We report improved methods for electroplating cadmium sulfide (CMS) films. Aprevious problem was cracking/flaking of films deposited from organic solutions of elemental sulfur; attempts to improve adhesion via bath additives reduced grain size. Aqueous baths of thiosulfate ions yield cadmium-richness at low T temperatures (T), long deposition times, and/or poor bath stability. Developments in our work to be discussed include (1) plating ofuniform, adherent, and stoichiometric CdS from tetraethylene baths of CdCl 2 and elemental sulfur at T \u3e70° C with minimal cracking/flaking, (2) improved uniformity/ adherence by use of CdL\u3e, and (3) swept voltage methods in aqueous thiosulfate baths to plate stoichiometric (vs. Cd-rich) films near room temperature

Aleem, Wasim

Barber, Chris

Engelken, Robert

Kemp, Brandon

Khan, Imran

Raza, Arif

ScholarWorks@UARK

Journal of the Arkansas Academy of ScienceVolume 50 Article 161996Improved Methods for Electroplating CadmiumSulfide Thin FilmsBrandon KempArkansas State UniversityRobert EngelkenArkansas State UniversityArif RazaArkansas State UniversityWasim AleemArkansas State UniversityImran KhanArkansas State UniversitySee next page for additional authorsFollow this and additional works at: http://scholarworks.uark.edu/jaasPart of the Electronic Devices and Semiconductor Manufacturing CommonsThis article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able toread, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking priorpermission from the publisher or the author.This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the Arkansas Academyof Science by an authorized editor of ScholarWorks@UARK. For more information, please contact scholar@uark.edu.Recommended CitationKemp, Brandon; Engelken, Robert; Raza, Arif; Aleem, Wasim; Khan, Imran; and Barber, Chris (1996) "Improved Methods forElectroplating Cadmium Sulfide Thin Films," Journal of the Arkansas Academy of Science: Vol. 50 , Article 16.Available at: http://scholarworks.uark.edu/jaas/vol50/iss1/16Improved Methods for Electroplating Cadmium Sulfide Thin FilmsAuthorsBrandon Kemp, Robert Engelken, Arif Raza, Wasim Aleem, Imran Khan, and Chris BarberThis article is available in Journal of the Arkansas Academy of Science: http://scholarworks.uark.edu/jaas/vol50/iss1/16i<(<ii<44«-79Improved Methods For Electroplating Cadmium Sulfide Thin FilmsBrandon Kemp, Robert Engelken, ArifRaza,Wasim Aleem, ImranKhan, and Chris BarberOptoelectronic Materials Research LaboratoryDepartment ofEngineeringArkansas State UniversityP.O. Box 1740State University, Arkansas 72467AbstractWe report improved methods for electroplating cadmium sulfide (CMS) films. A previous problem was cracking/flak-ing of films deposited from organic solutions of elemental sulfur; attempts to improve adhesion via bath additivesreduced grain size. Aqueous baths of thiosulfate ions yield cadmium-richness at low T temperatures (T), long depositiontimes, and/or poor bath stability. Developments in our work to be discussed include (1) plating ofuniform, adherent, andstoichiometric CdS from tetraethylene baths of CdCl 2 and elemental sulfur at T > 70° C with minimal cracking/flaking,(2) improved uniformity/ adherence by use of CdL>, and (3) swept voltage methods in aqueous thiosulfate baths to platestoichiometric (vs. Cd-rich) films near room temperature.IntroductionElectrodeposition has emerged as a commerciallyattractive method for depositing semiconductor thinfilms for optoelectronic device applications due to lowtemperature and atmospheric operation, low equipmentand supply costs, easy scale-up, lack ofhazardous reactantgases, and precision measurement and control via its elec-trical nature. It contrasts with other methods of deposi-tion such as vacuum evaporation/sputtering, chemicalvapor deposition (CVD), and molecular beam epitaxy(MBE), which require expensive, high vacuum, and/orgas-phase apparatuses. Standard chemical precipitationdeposition (CPD) techniques for the synthesis of metalsulfides utilize the very toxic sulfur source, thiourea,(NH2)2CS.Cadmium sulfide, CdS, is a leading semiconductormaterial used in optoelectronic devices. It has applica-tions in light detection and photovoltaic cells. There hasbeen much interest in the applications of n-CdS as thewindow material in CdS/CdTe and CdS/CuInSe 2 solarcells because of its 2.42 eV bandgap. CdS can be elec-trodeposited from organic solutions containing molecu-larly dissolved sulfur and acidic, aqueous based thiosul-fate ion, S9O.5-, solutions.In many organic solvents elemental sulfur dissolvesinto polyatomic, zero valent sulfur molecules Sx(0) (x=l to8). Film formation can proceed bya. Cd+^ + 2e =Cd(s) (1)b. Cd(8) +S x =CdS (s) + S x_] and/ora. Sx+ 2e = S-2 +Sx.] (2)b. S-2 + Cd+-;= CdS (s)where the former case is usually dominant unless themetal ions are heavily complexed.Electrodeposition of CdS from aqueous electrolytesutilizes thiosulfate as a low cost, low toxicity, and very sol-uble sulfur source. In acidic solutions, thiosulfate decom-poses to release sulfur:a. S 2O3-2(aq) =S(g) +SO3-2(aq) (3)The formation of CdS then proceeds on the cathode sur-face as in (1) and/or (2) above (with x = 1). Due to thereduction in cadmium atom activity provided through thechemical reaction with sulfur, it is possible to plate CdS atvoltages positive of that required to plate metallic cadmi-um. This is known as underpotential deposition(Engelken and Van Doren, 1985).Works describing the electrodeposition ofII-VIsemi-conductor compounds such as cadmium telluride, CdTe(Poole et al., 1994; Engelken and Van Doren, 1985:Engelken, 1988; Kemp et al., 1995), copper sulfide, CuxS(Engelken and McCloud, 1985), and tin sulfide, SnS(Mishra, Engelken, et al., 1989) have accumulated in thepast 15 years. There are two basic electrolytes specific tothe electrodeposition of CdS films: (1) organic elec-trolytes of moleculaiiy dissolved sulfur and ionically dis-solved metal salts (Baranski and Fawcett, 1980) and (2)aqueous solutions of the thiosulfate ion (McCann andSkyllas-Kazacos, 1981). Both tend to produce metal-richProceedings Arkansas Academy of Science, Vol.50, 199679Journal of the Arkansas Academy of Science, Vol. 50 [1996], Art. 16Published by Arkansas Academy of Science, 199680tfilms unless conditions are optimized. Pulsed depositioni' methods offer potential for overcoming this problem(Fatas et al., 1984). In fact, more recent work has focused* *on pulsed electrodeposition techniques for the prepara-tion of CdS/CdTe thin film solar cells (Das and Morris,T 1993; Das, 1993).Materials and MethodsThe apparatuses used in the experiments included an< > IBM EC 225 voltammetric analyzer, a Hewlettt Packard7047A x-y-trecorder, and a 1000 W quartz/tungsten halo-gen lamp for in-situ photoconductivity measurements.The anode was made of Poco pyrolyzed graphite and itssubmerged area was approximately 6 cm-. The referenceelectrode was a Fisher Ag/AgCl reference electrode. Thecathode clamp was constructed ofPoco Graphite, and thecathode substrates were Donnelly indium tin oxide (ITO)coated glass. Other submerged items included an alcoholthermometer and a Teflon covered stirring bar.Supplies and reagents used included ACS grade AlfaNa2S 2O3, 99+% Aldrich CdCl2, CdSO 4, and Cdl2, andT 99+% Alfa ethylenediaminetetraacetic acid (EDTA).Aldrich tetraethylene glycol of purity 99% was used asthe organic solvent. Distilled water was used for the aque-ous depositions.A Rigaku D-Max x-ray diffractometer was used tocharacterize crystallinity. A Perkin-Elmer Lambda 19UV/VIS/NIR spectrophotometer was used for opticalabsorbance spectroscopy.!? Films were deposited by swept deposition from acidicsolutions. The solutions contained 150 ml of water, with? 0.01 M CdSO 4, 0.02 M Na 2S2O,, and 0.03 MEDTA at apH of 2.0. The solutions were heated to 60-7()°C for? voltammetric analysis and filmdeposition. Thin filmswere deposited by application of a continuous wave, ramp? voltage. The sweep rate was 5 mV/s.Films were deposited from TEG at constant voltage at* 90-100 C. The solutions were saturated with sulfur andcontained 0.01 M CdCL, or Cdl2.Deposition was carried? out at -0.92 V with respect to the reference electrode for 4hours.[w Results and Discussionw TEG Based Solutions. —Several films were depositedfrom the tetraethylene glycol (TEG) baths until optimum? conditions were found. Golden (MS films were depositedby constant voltage electrodeposition. Uniformity and? adherence were much improved over those grown by usfrom ethylene glycol using similar methods. Figure 1? shows the plot of absorbance versus wavelength for one?film. The sharp inflection near 500 nm is indicative ofCdS. Figure 2 is the x-ray diffraction data for the film.The sharp intensity peaks corresponding to the overlaidCdS powder diffraction file card reveal that the sample isploycrystalline CdS.The TEG based solutions were used in initialexperi-ments to plate large area (i.e. 4 in. x 7 in.) films for lightdetection devices. The scaled-up method also producedvery uniform films. The replacement of CdCl2 with Cdl.jincreased uniformity. These films appeared more goldenorange in color, as opposed to golden yellow. Initialresults indicate that use of TEG as a CdS electrodeposi-tion electrolyte improves adherence and uniformity with-out sacrificing key material characteristics.Aqueous Solutions.—Photovoltammetry was conduct-ed for the aqueous solutions to determine the most effec-Fig. 1. Optical absorbance vs. wavelength for a CdS filmelectrodeposited from the TEG solution. Note theabsorption edge near 500 nm, consistent with CdS's 2.42eV bandgap.Fig. 2. X-ray diffraction data for the CdS filmelectrode-posited onto ITO-coated glass from the TEG solution.Vertical lines denote standard CdS powder diffractioncard [6-314].Proceedings Arkansas Academy of Science, Vol.50, 199680Journal of the Arkansas Academy of Science, Vol. 50 [1996], Art. 16http://scholarworks.uark.edu/jaas/vol50/iss1/1681Brandon Kemp, Dr. Robert Engelken, ArifRaza, Wasim Aleem, Imran Khan, and Chris Barbertive way of depositing photoconductive CdS. Figure 3shows a voltammogram (i.e. without pulsed illumination)of a typical aqueous CdS electrodeposition solution. Thelack of structure positive of the undei potential Cd platingwave indicates that CdS is not being cathodically plated atthese positive most voltages. Figure 4 shows a similar pho-tovoltammagram with anodic photocurrents appearingon the reverse (i.e. negative to positive voltage) sweep fol-lowing the cadmium stripping structure. Study of theseplots indicates that sulfur species released from the thio-sulfate ions are reacting at the cathode surface with plat-ed cadmium. The absence of a stripping wave more posi-tive than the cadmium plating range reveals that CdSthus formed remains on the substrate surface.Photovoltammetry was conducted periodically duringswept deposition filmgrowth. Figure 5 shows the initiasweep during a filmdeposition. As previously notedanodic photocurrent pulses are observed on the reversesweep. Figure 6 is a photovoltammogram conducted during the same filmdeposition but after several sweepsNotice the increase in photocurrent magnitude with thissweep. At this point it was observed that a thin, goldenyellow filmhad formed on the substrate surface. Anoptical absorbance vs. wavelength plot for this filmis shownin Fig. 7. The sharp rise in absorbance near 500 nm isconsistent with the bandgap ofCdS. As might be expect-ed with thin films deposited near room temperature, x-ray diffraction reveals poor crystallinity. Figure 8 showsthe pattern overlaid with a standard CdS powder diffrac-Fig. 3. Cyclic voltammogram (current-voltage curve) foran ITO coated glass cathode in an aqueous solution at60 °C with 0.01 M CdCU, 0.02 Na2S 2Os, and 0.03 MH4EDTA at pH =2. Note the lack of voltammetric struc-ture other than the cadmium plating and stripping peaks.Fig. 5. Cyclic voltammogram recorded during the initialsweep of a CdS filmdeposited by swept voltage methodsin a thiosulfate bath. Note the anodic photocurrents onthe reverse sweep and the absence of the cadmium strip-ping peak due to the more positive stopping potential.Fig. 4. Cyclic voltammogram as with Fig. 3 except thatpulsed light is incident on the substrate surface. Note theappearance ofanodic photocurrent pulses on the reversesweep following the stripping ofcadmium.Fig. 6. Cyclic voltammogram as with Fig. 5 except thefilmhad deposited for about 1 hour. Note the change incurrent scale and the increase in the magnitude of thephotocurrent pulses.Proceedings Arkansas Academy of Science, Vol.50, 199681Journal of the Arkansas Academy of Science, Vol. 50 [1996], Art. 16Published by Arkansas Academy of Science, 1996Improved Methods For Electroplating Cadmium Sulfide ThinFilmsAlso, the introduction of iodide ions into the solution fur-ther enhances uniformity.Electrodeposition of CdS from aqueous solutionscontaining thiosulfate ions was much improved by theimplementation of swept voltage deposition. This methodtends to produce stoichiometric CdS, as opposed to Cd-rich films. This technique is easily monitored by in-situvoltammetry.Future related work willinvolve further refinement ofthe electrodeposition of CdS from hot organic baths (i.e.T >100°C) and more detailed investigation of pulsed andswept methods for the electrodeposition of CdS fromaqueous solutions. Further experimentation willinclude amore quantitative approach to the growth and characteri-zation of films deposited from organic electrolytes con-taining CdL; Also, dithionite ion, S^O^"-, as a sulfursource in acidic solutions willbe investigated.Acknowledgements. —This work was jointly support-ed by the Arkansas Science Information Liaison OfficeUndergraduate Research Fellowship Program (Bo Ryall,Coordinator), the AR/NASA EPSCoR Program (Dr.Hameed Naseem, Arkansas Advanced Photovoltaic-Materials Research Center Director; Dr. GaylordNorthrop, State Coordinator; Dr. Dennis Flood, NASALewis Research Center, NASA Mentor), and InvoTek, Inc.of Alma, AR (Mr. Tom Jakobs, President) through aNational Institutes of Health Phase IISBIR Grant. Wealso gratefully acknowledge the ongoing support ofArkansas State University.Literature CitedBaranski, A.and W. Fawcett. 1980. J. Electrochem. Soc,127:766.Das, S. 1993. Characterization of CdCl2 treated elec-trodeposited CdS/CdTe thin filmsolar cells. SolarEnergy Materials and Solar Cells. 29:277-278.Das, S. and G. Morris. 1993. Preparation and propertiesof CdS/CdTe thin filmsolar cells produced by peri-odic pulse electrodeposition techniques. Solar EnergyMaterials and Solar Cells. 30:107-1 18.Engelken, R. 1988. Ionic electrodeposition ofII-VIandIII-Vcompounds -Part IV. J. Electrochem. Soc.135:834-839.Engelken, R. and H.McCloud. 1985. Electrodepositionand material characterization ofCu xS films. J.Electrochem. Soc. 132:567-573.Engelken, R. and T. Van Doren. 1985. Ionic electrodepo-sition ofII-VIand III-Vcompounds - Parts Iand II.J.Electrochem. Soc. 132:2904-2909, 2910-2919.Fatas, E., R. Duo, P. Herrasti, F. Arjono, and E. GarciaCamarero. 1984. J. Electrochem Soc. 131:2243.Kemp, B., R. Engelken, A. Raza, A. Siddiqui, and O.Fig. 7. Optical absorbance vs. wavelength spectrum for aCdS filmelectroplated by swept deposition characterizedby the photovoltammentry in Fig. 5 and Fig. 6. Note theabsorption edge near 500 nm,Fig. 8. XRD data for filmdescribed by current vs. voltageplots in Fig. 5 and Fig. 6. Note the absence of strongpeaks (except for those corresponding to ITO) which is agood indication that the filmis amorphous. The verticallines denote standard CdS powder diffraction peak cardtion file card. The CdS peaks are too small relative to the, background to be a conclusive match although the dis-tinct anodic photocurrents are strong indication ofgood,, albeit nearly amorphous, semiconducting CdS.Conclusions and Future PlansThe results discussed reveal improved methods forelectrodepositing CdS from tetraethylene glycol andacidic aqueous solutions. It has been found that the useof TEG improves the uniformity of films over thosegrown by similar techniques from other organic solvents.Proceedings Arkansas Academy of Science, Vol.50, 199682Journal of the Arkansas Academy of Science, Vol. 50 [1996], Art. 16http://scholarworks.uark.edu/jaas/vol50/iss1/16Brandon Kemp, Dr.Robert Engelken, ArifRaza, Wasim Aleem, Imian Khan, and Chris BarberMustafa. 1995. Diagnostics ofCdTe electrodepositionby rest potential voltammentry. Proceedings of the79th Annual Meeting of the Arkansas Academy ofScience. 49:97-93.McCann, J. and M.Skyllas-Kazacos. 1981. The electro-chemical deposition and formation of cadmium sul-phide thin filmelectrodes inaqueous electrolytes. J.Electroanal. Chem. 119:409-412.Mishra, K.,R. Engelken, K.Rajeshwar, A.Weiss, M.Murley,M.Slayton, and H.McCloud. 1989.Electrodeposition and characterization ofSnS thinfilms.J. Electrochem. Soc. 136:1915-1923.Poole, C, R. Engelken, B.Kemp, and J. Brannen. 1994.Tetraethylene glycol - based electrolytes forhightemperature electrochemical deposition of compoundsemiconductors. Proceedings of the 78th AnnualMeeting of the Arkansas Academy ofScience. 48:133-139.Proceedings Arkansas Academy of Science, Vol.50, 1996 483Journal of the Arkansas Academy of Science, Vol. 50 [1996], Art. 16Published by Arkansas Academy of Science, 1996

Improved Methods for Electroplating Cadmium Sulfide Thin Films

UARK (University of Arkansas )

Journal of the Arkansas Academy of Science Volume 50 Article 16 1996 Improved Methods for Electroplating Cadmium Sulfide Thin Films Brandon Kemp Arkansas State University Robert Engelken Arkansas State University Arif Raza Arkansas State University Wasim Aleem Arkansas State University Imran Khan Arkansas State University See next page for additional authors Follow this and additional works at: https://scholarworks.uark.edu/jaas  Part of the Electronic Devices and Semiconductor Manufacturing Commons Recommended Citation Kemp, Brandon; Engelken, Robert; Raza, Arif; Aleem, Wasim; Khan, Imran; and Barber, Chris (1996) "Improved Methods for Electroplating Cadmium Sulfide Thin Films," Journal of the Arkansas Academy of Science: Vol. 50, Article 16. Available at: https://scholarworks.uark.edu/jaas/vol50/iss1/16 This article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able to read, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking prior permission from the publisher or the author. This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the Arkansas Academy of Science by an authorized editor of ScholarWorks@UARK. For more information, please contact scholar@uark.edu, uarepos@uark.edu. Improved Methods for Electroplating Cadmium Sulfide Thin Films Authors Brandon Kemp, Robert Engelken, Arif Raza, Wasim Aleem, Imran Khan, and Chris Barber This article is available in Journal of the Arkansas Academy of Science: https://scholarworks.uark.edu/jaas/vol50/iss1/16 i<(<ii<44«-79Improved Methods For Electroplating Cadmium Sulfide Thin FilmsBrandon Kemp, Robert Engelken, ArifRaza,Wasim Aleem, ImranKhan, and Chris BarberOptoelectronic Materials Research LaboratoryDepartment ofEngineeringArkansas State UniversityP.O. Box 1740State University, Arkansas 72467AbstractWe report improved methods for electroplating cadmium sulfide (CMS) films. A previous problem was cracking/flak-ing of films deposited from organic solutions of elemental sulfur; attempts to improve adhesion via bath additivesreduced grain size. Aqueous baths of thiosulfate ions yield cadmium-richness at low T temperatures (T), long depositiontimes, and/or poor bath stability. Developments in our work to be discussed include (1) plating ofuniform, adherent, andstoichiometric CdS from tetraethylene baths of CdCl 2 and elemental sulfur at T > 70° C with minimal cracking/flaking,(2) improved uniformity/ adherence by use of CdL>, and (3) swept voltage methods in aqueous thiosulfate baths to platestoichiometric (vs. Cd-rich) films near room temperature.IntroductionElectrodeposition has emerged as a commerciallyattractive method for depositing semiconductor thinfilms for optoelectronic device applications due to lowtemperature and atmospheric operation, low equipmentand supply costs, easy scale-up, lack ofhazardous reactantgases, and precision measurement and control via its elec-trical nature. It contrasts with other methods of deposi-tion such as vacuum evaporation/sputtering, chemicalvapor deposition (CVD), and molecular beam epitaxy(MBE), which require expensive, high vacuum, and/orgas-phase apparatuses. Standard chemical precipitationdeposition (CPD) techniques for the synthesis of metalsulfides utilize the very toxic sulfur source, thiourea,(NH2)2CS.Cadmium sulfide, CdS, is a leading semiconductormaterial used in optoelectronic devices. It has applica-tions in light detection and photovoltaic cells. There hasbeen much interest in the applications of n-CdS as thewindow material in CdS/CdTe and CdS/CuInSe 2 solarcells because of its 2.42 eV bandgap. CdS can be elec-trodeposited from organic solutions containing molecu-larly dissolved sulfur and acidic, aqueous based thiosul-fate ion, S9O.5-, solutions.In many organic solvents elemental sulfur dissolvesinto polyatomic, zero valent sulfur molecules Sx(0) (x=l to8). Film formation can proceed bya. Cd+^ + 2e =Cd(s) (1)b. Cd(8)+S x=CdS (s) + S x_] and/ora. Sx+ 2e= S-2 +Sx.] (2)b. S-2 + Cd+-;= CdS (s)where the former case is usually dominant unless themetal ions are heavily complexed.Electrodeposition of CdS from aqueous electrolytesutilizes thiosulfate as a low cost, low toxicity, and very sol-uble sulfur source. In acidic solutions, thiosulfate decom-poses to release sulfur:a. S 2O3-2(aq)=S(g) +SO3-2(aq) (3)The formation of CdS then proceeds on the cathode sur-face as in (1) and/or (2) above (with x = 1). Due to thereduction in cadmium atom activity provided through thechemical reaction with sulfur, it is possible to plate CdS atvoltages positive of that required to plate metallic cadmi-um. This is known as underpotential deposition(Engelken and Van Doren, 1985).Works describing the electrodeposition ofII-VIsemi-conductor compounds such as cadmium telluride, CdTe(Poole et al., 1994; Engelken and Van Doren, 1985:Engelken, 1988; Kemp et al., 1995), copper sulfide, CuxS(Engelken and McCloud, 1985), and tin sulfide, SnS(Mishra, Engelken, et al., 1989) have accumulated in thepast 15 years. There are two basic electrolytes specific tothe electrodeposition of CdS films: (1) organic elec-trolytes of moleculaiiy dissolved sulfur and ionically dis-solved metal salts (Baranski and Fawcett, 1980) and (2)aqueous solutions of the thiosulfate ion (McCann andSkyllas-Kazacos, 1981). Both tend to produce metal-richProceedings Arkansas Academy of Science, Vol.50, 199679Journal of the Arkansas Academy of Science, Vol. 50 [1996], Art. 16Published by Arkansas Academy of Science, 199680tfilms unless conditions are optimized. Pulsed depositioni'methods offer potential for overcoming this problem(Fatas et al., 1984). In fact, more recent work has focused* *on pulsed electrodeposition techniques for the prepara-tion of CdS/CdTe thin film solar cells (Das and Morris,T 1993; Das, 1993).Materials and MethodsThe apparatuses used in the experiments included an< >IBM EC 225 voltammetric analyzer, a Hewlettt Packard7047A x-y-trecorder, and a 1000 W quartz/tungsten halo-gen lamp for in-situ photoconductivity measurements.The anode was made of Poco pyrolyzed graphite and itssubmerged area was approximately 6 cm-. The referenceelectrode was a Fisher Ag/AgCl reference electrode. Thecathode clamp was constructed ofPoco Graphite, and thecathode substrates were Donnelly indium tin oxide (ITO)coated glass. Other submerged items included an alcoholthermometer and a Teflon covered stirring bar.Supplies and reagents used included ACS grade AlfaNa2S 2O3, 99+% Aldrich CdCl2, CdSO 4, and Cdl2, andT 99+% Alfa ethylenediaminetetraacetic acid (EDTA).Aldrich tetraethylene glycol of purity 99% was used asthe organic solvent. Distilled water was used for the aque-ous depositions.A Rigaku D-Max x-ray diffractometer was used tocharacterize crystallinity. A Perkin-Elmer Lambda 19UV/VIS/NIR spectrophotometer was used for opticalabsorbance spectroscopy.!? Films were deposited by swept deposition from acidicsolutions. The solutions contained 150 ml of water, with? 0.01 M CdSO 4, 0.02 M Na 2S2O,, and 0.03 MEDTA at apH of 2.0. The solutions were heated to 60-7()°C for? voltammetric analysis and filmdeposition. Thin filmswere deposited by application of a continuous wave, ramp? voltage. The sweep rate was 5 mV/s.Films were deposited from TEG at constant voltage at*90-100 C. The solutions were saturated with sulfur andcontained 0.01 M CdCL, or Cdl2.Deposition was carried? out at -0.92 V with respect to the reference electrode for 4hours.[w Results and Discussionw TEG Based Solutions.—Several films were depositedfrom the tetraethylene glycol (TEG) baths until optimum? conditions were found. Golden (MS films were depositedby constant voltage electrodeposition. Uniformity and? adherence were much improved over those grown by usfrom ethylene glycol using similar methods. Figure 1? shows the plot of absorbance versus wavelength for one?film. The sharp inflection near 500 nm is indicative ofCdS. Figure 2 is the x-ray diffraction data for the film.The sharp intensity peaks corresponding to the overlaidCdS powder diffraction file card reveal that the sample isploycrystalline CdS.The TEG based solutions were used in initialexperi-ments to plate large area (i.e. 4 in. x 7 in.) films for lightdetection devices. The scaled-up method also producedvery uniform films. The replacement of CdCl2 with Cdl.jincreased uniformity. These films appeared more goldenorange in color, as opposed to golden yellow. Initialresults indicate that use of TEG as a CdS electrodeposi-tion electrolyte improves adherence and uniformity with-out sacrificing key material characteristics.Aqueous Solutions.—Photovoltammetry was conduct-ed for the aqueous solutions to determine the most effec-Fig. 1. Optical absorbance vs. wavelength for a CdS filmelectrodeposited from the TEG solution. Note theabsorption edge near 500 nm, consistent with CdS's 2.42eV bandgap.Fig. 2. X-ray diffraction data for the CdS filmelectrode-posited onto ITO-coated glass from the TEG solution.Vertical lines denote standard CdS powder diffractioncard [6-314].Proceedings Arkansas Academy of Science, Vol.50, 199680Journal of the Arkansas Academy of Science, Vol. 50 [1996], Art. 16https://scholarworks.uark.edu/jaas/vol50/iss1/1681Brandon Kemp, Dr. Robert Engelken, ArifRaza, Wasim Aleem, Imran Khan, and Chris Barbertive way of depositing photoconductive CdS. Figure 3shows a voltammogram (i.e. without pulsed illumination)of a typical aqueous CdS electrodeposition solution. Thelack of structure positive of the undei potential Cd platingwave indicates that CdS is not being cathodically plated atthese positive most voltages. Figure 4 shows a similar pho-tovoltammagram with anodic photocurrents appearingon the reverse (i.e. negative to positive voltage) sweep fol-lowing the cadmium stripping structure. Study of theseplots indicates that sulfur species released from the thio-sulfate ions are reacting at the cathode surface with plat-ed cadmium. The absence of a stripping wave more posi-tive than the cadmium plating range reveals that CdSthus formed remains on the substrate surface.Photovoltammetry was conducted periodically duringswept deposition filmgrowth. Figure 5 shows the initiasweep during a filmdeposition. As previously notedanodic photocurrent pulses are observed on the reversesweep. Figure 6 is a photovoltammogram conducted during the same filmdeposition but after several sweepsNotice the increase in photocurrent magnitude with thissweep. At this point it was observed that a thin, goldenyellow filmhad formed on the substrate surface. Anoptical absorbance vs. wavelength plot for this filmis shownin Fig. 7. The sharp rise in absorbance near 500 nm isconsistent with the bandgap ofCdS. As might be expect-ed with thin films deposited near room temperature, x-ray diffraction reveals poor crystallinity. Figure 8 showsthe pattern overlaid with a standard CdS powder diffrac-Fig. 3. Cyclic voltammogram (current-voltage curve) foran ITO coated glass cathode in an aqueous solution at60 °C with 0.01 M CdCU, 0.02 Na2S 2Os, and 0.03 MH4EDTA at pH =2. Note the lack of voltammetric struc-ture other than the cadmium plating and stripping peaks.Fig. 5. Cyclic voltammogram recorded during the initialsweep of a CdS filmdeposited by swept voltage methodsin a thiosulfate bath. Note the anodic photocurrents onthe reverse sweep and the absence of the cadmium strip-ping peak due to the more positive stopping potential.Fig. 4. Cyclic voltammogram as with Fig. 3 except thatpulsed light is incident on the substrate surface. Note theappearance ofanodic photocurrent pulses on the reversesweep following the stripping ofcadmium.Fig. 6. Cyclic voltammogram as with Fig. 5 except thefilmhad deposited for about 1 hour. Note the change incurrent scale and the increase in the magnitude of thephotocurrent pulses.Proceedings Arkansas Academy of Science, Vol.50, 199681Journal of the Arkansas Academy of Science, Vol. 50 [1996], Art. 16Published by Arkansas Academy of Science, 1996Improved Methods For Electroplating Cadmium Sulfide ThinFilmsAlso, the introduction of iodide ions into the solution fur-ther enhances uniformity.Electrodeposition of CdS from aqueous solutionscontaining thiosulfate ions was much improved by theimplementation of swept voltage deposition. This methodtends to produce stoichiometric CdS, as opposed to Cd-rich films. This technique is easily monitored by in-situvoltammetry.Future related work willinvolve further refinement ofthe electrodeposition of CdS from hot organic baths (i.e.T >100°C) and more detailed investigation of pulsed andswept methods for the electrodeposition of CdS fromaqueous solutions. Further experimentation willinclude amore quantitative approach to the growth and characteri-zation of films deposited from organic electrolytes con-taining CdL; Also, dithionite ion, S^O^"-, as a sulfursource in acidic solutions willbe investigated.Acknowledgements. —This work was jointly support-ed by the Arkansas Science Information Liaison OfficeUndergraduate Research Fellowship Program (Bo Ryall,Coordinator), the AR/NASA EPSCoR Program (Dr.Hameed Naseem, Arkansas Advanced Photovoltaic-Materials Research Center Director; Dr. GaylordNorthrop, State Coordinator; Dr. Dennis Flood, NASALewis Research Center, NASA Mentor), and InvoTek, Inc.of Alma, AR (Mr. Tom Jakobs, President) through aNational Institutes of Health Phase IISBIR Grant. Wealso gratefully acknowledge the ongoing support ofArkansas State University.Literature CitedBaranski, A.and W. Fawcett. 1980. J. Electrochem. Soc,127:766.Das, S. 1993. Characterization of CdCl2 treated elec-trodeposited CdS/CdTe thin filmsolar cells. SolarEnergy Materials and Solar Cells. 29:277-278.Das, S. and G. Morris. 1993. Preparation and propertiesof CdS/CdTe thin filmsolar cells produced by peri-odic pulse electrodeposition techniques. Solar EnergyMaterials and Solar Cells. 30:107-1 18.Engelken, R. 1988. Ionic electrodeposition ofII-VIandIII-Vcompounds -Part IV. J. Electrochem. Soc.135:834-839.Engelken, R. and H.McCloud. 1985. Electrodepositionand material characterization ofCu xS films. J.Electrochem. Soc. 132:567-573.Engelken, R. and T. Van Doren. 1985. Ionic electrodepo-sition ofII-VIand III-Vcompounds - Parts Iand II.J.Electrochem. Soc. 132:2904-2909, 2910-2919.Fatas, E., R. Duo, P. Herrasti, F. Arjono, and E. GarciaCamarero. 1984. J. Electrochem Soc. 131:2243.Kemp, B., R. Engelken, A. Raza, A. Siddiqui, and O.Fig. 7. Optical absorbance vs. wavelength spectrum for aCdS filmelectroplated by swept deposition characterizedby the photovoltammentry in Fig. 5 and Fig. 6. Note theabsorption edge near 500 nm,Fig. 8. XRD data for filmdescribed by current vs. voltageplots in Fig. 5 and Fig. 6. Note the absence of strongpeaks (except for those corresponding to ITO) which is agood indication that the filmis amorphous. The verticallines denote standard CdS powder diffraction peak cardtion file card. The CdS peaks are too small relative to the, background to be a conclusive match although the dis-tinct anodic photocurrents are strong indication ofgood,, albeit nearly amorphous, semiconducting CdS.Conclusions and Future PlansThe results discussed reveal improved methods forelectrodepositing CdS from tetraethylene glycol andacidic aqueous solutions. It has been found that the useof TEG improves the uniformity of films over thosegrown by similar techniques from other organic solvents.Proceedings Arkansas Academy of Science, Vol.50, 199682Journal of the Arkansas Academy of Science, Vol. 50 [1996], Art. 16https://scholarworks.uark.edu/jaas/vol50/iss1/16Brandon Kemp, Dr.Robert Engelken, ArifRaza, Wasim Aleem, Imian Khan, and Chris BarberMustafa. 1995. Diagnostics ofCdTe electrodepositionby rest potential voltammentry. Proceedings of the79th Annual Meeting of the Arkansas Academy ofScience. 49:97-93.McCann, J. and M.Skyllas-Kazacos. 1981. The electro-chemical deposition and formation of cadmium sul-phide thin filmelectrodes inaqueous electrolytes. J.Electroanal. Chem. 119:409-412.Mishra, K.,R. Engelken, K.Rajeshwar, A.Weiss, M.Murley,M.Slayton, and H.McCloud. 1989.Electrodeposition and characterization ofSnS thinfilms.J. Electrochem. Soc. 136:1915-1923.Poole, C, R. Engelken, B.Kemp, and J. Brannen. 1994.Tetraethylene glycol - based electrolytes forhightemperature electrochemical deposition of compoundsemiconductors. Proceedings of the 78th AnnualMeeting of the Arkansas Academy ofScience. 48:133-139.Proceedings Arkansas Academy of Science, Vol.50, 1996 483Journal of the Arkansas Academy of Science, Vol. 50 [1996], Art. 16Published by Arkansas Academy of Science, 1996

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Improved Methods for Electroplating Cadmium Sulfide Thin Films

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