32 <strong>E21</strong> <strong>Annual</strong> <strong>Report</strong> <strong>2011</strong>/<strong>2012</strong> Thin Film Alloying Studied by CDBS with the NEPOMUC Positron Beam Markus Reiner 1, 2 , Philip Pikart 1, 2 , Christoph Hugenschmidt 1, 2 1 Physik-Department <strong>E21</strong>, <strong>Technische</strong> Universität München, D-85748 Garching, Germany. 2 Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), <strong>Technische</strong> Universität München, D-85748 Garching, Germany. Coincident Doppler Broadening Spectroscopy (CDBS) enables the detection of high momenta of strongly bound core electrons and hence, reveals the chemical environment of the positron annihilation site on an atomic scale. Using the highintensity NEPOMUC positron beam at the FRM II enables CDBS as function of both implantation depth and temperature. This unique experimental technique offers a great potential for the investigation of the structure and kinematics in multilayer systems such as annealing, interdiffusion and thin film alloying. The element selectivity of these studies can be further increased by the ab-initio calculation of CDB spectra. Ab-initio calculation of CDB spectra For the calculation of CDB spectra, the momentum distribution ρ(p) of the annihilating electron-positron-pair ρ(p) = πr0c ∑ ∫ 2 2 u 2 j(0) ∣ dr e −ip·r ψ + (r)ψ j (r) ∣ (1) j is determined within a two-component density functional theory [1]. In the limit of a vanishing positron density, ψ + is obtained by solving the positron´s Schrödinger equation in the bulk. Its charge density is described by an atomic superposition method of electronic wavefunctions ψ j . Electronpositron correlations are described by the state-dependent enhancement factoru j (0) and modeled by a generalized gradient approximation. The measured momentum distribution ρ(p) is given as sum of all orbital momentum distributions; r 0 denotes the classical electron radius. The presented calculational method accounts for the annihilation with (semi-)core electrons. Hence, the calculated well describe the element-specific signature in the High Momentum Area (HMA) of CDB spectra. Au/Cu interdiffusion The vast range of applications of thin film systems and their continuous downscaling demand a detailed understanding of microscopic processes. For this, positrons with their high defect sensitivity reveal not only unique information about defect-related processes like annealing [2], but also about structural changes by the use of depth-dependent and insitu CDBS at elevated temperature. Hence, this experimental technique, which requires a high-intensity positron beam, is an outstanding tool for the investigation of thin film annealing and alloying at the interface on the same time. In a comprehensive study, the tempering of a vapor deposited bilayer Au (180 nm)/Cu (480 nm) on a Si substrate was investigated by depth-dependent and in-situ CDBS for the first time [3]. In both layers, the grain size was determined to be 30±10 nm by XRD-spectroscopy. During tempering, in-situ CDBS with an positron implantation energy of 9 keV (which corresponds to implantation of positrons on the topmost Au layer close to the Au/Cu interface) was performed at three different temperatures: 633 K, 683 K and 733 K. During tempering at 633 K, the following two stages were identified: In the first three hours, mainly annealing was detected and the initial grain boundary diffusion of Cu atoms into the Au film only slightly affected the measured spectra. In the following four hours, the spectra slowly approached that taken at 683 K and 733 K, where the sample was found to be in thermal equilibrium. The latter both measured spectra were compared with theoretical calculations. The CDB spectra for the disordered fcc phase of (Au,Cu) with a varying Au content between 20 % and 90 % were calculated (figure 1). At both temperatures, excellent agreement was found for an Au content of 70 %. Hence, it can be concluded that during the second tempering stage observed at 633 K a homogenization of Au and Cu atoms took place leading to the formation of Au 0.7 Cu 0.3 CDB ratio to Cu 0.8 0.7 0.6 0.5 0.4 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 Au content 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 Doppler shift (keV) 18 20 22 24 26 28 30 32 34 longitudinal momentum component (10 -3 m 0 c) Figure 1: HMA of the CDB ratio curve at 733 K at the Au/Cu interface. Calculational results display an Au content of around 70 %. Depth-dependent CDBS before and after tempering confirmed that the topmost Au layer was replaced by a homogeneous intermixing zone of Au 0.7 Cu 0.3 . Below this intermixing zone, a high amount of Cu was detected as well. Hence, both layers did not totally mix up. Furthermore, the kinematics of the observed process indicate that the initial movement of Cu atoms along grain boundaries accelerated the intermixing [3]. References [1] M. J. Puska and R. M. Nieminen, Rev. Mod. Phys. 66, 841 (1994). [2] M. Reiner, P. Pikart, and C. Hugenschmidt, Phys. Procedia 35, 104 (<strong>2012</strong>). [3] M. Reiner, P. Pikart, and C. Hugenschmidt, to be published.
Chapter 4. Radiography and Tomography 33 Chapter 4 Radiography and Tomography X-Ray and Neutron CT of Tektite (by K.-U. Hess, see page 38)
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