Chemistry Reference
In-Depth Information
energy of film-substrate adhesion using thermal desorption spectroscopy (TDS).
Other diagnostics include low-energy electron diffraction (LEED). In this case,
low-energy electrons (
100 eV) impinge perpendicularly onto the sample surface.
Thismethod reduces radiation-induced damage but theworking geometry precludes
real-time growth studies and the
in situ
experiments have to be performed in an
additional chamber attached to the growth chamber. Finally, many other vacuum
analysis techniques such as scanning tunnelling microscopy, XPS, Auger electron
spectroscopy, ellipsometry, etc. can also be employed in conjunction with OMBD
simply by installing the appropriate equipment in a chamber connected to the growth
chamber. Concerning the type of pumping used to achieve the UHV environment
necessary for OMBD systems, there is considerable variation. Due to the volatility
of some organic materials, high throughput pumps are generally employed. These
include turbomolecular pumps, ion pumps and cryopumps.
Several examples of OMBD-grown films will be discussed in Chapter 4 and
here we briefly study the case of highly ordered DIP films (Durr
et al.
, 2002a). DIP
was first purified by temperature gradient sublimation. All samples were prepared
on oxidized Si(100) substrates. DIP layers with varying thickness between 7 and
110 nmwere grown at
T
sub
<
418K and at a rate of 1.2 nmmin
−
1
under UHV condi-
10
−
9
mbar during deposition). The spatial coherence of the crystalline
order has been quantitatively determined by high-resolution XRD measurements.
In the so-called specular diffraction mode the momentum transfer
q
is perpendicu-
lar to the surface normal, thus probing the electron density profile along the surface
normal. Figure 3.17(a) displays a typical scan along the specular rod for a DIP thin
film (thickness
≤
×
tions (
2
20.6 nm) prepared under the conditions described above. For all
samples, Bragg reflections associated with the DIP film up to at least the seventh
order are visible, from which a lattice constant of 1
005 nm is derived.
The large number of Bragg reflections reveals the high structural order achieved in
the DIP films. Figure 3.17(b) shows a magnification of a small-
q
z
region of Fig.
3.17(a). The periodicity of the interference fringes, the so-called Kiesing fringes,
equals 2
.
660
±
0
.
divided by the film thickness. The almost undamped Laue oscillations
around the Bragg reflections are evidence for a laterally homogeneous coherent
thickness, e.g., a well-defined number of ordered MLs.
A recently developed alternative to the OMBD technique is the hyperthermal
molecular beam deposition (HMBD) method, which has been used to grow e.g.,
highly ordered pentacene films on metal surfaces (Casalis
et al.
, 2003). The tech-
nique is based on the acceleration of the impinging molecules to a few eV us-
ing seeded supersonic free jet expansion from a molecular beam source (typical
diameter of 0.1 mm) in which different inert gases (He, Ar and Kr) can be used as a
carrier gas. The fine control of the kinetic energy leads to high-quality films. While
π
