Atomic Layer Deposition in Nanoporous Catalyst Materials - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 168

7.1.1 Atomic Layer Deposition
The ALD technique, formerly known as ALE (atomic layer epitaxy) or ALCVD
(atomic layer chemical vapor deposition), was invented y and patented by
Suntola and co-workers in Finland in the 1970s. 2 Its application was initially
limited to the deposition of materials used in electroluminescent flat panel
displays. Driven by the decreasing device dimensions, ALD research inten-
sified in the 1990s, which has led to a major commercial breakthrough of
ALD in the semiconductor industry for growing high-k gate oxides. Already in
the 1990s, several authors also recognized the potential impact of ALD in the
area of catalyst preparation. 3-8 Since the early 2000s, an increasing number
of researchers are exploring ALD as a generic coating technique for all sorts
of nanostructures, as illustrated in a number of recent review papers. 9-14
Catalysis is an important potential application field, but also gas separation,
sensors, batteries, capacitors, fuel cells, photovoltaics and photonics are
explored by the growing ALD community.
Various materials that are relevant to catalysis including several oxides,
nitrides, sulfides and (noble) metals can be deposited via ALD. For an ex-
tensive overview of existing ALD processes, the reader is referred to a recent
review paper. 15 A classic example of an ALD process is the growth of Al 2 O 3 by
alternating exposure of the substrate to trimethylaluminium (TMA) and H 2 O
vapor. 16,17 The so-called ALD cycle is depicted in Figure 7.1. In part A of the
reaction cycle, the surface is exposed to TMA molecules which react with the
OH surface groups, resulting in strong Al-O bonds and a CH 3 -terminated
surface. Because TMA is inert towards the CH 3 surface groups, the reaction
will stop once all accessible z OH groups have been consumed. The TMA
pulse is followed by an evacuation of the growth chamber through pumping
or purging with an inert gas. In part B of the reaction cycle, H 2 O vapor
hydrolyses the residual CH 3 groups on the surface resulting in the formation
of a (sub)monolayer of Al 2 O 3 . The reaction is again self-limiting because H 2 O
does not react with the newly generated OH surface groups. Instead, after a
second evacuation of the chamber, these surface groups allow for the
repetition of the ALD cycle to continue Al 2 O 3 growth in a self-limited
(sub)monolayer-by-(sub)monolayer manner.
The basic chemical mechanism that offers ALD an exceptional growth
control is the self-saturation of the surface reactions. It is therefore of critical
importance that the precursors do not decompose upon adsorption on
the surface. This requirement sets an upper limit to the growth temperature
of ALD processes. On the other hand, the sample temperature must be
su ciently high to provide the activation energy required for the surface
reactions and to avoid condensation of precursor molecules as this would

d n 9 r 4 n g | 7

.

y

A similar type of process based on self-limited gas-solid reactions called 'molecular layering'
was independently developed by Aleskovskii and co-workers in the former Soviet Union in the
late 1960s. 1

z

Some OH groups are no longer available because of steric hindrance from chemisorbed
neighboring TMA molecules.