znxcd1-xs (x=0-1) thin films prepared by nano colloidal

The 2012 World Congress on Advances in Civil, Environmental, and Materials Research (ACEM’ 12)Seoul, Korea, August 26-30, 2012

ZnxCd1-xS (x=0-1) Thin Films Prepared by Nano Colloidal Suspension – Optical and Structural Analyses

Samia Hussain1), Saira Riaz1,2)* and Shahzad Naseem1)

1) Centre of Excellence in Solid State Physics, Punjab University, Lahore 54590, Pakistan

2) [email protected]

ABSTRACT

ZnxCd1-xS (x=0-1) nanocrystal semiconductor with a direct band gap is synthesized by following sol-gel technique for its potential application in photovoltaics. Research grade Zn(NO3)2.6H2O, Cd(NO3)2.4H2O and Na2S.9H2O were used as zinc, cadmium and sulphur sources. Organic solvents such as oleylamine and some alcohols were used during synthesis. ZnxCd1-xS solution was stirred mostly at 60oC for 30 minutes. Transparent sol was obtained after aging of 24 hours at room temperature. Transparent dark yellowish ZnxCd1-xS sol changed to light yellowish color by increasing the Zn content. Thin films with thickness of 80 - 200nm were deposited on to glass substrates by spinning the sol at different speeds for various time intervals. Effects of solvents, preparation conditions and compositions on the structure and optical properties of ZnxCd1−xS have been investigated by using X-ray Diffractometer (XRD) and spectroscopic ellipsometer. ZnxCd1−xS thin films comprised of nanostructures showing variation in absorption and electrical properties as a function of x as reported earlier. The direct band gap varies from 2.42eV to 3.11eV for x=0 to x=1 respectively. These results are comparable to those obtained for ZnxCd1-xS (x=0-1) thin films prepared by thermal evaporation technique. Decrease in lattice constant values whereas increase in optical band gap was observed by increasing the Zn content. Sol-gel technique used in this research work for the preparation of ZnxCd1-xS (x=0-1) proved to be low-cost application oriented technique for synthesis of photovoltaics related materials.

1. INTRODUCTION

Due to unique size dependent structural, electrical and optical properties, semiconductor nano-crystals recently have drawn much attention. Researchers are taking special interest in II-VI chalcogenide compound semiconductors nano crystals (SNCs) because they show potential applications in solar cells (Zia 2010), optical sensing (Jaiswal 2003), light emitting diodes (Jang 2003) and biological labeling (Jr 1998) etc. Reduction of size in nano science results in three dimensional confinement of large charge carriers in bulk state and correspondingly bulk energy bands transform to discrete energy states and hence band gap can be tuned to desired energy level by keeping the control on particle size during the synthesis of semiconductor nano-crystal. Tuning of physical and chemical properties by changing the particle size has also some disadvantage in many application because they can cause thermal / electrical instability

problems when ≤ 2nm (Li 2005). Further, due to pinching of size it is difficult to achieve high color transparency for binary SNCs. Very recently research on tunable emission spectra of nano-crystals by changing their constituent stochiometries in mixed ternary nano-crystals have been reported (Zhong 2003, Li 2005). Among II-VI SNCs zinc and cadmium have suitable nano-crystals for photovoltaic applications as they have direct band gap. CdS is most commonly studied material to be used as a window layer. The band gap of CdS can be increased by the addition of ZnS. ZnxCd1-xS alloy compound has great interest because the band gap can be tuned and lattice parameters can be varied (Zia 2010, Oladeji 2000, Kumar 2004). Yunchao et al (2005) reported the formation of tunable ZnxCd1-xS alloyed SNCs by a new approach relatively at low temperature and show composition dependent structural and optical properties. Hence it is a promising candidate for use as light emitting materials in nano devices.

ZnxCd1-xS thin films have been prepared previously by vacuum evaporation (Zia 2010), PLD (Singhal 2009), chemical bath deposition (Khare 2009), spray pyrolysis (Ilican 2007), thermal evaporation (Sebastian 2004), dip-coating method (Rafea 2009), molecular beam epitaxial growth (Telfar 2000), screen printing method (Kumar 1998) and e-beam evaporation (Mughal 1996). However, a very few reports are available on the formation of ZnxCd1-xdS thin films by sol-gel method. Moreover, these reports are about the preparation of nanoparticles by chemical route (Li 2005) rather than preparing a thin film. Sol-gel is a very simple process; in a single step and at very low temperature, as compared to the other chemical techniques, this process leads to the transformation of a molecule in a material that is ready for shaping. In this work thin films of ZnxCd1-xS (x=0-1) are prepared by simple and cost effective sol-gel method. Thin films of different thicknesses are prepared by varying the spinning speed and time. Optical and structural properties are presented as a function of composition and thickness. Objective of this work is to make highly efficient and cost effective thin films of ZnxCd1-xS for use in solar cells as a window layer.

2. EXPERIMENTAL 2.1 Synthesis of ZnxCd1-xS sol The sol used for the deposition of thin films was prepared by using

Zn(NO3)2.6H2O,Cd(NO3)2.4H2O and Na2S.9H2O as starting materials and ethanol as solvent. 30 ml of 0.1M solution of Na2S was introduced into Cd(NO3)2.4H2O solution to form CdS. Zn(NO3).6H2O solution was added drop wise to the as-prepared CdS solution to form ZnxCd1-xS, as shown in Fig. 1. Ten different composition were synthesized by varying x= 0-1 for fabrication of films. However, only x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0, compositions are presented in this paper.

Fig.1 synthesis of ZnxCd1-xS (x=0-1) sol

2.2 Preparation of ZnxCd1-xS Thin Films Films were deposited by Suss MicroTec delta 6RC spin coater on glass substrate.

The glass substrates were first thoroughly washed in de-ionized water and then successively degreased using acetone and isopropyl alcohol, while the beaker containing the slides was agitated in ultrasonic bath. The substrates were air dried before loading in the spin coater. Post deposited samples were than annealed in hot spot furnace for 1 hour at 300℃.

2.3 Characterization of ZnxCd1-xS Thin Films Crystallographic structure along with the phase variation and crystallite size of ZnxCd1-xS thin films was studied by Rigaku D-MAX/IIA X-ray diffractometer (XRD). CuKα (Ni filtered) radiations (λ=1.5405 Å) were used to obtain the XRD pattern. Optical properties in the UV-VIS-IR range were obtained by using JA Woollam’s variable angle spectroscopic ellipsometer (VASE).

3. RESULTS AND DISCUSSION 3.1 Optical Characterization of ZnxCd1-xS Thin Films Optical transmission of ZnxCd1-xS thin films was measured by spectroscopic

ellipsometer and is given in Figs. 1 – 6. Transmittance increases as Zn content increases and the values of the absorption edge shift towards shorter wavelengths with increasing Zn content. This might happen due to 2 major effects: 1) Increase in band gap of solid solution of ZnxCd1-xS for higher Zn content and 2) as Zn content increases, particle size decreases, resulting in higher band gap due to domination of quantum size effect in nano crystals. These results are in agreement with literature data (Khare 2009, Zia 2010, Ghoneim 2010, Zhigang 2010).

Fig. 1 Transmission plots of ZnxCd1-xS thin films; x=0

Fig. 2 Transmission plots of ZnxCd1-xS thin films; x=0.2

Rapid increase in transmission indicates the direct band gap of films whereas the oscillations of the transmittance are caused by the optical interference, indicating the smoothness of films surfaces (Naseem 2000). Figs. 1-6 show that transmission varies with thickness of films and transmission increases with increases Zn content except for x=0.2. For x = 0, 0.2, 0.4 ,0.6, 0.8, 1.0, the values of (α)2 are plotted as a function of hע. The values of the band gap are estimated by a solid line which represents the best square fit of the data points as shown in Fig. 7. Values of the band gap for CdS (x=0) films and ZnS(X=1) films are found to be 2.41eV and 3.17eV respectively and this is in good agreement with the ones reported in literature (Patidar 2008).

Fig. 7 α2 versus (hע) plots for determination of band gap of ZnxCd1-xS thin films

Variation in optical band gap of direct transition with Zn concentration ‘x’ is found to be almost linear and is found to be increase continuously with increasing x as shown in Fig. 7 above and this is in agreement with literature (Zia 2010, Zhigang 2010, Patidar 2008, Khare 2009). However, for x=0.8 and 1.0 the increase in absorption coefficient

values is not as sharp as for rest of the composition. This might be due to the presence of indirect band gap along with the direct gap transition and it needs further investigation.

Figs. 8 - 10 show refractive index (n) graphs with respect to wave length range from 200nm to 1100nm for x=0,0.4 and 1. For x=0 the maximum value of refractive index is approximately 2.1 and for x=0.4 it is found to be 1.84 and for x=1 it is approximately 2.0. The slight difference in n values from literature (Zia 2010, Zhigang 2010) might be because of the different preparation technique and solvent effect.

Fig. 8 Refractive index (n) for ZnxCd1-xS thin films; x=0

Fig. 9 Refractive index (n) for ZnxCd1-xS thin films; x=0.4

Fig. 10 Refractive index (n) for ZnxCd1-xS thin films; x=1.0

Figs. 11-13 show oscillatory behavior of extinction coefficient (k) for x=0, 0.4 and 1.0. The light absorbed by thin films is apparent from the rise and fall in the extinction coefficient. In case of thin films the light is absorbed at the boundaries of grain of polycrystalline material (Zia 2010).

Fig. 11 Extinction coefficient (k) for ZnxCd1-xS; x=0

Fig. 12 Extinction coefficient (k) for ZnxCd1-xS; x=0.4

Fig. 13 Extinction coefficient (k) for ZnxCd1-xS; x=1.0

3.2 Structural Properties of ZnXCd1-xS Thin Films Structural properties of sol-gel prepared ZnxCd1-xS thin films were measured by

RIGAKU D-MAX II-A XRD system by using CuKα radiations and are shown in Fig. 14. d values were calculated by measuring θ from X-ray peaks by using Bragg’s relation 2dsinθ=nλ, where λ =1.5045 Å for CuKα X-rays. For x=0 (CdS) we observed a prevailing peak at 33.5˚ of 2θ value. According to JPCDS data card number (6-0314) this is in consistent with a highly crystalline film with perfect orientation in (0 0 2) direction representing the hexagonal structure. For x=1, i.e. pure ZnS, dominant peak occurred at 36.2� 2θ value according to JCPDS card number (12-688) and was identified as reflection from (1 0 0) plane of ZnS. By increasing Zn content in CdS sol upto 80% (Zn0.8Cd0.2S) films showed similar structure to that of CdS hexagonal structure. The main peaks of all composition of ZnxCd1-xS (x=0-1) films appear between 33.5�-36.2� with slight increase in 2θ˚ values. Shifting of peak towards higher 2θ values by increasing Zn concentration (X) indicates a decrease in (0 0 2) lattice constant. This shifts to higher angles is because of mixing of Zn ions into CdS lattice and larger ionic radii of Cd+2 in comparison to Zn+2, Further, by increasing Zn content ZnxCd1-xS films moves toward higher crystallinity as indicated by increasing peak intensity.

Fig. 14 XRD plots of ZnxCd1-xS thin films

FWHM of the dominating ZnxCd1-xS peak is plotted in Fig. 15(a). Crystallite size is inversely related to the FWHM of the individual peak i.e. the more narrow the peak the larger the grain size. Crystallite size is found to be in range of 10 – 29.7nm, calculated by Scherer equation stated below and plotted in Fig. 15(b).

D = K /Bcosθ

Where K is a constant that is equal to 0.94, lambda is the X-ray wavelength = 1.5405Å and B is the full width at half maximum (FWHM) of the XRD selected diffraction peak on the 2 scale, is the diffraction angle.

Fig. 15 (a) FWHM and (b) crystallite size of ZnxCd1-xS thin films

CONCLUSION

We have prepared ZnxCd1-xS thin films using low cost sol-gel method for various compositions. Sol-gel prepared thin films showed high transmission and low absorption in the visible and infrared regions. Band gap increased with increasing Zn concentration for ZnxCd1-xS thin films. The band gap values of thin films lie in the range of 2.42-3.1 eV. XRD results reveal that binary compound CdS is completely transferred into ternary compound by adding Zn content in CdS sol under different conditions. Variation in optical constants especially in the extinction coefficient (k) with respect to wavelength is oscillatory.

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znxcd1-xs (x=0-1) thin films prepared by nano colloidal

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