Biomedical Engineering Reference
In-Depth Information
15.5
MOLECULAR BEAM EPIT
AXY
layered structures, superlattices, and layered
alloys with precise interfaces
[24]
. ALD can
be used to produce thin films with good confor-
mal coverage, and it has the ability to control
film thickness accurately at the subnanometer
level. Such distinctive advantages have made it
a potentially valuable tool for nanotechnology.
The same advantages make it useful for
bioreplication. Indeed, ALD has been used to
fabricate alumina replicas of the corneal layer of
the compound eyes of flies
[32]
and butterfly
wings
[33]
. For example, ALD was used to pro-
duce a 100-nm-thick alumina coating on a fly
eye, and then the biotemplate was removed by
pyrolysis. The resulting replica captured the
200-nm nipple-like features patterning the com-
pound eye
[32]
. ALD has also been used to
replicate the spines of the sea mouse
[34]
, to infil-
trate spider silk in order to toughen it
[35]
, and
to prepare photocatalytic replicas of the inner
membranes of avian eggshells
[36]
. Chapter 16
provides a detailed treatment of ALD for
biomimicry.
Molecular beam epitaxy
(MBE) is a technique
used to produce ultrathin films as high-quality
epitaxial layers with very sharp interfaces and
good control of thickness, doping, and composi-
tion
[37]
. Deposition usually takes place under
high or ultrahigh vacuum conditions (typically
below 10
−
10
Torr). Because of the high degree of
control possible with MBE and the possibility of
growing compound semiconductors, it is a valu-
able tool in the development of sophisticated
electronic and optoelectronic devices.
The MBE process can be considered a refined
form of evaporation. Ultrapure target materials
are placed in effusion cells (also called
Knudsen
cells
) and heated to their sublimation points
[8]
.
Molecular beams thereby produced are then
directed toward a single-crystal substrate, in the
vicinity of which they may react chemically with
each other or other gaseous species introduced
into the vacuum chamber and then condense as
a layer on the substrate.
Figure 15.9
shows the
FIGURE 15.9
Schematic of a typical system for molecular beam epitaxy (MBE). Solid target materials are heated in effusion
cells to produce molecular beams. The substrate is heated to the necessary temperature and, when needed, continuously rotated
to improve the growth homogeneity. A reflection high-energy-electron diffraction (RHEED) gun is used for
in situ
monitoring.
