Electrodeposition and growth mechanism of SnSe thin films

Applied Surface Science 257 (2011) 2944–2949 Contents lists available at ScienceDirect Applied Surface Science journa l homepage: www.e lsev ier .com Electro nS Mustafa Department of a r t i c l Article history: Received 11 M Received in re Accepted 19 O Available onlin Keywords: SnSe Thin films Electrodeposit Surface morph ctroch nd ED f Sn a (SEM chara ecrea ted fi e film eases nd fo the fi n Co 1. Introdu Tin chalc various optical and optoelectronic applications. These compounds are also used as sensor and laser materials, thin films polariz- ers and thermoelectric cooling materials [1]. SnSe is a narrow band gap binary IV–VI semiconductor with an orthorhombic crys- tal structure. The uses of SnSe are: as memory switching devices, holographic Various me oration [4], ablation [7] have been u methods, h materials, w tal problem to electrode tion is a sim films of goo SnSe thi trochemica [13]. ECALE of combini ples of atom Atomic laye at UPD in ∗ Correspon E-mail add enom al–ch tion more positive potentials than the deposition equilibrium potential (Nernst potential), the electrode surface is partially or completely (up to an atomic layer) covered by a deposit. However, the overpo- tential deposition (OPD or bulk deposition) process is determined by electrode potential (overpotential), deposit growth kinetics 0169-4332/$ – doi:10.1016/j. recording systems, and infrared electronic devices [2]. thods, including vacuum evaporation [3], flash evap- hot wall epitaxy [5], reactive evaporation [6], laser , brush plating [8], and chemical bath deposition [9] sed for the synthesis of SnSe thin films. Most of these owever, are quite expensive and employ highly toxic hich create number of technological and environmen- s. On the other hand, some reports have been devoted position of SnSe thin films [10–14]. The electrodeposi- ple, economical and viable technique, which produces d quality for device applications [15,16]. n films on gold substrates were deposited by elec- l atomic layer epitaxy (ECALE) at room temperature , developed by Stickney and coworkers, is the result ng underpotential deposition (UPD) with the princi- ic layer epitaxy (ALE) to form a deposition cycle [17]. rs of a compound’s component elements are deposited a cycle, to directly form a compound. UPD is a sur- ding author. Tel.: +90 264 2956063; fax: +90 264 2955950. ress: [email protected] (I˙. S¸is¸man). and mechanism (2D or 3D), electroactive species concentration, and deposit–substrate and deposit–deposit interactions. OPD takes place at more negative potentials than the Nernst equilibrium potential. Briefly, UPDmay involve deposition onto substratewhile OPDwould involve deposition onto a substrate surfacemodified by an atomic layer, which was formed during the UPD process. Gener- ally, deposits reach more than one atomic layer in the OPD regions [18]. The main advantage of ECALE is the possibility of deposi- tion on substrates of any shape and at room temperature, which implies minimum diffusion in thin films, an important advantage for instance in superlattices. However, this method is very time- consuming and produces a large amount of dilute wastewater due to the rinsing of the substrate after each deposition. SnSe thin films were electrodeposited from the single-source (codeposition) at 55 ◦C [10,12]. Traditionally, semiconductor thin films have been deposited by the so-called induced codeposi- tion mechanism, where both elements are deposited at the same time from the single solution [19]. Stoichiometry is maintained by having the more noble element as the limiting reagent. Codepo- sition holds great promise if greater control can be achieved. At present, the main points of control are solution composition and the deposition potential. Recently, Öznülüer et al. have developed see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. apsusc.2010.10.096 deposition and growth mechanism of S Bic¸er, I˙lkay S¸is¸man ∗ Chemistry, Arts and Sciences Faculty, Sakarya University, 54187, Sakarya, Turkey e i n f o ay 2010 vised form 19 October 2010 ctober 2010 e 27 October 2010 ion ology a b s t r a c t Tin selenide (SnSe) thinfilmswereele solution containing SnCl2, Na2SeO3, a iors and the codeposition potentials o (XRD), scanning electron microscopy tion spectroscopy were employed to increased, the Se content in the films d be obtained at −0.50V. The as-deposi (1 11) plane. The morphologies of SnS ticles as the deposition potential incr via nucleation, growth of film layer a The optical absorption study showed Crow ction ogenides offer a range of optical band gaps suitable for face ph physic interac / locate /apsusc e thin films emicallydepositedontoAu(111) substrates fromanaqueous TA at room temperature (25 ◦C). The electrochemical behav- nd Se were explored by cyclic voltammetry. X-ray diffraction ), energy dispersive spectroscopy (EDS), and UV–vis absorp- cterize the thin films. When the electrodeposition potential sed. Itwas found that the stoichiometric SnSe thin films could lms were crystallized in the preferential orientation along the s could be changed from spherical grains to platelet-like par- . The SEM investigations show that the film growth proceeds rmation of needle-like particles on the overlayer of the film. lm has direct transition with band gap energy of 1.3 eV. pyright © 2010 Published by Elsevier B.V. All rights reserved. enon, depending on the substrate structure, substrate emical characteristics, and deposit (adatom)–substrate s. As a result of the UPD process, which takes place at M. Bic¸er, I˙. S¸is¸man / Applied Surface Science 257 (2011) 2944–2949 2945 an electrodepositionmethod, based on codeposition from the same solution of the precursors of the target compound at a constant potential, which is determined from the UPD potential of each pre- cursor [20]. This method has been successfully applied to obtain thin films o CdS [23]. As reported ele under the c knowledge, of SnSe thin to the elect lack of syst the effect o of SnSe thin the codepo voltammetr a constant c of Sn and fo of codeposi investigated 2. Experim Cyclic v performed nected to a room temp lar to a ball- (Alfa-Johns by Hamelin with sheet for optical m reference e was used as Solution Prior to eac N2 gas. Solu measureme position of (SnCl2·2H2O Merck), and solution wa EDTA was u used, it cou precipitate determined ence of EDT to −0.60V. films were room temp Characte niques. The Advance Po angle betwe films were JEOL, JSM-6 troscopy, w UV–vis abs 2401 spectr 3. Results In order elements, c Au(111) s -0.5 C u rr en t d en si ty A13 a) Cyc mV/s ion of 1) su wn ned ears of Sn ond 42V ile th oy (th s sca r red Sn in rs at Au(1 27]. A nsta ), an atomic layer of Sn is deposited at the electrode. If the de potential is kept constant at a potential within a range of to −0.45V (alloy region), multilayer is deposited on the sub- When the electrode potential is shifted less potentials than V (OPD region), more atomic layers of Sn is deposited on the ate than alloy region. comparison with Fig. 1, the CVs for an Au(111) electrode M SnCl2, and 12mM EDTA solution, are shown in Fig. 2a. thodic peak 1 at about −0.50V and second cathodic peak .58V) correspond to the alloy and bulk deposition of Sn, tively. Compared to noncomplexing solution in Fig. 1, these tion peaks are shifted more negative potentials. This is a indication of the binding of Sn2+ by EDTA4−. A similar as been observed for the deposition of Cd on an Au elec- n the presence of triethanolamine and cysteine as chelating ts [28]. Fig. 2b shows a cyclic voltammetric curve of Se4+ on 1) in the presence of EDTA. Based on previous reports [29], signment of these voltammetric peaks (I–V) is as follows: otential deposition of Se (I–III), bulk deposition of selenium d formation of H2Se (V) in a direct reduction by 6 electrons . In the reverse scan, the observed peak A is attributed to the ation. A mass loss occurs via formation of H2Se at peak V. er, the redeposition of Se occurs on the electrode surface via f PbS [20], CdS [21], Bi2Te3−ySey [22], and nanowires of a different work on semiconductor thin films, we have ctrodepositionof thinfilmsof Bi1−xSbx andBi2−xSbxTe3 onditions of both UPD and OPD, recently [24]. To our there are only a few studies on the electrodeposition films. Furthermore, these studies show the possibility rochemical synthesis of SnSe thin films, but there is a ematic studies explaining the growth mechanism and f applied potential on the composition and morphology films. In this work, the electrochemical behaviors and sition potentials of Sn and Se were explored by cyclic y. The stoichiometric SnSe thin films were obtained by odeposition potential which is suited alloy deposition rmation of H2Se. The growth mechanism and the effect tionpotential on the composition andmorphologywere . ental oltammetry and electrodeposition experiments were using a PAR model 2273 potentiostat/galvanostat con- three-electrode cell (K0269A Faraday Cage, PAR) at erature (25 ◦C). The Au(111) working electrode, simi- shaped droplet, was (111)-oriented single-crystal gold on Matthey, 99.995%) prepared as previously described [25]. An indium tin oxide (ITO)-coated glass substrate resistance of 10� cm−2 was used as working electrode easurements. In all electrochemical experiments, the lectrode was an Ag/AgCl/3M NaCl and a platinum wire counter electrode. s were prepared with deionized water (i.e., >18M�). h experiment, the solutions were purged with purified tions were not stirred during all the electrochemical nts and depositions. The solution for the electrode- SnSe thin films contains a mixture of 10mM SnCl2 , Sigma–Aldrich), 12mMEDTA(C10H14N2Na2O8·2H2O, 2.5mM Na2SeO3 (Na2SeO3·5H2O, Fluka). The pH of the s adjusted to 2.5±0.1 by using 0.2M H2SO4 solution. sed to form a SnEDTA2− complex. If SnCl2 was directly ld be hydrolyzed in aqueous solution and produce a of Sn(OH)Cl [26]. The deposition potential for SnSe was from cyclic voltammetry data of Sn and Se in the pres- A. The deposition potentials were in the range of −0.20 After each electrodeposition, the obtained SnSe thin rinsed with deionized water and then dried in air at erature. rizationof thefilmswas carriedoutwithdifferent tech- XRD patterns for the films were recorded by Rigaku wder X-ray diffractometer (�=1.54050 A˚) in the span of en 20◦ and 60◦. Surface morphologies of the deposited observed using a scanning electron microscopy (SEM), 060LV. EDS was performed on an Oxford Inca spec- hich was attached to SEM for composition analysis. orption spectrum was recorded on a Shimadzu UV–vis ophotometer in thewavelength range of 400–1100nm. and discussion to determine the codeposition potentials for the yclic voltammetry experiments were performed on ubstrates. The cyclic voltammograms (CVs) for an Fig. 1. ( rate 100 UPD reg Au(11 are sho is scan C1 app (UPD) corresp (at −0. Sn, wh the all trode i anothe tion of 3 occu Sn on et al. [ kept co region electro −0.38 strate. −0.45 substr For in 10m The ca (at −0 respec deposi direct shift h trode i reagen Au(11 the as underp (IV), an of Se4+ Se oxid Howev -0.4 -0.3 -0.2 -0.1 0.10 0.2 Potential/V vs Ag/AgCl C1 (a) (b) 250 µA cm-2 4 1 2 lic voltammograms of Au(111) in 10mM SnCl2 +0.2M H2SO4, sweep . (b) The thin line shows a 10-fold magnification of the CV-curve in the Sn. bstrate in solution with 10mM SnCl2 and 0.2M H2SO4 in Fig. 1. When the potential of the Au(111) electrode between 0.20V and −0.38V, the broad cathodic peak between −0.14 and −0.32V for a monolayer deposition (Fig. 1b). On the reverse scan, the stripping peak A1 s to dissolution of Sn UPD. Subsequent cathodic peak 1 ) corresponds to multiatomic layer (alloy) deposition of at anodic peak 4 corresponds to dissolution of Sn from in line in Fig. 1a). If the potential of the Au(111) elec- nned through more negative potentials than −0.45V, uctive peak 2 appears at −0.48V for the bulk deposi- the OPD region, and the corresponding oxidative peak −0.44V (thick curve). The electrochemical behaviors of 11) in this work are similar to that reported by Mao ccording to the CVs, if the potential of the electrode is nt at a potentialwithin a range of−0.14 to−0.32V (UPD 2946 M. Bic¸er, I˙. S¸is¸man / Applied Surface Science 257 (2011) 2944–2949 -0.7 C u rr en t d en si ty (a) 150 µA cm -2 1 3 4 A Fig. 2. Cyclic 2.5) containing the chemica H2SeO3 +2 Thus the be balanced performed f 2.5 at poten between th be deposite is increased −0.40 to −0 In order the Sn–Se b sitions of th by EDS, and atomic perc shift of the of SnSe thi SnCl2 +2.5m weak signal signals Sn a cates that t 48% Se, wh different re the similar XRD patter for 3h are p observed at Table 1 Properties of S Potential (V) −0.20 −0.35 −0.45 −0.50 −0.60 Fig. 3. EDS spectrum of SnSe thin film. tion peaks observed at 2� =30.3◦ and 31.2◦ correspond to the and (400) planes of the SnSe orthorhombic phase (JCPDS o. 32-1382), respectively. As clearly seen in Fig. 4a–c, the osited films were crystallized in the preferential orienta- ong the (111) plane. XRD analyses of different regions of Se thin films gave the similar results. Therefore, it may be ered that the entire film is crystallized in this orientation. In n, all the peak intensities increase with the negative shift depositing potential. This increase confirms that the higher t densities lead to the formation of well-crystallized SnSe. stallite size (D) was calculated from the full-width at half- um (FWHM) (ˇ) by using the Debye–Scherrer equation .9� cos � (2) � is thewavelengthof theX-rayused,ˇ is the FWHM,D is the lite size value, and � is half the angle between the incident e scattered X-ray beams. The crystallite sizes of deposits are in Table 1. It can be seen that the crystallite size slightly es w -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.9 Potential/V vs Ag/AgCl (b) 2 III IIIIV V voltammograms of Au(111) electrode in 12mM EDTA solution (pH : (a) 10mM SnCl2 and (b) 2.5mM Na2SeO3. Scan rate: 100mV/s. l reaction [30]: H2Se ↔ 3Se + 3H2O (1) competition between these two processes appears to . Based on the above results, if the electrodeposition is rom10mMSnCl2 +2.5mMNa2SeO3 +12mMEDTA, pH tial range of −0.20 to −0.40V, which stands for a region e deposition of Sn UPD and Se bulk, Se-rich films will d at the electrode. However, the Sn content in the films when the electrodeposition potential is changed from .50V (multilayer deposition of Sn). to get more information about the reduction process in inary solution, the potential dependence of the compo- e films deposited at different potentials were evaluated the results are listed in Table 1. It can be seen that the entage of Sn in the films increases with the negative diffrac (111) card n as-dep tion al the Sn consid additio of the curren The cry maxim D = 0 ˇ where crystal and th shown increas depositing potential. The representative EDS spectrum n film deposited at −0.50V in a solution of 10mM M Na2SeO3 +12mM EDTA, pH 2.5 is shown in Fig. 3. A from the Au atoms was recorded along with the strong nd Se atoms. Quantitative analysis of the spectrum indi- he atomic composition of the thin film is 52% Sn and ich is very close to 1:1 stoichiometry. EDS analyses of gions of the SnSe thin film onto Au(111) substrate gave results. The deposits were also analyzed by XRD. The ns of the films deposited at −0.35, −0.45, and −0.50V resented in Fig. 4a–c, respectively. The diffraction peak 2� =38.2◦ is due to the Au(111) substrate. The other nSe thin films deposited at different potentials. Atomic (%) Crystallite size (nm) Thickness (nm) Sn Se 23 77 – – 31 69 42 215 43 57 48 436 52 48 57 602 67 33 – – 20 In te n si ty / a. u . Fig. 4. XRD pa tials: (a) −0.35 ith the negative shift of the depositing potential. These (1 11 ) (4 00 ) Au (b) (c) 30 40 50 60 2 / degree (a) tterns of SnSe thin films electrodeposited for 3h at different poten- , (b) −0.45, and (c) −0.50V. M. Bic¸er, I˙. S¸is¸man / Applied Surface Science 257 (2011) 2944–2949 2947 Fig. 5. SEM im −0.35, (b) −0.4 results prov particles siz Fig. 5 sho thin films d ages of SnSe thin films deposited for 3h at applied potentials of (a) 5, and (c) −0.50V. ed that larger current densities led to growth of larger es when deposition potentials became more negative. ws SEM images from the surfaces of the prepared SnSe eposited at different potentials. The shape of the films Fig. 6. SEM im times: (a) 0.5, could be ch cle as the d the particle potential. S posited CdS ages of SnSe thin films deposited at −0.50V for various deposition (b) 1, and (c) 1.5h. anged from a spherical grain to a platelet-like parti- eposition potential increases. It was also observed that size of the films increases with increasing deposition imilar observations of the particle size of the electrode- e films dependence on the applied potential have been 2948 M. Bic¸er, I˙. S¸is¸man / Applied Surface Science 257 (2011) 2944–2949 0.4 A b so rb an ce 2 9 2 a Fig. 7. The plo posited at ITO reported by than the cry tion. From t fact compac films were efficiency, d = QM �nFA where Q is A is the dep number an Table1. It ca ative shift o images of S at −0.50V f Au(111) su of 100nm. W nanoparticl We perform at higher de crystals afte sition proceeds further, the needle-like crystals were transformed to bigger platelet-like crystals in good uniformity (Fig. 5c). SEM images recorded as a function of deposition time clearly indicate that the growth of SnSe proceeds via nucleation, growth layer, for- of n es. Th and7 esw SnSe e ex opt nves wave bsorp tion =˛d A is t and lying o[h� h� is ant w ds. T 0 0.1 0.2 0.3 400 600 800 1000 Wavelength (nm) 3 b mation particl 26, 49, increas of the time, w The were i in the gooda absorp 2.303A where other h by app ˛h� =A where a const the ban 0 1 2 1.51 2 h (eV) ( h ) x 10 (e V /c m ) Eg = 1.3 eV ts of (a) absorption and (b) (˛h�)2 −h� of SnSe thin film electrode- -coated glass substrate for 2h. Shen et al. [31]. However, the particle sizes are larger stallite sizes calculatedusing theDebye–Scherrer equa- hese results it is revealed that the SnSe particles are in t agglomeration of small grains. The thicknesses of the calculated using Faraday’s law, assuming 100% current (3) the electric charge, M is the formula weight of SnSe, osition area, � is the density of SnSe, n is the charge d F is the Faraday constant. The results are shown in nbe seen that thefilmthickness increaseswith theneg- f the depositing potential. Fig. 6a–c presents the SEM nSe thin films on Au(111) substrates electrodeposited or 0.5, 1, and 1.5h, respectively. It can be seen in Fig. 6a, rface is covered by the nanoparticles with average size hen the deposition time is increased to 1h, numerous es were formed on the top of SnSe overlayer (Fig. 6b). ed SEM experiments to investigate the films forming position times. The film surface consists of needle-like r the electrodeposition for 1.5h (Fig. 6c). As the depo- rial and 2 fo to direct ba is found to b direct band of the filmw of (˛h�)2 ve This is in go 4. Conclus SnSe thi aqueous su SnCl2, 2.5m and the co by cyclic vo lower Se co be changed deposition nucleation, like nanopa particles. Th (111) direc band gap of Acknowled Sakarya support of t References [1] K. Zweibe [2] T. Lindgre [3] D.T. Quan [4] J.P. Singh [5] J.P. Singh [6] K.J. John, [7] R. Teghil, A. Mele, A [8] B. Subram (2002) 42 [9] Z. Zainal, B 107 (20 eedle-like on the overlayer, and growth of platelet-like e thicknessesof the thinfilms inFig. 6a–cwere foundbe 1nm, respectively. It canbe seen that thefilmthickness hile the deposition time increase. Because the thickness thin films will be dependent on the electrodeposition pect to have thicker SnSe films at longer times. ical properties of the SnSe thin film deposited for 2h tigated by the UV–vis absorption spectrum measured length range of 400–1100nm (Fig. 7a). The film shows tion in the visible region. From the absorbancedata, the coefficient ˛ was calculated using Lambert’s law [32] (4) he optical absorbance and d is the film thickness. On the , the optical band gap of the thin film was determined the Tauc relationship given by: −Eg]n (5) the photon energy, Eg is the band gap energy and Ao is hich is related to the effective masses associated with he value of n is equal to 1/2 for a direct band gap mate- r indirect gap. The absorption (˛≥104 cm−1) is related nd transitions [33]. The value of absorption coefficient e of the order of 104 cm−1 for the film that supports the gap nature of the semiconductor. The optical band gap as determined from the extrapolation of the linear plot rsus h� at ˛=0 and it was found to be 1.3 eV (Fig. 7b). od agreement with the value reported earlier [34]. ions n films were electrodeposited at room temperature in lfuric acid electrolyte solutions consisting of 10mM M Na2SeO3, and 12mM EDTA. The electrochemistry deposition potentials of Sn and Se were investigated ltammetry. More negative deposition potential yielded ntent in the films. The morphologies of the films could from spherical grains to platelet-like particles as the potential increases. The growth of the film proceeds via growth of the smooth film layer, formation of needle- rticles on the top of layer, and growth of platelet-like e SnSe films showed a preferred orientation along the tion. These single phase SnSe films showed an optical 1.3 eV. gements University is gratefully acknowledged for the financial his work. l, Sol. Energy Mater. Sol. Cells 63 (2000) 375. n, M. Larsson, S. Lindquist, Sol. Energy Mater. Sol. Cells 73 (2002) 377. , Thin Solid Films 149 (1987) 197. , R.K. Bedi, Thin Solid Films 199 (1991) 9. , R.K. Bedi, J. Appl. Phys. 68 (1990) 2776. B. Pradeep, E. Mathai, J. Mater. Sci. 29 (1994) 1581. A. Santagata, V. Marotta, S. Orlando, G. Pizzella, A. Giardini-Guidoni, ppl. Surf. 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