XPS and XRD study of photoconductive CdS films obtained by a chemical bath deposition process Mitko Stoev" and Atanes Katerski Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Cadmium sulfide layers have been obtained from CdCl,, CS(NH,),, NH,Cl, NH, and H,O solutions using the chemical bath deposition technique at 50 "C. After dipping in a solution of 0.7 mass% CdC1, in CH,OH and thermal treatment under Ar at 400 "C for 30 min, the CdS layers obtained acquire a photosensitivity amounting to a ratio, between photocurrent values in the light and in the dark, of four orders of magnitude. The chemical changes of the layers occurring, as evidenced by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), during their preparation in solution and after thermal treatment are discussed.The interest in CdS goes back to 1954 when the photovoltaic effect in CdS/Cu2S rectifiers was discovered.' Thin CdS films have been applied to the production of n-type window layers for heterojunction solar cells such as n-Cds/~-CdTe~,~ and n-CdS/p-CuInSe, .4 The cheapest and easiest preparation method of polycrystalline CdS films is via chemical bath deposition (CBD). The earliest investigations on the prep- aration of CdS thin films by chemical precipitation in solutions are associated with the use of cadmium salts, thiocarbamide and sodium hydr~xide.~.~ The optimum concentrations of the reaction mixtures and the conditions of chemical pFecipitation for the preparation of CdS films of up to 1OOOA thickness have been established. To enhance the CdS film thickness by a single dip, NH40H is used instead of NaOH.7 The kinetics and mechanism of CdS formation with the participation of ammonium thiocarbamide complexes' of cadmium has been elucidated and the properties'-'' of the resulting films have been determined.Since the CdS preparation process is difficult to control, a technique using the buffer effect of ammonium salts has been devel~ped.'~.~~ The application of a buffer solution technique14 to the preparation of CdS films by CBD has been discussed. It has been found that in the case of buffer solutions of ammonium salts the growth of CdS films is easier to control because the ammonium salt slows down the release of cadmium ions into the solution to form CdS ion-by-ion on the substrate.With the bath deposition method, the cadmium is in a complexed form. In addition to NH,OH, N( CH,CH,OH), (triethan~larnine)'~-'~has also been used as a complexing agent for the preparation of high-quality CdS thin films. When Na3C6H507 2H20 (sodium itr rate)'^'^^ was used, a citrate complex of cadmium was formed in solution, which yielded CdS thin films. Films of CdS have also been obtained using KCN as a complexing agent in which the [Cd(CN),I2- complex was formed in solution.21 The effect of CdCl, on the electronic properties of solar cells of the type CdS/CdTe has been inve~tigated.~~?~~ Treatment with a satu- rated solution of CdC1,-CH,OH leads to better properties of the CdS/CdTe interface, induced CdTe grain growth and significantly improved overall efficiency.XRD and XPS studies of the conversion of chemically deposited photosensitive CdS thin films to n-type materials by air annealing and ion- exchange reactions have been reported.,' In these cases, the conversion of the films to n-type materials was achieved by immersing the film in a 0.01 mol dm-, HgC1, solution for 15 min followed by air annealing for 1h at 200°C. The chemistry of formation of photosensitive CdS films obtained by the buffer solution technique and treatment with CdCl,-CH,OH solutions has not yet been elucidated, nor has any interpretation of the changes in contour of the XP spectra during the formation of photosensitive films been provided.The present study is a continuation of investigations on the preparation of CdS thin films by chemical method^.^^,^^ It is aimed at: (i) deposition of CdS films suitable for solar cells on glass substrates using the buffer solution technique; (ii) thermal treatment of CdS films in order to obtain photosensitive films, and (iii) investigation by XPS and XRD of the chemical changes occurring on the surface of CdS layers as a result of thermal treatment . Experimental Preparation of CdS thin films by chemical bath deposition The CdS thin films were obtained from solutions of 0.02 mol dm-, CdCl,, 0.065 mol dmP3 NH,C1, 0.15 mol dmP3 CS(NH,), and 0.4mol dm-, NH, at pH 10.3- 1 l.l.I3 The films were deposited on previously cleaned glass substrates with dimensions of 28 x 48 x 1.0 mm3 by dipping, four times, in a reaction bath at 50 "C for 30 min intervals.The thermal treatment of the films, which was aimed at achieving photosensitivity, was performed after dipping them into a solution of 0.7 mass% CdCl, in absolute CH30H. When the glass substrate, with a CdS film deposited on it at 25"C, was removed from the saturated methanol solution of CdCl,, an evaporation process began on the substrate surface. Methanol is an appropriate solvent since it is volatile and because cadmium chloride is only poorly soluble in it (a saturated methanol solution of CdC1, attained only 1.16 mass% CdCl,). A thin film of the crystalline solvate CdC1, -2CH,OH was formed on the CdS film surface.26 As the crystalline solvate is hygroscopic and absorbs moisture from the air, desolvation and hydration to the crystalline hydrates CdCl, * H20 and CdC1, 2.5H20 occurred.The thermal treat- ment was carried out in a quartz furnace under Ar at 400°C for 30 min. Under these conditions, the crystalline solvate and possibly formed crystalline hydrates were desolvated. As a result, anhydrous CdC1, appeared on the film surface, and this led to recrystallization of CdS. The substances used for the experiments were: CdC12.2.5H20 (A. R., Merck), CS(NH,), (A. R., Merck), NH4C1 (A. R.), aqueous NH, (A. R.), CH30H (A. R., Merck) and HzO (double distilled). The anhydrous CdCl, was obtained by thermal dehydrati~n~~ of CdC1, .2.5H2O, and the CH,OH used was dehydrated by a standard method.28 XPS and XRD measurements The XPS measurements were performed in the ultra-high vacuum chamber of an ESCALAB-Mk I1 (VG Scientific) J.Mater. Chem., 1996, 6(3), 377-380 377 electron spectrometer with a base pressure of 1x Pa. The spectra were taken at normal emission with A1-Ka excitation (hv= 1486.6 eV) and an analyser pass energy of 50 eV. The total instrumental resolution was 1.2eV as measured by the full width at half maximum (FWHM) of the Ag 3ds,, photoelec- tron peak. The energy scale was calibrated using C 1s (at 285 eV) of adventitious carbon as a reference. The CdS stan- dard was prepared by pelleting CdS powder (A. R., Merck, 99.999%) in air.The XP spectra were recorded numerically with an Apple I1 plus 8 bites personal computer and treated with original software provided by VG 1000-Scientific. The spectra were additionally transferred to an AT 486 DX2 instrument and processed with Fourier deconvolution (FD) Spectra TOO IS,^^^^* PHI-MATLAB V4 and Microcal Origin v.3.54 software. The atomic concentrations were calculated using normalized photoelectron intensities I/. where 0 is the photoelectron cross-~ection.~~ The X-ray analysis was carried out with a DRON-2 X-ray diffractometer using a cobalt anode, Ka radiation and a nickel filter for a radiation. Contacts of In, Au, Ag and C were deposited on the CdS layers by solder, vacuum evaporation and sputtering methods as well as in the form of paste.The layers were illuminated with a lamp having a heatable tungsten wire, at 100 mW crnp2. Results and Discussion XPS characteristics of CdS films The results from the XPS study are given in Fig. 1-7. Fig. 1 presents the XP spectrum of the CdS standard within the range 5-1005eV. Owing to the high purity of the CdS used for tabletting, lines for Cd and S are observed on the tablet surfaces, the C impurity being due to the oils of the vacuum pumps. The oxygen on the surface originates mainly from the atmosphere under which the CdS standard sample was pre- pared. The Cd: S : 0 ratio on the sample surface in atom% is 45.7 : 42.5 : 11.8. The oxygen on the surface at 532 eV is most probably chemisorbed oxygen.32 The peak positions (Cd 3d5,, at 405.3 eV and S 2p3,, at 161.7 eV) correspond to the com- pound CdS.33,34 Fig.2 presents the XP spectra of Cd 3d of standard (l),as-deposited (2) and heat-treated CdS (3). Both peaks (Cd 3d,,, and Cd 3d3,,) of the standard and as-deposited samples have FWHM = 1.7 eV and positions of the maxima at 405.4 and 412.1 eV, respectively, which correspond to Cd in the form of CdS. For sample 3 the FWHM of the Cd 3dS,, and Cd 3d3,, peaks is increased to 3 eV and the maxima are observed at 405.5 and 412.3 eV, respectively. Fig. 3 shows the results from a fit procedure with PHI-MATLAB V4 of a XP spectrum of Cd 3d5,, of heat-treated CdS. Here, 2.9 atom% Cd are bound as CdCl,, and 28.2 atom% Cd are present as CdS, CdO and Cd(OH),.Fig. 4 presents the S 2p XP spectra of as-deposited (1) and heat-treated (2) CdS. The CdS layer I I 1 I ri Cd 3dY2 200 400 Mx) 800 1000 binding energylev Fig. 1 XP spectrum of standard CdS I . I . 1 . 1 . 400 405 410 415 420 binding energylev Fig. 2 Cd 3d XP spectra of CdS films: 1, standard; 2, as-deposited; 3, heat-treated CdS- 16oooo -140000 -120000 -I ;loo000 -c. t3 8 8oooo-1 .-a UJ c.s--.-C 40000 -2oooo -0--2oooo' I I ' I400 ' 402 ' 404 ' 406 ' 408 ' 410 binding energyleV Fig.3 Cd 3d,,, XP spectrum of heat-treated CdS. 0, Experimental spectrum; -, fit spectrum; . . . , .., synthesized spectra; 0, residue (experimental spectrum minus fit spectrum). obtained by CBD in solution contains S in the form of SO4,-, the amount of S being 2.4 atom%.After thermal treatment under Ar, the sulfates disappear from the surface of the CdS film. Fig. 5 demonstrates the results after the application of the FD procedure to the S 2p spectrum of as-deposited CdS. The peak at 168.7eV corresponds to S present as SO4,-, probably as CdSO,. Fig. 6 illustrates the results from XP spectra of C1 2p of as-deposited (1) and heat-treated (2) CdS. The chlorine concentration in as-deposited CdS is 0.2 atom% and is due to the reagents CdC12*2.5H20 and NH,C1 used for the preparation of the CdS layer by CBD. On the surface of the heat-treated CdS, a concentration of 16.5 atom% C1 as CdC1, is established. Fig.7 shows the XP spectra of 0 1s of as-deposited (1) and heat-treated (2) CdS. The XP spectral contour of 0 1s for heat-treated CdS is difficult to interpret because on both sides of the main peak, which can be deconvol- uted into two peaks at 532 and 533.7eV, there are two new peaks. The surface oxygen is probably chemisorbed and in the 378 J. Muter. Chem., 1996, 6(3), 377-380 160 165 170 175 binding energy/eV Fig. 4 S 2p XP spectra of CdS films. 1, As-deposited CdS; 2, heat- treated CdS. -'-IY Ii! .-c3 I 1. 1 1 I 160 165 176 binding energy/eV Fig.5 S 2p XP spectra of an as-deposited CdS film (no. 4, K = 100-200). +, Experimental spectrum; . . . . .., deconvoluted spectrum. form of CdS04, CdO and Cd(OH)2. The larger oxygen content on the CdS surface after thermal treatment is due to the hygroscopicity of the CdC1, formed as a result of decompo- sition of the crystalline solvate CdC1, 2CH30H.The con- centrations of the elements in atom% are determined on the surfaces of the CdS layers, as follows: as-deposited CdS, Cd :S : C1: 0=48.9 : 25 :0.2 :25.9; heat-treated CdS, Cd:S:Cl:0=31.1: 10.6:16.5:41.8. XRD characteristics of CdS films Fig. 8 shows the results from X-ray analysis of a heat-treated CdS film in comparison with the standards35 a-and P-CdS. The as-deposited CdS layer obtained after precipitation in the reaction bath is amorphous. After dipping of the amorphous CdS layer in a 0.7 mass% CdCl, solution, and thermal treatment under Ar at 400 "C for 30 min, recrystallization sets in and a hexagonal CdS film is formed.195 200 205 210 binding energylev Fig. 6 C1 2p XP spectra of CdS films. 1, As-deposited CdS film (no. 4, K = 100-200); 2, heat-treated CdS film (no. 4,K = lo4). 525 530 535 540 binding energy/eV Fig. 7 0 1s XP spectra of CdS films. 1, As-deposited (no. 4, K = 100-200); 2, heat-treated (no. 4,K = lo4). Electrical characteristics of as-deposited and heat-treated CdS Table 1 summarizes the data on the electrical characteristics of as-deposited and heat-treated CdS films. All layers are deposited at the same concentration of substances participating in the reaction bath at 50 "C. The best photoconductivity ratio (K = lo4) of a CdS layer suitable for solar cells was exhibited by sample no.4 which was dipped in a solution of 0.7 mass% CdC1, in CH30H and heat treated under Ar at 400°C for 30 min. Conclusions Thermal treatment at 400°C under Ar of as-deposited amor- phous CdS films leads to their crystallization into hexagonal CdS. A combination of thermal treatment, pre-treatment of the CdS surface with a methanol solution of CdC1, and exposure of the layer to air results in enhancement of the J. Mater. Chem., 1996,6(3), 377-380 379 Table 1 Resistance characteristics of as-deposited and heat-treated CdS films pD (in !2 cm)=dark resistivity, pL (52 cm) =light resistivity, K (photoconductivity ratio) =pD/pL,no 1 two bath immersions, pH 11 3, 45 min, no 2 4 bath immersions, pH 11 14, 10 3, 10 3, 10 3, 25 min, no , 3 four bath immersions, pH 10 3, 20 min, no 4 4 bath immersions, pH 11 18, 10 3, 10 3, 10 3, 30 min heat-treated CdS as-deposited CdS 200°C, 30 min, Ar no PD PL PD PL 1 107 (3-6) x lo6 Kz3 3 x lo6 K=15 2 x lo6 2 6 7 x lo6 3 x lo6 5 x lo6 25x106 K=2 K=2 3 3 x lo6 12x 105 15x107 3 x lo6 Kz20 K=5 4 2 x los (1-2) x 103 K = 100-200 2 x 104 K = 3-10 2 x 103 6 7 8 9 10 11 12 13 14 15 16 17 dlA 18 Fig. 8 X-ray analysis of (a)a heat-treated CdS sample (no 4) obtained 19by the chemical bath deposition technique, compared with (b)a-CdS and (c)P-CdS standards 20 21 amount of oxygen on the film surface due to the hygroscopicity of anhydrous CdCl, Treatment of as-deposited CdS prelimin- 22 arily dipped in a methanol solution of 0 7 mass% CdCI, and 23 heat treated under Ar at 400°C for 30min makes the film photosensitive, the ratio between the photocurrents in the light 24 and in the dark being of four orders of magnitude 25 26 This work was supported by a grant from EC Brussels under 27 contract ERB JOU2-CT92-0241 28 29 References 30 1 D C Reynolds and G M Leies, Electr Eng, 1954,73,734 31 2 D Bonnet,M Carter, M Burgelman,N Romeo,A W Brinkman, 32 M Carter, E Ozsan, H Hogg and J Vedel, Comm Eur Communities, [Rep] EUR 1993, (EUR 15098) 59, Chem Abstr , 33 120 11645m 3 R W Buckley, E Kuetz, D Valik and F Steueter, Comm Eur 34 Communities [Rep] EUR 1993, (EUR 15098), 173, Chem Abstr , 119277036n 35 4 H W Schock, Sol Energy Mater Sol Cells 1-4,1994,34, 19 5 G A Kitaev, S G Mokrushin and A A Uriskaya, Kolloidn Zh (Russ ), 1965,28,51 380 J Muter Chem , 1996,6(3), 377-380 400"C, 30 min, Ar, 4OO0C, 30 min, Ar CdCl, PD PL PD PL 103 7 x 10, 14x 10' 4 x 104 K<2 K =300 5 x 103 103 4 x lo8 1O6 1o6 K=5 105 1os K =400 lo6 K=lO 14 x 107 105-1o6 K = 103 2 5 x 107 2 x 103 K 10-100 K = 104 G A Kitaev, A A Uriskaya and S G Mokrushin, Zh Kolloidn Khim (Russ ), 1965,34,2065 T G Leonova, T V Kramareva and V M Shulman, Zh Kolloidn Khim (Russ), 1968,30,61 T G Leonova and V I Kozbanov, Izv Sib Otd Akad Nauk SSSR Ser Khim Nauk (Russ ), 1977,7,108 N R Pavaskar, C A Menezes and A P B Sinha, J Electrochem SOC,1977,124,743 J M Doiia and J Herrera, J Electrochem SOC , 1992,139,2810 K I Grancharova, J G Bistrev, L J Bedikjan and G B Spasova, J Mater Sci Lett, 1993,12,852 R W Buckley, Phys Educ , 1992,27,323 R W Buckley, Proceeding of the 11th EC Photovoltaic Solar Energy Conference, Montreux, Switzerland, 1992, p 962 N G Dhere, D L Waterhouse, K B Sundaram, 0 Melendez, N R Parikh and B Patnaik, J Mater Sci Mater Electr, 1995, 6, 52 A Mondal, T K Chaudhuri and P Pramanik, Sol Energy Mater , 1983,7,431 P K Nair and M T 5' Nair, Sol Cells, 1987,22, 103 P K Nair, J Campos and M T S Nair, Semicond Sci Technol, 1988,3,134 J A Ugai, E M Averbah, A S Skuratov and T V Gavnkova, Zh Neorg Khim, 1990,352192 M T Nair, P K Nair, R A Zingaro and E A Meyers, J Appl Phys , 1993,74,1879 M T S Nair, P K Nair, R A Zingaro and E A Meyers, J Appl Phys, 1994,75,1557 R L Call, N K Jaber, K Seshan and J R Whyte, Jr , Sol Energy Mater , 1980,2,373 S A Ringel, A W Smith, M H MacDougal and A Rohatgi, J Appl Phys, 1991,70,881 A Rohatgi, R Suharsanan, S A Ringel and M H MacDougal, Sol Cells, 1991,30, 109 M Stoev and S Ruseva, Monatsh Chem , 1994,125,599 M Stoev, S Ruseva and B Keremidchieva, Monatsh Chem , 1994, 125,1215 M Stoev and I Zlateva, Z Anorg Allg Chem, 1988,558,223 C Dural, Anal Chim Acta, 1959,20,263 Purlfieation of Laboratory Chemicals, ed D D Perrin, W L F Armarego and D R Perrin, Pergamon, Oxford, 1980, p 252 T Kalkanjiev, V Petrov and J Nikolov, Appl Spectrosc, 1989, 43,44 V H Astinov and M Stoev, J Raman Spectrosc , 1994,25,381 J H Scofield, J Electron Spectrosc , 1977,8, 129 C A Estrada, P K Nair, M T S Nair, R A Zingaro and E A Meyers, J Electrochem SOC , 1994,141,801 S W Gaarenstroom and N Winograd, J Chem Phys, 1977, 67, 3 500 V G Bhide, S Salkalachen, A C Rastogi, C N Rao and M S Hegde, J Phys D, 1981,14,1647 Powder Diflraction File, International Center for Diffraction Data, Pennsylvania, 1979,6, 314, 1979, 10,454 Paper 5/05302A, Received 8th August, 1995