Photonic crystals in biology

Poster Session, Tuesday, June 15
Theme A1 - B702
Preparation and Characterization of Nanostructured ZnS Thin Films Grown on Glass and
Monocrystalline Si Substrates
R. Sahraei 1* , G. Nabiyouni 2 and A. Daneshfar 1
1 Department of Chemistry, University of Ilam, Ilam P.O. Box: 65315-516, Iran
2 Department of Physics, University of Arak, Arak, Iran
Abstract— Nanocrystalline zinc sulfide thin films were prepared by a new chemical bath deposition technique onto glass and
silicon (111) substrates. Deposition takes place at a temperature of 70 ºC and a pH of 6.0, from an aqueous solution containing
zinc acetate, thioacetamide, and ethylenediamine. Microstructure analysis using atomic force microscopy shows that the films
deposited on glass substrates contain 28-30 nm clusters, whereas much larger clusters (around 80-120 nm) comprise the films
deposited on silicon (111) substrate. X-ray diffraction analysis indicates that both the ZnS films deposited on glass and Si
substrates have cubic zincblende structure. Direct band gap energy for these samples was measured to be in the range of 3.97-
4.00 eV.
Recently, the II-VI compounds semiconductor thin films
have received an intensive attention due to their application in
thin film solar cells [1]. Among these metal chalcogenides,
ZnS is an important semiconductor material because of its
broad direct band gap energy (~3.6 eV) at room temperature
[2]. Various techniques have been employed to fabricate ZnS
thin films, such as, electrodeposition, pulsed-laser deposition,
chemical vapor deposition (CVD), and chemical bath
deposition (CBD) [3, 4].
In this work, we report deposition of nanocrystalline zinc
sulfide thin films on the glass and mono-crystalline Si
substrates using a weak acidic bath in which ethylenediamine
acts as a complexing agent and thioacetamide acts as a source
of sulfide ions. Atomic force microscopy (AFM), X-ray
diffraction (XRD), and UV-Vis spectrophotometery are used
to investigate the surface morphology, structural, and optical
properties of the nanostructured ZnS thin films. We show how
the morphology and surface roughness of the ZnS thin films
depend on the substrate type.
Figure 1. AFM images (two- dimensional (2D)) of CBD ZnS thin films
on (a) Si and glass substrate (b).
X-ray diffraction patterns of the ZnS film grown on glass
and monocrystalline Si substrate show three distinguished
peaks at the angles of 28.6º, 47.7º and 56.5º reveal a cubic
lattice structure and can be assigned to the (111), (220), and
(311) plans, respectively. Broadening of diffraction peaks in
the XRD pattern of the ZnS film is attributed to the
nanometer-sized crystallites. The calculated average size of
nanocrystallites, using Scherrer equation is found to be about
4.5 and 8 nm for the ZnS films deposited on glass and single
crystal Si substrates, respectively.
The average transmittance of ZnS films is calculated to be
84%, 78%, 74% and 71%, respectively, in the visible
wavelength region. As it is clear from spectra the films have a
steep optical absorption feature, indicating good homogeneity
in the shape and size of the nanocrystallites and low defect
density near the band edge [5]. The band gap energy (E g ) was
determined to be in the range of 3.97-4.00 eV for the ZnS
films with deposition times varying from 4 to 16 hours. These
values are rather larger than the literature value for the bulk
ZnS (~ 3.6 eV). The result could be attributed to the quantum
size effects as expected from the nanocrystalline nature of the
ZnS thin films [6, 7].
Figure 1 (a) and (b) illustrates two-dimensional AFM
images of the ZnS thin films deposited on monocrystalline Si
and commercial glass slide substrates, respectively. The thin
film deposited on Si substrate is made of aggregates (clusters)
with a square-like surface morphology, whereas much finer
aggregates with an isosceles triangular surface morphology
comprise the film deposited on glass substrate. As can be seen,
the films deposited on the glass substrate contain smaller
clusters (average grain size of around 28-30 nm in diameter)
and have more surface aggregates than those deposited on Si
substrate (average grain size of around 80-120 nm in
diameter).
In summary, we have successfully deposited the
nanocrystalline ZnS thin films onto glass and monocrystalline
Si substrates, from a chemical bath at temperature of 70 °C,
and using ethylenediamin as a complexing agent. The XRD
measurements indicate that the structure of the ZnS thin films
is cubic. In our experiment, based on the optical transmission
measurements, the band gap energies are calculated to be
between 3.97-4.00 eV for the ZnS films with different
thicknesses. Morphology and optical properties of the ZnS
films were characterized using AFM and UV-Visible
spectroscopy.
*Corresponding author: reza_sahrai@yahoo.com
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6th Nanoscience and Nanotechnology Conference, zmir, 2010 352