Journal of Electron Spectroscopy and Related Phenomena 104 (1999) 99–107 XPS and EELS study of the bismuth selenide * V.B. Nascimento,V.E. de Carvalho , R. Paniago, E.A. Soares, L.O. Ladeira, H.D. Pfannes ´ ˆ Departamento de Fısica, Instituto de Ciencias Exatas, Universidade Federal de Minas Gerais, C.P . 702, 30123-970, Belo Horizonte, MG Brazil Received 16 July 1998; received in revised form 11 February 1999; accepted 16 February 1999 Abstract A Bi Se crystal was studied by XPS (X-Ray Photoelectron Spectroscopy) and EELS (Electron Energy Loss 2 3 Spectroscopy). Al Ka radiation and an electron beam energy in the range of 0.1 to 2 keVwere used, respectively, to probe a Se-terminated (0001) surface. Samples of the constituent elements (Bi and Se) have also been measured with the same setup. The core level chemical shifts obtained show that a charge transfer occurs in Bi Se . The spectra in the valence band region 2 3 suggest that the density of states in the compound may be obtained by combining the spectra of the constituent elements, that the electronic states in the vicinity of the gap region consist of a mixture of the metal and calcogen p-orbitals and that the band arising from the valence s-orbitals occurs about 12 eV below the valence band maximum. The EELS spectra allow to identify the bulk plasmons for the three materials and the Bi5d3 and Bi5d5 interband transitions. Considerations of the energies of the Bi5d transitions as measured by XPS and EELS indicate that the bottom of the conduction band of the compound is 1.2 eVabove the Fermi level. The EELS results also shows evidence that the losses occurring at 6.4 eV in the compound and at 5.4(5.5) eV in Bi(Se) have their origins in some surface process. We suggest that they may be associated to a surface plasmon.  1999 Elsevier Science B.V. All rights reserved. Keywords: V VI compounds; Bi Se ; X-ray photoelectron spectroscopy (XPS); Electron energy loss spectroscopy (EELS); Plasmons; 2 3 2 3 Electronic transitions 1. Introduction ture. It crystallizes in a rhombohedral unit cell, where equivalent atoms are arranged in two dimen- The semiconducting V VI compounds have been sional, noncompact hexagonal layers perpendicular 2 3 studied for more than two decades, mainly because to the c-axis. Five such layers – in the sequence of 1 2 1 some of these compounds present properties that Se –Bi–Se –Bi–Se , where the Se atoms occupy make them important for technological applications two different sites – form a tight bound unit whereas in thermoelectric power conversion and in the fabri- these ‘sandwiches’ are kept together by a much 1 1 cation of Hall effect devices. Bismuth selenide, an weaker Se –Se interaction. The bonds inside these important member of these V VI compounds, is a ‘sandwiches’ are generally considered to be pre- 2 3 narrow gap semiconductor ( E 50.25 eV) [1] and dominantly covalent, and those between them, are gap exhibits an unusual and very anisotropic layer struc- due to van der Waals forces [2,3]. This structural arrangement leads to an easy cleavage perpendicular 1 to the c-axis showing a Se -terminated surface. *Corresponding author. Fax.: 155-31-499-5600. E-mail address: vagner@fisica.ufmg.br (V.E. de Carvalho) Although the Bi Se has been the subject of a large 2 3 0368-2048 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048(99)00012-2