Preparation of Semiconducting Materials in the Laboratory Production of CdS Thin Films and Estimation of Their Band Gap Energy Jorge G. lbanez1 Universidad Iberoamericana, Depto. Ing. y C. Quimicas, Prol Paseo Reforma 880, Mexico, DF 01210, Mexico Omar Solorza CINVESTAV-IPN, Depto. de Quimica, Ap. Postal 14-740, Mexico, DF 07000; Mexico Enrique Gomez-del-Campo Universidad Iberoamericana, Depto. de Fisica, Prol Paseo Reforma 880, Mexico, DF 01 210, Mexico Electronics plays an increasingly important role in mode m society. The applications of semiconducting electronic devices .eo . from transistorized radios to the most com~licutrd computational systems. The fundnmental principles of semiconductors rSC, have been widely di;icussed in this Journal (see for example, refs. 1-91 as have the theory and a ~ ~ l i c a t i oin n sthe conversion of radiant enerw to electricicy'l.5, fi, and in the producrion of photoel&ochemical reactions (8, 10-1.7,. Nc\wlhelcss, it is somewhat uncommon to finddiscussions and analysis of the different methods of production of SC in the educational literature (see for example refs. 7,141 and much less to find experimental procedures for the laboratory preparation of SC (15).In the experiments described here, thin films of semiconducting CdS are deposited on glass substrates by usingfour different techniques; in addition, the band gap energy, Eg (defined in the Theoretical section below) of CdS is estimated by analyzing its optical absorption spectra. Cadmium sulfide was chosen since its band gap energy lies in the visible spectrum (2.42 eV, which corresponds to h = 512 nm) (16) and it is a widely used SC in optoelectmnicdevices (17,181. These experiments may be used as teacher demonstrations under carefully controlled conditions(see the Experimental section) in lab courses such as inorganic, physical and electrochemistry,electronics, thin films, materials science, etc. Theory The interactions among atomic energy levels in a crystal lattice vield a series of allowed enerw -. bands: the hiehest .energy.bulld occuplcd by electrons is called u & n c e hand t V I % and the lowrst unoccu~icdband is called a mnductim band (CB) (4).The energy gap between these two bands is called band gap energy (EJ; for insulators, Egis very large (larger than about 3 eV) whereas for semiconductors it acquires an intermediate value. For metals, the highest energy band with significant electronic occupancy is partially filled. Electrons can he excited from the VB to the CB by the absorption of incident radiant energy only if this energy is equal to or larger than E,. Thus, absorption of light occurs when the energy of a quantum (hv)equals Eg (1,19):
where h (wavelength) is expressed in nm. This paper was presented at the 199th. National Meeting of the American Chemical Society. Division of Chemical Education, Boston. MA April, 1990. 'Author to whom correspondence should be addressed.
872
Journal of Chemical Education
In the experiments described herein, h g i s determined a t the point where there is an abrupt change in the slope of the absorbance versus wavelength mrve of a semiconducting CdS thin film. Experimental Thin films of CdS are deposited on regular laboratory glass slides by using four different techniques: (a)chemical bath deposition, (b) spray pyrolysis, (c) electroplating and (d)high vacuum deposition. The glass substrates must be very clean (use of a ultrasonic bath is recommended), and their absorption spectra should be obtained prior to the deposition procedures in order to use o~ticallysimilar slides when obtainina the absorption spectra ofthe tlun films produced wee thc last Dart of this Kx~erimentalswtion~.All rcaaents should be knalytical ~ r a d eor better. Due to the tokicity and suspected carcinogenic activity of many Cd-containing compounds (20,211, we recommend that these experiments be performed as teacher demonstrations under carefully controlled conditions. Caution:special care needs to be given to avoiding breathing of Cd-containing vapors or dusts as well as touching the reagents and the thin films produced. The teacher should use gloves and perform the demonstrations under a fume hood. For the disposal of small amounts of Cd compounds, the insoluble compounds can be mixed with wet sand, swept up, and treated as normal waste; the soluble salts can be mopped up with water and run to waste, diluting greatly with water (21). For larger amounts, it is recornmended to bury the solid CdS waste in a landfill site approved for the disposal of chemical hazardous wastes (20); the dissolved Cd compounds should be precipitated as the sulfide, adjusting the pH of the solution to seven to complete precipitation; then, filter the insoluhles and dispose of them in a hazardous waste site; destroy any excess sulfide with sodium hypochlorite, and neutralize the solution before flushing down the drain (20). Chemical Bath Deposition This technique is based upon the controlled precipitation reaction of an insoluble salt (22). In order to deposit CdS, the experimental set-up shown in Figure 1is used. Here, 10 mL of a 1M Cd2+solution (for example Cd(CH3C00)2or CdS04)is mixed with 20 mL of an 8 M NH3(,, solution and with 10 mL of a 1M thiourea solution, (NH212 CS in a 50 mL glass beaker at mom temperature. Such a mixture produces the complex ion Cd(NH&2t that supplies the necessary Cd2+ions for the formation of an insoluble thin film (22, 23); in addition, it is well-known that (NH2)2CS yields S2-ionsinbasic media (22).The beaker is then placed
Termraturn
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Controller
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rn stant Temperature Baih Figure 3. Potentiostated set up for electrodepositingsemiconducting thin films.
Figure 1. Chemical bath deposition of semiconducting thin films in a water bath at -75 "C and its contents stirred with a magnetic bar; at this point, one or more clean glass slides are introduced into the solution (Fig. 1). The temperature is kept constant (-75 "C) for approximately half an hour; during this time a stoichiometric, self-controlled precipitation of a thin and homogeneous film of CdS occurs (22). Then, the glass slides (with a yellowish film) are taken out, rinsed with distilled water and air-dried. Spray Pyrolysis This technique involvcs thesprayingovcra hot subfitrate of solution containine - - a chemical ~ ~ ~ ~ ~ solublc salts of atom8 of the compound to be deposited. k this way, every droplet reaching the hot surface decomposes endothermically and forms a solid deposit of the desired compound; the solvent and volatile byproducts evaporate (22).In the present case, an alcoholic solution of 0.1 M thiourea and 0.1M CdClz is sprayed during 2 min onto a hot glass slide that is kept at constant temperature (-400 "C) by maintaining it floating on a fused Sn bath, as shown in Figure 2 (24, 25). The solution is impelled by an inert gas (e.g. Nz) that flows at about 4 L m i d ; the flow rate of the solution is controlled by a rotameter at about 3.8 mL An-'. The CdS film thus obtained is allowed to cool down to room temperature, rinsed with distilled water and air-dried. ~~
~
Electroplating In order to obtain electrodeposits on glass substrates, these must be previously made conductive. This is achieved by using the spray pyrolysis technique described above in the following way: an alcoholic solution of 0.2M SnCh 6 H 2 0 and 0.5 M HF is sprayed with the aid of Nd-4 L
m i d ) at a flow rate of 3.8 mL m i d onto a hot (-425 "C) glass slide. In this way, the SnC146Hz0undergoes a hydrolysis reaction right before reaching the substrate and forms a thin film containing a mixture of SnOz, SnO and Sn902 " " (24). . . Some F ions are substitutionallv incoruorated into the lattice by displacing Ocions; this effect lowers the oxide resistivitv and the film becomes conductive. Caution:hy&ofluoric acid solutions inflict destructive and extremely painful burns on human tissue (21); therefore, special care needs to be taken to avoid skin and eye contact and breathing its vapors. First aid procedures involve the removal of all contaminated clothing and flooding the skin with large volumes of running water, followed by the application of a 2% calcium gluconate gel (sold in some pharmacies as H-F antidote gel) (21)or an iced 70% alcohol solution or an ice-cold saturated solution of magnesium sulfate (Epsom salt) for at least 30 minutes or longer if pain persists (26). The injured person should he taken immediately to a physician. Further first-aid procedures should be consulted before working with HF (21, 26). A standard three-electrode cell, with a capacity of -100 mL, is hooked to a standard potentiostatic system (see for example, refs. 18,2731)andset up as showninFigure 3. The whole deposition process is carried out in an aqueous solution of 2 x 103M CdS04and 0.1MNa2S203,at pH = 2.5; the pH is adjusted with HzS04The solution is prepared with bidistilled water and deaerated with Nz before each run and a Nzatmosphere is maintained inside the cell throughout the process. Under the conditions just described, the SzOa"ions are decomposed by the HzS04to form colloidal sulfur (IS), which then is cathodically co-reduced with the metal ion. Ignoring the fact that metal ions are frequently complexed with solvent molecules or anions and that the sulfur exists as a polyatomic species in solution, this process may be written as follows (32): cdZ++s + 2 e
+ cds
(2)
The working electrode (WE) is a conductive glass slide, ~ ~gauze, i s a and the reference the a u x i ~ i a ~ ; i e c t r o d c ( ~ Pt elcctrode(RF:~isa ~aturatedcalomelelectrode(SCE~. From the literature (18,331and our own experimental work, we have found that an appropriate set of conditions for the co-deposit of Cd and S to form CdS is: E = -0.84 V versus SCE, T = 60 "C andt = 2 h. ARer this time, the potentiostat is turned off, the glass slides are removed, rinsed with distilled water, and air-dried. High-Vacuum Deposition To Heder
-
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Figure 2. Spray pyrolysis technique.
In this technique, the desired SC is evaporated under high vacuum onto a given substrate. It is necessary for the compound to be evaporated, not to decompose under the experimental conditions. In the present case, 0.3 g of CdS are placed in a Mo crucible, and a high vacuum tom) Volume 68 Number 10 October 1991
873
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Water Inlet Figure 4. High vacuum deposition apparatus. is applied with a mechanical pump coupled to a diffusion pump (see Fig. 4) (34). A current of 300 A is then passed through the crucible, and sublimation of CdS starts; the current is allowed to pass during 3 min. Under these conditions, the CdS sample does not fuse but sublimes and deposits on the glass slide placed a few centimeters above th;? crucible. caution: since this is a high current, care should be taken to avoid the possibilities ofelectrical shock and of fire, specially in areas where flammable liquids, gases or dusts are involved (26).In addition, the whole setup should be placed under a closed fume hood; if this is not possible, the high vacuum chamber should be enclosed by a shatterproof safety screen (26) and placed next to an exhaust system. Once each deposit is obtained by these techniques, the glass slides are inserted one bv one in the cell com~artment of a VIS spectrophotometer, ;sing as a reference a glass slide that has undergone the same treatment as the sample in turn but with no SC deposit on it (a comparison spectrum using the sample and the reference slides should be obtain& previous todepositing the SC in order to asiiure that no si~mificantdifference in their absnrhances exia+).Then, a c&ve of absorbance versus wavelength is obtained and plotted for each sample (See Figs. 5 a, b, c, and d, which correspond to each one of the techniques described above, respectively). These results were obtained with a doublebeam Cecil Series 5000 UV-VIS spectrophotometer. Results and Discussion In order to obtainE, for each sample, h ,is obtained from each curve in Figure 5 by calculating the point where an abrupt change in slone occurs (I):this is done bv findine the ikersectFon of thetwo tangentlines involved6ee curve 5b). Even though this procedure is not verv accurate, the results thus ohiained ake satisfactory forouFpurposes; i.e., A, from Figures 5a, b, c, d are found to he 519,490,515, and 523 nm Innte that the thicker film ~ d )deposited , under high vacuum, requires a larger absorbance scale and that it yields an undulation due to light interference patterns). By usingeq 1,theE,'s calculated are 2.39,2.53,2.41, and 2.37 eV, which can be compared to the reported value of 2.42 eV (16, 19). In order to obtain more accurate values of ED the in the SC ke.. direct or nature of the o ~ t i c atransitions l indirect) wouldaneed to be elucidated by cilcuiating the functions (uhv )'R.(ahv12and (ahv) . . "3 where a = absorotiou coefficient, and them versus hv for each data point (23, 35); however, this procedure requires as input the a values, which, in turn, require knowledge of the thickness of each film deposited. This more accurate procedure should be used if at all possible. However, in all likelihood, the films of CdS that are formed will not be of uniform 874
Journal of Chemical Education
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Wavelength (nml Figure 5. Absorbance versus. wavelength curves of the samples obtained by: (a) chemical bath deposition, (b) spray pyrolysis. (c) electroplating and (d)high vacuum deposition. thickness, complicating any analysis. Nevertheless, the much simoler nrocedure used above orovides a eood and practical qualiiative understanding oithe theoreTica1 concept of E.. as well as a simple and semiauantitative technique fopits evaluation. 60 alternatiie prwcdures for evaluatingl?. in commerciallv available CdS ootoelectronic devices hivegeen reported this Journal (15). Conclusions I t is possible to deposit thin films of semiconducting materials on glass substrates bv using relativelv simple techniques. ~e concept of band-gap energy can be easily shown experimentally by obtaining the absorbance spectrum of a SC thin film. Additional projects could involve the morpholo~caland electrical characterization of the films, aswell a s the measurement of the film thicknesses, theestimationofabsorption coefficients, thc analysisofthe presence of pinholes, the use of different substrates, the production of heterogeneous multilayers, the production of binary SC other than CdS, the production of ternary compounds, e t ~ Less . toxic materials (e.g., ZnS, CuO, Fez03, Bi2S3)should be used if a t all possible. Acknowledgment We t h a n k Jaime Mimila a n d Arturo Maldonado (CINVESTAV-IPN) for helpful comments and suggestions. Experimental assistance by Adolfo Finck, Enrique Sanchez, Carmen Gonzalez-Mesa, Diana Lozano, Ivonne Konik, Alejandra Mugica, Flora Gomez and Samuel Macias (Univ. Iberoamericana) is gratefully acknowledged. This work was supported by PEMEX, CONACYT and the Science and Technology Research Program of the U. Literature Cited 1. Jwter, N. J. J Chem Educ lW, 40.48-96.
2. Weller, P. F. J Chem. Edve lMT, 44, 391393. 3. nunlap, W. C. J c h p m E ~ Z C1861.38, . ~8-241. 4. Gumoe, E. P J. C k m . Edm. 1869.46.8W5. 5. Mickey, C. D.J Chem. Edue. 1981,58,418-423. 6. J.Chem.Edue Staff J Chem. Edue. 1919,66,26&266 7 . Hinib, H. H. J. Chem. Educ 1986.63,966959. 8 . Finklea.H.0.J Chpm.Educ. 1983.60. 325-327. 9. Ad1er.D. J Chem Edvc 1980,57,560-564.
10. Wr@hton,M. S. J. Chem.Edue. 1983,60.871481. 11. McOe%tt,J.I. J. Chem. Edue. 1984.61.217-221. 12. Turner. J.A.J C h . Educ. 1983.60.327-329, 13. spitler,'~.~. J them. due. 1985,60,33~32. 14. Wold, k J Chem Educ IS&?, 57,531536. 15. Boudreau, S. M.; h u h . R. D.; Boudresu. R. A. J. Chem. Educ 1983 60,498499. 16. CRCHondbwk of Chemistry m d P h y a h , R.C . Weast, Editor. 63rd Ed. CRC Press Inc. : Boea R a m , FL, 198283, n ES9.
17. Mimn, F. M.Engino~r'eMini -Notebo& : Optml~ctmnlcCircuits: Radio ShackCo., Inc. Fort Worth, TX,1986;p 18-19. 18. Power,G.P.:Peggs, D. R. ; Parker,A. J Ekctmhim. Aclo 1981,26,681482. 19. Sturm,J . E.J. Chem.Educ 1989.66,1052-1053. 20. Aldnch Catalogmandbook of Fine Chemieals. Aldrich Chemical Co., Inc.. Milwankee, WI, 1988-89,p 291 and F 16. 21. Muir, G. D.Horarda in tho Chemlcol Lobomtory, 2nd Ed.; The Chemical Society : New York, 1977;p23,106,175. 22. Choprs,K.L.: K8inthla.R. C.;Pandya,D.K.;Thskom,A. PInPhysicsofThinFilm; Hase. G.: Francombe. M. H.;Vossen, J. L., Eds.;AcademieRess :NewYark, 1982:
26. Manufachldg Chemiata'As~s~siitiii.Gui& for Safely i n the Cham~aiLobOmfory, 2nd. Ed.;Van-Nostrand-Reinhold: New York, 1972:pp 102-107.274. 21. Kisainger, P.T.;Heineman, W. R. J Clum. Edue 1985.60.702-706. 28. Gunasingham, H.;Ang, K P.J. Ckem.Edvc 1985,62.810-612. 29. Maloy, J. T.J Chem. Edue 1889.60.285-289. 30. Roe,D.K.: Wenzhao,L.: Geri8eher.H.J Ekcfmaml. Cham. 1982,136.32H37. 31. Mondon,F. J Ekbmeham. Soc. 1985. 132,319325. 32. BaransLi,A. S.; Bennett, M. S.; FawceU,W. R. J ApplFhys 1983,54,619&6394. 33. Jayachandran, M.; Chokalingam, M. J.: Venkateaan,V. K J Mot. S c i I a I L , 1989, 8,563-565. 34. Chopra, K. L.Thin FilmPhammno ;Mc Graw-HI11 : NeuYork, 1969:p 55. 35. Prmnik,P.:Bhattaehsrya,S.J Mot.Sci. I a f f . 1989,8,181-782.
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