J. MATER. CHEM., 1993,3(12), 1295-1298 1295

Electrochemical Formation and Photoelectrochemical Characterization of (CdHg)Se Films

Kehar Singh* and Md. Rashid Tanveer Chemistry Department, Gorakhpur University, Gorakhpur, India

(CdHg)Se films of variable composition have been prepared using electrochemical codeposition. Photoelectric activity and current-voltage studies in the dark and under illumination revealed the formation of n- and p-type electrodeposits. The functional activity of these electrodeposits depends on the speed with which the current decreases during electrochemical codeposition under potentiostatic control. Photoaction spectral studies were used to investigate the effect of the inclusion of mercury on the bandgap of the electrodeposits. X-Ray diffraction and scanning electron microscopic studies have also been performed.

Keywords: Thin film ; Photoelectrochemistry; Semiconductor; Cadmium selenide

Cadmium selenide is endowed with attractive photoelectro- convertibility because of the compatibility of its absorption spectrum with that of the visible part of the solar spectrum.'*2 Adducts such as (CdHg)Te have been shown to be of consider- able interest from the point of view of some practical appli- c a t i o n ~ . ~ . ~ We have attempted to prepare (CdHg)Se films of variable composition by electrochemical means to investigate the effect of progressive inclusion of mercury on the bandgap of the electrodeposits. Photoelectrochemical studies and exam- ination of the current-voltage behaviour of the films in the dark and under illumination reveal the formation of both n- and p-type preparations. The nature of semiconductivity with which the (CdHg)Se electrodeposits are endowed depends on the composition of the electroplating solution and the depos- ition potential. Photoaction spectral studies indicate a lower- ing of bandgap with increased inclusion of mercury in the electrodeposits. X-Ray diffraction and scanning electron microscopic studies have also been carried out for characteriz- ation of the electrodeposits.

Experimental For electrochemical synthesis of (CdHg)Se films, titanium foils (M/S Titanium Equipment and Anode Manufacturing Co. Ltd. Madras) were polished with diamond paste and washed successively with acetone and distilled water. An experimental arrangement described e l s e ~ h e r e ~ . ~ was used for electrosyn- thetic work. CdS04 and HgC12 (CDH, India) and Se02 (Fluka Chemika, Switzerland) were used for the preparation of elec- troplating solutions. Variation of the deposition current with time during electrodeposition was studied to derive film thickne~ses.~ The electrodeposits were tested for their photo- electroactivity in 1 .O mol dmP3 (CH3C02)2Cd solution containing 0.1 mol dmP3 KI and 5mmol dmP3 I,. The curren t--voltage behaviour in the dark and under illumination was studied to characterise the electrodeposits. Photoaction spectral data were obtained using a photoirradiator system (fl3.4 Applied Photophysics, London) to study the variation of bandgap with composition of the electrodeposits.

Results and Discussion For electrochemical formation of a substance, identification of the optimal deposition potential is essential. To do this, the current-voltage behaviour of a solution containing 0.05 mol dm-3 CdS04 and 0.01 mol dmP3 SeO, was investi- gated, Fig. 1 shows that the electrochemical activity is confined

1.6

1.4

1.2

1 .o a E 4 0.8 I --.

0.6

0.4

0.2

0.0

-E VS. SCEN

Fig. 1 Current-voltage behaviour in 0.05 mol dm-j CdSO, and 0.01 mol dm- j SeO, solution

to - 0.40 to - 0.75 V us. SCE. Cadmium selenide electrodepo- sits within this range were prepared and tested for their photoactivity in 1.0 mol dm-3 (CH3C02),Cd, 0.1 mol dm-3 KI and 5 mmol dmP3 I2 solution, which had previously been found suitable for such studies.8 Fig. 2 shows that cadmium selenide films prepared using a deposition potential of -0.65 V us. SCE are endowed with maximum activity. Accordingly, all subsequent preparations were carried out using this deposition potential.

The experimental conditions under which electrochemical formation of (CdHg)Se films of variable composition were carried out are summarised in Table 1. Film thicknesses were estimated using the relation~hip:~

where h is the film thickness, Q is the charge in coulombs, We is the equivalent weight of the deposited material, A is the area of the electrode, p is the density of the depositing material and F is the Faraday constant. For the estimation of Q, the area under the I us. t curve was determined for the duration

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1296

210 > E

170. .- c. C a,

0 0 c

c

130 c1

90

50

300 1

'

J. MATER. CHEM., 1993, VOL. 3

>

c 0

200

180

160 1

0

0 0

4 I I I I I I

deposition potential ws. SCEN -0.60 -0.70 -0.80

Fig. 2 Identification of optimal deposition potential. Testing solution: 1.0 rnol dmP3 (CH3C02),Cd, 0.1 mol dm-3 KI and 5 mmol dm-3 I,.

of the deposition. The film thickness per coulomb h/Q (Table 1) is independent of the concentration of Hg2+ in the electroplat- ing solution. During electrochemical codeposition of (CdHg)Se, the deposition current decreases rapidly initially to a plateau. The relative speed with which the current decreases during electrodeposition (Fig. 3) is expected to depend on the quality of the deposit in terms of effective coverage of the substrate. Uniform, well bound, homogeneous electrodeposits are expected to have lower tl12 values, uiz. the time needed for the deposition current to reach half its initial value.g Accordingly, a correlation between photoresponse and t for electrodeposits is expected (Fig. 4). All the (CdHg)Se films were prepared under identical conditions using a deposition potential of -0.50 V us. SCE. The electrodeposits for which t1,2 is small exhibit an enhanced photoresponse. Therefore monitoring the variation of deposition current during electro- chemical codeposition of a photoelectroactive material gives useful information about its quality.

The formation of (CdHg)Se may be represented as

H2Se03 +Cd2+ +Hg2+ +4Hf +6e+(CdHg)Se+3H20

in the light of the established mechanism for CdSe formation by electrochemical codeposition. lo The above reaction may be visualized to occur in two steps:

H,Se0,+4Hf+4e+Se+3H20

Cd2 + + Hg2 + + Se + 2e+(CdHg)Se

Titanium-supported (CdHg)Se electrodeposits prepared using cadmium sulfate and selenium dioxide solutions contain- ing variable amounts of mercuric chloride, were tested for

Table 1 Deposition conditions used for the preparation of (CdHg)Se films

0.7

0.61

0.5

a E I \ +

I I I 1 ' I + 0.0 ' 20 40 60 80 100 180

tlmin

Fig. 3 Dependence of deposition current on time. Electroplating solution: 0.05 mol dm-3 CdSO,, 0.01 mol dm-3 SeO, and 5 x lop5 mol dm-3 HgC12. Deposition potential= -0.60 V us. SCE.

250 t

0 4 8 12 16 20 t:/s 2

Fig. 4 Photopotential and t1,2 values for different (CdHg)Se elec- trodes obtained using deposition potential of -0.50 V us. SCE. Electroplating solution = 0.05 rnol dm-3 CdS04, 0.01 mol dm-3 Se02 and 5 x lop5 rnol dm-3 HgCl,.

their photoresponse in a cadmium acetate solution in the presence of the 13//12 redox couple (Table 2). Inversion from p- to n-type semiconductivity with progressive inclusion of Hg in the electrodeposit occurs. Examination of the cur- rent-voltage behaviour in the dark and under illumination also supports this conclusion (Fig. 5 and 6). Photoaction spectral data were used to construct E i us. R plots to obtain bandgap values (Fig. 7). The bandgap depends on composition of the electroplating solution. Increasing the Hg content leads

[HgC12]/ 10 - initial steady rnol dm-3 current/mA current/mA

deposition film period/h thi~kness/lO-~ cm h Q-'/10-5 cm C-'

0 1 2 5

50 100

1.63 0.52 0.63 1 .oo 1.58 2.47

0.034 0.15 0.05 0.44 1.01 1.87

7.4 1.5 5.0 8.8

11.0 150

7.34 7.35 7.34 7.33 7.33 7.33

Deposition potential, -0.65 V us. SCE; electrolyte composition, 0.05 mol dm-3 CdSC4 0.01 rnol dmP3 SeO, and variable composition of HgCI,.

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Table 2 Photoresponse of (CdHg)Se films

-

1297

-24

0 1 2 5

50 100

-318 - 38 280 84 30 1 217

- 106 101 207 - 250 - 580 - 330 -133 - 335 - 202 -217 - 420 - 203

~ _ _ _ _ _ _ _ ~

Deposition potential, -0.65 V us. SCE; electroplating solution: 0.05 mol dmP3 CdS04, 0.01 mol dm-3 SeO, and variable composi- tion of HgCI,; testing solution: 1.0 mol dm-3 (CH3C02)2Cd, 0.1 mol dmP3 KI and 5 mmol dm-3 I,. ED, dark potential; EL, potential upon illumination; Ep, photopotential.

-14 E vs. SCEN I

b

Fig. 5 Current-voltage behaviour of p-(CdHg)Se: a, in dark; 0, under illumination

240 -

200 -

160- a 3 gJ 120- 3 0

80 -

40 -

-4Oc

Fig. 6 Current-voltage behaviour of n-(CdHg)Se: a, in dark; 0, under illumination

to a progressive lowering of the bandgap of (CdHg)Se elec- trodeposits (Fig. 8). The electrodeposits may be solid solutions of CdSe and HgSe. The covalent radii of Cd and Hg are comparable (ca. 1.48 A) and accordingly the adduct (CdHg)Se is expected to be a solid solution11 of CdSe and HgSe. The bandgap is not proportional to the concentration of Hg2+ ion (Fig. 8), possibly because the Hg content in the elec-

4000> 2000 0

700 720 740 760 780 800 820 A /nm

Fig. 7 Photoaction spectral data

1.0 I I 1 I 1

0 20 40 60 80 100 concentration of HgCI,/l 0-5 rnol dm-3

Fig. 8 Band-gap values for different (CdHg)Se films

trodeposit is different from the content of Hg2+ in the electroplating solution, as was found in similar cases.12 The inversion from p- to n-type semiconductivity with progressive inclusion of Hg in the electrodeposit possibly occurs because HgSe is an n-type semiconductor.

A typical X-ray diffraction pattern is shown in Fig. 9. The sharpness of the peaks shows the polycrystalline nature of the material and there is preferential orientation along (1 10) axes. The extra peaks obtained in the diffractogram are due to the titanium substrate. The lattice constant for the (CdHg)Se electrodeposit ( ~ ~ 6 . 3 6 7 A) is larger than that for CdSe (a= 4.309 A) owing to the incorporation of mercury in the lattice.

The scanning electron micrographs of the (CdHg)Se elec-

3oooc

10.0 30.0 50.0 70.0 90.0 2Bldegrees

Fig. 9 Typical X-ray diffractogram of (CdHg)Se using Cu-Kcr radiation

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1298 J. MATER. CHEM., 1993, VOL. 3

- 1 pm Fig. 10 Typical scanning electron micrograph of (CdHg)Se. Electroplating solution: 0.05 rnol dm-3 CdS04, 0.01 mol dm-3 SeOz and 5 x ~ O - ~ mol dm-3 HgCl,.

100

50

0

-50

-100

-1 50

-200

-250

-300

-350

T O

" Y \ deposition potential vs. SCEN -0.60 -0.70 -0.80

trodeposit (Fig. 10) show that the films are uniform with a grain size of ca. 1.5 pm.

The electrodeposited (CdHg)Se films prepared from an electroplating solution containing 0.05 mol dm- CdS04, 0.01 mol dm-3 SeOz and 5 x lo-' mol dm-3 HgCI2 using different deposition potentials were also tested for their photo- activity (Fig. 11). Electrodeposits with either p- or n-type semiconductivity were obtained at -0.60 V us. SCE. At lower deposition potentials, only p-type semiconducting films were obtained, while at higher deposition potentials, the elec- trodeposits exhibited only n-type semiconductivity. This poss- ibly arises due to the change in the composition of the electrodeposits when different deposition potentials are applied.

The studies here illustrate the possibility of preparing (CdHg)Se films of variable composition with n- or p-type semiconductivity, by electrochemical codeposition.

The authors are grateful to the Department of Non- Conventional Energy Sources, Government of India, for fin- ancial support. Thanks are also due to the Head, Chemistry Department, Gorakhpur University for providing laboratory facilities under the DRS and COSIST programmes of UGC.

References

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M. S. Wringhton, A. B. Ellis and S . W. Kaiser, Adv. Chem. Ser., 1977,163, 71. B. Miller, A. Heller, M. Robbius, S. Menezos, K. C. Chang and J. Thomson Jr., J . Electrochem. Soc., 1977,124,1019. M. Neumann-Spallart, G. Tamizhmani and C. L. Clement, J . Electrochem. SOC., 1990,137,3434. E. Mori, K. K. Mishra and K . Rajeshwar, J . Electrochem. SOC., 1990,137, 1 100. K. Singh and J . P . Rai, Mater. Sci. Lett . , 1985,4, 1401. K. Singh and J. P . Rai, Phys. Status Solidi A, 1987,99,257. K . Singh and R. K. Pathak, Ind. J . Chem. A , 1991,30,674. K . Singh, D. N. Upadhyay and J . P. Rai, Ind. J . Chem. A , 1986, 25, 3 14. K. Singh and A. K. Shukla, Solar Energy Mater., 1993,30, 169. K. K. Mishra and K . Rajeshwar, J . Electroanal. Chem., 1989, 273, 169. N. B. Hannay, in Solid State Chemistry, Prentice-Hall, Englewood Cliffs, 1967, p. 14. M. Neumann-Spallart and G. Tamizhmani, Thin Solid Films, 1989,169,313.

Paper 3/02803H; Received 17th May, 1993

0

Fig. 11 Variation of photoactivity with deposition potential. Testing solution: 1.0 rnol dmP3 (CH3CO2),Cd, 0.1 mol dm-3 KI and 5 mmol dm-3 I,. Electroplating solution: 0.05 mol dm-3 CdS04, 0.01 rnol dmP3 SeO, and 5 xlOP5 mol dmP3 HgCl,.

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