Zinc Selenide (ZnSe) is an II-VI semiconductor with wide band gap and these
semi conducting devices has wide range of applications in various optoelectronic
devices and in solar cells. This type of Semiconductor has a direct band gap
and in wide range of visible spectrum it is transparent. ZnSe thin films have
Polycrystalline nature and these materials are used in electroluminescent display
and window layers in solar cells. In recent years, considerable efforts have
been denoted to develop blue green laser diodes based on ZnSe and its alloys
so for these have not been realized, mainly because of the difficulty in controlling
electrical conduction. Preparation of conductive layer is essential for its
use light emitting devices of Zinc Selenide this films can be deposited by variety
of methods like laser assisted evaporation (Aksas et
al., 2006), Atomic Layer Deposition (ALD) (Choudhury
et al., 2004), spin coating method (Guziewicz et
al., 2004). In this study, brush plating method was used to deposit
zinc selenide thin films on FTO substrate, also seems to be a simple and inexpensive
method for fabricating semiconductor films and the films were characterized
for their structural and optical properties.
MATERIALS AND METHODS
All the parameters are optimized (deposition time 50 min, pH, concentration (1 N), deposition current (1 mA). Layers where brush plated on Fluorine doped Tin Oxide (FTO) coated conducting substrates. The negative terminal of power supply connected with a graphite rod wrapped in cotton wool served as the anode. The cotton wool behaves like a brushing tool. Prior to plating the stylus was wired to the power supply and the cotton wool was soaked in the electrolyte. The stylus was then made to contact with the FTO substrate and moved in one direction at uniform speed and repeat this step for 20 min. Whenever the stylus was contact with the substrate an electrical current was found to passing through the graphite. The electrolyte trapped within the cotton wool also associated with the acceleration of the ions which were subsequently forming the ZnSe layer on the substrate. The bath contained 1 g of Zinc Oxide and 0.05 g of Selenium di oxide (SeO2). In each case, the power unit was preset at voltage value 1 V and the electrolyte temperature was at room temperature. The films were annealed in vacuum at 300 and 500°C for 2 h.
The structural investigations of zinc Selenide thin films were done by X-ray
diffraction method. Structural analysis using XPERT-PRO diffractometer
with CuKα radiation (λ = 1.54060 ε). A Hitachi UV-Vis-NIR spectrometer
was used to carry out the optical studies. Photoluminescence measurements were
carried out at room temperature by a method (Cary Eclipse Win FLR) with 600
RESULTS AND DISCUSSION
Structural Investigations: Figure 1 shows XRD patterns of the zinc selenide thin film as-deposited coated on FTO substrates. Well defined peaks are observed in the XRD patterns suggesting that the films are crystalline in nature. The observed peaks are compared with the standard JCPDS values show that the as-deposited on FTO substrate film gives a structure matching with (JCPDS 02-0479), zinc blend Cubic structure was obtained for as deposited ZnSe thin film with lattice parameter a = 5.65 nm. The observed peak belong to ZnSe where detected at 2θ = 14.3°, 55.9°, 37.9° corresponding to interplanar distances of 6.5 ε, 2.5 ε, 2.4 ε, respectively.
When the film is annealed up to 300°C we observed more randomized orientation of the crystallites than in the as prepared film. The structural composition does not change due to increased annealing temperatures up to 300°C. Figure 2 and 3 shows the XRD patterns of the film annealed in vacuum at 300 and 500°C for 2 h. The observed new peaks at 2θ = 27.4°, 37.9°, 78.1° corresponding to interplanar distances of 3.2 ε, 2.4 ε, 1.2 ε this pattern matches the standard pattern of stoichiometric ZnSe with a = 3.996 nm (JCPDS 15-0105). The intensity of the peaks was increased as the annealing temperature increased.
The crystallite grain size of the zinc selenide thin films was calculated using the Scherrers formula:
D = 0.94λ/βcosθ (nm)
The grain size increased from 6-31 nm as the annealing temperature increased. The strain and dislocation density were calculated using the following relations:.
Strain (ε) = βcosθ/4
||Full Width Half Maximum of the XRD peak (FWHM):
Dislocation density (δ) = 15ε/aD
|| XRD pattern of As-deposited film
|| XRD pattern of annealed film at 300°C
|| XRD pattern of annealed film at 500°C
The value of strain increases from 2.48x10-2-3.60x10-2
as the annealing temperature increases.
||Transmittance spectra for, (a) As-deposited (b) Annealed at
300°C and (c) Annealed at 500°C
The observed dislocation density was increased from 0.017x1014 m-2-0.03x1014
m-2 (Choudhury et al., 2004) reported
the hkl indices of the film deposited by laser assisted evaporation had (111),
(220), (311), (400) and (331) which has good agreement with the present study.
Aksas et al. (2006) reported the presence of
(111), (220), (311) with indices of the film deposited by spin coating method.
Optical studies: The optical absorbance and transmittance spectra of
the films formed by brush plating method are shown in Fig. 4
and 5, respectively for the as-deposited and annealed (at
300 and 500°C) zinc selenide thin films. Annealing the film at 300 and 500°C
produced some changes in the optical absorbance and transmittance spectra of
the film. These changes are attributed to structural, compositional and crystalline
process as discussed before.
||Absorbance spectra for, (a) As-deposited and (b) Annealed
|| Direct band gap of As-deposited film
The band gap values for the films were estimated from the absorption spectra. The corrected transmittance values are obtained by following expression:
The transitions from the valance band to conduction band across the band gap at different wavelengths were calculated for obtaining the absorption coefficients (αg):
αg = (1/t) ln (100/Tcorr)
where, Tcorr is the corrected transmittance values and t is the film thickness. The direct band gap value is obtained from following graph drawn between absorption coefficient αg2 against the corresponding values of photon energy (hv).
The energy gaps were determined from the Fig. 6 and 7.
The as-deposited sample has a Egdir value of 2.75 eV and for the
annealed (300°C) has 3.38 eV.
|| Direct band gap of annealed film
||Photoluminescence spectra for ZnSe thin films, (a) The photons
are exited in the wavelength of 242 nm and (b) The emission of thin film
is at the wavelength of 484 nm
These band gap values are compared with earlier reported values. From this
characterization these materials have good chance to be used for solar cells.
Photoluminescence: The photoluminescence spectra of ZnSe thin films are shown in Fig. 8. From the spectra, the photons are excited in the wavelength of 242 nm. The intensity of the peak Decreases as the annealing temperature increases. From the spectra the emission of ZnSe thin films is at the wavelength 484 nm. The optical band gap is comparable with Photoluminescence spectra.
Zinc selenide thin films were successfully deposited onto FTO substrates by brush plating technique. The films were uniform and had good adherence to the substrate. The XRD of zinc selenide films compared with Standard JCPDS (02-0479) and (15-0105). The average grain size of the films were determined and it increases from 6-35 nm as the annealing temperature increases. The energy gap values of the films were determined and compared with the reported one. The emitted and excited wavelength were determined from the photoluminescence spectra. The Zinc selenide films have wide applications in photovoltaic cells and solar conversion cells.