Electrodeposition of CdTe by potentiostatic and periodic pulse techniques

Thin Solid Films, 202 (1991 ) 67-75 PREPARATION AND CHARACTERIZATION

67

E L E C T R O D E P O S I T I O N O F CdTe BY POTENT1OSTATIC A N D PERIODIC PULSE TECHNIQUES* S. MOORTHYBABU, R. DHANASEKARANAND P. RAMASAMY Crystal Growth Centre, Anna University, Madras 600025 (India)

(ReceivedJuly 20, 1990;revisedNovember23, acceptedJanuary 31, 1991)

Cadmium telluride was electrodeposited from an aqueous solution containing the species CdCI 2 and TeO 2 by potentiostatic and periodic pulse techniques. The effect of different parameters such as concentration of the individual species, hydrogen ion concentration, applied potential and temperature are analysed to obtain optimum conditions for codeposition. X-ray diffractograms, X-ray microprobe analysis, scanning electron microscopy and voltammetry were employed to understand the crystalline nature, elemental composition and surface properties of the deposited films. A theoretical model has been developed for the electrodeposition of CdTe to understand the mechanism involved in the process. The results are discussed.

1. INTRODUCTION The unique physical properties of CdTe have been much utilized for the construction of a number of devices 1. Two features, (i) the energy band gap of 1.45 eV which provides an optimal match with the solar spectrum and (ii) the direct mode of the main optical transition, are particularly attractive for photovoltaic conversion of sunlight. Electrodeposition methods have been successfully employed for the fabrication of CdTe thin films 2-1°. Theoretical work concerned with the deposition of CdTe has been discussed by few researchers 3' 11. Several reports are available on the deposition parameters, structural, morphological, compositional and electronic characterisation of the electrodeposited CdTe in the literature. All the papers are devoted to cathodic or anodic deposition under potentiostatic conditions. In this work, we have successfully electrodeposited CdTe by a periodic pulse technique in addition to potentiostatic conditions. It is generally known that the deposits obtained with the periodic pulse technique have reduced porosity and improved conductivity compared with deposits of the same thickness obtained by the potentiostatic method. The pulse-to-pause width ratio of the potential, which is an *Presented at the 17th International Conference on Metallurgical Coatings and 8th International Conferenceon Thin Films, San Diego,CA, U.S.A.,April 2-6, 1990. 0040-6090/91/$3.50

© ElsevierSequoia/Printedin The Netherlands

68

S . M . BABU, R. DHANASEKARAN, P. RAMASAMY

addition parameter in the periodic pulse technique, plays a major role in the uniformity of the surface and hence in the physical properties of the deposits. The effect of various parameters on the deposit uniformity has been studied. A theoretical model developed for the electrodeposition of binary compounds has been extended to CdTe. This model is based on the Butler-Volmer equation. The procedure for determining the total current density and hence the voltammogram has been explained. The influence of different parameters on the voltammogram is studied. 2.

EXPERIMENTAL DETAILS

Cadmium telluride thin films were cathodically deposited from an aqueous 1 M CdC12 and 10mM TeO2 solution. The usual three-electrode cell was used with titanium or copper as the working electrode, platinum being the counter electrode and a saturated calomel electrode (SCE) the reference electrode. The solubility of TeO 2 in aqueous medium is very low, so TeO2 was first dissolved in concentrated hydrochloric acid and then water was added to the solution medium. The pH of the solution was adjusted to 2 with dilute sulphuric acid. The deposition potential was varied from - 0 . 4 to - 0 . 8 V (SCE). All depositions were carried out at ambient temperature. The pulse-to-pause width ratio of the potential was also varied to provide a uniform deposit distribution without dissolution. An electrochemical system consisting of a PAR model 173 potentiostat-galvanostat and model 175 universal programmer was employed for electrochemical deposition of the material. The charge that passed between the working electrode and the counterelectrode was measured by a PAR model 179 digital coulometer. X-ray powder diffractogram data were obtained using a Rich Seifert MZ-III diffractometer with Cu K0t radiation. Xray microprobe analysis and scanning electron micrography for the deposits were carried out with a Philips model P505 energy-dispersive X-ray analysis unit. 3. THEORY The basic electrochemical reactions for the electrodeposition of CdTe and their corresponding Nernst relations are Cd z ÷ + 2e- ~ Cd; E~d = -- 0.645 V (SCE)

(1)

Ecd =--0.645--RTln ( aca )t 2F \acd2. J TeO 2 + 4H + + 4e- ~-- Te + 2H20; E~e = 0.351 V (SCE) aT~

(2)

RT

ETe= 0"351 --RTin [ p (a-~o~) H ] +4F --F According to Engelken and Van Doren 11, telluride or polytelluride anion formation from the six- or ten-electron reduction of tellurium (IV) ions is assumed negligible

ELECTRODEPOSITION OF

CdTe

69

with the deposition voltages used. So the deposit is formed by solid state reaction of plated cadmium and tellurium atoms rather than pecipitation of Te22-, Te 2-, H T e - or H2Te and Cd 2 ÷" Cd + Te ~ CdTe; AG ° = - 9.97 x 104 J mol- 1

(3)

The expressions for the current density due to cadmium and tellurium can be written using Butler-Volmer equations 1l-t 3 as

Jca = 2Fk~d

~C"exp -

2F~cd

fl

~,-

x

2Feca RT (E+0.645)

II+exp(fl'~RT)I~ J / L

[

1

M+

2

x 1 -I" 2Fkcaac":* . e x p (, ~ - -2F~c°'"~' ~ - - ~ tL-I- 0.6a5)

jTe =

Jc,lcd

l+exp(ff'/RT

1

}]

(4)

4F~Te

4Fk~((aT.o2a~+)~"+'i"°exp{--~-T--(E--0.35)}

4F~T, fl 2 M + l 2 -- e x p { ~ (E-- 0.35)} exp{~q-,~(2--~--~) }

I+~T)J L.l.o

IlL,

e x p , - ~ - ~ - - t t ~ - 0.35)

I

l+~RTJ

_.]/ (5)

with

M = 2jcd/JTe 1

(6) 2 The parameters in the above equations are defined in Appendix A. The current density due to the deposition of cadmium and tellurium has been simulated from eqns. (4)-(6). The method of simulation adopted for the present work is as follows. The individual current densities due to cadmium and tellurium have been calculated from eqns. (4) and (5) for a given applied potential and for various values of the constant M (from - 1/2 to 1/2). For each value of M, the obtained current densities are checked with the condition given by eqn. (6). Only those values ofjc d andjTe for which eqn. (6) is satisfied have been taken into account while constructing the voltammograms (potential - current relations). The total current density is the sum of the individual current densities. The same procedure is followed to determine the total current densities for various values of the applied potential so that the

70

S . M . BABU, R. DHANASEKARAN, P. RAMASAMY

voltammogram can be constructed. Similarly, the voltammograms have also been constructed for various values of the activities of the ions, hydrogen ion concentration, rate constants and temperature of the electrolyte. 4.

RESULTS AND DISCUSSION

Thin film CdTe was electrodeposited on titanium and copper substrates by potentiostatic and periodic pulse techniques. Figure l(a) shows the X-ray powder diffractogram data of the films deposited (from 1 M CdCI2 and 10mM TeO 2 solution at - 0 , 7 2 V (SCE)) on titanium substrates. Figure l(b) shows the X-ray powder diffractogram data of the films deposited (from 1 M CdC12 and 10 mM TeO 2 solution at -0.65 V (SCE)) on copper substrates. The diffractogram confirms the presence of CdTe and its crystalline nature. Figure 2 shows surface micrographs of

39.2

2.295

~Z.7

17a

e,r~5 I.495

i]lt=r 75

]l

~5.5

~

i l e l r l f J l , , J i l , 65

I. j l J t t l J l , , , t l , 55

I

, ill

35

,~, 25

J i ,it_L_ 15

2e

"

Ca)

•

i,]l~=,J

45

z~]

ZXj Z6 ~

39.Z 2.29a0 ! t IN)

44 Z.O~7 •

J 48

(b)

,

~2 7 Z r'~e • ' ¢H)

Ze.5

(H)

I 44

f

I

=

I

40

36

-

.J

J 32

I

I 28

t

I 24

=

J 20

L

I 16

2 o

Fig. 1. X-ray diffractograms ofelectrodeposited CdTe films from 1 M CdCl 2 and 10raM TeO 2 solution (a) on titanium (at E = - 0.72 V (SCE)) and (b) on copper (at E = - 0.65 V (SCE)) substrates: O, CdTe; (3, cadmium; &, tellurium; A, titanium; C], copper.

ELECTRODEPOSITIONOF CdTe

71

(a) (b) Fig. 2. Surfacemierographs of CdTe filmsdeposited by the potentiostatic method (from 1 M CdC12and lOmM TeO 2 solution on a titanium substrate at E = -0.72 V (SCE)): (a) inner surface;(b) edge of the substrate.

CdTe deposited by the potentiostauc method. They reveal that the deposit is porous and also that there was unwanted nucleation on the edges of the substrate and nonuniform vacant spaces at the inner surface of the substrates. Figure 3 shows a surface micrograph of a CdTe deposit obtained by the periodic pulse technique. Study of the micrograph reveals that the deposit is more compact and uniform. Depositions have been carried out for different pulse-to-pause width ratios (10min:5min, 10min:l min, 10min:5 s, 5 min:10 s, 5 min:5 s, 5 min:2 s, 1 min:10 s, 1 min:5 s, 1 rain:2 s etc). Uniform deposits of CdTe were obtained with pulse-to-pause width .ratios of 10min:5 s, 5rain:2 s and 1 rain:2 s. If the pause width is increased to 10s, for the 1 min:2 s pulse-to-pause width, porous deposits are obtained, whereas an increase in the pause width to 30 s leads to dissolution of the materials. Similarly, if the pulse width is increased to 5 min, for the 1 min:2 s pulse-to-pause width, more of a deposit is obtained without disturbing the uniformity of the surface. However, a larger value of the pulse leads to the potentiostatic condition. The reason that the best deposits are obtained using this periodic pulse technique can be explained in terms of the nucleation phenomenon 14. The nucleation occurs during the first pulse stage. The weak and unwanted nuclei disappear in the subsequent pause stage. The remaining stable nuclei alone then grow larger in the next pulse stage and there may be some

Fig. 3. SurfacemicrographofaCdTefilmdepositedbytheperiodicpulsetechnique(from 1 MCdC12and 10 mM TeO 2 solution on a titanium substrate).

72

s . M . BABU, R. DHANASEKARAN, P. RAMASAMY

more nuclei or embryos which are dissolved in the subsequent pause stages. Hence, uniform deposits can be obtained with a suitable pulse-to-pause width ratio of the potential in this periodic pulse technique. The thicknesses of the deposited films are 2-4 lam on titanium substrates of dimensions 1 cm x 1 cm. The grain size of the deposits on the electrode surface is measured using the scanning electron micrographs to be 10-20~tm. X-ray microprobe spectra of deposited films on titanium (1 M CdCI2 and 10mM TeO2 solution at -0.72 V (SCE)) and copper (1 M CdCI2 and 10mM TeO2 solution at - 0 . 6 5 ~ r (SCE)) substrates are shown in Figs. 4(a) 4(b) respectively. The spectra confirm the exclusive presence of cadmium and tellurium in the deposits. The relative composition of the deposits, Cd0.96Te1.04, was calculated from the spectra.

Cd L¢I

TeLd. Te

(a)

(b)

I 0

2.5

5 KeV

Fig. 4. X-ray microprobe spectra of electrodeposited CdTe thin films (from 1 M CdC12 and 10 m M TeO2 solution): (a) on titanium (at E = - 0,72 V (SCE)); (b) on copper (at E = - 0.65 V (SCE)).

Figure 5 shows plots of the calculated current density as a function of applied potential for various values of the hydrogen ion concentration. For pH 0.5 the total current density at E = 0.4 V is equal to the diffusion-limited current density of tellurium. The current density remains constant until the deposition voltage reaches 0.12 V. A further decrease in potential results in a distinct increase in current. This is due to cadmium deposition. Figure 6 shows the recorded voltammograms for cadmium, tellurium and CdTe. The plateau region of the CdTe curve is the underpotential deposition region for cadmium. The higher current value for CdTe deposition confirms the presence of currents due to cadmium and tellurium. Figure 7 shows voltammograms as a function of the activity of cadmium. The plateau width corresponding to the total current density value at the diffusion-limited current density of tellurium decreases with increasing cadmium activity. -

ELECTRODEPOSITION OF C d T e

73

260["

I

%

0.4

0.3

0.2

O.l

0.0

E ( Volts

-I

-0.I

I

-0.2

-0.3

-0.4

vs SCE )

Fig. 5. Calculated voltammograms of CdTe for various values of the hydrogen ion concentration: curve a, pH 0.5; curve b, pH 1.0; curve c, pH 1.5; curve d, pH 2.0; curve e, pH 2.5; curve f, pH 3.0; curve g, pH 3.5; curve h, pH 4.0.

10

B

--

~-.

/

z,,,4z:) L)

i..././

20 .....

./ -"~"

I 0

J

/ ~

~

/.

. . . . . . . . . . .

I -0.25

/

I

t -0.50

JJ/

/

I -075

E (Volts vs SCE)

Fig. 6. Voltammograms recorded experimentally for cadmium, tellurium and CdTe (from 1 M CdCl2 and 10 mM Te02 solution): - - - , cadmium; --, tellurium; - . . . -, CdTe.

74

s . M . BABU, R. DHANASEKARAN, P. RAMASAMY

140

150

120

"- 110 E 100

#

90

80

70 0,4

I

I

I

I

0.5

0.2

0,]

0.0

I

I

-0.]

-0.2

I _ _ -0.5

-0.4

E (Voti's vs SCE)

Fig. 7. Calculated voltammograms of CdTe for different values of the activity of cadmium: curve a, 0.009; curve b, 0.019; curve c, 0.029; curve d, 0.039; curve e, 0.049.

5. CONCLUSION

Thin film CdTe was electrodeposited from aqueous CdC1a and TeO2 solutions. The X-ray microprobe analysis confirms the presence of cadmium and tellurium in the deposited CdTe material. The X-ray powder diffractogram data reveal the crystalline nature and structure of the deposited CdTe material. Surface micrographs of the deposited CdTe reveal that the surface is compact and uniform when the periodic pulse technique is used. Uniform film thickness is significant for solar cell fabrication, since non-uniform films containing microvoids provide potential shorting paths when a heterojunction is formed over them 15. The porous nature of the films will short the active p-type layer formed over the n-type layer at the back contact; the pores also act as effective carrier trapping sites and reduce the carrier diffusion length and lifetime 16. These potential shorting paths, when a heterojunction is formed, are avoided with films deposited by the periodic pulse technique. The pulse-to-pause width ratio of the potential plays a major role in the deposit distribution. Linear sweep voltammograms have been recorded experimentally and constructed theoretically. The influence of the hydrogen ion concentration and the activity of cadmium on the deposition process has been studied voltammetrically. ACKNOWLEDGMENT

The authors are grateful to Dr. J. Kumar for taking the scanning electron

ELECTRODEPOSITION OF CdTe

75

micrographs and X-ray microprobe spectra of the samples at MASPEC, Parma, Italy. REFERENCES 1 K. Zanio, in R. K. Willardson and A. C. Beer (eds.), Semiconductor and Semimetals, Vol. 13, Academic Press, New York, 1978. 2 M . P . R . Panicker, M. Knaster and F. A. Kroger, J. Electrochem. Soc., 125 (1978) 566. 3 F.A. Kroger, J. Electrochem. Soc., 125 (1978) 2028. 4 G. Fulab, M. Doty, P. Meyers, J. Betz and C. H. Liu, Appl. Phys. Lett., 40 (1982) 327. 5 R.N. Bhattacharya, K. Rajeshwar and R. Noufi, J. Electrochem. Soc., 131 (1984) 939. 6 M. Takahashi, K. Uosaki and H. Kita, J. Electrochem. Soc., 131 (1984) 2305. 7 R.N. Bhattacharya and K. Rajeshwar, J. Electrochem. Soc., 132 (1985) 732. 8 M.W. Verbrugge and C. W. Tobias, J. Electrochem. Soc., 132 (1985) 1298. 9 Jesko Von Windheim and M. Cocivera, J. Electrochem. Soc., 134 (1987) 440. 10 J. Touskova, D. Kindl and J. Tousek, Sol. Energy Mater.,18 (1989) 392. 11 R.D. Engleken and T. P. Van Doren, J. Electrochem. Soc., 132 (1985) 2904. 12 L.I. Antropov, Theoretical Electrochemistry, Mir, New York, 1972, p. 378. 13 L. Durai, R. Dhanasekaran and P. Ramasamy, Bu/l. Electrochem., 3 (1987) 335. 14 S. Moorthy Babu, R. Dhanasekaran and P. Ramasamy, Bull. Electrochem., 6 (1990) 732. 15 L.M. Peter, Electrochim. Act& 23 (1978) 165. 16 A.C. Rastogi and K. S. Balakrishnan, Int. J. Sol. Energy, 1 (1983) 431. APPENDIX A: NOMENCLATURE

ai activity of the constituent ion i E applied potential Ei equilibrium potential of constituent ion i E~ standard reduction potential ofi F Faraday constant A G ° free energy Ji current density due to constituent ion i Jc,l, diffusion-limited current density of ion i ki rate constant o f i o n i M dimensionless current ratio variable R universal gas constant T temperature g cathodic transfer coefficient of ion i anodic transfer coefficient of ion i fl solid state activity coefficient parameter ~,

~I2-AG o