Evidence for an Ultrafast Breakdown of the BeO Band Structure Due to Swift Argon and Xenon Ions

Abstract

Auger-electron spectra associated with Be atoms in the pure metal lattice and in the stoichiometric oxide have been investigated for different incident charged particles. For fast incident electrons, for Ar 7þ and Ar 15þ ions as well as Xe 15þ and Xe 31þ ions at velocities of 6% to 10% the speed of light, there are strong differences in the corresponding spectral distributions of Be-K Auger lines. These differences are related to changes in the local electronic band structure of BeO on a femtosecond time scale after the passage of highly charged heavy ions. DOI: 10.1103/PhysRevLett.105.187603 PACS numbers: 79.20.Rf, 31.70.Hq, 32.80.Hd, 71.20.Ps The energy deposition of slow ions in solids may depend on details of the ion trajectory and threshold conditions are currently being investigated [1]. On the contrary, the local energy deposition of fast ions is relatively well understood and different theoretical concepts yield results that agree roughly with experiment and with each other [2,3]. The electronic energy dissipation and relaxation, however, is subject to intense investigations and especially effects of the high-energy density due to focused short lasers pulses [4] or individual swift heavy ions may give rise to unexpected results [5]. There have been speculations about a collective Coulomb explosion [6], a repulsive interaction among multiply ionized target atoms, and in fact evidence for this effect has been found for laser [4] and ion-beam interactions in some specific insulators [7]. For most materials, however, the corresponding nuclear-track potentials and fields are too low or do not last long enough to initiate materials modifications. Under these conditions, perturbative mechanisms such as the site-specific Auger-induced ionic desorption [8] or collective mechanisms driven by high kinetic electron energies may gain importance. For the latter case, either lattice instabilities [9] or electronic thermal-spike effects [10,11] may play a major role, where hot electrons at electron temperatures of nearly 100 000 K [12,13] transfer their energy to the lattice.Usually, the energy distribution dnðT e ; "Þ=d" of thermally equilibrated hot electrons is given by the product of total (occupied plus unoccupied) electronic density-ofstates (EDOS) Dð"Þ and the Fermi-Dirac distribution. The resulting Auger spectrum is given by the convolution of two of such densities, dn X =d" and dn Y =d" for all combinations of bands X and Y, weighted by the squared Auger matrix elements. It will be shown here, however, that Auger spectra for BeO are inconsistent with this simple factorization of dn=d".

Ultrafast electronic processes in an insulator: The Be and O sites in BeO

“…This Auger decay involves an initial K-shell vacancy that becomes filled by an electron resulting from the interaction of two weakly bound electrons. The measured spectra or the corresponding differentiated yields agree well among each other as well as with our measurements for a fully oxidized surface 29 . In this section, we try to provide more details on the Be-K Auger spectra by further analyzing the previously published data and by providing auxiliary x-ray data for a deeper understanding of electronic structure and dynamics of BeO.…”

“…As shown previously 29,32 , thin BeO films of about 3 nm thickness 33 34 . In the final ion-beam times presented here, we have used either pure oxygen exposure at about 10 -6 mbar or alternatively continuous 500-eV O-ion implantation inside the main chamber, in order to prevent any net erosion effect.…”

“…29 The electron-induced Be-K Auger-electron spectrum of bulk BeO samples and BeO surface films has been investigated previously also by several other groups 36,37,38,39,40,41 . This Auger decay involves an initial K-shell vacancy that becomes filled by an electron resulting from the interaction of two weakly bound electrons.…”

“…29,30 Hence, only a short description will be given here. Electron spectra have been taken with a modified electrostatic parallel-plate electron spectrometer involving optimized transmission and energy resolution.…”

“…Recently, Be Augerelectrons emitted from an ionic BeO surface film due to excitation by fast heavy ions have been investigated for the first time 29 . Here, the Be K-Auger spectra are analyzed in more detail using auxiliary x-ray data for quantitative comparison and K-Auger spectra related to the oxygen site of BeO are presented and analyzed for the first time.…”