Simultaneous polarographic determination of cadmium and tellurium in electro-deposited cadmium telluride thin films

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SIMULTANEOUS POLAROGRAPHIC DETERMINATION OF CADMIUM AND TELLURIUM IN ELECTRO-DEPOSITED CADMIUM TELLURIDE THIN FILMS ANDRZEJ DARKOWSKI? and MICHAEL COCIVERA* Centre for Graduate Work in Chemistry, University of Guelph, Guelph, Ontario, Canada

Guelph-Waterloo

(Receiced 29 March 1985. Accepted 6 August 1985)

Summary-A polarographic method has been developed for the simultaneous determination of cadmium and tellurium in thin-film cadmium telluride. The procedure involves dissolution of the film with concentrated mtrlc aad, which IS subsequently removed by evaporation. The Cd(H) and Te(IV) waves are well seoarated at nH 10. but sufficient ammonia must be present to prevent the precipitation of cadmium hydroxide. &

The cathodic electro-deposition of thin-film cadmium telluride has been reported by a number of workers.l-’ The interest in this material arises because it has an energy band-gap well suited to the solar spectrum, has a direct transition and absorbs strongly, and can be deposited as either n- or p-type. This last property is related to the composition of the film. An excess of cadmium results in n-type and an excess of tellurium in p-type CdTe.’ In a number of studies of electro-deposition from solutions containing Cd(H) and Te(IV), X-ray diffraction and electron microprobe analysis were used to determine the composition of the film in relation to its type of conductivity.3” Since these techniques cover small surface areas, polarography can act as an effective complement to them because it can provide the average composition of the whole film as well as the total amount of material deposited. In addition, polarography has the advantages of speed and cheapness. It has been very helpful in our studies of new electrochemical processes for the preparation of thinfilm cadmium telluride,’ with special regard to correlation of composition with deposition conditions. We have found that the non-aqueous cathodic electro-deposition of CdTe from solutions containing Cd(H) and tri-n-butylphosphine telluride results in a p-type film which contains a slight excess of cadmium.’ Consequently, since an excess of cadmium usually produces n-type CdTe, it would appear that the excess is not uniformly distributed throughout the films and may occur in small pockets and hence not affect the type of conductivity. EXPERIMENTAL The calibration procedure made use of standard solutions of Cd(H) and Te(IV), which were mixed to provide solu*Author to whom correspondence should be addressed. ton leave from Department of Chemistry, Warsaw Technical University.

tions containing various Cd/Te concentrations and ratios. The cadmium solutions were prepared from cadmium perchlorate, and the tellurium(IV) solutions by dissolving elemental tellurium in concentrated nitric acid, which was then removed by evaporation after addition of few ml of sulphuric acid. The pH of the mixed solution was raised to 10 by addition of ammonia solution, and 0.1M ammonium sulphate was used as the supporting electrolyte. The total volume of the solution was brought to 25 ml. At equimolar cadmium and tellurium concentrations of 5 x IOm4M or lower, the tellurium wave was not distorted, and the diffusion current for the tellurmm was exactly twice that for the cadmium (Table 1). Cadmium telluride films for analysis were dissolved in a few drops of concentrated nitric acid, after which a few ml of sulphuric acid were added and the excess of nitric acid was removed by evaporation. The solution was diluted to 20 ml. After addition of 0.125 g of ammonium sulphate and adjustment of the pH to 10 with ammonia solution, sufficient distilled water was added to adjust the cadmium and tellurium concentrations to close to lo-“M. As a check of this procedure, authentic samples of 99.99% pure CdTe were dissolved in the same manner and analysed. The polarographic analysis was performed with a PAR 174A polarographic analyser operating in the normal mode. A drop-time of 1 set and a potential scan of 5 mV/sec were employed. These conditions provided two clean waves that were well separated, with E,,, at -600 f 20 mV for Cd(I1) and -800 k 10 mV for Te(IV), vs. Ag/AgCl. A pool of mercury was used as the counter-electrode. RESULTS AND

DISCUSSION

Figure 1 shows the polarogram of a solution containing Cd(I1) and Te(IV), both at 1 x 10-4M concentration. Since the cadmium reduction involves two electrons, the fact that the tellurium reduction current is twice that of the cadmium indicates a four-electron reduction for the tellurium. Table 1 indicates that this current ratio holds for concentrations up to 5 x 10e4 M. Consequently it appears that Te(IV) is reduced to elemental tellurium under these conditions. The diffusion current was linearly proportional to concentration for both elements, the slopes of the calibration graphs 187

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IO-

B-

a i

I

I 500

I

I

I

800

900

I

700 -E (mV)

600

Fig. 1. Polarographic waves due to Cd(H) and Te(IV), both 10m4M, with E,,, at -610 vs. Ag/AgCl,

respectively.

Supporting

electrolyte

was O.lM ammonium

and sulphate.

-800

mV

Table 1. Polarography of standard solutions containing Cd(I1) and Te(IV) Diffusion

PWUI,

~e(Wl~

10-4M

10-4M

5.0 2.0 1.0 0.50 0.50 2.0

5.0 2.0 1.0 0.50 1.0 1.0

current,

PA Current

ratio,

Cadmium

Tellurium

GCd

1.32 0.52 0.275 0.132 0.134 0.50

2.64 1.06 0.55 0.270 0.563 0.52

2.0 2.0, 2.00 2.05 4.20 1.0,

being 26 and 52 mA. 1.mole-’ for cadmium and tellurium respectively. Cadmium concentrations larger than 5 x 10m4M are not practical because the increased ammonia concentration that is required to prevent precipitation of cadmium hydroxide results in poor separation between the Cd(I1) and Te(IV) reduction waves. Below 5 x 10-5M concentration the ratio of the diffusion currents for the two waves deviates from 2 and calibration curves must be employed. A CdTe film electro-deposited on a titanium plate was dissolved and the procedure applied to the solution; two waves were obtained, which were similar to those in Fig. 1. We have found no wave for Te(VI), for which & is quoted’ as - 1.34 V at pH 9.2 in the presence of ammonium chloride. As a control, the dissolution procedure was repeated with a bare titanium plate; no waves were observed over the scan range employed for the CdTe analysis. Table

2 lists some results of analyses of authentic samples of 99.99% pure CdTe. For ten replicate analyses the

Table 2. Polarographic analysis of 99.99% pure CdTe and electro-deposited thin-film CdTe Measured

concentrations,

10-4M

WTel, lo-4M

lCd(II)l

lTe(IV)l

1.50’ 1.001 0.75* 0.501 Film? Film?

1.5, 1.o, 0.77, 0.52, 0.51 0.91,

1.5, I .o, 0.78, 0.52, 0.58 0.98

Atomic ratto, Te/Cd 0.98 1.o, 1.0, 1.o, 1.1, 0.99,

*Prepared by dissolution of 99.99% pure CdTe according to the procedure in text. tDissolution of thin-film CdTe that had been electrodeposited on titanium.

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average Te/Cd ratio was 0.99, with a standard deviation of 0.01, and 95% confidence limits of f0.01. The close agreement between the measured concentration and that based on the mass of material dissolved indicates that the dissolution procedure causes little loss of tellurium by evolution of hydrogen telluride. For comparison, the results’ for two electro-deposited CdTe thin films are also listed. The advantage of this analytical procedure is that it is relatively quick because cadmium and tellurium are determined simultaneously with a minimum of handling. The polarographic analysis for tellurium in

the presence of 0. I M sodium hydroxide’ is less attractive because cadmium is precipitated under these conditions and, therefore, cannot be determined simultaneously with the tellurium. Likewise, polarographic analysis in dilute nitric acid is less attractive because two pre-waves occur before the tellurium

wave,“’ and we have found that the tellurium wave is sometimes distorted by hydrogen evolution. REFERENCES 1.

M. P. R. Panicker, M. Knaster and F. A. Kroger.

J. Electrochem. Sot., 1978, 125, 566. 2. G. Fulou. M. Dotv. P. Mevers and C. H. Liu. ADDI. .. Phys. Le’ri, 1982, ki, 327. 3. R. N. Bhattacharya, K. Rajeshwar and R. N. Not& J. Electrochem. Sot., 1984, 131, 939. 4. J. Llabres, ibid., 1984, 131, 464. 5. M. Takahashi, K. Uosaki and H. Kita, ibid., 1984, 131, 2304. 6. K. Uosaki, M. Takahashi and H. Kita. Elecrrochim. Acta, 1984, 29, 279. 7. A. Darkowski and M. Cocivera, unpublished work. 8. D. de Nobel, Philips Res. Rep., 1959, 14, 361. 9. M. Heyrovsky and P. Zuman, Practical Polarography,

p. 198. Academic Press, New York, 1968. 10. D. C. Whitnack, T. M. Donovan and M. H. Ritchte, Electroanal. Chem. Interfacial Chem., 1967, 14, 205.