80
COMPOSITIONAL
DEPTH PROFILING
analysis,
the
results obtained with
the
high lateral resolution employed
in
AES and SAM
mean
that
such changes
in
analysis depth occur
in the
analysis
of
parts
of the
specimen
with
different
orientations
to the
elec-
tron
analyser (because
of
specimen surface roughness). Such effects tend
to be
regarded
as
experimental artefacts
to be
circumvented
by the
Auger
microscopist,
and
have only recently become
the
subject
of
rig-
orous
scientific investigation.
If
we
consider
the
Beer-Lambert equation, discussed
in
Chapter
1, it
is
clear that
the
depth
of
analysis
is
dependent
on the
electron angle
of
emission,
9
s
. By
recording spectra with
good
angular resolution
at a
high value
of #, say 75°
(relative
to the
sample normal),
an
analysis
is
recorded which
is
extremely surface sensitive.
As
normal electron emis-
sion
is
approached
(0 = 0°) so the
analysis depth moves towards
the
limiting value
of
~3A. This value
is
often referred
to as the XPS
analysis
depth although
it is, of
course, more correctly described
by 3
A
cos 6. The
relative
sampling depths
at
different
take-off
angles
are
illustrated
sche-
matically
in
Figure
4.1
(a).
A
thin overlayer will give
a
characteristic
angular distribution predicted
by the
Beer-Lambert expression,
as
shown
in
Figure 4.1(b).
An
island-like distribution
will
show
no
angular
dependence, thus
it is
possible
to
distinguish between these
two
types
of
phase distribution with relative ease.
This manner
of
depth
profiling
is
invaluable
for
compositional
changes that occur very close
to the
surface
and has
been employed
for
studies
of
thin passive
films on
metals
and
surface segregation
in
polymers.
In
conventional angle resolved
XPS
(ARXPS),
the
angular
acceptance range
of the
spectrometer
is
reduced
by the
user
to
provide
the
required angular resolution. Clearly, there must
be a
compromise
between angular resolution
and
sensitivity (acquisition time). Spectra
are
then collected
at
each
of a
number
of
take-off
angles
by
tilting
the
specimen.
The
experimental method
is
illustrated
in
Figure
4.2
while
Figure
4.3
presents
an
ARXPS data
set for a
sample
of
GaAs with
a
thin
oxide layer
at its
surface.
It is
clear
from
the
montage
of As 3d
spectra that
the
oxide peak
is
dominant
at the
surface
whereas
the
peak
due to As in the
form
of
GaAs
is
more dominant
at
near normal analysis angles. This phenomenon
is
repeated
in the
gallium spectra (not shown).
5
According
to the
International Standard,
ISO
18115,
the
angle
of
emission
is
measured with
respect
to the
surface
normal while
the
take-off
angle
is
measured with respect
to the
plane
of
the
surface (see Glossary, page 185). Throughout this text
the
angle
of
emission
will
be
used.