International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 5, May 2013
ISSN 2319 - 4847
Effect of deposition angle on optical properties
of Selenium thin films
Raad Saied Abed1, Hanaa Saleh Sabaa2, Marwa Abdul Muhsien3
1,2,3
Department of Physics, College of Science,
Al-Mustansiriyah University, Baghdad, Iraq
ABSTRACT
The selenium thin films of obliquely and normal deposited were prepared using thermal evaporation method at pressure of 10- 6
torr on glass substrate at room temperature. The optical band gap of films has been determined from the transmission and
absorption spectra taken with the help of spectrophotometer in the wavelength range of 400-1080 nm at room temperature. The
optical band gap of as-grown films has been found to have direct band gap of ~1.8, 1.5, 1.4, 1.2 eV. The optical band gap of Se
thin films decreases with angle of deposition increases.
Keywords: Thin films, Selenium, Optical band gap, transmission analysis.
1. Introduction
Selenium thin films are very easy to obtain by physical vapour deposition and it should be interesting to use it for cheap
large-scale device productions. Growth and characterisation of selenium thin films has been active research subject of
considerable interest due to its potential commercial applications in photovoltaic and photoconductive devices. Its low
melting point, high vapour pressure and device applications like rectifiers, photocells, switching, memory and X-ray
photoconductor etc. [1] has made it attractive. Currently, there are many research activities devoted to the development
of selenium thin films and their structural and functional properties [2]. Their electrical and optical properties are
known to strongly depend on film preparation technique, deposition temperature and post-deposition annealing process
[3]. The obliquely deposited films are nearly continuous display of several unexpected solar optical properties. The
granular metal films characterized by continuous metal network interspersed with nano-size voids are well known but
their light transmission properties have proved to be difficult to explain. The obliquely deposited thin films are of
interest for the applications like angular selective window coatings, where angular selectivity involves variable
transmittance for the same angle of incidence as azimuthal angle changes.The selenium in solid state is known to exist
in the trigonal, monoclinic and amorphous forms in nature.The polarised memory effect observed on amorphous
selinium thin films [4]. In this paper the effect of deposition angle on optical properties of Se thin films is presented.
2. Experimental
Thin films were grown by vacuum evaporation of black selenium lumps (99.999%) using molybdenum boat in a high
vacuum of 10-6 torr on glass substrate kept at room temperature. Prior to deposition, the glass substrates were cleaned
with acetone and deionised water and dried. The quartz crystal thickness monitor was used for monitoring the film
thickness during evaporation. Selenium films were deposited at a deposition rate (10-15 Å/s). The optical transmission
(T %) and absorpation (A %) of the films were measured using a double beam UV-VIS-NIR spectrophotometer
(Shimadzu) in the wavelength range 400-1080 nm, at room temperature.
3. Results and discussion
Optical Properties:
Fig. 1 show the relation between transmission and wavelength of Se thin films deposited at angles Ө= (0º, 30º, 50º, 70º)
respectively and thicknesse t = (250) nm. It is clearly, that the increasing of the deposition angle has an obvious effect
on the transmission decreasing, and this is resulted from roughness increasing for film surface from obtained by
increasing the columnar growth with needle and rod like shape. This structure increases the film trapping efficiency
when rays falling on it and this reduce the transmission. On the other hand, the nature of the material is that its
transmission is low for the spectral range almost from (λ = 400-550) nm, and it increases rapidly between (λ= 550-650)
nm, then it increases slowly till the end of the studied range (λ= 1080) nm.
Volume 2, Issue 5, May 2013
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 5, May 2013
ISSN 2319 - 4847
Fig. 1: Transmission spectra of selenium thin films prepared at different deposition angle.
Fig. 2 shows the dependence of absorption spectra on the wavelength prepared at different deposition angle. The
value of absorption increases slowly at the long wavelength region that means that the Se thin films have higher
transmission for the wavelengths larger than 600 nm whereas it increases fast at short wavelength region smaller than
600 nm, which is due to the increase of grain size which provides numerous absorption cases [5]. It is noticed clearly in
the figure below the effect of deposition angle on the absorption values as the absorption increases with increasing of
deposition angle for the reasons referred to in the transmission subject.
Fig. 2: Absorption as a function of wavelength of selenium thin films prepared at different deposition angle.
Analysis of optical absorption spectra is one of the most productive tools for understanding and developing the band
structure and energy gap. The optical absorption coefficient
is related to the transmission T of a sample with
thickness d through the relation [6]:(1)
Where d is the film thickness.
Fig. 4 shows the absorption coefficient (α2) versus photon energy (hv) obtained at room temperature of as-grown
selenium films of thickness 250 nm. From the figure the absorption coefficient (α2) increases with increasing photon
energy for investigated thin films. We can evidently see that absorption coefficient having values (
which leads to increase the probability of occurrence direct transition. It can be seen that the plot is linear in the region
of strong absorption near the fundamental absorption edge. Thus, the absorption takes place through direct transition.
Fig. 4: Absorption coefficient (α2) versus photon energy of selenium thin films prepared at different deposition angle.
Volume 2, Issue 5, May 2013
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 5, May 2013
ISSN 2319 - 4847
The energy band gap of thin films was calculated with the help of transmission spectra using the following equation
[7]:
(2)
Where (hν) is the photon energy, (α) the absorption coefficient, (Eg) the band gap, (A) is the constant, (n) =1/2 for
direct band gap material and (n) =2 for indirect band gap.
Fig. 5(a): Photon energy (eV) versus (αhν)2 of selenium thin films prepared at different depodition angles.
The optical band gap shown in Fig. 5(a), was calculated from the plot between photon energy ( ) and
and we
find, that optical energy band gap decreases with the angle of deposition. The optical energy band gaps for different
angles θ = 0o, 30o, 50o and 70o were found to be 1.8, 1.5, 1.4 and 1.2 eV respectively. The decrease of optical energy
band gap can be explained on the basis of "self- shadowing effect" in the obliquely deposited thin films .It is assumed
that evaporated material leading to the substrate has enough surface mobility to form small islands instead of a uniform
thin films. This behaviour has been observed during deposition in the form of island growth of thin metal films [8]. The
self-shadowing effect was also observed in obliquely deposited thin films [9]. According to them small variation was
found in thickness of obliquely deposited thin films and it was observed that angle of deposition increases the porosity
of films. The increase in porosity was found due toself - shadowing effect. In this effect, the size of island increases
with angle of deposition. The inrease in island size may be responsible for the minor change in thickness of films. Due
to minor change in thickness, the optical band gap of thin films decreases as shown in Fig. 5 (b). The island size
reduces the reflectivity of incident photon and increases the absorption due to the multiple reflections on film surface.
These results also well agree with Koonig and Holvoving [10] and Holland [11]. They had observed poor reflectance
and enhanced optical absorption in obliquely deposited thin film.
Fig. 5 (b): Variation of optical band gap with deposition angle of selenium thin films.
4. Conclousions
Selenium thin films were prepared by thermal evaporation method. The optical band gap of Se thin film has been
determined from the transmission and absorption spectra taken with the help of spectrophotometer in the wavelength
Volume 2, Issue 5, May 2013
Page 191
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 5, May 2013
ISSN 2319 - 4847
range of 400-1080 nm at room temperature. The optical band gap of as-grown films has been found to have direct band
gap of ~1.8, 1.5, 1.4, 1.2 eV prepared at different deposition angle.
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