PHYSICAL REVIE% 8 VOLUME 29, NUMBER 10 Samarium chemisorption on group-IV semiconductors A. Franclosl Department of Chemical Engineering and Material Science, Uniuersity of Minnesota, Minneapolis, Minnesota 55455 P. Perfetti* and A. D. Katnani Department of Physics, Uniuersity of Wisconsin, Madison, Wisconsin 53706 J. H. %eaver Department of Chemical Engineering and Material Science, Uniuersity of Minnesota, Minneapolis, Minnesota 55455 G. Margaritondo Department of Physics, Uniuersity of Wisconsin, Madison, Wisconsin 53706 (Received 25 July 1983; revised manuscript received 21 December 1983) Samarium is an excellent test system with which to study thc interaction between cheIIlisorbcd metallic overlaycrs Rnd different kinds of substrates because the 4f configuration is sensitive to the SIYl valcncc state. %C Used this Unique fcRtUI'c to stUdy thc chemisorption of Sm oIl cleaved Gc(111) and Si(111) surfaces. In both cases, synchrotron radiation photoemission spectra exhibited only di- valent Sm features at covcrages below 2 — 3 A and a mixture of trivalent and divalent Sm features at higher coverages. This transition was accompanied by strong adatorn-substrate cheImcal interac- tions as revealed by large coI'c-lcvcl shIfts. INTRODUCTION The chemical and physical phenomena underlying the formation of metal overlayers on semiconducting sub- strates are of great fundamental and technological in- terest. ' For example, the chemisorption of simple metals oil Si surfaces provided fhc first diicct test of tllc BRldccI1 model for Schottky barriers. The many experiments per- formed siIicc thcii liavc given illcrcasiIlg cvidcilcc tliRt tlm adatom-substrate interaction is generally rather complicated — and at the same time is of crucial impor- tance in determining metal-semiconductor — interface properties. ' The onset of a chemical reaction at an interface is diffi- cult to observe in most cases because the changes of the chemical state of the interface components are small. However» rare-earth atoms» 1n gcncral» and Sm atoms» ln particular, exhibit 4f spectral "fingerprints" sensitive to the atomic valence configuration. These features are well known and for Sm have been used to identify the valence state in bulk compounds, surfaces, and clusters. In this paper me use a rare-earth — atom probe to study the rnul- tistep formation process of metal-semiconductor inter- faces. ' Surface-sensitive synchrotron-radiation photoemission studies have enabled us to identify the coverage-dependent valence state of Sm atoms chemisorbed on cleaved co- valent semiconductor surfaces. Results for the Si(111)-Sm system have been pf escnted elsewhere. This paper focuses on the Ge(111)-Sm junction, while drawing from Ref. 6 for discussion of differences and similarities of the Ge-Sm and Si-Sm results. We have observed several distinct chemisorption stages for Sm both on Ge and Si surfaces. For coverages below 2 — 3 A, Sm appears in a purely divalent form, but, at the onset of chemical reaction, trivalent features appear and they ultimately become dominant for coverages greater than 10 — 15 A. Studies of the semiconductor cores have revealed %'cak substrate"Rdatolll chemical 1ntcractIons Rt coverages for which only divalent Sm was observed. However, the appearance of the spectral features of trivalent Sm corresponded to strong chemical interactions, as revealed by large energy shifts of the metal and semi- conductor core levels. The relative concentration of the trivalent to divalent species increases for coverage up to 8 — 12 A on Ge (10 — 15 A for Si). At higher coverages the ratio decreases toward the samarium-metal value, when unreacted Sm forms on top of the fully reacted interface I cglon. We conclude that R qualitatively MImlar Inultistcp interface-formation process takes place both for Si and Ge, although quantitative differences exist in the charac- tcrlstlc coverage I'angcs. EXPERIMENTAL The experiments consisted of taking photoemission spectra on cleaved (111) semiconductor surfaces before and after deposition of Sm overlayers of increasing thick- ness. The samples were n-type single crystals of Ge (n =2. 3X10'7 cm '), which were cleaved in situ at working pressures of 5)&10 " Torr. Sm overlayers were deposited from a tungsten crucible, and their thickness was measured by a quartz microbalance (pressure & 6 && 10 Io Torr during evaporation). Synchrotron-radiation photoemission experiments were performed at the VAsconsIn Storage RIng» Tantalus» usIng a grazing-incidence "grasshopper" monochromator and a 3-m toroidal-grating monochromator (10&hv& 140 CV). 1984 Thc American Physical Society