- Email: [email protected]
Structural and optical studies on thermal-annealed In2S3 films prepared by the chemical bath deposition technique
Thin Solid Films 472 (2005) 5 – 10 www.elsevier.com/locate/tsf
Structural and optical studies on thermal-annealed In2S3 films prepared by the chemical bath deposition technique M.G. Sandoval-Paz a, M. Sotelo-Lerma a, J.J. Valenzuela-Ja´uregui b, M. Flores-Acosta b,c, R. Ramı´rez-Bon b,* a b
Centro de Investigacio´n en Polı´meros y Materiales, Universidad de Sonora, Apdo. Postal 130, Hermosillo, Son. 83190, Mexico Centro de Investigacio´n y Estudios Avanzados del IPN, Unidad Quere´taro, Apdo. Postal 1-798, Quere´taro, Qro. 76001, Mexico c Centro de Investigacio´n en Fı´sica, Universidad de Sonora, Apdo. Postal 5-88, Hermosillo, Son. 83190, Mexico Received 4 November 2003; received in revised form 21 May 2004; accepted 21 May 2004 Available online 3 August 2004
Abstract Polycrystalline In2S3 films were grown on glass substrates by means of the chemical bath method and subsequently thermal-annealed in an Ar atmosphere at temperatures from 200 to 450 jC. The optical and structural properties of the films were studied as a function of the annealing temperature. The experimental results show that the as-deposited films are composed by a mixture of both cubic a and h crystalline phases, with some fraction of tetragonal phase. The thermal annealing on the films produces the conversion of the cubic crystalline phases to the tetragonal h one and a crystalline reorientation of the latter phase. Two energy band gaps were determined for all the films: one indirect and other direct at higher energy. The structural modifications of the films are accompanied by changes in the two energy band gaps of the films. D 2004 Elsevier B.V. All rights reserved. PACS: 81.16.Be; 81.40. z; 78.66.Li Keywords: Indium sulfide; Thermal annealing; Thin films; Structural transition; Deposition process; Optical properties; Structural properties; Chalcogens; Chemical deposition; Chalcogenides
1. Introduction Solar cells based on heterostructures using CdS as the window buffer layer have been the focus of intensive research in the last few years because they have attained very promising conversion efficiencies. Typically, the CdS layer is deposited by means of the chemical bath deposition (CBD) technique, which is very suitable for large area substrate deposition. However, the application of CBD to the growth of the CdS layer in large-scale production of solar cells represents a serious environmental problem because of the great amount of Cd-containing waste and the utilisation of ammonia in the process [1 –3]. There are several recent papers that report important advancements about the partial solution of this environmental problem [1– 6]. For these reasons, it is important to investigate
* Corresponding author. Tel.: +52-442-4-41-49-06; fax: +52-442-4-4149-39. E-mail address: [email protected] (R. Ramı´rez-Bon). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.05.096
about other semiconductor materials with the appropriate properties to substitute the CdS buffer layer in the solar cell heterostructure. Several semiconductor materials have been investigated for the purpose of replacing the CdS buffer layer [7– 8]. One of the most promising semiconductor materials studied is In2S3. There are reports about solar cells based on Cu(In,Ga)Se2 – In2S3 heterostructures that have achieved efficiencies as high as 15.7% [9,10]. Like CdS, In2S3 films can be prepared by the CBD method, keeping the possibility of large area deposition. Several crystalline phases have been reported for In2S3 films (a, h, and g) deposited by several techniques, with the tetragonal h phase being the most stable at room temperature. This is the most common crystalline phase observed in In2S3 films [11 – 13]. The higher temperature cubic a phase has been observed in chemically deposited In2S3 films onto glass substrates as a mixture with the cubic h phase [14]. Amorphous In2S3 films onto glass substrates have also been obtained by the CBD method [15]. On the other hand, about the optical properties of In2S3 films, the energy band gap takes values between 2.0
6
M.G. Sandoval-Paz et al. / Thin Solid Films 472 (2005) 5–10
and 2.75 eV, depending on the composition and deposition parameters [8,15]. There are reports on direct gap and indirect gap transitions. h-In2S3 films present a direct energy band gap with a value about 2 eV [12,13]. However, there are reports that this energy gap is indirect with a value about 2.2 eV [11]. In this work, we have deposited polycrystalline In2S3 films by means of the CBD technique. The films were thermalannealed in an Ar atmosphere at temperatures from 200 to 450 jC. We report here the structural and optical modifications undergone by the films as consequence of the annealing process.
2. Experimental details The deposition of In2S3 films was done in a reactive solution prepared in a 100-ml beaker by the sequential addition of 10 ml of 0.1 M InCl3, 20 ml of 0.5 M acetic acid, and 20 ml of 1 M thioacetamide. This solution was also added with distilled water to complete a total volume of 100 ml. The initial pH of the solution was about 2.5. Commercial glass slide substrates were placed in the reaction solution at a temperature of 30 jC, and it was found that the optimum deposition time was 48 h. The resulting In2S3 film cases were homogeneous, yellowish, hard, specularly reflecting, and very well adhered to the substrate. The measured thickness of the films was about 150 nm. The thermal annealing was performed on sample pieces with an area of about 1 cm2. The pieces were placed in a quartz tube of a hot wall furnace during 1 h at temperatures from 200 to 450 jC. During the annealing process, a flow of argon was passed through the tube. The thickness of the films was measured with a Dektar II surface profile measuring system. The crystalline structure of the as-deposited and annealing films was studied by X-ray diffraction (XRD) patterns measured in a Rigaku D/max-2100 X-ray diffractometer. The transmission and reflection optical spectra of the samples were measured with a Film Tekk 3000 spectrometer. The surface morphology of the samples was investigated by scanning electron microscopy (SEM) using an XL 30 ESEM Phillips microscope.
3. Results and discussion The as-deposited films were polycrystalline as shown by the XRD pattern at the bottom of Fig. 1, where the XRD patterns of annealed films at temperatures from 200 to 450 jC are also shown. The pattern of the as-deposited film displays broad diffraction peaks at about 28j, 34j, and 48j. These peaks coincide with the most intense diffraction signals of three different In2S3 crystalline phases. The diffraction peaks could be produced by the (109), (0012), and (2212) crystalline planes of the tetragonal h-In2S3 phase (JCPDS no. 25-0390 [16]) or by the (311), (400), and (440)
Fig. 1. X-ray diffraction patterns of as-deposited and thermal-annealed In2S3 films. The indices correspond to the crystalline planes of the tetragonal h phase.
crystalline planes of the cubic h-In2S3 phase (JCPDS no. 32-0456 [17]), or by the (111), (200), and (220) crystalline planes of the cubic a-In2S3 phase (JCPDS no. 05-0731 [18]). In Table 1, the 2h values for the diffraction peaks observed in the diffraction patterns of the as-deposited film and of the In2S3 films annealed at 300 and 400 jC are shown. For comparison, in this table, the relative intensity, the (hkl) crystalline plane indices, and the 2h values corresponding to the diffraction peaks in the powder XRD patterns of the a-cubic, h-cubic, and h-tetragonal In2S3 crystalline phases are also shown. Only the diffraction peaks of the three crystalline phases, which could be assigned to some of the peaks in the XRD patterns of the In2S3 films, are included in this table. Although the tetragonal h phase is the room temperature stable phase, it has been suggested that chemically deposited In2S3 films are composed by a mixture of cubic a and h phases [19]. Recent electron diffraction studies on this type of In2S3 films have shown that they are indeed constituted by the mixture of both cubic phases [14]. In our case, the 2h values for the three peaks in the pattern of the as-deposited film match better with those of the a-cubic phase, as can be seen in Table 1. However, the wide broadness of the three diffraction peaks, mainly the peak at about 28j, could indicate also the presence of the hcubic and h-tetragonal phases in the as-deposited films. The annealing of the films at 200, 300, and 350 jC produces the appearance of an intense diffraction peak in their XRD patterns at about 11j, which coincides with the diffraction signal produced by the (004) crystalline planes of the tetragonal h phase (see Table 1). The high relative intensity
M.G. Sandoval-Paz et al. / Thin Solid Films 472 (2005) 5–10
7
Table 1 Values of 2h for the diffraction peaks in the XRD patterns of the as-deposited In2S3 film and of the In2S3 films annealed at 300 and 400 jC, and intensity, crystalline plane indices (hkl), and 2h values for the corresponding diffraction peaks of the a-cubic, h-cubic, and h-tetragonal phases of powder In2S3 a-Cubic
In2S3 films
h-Cubic
h-Tetragonal
As-deposited [j]
300 jC [j]
400 jC [j]
Intensity
(hkl)
2h [j]
Intensity
(hkl)
2h [j]
Intensity
(hkl)
2h [j]
– – 28.27 33.60 – 48.36
10.84 – 28.27 33.56 – 48.12
– 23.24 27.64 33.28 43.60 47.84
– – 40 60 10 100
– – 111 200 – 220
– – 28.803 33.445 43.848 47.995
– 35 65 50 75 100
– 220 311 400 511 440
– 23.713 27.532 33.394 43.781 47.914
4 18 100 50 45 65
004 116 109 0012 1015 2212
10.927 23.322 27.429 33.228 43.604 47.700
of this diffraction peak shows that the h-tetragonal phase in these films has a high preferred orientation along the [004] crystalline direction, induced by the thermal annealing. None of the cubic phases presents a diffraction peak at this value of 2h. This is a first indication that thermal annealing induces a structural transition on the In2S3 films from the cubic to the tetragonal phase and a crystalline reorientation of the latter phase. The XRD patterns of the films annealed at 400 and 450 jC have sharper diffraction peaks and do not display the (004) diffraction line. It is also observed in these patterns that the center of the diffraction peaks at 28j, 34j, and 48j shifts slightly to lower values of 2h. The 2h values for the peaks in the pattern of the film annealed at 400 jC are shown in Table 1. Since the diffraction lines of the tetragonal h phase at about 28j, 34j, and 48j lie at lower values of 2h than the corresponding diffraction lines of both cubic phases, as shown in Table 1, it could be assumed that the shift of these diffraction peaks is produced by the structural transition of In2S3 films from the cubic phase to the tetragonal one. In addition, very weak diffraction signals
Fig. 2. Transmission and reflection spectra of as-deposited and thermalannealed In2S3 films.
at about 14j, 23.2j, and 44j corresponding to the tetragonal h-In2S3 phase can be seen in these patterns. The 2h data in Table 1 for the film annealed at 400 jC match quite well with the corresponding data for the h-tetragonal In2S3 crystalline phase. A preferred crystalline orientation in this film is not observed, neither in the film annealed at 450 jC. In Fig. 2 are shown the transmission (T) and reflection (R) spectra of as-deposited and annealed In2S3 films. The optical transmission spectrum of the as-deposited film displays its absorption edge at about 500 nm, and it shifts to higher wavelength for the annealed films. A second absorption edge can be seen as a shoulder in some transmission spectra at about 420 – 440 nm. The transmission of all the films at wavelengths larger than the absorption edge has values between 65% and 85%. On the other hand, the reflection of all the films is between 5% and 25% in all the wavelength ranges. In the reflection spectra, the absorption edge of the films and its shifting to higher wavelength are also manifested. The two absorption edges can be more readily seen in the spectra given by the first numerical derivative of the transmission spectra, shown in Fig. 3. In this graph, the two absorption edges are defined as two broad peaks centered at
Fig. 3. First numerical derivative of the transmission spectra of Fig. 2.
8
M.G. Sandoval-Paz et al. / Thin Solid Films 472 (2005) 5–10
about 470 and 390 nm in the spectrum of the as-deposited film. The position of both peaks shifts to higher wavelength in the spectra of the annealed films. The peak corresponding to the second absorption edge is not very well defined in the spectra of the films annealed at 400 and 450 jC. Figs. 1 and 2 show that thermal annealing produces a shift to lower energy in the absorption edges of In2S3 films, which can be related to the change in their energy band gap. The energy band gap Eg of all the films was determined from their transmission spectra. Since there are reports about direct and indirect energy gaps for In2S3 films, we applied the models for both direct and indirect allowed transitions between parabolic energy bands [20]. For this, we plotted the spectra (a*E)2 (direct transitions) and (a*E)1/2 (indirect transitions) vs. E, and fitted the spectra to a straight line in the region of the absorption edge. Here, a is the absorption coefficient and E is the photon energy. The results are shown in Fig. 4 where the (a) (a*E)1/2 and (b) (a*E)2 vs. E spectra for the as-deposited films and films annealed at 350 jC are plotted as solid lines. The dotted lines in both graphs represent the best fitting to the theoretical models. From these results, it can be concluded that the first absorption edge in the In2S3 films corresponds to an indirect energy gap, and the second absorption edge at higher energy corresponds to a direct energy gap. Both energy gaps shift to lower energy as a consequence of the thermal annealing on the films, as observed in this figure. The dependence of the energy band gaps on the annealing temperature is shown in Fig. 5. The
Fig. 4. (a) (a*E)1/2 vs. E spectra and (b) (a*E)2 vs. E spectra for asdeposited and In2S3 films annealed at 350 jC.
Fig. 5. Dependence on annealing temperature of direct and indirect energy band gaps of In2S3 films.
temperature assigned to the as-deposited films was the deposition temperature, which is 30 jC. For the as-deposited In2S3 film, the indirect band gap is 2.12 eV and the direct one is 2.65 eV; these values decrease for films annealed at 200– 350 jC and then increase for films annealed at 400 –450jC. For the film annealed at 450 jC, the values of indirect and direct energy band gaps are 1.98 and 2.53 eV, respectively. Indirect energy band gaps of 2.2 and 1.85 eV have been determined from optical absorption measurements in tetragonal h-In2S3 films deposited by spray pyrolysis [11] and by chalcogenization of indium layers, respectively [13]. However, there are other reports indicating that tetragonal h-In2S3 films have a direct energy band gap of about 2.0 eV [12,13]. About the direct energy band gap observed at higher energy in our films, its value is similar to the direct energy band gap of 2.75 and 2.3 eV determined in amorphous In2S3 films deposited on glass substrates by the CBD and successive ionic layer adsorption and reaction techniques, respectively [15,21]. The decrease of the energy band gap with annealing temperature down to a minimum value and its subsequent increase at higher annealing temperatures have been observed in chemically deposited CdS films [22,23]. As-deposited CdS films with cubic crystalline structure undergo a structural transition to the most stable hexagonal phase when annealed at temperatures from 200 to 400 jC. The annealing temperature, 300 jC, at which the CdS films have the minimum value of energy band gap, has been considered the transition temperature. The CdS films annealed at 400 jC have the hexagonal crystalline structure. The behaviour with annealing temperature of the indirect energy band gap of In2S3 films is similar to that observed during the structural transition of CdS films. In the case of In2S3 films, considering the XRD information, the as-deposited films have a cubic structure (mixture of a and h phases), perhaps with a small fraction of the h-tetragonal phase, and the films annealed at 400 and
M.G. Sandoval-Paz et al. / Thin Solid Films 472 (2005) 5–10
9
about 2– 3, indicating the good stoichiometry of the films. The composition of the films was not affected by the thermal annealing. The surface morphology of the as-deposited and annealed films at 350 and 400jC, obtained by SEM, is shown in Fig. 6. The surface of the as-deposited film shows a uniform granular structure with very-well-defined grain boundaries and with some larger grains dispersed on the film surface. The grains have an irregular round shape. The film covers the entire substrate surface, and pinholes are not observed on the film. The surface of the film annealed at 350 jC is also uniform and displays a better-defined granular structure. The grains have a similar shape than those in the asdeposited film. The surface morphology of the film annealed at 400 jC is quite different. This surface presents larger grains, some of them with elongated shape. The different grain shape in this film can be due to the different crystalline structures [23]. Thus, the grain shape evolution with thermal annealing observed in these images supports the conclusion about the structural transition induced in the films.
4. Conclusions
Fig. 6. SEM images of the surface of In2S3 films: (a) as-deposited, (b) annealed at 350 jC, and (c) annealed at 400 jC.
450 jC have the tetragonal h structure. From the Eg vs. annealing temperature graph, it can be inferred that the structural transition temperature is between 200 and 350 jC because at these annealing temperatures, Eg takes the minimum values. In this case, the transition temperature is not as well defined as in the case of CdS films. The behaviour of the energy band gap as a function of temperature suggests that the structural transition of In2S3 films might be an order – disorder transition. Thus, the decrease of the energy band gap could be produced by an increase in the structural disorder induced by the structural change from cubic to tetragonal phase of In2S3 films. The minimum value of the energy band gap is attained at the maximum of structural disorder, which corresponds to films constituted by a mixture of both crystalline phases. The energy band gap increases for the films annealed at higher temperatures, which are composed mainly by the tetragonal h-phase. The composition of the films was determined from energy-dispersive X-ray analysis at several points. The results showed an average ratio of atomic percentage of In to S of
In this paper, we report the structural and optical properties of chemically deposited In2S3 polycrystalline films and their modifications produced by thermal annealing in an inert atmosphere. The optical transmission spectra of the films and its first derivative clearly exhibit two absorption edges. The fitting to the models of transitions between parabolic bands shows that the first absorption edge is produced by an indirect energy band gap and the second one at higher energy is due to a direct energy band gap. The XRD and SEM measurements, and the behaviour of the indirect energy band gap of the films with annealing temperature show that the annealing process induces a structural transition from a mixture of cubic and tetragonal phases to the tetragonal h one. In this transition process, the crystalline reorientation of the tetragonal phase is also observed. Acknowledgements We acknowledge the helpful technical assistance of J.E. Urbina-Alva´rez, M.A. Herna´ndez-Landaverde, and R. Flores-Farı´as. This work was partially supported by CONACyT (project no. 34514-U). References [1] B. Canava, J.F. Guillemoes, E.B. Yousfi, P. Cowache, H. Kerber, A. Schock, H.W. Schock, M. Powalla, D. Ariskos, D. Lincot, Thin Solid Films 361 – 362 (2000) 187. [2] D. Hariskos, M. Powalla, N. Chevaldonnet, D. Lincot, A. Schindler, B. Dimmler, Thin Solid Films 387 (2000) 179. [3] D.S. Boyle, A. Bayer, M.R. Heinrich, O. Robbe, P.O. O’Brien, Thin Solid Films 361 – 362 (2000) 150.
10
M.G. Sandoval-Paz et al. / Thin Solid Films 472 (2005) 5–10
[4] M. Kostoglu, N. Andritsos, A.J. Karabelas, Thin Solid Films 387 (2001) 115. [5] P. Nemec, I. Nemec, P. Nahalkova, K. Knizek, P. Maly, J. Cryst. Growth 240 (2002) 484. [6] M.B. Ortun˜o-Lo´pez, J.J. Valenzuela-Ja´uregui, M. Sotelo-Lerma, A. Mendoza-Galva´n, R. Ramı´rez-Bon, Thin Solid Films 429 (2003) 34. [7] J. Herrero, M.T. Gutierrez, C. Guille´n, J.M. Don˜a, M.A. Martı´nez, A.M. Chaparro, R. Bayo´n, Thin Solid Films 361 – 362 (2000) 28. [8] N. Barreau, S. Marsillac, D. Albertini, J.C. Bernede, Thin Solid Films 403 – 404 (2002) 331. [9] D. Hariskos, M. Ruckh, U. Ruhle, T. Walter, H. Werner Shock, J. Hedstrom, L. Stolt, Solar Energy Mater. Solar Cells 41 – 42 (1996) 345. [10] S. Spiering, D. Hariskos, M. Powalla, N. Naghavi, D. Lincot, Thin Solid Films 431 – 432 (2003) 359. [11] W.T. Kim, C.D. Kim, J. Appl. Phys. 60 (1986) 2631. [12] J. George, K.S. Joseph, B. Pradeep, T.I. Palson, Phys. Status Solidi, A 106 (1988) 123. [13] J. Herrero, J. Ortega, Solar Energy Mater. 17 (1988) 357.
[14] R. Bayon, J. Herrero, Appl. Surf. Sci. 158 (2000) 49. [15] C.D. Lockande, A. Ennaoui, P.S. Patil, M. Giersig, K. Diesner, M. Muller, H. Tributsch, Thin Solid Films 340 (1999) 18. [16] Powder Diffraction File, Joint Committee on Powder Diffraction Standards, ASTM, Philadelphia, PA, 1967, Card 250390. [17] Powder Diffraction File, Joint Committee on Powder Diffraction Standards, ASTM, Philadelphia, PA, 1967, Card 320456. [18] Powder Diffraction File, Joint Committee on Powder Diffraction Standards, ASTM, Philadelphia, PA, 1967, Card 050731. [19] G.A. Kitaev, V.Y. Dvoinin, A.V. Ust’yanseva, M.N. Belayeva, L.G. Skornyakov, Inorg. Mater. 12 (1976) 1448. [20] J.I. Pankove, Optical Processes in Semiconductors, Dover Publications, New York, 1971. [21] R.S. Mane, C.D. Lockande, Mater. Chem. Phys. 78 (2002) 15. [22] O. Zelaya-Angel, A.E. Esparza-Garcı´a, C. Falcony, R. LosadaMorales, R. Ramı´rez-Bon, Solid State Commun. 94 (1995) 81. [23] R. Ramı´rez-Bon, N.C. Sandoval-Inda, F.J. Espinoza-Beltra´n, M. Sotelo-Lerma, O. Zelaya-Angel, C. Falcony, J. Phys.: Condens. Matter 9 (1997) 10051.
Structural and optical studies on thermal-annealed In2S3 films prepared by the chemical bath deposition technique M.G. Sandoval-Paz a, M. Sotelo-Lerma a, J.J. Valenzuela-Ja´uregui b, M. Flores-Acosta b,c, R. Ramı´rez-Bon b,* a b
Centro de Investigacio´n en Polı´meros y Materiales, Universidad de Sonora, Apdo. Postal 130, Hermosillo, Son. 83190, Mexico Centro de Investigacio´n y Estudios Avanzados del IPN, Unidad Quere´taro, Apdo. Postal 1-798, Quere´taro, Qro. 76001, Mexico c Centro de Investigacio´n en Fı´sica, Universidad de Sonora, Apdo. Postal 5-88, Hermosillo, Son. 83190, Mexico Received 4 November 2003; received in revised form 21 May 2004; accepted 21 May 2004 Available online 3 August 2004
Abstract Polycrystalline In2S3 films were grown on glass substrates by means of the chemical bath method and subsequently thermal-annealed in an Ar atmosphere at temperatures from 200 to 450 jC. The optical and structural properties of the films were studied as a function of the annealing temperature. The experimental results show that the as-deposited films are composed by a mixture of both cubic a and h crystalline phases, with some fraction of tetragonal phase. The thermal annealing on the films produces the conversion of the cubic crystalline phases to the tetragonal h one and a crystalline reorientation of the latter phase. Two energy band gaps were determined for all the films: one indirect and other direct at higher energy. The structural modifications of the films are accompanied by changes in the two energy band gaps of the films. D 2004 Elsevier B.V. All rights reserved. PACS: 81.16.Be; 81.40. z; 78.66.Li Keywords: Indium sulfide; Thermal annealing; Thin films; Structural transition; Deposition process; Optical properties; Structural properties; Chalcogens; Chemical deposition; Chalcogenides
1. Introduction Solar cells based on heterostructures using CdS as the window buffer layer have been the focus of intensive research in the last few years because they have attained very promising conversion efficiencies. Typically, the CdS layer is deposited by means of the chemical bath deposition (CBD) technique, which is very suitable for large area substrate deposition. However, the application of CBD to the growth of the CdS layer in large-scale production of solar cells represents a serious environmental problem because of the great amount of Cd-containing waste and the utilisation of ammonia in the process [1 –3]. There are several recent papers that report important advancements about the partial solution of this environmental problem [1– 6]. For these reasons, it is important to investigate
* Corresponding author. Tel.: +52-442-4-41-49-06; fax: +52-442-4-4149-39. E-mail address: [email protected] (R. Ramı´rez-Bon). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.05.096
about other semiconductor materials with the appropriate properties to substitute the CdS buffer layer in the solar cell heterostructure. Several semiconductor materials have been investigated for the purpose of replacing the CdS buffer layer [7– 8]. One of the most promising semiconductor materials studied is In2S3. There are reports about solar cells based on Cu(In,Ga)Se2 – In2S3 heterostructures that have achieved efficiencies as high as 15.7% [9,10]. Like CdS, In2S3 films can be prepared by the CBD method, keeping the possibility of large area deposition. Several crystalline phases have been reported for In2S3 films (a, h, and g) deposited by several techniques, with the tetragonal h phase being the most stable at room temperature. This is the most common crystalline phase observed in In2S3 films [11 – 13]. The higher temperature cubic a phase has been observed in chemically deposited In2S3 films onto glass substrates as a mixture with the cubic h phase [14]. Amorphous In2S3 films onto glass substrates have also been obtained by the CBD method [15]. On the other hand, about the optical properties of In2S3 films, the energy band gap takes values between 2.0
6
M.G. Sandoval-Paz et al. / Thin Solid Films 472 (2005) 5–10
and 2.75 eV, depending on the composition and deposition parameters [8,15]. There are reports on direct gap and indirect gap transitions. h-In2S3 films present a direct energy band gap with a value about 2 eV [12,13]. However, there are reports that this energy gap is indirect with a value about 2.2 eV [11]. In this work, we have deposited polycrystalline In2S3 films by means of the CBD technique. The films were thermalannealed in an Ar atmosphere at temperatures from 200 to 450 jC. We report here the structural and optical modifications undergone by the films as consequence of the annealing process.
2. Experimental details The deposition of In2S3 films was done in a reactive solution prepared in a 100-ml beaker by the sequential addition of 10 ml of 0.1 M InCl3, 20 ml of 0.5 M acetic acid, and 20 ml of 1 M thioacetamide. This solution was also added with distilled water to complete a total volume of 100 ml. The initial pH of the solution was about 2.5. Commercial glass slide substrates were placed in the reaction solution at a temperature of 30 jC, and it was found that the optimum deposition time was 48 h. The resulting In2S3 film cases were homogeneous, yellowish, hard, specularly reflecting, and very well adhered to the substrate. The measured thickness of the films was about 150 nm. The thermal annealing was performed on sample pieces with an area of about 1 cm2. The pieces were placed in a quartz tube of a hot wall furnace during 1 h at temperatures from 200 to 450 jC. During the annealing process, a flow of argon was passed through the tube. The thickness of the films was measured with a Dektar II surface profile measuring system. The crystalline structure of the as-deposited and annealing films was studied by X-ray diffraction (XRD) patterns measured in a Rigaku D/max-2100 X-ray diffractometer. The transmission and reflection optical spectra of the samples were measured with a Film Tekk 3000 spectrometer. The surface morphology of the samples was investigated by scanning electron microscopy (SEM) using an XL 30 ESEM Phillips microscope.
3. Results and discussion The as-deposited films were polycrystalline as shown by the XRD pattern at the bottom of Fig. 1, where the XRD patterns of annealed films at temperatures from 200 to 450 jC are also shown. The pattern of the as-deposited film displays broad diffraction peaks at about 28j, 34j, and 48j. These peaks coincide with the most intense diffraction signals of three different In2S3 crystalline phases. The diffraction peaks could be produced by the (109), (0012), and (2212) crystalline planes of the tetragonal h-In2S3 phase (JCPDS no. 25-0390 [16]) or by the (311), (400), and (440)
Fig. 1. X-ray diffraction patterns of as-deposited and thermal-annealed In2S3 films. The indices correspond to the crystalline planes of the tetragonal h phase.
crystalline planes of the cubic h-In2S3 phase (JCPDS no. 32-0456 [17]), or by the (111), (200), and (220) crystalline planes of the cubic a-In2S3 phase (JCPDS no. 05-0731 [18]). In Table 1, the 2h values for the diffraction peaks observed in the diffraction patterns of the as-deposited film and of the In2S3 films annealed at 300 and 400 jC are shown. For comparison, in this table, the relative intensity, the (hkl) crystalline plane indices, and the 2h values corresponding to the diffraction peaks in the powder XRD patterns of the a-cubic, h-cubic, and h-tetragonal In2S3 crystalline phases are also shown. Only the diffraction peaks of the three crystalline phases, which could be assigned to some of the peaks in the XRD patterns of the In2S3 films, are included in this table. Although the tetragonal h phase is the room temperature stable phase, it has been suggested that chemically deposited In2S3 films are composed by a mixture of cubic a and h phases [19]. Recent electron diffraction studies on this type of In2S3 films have shown that they are indeed constituted by the mixture of both cubic phases [14]. In our case, the 2h values for the three peaks in the pattern of the as-deposited film match better with those of the a-cubic phase, as can be seen in Table 1. However, the wide broadness of the three diffraction peaks, mainly the peak at about 28j, could indicate also the presence of the hcubic and h-tetragonal phases in the as-deposited films. The annealing of the films at 200, 300, and 350 jC produces the appearance of an intense diffraction peak in their XRD patterns at about 11j, which coincides with the diffraction signal produced by the (004) crystalline planes of the tetragonal h phase (see Table 1). The high relative intensity
M.G. Sandoval-Paz et al. / Thin Solid Films 472 (2005) 5–10
7
Table 1 Values of 2h for the diffraction peaks in the XRD patterns of the as-deposited In2S3 film and of the In2S3 films annealed at 300 and 400 jC, and intensity, crystalline plane indices (hkl), and 2h values for the corresponding diffraction peaks of the a-cubic, h-cubic, and h-tetragonal phases of powder In2S3 a-Cubic
In2S3 films
h-Cubic
h-Tetragonal
As-deposited [j]
300 jC [j]
400 jC [j]
Intensity
(hkl)
2h [j]
Intensity
(hkl)
2h [j]
Intensity
(hkl)
2h [j]
– – 28.27 33.60 – 48.36
10.84 – 28.27 33.56 – 48.12
– 23.24 27.64 33.28 43.60 47.84
– – 40 60 10 100
– – 111 200 – 220
– – 28.803 33.445 43.848 47.995
– 35 65 50 75 100
– 220 311 400 511 440
– 23.713 27.532 33.394 43.781 47.914
4 18 100 50 45 65
004 116 109 0012 1015 2212
10.927 23.322 27.429 33.228 43.604 47.700
of this diffraction peak shows that the h-tetragonal phase in these films has a high preferred orientation along the [004] crystalline direction, induced by the thermal annealing. None of the cubic phases presents a diffraction peak at this value of 2h. This is a first indication that thermal annealing induces a structural transition on the In2S3 films from the cubic to the tetragonal phase and a crystalline reorientation of the latter phase. The XRD patterns of the films annealed at 400 and 450 jC have sharper diffraction peaks and do not display the (004) diffraction line. It is also observed in these patterns that the center of the diffraction peaks at 28j, 34j, and 48j shifts slightly to lower values of 2h. The 2h values for the peaks in the pattern of the film annealed at 400 jC are shown in Table 1. Since the diffraction lines of the tetragonal h phase at about 28j, 34j, and 48j lie at lower values of 2h than the corresponding diffraction lines of both cubic phases, as shown in Table 1, it could be assumed that the shift of these diffraction peaks is produced by the structural transition of In2S3 films from the cubic phase to the tetragonal one. In addition, very weak diffraction signals
Fig. 2. Transmission and reflection spectra of as-deposited and thermalannealed In2S3 films.
at about 14j, 23.2j, and 44j corresponding to the tetragonal h-In2S3 phase can be seen in these patterns. The 2h data in Table 1 for the film annealed at 400 jC match quite well with the corresponding data for the h-tetragonal In2S3 crystalline phase. A preferred crystalline orientation in this film is not observed, neither in the film annealed at 450 jC. In Fig. 2 are shown the transmission (T) and reflection (R) spectra of as-deposited and annealed In2S3 films. The optical transmission spectrum of the as-deposited film displays its absorption edge at about 500 nm, and it shifts to higher wavelength for the annealed films. A second absorption edge can be seen as a shoulder in some transmission spectra at about 420 – 440 nm. The transmission of all the films at wavelengths larger than the absorption edge has values between 65% and 85%. On the other hand, the reflection of all the films is between 5% and 25% in all the wavelength ranges. In the reflection spectra, the absorption edge of the films and its shifting to higher wavelength are also manifested. The two absorption edges can be more readily seen in the spectra given by the first numerical derivative of the transmission spectra, shown in Fig. 3. In this graph, the two absorption edges are defined as two broad peaks centered at
Fig. 3. First numerical derivative of the transmission spectra of Fig. 2.
8
M.G. Sandoval-Paz et al. / Thin Solid Films 472 (2005) 5–10
about 470 and 390 nm in the spectrum of the as-deposited film. The position of both peaks shifts to higher wavelength in the spectra of the annealed films. The peak corresponding to the second absorption edge is not very well defined in the spectra of the films annealed at 400 and 450 jC. Figs. 1 and 2 show that thermal annealing produces a shift to lower energy in the absorption edges of In2S3 films, which can be related to the change in their energy band gap. The energy band gap Eg of all the films was determined from their transmission spectra. Since there are reports about direct and indirect energy gaps for In2S3 films, we applied the models for both direct and indirect allowed transitions between parabolic energy bands [20]. For this, we plotted the spectra (a*E)2 (direct transitions) and (a*E)1/2 (indirect transitions) vs. E, and fitted the spectra to a straight line in the region of the absorption edge. Here, a is the absorption coefficient and E is the photon energy. The results are shown in Fig. 4 where the (a) (a*E)1/2 and (b) (a*E)2 vs. E spectra for the as-deposited films and films annealed at 350 jC are plotted as solid lines. The dotted lines in both graphs represent the best fitting to the theoretical models. From these results, it can be concluded that the first absorption edge in the In2S3 films corresponds to an indirect energy gap, and the second absorption edge at higher energy corresponds to a direct energy gap. Both energy gaps shift to lower energy as a consequence of the thermal annealing on the films, as observed in this figure. The dependence of the energy band gaps on the annealing temperature is shown in Fig. 5. The
Fig. 4. (a) (a*E)1/2 vs. E spectra and (b) (a*E)2 vs. E spectra for asdeposited and In2S3 films annealed at 350 jC.
Fig. 5. Dependence on annealing temperature of direct and indirect energy band gaps of In2S3 films.
temperature assigned to the as-deposited films was the deposition temperature, which is 30 jC. For the as-deposited In2S3 film, the indirect band gap is 2.12 eV and the direct one is 2.65 eV; these values decrease for films annealed at 200– 350 jC and then increase for films annealed at 400 –450jC. For the film annealed at 450 jC, the values of indirect and direct energy band gaps are 1.98 and 2.53 eV, respectively. Indirect energy band gaps of 2.2 and 1.85 eV have been determined from optical absorption measurements in tetragonal h-In2S3 films deposited by spray pyrolysis [11] and by chalcogenization of indium layers, respectively [13]. However, there are other reports indicating that tetragonal h-In2S3 films have a direct energy band gap of about 2.0 eV [12,13]. About the direct energy band gap observed at higher energy in our films, its value is similar to the direct energy band gap of 2.75 and 2.3 eV determined in amorphous In2S3 films deposited on glass substrates by the CBD and successive ionic layer adsorption and reaction techniques, respectively [15,21]. The decrease of the energy band gap with annealing temperature down to a minimum value and its subsequent increase at higher annealing temperatures have been observed in chemically deposited CdS films [22,23]. As-deposited CdS films with cubic crystalline structure undergo a structural transition to the most stable hexagonal phase when annealed at temperatures from 200 to 400 jC. The annealing temperature, 300 jC, at which the CdS films have the minimum value of energy band gap, has been considered the transition temperature. The CdS films annealed at 400 jC have the hexagonal crystalline structure. The behaviour with annealing temperature of the indirect energy band gap of In2S3 films is similar to that observed during the structural transition of CdS films. In the case of In2S3 films, considering the XRD information, the as-deposited films have a cubic structure (mixture of a and h phases), perhaps with a small fraction of the h-tetragonal phase, and the films annealed at 400 and
M.G. Sandoval-Paz et al. / Thin Solid Films 472 (2005) 5–10
9
about 2– 3, indicating the good stoichiometry of the films. The composition of the films was not affected by the thermal annealing. The surface morphology of the as-deposited and annealed films at 350 and 400jC, obtained by SEM, is shown in Fig. 6. The surface of the as-deposited film shows a uniform granular structure with very-well-defined grain boundaries and with some larger grains dispersed on the film surface. The grains have an irregular round shape. The film covers the entire substrate surface, and pinholes are not observed on the film. The surface of the film annealed at 350 jC is also uniform and displays a better-defined granular structure. The grains have a similar shape than those in the asdeposited film. The surface morphology of the film annealed at 400 jC is quite different. This surface presents larger grains, some of them with elongated shape. The different grain shape in this film can be due to the different crystalline structures [23]. Thus, the grain shape evolution with thermal annealing observed in these images supports the conclusion about the structural transition induced in the films.
4. Conclusions
Fig. 6. SEM images of the surface of In2S3 films: (a) as-deposited, (b) annealed at 350 jC, and (c) annealed at 400 jC.
450 jC have the tetragonal h structure. From the Eg vs. annealing temperature graph, it can be inferred that the structural transition temperature is between 200 and 350 jC because at these annealing temperatures, Eg takes the minimum values. In this case, the transition temperature is not as well defined as in the case of CdS films. The behaviour of the energy band gap as a function of temperature suggests that the structural transition of In2S3 films might be an order – disorder transition. Thus, the decrease of the energy band gap could be produced by an increase in the structural disorder induced by the structural change from cubic to tetragonal phase of In2S3 films. The minimum value of the energy band gap is attained at the maximum of structural disorder, which corresponds to films constituted by a mixture of both crystalline phases. The energy band gap increases for the films annealed at higher temperatures, which are composed mainly by the tetragonal h-phase. The composition of the films was determined from energy-dispersive X-ray analysis at several points. The results showed an average ratio of atomic percentage of In to S of
In this paper, we report the structural and optical properties of chemically deposited In2S3 polycrystalline films and their modifications produced by thermal annealing in an inert atmosphere. The optical transmission spectra of the films and its first derivative clearly exhibit two absorption edges. The fitting to the models of transitions between parabolic bands shows that the first absorption edge is produced by an indirect energy band gap and the second one at higher energy is due to a direct energy band gap. The XRD and SEM measurements, and the behaviour of the indirect energy band gap of the films with annealing temperature show that the annealing process induces a structural transition from a mixture of cubic and tetragonal phases to the tetragonal h one. In this transition process, the crystalline reorientation of the tetragonal phase is also observed. Acknowledgements We acknowledge the helpful technical assistance of J.E. Urbina-Alva´rez, M.A. Herna´ndez-Landaverde, and R. Flores-Farı´as. This work was partially supported by CONACyT (project no. 34514-U). References [1] B. Canava, J.F. Guillemoes, E.B. Yousfi, P. Cowache, H. Kerber, A. Schock, H.W. Schock, M. Powalla, D. Ariskos, D. Lincot, Thin Solid Films 361 – 362 (2000) 187. [2] D. Hariskos, M. Powalla, N. Chevaldonnet, D. Lincot, A. Schindler, B. Dimmler, Thin Solid Films 387 (2000) 179. [3] D.S. Boyle, A. Bayer, M.R. Heinrich, O. Robbe, P.O. O’Brien, Thin Solid Films 361 – 362 (2000) 150.
10
M.G. Sandoval-Paz et al. / Thin Solid Films 472 (2005) 5–10
[4] M. Kostoglu, N. Andritsos, A.J. Karabelas, Thin Solid Films 387 (2001) 115. [5] P. Nemec, I. Nemec, P. Nahalkova, K. Knizek, P. Maly, J. Cryst. Growth 240 (2002) 484. [6] M.B. Ortun˜o-Lo´pez, J.J. Valenzuela-Ja´uregui, M. Sotelo-Lerma, A. Mendoza-Galva´n, R. Ramı´rez-Bon, Thin Solid Films 429 (2003) 34. [7] J. Herrero, M.T. Gutierrez, C. Guille´n, J.M. Don˜a, M.A. Martı´nez, A.M. Chaparro, R. Bayo´n, Thin Solid Films 361 – 362 (2000) 28. [8] N. Barreau, S. Marsillac, D. Albertini, J.C. Bernede, Thin Solid Films 403 – 404 (2002) 331. [9] D. Hariskos, M. Ruckh, U. Ruhle, T. Walter, H. Werner Shock, J. Hedstrom, L. Stolt, Solar Energy Mater. Solar Cells 41 – 42 (1996) 345. [10] S. Spiering, D. Hariskos, M. Powalla, N. Naghavi, D. Lincot, Thin Solid Films 431 – 432 (2003) 359. [11] W.T. Kim, C.D. Kim, J. Appl. Phys. 60 (1986) 2631. [12] J. George, K.S. Joseph, B. Pradeep, T.I. Palson, Phys. Status Solidi, A 106 (1988) 123. [13] J. Herrero, J. Ortega, Solar Energy Mater. 17 (1988) 357.
[14] R. Bayon, J. Herrero, Appl. Surf. Sci. 158 (2000) 49. [15] C.D. Lockande, A. Ennaoui, P.S. Patil, M. Giersig, K. Diesner, M. Muller, H. Tributsch, Thin Solid Films 340 (1999) 18. [16] Powder Diffraction File, Joint Committee on Powder Diffraction Standards, ASTM, Philadelphia, PA, 1967, Card 250390. [17] Powder Diffraction File, Joint Committee on Powder Diffraction Standards, ASTM, Philadelphia, PA, 1967, Card 320456. [18] Powder Diffraction File, Joint Committee on Powder Diffraction Standards, ASTM, Philadelphia, PA, 1967, Card 050731. [19] G.A. Kitaev, V.Y. Dvoinin, A.V. Ust’yanseva, M.N. Belayeva, L.G. Skornyakov, Inorg. Mater. 12 (1976) 1448. [20] J.I. Pankove, Optical Processes in Semiconductors, Dover Publications, New York, 1971. [21] R.S. Mane, C.D. Lockande, Mater. Chem. Phys. 78 (2002) 15. [22] O. Zelaya-Angel, A.E. Esparza-Garcı´a, C. Falcony, R. LosadaMorales, R. Ramı´rez-Bon, Solid State Commun. 94 (1995) 81. [23] R. Ramı´rez-Bon, N.C. Sandoval-Inda, F.J. Espinoza-Beltra´n, M. Sotelo-Lerma, O. Zelaya-Angel, C. Falcony, J. Phys.: Condens. Matter 9 (1997) 10051.