitution of metal or chalcogenide provide nearly ideal isostructural model systems for the study of basic metal/substrate interactions. The results * Permanent address: Hahn-Meitner-Institut, Glienicker Str. 100, loo0 Berlin 39. FRG. obtained for Cu, Ag and Au deposition on SnS, and WS2 are published in detail The data of the Cu, Ag, Au/SnSe, and Cu/ZrS, , MoS, systems will be presented in this paper for the first time and possible reasons for the differences observed in metal semicon- ductor reactivity will be discussed. EXPERIMENTAL The x-ray photoelectron spectroscopy measurements described here were performed in an UHV multi- technique chamber with monochromatic x-rays (A1 zy Ka). The XPS system was constructed by Surface Science Lab Inc. and was equipped with a computer system (HP9836) for data acquisition and manipulation. Further details of the system configuration are given el~ewhere.~ The spectrometer was calibrated with clean evaporated Au and Cu samples. The resolution achieved was about zyxw 0.6 eV (FWHM of W 4f7,,). LEED experiments were performed with a Varian 3 grid LEED system with electrons of about 100 eV energy. Single crystals of the layered chalcogenides, grown in quartz ampoules via chemical vapor transport, were attached with one van der Waals face to an A1 sample holder via conductive Ag epoxy. Clean (001) faces were produced by cleaving the crystals in UHV. The opti- cally smooth surfaces obtained were free from contami- nations as judged from XPS and LEED. Metal atoms were deposited onto the cleaved surfaces from high temperature metal sublimation sources. All metal coverages herein are expressed in monolayers (ML) estimated from XPS peak intensities for different exposure times. The core level intensities used were cor- rected by photoionisation cross-sections given for the spectrometer ~ y s t e m . ~ The spectra were evaluated by a peak fitting program’ assuming a mixed Lorentzian- Gaussian peak form. 0142-242 1/88/24029344 S05.M) zyxwvutsrqponm 0 1988 by John Wiley & Sons Ltd