Potentiostatic cathodic electrodeposition of ZnTe was investigated from the ...
Deposition of three kinds of deposits, i.e., polycrystalline ZnTe with closely.
Materials Transactions, Vol. 45, No. 2 (2004) pp. 277 to 280 #2004 The Japan Institute of Metals
Effect of pH on the Electrodeposition of ZnTe Film from a Citric Acid Solution*1 Takahiro Ishizaki*2 , Takeshi Ohtomo*2 , Yusuke Sakamoto*3 and Akio Fuwa Department of Material Science and Engineering, School of Science and Engineering, Waseda University, Tokyo 169-8555, Japan Potentiostatic cathodic electrodeposition of ZnTe was investigated from the viewpoint of the effect of pH on the deposits’ composition and crystallinity using citric acidic electrolytic baths, in which Zn(II) and Te(IV) species were dissolved to form ZnH2 Citþ , ZnHCit, ZnCit , Zn(Cit)2 4 and HTeO2 þ , HTeO3 , respectively (Cit: citrate) at various pH. The complex equilibrium calculation was carried out to examine the most predominant complex ion for Zn-Cit system at different pH. Deposition of three kinds of deposits, i.e., polycrystalline ZnTe with closely stoichiometric composition, crystalline Te, and the mixed crystal due to Te and ZnTe, can be controlled by changing the pH and [Zn(II)]/ [Te(IV)] concentration ratio of the baths. All the deposits obtained at pH 4.0 were well crystallized with a ZnTe cubic preferential orientation along the (111) plane without any post-treatment. The difference of the electrodeposition behavior at various pH was also discussed. (Received October 27, 2003; Accepted November 27, 2003) Keywords: electrochemical deposition process; Zinc telluride; pH; citrate
1.
Introduction
Thin film ZnTe compound semiconductor has found application as window material in multijunction solar cells based on chalcogenide and chalcopyrite type semiconductors due to its optimum energy gap 2.2 eV1) and low affinity 3.5 eV.2) The low electronic affinity and its p-type conductivity characteristics have improved the ohmic contact on CdTe or GaAs p-type semiconductors, which are abssorber material in high-efficiency photovoltaic devices. ZnTe thin films have been prepared by several techniques including molecular beam epitaxy (MBE),3,4) vacuum evaporation,5) r.f. sputtering6) and electrodeposition.7–9) Among these techniques, electrodeposition offers several advantages: it is relatively economical; it can be used on a large scale; and it is conducted at low-temperature. Although there has recently been a growing interest in the electrodeposition of ZnTe film due to these advantages, two issues have concerned us. The first is that heat-treatment is necessary after deposition in order to adjust the Zn/Te stoichiometry, thus forfeiting the advantages of a low-temperature process. The second is that electrodeposition is often performed at a relatively negative potential, indicating that a sub-reaction of hydrogen evolution reduction could arise, leading to a reduction in current efficiency.10) For these reasons, we have been strongly interested in achieving a single electrodeposition of ZnTe film at a low overpotential (i.e., a more positive potential) and without heat-treatment. Recently, we have overcome these two issues and have successfully deposited ZnTe films at pH 4.0 from a solution containing ZnSO4 , TeO2 , H3 Cit and Na3 Cit (Cit = C6 H5 O7 3 ). It is well known that Zn and citrate ions form the complex as ZnH2 Citþ , ZnHCit, ZnCit and Zn(Cit)2 4 as shown in the literature.11) In these complex ion species, the relatively predominant ion species in the electrolytic solution depends on pH of the solution. It is considered that the variations of pH could affect the electrodeposition behavior
of ZnTe. Thus, it is necessary to investigate the effect of pH on the electrodeposition of ZnTe. In this study, the electrodeposition from a citric acidic solution was carried out to examine the effect of pH on the electrodeposition behavior and the properties of the resulting ZnTe films. 2.
Experimental
Citric acid aqueous electrolytes containing 99.99% pure ZnSO4 (Kanto Kagaku), 99.0% TeO2 (Kanto Kagaku), C6 H8 O7 H2 O (H3 CitH2 O; Kanto Kagaku) and Na3 C6 H5 O7 2H2 O (Na3 Cit2H2 O; Kanto Kagaku) were employed for electrodeposition of ZnTe. All the chemicals were of reagent grade and were used without pretreatment. Deionized water (7:5 106 cm) was obtained from an Autostill water system (YAMATO Co., Ltd. WG25). Table 1 summarizes the concentrations of Zn and Te, and the pH of the citrate electrolytes employed in this study. Each Zn-Tecitrate solution was obtained by dissolving the required amounts of Zn and Te for their respective experimental conditions. The pH of the solutions was adjusted by changing the mixed ratio of citrate and sodium citrate, i.e., [Na3 Cit]/ [H3 Cit]. The temperature was maintained constant by a rubber heater at 363 K. Cathodic electrodeposition was performed under potentiostatic conditions using a conven-
Table 1 List of aqueous electrolytes containing citrate (Cit) employed for potentiostatic electrodeposition of ZnTe. (M = kmol m3 ) [Zn(II)] (mM)
[Na3 Cit] (M) 0
[Cit]total (M) 0.50
50
20
1.5
[H3 Cit] (M) 0.50
a1
b1
c1
2.6
0.45
0.05
0.50
a2
b2
c2
2.75
0.075
0.425
0.50
a3
b3
c3
4.0
0.25
0.25
0.50
a4
b4
c4
*1This Paper was Presented at the Autumn Meeting of the Japan Institute of
5.6
0.05
0.45
0.50
a5
b5
c5
Metals, held in Sapporo, on October 13, 2003. Student, Waseda University. *3Undergraduate Student, Waseda University.
Cathode potential was 0:65 V vs. Ag/AgCl. Te(IV) concentrations were 0.16 mM (M = kmol m3 ) under all conditions.
*2Graduate
pH
10
T. Ishizaki, T. Ohtomo, Y. Sakamoto and A. Fuwa
tional three-electrode setup comprised of a potentiostat (Hokuto Denko HA-501) connected to a function generator (Hokuto Denko HB-104) and a coulometer (Fuso Seisakujo HECS-343B). An Ag/AgCl electrode immersed in saturated KCl was used as a reference. A gold-plated copper sheet (20 mm 40 mm) and a platinum sheet (50 mm 50 mm) were employed for the working and counter electrodes, respectively. The gold-plating (thickness ca. 2 mm) was carried out using a gold-plating aqueous solution (EEJA microfab Au310) under galvanostatic conditions of 4 mA cm2 at 323 K. Before gold-plating, the copper sheet was polished with 6, 1 and 0.3 mm diamond paste, cleaned with distilled water and degreased. The electrolytic solution was agitated at 400 rpm with a magnetic stirring unit for all runs. In order to avoid any influence from dissolved oxygen, the electrolyte was deaerated by bubbling pure argon gas through it for about 20 min, following which the flux was kept over the solution. Film deposition was carried out potentiostatically with the total quantity of electricity being 1.0 C cm2 . The composition of the films was determined quantitatively using energy dispersive X-ray analysis (EDX) (HORIBA EX200). X-ray diffraction (XRD) (RIGAKU RAD-IIC) was conducted to examine the crystal structure of the Zn-Te compound film. XRD data were obtained using a powder diffractometer with CuK radiation. The XRD peaks were assigned based on JCPDS data. 3.
Table 2 Formation constants and dissociation constants K used for calculation of concentration distribution of chemical species in Zn(II)citrate bath. Formation constants of Zn(II)-citrate complexes log 1 ¼ 1:25 Zn2þ + H2 Cit = ZnH2 Citþ
Ref. 11
Zn2þ + HCit2 = ZnHCit
log 2 ¼ 2:98
11
Zn2þ + Cit3 = ZnCit
log 3 ¼ 4:98
11
Zn2þ + 2Cit3 = Zn(Cit)2 4
log 4 ¼ 5:90
11
H3 Cit = H2 Cit + Hþ
log K1 ¼ 2:87
11
H2 Cit = HCit2 + Hþ
log K2 ¼ 4:35
11
HCit2 = Cit3 + Hþ
log K3 ¼ 5:69
11
Dissociation constants of citric acid
1.0
H3Cit Zn
Repartition of Species
278
Zn(Cit)2
2+
4-
0.8
Cit H2Cit
-
3-
2-
HCit
0.6 ZnHCit
0.4 ZnH 2 Cit
+
0.2 ZnCit
-
0.0 0
2
4 pH
6
8
Fig. 1 Distribution of soluble species in the zinc-citrate system as a function of the pH of the solution at room temperature.
Solution Chemistry
In order to examine the relatively predominant complex ion for Zn-Cit system at different pH, the complex equilibrium calculations were carried out. In aqueous solution, Zn(II) soluble species form various complexes with citrate ligands (expressed as Cit3 = C6 H5 O7 3 ). The total amounts of Zn(II) species, CZn , and citrate, CCit , in the electrolytic solution are expressed by the following equations CZn ¼ ½Zn2þ þ ½ZnH2 Citþ þ ½ZnHCit þ ½ZnCit þ ½Zn(Cit)2 4
4.
Results and discussion
ð1Þ
and CCit ¼ ½H3 Cit þ ½H2 Cit þ ½HCit2 þ ½Cit3 þ ½ZnH2 Citþ þ ½ZnHCit þ ½ZnCit þ 2½Zn(Cit)2 4
between 0 and 5.45, 5.45 and 7, respectively.11) Although it is considered that Te soluble species could form a complex with citrate ligands,12) there is no available complex formation data for them. Thus, the reduction reactions for Te were treated as the reaction used in Pourbaix-type diagram.11) Consequently, the pH used in this study were 1.5, 2.6, 2.75, 4 and 5.6.
ð2Þ
The concentration of each species can be calculated using eqs. (1) and (2), formation constant, , of the Zn2þ -citrate complexes, the dissociation constant, K, of citric acid, the total amounts of Zn(II), CZn and citrate, CCit , and the pH value. The values for and K are listed in Table 2. Figure 1 indicates the results obtained with the Zn(II)citrate bath. The relative predominant ion species are Zn2þ , ZnH2 Citþ , ZnHCit, Zn(Cit)2 4 at pH from 0 to 2, pH from 2 to 2.7, pH from 2.7 to 2.8, and at pH > 2.8, respectively. As the contribution of other ion specie, i.e., ZnCit , is less than 10% at pH between 0 and 7, it is assumed that this ZnCit specie does not contribute the main electrodeposition behavior for the reduction of Zn due to the formation of ZnTe. The relative predominant ion species for tellurium in the acidic electrolytic solution are HTeO2 þ , HTeO3 at pH
Our recent study concluded that citric acid bath of pH 4.0 with composition: [Zn(II)] = 5–50 mM (M = kmol m3 ), [Te(IV)] = 0.16 mM, and [Cit]toatal = 0.5 M, gave ZnTe deposits of close stoichiometry under potentiostatic electrodeposition potential at 0:65 V.12) The deposition behavior is inferred to be (i) cathodic electrodeposition of Te atoms, followed by (ii) underpotential deposition of the Zn(II) ions to form ZnTe. It is necessary for the formation of stoichiometric ZnTe that the depositions of Te atoms are quantitatively followed by that of Zn atoms. Thus, the deposition of Zn should occur immediately after the deposition of Te to prevent codeposition of bulk Te. As stated above, however, the relative predominant ion species in the citric acidic solution depend on the pH. If the predominant species in the bath change, it is predicted that the reduction rate of the deposition could be different. According to the complex equilibrium calculations, it was indicated that the concentration of relatively predominant ion specie was considerably higher than that of other ion species. Thus, assuming that the reactions of relatively predominant ion species proceed mainly, the reduction reactions for
Effect of pH on the Electrodeposition of ZnTe Film from a Citric Acid Solution
tellurium can be as follows:
[Te(IV)] = 0.16 mM, [Cit] total = 0.5 M (M = kmol m-3) pH 2.75 4.0 1.5 2.6 5.6
HTeO2 þ þ 3Hþ þ 4e ¼ Te þ 2H2 O ð0 pH 5:45Þ
ð3Þ
HTeO3 þ 5H þ 4e ¼ Te þ 3H2 O ð5:45 pH 7Þ
ð8Þ
It can be considered that the differences of complex ion species have an effect on the deposition amount of Zn, since the amounts of Zn2þ ions concentration in the bath, which are necessary to deposit Zn atom, are different. The effects of the difference in these reaction paths on the electrodeposited film’s composition and crystallization were then investigated. Figure 2 shows the XRD diffraction patterns of the deposits obtained at 0:65 V from a solution containing Zn(II) 20 mM, Te(IV) 0.16 mM, and total citrate 0.5 M at various pH (i.e., sample b1–b5). In addition to diffraction peaks attributed to the substrate materials, i.e., Au13) and Cuð13Þ , there were three peaks related to ZnTeð13Þ at around 2 of 25 , 41 and 49 , indicating that the deposits comprised crystalline ZnTe, except the deposit obtained at pH 5.6. The deposit obtained at pH 5.6 gave a diffraction peak corresponding to Teð13Þ and had a Te-rich composition: 99 at % Te1 at % Zn. This means that the Zn deposition can be suppressed due to that the increase of pH leads to a negative shift in the Zn deposition potential, leading to that Te deposition proceeds preferentially at this pH. In contrast, the deposits obtained from samples b1-b4 were well crystallized with a ZnTe cubic preferential orientation along the (111) plane. Although the Au
-1
Intensity, i / s
: ZnTe
Au
Au2Te 1000
0
Te
Te
(111)
Cu
Te (e) (d) (c) (b) (a)
ZnTe: JCPDS Card
(200) No. 15-0746 (220) 20°
Te
30°
2θ
40°
(222)
(311) 50°
40.9
41.7
46.3
0
49.1
45.5
0.5
48.7
53.2
60°
Fig. 2 XRD patterns of the deposits obtained at 343 K from citric acidic bath ([Zn(II)] = 20 mM (M = kmol m3 ), [Te(IV)] = 0.16 mM, [Cit]total = 0.5 M) of (a) pH 1.5, (b) pH 2.6 (c) pH 2.75 (d) pH 4.0, and (e) pH 5.6. Cathode potential was 0:65 V vs. Ag/AgCl. Total quantity of charge was 1.0 C cm2 .
1000 500
ð6Þ
ð2:7 pH 2:8Þ ð7Þ þ þ 4H þ 2e ¼ ZnTe þ 2H2 Cit
Te
40.7
20 25 20° 25° 30 30°
ð2:8 pHÞ
2000
1.6
50
Te þ ZnHCit þ Hþ þ 2e ¼ ZnTe þ H2 Cit
pH (a) 1.5 (b) 2.6 (c) 2.75 (d) 4.0 (e) 5.6
46.7
20
ð5Þ
Te þ ZnH2 Citþ þ Hþ þ 2e ¼ ZnTe þ H3 Cit ð2 pH 2:7Þ
3000
44.5
2θ
0
Intensity, i / s -1
Te þ Zn2þ þ 2e ¼ ZnTe ð0 pH 2Þ
Te þ Zn(Cit)2
30.3
ð4Þ
The reduction reactions for the formation of ZnTe can be given by
4
37.3 10 [Zn(II)] / mM
þ
279
Fig. 3 XRD and Zn content (at %) of the deposits at 363 K from citric acidic baths summarized in Table 1: [Zn(II)] = 10-50 mM, [Te(IV)] = 0.16 mM, [Cit]total = 0.5 M, pH range 1.5–5.6. Cathode potential was 0:65 V vs. Ag/AgCl. Total quantity of charge was 1.0 C cm2 .
XRD patterns for each of these film were obtained in the range 2 = 20–60 , most of the diffraction peaks appearing in this range were attributable to the Au plating of the substrate. Hence, a part of the XRD patterns, 2 = 20–30 , is summarized in the Fig. 3. Figure 3 shows a part of the XRD patterns of the deposits obtained from various electrolytes. Compositions (at. % Zn) of the deposits determined by EDX are also summarized in this figure. In this 2 range, diffractions from ZnTe(111) (2 ¼ 25:259 for CuK) and/ or Te(101) (2 ¼ 27:562 ) planes may appear. Based on the XRD patterns, these films can be classified into three categories, which gave (i) relatively sharp diffraction from ZnTe(111) plane, (ii) sharp diffraction from Te(101) plane, and (iii) a sharp and rather broad peak ranging between 25 and 28 due to ZnTe(111) and Te(101) planes. For example, all the films obtained from the electrolytes at pH 5.6 (samples a5, b5 and c5) fall into category (ii), while those from the electrolytes with pH 4.0 (samples a4, b4 and c4) belong to category (i). The relatively sharp and broad peak of the films of category (iii), which resulted from the electrolytes with relatively low pH and Zn(II) concentrations, e.g., samples b1b3 and c1-c3, indicates that the films were not a single phase from ZnTe but the mixed crystals due to ZnTe and Te. It is worth noting that we can recognize the same categories in Fig. 3 as in the composition of the film: the Zn contents xZn (at %) of the films in categories (i), (ii), and (iii) were 45 xZn 55, xZn < 5, and 30 < xZn < 45, respectively, within the [Zn(II)] concentrations and the pH ranges investigated. At pH 1.5, 2.6, and 2.75, the content of Zn in the deposit increased with the increase of [Zn(II)] concentration in the bath. As the [Zn(II)] concentration in the bath increases, the ions related to the reduction reaction of Zn, i.e., Zn2þ , ZnH2 Citþ , ZnHCit, and Zn(Cit)2 4 , increases. From a thermodynamic point of view, increasing the ion concentrations related to the reduction reaction of Zn corresponds to a positive shift of the Nernst potential for the deposition of Zn atoms on Te to form ZnTe according to reaction (5)–(8). For example, the potential of the Zn deposition in reaction (6) is given by the following equation:
280
T. Ishizaki, T. Ohtomo, Y. Sakamoto and A. Fuwa
EðVÞ ¼ 0:729 0:0295pH 0:0295 log½H3 Cit þ 0:0295 log½ZnH2 Citþ
ð9Þ þ
This suggests that not only increasing [ZnH2 Cit ] but also decreasing citrate concentration leads to a positive shift of the potential and to a promotion of the zinc deposition. On the other hand, at pH 4.0, the deviation from the stoichiometric composition of the deposits was hardly observed despite the increase of the [Zn(II)] concentration. The deposits had a diffraction peak only from ZnTe(111). According to these results, it is revealed that the difference of the electrodeposition behavior at various pH has an effect on the composition and crystallinity of the electrodeposited films. This can be ascribed to that Zn(II) soluble species form various complexes ion with citrate ligands and their dissociation degrees from solvated to unsolvated ions are different. It is concluded that the electrodepositions at pH 4.0 are the most suitable for the crystalline ZnTe film with the closely stoichiometric composition within the pH range investigated. 5.
Conclusions
The effect of pH on cathodic electrodeposition of ZnTe from citric acidic solution was investigated and the complex equilibrium calculation was carried out to examine the most predominant complex ion at different pH. Based on the condition established for ZnTe deposition, the pH and Zn(II)/Te(II) concentration ratio were optimized so as to electrodeposit ZnTe with close stoichiometric composition. There was a tendency for the Zn content of the deposit from
citric acidic baths to be higher with the increase of the [Zn(II)] concentration. The results of composition and crystallinity suggest that the deposition behavior changes within the pH range investigated, since the relative predominant species in the baths changes. Consequently, polycrystalline ZnTe can be deposited potentiostatically by control of the pH and Zn(II)/Te(II) concentration ratio. REFERENCES 1) A. Kaneta and S. Adachi: J. Phys. D: Appl. Phys. 33 (2000) 901–905. 2) A. Pistone, A. S. Arico, P. L. Antonucci, D. Silvestro and V. Antonucci: Solar Energy Mater. Solar Cells. 53 (1998) 255–267. 3) R. L. Gunshor, L. A. Koladziejski, N. Otsuka and S. Datta: Surf. Sci. 174 (1986) 522–533. 4) R. N. Bicknell-Tassius, T. A. Kuhn and W. Ossau: App. Surf. Sci. 36 (1989) 95–101. 5) U. Pal, S. Saha, A. K. Chaudhuri, V. V. Rao and H. D. Banerjee: J. Phys. D: Appl. Phys. 22 (1989) 965–970. 6) H. Bellakhder, A. Outzourhit and E. L. Ameziane: Thin Solid Films 382 (2001) 30–33. 7) M. Neumann-Spallart and Ch. Ko¨nigstein: Thin Solid Films 265 (1995) 33–39. 8) Ch. Ko¨nigstein and M. Neumann-Spallart: J. Electrochem. Soc. 145 (1998) 337–343. 9) A. B. Kashyout, A. S. Arico´, P. L. Antonucci, F. A. Mohamed and V. Antonucci: Mater. Chem. Phys. 51 (1997) 130–134. 10) T. Mahalingam, V. S. John, S. Rajendran, G. Ravi and P. J. Sebastian: Surf. Coat. Tech. 155 (2002) 245–249. 11) M. Pourbaix: Atlas of Electrochemical Equilibria in Aqueous Solutions, (Pregamon Press, Oxford, 1966) pp. 560–571. 12) T. Ishizaki, T. Ohtomo and A. Fuwa: J. Electrochem. Soc. in press. 13) JCPDS Data Base, Card No. 04-0784, 04-0836, 15-0746, 36-1452 (unindexed peaks), JCPDS, Swarthmore, PA.
