Resonant-photoemission study ofSnO2: Cationic origin of the defect band-gap states

Abstract

The electronic properties of tin dioxide single-crystalline (110) surfaces have been studied in correlation with their structure by low-energy electron diffraction, angle-integrated and resonant photoemission using synchrotron radiation [ultraviolet photoemission spectroscopy (UPS)]. Energy distribution curves were measured from the Sn 4d core levels and from the valence band. The experimental valence band is compared with the theoretical density of states (DOS) from perfect and defective surfaces. UPS difference curves, normalized to the Sn 4d intensity, reflect mainly the increase in the oxygen partial DOS when the sample is annealed at increasing temperatures up to 1000 K after sputtering. Their comparison with simulated theoretical difference curves favors a bridging oxygen termination for annealing temperatures above 900 K. After argon-ion bombardment, bandgap defect states that are not predicted by the calculations are found at a maximum density 1.4 eV above the valence-band maximum (VBM). Various degrees of resonant enhancement occur throughout the valence band when the photon energy crosses the Sn 4d~5p photoabsorption threshold, and these are used to establish the tin-derived character of the gap states, for which a tin Ss origin is proposed. Partial-yield spectra allow the localization of unoccupied Sn 5p states in the conduction band starting from 8 eV above the VBM with a maximum at 10 eV. The Sn 4d~5p absorption threshold also shows possible core exciton formation for sputtered surfaces only.

The structure of the stoichiometric and reduced SnO2(110) surface

“…The structure we are assuming here is the one deduced from a wide range of measurements (see e.g. [9,12,14]). The terminology we shall use when discussing the surface structure is as follows.…”

“…These applications have stimulated intensive study of its surface properties [4,5,6,7,8,9,10,11,12,13,14,15,16,17], concentrating mainly on the (110) surface (see fig. 1), which is the most stable.…”

“…Because of the variable valence of Sn, the material readily loses surface oxygen, and there have been a number of studies of the atomic structure, electronic structure and electrical properties of the (110) surface as a function of temperature and oxygen partial pressure. The existence of a rather complicated series of surface reconstructions [4,10,13], marked changes of electronic structure in the band gap [5,9,12], and strong variation of the surface electrical conductivity with deviation from stoichiometry [9,13] are all well documented. In spite of this work, many questions remain unresolved.…”

Energetics and approximate quasiparticle electronic structure of low-index surfaces of SnO2

“…The possibility of Sn being either Sn 4+ or Sn 2+ may cause a variety of surface structures 18 and homologous compounds such as Sn 2 O 3 or Sn 3 O 4 . 19 Even though numerous experimental [20][21][22][23][24] as well as firstprinciples studies 18,21,[25][26][27][28][29] have been carried out for surfaces of SnO 2 , several open questions remain. While the oxygen terminated (110) surface is generally considered to be the most stable one, the energetic order of surfaces with different orientation is still being debated.…”

“…In general, little is known about the electronic structures, especially of the non-stoichiometric surfaces: While photoemission spectroscopy (PES) studies 6,20,23 focus on the (110) surface of SnO 2 , previous theoretical works 18,[26][27][28] suffer from the significant underestimation of the fundamental gaps (by more than 2 eV) that can be attributed to the use of density functional theory (DFT). In addition, also the surface energy barriers for the emission (ionization energy) or the escape (electron affinity) of electrons are not well understood (see Ref.…”