Optical and structural study of electrodeposited zinc selenide thin films

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Materials Chemistry and Physics 106 (2007) 215–221

Optical and structural study of electrodeposited zinc selenide thin films A. Kathalingam a,∗ , T. Mahalingam b , C. Sanjeeviraja b a

Department of Physics, Periyar Maniammai College of Technology for Women, Vallam, Thanjavur 613403, Tamilnadu, India b Department of Physics, Alagappa University, Karaikudi 630003, India Received 24 November 2006; received in revised form 11 May 2007; accepted 21 May 2007

Abstract Electrochemical deposition and characterization of zinc selenide (ZnSe) thin films deposited onto tin oxide (SnO2 ) coated conducting glass plates from an aqueous bath containing ZnSO4 and SeO2 is discussed in this paper. The effect of electrolyte composition, deposition potential, pH and temperature on the properties of ZnSe films has been studied. The deposited ZnSe films have been characterized by X-ray diffraction (XRD), energy dispersive X-ray (EDX), scanning electron microscope (SEM) and optical absorption studies for their structural, compositional and optical properties. Raman spectroscopic and photoluminescence studies were also carried out and the results are discussed. The electrolyte composition plays a major role in the production of stoichiometric ZnSe films. Inclusion of excess elemental selenium in the film is unavoidable, however annealing improves the stoichiometry of the film. © 2007 Elsevier B.V. All rights reserved. Keywords: Semiconductor; Thin films; Zinc selenide; Electrodeposition; II–VI Compounds

1. Introduction Wide band gap semiconductor thin film heterostructures are extensively studied for optoelectronic applications, such as, light emitting and laser diodes. Particularly, ZnSe is an interesting II–VI compound semiconducting material, widely used in optoelectronic devices, because its band gap (2.7 eV) belongs to the visible region [1]. Therefore, there is currently a major interest in ZnSe based materials suitable for the fabrication of lightemitting devices operating in the blue-green region [2] and in the manufacture of optical components, mirrors, lenses etc. for IR lasers [3,4]. A number of methodologies are employed in the formation of high quality ZnSe thin films, including chemical vapor deposition [5], molecular beam epitaxy [6,7], atomic layer epitaxy [8], pulsed laser, metalorganic chemical vapor deposition [9], sputtering [10], and other advanced techniques [11–13]. However, there is an interest to investigate other approaches too, which could open new or supplementary possibilities in terms of device properties, structure or engineering. Electrodeposition and chemical bath depositions [14,15] are the alternative methods that are particularly adapted for the deposition of chalcogenide materials. Chemical bath depositions of

∗

Corresponding author. E-mail address: kathu [email protected] (A. Kathalingam).

0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.05.051

sulfides and selenides are already used for producing interfacial buffer layers in high efficiency thin film solar cells based in copper indium gallium diselenide. Among the direct wide-band semiconducting materials, the zinc chalcogenide compounds have been the objects of numerous studies concerning thin film electrodeposition from aqueous solutions. Recently, electrodeposition has emerged as a simple, economical, low temperature and viable technique, which could produce films of good quality for device applications [16]. The attractive features of this method are the convenience for producing large area devices and possibility to control the film thickness, morphology and stoichiometry of the films by readily adjusting the electrical parameters, as well as the composition of the electrolyte solution. The works available concerning the optical study of electrodeposited ZnSe films are less in number. Therefore we decided to deposit ZnSe thin films by electrodeposition and to study their properties. In this study, we have prepared a series of ZnSe thin films by electrodeposition from an aqueous bath containing ZnSO4 and SeO2 . The influence of growth conditions such as deposition potential, temperature and concentration of the constituents of the bath on crystallinity and composition of the film was studied. In order to characterize the ZnSe thin films for their structural and optical properties, scanning electron microscopy (SEM), X-ray diffraction (XRD, energy dispersive X-ray (EDX), photoluminescence (PL), Raman spectroscopy and optical absorption techniques were employed. Both absorp-

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tion and photoluminescence techniques were used to find out the optical band gap energy of the ZnSe films. 2. Experimental details A series of ZnSe films was prepared electrochemically by co-deposition of zinc and selenium using aqueous solutions of ZnSO4 and SeO2. Fluorine doped tin oxide (SnO2 ) covered conducting glass plates of resistance 15  cm−1 where used as substrate in this deposition. These conducting glasses of approximately 1 cm2 area were cleaned with detergent, dried and degreased with acetone and distilled water. The electrolyte bath contained aqueous solutions of 50–300 mM of ZnSO4 and 0.2–2 mM of SeO2 . The chemicals used in this deposition were of Analar® grade and they were used without further purification. Deposition of the films was carried out cathodically using a potentiostat (EG & G Princeton Applied Research, Model 362) with standard three-electrode system. ZnSe films were deposited at different potentials using graphite as counter electrode and saturated calomel electrode (SCE) as reference electrode. The deposition potential was varied in the range between −0.6 and −1.2 V versus SCE with different bath temperatures and pH between 2 and 3. Thickness of the film was measured using weight gain and ellipsometric methods. Structure of the films was analyzed using X-ray diffraction (XRD) by Bruker Discover D8 diffractometer using Cu k␣ radiation (λ = 0.15418 nm) and Philips scanning electron microscope (SEM) attached with energy dispersive X-ray (EDX). Optical absorption, photoluminescence, Raman spectra and compositional analysis were also carried out for the films grown at optimized condition. Optical absorption study was performed with a UV–vis–NIR Spectrophotometer (Hitachi) and room temperature Raman spectra were obtained by using He–Ne laser as an excitation source with a maximum power of 25 mW. Photoluminescence measurements were carried out in a closed cycle refrigeration system (Janis) using He–Cd laser as an excitation source. The luminescent radiation was collected and directed into a spectrometer and recorded by a photon counting system.

Fig. 1. Variation of film thickness with concentration of ZnSO4 , (a) 0.1 M, (b) 0.2 M, (c) 0.3 M and (d) 0.4 M. Bath, 1 mM SeO2 ; potential, −0.8 V; pH, 2.5; temperature, 70 ◦ C.

the adsorption of Zn2+ ion is playing vital role in obtaining stoichiometric films. If the adsorption sites were occupied by other species (i.e. H2 SeO3 or any) the rate of Zn2+ reduction was reduced compared to SeO2 reduction, thus Se clusters could be formed. If once the Se clusters have reached a certain size they react slowly to give ZnSe. Low concentration of selenous acid was found to give stoichiometric and good ZnSe films. However, too low concentration of the selenous acid (<0.5 mM) gives low deposition rate and poor quality of the film. 3.2. Structural studies

3. Result and discussion 3.1. Growth kinetics The films deposited with the concentrations around 300 mM ZnSO4 and 1 mM SeO2 at the potential around −0.8 V versus SCE, current density 1 mA cm−2 and temperature 70 ◦ C resulted in uniform and good quality films. The pH of the electrolyte solution played a major role in the deposition of the film and it was found that the pH value around 3 was suitable for the electrodeposition of ZnSe thin films. In the case of electrodeposition of ZnSe, the use of low concentration of selenous acid and high concentration of zinc salt is the usual approach [17], because the zinc is a less noble constituent of the compound. In this way, the rate of the following reaction can be lowered, which is one of the paths for the incorporation of excess elemental selenium in the film: H2 SeO3 + 4H+ + 4e− → Se + 3H2 O

X-ray diffraction studies were done in order to identify crystallinity and phases of the grown films. From the XRD profile, the inter planer spacing dh k l was calculated for the (1 1 1) plane using the Bragg’s relation: dh k l =

nλ 2 sin θ

(2)

where λ is the wavelength of the X-ray used, d the lattice spacing, n the order number and θ is the Bragg’s angle. The factor d is related to (h k l) indices of the plane and the dimension of the unit cells. The crystallite size (D) of the films were calculated from Scherrer’s formula from the full width at half maximum

(1)

The influence of bath concentration of ZnSO4 and SeO2 on the film thickness is shown in Figs. 1 and 2, respectively. The film thickness linearly increases with concentration of both precursors and deposition time. However the increase is restricted to a very low concentration range and it gets saturated at higher concentrations of both precursors. We observed less adherent, uncontrolled and non-reproducible growth rate for the concentrations of ZnSO4 and SeO2 above 400 and 2 mM, respectively. EDX measurements showed that the excess inclusion of elemental selenium in the deposited film is unavoidable. This is because,

Fig. 2. Variation of film thickness with concentration of SeO2 , (a) 0.5 mM, (b) 1.0 mM, (c) 1.5 mM, (d) 2.0 mM and (e) 2.5 mM. Bath; 0.3 M ZnSO4 ; potential, −0.8 V; pH, 2.5; temperature, 70 ◦ C.

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ratio of the films was determined by EDX analysis, it shows an excess of elemental selenium in all the films. Since the XRD patterns did not show any peak corresponding to selenium, it probably was in an amorphous phase. The best stoichiometry of the film was obtained at the deposition potential close to −0.8 V. This study shows that the deposition potential does not change the structural phase of the ZnSe films. However, an increase in the crystallinity of the films with deposition potential up to −0.8 V versus SCE is indicated. Therefore the deposition potential is fixed as −0.8 V versus SCE to deposit zinc selenide thin films.

(3)

3.2.2. Effect of bath composition The X-ray diffraction studies were also carried out on electrodeposited ZnSe thin films to understand the effect of concentration of selenium dioxide in the solution bath on the structural properties of thin films. The XRD patterns obtained for ZnSe thin films prepared under the selenium dioxide concentrations 10, 5 and 1 mM are shown in Fig. 4(a–c), respectively. It is clear from the Fig. 4(a) that the films prepared with 10 mM selenium dioxide concentration exhibits three peaks corresponding to the cubic phase of zinc selenide. In addition to these peaks, one peak corresponding to ZnSe hexagonal phase and one peak corresponding to ZnSeO3 are also present. A Se peak is always present for all the samples. The reduction of concentration of selenium dioxide to 5 mM does not alter the pattern much but change in the intensities of the peaks is observed. An additional ZnSe cubic peak of higher ‘2θ’ is also observed. At the concentration of 1 mM only

3.2.1. Effect of deposition potential Cyclic voltammetric studies revealed that zinc selenide films could be electrodeposited in the potential range −0.5 to −1.2 V versus SCE. Hence, zinc selenide thin films were prepared in this potential range. XRD patterns were obtained for zinc selenide thin films electrodeposited from a bath with composition of 300 mM zinc sulphate and 1 mM selenium dioxide under various deposition potentials. Fig. 3 shows typical X-ray diffraction patterns of the films deposited at −0.6, −0.7, −0.8, and −0.9 V versus SCE with bath temperature 70 ◦ C. The 2θ values of diffraction peaks observed at 27.3◦ , 45.3◦ , and 53.7◦ and 65.8◦ are attributed to the (1 1 1), (2 2 0), (3 1 1) and (4 0 0) cubic ZnSe planes, respectively. This confirms that polycrystalline ZnSe films are deposited in this work by comparing the peak positions 27.3◦ (d = 3.272), 45.3◦ (d = 2.0048), 53.7◦ (d = 1.7094) and 65.8◦ (d = 1.478) of the XRD patterns with the standard X-ray powder diffraction data file (card no. 5-522). All the peak intensities are found to increase when the deposition potentials decrease down to −0.8 V. When the deposition potential is further decreased to −0.9 V versus SCE, ZnSe diffraction peaks are decreased. This indicates that ZnSe thin films with polycrystalline morphology are formed in the potential range between −0.7 and −0.9 V versus SCE. The XRD peaks are sharpened and more intense at the deposition potential of −0.8 V, indicating the formation of polycrystalline morphology of good quality ZnSe films. No peaks corresponding to other impurities such as Zn or Se can be detected. The Zn/Se

Fig. 4. XRD patterns of the ZnSe films with various SeO2 concentration, (a) 10 mM, (b) 5 mM, and (c) 1 mM. Bath, 0.3 M ZnSO4 , 1 mM SeO2 ; potential, 0.8 V; pH, 2.5; temperature, 75 ◦ C; plating time, 1 h.

Fig. 3. XRD patterns of the films deposited at different potentials, (a) −0.6 V, (b) −0.7 V, (c) −0.8 V and (d) −0.9 V. Bath, 0.3 M ZnSO4 , 1 mM SeO2 ; pH, 2.5; temperature, 70 ◦ C; plating time, 1 h.

(β) of the peaks expressed in radians: D=

Kλ β cos θ

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Fig. 5. XRD patterns of the films with various bath temperature, (a) 40 ◦ C, (b) 50 ◦ C, (c) 60 ◦ C and (d) 70 ◦ C. Bath, 0.3 M ZnSO4 , 1 mM SeO2 ; potential, 0.8 V; pH, 2.5; plating time, 1 h.

cubic ZnSe peaks are observed. However, the pattern showed the selenium presence for all concentrations of SeO2 . These studies reveal that selenium dioxide concentration plays a key role in the formation of ZnSe thin films by electrodeposition method. Subsequent films were obtained with concentration of selenium dioxide with 1 mM. The increase of concentration of zinc sulphate concentration in the solution bath from 10 to 400 mM does not produce significant effect on the structure of zinc selenide thin films. The preferential orientation of these films is mostly along (1 1 1) and (2 2 2) planes. The (h h h) type of orientations is found to be predominant for all the selenium dioxide concentrations studied. 3.2.3. Effect of bath temperature The XRD patterns obtained for ZnSe thin films with a bath composition of 300 mM zinc sulphate and 1 mM selenium dioxide at a deposition potential of −0.8 V versus SCE under various bath temperatures are shown in Fig. 5(a–d). The pattern shown in Fig. 5(a) corresponds to 40 ◦ C bath temperature, which indicates the amorphous nature of the films. Films deposited at 50, 60, and 70 ◦ C are found to be crystalline in nature. Fig. 5(b–d) shows that the intensity of the peaks corresponding to zinc selenide cubic phase is increased with the rise of bath temperature. The

Fig. 6. XRD patterns of the films annealed at different temperatures, (a) as grown, (b) 200 ◦ C, (c) 300 ◦ C and (d) 400 ◦ C. Bath, 0.3 M ZnSO4 , 1 mM SeO2 ; potential, −0.8 V; pH, 2.5, temperature, 70 ◦ C; plating time, 1 h.

selenium peak begins to appear at a temperature above 70 ◦ C. This indicates that the excess selenium found in these films crystallizes above 70 ◦ C. At lower temperature the excess selenium presented may not be crystalline and hence the selenium peaks could not be detected. 3.2.4. Effect of annealing Fig. 6 shows the XRD pattern of the film annealed at various temperatures. The FWHM of the prominent peak was measured; using this, the variation of crystallite size was calculated, and it was found to increase in size with annealing temperature. Annealing the films at about 400 ◦ C for nearly 1 h showed significant change in the crystalline quality and stoichiometric ratio of the films. For example, the stoichiometric ratio 31/69 (Zn/Se) of the as grown ZnSe improved to 39/61 after annealing at 400 ◦ C for 1 h. The XRD patterns of all ZnSe thin films showed a preferred orientation along [1 1 1] plane. The [1 1 1] direction is the close-packing direction of the zinc blende structure. The crystallite size (D) of the as deposited and the annealed films were determined from the (1 1 1) peak using the Scherrer formula, and it is shown in the Table 1. This shows that there is an enhancement of crystallite size with annealing temperature.

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Table 1 Particle size with annealing temperature Annealing temperature (◦ C)

Crystallite size (nm)

As grown 200 300 400

26.5 28.0 31.0 33.5

3.3. Optical absorption studies Optical constants such as absorption coefficient, band gap energy play an important role in understanding the optoelectronic properties of semiconducting materials. The absorption coefficient (in cm−1 ) of the strong absorption region of thin films is calculated from their optical transmittance and reflectance values using the formula: α=

ln[T/(1 − R)2 ] d

(4)

where α is the absorption coefficient (cm−1 ), T the transmittance (%), R the reflectance(%), and d is the thickness of the film (cm). The type of the band-gap is identified using the absorption coefficient values from the following equation: hνα = A(hν − Eg)m

(5)

where Eg-band gap of the material, hν-photon energy and A is an energy dependent constant. Substituting the value of ‘m’ as 1/2, 2, 3/2 and 3 the direct allowed, indirect allowed, forbidden direct and forbidden indirect transitions, respectively are calculated. The plots drawn for (αhν)2 versus hν, (αhν)1/2 versus hν, (αhν)2/3 versus hν and (αhν)1/3 versus hν give an intercept with hν axis, which directly gives band gap energy of the respective transition. In order to characterize the optical quality of the grown ZnSe films, the room temperature optical absorption was measured in the optical edge region. The absorption edge is very sharp and is located at about 465 nm with the transmission around 50%, it is found to be in good agreement with the energy band gap of 2.67 eV corresponding to that the reported value [18]. It also reveals a high transmission of 90% in the infrared region (≥700 nm) and a low transmission of 40% at 400 nm. Annealing of the films produces a little effect on the energy of the absorption edge (result not shown); it shifts the absorption edge to the longer wavelengths, which must be attributed to the growth of ZnSe crystal grains and consequent decrease of the quantum size effects. Fig. 7 shows the α2 versus hν plot of ZnSe film. The linear dependence shown by α2 with hν indicates that this transition is direct. The band gap (Eg) value of the grown film has been calculated from the linear fit of the α2 versus hν plot, and it gives a value of 2.66 eV [19]. It is known that the optical band gap of polycrystalline ZnSe semiconducting thin film can be higher than that of bulk ZnSe (Eg = 2.58 eV) due to quantum size effects [20]. As the film grows thicker there is ZnSe particulate adsorption at the film surface-giving rise to scattering loses, which is increasing with decreasing of wavelength.

Fig. 7. Variation of α2 vs. hν for ZnSe films.

3.4. Photoluminescence and Raman spectroscopical studies Photoluminescence measurement is a powerful and sensitive tool for studying the effects of purification and contamination of the film in the growth process [21]. Photoluminescence study was carried out at low temperature (10 K) for the films using He–Cd laser as an exciting source and a grating monochromator. The actual power used is less than 10 mW in order to avoid heating of the samples. Fig. 8 Shows the PL spectra measured on the films of about 1.2 ␮m thicknesses with various Zn/Se ratios in the energy range 2.75–2.8 eV. In the excitonic region, the luminescence of free exciton (EF ), the donor bound exci-

Fig. 8. Typical photoluminescence spectra of ZnSe film at the temperature of 10 K with different Zn/Se ratios 0.8 and 1.0.

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Fig. 9. Raman spectra of ZnSe film with various thickness (450, 750, 920 and 1200 nm).

ton (I2 ), the acceptor bound exciton (I1 ) and an exciton bound to a deep acceptor (I1c ) emissions are observed [22]. The free exciton peak of 442.5 nm is equal to the value of 2.8015 eV, The observed strong peaks I2 at 443 nm (2.7980 eV) and I1 at 444 nm (2.7935 eV) are believed to be the peaks bounded to the neutral donor. Each of the peaks I2 and I1 represents the upper and lower polaritons, respectively [23]. With decreasing Zn/Se ratio, the line (I1c ) is boosted, demonstrating that this deeply bound exciton line correlated with Zn vacancies. Raman spectra of ZnSe for different thickness are presented in Fig. 9; the ZnSe LO phonon frequency is blue shifted [24] upto the thickness of 920 nm relative to that of bulk ZnSe but after 920 nm no shift of peak is observed. This shift of Raman peak with the increase of film thickness may be due the change in interference because of increased thickness and lattice deformation of the film. Raman spectra of the as-grown and annealed ZnSe films were also obtained (result not shown). It presented a peak observed at 252 cm−1 ; it is due to the longitudinal optical (LO) phonon of ZnSe [25]. It confirms the formation of polycrystalline ZnSe films. The high sharp peak obtained for film annealed at 300 ◦ C shows reduction of full width at half maximum (FWHM). It attributes the increase of crystallite size due to annealing. 3.5. Morphological study The microstructure of the films was investigated by scanning electron microscopy (SEM). In the optimized condition the surface of the film is smooth showing grains well covering the substrate. The cross sectional view (Fig. 10a) of the ZnSe film does not show presence of any voids or pinholes. The as grown films do not present well-defined grain edges due to the excess selenium (Fig. 10b). Isolated islands are found, some larger spots are also observed but actually they are the aggregation of the smaller islands. Differences in terms of chemical composition are also observed in the morphology of the films.

Fig. 10. SEM picture of ZnSe film, (a) cross sectional view, (b) Zn/Se ratio 0.9, (c) Zn/Se ratio 1.2.

Fig. 10c shows the film deposited at −1.0 V with Zn/Se ratio 1.2 in the film, it has presented a needle like morphology with dendrite growth pattern. 4. Conclusion It was possible to grow ZnSe thin films from solution by appropriate selection of the growth parameters. The as grown films presented excellent adherence and relatively good mor-

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phological and crystalline properties as inferred from SEM and XRD analysis The bath temperature did not show any significant change in the XRD pattern. Hence it is attributed that there is no effect of deposition temperature in the crystalline size. It has been found that the ratio of [Zn2+ ]:[SeO2 ] in the bath should be high in order improve Zn/Se ratio in the film. Annealing has resulted improved stoichiometry of the films. PL excitonic emission and LO phonon in Raman spectrum confirmed the compound formation of ZnSe. The grown ZnSe thin film has a blue emission characterization and hence it can be used to short wavelength optoelectronic devices. The incorporation of elemental selenium is unavoidable but annealing allows its elimination for some extent and thus improves the Zn/Se ratio. Energy band gap of the film is 2.66 eV and it is quite close to the reported value of 2.7 eV. References [1] Huanyoug Li, Wanqi Jie, J. Cryst. Growth 257 (2003) 110. [2] S. Guha, H. Munekata, F.K. LeGouse, L. Lchang, Appl. Phys. Lett. 60 (1992) 3220. [3] Guozhen Shen, Dichen, Kaibin Tang, Yitai Qian, J. Cryst. Growth 257 (2003) 276. [4] G.J. Yi, Radomsky, G.F. Neumark, J. Cryst. Growth 138 (1994) 208. [5] A. Rumberg, Ch. Sommerhalter, M. Toplak, A. Jager-Waldau, M.Ch. LuxSteiner, Thin Solid Films 361/362 (2000) 172. [6] L. Vanzetti, A. Bonanni, G. Bratina, L. Sorba, A. Franciosi, M. Lomascolo, D. Greco, R. Cingolani, J. Cryst. Growth 150 (1995) 765. [7] Ziqiang Zhu, Kazuhisa Takebayashi, Takafumi Yao, Yasumasa Okada, J. Cryst. Growth 150 (1995) 797–802.

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