Fluorinedoped tin oxide films prepared by spray pyrolysis were subjected to chemical etching using

hydrazine hydrate, mechanical polishing and electrochemical etching (cathodic), and the consequent

changes in the sheet resistance and optical transmiss~on of the films are reported. Scanning electron

microscopic examination revealed a change in the surface morphology of the film

Krishnakumar, R.

Lakshmanan, A.S.

Subramanian, V.

Suryanarayana, C.V.

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Bulletin of Electrochemistry 1 (5) Sep.-Oct. 1985, pp. 499-501 SURFACE TREATMENTS OF FLUOKINE DOPED TIN OXIDE FILMS R KRISHNAKUMAR, V SUBRAMANIAN, AS'LAKSHMANAN Central Electrochemical Research Institute, Karaikudi - 623 006 and C V SURYANARA YANA Emeritus Scientist, Anna University, Madras ABSTRACT Fluorinedoped tin oxide films prepared by spray pyrolysis were subjected to chemical etching using hydrazine hydrate, mechanical polishing and electrochemical etching (cathodic), and the consequent changes in the sheet resistance and optical transmiss~on of the films are reported. Scanning electron microscopic examination revealed a change in the surface morphology of the films. Key words: Fluorine doped tin oxide, spray pyrolysis, surface treatment INTRODUCTION T in oxide (SnOz) and indium-tin oxide (ITO) semiconductor films are useful as highly electrically conducting and transparent substrates for photovoltaic films in solar cells. They are also used as antireflection coatings. l l e se  films can be fabricated even over large areas by simple and mexpensive processes like spray pyrolysis. This paper deals with the studies Dn the effect of surface chemical treatments, mechanical polishing and electrochemical etching (cathodic) on fluorine-doped SnOz films prepared [ I ]  on glass substrates for photoanodic layers of cadmium chalcogenides and related compounds. The optical transmission and electrical conductivity of these layers are also discussed in this paper. EXPERIMENTAL Chemical etching Hydrazine hydrate treatmentoof SnOz films from the point of view of :hemical etching appears not to have been studied so far. SnOz films were treated with pure hydrazine hydrate and hydrazine-water mixtures (50-50 by volume) for different durations by dipping the samples in these solutions : the high resistance (2.5-3.0 kohm/square) SnOz films being treated for longer duration and the low resistance (- 80-100ohm/square) SnOl films lor shorter duration. After the treatment, the plates were cleaned using teepol and distilled water and thoroughly dried before measurements. Mechanical polirhing Mechanical' polishing was found to introduce an anomalous 'visible photoresponse extending to 600nm in the case of strontium titanate photoanodes [2,3). Hence thin films of SnOl were subjected to mechanical polishing with luster Ronuk compound 'Tripol A'. These were then degreased, washed in dilute HCI, cleaned with distilled water and dried. Electrochemical etching Electrochemical etching of SnOz has been studied earlier 141. The material being an oxide, the electrochemical etching has to be carried out only cathodically, since this involvea a reduction reaction influencing the oxide layer. It has been reported that the etch rate is independent of dilution between HCI and water in the ratios of 1 : 3 and 1 : 10 [4]. Higher concentrations of acid reaulted in lower etch rates. In the present study, etching has been done in dil. HCI (1 : 10) at room temperature for five minutes. The SnOz film acted as cathode while a platinum foil was used as anode. A d.c. power supply was employed and experiments were carried out at different current densities and for different durations. The sheet resistance of the films were measured by a two probe d.c. method and the optical absorption measurements were made using a Beckmann DU Spectrophotometer. RESULTS AND DISCUSSION Chemical etching Table I shows the decrease in the surface resistance values with duration of chemical etching treatment for the high resistance films in pure hydrazine hydrate as well as in 50-50 mixture of hydrazine hydrate and water; the decrease being more rapid in pure hydrazine hydrate. Table I :  Effect of concentmtion of chemical etchant on the sheet wsistance of SnOz films Initial resistance Resistance (kohmlsq) after treatment for k o h d s q  different durations (hr) (A) Pure hydrazine hydrate 2.95 - 0.92 - 0.79 0.68 - 0.69 - (B) Hydrazine hydrate-water mixture (50-50) 2.48 2 .24 -  2 .28 -  - 1 .78-  1.79 Figure 1 depicts the lowering of surface resistance values for two low resistance film samples A and B, in pure hydrazine hydrate for different durations. The optical transmission of the film A is shown in figure 2. It can be seen that the optical transmission increases from 58 % to 79% for 700nm (peak values). Thus, it has been found that treating in pure hydrazine hydrate not only decreases the surface resistance but also improves the optical transmission. These properties were monitored over a period of time (about one month) and found to remain constant. The films are generally known to be sensitive to changes of surface resistance due to adsorption. The influence of hydrazine hydrate on any adsorbed species during the preparation of the layers might in consequence influence the resistivity. Further studies may be necessary to understand the behaviour [5]. Krishnakumar et al- Fluorine domd tin oxide films o FILM A FILM B TfWE . MINUTES fig 1 Varlatlon of surface reslslance wlth duratlon ol trealmenl wlth pure hydrame hydrate for low resstance ( - 80-120 ohmkquare) fllms. Fw. 2 : Transmission spectrum of the film wrth chemical etching (1 8 2) end mechanical polishing (3&4) 183 = bstore Ireatmenl; 2 h 4  = after treatment Surface morphology of the SnB2 filma before and after hydrazine hydrete treatment is ahown in figure 3 (a) and (b) respectively. It can be seen that bigger crystah a m  discernible after the treatment. The surface resiataace of Sn02  Alma before and pftermechPnlcd poliahingis shown in Table 11. The optical tranarnission of the SnOz Film before and after treatment is given in figure 2. As a result of the polishing prwesa, the undisturbed polycryatdline surface becomes more and more disturbed and fragmented. The hifi local ternperaturea generated by the mechanical plishing may m u l t  in a &ally amorphous surface in the case of materials not having high melting points. Such a cahdition may be responsible for the increase in resistance of SnOz films after mechanical polishing [ti]. Fly 3(a) : SEM picture of the SnOn lllm in the 8s prepared candit~on Fig. 3 (b) : SEM picture of the SnO2 film after hydrame treatment Table I1 : Surface resistance of SnO, lllms before and after mechanical polishing. Before polishing 12.5 82.9 99 33 After polirhing 31.2 210 130.9 102 (ohmfsquare) - Surface morphology of the SnOa film after mechanical polishing is shown in figure 4. Though there is an increme in surface resistance, optical transmission improver considerably. It has been suggested [4] that during electrochemical etching, SnOz is reduced to me:allic tin by the naacent hydrogen produced at the cathode by the parsage of current and with the metallic tln dissolving in the electrolyte. In the present study, a grey layer has been observed to form which awn dissolves in the electrolyte. - ~ - - -  -- -~ - io0 Bulletin of Electrochemistry 1 (5) Sep.-Oct. 1985 Krishnakumar el al-fluonne doped tin oxide films Fig, 4: SEM picture of Ihe SnOz f~ lm sublected to merhanlcal pollshmg Tablea IU and IV show the changes in the surfare resirtame and optical transrnisaion o i  SnOz films before and d t e r  electrochemical treatment. Table Ill : Variation of surface resistance with current denslty on electro- chemical etching In dilute HCI (1 : 10) Duration = 5 mta Current density Sheet resistance (ohm/aquare) r n ~ .  crn-' Before etching Aftet etching Table IV: Varlation In transmission spectrum of tln oxide films subjected to eleclrochemical etching in dilute HCI (1 : 10) Dumtlon = 5 mts Wavelength Current denslity of Current density of v 20 m~.cm- '  40 rn~.crn-' M o r e  After Before After etching etching etching etching (96) ( % )  Rtduction in surface resistance is greatest at a current density of 2 0 m ~ . c m - ~ .  Bq~t the optical transmission impmves in direct proportion to the c u m n t  dmsity (Table W ) .  FIQ. 5: SEM picture of the SnOz lilm alter electrochem~cal elchi ig (cathodrc) Surface morphology oi the film after e l ~ t r o c h m i c a i  etching is shown in figure 5 where bigger crystallites are reen. CONCLUSIONS Treatment with hydrazine at  room temperature for five minutes improves both the electrical conductivity and optical transmission of  the S n 0 2  films. Mmhanical polishing impmves the optical transmission but results in an increase iq the surface resistance. Electrochemical etching at  a c u m n t  density of 4 0 m ~ . c m - '  improves the optical transmission. There is no marked change in the surface resistance. Rut at  a current density of 20 mA.cm-' the surface resistance decreases enormously while there is no change in the optical transmission. Ackiwwlcdycnttnts: The authors thank Dr V K Venkatcsan, Head of  the -Division of Electrochemical Materials acience and Electrophysics for his keen interest and encouragement and Mr Y Mahadeva Iyer and Mr K R  Ramakrishnan for providing the SEM pictures of the samples. REFERENCES 1. R Krishnakumar, K Ramakrishnan, V Subramanim, A S  Lakshmman and C V Suryanarayana, B d I  Elrcfrochcm 1-2 (1YR5)  195 2:B.T Chang, G Campet, P Hagenmuller and J B Gocrdenough, ISoiid Stole Chm 48 (1983) 247 3. P Salvador, I Electrochem Soc, 131 (1984) 5.50 4. B Jayant B a l i p  and Sorab K Gandhi, ~Etactrvchcm Soc, I14 (1977) 1059 5. BE Conway, Techniques ofElsc~rochsmirby, Vol 1 ,  Eda. Ernest Yeager and Alvind J Salkind, Wiley Interscience. New York (1972) p 542 6. S F  Page, Finc Sufart Fhinirh. Chapman and H a l l  Ltd., London (1948) p t i 1  Bulletin of Electrochemistry 1 (5) Sep.-Oct. 1985 501 

Surface treatments of fluorine doped tin oxide films

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