- Email: [email protected]
Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD
Materials Chemistry and Physics xxx (2015) 1e6
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD E.A. Sanchez-Ramirez a, M.A. Hernandez-Perez a, *, J.R. Aguilar-Hernandez b, G. Contreras-Puente b a b
Escuela Superior de Ingeniería Química e Industrias Extractivas, Instituto Polit ecnico Nacional, CP 07738 M exico D.F., Mexico ticas, Instituto Polit Escuela Superior de Física y Matema ecnico Nacional, CP 07738 M exico D.F., Mexico
h i g h l i g h t s CdS1xSex ternary alloy thin films with x ¼ 0.5 ± 0.05 can be grown by CBD at 75 C. CdS1xSex nanocrystals are well arranged in a “worm” structure from 30 min and x 0.25. The Eg of (002) oriented hexagonal film is strongly affected by x and crystal size. Films with x ¼ 0.5 are obtained from 30 min using a Cd:S:Se ¼ 0.76:0.5:0.6 bath ratio. Consumption rate has the same behavior that growth rate, changing around 60 min.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 November 2014 Received in revised form 11 August 2015 Accepted 4 September 2015 Available online xxx
Chemical Bath Deposition (CBD) was used to grow CdS1xfSexf (xf ¼ 0.5) thin films on Corning glass substrates at 75 C. The atomic composition of the bath was varied until an xf of 0.5 was obtained, maintaining the deposition time at 120 min. Then the deposition time was modified from 5 to 360 min. The structural and optical properties of the films were analyzed by Scanning Electron Microscopy, Energy Dispersive Spectroscopy, X-Ray Diffraction, UVeVis Spectroscopy, Profilometry and Inductive Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). A bath atomic composition of Cd:S:Se equal to 0.76:0.55:0.45 was employed to obtain a film of xf ¼ 0.5. The films are constituted by clusters of semispherical nanoparticles (fav ¼ 15 nm), which are well-arranged in a “nanoworm” structure. The nucleation time of the particles is lower than 5 min. All the films are polycrystalline with hexagonal phase and preferentially orientated on the (002) plane. The crystal size (11e6 nm) and the band gap (2.17 e1.99 eV) decrease with the content of Se and remain constant with the deposition time. The composition xf ¼ 0.5 is achieved at different times to the heterogeneous (60 min) and homogeneous reactions (15 min). The kinetics of deposition and the consumption rate of Se change in a similar way, reaching the stability after 60 min. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Semiconductors Chemical synthesis Optical properties
1. Introduction Cadmium Selenosulphide (CdS1xSex) belongs to the IIeVI semiconductor group, it can be employed in a wide range of applications, especially in optoelectronic, photovoltaic and light emitting devices, solar cells and field effect transistors [1e4]. The interest is focused on the modulation of the properties by
xico D.F., Mexico. * Corresponding author. ESIQIE-IPN, CP 07738 Me E-mail addresses: [email protected], [email protected] (M.A. Hernandez-Perez).
modifying the composition x [4e8]. Ternary semiconductor films like CdSSe, CdZnS and ZnSSe, can be prepared by several techniques such as sputtering [1], screen printing [3], spray pyrolysis [4], laser ablation [9,10], and chemical bath deposition (CBD) [11,12]. CBD allows the control of the films properties by modifying the experimental parameters such as type and concentration of the reactants, temperature, pH and deposition time. In CBD two reactions occur simultaneously; the homogeneous reaction which takes place in the bulk solution and produces powders of CdS1xpSexp, and the heterogeneous reaction that results in a CdS1xfSexf film deposited on the surface of the substrate. The film is formed due to
http://dx.doi.org/10.1016/j.matchemphys.2015.09.005 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: E.A. Sanchez-Ramirez, et al., Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/ j.matchemphys.2015.09.005
2
E.A. Sanchez-Ramirez et al. / Materials Chemistry and Physics xxx (2015) 1e6
the slow release of the ions, from the anions hydrolysis and cationic complex (frequently with NH3), on the substrate surface [5e7]. Concerning ternary semiconductors, the films atomic composition is the result of the combined effect of the binary chalcogenides physical properties (mainly the solubility product, Ksp) and the kinetic factors of the entire reaction system (homogeneous and heterogeneous reactions) [13]. Notable achievements have been reported in the synthesis and characterization of CdS1xfSexf thin films, most of them focused on the improvement of optical, electrical and structural properties and on their technological application. However, the understanding of compositional properties and the kinetic factors is in an early stage. In this work, we report the deposition of CdS1xfSexf (xf ¼ 0.5) films at 75 C, the atomic composition (xf) equal 0.5 was fixed by varying the atomic composition of Cd, S and Se (xb) reactants in the bath. The influence of the deposition time on structural, optical and compositional properties is analyzed. Special attention was focused on the relationship between the growth rate of the films and the reactants consumption rate.
2.2. Film characterization The structural properties were analyzed by X-ray diffraction (XRD), the spectra were recorded in a Bruker Advanced diffractometer (Cu Ka ¼ 1.5406 Å) operating in grazing angle (0.3e2 ) configuration. The surface morphology of the films was analyzed with a high resolution SEM JEOL JSM6701F equipped with an Energy Dispersive Spectroscopy (EDS) module. Optical absorption studies were performed using a PerkineElmer UVeVis spectrometer Lambda35. The thickness was measured with a Sloan Dektak Profilometer. A Perkin Elmer ICP-OES Optima 8300 was used in order to evaluate the depletion of the reactants in the bath in function of the deposition time. For that, an aliquot of 1 mL was taken from the reactor at different time; the consumption rate was calculated considering the change of the volume. 3. Results and discussion 3.1. Setting the atomic composition and properties of the films grown during 120 min
2. Experimental details 2.1. Deposition CdS1xfSexf thin films were grown at 75 C onto glass substrates employing CBD method. Solutions of CdCl2 (0.1 M), CS(NH2)2 (0.1 M) and Na2SeSO3 (0.1 M) were used as reactants, all of them were prepared from analytical grade reagents. Na2SeSO3 (0.1 M) was prepared following the procedure reported by [14]. In order to promote the homogeneity of the films, the solutions were heated in separated beakers and mixed when they reached 72 C. The alkaline pH of the bath was maintained between 10 and 11 by the addition of NH4OH (4 M) and NH4Cl (0.2 M). At the end of each experiment, the precipitated powder and the deposited film were rinsed with deionized water and dried at room temperature. The adjustment of xf was performed in two stages, as presented in Table 1, the deposition time was maintained constant at 120 min. At the beginning, Se and S atomic compositions in the bath were modified until the xf closest to 0.5 was obtained, and the atomic composition of Cd kept constant. In the second stage, the atomic composition of Cd was diminished and Se and S atomic compositions were constant. Subsequently, deposition time was varied from 5 to 360 min in order to analyze its influence on the atomic composition of the films and powders, on the thickness and growth rate of the films, as well as on the depletion of the reactants. For these experiments, the employed bath composition was that obtained from the set values.
Fig. 1 shows the effect of the bath atomic composition (xb) in the atomic composition of the films (xf) when the atomic composition of Cd in the bath was constant, the SEM micrographs show the surface morphology of some films. For the studied range, the relationship seems to be linear. When the atomic composition of Se in the bath is increased, the quantity of Se incorporated to the film also increases. The film prepared with the stoichiometric reactants composition Cd:S:Se ¼ 1:0.5:0.5 (circled point), had an elemental atomic composition of Cd:S:Se ¼ 1.24:0.68:0.32. This means that the precipitation of Se is slower than that of S resultant in a film with a deficit of Se (xf ¼ 0.32) and S in excess (1xf ¼ 0.68). Therefore, it was necessary to use an excess of Se in the bath solution in order to enhance its incorporation to the film. According to the plot, xf ¼ 0.5 can be obtained using and xb of 0.58. Table 1 is the summary of the studied bath atomic compositions and the resulting film atomic compositions. Although the employed Cd quantity was constant, the films contain an excess of Cd that vary randomly. The atomic composition of Cd was estimated in basis to the total atomic composition of chalcogen ions (SeþS ¼ xf þ 1xf). In general, semispherical clusters of particles form the films, but flakes-
Table 1 Cd, S and Se atomic composition of the bath and films. Bath
First Stage
Second Stage
Films
Cd
S 1xb
Se xb
Cd
S 1xf
Se xf
1 1 1 1 1 1 1 0.86 0.76
0.50 0.47 0.45 0.40 0.37 0.33 0.45 0.45 0.45
0.50 0.53 0.55 0.60 0.63 0.67 0.55 0.55 0.55
1.24 1.48 1.21 1.38 1.33 1.15 1.21 1.47 1.1
0.68 0.62 0.55 0.46 0.39 0.33 0.55 0.57 0.51
0.32 0.38 0.45 0.54 0.61 0.67 0.45 0.43 0.49
0.66
0.45
0.55
0.85
0.5
0.5
Gray shadow indicates the best bath atomic composition to reach xf ¼ 0.5 and the Cd composition closest to 1.
Fig. 1. Effect of the bath atomic composition (xb) on the atomic composition of the films (xf) grown at 75 C during 120 min.
Please cite this article in press as: E.A. Sanchez-Ramirez, et al., Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/ j.matchemphys.2015.09.005
E.A. Sanchez-Ramirez et al. / Materials Chemistry and Physics xxx (2015) 1e6
like structures appear because of the excess of Cd as reported by Mane et al. [15]. Due to these results, the atomic composition of Cd in the bath was reduced in the second stage as presented in Table 1. Even though the Cd excess in the films is reduced, a clear relationship with the quantity of Cd in the bath is not observed. The xf ¼ 0.5 ± 0.02 can be obtained using an atomic composition of Cd:S:Se in the bath equal to 0.76:0.45:0.55. Fig. 2 shows the XRD spectra of CdS1xfSexf films with different atomic composition. The films are polycrystalline, the peaks corresponding to (100), (002), (110) and (112) planes of the hexagonal phase are observed, according to JCPDS files No. 491459, 893682, 500720 and 500721. The peaks are shifted to lower degrees when xf increases as a result of the bigger ionic radius of Se and its incorporation to the solid solution [16]. Preferential orientation on (002) plane is observed as reported in literature [7,14]. The films show other peaks which, according to the JCPDS data, could be related to different Cd compounds such as CdCN2 (360657) [8], CdSeO3 (82e1208), CdC2O4 (14e0712) and CdCO3 (850989). The quantity of these impurities seems to be associated with the Cd excess since the film with the lowest Cd atomic composition is free of impurities. The band gap value (Eg) was estimated from optical absorption measurement using Tauc's relation: ahy ¼ Aðhy Eg Þn , where a is the absorption coefficient, A is a constant and n ¼ 1/2 for direct band gap semiconductors. Extrapolation to a ¼ 0 in the (ahy)2 vs hy plot (Fig. 3a) gives the value of Eg [3e8,14]. A redshift of the Eg is observed when xf increases, however Eg of the films with xf from 0.59 to 0.67 remains almost constant. The optical properties of the films are not affected by the presence of impurities. The relationship between the crystal size and the band gap as a function of the films atomic composition is plotted in Fig. 3b. The relationship between Eg and xf seems to be linear. The theoretical Eg of CdS0.5Se0.5 material (circled point) was calculated as 2.06 eV using Vegard's Law: Eg CdSð1xÞ Sex ¼ ð1 xÞEg CdS þ xEg CdSe bxð1 xÞ where x is the Se atomic composition, ð1 xÞEg CdS and xEg CdSe are the individual contributions of CdS and CdSe to the band gap and b is the bowing factor (b ¼ 0 for linear relation). The crystal size was calculated according to Scherrer's equation using the FWHM of (002) plane. In general, crystal size decreases from 11 to 5 nm as a function of the atomic composition, in a nonlinear relationship. Similar and bigger crystal size has been reported for CdS1xSex films prepared by different techniques [3,9,17]. The experimental Eg of the film with xf ¼ 0.49 is bigger than that of the theoretical value,
Fig. 2. XRD spectra of CdS1xfSexf films with different atomic composition deposited at 75 C during 120 min.
3
this difference can be attributed to the small crystal size and its quantum effect [18e20]. The crystal size seems to affect also the Eg of the films with xf 0.59; for which a decrease is expected, however it remains almost constant as well as crystal size does. 3.2. Effect of the deposition time The effect of the deposition time on the properties of the films was studied from 5 to 360 min maintaining constant the bath atomic composition of Cd:S:Se ¼ 0.76:0.45:55. Fig. 4 shows the XRD patterns of CdS1xfSexf films deposited at different times, they were normalized in order to improve the comparison. The patterns of the films grown at 5 and 15 min show a strong contribution of the amorphous substrate, around 15e30 , this is attributed to the small thickness (20e40 ± 5 nm) and to a low degree of crystallinity. At longer deposition time, the characteristic peaks of hexagonal structure of CdS1xSex are observed: (100), (002), (110) and (200) planes (JCPDS 491459, 491460 and 893682). The films have preferential orientation on (002) plane as reported previously [3,7,9]. As the deposition time increases the crystalline quality is improved and the noise signal of the XRD patterns becomes better. For the films grown at deposition times 60 min it is observed a slight shift of the peaks to lower degrees which could be related with a small change of the atomic composition. However, the films prepared at higher deposition time do not show a perceivable change on the position of the peaks, suggesting an atomic composition almost constant. The films deposited from 5 to 60 min present impurities related with an excess of Cd. The crystal size remains almost constant with the deposition time; the estimated values (only for the films with well-defined peaks) were 7 ± 2 nm. The Eg does not change significantly (2.13 ± 0.03 eV, results not shown), whereby it can be assumed that Eg of the films deposited at different times is dependent on the slight variation of the crystal size and small change of the atomic composition observed, from the position of the peaks in XRD patterns. The morphology is affected by the deposition time which can be seen in the high resolution SEM images presented in Fig. 5. The density of the films increases with time as well as the size of the nanostructured clusters from 100 to 600 nm. Fast nucleation happens since the beginning of the reaction producing small nuclei which grow reaching an average size of 150 nm in diameter as observed in the film deposited at 5 min [20]. As the film becomes thicker, the nanoparticles that constitute the clusters are arranged in some kind of “nanoworm” structures. The nanoparticles size (~15 nm) is of the same order of magnitude than crystal size calculated from XRD results. In particular, the films deposited at 30 and 60 min present two morphologies, the semispherical clusters and the flakes-like structure observed for films with Cd excess [15]. In order to quantify the atomic composition of Cd, S and Se of the films and powders (obtained from the same experiment), the samples prepared at different deposition time were analyzed by EDS, the results are plotted in Fig. 6. The measurement error is higher for the films deposited at low deposition time because of their small thickness. The quantity of Se increases whereas that of S decreases until an equilibrium composition is reached. In the case of the heterogeneous reaction (film) the atomic composition seems to reach the equilibrium state around 60 min but for the homogeneous reaction (powder) the composition remains almost constant at 15 min. This could indicate that the homogeneous is faster than heterogeneous reaction, because of the high number of collisions between the ions. According to the plot, the growth of the films begins with the formation of a solid solution with a small atomic composition of Se (xf ¼ 0.2 ± 0.1) and subsequently some S2 are substituted by Se2 ions [5,6]. Therefore, the chalcogenide ions incorporate to the film at different rate. This behavior can be
Please cite this article in press as: E.A. Sanchez-Ramirez, et al., Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/ j.matchemphys.2015.09.005
4
E.A. Sanchez-Ramirez et al. / Materials Chemistry and Physics xxx (2015) 1e6
Fig. 3. a) Plot of (ahy)2 vs (hy) and b) variation of crystal size and Eg with the atomic composition of the CdS1xfSexf films.
Fig. 4. XRD spectra of CdS1xfSexf films grown at 75 C and different deposition time.
related to the difference of the free energy, the solubility product and the kinetic of the deposition of CdS and CdSe. The growth rate of CdS is higher than that of CdSe as it has been reported in bibliography [5,13]. Regarding the Cd composition, first it increases then decreases with time until reaching the stability at 120 min. The high excess of Cd (>1.2) in the films grown at deposition time 60 min is consistent with the formation of the flakes-like structure observed by SEM. These results (considering the measurement error) indicate that the atomic composition of the films changes with the deposition time, even though this change is not observed in the Eg of the films and it is barely perceivable from the shift of the reflexions in XRD patterns. The expected Eg reduction as a function of xf is not observed because of the constant crystal size. The thickness of the films increases from 20 to 180 nm as a function of the deposition time as observed in Fig. 7a. The crosssectional SEM images of the films deposited at 5 and 120 min presented in Fig. 7b and c confirm that the films prepared at short deposition time (5e15 min) are very thin. Fig. 8a presents the relationship between the growth rate of the films and the deposition time, the grow rate was estimated as the quotient of the thickness and the deposition time. The typical shape of growth rate curve for films deposited by CBD is observed. The growth rate decreases showing two stages, however the induction stage is not observed because the nucleation happens rapidly, before 5 min as seen by SEM in Figs. 5 and 7b. Although the growth rate tends to the equilibrium at ~60 min, the thickness continuous increasing because of sufficient quantity of reactants remains in the bath. The consumption rate of Se in the bath was estimated from ICP-OES analysis in order to understand the kinetics of deposition, Fig. 8b shows the results. Se consumption rate represents the global reaction system because it depends on the simultaneous occurrence of the heterogeneous and homogeneous reactions. At deposition times <30 min the consumption rate strongly decreases, after that the slope decays considerably, this behavior is in agreement with the growth rate tendency. The change of the slope for the consumption and growth rate plots occurs at the same time thereby, it can be assumed that the bath system is more stable after 60 min of reaction. 4. Conclusion
Fig. 5. High resolution SEM micrographs of CdS1xfSexf films grown at 75 C and different deposition time.
CBD allows to grow CdS1xfSexf alloys thin films with tunable xf by controlling the bath atomic composition. xf ¼ 0.5 ± 0.02 was obtained using an atomic composition of Se 20% higher than that of
Please cite this article in press as: E.A. Sanchez-Ramirez, et al., Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/ j.matchemphys.2015.09.005
E.A. Sanchez-Ramirez et al. / Materials Chemistry and Physics xxx (2015) 1e6
5
Fig. 6. Variation of the atomic composition of Cd, S and Se as a function of the deposition time for CdS1xSex a) films and b) powders.
Fig. 7. a) Variation of the thickness as a function of the deposition time, and cross sectional micrographs of the CdS1xfSexf films deposited at b) 5 min and c) 120 min.
Fig. 8. Effect of the deposition time on a) Growth rate, and b) Se consumption rate.
S in the bath. Cd excess of the films favors the formation of flakeslike structures and Cd impurities. Eg and crystal size decrease with the increase of xf. In the case of the films with xf 0.5 the Eg value
barely changes by due to the small crystal size. The films are constituted by clusters of well arranged nano-particles in a “wormlike” structure. All the films are polycrystalline and exhibit
Please cite this article in press as: E.A. Sanchez-Ramirez, et al., Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/ j.matchemphys.2015.09.005
6
E.A. Sanchez-Ramirez et al. / Materials Chemistry and Physics xxx (2015) 1e6
hexagonal phase with preferential orientation on (002) plane. The atomic composition of powders and films increases as a function of the deposition time, the growth starts with an excess of S. The homogeneous reaction reaches an equilibrium state of constant composition faster than the heterogeneous reaction. The nucleation takes place on the first 5 min. The consumption rate has the same behavior than the growth rate. The stability of the entire bath system is attained after 60 min. Acknowledgments This work was supported by the IPN-SIP Contracts 20151801 and 20144355, E.A.S.R thanks the CONACYT for a PhD grant and to IPN for BEIFI grant. References [1] A.M. Saad, A.K. Fedotov, A.V. Mazanik, M.I. Tarasik, A.M. Yanchenko, A.S. Posedko, L.Y. Survilo, Y.V. Trofimov, N.F. Kurilovich, Thin Solid Films 487 (2005) 202. [2] Z. Pi, L. Wang, X. Tian, C. Yang, J. Zheng, Matter. Lett. 61 (2007) 4857. [3] V. Kumar, D.K. Dwivedi, Optik 124 (2013) 2345.
[4] A.A. Yadav, E.U. Masumdar, J. Alloys Compd. 505 (2010) 787. [5] G.S. Shahane, B.M. More, C.B. Rotti, L.P. Deshmukh, Mater. Chem. Phys. 47 (1997) 263. [6] R. Mariappan, V. Ponnuswamy, M. Ragavendar, Optik 123 (2012) 1196. [7] J.B. Chaudhari, N.G. Deshpande, Y.G. Gudage, A. Ghosh, V.B. Huse, R. Sharma, Appl. Surf. Sci. 254 (2008) 6810. [8] R. Xie, J. Su, Y. Lui, L. Guo, Int. J. Hydrogen Energy 39 (2014) 3517. [9] G. Perna, S. Pagliara, V. Capozzi, M. Ambrico, T. Ligonzo, Thin Solid Films 349 (1999) 220. [10] S. Pagliara, L. Sangaletti, L.E. Depero, V. Capozzi, G. Perna, Appl. Surf. Sci. 186 (2002) 527. [11] R. Mariappan, M. Ragavendar, V. Ponnuswamy, J. Alloys Compd. 509 (2011) 7337. [12] H.K. Sadekar, A.V. Ghule, R. Sharma, J. Alloys Compd. 509 (2011) 5525. [13] G. Hodes (Ed.), Chemical Solution Deposition of Semiconductor Films, Marcel Dekker, Inc, New York, 2002. [14] M.A. Hernandez-Perez, J. Aguilar-Hernandez, G. Contreras-Puente, J.R. VargasGarcia, E. Rangel-Salinas, Phys. E 40 (2008) 2535. [15] R.S. Mane, C.D. Lokhande, V.V. Todkar, H. Chung, M.Y. Yoon, S.H. Ha, Appl. Surf. Sci. 253 (2007) 3922. [16] E.A. Sanchez-Ramirez, M.A. Hernandez-Perez, J. Aguilar-Hernandez, E. RangelSalinas, J. Alloys Compd. 615 (2014) s511. [17] S. Agrawal, A. Khare, Arab. J. Chem. 8 (2015) 450. [18] S.K. Apte, B.B. Kale, R.S. Sonawane, S.D. Naik, S.S. Bodhale, B.K. Das, Mater. Lett. 60 (2006) 499. [19] A.A. Ziabari, F.E. Ghodsi, Sol. Energy Mater. Sol. Cells 105 (2012) 249e262. [20] S.O. Oluwafemi, N. Revaprasadu, Phys. B 404 (2009) 1204.
Please cite this article in press as: E.A. Sanchez-Ramirez, et al., Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/ j.matchemphys.2015.09.005
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD E.A. Sanchez-Ramirez a, M.A. Hernandez-Perez a, *, J.R. Aguilar-Hernandez b, G. Contreras-Puente b a b
Escuela Superior de Ingeniería Química e Industrias Extractivas, Instituto Polit ecnico Nacional, CP 07738 M exico D.F., Mexico ticas, Instituto Polit Escuela Superior de Física y Matema ecnico Nacional, CP 07738 M exico D.F., Mexico
h i g h l i g h t s CdS1xSex ternary alloy thin films with x ¼ 0.5 ± 0.05 can be grown by CBD at 75 C. CdS1xSex nanocrystals are well arranged in a “worm” structure from 30 min and x 0.25. The Eg of (002) oriented hexagonal film is strongly affected by x and crystal size. Films with x ¼ 0.5 are obtained from 30 min using a Cd:S:Se ¼ 0.76:0.5:0.6 bath ratio. Consumption rate has the same behavior that growth rate, changing around 60 min.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 November 2014 Received in revised form 11 August 2015 Accepted 4 September 2015 Available online xxx
Chemical Bath Deposition (CBD) was used to grow CdS1xfSexf (xf ¼ 0.5) thin films on Corning glass substrates at 75 C. The atomic composition of the bath was varied until an xf of 0.5 was obtained, maintaining the deposition time at 120 min. Then the deposition time was modified from 5 to 360 min. The structural and optical properties of the films were analyzed by Scanning Electron Microscopy, Energy Dispersive Spectroscopy, X-Ray Diffraction, UVeVis Spectroscopy, Profilometry and Inductive Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). A bath atomic composition of Cd:S:Se equal to 0.76:0.55:0.45 was employed to obtain a film of xf ¼ 0.5. The films are constituted by clusters of semispherical nanoparticles (fav ¼ 15 nm), which are well-arranged in a “nanoworm” structure. The nucleation time of the particles is lower than 5 min. All the films are polycrystalline with hexagonal phase and preferentially orientated on the (002) plane. The crystal size (11e6 nm) and the band gap (2.17 e1.99 eV) decrease with the content of Se and remain constant with the deposition time. The composition xf ¼ 0.5 is achieved at different times to the heterogeneous (60 min) and homogeneous reactions (15 min). The kinetics of deposition and the consumption rate of Se change in a similar way, reaching the stability after 60 min. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Semiconductors Chemical synthesis Optical properties
1. Introduction Cadmium Selenosulphide (CdS1xSex) belongs to the IIeVI semiconductor group, it can be employed in a wide range of applications, especially in optoelectronic, photovoltaic and light emitting devices, solar cells and field effect transistors [1e4]. The interest is focused on the modulation of the properties by
xico D.F., Mexico. * Corresponding author. ESIQIE-IPN, CP 07738 Me E-mail addresses: [email protected], [email protected] (M.A. Hernandez-Perez).
modifying the composition x [4e8]. Ternary semiconductor films like CdSSe, CdZnS and ZnSSe, can be prepared by several techniques such as sputtering [1], screen printing [3], spray pyrolysis [4], laser ablation [9,10], and chemical bath deposition (CBD) [11,12]. CBD allows the control of the films properties by modifying the experimental parameters such as type and concentration of the reactants, temperature, pH and deposition time. In CBD two reactions occur simultaneously; the homogeneous reaction which takes place in the bulk solution and produces powders of CdS1xpSexp, and the heterogeneous reaction that results in a CdS1xfSexf film deposited on the surface of the substrate. The film is formed due to
http://dx.doi.org/10.1016/j.matchemphys.2015.09.005 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: E.A. Sanchez-Ramirez, et al., Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/ j.matchemphys.2015.09.005
2
E.A. Sanchez-Ramirez et al. / Materials Chemistry and Physics xxx (2015) 1e6
the slow release of the ions, from the anions hydrolysis and cationic complex (frequently with NH3), on the substrate surface [5e7]. Concerning ternary semiconductors, the films atomic composition is the result of the combined effect of the binary chalcogenides physical properties (mainly the solubility product, Ksp) and the kinetic factors of the entire reaction system (homogeneous and heterogeneous reactions) [13]. Notable achievements have been reported in the synthesis and characterization of CdS1xfSexf thin films, most of them focused on the improvement of optical, electrical and structural properties and on their technological application. However, the understanding of compositional properties and the kinetic factors is in an early stage. In this work, we report the deposition of CdS1xfSexf (xf ¼ 0.5) films at 75 C, the atomic composition (xf) equal 0.5 was fixed by varying the atomic composition of Cd, S and Se (xb) reactants in the bath. The influence of the deposition time on structural, optical and compositional properties is analyzed. Special attention was focused on the relationship between the growth rate of the films and the reactants consumption rate.
2.2. Film characterization The structural properties were analyzed by X-ray diffraction (XRD), the spectra were recorded in a Bruker Advanced diffractometer (Cu Ka ¼ 1.5406 Å) operating in grazing angle (0.3e2 ) configuration. The surface morphology of the films was analyzed with a high resolution SEM JEOL JSM6701F equipped with an Energy Dispersive Spectroscopy (EDS) module. Optical absorption studies were performed using a PerkineElmer UVeVis spectrometer Lambda35. The thickness was measured with a Sloan Dektak Profilometer. A Perkin Elmer ICP-OES Optima 8300 was used in order to evaluate the depletion of the reactants in the bath in function of the deposition time. For that, an aliquot of 1 mL was taken from the reactor at different time; the consumption rate was calculated considering the change of the volume. 3. Results and discussion 3.1. Setting the atomic composition and properties of the films grown during 120 min
2. Experimental details 2.1. Deposition CdS1xfSexf thin films were grown at 75 C onto glass substrates employing CBD method. Solutions of CdCl2 (0.1 M), CS(NH2)2 (0.1 M) and Na2SeSO3 (0.1 M) were used as reactants, all of them were prepared from analytical grade reagents. Na2SeSO3 (0.1 M) was prepared following the procedure reported by [14]. In order to promote the homogeneity of the films, the solutions were heated in separated beakers and mixed when they reached 72 C. The alkaline pH of the bath was maintained between 10 and 11 by the addition of NH4OH (4 M) and NH4Cl (0.2 M). At the end of each experiment, the precipitated powder and the deposited film were rinsed with deionized water and dried at room temperature. The adjustment of xf was performed in two stages, as presented in Table 1, the deposition time was maintained constant at 120 min. At the beginning, Se and S atomic compositions in the bath were modified until the xf closest to 0.5 was obtained, and the atomic composition of Cd kept constant. In the second stage, the atomic composition of Cd was diminished and Se and S atomic compositions were constant. Subsequently, deposition time was varied from 5 to 360 min in order to analyze its influence on the atomic composition of the films and powders, on the thickness and growth rate of the films, as well as on the depletion of the reactants. For these experiments, the employed bath composition was that obtained from the set values.
Fig. 1 shows the effect of the bath atomic composition (xb) in the atomic composition of the films (xf) when the atomic composition of Cd in the bath was constant, the SEM micrographs show the surface morphology of some films. For the studied range, the relationship seems to be linear. When the atomic composition of Se in the bath is increased, the quantity of Se incorporated to the film also increases. The film prepared with the stoichiometric reactants composition Cd:S:Se ¼ 1:0.5:0.5 (circled point), had an elemental atomic composition of Cd:S:Se ¼ 1.24:0.68:0.32. This means that the precipitation of Se is slower than that of S resultant in a film with a deficit of Se (xf ¼ 0.32) and S in excess (1xf ¼ 0.68). Therefore, it was necessary to use an excess of Se in the bath solution in order to enhance its incorporation to the film. According to the plot, xf ¼ 0.5 can be obtained using and xb of 0.58. Table 1 is the summary of the studied bath atomic compositions and the resulting film atomic compositions. Although the employed Cd quantity was constant, the films contain an excess of Cd that vary randomly. The atomic composition of Cd was estimated in basis to the total atomic composition of chalcogen ions (SeþS ¼ xf þ 1xf). In general, semispherical clusters of particles form the films, but flakes-
Table 1 Cd, S and Se atomic composition of the bath and films. Bath
First Stage
Second Stage
Films
Cd
S 1xb
Se xb
Cd
S 1xf
Se xf
1 1 1 1 1 1 1 0.86 0.76
0.50 0.47 0.45 0.40 0.37 0.33 0.45 0.45 0.45
0.50 0.53 0.55 0.60 0.63 0.67 0.55 0.55 0.55
1.24 1.48 1.21 1.38 1.33 1.15 1.21 1.47 1.1
0.68 0.62 0.55 0.46 0.39 0.33 0.55 0.57 0.51
0.32 0.38 0.45 0.54 0.61 0.67 0.45 0.43 0.49
0.66
0.45
0.55
0.85
0.5
0.5
Gray shadow indicates the best bath atomic composition to reach xf ¼ 0.5 and the Cd composition closest to 1.
Fig. 1. Effect of the bath atomic composition (xb) on the atomic composition of the films (xf) grown at 75 C during 120 min.
Please cite this article in press as: E.A. Sanchez-Ramirez, et al., Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/ j.matchemphys.2015.09.005
E.A. Sanchez-Ramirez et al. / Materials Chemistry and Physics xxx (2015) 1e6
like structures appear because of the excess of Cd as reported by Mane et al. [15]. Due to these results, the atomic composition of Cd in the bath was reduced in the second stage as presented in Table 1. Even though the Cd excess in the films is reduced, a clear relationship with the quantity of Cd in the bath is not observed. The xf ¼ 0.5 ± 0.02 can be obtained using an atomic composition of Cd:S:Se in the bath equal to 0.76:0.45:0.55. Fig. 2 shows the XRD spectra of CdS1xfSexf films with different atomic composition. The films are polycrystalline, the peaks corresponding to (100), (002), (110) and (112) planes of the hexagonal phase are observed, according to JCPDS files No. 491459, 893682, 500720 and 500721. The peaks are shifted to lower degrees when xf increases as a result of the bigger ionic radius of Se and its incorporation to the solid solution [16]. Preferential orientation on (002) plane is observed as reported in literature [7,14]. The films show other peaks which, according to the JCPDS data, could be related to different Cd compounds such as CdCN2 (360657) [8], CdSeO3 (82e1208), CdC2O4 (14e0712) and CdCO3 (850989). The quantity of these impurities seems to be associated with the Cd excess since the film with the lowest Cd atomic composition is free of impurities. The band gap value (Eg) was estimated from optical absorption measurement using Tauc's relation: ahy ¼ Aðhy Eg Þn , where a is the absorption coefficient, A is a constant and n ¼ 1/2 for direct band gap semiconductors. Extrapolation to a ¼ 0 in the (ahy)2 vs hy plot (Fig. 3a) gives the value of Eg [3e8,14]. A redshift of the Eg is observed when xf increases, however Eg of the films with xf from 0.59 to 0.67 remains almost constant. The optical properties of the films are not affected by the presence of impurities. The relationship between the crystal size and the band gap as a function of the films atomic composition is plotted in Fig. 3b. The relationship between Eg and xf seems to be linear. The theoretical Eg of CdS0.5Se0.5 material (circled point) was calculated as 2.06 eV using Vegard's Law: Eg CdSð1xÞ Sex ¼ ð1 xÞEg CdS þ xEg CdSe bxð1 xÞ where x is the Se atomic composition, ð1 xÞEg CdS and xEg CdSe are the individual contributions of CdS and CdSe to the band gap and b is the bowing factor (b ¼ 0 for linear relation). The crystal size was calculated according to Scherrer's equation using the FWHM of (002) plane. In general, crystal size decreases from 11 to 5 nm as a function of the atomic composition, in a nonlinear relationship. Similar and bigger crystal size has been reported for CdS1xSex films prepared by different techniques [3,9,17]. The experimental Eg of the film with xf ¼ 0.49 is bigger than that of the theoretical value,
Fig. 2. XRD spectra of CdS1xfSexf films with different atomic composition deposited at 75 C during 120 min.
3
this difference can be attributed to the small crystal size and its quantum effect [18e20]. The crystal size seems to affect also the Eg of the films with xf 0.59; for which a decrease is expected, however it remains almost constant as well as crystal size does. 3.2. Effect of the deposition time The effect of the deposition time on the properties of the films was studied from 5 to 360 min maintaining constant the bath atomic composition of Cd:S:Se ¼ 0.76:0.45:55. Fig. 4 shows the XRD patterns of CdS1xfSexf films deposited at different times, they were normalized in order to improve the comparison. The patterns of the films grown at 5 and 15 min show a strong contribution of the amorphous substrate, around 15e30 , this is attributed to the small thickness (20e40 ± 5 nm) and to a low degree of crystallinity. At longer deposition time, the characteristic peaks of hexagonal structure of CdS1xSex are observed: (100), (002), (110) and (200) planes (JCPDS 491459, 491460 and 893682). The films have preferential orientation on (002) plane as reported previously [3,7,9]. As the deposition time increases the crystalline quality is improved and the noise signal of the XRD patterns becomes better. For the films grown at deposition times 60 min it is observed a slight shift of the peaks to lower degrees which could be related with a small change of the atomic composition. However, the films prepared at higher deposition time do not show a perceivable change on the position of the peaks, suggesting an atomic composition almost constant. The films deposited from 5 to 60 min present impurities related with an excess of Cd. The crystal size remains almost constant with the deposition time; the estimated values (only for the films with well-defined peaks) were 7 ± 2 nm. The Eg does not change significantly (2.13 ± 0.03 eV, results not shown), whereby it can be assumed that Eg of the films deposited at different times is dependent on the slight variation of the crystal size and small change of the atomic composition observed, from the position of the peaks in XRD patterns. The morphology is affected by the deposition time which can be seen in the high resolution SEM images presented in Fig. 5. The density of the films increases with time as well as the size of the nanostructured clusters from 100 to 600 nm. Fast nucleation happens since the beginning of the reaction producing small nuclei which grow reaching an average size of 150 nm in diameter as observed in the film deposited at 5 min [20]. As the film becomes thicker, the nanoparticles that constitute the clusters are arranged in some kind of “nanoworm” structures. The nanoparticles size (~15 nm) is of the same order of magnitude than crystal size calculated from XRD results. In particular, the films deposited at 30 and 60 min present two morphologies, the semispherical clusters and the flakes-like structure observed for films with Cd excess [15]. In order to quantify the atomic composition of Cd, S and Se of the films and powders (obtained from the same experiment), the samples prepared at different deposition time were analyzed by EDS, the results are plotted in Fig. 6. The measurement error is higher for the films deposited at low deposition time because of their small thickness. The quantity of Se increases whereas that of S decreases until an equilibrium composition is reached. In the case of the heterogeneous reaction (film) the atomic composition seems to reach the equilibrium state around 60 min but for the homogeneous reaction (powder) the composition remains almost constant at 15 min. This could indicate that the homogeneous is faster than heterogeneous reaction, because of the high number of collisions between the ions. According to the plot, the growth of the films begins with the formation of a solid solution with a small atomic composition of Se (xf ¼ 0.2 ± 0.1) and subsequently some S2 are substituted by Se2 ions [5,6]. Therefore, the chalcogenide ions incorporate to the film at different rate. This behavior can be
Please cite this article in press as: E.A. Sanchez-Ramirez, et al., Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/ j.matchemphys.2015.09.005
4
E.A. Sanchez-Ramirez et al. / Materials Chemistry and Physics xxx (2015) 1e6
Fig. 3. a) Plot of (ahy)2 vs (hy) and b) variation of crystal size and Eg with the atomic composition of the CdS1xfSexf films.
Fig. 4. XRD spectra of CdS1xfSexf films grown at 75 C and different deposition time.
related to the difference of the free energy, the solubility product and the kinetic of the deposition of CdS and CdSe. The growth rate of CdS is higher than that of CdSe as it has been reported in bibliography [5,13]. Regarding the Cd composition, first it increases then decreases with time until reaching the stability at 120 min. The high excess of Cd (>1.2) in the films grown at deposition time 60 min is consistent with the formation of the flakes-like structure observed by SEM. These results (considering the measurement error) indicate that the atomic composition of the films changes with the deposition time, even though this change is not observed in the Eg of the films and it is barely perceivable from the shift of the reflexions in XRD patterns. The expected Eg reduction as a function of xf is not observed because of the constant crystal size. The thickness of the films increases from 20 to 180 nm as a function of the deposition time as observed in Fig. 7a. The crosssectional SEM images of the films deposited at 5 and 120 min presented in Fig. 7b and c confirm that the films prepared at short deposition time (5e15 min) are very thin. Fig. 8a presents the relationship between the growth rate of the films and the deposition time, the grow rate was estimated as the quotient of the thickness and the deposition time. The typical shape of growth rate curve for films deposited by CBD is observed. The growth rate decreases showing two stages, however the induction stage is not observed because the nucleation happens rapidly, before 5 min as seen by SEM in Figs. 5 and 7b. Although the growth rate tends to the equilibrium at ~60 min, the thickness continuous increasing because of sufficient quantity of reactants remains in the bath. The consumption rate of Se in the bath was estimated from ICP-OES analysis in order to understand the kinetics of deposition, Fig. 8b shows the results. Se consumption rate represents the global reaction system because it depends on the simultaneous occurrence of the heterogeneous and homogeneous reactions. At deposition times <30 min the consumption rate strongly decreases, after that the slope decays considerably, this behavior is in agreement with the growth rate tendency. The change of the slope for the consumption and growth rate plots occurs at the same time thereby, it can be assumed that the bath system is more stable after 60 min of reaction. 4. Conclusion
Fig. 5. High resolution SEM micrographs of CdS1xfSexf films grown at 75 C and different deposition time.
CBD allows to grow CdS1xfSexf alloys thin films with tunable xf by controlling the bath atomic composition. xf ¼ 0.5 ± 0.02 was obtained using an atomic composition of Se 20% higher than that of
Please cite this article in press as: E.A. Sanchez-Ramirez, et al., Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/ j.matchemphys.2015.09.005
E.A. Sanchez-Ramirez et al. / Materials Chemistry and Physics xxx (2015) 1e6
5
Fig. 6. Variation of the atomic composition of Cd, S and Se as a function of the deposition time for CdS1xSex a) films and b) powders.
Fig. 7. a) Variation of the thickness as a function of the deposition time, and cross sectional micrographs of the CdS1xfSexf films deposited at b) 5 min and c) 120 min.
Fig. 8. Effect of the deposition time on a) Growth rate, and b) Se consumption rate.
S in the bath. Cd excess of the films favors the formation of flakeslike structures and Cd impurities. Eg and crystal size decrease with the increase of xf. In the case of the films with xf 0.5 the Eg value
barely changes by due to the small crystal size. The films are constituted by clusters of well arranged nano-particles in a “wormlike” structure. All the films are polycrystalline and exhibit
Please cite this article in press as: E.A. Sanchez-Ramirez, et al., Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/ j.matchemphys.2015.09.005
6
E.A. Sanchez-Ramirez et al. / Materials Chemistry and Physics xxx (2015) 1e6
hexagonal phase with preferential orientation on (002) plane. The atomic composition of powders and films increases as a function of the deposition time, the growth starts with an excess of S. The homogeneous reaction reaches an equilibrium state of constant composition faster than the heterogeneous reaction. The nucleation takes place on the first 5 min. The consumption rate has the same behavior than the growth rate. The stability of the entire bath system is attained after 60 min. Acknowledgments This work was supported by the IPN-SIP Contracts 20151801 and 20144355, E.A.S.R thanks the CONACYT for a PhD grant and to IPN for BEIFI grant. References [1] A.M. Saad, A.K. Fedotov, A.V. Mazanik, M.I. Tarasik, A.M. Yanchenko, A.S. Posedko, L.Y. Survilo, Y.V. Trofimov, N.F. Kurilovich, Thin Solid Films 487 (2005) 202. [2] Z. Pi, L. Wang, X. Tian, C. Yang, J. Zheng, Matter. Lett. 61 (2007) 4857. [3] V. Kumar, D.K. Dwivedi, Optik 124 (2013) 2345.
[4] A.A. Yadav, E.U. Masumdar, J. Alloys Compd. 505 (2010) 787. [5] G.S. Shahane, B.M. More, C.B. Rotti, L.P. Deshmukh, Mater. Chem. Phys. 47 (1997) 263. [6] R. Mariappan, V. Ponnuswamy, M. Ragavendar, Optik 123 (2012) 1196. [7] J.B. Chaudhari, N.G. Deshpande, Y.G. Gudage, A. Ghosh, V.B. Huse, R. Sharma, Appl. Surf. Sci. 254 (2008) 6810. [8] R. Xie, J. Su, Y. Lui, L. Guo, Int. J. Hydrogen Energy 39 (2014) 3517. [9] G. Perna, S. Pagliara, V. Capozzi, M. Ambrico, T. Ligonzo, Thin Solid Films 349 (1999) 220. [10] S. Pagliara, L. Sangaletti, L.E. Depero, V. Capozzi, G. Perna, Appl. Surf. Sci. 186 (2002) 527. [11] R. Mariappan, M. Ragavendar, V. Ponnuswamy, J. Alloys Compd. 509 (2011) 7337. [12] H.K. Sadekar, A.V. Ghule, R. Sharma, J. Alloys Compd. 509 (2011) 5525. [13] G. Hodes (Ed.), Chemical Solution Deposition of Semiconductor Films, Marcel Dekker, Inc, New York, 2002. [14] M.A. Hernandez-Perez, J. Aguilar-Hernandez, G. Contreras-Puente, J.R. VargasGarcia, E. Rangel-Salinas, Phys. E 40 (2008) 2535. [15] R.S. Mane, C.D. Lokhande, V.V. Todkar, H. Chung, M.Y. Yoon, S.H. Ha, Appl. Surf. Sci. 253 (2007) 3922. [16] E.A. Sanchez-Ramirez, M.A. Hernandez-Perez, J. Aguilar-Hernandez, E. RangelSalinas, J. Alloys Compd. 615 (2014) s511. [17] S. Agrawal, A. Khare, Arab. J. Chem. 8 (2015) 450. [18] S.K. Apte, B.B. Kale, R.S. Sonawane, S.D. Naik, S.S. Bodhale, B.K. Das, Mater. Lett. 60 (2006) 499. [19] A.A. Ziabari, F.E. Ghodsi, Sol. Energy Mater. Sol. Cells 105 (2012) 249e262. [20] S.O. Oluwafemi, N. Revaprasadu, Phys. B 404 (2009) 1204.
Please cite this article in press as: E.A. Sanchez-Ramirez, et al., Bath atomic composition and deposition time influence on the properties of nanostructured CdS0.5Se0.5 thin films synthesized by CBD, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/ j.matchemphys.2015.09.005