Electrical and optical properties of organic materials are closely related to its
molecular orientation. SE is employed in the understanding of molecular
orientation of an anisotropic organic material with and without pre-deposition
aligning treatment. By taking SE measurements at varying sample rotation angles
(rotation along the sample normal), we observed that the organic thin film
deposited on rubbed monolayer displays not only out-of-plane anisotropy but also
in-plane anisotropy. However, thin film grown on un-rubbed monolayer does not
show any orientation (sample rotation) dependence.
Nanostructures and Molecular Orientation Studied by Spectroscopic Ellipsometry
Shih-Hsin Hsu1, Yia-Chung Chang1, Yuh-Jen Cheng1, Ching-Hua Chiu2, Hao-Chung Kuo2, Tien-Chang Lu2,
Shing-Chung Wang2, Chih-Wei Huang3, and Yu-Tai Tao3
1Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan2Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan
3Institute of Chemistry, Academia Sinica, Taipei, Taiwan
Spectroscopic ellipsometry (SE) is a nondestructive optical technique and conventionally used for
characterizing thin films and bulk materials. Here, we extend its applications to the
characterization of nanorods and the study of molecular orientation of organic thin films.
Figure 6. Layer structure of the
organic thin film deposited on
top of a monolayer on a SiO2/Si
substrate.
GaN Nanorods
Molecular Orientation
Figure 7. SE measurement of organic thin films with (left) and without rubbing pre-
treatment (right).
The refractive index profiles of GaN nanorods extracted from the SE analysis
suggest the broad spectral and angular antireflection is mainly attributed to the
gradually varying porous structure. The orientation-resolved SE measurement of
organic thin films clearly reveals the anisotropic property and the pre-deposition
treatment is crucial to its orientation. This study demonstrates that SE could be a
useful and non-contact tool for the characterization of structures in nanometer
scale and potentially in molecular level.
Summary
Experimental Data
Angle of Incidence (degree)20 30 40 50 60 70 80
Ref
lect
ion
0.00
0.05
0.10
0.15
0.20
0.25pR, 400nmpR, 632.8nmsR, 400nmsR, 632.8nm
Experimental Data
Angle of Incidence (degree)20 30 40 50 60 70 80
Ref
lect
ion
0.00
0.10
0.20
0.30
0.40
0.50
pR, 400nmpR, 640nmpR, 880nmsR, 400nmsR, 640nmsR, 880nm
Experimental Data
Angle of Incidence (degree)20 30 40 50 60 70 80
Ref
lect
ion
0.00
0.10
0.20
0.30
0.40
0.50pR, 400nmpR, 640nmpR, 1120nmsR, 400nmsR, 640nmsR, 1120nm
Figure 4. Measurement data and model fitting by describing the GaN nanorods as a 3-
node graded EMA layer.
GaN nanorods demonstrating broad angular and spectral antireflection were
characterized by SE. The GaN samples were first epitaxially grown by MOCVD
on c-plane sapphire substrates. Thin Ni films with various thicknesses ranging
from 5 to 20 nm were subsequently evaporated, and followed by a rapid thermal
annealing process under N2 gas to form Ni nano-dots of different sizes, which
served as the etching masks. After being etched by an RIE process, the samples
were dipped into a heated HNO3 to remove the residual Ni. Optical reflection
measurements show the reflectance for both p- and s-polarizations is held well
below 10% from UV to IR wavelengths and at incident angles up to 60º.
Effective medium approximation (EMA) theory was employed in the analysis and
the nanorods were modeled as a graded-index layer, in which each sub-layer is
modeled as a mixture of uniaxial GaN and voids with varying porosity fraction.
The model fitting based on a 3-node, graded EMA layer model works well in the
IR region, and can be extended to visible region for samples with smaller rod
sizes.
Figure 3. Optical reflection measurements of GaN nanorods. Left to right panels
correspond to the samples shown in Fig. 2 (a), (b), (c), respectively.
Figure 1. Fabrication process of GaN nanorods.
RTA RIE
Figure 2. SEM images of the GaN nanorods
samples: top view (a, b, c) with a scale bar
corresponding to 200 nm; cross-sectional view (d)
with a scale bar corresponding to 1 m.
a b c
d
Figure 5. The depth profiles for the refractive index of GaN nanorods at 632.8 nm
(left) and 1000 nm (right).
rubbed or un-rubbed
-9
-6
-3
0
3
6
varia
tion
of
60o
65o
0 90 180 270 360-18
-12
-6
0
6
Angle (degree)
varia
tion
of
-1
0
1
varia
tion
of
60o
65o
0 90 180 270 360
-2
0
2
Angle (degree)
varia
tion
of
Depth Profile at 632.8nm
Distance from Substrate in nm0 500 1000 1500 2000
Ref
ract
ive
Inde
x n
1.0
1.4
1.8
2.2
2.6
none
Depth Profile at 1000 nm
Distance from Substrate in nm0 500 1000 1500 2000
Ref
ract
ive
Inde
x n
1.0
1.4
1.8
2.2
2.6
none
GaN nanorods GaN nanorodsGaN film GaN film