Annual Report 2011 / 2012 - E21 - Technische Universität München

32 E21 Annual Report 2011/2012
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 E21, Technische Universität München, D-85748 Garching, Germany.
2 Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Technische 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 (2012).
[3] M. Reiner, P. Pikart, and C. Hugenschmidt, to be published.