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Electrodeposition of stoichiometric polycrystalline ZnTe on n+-GaAs and Ni–P
Materials Chemistry and Physics 66 (2000) 219–228
Electrodeposition of stoichiometric polycrystalline ZnTe on n+-GaAs and Ni–P B. Bozzini a,∗ , C. Lenardi b , N. Lovergine a a
Dipartimento di Ingegneria dell’Innovazione, Istituto Nazionale per la Fisica della Materia (INFM), Università di Lecce, Via Arnesano, I-73100 Lecce, Italy b Commission of the European Union, Joint Research Centre, I-21020 Ispra, Italy
Abstract An acid aqueous sulphate electrolyte is proposed for the low temperature direct electrochemical growth of single-phase polycrystalline ZnTe. Single-crystal n+ -GaAs and amorphous electroless Ni–P were used as substrates. The relationship between electrochemical growth conditions and the crystalline structure of as-deposited ZnTe were disclosed and correlated to the cathode chemistry during the growth process. Under suitable plating conditions the removal of tellurium (Te) excess from the deposit can be achieved, resulting in stoichiometric ZnTe. The nucleation of ZnTe was assessed through morphology observations by scanning electron microscopy: an instantaneous type prevails on GaAs and a progressive one on amorphous Ni–P. Chemical depth profiles of Zn and Te were investigated by X-ray photoelectron spectrometry measurements. © 2000 Elsevier Science S.A. All rights reserved. Keywords: ZnTe; Electrodeposition; XPS
1. Introduction Polycrystalline ZnTe films are of potential interest for the low temperature growth of transparent conductive windows in solar cell fabrication. A comprehensive list of other perspective applications of ZnTe has been reviewed in [1,2]. Additionally, electrodeposited polycrystalline ZnTe can be used as a passivating layer in CdTe-based bulk crystals for RT X-ray detectors: as electrodeposition from aqueous electrolytes is a low-temperature process, this makes it suitable for the above use. However, the low temperature advantage is lost if the material needs to be heat-treated after deposition to adjust the Zn/Te stoichiometry, as pointed out in [1]; it is, therefore, strongly advisable to achieve the direct deposition of ZnTe without an annealing stage. Furthermore, the electrochemical process allows the coating over large areas with good lateral control of the material quality; whilst such characteristic is readily achievable by electrodeposition, it is not yet possible or difficult with other state-of-the-art low temperature methods. We report on the possibility of obtaining polycrystalline ZnTe films by a single-step electrodeposition process from a highly acidic solution of TeO2 , ZnO and H2 SO4 . With the proposed electrochemical process it is not necessary to work with the high pH (≈4) solutions, as proposed in [1,2]. ∗ Corresponding author. Tel.: +39-832-320325; fax: +39-832-320525. E-mail address: [email protected] (B. Bozzini).
In this latter case, the bath displays unstable precipitation. This study is complemented by the results of X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS) and energy dispersive X-ray spectrometry (EDX) measurements performed on the present ZnTe samples.
2. Materials and experimental methods 2.1. Bath A stock solution of Te4+ was obtained by dissolving 20 mg of TeO2 in 125 ml of 96% H2 SO4 . After the dissolution process was carried out with heating at 60◦ C and stirring overnight, the solution was subsequently diluted with distilled and deionised water to 500 ml (nominal [Te4+ ]=0.25×10−3 mol dm−3 ). This highly acidic stock solution is extremely stable against precipitation of Te compounds after complete dissolution; the solution was tested at intervals of 15 days for 6 months with a Coulter photon autocorrelation spectrometer and no differences were observed between the Te4+ -bath and a blank solution containing H2 SO4 and water. This procedure is extremely simple and reproducible. The stock solution of Zn2+ was an aqueous solution 2 mol dm−3 in ZnO and 2.25 mol dm−3 in H2 SO4 . The ZnTe solution was prepared by blending suitable amounts of the Te4+ and Zn2+ stock solutions and diluting with water to a final nominal concentration of [Te4+ ]=25×10−6 mol dm−3
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and [Zn2+ ]=20×10−3 mol dm−3 . The working temperature of the bath was 90±0.5◦ C. 2.2. Electrodes The working electrodes we used in this work were: amorphous Ni–P (P 9%) coatings obtained by electroless plating (∼20 m) from a bath described elsewere [3] and single-crystal GaAs (1 0 0) n+ wafers. The counter electrode was a platinised Ti expanded mesh electrode. The reference electrode was Ag/AgCl, all potentials are reported against the Ag/AgCl electrode. 2.3. Electrochemical experiments Potentiostatic and galvanostatic electrochemical growth procedures were used for the preparation of the samples. All the experiments were carried out with a natural-convection cell. The hydrodynamic behaviour and current distribution were established in previous work [4]. Deposit thicknesses for XRD investigations were typically ∼5 m. 2.4. Characterisation of the films The morphology of nuclei and fully developed films was studied by SEM, whereas the structure of ZnTe deposits was studied by XRD using a thin film and powder diffractometers. The thickness of the samples was measured by laser interference profilometry. The chemical environment of Zn and Te in the coatings was investigated by XPS analysis. The measurements were performed with a Riber Nanoscan using the Al K␣ line (1486.6 eV) with an overall energy resolution of 0.5 eV. All spectra were collected under the same operating conditions. All the quoted peak positions were determined by non-linear least squares fitting.
3. Results and discussion 3.1. Electrochemical growth and crystallographic structure Electrochemical growth experiments can be carried out both potentiostatically and galvanostatically, since a single-valued c.d. versus potential curve was obtained. Control of the electrodeposition process is in principle easier in the potentiostatic mode, since the growth process shows a form of cathodic inhibition; nevertheless, this growth mode can be applied reliably only if low-resistance cathodic substrates are employed, which is not the case for many of the semiconducting substrates of interest. Thus both approaches were adopted in this work. The main electrochemical processes affecting the structure of the deposited film are listed in the following sequence which occurs as the cathode potential is increased:
1. Overpotential electrodeposition of Te at potentials low enough to rule out Zn underpotential deposition (about >−50 mV). 2. Overpotential electrodeposition of Te with concomitant Zn underpotential deposition, but without H2 Te evolution (about −550 to −50 mV). 3. Same as above, but with concomitant H2 Te evolution (about −950 to −550 mV). 4. Overpotential electrodeposition of Te, H2 Te and Zn (about <−1000 mV). The different processes give rise to different deposit compositions and structures: 1. Single-phase hexagonal Te. 2. Two-phase cubic ZnTe and hexagonal Te. 3. Single-phase cubic ZnTe. 4. Two-phase cubic ZnTe and hexagonal Zn. Examples of XRD structures of films grown in these processes are reported in Figs. 1, 2 and 4. In Fig. 1, we report the XRD spectrum of a film grown potentiostatically at −450 mV (process 2): reflections from the two relevant phases can be observed together with the amorphous peak around the Ni (1 1 1) reflection, as it is typical for the Ni–P substrate. In Fig. 2, we show the XRD spectrum of a film grown galvanostatically at 1.5 mA cm−2 (process 3), a minor amount of Te can be observed, but this-as further proved in Section 3.2 by XPS-is a surface effect due to the etching by the aggressive plating solution leading to a selective dissolution of Zn, i.e. a sort of ‘dezincification’, a typical corrosion form of Zn alloys with more noble elements. This conclusion is consistent with literature work on acidic ZnTe etching [5–7] performed by XPS and AFM. Powder XRD of the same sample (Fig. 3) shows only traces of the hexagonal Te phase (together with the amorphous Ni–P peak and a Cu peak due to the substrate onto which Ni–P was plated), which supports the view that the Te phase formation is a surface effect. In Fig. 4, we show the XRD spectrum of a film grown galvanostatically at 2.5 mA cm−2 (process 4), reflections from the two relevant phases above can be observed, with a marked predominance of the hexagonal Zn phase. The nucleation of electrochemically grown ZnTe onto Ni–P and GaAs was studied by performing short (2 min) and prolonged (30 min) galvanostatic electrodepositions at 1.5 mA cm−2 and comparing the resulting growth morphologies. SEM images are reported in Figs. 5–8. Figs. 5 and 7 correspond to short deposition times, whilst Figs. 6 and 8 correspond to prolonged deposition times for the Ni–P and GaAs cathodes, respectively. Transient potentiostatic measurements (for a presentation of the method see, e.g., [8]), which will be reported elsewere, support the interpretation that the ZnTe nucleation is of the instantaneous type for n+ -GaAs and polycrystalline Cu substrates, while it is of the progressive type for amorphous Ni–P substrates (which are covered by a layer of environmentally formed p-type semiconducting NiO).
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Fig. 5. SEM micrograph of a ZnTe deposit obtained galvanostatically at 1.5 mA cm−2 , deposition time: 2 min, substrate: electroless Ni–P.
Very tiny spheroidal nuclei form onto Ni–P, the surface coverage being not complete after 2 min of electrodeposition (Fig. 5), in this case laser interferometry does not yield a quantitative value for the deposit thickness. However, after 30 min the surface coverage is complete and thickness builds up (∼5 m), the shape and dimensions of nuclei remaining unchanged (Fig. 6). A comparatively lower number of very fibrous nuclei forms instantaneously on GaAs (Fig. 7)-laser interferometry gives a very rough profile with thicknesses of 3±2 m in this case, and subsequently grows (Fig. 8) up to a thickness of ∼4.25±2.5 m after 30 min; in this case, the overall
morphology of the layers seems to be due to the growth and clustering of the initial nuclei. Apparently, the surface of Ni–P and GaAs offers a very different number of nucleation centres for the electrodeposition of ZnTe; a more thorough understanding of the nature of the respective native oxides and of the crystalline nature of the nuclei is needed for a proper explanation. The profoundly different morphologies (uncorrelated spherules and bunches of fibres) can in principle both stem from the fact that lateral growth is hindered, possibly by the simultaneous formation of gaseous H2 Te; TEM evidence of the structure of spherules and fibres would be required for a full insight in these phenomena.
Fig. 6. SEM micrograph of a ZnTe deposit obtained galvanostatically at 1.5 mA cm−2 , deposition time: 30 min, substrate: electroless Ni–P.
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Fig. 7. SEM micrograph of a ZnTe deposit obtained galvanostatically at 1.5 mA cm−2 , deposition time: 2 min, substrate: electroless n+ -GaAs.
3.2. XPS XPS data show that Te 3d5/2 peak position of the native surface shifts from 573.05 to 572.75 eV for the sputter etched surface (Ar ions, 3 keV, 0.20 A, etch. time 150 min), where the contribution of TeO2 at 576.2 eV is almost completely removed. The Te 3d5/2 XPS spectra are reported in Fig. 9. The metallic component of Te results to be more present in the outer layers and decreases with depth, at which the intermetallic compound becomes predominant. XPS data on elemental tellurium and bulk ZnTe were reported by [9,10]. The trend we observed as a function of the sputtering time
is consistent with data presented in [10]; a proper trend can not be assessed from the data of [9] due to the reported error bars associated with the peak positions. No appreciable peak shifts were observed for Zn 2p3/2 as shown in Fig. 10. Even without a rigorous quantitative analysis it is obvious that the Zn content in the native surface is markedly lower than in the bulk. Peak positions suggest Zn is present as ZnO on the surface, whilst in the bulk Zn is principally bonded to Te [11]. In Fig. 11, we report the O 1s XPS spectra as a function of sputtering time. The surface oxygen peak is shifted to higher binding energy with respect to the bulk. This is coherent with phenomenological results on the chemical etching of
Fig. 8. SEM micrograph of a ZnTe deposit obtained galvanostatically at 1.5 mA cm−2 , deposition time: 30 min, substrate: electroless n+ -GaAs.
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Fig. 9. Te 3d5/2 XPS spectra of native and Ar+ -etched ZnTe film grown galvanostatically at 1.5 mA cm−2 .
Fig. 10. Zn 2p3/2 XPS spectra of native and Ar+ -etched ZnTe film grown galvanostatically at 1.5 mA cm−2 .
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which TeO2 and ZnO are dissolved. With the electrochemical techniques described in this work, it is not necessary to work at higher pH, where the bath displays an unstable precipitation behaviour, in order to achieve the desired structure and composition. The electrochemical growth of single-phase ZnTe was carried out onto conducting (amorphous Ni–P) and semiconducting (n+ -GaAs) cathodes. The presence of elemental Te in the ZnTe deposits obtained under optimal plating conditions can be detected, but it is confined to the top layer owing to an etching by the bath; this conclusion was based on XRD, XPS and EDX data. References
Fig. 11. O 1s XPS spectra of native and Ar+ -etched ZnTe film grown galvanostatically at 1.5 mA cm−2 .
ZnTe reported in [7,12] and can be interpreted in terms of hydrous oxides present on the surface [11].
4. Conclusions In this study, we reported of the possibility of obtaining polycrystalline ZnTe films by a single-step electrodeposition process from a highly acidic aqueous solution H2 SO4 in
[1] M. Neumann-Spallart, Ch. Königstein, Thin Solid Films 265 (1995) 33–39. [2] Ch. Königstein, M. Neumann-Spallart, J. Electrochem. Soc. 145 (1998) 337–343. [3] B. Bozzini, Ph.D. Thesis, Electrochemical Engineering, Politecnico di Milano, Italy, 1994. [4] B. Bozzini, F. Pavan, P.L. Cavallotti, G. Bollini, Trans. IMF 75 (1997) 175–181. [5] A. Kita, M. Ozawa, C.D. Gutleben, Appl. Surface Sci. 101 (1996) 652–655. [6] C.M. Sotomayor-Torres, A.P. Smart, M. Watt, M.A. Foad, K. Tsutsui, C.D.W. Wilkinson, J. Electron. Mater. 23 (1994) 289–298. [7] M.A. George, M. Azoulay, H.N. Jayatirtha, A. Burger, W.E. Collins, E. Silberman, Surf. Sci. 296 (1993) 231. [8] T. Vargas, R. Varma. in: J.R. Selman (Eds.), Techniques for Characterization of Electrodes and Electrochemical Processes, Wiley, New York, 1991, pp. 717–760. [9] W.E. Swartz, K.J. Wynne, D.M. Hercules, Anal. Chem. 43 (1971) 1884. [10] D.W. Langer, C.J. Vesely, Phys. Rev. B. 2 (1970) 4885. [11] D. Briggs, M. P. Seah, Practical Surface Analysis, Vol. 1, Wiley, Chichester, UK, 1990, pages 501 and 608. [12] A. Kita, M. Ozawa, C.D. Gutleben, Appl. Surf. Sci. 101 (1996) 652–655.
Electrodeposition of stoichiometric polycrystalline ZnTe on n+-GaAs and Ni–P B. Bozzini a,∗ , C. Lenardi b , N. Lovergine a a
Dipartimento di Ingegneria dell’Innovazione, Istituto Nazionale per la Fisica della Materia (INFM), Università di Lecce, Via Arnesano, I-73100 Lecce, Italy b Commission of the European Union, Joint Research Centre, I-21020 Ispra, Italy
Abstract An acid aqueous sulphate electrolyte is proposed for the low temperature direct electrochemical growth of single-phase polycrystalline ZnTe. Single-crystal n+ -GaAs and amorphous electroless Ni–P were used as substrates. The relationship between electrochemical growth conditions and the crystalline structure of as-deposited ZnTe were disclosed and correlated to the cathode chemistry during the growth process. Under suitable plating conditions the removal of tellurium (Te) excess from the deposit can be achieved, resulting in stoichiometric ZnTe. The nucleation of ZnTe was assessed through morphology observations by scanning electron microscopy: an instantaneous type prevails on GaAs and a progressive one on amorphous Ni–P. Chemical depth profiles of Zn and Te were investigated by X-ray photoelectron spectrometry measurements. © 2000 Elsevier Science S.A. All rights reserved. Keywords: ZnTe; Electrodeposition; XPS
1. Introduction Polycrystalline ZnTe films are of potential interest for the low temperature growth of transparent conductive windows in solar cell fabrication. A comprehensive list of other perspective applications of ZnTe has been reviewed in [1,2]. Additionally, electrodeposited polycrystalline ZnTe can be used as a passivating layer in CdTe-based bulk crystals for RT X-ray detectors: as electrodeposition from aqueous electrolytes is a low-temperature process, this makes it suitable for the above use. However, the low temperature advantage is lost if the material needs to be heat-treated after deposition to adjust the Zn/Te stoichiometry, as pointed out in [1]; it is, therefore, strongly advisable to achieve the direct deposition of ZnTe without an annealing stage. Furthermore, the electrochemical process allows the coating over large areas with good lateral control of the material quality; whilst such characteristic is readily achievable by electrodeposition, it is not yet possible or difficult with other state-of-the-art low temperature methods. We report on the possibility of obtaining polycrystalline ZnTe films by a single-step electrodeposition process from a highly acidic solution of TeO2 , ZnO and H2 SO4 . With the proposed electrochemical process it is not necessary to work with the high pH (≈4) solutions, as proposed in [1,2]. ∗ Corresponding author. Tel.: +39-832-320325; fax: +39-832-320525. E-mail address: [email protected] (B. Bozzini).
In this latter case, the bath displays unstable precipitation. This study is complemented by the results of X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS) and energy dispersive X-ray spectrometry (EDX) measurements performed on the present ZnTe samples.
2. Materials and experimental methods 2.1. Bath A stock solution of Te4+ was obtained by dissolving 20 mg of TeO2 in 125 ml of 96% H2 SO4 . After the dissolution process was carried out with heating at 60◦ C and stirring overnight, the solution was subsequently diluted with distilled and deionised water to 500 ml (nominal [Te4+ ]=0.25×10−3 mol dm−3 ). This highly acidic stock solution is extremely stable against precipitation of Te compounds after complete dissolution; the solution was tested at intervals of 15 days for 6 months with a Coulter photon autocorrelation spectrometer and no differences were observed between the Te4+ -bath and a blank solution containing H2 SO4 and water. This procedure is extremely simple and reproducible. The stock solution of Zn2+ was an aqueous solution 2 mol dm−3 in ZnO and 2.25 mol dm−3 in H2 SO4 . The ZnTe solution was prepared by blending suitable amounts of the Te4+ and Zn2+ stock solutions and diluting with water to a final nominal concentration of [Te4+ ]=25×10−6 mol dm−3
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and [Zn2+ ]=20×10−3 mol dm−3 . The working temperature of the bath was 90±0.5◦ C. 2.2. Electrodes The working electrodes we used in this work were: amorphous Ni–P (P 9%) coatings obtained by electroless plating (∼20 m) from a bath described elsewere [3] and single-crystal GaAs (1 0 0) n+ wafers. The counter electrode was a platinised Ti expanded mesh electrode. The reference electrode was Ag/AgCl, all potentials are reported against the Ag/AgCl electrode. 2.3. Electrochemical experiments Potentiostatic and galvanostatic electrochemical growth procedures were used for the preparation of the samples. All the experiments were carried out with a natural-convection cell. The hydrodynamic behaviour and current distribution were established in previous work [4]. Deposit thicknesses for XRD investigations were typically ∼5 m. 2.4. Characterisation of the films The morphology of nuclei and fully developed films was studied by SEM, whereas the structure of ZnTe deposits was studied by XRD using a thin film and powder diffractometers. The thickness of the samples was measured by laser interference profilometry. The chemical environment of Zn and Te in the coatings was investigated by XPS analysis. The measurements were performed with a Riber Nanoscan using the Al K␣ line (1486.6 eV) with an overall energy resolution of 0.5 eV. All spectra were collected under the same operating conditions. All the quoted peak positions were determined by non-linear least squares fitting.
3. Results and discussion 3.1. Electrochemical growth and crystallographic structure Electrochemical growth experiments can be carried out both potentiostatically and galvanostatically, since a single-valued c.d. versus potential curve was obtained. Control of the electrodeposition process is in principle easier in the potentiostatic mode, since the growth process shows a form of cathodic inhibition; nevertheless, this growth mode can be applied reliably only if low-resistance cathodic substrates are employed, which is not the case for many of the semiconducting substrates of interest. Thus both approaches were adopted in this work. The main electrochemical processes affecting the structure of the deposited film are listed in the following sequence which occurs as the cathode potential is increased:
1. Overpotential electrodeposition of Te at potentials low enough to rule out Zn underpotential deposition (about >−50 mV). 2. Overpotential electrodeposition of Te with concomitant Zn underpotential deposition, but without H2 Te evolution (about −550 to −50 mV). 3. Same as above, but with concomitant H2 Te evolution (about −950 to −550 mV). 4. Overpotential electrodeposition of Te, H2 Te and Zn (about <−1000 mV). The different processes give rise to different deposit compositions and structures: 1. Single-phase hexagonal Te. 2. Two-phase cubic ZnTe and hexagonal Te. 3. Single-phase cubic ZnTe. 4. Two-phase cubic ZnTe and hexagonal Zn. Examples of XRD structures of films grown in these processes are reported in Figs. 1, 2 and 4. In Fig. 1, we report the XRD spectrum of a film grown potentiostatically at −450 mV (process 2): reflections from the two relevant phases can be observed together with the amorphous peak around the Ni (1 1 1) reflection, as it is typical for the Ni–P substrate. In Fig. 2, we show the XRD spectrum of a film grown galvanostatically at 1.5 mA cm−2 (process 3), a minor amount of Te can be observed, but this-as further proved in Section 3.2 by XPS-is a surface effect due to the etching by the aggressive plating solution leading to a selective dissolution of Zn, i.e. a sort of ‘dezincification’, a typical corrosion form of Zn alloys with more noble elements. This conclusion is consistent with literature work on acidic ZnTe etching [5–7] performed by XPS and AFM. Powder XRD of the same sample (Fig. 3) shows only traces of the hexagonal Te phase (together with the amorphous Ni–P peak and a Cu peak due to the substrate onto which Ni–P was plated), which supports the view that the Te phase formation is a surface effect. In Fig. 4, we show the XRD spectrum of a film grown galvanostatically at 2.5 mA cm−2 (process 4), reflections from the two relevant phases above can be observed, with a marked predominance of the hexagonal Zn phase. The nucleation of electrochemically grown ZnTe onto Ni–P and GaAs was studied by performing short (2 min) and prolonged (30 min) galvanostatic electrodepositions at 1.5 mA cm−2 and comparing the resulting growth morphologies. SEM images are reported in Figs. 5–8. Figs. 5 and 7 correspond to short deposition times, whilst Figs. 6 and 8 correspond to prolonged deposition times for the Ni–P and GaAs cathodes, respectively. Transient potentiostatic measurements (for a presentation of the method see, e.g., [8]), which will be reported elsewere, support the interpretation that the ZnTe nucleation is of the instantaneous type for n+ -GaAs and polycrystalline Cu substrates, while it is of the progressive type for amorphous Ni–P substrates (which are covered by a layer of environmentally formed p-type semiconducting NiO).
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Fig. 5. SEM micrograph of a ZnTe deposit obtained galvanostatically at 1.5 mA cm−2 , deposition time: 2 min, substrate: electroless Ni–P.
Very tiny spheroidal nuclei form onto Ni–P, the surface coverage being not complete after 2 min of electrodeposition (Fig. 5), in this case laser interferometry does not yield a quantitative value for the deposit thickness. However, after 30 min the surface coverage is complete and thickness builds up (∼5 m), the shape and dimensions of nuclei remaining unchanged (Fig. 6). A comparatively lower number of very fibrous nuclei forms instantaneously on GaAs (Fig. 7)-laser interferometry gives a very rough profile with thicknesses of 3±2 m in this case, and subsequently grows (Fig. 8) up to a thickness of ∼4.25±2.5 m after 30 min; in this case, the overall
morphology of the layers seems to be due to the growth and clustering of the initial nuclei. Apparently, the surface of Ni–P and GaAs offers a very different number of nucleation centres for the electrodeposition of ZnTe; a more thorough understanding of the nature of the respective native oxides and of the crystalline nature of the nuclei is needed for a proper explanation. The profoundly different morphologies (uncorrelated spherules and bunches of fibres) can in principle both stem from the fact that lateral growth is hindered, possibly by the simultaneous formation of gaseous H2 Te; TEM evidence of the structure of spherules and fibres would be required for a full insight in these phenomena.
Fig. 6. SEM micrograph of a ZnTe deposit obtained galvanostatically at 1.5 mA cm−2 , deposition time: 30 min, substrate: electroless Ni–P.
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Fig. 7. SEM micrograph of a ZnTe deposit obtained galvanostatically at 1.5 mA cm−2 , deposition time: 2 min, substrate: electroless n+ -GaAs.
3.2. XPS XPS data show that Te 3d5/2 peak position of the native surface shifts from 573.05 to 572.75 eV for the sputter etched surface (Ar ions, 3 keV, 0.20 A, etch. time 150 min), where the contribution of TeO2 at 576.2 eV is almost completely removed. The Te 3d5/2 XPS spectra are reported in Fig. 9. The metallic component of Te results to be more present in the outer layers and decreases with depth, at which the intermetallic compound becomes predominant. XPS data on elemental tellurium and bulk ZnTe were reported by [9,10]. The trend we observed as a function of the sputtering time
is consistent with data presented in [10]; a proper trend can not be assessed from the data of [9] due to the reported error bars associated with the peak positions. No appreciable peak shifts were observed for Zn 2p3/2 as shown in Fig. 10. Even without a rigorous quantitative analysis it is obvious that the Zn content in the native surface is markedly lower than in the bulk. Peak positions suggest Zn is present as ZnO on the surface, whilst in the bulk Zn is principally bonded to Te [11]. In Fig. 11, we report the O 1s XPS spectra as a function of sputtering time. The surface oxygen peak is shifted to higher binding energy with respect to the bulk. This is coherent with phenomenological results on the chemical etching of
Fig. 8. SEM micrograph of a ZnTe deposit obtained galvanostatically at 1.5 mA cm−2 , deposition time: 30 min, substrate: electroless n+ -GaAs.
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Fig. 9. Te 3d5/2 XPS spectra of native and Ar+ -etched ZnTe film grown galvanostatically at 1.5 mA cm−2 .
Fig. 10. Zn 2p3/2 XPS spectra of native and Ar+ -etched ZnTe film grown galvanostatically at 1.5 mA cm−2 .
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which TeO2 and ZnO are dissolved. With the electrochemical techniques described in this work, it is not necessary to work at higher pH, where the bath displays an unstable precipitation behaviour, in order to achieve the desired structure and composition. The electrochemical growth of single-phase ZnTe was carried out onto conducting (amorphous Ni–P) and semiconducting (n+ -GaAs) cathodes. The presence of elemental Te in the ZnTe deposits obtained under optimal plating conditions can be detected, but it is confined to the top layer owing to an etching by the bath; this conclusion was based on XRD, XPS and EDX data. References
Fig. 11. O 1s XPS spectra of native and Ar+ -etched ZnTe film grown galvanostatically at 1.5 mA cm−2 .
ZnTe reported in [7,12] and can be interpreted in terms of hydrous oxides present on the surface [11].
4. Conclusions In this study, we reported of the possibility of obtaining polycrystalline ZnTe films by a single-step electrodeposition process from a highly acidic aqueous solution H2 SO4 in
[1] M. Neumann-Spallart, Ch. Königstein, Thin Solid Films 265 (1995) 33–39. [2] Ch. Königstein, M. Neumann-Spallart, J. Electrochem. Soc. 145 (1998) 337–343. [3] B. Bozzini, Ph.D. Thesis, Electrochemical Engineering, Politecnico di Milano, Italy, 1994. [4] B. Bozzini, F. Pavan, P.L. Cavallotti, G. Bollini, Trans. IMF 75 (1997) 175–181. [5] A. Kita, M. Ozawa, C.D. Gutleben, Appl. Surface Sci. 101 (1996) 652–655. [6] C.M. Sotomayor-Torres, A.P. Smart, M. Watt, M.A. Foad, K. Tsutsui, C.D.W. Wilkinson, J. Electron. Mater. 23 (1994) 289–298. [7] M.A. George, M. Azoulay, H.N. Jayatirtha, A. Burger, W.E. Collins, E. Silberman, Surf. Sci. 296 (1993) 231. [8] T. Vargas, R. Varma. in: J.R. Selman (Eds.), Techniques for Characterization of Electrodes and Electrochemical Processes, Wiley, New York, 1991, pp. 717–760. [9] W.E. Swartz, K.J. Wynne, D.M. Hercules, Anal. Chem. 43 (1971) 1884. [10] D.W. Langer, C.J. Vesely, Phys. Rev. B. 2 (1970) 4885. [11] D. Briggs, M. P. Seah, Practical Surface Analysis, Vol. 1, Wiley, Chichester, UK, 1990, pages 501 and 608. [12] A. Kita, M. Ozawa, C.D. Gutleben, Appl. Surf. Sci. 101 (1996) 652–655.