- Email: [email protected]
Synthesis, characterization of chemically deposited indium selenide thin films at room temperature
ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 69 (2008) 249–254 www.elsevier.com/locate/jpcs
Synthesis, characterization of chemically deposited indium selenide thin films at room temperature M.R. Asabea, P.A. Chatea, S.D. Delekara, K.M. Garadkarb, I.S. Mullac, P.P. Hankarea, a
Solid State Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416004, Maharashtra, India b University of Pune, Pune 411007, Maharashtra, India c National Chemical Laboratory, Pune 411007, Maharashtra, India Received 28 June 2007; received in revised form 27 August 2007; accepted 31 August 2007
Abstract Polycrystalline In2Se3 semiconducting thin films were prepared by using relatively simple chemical bath deposition method at room temperature by the reaction between indium chloride, tartaric acid, hydrazine hydrate and sodium selenosulphate in an aqueous alkaline medium. Various preparative conditions of thin film deposition are outlined. The as grown films were found to be transparent, uniform, well adherent and red in color. The films were characterized using X-ray diffraction (XRD), scanning electron microscopy, atomic absorption spectroscopy and energy dispersive atomic X-ray diffraction (EDAX). The XRD analysis of the film showed the presence of polycrystalline nature with hexagonal crystal structure. SEM study revels that the grains are homogenous, without cracks or pinholes and well covers the glass substrate. The optical absorption and electrical conductivity was measured. The direct optical band gap value for the films was found to be of the order of 2.35 eV at room temperature and have specific electrical conductivity of the order of 102 (O cm)1 showing n-type conduction mechanism. The utility of the adapted technique is discussed from the view-point of applications considering the optoelectric and structural data. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Chemical synthesis; B. Semiconductor
1. Introduction Binary semiconductors are considered as important technological materials because of their prime applications in various optical and electronic devices [1]. Indium selenide has two crystalline surfaces exhibiting very different physical properties. The cleavage surface perpendicular to the z-axis consists of selenium atom bounded together with covalent bonds. The other surface parallel to the z-axis is made up of selenium atoms of adjacent layers being bounded by Van der Waal forces [2,3]. It shows very significant properties for photovoltaic and photochemical applications. This is because of its high absorption coefficient associated with an energy band gap in the optimum range for solar energy conversion [4–6]. The Corresponding author. Tel.: +91 231 2609381; fax: +91 231 2605271.
E-mail address: [email protected] (P.P. Hankare). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.08.070
molecular beam epitaxy, vapor deposition, spray pyrolysis, evaporation technique, electrochemical atomic layer epitaxy are some of the methods used for the growth of III–VI materials like In2Se3 [7–17]. But chemical bath deposition method is an alternative, low-cost method which can operate at low processing temperature and provides large area deposition. The method consist of complexed metal ion of interest, source of chalcogen ions, the stability equilibrium of which provide a concentration of ions small enough for controlled homogenous precipitation of material in the thin film form on substrate [18–21]. The various chalcogenide thin films like CdS, CdSe, SnSe, Bi2Se3, Bi2S3, MoSe2 have been so far synthesized by chemical bath deposition techniques and characterized for their structural, optical and electrical studies [22–25]. But very few attempts have been made for the investigation of In2Se3 films prepared by using chemical bath deposition method at room temperature. The various preparative
ARTICLE IN PRESS 250
M.R. Asabe et al. / Journal of Physics and Chemistry of Solids 69 (2008) 249–254
parameters, structural, compositional, morphological characterization and optical and electrical properties of these films are reported here. 2. Experimental 2.1. Substrate cleaning The substrates were cleaned by boiling them in chromic acid for 1 h which was followed by washing successively with detergent and alcohol. They were finally stored in double distilled water before use. 2.2. Reagents and preparation of solutions The chemicals used for preparation of thin films were AR grade indium trichloride, tartaric acid, hydrazine hydrate, sodium sulphite and selenium. The solutions were prepared in double distilled water. Sodium selenosulphate (0.25 M) was used as a source of selenium. The solution was prepared by refluxing 5 g of selenium powder with 15 g sodium sulphite in 200 mL double distilled water for 9 h at 363 K. The solution was cooled, filtered to remove undissolved selenium and stored in an airtight container. 2.3. Synthesis of In2Se3 thin films The deposition of In2Se3 thin films was made in a reactive solution obtained by mixing 10 mL (0.2 M) indium trichloride, 2.5 mL (1 M) tartaric acid, 10 mL (10%) hydrazine hydrate and 15 mL (0.25 M) sodium selenosulphate. The total volume of the reactive mixture was made upto 100 mL by adding double distilled water. The beaker containing the reactive solution was transferred to an ice bath at 278 K temperature. The pH of the resulting solution was found to be 11.8070.05. To obtain the film, four glass substrates were positioned vertically on a specially designed substrate holder and rotated in a reactive solution with a speed of 5572 rpm. The temperature of the solution was then allowed to rise slowly to 293 K. The substrates were subsequently removed from the beaker after 2 h of deposition. The films obtained were washed with distilled water, dried in air and kept in a desiccator.
along the length of a film performed thermoelectric power measurements were made. The potential difference between the two points of contact separated by 1 cm was recorded with a digital microvoltmeter. A calibrated thermocouple (chromel–alumel, 24 gauge) with a digital indicator was used to sense the working temperature. The optical absorption measurements were made in the wavelength range 400–800 nm by using a Hitachi-330 (Japan) UV– VIS–NIR double beam spectrophotometer at room temperature. An identical, uncoated glass substrate in the reference beam made a substrate absorption correction. The analysis of the spectrum was carried out by computing the values of absorption at every step of 2 nm. A 250MKIII Stereoscan (USA) scanning electron microscope (SEM) was used for the microscopic observations. Compositional analysis of indium was done with an atomic absorption spectrophotometer (Perkin-Elmer Model 3030) and EDAX. 4. Results and discussions 4.1. Growth mechanism and film thickness In the reaction bath, In+3 ions are complexed with tartaric acid in the form of water-soluble In-tartarate complex and thus control In+3 ion concentration. The dissociation of sodium selenosulphate as well as Intartarate complex in alkaline medium takes place. At 278 K, it forms clear solution and no film or precipitate is observed when solution was kept for a long time. The metal ions are in stable complex state [In(A)n]. As temperature increases slowly formation of selenosulphate and metal complex take place in alkaline medium, favoring the formation of In2Se3 thin film. The deposition process is based on slow release of In+3 and Se2 ions in the solution on the ion-by-ion basis on the glass substrate. The kinetics of growth of the films can be understood from the following reaction. Inþ3 þ nA2 2½InðAÞn;
(1.1)
Na2 SeO3 þ OH ! Na2 SO4 þ HSe ;
(1.2)
HSe þ OH ! H2 O þ Se2 ;
(1.3)
3. Characterization of In2Se3 thin films
½InðAÞn þ Se2 ! In2 Se3 þ nA:
(1.4)
The X-ray diffraction (XRD) study of In2Se3 film was carried out in the range of the diffraction angle 101–801 with Cu Ka1 radiation using Philips PW-1710 diffractometer (l=1.54056 A˚). The layer thickness of the film was estimated by the weight difference method. The electrical conductivity of In2Se3 thin film was measured using a ‘dc’ two-probe method. A quick drying silver paste was applied at the ends of the film for good ohmic contacts. For the measurements of conductivity, a constant voltage of 30 V was applied across the sample. The current was noted at different temperatures. Maintaining a temperature gradient
Hydrazine hydrate acts as complementary complexant, which improves compactness and adherence of the film. Speed of rotation of 5572 rpm was selected to deposit In2Se3 thin films. At higher speeds very thin film is deposited. At lower speed, thick non-adherent films are deposited. The films obtained are uniform, well adherent, red and transparent. The thickness of In2Se3 film was measured by weight difference method by using sensitive microbalance. The thickness was measured every 30 min and is plotted against time (Fig. 1). The thickness increases linearly with time
ARTICLE IN PRESS M.R. Asabe et al. / Journal of Physics and Chemistry of Solids 69 (2008) 249–254
251
Thickness (μm)
0.6
0.3
0
0
100
50
150
Time (min) Fig. 1. Variation of the film thickness with deposition time.
0.6
Fig. 3. XRD pattern of In2Se3 thin film.
Table 1 Structural and morphological data of In2Se3 thin film
Thickness (μm )
Film
In2Se3
0.3
0 275
290
305
Temperature (K) Fig. 2. Variation of the film thickness with deposition temperature.
upto 120 min and then decreases slightly. This is due to decreasing concentration of reactive species. The thickness was also measured as a function of temperature as shown in Fig. 2. The thickness increases linearly with temperature upto 298 K and then decreases. Such a result is observed due to faster release of In+3 and Se2 ions forming precipitate instead of film formation. The terminal thickness was found to be 0.55 mm. 4.2. XRD and morphological characterization To study the crystal structure of In2Se3 film, X-ray diffractogram of the film was examined. The XRD pattern of In2Se3 deposited on glass substrate is shown in Fig. 3. The films are polycrystalline in nature. The crystallographic data for In2Se3 is shown in Table 1. The XRD analysis reveals that the obtained films were monophased and crystallized in the hexagonal phase (ASTM Diff. File
d-Values (A0)
Planes hkl
Observed
ASTM
3.5001 3.1130 2.0864 1.8131 1.3380 1.2907
3.4964 3.1130 2.0970 1.8130 1.3369 1.2938
111 113 124 305 139 236
Grain size (A0) XRD
SEM
210
218
No. 71-0250). The broad hump (2y ¼ 201–401) is due to amorphous glass substrate. In2Se3 thin film shows prominent (1 1 1) (1 1 3) (1 2 4) (3 0 5) (1 3 9) (2 3 6) peaks. The lattice parameter and hkl planes are in fairly good agreement with standard values. The average grain size of the material was determined by using the Scherrer formula: D ¼ Kl=b cos y,
(1.5)
where D is crystallite size, l is the X-ray wavelength used in A˚, b is the angular line width at half the maximum intensity, y is Bragg’s diffraction angle and K is constant. The average grain size was calculated by resolving the highest intensity peak. The average crystallite size of the as deposited In2Se3 thin film was found to be 210 A˚. The surface morphology of indium selenide thin films was analyzed by using SEM. SEM images of indium selenide films are shown in Fig. 4. It is observed that the indium selenide thin film is homogenous, without cracks or pinholes and it well covers the glass substrate. It also suggests that the film is composed of minute grains, was uniformly distributed over a smooth homogenous background that may correspond to some amorphous phase of indium selenide thin film. The presence of fine background
ARTICLE IN PRESS M.R. Asabe et al. / Journal of Physics and Chemistry of Solids 69 (2008) 249–254
252
Fig. 4. SEM micrographs of In2Se3 thin film.
is an indication of one step growth by multiple nucleations. The average grain size of indium selenide sample is reported in Table 1. 4.3. Compositional analysis
4.4. Optical and electrical studies The optical absorption spectra of indium selenide film deposited onto glass substrate were taken at in the wavelength range of 400–800 nm.Fig. 6 shows variation of optical absorption with wavelength. The optical study shows that the films are highly absorptive (a 104 cm1). Indium selenide is a direct band gap semiconductor [27]. For an allowed direct band gap transition the absorption
3
Absorbance
Atomic absorption spectroscopy was used to study compositional analysis by calibration curve method. The weighed quantity of sample was dissolved in the minimum quantity of conc. HNO3. Below pH ¼ 7, the selenium was precipitated as free element [26]. While nitrates of indium remain in the solution. The precipitate was filtered through a Gooch crucible and subjected to selenium estimation using a standard gravimetric method. The filtrate containing indium nitrate was diluted to suitable dilution and estimated by AAS. The standard solution used for obtaining the calibration curve was made by diluting commercial standards to concentration 0.4, 0.8, 1.2, 1.6, 2.0 mg/mL for indium. The compositional analysis of the sample using AAS gave 39.53% indium and 60.47% selenium, showing sample is indium deficient. The composition of the sample is also confirmed by EDAX. It suggests that 37.64% indium and 62.36% selenium are present in the sample. The EDAX of the In2Se3 sample is shown in Fig. 5.
Fig. 5. EDAX of In2Se3 thin film.
2
1
0 400
500
600
700
800
Wavelength (nm) Fig. 6. Absorption spectrum of In2Se3 thin film.
coefficient a can be related to the photon energy hn by ðahnÞ2 ¼ Aðhn E g Þ,
(1.6)
where A is a constant and Eg is the energy band gap. For a direct band gap semiconductor the (ahn)2 vs. hn characteristic is predicted to be a straight line with a photoenergy axis intercept indicative for the band gap [28,29]. This is illustrated in Fig. 7, where a band gap of 2.35 eV can be obtained.
ARTICLE IN PRESS M.R. Asabe et al. / Journal of Physics and Chemistry of Solids 69 (2008) 249–254
9000
-1 1.5
2
253
2.5
3
3.5
-1.1
(αhv)2 X 108
log (conductivity
6000
3000
0
-1.2
-1.3
-1.4
2
2.5 3 photon energy (hv)
1000/T (per K)
3.5
Fig. 8. The variations of log (conductivity) with inverse temperature.
2
Fig. 7. Plots of (ahn) vs. photon energy.
Table 2 Data on optical and electrical properties of In2Se3 thin film Film
In2Se3
Band gap (eV)
2.35
Activation energy (eV)
Specific conductance (O cm)1
HT
LT
300 K
525 K
0.143
0.021
9.67 102
1.02 102
The dark electrical conductivity of In2Se3 film on nonconducting glass slide was determined by using a dc two probe method, in the temperature range 300–525 K. At room temperature the specific conductance was found to be of the order of 102 (O cm)1, which agrees well with the earlier reported value [30]. The values of specific conductance at 300 and 525 K are reported in Table 2. It is observed that the conductivity on the film increases with increasing in temperature which indicates the semiconducting nature of the thin film. The electrical conductivity with temperature during heating and cooling cycles was found to be different and this shows that the ‘as deposited’ films undergo an irreversible change due to annealing out of nonequilibrium defects during first heating. A plot of log s (conductivity) vs. inverse temperature for the cooling cycle is shown in the Fig. 8. The plot shows that electrical conductivity has two linear regions, an intrinsic region setting at low temperature, characterized by small slope (300–350 K). High temperature region is associated with extrinsic conduction due to the presence donor states. The activation energy is calculated using the Arrhenius equation s ¼ s0 expðEa=kTÞ,
(1.7)
where the terms have usual meaning. The activation energies are 0.021 and 0.143 eV for low and high temperature regions, respectively.
In thermoelectric power measurements, the open circuit thermovoltage generated by the sample, when a temperature gradient is applied across a length of the sample, was measured using a digital microvoltmeter. The temperature difference between the two ends of the samples causes the transport of carriers from the hot to cold end, thus creating an electric field, which gives rise to thermovoltage across the ends. The thermovoltage generated is directly proportional to temperature gradient maintained across the ends of the sample. From the sign of the potentiometer terminal connected at the cold end, one can deduce the sign of predominant charge carries. In the case of In2Se3 thin film, the negative terminal was connected to the cold end, therefore, the film shows n-type conductivity [31]. 5. Conclusion Homogenous and uniform films of indium selenide (In2Se3) have been successfully deposited using chemical bath deposition method. The film formation takes place by ion-by-ion growth mechanism. Crystallographic and micrographic studies revealed the polycrystalline nature of the films. Optical studies show that, indium selenide films have high optical absorption coefficient and direct band-toband type optical transition. Temperature dependence of electrical conductivity showed the semiconducting nature of the film. Thermoelectric power measurement shows n-type conduction for In2Se3 thin film. References [1] S.T. Lakshmikumar, A.C. Rastogi, Thin Solid Films 256 (1995) 150. [2] H.M. Pathan, S.S. Kulkarni, R.S. Mane, C.D. Lokhande, Mater. Chem. Phys. 93 (2005) 16. [3] M. Eddiet, C. Julien, M. Balkanski, Mater. Lett. 2 (1984) 432. [4] H. Bouzuita, N. Bouguila, S. Duchemin, S. Fiechter, A. Dhouib, Renew. Energy 25 (2002) 131.
ARTICLE IN PRESS 254
M.R. Asabe et al. / Journal of Physics and Chemistry of Solids 69 (2008) 249–254
[5] J.C. Former, J.A. Turner, R. Noufi, D. Cohen, J. Electrochem. 132 (1985) 1319. [6] J. Herrero, J. Ortega, Sol. Energy Mater. 16 (1987) 477. [7] T. Ohtsuka, T. Okamoto, A. Yamada, M. Konagai, Comp. Semicond. 156 (1997) 183. [8] T. Ohtsuka, K. Nakanishi, T. Okamoto, A. Yamada, M. Konagai, U. Jahn, Jpn. J. Appl. Phys. 40 (2001) 509. [9] T. Okamoto, A. Yamada, M. Konagai, J. Cryst. Growth 175 (1997) 1045. [10] J. Cheon, J. Arnold, K.M. Yu, E.D. Bourret, Chem. Mater. 7 (1995) 2273. [11] M. Emziane, R. Le Ny, J. Phys. D: Appl. Phys. 32 (1999) 1319. [12] M. Emziane, S. Marsillac, J.C. Bernede, Mater. Chem. Phys. 62 (2000) 84. [13] S. Marsillac, J.C. Bernede, R. Leny, A. Conan, Vacuum 46 (1995) 997. [14] A.F. Qasrawi, M. Parlak, C. Ercelehi, I. Gunal, J. Mater. Sci: Mater. Electron. 12 (2001) 473. [15] S. Marsillac, J.C. Bernede, M. Emziane, J. Wery, E. Faulques, P. Ray, Appl. Surf. Sci. 151 (1999) 171. [16] M. Emziane, R. Le Ny, Thin Solid Films 385 (2001) 312. [17] R. Vaidyanathan, J.L. Stickney, S.M. Cox, S.P. Compton, U. Happek, J. Electroanal. Chem. 559 (2003) 55.
[18] C.D. Lokhande, P.S. Patil, H. Tributsch, A. Ennaoui, Sol. Energy Mat. Sol. Cells 55 (1998) 379. [19] M. Ganchev, N. Stratieva, E. Tzvetkova, J. Mater. Sci. Mater. Electron. 14 (2003) 847. [20] R.K. Pandey, A.J.N. Rooz, R.B. Gore, Semicond. Sci. Tech. 3 (1988) 733. [21] P.P. Hankare, P.A. Chate, M.R. Asabe, S.D. Delekar, I.S. Mulla, K.M. Garadkar, J. Mater. Sci. Mater. Electron. 17 (2006) 1055. [22] S. Chandra, D.P. Singh, D.C. Srivastava, S.N. Sahu, J. Phys. D: Appl. Phys. 17 (1984) 2125. [23] R. Bichsel, F. Levy, H.J. Mathieu, Thin Solid Films 131 (1985) 87. [24] M. Ganchev, N. Strahieva, E. Tzvetkova, I. Gadjov, J. Mater. Sci.: Mater. Electron. 14 (2003) 847. [25] P.P. Hankare, P.A. Chate, S.D. Delekar, V.M. Bhuse, M.R. Asabe, B.V. Jadhav, K.M. Garadkar, Cryst. Growth 291 (2006) 40. [26] P.P. Hankare, P.A. Chate, S.D. Delekar, M.R. Asabe, I.S. Mulla, J. Phys. Chem. Solid 67 (2006) 2310. [27] C. Julien, A. Chevy, D. Siapkas, Phys. Stat. Sol. (a) 118 (1990) 553. [28] K. Kambas, Physica B 166 (1989) 103. [29] V.P. Munshinskii, V.I. Kobolev, Sov. Phys. Semicond. 5 (1971) 1104. [30] J.C. Bernede, S. Marsillac, A. Conan, Mater. Chem. Phys. 48 (1997) 5. [31] C.N.R. Rao, Modern Aspects of Solid State Chemistry, Plenum Press, New York, 1970, p. 531.
Journal of Physics and Chemistry of Solids 69 (2008) 249–254 www.elsevier.com/locate/jpcs
Synthesis, characterization of chemically deposited indium selenide thin films at room temperature M.R. Asabea, P.A. Chatea, S.D. Delekara, K.M. Garadkarb, I.S. Mullac, P.P. Hankarea, a
Solid State Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416004, Maharashtra, India b University of Pune, Pune 411007, Maharashtra, India c National Chemical Laboratory, Pune 411007, Maharashtra, India Received 28 June 2007; received in revised form 27 August 2007; accepted 31 August 2007
Abstract Polycrystalline In2Se3 semiconducting thin films were prepared by using relatively simple chemical bath deposition method at room temperature by the reaction between indium chloride, tartaric acid, hydrazine hydrate and sodium selenosulphate in an aqueous alkaline medium. Various preparative conditions of thin film deposition are outlined. The as grown films were found to be transparent, uniform, well adherent and red in color. The films were characterized using X-ray diffraction (XRD), scanning electron microscopy, atomic absorption spectroscopy and energy dispersive atomic X-ray diffraction (EDAX). The XRD analysis of the film showed the presence of polycrystalline nature with hexagonal crystal structure. SEM study revels that the grains are homogenous, without cracks or pinholes and well covers the glass substrate. The optical absorption and electrical conductivity was measured. The direct optical band gap value for the films was found to be of the order of 2.35 eV at room temperature and have specific electrical conductivity of the order of 102 (O cm)1 showing n-type conduction mechanism. The utility of the adapted technique is discussed from the view-point of applications considering the optoelectric and structural data. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Chemical synthesis; B. Semiconductor
1. Introduction Binary semiconductors are considered as important technological materials because of their prime applications in various optical and electronic devices [1]. Indium selenide has two crystalline surfaces exhibiting very different physical properties. The cleavage surface perpendicular to the z-axis consists of selenium atom bounded together with covalent bonds. The other surface parallel to the z-axis is made up of selenium atoms of adjacent layers being bounded by Van der Waal forces [2,3]. It shows very significant properties for photovoltaic and photochemical applications. This is because of its high absorption coefficient associated with an energy band gap in the optimum range for solar energy conversion [4–6]. The Corresponding author. Tel.: +91 231 2609381; fax: +91 231 2605271.
E-mail address: [email protected] (P.P. Hankare). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.08.070
molecular beam epitaxy, vapor deposition, spray pyrolysis, evaporation technique, electrochemical atomic layer epitaxy are some of the methods used for the growth of III–VI materials like In2Se3 [7–17]. But chemical bath deposition method is an alternative, low-cost method which can operate at low processing temperature and provides large area deposition. The method consist of complexed metal ion of interest, source of chalcogen ions, the stability equilibrium of which provide a concentration of ions small enough for controlled homogenous precipitation of material in the thin film form on substrate [18–21]. The various chalcogenide thin films like CdS, CdSe, SnSe, Bi2Se3, Bi2S3, MoSe2 have been so far synthesized by chemical bath deposition techniques and characterized for their structural, optical and electrical studies [22–25]. But very few attempts have been made for the investigation of In2Se3 films prepared by using chemical bath deposition method at room temperature. The various preparative
ARTICLE IN PRESS 250
M.R. Asabe et al. / Journal of Physics and Chemistry of Solids 69 (2008) 249–254
parameters, structural, compositional, morphological characterization and optical and electrical properties of these films are reported here. 2. Experimental 2.1. Substrate cleaning The substrates were cleaned by boiling them in chromic acid for 1 h which was followed by washing successively with detergent and alcohol. They were finally stored in double distilled water before use. 2.2. Reagents and preparation of solutions The chemicals used for preparation of thin films were AR grade indium trichloride, tartaric acid, hydrazine hydrate, sodium sulphite and selenium. The solutions were prepared in double distilled water. Sodium selenosulphate (0.25 M) was used as a source of selenium. The solution was prepared by refluxing 5 g of selenium powder with 15 g sodium sulphite in 200 mL double distilled water for 9 h at 363 K. The solution was cooled, filtered to remove undissolved selenium and stored in an airtight container. 2.3. Synthesis of In2Se3 thin films The deposition of In2Se3 thin films was made in a reactive solution obtained by mixing 10 mL (0.2 M) indium trichloride, 2.5 mL (1 M) tartaric acid, 10 mL (10%) hydrazine hydrate and 15 mL (0.25 M) sodium selenosulphate. The total volume of the reactive mixture was made upto 100 mL by adding double distilled water. The beaker containing the reactive solution was transferred to an ice bath at 278 K temperature. The pH of the resulting solution was found to be 11.8070.05. To obtain the film, four glass substrates were positioned vertically on a specially designed substrate holder and rotated in a reactive solution with a speed of 5572 rpm. The temperature of the solution was then allowed to rise slowly to 293 K. The substrates were subsequently removed from the beaker after 2 h of deposition. The films obtained were washed with distilled water, dried in air and kept in a desiccator.
along the length of a film performed thermoelectric power measurements were made. The potential difference between the two points of contact separated by 1 cm was recorded with a digital microvoltmeter. A calibrated thermocouple (chromel–alumel, 24 gauge) with a digital indicator was used to sense the working temperature. The optical absorption measurements were made in the wavelength range 400–800 nm by using a Hitachi-330 (Japan) UV– VIS–NIR double beam spectrophotometer at room temperature. An identical, uncoated glass substrate in the reference beam made a substrate absorption correction. The analysis of the spectrum was carried out by computing the values of absorption at every step of 2 nm. A 250MKIII Stereoscan (USA) scanning electron microscope (SEM) was used for the microscopic observations. Compositional analysis of indium was done with an atomic absorption spectrophotometer (Perkin-Elmer Model 3030) and EDAX. 4. Results and discussions 4.1. Growth mechanism and film thickness In the reaction bath, In+3 ions are complexed with tartaric acid in the form of water-soluble In-tartarate complex and thus control In+3 ion concentration. The dissociation of sodium selenosulphate as well as Intartarate complex in alkaline medium takes place. At 278 K, it forms clear solution and no film or precipitate is observed when solution was kept for a long time. The metal ions are in stable complex state [In(A)n]. As temperature increases slowly formation of selenosulphate and metal complex take place in alkaline medium, favoring the formation of In2Se3 thin film. The deposition process is based on slow release of In+3 and Se2 ions in the solution on the ion-by-ion basis on the glass substrate. The kinetics of growth of the films can be understood from the following reaction. Inþ3 þ nA2 2½InðAÞn;
(1.1)
Na2 SeO3 þ OH ! Na2 SO4 þ HSe ;
(1.2)
HSe þ OH ! H2 O þ Se2 ;
(1.3)
3. Characterization of In2Se3 thin films
½InðAÞn þ Se2 ! In2 Se3 þ nA:
(1.4)
The X-ray diffraction (XRD) study of In2Se3 film was carried out in the range of the diffraction angle 101–801 with Cu Ka1 radiation using Philips PW-1710 diffractometer (l=1.54056 A˚). The layer thickness of the film was estimated by the weight difference method. The electrical conductivity of In2Se3 thin film was measured using a ‘dc’ two-probe method. A quick drying silver paste was applied at the ends of the film for good ohmic contacts. For the measurements of conductivity, a constant voltage of 30 V was applied across the sample. The current was noted at different temperatures. Maintaining a temperature gradient
Hydrazine hydrate acts as complementary complexant, which improves compactness and adherence of the film. Speed of rotation of 5572 rpm was selected to deposit In2Se3 thin films. At higher speeds very thin film is deposited. At lower speed, thick non-adherent films are deposited. The films obtained are uniform, well adherent, red and transparent. The thickness of In2Se3 film was measured by weight difference method by using sensitive microbalance. The thickness was measured every 30 min and is plotted against time (Fig. 1). The thickness increases linearly with time
ARTICLE IN PRESS M.R. Asabe et al. / Journal of Physics and Chemistry of Solids 69 (2008) 249–254
251
Thickness (μm)
0.6
0.3
0
0
100
50
150
Time (min) Fig. 1. Variation of the film thickness with deposition time.
0.6
Fig. 3. XRD pattern of In2Se3 thin film.
Table 1 Structural and morphological data of In2Se3 thin film
Thickness (μm )
Film
In2Se3
0.3
0 275
290
305
Temperature (K) Fig. 2. Variation of the film thickness with deposition temperature.
upto 120 min and then decreases slightly. This is due to decreasing concentration of reactive species. The thickness was also measured as a function of temperature as shown in Fig. 2. The thickness increases linearly with temperature upto 298 K and then decreases. Such a result is observed due to faster release of In+3 and Se2 ions forming precipitate instead of film formation. The terminal thickness was found to be 0.55 mm. 4.2. XRD and morphological characterization To study the crystal structure of In2Se3 film, X-ray diffractogram of the film was examined. The XRD pattern of In2Se3 deposited on glass substrate is shown in Fig. 3. The films are polycrystalline in nature. The crystallographic data for In2Se3 is shown in Table 1. The XRD analysis reveals that the obtained films were monophased and crystallized in the hexagonal phase (ASTM Diff. File
d-Values (A0)
Planes hkl
Observed
ASTM
3.5001 3.1130 2.0864 1.8131 1.3380 1.2907
3.4964 3.1130 2.0970 1.8130 1.3369 1.2938
111 113 124 305 139 236
Grain size (A0) XRD
SEM
210
218
No. 71-0250). The broad hump (2y ¼ 201–401) is due to amorphous glass substrate. In2Se3 thin film shows prominent (1 1 1) (1 1 3) (1 2 4) (3 0 5) (1 3 9) (2 3 6) peaks. The lattice parameter and hkl planes are in fairly good agreement with standard values. The average grain size of the material was determined by using the Scherrer formula: D ¼ Kl=b cos y,
(1.5)
where D is crystallite size, l is the X-ray wavelength used in A˚, b is the angular line width at half the maximum intensity, y is Bragg’s diffraction angle and K is constant. The average grain size was calculated by resolving the highest intensity peak. The average crystallite size of the as deposited In2Se3 thin film was found to be 210 A˚. The surface morphology of indium selenide thin films was analyzed by using SEM. SEM images of indium selenide films are shown in Fig. 4. It is observed that the indium selenide thin film is homogenous, without cracks or pinholes and it well covers the glass substrate. It also suggests that the film is composed of minute grains, was uniformly distributed over a smooth homogenous background that may correspond to some amorphous phase of indium selenide thin film. The presence of fine background
ARTICLE IN PRESS M.R. Asabe et al. / Journal of Physics and Chemistry of Solids 69 (2008) 249–254
252
Fig. 4. SEM micrographs of In2Se3 thin film.
is an indication of one step growth by multiple nucleations. The average grain size of indium selenide sample is reported in Table 1. 4.3. Compositional analysis
4.4. Optical and electrical studies The optical absorption spectra of indium selenide film deposited onto glass substrate were taken at in the wavelength range of 400–800 nm.Fig. 6 shows variation of optical absorption with wavelength. The optical study shows that the films are highly absorptive (a 104 cm1). Indium selenide is a direct band gap semiconductor [27]. For an allowed direct band gap transition the absorption
3
Absorbance
Atomic absorption spectroscopy was used to study compositional analysis by calibration curve method. The weighed quantity of sample was dissolved in the minimum quantity of conc. HNO3. Below pH ¼ 7, the selenium was precipitated as free element [26]. While nitrates of indium remain in the solution. The precipitate was filtered through a Gooch crucible and subjected to selenium estimation using a standard gravimetric method. The filtrate containing indium nitrate was diluted to suitable dilution and estimated by AAS. The standard solution used for obtaining the calibration curve was made by diluting commercial standards to concentration 0.4, 0.8, 1.2, 1.6, 2.0 mg/mL for indium. The compositional analysis of the sample using AAS gave 39.53% indium and 60.47% selenium, showing sample is indium deficient. The composition of the sample is also confirmed by EDAX. It suggests that 37.64% indium and 62.36% selenium are present in the sample. The EDAX of the In2Se3 sample is shown in Fig. 5.
Fig. 5. EDAX of In2Se3 thin film.
2
1
0 400
500
600
700
800
Wavelength (nm) Fig. 6. Absorption spectrum of In2Se3 thin film.
coefficient a can be related to the photon energy hn by ðahnÞ2 ¼ Aðhn E g Þ,
(1.6)
where A is a constant and Eg is the energy band gap. For a direct band gap semiconductor the (ahn)2 vs. hn characteristic is predicted to be a straight line with a photoenergy axis intercept indicative for the band gap [28,29]. This is illustrated in Fig. 7, where a band gap of 2.35 eV can be obtained.
ARTICLE IN PRESS M.R. Asabe et al. / Journal of Physics and Chemistry of Solids 69 (2008) 249–254
9000
-1 1.5
2
253
2.5
3
3.5
-1.1
(αhv)2 X 108
log (conductivity
6000
3000
0
-1.2
-1.3
-1.4
2
2.5 3 photon energy (hv)
1000/T (per K)
3.5
Fig. 8. The variations of log (conductivity) with inverse temperature.
2
Fig. 7. Plots of (ahn) vs. photon energy.
Table 2 Data on optical and electrical properties of In2Se3 thin film Film
In2Se3
Band gap (eV)
2.35
Activation energy (eV)
Specific conductance (O cm)1
HT
LT
300 K
525 K
0.143
0.021
9.67 102
1.02 102
The dark electrical conductivity of In2Se3 film on nonconducting glass slide was determined by using a dc two probe method, in the temperature range 300–525 K. At room temperature the specific conductance was found to be of the order of 102 (O cm)1, which agrees well with the earlier reported value [30]. The values of specific conductance at 300 and 525 K are reported in Table 2. It is observed that the conductivity on the film increases with increasing in temperature which indicates the semiconducting nature of the thin film. The electrical conductivity with temperature during heating and cooling cycles was found to be different and this shows that the ‘as deposited’ films undergo an irreversible change due to annealing out of nonequilibrium defects during first heating. A plot of log s (conductivity) vs. inverse temperature for the cooling cycle is shown in the Fig. 8. The plot shows that electrical conductivity has two linear regions, an intrinsic region setting at low temperature, characterized by small slope (300–350 K). High temperature region is associated with extrinsic conduction due to the presence donor states. The activation energy is calculated using the Arrhenius equation s ¼ s0 expðEa=kTÞ,
(1.7)
where the terms have usual meaning. The activation energies are 0.021 and 0.143 eV for low and high temperature regions, respectively.
In thermoelectric power measurements, the open circuit thermovoltage generated by the sample, when a temperature gradient is applied across a length of the sample, was measured using a digital microvoltmeter. The temperature difference between the two ends of the samples causes the transport of carriers from the hot to cold end, thus creating an electric field, which gives rise to thermovoltage across the ends. The thermovoltage generated is directly proportional to temperature gradient maintained across the ends of the sample. From the sign of the potentiometer terminal connected at the cold end, one can deduce the sign of predominant charge carries. In the case of In2Se3 thin film, the negative terminal was connected to the cold end, therefore, the film shows n-type conductivity [31]. 5. Conclusion Homogenous and uniform films of indium selenide (In2Se3) have been successfully deposited using chemical bath deposition method. The film formation takes place by ion-by-ion growth mechanism. Crystallographic and micrographic studies revealed the polycrystalline nature of the films. Optical studies show that, indium selenide films have high optical absorption coefficient and direct band-toband type optical transition. Temperature dependence of electrical conductivity showed the semiconducting nature of the film. Thermoelectric power measurement shows n-type conduction for In2Se3 thin film. References [1] S.T. Lakshmikumar, A.C. Rastogi, Thin Solid Films 256 (1995) 150. [2] H.M. Pathan, S.S. Kulkarni, R.S. Mane, C.D. Lokhande, Mater. Chem. Phys. 93 (2005) 16. [3] M. Eddiet, C. Julien, M. Balkanski, Mater. Lett. 2 (1984) 432. [4] H. Bouzuita, N. Bouguila, S. Duchemin, S. Fiechter, A. Dhouib, Renew. Energy 25 (2002) 131.
ARTICLE IN PRESS 254
M.R. Asabe et al. / Journal of Physics and Chemistry of Solids 69 (2008) 249–254
[5] J.C. Former, J.A. Turner, R. Noufi, D. Cohen, J. Electrochem. 132 (1985) 1319. [6] J. Herrero, J. Ortega, Sol. Energy Mater. 16 (1987) 477. [7] T. Ohtsuka, T. Okamoto, A. Yamada, M. Konagai, Comp. Semicond. 156 (1997) 183. [8] T. Ohtsuka, K. Nakanishi, T. Okamoto, A. Yamada, M. Konagai, U. Jahn, Jpn. J. Appl. Phys. 40 (2001) 509. [9] T. Okamoto, A. Yamada, M. Konagai, J. Cryst. Growth 175 (1997) 1045. [10] J. Cheon, J. Arnold, K.M. Yu, E.D. Bourret, Chem. Mater. 7 (1995) 2273. [11] M. Emziane, R. Le Ny, J. Phys. D: Appl. Phys. 32 (1999) 1319. [12] M. Emziane, S. Marsillac, J.C. Bernede, Mater. Chem. Phys. 62 (2000) 84. [13] S. Marsillac, J.C. Bernede, R. Leny, A. Conan, Vacuum 46 (1995) 997. [14] A.F. Qasrawi, M. Parlak, C. Ercelehi, I. Gunal, J. Mater. Sci: Mater. Electron. 12 (2001) 473. [15] S. Marsillac, J.C. Bernede, M. Emziane, J. Wery, E. Faulques, P. Ray, Appl. Surf. Sci. 151 (1999) 171. [16] M. Emziane, R. Le Ny, Thin Solid Films 385 (2001) 312. [17] R. Vaidyanathan, J.L. Stickney, S.M. Cox, S.P. Compton, U. Happek, J. Electroanal. Chem. 559 (2003) 55.
[18] C.D. Lokhande, P.S. Patil, H. Tributsch, A. Ennaoui, Sol. Energy Mat. Sol. Cells 55 (1998) 379. [19] M. Ganchev, N. Stratieva, E. Tzvetkova, J. Mater. Sci. Mater. Electron. 14 (2003) 847. [20] R.K. Pandey, A.J.N. Rooz, R.B. Gore, Semicond. Sci. Tech. 3 (1988) 733. [21] P.P. Hankare, P.A. Chate, M.R. Asabe, S.D. Delekar, I.S. Mulla, K.M. Garadkar, J. Mater. Sci. Mater. Electron. 17 (2006) 1055. [22] S. Chandra, D.P. Singh, D.C. Srivastava, S.N. Sahu, J. Phys. D: Appl. Phys. 17 (1984) 2125. [23] R. Bichsel, F. Levy, H.J. Mathieu, Thin Solid Films 131 (1985) 87. [24] M. Ganchev, N. Strahieva, E. Tzvetkova, I. Gadjov, J. Mater. Sci.: Mater. Electron. 14 (2003) 847. [25] P.P. Hankare, P.A. Chate, S.D. Delekar, V.M. Bhuse, M.R. Asabe, B.V. Jadhav, K.M. Garadkar, Cryst. Growth 291 (2006) 40. [26] P.P. Hankare, P.A. Chate, S.D. Delekar, M.R. Asabe, I.S. Mulla, J. Phys. Chem. Solid 67 (2006) 2310. [27] C. Julien, A. Chevy, D. Siapkas, Phys. Stat. Sol. (a) 118 (1990) 553. [28] K. Kambas, Physica B 166 (1989) 103. [29] V.P. Munshinskii, V.I. Kobolev, Sov. Phys. Semicond. 5 (1971) 1104. [30] J.C. Bernede, S. Marsillac, A. Conan, Mater. Chem. Phys. 48 (1997) 5. [31] C.N.R. Rao, Modern Aspects of Solid State Chemistry, Plenum Press, New York, 1970, p. 531.