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Growth of copper sulphide thin films by successive ionic layer adsorption and reaction (SILAR) method
Materials Chemistry and Physics 65 (2000) 63–67
Growth of copper sulphide thin films by successive ionic layer adsorption and reaction (SILAR) method S.D. Sartale, C.D. Lokhande∗ Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India Received 6 July 1999; received in revised form 30 November 1999; accepted 15 December 1999
Abstract The copper sulphide (Cux S) thin films were deposited using relatively simple and new successive ionic layer adsorption and reaction (SILAR) method using copper sulphate and thiourea solutions as cationic and anionic precursors, respectively. The films were deposited on glass and Si (1 1 1) wafer substrates. To obtain good quality Cux S thin films, preparative conditions such as concentration, pH and temperature of cationic and anionic precursor solutions adsorption, reaction and rinsing time durations etc. were optimised. The characterisation of the films was carried out by using X-ray diffraction, scanning electron microscopy, optical absorption, electrical resistivity and thermoemf techniques. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Copper sulphide thin films; SILAR method; Preparation and characterisation
1. Introduction Copper sulphide (Cux S) thin films have received particular attention since the discovery of the CdS/Cux S heterojunction solar cell in 1954 by Reynolds et al. [1]. The Cux S thin films are effectively used in solid junction solar cells which have many applications as it is a direct energy conversion device. It is well known that Cux S has distinct compositions because of the variation in x, 1≤x≤2 with different stoichiometry and that oxidation and temperature is responsible for a change from one composition to another [2–4]. It is interesting that these distinct compositions are not remarkable for the change in crystalline structure but in the variation of electrical resistivity [5] and significant variation in optical band gap [6]. The CdS/Cux S heterojunction solar cell involves the stability of short circuit current (Isc ). The generated current in CdS/Cux S heterojunction solar cell is mainly due to photon absorption by the Cux S layer. Electron–hole pairs are created in this layer and are diffused towards the junction. It is known that Isc is a function of x in Cux S as x increases, Isc also increases and attains a maximum value at x=2 [2]. The Cux S thin films have been found to possess near ideal solar control characteristics. Transmittance in the infrared region, low reflectance <10% in the visible region so as to avoid glare and relatively high reflectance >15% in the near infrared region. The films can also be used in laminated glaz∗
Corresponding author.
ing. The Cux S thin films have been used in air–glass tubular solar collectors as absorber coating, in photo-detectors and photovoltaic applications. Numerous techniques for producing Cux S thin films have been investigated. These include vacuum evaporation [7], activated reactive evaporation [8], reactive magnetron sputtering [9], spray pyrolysis [10], sulphurisation of copper foils and films [11] and slurry technique [12]. Chemical bath deposition of Cux S thin films from alkaline media has been investigated [13–16]. Gadave and Lokhande have formed these films through chemical bath deposition from acidic medium [17]. Das et al. have prepared these films by a solid state reaction between CdS and CuCl films in the temperature range 200–250◦ C [18]. Nair and Nair have reported that absorption greater than 90% can be realised in chemically deposited SnS–Cux S thin films [19]. These films have wide applications in glass evacuated tube solar collectors. Chain and Vook have studied reflection high energy electron diffraction and transmission electron microscopy of Cux S on single crystal copper films [20]. Fernandez and Nair have prepared Cux S and PbS–Cux S films which have been used for transparent polymer coating, primarily to offer protection during window cleaning [21]. Relatively simple, quick, economical and suitable for large area deposition of any configuration, successive ionic layer adsorption and reaction (SILAR) method was reported in mid-1980s [22]. It does not require sophisticated instruments, the substrate need not be conductive and have a high
0254-0584/00/$ – see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 2 0 7 - 8
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S.D. Sartale, C.D. Lokhande / Materials Chemistry and Physics 65 (2000) 63–67
melting point. The SILAR method is based on immersion of substrate into different cationic and anionic precursor solutions and rinsing before every immersion with highly purified deionised water to remove the loosely bounded species. Thus, single SILAR deposition cycle consists of adsorption of cations, rinsing with water, adsorption and reaction of anions and again rinsing with water. The time durations for adsorption, reaction and rinsing can be experimentally calculated. The growth of the film can be controlled at an accuracy of one SILAR deposition cycle [23,24]. The SILAR method has been applied for the deposition of CdS, ZnS, Cd–ZnS, PbS, ZnO, Sb2 S3 etc. thin films on various substrates [25–31]. Literature survey shows that there is no attempt to deposit Cux S thin films using SILAR method. Keeping this point in mind Cux S thin films were deposited on glass and single crystal Si (1 1 1) wafer substrates using SILAR method. These films were characterised for their structural, optical and electrical properties by using X-ray diffraction (XRD), scanning electron microscopy (SEM), optical absorption, electrical resistivity and thermoemf measurements.
2. Experimental 2.1. Substrate cleaning Corning glass slides of 26 mm×76 mm×1 mm dimension and single crystal Si (1 1 1) wafer (n-type, electrical resistivity, 0.005–0.015 cm) with dimension 20 mm×20 mm×0.5 mm were used as substrates. Corning glass slides were boiled in concentrated chromic acid for 30 min and ultrasonically cleaned with double distilled water and dried with hot air. The single crystal Si (1 1 1) wafer was etched in 40% NH4 F solution at 10◦ C for 10 min and then rinsed in double distilled water to make it hydrogen terminated. 2.2. Cux S thin film for nation Loba analytical reagent grade copper sulphate and thiourea were used in the deposition of Cux S thin films. The cationic precursor was 0.1 M copper sulphate solution. The pH was adjusted to 10 by adding excess ammonia. The source of sulphur ions was 0.1 M thiourea solution (pH∼6). Prepared solutions were taken into beakers of capacity 50 ml each. For rinsing purpose ample quantity of double distilled water (resistivity∼18 M cm) was used. The deposition was carried out at room temperature (27◦ C) using unstirred solutions. In SILAR method concentration, pH and temperature of precursor solutions and the time durations for adsorption, reaction and rinsing are important. By making several trials, Cux S thin film deposition conditions were optimized. The substrate was immersed in 0.1 M copper sulphate solution for 20 s. Then the Cu2+ ions were adsorbed to the
substrate. Then rinsing of the substrate with double distilled water for 30 s removed the desorbed ions. Further, during immersion of the substrate in 0.1 M thiourea solution for 20 s, the S2− ions were adsorbed and reacted with Cu2+ ions on the substrate to form Cux S. The unreacted S2− ions removed by rinsing of the substrate in double distilled water for 30 s. Thus, single SILAR deposition cycle was made up of 20 s adsorption of Cu2+ ions, 30 s rinsing with double distilled water, 20 s adsorption and reaction of S2− ions and 30 s rinsing with double distilled water. By repeating such SILAR deposition cycles for 25 times, we have obtained Cux S thin films of thickness of about 4000 Å. 2.3. Characterisation of the films Film thickness of Cux S was determined by gravimetric weight difference method. For this, a sensitive microbalance was utilised and film density was assumed as the bulk density of Cu2 S (5.6 g cm−3 ). The structural characterisation of Cux S thin films deposited on glass and Si (1 1 1) wafer substrates was carried out by analysing the XRD patterns, obtained using a Philips PW-1710 X-ray diffractometer using Cu K␣ radiations (λ=1.5405 Å). For surface morphology studies of the film deposited on glass substrate, scanning electron microscopy technique was used. The films were coated with gold–palladium by a polaron sputter coating unit E-2500. The coating thickness was 10 nm. The films were loaded in the sample holder of Cambridge stereoscan 250-MK-3 unit for SEM analysis. Optical absorption studies were carried out using a UV–VIS–NIR spectrophotometer (Hitachi, model 330, Japan) in the wavelength range 350–850 nm. The optical spectra were recorded for the film deposited on glass substrates. The light beams were incident from the film side. The glass substrate was the reference while recording the absorption spectra. To study the electrical characterisation of the films, electrical resistivity measurement was carried out using the DC two point probe method in the temperature range 300–500 K. The electrical contacts to the film surface were made through a pair of silver paint electrodes of 5 mm length printed at a separation of 1 cm. To study temperature dependent resistivity measurements, strip heaters were used to heat samples and Chromel–Alumel thermocouple was used to measure the temperature of the samples. The type of electrical conductivity of the films was determined by thermoemf polarity of the thermally generated voltage at the hot end of the film.
3. Results and discussion 3.1. Film thickness Thickness of Cux S thin films for different number of SILAR deposition cycles on glass substrate was measured
S.D. Sartale, C.D. Lokhande / Materials Chemistry and Physics 65 (2000) 63–67
65
Fig. 1. Plot of film thickness against number of SILAR deposition cycles.
using weight difference method as t=
m Aρ
(1)
where, m is the mass of the film, A the area of the film and ρ is the density of the film material (for Cu2 S, ρ=5.6 gm cm−3 ). Fig. 1 shows the variation of film thickness with number of SILAR deposition cycles with optimized deposition conditions. The terminal thickness as 4000 Å was found for 25 SILAR deposition cycles. The growth rate per five SILAR deposition cycles was found to be 800 Å. After 25 SILAR deposition cycles, the film peels off from the substrate. 3.2. Structural studies Fig. 2 (a and b) shows X-ray diffractograms of the films deposited on (a) glass and (b) single crystal Si (1 1 1) wafer substrates. These films on glass substrates are amorphous or consisting of fine grains. The broad hump is due to amorphous glass substrate. The film on Si (1 1 1) wafer substrate shows improvement in crystallinity. Many-fold increase in crystallinity for Si (1 1 1) wafer substrate is attributed to the single crystalline nature of Si (1 1 1) wafer substrate. Such type of improvement in crystallinity by using single crystal substrates has been reported earlier [32]. By comparison
Fig. 2. X-ray diffractograms of SILAR grown Cux S thin films on (a) glass and (b) Si (1 1 1) wafer substrates.
of observed and standard [33] d values, it is concluded that the formed compound is mixture of Cux S with Cux S (1.83≤x≤1.96) and Cu2 S phases with hexagonal crystal structure. The comparison of observed and standard [33] d values is made in Table 1. An SEM image of 4000 Å thick Cux S film on glass substrate at ×30,000 magnification is shown in Fig. 3. The scale bar length is 1 m. It can be seen that the film is dense, smooth and homogeneous without visible pores. The film surface look smooth and uniform. 3.3. Optical absorption studies Fig. 4 shows the plot of optical absorption coefficient (α) against wavelength (λ) for Cux S film on Corning glass substrate. The nature of the transition (direct or indirect) is determined by using the relation
Table 1 Comparison of observed and standard [33] d values Substrate
Observed d values (Å)
Standard d values (Å)
Reflection plane (h k l)
Observed Cux S phase
Glass
2.39 1.39 3.18 1.54 1.14
2.40 1.39 3.16 1.54 1.14
102 5 6 1/4 6 4 033 460 300
Cu2 S Cux S (1.83≤x≤1.96) Cux S (1.83≤x≤1.96) Cux S (1.83≤x≤1.96) Cu2 S
Si (1 1 1) wafer
66
S.D. Sartale, C.D. Lokhande / Materials Chemistry and Physics 65 (2000) 63–67
Fig. 3. A SEM image of 4000 Å thick SILAR grown Cux S thin film on glass substrate with ×30,000 magnification. The scale bar length is 1 m.
α=
A(hν − Eg )n hν
(2)
where hν is the photon energy and Eg the optical bandgap. A is the constant which is related to the effective masses associated with the valence and conduction bands. For allowed direct transitions, n=1/2 and allowed indirect transitions, n=2. Fig. 5 shows the plot of (αhν)2 against hν. The variation of (αhν)2 with hν is linear which indicates that the direct transition is present. Extrapolating the straight line portion of the plot of (αhv)2 against hν to energy axis for zero absorption coefficient give optical bandgap energy value as 2.36 eV, which is comparable to the value reported earlier [17].
Fig. 4. Plot of optical absorption coefficient (α) for the SILAR grown Cux S thin film on glass substrate.
Fig. 5. Plot of (αhν)2 against hν for the SILAR grown Cux S thin film on glass substrate.
3.4. Electrical characterisations Fig. 6 shows the variation of logarithm of resistivity with reciprocal of temperature for the film on glass substrate. The resistivity decreases with increase in temperature indicating the semiconducting behaviour of the Cux S film. It is also observed that the variation is non-linear. The plot shows two regions corresponding to low and high temperature regions. Similar types of results are reported earlier [17]. The resistivity at room temperature (27◦ C) was found to be of
Fig. 6. Plot of logarithm of resistivity against reciprocal of temperature for the SILAR grown Cux S thin film on glass substrate.
S.D. Sartale, C.D. Lokhande / Materials Chemistry and Physics 65 (2000) 63–67
the order of 10−2 cm. From polarity of thermoemf, p-type conductivity of Cux S thin films is confirmed. Same nature of conductivity of Cux S thin films is reported earlier [17]. 4. Conclusions In this paper, we have reported the simple successive ionic layer adsorption and reaction (SILAR) method for production of Cux S thin films using copper sulphate and thiourea solutions as cationic and anionic precursors, respectively. The films were amorphous or consisting of fine grains on glass substrate, whereas many-fold increase in crystallinity was observed for Si (1 1 1) wafer substrate. From the SEM image, the film surface looks smooth and uniform. From optical absorption study, direct band gap was found to be 2.36 eV. The resistivity at room temperature (27◦ C) was found to be of the order of 10−2 cm. The films were p-type and semiconducting as evidenced by thermoemf and electrical resistivity studies, respectively.
Acknowledgements We thank the University Grants Commission, New Delhi (India) for the financial support through the research project No. F. 10-7/97 (SR-I). References [1] D.C. Reynolds, G. Leies, L.T. Antes, R.E. Margurber, Phys. Rev. 96 (1954) 535. [2] R. Plaz, J. Besson, T.N. Dug, J. Vedel, in: Proc. 10th IEEE Photovoltaic specialists Conf., IEEE, New York, 1973, p. 69. [3] J.J. Loferski, J. Shewchun, S.D. Miltleman, E.A. De Meo, R. Arnott, H.L. Hwang, R. Beauliew, G. Champman, Solar Energy Mater. 1 (1979) 157. [4] M. Saveli, J. Bougnot, Solar Energy Conversion, in: B. O. Seraphin (Ed.), Topics in Applied Physics, Vol. 31, Springer, Berlin, 1979, p. 213. [5] K. Otamoto, S. Kawai, Jpn. J. Appl. Phys. 21 (1973) 1130.
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[6] S. Couve, L. Gouskov, L. Szepessy, J. Vedel, E. Castal, Thin Solid Films 15 (1973) 233. [7] B. Bezig, S. Duchemin, F. Guastavino, Sol. Energy Mater. 2 (1979) 53. [8] H.S. Randhawa, R.E. Bunshah, D.G. Brock, B.M. Basol, J.B. Philips, Sol. Energy Mater. 6 (1982) 445. [9] J.A. Thronton, D.G. Cornog, W.W. Anderson, R.B. Hall, J.E. Philips, in: Proc. 16th IEEE Photovoltaic Specialists Conf., San Diego, 1982, p. 737. [10] J. Marucchi, M. Protin, Oudeacoumar, M. Savelli, in: Proc. 13th IEEE Photovoltaic Specialists Conference, Washington, DC, IEEE, NJ, 1978, p. 298. [11] K.W. Boer, J.D. Meakin, in: Proc. Int. Workshop on Cadmium Solar Cells and other Abrupt Heterojunctions, University of Delaware, 1975. [12] CdS Solar Cells Development, NASA Rep. (R.-7296), Final Report, June 1971. [13] E. Fatas, T. Garcia, C. Montemayor, A. Medina, E.G. Ganarero, F. Arjona, Mater. Chem. Phys. 12 (1985) 121. [14] P. Pramanik, M.A. Akther, P.K. Basu, J. Mater. Sci. Lett. 6 (1987) 1277. [15] A.J. Varkey, Solar Energy Mater. 19 (1989) 415. [16] R.N. Bhattacharya, P. Pramanik, Bull. Mater. Sci. Lett. 3 (1988) 407. [17] K.M. Gadave, C.D. Lokhande, Thin Solid Films 229 (1993) 1. [18] S.R. Das, V.D. Vankar, P. Nath, K.L. Chopra, Thin Solid Films 51 (1978) 227. [19] P.K. Nair, M.T.S. Nair, J. Appl. Phys. D: Appl. Phys. 24 (1991) 83. [20] O.J. Chain, R.W. Vook, Thin Solid Films 51 (1978) 373. [21] A.M. Fernandez, P.K. Nair, Thin Solid Films 204 (1991) 459. [22] M. Ristov, G.J. Saindinovski, I. Grozdanov, Thin Solid Films 123 (1985) 63. [23] Y.F. Nicolau, Application Surf. Sci. 22/23 (1985) 1061. [24] Y.F. Nicolau, U.S. Patent, 4675207 (1987). [25] Y.F. Nicolau, M. Dupuy, J. Electrochem. Soc. 137 (1990) 2915. [26] Y.F. Nicolau, J.C. Menard, J. Crystal Growth 92 (1988) 128. [27] M.P. Valkonen, T. Kannianen, S. Lindroos, M. Leskela, E. Rauhala, Appl. Surf. Sci. 115 (1997) 386. [28] B.R. Sankapal, R.S. Mane, C.D. Lokhande, Mater. Res. Bull., in press. [29] R. Resch, G. Friedbacher, M. Grasserbaur, T. Kanniainen, S. Lindroos, L. Niinisto, Appl. Surf. Sci. 120 (1997) 51. [30] A.E. Jimenez-Gonzailez, P.K. Nair, Semicond. Sci. Technol. 10 (1995) 1277. [31] B.R. Sankapal, R.S. Mane, C.D. Lokhande, J. Mater. Sci. Lett. 18 (1999) 1453. [32] M.P. Valkonen, S. Lindroos, R. Resch, M. Leskela, G. Friedbacher, M. Grasserbaur, Appl. Surf. Sci. 136 (1998) 131. [33] ASTM X-ray data files, 23-958 and 24-57.
Growth of copper sulphide thin films by successive ionic layer adsorption and reaction (SILAR) method S.D. Sartale, C.D. Lokhande∗ Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India Received 6 July 1999; received in revised form 30 November 1999; accepted 15 December 1999
Abstract The copper sulphide (Cux S) thin films were deposited using relatively simple and new successive ionic layer adsorption and reaction (SILAR) method using copper sulphate and thiourea solutions as cationic and anionic precursors, respectively. The films were deposited on glass and Si (1 1 1) wafer substrates. To obtain good quality Cux S thin films, preparative conditions such as concentration, pH and temperature of cationic and anionic precursor solutions adsorption, reaction and rinsing time durations etc. were optimised. The characterisation of the films was carried out by using X-ray diffraction, scanning electron microscopy, optical absorption, electrical resistivity and thermoemf techniques. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Copper sulphide thin films; SILAR method; Preparation and characterisation
1. Introduction Copper sulphide (Cux S) thin films have received particular attention since the discovery of the CdS/Cux S heterojunction solar cell in 1954 by Reynolds et al. [1]. The Cux S thin films are effectively used in solid junction solar cells which have many applications as it is a direct energy conversion device. It is well known that Cux S has distinct compositions because of the variation in x, 1≤x≤2 with different stoichiometry and that oxidation and temperature is responsible for a change from one composition to another [2–4]. It is interesting that these distinct compositions are not remarkable for the change in crystalline structure but in the variation of electrical resistivity [5] and significant variation in optical band gap [6]. The CdS/Cux S heterojunction solar cell involves the stability of short circuit current (Isc ). The generated current in CdS/Cux S heterojunction solar cell is mainly due to photon absorption by the Cux S layer. Electron–hole pairs are created in this layer and are diffused towards the junction. It is known that Isc is a function of x in Cux S as x increases, Isc also increases and attains a maximum value at x=2 [2]. The Cux S thin films have been found to possess near ideal solar control characteristics. Transmittance in the infrared region, low reflectance <10% in the visible region so as to avoid glare and relatively high reflectance >15% in the near infrared region. The films can also be used in laminated glaz∗
Corresponding author.
ing. The Cux S thin films have been used in air–glass tubular solar collectors as absorber coating, in photo-detectors and photovoltaic applications. Numerous techniques for producing Cux S thin films have been investigated. These include vacuum evaporation [7], activated reactive evaporation [8], reactive magnetron sputtering [9], spray pyrolysis [10], sulphurisation of copper foils and films [11] and slurry technique [12]. Chemical bath deposition of Cux S thin films from alkaline media has been investigated [13–16]. Gadave and Lokhande have formed these films through chemical bath deposition from acidic medium [17]. Das et al. have prepared these films by a solid state reaction between CdS and CuCl films in the temperature range 200–250◦ C [18]. Nair and Nair have reported that absorption greater than 90% can be realised in chemically deposited SnS–Cux S thin films [19]. These films have wide applications in glass evacuated tube solar collectors. Chain and Vook have studied reflection high energy electron diffraction and transmission electron microscopy of Cux S on single crystal copper films [20]. Fernandez and Nair have prepared Cux S and PbS–Cux S films which have been used for transparent polymer coating, primarily to offer protection during window cleaning [21]. Relatively simple, quick, economical and suitable for large area deposition of any configuration, successive ionic layer adsorption and reaction (SILAR) method was reported in mid-1980s [22]. It does not require sophisticated instruments, the substrate need not be conductive and have a high
0254-0584/00/$ – see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 2 0 7 - 8
64
S.D. Sartale, C.D. Lokhande / Materials Chemistry and Physics 65 (2000) 63–67
melting point. The SILAR method is based on immersion of substrate into different cationic and anionic precursor solutions and rinsing before every immersion with highly purified deionised water to remove the loosely bounded species. Thus, single SILAR deposition cycle consists of adsorption of cations, rinsing with water, adsorption and reaction of anions and again rinsing with water. The time durations for adsorption, reaction and rinsing can be experimentally calculated. The growth of the film can be controlled at an accuracy of one SILAR deposition cycle [23,24]. The SILAR method has been applied for the deposition of CdS, ZnS, Cd–ZnS, PbS, ZnO, Sb2 S3 etc. thin films on various substrates [25–31]. Literature survey shows that there is no attempt to deposit Cux S thin films using SILAR method. Keeping this point in mind Cux S thin films were deposited on glass and single crystal Si (1 1 1) wafer substrates using SILAR method. These films were characterised for their structural, optical and electrical properties by using X-ray diffraction (XRD), scanning electron microscopy (SEM), optical absorption, electrical resistivity and thermoemf measurements.
2. Experimental 2.1. Substrate cleaning Corning glass slides of 26 mm×76 mm×1 mm dimension and single crystal Si (1 1 1) wafer (n-type, electrical resistivity, 0.005–0.015 cm) with dimension 20 mm×20 mm×0.5 mm were used as substrates. Corning glass slides were boiled in concentrated chromic acid for 30 min and ultrasonically cleaned with double distilled water and dried with hot air. The single crystal Si (1 1 1) wafer was etched in 40% NH4 F solution at 10◦ C for 10 min and then rinsed in double distilled water to make it hydrogen terminated. 2.2. Cux S thin film for nation Loba analytical reagent grade copper sulphate and thiourea were used in the deposition of Cux S thin films. The cationic precursor was 0.1 M copper sulphate solution. The pH was adjusted to 10 by adding excess ammonia. The source of sulphur ions was 0.1 M thiourea solution (pH∼6). Prepared solutions were taken into beakers of capacity 50 ml each. For rinsing purpose ample quantity of double distilled water (resistivity∼18 M cm) was used. The deposition was carried out at room temperature (27◦ C) using unstirred solutions. In SILAR method concentration, pH and temperature of precursor solutions and the time durations for adsorption, reaction and rinsing are important. By making several trials, Cux S thin film deposition conditions were optimized. The substrate was immersed in 0.1 M copper sulphate solution for 20 s. Then the Cu2+ ions were adsorbed to the
substrate. Then rinsing of the substrate with double distilled water for 30 s removed the desorbed ions. Further, during immersion of the substrate in 0.1 M thiourea solution for 20 s, the S2− ions were adsorbed and reacted with Cu2+ ions on the substrate to form Cux S. The unreacted S2− ions removed by rinsing of the substrate in double distilled water for 30 s. Thus, single SILAR deposition cycle was made up of 20 s adsorption of Cu2+ ions, 30 s rinsing with double distilled water, 20 s adsorption and reaction of S2− ions and 30 s rinsing with double distilled water. By repeating such SILAR deposition cycles for 25 times, we have obtained Cux S thin films of thickness of about 4000 Å. 2.3. Characterisation of the films Film thickness of Cux S was determined by gravimetric weight difference method. For this, a sensitive microbalance was utilised and film density was assumed as the bulk density of Cu2 S (5.6 g cm−3 ). The structural characterisation of Cux S thin films deposited on glass and Si (1 1 1) wafer substrates was carried out by analysing the XRD patterns, obtained using a Philips PW-1710 X-ray diffractometer using Cu K␣ radiations (λ=1.5405 Å). For surface morphology studies of the film deposited on glass substrate, scanning electron microscopy technique was used. The films were coated with gold–palladium by a polaron sputter coating unit E-2500. The coating thickness was 10 nm. The films were loaded in the sample holder of Cambridge stereoscan 250-MK-3 unit for SEM analysis. Optical absorption studies were carried out using a UV–VIS–NIR spectrophotometer (Hitachi, model 330, Japan) in the wavelength range 350–850 nm. The optical spectra were recorded for the film deposited on glass substrates. The light beams were incident from the film side. The glass substrate was the reference while recording the absorption spectra. To study the electrical characterisation of the films, electrical resistivity measurement was carried out using the DC two point probe method in the temperature range 300–500 K. The electrical contacts to the film surface were made through a pair of silver paint electrodes of 5 mm length printed at a separation of 1 cm. To study temperature dependent resistivity measurements, strip heaters were used to heat samples and Chromel–Alumel thermocouple was used to measure the temperature of the samples. The type of electrical conductivity of the films was determined by thermoemf polarity of the thermally generated voltage at the hot end of the film.
3. Results and discussion 3.1. Film thickness Thickness of Cux S thin films for different number of SILAR deposition cycles on glass substrate was measured
S.D. Sartale, C.D. Lokhande / Materials Chemistry and Physics 65 (2000) 63–67
65
Fig. 1. Plot of film thickness against number of SILAR deposition cycles.
using weight difference method as t=
m Aρ
(1)
where, m is the mass of the film, A the area of the film and ρ is the density of the film material (for Cu2 S, ρ=5.6 gm cm−3 ). Fig. 1 shows the variation of film thickness with number of SILAR deposition cycles with optimized deposition conditions. The terminal thickness as 4000 Å was found for 25 SILAR deposition cycles. The growth rate per five SILAR deposition cycles was found to be 800 Å. After 25 SILAR deposition cycles, the film peels off from the substrate. 3.2. Structural studies Fig. 2 (a and b) shows X-ray diffractograms of the films deposited on (a) glass and (b) single crystal Si (1 1 1) wafer substrates. These films on glass substrates are amorphous or consisting of fine grains. The broad hump is due to amorphous glass substrate. The film on Si (1 1 1) wafer substrate shows improvement in crystallinity. Many-fold increase in crystallinity for Si (1 1 1) wafer substrate is attributed to the single crystalline nature of Si (1 1 1) wafer substrate. Such type of improvement in crystallinity by using single crystal substrates has been reported earlier [32]. By comparison
Fig. 2. X-ray diffractograms of SILAR grown Cux S thin films on (a) glass and (b) Si (1 1 1) wafer substrates.
of observed and standard [33] d values, it is concluded that the formed compound is mixture of Cux S with Cux S (1.83≤x≤1.96) and Cu2 S phases with hexagonal crystal structure. The comparison of observed and standard [33] d values is made in Table 1. An SEM image of 4000 Å thick Cux S film on glass substrate at ×30,000 magnification is shown in Fig. 3. The scale bar length is 1 m. It can be seen that the film is dense, smooth and homogeneous without visible pores. The film surface look smooth and uniform. 3.3. Optical absorption studies Fig. 4 shows the plot of optical absorption coefficient (α) against wavelength (λ) for Cux S film on Corning glass substrate. The nature of the transition (direct or indirect) is determined by using the relation
Table 1 Comparison of observed and standard [33] d values Substrate
Observed d values (Å)
Standard d values (Å)
Reflection plane (h k l)
Observed Cux S phase
Glass
2.39 1.39 3.18 1.54 1.14
2.40 1.39 3.16 1.54 1.14
102 5 6 1/4 6 4 033 460 300
Cu2 S Cux S (1.83≤x≤1.96) Cux S (1.83≤x≤1.96) Cux S (1.83≤x≤1.96) Cu2 S
Si (1 1 1) wafer
66
S.D. Sartale, C.D. Lokhande / Materials Chemistry and Physics 65 (2000) 63–67
Fig. 3. A SEM image of 4000 Å thick SILAR grown Cux S thin film on glass substrate with ×30,000 magnification. The scale bar length is 1 m.
α=
A(hν − Eg )n hν
(2)
where hν is the photon energy and Eg the optical bandgap. A is the constant which is related to the effective masses associated with the valence and conduction bands. For allowed direct transitions, n=1/2 and allowed indirect transitions, n=2. Fig. 5 shows the plot of (αhν)2 against hν. The variation of (αhν)2 with hν is linear which indicates that the direct transition is present. Extrapolating the straight line portion of the plot of (αhv)2 against hν to energy axis for zero absorption coefficient give optical bandgap energy value as 2.36 eV, which is comparable to the value reported earlier [17].
Fig. 4. Plot of optical absorption coefficient (α) for the SILAR grown Cux S thin film on glass substrate.
Fig. 5. Plot of (αhν)2 against hν for the SILAR grown Cux S thin film on glass substrate.
3.4. Electrical characterisations Fig. 6 shows the variation of logarithm of resistivity with reciprocal of temperature for the film on glass substrate. The resistivity decreases with increase in temperature indicating the semiconducting behaviour of the Cux S film. It is also observed that the variation is non-linear. The plot shows two regions corresponding to low and high temperature regions. Similar types of results are reported earlier [17]. The resistivity at room temperature (27◦ C) was found to be of
Fig. 6. Plot of logarithm of resistivity against reciprocal of temperature for the SILAR grown Cux S thin film on glass substrate.
S.D. Sartale, C.D. Lokhande / Materials Chemistry and Physics 65 (2000) 63–67
the order of 10−2 cm. From polarity of thermoemf, p-type conductivity of Cux S thin films is confirmed. Same nature of conductivity of Cux S thin films is reported earlier [17]. 4. Conclusions In this paper, we have reported the simple successive ionic layer adsorption and reaction (SILAR) method for production of Cux S thin films using copper sulphate and thiourea solutions as cationic and anionic precursors, respectively. The films were amorphous or consisting of fine grains on glass substrate, whereas many-fold increase in crystallinity was observed for Si (1 1 1) wafer substrate. From the SEM image, the film surface looks smooth and uniform. From optical absorption study, direct band gap was found to be 2.36 eV. The resistivity at room temperature (27◦ C) was found to be of the order of 10−2 cm. The films were p-type and semiconducting as evidenced by thermoemf and electrical resistivity studies, respectively.
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