Chemistry Reference
In-Depth Information
Blodgett and Langmuir-Schaefer films to the more recent study of ultrathin layers
evaporated in ultrahigh vacuum.
2 Optics for Organics
The very first example of an optical spectroscopy applied to the investigation of the
growth of an organic layer dates back to the 1990s [
3
]. At that time, the study of thin
layers of organic molecules deposited on solid substrates began to be a pressing
necessity for research. In fact, despite the well-established application of optical
spectroscopy to detect surface states of metal and semiconductor surfaces [
4
], the
extension of optics to organics was motivated by the occurrence of a scientific and
technological issue related to the development of organic molecular beam epitaxy
(OMBE) [
5
], also known as organic molecular beam deposition (OMBD). As it was
clearly reported since the very early papers published on this subject, OMBE
invention was a consequence of the preceding success of the inorganic counterpart
(molecular beam epitaxy (MBE)) and of the increasing interest for a method able to
control also for organics the growth with subnanometer accuracy from the very
early steps of deposition, in view of the final device architecture. Several technical
details were just taken from the inorganic case, sometimes without considering
correctly the peculiarities of organics: to mention a significant example, the use of
high-energy electrons to monitor in real time the deposition process often produced
defects in the soft organic samples investigated (in reflected high-energy electron
diffraction (RHEED), commonly used in MBE, the electron energy usually is raised
up to 2-3 keV). The OMBE inventor himself wrote that “leaving the (RHEED
electron) beam on during deposition (of the organic thin film), resulted in the
development of broad rings characteristic of amorphous film growth. These features
are possibly due to disruption of the grown layer due to charging and/or heating of
the relatively insulating film, or even due to damage of the molecules by high
energy electrons” [
6
]. The softness of the organic matter and the relative weakness
of the related bonding made then necessary the use of a more
gentle
probe, but able
to investigate the electronic and structural properties of the growing layer and to
give information about the augmenting thickness.
Optics, mainly in the near ultraviolet (UV)-visible-near-infrared (IR) range
(300-1,000 nm), offered the possible alternative to electronic diffraction tech-
niques. At the end of the 1990s, three optical spectroscopies were more commonly
used to investigate surface states (i.e., with the necessary sensitivity to very tiny
quantities of organic material): surface differential spectroscopy (SDR),
ellipsometry, and reflectance anisotropy spectroscopy (RAS).
In this review, ellipsometry will just be mentioned. It is a very powerful and
sensitive technique, giving complete information on the optical properties of
materials (isotropic and anisotropic), multilayers, and in general samples whose
characteristics vary with thickness [
7
]. Ellipsometry has been also used for in situ
and real-time growth control in MBE systems [
8
] and is therefore appropriate for
