Electrodeposition technique and properties of semiconducting cadmium chalcogenide thin films from aprotic electrolytes

Thm Sohd Ftlms, 163 (1988) 279 284

279 E L E C T R O D E P O S I T I O N T E C H N I Q U E A N D P R O P E R T I E S OF SEMICONDUCTING CADMIUM CHALCOGENIDE THIN FILMS FROM APROTIC ELECTROLYTES* K

S.

BALAKRISHNANAND A.

C.

RASTOGI

National Phystcal Laboratory. Dr K S Krtshnan Road. New Delht 110012 (India)

A cathodic electrodeposltlon technique for fabricating stoichiometric coherent and oriented crystalline CdS thin films from aprotic electrolytes is described. Microstructural and X-ray diffraction studies show that the orientation of the c axis is normal to the film plane and that good nodular growth is preferred for deposition at current densities of about 5 mA c m - z m electrolytes with excess Cd 2 ÷ ions. The stoIchiometry of the films is found to be nearly independent of deposition variables. The growth kinetics show non-linear behavlour attributed essentially to extrinsic surface reactions on the depositing CdS films. ,

1. INTRODUCTION Semlconducting cadmium chalcogenides such as CdTe, CdSe and CdS in thin film form are potential materials for applications in low cost solar cells, photoelectrochemical cells and optoelectronic devices. Electrochemical techniques for the deposition of stoichiometnc thin films of these materials for electronic applications with large-size grains and oriented crystalhte structures have emerged as the subject of considerable experimental research. Several electrodeposition techniques based on different physicochemical principles such as anodic oxidation I of metal (cadmium) films in chalcogen-containmg solution and cathodic co-reduction 2 of cadmium and chalcogen compounds m acidic aqueous solutions have been developed. In the latter technique, the complex electrode reactions first require the reduction of the chalcogen c o m p o u n d to match its corresponding dianion. In another technique such reactions have been excluded by using elemental chalcogen m suitable solvents 3,4. We have adopted this approach and deposited CdS and CdTe thin films m non-aqueous aprotic electrolytes and have studied in detail the effect of the composition of the electrolyte and the electroplating current on the growth kinetics, stolchiometry, microstructure, crystalhne orientation and electrical properties of these films. In this paper we present results of such investigations on CdS thin films.

* Paper presented at the 7th International Conferenceon Thin Fdms, New Delhi, India, December7- 11, 1987 0040-6090/88/$3 50

© ElsevierSequom/Prmted in The Netherlands

280

K. S BALAKRISHNAN, A C. RASTOGI

2. EXPERIMENTAL PROCEDURE

Thin CdS films are deposited in a conventional electrochemical cell onto vacuum-deposited Cr + Ag glass, tin-oxide-coated conducting glass and polished stainless steel substrates. Platinum served as the counterelectrode The electrolyte is a solution of AR grade 1.0g CdCl 2 and 0.6g sulphur powder in 100ml of dimethylsulphoxlde solvent The deposition is carried out by passing a constant current through the medium kept at 140 '~C for various periods of time which forms films of different thicknesses. Film thicknesses are measured by a Talysurf 5-60 Instrument Mlcrostructure and X-ray dispersive microanalysis is carried out in a scanning electron microscope equipped with an energy-dispersive X-ray attachment The crystalline structure is identified with an X-ray dlffractometer using Cu K s monochromatic X-rays 3. RESULTS AND DISCUSSION

3 1 Physlcalpropertles Electrodeposlted CdS thin films appear uniform and smooth and adhere strongly to substrates The morphologies of typical CdS films on conducting glass substrates electrodeposlted from the electrolyte containing higher (about 0.075 M) and lower (about 0.025 M) concentrations of Cd 2+ are shown in Fig. 1 A smooth glass-like but cracked microstructure gives way to a nodular but coherent, pinholefree and uncracked structure when the Cd 2 + cationic content IS increased. The average grain size is large (greater than or equal to 2 jam) In the former case compared with about 0 5-0.6 ~m in the latter ~ase. The appearance of cracks in electrodeposlted binary compound thin films has been a major problem in their application, particularly in solar cells where they are potential shorting paths and carrier trapping sites 5. Their formation IS attributed to occlusion of the solvent during deposition. Subsequently the evaporative extraction of the solvent from the film leads to mlcrostructural changes 4'6. Internal stresses have also been suggested as the cause of such cracks 3 Our studies show 7 that the tendency to form cracks decreases and uniform films can be deposited by using higher current densities

(a) (b) Fig 1 Scanningelectron mlcrographsof CdS thin filmselectrodeposlted at 2 mAcm 2from electrolytic baths havingcadmmm-to-sulphurratios of{a)0 46([Cd2-] = 0025 M) and (bj 1 5 ([Cd2+] - 0 075 Mt

ELECTRODEPOSITION AND PROPERTIES OF

CdS

FILMS

281

and/or a higher Cd 2 + ionic concentration in the electrolytes. This implies that films deposited at higher rates are likely to remain coherent. Thus secondary effects such as lateral displacement of the deposited ions on the substrate plane during deposition (for the growth of bigger grains) compared with that in the deposition direction (normal to the substrate plane) appear to be responsible for the occurrence of these cracks. In Figs. 2 and 3 we present X-ray dlffractograms of the films grown under different electroplatmg conditions. Figure 2 shows the effect of Cd 2 + Ion concentration in the electrolyte and Fig. 3 that of the plating current density. A comparison with the standard CdS powder pattern reveals that a series of intense lines, of crystallographic indices 75 (100), 57 (110) and 45 (112), are missing. Instead, the relatively weak lines 17 (200), 25 (102), 42 (103) and 59 (002) are present. By considering the relative intensities of the lines of the standard powder samples it is apparent from Fig 2 that higher Cd 2 + ion concentration in the electrolyte promotes a preferred orientation of the crystalhtes The c axis of a large number of crystalhtes is oriented perpendicular to the film plane while a small number of the crystalhtes are oriented with their (101) plane parallel to the substrate Thus, for these, the c axis is inclined at 28 ° from the vertical direction With reduced Cd 2 ÷ concentration (less than about 0.055 M), the c axis is predominantly inclined at 28 ° and with even less Cd 2+ (less than about 0025 M) randomly oriented crystalhtes grow. A similar inference is drawn from Ftg. 3. Higher current densities exert a strong orienting influence on the crystalhtes Films deposited with current densities of about 5 mA cm - 2 exhibit a c axis orientation normal to plane of the film. A reduction In current density progressively changes their orientation to 28 ° from the vertical.

3 2 Electrlcalpropertws Typically the electrical resistivity of the electrodeposlted CdS films is very high and varies between 10 7 and 5 x 104 f~ cm. Annealing of the films at about 250 °C for periods of 30 60 min in vacuum results in only a nominal decrease in their resistivity, indicating that these films are extremely stable with respect to the structure and Stolchtometry. The reslstlvltles of as-deposited films are, however, strongly affected by the electrodeposition variables. Generally the resistivity decreases with the

EFFECT

OF Cd IN BATH C~/', 46 Cd/S ~ C (103) Ca/S ~ 4

(102)

I

I101)

I2001 ~]

55

:

~I J0

41o

410

2e

35

30

25

Fig 2 X-ray d~ffractlon patterns of CdS films showing the effect o f c a d m m m mn content m the electrolyte on the crystalhne orientation Cadmmm-to-sulphur ranos are 0 46, 0 10 and 1 5 corresponding to Cd 2 + ion concentrations of 0 025 M, 0 055 M and 0 075 M respectively

282

K. S. B A L A K R I S H N A N , A C RASTOGI

EFFECT OF PLATING CURRENT

(103)

-

(102)

t mA/Cm? 2 mA/Cmz 5 mA/Cm2

(200)

55

I

45

50

410

510

515

2~

20

Fig 3 X-ray diffraction patterns of CdS films showing the effect ofelectroplatlng current density on the crystalline orientation Cadmium-to-sulphur ratio IS 1 0 corresponding to Cd 2 + Ion concentration 0 055 M m c r e a s e m p l a t i n g current density. O f particular mterest is its d e p e n d e n c e on the t h i c k n e s s of the C d S films. T h i s is s h o w n in Fig. 4 as a f u n c t i o n of the tonic c o m p o s i t i o n of the electrolyte. T h e reslsttvtty of the film decreases w i t h increase m the t h i c k n e s s ' the decrease for a s t o l c h l o m e t r l c electrolyte c o m p o s i t i o n c o u l d be by as m u c h as t w o orders o f m a g n i t u d e . T h e p e r c e n t a g e c h a n g e is generally s m a l l for films prepared m electrolytes w i t h higher C d z + ion c o n c e n t r a t i o n s .

3.3. Growth kinetics T h e effect of the v a r i a t i o n in the C d 2 + - t o - S ratio in the electrolyte on the g r o w t h k m e t i c s of the e l e c t r o d e p o s l t e d C d S thin films is s h o w n in Fig. 5. A conststent d e p e n d e n c e of the film t h i c k n e s s on the i o n i c ratio has not e m e r g e d f r o m these g r o w t h curves F i l m s g r o w n f r o m baths c o n t a i n m g C d z + c o n c e n t r a t i o n s of Cd

•

TO S MOLAR RATIOIN ELECTROLYTE

--

A--

046

o--

I0

D - - ~ 4

x--40

FILM THICKNESS { u r n )

Fig 4 Thickness dependence of the electrical resistivity ofelectrodeposlted CdS thin films as a f u n c t i o n of electrolyte composition The electroplatlng current density in all cases is 2 mA cm z Cadmium-tos u l p h u r r a t i o s are 0 4 6 , 1 0, and 1 5 c o r r e s p o n d i n g

0 075 M respectively,

to C d 2 . ion c o n c e n t r a t i o n s

of 0 0 2 5 M, 0 0 5 5 M a n d

E L E C T R O D E P O S I T I O N A N D P R O P E R T I E S OF

CdS

FILMS

283

0 075 M or more exhibit a linear growth behavtour. In contrast, for lower Cd 2 + concentrations the initial growth is non-linear, becoming linear subsequently. The linear growth region of the stoichiometric bath obeys Faraday's law, conforming to a simple bl-electronic transfer process, C d 2 + q- S q- 2e --* CdS, at the cathode. The deviant linear region for the other bath composition corresponds to the effective participation of 8/3 electrons, implying higher oxidation states of sulphur. Optical absorption spectra of the electrolyte show v absorption bands at 490 nm and 618 nm, suggesting the presence of S 2 and S~ - ions respectively in the electrolyte, probably through the reactmns 8 ~0S8+2 e --' S 2 andS8+3Se ~ $ 46 2 ,S 2 + 2 e ~2S23 . Despite the varied deposition parameters our X-ray energy dispersion results show no change in the CdS film stoichiometry, suggesting that deposition takes place by a reaction at cathode between cadmium and sulphur which remains largely unaffected by relative ion concentration and transport. The growth behaviour in Fig. 5 changes with CdS film thickness which suggests that extrinsic reactions such as physicoadsorption 11 of sulphur on CdS film which inhibits further cadmium reduction and co-adsorption of C1 runs which act as attachment sites for further Cd z+ adsorption 12 are responsible for observed growth kinetics. We also find 7'8 that S 2 and $3 2 ions are more stable than $8 2 Thus the relative change in the concentrations of these ions m the electrolyte during deposition can effect the change in the deposition mechanism with deposition time, as indeed is observed

10I 4

z / S

i'~

..,./~

o"

4

4

~.,, ~ .~T,o

/I

D

~0

2

Q--

4

6

~©'

DEPOSITION

14

2

4

6

TiME ( mm )

Fig 5 Growth kinetics of CdS thin films as a function of electrolyte composition The e]ectrop]atlng c u r r e n t d e n s i t y In all cases is 2 m A c m 2 , theoretical growth behaviour based on a blelectromc t r a n s f e r p r o c e s s , - - - - , t h e o r e t i c a l g r o w t h b e h a v l o u r for a t r a n s f e r p r o c e s s i n v o l v i n g 8/3 e l e c t r o n s C a d m i u m - t o - s u l p h u r ratios a r e 0 46, 1 0 a n d 1 5 c o r r e s p o n d i n g to C d 2 + i o n c o n c e n t r a t i o n s of 0 025 M, 0 055 M a n d 0 075 M respectively

4 CONCLUSIONS In conclusion we show that structurally suitable coherent stoichlometrlc CdS thin films with oriented crystalline structure can be prepared by the electrodeposition technique. A linear film growth behavlour promoted by high C d 2 + IOnic

284

K. S B A L A K R I S H N A N , A. C RASTOGI

concentration and electroplatmg current density is most desirable for obtaining coherent CdS films with the preferred c axis orientation normal to the film plane. ACKNOWLEDGMENTS

We thank the Director, National Physical Laboratory, for permitting us to publish this work Thanks are also due to the X-ray and Electronmlcroscopy Groups of the National Physical Laboratory for their assistance REFERENCES

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