Surface electronic structure of metastable FeSi„CsCl…„111… epitaxially grown on Si„111… J. J. Hinarejos, G. R. Castro, P. Segovia, J. Alvarez, E. G. Michel, R. Miranda, A. Rodrı ´ guez-Marco, D. Sa ´ nchez-Portal, E. Artacho, and F. Yndura ´ in Dto. de Fı ´sica de la Materia Condensada and Instituto Nicola ´s Cabrera, Universidad Auto ´noma, 28049 Madrid, Spain S. H. Yang Naval Research Laboratory, Washington, D.C. 20375-5345 P. Ordejo ´ n Departamento de Fı ´sica, Universidad de Oviedo, Calvo Sotelo s/n, 33007 Oviedo, Spain J. B. Adams Chemical, Bio, and Materials Department, Arizona State University, P.O. Box 876006, Tempe, Arizona 85287-6006 Received 3 February 1997 We report an investigation on the electronic structure of metastable, epitaxial FeSi films grown on Si111. The electronic structure of the metastable silicides was probed with angle-resolved photoemission, and com- pared with theoretical calculations. We identify the silicide as FeSi crystallizing in the CsCl structure. Its surface is Fe terminated, and presents a prominent, strongly dispersing surface state at a binding energy of -3.5 eV in  ¯ . Its origin lies in the truncation of Fe bonds at the surface, and thus it has a major Fe d z 2 content. S0163-18299750424-3 Iron silicides have been investigated during the last years due to their interesting properties, both from fundamental and applied points of view. 1–9 One of the most relevant fea- tures of these materials is that, in addition to bulk phases, 10 metastable phases can be stabilized through epitaxial growth on a Si substrate. 4 The stabilization of metastable phases is due to their better lattice matching with the substrate. This reduces the interface energy, and favors an otherwise un- stable crystalline structure. Obviously, these phases may ex- hibit interesting properties not found in the bulk compounds. Growth of FeSi crystallizing in a CsCl lattice, 4 and of FeSi 2 in a fluorite structure (  -FeSi 2 ), 5,6 have been re- ported. The lattice mismatch of FeSi CsCl with Si111 is 2%, 7 and this structure appears in a wide range of Fe con- centrations from FeSi to FeSi 2 ). The crystalline structure of FeSi CsCl is very close to the DO 3 structure of Fe 3 Si, which is ferromagnetic. This feature, and the use of iron silicides as spacers in Fe multilayers exhibiting magnetic ordering, 8 have prompted a great interest on the electronic and/or magnetic properties of this and other metastable phases. 9 In addition to interesting properties, metastable ep- itaxial phases have the basic interest of being crystallo- graphically simpler than the bulk phases. They often repre- sent ideal phases prior to instability driven deformations. In this paper we study the electronic structure of meta- stable iron silicides grown epitaxially on Si111. We iden- tify a 1 1 phase as quasi-stoichiometric FeSi CsCl. 1 1-FeSi CsCl films are Fe terminated, and present elec- tronic states which agree well with theoretical calculations for this compound. A band gap of the projected bulk band structure is predicted between 2.5- and 4.0-eV binding en- ergy BE, in agreement with the experiment. In this area we observe a strongly dispersing surface state, which can also be well reproduced in the theoretical calculation. The surface state has a major Fe-d z 2 content, in agreement with the Fe surface termination found. The experiments have been carried out in an ultrahigh vacuum UHV chamber permanently located at the exit of a Seya-1m monochromator receiving synchrotron light from the DORIS III storage ring of the Hamburger Synchrotron- strahlungslabor HASYLAB at Deutsches Elektronensyn- chrotron. It was equipped with facilities for low-energy elec- tron diffraction LEED, Auger electron spectroscopy AES, and angle-resolved ultraviolet-photoemission spectroscopy ARUPS. The ARUPS spectra were recorded with a hemi- spherical analyzer mounted on an UHV goniometer.The base pressure of the system was 2 10 -10 Torr. The Si111 n -doped, mirror-polished single-crystalline samples resistiv- ity 0.2  cm were cleaned by annealing at 1150 °C after long-time degassing at lower temperatures. All surfaces pre- pared as described above consistently displayed intense sur- face states 11 and a sharp 7 7 LEED pattern. Iron silicide films were prepared by solid-phase epitaxy. Fe coverages were measured with a quartz balance, and cross checked by AES. 12 All cited coverages are subject to an estimated error of 20%. Ion scattering spectroscopy ISS experiments were performed using an angle-integrating hemispherical analyzer for a He + energy of 500 eV. We estimate a 90% sensitivity to the topmost surface layer under our ex- perimental conditions. The electronic structures of FeSi CsCl and  -FeSi 2 were calculated using a first-principles method based on the local-density approximation LDA to density-functional theory. 13 The core electrons are replaced by norm-conserving pseudopotentials. 14 One-electron wave-functions are ex- panded in the nonorthogonal basis given by finite-range pseudoatomic orbitals, 15 including s , p , and d orbitals, both RAPID COMMUNICATIONS PHYSICAL REVIEW B 15 JUNE 1997-II VOLUME 55, NUMBER 24 55 0163-1829/97/5524/160654/$10.00 R16 065 © 1997 The American Physical Society