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Behavior of chemically deposited PbS thin films subjected to two different routes of post deposition annealing
Materials Science in Semiconductor Processing 16 (2013) 605–611
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Behavior of chemically deposited PbS thin films subjected to two different routes of post deposition annealing K.C. Preetha a,b,n, T.L. Remadevi a,c a b c
School of Pure and Applied Physics, Kannur University, Kerala-670327, India Sree Narayana College, Kannur, Kerala, India Pazhassi Raja N.S.S. College, Mattannur, Kerala, India
a r t i c l e i n f o
abstract
Available online 21 January 2013
Lead sulfide (PbS) thin films were prepared on soda lime glass substrates at room temperature by Chemical Bath Deposition (CBD) technique. This paper reports a comparative study of characteristic properties of as-prepared PbS thin films after thermal treatment through two different routes. Studies were carried out for as-prepared as well as rapidly and gradually annealed samples at 100, 200 and 300 1C. The characterizations of the films were carried out using X-ray diffraction, scanning electron microscopy and optical measurement techniques. The structural studies confirmed the polycrystalline nature and the cubic structure of the films. As-deposited films partly transformed to Pb2O3 when gradually annealed to 300 1C. The presence of nano crystallites was revealed by structural and optical absorption measurements. The values of average crystallite size were found to be in the range 18–20 nm. The variation in the microstructure, thickness, grain size, micro strain and optical band gap on two types of annealing were compared and analyzed. Data showed that post deposition parameters and thermal treatment strongly influence the optical properties of PbS films. Optical band gap of the film gets modified remarkably on annealing. Direct band gap energy values for rapidly and gradually annealed samples varied in the range of 1.68–2.01 eV and 1.68–2.12 eV respectively. Thus we were succeeded in tailoring direct band gap energies by post deposition annealing method. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Lead sulfide Thin film Micro strain CBD Annealing
1. Introduction The preparation of polycrystalline chalcogenide compound semiconductors through chemical methods is significant in view of wide applications in diverse fields. Nanocrystalline materials have drawn extensive interest due to variation in the properties from their bulk, which leads to novel functionalities. PbS with direct band gap of 0.4 eV and absorption coefficient continuously increasing from the infrared through the visible region, has been
n Correspondence to: Symphony, Kannur (dt), Kerala-670011, India. Tel.: þ 91 9895 262 736; fax: þ 91 490 2731400. E-mail address: [email protected] (K.C. Preetha).
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.11.004
used in infrared detectors [1,2]. The chemical deposition technique for PbS thin films, known since 1910 [3] was initially developed in the late 1940s for IR detection applications [4]. For solar control and other applications PbS thin films were deposited on glass substrates in single layer [5–7]. CBD has been used to deposit films of metal sulfides, selenides and oxides since long back [8–11]. It has been found that the properties of chemically deposited PbS thin films depend strongly on growth conditions, It is the most appropriate method for the preparation of polycrystalline thin films [12–14]. By this method pin hole free thin films can be grown easily since basic building blocks are ions instead of atoms [15]. Considering the current interest in nanoparticles, CBD is an excellent technique to deposit nanocrystalline thin
606
K.C. Preetha, T.L. Remadevi / Materials Science in Semiconductor Processing 16 (2013) 605–611
films. The most obvious manifestation of nanoparticles is the increase in optical band gap with decrease in crystal size [16–18]. Thus CBD meets the norms of cheap, reproducible and relatively simple process both for the synthesis of nanocrystalline thin films and particles. Annealing is one of the steps in sensitization of PbS films in the preparation of efficient IR detectors and as such its effect on the crystallite size and structure is of considerable importance [18]. In this context the goal of this work was to synthesize PbS thin films in a cost effective way to modify some of the characteristic properties of as-deposited samples on annealing and also to compare two routes of heat treatments. The improvement of optoelectronic properties of as-deposited PbS thin films by thermal treatment was highlighted in this work. 2. Experimental details PbS thin films were deposited on clean, spectroscopic grade soda lime glass substrate at room temperature by chemical bath deposition technique. All the reagents used were of analytical grade (Merck). The reactive solution was prepared by the sequential addition of 0.3 M lead acetate, 3.7 M Triethanolamine (TEA) and 0.3 M thioacetamide in a 100 ml beaker. As in our previous work here also we have employed TEA as complexing agent which forms the [Pb(TEA)]2 þ complex and thereby lowering the concentration of free Pb2 þ ions below the level required for the precipitation of solid phase Pb(OH)2 [19]. Presence of Pb(OH)2 in the freshly prepared bath is essential for homogeneous nucleation process on the glass substrate leading to the formation of a thin layer of PbS film. Thin film formation was due to the reaction between slowly released Pb2 þ ions from complexed metal source and S2 ions from thioacetamde. Thin grayish black, highly adherent PbS films were taken out from the reaction bath after 3 h and the as-prepared samples were named as RT. Then we have carried out two different ways of annealing. One was rapid annealing in which samples were suddenly introduced into the hot air oven just at the particular temperature and then taken out immediately after 1 h. We have selected 100 1C, 200 1C and 300 1C for rapid annealing and were grouped under Type I with sample names 100RAN, 200RAN and 300RAN respectively. In other case samples were annealed gradually from room temperature to the above mentioned temperatures for same interval and taken out from the oven after attaining the initial temperature. These samples were grouped as Type II and assigned names as 100GAN, 200GAN and 300GAN respectively. The heat treatment changed the color of the films from grayish black to pale brown. The variations in crystallinity, orientation, grain size, thickness, microstructure, strain and optical band gap etc. were analyzed and compared from structural, morphological and optical studies. The crystallite structure and crystallographic orientation of the films were characterized by X-ray diffraction technique using Burker AXS D8 Advance with Cu-Ka line ˚ Morphology of the films was examined by (l ¼1.5406 A). scanning electron microscopy (SEM) taken by JEOL Model JSM 6490. The thickness of the samples was determined by
gravimetric analysis. The optical characterization was done by using a Hitachi-U-3410 UV–vis–NIR spectrophotometer. 3. Results and discussion 3.1. Structural properties The intensity distribution of diffraction peaks for as-prepared samples along with Type I and Type II samples are shown in Figs. 1 and 2 respectively. Type I films are polycrystalline with presumably face centered cubic structure (JCPDS. no.-5-592). In order to examine the preferred orientation, the ratio of the relative diffraction peak intensities (I200/I111) and (I220/I111) for all the PbS thin films are calculated and datas are shown in Table 1. Since these two ratios are less than 1, we could claim that for all the samples preferential orientation is along (111) plane, indicating three dimensional growth of the CBD films [13]. On annealing upto 200 1C, the intensity of the preferentially oriented peak decreases as depicted in Table 1. The reduction in intensity at 200 1C may be due to the presence of elemental sulfur at this temperature and which has dominant effect on micro strain, which will be discussed in detail at the end of this section. Further rapid annealing at 300 1C improves crystallinity without any phase change. Thus chemically deposited PbS thin film is highly stable to rapid annealing. In gradual annealing upto 200 1C, the preferential orientation of the films remains the same as in Type1
Fig. 1. XRD patterns of sample RT and Type I samples.
Fig. 2. XRD patterns of sample RT and Type II samples.
K.C. Preetha, T.L. Remadevi / Materials Science in Semiconductor Processing 16 (2013) 605–611
607
Table 1 Structural and optoelectronic parameters of PbS thin films annealed at different temperatures. Sample
Temperature (1C)
hkl
Intensity (cps)
2y (deg.)
d (nm)
Type I
27
111 200 220 111 200 220 111 200 220 111 200 220
704 474 188 205 155 67 92 59 19 229 144 62
26.314 30.436 43.414 26.302 30.420 43.369 26.281 30.374 43.493 26.311 30.406 43.384
3.384 2.935 2.063 3.385 2.936 2.085 3.388 2.940 2.082 3.384 2.937 2.085
111 200 220 111 200 220 111 200 220
289 220 99 219 128 43 63 32 15
26.304 30.418 43.410 26.302 30.420 43.604 26.281 30.374 44.059
3.385 2.936 2.083 3.383 2.935 2.082 3.414 2.965 2.079
100
200
300
Type II
100
200
300
samples, but crystallinity is found to be improved than rapid annealing. Further annealing to 300 1C results in tetragonal Pb2O3 (JCPDS no.-85-0859) along with very small traces of elemental sulfur peaks also (JCPDS no.-77-0228). This conversion may be due to replacement of sulfur with the chemisorbed oxygen owing to very slow annealing at higher temperature. Moreover excess strain developed in the film at this particular annealing temperature also results in thermal decomposition of PbS. These two factors lead to considerable reduction in crystallinity for this thin film. Growth of crystallites during annealing are controlled by several factors, among these are the absolute temperature of annealing, duration of annealing, degree to which the crystallite has recovered after deformation, orientation of crystallites with respect to each other. As reported by researchers, for PbS annealing has very little influence on grain growth [20]. CBD films are annealed not simply to increase crystal size but rather to improve certain properties. Annealing may have other important effects on CBD films besides change in crystal size. We have calculated crystallite size by Debye scherrer formula and the values are presented in Table 1. In this work we focused our effort to improve optical properties, mainly to enhance optical transmittance and band gap. To examine the effect of annealing on microstrain developed in PbS thin films, the average internal strain (e) is calculated from Williamson–Hall plot (W–H plot) by plotting reduced broadening bn ¼(bcos y)/l against the scattering vector length s ¼(4sin y)/l where b is the full width at half maximum. The slopes of the curves as depicted in Fig. 3(a) and (b) characterize the internal strain developed in samples. The standard deviation (SD) is shown in Table 1. For Type I samples annealing enhances strain and it becomes maximum at 200 1C, which reduces the crystallinity upto 200 1C. Then the strain decreases at 300 1C and thereby improves
D (nm)
19.76
19.62
18.68
19.12
18.30
19.52
20.46
I/I0 1 0.67 0.27 1 0.75 0.33 1 0.64 0.21 1 0.63 0.27 1 0.76 0.34 1 0.58 0.19 1 0.51 0.24
e
SD
Eg (eV)
0.0036
0.00243
1.68
0.0996
0.00203
1.86
1.0070
0.00956
2.01
0.9980
0.00428
1.96
1.0290
0.08508
2.06
1.0409
0.07768
1.89
1.0889
0.08345
2.12
crystallinity to a small extent at 300 1C. But for Type II samples it increases continuously and at 300 1C higher microstrain results in tetragonal Pb2O3 formation. In Type II samples, the crystallinity continuously decreases due to excess strain developed in the samples at higher annealing temperatures. Thus in both routes of annealing, annealed samples with traces of sulfur exhibit maximum strain. 3.2. Morphological properties Figs. 4 and 5 illustrate the morphology of PbS thin films annealed through two routes along with sample RT for comparison. The formation of spherical shaped grains with a compact texture with well defined boundaries is observed for as deposited samples (Fig. 4a). For Type I samples, annealing at 100 1C results in spherical granules similar to as deposited samples (Fig. 4b). Samples at low temperature display sharp and well defined boundary. At higher annealing temperature surface becomes less dense and grains are non-uniform in size with irregular boundary (Fig. 4c and Fig. 4d). Thus with respect to crystallinity, there is very good agreement between film morphology of the annealed samples and structural properties. In comparison with Type I samples, Type II samples retain orderness and spherical nature of grains upto 200 1C (Fig. 5a and b). Annealing at 300 1C results in discrete elongated grains characteristic of new phase (Fig. 5c). The entirely different morphology at this temperature is due to tetragonal Pb2O3 formation and enhanced micro strain developed in the film. 3.3. Optical properties Fig. 6a and b depict the variation of absorbance with wavelength for the PbS samples. The spectra show two regions, one for higher wavelength showing practically
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low absorption and other for lower wavelength in which absorption increases rapidly for all samples. Similar result was obtained for SILAR deposited PbS thin films in our previous work [19]. Bulk PbS has an absorption onset at 3020 nm corresponds to a small band gap of 0.41 eV at 298 K. Observations of a higher band-gap may be due to oxygen inclusion, quantum size effects and more recently stoichiometry change [21]. Annealing may results in higher oxygen content and hence a change in chemical composition. Consequently leading the broadening of band gap. The strong blue shift in the absorption threshold from the bulk is due to strong quantum confinement [16]. According to this, the calculated particle size around Bohr radius of PbS (20 nm) may be one of the reasons for the observed blue shift. Type I samples show higher absorbance compared to Type II samples. In both types annealed samples have low absorbance compared to asprepared samples. For the parabolic band structure, the absorption coefficient (a) is related to the band gap of the material by the equation [22]
ahn ¼ A hnEg n ,
ð1Þ
where n¼1/2 for allowed direct transition and n¼2 for indirect transition. ‘A’ is the parameter which depends on the transition probability. The absorption coefficient can be deduced from the absorption spectra using the relation [23]
a ¼ 2:303 A=t
Fig. 3. Williamson–Hall plot of micro strain for Type I and Type II samples.
ð2Þ
where ‘t’ is the thickness of the as deposited PbS thin film. We adopted weight difference method for measuring PbS thin film thickness. This method is one of the classic
Fig. 4. SEM Images of samples: (a) RT, (b) 100RAN, (c) 200RAN, and (d) 300 RAN.
K.C. Preetha, T.L. Remadevi / Materials Science in Semiconductor Processing 16 (2013) 605–611
609
Fig. 6. The variation of optical absorbance with wavelength for (a) sample RT along with Type I samples and (b) Type II samples.
Fig. 5. SEM Images of samples: (a) 100 GAN, (b) 200 GAN, and (c) 300 GAN.
methods in measuring thickness using sensitive electrical balance. In our present work we have obtained the film thickness by gravimetric method [24] in the range 0.82 mm and 0.72 mm for Type I samples, 0.82 mm and 0.60 mm for Type II samples. The optical absorption data were analyzed by plotting (ahn)n versus hn, for n¼ 2 for allowed direct transition. The intercept on energy axis gives band gap energy. The direct band gap energy plots for Type I and Type II samples are shown in Fig. 7a and b respectively. Direct band gap energy values for quickly annealed samples and slowly annealed samples are in the range of 1.68–2.01 eV and 1.68–2.12 eV respectively. Thus we were able to tailor the direct band gap by post deposition annealing process through two different routes. The effect of annealing temperature on direct band gap energies for Type I and Type II samples are depicted in Fig. 8(a) and (b) respectively. In type I samples, band gap energies increase with annealing temperature upto 200 1C and there after a slight decrease is noticed. But in Type II samples, band gap energy increases continuously for
all the range of annealing temperatures except at 200 1C. The energy gap broadening on annealing films may also be related to the existence within the band gap of a high density levels with energies near the bands which can give rise to band tailing, as has been suggested for other polycrystalline materials [25]. These levels should be associated with the electronic states at grain boundaries and their density should decrease markedly with heat treatment. In both type of annealing samples under highest micro strains (200RAN and 300GAN) have maximum optical band gap energies. Broadening of optical band gap at 300 1C for type II samples may be due to higher oxygen content and sulfur deficiency at this temperature range. These results are in consistent with reduction in crystallinity as observed in XRD and SEM results at these temperatures also. Thus we could be able to correlate the structural, morphological and optoelectronic properties of the PbS samples with annealing temperatures. The band gap obtained is within the solar spectral region and hence the films find potential applications in solar cell fabrication. Detailed variations in structural and optoelectronic parameters such as interplanar spacing ‘d’, crystallite size (D), microstrain (E) and band gap (Eg) on annealing by two different ways are depicted in Table 1. Fig. 9(a) and (b) shows the effect of annealing temperature on spectral transmittance, the percentage transmittance increases with increasing annealing temperatures. Spectra
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K.C. Preetha, T.L. Remadevi / Materials Science in Semiconductor Processing 16 (2013) 605–611
Fig. 7. Direct band gap energies for samples (a) RT and Type I samples, and (b) Type II samples.
Fig. 9. The variation of optical transmittance with wavelength for (a) sample RT along with Type I samples, and (b) Type II samples.
Fig. 8. The variation of direct band gap energy with annealing temperatures.
The reflectance plots as shown in Fig. 10(a) and (b) indicate a fall in reflectance at higher annealing temperatures and these results are in consistence with transmission results on annealing at higher temperatures. All the samples annealed upto 200 1C by both routes of annealing can be used as solar control coatings as they have very low optical transmittance in visible region and appreciable reflection in NIR region [6]. Those characteristics will assure an adequate illumination of the buildings inside, and in the same time, reflecting the incident NIR radiation will stop the increase of the temperature inside the buildings, thus reducing the necessary costs for cooling. In summary, air annealing leads to significant changes in the structural, morphological, and optical properties of PbS thin films.
also illustrate a sharp optical transmission cut-off near UV–vis region corresponding to the band gap of PbS. The film shows good homogeneity with a maximum transmission of 32% at 1200 nm for RAN200 and 50% for GAN200, where as transmittance of RT is only 22% at this wavelength. The transmittance in visible range is less than 10% for all samples except GAN 300, which shows 40% transmittance in visible region and it rises and stabilized to 65% in IR region, where as transmittance of RAN 300 in IR region is only 36%. The success of our work is that we were able to tailor the optical band gap over a wide range and also the optical transmittance has been increased considerably in Type I samples, without any phase change on annealing.
4. Conclusion The effect of annealing temperature on structural, morphological and optical properties of PbS thin films prepared by chemical bath deposition technique was investigated. The results showed that the as-prepared samples were polycrystalline single phase cubic structure with a preferential orientation of (111) plane. Gradual annealing at 300 1C leads partial conversion of PbS to Pb2O3. Micro strain enhanced considerably at higher annealing temperatures and results in reduction in crystallinity. As grown PbS thin films at room temperature has high absorption coefficient in the visible light and
K.C. Preetha, T.L. Remadevi / Materials Science in Semiconductor Processing 16 (2013) 605–611
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films. This study point out that the annealed temperature was one of the criteria that strongly influence film structure, stoichiometry, microstructure, crystallinity and consequently optical properties of PbS thin films.
Acknowledgment The financial assistance from the University Grants Commission of India is gratefully acknowledged. We express our sincere thanks to STIC, Cochin, Kerala, India for offering technical support. References
Fig. 10. The variation of optical reflectance with wavelength for (a) sample RT along with Type I samples, and (b) Type II samples.
it decreases on post deposition annealing. The result of optical energy gap shows that PbS thin films have allowed direct transition, the optical energy gap of PbS thin films is affected by annealing process and increases with increase in annealing temperatures, it increases from 1.68 to 2.01 eV and 1.68–2.12 eV respectively for Type I and Type II samples. Thus We were able to tailor the direct band gap energies by post deposition annealing process through two entirely different routes. The as-prepared PbS thin films become more transparent after post thermal treatment and transparency increase from 22%–36% for Type I samples and 22%–65% for Type II samples. These properties confirm that PbS layers were suitable to be employed as material for various photovoltaic applications. Deposition parameters and thermal treatment strongly influence the optical properties of PbS
[1] Harumi Moreno-Garcı´a, M.T.S. Nair, P.K. Nair, Thin Solid Films 519 (2011) 2287. [2] Alex P. Gaiduk, Peter I. Gaiduk, Arne Nylandsted Larsen, Thin Solid Films 516 (2008) 3791. [3] H.M. Pathan, C.D. Lokhande, Bullettin of Material Science 27 (2004) 85. [4] D.E. Bode, in: G. Hass, R.E. Thun (Eds.), Physics of Thin Films, vol. 3, Academic Press, New York, 1966., p. 275. [5] P.K. Nair, M. Ocampo, A. Fernadez, M.T.S. Nair, Solar Energy Material 20 (1990) 235. [6] P.K. Nair, M.T.S. Nair, Journal of Physics D: Applied Physics 23 (1990) 150. [7] P.K. Nair, V.M. Garcia, A.B. Hrenandez, M.T.S. Nair, Journal of Physics D: Applied Physics 24 (1991) 1466. [8] G. Hodes, Physical Chemistry Chemical Physics 9 (2007) 2181. [9] K.M. Gadave, S.A. Jodgudri, C.D. Lokhande, Thin Solid Films 245 (1994) 7. [10] Yu Jun Yang, Shengshui Hu, Thin Solid Films 516 (2008) 6048. [11] V.V. Valenzuela-Jauregui, R. Ramrez.Bon, A. Mendoza-Galvan, M. Soleto-Lerma, Thin Solid Films 441 (2003) 104. [12] R.S. Patil, H.M. Pathan, T.P. Gujar, C.D. Lokhande, Journal of Materials Science 41 (2006) 5723. [13] S. Seghaier, N. Kamoun, R. Brini, A.B. Amara, Materials Chemistry and Physics 97 (2006) 71. [14] E.M. Larramendi, O. Calzadilla, A. Gonzalez-Arias, E. Hernandez, Thin Solid Films 389 (2001) 301. [15] R.S. Mane, C.D. Lokhande, Materials Chemistry and Physics 65 (2000) 1. [16] K.Suresh Babu, C. Vijayan, Prathap Haridoss, Materials Science and Engineering C 27 (2001) 922. [17] A. Popa, M. Lisca, V. Stancu, Journal of Optoelectronics and Advanced Materials 8 (2006) 43. [18] G.P. Kothiyal, B. Ghosh, R.Y. Deshpande, Journal of Physics D: Applied Physics 13 (1980) 869. [19] K.C. Preetha, K.V. Murali, A.J. Rajina, K. Deepa, T.L Remadevi, Current Applied Physics 12 (2011) 53. [20] R. Bruce, Contribution to Mineralogy and Petrology 64 (1977) 149. [21] K.S.A. Butcher, T.L. Tansley, Superlattices and Microstructures 38 (2005) 1. [22] J. Tauc, Amorphous and Liquid Semiconductor, Plenum Press, New York, 1974, p. 159. [23] F.N. Dultsev, L.L. Vasilieva, S.M. Maroshina, L.D. Pokrovsky, Thin Solid Films 510 (2006) 255–259. [24] R. Das, Rajesh kumar, Materials Research Bulletin 47 (2011) 239. [25] Anderson Janotti, Chris G. Van de Wall, Reports on Progress in Physics 72 (2009) 126501.
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Behavior of chemically deposited PbS thin films subjected to two different routes of post deposition annealing K.C. Preetha a,b,n, T.L. Remadevi a,c a b c
School of Pure and Applied Physics, Kannur University, Kerala-670327, India Sree Narayana College, Kannur, Kerala, India Pazhassi Raja N.S.S. College, Mattannur, Kerala, India
a r t i c l e i n f o
abstract
Available online 21 January 2013
Lead sulfide (PbS) thin films were prepared on soda lime glass substrates at room temperature by Chemical Bath Deposition (CBD) technique. This paper reports a comparative study of characteristic properties of as-prepared PbS thin films after thermal treatment through two different routes. Studies were carried out for as-prepared as well as rapidly and gradually annealed samples at 100, 200 and 300 1C. The characterizations of the films were carried out using X-ray diffraction, scanning electron microscopy and optical measurement techniques. The structural studies confirmed the polycrystalline nature and the cubic structure of the films. As-deposited films partly transformed to Pb2O3 when gradually annealed to 300 1C. The presence of nano crystallites was revealed by structural and optical absorption measurements. The values of average crystallite size were found to be in the range 18–20 nm. The variation in the microstructure, thickness, grain size, micro strain and optical band gap on two types of annealing were compared and analyzed. Data showed that post deposition parameters and thermal treatment strongly influence the optical properties of PbS films. Optical band gap of the film gets modified remarkably on annealing. Direct band gap energy values for rapidly and gradually annealed samples varied in the range of 1.68–2.01 eV and 1.68–2.12 eV respectively. Thus we were succeeded in tailoring direct band gap energies by post deposition annealing method. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Lead sulfide Thin film Micro strain CBD Annealing
1. Introduction The preparation of polycrystalline chalcogenide compound semiconductors through chemical methods is significant in view of wide applications in diverse fields. Nanocrystalline materials have drawn extensive interest due to variation in the properties from their bulk, which leads to novel functionalities. PbS with direct band gap of 0.4 eV and absorption coefficient continuously increasing from the infrared through the visible region, has been
n Correspondence to: Symphony, Kannur (dt), Kerala-670011, India. Tel.: þ 91 9895 262 736; fax: þ 91 490 2731400. E-mail address: [email protected] (K.C. Preetha).
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.11.004
used in infrared detectors [1,2]. The chemical deposition technique for PbS thin films, known since 1910 [3] was initially developed in the late 1940s for IR detection applications [4]. For solar control and other applications PbS thin films were deposited on glass substrates in single layer [5–7]. CBD has been used to deposit films of metal sulfides, selenides and oxides since long back [8–11]. It has been found that the properties of chemically deposited PbS thin films depend strongly on growth conditions, It is the most appropriate method for the preparation of polycrystalline thin films [12–14]. By this method pin hole free thin films can be grown easily since basic building blocks are ions instead of atoms [15]. Considering the current interest in nanoparticles, CBD is an excellent technique to deposit nanocrystalline thin
606
K.C. Preetha, T.L. Remadevi / Materials Science in Semiconductor Processing 16 (2013) 605–611
films. The most obvious manifestation of nanoparticles is the increase in optical band gap with decrease in crystal size [16–18]. Thus CBD meets the norms of cheap, reproducible and relatively simple process both for the synthesis of nanocrystalline thin films and particles. Annealing is one of the steps in sensitization of PbS films in the preparation of efficient IR detectors and as such its effect on the crystallite size and structure is of considerable importance [18]. In this context the goal of this work was to synthesize PbS thin films in a cost effective way to modify some of the characteristic properties of as-deposited samples on annealing and also to compare two routes of heat treatments. The improvement of optoelectronic properties of as-deposited PbS thin films by thermal treatment was highlighted in this work. 2. Experimental details PbS thin films were deposited on clean, spectroscopic grade soda lime glass substrate at room temperature by chemical bath deposition technique. All the reagents used were of analytical grade (Merck). The reactive solution was prepared by the sequential addition of 0.3 M lead acetate, 3.7 M Triethanolamine (TEA) and 0.3 M thioacetamide in a 100 ml beaker. As in our previous work here also we have employed TEA as complexing agent which forms the [Pb(TEA)]2 þ complex and thereby lowering the concentration of free Pb2 þ ions below the level required for the precipitation of solid phase Pb(OH)2 [19]. Presence of Pb(OH)2 in the freshly prepared bath is essential for homogeneous nucleation process on the glass substrate leading to the formation of a thin layer of PbS film. Thin film formation was due to the reaction between slowly released Pb2 þ ions from complexed metal source and S2 ions from thioacetamde. Thin grayish black, highly adherent PbS films were taken out from the reaction bath after 3 h and the as-prepared samples were named as RT. Then we have carried out two different ways of annealing. One was rapid annealing in which samples were suddenly introduced into the hot air oven just at the particular temperature and then taken out immediately after 1 h. We have selected 100 1C, 200 1C and 300 1C for rapid annealing and were grouped under Type I with sample names 100RAN, 200RAN and 300RAN respectively. In other case samples were annealed gradually from room temperature to the above mentioned temperatures for same interval and taken out from the oven after attaining the initial temperature. These samples were grouped as Type II and assigned names as 100GAN, 200GAN and 300GAN respectively. The heat treatment changed the color of the films from grayish black to pale brown. The variations in crystallinity, orientation, grain size, thickness, microstructure, strain and optical band gap etc. were analyzed and compared from structural, morphological and optical studies. The crystallite structure and crystallographic orientation of the films were characterized by X-ray diffraction technique using Burker AXS D8 Advance with Cu-Ka line ˚ Morphology of the films was examined by (l ¼1.5406 A). scanning electron microscopy (SEM) taken by JEOL Model JSM 6490. The thickness of the samples was determined by
gravimetric analysis. The optical characterization was done by using a Hitachi-U-3410 UV–vis–NIR spectrophotometer. 3. Results and discussion 3.1. Structural properties The intensity distribution of diffraction peaks for as-prepared samples along with Type I and Type II samples are shown in Figs. 1 and 2 respectively. Type I films are polycrystalline with presumably face centered cubic structure (JCPDS. no.-5-592). In order to examine the preferred orientation, the ratio of the relative diffraction peak intensities (I200/I111) and (I220/I111) for all the PbS thin films are calculated and datas are shown in Table 1. Since these two ratios are less than 1, we could claim that for all the samples preferential orientation is along (111) plane, indicating three dimensional growth of the CBD films [13]. On annealing upto 200 1C, the intensity of the preferentially oriented peak decreases as depicted in Table 1. The reduction in intensity at 200 1C may be due to the presence of elemental sulfur at this temperature and which has dominant effect on micro strain, which will be discussed in detail at the end of this section. Further rapid annealing at 300 1C improves crystallinity without any phase change. Thus chemically deposited PbS thin film is highly stable to rapid annealing. In gradual annealing upto 200 1C, the preferential orientation of the films remains the same as in Type1
Fig. 1. XRD patterns of sample RT and Type I samples.
Fig. 2. XRD patterns of sample RT and Type II samples.
K.C. Preetha, T.L. Remadevi / Materials Science in Semiconductor Processing 16 (2013) 605–611
607
Table 1 Structural and optoelectronic parameters of PbS thin films annealed at different temperatures. Sample
Temperature (1C)
hkl
Intensity (cps)
2y (deg.)
d (nm)
Type I
27
111 200 220 111 200 220 111 200 220 111 200 220
704 474 188 205 155 67 92 59 19 229 144 62
26.314 30.436 43.414 26.302 30.420 43.369 26.281 30.374 43.493 26.311 30.406 43.384
3.384 2.935 2.063 3.385 2.936 2.085 3.388 2.940 2.082 3.384 2.937 2.085
111 200 220 111 200 220 111 200 220
289 220 99 219 128 43 63 32 15
26.304 30.418 43.410 26.302 30.420 43.604 26.281 30.374 44.059
3.385 2.936 2.083 3.383 2.935 2.082 3.414 2.965 2.079
100
200
300
Type II
100
200
300
samples, but crystallinity is found to be improved than rapid annealing. Further annealing to 300 1C results in tetragonal Pb2O3 (JCPDS no.-85-0859) along with very small traces of elemental sulfur peaks also (JCPDS no.-77-0228). This conversion may be due to replacement of sulfur with the chemisorbed oxygen owing to very slow annealing at higher temperature. Moreover excess strain developed in the film at this particular annealing temperature also results in thermal decomposition of PbS. These two factors lead to considerable reduction in crystallinity for this thin film. Growth of crystallites during annealing are controlled by several factors, among these are the absolute temperature of annealing, duration of annealing, degree to which the crystallite has recovered after deformation, orientation of crystallites with respect to each other. As reported by researchers, for PbS annealing has very little influence on grain growth [20]. CBD films are annealed not simply to increase crystal size but rather to improve certain properties. Annealing may have other important effects on CBD films besides change in crystal size. We have calculated crystallite size by Debye scherrer formula and the values are presented in Table 1. In this work we focused our effort to improve optical properties, mainly to enhance optical transmittance and band gap. To examine the effect of annealing on microstrain developed in PbS thin films, the average internal strain (e) is calculated from Williamson–Hall plot (W–H plot) by plotting reduced broadening bn ¼(bcos y)/l against the scattering vector length s ¼(4sin y)/l where b is the full width at half maximum. The slopes of the curves as depicted in Fig. 3(a) and (b) characterize the internal strain developed in samples. The standard deviation (SD) is shown in Table 1. For Type I samples annealing enhances strain and it becomes maximum at 200 1C, which reduces the crystallinity upto 200 1C. Then the strain decreases at 300 1C and thereby improves
D (nm)
19.76
19.62
18.68
19.12
18.30
19.52
20.46
I/I0 1 0.67 0.27 1 0.75 0.33 1 0.64 0.21 1 0.63 0.27 1 0.76 0.34 1 0.58 0.19 1 0.51 0.24
e
SD
Eg (eV)
0.0036
0.00243
1.68
0.0996
0.00203
1.86
1.0070
0.00956
2.01
0.9980
0.00428
1.96
1.0290
0.08508
2.06
1.0409
0.07768
1.89
1.0889
0.08345
2.12
crystallinity to a small extent at 300 1C. But for Type II samples it increases continuously and at 300 1C higher microstrain results in tetragonal Pb2O3 formation. In Type II samples, the crystallinity continuously decreases due to excess strain developed in the samples at higher annealing temperatures. Thus in both routes of annealing, annealed samples with traces of sulfur exhibit maximum strain. 3.2. Morphological properties Figs. 4 and 5 illustrate the morphology of PbS thin films annealed through two routes along with sample RT for comparison. The formation of spherical shaped grains with a compact texture with well defined boundaries is observed for as deposited samples (Fig. 4a). For Type I samples, annealing at 100 1C results in spherical granules similar to as deposited samples (Fig. 4b). Samples at low temperature display sharp and well defined boundary. At higher annealing temperature surface becomes less dense and grains are non-uniform in size with irregular boundary (Fig. 4c and Fig. 4d). Thus with respect to crystallinity, there is very good agreement between film morphology of the annealed samples and structural properties. In comparison with Type I samples, Type II samples retain orderness and spherical nature of grains upto 200 1C (Fig. 5a and b). Annealing at 300 1C results in discrete elongated grains characteristic of new phase (Fig. 5c). The entirely different morphology at this temperature is due to tetragonal Pb2O3 formation and enhanced micro strain developed in the film. 3.3. Optical properties Fig. 6a and b depict the variation of absorbance with wavelength for the PbS samples. The spectra show two regions, one for higher wavelength showing practically
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low absorption and other for lower wavelength in which absorption increases rapidly for all samples. Similar result was obtained for SILAR deposited PbS thin films in our previous work [19]. Bulk PbS has an absorption onset at 3020 nm corresponds to a small band gap of 0.41 eV at 298 K. Observations of a higher band-gap may be due to oxygen inclusion, quantum size effects and more recently stoichiometry change [21]. Annealing may results in higher oxygen content and hence a change in chemical composition. Consequently leading the broadening of band gap. The strong blue shift in the absorption threshold from the bulk is due to strong quantum confinement [16]. According to this, the calculated particle size around Bohr radius of PbS (20 nm) may be one of the reasons for the observed blue shift. Type I samples show higher absorbance compared to Type II samples. In both types annealed samples have low absorbance compared to asprepared samples. For the parabolic band structure, the absorption coefficient (a) is related to the band gap of the material by the equation [22]
ahn ¼ A hnEg n ,
ð1Þ
where n¼1/2 for allowed direct transition and n¼2 for indirect transition. ‘A’ is the parameter which depends on the transition probability. The absorption coefficient can be deduced from the absorption spectra using the relation [23]
a ¼ 2:303 A=t
Fig. 3. Williamson–Hall plot of micro strain for Type I and Type II samples.
ð2Þ
where ‘t’ is the thickness of the as deposited PbS thin film. We adopted weight difference method for measuring PbS thin film thickness. This method is one of the classic
Fig. 4. SEM Images of samples: (a) RT, (b) 100RAN, (c) 200RAN, and (d) 300 RAN.
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Fig. 6. The variation of optical absorbance with wavelength for (a) sample RT along with Type I samples and (b) Type II samples.
Fig. 5. SEM Images of samples: (a) 100 GAN, (b) 200 GAN, and (c) 300 GAN.
methods in measuring thickness using sensitive electrical balance. In our present work we have obtained the film thickness by gravimetric method [24] in the range 0.82 mm and 0.72 mm for Type I samples, 0.82 mm and 0.60 mm for Type II samples. The optical absorption data were analyzed by plotting (ahn)n versus hn, for n¼ 2 for allowed direct transition. The intercept on energy axis gives band gap energy. The direct band gap energy plots for Type I and Type II samples are shown in Fig. 7a and b respectively. Direct band gap energy values for quickly annealed samples and slowly annealed samples are in the range of 1.68–2.01 eV and 1.68–2.12 eV respectively. Thus we were able to tailor the direct band gap by post deposition annealing process through two different routes. The effect of annealing temperature on direct band gap energies for Type I and Type II samples are depicted in Fig. 8(a) and (b) respectively. In type I samples, band gap energies increase with annealing temperature upto 200 1C and there after a slight decrease is noticed. But in Type II samples, band gap energy increases continuously for
all the range of annealing temperatures except at 200 1C. The energy gap broadening on annealing films may also be related to the existence within the band gap of a high density levels with energies near the bands which can give rise to band tailing, as has been suggested for other polycrystalline materials [25]. These levels should be associated with the electronic states at grain boundaries and their density should decrease markedly with heat treatment. In both type of annealing samples under highest micro strains (200RAN and 300GAN) have maximum optical band gap energies. Broadening of optical band gap at 300 1C for type II samples may be due to higher oxygen content and sulfur deficiency at this temperature range. These results are in consistent with reduction in crystallinity as observed in XRD and SEM results at these temperatures also. Thus we could be able to correlate the structural, morphological and optoelectronic properties of the PbS samples with annealing temperatures. The band gap obtained is within the solar spectral region and hence the films find potential applications in solar cell fabrication. Detailed variations in structural and optoelectronic parameters such as interplanar spacing ‘d’, crystallite size (D), microstrain (E) and band gap (Eg) on annealing by two different ways are depicted in Table 1. Fig. 9(a) and (b) shows the effect of annealing temperature on spectral transmittance, the percentage transmittance increases with increasing annealing temperatures. Spectra
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Fig. 7. Direct band gap energies for samples (a) RT and Type I samples, and (b) Type II samples.
Fig. 9. The variation of optical transmittance with wavelength for (a) sample RT along with Type I samples, and (b) Type II samples.
Fig. 8. The variation of direct band gap energy with annealing temperatures.
The reflectance plots as shown in Fig. 10(a) and (b) indicate a fall in reflectance at higher annealing temperatures and these results are in consistence with transmission results on annealing at higher temperatures. All the samples annealed upto 200 1C by both routes of annealing can be used as solar control coatings as they have very low optical transmittance in visible region and appreciable reflection in NIR region [6]. Those characteristics will assure an adequate illumination of the buildings inside, and in the same time, reflecting the incident NIR radiation will stop the increase of the temperature inside the buildings, thus reducing the necessary costs for cooling. In summary, air annealing leads to significant changes in the structural, morphological, and optical properties of PbS thin films.
also illustrate a sharp optical transmission cut-off near UV–vis region corresponding to the band gap of PbS. The film shows good homogeneity with a maximum transmission of 32% at 1200 nm for RAN200 and 50% for GAN200, where as transmittance of RT is only 22% at this wavelength. The transmittance in visible range is less than 10% for all samples except GAN 300, which shows 40% transmittance in visible region and it rises and stabilized to 65% in IR region, where as transmittance of RAN 300 in IR region is only 36%. The success of our work is that we were able to tailor the optical band gap over a wide range and also the optical transmittance has been increased considerably in Type I samples, without any phase change on annealing.
4. Conclusion The effect of annealing temperature on structural, morphological and optical properties of PbS thin films prepared by chemical bath deposition technique was investigated. The results showed that the as-prepared samples were polycrystalline single phase cubic structure with a preferential orientation of (111) plane. Gradual annealing at 300 1C leads partial conversion of PbS to Pb2O3. Micro strain enhanced considerably at higher annealing temperatures and results in reduction in crystallinity. As grown PbS thin films at room temperature has high absorption coefficient in the visible light and
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films. This study point out that the annealed temperature was one of the criteria that strongly influence film structure, stoichiometry, microstructure, crystallinity and consequently optical properties of PbS thin films.
Acknowledgment The financial assistance from the University Grants Commission of India is gratefully acknowledged. We express our sincere thanks to STIC, Cochin, Kerala, India for offering technical support. References
Fig. 10. The variation of optical reflectance with wavelength for (a) sample RT along with Type I samples, and (b) Type II samples.
it decreases on post deposition annealing. The result of optical energy gap shows that PbS thin films have allowed direct transition, the optical energy gap of PbS thin films is affected by annealing process and increases with increase in annealing temperatures, it increases from 1.68 to 2.01 eV and 1.68–2.12 eV respectively for Type I and Type II samples. Thus We were able to tailor the direct band gap energies by post deposition annealing process through two entirely different routes. The as-prepared PbS thin films become more transparent after post thermal treatment and transparency increase from 22%–36% for Type I samples and 22%–65% for Type II samples. These properties confirm that PbS layers were suitable to be employed as material for various photovoltaic applications. Deposition parameters and thermal treatment strongly influence the optical properties of PbS
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