Biomedical Engineering Reference
In-Depth Information
The electric field intensity can reach very high values of the order of 1 GV/m due to
the reduced dimensions of the AFM tip (below 10 nm). Initially, at relative humidity
(RH) values above zero and in thermal equilibrium, several MLs of water molecules
are chemically absorbed on both substrate and AFM tip surfaces. When a strong
electric field is applied, it induces the formation of a water bridge with nanometric
dimensions that can be seen as a nanometric electrochemical cell. By applying a
negative voltage above a threshold value (
V
th
) to the tip while the substrate is set to
ground [
58
], oxidation and reduction processes take place on the substrate surface
and AFM tip, respectively.
The eventual oxide size/shape and the oxidation kinetics depend mainly on
the electric field intensity distribution on the substrate surface (i.e., oxidation
experimental parameters). Given that the formation of the oxide structure is spatially
confined within the water bridge, the maximum lateral extension of the oxide can be
controlled by the AFM working conditions in non-contact-mode [
59
]. On the other
hand, reproducibility in size/shape of the formed oxides depends strongly on the
stability and uniformity of the water bridge during the oxidation process, which
mainly depends on the surface roughness. In this regard, epitaxial GaAs (0 0 1)
substrates are more convenient as they present a flat surface at the atomic level as
measured by AFM (root mean square, RMS
∼
0.16 nm). In this work these surfaces
were obtained by growing a 0.5-
m-thick GaAs layer by MBE at a growth rate of
1MLs
−
1
, substrate temperature of 580
◦
C, and a V(As)/III(Ga) flux ratio of 8.
In the case of AFM-LAO patterning of GaAs (0 0 1) surfaces, the resulting
oxides are mainly composed of Ga
2
O
3
and As
2
O
3
species through the reaction
2GaAs
6h
+
[
60
]. The volume per mol values
for GaAs, Ga
2
O
3,
and As
2
O
3
are 24.77, 65.74, and 68.04 cm
3
mol
−
1
, respectively.
Hence, the oxide volume is larger than that of the GaAs material and consequently,
the oxides emerge above the substrate surface level. In order to obtain patterned
substrates with nanoholes, these oxides can be easily removed by selective wet
chemical etching with HF (49%). In Fig.
1.13
, square arrays of oxide dots and
the resulting nanoholes for applied voltages of
V
ox
=
−
12h
+
+
6OH
-
+
→
Ga
2
O
3
+
As
2
O
3
+
8V(Fig.
1.13
a-c), and
V
ox
=
−
14 V (Fig.
1.13
d-f) are shown. The oxides were fabricated in AFM non-
contact-mode (RH
40 N m
−
1
). A fixed oxidation
=
30%) using n-doped Si tips (
K
=
time of 500 ms was used. For
V
ox
=
8 V the resulting oxides show a “simple conical
structure” (Fig.
1.13
c), whereas for
V
ox
=
−
14 V, an oxide with a broad and low base
crown with a narrow spike is obtained (hereafter referred as oxides with “double
structure”) (Fig.
1.13
f). This special morphology obtained at high applied voltages
is ascribed to the vertical growth saturation of the oxide. Interestingly, the size/shape
of the resulting nanoholes exactly replicates the size/shape of the emerging oxide
with respect to the surface. Hence, customized nanoholes can be designed just by
controlling the size/shape of the oxide structures, which eventually depends on the
experimental oxidation parameters [
61
].
