- Email: [email protected]
XPS investigation of the reaction of SnS2(0001) with Cu
Surface Science 194 (1988) L69-L76 North-Holland, Amsterdam
L69
SURFACE SCIENCE LETTERS
XPS INVESTIGATION OF T H E REACTION OF S n S 2 ( ~ I ) WITH Cu F.S. OHUCHI, W. J A E G E R M A N N * and B.A. PARKINSON E.I. du Pont de Nemours & Co, Central Research and Development Department, Experimental Station, Wilmington, DE 19898, USA Received 30 July 1987; accepted for publication 23 October 1987
The reactivity of Cu with UHV-cleaved (0001) surfaces of SnS 2 is investigated by X-ray photoelectron and Auger spectroscopy. Deposition of Cu results m an tmmediate reaction to form a new Cu containing phase of the stoichiometry CuSnS 2. Further deposited Cu diffuses through this phase evidently leading to a bulk reaction. We suggest the formation of an intercalation compound of Cu with SnS2.
The reactivity of semiconductor surfaces with metals has been the subject of intense study in the last decade [1,2]. UHV surface investigations contribute to a microscopic understanding of the chemistry involved which is the basis for the control of Schottky barrier and ohmic contact formation. Metal atoms adsorbed on surfaces may form either metal layers, clusters e- r,:act with the substrate. Factors determining the occurrence of a specific pr "~ss are still of interest especially for the more complex chemical sy~Lems of compound semiconductors [1.2], We report here photoelectron measurements o~ the depositio~ of Cu on SnS2 van der Waals faces. SnS 2, a layered semiconductor with a 2.2 eV bandgap, belongs to a class of layered dichalcogenides which have potential application in intercalation battery systems [3] m~d photoelectroehemical ~olar cells [4]. The special feature of these materials is their pronounced crystalline anisotropy resulting from strong chemical bonds in two dimensions. The crystals are in the form of sheets of X - M - X layers bound to each other via weak van der Waals interactions [5]. The van der Waals surfaces consist of close packed chalcogenide atoms with saturated chemical bonds, considered to be the reason for the chemical haertness of these materials. This inertness and the lack of dangling bonds, which may be surface recombination centers for charge carriers, exhibit special advantages for use in . . . . . . -1. . . . . . . . ' . . . . . lyre junctions for solar energy conversion [4] or dye sensitization {6). Solid state junctions have also been prepared from such semiconductors [7]. * Permanent address: Hahn Meitner lnstitut, Glienicker strasse 100, D-10(O Berlin 39, Germany.
0039-6028/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L70
F.S. Ohuchi et ai. / The reaction of SnS,(O001) with Cu
These materials are very attractive for surface science studies, as renewable, clean and undisturbed surfaces can easily be prepared by cleaving the crystals in UHV. Therefore the interaction of many different adsorbates with inert surfaces can be investigated. For example it has been shown that halogens, 02 or H 2 0 adsorbed on MoX2(0001) (X = S, Se) [8,91 or 02 on SnS2 or ZrSe2(0001) [10] do not react with the substrate. Herein we investigate the reactivity of SnS2 with Cu atoms. Due to the two dimensionality of the substrate and the resulting chemical features (intercalation reactions, lack of dangling bonds) these materials provide model systems for the study of basic metal/substrate interactions. The X-ray photoelectron spectroscopy measurements described here were performed in an UHV multitechnique chamber with monochromatized X-rays (AI Ka). The XPS system was constructed by Surface Science Lab Inc. and was equipped with a computer system (HP 9836) for data aquisition and manipulation. One special advantage of this system is the capability to probe small areas (300 ~m). Further details of the system configuration are given elsewhere [11]. The spectrometer was calibrated with clean evaporated Au and Cu samples, respectively. The resolution achieved for Au 3d5/e was about 0.9 eV.
Auger electron spectra were obtained from a single pass cylindrical mirror analyzer (PHI-10/155). A 2000 eV electron beam was used as excitation source.
Single crystals of n-SnS 2 (phosphorus doped) prepared by gas transport reaction with i2 [12] were attached with one van der Waals face to an AI ~ample holder via conductive Ag-epoxy and dried at 100 ° C for 12 h to remove organic residues. Clean (000i) van der Waalv, surfaces were produced by ~ieaving the crystals in ~X~, ~ via a ~ticky tape tipped to the exposea surtace z ad peeling off a sheet of the crystal. The optically smooth surfaces obtained were free from contamination, as judged from XPS and AES. Cu atoms were deposited "in-situ" onto the cleaved surfaces from a high temperature Cu sublimation source. The deposition rate was estimated by AES and XPS with different exposure times. AU Cu coverages are expressed herein in deposition times (seconds). A deposition time of 50 s corresponds to about one monolayer. Photoelectron spectra obtained for different dosages of Cu are shown in figs. 1, 2 and 4. For low dosages (2-5 s Cu) the Sn photoemission lines first show a shift of their fines to lower binding energies (Sn(4ds/z) from 26.0 to 25.7 eV, Sn(3d 5/2) from 486.9 to 486.5 eV) w'.'.th no pronounced broaderfing of the enfission peaks. After depositing Cu for 10 s a reaction sets in as deduced from the triplet pattern developed for the Sn(4d) line (fig. 1) and the broadening of the Sn(3d) emission (FWHM changes from 0.9 to 1.3 eV). After
L71
F.S. Ohuchi et aL / The reaction of SnS2(O001) with Cu |
Sn(4d) : CulSnS2(0001).
Cu deposition (see| 500 202 62
i
20 10
5
/ m 30
I 29
!
.
28 Elec|ron
\ I
.I
27
26
.....
I
2S
I 24
I 23
22
Binding Energy(eV)
Fig. 1. Changes of the Sn(4d) photoemission peak in the course of Cu deposition. The Cu deposition time is indicated.
deposition of Cu for 60 s the surface has been completely reorganized as is indicated by the o_,A.~o,t.,uj1,~,,,1 again o......."r''"~""-.e,th,~......~/'9-'~/9 . . . . . . . . .m,!tip!et .. with a different binding energy (EB(3d5/,_)= 24.9 eV). Also the halfwidth of the Sn(3ds/a) peak is again reduced and shows the shift in binding energy (E~ = 485.9 eV). A further deposition of Cu (even for orders of magnitude greater dosages) no longer changes the spectral features. A peculiar and
L72
F.$. Ohuchi et aL / The reaction of SnSz(O001) with Cu
'1
I
I'
i
IA
I
I
I
i
S(2p) : CulSnS2( 0001) /,,,\ Cu Deposition (sec)
500
202 62 m
fb
20 e~ 10
/ i 16~
I
J
I
t
I
i
I
I
165
15z,
163
162
161
160
'~.~9
158
E,e©tron B|ndln9 ~=nergy(eV) Fig. 2. Cnanges of the S(2p) photoemissionpeak in the course of Cu deposition. The Cu depositiontimeis indicated. remarkable result is the formation of a clear "isosbestic" poLnt du6.ng the course of the reaction, indicating that only two Sn-species are involved. The S(2p) peak and :hange due to Cu deposition is shown in fig. 2. ParalleUing the irfitial sh of the Sn peaks is a shift of the sulfur peaks to lower binding energies ( E B = 161.9 eV to E B = 161.7 eV for 5 s Cu). For additional Cu deposition only small changes of the S emission peak are
F.S. Ohuchi et al. / The reaction of SnS2(O001) with Cu
L73
Equivalent ML 1.0
0
i
1
2.0
3.0
[
|
2.2 2.OK 3Q-.D-13'~"
D ......
Q
O
O.------
S/Sn atomic ratio
1.8
i
1.6 1.4 1.2 0
1.0 if ~'''~
.8
@
Cu/Sn atomic ratio
.6
.2 ,
0
L__L__L__J__I
50
_~m~___L__L__~
100
L___~__L
l 150
Cu Deposition Time ~see! Fig. 3. Change of the corrected Cut2p,/2)/Sn(3ds/2) intensity ratio in the course of Cu deposilion.
measured. However the intensity is slightly reduced and an additional shift of 0.1 eV to lower binding energies occurs for very high Cu doses ( > 202 s). The Cu emission peak is measured at a c~nstant b[ndLng e~ergy th~oug~hout the whole adsorption experiment (EB(Cu(3p3/2)) = 932.35 eV). No shake-up satelfites are observed indicating the presence of Cu 1+ or ( u °. A gradual, correspondent increase of the Cu emission intensity is observe t up to dosages of 20 s (equivalent to 0.4 ML Cu) after which further dep,~sition of Cu is not reflected in changes of the Cu peak intensity. The change of the Cu(2p3/2)/Sn(3d 5/2) photoemAssion intensity ratio (corrected by photoen'fission cross sections) during the course of Cu deposition is shown in fig. 3. The curve is characterized by a fast ~mtial growth of the Cu emission peak combined with a fast attenuation of the Sn emission which saturates after an exposure to an equivalent of 0.4 ML. After this the relative intensities remain nearly unchanged indicating that the deposited Cu is
ES. Ohuchi et al. / The reaction of SnS2(O001) with Cu
L74
Valence : CulSnS2(0001)
Cu Deposition time Isec)
=k,
62
m
m
l= c:
m
(xl/2)
Ixl/2l 16
15 1l.
13
12 11 10
9
8
7
6
5
4
3
2
1
EF
Electron BlndBn9 EnergyleV) Fig. 4. Changes of the valence band spectra in the course of Cu deposition, The Cu deposition time is indicated. No further changes occur for higher deposition times.
migrating into the bulk of the crystal. A second experiment, measuring the change of Auger intensities of the Cu(LMM) a,d Sn(MNN) transitions, resulted in a very similar curve. Changes of the valence band spectra are shown in fig. 4. The spectrum of clean SnS2 shows four spectral features at 2.8, 4.9, 6.6 and 9.1 eV all related to the S(3p) derived energy bands [13,14] and a broad feature with a maximum at
F.S. Ohuchi et ai. / The reaction of SnS2(O001) with Cu
L75
13.6 eV originating from the S(3s) electron state. The difference of the valence band maximum to EF is 1.7 eV, in reasonable agreement with the n-type semiconducting behavior of the phosphorus doped crystals [151.Addition of Cu gradually changes the valence band spectrum manifested by the intense emission of the Cu(3d) electron states. The valence band m a x i m u m shifts towards E F with Cu deposition. The final spectrum shows an intense feature of the Cu d-emission at 3.1 eV with a shoulder towards lower En values (E n = 1.5 eV). The S(3s) levelstillshows up as a peak at 13.6 eV but no Fermi edge indicative of a metallic Cu layer is developed. The above spectral resultssuggest the following mechanism for Cu deposition and subsequent reaction.For low coverages Cu transferselectron density to the semiconductor surface atoms shifting their binding energies to lower values. Further additions of copper resultin charge transfermostly to Sn, as is indicated by a comparison of its measured binding energy values to literature data [16]. As a consequence the surface is reorganized to form a CuxSnS 2 phase. The valence band spectrum is in good agreement with the published spectrum for CueS or similar Cu + compounds with the Cu + ion in tctrahcdral sulfur environment [17].W e therefore suggest that the deposited Cu atoms are oxidized and intercalatedinto the tetrahcdralholes in the van dcr Waals layers of SnS 2. The formation of the "isosbestic" points and the occurrence of the saturation value in fig. 3 suggests that a stable phase is formed. As indicated by the valence band spectrum this new phase is a p-type semiconductor. The stoichiometricratio(Cu/Sn) which is obtained from the photocmission peak areas corrected by photoemission cross sections reaches values of about 1 in the saturation region of fig. 3. The Sn/S ratio remains nearly unchangecl, therefore the stoichiometry for the stable phase is CuSnS2. Sn is reduced to a formal oxidation state of +3. To our knowledge no compound of this stoichiometry has been reported, but similar compounds with layered structures such as CuxNbS2 [181 or AgCrS2 [191 are known. Intercalated ions axe usually N~tly mobile within the host rnatc~al [31. Therefore once a surface layer is formed a subsequent evaporation of Cu atoms to the surface does not form a metallic Cu layer but instead Cu diffuses through this layer and further reaction with SnS2 occurs. It is not clear to us if the immediate reaction is due to a transport of Cu + through the S - S n - S sheets, which may be possible through tetrahedral hole connections existing within the S - S n - S layers, or whether a high mobility of Cu + on the surfaces and within the layers allows reasonable transport rates in the c-direction via stacking facts, screw dislocations and oqler defects. The junction between SnS2 and the ,eacted surface layer has to be investigated with respect to its electrical properties. Future experiments wi!l Mso concentrate on structural and optical investigations of the new pha~ ~ In
F.S. Ohuchi et al. / The reaction of SnS2(O001) with Cu
L76
addition different metal adsorbates and other types of layered compound substrates will be studied. We thank J. Koston for his help in crystal growth.
References [1] R.J. Nemanich, P.S. Ho and S,S. Lau, Eds, Thin Films - Interfaces and Phenomena, MRS-series, Vol 54 (1986). [2] G. Le Lay, J. Derrien and C.A. S6benne, Eds., Proc. Intern. Conf. on the formation of Semiconductor Interfaces, Surface Sci. 168 (1986). [3] M.S. Whittingham and A.J. Jacobsen, Eds., Intercalation Chemistry (Academic Press, New York, 1982). [4] H. Tributsch, Struct. Bonding 49 (1982) 127. [5] E. Mooser, Ed., Physics and Chemist~ of Materials with Layered Structure, Vols. 1-5 (Reidel, Dordrecht, 1976). [6] M. Spitler and B.A. Parkinson, Langmuir 2 (1986) 549. [7] E. Bucher, Appl. Phys. 17 (1978) 1. [8] W. Jaegerrnaun, Chem. Phys. Letters 126 (1986) 301; J. Chem. Phys., submitted; W. Jaegermann and D. Schmeisser, to be published. [9] J.L. Stickney, S.D. Rosasco, B.C. Schardt, T. Solomun, A.T. Hubbard and B.A. Parldnson, Surface Sci. 136 (1984) 15. [10] W. Jaegermann, F.S. Ohuchi, unpublished results. [11] R.L. Chaney, Recent Developments in Spatially Resolved ESCA (Surface Sci. Lab. Publication, Mountain View, CA, 1985). [12] B.A. Parkinson, to be published. [13] R.~. Williams, R.B. Murray, D.W. Govan, J.M. Thomas and E.L. Evans, J. Phys. C6 (1973) 3631. [14] P.M. Williams, in ref. [5], Vol. 4, p. 273. [15] B. Fotouhi, A. Katty and R. Parsons, J. Electroanal. Chem. 183 (1985) 303. [16] G.E. Muidenberg, Ed., Handbook of X-ray Photoelectron Spectroscopy (Perkin-Elmer, Eden Prairie, MN, 1978). [17] J.C.W. Folmer and F. JeUinek, J. Less-Common Metals 76 (1980) 153. [18] N. LeNagard, G. Collin and O. Goroehov, Mater. Res. Bull. 14 (1979) 1411. [19] T. Hibma, Solid State Commun. 33 (1980) 445.
L69
SURFACE SCIENCE LETTERS
XPS INVESTIGATION OF T H E REACTION OF S n S 2 ( ~ I ) WITH Cu F.S. OHUCHI, W. J A E G E R M A N N * and B.A. PARKINSON E.I. du Pont de Nemours & Co, Central Research and Development Department, Experimental Station, Wilmington, DE 19898, USA Received 30 July 1987; accepted for publication 23 October 1987
The reactivity of Cu with UHV-cleaved (0001) surfaces of SnS 2 is investigated by X-ray photoelectron and Auger spectroscopy. Deposition of Cu results m an tmmediate reaction to form a new Cu containing phase of the stoichiometry CuSnS 2. Further deposited Cu diffuses through this phase evidently leading to a bulk reaction. We suggest the formation of an intercalation compound of Cu with SnS2.
The reactivity of semiconductor surfaces with metals has been the subject of intense study in the last decade [1,2]. UHV surface investigations contribute to a microscopic understanding of the chemistry involved which is the basis for the control of Schottky barrier and ohmic contact formation. Metal atoms adsorbed on surfaces may form either metal layers, clusters e- r,:act with the substrate. Factors determining the occurrence of a specific pr "~ss are still of interest especially for the more complex chemical sy~Lems of compound semiconductors [1.2], We report here photoelectron measurements o~ the depositio~ of Cu on SnS2 van der Waals faces. SnS 2, a layered semiconductor with a 2.2 eV bandgap, belongs to a class of layered dichalcogenides which have potential application in intercalation battery systems [3] m~d photoelectroehemical ~olar cells [4]. The special feature of these materials is their pronounced crystalline anisotropy resulting from strong chemical bonds in two dimensions. The crystals are in the form of sheets of X - M - X layers bound to each other via weak van der Waals interactions [5]. The van der Waals surfaces consist of close packed chalcogenide atoms with saturated chemical bonds, considered to be the reason for the chemical haertness of these materials. This inertness and the lack of dangling bonds, which may be surface recombination centers for charge carriers, exhibit special advantages for use in . . . . . . -1. . . . . . . . ' . . . . . lyre junctions for solar energy conversion [4] or dye sensitization {6). Solid state junctions have also been prepared from such semiconductors [7]. * Permanent address: Hahn Meitner lnstitut, Glienicker strasse 100, D-10(O Berlin 39, Germany.
0039-6028/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L70
F.S. Ohuchi et ai. / The reaction of SnS,(O001) with Cu
These materials are very attractive for surface science studies, as renewable, clean and undisturbed surfaces can easily be prepared by cleaving the crystals in UHV. Therefore the interaction of many different adsorbates with inert surfaces can be investigated. For example it has been shown that halogens, 02 or H 2 0 adsorbed on MoX2(0001) (X = S, Se) [8,91 or 02 on SnS2 or ZrSe2(0001) [10] do not react with the substrate. Herein we investigate the reactivity of SnS2 with Cu atoms. Due to the two dimensionality of the substrate and the resulting chemical features (intercalation reactions, lack of dangling bonds) these materials provide model systems for the study of basic metal/substrate interactions. The X-ray photoelectron spectroscopy measurements described here were performed in an UHV multitechnique chamber with monochromatized X-rays (AI Ka). The XPS system was constructed by Surface Science Lab Inc. and was equipped with a computer system (HP 9836) for data aquisition and manipulation. One special advantage of this system is the capability to probe small areas (300 ~m). Further details of the system configuration are given elsewhere [11]. The spectrometer was calibrated with clean evaporated Au and Cu samples, respectively. The resolution achieved for Au 3d5/e was about 0.9 eV.
Auger electron spectra were obtained from a single pass cylindrical mirror analyzer (PHI-10/155). A 2000 eV electron beam was used as excitation source.
Single crystals of n-SnS 2 (phosphorus doped) prepared by gas transport reaction with i2 [12] were attached with one van der Waals face to an AI ~ample holder via conductive Ag-epoxy and dried at 100 ° C for 12 h to remove organic residues. Clean (000i) van der Waalv, surfaces were produced by ~ieaving the crystals in ~X~, ~ via a ~ticky tape tipped to the exposea surtace z ad peeling off a sheet of the crystal. The optically smooth surfaces obtained were free from contamination, as judged from XPS and AES. Cu atoms were deposited "in-situ" onto the cleaved surfaces from a high temperature Cu sublimation source. The deposition rate was estimated by AES and XPS with different exposure times. AU Cu coverages are expressed herein in deposition times (seconds). A deposition time of 50 s corresponds to about one monolayer. Photoelectron spectra obtained for different dosages of Cu are shown in figs. 1, 2 and 4. For low dosages (2-5 s Cu) the Sn photoemission lines first show a shift of their fines to lower binding energies (Sn(4ds/z) from 26.0 to 25.7 eV, Sn(3d 5/2) from 486.9 to 486.5 eV) w'.'.th no pronounced broaderfing of the enfission peaks. After depositing Cu for 10 s a reaction sets in as deduced from the triplet pattern developed for the Sn(4d) line (fig. 1) and the broadening of the Sn(3d) emission (FWHM changes from 0.9 to 1.3 eV). After
L71
F.S. Ohuchi et aL / The reaction of SnS2(O001) with Cu |
Sn(4d) : CulSnS2(0001).
Cu deposition (see| 500 202 62
i
20 10
5
/ m 30
I 29
!
.
28 Elec|ron
\ I
.I
27
26
.....
I
2S
I 24
I 23
22
Binding Energy(eV)
Fig. 1. Changes of the Sn(4d) photoemission peak in the course of Cu deposition. The Cu deposition time is indicated.
deposition of Cu for 60 s the surface has been completely reorganized as is indicated by the o_,A.~o,t.,uj1,~,,,1 again o......."r''"~""-.e,th,~......~/'9-'~/9 . . . . . . . . .m,!tip!et .. with a different binding energy (EB(3d5/,_)= 24.9 eV). Also the halfwidth of the Sn(3ds/a) peak is again reduced and shows the shift in binding energy (E~ = 485.9 eV). A further deposition of Cu (even for orders of magnitude greater dosages) no longer changes the spectral features. A peculiar and
L72
F.$. Ohuchi et aL / The reaction of SnSz(O001) with Cu
'1
I
I'
i
IA
I
I
I
i
S(2p) : CulSnS2( 0001) /,,,\ Cu Deposition (sec)
500
202 62 m
fb
20 e~ 10
/ i 16~
I
J
I
t
I
i
I
I
165
15z,
163
162
161
160
'~.~9
158
E,e©tron B|ndln9 ~=nergy(eV) Fig. 2. Cnanges of the S(2p) photoemissionpeak in the course of Cu deposition. The Cu depositiontimeis indicated. remarkable result is the formation of a clear "isosbestic" poLnt du6.ng the course of the reaction, indicating that only two Sn-species are involved. The S(2p) peak and :hange due to Cu deposition is shown in fig. 2. ParalleUing the irfitial sh of the Sn peaks is a shift of the sulfur peaks to lower binding energies ( E B = 161.9 eV to E B = 161.7 eV for 5 s Cu). For additional Cu deposition only small changes of the S emission peak are
F.S. Ohuchi et al. / The reaction of SnS2(O001) with Cu
L73
Equivalent ML 1.0
0
i
1
2.0
3.0
[
|
2.2 2.OK 3Q-.D-13'~"
D ......
Q
O
O.------
S/Sn atomic ratio
1.8
i
1.6 1.4 1.2 0
1.0 if ~'''~
.8
@
Cu/Sn atomic ratio
.6
.2 ,
0
L__L__L__J__I
50
_~m~___L__L__~
100
L___~__L
l 150
Cu Deposition Time ~see! Fig. 3. Change of the corrected Cut2p,/2)/Sn(3ds/2) intensity ratio in the course of Cu deposilion.
measured. However the intensity is slightly reduced and an additional shift of 0.1 eV to lower binding energies occurs for very high Cu doses ( > 202 s). The Cu emission peak is measured at a c~nstant b[ndLng e~ergy th~oug~hout the whole adsorption experiment (EB(Cu(3p3/2)) = 932.35 eV). No shake-up satelfites are observed indicating the presence of Cu 1+ or ( u °. A gradual, correspondent increase of the Cu emission intensity is observe t up to dosages of 20 s (equivalent to 0.4 ML Cu) after which further dep,~sition of Cu is not reflected in changes of the Cu peak intensity. The change of the Cu(2p3/2)/Sn(3d 5/2) photoemAssion intensity ratio (corrected by photoen'fission cross sections) during the course of Cu deposition is shown in fig. 3. The curve is characterized by a fast ~mtial growth of the Cu emission peak combined with a fast attenuation of the Sn emission which saturates after an exposure to an equivalent of 0.4 ML. After this the relative intensities remain nearly unchanged indicating that the deposited Cu is
ES. Ohuchi et al. / The reaction of SnS2(O001) with Cu
L74
Valence : CulSnS2(0001)
Cu Deposition time Isec)
=k,
62
m
m
l= c:
m
(xl/2)
Ixl/2l 16
15 1l.
13
12 11 10
9
8
7
6
5
4
3
2
1
EF
Electron BlndBn9 EnergyleV) Fig. 4. Changes of the valence band spectra in the course of Cu deposition, The Cu deposition time is indicated. No further changes occur for higher deposition times.
migrating into the bulk of the crystal. A second experiment, measuring the change of Auger intensities of the Cu(LMM) a,d Sn(MNN) transitions, resulted in a very similar curve. Changes of the valence band spectra are shown in fig. 4. The spectrum of clean SnS2 shows four spectral features at 2.8, 4.9, 6.6 and 9.1 eV all related to the S(3p) derived energy bands [13,14] and a broad feature with a maximum at
F.S. Ohuchi et ai. / The reaction of SnS2(O001) with Cu
L75
13.6 eV originating from the S(3s) electron state. The difference of the valence band maximum to EF is 1.7 eV, in reasonable agreement with the n-type semiconducting behavior of the phosphorus doped crystals [151.Addition of Cu gradually changes the valence band spectrum manifested by the intense emission of the Cu(3d) electron states. The valence band m a x i m u m shifts towards E F with Cu deposition. The final spectrum shows an intense feature of the Cu d-emission at 3.1 eV with a shoulder towards lower En values (E n = 1.5 eV). The S(3s) levelstillshows up as a peak at 13.6 eV but no Fermi edge indicative of a metallic Cu layer is developed. The above spectral resultssuggest the following mechanism for Cu deposition and subsequent reaction.For low coverages Cu transferselectron density to the semiconductor surface atoms shifting their binding energies to lower values. Further additions of copper resultin charge transfermostly to Sn, as is indicated by a comparison of its measured binding energy values to literature data [16]. As a consequence the surface is reorganized to form a CuxSnS 2 phase. The valence band spectrum is in good agreement with the published spectrum for CueS or similar Cu + compounds with the Cu + ion in tctrahcdral sulfur environment [17].W e therefore suggest that the deposited Cu atoms are oxidized and intercalatedinto the tetrahcdralholes in the van dcr Waals layers of SnS 2. The formation of the "isosbestic" points and the occurrence of the saturation value in fig. 3 suggests that a stable phase is formed. As indicated by the valence band spectrum this new phase is a p-type semiconductor. The stoichiometricratio(Cu/Sn) which is obtained from the photocmission peak areas corrected by photoemission cross sections reaches values of about 1 in the saturation region of fig. 3. The Sn/S ratio remains nearly unchangecl, therefore the stoichiometry for the stable phase is CuSnS2. Sn is reduced to a formal oxidation state of +3. To our knowledge no compound of this stoichiometry has been reported, but similar compounds with layered structures such as CuxNbS2 [181 or AgCrS2 [191 are known. Intercalated ions axe usually N~tly mobile within the host rnatc~al [31. Therefore once a surface layer is formed a subsequent evaporation of Cu atoms to the surface does not form a metallic Cu layer but instead Cu diffuses through this layer and further reaction with SnS2 occurs. It is not clear to us if the immediate reaction is due to a transport of Cu + through the S - S n - S sheets, which may be possible through tetrahedral hole connections existing within the S - S n - S layers, or whether a high mobility of Cu + on the surfaces and within the layers allows reasonable transport rates in the c-direction via stacking facts, screw dislocations and oqler defects. The junction between SnS2 and the ,eacted surface layer has to be investigated with respect to its electrical properties. Future experiments wi!l Mso concentrate on structural and optical investigations of the new pha~ ~ In
F.S. Ohuchi et al. / The reaction of SnS2(O001) with Cu
L76
addition different metal adsorbates and other types of layered compound substrates will be studied. We thank J. Koston for his help in crystal growth.
References [1] R.J. Nemanich, P.S. Ho and S,S. Lau, Eds, Thin Films - Interfaces and Phenomena, MRS-series, Vol 54 (1986). [2] G. Le Lay, J. Derrien and C.A. S6benne, Eds., Proc. Intern. Conf. on the formation of Semiconductor Interfaces, Surface Sci. 168 (1986). [3] M.S. Whittingham and A.J. Jacobsen, Eds., Intercalation Chemistry (Academic Press, New York, 1982). [4] H. Tributsch, Struct. Bonding 49 (1982) 127. [5] E. Mooser, Ed., Physics and Chemist~ of Materials with Layered Structure, Vols. 1-5 (Reidel, Dordrecht, 1976). [6] M. Spitler and B.A. Parkinson, Langmuir 2 (1986) 549. [7] E. Bucher, Appl. Phys. 17 (1978) 1. [8] W. Jaegerrnaun, Chem. Phys. Letters 126 (1986) 301; J. Chem. Phys., submitted; W. Jaegermann and D. Schmeisser, to be published. [9] J.L. Stickney, S.D. Rosasco, B.C. Schardt, T. Solomun, A.T. Hubbard and B.A. Parldnson, Surface Sci. 136 (1984) 15. [10] W. Jaegermann, F.S. Ohuchi, unpublished results. [11] R.L. Chaney, Recent Developments in Spatially Resolved ESCA (Surface Sci. Lab. Publication, Mountain View, CA, 1985). [12] B.A. Parkinson, to be published. [13] R.~. Williams, R.B. Murray, D.W. Govan, J.M. Thomas and E.L. Evans, J. Phys. C6 (1973) 3631. [14] P.M. Williams, in ref. [5], Vol. 4, p. 273. [15] B. Fotouhi, A. Katty and R. Parsons, J. Electroanal. Chem. 183 (1985) 303. [16] G.E. Muidenberg, Ed., Handbook of X-ray Photoelectron Spectroscopy (Perkin-Elmer, Eden Prairie, MN, 1978). [17] J.C.W. Folmer and F. JeUinek, J. Less-Common Metals 76 (1980) 153. [18] N. LeNagard, G. Collin and O. Goroehov, Mater. Res. Bull. 14 (1979) 1411. [19] T. Hibma, Solid State Commun. 33 (1980) 445.