Chemical synthesis and characterization of CdSe thin films deposited by SILAR technique for optoelectronic applications

Abstract CdSe thin films were deposited on the glass substrate by successive ionic layer adsorption and reaction (SILAR) method. Different sets of the film are prepared by changing the number of immersion cycles as 30, 40, 50 and 60. Further the effect of a number of immersion cycles on the characteristic structural, morphological, optical and electrical properties of the films are studied. The XRD studies revealed that the deposited films showed hexagonal structure with most prominent reflection along (1 0 1) plane. Moreover, the peak intensity of (1 0 1) plane is found to be increased as the number of immersion cycles is increased. All the thin films look relatively smooth and homogeneous covering the entire surface area in FESEM image. Optical properties of the CdSe thin films for a different number of immersion cycles were studied, which indicates that the absorbance increases with the increase in the immersion cycles. Furthermore, the optical band-gap in conjunction with the electrical resistivity was found to get decreased with increase in the immersion cycles. A good correlation between the number of immersion cycles and the physical properties indicates a simple method to manipulate the CdSe material properties for optoelectronic applications.


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Introduction
Thin films of metal chalcogenides have been studied extensively in view of their potential industrial applications [1,2]. Notwithstanding to this, these materials are also important both academically as well as scientifically. In particular, Cadmium selenide (CdSe) is among the metal chalcogenide materials, which has remarkable optoelectronic properties that make it suitable for various applications in the field of low-cost optoelectronic devices such as solid-state solar cells [3,4], photoconductors [5], photoelectrochemical cells [6] and solar control coatings [7] etc. Major attention has been given in recent years to the investigation of electrical and optical properties of CdSe thin films in order to improve the performance of the devices and also for finding new applications [8]. CdSe is an n-type material with its band-gap lying in close range with the maximum theoretical range that is attainable for energy conversion efficiency.
Moreover, CdSe can be grown with either hexagonal, cubic or mixed (hexagonal-cubic) crystal structures. Accordingly, the optical band-gap can be defined for each structure, which could be suitable for different applications such as solar cells, thin film transistors, sensors, lasers, photoconductors, gamma ray detectors [9,10].
Till date, thin films of CdSe have been deposited by various techniques such as chemical bath deposition (CBD) [11,12], electrodeposition [13], cathodic electrodeposition [14], physical vapour deposition [15], spray pyrolysis [16], vacuum evaporation technique [17] etc. Preparation of thin films by a simple SILAR method is currently attracting considerable attention as it is simple, cost-effective and reproducible [18,19]. Importantly, with this method, one can avoid fast precipitation and the deposition can be done in a controlled manner, which is what rather difficult in other methods, especially, CBD. In concern to this, only a few reports are available on the deposition of CdSe thin films by SILAR method. SEM studies they showed that the thin films have a polycrystalline structure with preferential orientation along (0 0 2) plane and the crystalline and surface properties of the prepared films were improved by increasing film thickness. Therefore, from the aforementioned studies, it could be seen that the characteristic properties of the CdSe thin films and their applications in the field of optoelectronics are closely related to the crystallinity, orientation, grain size, optical band-gap and electrical resistivity that are affected by the film thickness, which will be controlled by varying the immersion cycles in the SILAR deposition method. And hence in this paper, we report the synthesis of CdSe thin film by SILAR technique. The effect of immersion cycles on the structural, morphological, optical and electrical properties of SILAR deposited CdSe thin films was studied.

Preparation of CdSe thin film
In this work, CdSe thin films were deposited on a glass substrate using SILAR method at room temperature and ambient conditions. To deposit CdSe thin films, 0.2M cadmium chloride (CdCl 2 .H 2 O) solution at pH ~ 8 and freshly prepared 0.1M sodium selenosulphite (Na 2 SeSO 3 ) at pH~11.3 were used as cationic and anionic precursor solutions, respectively. EDTA was used as a complexing agent and ammonia is used to control the pH of the cationic precursor solution to 8. Before, the actual deposition, the glass substrates were thoroughly washed with detergent & chromic acid, rinsed with acetone and finally ultrasonically cleaned with double distilled water.
The following procedure was adopted to deposit CdSe thin films, one SILAR growth cycle involving four steps (see Fig. 1  This completes one SILAR immersion cycle of CdSe deposition. The scheme for the deposition of CdSe films by SILAR method is represented in Fig. 1. Hence, several repeated immersion cycles can result in the required CdSe compound of desired thickness. The uniqueness of this SILAR method lies in the easy control of the parameters [18]. This allows one to properly control the thickness necessary for various device applications.

Characterization of the films
To investigate the effect of SILAR immersion cycles on the properties of the CdSe thin films, XRD, FESEM, optical absorption measurements and the two-point-probe methods were used.
The XRD pattern of the films were recorded on a Bruker AXS, Germany (D8 Advanced) diffractometer in the scanning range 2θ =20-80 o using Cu K α radiations with wavelength 1.5405 Ǻ. S-4800 Type-II (HITACHI HIGH TECHNOLOGY CORPORATION Tokyo, Japan) field emission scanning electron microscope (FESEM) with an energy dispersive spectrometer (EDS) attachment was used for the determination of morphology and elemental chemical composition of the sample. To study the optical characteristics of the films, absorbance spectra were recorded in the range 450-900 nm by means of JASCO UV-VIS spectrophotometer (V-630). The resistivity of the CdSe thin films was determined by the standard two-probe method.

Film thickness
In order to study the growth rate, SILAR coated CdSe thin films were deposited for various immersion cycles on glass substrates. For this particular study, we have deposited CdSe thin films with different immersion cycles i.e., 30, 40, 50 and 60 SILAR immersion cycles. Fig. 2 represents CdSe film thickness as a function of the immersion cycles for optimized concentrations of CdCl 2 and Na 2 SeSO 3 . It is found that the film thickness increases with the immersion cycles. The CdSe film has a maximum terminal thickness of the order of 370 nm at 60 SILAR immersion cycles. where K is constant (0.9), λ is the wavelength of X-ray, β is the full width at half of the peak maximum in radians and θ is Bragg's angle. It is observed that the crystallite size increases from 3.07 nm to 3.98 nm as immersion cycle increases from 30 to 60.

Structural analysis
Further, to have more information on the amount of defects in the synthesized thin films, the dislocation density (ߜ) was calculated from Williamson Smallman's formula as given below [24]: where 'n' is a factor, which when equal to unity gives the minimum dislocation density and 'D' is the average crystallite size.
The average microstrain developed in the prepared thin films is defined as disarrangement of lattice and was calculated by using the relation as given below [24]:

Morphological properties
The surface morphology of CdSe thin films was studied using FESEM.

Elemental analysis
The elemental analysis of CdSe thin films deposited on the glass substrate was performed using EDS analysis. The typical EDS spectra for the 60 SILAR immersion cycles deposited CdSe thin film is shown in Fig. 6. It is observed that the emission lines of 'Cd' and 'Se' are present in the EDS spectra indicating the formation of CdSe thin films. Fig. 7 shows the average atomic ratio of Cd/Se as a function of SILAR immersion cycles. It is observed that the 'Cd' and 'Se' ratio is found to be decreased (reaching 1.05) with the increase of immersion cycles, which indicates the stoichiometric CdSe formation. This is also in conjunction with the thickness measurements ( Fig. 2), where the thickness starts to saturate indicating the lowering of the CdSe compound on the thin film surface [25]. Fig. 8 shows the optical absorption spectra of CdSe thin films deposited with different SILAR immersion cycles. It can be observed that the absorption edge of the spectra shifts towards longer wavelength in the higher immersion cycles. Also, the absorbance was found to be increased with M A N U S C R I P T

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the increase in the immersion cycles. This might be due to the simultaneous increase in the thickness that is being observed.
The theory of optical absorption gives the relation between the absorption coefficient α and the photon energy hν, especially, for direct allowed transition as, where hν is the photon energy, E g is the optical band-gap, A is a constant.
A typical plot of (αhν) 2 versus hν for 30 SILAR immersion cycles deposited CdSe thin films is as shown in Fig. 9 (a). The linear fit of the plot indicates the existence of the allowed direct band-gap transition. The band gap was found within the range 1.79-1.88 eV for CdSe thin film.
These band-gap values were in good agreement with the earlier reported values of band-gap for CdSe nanocrystalline thin films deposited by CBD technique [11]. The direct band-gap of CdSe thin films deposited with various SILAR immersion cycles is determined and is shown in the Fig. 9 (b). It is obvious from the results that the optical band-gap decreases with the increase in the SILAR immersion cycles, which may be due to the quantum size effect, improvement of the crystallization and variation in the stoichiometry of the film.

Electrical properties
The

Conclusion
CdSe thin films were deposited successfully using SILAR technique with different immersion cycles. From XRD studies, it is confirmed that obtained films have a hexagonal phase with M A N U S C R I P T

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(1 0 1) as preferential orientation and films are nanocrystalline in nature, which is in corroboration with the FESEM data. The optical band-gap, as well as electrical resistivity decreases with the increase of immersion cycles indicating that the thin films can be easily tailored by simply SILAR immersion cycles.

Acknowledgment
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