PHYSICAL REVIEW B 81, 205317 ͑2010͒ 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. Hämäläinen1 1Department of Physics, P. O. Box 64, FI-00014, University of Helsinki, Finland 2Fakultät Physik/DELTA, TU Dortmund, D-44221 Dortmund, Germany 3Deutsches Elektronen-Synchrotron (DESY), D-22603 Hamburg, Germany 4Institute for Materials Science, Darmstadt University of Technology, 64287 Darmstadt, Germany 5Department of Physics, University of Washington, Seattle, Washington 98195, USA ͑Received 16 February 2010; revised manuscript received 30 April 2010; published 21 May 2010͒ The microscopic structure of disproportionating amorphous silicon monoxide is studied by inelastic x-ray scattering at the silicon LII,III edge. This material arranges into nanocrystalline regions of Si embedded in amorphous SiO2 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 ͑ᐉDOS͒ demonstrate that the most significant differences are found in the p-type ᐉDOS. DOI: 10.1103/PhysRevB.81.205317 PACS number͑s͒: 68.35.Ct, 78.67.Bf, 78.70.Ck I. INTRODUCTION ͑XRS͒, are important probes of the local electronic structure of a wide range of materials.13 XRS, i.e., inelastic x-ray scat- Silicon oxides are important materials for present day tering from inner-shell electronic excitations, in particular, is semiconductor technology and for various potential micro- well suited for studying the electronic excitations of disor- electronic and optoelectronic applications.1–3 Amorphous dered materials due to its bulk sensitivity, applicability under silicon monoxide ͑a-SiO͒ is of particular interest because its extreme experimental conditions, and the unique information black coal-like modification shows phase separation, i.e., dis- provided by the momentum-transfer dependency.14 Our ap- proportionation to nanocrystalline regions of Si embedded in proach is based on separating the XRS spectrum into contri- 4 a-SiO2 under annealing. It is well acknowledged that em- butions from excitations at nanocrystalline Si regions, at bedding of crystalline silicon nanoclusters into silicon diox- amorphous SiO2 regions, and at their interfaces. We employ ide can significantly improve its luminescence properties.5,6 molecular-dynamics ͑MD͒ simulations and density- Unlike bulk silicon with an indirect band gap, Si nanoclus- functional theory ͑DFT͒ calculations to analyze the interface ters embedded in SiO2 show intense visible photolumines- contribution in detail. The annealing temperature depen- cence with wavelengths depending on the size of the dency of the disproportionation shows significant structural nanoclusters.7 The origin of the phenomenon is intensively changes at around 900 °C and a high amount of suboxides in debated and various explanations have been proposed.8–10 In the samples is found. Furthermore, the presented framework / any case the important role of the Si SiO2 interface is ac- provides a method to calculate XRS spectra from MD simu- knowledged and its structural characterization is of particular lations that can be directly compared with experiments and importance. therefore it can be used to verify the theory by experiment. According to the interface clusters mixture ͑ICM͒ model, ͑ ͒ the native i.e., not annealed a-SiO consists of amorphous II. X-RAY RAMAN SCATTERING regions of Si and SiO2 that have diameters of less than 2 nm and where a significant proportion of atoms are located in the In an inelastic x-ray scattering process a photon scatters interfaces of these regions.4 The silicon atoms within these from the sample causing an elementary excitation in the ma- interfaces can have a variable number of oxidation states and terial. Within the first-order Born approximation the experi- they are thus called suboxides. Under annealing at elevated mental spectrum is proportional to the dynamic structure fac- ͑ ␻͒ ␻ temperatures the Si and SiO2 clusters grow along with inter- tor S q, , where q and are the momentum and the energy face obliteration and eventually Si nanocrystals form. This transferred to the sample, respectively ͑we use atomic units ប structural model is supported by several experimental in this work, i.e., me = =e=1, except for the numerical val- observations4,11,12,23 but the knowledge of the detailed struc- ues͒. In an XRS process an inner-shell electron is excited ture especially at the interfaces remains ambiguous. from the core state to one of the unoccupied electronic states. In this work we apply near-edge spectroscopy to study the The cross section ͑and therefore the dynamic structure fac- atomic structure of the internal suboxide interfaces in dispro- tor͒ carries information on the transition rates between the portionating a-SiO. Near-edge spectroscopies, such as x-ray core and unoccupied states. The highly symmetric and local- absorption spectroscopy ͑XAS͒ and electron energy-loss ized nature of the inner-shell state enables the interpretation spectroscopy ͑EELS͒ as well as x-ray Raman scattering of the spectrum in terms of the symmetry properties of the 1098-0121/2010/81͑20͒/205317͑7͒ 205317-1 ©2010 The American Physical Society SAKKO et al. PHYSICAL REVIEW B 81, 205317 ͑2010͒ unoccupied states in the immediate vicinity of the excited (a) (b) 15 atomic site. One can also obtain detailed structural infor- 1200 °C mation on the sample using computational methods that al- 1200 °C low an accurate interpretation of the near-edge spectrum. native These properties have made XRS, and near-edge spec- native troscopies in general, very powerful tools for the study of [arb. units] ) ) [arb. units] ω ω numerous problems in materials science. , The similarity of XRS and XAS is often acknowledged: at (q, (q S S low scattering angles both XRS and XAS probe dipole tran- sitions and provide similar information on the sample. At high scattering angles, that is, high-momentum transfers, XRS probes also nondipole transitions and a more complete 100 105 110 115 100 105 110 115 picture of the electronic structure can be obtained. Because energy transfer [eV] energy transfer [eV] the XRS experiments are carried out with hard x-rays ͑the FIG. 1. ͑Color online͒ The experimentally measured dynamic incoming energy is typically around 10 keV͒, the method is structure factors for the different samples at ͑a͒ q=2.4 Å−1 and ͑b͒ insensitive to surface effects and no vacuum is required in q=9.85 Å−1. The changes between 101 and 110 eV are due to the the experiments which also makes the measurements at ex- decreasing suboxide content with the annealing temperature. The 16 treme conditions feasible. Even though the low scattering high-momentum-transfer spectra were published originally in Ref. cross section means that the experimental work has to be 23. carried out at synchrotron-radiation facilities, XRS has be- come a powerful tool for numerous studies where experi- atoms are in a suboxide state. This is in line with the ICM ments using other techniques would be problematic.17–22 model, as was pointed out in Ref. 11. The suboxidic silicon Concerning this work, the relatively low energy atoms lie at the interfaces and have oxidation states between ͑ϳ100 eV͒ of the Si L edge would make its measure- II,III +1 and +3. Since XRS is a local probe, the admixture of ment using soft x-ray absorption surface sensitive which can different phases results in proportional admixture of spectra obscure the information on bulk properties of the system. so that the experimentally measured dynamic structure factor can then be written as A. Experiments S ͑q,␻͒ = ͑1−a͒ ϫ S ͑q,␻͒ + a ϫ S ͑q,␻͒, exp Si+SiO2 SiOx The details of the sample preparation and the experimen- ͑1͒ tal work are given in another publication23 and therefore only a short review is presented here. The samples were prepared where a is the proportion of the suboxidic Si atoms in the by evaporation of a mixture of Si and SiO at 1400 °C and sample, S ͑q,␻͒ is the spectrum of the suboxides, and 2 SiOx subsequent condensation at about 600 °C. The resulting na- S ͑q,␻͒ is the sum of the spectra of bulk Si and bulk Si+SiO2 tive a-SiO samples were then annealed at temperatures T 26 a SiO2, normalized to the same value. Our approach bases on between 725 and 1200 °C, grounded to powder and finally the assumption that the amount of silicon atoms at Si and pressed into pellets. The XRS experiments were carried out SiO2 regions is equal, which implies that the average oxida- at beamline XOR/PNC 20-ID of the Advanced Photon tion state in the suboxide regions is +2. If this would not Source using the LERIX spectrometer24 with an energy res- hold the Eq. ͑1͒ would need to be generalized. We discuss olution of 0.6 eV. The incident-beam energy was 9.89 keV this case at Sec. III. Because the interface contribution be- and a set of spectra were measured with momentum transfers comes negligible at high annealing temperatures when the between 2.4 and 10.1 Å−1 ͑series A͒. In an earlier ͑ ␻͒ nanocrystalline regions grow larger, the SSi+SiO q, can be 25 2 experiment we measured spectra for momentum transfer approximated by the spectrum of the sample annealed at T −1 a q=10.1 Å for native and annealed SiO with an energy =1200 °C.23 resolution of 1.4 eV ͑series B͒.