Suboxide interface in disproportionating a-SiO studied by x-ray Raman scattering A. Sakko, 1, * C. Sternemann, 2 Ch. J. Sahle, 2 H. Sternemann, 2 O. M. Feroughi, 2 H. Conrad, 2,3 F. Djurabekova, 1 A. Hohl, 4 G. T. Seidler, 5 M. Tolan, 2 and K. Hämäläinen 1 1 Department of Physics, P. O. Box 64, FI-00014, University of Helsinki, Finland 2 Fakultät Physik/DELTA, TU Dortmund, D-44221 Dortmund, Germany 3 Deutsches Elektronen-Synchrotron (DESY), D-22603 Hamburg, Germany 4 Institute for Materials Science, Darmstadt University of Technology, 64287 Darmstadt, Germany 5 Department of Physics, University of Washington, Seattle, Washington 98195, USA Received 16 February 2010; revised manuscript received 30 April 2010; published 21 May 2010 The microscopic structure of disproportionating amorphous silicon monoxide is studied by inelastic x-ray scattering at the silicon L II,III edge. This material arranges into nanocrystalline regions of Si embedded in amorphous SiO 2 at proper annealing temperatures and in this work we demonstrate how the contribution of the suboxide interfaces between these regions can be extracted from the experimental data. The resulting near-edge spectra are analyzed in detail using a computational framework that combines molecular-dynamics simulations and density-functional theory calculations. The results indicate that the amount of silicon atoms with oxidation states between +1 and +3 is significant and depends strongly on the annealing temperature. Furthermore, the presented s, p, and d-type local densities of states DOS demonstrate that the most significant differences are found in the p-type DOS. DOI: 10.1103/PhysRevB.81.205317 PACS numbers: 68.35.Ct, 78.67.Bf, 78.70.Ck I. INTRODUCTION Silicon oxides are important materials for present day semiconductor technology and for various potential micro- electronic and optoelectronic applications. 1–3 Amorphous silicon monoxide a-SiO is of particular interest because its black coal-like modification shows phase separation, i.e., dis- proportionation to nanocrystalline regions of Si embedded in a-SiO 2 under annealing. 4 It is well acknowledged that em- bedding of crystalline silicon nanoclusters into silicon diox- ide can significantly improve its luminescence properties. 5,6 Unlike bulk silicon with an indirect band gap, Si nanoclus- ters embedded in SiO 2 show intense visible photolumines- cence with wavelengths depending on the size of the nanoclusters. 7 The origin of the phenomenon is intensively debated and various explanations have been proposed. 8–10 In any case the important role of the Si / SiO 2 interface is ac- knowledged and its structural characterization is of particular importance. According to the interface clusters mixture ICM model, the native i.e., not annealed a-SiO consists of amorphous regions of Si and SiO 2 that have diameters of less than 2 nm and where a significant proportion of atoms are located in the interfaces of these regions. 4 The silicon atoms within these interfaces can have a variable number of oxidation states and they are thus called suboxides. Under annealing at elevated temperatures the Si and SiO 2 clusters grow along with inter- face obliteration and eventually Si nanocrystals form. This structural model is supported by several experimental observations 4,11,12,23 but the knowledge of the detailed struc- ture especially at the interfaces remains ambiguous. In this work we apply near-edge spectroscopy to study the atomic structure of the internal suboxide interfaces in dispro- portionating a-SiO. Near-edge spectroscopies, such as x-ray absorption spectroscopy XAS and electron energy-loss spectroscopy EELS as well as x-ray Raman scattering XRS, are important probes of the local electronic structure of a wide range of materials. 13 XRS, i.e., inelastic x-ray scat- tering from inner-shell electronic excitations, in particular, is well suited for studying the electronic excitations of disor- dered materials due to its bulk sensitivity, applicability under extreme experimental conditions, and the unique information provided by the momentum-transfer dependency. 14 Our ap- proach is based on separating the XRS spectrum into contri- butions from excitations at nanocrystalline Si regions, at amorphous SiO 2 regions, and at their interfaces. We employ molecular-dynamics MD simulations and density- functional theory DFT calculations to analyze the interface contribution in detail. The annealing temperature depen- dency of the disproportionation shows significant structural changes at around 900 ° C and a high amount of suboxides in the samples is found. Furthermore, the presented framework provides a method to calculate XRS spectra from MD simu- lations that can be directly compared with experiments and therefore it can be used to verify the theory by experiment. II. X-RAY RAMAN SCATTERING In an inelastic x-ray scattering process a photon scatters from the sample causing an elementary excitation in the ma- terial. Within the first-order Born approximation the experi- mental spectrum is proportional to the dynamic structure fac- tor Sq , , where q and  are the momentum and the energy transferred to the sample, respectively we use atomic units in this work, i.e., m e =  = e = 1, except for the numerical val- ues. In an XRS process an inner-shell electron is excited from the core state to one of the unoccupied electronic states. The cross section and therefore the dynamic structure fac- tor carries information on the transition rates between the core and unoccupied states. The highly symmetric and local- ized nature of the inner-shell state enables the interpretation of the spectrum in terms of the symmetry properties of the PHYSICAL REVIEW B 81, 205317 2010 1098-0121/2010/8120/2053177 ©2010 The American Physical Society 205317-1