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X-ray photoelectron spectra of crystal and thin film cadmium sulphide
Surface Science 154 (1985) L251-L254 North-Holland, Amsterdam
L251
SURFACE SCIENCE LETTERS X-RAY PHOTOELECTRON SPECTRA OF CRYSTAL AND THIN FILM CADMIUM SULPttIDE M. MARYCHURCH and G.C. MORRIS Department of Chemistry, University of Queensland, Brisbane 4067, Australia Received 2 January 1985
Normal incidence and angle resolved X-ray photoelectron spectra of film and single crystal CdS are reported. In the spectral region for S there is a S 2- peak (162.6 eV) and after surface oxidation a SO42- peak (169.0 eV) which can be removed by washing. Using MgKa excitation, ghost Cd peaks, excited by stray AIKa radiation, occur in the S spectral region (172.6 and 179.4 eV). Cd peaks are 405.2+0.2 and 412.0+0.2 eV. Temperature dependence studies of the Cd:S ratio show that heating in ultrahigh vacuum leaves the surface S rich.
X-ray photoelectron spectra (XPS) have been used to probe processes occurring during manufacture of thin film n-CdS/p-CdTe heterojunction solar cells [1]. These studies using photoconductive CdS films have displayed XPS spectral features which have been previously observed and interpreted in several ways [2-4]. This letter gives XPS data for both crystal and thin film CdS, provides an unambiguous interpretation of that data and rationalizes the previous different interpretations. XPS data were recorded with a Perkin-Elmer Model 560 double pass CMA with an angle resolvable aperture using either MgKa or A1Ka radiation from a dual anode X-ray source operated at 300 W, 15 kV. Band pass energy was 25 eV. The spectrometer was calibrated using the Au 4f7/2 line (83.8 eV) with the C ls line (284.6 eV) as an internal standard. All scans were simultaneously run over the regions for S (158-183 eV), Cd (400-420 eV), O (525-545 eV) and C (280-300 eV) using the Model 560 MACS Version VI software. Samples used were photoconductive polycrystalline films made by chemical deposition onto ITO-glass substrates [5]; polycrystal cadmium sulfate (ESPI, USA, 99.99 + % pure) and a single crystal n-CdS doped with 1016 cm -3 indium (Materials Research Corp., USA). Both crystal and film were conductive and electron irradiation to prevent charging was unnecessary, a useful precaution in view of the reported electron irradiation effects on CdS [6,7]. Temperature dependent studies were made using a hot/cold probe as the sample mount (Perkin-Elmer Model 02-122). In earlier measurements, Amalnerkar et al. [2,3] obtained three peaks in the S region using MgKa radiation from a dual anode source and interpreted these 0039-6028/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M. Marychurch, G.C. Morris / XPS of crystal and thin film CdS
L252
Table 1 Peak positions (eV) in X-ray photoelectron spectra of CdS (film and crystal) and CdSO 4 (polycrystal) Single crystal
Thin film
S(0001) face
Cd(0001) face
Unannealed
Annealed
161.6_+0,2 169.0_+0,3 172.2_+0.2 179.0_+0.2 405.2_+0.2 412.0+0.2 531.7_+0.3
161.6_+0.2 169.0_+0.3 172.2_+0.2 179.0+_0.2 405.2+_0.2 412.0+_0.2 531.7+_0.3
161.4_+0.2 168.6_+0.3 172.0-+0.2 178.8_+0.2 405.0_+0.2 411.8_+0.2 531.4+_0.3
161,2_+0.2 169.0_+0.2 172.0-+0.4 178.8_+0.4 405.0_+0.2 411.8-+0.2 531.4_+0.3
CdSO 4
Assignment
161.6+0.4 168.6_+0.2 172.4_+0.4 179.0_+0.4 405.4_+0.2 412.2-+0.2 531.6-+0.4
S2p S 2S 2p SO42 Cd 3d5/z ghost ") Cd 3d3/2 ghost ~ Cd 3d5/2 Cd 3d~/2 O ls
~ Present only under Mg radiation.
t <
>..
b
i
-180
i
,
,
•
-
-171 -162 Binding Energy, EV
-
180
-171 -162 B~ndtng Energy, EV
Fig. 1. X-ray photoelectron spectra in the S spectral region. (a) CdS crystal (S face (0001)) under MgKa radiation. (b) CdS crystal (S face (0001)) under A1Ka radiation. (c) CdS crystal (S face (0001)), angle resolved (grazing angle) under A1K a radiation. (d) CdSO 4 polycrystal under MgK a radiation. Fig. 2. X-ray photoelectron spectra of photoconductive CdS film in the S spectral region: (a) under MgKa radiation; (b) under AIKa radiation.
M. Marychurch, G.C. Morris / XPS of crystal and thin film CdS
L253
as S2- (162.6 eV), SO~- (172.8 eV) and SOz adsorbate (179.7 eV). Their published spectra also showed a peak near 169 eV although that peak was not discussed. The peaks (172.8 and 179.7 eV) were substantially reduced in intensity by thermal desorption as was the sample photoconductivity and the data were used to suggest a causal relationship between the existence of the S-O species and the extent of photoconductivity. However, Lichtensteiger and Webb [4] reported peaks in the S region for CdS films at 162 and 169 eV only and suggested that the peaks at 172.8 and 179.7 eV observed by Amalnerkar et al. [3] are Cd ghost peaks excited by stray AIKc~ radiation. That view was rejected by Amalnerkar et al. [3] on two grounds, viz.: (a) the reduction of the peak intensities with oxygen desorption suggested that they arose from S-O species; (b) the binding energy value of metal-SO 2 species determined by Furuyama et al. [9] and quoted by Amalnerkar et al. as 176.7 eV was consistent with the observed value of - 172-179 eV. Our data show that the peaks at 172.8 and 179 eV are indeed Cd ghosts excited by stray A1Ka radiation and that the rejection of that view by Amalnerkar et al. was based on false premises in that: (i) the XPS peaks for metal-SO 2 species observed by Furuyama et al. is near 167.6 eV as seen in their figs 1-4 and their table 1, not the printing error value 176.6 eV which appears on page 65 of their article and is used by Amalnerkar et al.: (ii) the reduction in the peak intensities thought to be associated with oxygen desorption is caused by the outdiffusion of Cd upon heating so that ghost Cd lines are reduced in intensity. The evidence to support our view is now presented. Fig. 1 shows spectra of a (0001) face of a single crystal CdS under MgKa and AIKa radiation. Data from the (0001) S face are shown: the data from the (0001) Cd face are different only in the signal intensity with angle resolved (AR) spectra. It is clear from fig. 1 that the peaks at 172.6 and 179.4 eV are present only when MgKa is the radiation source. In table 1, the peak positions and their interpretations are given for these surface spectra. The peaks at 172.6 and 179.4 eV are "ghosts" from indeterminate A1Ka excitation of Cd being displayed downfield by the energy difference of the AIKa (1486.6 eV) - MgKa (1253.6 eV). Also shown in fig. 1 is the spectrum of CdSO4 under MgKa irradiation which allows identification of the 169.2 eV peak as SO~-. In this spectrum, similar "ghost" Cd peaks as seen in CdS are clearly observed. Angle resolved spectra under either MgKa or A1Ka radiation showed that the 169.0 eV peak was the most intense for only shallow angle emission emphasizing the surface nature of the species. Again the MgKa radiation gave strong ghost peaks which were absent under A1Ka radiation. In fig. I is shown the angle resolved spectra (grazing angle) under A1Ka radiation showing the SO42- and S z- peaks. The size of the 169.0 eV peak can be altered by treatment of the single
L254
M. Marychurch, G. C. Morris / XPS of crystal and thin film CdS
crystal surface. After etching with HC1 (conc. 20 s), or after ion etching the outer surface layers, there was no peak at 169.0 eV but it developed after the CdS was exposed to the air. Washing with Millipore water for 5 min, rinsing with methanol and drying the surface with a stream of purified nitrogen removed the peak at 169.0 eV. The XPS spectra for thin film CdS are shown in fig. 2 and the data given in table 1. Angle resolved spectra of the film differed little from the normal multiplex, presumably because of surface roughness of the film. Thin film CdS annealed at 600°C (in air) had the XPS peak at 169.0 eV as the most prominent. Thin film CdS with an oxidised surface when heated in the ultra high v a c u u m c h a m b e r ( < 10-9 Torr) from r o o m temperature shows a decrease of the Cd : S ratio with temperature increase over the range 300-600°C. This result indicates that Cd outdiffuses from the surface which was left S rich. With temperature increase, the SO42- peak (169.0 eV) diminished in intensity with an apparent activation energy of 0.3 eV. W h e n the air contaminated crystal is subjected to oxygen a d s o r p t i o n - d e sorption cycles, the peaks in the S region alter little in intensity. The SO 2 peak can be removed as discussed above by washing the surface or by heating in ultra high vacuum. Such heating also reduces the intensities of the " g h o s t " peaks because the crystal surface becomes S rich not because of removal of oxygen species as stated by Amalnerkar et al. [2,3]. In conclusion, the X-ray photoelectron spectra of CdS in the S region shows that lattice S 2- appears at 162.6 eV and after surface oxidation SO42- appears at 169.0 eV. Stray A1Ka radiation can produce " g h o s t " Cd peaks at 172.6 and 179.4 eV under M g K a radiation. Financial support came from the Australian Research Grants Scheme. We thank Dr. W. D a n a h e r for preparing the thin film CdS and for discussions.
References [1] D. Horton, J. Keyes, L.E. Lyons and G.C. Morris, J. Electroanal. Chem. 168 (1984) 101. W. Danaher, L.E. Lyons and G.C. Morris, Appl. Surface Sci., in press. [2] D.P. Amalnerkar, S. Badadrinarayanan, S.K. Date and A.P.B. Sinha, Appl. Phys. Letters 41 (1982) 270. [3] D.P. Amalnerkar, S. Badrinarayanan, S.K. Date and A.P.B. Sinha, J. Appl. Phys. 54 (1983) 2881. [4] M. Lichtensteiger and C. Webb, J. Appl. Phys. 54 (1983) 2127. [5] W. Danaher, L.E. Lyons and G.C. Morris, Solar Energy Mater., in press. [6] M. Lichtensteiger, C. Webb and J. Lagowski, Surface Sci. 97 (1980) L375. [7] J. Lagowski, M. Lichtensteiger and P.M. Williams, Surface Sci. 84 (1979) L223. [8] D. Lichtman, H. Craig, V. Sailor and M. Drinkwine, Appl. Surface Sci. 7 (1981) 325. [91 M. Furuyama, K. Kishi and S. Ikea, J. Electron Spectrosc. Related Phenomena 13 (1978) 59. [10] S. Kolhe, S.K. Kulkarni, A.S. Nigavekar and S.K. Sharma, Solar Energy Mater. 10 (1984) 47.
L251
SURFACE SCIENCE LETTERS X-RAY PHOTOELECTRON SPECTRA OF CRYSTAL AND THIN FILM CADMIUM SULPttIDE M. MARYCHURCH and G.C. MORRIS Department of Chemistry, University of Queensland, Brisbane 4067, Australia Received 2 January 1985
Normal incidence and angle resolved X-ray photoelectron spectra of film and single crystal CdS are reported. In the spectral region for S there is a S 2- peak (162.6 eV) and after surface oxidation a SO42- peak (169.0 eV) which can be removed by washing. Using MgKa excitation, ghost Cd peaks, excited by stray AIKa radiation, occur in the S spectral region (172.6 and 179.4 eV). Cd peaks are 405.2+0.2 and 412.0+0.2 eV. Temperature dependence studies of the Cd:S ratio show that heating in ultrahigh vacuum leaves the surface S rich.
X-ray photoelectron spectra (XPS) have been used to probe processes occurring during manufacture of thin film n-CdS/p-CdTe heterojunction solar cells [1]. These studies using photoconductive CdS films have displayed XPS spectral features which have been previously observed and interpreted in several ways [2-4]. This letter gives XPS data for both crystal and thin film CdS, provides an unambiguous interpretation of that data and rationalizes the previous different interpretations. XPS data were recorded with a Perkin-Elmer Model 560 double pass CMA with an angle resolvable aperture using either MgKa or A1Ka radiation from a dual anode X-ray source operated at 300 W, 15 kV. Band pass energy was 25 eV. The spectrometer was calibrated using the Au 4f7/2 line (83.8 eV) with the C ls line (284.6 eV) as an internal standard. All scans were simultaneously run over the regions for S (158-183 eV), Cd (400-420 eV), O (525-545 eV) and C (280-300 eV) using the Model 560 MACS Version VI software. Samples used were photoconductive polycrystalline films made by chemical deposition onto ITO-glass substrates [5]; polycrystal cadmium sulfate (ESPI, USA, 99.99 + % pure) and a single crystal n-CdS doped with 1016 cm -3 indium (Materials Research Corp., USA). Both crystal and film were conductive and electron irradiation to prevent charging was unnecessary, a useful precaution in view of the reported electron irradiation effects on CdS [6,7]. Temperature dependent studies were made using a hot/cold probe as the sample mount (Perkin-Elmer Model 02-122). In earlier measurements, Amalnerkar et al. [2,3] obtained three peaks in the S region using MgKa radiation from a dual anode source and interpreted these 0039-6028/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M. Marychurch, G.C. Morris / XPS of crystal and thin film CdS
L252
Table 1 Peak positions (eV) in X-ray photoelectron spectra of CdS (film and crystal) and CdSO 4 (polycrystal) Single crystal
Thin film
S(0001) face
Cd(0001) face
Unannealed
Annealed
161.6_+0,2 169.0_+0,3 172.2_+0.2 179.0_+0.2 405.2_+0.2 412.0+0.2 531.7_+0.3
161.6_+0.2 169.0_+0.3 172.2_+0.2 179.0+_0.2 405.2+_0.2 412.0+_0.2 531.7+_0.3
161.4_+0.2 168.6_+0.3 172.0-+0.2 178.8_+0.2 405.0_+0.2 411.8_+0.2 531.4+_0.3
161,2_+0.2 169.0_+0.2 172.0-+0.4 178.8_+0.4 405.0_+0.2 411.8-+0.2 531.4_+0.3
CdSO 4
Assignment
161.6+0.4 168.6_+0.2 172.4_+0.4 179.0_+0.4 405.4_+0.2 412.2-+0.2 531.6-+0.4
S2p S 2S 2p SO42 Cd 3d5/z ghost ") Cd 3d3/2 ghost ~ Cd 3d5/2 Cd 3d~/2 O ls
~ Present only under Mg radiation.
t <
>..
b
i
-180
i
,
,
•
-
-171 -162 Binding Energy, EV
-
180
-171 -162 B~ndtng Energy, EV
Fig. 1. X-ray photoelectron spectra in the S spectral region. (a) CdS crystal (S face (0001)) under MgKa radiation. (b) CdS crystal (S face (0001)) under A1Ka radiation. (c) CdS crystal (S face (0001)), angle resolved (grazing angle) under A1K a radiation. (d) CdSO 4 polycrystal under MgK a radiation. Fig. 2. X-ray photoelectron spectra of photoconductive CdS film in the S spectral region: (a) under MgKa radiation; (b) under AIKa radiation.
M. Marychurch, G.C. Morris / XPS of crystal and thin film CdS
L253
as S2- (162.6 eV), SO~- (172.8 eV) and SOz adsorbate (179.7 eV). Their published spectra also showed a peak near 169 eV although that peak was not discussed. The peaks (172.8 and 179.7 eV) were substantially reduced in intensity by thermal desorption as was the sample photoconductivity and the data were used to suggest a causal relationship between the existence of the S-O species and the extent of photoconductivity. However, Lichtensteiger and Webb [4] reported peaks in the S region for CdS films at 162 and 169 eV only and suggested that the peaks at 172.8 and 179.7 eV observed by Amalnerkar et al. [3] are Cd ghost peaks excited by stray AIKc~ radiation. That view was rejected by Amalnerkar et al. [3] on two grounds, viz.: (a) the reduction of the peak intensities with oxygen desorption suggested that they arose from S-O species; (b) the binding energy value of metal-SO 2 species determined by Furuyama et al. [9] and quoted by Amalnerkar et al. as 176.7 eV was consistent with the observed value of - 172-179 eV. Our data show that the peaks at 172.8 and 179 eV are indeed Cd ghosts excited by stray A1Ka radiation and that the rejection of that view by Amalnerkar et al. was based on false premises in that: (i) the XPS peaks for metal-SO 2 species observed by Furuyama et al. is near 167.6 eV as seen in their figs 1-4 and their table 1, not the printing error value 176.6 eV which appears on page 65 of their article and is used by Amalnerkar et al.: (ii) the reduction in the peak intensities thought to be associated with oxygen desorption is caused by the outdiffusion of Cd upon heating so that ghost Cd lines are reduced in intensity. The evidence to support our view is now presented. Fig. 1 shows spectra of a (0001) face of a single crystal CdS under MgKa and AIKa radiation. Data from the (0001) S face are shown: the data from the (0001) Cd face are different only in the signal intensity with angle resolved (AR) spectra. It is clear from fig. 1 that the peaks at 172.6 and 179.4 eV are present only when MgKa is the radiation source. In table 1, the peak positions and their interpretations are given for these surface spectra. The peaks at 172.6 and 179.4 eV are "ghosts" from indeterminate A1Ka excitation of Cd being displayed downfield by the energy difference of the AIKa (1486.6 eV) - MgKa (1253.6 eV). Also shown in fig. 1 is the spectrum of CdSO4 under MgKa irradiation which allows identification of the 169.2 eV peak as SO~-. In this spectrum, similar "ghost" Cd peaks as seen in CdS are clearly observed. Angle resolved spectra under either MgKa or A1Ka radiation showed that the 169.0 eV peak was the most intense for only shallow angle emission emphasizing the surface nature of the species. Again the MgKa radiation gave strong ghost peaks which were absent under A1Ka radiation. In fig. I is shown the angle resolved spectra (grazing angle) under A1Ka radiation showing the SO42- and S z- peaks. The size of the 169.0 eV peak can be altered by treatment of the single
L254
M. Marychurch, G. C. Morris / XPS of crystal and thin film CdS
crystal surface. After etching with HC1 (conc. 20 s), or after ion etching the outer surface layers, there was no peak at 169.0 eV but it developed after the CdS was exposed to the air. Washing with Millipore water for 5 min, rinsing with methanol and drying the surface with a stream of purified nitrogen removed the peak at 169.0 eV. The XPS spectra for thin film CdS are shown in fig. 2 and the data given in table 1. Angle resolved spectra of the film differed little from the normal multiplex, presumably because of surface roughness of the film. Thin film CdS annealed at 600°C (in air) had the XPS peak at 169.0 eV as the most prominent. Thin film CdS with an oxidised surface when heated in the ultra high v a c u u m c h a m b e r ( < 10-9 Torr) from r o o m temperature shows a decrease of the Cd : S ratio with temperature increase over the range 300-600°C. This result indicates that Cd outdiffuses from the surface which was left S rich. With temperature increase, the SO42- peak (169.0 eV) diminished in intensity with an apparent activation energy of 0.3 eV. W h e n the air contaminated crystal is subjected to oxygen a d s o r p t i o n - d e sorption cycles, the peaks in the S region alter little in intensity. The SO 2 peak can be removed as discussed above by washing the surface or by heating in ultra high vacuum. Such heating also reduces the intensities of the " g h o s t " peaks because the crystal surface becomes S rich not because of removal of oxygen species as stated by Amalnerkar et al. [2,3]. In conclusion, the X-ray photoelectron spectra of CdS in the S region shows that lattice S 2- appears at 162.6 eV and after surface oxidation SO42- appears at 169.0 eV. Stray A1Ka radiation can produce " g h o s t " Cd peaks at 172.6 and 179.4 eV under M g K a radiation. Financial support came from the Australian Research Grants Scheme. We thank Dr. W. D a n a h e r for preparing the thin film CdS and for discussions.
References [1] D. Horton, J. Keyes, L.E. Lyons and G.C. Morris, J. Electroanal. Chem. 168 (1984) 101. W. Danaher, L.E. Lyons and G.C. Morris, Appl. Surface Sci., in press. [2] D.P. Amalnerkar, S. Badadrinarayanan, S.K. Date and A.P.B. Sinha, Appl. Phys. Letters 41 (1982) 270. [3] D.P. Amalnerkar, S. Badrinarayanan, S.K. Date and A.P.B. Sinha, J. Appl. Phys. 54 (1983) 2881. [4] M. Lichtensteiger and C. Webb, J. Appl. Phys. 54 (1983) 2127. [5] W. Danaher, L.E. Lyons and G.C. Morris, Solar Energy Mater., in press. [6] M. Lichtensteiger, C. Webb and J. Lagowski, Surface Sci. 97 (1980) L375. [7] J. Lagowski, M. Lichtensteiger and P.M. Williams, Surface Sci. 84 (1979) L223. [8] D. Lichtman, H. Craig, V. Sailor and M. Drinkwine, Appl. Surface Sci. 7 (1981) 325. [91 M. Furuyama, K. Kishi and S. Ikea, J. Electron Spectrosc. Related Phenomena 13 (1978) 59. [10] S. Kolhe, S.K. Kulkarni, A.S. Nigavekar and S.K. Sharma, Solar Energy Mater. 10 (1984) 47.