Solid State Sciences 11 (2009) 1226–1228
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CdS thin ﬁlm: Synthesis and characterization P.P. Hankare a, P.A. Chate b, *, D.J. Sathe c a
Department of Chemistry, Shivaji University, Kolhapur (M.S.), India Department of Chemistry, J.S.M. College, Alibag (M.S.), India c Department of Chemistry, KIT’s college of Engineering, Kolhapur (M.S.), India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 January 2009 Received in revised form 19 March 2009 Accepted 23 March 2009 Available online 2 April 2009
CdS thin ﬁlms have been deposited by dip technique using succinic acid as a complexing agent. The structural characterizations of ﬁlms have been studied by X-ray diffraction. X-ray diffraction pattern prove crystallinity of the deposited ﬁlms that crystallize in the cubic phase of CdS. The ﬁlms show high absorption and band gap value which were found to be 2.58 eV. The speciﬁc conductivity of the ﬁlm was found to be in the order of 107 (U cm)1. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Semiconductor Thin ﬁlms Chemical synthesis X-ray diffraction
1. Introduction The special optoelectronic properties of II–VI semiconductors make basic physics and device physics interesting [1,2]. Cadmium sulﬁde (CdS), due to its wide band gap, photoconductivity, and high electron afﬁnity, is known to be an excellent heterojunction partner for p-type cadmium telluride, p-type copper indium diselenide. It has been widely used as a window material in high efﬁciency thin ﬁlm solar cells based on cadmium telluride or copper indium diselenide [3–5]. CdS is a candidate semiconducting material for printed electronics . It is an interesting crystal material in the area of photodetectors, semiconductor lasers, and nonlinear integrated optical devices. The importance of wide band gap materials is related to the possibility of fabricating light emitting diodes or laser heterostructures for emission in the visible spectral range. These devices are important for many applications. For example, they are useful in medical diagnosis and for fabricating red–green– blue display [7–10]. CdS thin ﬁlms can be deposited by different deposition techniques such as chemical bath deposition, vacuum evaporation, spray pyrolysis, thermal evaporation, chemical vapor deposition, metal organic vapor-phase epitaxy, close space vapor transport, photochemical deposition, radio frequency sputtering, vapor transport deposition, screen printing, electro deposition, pulsed laser deposition, etc. [11–19]. Several ligands have been
* Tel.: þ91 0231 2692258. E-mail address: [email protected]
(P.A. Chate). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.03.016
utilized in the deposition of CdS, such as NH3 [20–23], triethanolamine , ethylenediamine , ethylenediaminetetraacetic acid , nitrilotriacetic acid [27,28], cyano-complex , citratocomplex , and more recently tartaric acid . In the present work, we ﬁrst time reported the deposition of good quality CdS thin ﬁlm by a new approach using dip method at room temperature. For better cell efﬁciency, the junction of a solar cell should be capable of depositing at low temperature to avoid grain boundary diffusion. Moreover, the depletion should be in bulk of host material. Low temperature is also beneﬁcial for minimizing interface diffusion between ﬁlms and substrate. The ﬁlm has been characterized by X-ray diffraction, optical and electrical properties. 2. Experimental details All the chemicals used for the deposition were of analytical grade. It includes cadmium sulfate, succinic acid, liquor ammonia, and thiourea. All the solutions were prepared in double distilled water. In actual experimentation, 10 mL (0.2 M) cadmium sulfate solution was taken in 100 mL beaker. 2.5 mL (1 M) succinic acid, 25 mL (2.8 M) ammonia and 10 mL (0.2 M) thiourea were added in the same reaction bath. The pH of the reactive mixture is 10.06. The temperature of the bath was maintained at 278 K using ice bath. Individual solutions were cooled at 278 K and mixed to avoid precipitation. The solution was stirred vigorously before dipping non-conducting glass substrates. The substrate was kept vertically slightly tilted in a reactive bath. The temperature of the bath was then allowed to increase up to 298 K very slowly. After 5 h, the
P.P. Hankare et al. / Solid State Sciences 11 (2009) 1226–1228
Cd2D D Succinicacid / ½Cd succinate SCðNH2 Þ2 D 2OH / S2L D H2 NCN D 2H2 O
Thickness = 0.58μm Absorbance
slides were removed and washed several times with double distilled water. The ﬁlm was dried naturally preserved in dark desiccators over anhydrous CaCl2. The ﬁlm was deposited on both sides of slides. Overall chemical reaction is as follows
½Cd succinate D S2L / CdS D ½succinate2L As-deposited samples were used to investigate their structural, optical and electrical properties. Crystallographic studies of cadmium sulphide thin ﬁlm were characterized by using a Phillips PW-1710 X-ray diffractometer with Cu Ka1 line (l ¼ 1.54056 Å) in 2q range from 10 to 80 . The optical properties were studied by taking absorption spectra of ﬁlms using a Hitachi-330 (Japan) double beam spectrophotometer in range from 300 to 800 nm at room temperature. The dark electrical conductivity of ‘as-deposited’ CdS ﬁlm on non-conducting glass slide was determined by using a ‘dc’ two probe method, in the temperature range 300– 525 K. The thickness of the ﬁlms has been determined by weight difference density method. It was found to be 0.58 mm. 3. Results and discussion It has been reported that CdS may have either cubic or hexagonal structure, depending on the synthesis conditions such as deposition temperature and precursor concentration. The X-ray diffraction pattern of ‘as-deposited’ CdS ﬁlm is shown in Fig. 1. A comparison between our data with standard data from JCPDS shows that CdS ﬁlms obtained in this study have the cubic structure . Three major diffraction peaks were observed. The X-ray diffraction pattern shows the highest reﬂection peak at d ¼ 3.360 Å (111). The diffraction angles were (26.50) (111), (44.43) (220) and (52.16) (311). The lattice parameter of cubic phase was calculated by using a standard formula. The lattice parameter was found to be 5.795 0.01 Å. The crystallite size of CdS thin ﬁlms was calculated by using Scherrer’s formula. The average crystallite size was calculated by resolving the highest intense peak. It was found to be 256 2 Å. The optical absorption spectrum of the as-deposited thin ﬁlm onto glass substrate was studied at room temperature in the
Wavelength (nm) Fig. 2. Absorption spectrum of as-deposited CdS thin ﬁlm.
wavelength 300–800 nm. Fig. 2 shows variation of optical absorption with wavelength. The optical studies show that the ﬁlms are highly absorptive. The absorptivity was found in the range of 104 cm1. Based on obtained optical absorbance, the square of absorption co-efﬁcient (a2) is plotted as a function of photon energy (hn) in Fig. 3. It can be seen that the ﬁlms have a steep optical absorption feature, indicating good homogeneity in the shape and size of the grains and lower defect density near the band edge. As can be seen, a2 varies almost linearly with hn above band gap energy. According to the following equation for direct inter-band transition can be applied ;
a2 ¼ Aðhn EgÞ where A is a constant. The band gap energy is obtained by extrapolating the straight line portion of the curve to zero absorption co-efﬁcient. The band gap value of the as-deposited CdS was found to be 2.58 eV. The observed value is greater than standard band gap (2.48 eV) of the CdS material,  showing a ‘blue shift’ of 0.10 eV. This is due to localization of electrons and holes in conﬁned volume of the semiconductor materials. At room temperature the speciﬁc conductance was found to be in the order of 107 (U cm)1, which agrees well with the earlier reported value . The low value of conductivity may be due to low crystallinity and small thickness of the ﬁlm. The electrical properties of polycrystalline thin ﬁlms are mainly dependent upon their structural characteristics and composition . It is observed
(α)2 x 108 cm-2
Two Theta (deg) Fig. 1. XRD pattern of as-deposited CdS thin ﬁlm.
Photon energy (eV) Fig. 3. Plot of (a)2 with respect to photon energy.
P.P. Hankare et al. / Solid State Sciences 11 (2009) 1226–1228
-2 (a)-heating (b)-cooling -4
1000/T (K-1) Fig. 4. The variations of log (conductivity) with inverse temperature.
that the conductivity on the ﬁlm increases with increase in temperature. This indicates the semiconducting behavior of the thin ﬁlm. The electrical conductivity variation with temperature during heating and cooling cycles was found to be different and this shows that the ‘as-deposited’ ﬁlms undergo an irreversible change due to annealing out of non-equilibrium defects during ﬁrst heating. A plot of log (conductivity) versus inverse absolute temperature for the cooling and heating curve is shown in Fig. 4. A plot shows that electrical conductivity has two linear regions, an intrinsic region setting at low temperature, characterized by small slope (300–350 K). High temperature region is associated with extrinsic conduction due to the presence of donor states. The activation energy is calculated using exponential form of Arrhenius equation. The activation energies are 0.065 and 0.62 eV for low and high temperature regions, respectively.
4. Conclusion Cadmium sulphide thin ﬁlm can be deposited easily by a room temperature dip type method. XRD indicates that ﬁlm exists in cubic phase. Optical study shows that the ﬁlms have high absorption co-efﬁcient. Temperature dependent electrical conductivity showed the semiconducting nature of the ﬁlm.
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