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52 3. Experimental details
hard x-rays
e −
z
5 nm
10 nm
Figure 3.17.: Schematics of hard X-ray photoelectron
excitation and damping in a threelayer
heterostructure. Taking advantage of
the largely tunable excitation energy of the
hard X-ray synchrotron radiation, the resulting
information depth is the essential
parameter for a depth-selective probe of
multilayer structures.
As an example, the excitation takes place in
the buried magnetic oxide and the electron
yield is damped by the metallic overlayer.
An essential application of HAXPES is the determination of non-destructive depth profiles
of specific elements with Ångström resolution. Photoelectron excitation and damping in dependence
on the depth z is illustrated in Fig. 3.17. The detected intensity I can be expressed
in dependence on the density n of an element x which reveals a profile in z,
I ∞ ∫ b
x
I x =
Nx ∞ λ(E kin ) cosθ ·
z
n x (z) exp−
a
λ(E kin ) cosθ
dz, (3.11)
where N ∞ x denotes the density of an element x in the bulk and n x (z) the density profile of x.
The function n x (z) can be unraveled by a series of photoemission measurements of element
x with different kinetic energies of the photoelectrons (procedure conducted e. g. by Rubio-
Zuazo and Castro (2008)). 137
In this work, however, the determination of a continuous density profile is replaced by two
representative values. First, bulk-sensitive measurements are conducted by using a large kinetic
energy of the photoelectrons, thus λ is large, too. As a complementary second measurement,
the kinetic energy is tuned down by using the minimum excitation energy provided
by the beamline, thus the information depth is tuned to a minimum and the experiment is
surface-sensitive. Alternatively, changing the emission geometry of the photoelectrons to a
large off-normal emission angle also reduces the mean escape depth according to λ(E kin )cosα
(see Fig. 3.18).
Thickness determination of buried layers by HAXPES
The depth sensitivity of HAXPES not only allows the determination of the chemical state
of a thin buried layer, but also its thickness. The thickness of buried oxide tunnel barriers,
though, is a key parameter for oxide spintronics. 85 Considering a layer stack, in which a
toplayer of thickness c is of interest on top of a substrate, a simple formula can be derived 138
and applied 139 for such a two-layer system. As an example, the thickness of a silicon surface