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