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Structural and optical properties of electrosynthesized ZnSe thin films
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ARTICLE IN PRESS Optik xxx (2012) xxx–xxx
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Structural and optical properties of electrosynthesized ZnSe thin films V. Dhanasekaran a , T. Mahalingam a,∗ , Jin-Koo Rhee b , J.P. Chu c a b c
Department of Physics, Alagappa University, Karaikudi 630 003, India Millimeter-wave INnovation Technology Research Center (MINT), Dongguk University, Seoul 100-715, Republic of Korea Department of Polymer Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan
a r t i c l e
i n f o
Article history: Received 3 July 2011 Accepted 18 November 2011 Available online xxx Keywords: Thin films Electrodeposition Structural studies Electrochemical reaction Optical properties
a b s t r a c t Structural properties of electrodeposited ZnSe thin films were studied using X-ray diffraction (XRD) and the electrodeposited films are found to be polycrystalline in nature with face centered cubic structure. Optical constants of ZnSe thin films for various deposition potentials were determined in the spectral range 400–1200 nm using the optical absorption and transmittance measurements. The Tauc’s plot was drawn to determine the energy band gap values of the deposited films and is estimated to be in the range between 2.52 eV and 2.61 eV respectively. Their optical constants like refractive index (n), dielectric constants (ε), optical conductivity (), average excitation energy (E0 ), oscillator strength (Ed ), effective mass (m* ), plasma frequency (ωp ), static dielectric constant (ε∞ ) and carrier concentration (N) were estimated and reported. The room temperature photo luminescence studies were also performed. The near band edge luminescence emission has been observed and reported. © 2011 Elsevier GmbH. All rights reserved.
1. Introduction Zn based nanostructures have been widely investigated recently due to its potential applications. Zinc selenide (ZnSe) shows unique optical properties exhibiting some potential applications, such as blue-green light emitting diodes, photo-luminescent and electro-luminescent devices, lasers, thin film solar cell, nonlinear optical crystal and infrared optical material [1,2]. ZnSe and its lattice matched ternary alloys have been regarded as useful II–VI compound semiconductors for optoelectronic and photoelectronic devices with the energy range from visible to ultraviolet ever since the first manifestation of the blue-green laser on ZnSe based material structures in 1991 [3]. ZnSe has been a material of choice for blue diode lasers and photovoltaic solar cells since its bulk band gap is 2.67 eV (460 nm) which can be tuned by adding impurities. Out of varieties of applications, ZnSe can be used as optically controlled switching devices [4,5]. Hence it is of great interest as a model material as thin film, quantum wells, bulk crystals and nanodots [6]. Since last few decades the nanosized materials have been subject of great interest due to their unique physical and chemical properties. Thus the strong, size-dependent optical emission of many semiconductor nanostructures makes them promising candidates for use as fluorescent tags in the study of biological systems. High quality of thin films can be obtained from a number of techniques [7,8] are such as, chemical vapor deposition, molecular beam epitaxy, pulsed laser, evaporation, and sputtering. However,
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (T. Mahalingam).
there is an interest to investigate other approaches, which could open new or supplementary possibilities in terms of device properties, structure or engineering. Chemical bath deposition [9,10] and electrodeposition techniques are belonging to these alternative methods that could also produce high quality films of chalcogenide materials. High efficient devices can be obtained by electrodeposition such as electrodeposited cadmium telluride solar cells or electrodeposited wide band gap sulfide or oxides buffer/window layers. Within direct wide-band semiconductor materials, the zinc chalcogenides compounds have been the objects of numerous studies concerning thin film electrodeposition from aqueous solutions [11,12]. Zinc selenide (ZnSe) is considered as an important technological material due to their potential applications in various optical and electronic devices. ZnSe thin film has been used as n-type window layer for thin film heterojunction solar cells. Moreover, the interest in ZnSe–GaAs heterojunction has greatly increased in recent years because of possible potential applications in a number of high speed and optoelectronic devices [13]. The buffer layer determines properties of thin film solar cells in the absorber interfacial states and electronic bands alignment. It is also involved in the long-term stability of the cells and light soaking effect [14]. ZnSe is used as materials for production of optical elements for IR range including the passive laser optics elements since it posses the high optical transmission with extremely low bulk loses from scatter and absorption [15]. Our group has considerable experience since 1997 in the electrodeposition of ZnSe thin films for the fabrication of photoelectrochemical solar cells [16]. In the present work, face centered cubic ZnSe thin films have been synthesized on indium doped tin oxide (ITO) coated conducting glass substrates at various deposition potential from aqueous
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Please cite this article in press as: V. Dhanasekaran, et al., Structural and optical properties of electrosynthesized ZnSe thin films, Optik - Int. J. Light Electron Opt. (2012), doi:10.1016/j.ijleo.2011.11.063
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electrolytic bath by using cathodic electrodeposition technique. The ZnSe films deposited for various deposition potentials are to be characterized for electrochemical studies, structural studies, and optical properties. 2. Experimental Electrochemical experiments were carried out using a Potentiostat/Galvanostat (EG&G Princeton Applied Research, USA Model 362A). Electrodeposition was performed in a conventional three electrode cell with ITO substrates as cathode and graphite rod as anode and a saturated calomel electrode as reference electrode. ZnSe thin films were prepared using an electrolyte containing zinc sulphate ZnSO4 and SeO2 . ZnSO4 and SeO2 were taken with 0.02 M and 0.005 M, respectively. Electrodeposition was performed with deposition potential varying from −0.650 V to −0.950 V insteps of 100 mV with respect to SCE. The electrolytic bath temperature and deposition time were maintained at 75 ◦ C and 30 min, respectively. Cyclic voltammetric studies were carried out to select the deposition parameters. An X-ray diffractometer [X’PERT PRO PANalytical, Netherlands] with Cuk␣ radiation ( = 0.1540 nm) was used to identify the crystal structure of the films. Optical properties of the deposited samples were analyzed using a UV–vis–NIR double beam spectrophotometer (HR – 2000, M/S Ocean Optics, USA). 3. Results and discussion 3.1. Electrochemical and structural studies Cyclic voltammetric studies were carried out in a standard three compartment cells comprising of indium doped tin oxide coated conducting glass substrate as cathode, platinum electrode as anode and saturated calomel electrode as reference electrode, respectively. The scan rate employed was 20 mV/s. The voltammetric curves were scanned in the potential range from −1500 to +1500 mV vs SCE. A typical cyclic voltammogram recorded for ITO glass electrode in an aqueous solution mixture containing 0.02 M ZnSO4 and 0.005 M SeO2 is shown in Fig. 1. This can be an effect of the irreversibility of reaction (1), albeit the scan was not followed to more positive potentials because of the intense dissolution of the film from the substrate. It is observed that the oxidation and reduction peaks have emerged in the region −0.5 to −0.6 V vs SCE. Also another one broad oxidation peak is also present in positive potential which may be due to the negative standard electrode potential. In this region Zn is in adsorbed state and the Se is reduced to Se2− state, because at these potentials more negative than Se reduction to Se0 (i.e. −540 mV vs SCE), Se forms H2 Se, which is highly reactive.
Fig. 1. Cyclic voltammogram of ZnSe thin film obtained from an aqueous electrolytic bath consists of 0.02 M ZnSO4 and 0.005 M SeO2 .
This immediately reacts with adsorbed Zn2+ and forms ZnSe. The plateau region gives the co-deposition range for ZnSe with in −700 and −900 mV vs SCE. Hence, the deposition potential is fixed as −850 mV vs SCE, in order to obtain ZnSe thin films. For the SeO2 dissolved in water form H2 SeO3 solution which is associated with selenium deposition according to the reaction: H2 SeO3 + 4H+ + 4e− = Se + 3H2 O
(1)
In the cyclic voltammogram obtained this effect is masked probably might have been superimposed by hydrogen evolution. Reduction of the Zn2+ with the selenium precursor in the electrolyte starts at −1.5 V vs SCE according to the reaction (2): Zn2+ + 2e− = Zn
(2)
and it is manifested by a steep increase in cathodic current in the voltammogram. Hydrogen evolution was probably inhibited by the under potential deposited zinc on the negative electrode, when this voltammogram was compared with the one obtained in the solution without zinc ions. During the reverse scan an anodic peak appeared in the voltammogram, connected with the dissolution of deposited zinc. When the substrate is immersed in Zn containing solution, Zn ions are adsorbed on the surface of the conducting glass substrate. After immersion of such substrate in Se ion containing solution, following reaction takes place on the surface of conducting glass substrate: Zn + Se = ZnSe
(3)
The structural properties of electrodeposited ZnSe thin films were investigated by X-ray diffraction using Cuk␣ radiation with = 0.154 nm. X-ray diffraction patterns are recorded for ZnSe thin films obtained at different deposition potentials ranging from −650 mV to −950 mV vs SCE by maintaining the deposition time and bath temperature at 30 min and 75 ◦ C, respectively shown in Fig. 2. X-ray diffraction studies revealed that as-deposited films were polycrystalline nature. X-ray diffraction patterns also show that various diffraction peaks at 2 values 27.25◦ , 31.43◦ , 45.10◦ , 53.65◦ , 65.71◦ and 72.11◦ , were identified to originate from (1 1 1), ៝ 1 1), (2 0 0), (2 2 0), (3 1 1), (4 0 0) and (3 3 1) planes, respec(1 tively, which correspond to face centered cubic phase of ZnSe. The
Fig. 2. X-ray diffractogram of ZnSe thin films at various deposition potentials (a) −650 mV, (b) −750 mV, (c) −850 mV, and (d) −950 mV vs SCE.
Please cite this article in press as: V. Dhanasekaran, et al., Structural and optical properties of electrosynthesized ZnSe thin films, Optik - Int. J. Light Electron Opt. (2012), doi:10.1016/j.ijleo.2011.11.063
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observed peaks in the diffraction patterns were indexed and the corresponding values of lattice spacing “d” were calculated and compared with JCPDS standards no. 88-2345 for ZnSe. The strongest diffraction peak appears 27.25◦ , corresponds to (1 1 1) planes. This peak has shifted towards higher diffraction angles for ZnSe thin films when the deposition potential increases. The line width (1 1 1) peak, 0.18◦ for −650 mV vs SCE prepared film, increases largely with increasing deposition potential and reaches 0.21◦ , implying that the grain size in the ZnSe films is increased as the deposition potential is increased. For further decrease (increase) in deposition potential, the films show improved crystalline structure and the intensity of (1 1 1) plane increases to maximum up to potential −950 mV vs SCE. Above this precursor deposition potential, the intensity (3 1 1) peak is reduced and the (3 3 1) peak is slightly increased however both the peaks seems to be less than the (1 1 1) peak. The average grain size is found to be 44 nm for the maximum deposition potential and 39 nm for the more negative potential. Moreover, the increase in the intensity and broadening of the peak (1 1 1) with decrease in the deposition potential implies that the grain size along (1 1 1) plane and the crystallinity decrease as the deposition potential becomes more negative. All the figures reveal that the wurtzite cubic structure of ZnSe is conserved. However, one ZnSe satellite peak is (2 0 0) emerged in addition to the conventional ZnSe peaks (1 1 1) and (3 1 1). These satellite peaks show high crystallinity for increasing deposition potential. The presence of (1 1 1) peak in the all the films indicates that the films were highly oriented. 3.2. Optical studies Optical transmission spectra were recorded at room temperature in air to obtain information on the optical properties of zinc selenide thin films. Fig. 3 shows the transmittance spectrum obtained for ZnSe films prepared by electrodeposition at different deposition potentials, −650 mV, −750 mV, −850 mV and −950 mV vs SCE, respectively. Transmission spectra, in the wavelength range 400–1200 nm, at normal incidence, of ZnSe thin films of various deposition potential were recorded. The transmittance spectrum shows the surface plasmons resonance indicating the crystallite sizes in the nanometre range. The transmittance increases as the deposition potential decreases and it tends to increase in the IR region for all the films. The increase in the transmittance is due to an increase in the band gap with carrier concentration. It is observed that transmittance increases with the wavelength from 40 to 80%. The maximum range of deposition potential film displays the spectra with lesser transmittance. The nature of transition is determined using the following Eq. (4) [17], ˛h = A(h − Eg )
n
(4)
Fig. 3. Optical transmittance spectra of ZnSe thin films as a function of wavelength.
3
Fig. 4. Band gap variation of ZnSe thin films with deposition potentials.
where ˛ is absorption coefficient in cm−1 , h is photon energy, Eg is an energy gap, A is energy dependent constant and n is an integer depending on the nature of electronic transitions. For the direct allowed transitions, n has a value of 1/2 while for the indirect allowed transitions, n = 2. The band gap variation of ZnSe thin films with deposition potential is as shown in Fig. 4. The energy band gap of films deposited at different deposition potential was found to be in the range 2.52–2.61 eV using Tauc’s plot. It is clear that as the thickness increases the energy band gap increases. This increase in the energy band gap with increase of film thickness may be due to quantum confinement. We have further investigated the experimental data of optical transmittance of electrodeposited ZnSe thin film to calculate the refractive index and the extinction coefficient. The refractive index was calculated using the Swanepoel’s extrapolated wavelength method as have been used by many authors [18]. The refractive index tends to decrease for both films as we approach longer wavelength regime. The refractive index was found to decrease when increasing the deposition potential is increased from negative to positive region. The refractive index and extinction coefficient of the films decrease with the increasing deposition potential as shown in Fig. 5. The increase of ‘n’ with ‘k’ indicated a rapid change in the absorption energy at the comparatively positive deposition potentials. Fig. 6 shows the variation of dielectric coefficient of electrodeposited ZnSe thin films deposited on indium doped tin oxide coated glass substrates with different deposition. It is observed that in all the cases of ZnSe films, the real part of dielectric constants increases with increase of wavelength. The complex dielectric constant is known to be a fundamental intrinsic material property. The real part of dielectric constant is associated with the property of slowing
Fig. 5. Refractive index and extinction coefficient variation of ZnSe thin films with various deposition potentials.
Please cite this article in press as: V. Dhanasekaran, et al., Structural and optical properties of electrosynthesized ZnSe thin films, Optik - Int. J. Light Electron Opt. (2012), doi:10.1016/j.ijleo.2011.11.063
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Fig. 6. Real and imaginary part of dielectric constant variation of ZnSe thin films with various deposition potentials. Fig. 8. Plot of ε1 vs 1/ω2 of ZnSe thin films.
down the speed of light in the material. The real and imaginary parts of the dielectric constant were determined using the relation [19] ε = εr + εi = (n + ik)
2
(5)
The spectral dependence of the refractive index of many semiconductors can be evaluated by using the single oscillator model proposed by Wemple and Di Domenico viz;
E0 Ed
where ‘εr ’ and ‘εi ’ are the real and imaginary parts of the dielectric constant respectively and are given by
n2 = 1 +
εr = n2 − k2
where n is the refractive index, E0 is the average excitation energy known as the oscillator energy, Ed is the dispersion energy called the oscillator strength, and h is the incident photon energy. To find out the dispersive parameters such as average excitation energy (E0 ), dispersion energy (Eb ), and the moments of the optical spectra M−1 and M−3 we have followed the procedure [20,21]. To evaluate the
(6)
and εi = 2nk
(7)
The imaginary part of the dielectric constant also showed the same behaviour as that of the real part only thing is that their values seem to be very less compared to than that of real dielectric constant values. The imaginary part of the dielectric constant also showed the same behaviour as that of the real part, only thing is that their values seem to be very less compared to that of real dielectric constant values. The dielectric constants are linearly decreasing with increase deposition potentials. The optical conductivity is determined using the usual relation = ˛nc
(8)
Fig. 7 displays the variation of conductivity of ZnSe thin films deposited onto conducting glass substrate as a function of wavelength. The optical conductivity of the films is estimated using Eq. (8). In this case ZnSe thin films optical conductivity decrease with the increase in wavelength. The same behaviour is exhibited in all deposition potentials and is as shown in Fig. 7. This is may be due to the increase of band gap that increases with the doping concentration.
E02 − (h)
(9)
2
−1
oscillator parameters, a graph of (n2 − 1) against h2 was plotted in Fig. 8. The moments of the optical dispersion spectra M−1 and M−3 , can be evaluated using the relationships; E02 = Ed2 =
M−1 M3
(10)
3 M−1
(11)
M3
The values of the dispersion parameters and band gaps of the films are presented in Table 1. Finally, we justify the above optical results with deposition potential effects on structural and optical properties of ZnSe thin films. The carrier concentration has been calculated from the plasma frequency whose expression is given below as shown in Fig. 9 [22,23] ε = ε∞ −
ε∞ ωp2
(12)
ω2
In the above Eq. (10) we can calculate carrier concentration of the ZnSe thin films. If the carrier concentration is known, the effective mass of the charge carriers could be found out from the plasma frequency. It is observed from the table that and all the values tend to increase when the band gap energies increase due to the decrease in crystallite size. The effective mass and carrier concentration of the films are linearly charge dependent and hence depends upon Table 1 Various dispersion parameter of electrodeposited ZnSe thin films for various deposition potentials.
Fig. 7. Optical conductivity of ZnSe thin films as a function of wavelength.
Deposition potential (mV vs SCE)
E0
Ed
M−1
M−3
Eg (eV)
−650 −750 −850 −950
5.656 5.058 5.335 5.294
18.81 17.53 20.00 20.83
3.326 3.466 3.749 3.935
0.103 0.125 0.131 0.140
2.61 2.58 2.54 2.52
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4. Conclusions
Fig. 9. Plot of (n2 − 1)−1 vs (h)2 of ZnSe thin films.
Table 2 Static dielectric constant, angular frequency, effective mass and carrier concentration of electrodeposited ZnSe thin films.
Thin films of ZnSe were electrodeposited onto indium doped tin oxide coated conducting glass substrate from an aqueous electrolytic bath using potentiostatic electrodeposition technique. X-ray diffraction studies were carried out in order to study the crystalline nature of the deposited films. The presence of the XRD pattern represents that the deposited films was found to exhibit poly crystalline in nature with cubic structure with preferential orientation along (1 1 1) plane. We can easily find that the transmittance increases with wavelength from 40 to 80%. Optical absorption measurements represents that the deposited films have a direct band gap in the range between 2.52 and 2.61 eV which in turn confirms the stoichiometric formation of well crystallized ZnSe thin films. From the optical transmittance and absorption studies optical constants were evaluated and reported. The parameters like plasma frequency, carrier concentration and effect mass were estimated to be 9.3 × 1014 s−1 , 1.34 × 1018 cm−3 and 0.955, respectively for optimized deposition potential.
Deposition potential (mV vs SCE)
ε∞
ωp × 10 14 s−1
m*
N × 1018 cm−3
References
−650 −750 −850 −950
5.436 5.287 5.033 4.823
6.306 6.675 9.304 8.342
0.9565 0.9560 0.9556 0.951
0.9364 1.0198 1.8846 1.4513
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the deposition potential. The obtained optical constants are tabulated in Table 2. It is found from the optical studies that there occurs an increase in energy band gap with increasing deposition potential of ZnSe films and this can be attributed to the enhancement of crystallite size and hence exciton confinement effect which was contributed by deposition potential. Photoluminescence (PL) spectra were recorded at room temperature using an excitation wavelength of 441 nm. The spectra show peaks at 2.4 and 2.56 eV (Fig. 10) respectively and the PL intensity was found to increase with decrease deposition potential. These peaks are assigned to the near band emission. Self activated centers arising from complexes of zinc vacancies and shallow donors (selenium interstitials) would occur around 2.5 eV [24]. In this peaks are associated with band gap values of ZnSe thin films estimated by conventional method. The PL emission from the undoped ZnSe has been attributed to the presence of native defects like zinc and selenium vacancies or interstitials, which are likely to be introduced during the growth process [25,26].
Fig. 10. Photoluminescence spectra of ZnSe thin film prepared at −850 mV vs SCE.
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[24] K.R. Murali, A. Austine, D.C. Trivedi, Properties of ZnSe films brush plated on high temperature substrates, Mater. Lett. 59 (2005) 2621–2624. [25] T. Yodo, R. Ueda, K. Morio, R. Yamasita, S. Tanaka, Photoluminescence study of Li- and Na-implanted ZnSe epitaxial layers grown by atmospheric pressure metalorganic vapor-phase epitaxy, J. Appl. Phys. 68 (1990) 3212–3218. [26] F. Sakurai, K. Suto, S. Sanda, J. Nishizawa, Photoluminescence and photocapacitance of Se vapor-pressure-dependent deep levels in Ga-diffused layers formed in p-type ZnSe substrates, J. Electrochem. Soc. 149 (2002) G100–G102.
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Structural and optical properties of electrosynthesized ZnSe thin films V. Dhanasekaran a , T. Mahalingam a,∗ , Jin-Koo Rhee b , J.P. Chu c a b c
Department of Physics, Alagappa University, Karaikudi 630 003, India Millimeter-wave INnovation Technology Research Center (MINT), Dongguk University, Seoul 100-715, Republic of Korea Department of Polymer Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan
a r t i c l e
i n f o
Article history: Received 3 July 2011 Accepted 18 November 2011 Available online xxx Keywords: Thin films Electrodeposition Structural studies Electrochemical reaction Optical properties
a b s t r a c t Structural properties of electrodeposited ZnSe thin films were studied using X-ray diffraction (XRD) and the electrodeposited films are found to be polycrystalline in nature with face centered cubic structure. Optical constants of ZnSe thin films for various deposition potentials were determined in the spectral range 400–1200 nm using the optical absorption and transmittance measurements. The Tauc’s plot was drawn to determine the energy band gap values of the deposited films and is estimated to be in the range between 2.52 eV and 2.61 eV respectively. Their optical constants like refractive index (n), dielectric constants (ε), optical conductivity (), average excitation energy (E0 ), oscillator strength (Ed ), effective mass (m* ), plasma frequency (ωp ), static dielectric constant (ε∞ ) and carrier concentration (N) were estimated and reported. The room temperature photo luminescence studies were also performed. The near band edge luminescence emission has been observed and reported. © 2011 Elsevier GmbH. All rights reserved.
1. Introduction Zn based nanostructures have been widely investigated recently due to its potential applications. Zinc selenide (ZnSe) shows unique optical properties exhibiting some potential applications, such as blue-green light emitting diodes, photo-luminescent and electro-luminescent devices, lasers, thin film solar cell, nonlinear optical crystal and infrared optical material [1,2]. ZnSe and its lattice matched ternary alloys have been regarded as useful II–VI compound semiconductors for optoelectronic and photoelectronic devices with the energy range from visible to ultraviolet ever since the first manifestation of the blue-green laser on ZnSe based material structures in 1991 [3]. ZnSe has been a material of choice for blue diode lasers and photovoltaic solar cells since its bulk band gap is 2.67 eV (460 nm) which can be tuned by adding impurities. Out of varieties of applications, ZnSe can be used as optically controlled switching devices [4,5]. Hence it is of great interest as a model material as thin film, quantum wells, bulk crystals and nanodots [6]. Since last few decades the nanosized materials have been subject of great interest due to their unique physical and chemical properties. Thus the strong, size-dependent optical emission of many semiconductor nanostructures makes them promising candidates for use as fluorescent tags in the study of biological systems. High quality of thin films can be obtained from a number of techniques [7,8] are such as, chemical vapor deposition, molecular beam epitaxy, pulsed laser, evaporation, and sputtering. However,
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (T. Mahalingam).
there is an interest to investigate other approaches, which could open new or supplementary possibilities in terms of device properties, structure or engineering. Chemical bath deposition [9,10] and electrodeposition techniques are belonging to these alternative methods that could also produce high quality films of chalcogenide materials. High efficient devices can be obtained by electrodeposition such as electrodeposited cadmium telluride solar cells or electrodeposited wide band gap sulfide or oxides buffer/window layers. Within direct wide-band semiconductor materials, the zinc chalcogenides compounds have been the objects of numerous studies concerning thin film electrodeposition from aqueous solutions [11,12]. Zinc selenide (ZnSe) is considered as an important technological material due to their potential applications in various optical and electronic devices. ZnSe thin film has been used as n-type window layer for thin film heterojunction solar cells. Moreover, the interest in ZnSe–GaAs heterojunction has greatly increased in recent years because of possible potential applications in a number of high speed and optoelectronic devices [13]. The buffer layer determines properties of thin film solar cells in the absorber interfacial states and electronic bands alignment. It is also involved in the long-term stability of the cells and light soaking effect [14]. ZnSe is used as materials for production of optical elements for IR range including the passive laser optics elements since it posses the high optical transmission with extremely low bulk loses from scatter and absorption [15]. Our group has considerable experience since 1997 in the electrodeposition of ZnSe thin films for the fabrication of photoelectrochemical solar cells [16]. In the present work, face centered cubic ZnSe thin films have been synthesized on indium doped tin oxide (ITO) coated conducting glass substrates at various deposition potential from aqueous
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Please cite this article in press as: V. Dhanasekaran, et al., Structural and optical properties of electrosynthesized ZnSe thin films, Optik - Int. J. Light Electron Opt. (2012), doi:10.1016/j.ijleo.2011.11.063
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electrolytic bath by using cathodic electrodeposition technique. The ZnSe films deposited for various deposition potentials are to be characterized for electrochemical studies, structural studies, and optical properties. 2. Experimental Electrochemical experiments were carried out using a Potentiostat/Galvanostat (EG&G Princeton Applied Research, USA Model 362A). Electrodeposition was performed in a conventional three electrode cell with ITO substrates as cathode and graphite rod as anode and a saturated calomel electrode as reference electrode. ZnSe thin films were prepared using an electrolyte containing zinc sulphate ZnSO4 and SeO2 . ZnSO4 and SeO2 were taken with 0.02 M and 0.005 M, respectively. Electrodeposition was performed with deposition potential varying from −0.650 V to −0.950 V insteps of 100 mV with respect to SCE. The electrolytic bath temperature and deposition time were maintained at 75 ◦ C and 30 min, respectively. Cyclic voltammetric studies were carried out to select the deposition parameters. An X-ray diffractometer [X’PERT PRO PANalytical, Netherlands] with Cuk␣ radiation ( = 0.1540 nm) was used to identify the crystal structure of the films. Optical properties of the deposited samples were analyzed using a UV–vis–NIR double beam spectrophotometer (HR – 2000, M/S Ocean Optics, USA). 3. Results and discussion 3.1. Electrochemical and structural studies Cyclic voltammetric studies were carried out in a standard three compartment cells comprising of indium doped tin oxide coated conducting glass substrate as cathode, platinum electrode as anode and saturated calomel electrode as reference electrode, respectively. The scan rate employed was 20 mV/s. The voltammetric curves were scanned in the potential range from −1500 to +1500 mV vs SCE. A typical cyclic voltammogram recorded for ITO glass electrode in an aqueous solution mixture containing 0.02 M ZnSO4 and 0.005 M SeO2 is shown in Fig. 1. This can be an effect of the irreversibility of reaction (1), albeit the scan was not followed to more positive potentials because of the intense dissolution of the film from the substrate. It is observed that the oxidation and reduction peaks have emerged in the region −0.5 to −0.6 V vs SCE. Also another one broad oxidation peak is also present in positive potential which may be due to the negative standard electrode potential. In this region Zn is in adsorbed state and the Se is reduced to Se2− state, because at these potentials more negative than Se reduction to Se0 (i.e. −540 mV vs SCE), Se forms H2 Se, which is highly reactive.
Fig. 1. Cyclic voltammogram of ZnSe thin film obtained from an aqueous electrolytic bath consists of 0.02 M ZnSO4 and 0.005 M SeO2 .
This immediately reacts with adsorbed Zn2+ and forms ZnSe. The plateau region gives the co-deposition range for ZnSe with in −700 and −900 mV vs SCE. Hence, the deposition potential is fixed as −850 mV vs SCE, in order to obtain ZnSe thin films. For the SeO2 dissolved in water form H2 SeO3 solution which is associated with selenium deposition according to the reaction: H2 SeO3 + 4H+ + 4e− = Se + 3H2 O
(1)
In the cyclic voltammogram obtained this effect is masked probably might have been superimposed by hydrogen evolution. Reduction of the Zn2+ with the selenium precursor in the electrolyte starts at −1.5 V vs SCE according to the reaction (2): Zn2+ + 2e− = Zn
(2)
and it is manifested by a steep increase in cathodic current in the voltammogram. Hydrogen evolution was probably inhibited by the under potential deposited zinc on the negative electrode, when this voltammogram was compared with the one obtained in the solution without zinc ions. During the reverse scan an anodic peak appeared in the voltammogram, connected with the dissolution of deposited zinc. When the substrate is immersed in Zn containing solution, Zn ions are adsorbed on the surface of the conducting glass substrate. After immersion of such substrate in Se ion containing solution, following reaction takes place on the surface of conducting glass substrate: Zn + Se = ZnSe
(3)
The structural properties of electrodeposited ZnSe thin films were investigated by X-ray diffraction using Cuk␣ radiation with = 0.154 nm. X-ray diffraction patterns are recorded for ZnSe thin films obtained at different deposition potentials ranging from −650 mV to −950 mV vs SCE by maintaining the deposition time and bath temperature at 30 min and 75 ◦ C, respectively shown in Fig. 2. X-ray diffraction studies revealed that as-deposited films were polycrystalline nature. X-ray diffraction patterns also show that various diffraction peaks at 2 values 27.25◦ , 31.43◦ , 45.10◦ , 53.65◦ , 65.71◦ and 72.11◦ , were identified to originate from (1 1 1), ៝ 1 1), (2 0 0), (2 2 0), (3 1 1), (4 0 0) and (3 3 1) planes, respec(1 tively, which correspond to face centered cubic phase of ZnSe. The
Fig. 2. X-ray diffractogram of ZnSe thin films at various deposition potentials (a) −650 mV, (b) −750 mV, (c) −850 mV, and (d) −950 mV vs SCE.
Please cite this article in press as: V. Dhanasekaran, et al., Structural and optical properties of electrosynthesized ZnSe thin films, Optik - Int. J. Light Electron Opt. (2012), doi:10.1016/j.ijleo.2011.11.063
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observed peaks in the diffraction patterns were indexed and the corresponding values of lattice spacing “d” were calculated and compared with JCPDS standards no. 88-2345 for ZnSe. The strongest diffraction peak appears 27.25◦ , corresponds to (1 1 1) planes. This peak has shifted towards higher diffraction angles for ZnSe thin films when the deposition potential increases. The line width (1 1 1) peak, 0.18◦ for −650 mV vs SCE prepared film, increases largely with increasing deposition potential and reaches 0.21◦ , implying that the grain size in the ZnSe films is increased as the deposition potential is increased. For further decrease (increase) in deposition potential, the films show improved crystalline structure and the intensity of (1 1 1) plane increases to maximum up to potential −950 mV vs SCE. Above this precursor deposition potential, the intensity (3 1 1) peak is reduced and the (3 3 1) peak is slightly increased however both the peaks seems to be less than the (1 1 1) peak. The average grain size is found to be 44 nm for the maximum deposition potential and 39 nm for the more negative potential. Moreover, the increase in the intensity and broadening of the peak (1 1 1) with decrease in the deposition potential implies that the grain size along (1 1 1) plane and the crystallinity decrease as the deposition potential becomes more negative. All the figures reveal that the wurtzite cubic structure of ZnSe is conserved. However, one ZnSe satellite peak is (2 0 0) emerged in addition to the conventional ZnSe peaks (1 1 1) and (3 1 1). These satellite peaks show high crystallinity for increasing deposition potential. The presence of (1 1 1) peak in the all the films indicates that the films were highly oriented. 3.2. Optical studies Optical transmission spectra were recorded at room temperature in air to obtain information on the optical properties of zinc selenide thin films. Fig. 3 shows the transmittance spectrum obtained for ZnSe films prepared by electrodeposition at different deposition potentials, −650 mV, −750 mV, −850 mV and −950 mV vs SCE, respectively. Transmission spectra, in the wavelength range 400–1200 nm, at normal incidence, of ZnSe thin films of various deposition potential were recorded. The transmittance spectrum shows the surface plasmons resonance indicating the crystallite sizes in the nanometre range. The transmittance increases as the deposition potential decreases and it tends to increase in the IR region for all the films. The increase in the transmittance is due to an increase in the band gap with carrier concentration. It is observed that transmittance increases with the wavelength from 40 to 80%. The maximum range of deposition potential film displays the spectra with lesser transmittance. The nature of transition is determined using the following Eq. (4) [17], ˛h = A(h − Eg )
n
(4)
Fig. 3. Optical transmittance spectra of ZnSe thin films as a function of wavelength.
3
Fig. 4. Band gap variation of ZnSe thin films with deposition potentials.
where ˛ is absorption coefficient in cm−1 , h is photon energy, Eg is an energy gap, A is energy dependent constant and n is an integer depending on the nature of electronic transitions. For the direct allowed transitions, n has a value of 1/2 while for the indirect allowed transitions, n = 2. The band gap variation of ZnSe thin films with deposition potential is as shown in Fig. 4. The energy band gap of films deposited at different deposition potential was found to be in the range 2.52–2.61 eV using Tauc’s plot. It is clear that as the thickness increases the energy band gap increases. This increase in the energy band gap with increase of film thickness may be due to quantum confinement. We have further investigated the experimental data of optical transmittance of electrodeposited ZnSe thin film to calculate the refractive index and the extinction coefficient. The refractive index was calculated using the Swanepoel’s extrapolated wavelength method as have been used by many authors [18]. The refractive index tends to decrease for both films as we approach longer wavelength regime. The refractive index was found to decrease when increasing the deposition potential is increased from negative to positive region. The refractive index and extinction coefficient of the films decrease with the increasing deposition potential as shown in Fig. 5. The increase of ‘n’ with ‘k’ indicated a rapid change in the absorption energy at the comparatively positive deposition potentials. Fig. 6 shows the variation of dielectric coefficient of electrodeposited ZnSe thin films deposited on indium doped tin oxide coated glass substrates with different deposition. It is observed that in all the cases of ZnSe films, the real part of dielectric constants increases with increase of wavelength. The complex dielectric constant is known to be a fundamental intrinsic material property. The real part of dielectric constant is associated with the property of slowing
Fig. 5. Refractive index and extinction coefficient variation of ZnSe thin films with various deposition potentials.
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Fig. 6. Real and imaginary part of dielectric constant variation of ZnSe thin films with various deposition potentials. Fig. 8. Plot of ε1 vs 1/ω2 of ZnSe thin films.
down the speed of light in the material. The real and imaginary parts of the dielectric constant were determined using the relation [19] ε = εr + εi = (n + ik)
2
(5)
The spectral dependence of the refractive index of many semiconductors can be evaluated by using the single oscillator model proposed by Wemple and Di Domenico viz;
E0 Ed
where ‘εr ’ and ‘εi ’ are the real and imaginary parts of the dielectric constant respectively and are given by
n2 = 1 +
εr = n2 − k2
where n is the refractive index, E0 is the average excitation energy known as the oscillator energy, Ed is the dispersion energy called the oscillator strength, and h is the incident photon energy. To find out the dispersive parameters such as average excitation energy (E0 ), dispersion energy (Eb ), and the moments of the optical spectra M−1 and M−3 we have followed the procedure [20,21]. To evaluate the
(6)
and εi = 2nk
(7)
The imaginary part of the dielectric constant also showed the same behaviour as that of the real part only thing is that their values seem to be very less compared to than that of real dielectric constant values. The imaginary part of the dielectric constant also showed the same behaviour as that of the real part, only thing is that their values seem to be very less compared to that of real dielectric constant values. The dielectric constants are linearly decreasing with increase deposition potentials. The optical conductivity is determined using the usual relation = ˛nc
(8)
Fig. 7 displays the variation of conductivity of ZnSe thin films deposited onto conducting glass substrate as a function of wavelength. The optical conductivity of the films is estimated using Eq. (8). In this case ZnSe thin films optical conductivity decrease with the increase in wavelength. The same behaviour is exhibited in all deposition potentials and is as shown in Fig. 7. This is may be due to the increase of band gap that increases with the doping concentration.
E02 − (h)
(9)
2
−1
oscillator parameters, a graph of (n2 − 1) against h2 was plotted in Fig. 8. The moments of the optical dispersion spectra M−1 and M−3 , can be evaluated using the relationships; E02 = Ed2 =
M−1 M3
(10)
3 M−1
(11)
M3
The values of the dispersion parameters and band gaps of the films are presented in Table 1. Finally, we justify the above optical results with deposition potential effects on structural and optical properties of ZnSe thin films. The carrier concentration has been calculated from the plasma frequency whose expression is given below as shown in Fig. 9 [22,23] ε = ε∞ −
ε∞ ωp2
(12)
ω2
In the above Eq. (10) we can calculate carrier concentration of the ZnSe thin films. If the carrier concentration is known, the effective mass of the charge carriers could be found out from the plasma frequency. It is observed from the table that and all the values tend to increase when the band gap energies increase due to the decrease in crystallite size. The effective mass and carrier concentration of the films are linearly charge dependent and hence depends upon Table 1 Various dispersion parameter of electrodeposited ZnSe thin films for various deposition potentials.
Fig. 7. Optical conductivity of ZnSe thin films as a function of wavelength.
Deposition potential (mV vs SCE)
E0
Ed
M−1
M−3
Eg (eV)
−650 −750 −850 −950
5.656 5.058 5.335 5.294
18.81 17.53 20.00 20.83
3.326 3.466 3.749 3.935
0.103 0.125 0.131 0.140
2.61 2.58 2.54 2.52
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4. Conclusions
Fig. 9. Plot of (n2 − 1)−1 vs (h)2 of ZnSe thin films.
Table 2 Static dielectric constant, angular frequency, effective mass and carrier concentration of electrodeposited ZnSe thin films.
Thin films of ZnSe were electrodeposited onto indium doped tin oxide coated conducting glass substrate from an aqueous electrolytic bath using potentiostatic electrodeposition technique. X-ray diffraction studies were carried out in order to study the crystalline nature of the deposited films. The presence of the XRD pattern represents that the deposited films was found to exhibit poly crystalline in nature with cubic structure with preferential orientation along (1 1 1) plane. We can easily find that the transmittance increases with wavelength from 40 to 80%. Optical absorption measurements represents that the deposited films have a direct band gap in the range between 2.52 and 2.61 eV which in turn confirms the stoichiometric formation of well crystallized ZnSe thin films. From the optical transmittance and absorption studies optical constants were evaluated and reported. The parameters like plasma frequency, carrier concentration and effect mass were estimated to be 9.3 × 1014 s−1 , 1.34 × 1018 cm−3 and 0.955, respectively for optimized deposition potential.
Deposition potential (mV vs SCE)
ε∞
ωp × 10 14 s−1
m*
N × 1018 cm−3
References
−650 −750 −850 −950
5.436 5.287 5.033 4.823
6.306 6.675 9.304 8.342
0.9565 0.9560 0.9556 0.951
0.9364 1.0198 1.8846 1.4513
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the deposition potential. The obtained optical constants are tabulated in Table 2. It is found from the optical studies that there occurs an increase in energy band gap with increasing deposition potential of ZnSe films and this can be attributed to the enhancement of crystallite size and hence exciton confinement effect which was contributed by deposition potential. Photoluminescence (PL) spectra were recorded at room temperature using an excitation wavelength of 441 nm. The spectra show peaks at 2.4 and 2.56 eV (Fig. 10) respectively and the PL intensity was found to increase with decrease deposition potential. These peaks are assigned to the near band emission. Self activated centers arising from complexes of zinc vacancies and shallow donors (selenium interstitials) would occur around 2.5 eV [24]. In this peaks are associated with band gap values of ZnSe thin films estimated by conventional method. The PL emission from the undoped ZnSe has been attributed to the presence of native defects like zinc and selenium vacancies or interstitials, which are likely to be introduced during the growth process [25,26].
Fig. 10. Photoluminescence spectra of ZnSe thin film prepared at −850 mV vs SCE.
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