Association

48 3. Experimental details
coordinates q i are converted into real space distances with Miller’s indices m i ,
⎧
d i = 1 ⎪⎨ h or k, for i parallel,
·m
q i , m i =
i
⎪⎩ l, for i perpendicular.
(3.9)
In this work, all X-ray diffraction experiments are conducted with a four-cycle Phillips MRD
Pro using monochromatized Cu Kα radiation.
3.4.3. Hard X-ray photoemission spectroscopy (HAXPES)
Quantifying the element-specific electronic properties of a multilayer system is impossible
when using surface-sensitive soft X-ray photoemission due to its limited probing depth. The
major limiting factor is the inelastic scattering of free electrons in solids. For an electron with
energy E kin = 2 k 2
2m , the inelastic mean free path (IMFP) can be expressed as λ = ( )
k
m
τinel. ,
where τ inel. denotes the lifetime due to inelastic scattering processes. A removal of unwanted
top layers with argon etching and subsequent soft X-ray PES provides access to buried layers,
however, this approach significantly changes the surface morphology and hampers a further
investigation of highly reactive functional layers such as EuO.
Since the last decade, however, the hard X-ray regime (hν 2 keV) has become available to
photoemission – an energy range complementary to the established soft X-rays. This extends
the escape depth of the photoelectrons up to several tens of nanometers according to the estimation
λ ∝ Ekin 0.78,
as depicted in Fig. 2.18.84,85 Thus, HAXPES with photon energies up to
10 keV is perfectly suited to investigate the electronic properties of buried interface or bulk
properties. Hard X-ray excitation, however, implies also drawbacks such as reduced photoemission
cross-sections and Debye-Waller factors, as discussed in Ch. 2.4.3. Fortunately, since
Figure 3.13.: An extra-wide photoemission spectrum by HAXPES. Deep core-levels are accessible and
valence-like levels have reduced cross-sections, as illustrated in a survey spectrum taken at hν =
4 keV. The spectrum of a buried SiO x interface layer (inset) varies in observable SiO x fraction
with excitation energy.