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Novel chemical synthetic route and characterization of zinc selenide thin films
ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 67 (2006) 2310–2315 www.elsevier.com/locate/jpcs
Novel chemical synthetic route and characterization of zinc selenide thin films P.P. Hankarea,, P.A. Chatea, S.D. Delekara, M.R. Asabea, I.S. Mullab a
Solid State Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra 416 004, India b National Chemical Laboratory, Pune 411 007, India Received 11 January 2006; received in revised form 7 May 2006; accepted 11 May 2006
Abstract Zinc selenide (ZnSe) thin film have been deposited using chemical bath method on non-conducting glass substrate in a tartarate bath containing zinc sulfate, ammonia, hydrazine hydrate, sodium selenosulfate in an aqueous alkaline medium at 333 K. The deposition parameter of the ZnSe thin film is interpreted in the present investigation. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), optical absorption, electrical measurements, atomic absorption spectroscopy (AAS). The ZnSe thin layers grown with polycrystalline zinc blende system along with some amorphous phase present in ZnSe film. The direct optical band gap ‘Eg’ for the film was found to be 2.81 eV and electrical conductivity in the order of 108(O cm)1 with n-type conduction mechanism. r 2006 Elsevier Ltd. All rights reserved. Keywords: A. Chalcogenides; A. Electronic materials
1. Introduction The wide band gap candidates are ubiquitous component of modern technology with considerable interest in the electronic and optical devices [1,2]. Zinc selenide (ZnSe) is one of the important material as a buffer layer in copper indium selenide (CIS)-based solar cells, compare to CdS buffer layer because of better conformity of lattice parameter and non-toxicity [3,4]. Also, it has potential applications in red-, blue- and green-light-emitting diodes, photovoltaic, laser screens, thin-film transistor, photoelectrochemical cells [5–12]. Hence, we are focused towards obtaining ZnSe in the thin film form. Thin films of ZnSe have been deposited using molecular beam epitaxy, electron beam evaporation, chemical deposition, electrodeposition, vacuum evaporation, successive ionic layer adsorption and reaction (SILAR) technique [13–18]. Optical and electrical studies on chemically deposited ZnSe thin film have been made by Kale and Lokhande [18]. Optical reflectance spectra of chemically deposited ZnSe Corresponding author. Tel.: +91 269 225 8.
E-mail addresses: [email protected] (P.P. Hankare), [email protected] (P.A. Chate). 0022-3697/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2006.05.001
thin film are reported by Lokhande et al. [3]. The Raman spectra and photoluminescence studied have been reported by Jan et al. [2]. Optical selection rule is reported by Mitsumori et al. [19]. A convenient method for deposition of ZnSe thin film is chemical bath deposition. A very attractive method for producing ZnSe thin films, due to the possibility of large area deposition at low cost is the chemical bath deposition method. The method requires the presence of reagents that act as a source of chalcogenide and complexion of metal ions of interest whose stability equilibrium provide a concentration of metal cation small enough to produce the control homogenous precipitation of the film on the solid substrate. Films prepared by chemical bath deposition can be used in optoelectronic devices due to high purity of deposited material. The deposition of ZnSe is based on reaction between Se2 ions and zinc ions with controlled precipitation in equilibrium with the products. Also, ZnSe and related ternary alloys like Zn1xCdxSe, ZnSe1ySy are best materials for optoelectronic device technology in the blue region of visible spectrum [20]. In the present investigation, we describe the novel synthesis of ZnSe thin film on non-conducting glass slides from aqueous alkaline medium. The preparation
ARTICLE IN PRESS P.P. Hankare et al. / Journal of Physics and Chemistry of Solids 67 (2006) 2310–2315
parameters such as pH, deposition time, deposition temperature, etc have been reported in order to obtain good quality ZnSe films. The ZnSe films have characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic absorption spectroscopy (AAS), etc. Optical and electrical properties have studied and reported. 2. Experimental details All the chemicals used for the deposition were analytical grade. It includes zinc sulfate heptahydrate, tartaric acid, liquor ammonia, hydrazine hydrate, sodium sulfite and selenium powder. All the solutions were prepared in double distilled water. Sodium selenosulfate (Na2SeSO3) solution was prepared by refluxing 5 g selenium powder and 15 g sodium sulfite in 200 mL double distilled water for 9 h at 333 K. The commercially available, non-conducting glass slides of dimensions 26 76 2 mm were cleaned by washing with chromic acid (non-ionic liquid detergent/ surfactant) followed by rinsing in acetone and finally with double distilled water before use. In actual experimentation, 10 mL (0.2 M) zinc sulfate heptahydrate solution was taken in 100 mL beaker. A total of 2.5 mL (1 M) tartaric acid, 25 mL (2.8 M) ammonia, 25 mL (2%) hydrazine hydrate and 10 mL (0.25 M) sodium selenosulfate were added in the reaction bath at room temperature. The pH of the reactive mixture is 11.45. The beaker was kept in oil bath. The non-conducting glass substrate were mounted vertically on a specially designed substrate holder and rotated in the reaction mixture with a speed of 5572 rpm. The temperature of the bath was then allowed to increases slowly upto 333 K. After 120 min, the slides were removed, washed several times with double distilled water, dried naturally preserved in a dark dessicator over anhydrous CaCl2. The resultant films were homogenous, well adherent to glass substrate. 3. Sample characterizations The XRD study of ZnSe film was carried out in the range of the diffraction angle 101–801 with CuKa1 radiation using Philips PW-1710 diffractometer ˚ (l ¼ 1:54056 A). The layer thickness of the film was estimated by the weight difference method. The electrical conductivity of ZnSe thin film was measured using a ‘dc’ two-probe method. A quick drying silver paste was applied at the ends of the film for ohmic contact purpose. For the measurements of conductivity, a constant voltage of 30 V was applied across the sample. The current was noted at different temperature. Maintaining a temperature gradient along the length of a film performed thermoelectric power measurements and the potential difference between the points separated by a 1 cm was recorded with a digital microvoltmeter. A calibrated thermocouple probe (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
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350–850 nm by using a Hitachi-330 (Japan) UV–vis–NIR double beam spectrophotometer at room temperature. Placing 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) SEM was used for the microscopic observations. Compositional analysis for zinc was carried out with an atomic absorption spectrophotometer using Perkin-Elmer 3030, USA. 4. Results and discussions 4.1. Kinetics and growth mechanism In the reaction bath, Zn+2 ions are complexed with tartaric acid in the form of water-soluble Zn-tartarate complex and thus control Zn+2 concentration. The dissociation of sodium selenosulfate as well as Zn-tartarate complex in alkaline medium takes place. At room temperature, it forms clear solutions and no film or precipitate is observed. At low temperature, lower the kinetic energy of ions and avoids the precipitate. As the temperature increases slowly, the kinetic energy increases as the result decomposition of sodium selenosulfate and metal complex take place in alkaline medium favors the formation of ZnSe thin film. The deposition process is based on the slow release of Zn+2 ions and Se2 ions in the solution by ion-by-ion basis on the glass substrate. The deposition take place when the ionic product of Zn+2 and Se2 greater then solubility product. [K sp ¼ 1031 ]. The kinetics growth of film can be understood from the following: Znþ2 þ nA2 2½ZnðAÞn,
(1.1)
Na2 SeO3 þ OH ! Na2 SO4 þ HSe ,
(1.2)
HSe þ OH ! H2 O þ Se2 ,
(1.3)
½ZnðAÞn þ Se2 ! ZnSe þ nA.
(1.4)
There are several soluble and insoluble species of Zn+2 possible in a reaction bath containing OH. The pH of reactive mixture is less than 7.5 or greater than 13.7, soluble species such as Zn+2 and ZnO2 are present, respectively. If the pH of the bath is in between 7.5 and 13.7, then insoluble Zn(OH)2 may be present [21]. The presence of Zn(OH)2 in the reaction mixture is unavoidable due to aqueous alkaline nature of the bath. The amount of Zn(OH)2 increases with increases temperature of the bath. This results in the inclusion of Zn (OH)2 in the ZnSe film resulting in the formation of Znx(Se,OH)y thin film rather than ZnSe film [22]. An increase in deposition temperature favors the homogenous precipitation rather than the film formation, which causes saturation to occur. Both hydrazine hydrate and ammonia are necessary for the formation of ZnSe thin films and hydrazine hydrate might be
ARTICLE IN PRESS P.P. Hankare et al. / Journal of Physics and Chemistry of Solids 67 (2006) 2310–2315
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playing a complexing and/or catalytic role in the film process [3], which improves compactness and adherence of the films. In growth process, no film formation occurs within the first half an hour. This is the induction period required to from nucleation centers on the substrate. The presence of induction period suggests that ion-by-ion growth mechanism instead of cluster-by-cluster. The Fig. 1(a) shows the thickness against deposition temperature. The duration of film deposition the glass substrate was studied. This study revealed that homogenous ZnSe films have been deposited at 120 min. The thickness was measured every 30 min and plotted against time as shown in Fig. 1(b). Speed of rotation was 5572 rpm was selected to deposit ZnSe thin films. Above higher speed very thin film was deposited. At lower speed, thick non-adherent films deposited. The terminal thickness is found to be 0.2 mm. The best conditions in the deposition process for yielding good quality film at 333 K and 120 min respectively.
ment in crystallinity is due to increased in grain size into effectively crystals after annealing. The crystalline phase dominates over the mixture of amorphous phase. The lattice parameters of cubic phase was calculated by using a ¼ dðh2 þ l 2 þ k2 Þ1=2 .
(1.5)
The lattice parameter ‘a’ of annealed films is found to be 5.6601 A˚. The crystallite size of ZnSe thin films was calculated by using Scherrer’s formula D ¼ Kl=b cos y,
(1.6)
where, D is crystallite size, l is the X-ray wavelength used, b is the angular line width of half maximum intensity, y is Bragg’s diffraction angle and K is constant, 0.94 for ZnSe. The average crystallite size was calculated by resolving the
(111)
4.2. X-ray diffraction
Intensity (a.u.)
(200)
The crystallographic study of deposited ZnSe thin films were examined by X-ray diffractometer. Literature survey revealed that ZnSe has two structural phases such as hexagonal wurtzite, cubic zinc blende type. The XRD pattern of ‘as deposited’ films shows very poor crystallinity. The XRD pattern of ‘annealed’ ZnSe film is shown in Fig. 2. Comparison of observed ‘d’ with standard ‘d’ values confirms that chemically deposited film shows cubic structure (JCPDS-1463). The XRD pattern shows the highest intensity reflection peak at d ¼ 3:271 A˚ (1 1 1). The diffused background is due to amorphous glass substrate and also to some amorphous phase present in the ZnSe thin films. Along with (1 1 1) plane, (2 0 0), (2 2 0), (3 1 1), (2 2 2) peaks also observed. The significant improve-
(220)
(311) (222)
10
20
30
40 50 60 Two Thetha (Degree)
70
Fig. 2. XRD pattern of ‘annealed’ ZnSe thin film.
0.25 0.25 0.2 Thickness (µm)
Thickness (µm)
0.2 0.15 0.1
(a)
0.1
0.05
0.05 0 310
0.15
0 320
330
340
Temperature (K)
0
350
(b)
50
100
150
Time (min)
Fig. 1. (a) Variation of the film thickness with deposition temperature and (b) variation of the film thickness with deposition time.
80
ARTICLE IN PRESS P.P. Hankare et al. / Journal of Physics and Chemistry of Solids 67 (2006) 2310–2315
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Table 1 Structural characterization of ZnSe thin film Film d-values (A˚)
h k l planes Grain size (A˚)
Observed ASTM ZnSe 3.271 2.8537 1.9987 1.6912 1.6356 — —
3.271 2.837 2.004 1.708 1.636 1.4169 1.3
111 200 220 311 222 400 331
XRD
SEM
158
162
Cell parameter (A˚)
5.6601
Table 2 Compositional analysis Sr. no.
1
Composition
ZnSe
Bath content (ppm)
Film content (ppm)
Zn
Se
Zn
Se
523.3
760
260
376
4.4. Compositional analysis Atomic Absorption Spectroscopy was used to study compositional analysis by calibration curve method. Previously weighed minute sample was dissolved in the minimum quantity of Conc. HNO3 to yield the products as shown in the reactions: ZnSe þ 2HNO3 ) ZnðNO3 Þ2 þ Se þ H2 O: Fig. 3. SEM mircogrpahs of: (a) ‘as deposited’ and (b) ‘annealed’ ZnSe thin film.
highest intensity peak. The average crystallite size of annealing ZnSe thin film at 473 K was found to be 158 A˚. 4.3. Scanning electron microscope (SEM) SEM is an excellent method to study morphology of the sample. The SEM micrograph of ‘as deposited’ and those ‘annealed’at 473 K are shown in Fig. 3(a) and (b), respectively, at 10,000 magnification. ‘As deposited’ ZnSe thin film is homogenous, without cracks or pinholes and well cover to the glass substrate. From the micrograph, it is clearly seen that the film, composed of minute grains, was uniformly distributed over a smooth homogenous background that may correspond to amorphous phase of ZnSe thin film. In annealed film, the grains are more distinct and of bigger size. The increase in grain size leads to decrease in the grain boundaries. The presence of fine background is an indication of one-step growth by multiple nucleations. The average grain size of ‘as deposited’ as well as annealed samples are reported in Table 1.
(1.7)
Below pH 7, the selenium was precipitated as free metal [23]. While nitrates of zinc remain in the solution state. The precipitated was filtered through a Gooch crucible and subjected to selenium estimation using a standard gravimetric method. The filtrate containing zinc 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 zinc. The observed concentration of zinc and selenium in the film are reported in the Table 2. The compositional analysis of the sample using AAS gave 49.17% zinc and 50.83% selenium, showing samples is zinc deficient. 4.5. Optical studies The optical absorption spectra of ‘as deposited’ ZnSe film onto non-conducting glass substrate was studied at the room temperature in the wavelength of 350–850 nm without considering transmission and reflection. Fig. 4(a) shows the variation of optical absorbance with wavelength. The optical studies shows that the films are absorptive (a 104 cm1). The value of absorption coefficient is depends upon radiation energy as well as the composition of films. The data were properly studied in the vicinity of
ARTICLE IN PRESS P.P. Hankare et al. / Journal of Physics and Chemistry of Solids 67 (2006) 2310–2315
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8000
(αhυ)2 X 107(eV/cm)2
Absorbance (cm-1)
1.5 1.2 0.9 0.6 0.3
6000
4000
2000
0 1.4
0 350 (a)
450
550
650
750
850
Wavelength (nm)
1.8
2.2
2.6
3
3.4
Photon energy (eV)
(b)
Fig. 4. (a) Absorption spectrum of ZnSe thin film and (b) plots of (ahn)2 with respect to photon energy.
an absorption edge on the basis of three-dimensional model to estimate the ‘Eg’ value. The simplest forms of the equation obeyed near and above absorption edge are [24]:
Sample
(1.8)
where hn is the photon energy, Eg is the band gap, A and n are constants. ‘A’ is depending upon the temperature phonon energies, etc. For allowed direct transition n ¼ 1=2 and for allowed indirect transition n ¼ 2. A plot of (ahn)2 versus hn is shown in the Fig. 4(b). The linear nature of plot shows that the existence of the direct transitions. Extrapolation of the linear portion of the curve to a2 ¼ 0 gives the optical band gap, which is about 2.81 eV. The value of band gap obtained agrees well with the results of previously reported Znse using chemical bath deposition method [25]. The observed value is greater than standard band gap (2.7 eV) of the ZnSe material, [26] showing a ‘blue shift’ of 0.11 eV. This is attributed due to size-quantization in polycrystalline semiconductor. This is attributed due to size-quantization occurs due to localization of electrons and holes in confined volume of the semiconductor materials. 4.6. Electrical conductivity and thermoelectrical studies The dark electrical conductivity of ‘as deposited’ ZnSe film on non-conducting 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 in the order of 108 (O 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 3.The low value of conductivity may be due to low crystallinity and small thickness of the film. The electrical properties of polycrystalline thin films are mainly depends upon their structural characteristics and composition [28–29]. It is observed that the conductivity on the film increases with increasing in temperature. This indicates the semiconducting behavior of the thin film. The electrical conductivity variation with temperature during heating
ZnSe
Band gap Activation energy (eV) (eV)
2.81
1.75
Specific conductance (O cm)1
HT
LT
300 K
0.61
0.022
5.836 108 6.1421 105
2.25
2.75
525 K
3.25
-4.5
-5.5 log σ
a ¼ ðA=hnÞðhn E g Þn ,
Table 3 Optical and electrical characterization of ZnSe thin film
-6.5
-7.5 1000 / T (perK) Fig. 5. The variations of log (conductivity) with inverse temperature.
and cooling cycles were found to be different and this shows that the ‘as deposited’ films undergo an irreversible changes due to annealing out of non-equilibrium defects during first heating. A plot of log (conductivity) versus inverse absolute temperature for the cooling curve is shown in the Fig. 5. A plot shows that electrical conductivity has
ARTICLE IN PRESS P.P. Hankare et al. / Journal of Physics and Chemistry of Solids 67 (2006) 2310–2315
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 exponential form of Arrhenius equation s ¼ s0 expðE a =KTÞ,
(1.9)
where the terms have usual meaning. The activation energies are 0.022 and 0.61 eV for low and high temperature region, 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 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 semiconductor ends. 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 ZnSe thin film, the negative terminal was connected to the cold end, therefore, the film shows n-type conductivity [30]. 5. Conclusions (1) Zinc selenide (ZnSe) thin films have been successfully deposited using this method. (2) Crystallographic studies revealed ZnSe deposited in cubic form with presence of some amorphous phase. (3) Morphological studies found ‘as deposited’ films are uniform, homogenous minute grains where as annealed samples observed improvement in crystallinity. (4) The optical absorption study showed ZnSe samples have direct band gap 2.81 eV with direct type of transition. (5) The room temperature electrical conductivity of ZnSe thin film is of the order of 108(O cm)1. Thermoelectric power measurement observed n-type conduction for ZnSe thin film.
Acknowledgments Authors are thankful to the Director, IISc, Bangalore for providing SEM facility and also Prof. Deshmukh, M.B.,
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Head, Department of Chemistry, Shivaji University, Kolhapur for giving the laboratory facilities.
References [1] C. Wang, X.F. Qian, W.X. Zhang, X.M. Zhang, Y. Xie, Y.T. Qian, Mater. Res. Bull. 34 (1999) 1637. [2] J.-C. Jan, S.-Y. Kuo, S.-B. Yin, W.-F. Hsich, Chin. J. Phys. 39 (2001) 90. [3] C.D. Lokhande, P.S. Patil, H. Tributsch, A. Ennaoui, Sol. Energy Mater. Sol. Cells 55 (1998) 379. [4] M. Ganchev, N. Strahieva, E. Tzvetkova, I. Gadjov, J. Mater. Sci.: Mater. Electron. 14 (2003) 847. [5] O. Akira, S. Noriyoshi, S. Zembutsu, J. Appl. Phys. 64 (1988) 654. [6] A. Schmidt, J. Crystal Growth 101 (1990) 758. [7] S.Y. Wang, Appl. Phys. Lett. 61 (1992) 506. [8] H.S. Akbar, A.J. Abdulkadir, M.S. Bani, J. Sol. Energy Res. 6 (1988) 49. [9] A. Ennaoui, S. Siebntritt, M.I. Lux-Steiner, W. Riedl, F. Karg, Sol. Energy Mater. Sol. Cells 67 (2001) 31. [10] M. Kulapov, G.A. Murowick, V.N. Ulasyuk, IZV Akad. Nauk. SSR Neorg. Mater. 19 (1983) 1807. [11] R.R. Alfand, O.Z. Wang, J. Jumbo, B. Bhargava, J. Phys. Res. A 35 (1987) 459. [12] A. Rumberg, C. Sommerhalter, M. Toplak, A. Jager-Waldau, M.C. Lux-steiner, Thin Solid Films 361 (2001) 172. [13] N. Katsumura, K. Maemura, T. Mori, J. Saraie, J. Crystal Growth 159 (1996) 85. [14] R. Islam, D.R. Rao, J. Mater. Sci. Lett. 13 (1994) 1637. [15] A.M. Chaparro, M.T. Gutierez, J. Herero, Electrochim. Acta 47 (2001) 977. [16] A. Chandramohan, T. Mahalingam, J.P. Chu, P.J. Sebastian, Sol. Energy Mater. Sol. Cells 81 (2004) 371. [17] P.K.R. Kalita, B.K. Sarma, H.L. Das, Bull. Mater. Sci. 23 (2000) 313. [18] R.B. Kale, C.D. Lokhande, Mater. Res. Bull. 39 (2004) 1829. [19] Y. Mitsumori, Y. Ohkubo, A. Hasegawa, M. Sasaki, F. Minami, J. Luminescence 108 (2004) 259. [20] H. Luo, T.K. Furdyna, Semicond. Sci. Technol. 10 (1995) 1041. [21] C.A. Estrada, P.K. Nair, M.T.S. Nair, R.A. Zingaro, E.S. Meyers, J. Electrochem. Soc. 141 (1994) 802. [22] B. Mokili, Y. Charreire, R. Cortes, D. Lincot, Thin Solid Films 288 (1996) 21. [23] P.P. Hankare, S.D. Delekar, P.A. Chate, S.D. Sabane, K.M. Garadkar, V.M. Bhuse, Semicond. Sci. Technol. 20 (2005) 257. [24] D.S. Sutrave, G.S. Shahane, V.B. Patil, L.P. Deshmukh, Mater. Chem. Phys. 65 (2000) 298. [25] J. Dona, J. Herrero, J. Electrochem. Soc. 142 (1995) 764. [26] P. Pramanik, S. Biswas, J. Electrochem. Soc. 133 (1980) 350. [27] J. Krustok, P.E. Kukk, Mater. Sci. 25 (1989) 43. [28] K.L. Chopra, Thin Film Phenomenon, McGraw-Hill, New York, 1969. [29] G.I. Rusu, M.E. Popa, G.S. Rusu, I. Salaoru, Appl. Surf. Sci. 218 (2003) 222. [30] C.N. Rao, Modern Aspects of Solid State Chemistry, Plenum Press, New York, 1970, p. 531.
Journal of Physics and Chemistry of Solids 67 (2006) 2310–2315 www.elsevier.com/locate/jpcs
Novel chemical synthetic route and characterization of zinc selenide thin films P.P. Hankarea,, P.A. Chatea, S.D. Delekara, M.R. Asabea, I.S. Mullab a
Solid State Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, Maharashtra 416 004, India b National Chemical Laboratory, Pune 411 007, India Received 11 January 2006; received in revised form 7 May 2006; accepted 11 May 2006
Abstract Zinc selenide (ZnSe) thin film have been deposited using chemical bath method on non-conducting glass substrate in a tartarate bath containing zinc sulfate, ammonia, hydrazine hydrate, sodium selenosulfate in an aqueous alkaline medium at 333 K. The deposition parameter of the ZnSe thin film is interpreted in the present investigation. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), optical absorption, electrical measurements, atomic absorption spectroscopy (AAS). The ZnSe thin layers grown with polycrystalline zinc blende system along with some amorphous phase present in ZnSe film. The direct optical band gap ‘Eg’ for the film was found to be 2.81 eV and electrical conductivity in the order of 108(O cm)1 with n-type conduction mechanism. r 2006 Elsevier Ltd. All rights reserved. Keywords: A. Chalcogenides; A. Electronic materials
1. Introduction The wide band gap candidates are ubiquitous component of modern technology with considerable interest in the electronic and optical devices [1,2]. Zinc selenide (ZnSe) is one of the important material as a buffer layer in copper indium selenide (CIS)-based solar cells, compare to CdS buffer layer because of better conformity of lattice parameter and non-toxicity [3,4]. Also, it has potential applications in red-, blue- and green-light-emitting diodes, photovoltaic, laser screens, thin-film transistor, photoelectrochemical cells [5–12]. Hence, we are focused towards obtaining ZnSe in the thin film form. Thin films of ZnSe have been deposited using molecular beam epitaxy, electron beam evaporation, chemical deposition, electrodeposition, vacuum evaporation, successive ionic layer adsorption and reaction (SILAR) technique [13–18]. Optical and electrical studies on chemically deposited ZnSe thin film have been made by Kale and Lokhande [18]. Optical reflectance spectra of chemically deposited ZnSe Corresponding author. Tel.: +91 269 225 8.
E-mail addresses: [email protected] (P.P. Hankare), [email protected] (P.A. Chate). 0022-3697/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2006.05.001
thin film are reported by Lokhande et al. [3]. The Raman spectra and photoluminescence studied have been reported by Jan et al. [2]. Optical selection rule is reported by Mitsumori et al. [19]. A convenient method for deposition of ZnSe thin film is chemical bath deposition. A very attractive method for producing ZnSe thin films, due to the possibility of large area deposition at low cost is the chemical bath deposition method. The method requires the presence of reagents that act as a source of chalcogenide and complexion of metal ions of interest whose stability equilibrium provide a concentration of metal cation small enough to produce the control homogenous precipitation of the film on the solid substrate. Films prepared by chemical bath deposition can be used in optoelectronic devices due to high purity of deposited material. The deposition of ZnSe is based on reaction between Se2 ions and zinc ions with controlled precipitation in equilibrium with the products. Also, ZnSe and related ternary alloys like Zn1xCdxSe, ZnSe1ySy are best materials for optoelectronic device technology in the blue region of visible spectrum [20]. In the present investigation, we describe the novel synthesis of ZnSe thin film on non-conducting glass slides from aqueous alkaline medium. The preparation
ARTICLE IN PRESS P.P. Hankare et al. / Journal of Physics and Chemistry of Solids 67 (2006) 2310–2315
parameters such as pH, deposition time, deposition temperature, etc have been reported in order to obtain good quality ZnSe films. The ZnSe films have characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic absorption spectroscopy (AAS), etc. Optical and electrical properties have studied and reported. 2. Experimental details All the chemicals used for the deposition were analytical grade. It includes zinc sulfate heptahydrate, tartaric acid, liquor ammonia, hydrazine hydrate, sodium sulfite and selenium powder. All the solutions were prepared in double distilled water. Sodium selenosulfate (Na2SeSO3) solution was prepared by refluxing 5 g selenium powder and 15 g sodium sulfite in 200 mL double distilled water for 9 h at 333 K. The commercially available, non-conducting glass slides of dimensions 26 76 2 mm were cleaned by washing with chromic acid (non-ionic liquid detergent/ surfactant) followed by rinsing in acetone and finally with double distilled water before use. In actual experimentation, 10 mL (0.2 M) zinc sulfate heptahydrate solution was taken in 100 mL beaker. A total of 2.5 mL (1 M) tartaric acid, 25 mL (2.8 M) ammonia, 25 mL (2%) hydrazine hydrate and 10 mL (0.25 M) sodium selenosulfate were added in the reaction bath at room temperature. The pH of the reactive mixture is 11.45. The beaker was kept in oil bath. The non-conducting glass substrate were mounted vertically on a specially designed substrate holder and rotated in the reaction mixture with a speed of 5572 rpm. The temperature of the bath was then allowed to increases slowly upto 333 K. After 120 min, the slides were removed, washed several times with double distilled water, dried naturally preserved in a dark dessicator over anhydrous CaCl2. The resultant films were homogenous, well adherent to glass substrate. 3. Sample characterizations The XRD study of ZnSe film was carried out in the range of the diffraction angle 101–801 with CuKa1 radiation using Philips PW-1710 diffractometer ˚ (l ¼ 1:54056 A). The layer thickness of the film was estimated by the weight difference method. The electrical conductivity of ZnSe thin film was measured using a ‘dc’ two-probe method. A quick drying silver paste was applied at the ends of the film for ohmic contact purpose. For the measurements of conductivity, a constant voltage of 30 V was applied across the sample. The current was noted at different temperature. Maintaining a temperature gradient along the length of a film performed thermoelectric power measurements and the potential difference between the points separated by a 1 cm was recorded with a digital microvoltmeter. A calibrated thermocouple probe (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
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350–850 nm by using a Hitachi-330 (Japan) UV–vis–NIR double beam spectrophotometer at room temperature. Placing 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) SEM was used for the microscopic observations. Compositional analysis for zinc was carried out with an atomic absorption spectrophotometer using Perkin-Elmer 3030, USA. 4. Results and discussions 4.1. Kinetics and growth mechanism In the reaction bath, Zn+2 ions are complexed with tartaric acid in the form of water-soluble Zn-tartarate complex and thus control Zn+2 concentration. The dissociation of sodium selenosulfate as well as Zn-tartarate complex in alkaline medium takes place. At room temperature, it forms clear solutions and no film or precipitate is observed. At low temperature, lower the kinetic energy of ions and avoids the precipitate. As the temperature increases slowly, the kinetic energy increases as the result decomposition of sodium selenosulfate and metal complex take place in alkaline medium favors the formation of ZnSe thin film. The deposition process is based on the slow release of Zn+2 ions and Se2 ions in the solution by ion-by-ion basis on the glass substrate. The deposition take place when the ionic product of Zn+2 and Se2 greater then solubility product. [K sp ¼ 1031 ]. The kinetics growth of film can be understood from the following: Znþ2 þ nA2 2½ZnðAÞn,
(1.1)
Na2 SeO3 þ OH ! Na2 SO4 þ HSe ,
(1.2)
HSe þ OH ! H2 O þ Se2 ,
(1.3)
½ZnðAÞn þ Se2 ! ZnSe þ nA.
(1.4)
There are several soluble and insoluble species of Zn+2 possible in a reaction bath containing OH. The pH of reactive mixture is less than 7.5 or greater than 13.7, soluble species such as Zn+2 and ZnO2 are present, respectively. If the pH of the bath is in between 7.5 and 13.7, then insoluble Zn(OH)2 may be present [21]. The presence of Zn(OH)2 in the reaction mixture is unavoidable due to aqueous alkaline nature of the bath. The amount of Zn(OH)2 increases with increases temperature of the bath. This results in the inclusion of Zn (OH)2 in the ZnSe film resulting in the formation of Znx(Se,OH)y thin film rather than ZnSe film [22]. An increase in deposition temperature favors the homogenous precipitation rather than the film formation, which causes saturation to occur. Both hydrazine hydrate and ammonia are necessary for the formation of ZnSe thin films and hydrazine hydrate might be
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playing a complexing and/or catalytic role in the film process [3], which improves compactness and adherence of the films. In growth process, no film formation occurs within the first half an hour. This is the induction period required to from nucleation centers on the substrate. The presence of induction period suggests that ion-by-ion growth mechanism instead of cluster-by-cluster. The Fig. 1(a) shows the thickness against deposition temperature. The duration of film deposition the glass substrate was studied. This study revealed that homogenous ZnSe films have been deposited at 120 min. The thickness was measured every 30 min and plotted against time as shown in Fig. 1(b). Speed of rotation was 5572 rpm was selected to deposit ZnSe thin films. Above higher speed very thin film was deposited. At lower speed, thick non-adherent films deposited. The terminal thickness is found to be 0.2 mm. The best conditions in the deposition process for yielding good quality film at 333 K and 120 min respectively.
ment in crystallinity is due to increased in grain size into effectively crystals after annealing. The crystalline phase dominates over the mixture of amorphous phase. The lattice parameters of cubic phase was calculated by using a ¼ dðh2 þ l 2 þ k2 Þ1=2 .
(1.5)
The lattice parameter ‘a’ of annealed films is found to be 5.6601 A˚. The crystallite size of ZnSe thin films was calculated by using Scherrer’s formula D ¼ Kl=b cos y,
(1.6)
where, D is crystallite size, l is the X-ray wavelength used, b is the angular line width of half maximum intensity, y is Bragg’s diffraction angle and K is constant, 0.94 for ZnSe. The average crystallite size was calculated by resolving the
(111)
4.2. X-ray diffraction
Intensity (a.u.)
(200)
The crystallographic study of deposited ZnSe thin films were examined by X-ray diffractometer. Literature survey revealed that ZnSe has two structural phases such as hexagonal wurtzite, cubic zinc blende type. The XRD pattern of ‘as deposited’ films shows very poor crystallinity. The XRD pattern of ‘annealed’ ZnSe film is shown in Fig. 2. Comparison of observed ‘d’ with standard ‘d’ values confirms that chemically deposited film shows cubic structure (JCPDS-1463). The XRD pattern shows the highest intensity reflection peak at d ¼ 3:271 A˚ (1 1 1). The diffused background is due to amorphous glass substrate and also to some amorphous phase present in the ZnSe thin films. Along with (1 1 1) plane, (2 0 0), (2 2 0), (3 1 1), (2 2 2) peaks also observed. The significant improve-
(220)
(311) (222)
10
20
30
40 50 60 Two Thetha (Degree)
70
Fig. 2. XRD pattern of ‘annealed’ ZnSe thin film.
0.25 0.25 0.2 Thickness (µm)
Thickness (µm)
0.2 0.15 0.1
(a)
0.1
0.05
0.05 0 310
0.15
0 320
330
340
Temperature (K)
0
350
(b)
50
100
150
Time (min)
Fig. 1. (a) Variation of the film thickness with deposition temperature and (b) variation of the film thickness with deposition time.
80
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Table 1 Structural characterization of ZnSe thin film Film d-values (A˚)
h k l planes Grain size (A˚)
Observed ASTM ZnSe 3.271 2.8537 1.9987 1.6912 1.6356 — —
3.271 2.837 2.004 1.708 1.636 1.4169 1.3
111 200 220 311 222 400 331
XRD
SEM
158
162
Cell parameter (A˚)
5.6601
Table 2 Compositional analysis Sr. no.
1
Composition
ZnSe
Bath content (ppm)
Film content (ppm)
Zn
Se
Zn
Se
523.3
760
260
376
4.4. Compositional analysis Atomic Absorption Spectroscopy was used to study compositional analysis by calibration curve method. Previously weighed minute sample was dissolved in the minimum quantity of Conc. HNO3 to yield the products as shown in the reactions: ZnSe þ 2HNO3 ) ZnðNO3 Þ2 þ Se þ H2 O: Fig. 3. SEM mircogrpahs of: (a) ‘as deposited’ and (b) ‘annealed’ ZnSe thin film.
highest intensity peak. The average crystallite size of annealing ZnSe thin film at 473 K was found to be 158 A˚. 4.3. Scanning electron microscope (SEM) SEM is an excellent method to study morphology of the sample. The SEM micrograph of ‘as deposited’ and those ‘annealed’at 473 K are shown in Fig. 3(a) and (b), respectively, at 10,000 magnification. ‘As deposited’ ZnSe thin film is homogenous, without cracks or pinholes and well cover to the glass substrate. From the micrograph, it is clearly seen that the film, composed of minute grains, was uniformly distributed over a smooth homogenous background that may correspond to amorphous phase of ZnSe thin film. In annealed film, the grains are more distinct and of bigger size. The increase in grain size leads to decrease in the grain boundaries. The presence of fine background is an indication of one-step growth by multiple nucleations. The average grain size of ‘as deposited’ as well as annealed samples are reported in Table 1.
(1.7)
Below pH 7, the selenium was precipitated as free metal [23]. While nitrates of zinc remain in the solution state. The precipitated was filtered through a Gooch crucible and subjected to selenium estimation using a standard gravimetric method. The filtrate containing zinc 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 zinc. The observed concentration of zinc and selenium in the film are reported in the Table 2. The compositional analysis of the sample using AAS gave 49.17% zinc and 50.83% selenium, showing samples is zinc deficient. 4.5. Optical studies The optical absorption spectra of ‘as deposited’ ZnSe film onto non-conducting glass substrate was studied at the room temperature in the wavelength of 350–850 nm without considering transmission and reflection. Fig. 4(a) shows the variation of optical absorbance with wavelength. The optical studies shows that the films are absorptive (a 104 cm1). The value of absorption coefficient is depends upon radiation energy as well as the composition of films. The data were properly studied in the vicinity of
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8000
(αhυ)2 X 107(eV/cm)2
Absorbance (cm-1)
1.5 1.2 0.9 0.6 0.3
6000
4000
2000
0 1.4
0 350 (a)
450
550
650
750
850
Wavelength (nm)
1.8
2.2
2.6
3
3.4
Photon energy (eV)
(b)
Fig. 4. (a) Absorption spectrum of ZnSe thin film and (b) plots of (ahn)2 with respect to photon energy.
an absorption edge on the basis of three-dimensional model to estimate the ‘Eg’ value. The simplest forms of the equation obeyed near and above absorption edge are [24]:
Sample
(1.8)
where hn is the photon energy, Eg is the band gap, A and n are constants. ‘A’ is depending upon the temperature phonon energies, etc. For allowed direct transition n ¼ 1=2 and for allowed indirect transition n ¼ 2. A plot of (ahn)2 versus hn is shown in the Fig. 4(b). The linear nature of plot shows that the existence of the direct transitions. Extrapolation of the linear portion of the curve to a2 ¼ 0 gives the optical band gap, which is about 2.81 eV. The value of band gap obtained agrees well with the results of previously reported Znse using chemical bath deposition method [25]. The observed value is greater than standard band gap (2.7 eV) of the ZnSe material, [26] showing a ‘blue shift’ of 0.11 eV. This is attributed due to size-quantization in polycrystalline semiconductor. This is attributed due to size-quantization occurs due to localization of electrons and holes in confined volume of the semiconductor materials. 4.6. Electrical conductivity and thermoelectrical studies The dark electrical conductivity of ‘as deposited’ ZnSe film on non-conducting 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 in the order of 108 (O 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 3.The low value of conductivity may be due to low crystallinity and small thickness of the film. The electrical properties of polycrystalline thin films are mainly depends upon their structural characteristics and composition [28–29]. It is observed that the conductivity on the film increases with increasing in temperature. This indicates the semiconducting behavior of the thin film. The electrical conductivity variation with temperature during heating
ZnSe
Band gap Activation energy (eV) (eV)
2.81
1.75
Specific conductance (O cm)1
HT
LT
300 K
0.61
0.022
5.836 108 6.1421 105
2.25
2.75
525 K
3.25
-4.5
-5.5 log σ
a ¼ ðA=hnÞðhn E g Þn ,
Table 3 Optical and electrical characterization of ZnSe thin film
-6.5
-7.5 1000 / T (perK) Fig. 5. The variations of log (conductivity) with inverse temperature.
and cooling cycles were found to be different and this shows that the ‘as deposited’ films undergo an irreversible changes due to annealing out of non-equilibrium defects during first heating. A plot of log (conductivity) versus inverse absolute temperature for the cooling curve is shown in the Fig. 5. A plot shows that electrical conductivity has
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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 exponential form of Arrhenius equation s ¼ s0 expðE a =KTÞ,
(1.9)
where the terms have usual meaning. The activation energies are 0.022 and 0.61 eV for low and high temperature region, 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 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 semiconductor ends. 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 ZnSe thin film, the negative terminal was connected to the cold end, therefore, the film shows n-type conductivity [30]. 5. Conclusions (1) Zinc selenide (ZnSe) thin films have been successfully deposited using this method. (2) Crystallographic studies revealed ZnSe deposited in cubic form with presence of some amorphous phase. (3) Morphological studies found ‘as deposited’ films are uniform, homogenous minute grains where as annealed samples observed improvement in crystallinity. (4) The optical absorption study showed ZnSe samples have direct band gap 2.81 eV with direct type of transition. (5) The room temperature electrical conductivity of ZnSe thin film is of the order of 108(O cm)1. Thermoelectric power measurement observed n-type conduction for ZnSe thin film.
Acknowledgments Authors are thankful to the Director, IISc, Bangalore for providing SEM facility and also Prof. Deshmukh, M.B.,
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Head, Department of Chemistry, Shivaji University, Kolhapur for giving the laboratory facilities.
References [1] C. Wang, X.F. Qian, W.X. Zhang, X.M. Zhang, Y. Xie, Y.T. Qian, Mater. Res. Bull. 34 (1999) 1637. [2] J.-C. Jan, S.-Y. Kuo, S.-B. Yin, W.-F. Hsich, Chin. J. Phys. 39 (2001) 90. [3] C.D. Lokhande, P.S. Patil, H. Tributsch, A. Ennaoui, Sol. Energy Mater. Sol. Cells 55 (1998) 379. [4] M. Ganchev, N. Strahieva, E. Tzvetkova, I. Gadjov, J. Mater. Sci.: Mater. Electron. 14 (2003) 847. [5] O. Akira, S. Noriyoshi, S. Zembutsu, J. Appl. Phys. 64 (1988) 654. [6] A. Schmidt, J. Crystal Growth 101 (1990) 758. [7] S.Y. Wang, Appl. Phys. Lett. 61 (1992) 506. [8] H.S. Akbar, A.J. Abdulkadir, M.S. Bani, J. Sol. Energy Res. 6 (1988) 49. [9] A. Ennaoui, S. Siebntritt, M.I. Lux-Steiner, W. Riedl, F. Karg, Sol. Energy Mater. Sol. Cells 67 (2001) 31. [10] M. Kulapov, G.A. Murowick, V.N. Ulasyuk, IZV Akad. Nauk. SSR Neorg. Mater. 19 (1983) 1807. [11] R.R. Alfand, O.Z. Wang, J. Jumbo, B. Bhargava, J. Phys. Res. A 35 (1987) 459. [12] A. Rumberg, C. Sommerhalter, M. Toplak, A. Jager-Waldau, M.C. Lux-steiner, Thin Solid Films 361 (2001) 172. [13] N. Katsumura, K. Maemura, T. Mori, J. Saraie, J. Crystal Growth 159 (1996) 85. [14] R. Islam, D.R. Rao, J. Mater. Sci. Lett. 13 (1994) 1637. [15] A.M. Chaparro, M.T. Gutierez, J. Herero, Electrochim. Acta 47 (2001) 977. [16] A. Chandramohan, T. Mahalingam, J.P. Chu, P.J. Sebastian, Sol. Energy Mater. Sol. Cells 81 (2004) 371. [17] P.K.R. Kalita, B.K. Sarma, H.L. Das, Bull. Mater. Sci. 23 (2000) 313. [18] R.B. Kale, C.D. Lokhande, Mater. Res. Bull. 39 (2004) 1829. [19] Y. Mitsumori, Y. Ohkubo, A. Hasegawa, M. Sasaki, F. Minami, J. Luminescence 108 (2004) 259. [20] H. Luo, T.K. Furdyna, Semicond. Sci. Technol. 10 (1995) 1041. [21] C.A. Estrada, P.K. Nair, M.T.S. Nair, R.A. Zingaro, E.S. Meyers, J. Electrochem. Soc. 141 (1994) 802. [22] B. Mokili, Y. Charreire, R. Cortes, D. Lincot, Thin Solid Films 288 (1996) 21. [23] P.P. Hankare, S.D. Delekar, P.A. Chate, S.D. Sabane, K.M. Garadkar, V.M. Bhuse, Semicond. Sci. Technol. 20 (2005) 257. [24] D.S. Sutrave, G.S. Shahane, V.B. Patil, L.P. Deshmukh, Mater. Chem. Phys. 65 (2000) 298. [25] J. Dona, J. Herrero, J. Electrochem. Soc. 142 (1995) 764. [26] P. Pramanik, S. Biswas, J. Electrochem. Soc. 133 (1980) 350. [27] J. Krustok, P.E. Kukk, Mater. Sci. 25 (1989) 43. [28] K.L. Chopra, Thin Film Phenomenon, McGraw-Hill, New York, 1969. [29] G.I. Rusu, M.E. Popa, G.S. Rusu, I. Salaoru, Appl. Surf. Sci. 218 (2003) 222. [30] C.N. Rao, Modern Aspects of Solid State Chemistry, Plenum Press, New York, 1970, p. 531.