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Optical and structural properties of chemically deposited CdS thin films on polyethylene naphthalate substrates
Thin Solid Films 520 (2011) 999–1004
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Optical and structural properties of chemically deposited CdS thin films on polyethylene naphthalate substrates M.G. Sandoval-Paz a,⁎, R. Ramírez-Bon b a b
Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Concepción, Casilla 160-C, Concepción, Chile Centro de Investigación y Estudios Avanzados del IPN, Unidad Querétaro, Apdo. Postal 1-798, 76001 Querétaro, Qro., Mexico
a r t i c l e
i n f o
Article history: Received 25 November 2010 Received in revised form 20 July 2011 Accepted 3 August 2011 Available online 7 August 2011 Keywords: Cadmium sulfide Chemical bath deposition Plastic substrates
a b s t r a c t CdS thin films were deposited on polyethylene naphthalate substrates by means of the chemical bath deposition technique in an ammonia-free cadmium–sodium citrate system. Three sets of CdS films were grown in precursor solutions with different contents of Cd and thiourea maintaining constant the concentration ratios [Cd]/[thiourea] and [Cd]/[sodium citrate] at 0.2 and 0.1 M/M, respectively. The concentrations of cadmium in the reaction solutions were 0.01, 7.5 × 10 −3 and 6.8 × 10−3 M, respectively. The three sets of CdS films were homogeneous, hard, specularly reflecting, yellowish and adhered very well to the plastic substrates, quite similar to those deposited on glass substrates. The structural and optical properties of the CdS films were determined from X-ray diffraction, optical transmission and reflection spectroscopy and atomic force microscopy measurements. We found that the properties of the films depend on both the amount of Cd in the growth solutions and on the deposition time. The increasing of Cd concentration in the reaction solution yield to thicker CdS films with smaller grain size, shorter lattice constant, and higher energy band gap. The energy band gap of the CdS films varied in the range 2.42–2.54 eV depending on the precursor solution. The properties of the films were analyzed in terms of the growth mechanisms during the chemical deposition of CdS layers. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The emerging technology of high efficiency thin film solar cells has increased interest in CdS thin films, which have the role of window layers in this type of solar cells with CdTe or Cu(In,Ga)Se2 (CIGS) as the absorber layers. The chemical bath deposition (CBD) is one of the deposition techniques most widely employed to achieve CdS films with good window layer properties. The reasons for this are the simplicity, large substrate area compatibility and cost-effectiveness of this technique. In addition, CBD is a low temperature process because deposition temperature is limited by the water boiling temperature of the precursor reaction solution. This particularity of CBD makes it very convenient for the deposition of semiconductor thin films on plastic substrates, which low thermal resistance requires low temperature processing. In the last few years, studies of the deposition of semiconductor thin films on plastic substrates are of increasing interest. This is because the advantages of light weight, transparency, flexibility and low cost for the development of flexible devices [1–4]. Semiconductor films have been deposited on several types of plastic substrates such as polyethylene naphthalate (PEN), polyethylene terephthalate, polycarbonate, acrylic, etc. [5–11]. Among these, PEN is one of the plastic substrates most often employed
⁎ Corresponding author. Tel.: + 56 41 220 73 40; fax: + 56 41 222 45 20. E-mail address: [email protected] (M.G. Sandoval-Paz). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.08.006
because of its mechanical and optical properties. It is a semicrystalline transparent polymeric material with glass transition temperature around 125 °C and working temperature up to 155 °C. CdS thin films have been deposited on flexible commercial viewfoil substrates by CBD and pulsed laser deposition [1,4]. In both cases, adherent, homogeneous CdS polycrystalline films with hexagonal structure were deposited. Furthermore, despite the amorphous nature of the substrate and the low temperature processing, the CdS films had preferred crystalline orientation along the (002) direction. These facts ensure that CdS thin films deposited on plastic substrates can attain similar characteristics to those of films deposited on conventional rigid substrates such as glass, silicon, etc. Therefore, CdS thin films deposited on plastic substrates can provide alternatives to take advantage of the properties of this semiconductor material, where low weight and flexibility are of major importance. In this work we have studied the properties of CdS thin films deposited by CBD on PEN substrates. For this, we employed a very convenient ammonia-free CBD process based on sodium citrate as the complexing agent, which results in highly oriented CdS polycrystalline films when deposited on glass substrates [12,13]. In recent papers we have shown that these ammonia-free chemically deposited CdS films perform reasonably well as window layers in CdS– CdTe thin film solar cells [14] and as active layers in field effect thin film transistors [15]. The aim of this work was first to explore the formation and adherence of the CdS films to the plastic substrate and after these issues were ensured, to determine the optical and structural properties
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of the films as a function of the cadmium content in the precursor solution.
a) M1 films 80
[Cd]=0.01M
T
2. Experimental details
3. Results and discussion In Fig. 1 are shown the optical reflectance (R) and transmittance (T) spectra of a) CdS M1 and b) M2 films, respectively, deposited on PEN substrates at different deposition times. These optical spectra display similar characteristics to those corresponding to CdS films deposited on glass substrates [13], with absorption edge at about 480–500 nm in the T spectra. The transmittance of the film-substrate system is limited by the transmittance of the PEN substrate, which is about 85% in the entire visible spectral region [11]. The sharp absorption edge observed at about 400 nm in the T spectra is produced by the optical absorption of PEN. The T values at wavelengths larger than the absorption edge are between 67 and 85% for M1 films and 68–78% for M2 films. It can be seen that the reflectance in both types of films is from 6 to 31% in all the wavelength range. It is evident from R and T spectra that the thicknesses of M2 films are smaller than the corresponding ones in M1 films, indicated by the wavelength values of maxima and minima at the R spectra. On the other hand, all R and T spectra of M3 films deposited at different times were very similar (not shown herein), revealing that their thicknesses are about the same and that the final thickness is reached at very short deposition times for this reaction solution. In Fig. 2 are shown the R and T spectra of the M1, M2 and M3 CdS films deposited for 30 min on glass and PEN substrates. It can be observed similar behavior in the transmittance and reflectance spectra of the films deposited on the different types of substrates. The
15 min 30 min 60 min 90 min 120 min
T, R (%)
60
40
R
20
0 80
b) M2 films
T
[Cd]=7.5x10-3M
T, R (%)
60
40
R
20
0 300
400
500
600
700
800
Wavelength (nm) Fig. 1. Optical transmittance (T) and reflectance (R) spectra of a) CdS M1 and b) M2 films deposited at different times.
wavelength values of maxima and minima of interference fringes in R and T spectra roughly coincide for the same type of films on both glass and PEN substrates. Since these maxima and minima of interference directly provide the film thickness, according to reference [17], it is 100 CdS 30 min
1
80
2) M2, [Cd]=7.5x10-3M
2
3) M3, [Cd]=6.8x10-3M
3 3
60
40
PEN glass best fit R
3
3 2 20
2 1
0 200
T
1
1) M1, [Cd]=0.01M
T, R (%)
Three sets of CdS films were deposited on PEN substrates labeled as M1, M2 and M3, respectively. The recipes for preparing the M1 and M2 films are the same as those described in Ref. [16] for the CdS films labeled as AF1 and AF2, respectively, and consist of mixtures of aqueous solutions of CdCl2, sodium citrate, KOH, buffer pH 10 and thiourea, with lower amounts of cadmium, sodium citrate and thiourea in M2 precursor solution respect to that of M1 one. M3 films were obtained from a solution with even less cadmium and thiourea with respect to M2 ones. The concentrations of cadmium in reaction solutions M1, M2 and M3 were 0.01, 7.5× 10−3 and 6.8 × 10 −3 M, respectively. The ratios of concentrations [Cd]/[thiourea] and [Cd]/[Citrate] were 0.2 and 0.1 M/M, respectively, in each one of baths. When lower cadmium concentration was used in the solution than that for M3 films, there was no formation of a well adhered CdS film to the PEN substrates. The temperature of the reaction solution was set at 70 °C and there was no stirring during the deposition process. Each set of films was obtained by placing five 1 × 3 square inches PEN substrates in the reaction beaker and subsequently removing them from the solution after 15, 30, 60, 90 and 120 min, respectively. For comparison and for film thickness studies the same sets of films were also grown on glass substrates. All the CdS films were homogeneous, specularly reflecting, yellowish and adhered very well to the PEN substrates. That is, the formation of the CdS films on the plastic substrates and their adherence were very similar to the corresponding on glass substrates. The crystalline structure of the films was studied by X-ray diffraction (XRD) measurements obtained with a Rigaku D/max2100 X-ray diffractometer (with a Co source, λ = 1.78899 Å), employing a thin film attachment in order to measure in the glancing angle mode equivalent to the asymmetric Bragg-Bretano geometry. The transmission and reflection optical spectra of the films were obtained under near-normal incidence with a Film Tek™ 3000 spectrometer in the UV– Visible spectral range. The surface morphology of the samples was analyzed by atomic force microscopy (AFM) images by using an Instrument Veeco Dimension 3100 System, Nanoscope IV. The AFM images were measured in the tapping mode on 1 × 1 μm2 areas of the surface samples.
300
400
500
600
1
700
800
Wavelength (nm) Fig. 2. T and R spectra of CdS thin films deposited on PEN (dashed lines) and glass (solid lines) substrates at 30 min. Dotted lines represent the best fit to the two layers model.
M.G. Sandoval-Paz, R. Ramírez-Bon / Thin Solid Films 520 (2011) 999–1004
−αd
T = ð1−RÞe
ð1Þ
where d is the film thickness, and it is considered that the substrate is a non-absorbent material. In order to estimate the energy band gap, Eg, of the films, we used the model for allowed direct transitions between parabolic energy bands, given by [20] 1 = 2 α ðEÞ = A E−Eg
ð2Þ
with E the incident radiation energy and A is a constant. According to the model, the energy gap is obtained by extrapolating the linear portion of α 2 versus E plot to E = 0. In Fig. 4 the α 2 versus E plots, for M1, M2 and M3 films deposited during 60 min are shown. The solid lines correspond to experimental data and dotted ones to the best fit to a straight line, according to the theoretical model. The energy band gap of all films was determined from these adjustments and the average value of Eg for each set of CdS films was 2.51, 2.43 and 2.44 eV
1.0
CdS 60 min 0.8
0.6
2
2.4
for M1, M2 and M3 films, respectively. The energy band gap in M2 and M3 films decreased with respect to M1 films. Variations in the energy band gap of chemically deposited CdS films have been associated with variations in either their lattice constant or grain size [21,22]. For example, A.E. Rakhshani and A.S. Al-Azab reported an increase in the average grain size and a decrease in the energy band gap with increasing film thickness, so the thickness dependence of the band gap energy is attributed to the effect of lattice strain [22]. In Fig. 5 it is shown the variation of the energy band gap with the thickness for M1, M2 and M3 films, where it is observed a clear trend of increasing energy band gap with increasing film thickness, in contrast to the results of Rakhshani and Al-Azab. Similar results were obtained in previous work where the energy band gap energy is plotted as a function of deposition time in CdS films deposited on glass substrates [12]. In order to correlate the values of energy band gap with the structural properties of the CdS films, X-ray diffraction patterns were obtained for all the films. In Fig. 6 are shown the XRD patterns of CdS M1 and M2 films at different deposition times. All these patterns display only one diffraction peak at about 31°, which is related with the (002) diffraction line of the CdS hexagonal crystalline phase, according to the results obtained in films deposited on glass substrates [16]. The broad
CdS/PEN fims M1, [Cd]=0.01M M2, [Cd]=7.5x10-3M M3, [Cd]=6.8x10-3M
2.52
Energy band gap (eV)
160
100
2.8
Fig. 4. Energy dependence of α2 for CdS films deposited on PEN substrates for 60 min.
2.54
120
2.6
Energy (eV)
M2, [Cd]=7.5x10-3M
140
1
3 0.4
0.0
M3, [Cd]=6.8x10-3M
Thickness (nm)
-3 2) M2, [Cd]=7.5x10 M, Eg=2.43 eV
0.2
M1, [Cd]=0.01M
180
1) M1, [Cd]=0.01M,Eg=2.52 eV 3) M3, [Cd]=6.8x10-3M, Eg=2.44 eV
α 2 (x1010 cm-2)
expected that CdS films have similar thickness when deposited on PEN or on glass substrates. The thickness of the M1, M2 and M3 films was determined by fitting the T and R spectra of the films using a model of two layers which represents the CdS layer on the glass substrate and a roughened external layer. The CdS optical constants were represented by a dispersion relation which is a generalized version of Lorenz Oscillator Model and the roughened layer was modeled by an effective medium approximation as described in reference [18]. The best fits are plotted as dotted lines in Fig. 2. It can be seen that the modeled transmittance and reflectance spectra reproduce the measured spectra over the full wavelength range. The direct adjustment of T and R spectra of the films on PEN substrates was not possible because the PEN is an anisotropic material [19] whose optical properties cannot be simulated with Film Wizard™ software used. It is therefore assumed that the thickness obtained in the films deposited on glass substrates is similar to that of the films on PEN substrates. The thickness values obtained from the fitting procedure are plotted as a function of deposition time in Fig. 3. The film thickness decreases as the reagents concentration in reaction solution decreases as seen for the M1 to M3 films. In addition, the films M2 and M3 have very short effective termination times, about 30 min, and the final thicknesses for M1 to M3 films were 175, 100 and 73 nm, respectively. The optical absorption coefficient, α, of CdS films deposited on PEN substrates was determined from measured transmittance and reflectance spectra, using the relation
1001
2.50
2.48
2.46
2.44 80 2.42 60 0
30
60
90
120
Deposition time (min) Fig. 3. Growth kinetics of the CdS M1, M2 and M3 films.
60
80
100
120
140
160
180
Film thickness (nm) Fig. 5. Band gap energy versus film thickness of the CdS films deposited on PEN substrates.
M.G. Sandoval-Paz, R. Ramírez-Bon / Thin Solid Films 520 (2011) 999–1004
(a) M1 films [Cd]=0.01M
Intensity (Arb. units)
8000
6000 120 min 90 min
4000
60 min 2000
30 min 15 min
0 8000
(b) M2 films
Intensity (Arb. units)
[Cd]=7.5x10-3M 6000 120 min 90 min
4000
60 min 30 min
2000
15 min 0 25
30
35
40
2 Theta (Degrees) Fig. 6. X-ray diffraction patterns of CdS M1 and M2 films deposited on PEN substrates at different deposition times.
intense shoulder at the left of the (002) diffraction peaks of CdS is produced by the PEN substrates. These patterns show that CdS films on PEN glass substrates are highly oriented along the (002) crystalline direction. The diffraction peak in the XRD patterns of all the films was fit to a Gaussian function to determine the Bragg angle and full width at half maximum (FWHM) of the peak, from these parameters were determined the values of the lattice constant c and the averaged grain size of the films. The values of the lattice constant were calculated by means of the equation for the interplanar distances for a hexagonal crystalline system [23]. As the grains in the films are preferentially oriented in the [002] direction, the lattice parameter reduces to c = 2 d002, and by using the Bragg's equation we have c = λ/sin θB, where λ is the X-rays wavelength and θB is the Bragg's angle. The grain size L of the films was calculated from the FWHM of the peaks by means of the Scherrer formula, L = 0.9λ/B cos θB, where B is the FWHM. The Scherrer's equation is widely used for estimating the size of crystallites. The value of L represents the diameter of crystallites, perpendicular to the plane which corresponds to the measured diffraction peak. If the diameter of crystallites becomes small, for example, down to about 50 nm, the peak broadening can be appreciable. To be more precise, what is measured is not necessarily crystal size but coherence length, the length over which the periodicity of the crystal is complete [24]. Other causes for XRD peaks being broader than expected based on crystal size is the presence of strain in the crystals or other defects, such as dislocations, which destroy the long-range lattice order. Separation between crystal size and strain can be made if several different peaks are present, since the angular dependences of the two factors are different [25]. In the case of our films, this separation cannot be done because only one diffraction peak appears in the pattern (Fig. 6). For the cases where the size of crystallites in a sample of interest lies in the range about 5 nm below 50 nm, this can be computed within the error of about ± several percent when using the Scherrer formula, this is probably due to the so-called instrumental broadening factor, as well as
the fact that the FWHM determination is made by fitting a Gaussian function. Thus, the interpretation of XRD peak broadening should be carried out with care and preferably using complementary TEM measurements. However, the valuesof L obtained from the XRD peaks are in agreement with those reported in the literature for CdS films obtained by CBD [24]. In Fig. 7 is shown the thickness dependence of averaged grain size L for CdS M1, M2 and M3 films. It can be seen that for each set of samples, the grain size decreases as the film thickness increases. There is a general trend of the grain size to decrease with increasing film thickness. It can also be seen that the larger grains are obtained in formulations with lower concentrations of reactants in reaction solution, i.e. M3 films have larger grains than M1 ones. Larger crystal sizes are related to an ion by ion growth mechanism, which happens slower than the cluster by cluster mechanism [24]. It sounds reasonable that the growth of M2 and M3 films occur slower bearing in mind that the formulations M2 and M3 are more diluted (lower concentrations of cadmium and thiourea) respect to M1. In fact, the growth rate observed in Fig. 3 is a clear evidence of it. Furthermore, in a previous work we studied the growth kinetics of M1 and M2 films deposited on glass substrates [16] and it was shown that in the early stages, the growth mechanism is of type ion by ion, while at larger times the film deposition is through the aggregation mechanism. The latter mechanism promotes homogeneous precipitation, thus it is consistent to expect that films with larger thickness deposited at larger times have smaller grains. In addition to grain size, the c lattice constant and the strain of the CdS thin films were also determined from XRD measurements. The values of the c lattice constant in our films are smaller than those of the material in bulk (co = 6.713 Å), and they vary from one formulation to another. The shrinkage of the c lattice constant which is oriented along the (002) crystalline direction, perpendicular to the substrate surface, indicates that the crystalline lattice of the (002) oriented CdS films is under tensile strain. The amount of tensile strain depending on the amount of cadmium in the precursor solution, as shown by the c lattice constant values which are smaller for CdS M1 films than for M2 and M3 films. The percentage of variation of the lattice constant Δc/co was calculated and plotted as a function of the grain size, shown in Fig. 8. A clear trend to decrease of the Δc/co parameter with the increasing of grain size can be seen in this figure. This behavior is reasonable if it is considered that in the CdS films with larger grains, the slower deposition rate relaxes the tensile strain. Therefore, the results about the crystalline parameters determined from XRD measurements provided information about the chemical deposition mechanisms of CdS layers on PEN substrates, which
CdS/PEN films 30
Averaged grain size (nm)
1002
M1, [Cd]=0.01M M2, [Cd]=7.5x10-3M M3, [Cd]=6.8x10-3M
25
20
15 60
80
100
120
140
160
180
Film thickness (nm) Fig. 7. Averaged grain size versus film thickness of the CdS films deposited on PEN substrates.
M.G. Sandoval-Paz, R. Ramírez-Bon / Thin Solid Films 520 (2011) 999–1004
1.0
CdS/PEN films
0.9
M1, [Cd]=0.01M M2, [Cd]=7.5x10-3M M3, [Cd]=6.8x10-3M
Δc /c0 (%)
0.8 0.7 0.6 0.5 0.4 0.3 0.2 18
24
30
Grain size (nm)
1003
the fact that the energy band gap variation is not produced only by the grain size variation but also by the lattice constant variation. In any case, our results show that the energy band gap of the CdS films on PEN substrates can be tuned in the range 2.42–2.54 eV by adjusting the concentration of the reagents in the precursor reaction solution. In Fig. 10 are shown the two-dimensional AFM images of the surface morphology of a) M1, b) M2 and c) M3 films deposited during 60 min. The surface of the three films is flat, smooth and compact with a granular structure. The rms roughness values determined from these images were 10.4, 15 and 19.7 nm for M1, M2 and M3 films, respectively. The difference in grains size is appreciable in this figure, in accordance with the results obtained from XRD. On the other hand, the grain shape of M1 films is more likely a cauliflower also observed in other chemically deposited CdS films [30], while the grain morphology of M2 and M3 films is rather different to that of the M1 films. The granular structure of M2 and M3 films, deposited with lower reactants concentration, displays better defined grains. 4. Conclusions
Fig. 8. Percentage of variation of the lattice parameter Δc/c0 as a function the grain size for CdS M1, M2 and M3 films deposited on PEN substrates.
are consistent with those observed in similar CdS-CBD processes on glass substrates [16,26]. As stated above, the variations in the energy band gap of chemically deposited CdS films have been related with variations in either their grain size or lattice constant. The effect of grain size on the energy band gap of many chemically deposited chalcogenide films has been explained in terms of quantum confinement due to the reduced size of the crystallites [27,28]. On the other hand, tensile strain on grains, parallel to the substrate surface, produces shrinkage of the lattice constant c along the [002] direction, perpendicular to the substrate in highly oriented films. In Fig. 9 are plotted the energy band gap of the CdS films as a function of the grain size (empty symbols) and lattice constant (filled symbols), respectively. A trend is observed of the energy band gap of the films to increase with reducing either the grain size or lattice constant. In this figure, c0 indicates the lattice constant of bulk CdS. These results suggest that both crystalline parameters influence the energy band gap of the CdS films. In the case of the energy band gap versus grain size behavior the expected r −2 trend predicted by some theoretical models is not clear, where r is the particle size of the semiconductor material [29]. This could be due to
In this work we have deposited homogeneous CdS thin films which adhered very well to PEN substrates, by means of an ammonia-free CBD process. We determined the influence of the Cd concentration in the
51.4 nm
a)
0 nm 54.9 nm
b)
Grain size (nm) 2.56
16
18
20
22
24
28
30
32
M1, [Cd]=0.01M M2, [Cd]=7.5X10-3M M3, [Cd]=6.8x10-3M
2.54
Energy band gap (eV)
26
2.52
0 nm 2.50
c0 2.48
55.9 nm
c)
2.46 2.44 2.42 6.65
6.66
6.67
6.68
6.69
6.70
6.71
Lattice parameter (°A) Fig. 9. Energy band gap of the CdS films as a function of grain size (empty symbols) and lattice constant (filled symbols).
0 nm Fig. 10. AFM surface images of a) M1, b) M2, and c) M3 films deposited during 60 min.
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M.G. Sandoval-Paz, R. Ramírez-Bon / Thin Solid Films 520 (2011) 999–1004
precursor reaction solution on the optical and structural properties of the films. The CdS films were polycrystalline with hexagonal structure and preferred crystalline orientation along the (002) direction. We found that when the reaction solution contains higher amount of Cd, thicker CdS films with smaller grain size and shorter lattice constant are deposited. The variation of these crystalline parameters produces a variation of the energy band gap of the CdS films in the range 2.42–2.54 eV, which depends on the reactive concentrations in the precursor solution. Acknowledgments This work was partially supported by CONACYT-Mexico (project No. CB-2008-01-106952) and partially by FONDECYT-Chile (project No. 11090434). The technical assistance of M.A. Hernández-Landaverde and C. A. Avila-Herrera is also acknowledged. References [1] M.S. Shur, S. Rumyantsev, R. Gaska, B.Q. Wei, R. Vajtai, P.M. Ajayan, J. Sinius, Solid State Electron. 46 (2002) 1417. [2] K. Song, J. Hu, R. Shrestha, F. Zhu, S. Jin Chua, Thin. Solid. Films 477 (2005) 32. [3] S. Richter, M. Ploetner, W.J. Fischer, M. Schneider, P.T. Nguyen, W. Plieth, N. Kiriy, H.-J.P. Adler, Thin. Solid. Films 477 (2005) 140. [4] K.P. Acharya, J.R. Skuza, R.A. Lukaszew, C. Liyanage, B. Ullrich, J. Phys. Condens. Matter 19 (2007) 196221. [5] J.H. Shin, S.H. Shin, J.I. Park, H.H. Kim, J. Appl. Phys. 89 (2001) 5199. [6] F.L. Wong, M.K. Fung, S.W. Tong, C.S. Lee, S.T. Lee, Thin Solid Films 466 (2004) 225. [7] H. Kim, J.S. Horwitz, G.P. Kushto, Z.H. Kafafi, D.B. Chrisey, Appl. Phys. Lett. 79 (2001) 284. [8] P.F. Carcía, R.S. McLean, M.H. Reilly, Z.G. Li, L.J. Pillione, R.F. Messier, J. Vac. Sci. Technol. A 21 (2003) 745.
[9] M. Fonrodona, J. Escarre, F. Villar, D. Soler, J.M. Asensi, J. Bertomeu, J. Andreu, Sol. Energy Mater. Sol. Cells 89 (2005) 37. [10] J.M. Lee, C.P. Judge, S.W. Wright, Solid State Electron. 44 (2000) 1431. [11] M.G. Sandoval-Paz, R. Ramírez-Bon, Thin Solid Films 517 (2009) 2596. [12] M.B. Ortuño-López, J.J. Valenzuela-Jáuergui, M. Sotelo-Lerma, A. Mendoza-Galván, R. Ramírez-Bon, Thin Solid Films 429 (2003) 34. [13] M.B. Ortuño-López, M. Sotelo-Lerma, A. Mendoza-Galván, R. Ramírez-Bon, Thin Solid Films 457 (2004) 284. [14] R. Ochoa-Landín, J. Sastre-Hernández, O. Vigil-Galanand, R. Ramírez-Bon, Sol. Energy 84 (2010) 208. [15] G. Arreola-Jardón, L.A. González, L.A. García-Cerda, B. Gnade, M.A. QuevedoLópezand, R. Ramírez-Bon, Thin Solid Films 519 (2010) 517. [16] M.G. Sandoval-Paz, R. Ramírez-Bon, Thin Solid Films 517 (2009) 6747. [17] J.C. Manifacer, J. Gasiot, J.C. Fillard, J. Phys. E Sci. Instrum. 9 (1976) 1002. [18] M.G. Sandoval-Paz, R. Ramírez-Bon, in: R. Ramírez-Bon, F.J. Espinoza-Beltrán (Eds.), Deposition, Characterization and Applications of Semiconductor Thin Films, Research Signpost, Kerala, 2009, p. 21. [19] A. Laskarakis, S. Logothetidis, Appl. Surf. Sci. 253 (2006) 52. [20] J.I. Pankove, Optical Processes in Semiconductors, Dover Publications Inc., New York, 1971. [21] M.B. Ortuño López, J.J. Valenzuela Jáuregui, M. Sotelo Lerma, A. Mendoza Galván, Vacuum 76 (2004) 181. [22] A.E. Rakhshani, A.S. Al-Azab, J. Phys. Condens. Matter 12 (2000) 8745. [23] B.D. Cullity, Elements of X-Ray Diffraction, 2nd Ed. Addison Wesley Publishing Co., Inc., Massachussets, 1978. [24] G. Hodes, Chemical solution deposition of semiconductor films, 1st Ed. Marcel Dekker Inc., New York, 2003. [25] Y. Waseda, E. Matsubara, K. Shinoda, X-Ray Diffraction Crystallography, Introduction, examples and solved problems, 1st Ed. Springer, 2011. [26] M.G. Sandoval-Paz, M. Sotelo-Lerma, A. Mendoza-Galván, R. Ramírez-Bon, Thin Solid Films 515 (2007) 3356. [27] S. Gorer, G. Hodes, J. Phys. Chem. 98 (1994) 5338. [28] P.J. George, A. Sánchez, P.K. Nair, M.T.S. Nair, Appl. Phys. Lett. 66 (1995) 3624. [29] L. Efros, A. Efros, Sov. Phys. Semicond. 16 (1982) 772. [30] J.N. Ximello-Quiebras, G. Contreras-Puente, J. Aguilar-Hernández, G. SantanaRodríguez, A.A.C. Readigos, Sol. Energy Mater. Sol. Cells 82 (2004) 263.
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Optical and structural properties of chemically deposited CdS thin films on polyethylene naphthalate substrates M.G. Sandoval-Paz a,⁎, R. Ramírez-Bon b a b
Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Concepción, Casilla 160-C, Concepción, Chile Centro de Investigación y Estudios Avanzados del IPN, Unidad Querétaro, Apdo. Postal 1-798, 76001 Querétaro, Qro., Mexico
a r t i c l e
i n f o
Article history: Received 25 November 2010 Received in revised form 20 July 2011 Accepted 3 August 2011 Available online 7 August 2011 Keywords: Cadmium sulfide Chemical bath deposition Plastic substrates
a b s t r a c t CdS thin films were deposited on polyethylene naphthalate substrates by means of the chemical bath deposition technique in an ammonia-free cadmium–sodium citrate system. Three sets of CdS films were grown in precursor solutions with different contents of Cd and thiourea maintaining constant the concentration ratios [Cd]/[thiourea] and [Cd]/[sodium citrate] at 0.2 and 0.1 M/M, respectively. The concentrations of cadmium in the reaction solutions were 0.01, 7.5 × 10 −3 and 6.8 × 10−3 M, respectively. The three sets of CdS films were homogeneous, hard, specularly reflecting, yellowish and adhered very well to the plastic substrates, quite similar to those deposited on glass substrates. The structural and optical properties of the CdS films were determined from X-ray diffraction, optical transmission and reflection spectroscopy and atomic force microscopy measurements. We found that the properties of the films depend on both the amount of Cd in the growth solutions and on the deposition time. The increasing of Cd concentration in the reaction solution yield to thicker CdS films with smaller grain size, shorter lattice constant, and higher energy band gap. The energy band gap of the CdS films varied in the range 2.42–2.54 eV depending on the precursor solution. The properties of the films were analyzed in terms of the growth mechanisms during the chemical deposition of CdS layers. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The emerging technology of high efficiency thin film solar cells has increased interest in CdS thin films, which have the role of window layers in this type of solar cells with CdTe or Cu(In,Ga)Se2 (CIGS) as the absorber layers. The chemical bath deposition (CBD) is one of the deposition techniques most widely employed to achieve CdS films with good window layer properties. The reasons for this are the simplicity, large substrate area compatibility and cost-effectiveness of this technique. In addition, CBD is a low temperature process because deposition temperature is limited by the water boiling temperature of the precursor reaction solution. This particularity of CBD makes it very convenient for the deposition of semiconductor thin films on plastic substrates, which low thermal resistance requires low temperature processing. In the last few years, studies of the deposition of semiconductor thin films on plastic substrates are of increasing interest. This is because the advantages of light weight, transparency, flexibility and low cost for the development of flexible devices [1–4]. Semiconductor films have been deposited on several types of plastic substrates such as polyethylene naphthalate (PEN), polyethylene terephthalate, polycarbonate, acrylic, etc. [5–11]. Among these, PEN is one of the plastic substrates most often employed
⁎ Corresponding author. Tel.: + 56 41 220 73 40; fax: + 56 41 222 45 20. E-mail address: [email protected] (M.G. Sandoval-Paz). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.08.006
because of its mechanical and optical properties. It is a semicrystalline transparent polymeric material with glass transition temperature around 125 °C and working temperature up to 155 °C. CdS thin films have been deposited on flexible commercial viewfoil substrates by CBD and pulsed laser deposition [1,4]. In both cases, adherent, homogeneous CdS polycrystalline films with hexagonal structure were deposited. Furthermore, despite the amorphous nature of the substrate and the low temperature processing, the CdS films had preferred crystalline orientation along the (002) direction. These facts ensure that CdS thin films deposited on plastic substrates can attain similar characteristics to those of films deposited on conventional rigid substrates such as glass, silicon, etc. Therefore, CdS thin films deposited on plastic substrates can provide alternatives to take advantage of the properties of this semiconductor material, where low weight and flexibility are of major importance. In this work we have studied the properties of CdS thin films deposited by CBD on PEN substrates. For this, we employed a very convenient ammonia-free CBD process based on sodium citrate as the complexing agent, which results in highly oriented CdS polycrystalline films when deposited on glass substrates [12,13]. In recent papers we have shown that these ammonia-free chemically deposited CdS films perform reasonably well as window layers in CdS– CdTe thin film solar cells [14] and as active layers in field effect thin film transistors [15]. The aim of this work was first to explore the formation and adherence of the CdS films to the plastic substrate and after these issues were ensured, to determine the optical and structural properties
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of the films as a function of the cadmium content in the precursor solution.
a) M1 films 80
[Cd]=0.01M
T
2. Experimental details
3. Results and discussion In Fig. 1 are shown the optical reflectance (R) and transmittance (T) spectra of a) CdS M1 and b) M2 films, respectively, deposited on PEN substrates at different deposition times. These optical spectra display similar characteristics to those corresponding to CdS films deposited on glass substrates [13], with absorption edge at about 480–500 nm in the T spectra. The transmittance of the film-substrate system is limited by the transmittance of the PEN substrate, which is about 85% in the entire visible spectral region [11]. The sharp absorption edge observed at about 400 nm in the T spectra is produced by the optical absorption of PEN. The T values at wavelengths larger than the absorption edge are between 67 and 85% for M1 films and 68–78% for M2 films. It can be seen that the reflectance in both types of films is from 6 to 31% in all the wavelength range. It is evident from R and T spectra that the thicknesses of M2 films are smaller than the corresponding ones in M1 films, indicated by the wavelength values of maxima and minima at the R spectra. On the other hand, all R and T spectra of M3 films deposited at different times were very similar (not shown herein), revealing that their thicknesses are about the same and that the final thickness is reached at very short deposition times for this reaction solution. In Fig. 2 are shown the R and T spectra of the M1, M2 and M3 CdS films deposited for 30 min on glass and PEN substrates. It can be observed similar behavior in the transmittance and reflectance spectra of the films deposited on the different types of substrates. The
15 min 30 min 60 min 90 min 120 min
T, R (%)
60
40
R
20
0 80
b) M2 films
T
[Cd]=7.5x10-3M
T, R (%)
60
40
R
20
0 300
400
500
600
700
800
Wavelength (nm) Fig. 1. Optical transmittance (T) and reflectance (R) spectra of a) CdS M1 and b) M2 films deposited at different times.
wavelength values of maxima and minima of interference fringes in R and T spectra roughly coincide for the same type of films on both glass and PEN substrates. Since these maxima and minima of interference directly provide the film thickness, according to reference [17], it is 100 CdS 30 min
1
80
2) M2, [Cd]=7.5x10-3M
2
3) M3, [Cd]=6.8x10-3M
3 3
60
40
PEN glass best fit R
3
3 2 20
2 1
0 200
T
1
1) M1, [Cd]=0.01M
T, R (%)
Three sets of CdS films were deposited on PEN substrates labeled as M1, M2 and M3, respectively. The recipes for preparing the M1 and M2 films are the same as those described in Ref. [16] for the CdS films labeled as AF1 and AF2, respectively, and consist of mixtures of aqueous solutions of CdCl2, sodium citrate, KOH, buffer pH 10 and thiourea, with lower amounts of cadmium, sodium citrate and thiourea in M2 precursor solution respect to that of M1 one. M3 films were obtained from a solution with even less cadmium and thiourea with respect to M2 ones. The concentrations of cadmium in reaction solutions M1, M2 and M3 were 0.01, 7.5× 10−3 and 6.8 × 10 −3 M, respectively. The ratios of concentrations [Cd]/[thiourea] and [Cd]/[Citrate] were 0.2 and 0.1 M/M, respectively, in each one of baths. When lower cadmium concentration was used in the solution than that for M3 films, there was no formation of a well adhered CdS film to the PEN substrates. The temperature of the reaction solution was set at 70 °C and there was no stirring during the deposition process. Each set of films was obtained by placing five 1 × 3 square inches PEN substrates in the reaction beaker and subsequently removing them from the solution after 15, 30, 60, 90 and 120 min, respectively. For comparison and for film thickness studies the same sets of films were also grown on glass substrates. All the CdS films were homogeneous, specularly reflecting, yellowish and adhered very well to the PEN substrates. That is, the formation of the CdS films on the plastic substrates and their adherence were very similar to the corresponding on glass substrates. The crystalline structure of the films was studied by X-ray diffraction (XRD) measurements obtained with a Rigaku D/max2100 X-ray diffractometer (with a Co source, λ = 1.78899 Å), employing a thin film attachment in order to measure in the glancing angle mode equivalent to the asymmetric Bragg-Bretano geometry. The transmission and reflection optical spectra of the films were obtained under near-normal incidence with a Film Tek™ 3000 spectrometer in the UV– Visible spectral range. The surface morphology of the samples was analyzed by atomic force microscopy (AFM) images by using an Instrument Veeco Dimension 3100 System, Nanoscope IV. The AFM images were measured in the tapping mode on 1 × 1 μm2 areas of the surface samples.
300
400
500
600
1
700
800
Wavelength (nm) Fig. 2. T and R spectra of CdS thin films deposited on PEN (dashed lines) and glass (solid lines) substrates at 30 min. Dotted lines represent the best fit to the two layers model.
M.G. Sandoval-Paz, R. Ramírez-Bon / Thin Solid Films 520 (2011) 999–1004
−αd
T = ð1−RÞe
ð1Þ
where d is the film thickness, and it is considered that the substrate is a non-absorbent material. In order to estimate the energy band gap, Eg, of the films, we used the model for allowed direct transitions between parabolic energy bands, given by [20] 1 = 2 α ðEÞ = A E−Eg
ð2Þ
with E the incident radiation energy and A is a constant. According to the model, the energy gap is obtained by extrapolating the linear portion of α 2 versus E plot to E = 0. In Fig. 4 the α 2 versus E plots, for M1, M2 and M3 films deposited during 60 min are shown. The solid lines correspond to experimental data and dotted ones to the best fit to a straight line, according to the theoretical model. The energy band gap of all films was determined from these adjustments and the average value of Eg for each set of CdS films was 2.51, 2.43 and 2.44 eV
1.0
CdS 60 min 0.8
0.6
2
2.4
for M1, M2 and M3 films, respectively. The energy band gap in M2 and M3 films decreased with respect to M1 films. Variations in the energy band gap of chemically deposited CdS films have been associated with variations in either their lattice constant or grain size [21,22]. For example, A.E. Rakhshani and A.S. Al-Azab reported an increase in the average grain size and a decrease in the energy band gap with increasing film thickness, so the thickness dependence of the band gap energy is attributed to the effect of lattice strain [22]. In Fig. 5 it is shown the variation of the energy band gap with the thickness for M1, M2 and M3 films, where it is observed a clear trend of increasing energy band gap with increasing film thickness, in contrast to the results of Rakhshani and Al-Azab. Similar results were obtained in previous work where the energy band gap energy is plotted as a function of deposition time in CdS films deposited on glass substrates [12]. In order to correlate the values of energy band gap with the structural properties of the CdS films, X-ray diffraction patterns were obtained for all the films. In Fig. 6 are shown the XRD patterns of CdS M1 and M2 films at different deposition times. All these patterns display only one diffraction peak at about 31°, which is related with the (002) diffraction line of the CdS hexagonal crystalline phase, according to the results obtained in films deposited on glass substrates [16]. The broad
CdS/PEN fims M1, [Cd]=0.01M M2, [Cd]=7.5x10-3M M3, [Cd]=6.8x10-3M
2.52
Energy band gap (eV)
160
100
2.8
Fig. 4. Energy dependence of α2 for CdS films deposited on PEN substrates for 60 min.
2.54
120
2.6
Energy (eV)
M2, [Cd]=7.5x10-3M
140
1
3 0.4
0.0
M3, [Cd]=6.8x10-3M
Thickness (nm)
-3 2) M2, [Cd]=7.5x10 M, Eg=2.43 eV
0.2
M1, [Cd]=0.01M
180
1) M1, [Cd]=0.01M,Eg=2.52 eV 3) M3, [Cd]=6.8x10-3M, Eg=2.44 eV
α 2 (x1010 cm-2)
expected that CdS films have similar thickness when deposited on PEN or on glass substrates. The thickness of the M1, M2 and M3 films was determined by fitting the T and R spectra of the films using a model of two layers which represents the CdS layer on the glass substrate and a roughened external layer. The CdS optical constants were represented by a dispersion relation which is a generalized version of Lorenz Oscillator Model and the roughened layer was modeled by an effective medium approximation as described in reference [18]. The best fits are plotted as dotted lines in Fig. 2. It can be seen that the modeled transmittance and reflectance spectra reproduce the measured spectra over the full wavelength range. The direct adjustment of T and R spectra of the films on PEN substrates was not possible because the PEN is an anisotropic material [19] whose optical properties cannot be simulated with Film Wizard™ software used. It is therefore assumed that the thickness obtained in the films deposited on glass substrates is similar to that of the films on PEN substrates. The thickness values obtained from the fitting procedure are plotted as a function of deposition time in Fig. 3. The film thickness decreases as the reagents concentration in reaction solution decreases as seen for the M1 to M3 films. In addition, the films M2 and M3 have very short effective termination times, about 30 min, and the final thicknesses for M1 to M3 films were 175, 100 and 73 nm, respectively. The optical absorption coefficient, α, of CdS films deposited on PEN substrates was determined from measured transmittance and reflectance spectra, using the relation
1001
2.50
2.48
2.46
2.44 80 2.42 60 0
30
60
90
120
Deposition time (min) Fig. 3. Growth kinetics of the CdS M1, M2 and M3 films.
60
80
100
120
140
160
180
Film thickness (nm) Fig. 5. Band gap energy versus film thickness of the CdS films deposited on PEN substrates.
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(a) M1 films [Cd]=0.01M
Intensity (Arb. units)
8000
6000 120 min 90 min
4000
60 min 2000
30 min 15 min
0 8000
(b) M2 films
Intensity (Arb. units)
[Cd]=7.5x10-3M 6000 120 min 90 min
4000
60 min 30 min
2000
15 min 0 25
30
35
40
2 Theta (Degrees) Fig. 6. X-ray diffraction patterns of CdS M1 and M2 films deposited on PEN substrates at different deposition times.
intense shoulder at the left of the (002) diffraction peaks of CdS is produced by the PEN substrates. These patterns show that CdS films on PEN glass substrates are highly oriented along the (002) crystalline direction. The diffraction peak in the XRD patterns of all the films was fit to a Gaussian function to determine the Bragg angle and full width at half maximum (FWHM) of the peak, from these parameters were determined the values of the lattice constant c and the averaged grain size of the films. The values of the lattice constant were calculated by means of the equation for the interplanar distances for a hexagonal crystalline system [23]. As the grains in the films are preferentially oriented in the [002] direction, the lattice parameter reduces to c = 2 d002, and by using the Bragg's equation we have c = λ/sin θB, where λ is the X-rays wavelength and θB is the Bragg's angle. The grain size L of the films was calculated from the FWHM of the peaks by means of the Scherrer formula, L = 0.9λ/B cos θB, where B is the FWHM. The Scherrer's equation is widely used for estimating the size of crystallites. The value of L represents the diameter of crystallites, perpendicular to the plane which corresponds to the measured diffraction peak. If the diameter of crystallites becomes small, for example, down to about 50 nm, the peak broadening can be appreciable. To be more precise, what is measured is not necessarily crystal size but coherence length, the length over which the periodicity of the crystal is complete [24]. Other causes for XRD peaks being broader than expected based on crystal size is the presence of strain in the crystals or other defects, such as dislocations, which destroy the long-range lattice order. Separation between crystal size and strain can be made if several different peaks are present, since the angular dependences of the two factors are different [25]. In the case of our films, this separation cannot be done because only one diffraction peak appears in the pattern (Fig. 6). For the cases where the size of crystallites in a sample of interest lies in the range about 5 nm below 50 nm, this can be computed within the error of about ± several percent when using the Scherrer formula, this is probably due to the so-called instrumental broadening factor, as well as
the fact that the FWHM determination is made by fitting a Gaussian function. Thus, the interpretation of XRD peak broadening should be carried out with care and preferably using complementary TEM measurements. However, the valuesof L obtained from the XRD peaks are in agreement with those reported in the literature for CdS films obtained by CBD [24]. In Fig. 7 is shown the thickness dependence of averaged grain size L for CdS M1, M2 and M3 films. It can be seen that for each set of samples, the grain size decreases as the film thickness increases. There is a general trend of the grain size to decrease with increasing film thickness. It can also be seen that the larger grains are obtained in formulations with lower concentrations of reactants in reaction solution, i.e. M3 films have larger grains than M1 ones. Larger crystal sizes are related to an ion by ion growth mechanism, which happens slower than the cluster by cluster mechanism [24]. It sounds reasonable that the growth of M2 and M3 films occur slower bearing in mind that the formulations M2 and M3 are more diluted (lower concentrations of cadmium and thiourea) respect to M1. In fact, the growth rate observed in Fig. 3 is a clear evidence of it. Furthermore, in a previous work we studied the growth kinetics of M1 and M2 films deposited on glass substrates [16] and it was shown that in the early stages, the growth mechanism is of type ion by ion, while at larger times the film deposition is through the aggregation mechanism. The latter mechanism promotes homogeneous precipitation, thus it is consistent to expect that films with larger thickness deposited at larger times have smaller grains. In addition to grain size, the c lattice constant and the strain of the CdS thin films were also determined from XRD measurements. The values of the c lattice constant in our films are smaller than those of the material in bulk (co = 6.713 Å), and they vary from one formulation to another. The shrinkage of the c lattice constant which is oriented along the (002) crystalline direction, perpendicular to the substrate surface, indicates that the crystalline lattice of the (002) oriented CdS films is under tensile strain. The amount of tensile strain depending on the amount of cadmium in the precursor solution, as shown by the c lattice constant values which are smaller for CdS M1 films than for M2 and M3 films. The percentage of variation of the lattice constant Δc/co was calculated and plotted as a function of the grain size, shown in Fig. 8. A clear trend to decrease of the Δc/co parameter with the increasing of grain size can be seen in this figure. This behavior is reasonable if it is considered that in the CdS films with larger grains, the slower deposition rate relaxes the tensile strain. Therefore, the results about the crystalline parameters determined from XRD measurements provided information about the chemical deposition mechanisms of CdS layers on PEN substrates, which
CdS/PEN films 30
Averaged grain size (nm)
1002
M1, [Cd]=0.01M M2, [Cd]=7.5x10-3M M3, [Cd]=6.8x10-3M
25
20
15 60
80
100
120
140
160
180
Film thickness (nm) Fig. 7. Averaged grain size versus film thickness of the CdS films deposited on PEN substrates.
M.G. Sandoval-Paz, R. Ramírez-Bon / Thin Solid Films 520 (2011) 999–1004
1.0
CdS/PEN films
0.9
M1, [Cd]=0.01M M2, [Cd]=7.5x10-3M M3, [Cd]=6.8x10-3M
Δc /c0 (%)
0.8 0.7 0.6 0.5 0.4 0.3 0.2 18
24
30
Grain size (nm)
1003
the fact that the energy band gap variation is not produced only by the grain size variation but also by the lattice constant variation. In any case, our results show that the energy band gap of the CdS films on PEN substrates can be tuned in the range 2.42–2.54 eV by adjusting the concentration of the reagents in the precursor reaction solution. In Fig. 10 are shown the two-dimensional AFM images of the surface morphology of a) M1, b) M2 and c) M3 films deposited during 60 min. The surface of the three films is flat, smooth and compact with a granular structure. The rms roughness values determined from these images were 10.4, 15 and 19.7 nm for M1, M2 and M3 films, respectively. The difference in grains size is appreciable in this figure, in accordance with the results obtained from XRD. On the other hand, the grain shape of M1 films is more likely a cauliflower also observed in other chemically deposited CdS films [30], while the grain morphology of M2 and M3 films is rather different to that of the M1 films. The granular structure of M2 and M3 films, deposited with lower reactants concentration, displays better defined grains. 4. Conclusions
Fig. 8. Percentage of variation of the lattice parameter Δc/c0 as a function the grain size for CdS M1, M2 and M3 films deposited on PEN substrates.
are consistent with those observed in similar CdS-CBD processes on glass substrates [16,26]. As stated above, the variations in the energy band gap of chemically deposited CdS films have been related with variations in either their grain size or lattice constant. The effect of grain size on the energy band gap of many chemically deposited chalcogenide films has been explained in terms of quantum confinement due to the reduced size of the crystallites [27,28]. On the other hand, tensile strain on grains, parallel to the substrate surface, produces shrinkage of the lattice constant c along the [002] direction, perpendicular to the substrate in highly oriented films. In Fig. 9 are plotted the energy band gap of the CdS films as a function of the grain size (empty symbols) and lattice constant (filled symbols), respectively. A trend is observed of the energy band gap of the films to increase with reducing either the grain size or lattice constant. In this figure, c0 indicates the lattice constant of bulk CdS. These results suggest that both crystalline parameters influence the energy band gap of the CdS films. In the case of the energy band gap versus grain size behavior the expected r −2 trend predicted by some theoretical models is not clear, where r is the particle size of the semiconductor material [29]. This could be due to
In this work we have deposited homogeneous CdS thin films which adhered very well to PEN substrates, by means of an ammonia-free CBD process. We determined the influence of the Cd concentration in the
51.4 nm
a)
0 nm 54.9 nm
b)
Grain size (nm) 2.56
16
18
20
22
24
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32
M1, [Cd]=0.01M M2, [Cd]=7.5X10-3M M3, [Cd]=6.8x10-3M
2.54
Energy band gap (eV)
26
2.52
0 nm 2.50
c0 2.48
55.9 nm
c)
2.46 2.44 2.42 6.65
6.66
6.67
6.68
6.69
6.70
6.71
Lattice parameter (°A) Fig. 9. Energy band gap of the CdS films as a function of grain size (empty symbols) and lattice constant (filled symbols).
0 nm Fig. 10. AFM surface images of a) M1, b) M2, and c) M3 films deposited during 60 min.
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precursor reaction solution on the optical and structural properties of the films. The CdS films were polycrystalline with hexagonal structure and preferred crystalline orientation along the (002) direction. We found that when the reaction solution contains higher amount of Cd, thicker CdS films with smaller grain size and shorter lattice constant are deposited. The variation of these crystalline parameters produces a variation of the energy band gap of the CdS films in the range 2.42–2.54 eV, which depends on the reactive concentrations in the precursor solution. Acknowledgments This work was partially supported by CONACYT-Mexico (project No. CB-2008-01-106952) and partially by FONDECYT-Chile (project No. 11090434). The technical assistance of M.A. Hernández-Landaverde and C. A. Avila-Herrera is also acknowledged. References [1] M.S. Shur, S. Rumyantsev, R. Gaska, B.Q. Wei, R. Vajtai, P.M. Ajayan, J. Sinius, Solid State Electron. 46 (2002) 1417. [2] K. Song, J. Hu, R. Shrestha, F. Zhu, S. Jin Chua, Thin. Solid. Films 477 (2005) 32. [3] S. Richter, M. Ploetner, W.J. Fischer, M. Schneider, P.T. Nguyen, W. Plieth, N. Kiriy, H.-J.P. Adler, Thin. Solid. Films 477 (2005) 140. [4] K.P. Acharya, J.R. Skuza, R.A. Lukaszew, C. Liyanage, B. Ullrich, J. Phys. Condens. Matter 19 (2007) 196221. [5] J.H. Shin, S.H. Shin, J.I. Park, H.H. Kim, J. Appl. Phys. 89 (2001) 5199. [6] F.L. Wong, M.K. Fung, S.W. Tong, C.S. Lee, S.T. Lee, Thin Solid Films 466 (2004) 225. [7] H. Kim, J.S. Horwitz, G.P. Kushto, Z.H. Kafafi, D.B. Chrisey, Appl. Phys. Lett. 79 (2001) 284. [8] P.F. Carcía, R.S. McLean, M.H. Reilly, Z.G. Li, L.J. Pillione, R.F. Messier, J. Vac. Sci. Technol. A 21 (2003) 745.
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