J Mater Sci: Mater Electron (2008) 19:1252–1257 DOI 10.1007/s10854-008-9585-5
Structural, optical and microscopic properties of chemically deposited In2Se3 thin films P. P. Hankare Æ M. R. Asabe Æ P. A. Chate Æ K. C. Rathod
Received: 7 June 2007 / Accepted: 14 January 2008 / Published online: 12 February 2008 Ó Springer Science+Business Media, LLC 2008
Abstract Solar cell technologically important binary indium selenide thin film has been developed by relatively simple chemical method. The reaction between indium chloride, tartaric acid, hydrazine hydrate and sodium selenosulphate in an aqueous alkaline medium at room temperature gives deposits In2Se3 thin film. Various preparative parameters are discussed. The as grown films were found to be transparent, uniform, well adherent, red in color. The prepared films were studied using X-ray diffraction, scanning electron microscopy, atomic absorption spectroscopy, Energy dispersive atomic X-ray diffraction, optical absorption and electrical conductivity properties. The direct optical band gap value Eg for the films was found to be as the order of 2.35 eV at room temperature and having specific electrical conductivity of the order of 10-2 (X cm)-1 showing n-type conduction mechanism. The utility of the adapted technique is discussed from the point of view of applications considering the optoelectric and structural data obtained.
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 surface exhibiting very different physical properties. The cleavage surface perpendicular to the z-axis consists of selenium atom bounded together with
covalent forces. The other surface parallel to the z-axis is made a selenium atom of adjacent layers being bounded by Vander Waal forces. The indices of cleavage planes are (111) (113) [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]. Techniques such as 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. Similarly, it provides large deposition area. 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 synthesized by chemical bath deposition and characterized for structural, optical and electrical studies [22–25]. But very few attempt have been made for investigation of In2Se3 by using chemical bath deposition method at room temperature. The various preparative parameter as well as the structural, compositional, morphological, optical and electrical properties are studied.
2 Experimental details P. P. Hankare (&) M. R. Asabe P. A. Chate K. C. Rathod Solid State Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra 416 004, India e-mail:
[email protected]
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2.1 Synthesis of In2Se3 thin films The substrates were cleaned by boiling them in chromic acid for 1 h which was followed by washing successively
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with detergent and alcohol. They were finally stored in double distilled water before use. 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 was prepared by the following the method reported earlier [25]. In actual experimentation, 10 mL (0.2M) indium trichloride was taken in 100 mL beaker. 2.5 mL (1M) tartaric acid, 10mL (10%) hydrazine hydrate and 15 mL (0.25M) sodium selenosulphate were added in the reaction bath. 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 11.80 ± 0.05. To obtain the film, four glass substrate were positioned vertically on a specially designed substrate holder and rotated in a reactive solution with a speed of 55 ± 2 rpm. The temperature of the solution was then allowed to rise slowly to 293K. 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.
2.2 Characterization of In2Se3 thin films The X-ray diffraction study of In2Se3 film was carried out in the range of the diffraction angle 10°–80° with Cu Ka1 radiation using Philips PW-1710 diffractometer ˚ ). The layer thickness of the film was (k = 1.54056 A 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 along the length of a film performed thermoelectric power measurements were made. The potential difference between the two points of contact separated by 1cm 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 250MK-III Stereoscan (USA) scanning electron microscope (SEM)
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was used for the microscopic observations. Compositional analysis of indium was done with an atomic absorption spectrophotometer (Perkin-Elmer Model 3030) and EDAX.
3 Results and discussions 3.1 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 In-tartarate 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 Se-2 ions in the solution on the ionby-ion basis on the glass substrate. The kinetics of growth of the films can be understood from the following reaction. Inþ3 + nA2 $ [In(A)n]
ð1:1Þ
Na2 SeO3 + OH ! Na2 SO4 + HSe
ð1:2Þ
HSe + OH ! H2 O + Se2
ð1:3Þ
[In(A)n] + Se2 ! In2 Se3 + nA
ð1:4Þ
Hydrazine hydrate acts as complementary complexant, which improves compactness and adherence of the film. Speed of rotation of 55 ± 2 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 increase linearly with time 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 Se-2 ions forming precipitate instead of film formation. The terminal thickness was found to be 0.55 lm.
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Thickness (µm)
0.6
0.3
0 0
50
100
150
Time (min) Fig. 3 XRD pattern of In2Se3 thin film
Fig. 1 Variation of the film thickness with deposition time
Table 1 Structural and morphological data of In2Se3 thin film Film
0.6
Thickness (µm)
In2Se3
0.3
0 275
290
305
Temperature (K) Fig. 2 Variation of the film thickness with deposition temperature
˚) d-Values (A
hkl planes
Observed
ASTM
3.5001
3.4964
111
3.113
3.113
113
2.0864
2.097
124
1.8131
1.813
305
1.338
1.3369
139
1.2907
1.2938
236
˚) Grain size (A XRD
SEM
210
218
where ‘D’ is crystallite size, ‘k’ is the x-ray wavelength ˚ , ‘b’ is the angular line width at half the maxiused in A mum intensity, ‘h’ 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
3.2 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 No. 71-0250). The broad hump (2h = 20°–40°) is due to amorphous glass substrate. In2Se3 thin film shows prominent (111) (113) (124) (305) (139) (236) 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 = Kk/bcosh
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ð1:5Þ
3.3 Microscopic studies 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 is an indication of one step growth by multiple nucleations. The average grain size of indium selenide sample is reported in Table 1. The grain size calculated from SEM is matchable with grain size calculated by XRD.
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1255 9000
2
(α hv) X 10
8
6000
3000
0 2
2.5
3
3.5
photon energy (hv)
Fig. 4 SEM micrographs of In2Se3 thin film
Fig. 6 Plots of (ahm)2 vs. photon energy
3.4 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. Figure 5 shows variation of optical absorption with wavelength. The optical study shows that the films are highly absorptive (a 9 104 cm-1). Indium selenide is a direct band gap semiconductor [26]. For an allowed direct band gap transition the absorption coefficient a can be related to the photon energy hm by ðahmÞ2 ¼ A(hm Eg Þ
ð1:6Þ
where A is a constant and Eg is the energy band gap. For a direct band gap semiconductor the (ahm)2 versus hm characteristic is predicted to be a straight line with a photoenergy axis intercept indicative for the band gap. This
3
Absorbance
2
is illustrated in Fig. 6, where a band gap of 2.35 eV can be obtained. 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 10-2 (X cm)-1, which agrees well with the earlier reported value [27]. 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 non-equilibrium defects during first heating. A plot of log r (conductivity) vs. inverse temperature for the cooling cycle is shown in the Fig. 7. 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
1
Table 2 Data on optical and electrical properties of In2Se3 thin film Film 0 400
500
600
700
Band gap (eV)
Specific conductance (X cm)-1
HT
At 300 K
800
Wavelength (nm) Fig. 5 Absorption spectrum of In2Se3 thin film
Activation energy (eV)
In2Se3
2.35
0.143
LT 0.021
9.67 9 10
At 525 K -2
1.02 9 10-2
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2
2.5
3
3.5
lo g (co n d u ctiv ity)
-1.1
-1.2
-1.3
-1.4
1000/T (per K) Fig. 7 The variations of log (Conductivity) with inverse temperature
r ¼ r0 exp ðEa/kTÞ
ð1:7Þ
where, the terms have usual meaning. The activation energies are 0.021 eV 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 [28].
3.5 Compositional analysis 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 = 7, the selenium was precipitated as free element [29]. 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
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Fig. 8 EDAX of In2Se3 thin film
commercial standards to concentration 0.4, 0.8, 1.2, 1.6, 2.0 lg/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. 8.
4 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. Acknowledgement Author gratefully acknowledges financial support from UGC New Delhi, India.
J Mater Sci: Mater Electron (2008) 19:1252–1257
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