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Application of novel photochemical deposition technique for the deposition of indium sulfide
Materials Science and Engineering B96 (2002) 37 /42 www.elsevier.com/locate/mseb
Application of novel photochemical deposition technique for the deposition of indium sulfide R. Kumaresan a, M. Ichimura a,*, N. Sato a, P. Ramasamy b a
Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b Crystal Growth Centre, Anna University, Chennai 600 025, India Received 24 May 2002; accepted 28 June 2002
Abstract Indium sulfide thin films were grown by photochemical deposition technique from an aqueous solution by means of UV illumination. Both the as-grown and annealed films were studied by different analysis tools. The X-ray diffraction analysis confirmed the initial amorphous nature of as-deposited InS film and phase transition into crystalline In2S3 form upon annealing at 500 8C. The structural phase transition upon annealing has also been revealed by the Raman spectroscopic analysis. The compositional analysis by Auger electron spectroscopy indicated that the InS film contains oxygen as the impurity element. The bandgap energy of the as-deposited and annealed films was analyzed by means of optical transmission study. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Indium sulfide; X-ray diffraction; Optical transmission; Raman spectroscopy; Photochemical deposition
1. Introduction Recently, there has been increasing interest in III /VI materials, which find applications in optoelectronic and photovoltaic industries [1]. Indium sulfide is such a III / VI group compound semiconductor, which has a bandgap energy of about 1.9 eV [2]. Also, indium sulfide can be a binary base material in the deposition of compound semiconductors such as CuInS2, a popular absorber material in hetero-junction solar cell device structures [3]. InS has a layer structure with a three dimensional network in which In atoms have a tetrahedral coordination (three S atoms and one In atom), the two S atoms and one indium atom being in one layer and the third S atom in the neighboring layer [4]. Indium hydroxy sulfide In(OH)x Sy also attracts attention since it can be a good buffer layer for CuInS2 based solar cells [5,6]. In2S3 is another phase of indium sulfide with a direct bandgap energy of 2.0 /2.3 eV [7 /9] and finds application as buffer layers in photovoltaic solar cells
* Corresponding author. Fax: /81-52-735-5453 E-mail address: [email protected] (M. Ichimura).
[10,11]. Also, it is reported that In2S3 finds applications in photoelectrochemical solar cell devices [12]. Generally it is always preferable technically to form the desired thin film by a simple, inexpensive, less time consuming and environment friendly method. Deposition from aqueous solution is found suitable for this desirability, and interest has always been shown in developing new thin film technology with superior advantages. There are only a few reports in literature on the deposition of InS thin film by different techniques such as thermal evaporation [1]. There are also some reports seen in literature on the aqueous solution deposition of In2S3 [10,11] and In(OH)x Sy [5]. In our present research, we have deposited InS thin film for the first time by a recently established novel technique, namely, photochemical deposition (PCD) technique [13 /15]. As a first example, earlier we reported on the deposition and further characterization of CdS thin films (a II /VI compound semiconductor) by this novel technique [13,14]. PCD is a very low cost technique for the deposition of thin films, and it can be easily scaled up for industrial production. It is more advantageous than the conventional solution deposition techniques such as electrochemical deposition or chemical bath deposition by means of its applicability for the
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 3 2 2 - 7
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use of any substrates, either conducting or non-conducting, with a good control over the deposition nature. The present article deals with the deposition of InS, a III /VI compound semiconductor, by PCD. It also describes the various characteristic properties of the photochemically deposited InS thin films, the effects of annealing */leading to the phase transition from InS to In2S3 phase etc., as analyzed by different techniques.
2. Experimental 2.1. Photochemical deposition of InS thin film In PCD of InS compound semiconductor, the deposition is carried out from an aqueous solution containing Na2S2O3 and In2(SO4)3 made of de-ionized water. Either glass slide or indium /tin-oxide (ITO) coated glass was used as the substrate for deposition, which provides the nucleation centers needed for the film growth. Fig. 1 shows the schematic sketch of the PCD set-up and Fig. 2 shows the actual instrument used for PCD. It consists of a high-pressure mercury lamp with a provision for UV light irradiation. The deposition solution is taken in a glass container and the substrate used for deposition is kept immersed in the solution at a few mm depth from the surface of the solution. The deposition solution above the substrate was illuminated by UV irradiation from the top as shown in the figures. The formation of the InS is based on the excitation of S2O2 ions (present 3 in the solution) upon UV irradiation, which release sulfur atoms and electrons. These sulfur atoms and electrons react with the indium metal ions present in the solution to form InS compound. The photochemically formed InS gets deposited on the substrate under uniform stirring. The PCD of InS was carried out by
Fig. 2. PCD equipment.
varying the basic deposition factors such as the bath composition, pH of the solution, bath temperature. The grown films were dried in open air and were annealed at different temperatures up to 500 8C, in nitrogen ambient, for a period of 30 min. 2.2. Characterization of photochemically deposited InS film The various characteristics including the structural, compositional, optical properties of the deposited films were analyzed by different techniques. The structural analysis of the as-deposited and annealed films was carried out by X-ray diffraction (XRD) analysis. The Raman spectroscopic analysis was carried out in the frequency range of 100 /500 cm 1 using an Ar laser (488 nm) as the excitation source. The composition of films was determined by Auger electron spectroscopic (AES) technique. The thickness of the films was determined by using the thickness profile meter. By means of scanning electron microscopic (SEM) technique, the surface morphology of the deposited films was analyzed. The optical analysis was carried out by recording transmission spectra of the samples, using the substrate as the reference.
3. Results and discussion 3.1. PCD conditions and reactions
Fig. 1. Principle sketch of the PCD experimental set up.
The various deposition parameters were varied, and the optimum conditions for the PCD of InS thin films were identified. InS was deposited from a bath containing 1 /6 mM In2(SO4)3 and 100 mM Na2S2O3. The pH of the solution was in the range of 3.2 /4.5. The optimum condition for the uniform deposition of InS thin film was identified as: 1 mM In2(SO4)3; 100 mM Na2S2O3, the original pH of the bath being 4.2. When
R. Kumaresan et al. / Materials Science and Engineering B96 (2002) 37 /42
the thiosulphate concentration was decreased by an order, the pH of the bath solution was shifted to the basic side and no deposition occurred. On increasing the thiosulphate concentration twice, it was observed that the bath pH shifted to more acidic side and resulting in no deposition. By increasing the indium salt concentration by one order higher than the optimum condition, the pH shifted to more acidic side and precipitation occurred resulting in no deposition. This precipitate was identified as sulfur, as its Raman spectrum matched with the Raman active peaks for sulfur. When the pH of the bath was maintained more acidic than the optimized condition (pH of 3.2 /4.5), sulfur precipitation was found to occur and no deposition obtained. It is expected that the photochemical formation of InS takes place via. the following reactions: The S2O2 ions 3 present in the solution absorb UV irradiation and release S according to the following equation:
39
Fig. 3. XRD pattern depicting amorphous nature of as-deposited and 300 8C annealed InS film, its phase transition upon 500 8C annealing.
The as-deposited InS films (amorphous nature as discussed in the next section) were yellowish-white in color. Upon annealing at 300 8C, the color of the film changed into reddish-brown and upon annealing at 500 8C, the color became more intense (typical color of In2S3).
is consistent with the literature report [1]. Upon annealing at 100 and 200 8C, there is no change observed in the XRD pattern of the respective films. The 300 8C annealed film still does not exhibit any peaks assigned to InS, as shown in Fig. 3 (for this sample, we used an ITOcoated glass sheet as a substrate, and several sharp peaks due to ITO are observed). In the previous report also, the InS thin films were amorphous even after annealing at 300 8C [1]. The peaks corresponding to elemental sulfur disappeared, which indicated the evaporation of elemental sulfur present in the film upon annealing. The further annealing at 500 8C resulted in the generation of new peaks different from the 300 8C annealed film, as shown in Fig. 3. These peaks correspond to In2S3, and hence the XRD analysis confirms that upon annealing at 500 8C the phase transition occurs from InS to In2S3 phase. This kind of phase transition has been reported earlier in literature [16]. In general, In2S3 exists in many different phases, namely a, cubic-b, tetragonal-b, and g, etc., depending upon the preparation conditions [17]. The 500 8C annealed film is identified as tetragonal-b phase [7,18]. The XRD pattern of the as-deposited films grown from a bath containing a higher In2(SO4)3 concentration (up to 6 mM) does not exhibit any change, which is similar to the XRD pattern of the film grown from 1 mM In2(SO4)3 bath.
3.2. X-ray diffraction analysis
3.3. Raman analysis
The XRD pattern of the as-deposited film is shown in Fig. 3 which exhibited a broad hump in the 2u range of around 20 /408, and this indicated that the as-deposited film is amorphous in nature. Also the XRD pattern exhibited peaks corresponding to elemental S. As described below, results of the Raman analysis shows that the as-deposited film consists of InS and S. Our result of amorphous nature of the as-deposited InS film
The Raman analysis of the as-deposited film confirmed the formation of In/S phase by exhibiting various Raman active modes, as shown in Fig. 4. According to [4], the peaks observed for the asdeposited film can be assigned as follows: 152 cm 1: A1g (In /In stretching mode); 218 cm 1: A4g (In /S bending mode); 322 cm 1: A2g (In /S stretching mode). However, elemental sulfur also exhibit active modes
2 S2 O2 3 hn SSO3
(1)
In a more acidic medium (when pH was adjusted to be below 3.2), the precipitation occurred as explained in previous section. This spontaneous release of S in a more acidic medium takes place according to the equation: S2 O2 3 2H SH2 SO3
(2)
S2O2 3
Also the ions get excited by absorbing the UV irradiation and release electrons according to the equations: 2 2S2 O2 3 hn S4 O6 2e 2 2 SO2 3 S2 O3 hnS3 O6 2e
(3) (4)
The so formed sulfur atoms and electrons combine with the In metal ions present in the solution, to form InS according to the equation given below: In Se InS
(5)
R. Kumaresan et al. / Materials Science and Engineering B96 (2002) 37 /42
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Fig. 4. Raman spectrum exhibiting formation of InS, its progressive phase transition into In2S3 upon annealing.
near 150 and 220 cm 1, and therefore, the former two modes may be attributed to S since the presence of elemental S is confirmed by XRD. Annealing the asdeposited film at and above 200 8C leads to the vanishing of these two modes as shown in Fig. 4. This would be due to evaporation of S during the annealing. After the annealing at 500 8C, some new peaks are observed, and they correspond to the In2S3 phase. For the 400 8C-annealed sample, a very broad and weak peak is observed. This anomaly is due to the phase transition from InS to In2S3 form, which might involve some unidentified complex intermediate stages. 3.4. Thickness of the film The PCD of InS thin film exhibited a growth rate of 3 mm over a deposition period of 90 min. Upon annealing at higher temperatures the film thickness decreased as shown in Fig. 5. Upon annealing at 200 8C the thickness of the film decreased significantly to a value of 2 mm, which is due to the evaporation of elemental sulfur and the sintering of InS particles. Further annealing at 300 8C does not make a significant
Fig. 6. Scanning electron micrograph of (a) as-deposited, (b) 300 8Cannealed, and (c) 500 8C-annealed films.
Fig. 5. Effect of annealing temperature on film thickness.
change in the thickness. Upon annealing at 500 8C, the thickness was found to decrease further to about 1 mm.
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3.5. Surface morphology Fig. 6(a /c) exhibit the surface morphology of the asdeposited and annealed films as revealed by the SEM technique. The as-deposited (Fig. 6(a)) and 300 8Cannealed films (Fig. 6(b)) seem to consist of small grains and crust on it. After the 500 8C annealing, the crust part is removed and only the grains remain (Fig. 6(c)). This may corresponds to thickness decrease shown in Fig. 5. 3.6. Compositional analysis by AES The compositions of the as-grown and annealed films were analyzed by the AES technique, and Fig. 7 exhibits a typical AES spectrum for the 300 8C annealed InS film. The AES analysis indicated that the photochemically as-deposited InS film (similar to the 300 8C annealed film, but not shown in this article) is constituted upon the main elements, indium and sulfur. Also it indicated that oxygen is present in the film as an impurity element. Even after argon sputtering of the surface of the film during the AES analysis, the presence of oxygen was detected. Hence it can be predicted that the oxygen in the film is not only the surface oxidation, but also present in the film. Although the quantitative analysis of the as-deposited film indicates a stoichiometric composition (In/S /1), it is also certain that all the indium is not in the bonded form with the sulfur: certain ratio of the indium is expected to be present in the form of In2O3 and/or In(OH)x (which cannot be determined by the AES technique). Also, this is consistent with the presence of elemental sulfur in the asdeposited film, as indicated by the XRD analysis. It has been observed from the AES analysis that the sulfur to oxygen ratio present in the film is about S:O /6:1. The AES analysis showed (within the analysis limit) that upon annealing at 300 8C the InS film becomes slightly rich in indium (In/S $/1.13). This could be due to the escaping of elemental sulfur present in the film upon annealing at high temperature, as confirmed by the XRD analysis. In spite of the absence of elemental S, the oxygen present in the as-deposited film still existed after annealing, which strengthens the idea of the presence of oxygen in the form of In2O3. The sulfur to oxygen ratio
Fig. 7. A typical AES spectrum for the 300 8C annealed InS film.
41
after 300 8C annealing is observed to be as S:O /4:1. Annealing at 500 8C lead to phase transition from InS to In2S3 phase as confirmed by X-ray and Raman analysis, and this sample exhibited a S/In ratio of about 1, which is sulfur deficient (vs. standard In2S3 composition ratio of S/In $/1.5). Bayon et al. also has reported on a similar observation of the formation of In2S3 with a sulfur deficiency [6]. It is reported in literature that In2S3 is capable to incorporate an excess of indium, probably in the form of In , in its lattice assuming the composition of In2x S3 [16]. Even after a higher temperature annealing at 500 8C, oxygen was found to be present in the film. Barreau et al. has reported on the formation of In2S33x O3x by evaporation technique [19]. In the present case, during annealing at higher temperature (500 8C), there is a possibility for the formation of In2S33x O3x , since oxygen is identified in the film by AES technique. From the AES analysis, it is determined that the S to O ratio is about 9:1.
3.7. Optical analysis The optical transmission spectra of as-deposited and annealed films are shown in Fig. 8. As seen in the figure, the as-deposited InS film shows a widely increasing transmission in the long wavelength region (500 /800 nm), and it does not exhibit a clear absorption edge. This could be due to the amorphous nature of the asdeposited film and its associated presence of elemental sulfur. The transmission spectrum has a clear absorption edge upon annealing at 300 8C. The In2S3 phase formed after 500 8C annealing exhibited an absorption edge at an almost the same wavelength as for the 300 8C annealed film. The energy bandgap of the films was determined by calculating absorption coefficient a from the optical transmission spectra. Single crystal InS has an indirect band gap of about 1.9 eV, [2] whereas the In2S3 phase has a direct band gap in the range of 2.0 /2.3 eV [7 /9]. In the present study, both the films resulting upon 300 and 500 8C annealing exhibited a direct bandgap of about 2.1 eV, which closely agrees with the literature
Fig. 8. Optical transmission spectra of as-deposited and annealed films.
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R. Kumaresan et al. / Materials Science and Engineering B96 (2002) 37 /42
value. It was reported that the bandgap energy increases with oxygen content in indium oxysulfide (In2S33x O3x ) [19]. Although oxygen is included in the present films, its effects on the bandgap do not seem considerable.
4. Summary and conclusion Indium sulfide thin films were grown by the recently established PCD technique. The as-deposited InS films were amorphous in nature. Upon annealing at 500 8C, phase transition occurred from InS to In2S3 phase. Raman spectroscopic analysis supported the structural phase transition upon annealing. The composition of the different phases was analyzed by the AES technique, and it was observed that the phase transition resulted in a sulfur deficient In2S3 phase. Optical properties of the films were analyzed by recording the transmission spectra. The annealed films have a direct band gap of about 2.1 eV.
Acknowledgements One of the authors R. Kumaresan thanks the Japan Government for the award of Monbusho fellowship to carry out the research work. The partial financial support of Marubun Research Promotion Foundation and The Hori Information Science Promotion Foundation, Japan, to carry out the research work is highly acknowledged.
References [1] M.A.M. Seyam, Vacuum 63 (2001) 441. [2] N.M. Gasanly, A. Aydinli, Solid State Commun. 101 (1997) 797. [3] T. Yukawa, K. Kuwabara, K. Koumoto, Thin Solid Films 286 (1996) 151. [4] N.M. Gasanly, H. Ozkan, A. Aydinli, I. Yilmaz, Solid State Commun. 110 (1999) 231. [5] D. Braunger, D. Hariskos, T. Walter, H.W. Schock, Solar Energy Mater. Solar Cells 40 (1996) 97. [6] R. Bayon, J. Herrero, Appl. Surf. Sci. 158 (2000) 49. [7] W. Kim, C. Kim, J. Appl. Phys. 60 (1986) 2631. [8] J. George, K.S. Joseph, B. Pradeep, T.I. Palson, Phys. Status Solid A106 (1988) 123. [9] T. Asikainein, N. Ritala, N. Leskela, Appl. Surf. Sci. 82/83 (1994) 122. [10] C.D. Lokhande, A. Ennaoui, P.S. Patil, M. Giersig, K. Diesner, M. Muller, H. Tributsch, Thin Solid Films 340 (1999) 18. [11] D. Ohashi, T. Nakada, A. Kunioka, Solar Energy Mater. Solar Cells 67 (2001) 261. [12] K. Hara, K. Sayama, H. Arakawa, Solar Energy Mater. Solar Cells 62 (2000) 441. [13] M. Ichimura, F. Goto, E. Arai, J. Electrochem. Soc. 146 (1999) 1028. [14] R. Kumaresan, M. Ichimura, K. Takahashi, K. Takeuchi, F. Goto, E. Arai, Jpn. J. Appl. Phys. 40 (2001) 3161. [15] M. Ichimura, A. Nakamura, K. Takeuchi, E. Arai, Thin Solid Films 384 (2001) 157. [16] A.N. MacInnes, M.B. Power, A.F. Hepp, A.R. Barron, J. Organometal. Chem. 449 (1993) 95. [17] R. Diehl, R. Nitsche, J. Cryst. Growth 28 (1975) 306. [18] S. Yu, L. Shu, Y. Qian, Y. Xie, J. Yang, L. Yang, Mater. Res. Bull. 33 (1998) 717. [19] N. Barreau, J.C. Bernede, S. Marsillac, A. Mokrani, J. Cryst. Growth 235 (2002) 439.
Application of novel photochemical deposition technique for the deposition of indium sulfide R. Kumaresan a, M. Ichimura a,*, N. Sato a, P. Ramasamy b a
Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b Crystal Growth Centre, Anna University, Chennai 600 025, India Received 24 May 2002; accepted 28 June 2002
Abstract Indium sulfide thin films were grown by photochemical deposition technique from an aqueous solution by means of UV illumination. Both the as-grown and annealed films were studied by different analysis tools. The X-ray diffraction analysis confirmed the initial amorphous nature of as-deposited InS film and phase transition into crystalline In2S3 form upon annealing at 500 8C. The structural phase transition upon annealing has also been revealed by the Raman spectroscopic analysis. The compositional analysis by Auger electron spectroscopy indicated that the InS film contains oxygen as the impurity element. The bandgap energy of the as-deposited and annealed films was analyzed by means of optical transmission study. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Indium sulfide; X-ray diffraction; Optical transmission; Raman spectroscopy; Photochemical deposition
1. Introduction Recently, there has been increasing interest in III /VI materials, which find applications in optoelectronic and photovoltaic industries [1]. Indium sulfide is such a III / VI group compound semiconductor, which has a bandgap energy of about 1.9 eV [2]. Also, indium sulfide can be a binary base material in the deposition of compound semiconductors such as CuInS2, a popular absorber material in hetero-junction solar cell device structures [3]. InS has a layer structure with a three dimensional network in which In atoms have a tetrahedral coordination (three S atoms and one In atom), the two S atoms and one indium atom being in one layer and the third S atom in the neighboring layer [4]. Indium hydroxy sulfide In(OH)x Sy also attracts attention since it can be a good buffer layer for CuInS2 based solar cells [5,6]. In2S3 is another phase of indium sulfide with a direct bandgap energy of 2.0 /2.3 eV [7 /9] and finds application as buffer layers in photovoltaic solar cells
* Corresponding author. Fax: /81-52-735-5453 E-mail address: [email protected] (M. Ichimura).
[10,11]. Also, it is reported that In2S3 finds applications in photoelectrochemical solar cell devices [12]. Generally it is always preferable technically to form the desired thin film by a simple, inexpensive, less time consuming and environment friendly method. Deposition from aqueous solution is found suitable for this desirability, and interest has always been shown in developing new thin film technology with superior advantages. There are only a few reports in literature on the deposition of InS thin film by different techniques such as thermal evaporation [1]. There are also some reports seen in literature on the aqueous solution deposition of In2S3 [10,11] and In(OH)x Sy [5]. In our present research, we have deposited InS thin film for the first time by a recently established novel technique, namely, photochemical deposition (PCD) technique [13 /15]. As a first example, earlier we reported on the deposition and further characterization of CdS thin films (a II /VI compound semiconductor) by this novel technique [13,14]. PCD is a very low cost technique for the deposition of thin films, and it can be easily scaled up for industrial production. It is more advantageous than the conventional solution deposition techniques such as electrochemical deposition or chemical bath deposition by means of its applicability for the
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 3 2 2 - 7
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use of any substrates, either conducting or non-conducting, with a good control over the deposition nature. The present article deals with the deposition of InS, a III /VI compound semiconductor, by PCD. It also describes the various characteristic properties of the photochemically deposited InS thin films, the effects of annealing */leading to the phase transition from InS to In2S3 phase etc., as analyzed by different techniques.
2. Experimental 2.1. Photochemical deposition of InS thin film In PCD of InS compound semiconductor, the deposition is carried out from an aqueous solution containing Na2S2O3 and In2(SO4)3 made of de-ionized water. Either glass slide or indium /tin-oxide (ITO) coated glass was used as the substrate for deposition, which provides the nucleation centers needed for the film growth. Fig. 1 shows the schematic sketch of the PCD set-up and Fig. 2 shows the actual instrument used for PCD. It consists of a high-pressure mercury lamp with a provision for UV light irradiation. The deposition solution is taken in a glass container and the substrate used for deposition is kept immersed in the solution at a few mm depth from the surface of the solution. The deposition solution above the substrate was illuminated by UV irradiation from the top as shown in the figures. The formation of the InS is based on the excitation of S2O2 ions (present 3 in the solution) upon UV irradiation, which release sulfur atoms and electrons. These sulfur atoms and electrons react with the indium metal ions present in the solution to form InS compound. The photochemically formed InS gets deposited on the substrate under uniform stirring. The PCD of InS was carried out by
Fig. 2. PCD equipment.
varying the basic deposition factors such as the bath composition, pH of the solution, bath temperature. The grown films were dried in open air and were annealed at different temperatures up to 500 8C, in nitrogen ambient, for a period of 30 min. 2.2. Characterization of photochemically deposited InS film The various characteristics including the structural, compositional, optical properties of the deposited films were analyzed by different techniques. The structural analysis of the as-deposited and annealed films was carried out by X-ray diffraction (XRD) analysis. The Raman spectroscopic analysis was carried out in the frequency range of 100 /500 cm 1 using an Ar laser (488 nm) as the excitation source. The composition of films was determined by Auger electron spectroscopic (AES) technique. The thickness of the films was determined by using the thickness profile meter. By means of scanning electron microscopic (SEM) technique, the surface morphology of the deposited films was analyzed. The optical analysis was carried out by recording transmission spectra of the samples, using the substrate as the reference.
3. Results and discussion 3.1. PCD conditions and reactions
Fig. 1. Principle sketch of the PCD experimental set up.
The various deposition parameters were varied, and the optimum conditions for the PCD of InS thin films were identified. InS was deposited from a bath containing 1 /6 mM In2(SO4)3 and 100 mM Na2S2O3. The pH of the solution was in the range of 3.2 /4.5. The optimum condition for the uniform deposition of InS thin film was identified as: 1 mM In2(SO4)3; 100 mM Na2S2O3, the original pH of the bath being 4.2. When
R. Kumaresan et al. / Materials Science and Engineering B96 (2002) 37 /42
the thiosulphate concentration was decreased by an order, the pH of the bath solution was shifted to the basic side and no deposition occurred. On increasing the thiosulphate concentration twice, it was observed that the bath pH shifted to more acidic side and resulting in no deposition. By increasing the indium salt concentration by one order higher than the optimum condition, the pH shifted to more acidic side and precipitation occurred resulting in no deposition. This precipitate was identified as sulfur, as its Raman spectrum matched with the Raman active peaks for sulfur. When the pH of the bath was maintained more acidic than the optimized condition (pH of 3.2 /4.5), sulfur precipitation was found to occur and no deposition obtained. It is expected that the photochemical formation of InS takes place via. the following reactions: The S2O2 ions 3 present in the solution absorb UV irradiation and release S according to the following equation:
39
Fig. 3. XRD pattern depicting amorphous nature of as-deposited and 300 8C annealed InS film, its phase transition upon 500 8C annealing.
The as-deposited InS films (amorphous nature as discussed in the next section) were yellowish-white in color. Upon annealing at 300 8C, the color of the film changed into reddish-brown and upon annealing at 500 8C, the color became more intense (typical color of In2S3).
is consistent with the literature report [1]. Upon annealing at 100 and 200 8C, there is no change observed in the XRD pattern of the respective films. The 300 8C annealed film still does not exhibit any peaks assigned to InS, as shown in Fig. 3 (for this sample, we used an ITOcoated glass sheet as a substrate, and several sharp peaks due to ITO are observed). In the previous report also, the InS thin films were amorphous even after annealing at 300 8C [1]. The peaks corresponding to elemental sulfur disappeared, which indicated the evaporation of elemental sulfur present in the film upon annealing. The further annealing at 500 8C resulted in the generation of new peaks different from the 300 8C annealed film, as shown in Fig. 3. These peaks correspond to In2S3, and hence the XRD analysis confirms that upon annealing at 500 8C the phase transition occurs from InS to In2S3 phase. This kind of phase transition has been reported earlier in literature [16]. In general, In2S3 exists in many different phases, namely a, cubic-b, tetragonal-b, and g, etc., depending upon the preparation conditions [17]. The 500 8C annealed film is identified as tetragonal-b phase [7,18]. The XRD pattern of the as-deposited films grown from a bath containing a higher In2(SO4)3 concentration (up to 6 mM) does not exhibit any change, which is similar to the XRD pattern of the film grown from 1 mM In2(SO4)3 bath.
3.2. X-ray diffraction analysis
3.3. Raman analysis
The XRD pattern of the as-deposited film is shown in Fig. 3 which exhibited a broad hump in the 2u range of around 20 /408, and this indicated that the as-deposited film is amorphous in nature. Also the XRD pattern exhibited peaks corresponding to elemental S. As described below, results of the Raman analysis shows that the as-deposited film consists of InS and S. Our result of amorphous nature of the as-deposited InS film
The Raman analysis of the as-deposited film confirmed the formation of In/S phase by exhibiting various Raman active modes, as shown in Fig. 4. According to [4], the peaks observed for the asdeposited film can be assigned as follows: 152 cm 1: A1g (In /In stretching mode); 218 cm 1: A4g (In /S bending mode); 322 cm 1: A2g (In /S stretching mode). However, elemental sulfur also exhibit active modes
2 S2 O2 3 hn SSO3
(1)
In a more acidic medium (when pH was adjusted to be below 3.2), the precipitation occurred as explained in previous section. This spontaneous release of S in a more acidic medium takes place according to the equation: S2 O2 3 2H SH2 SO3
(2)
S2O2 3
Also the ions get excited by absorbing the UV irradiation and release electrons according to the equations: 2 2S2 O2 3 hn S4 O6 2e 2 2 SO2 3 S2 O3 hnS3 O6 2e
(3) (4)
The so formed sulfur atoms and electrons combine with the In metal ions present in the solution, to form InS according to the equation given below: In Se InS
(5)
R. Kumaresan et al. / Materials Science and Engineering B96 (2002) 37 /42
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Fig. 4. Raman spectrum exhibiting formation of InS, its progressive phase transition into In2S3 upon annealing.
near 150 and 220 cm 1, and therefore, the former two modes may be attributed to S since the presence of elemental S is confirmed by XRD. Annealing the asdeposited film at and above 200 8C leads to the vanishing of these two modes as shown in Fig. 4. This would be due to evaporation of S during the annealing. After the annealing at 500 8C, some new peaks are observed, and they correspond to the In2S3 phase. For the 400 8C-annealed sample, a very broad and weak peak is observed. This anomaly is due to the phase transition from InS to In2S3 form, which might involve some unidentified complex intermediate stages. 3.4. Thickness of the film The PCD of InS thin film exhibited a growth rate of 3 mm over a deposition period of 90 min. Upon annealing at higher temperatures the film thickness decreased as shown in Fig. 5. Upon annealing at 200 8C the thickness of the film decreased significantly to a value of 2 mm, which is due to the evaporation of elemental sulfur and the sintering of InS particles. Further annealing at 300 8C does not make a significant
Fig. 6. Scanning electron micrograph of (a) as-deposited, (b) 300 8Cannealed, and (c) 500 8C-annealed films.
Fig. 5. Effect of annealing temperature on film thickness.
change in the thickness. Upon annealing at 500 8C, the thickness was found to decrease further to about 1 mm.
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3.5. Surface morphology Fig. 6(a /c) exhibit the surface morphology of the asdeposited and annealed films as revealed by the SEM technique. The as-deposited (Fig. 6(a)) and 300 8Cannealed films (Fig. 6(b)) seem to consist of small grains and crust on it. After the 500 8C annealing, the crust part is removed and only the grains remain (Fig. 6(c)). This may corresponds to thickness decrease shown in Fig. 5. 3.6. Compositional analysis by AES The compositions of the as-grown and annealed films were analyzed by the AES technique, and Fig. 7 exhibits a typical AES spectrum for the 300 8C annealed InS film. The AES analysis indicated that the photochemically as-deposited InS film (similar to the 300 8C annealed film, but not shown in this article) is constituted upon the main elements, indium and sulfur. Also it indicated that oxygen is present in the film as an impurity element. Even after argon sputtering of the surface of the film during the AES analysis, the presence of oxygen was detected. Hence it can be predicted that the oxygen in the film is not only the surface oxidation, but also present in the film. Although the quantitative analysis of the as-deposited film indicates a stoichiometric composition (In/S /1), it is also certain that all the indium is not in the bonded form with the sulfur: certain ratio of the indium is expected to be present in the form of In2O3 and/or In(OH)x (which cannot be determined by the AES technique). Also, this is consistent with the presence of elemental sulfur in the asdeposited film, as indicated by the XRD analysis. It has been observed from the AES analysis that the sulfur to oxygen ratio present in the film is about S:O /6:1. The AES analysis showed (within the analysis limit) that upon annealing at 300 8C the InS film becomes slightly rich in indium (In/S $/1.13). This could be due to the escaping of elemental sulfur present in the film upon annealing at high temperature, as confirmed by the XRD analysis. In spite of the absence of elemental S, the oxygen present in the as-deposited film still existed after annealing, which strengthens the idea of the presence of oxygen in the form of In2O3. The sulfur to oxygen ratio
Fig. 7. A typical AES spectrum for the 300 8C annealed InS film.
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after 300 8C annealing is observed to be as S:O /4:1. Annealing at 500 8C lead to phase transition from InS to In2S3 phase as confirmed by X-ray and Raman analysis, and this sample exhibited a S/In ratio of about 1, which is sulfur deficient (vs. standard In2S3 composition ratio of S/In $/1.5). Bayon et al. also has reported on a similar observation of the formation of In2S3 with a sulfur deficiency [6]. It is reported in literature that In2S3 is capable to incorporate an excess of indium, probably in the form of In , in its lattice assuming the composition of In2x S3 [16]. Even after a higher temperature annealing at 500 8C, oxygen was found to be present in the film. Barreau et al. has reported on the formation of In2S33x O3x by evaporation technique [19]. In the present case, during annealing at higher temperature (500 8C), there is a possibility for the formation of In2S33x O3x , since oxygen is identified in the film by AES technique. From the AES analysis, it is determined that the S to O ratio is about 9:1.
3.7. Optical analysis The optical transmission spectra of as-deposited and annealed films are shown in Fig. 8. As seen in the figure, the as-deposited InS film shows a widely increasing transmission in the long wavelength region (500 /800 nm), and it does not exhibit a clear absorption edge. This could be due to the amorphous nature of the asdeposited film and its associated presence of elemental sulfur. The transmission spectrum has a clear absorption edge upon annealing at 300 8C. The In2S3 phase formed after 500 8C annealing exhibited an absorption edge at an almost the same wavelength as for the 300 8C annealed film. The energy bandgap of the films was determined by calculating absorption coefficient a from the optical transmission spectra. Single crystal InS has an indirect band gap of about 1.9 eV, [2] whereas the In2S3 phase has a direct band gap in the range of 2.0 /2.3 eV [7 /9]. In the present study, both the films resulting upon 300 and 500 8C annealing exhibited a direct bandgap of about 2.1 eV, which closely agrees with the literature
Fig. 8. Optical transmission spectra of as-deposited and annealed films.
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value. It was reported that the bandgap energy increases with oxygen content in indium oxysulfide (In2S33x O3x ) [19]. Although oxygen is included in the present films, its effects on the bandgap do not seem considerable.
4. Summary and conclusion Indium sulfide thin films were grown by the recently established PCD technique. The as-deposited InS films were amorphous in nature. Upon annealing at 500 8C, phase transition occurred from InS to In2S3 phase. Raman spectroscopic analysis supported the structural phase transition upon annealing. The composition of the different phases was analyzed by the AES technique, and it was observed that the phase transition resulted in a sulfur deficient In2S3 phase. Optical properties of the films were analyzed by recording the transmission spectra. The annealed films have a direct band gap of about 2.1 eV.
Acknowledgements One of the authors R. Kumaresan thanks the Japan Government for the award of Monbusho fellowship to carry out the research work. The partial financial support of Marubun Research Promotion Foundation and The Hori Information Science Promotion Foundation, Japan, to carry out the research work is highly acknowledged.
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