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The influence of oxygen deficiency and Sb doping on inverse photoemission spectra of SnO2
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Surface Science 280 (1993) 393-397 North-Holland
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_
The influence of oxygen deficiency and Sb doping on inverse photoemission spectra of SnO, P.C. Hollamby, P.S. Aldridge, G. Moretti ‘, R.G. Egdell Inorganic Chemistry Laboratory, South Parks Road, Oxford OX2 3QR, UK
and W.R. Flaveil Department of Chemistry, VMIST, P.O. Box 88, Manchester M60 lQD, UK
Received 14 July 1992; accepted for publication 22 September 1992
The changes in the empty electronic states in SnO, produced by ion-beam induced oxygen deficiency and by Sb doping have been studied by inverse photoemission spectroscopy. Inverse photoemission in SnO, itself is dominated by peaks 4 and 12 eV above the Fermi level, the former associated with empty states of dominant Sn 5p atomic character. Sb doping populates states in the Sn 5s conduction band, shifting the empty state structure closer to the Fermi energy. By contrast oxygen deficiency introduces new states above the main Sn5p peak. These are tentatively described as 5s-5p hybrids pushed up in energy from the Sp band by mixing between atomic orbitals of different parity in the non-centrosymmetric cation environment of oxygen deficient SnO,.
1. Introduction
Stoichiometric tin(W) oxide is a semiconductor with a direct forbidden bandgap of 3.6 eV adopting the tetragonal rutile structure [l-4]. Due to the essentially centrosymmetric environment of the tin cations within the rutile structure, there is little mixing between Sn5s and 5p states at the zone centre, and the lowest energy states are therefore of almost pure SnSs atomic character: the maximum in the Sn 5p partial density of states is at much higher energy [5,61. Electrons may be introduced into the conduction band by substitutional replacement of Sn with Sb or of 0 with F, the metal-to-non-metal transition occurring at carrier concentrations around 1.8 x 10”/cm3 [41. ’ On leave from: Centro di Studio de1 CNR “SASCO”, Dipartimento di Chimica, Universita “La Sapienza, Piazzale Aldo Moro 5, 00185 Roma, Italy. 0039-6028/93/$06.00
There have been several previous studies of polycrystalline and single crystal tin(W) oxide by photoemission techniques [7-g], including our own direct observations of the conduction electrons in Sb-doped material [lo-121. Photoemission shows that oxygen deficiency in SnO, produced either by removal of top-atomic-layer bridging oxygen atoms at SnO, (110) surfaces by thermal annealing in UHV [7-91 or by preferential sputtering of oxygen at single-crystal or polycrystalline surfaces under argon ion bombardment [133 does not introduce electrons into the SnSs conduction band. Instead, new occupied states are observed just above the 02p valence band edge, some 3 eV below the bottom of the conduction band. The stabilisation of these states has been discussed previously in terms of crystal field mixing between SnSs and 5p levels at noncentrosymmetric sites at the surface or adjacent to an 0 vacancy [8,13], giving a local density of
0 1993 - Elsevier Science Publishers B.V. All rights reserved
states similar to that of SnO where there is pronounced mixing of this sort [14]. Fig. 1 shows a schematic view of the pattern of ionic energy levels expected at centrosymmetric and noncentros~metric sites. Recent resonant photoemission studies have confirmed the cationic nature of the bandgap states [9], but the idea of 5s-5p mixing has been called into question. In the present communication we extend previous work on filled electronic states in SnO, to the study of empty states, using the technique of inverse photoemission spectroscopy. Comparison between the effects on the spectra of Sb doping and ion beam induced oxygen deficiency confirms the essential features of the levels illustrated in fig. 1.
2. Experimental Experiments were performed in a diffusion and titanium sublimation pumped UHV chamber (base pressure 1.5 x 10ei" mbar) equipped with a 50 mm-mean-radius spherical-sector electron energy analyser and a 5 keV electron gun for measurement of N(E) Auger spectra and energy loss spectra. Electrons for the IPES experiments were produced by a low energy electron gun with an
(a)
W
Fig. 1. Schematic ionic energy level diagram for Sn cations (a) in a centros~m~tric coordination environment and (b) in a non-centrosymmetric environment. Note strong mixing between 5s and 5p levels in the latter.
indirectly heated BaO/W dispenser cathode whose design was similar to that of Stoffel and Johnson [15]. With the aid of the electron energy analyser, it was established that the energy hal~idth in the emission from the electron gun under typical operating conditions was of the order 0.25-0.30 eV. Photons were detected with a bandpass detector comprising a 2 mm thick CaF, window 25 mm in diameter and a grounded Ta truncated conical photoemitter foil [Xl. Electrons emitted from the foil were counted with a Gahleo CEM4503 channeltron operating under 3 kV bias. The bandpass of this design of detector is centred around hv = 9.8 eV, with a theoretical energy halfwidth in the bandpass of 0.8 eV [17]. Electrons were incident normally on the surfaces of the polyc~stalIine samples studied in the present work and the photon detector was mounted at 4.5” to the axis of the electron gun. Channeltron count rates were of the order 500 s-l for incident gun currents of around 1 PA. The position of the Fermi energy was established from measurements on a polyc~stalline tantalum foil in electrical contact with the sample pellets. The full width at half maximum height of the differentiated Fermi edge signal was 1.2 eV. Comparison between IPE spectra of the metallic conductor Sn,,,Sb,,O, and semiconducting SnO, indicated that the conductivity of the latter was high enough to prevent problems in the calibration associated with sample charging. Polycrystalline SnO, (Johnson Matthey Specpure Grade) was pressed at 10 tonnes/cm2 into a 13 mm disc and sintered for several days at 1000°C to give a mechanically robust ceramic pellet. Similar ceramic samples of the doped material Sn 0,97Sb0.0302were prepared by coprecipitation of Sn and Sb from a solution with aqueous NH,, as described in detail elsewhere [lO,lS]. The undoped tin oxide samples were cleaned by cycles of bombardment with 500 eV argon ions and annealing in UHV or lop5 mbar oxygen at temperatures up to 500°C until Auger spectra were free of signals due to carbon contamination and the ratio between 0 KLL and Sn M,N,,N,, peak heights assumed a constant value of 0.84. The doped sample was cleaned by annealing alone. A very weak CKLL Auger feature re-
P.C. Hollamby et al. / The injluence of 0 and Sb on IPE spectra of SnO,
395
monitored by the progressive decrease O/Sri Auger ratio, as shown in fig. 3.
in the
3. Results and discussion The inverse photoemission spectrum of SnO, shown in fig. 4 contains two broad peaks on a rising background respectively 4 and 12 eV above the Fermi energy. The intensity of the structure immediately above the Fermi energy is very low, appearing as a broad and ill-defined shoulder to the low energy side of the 4 eV peak. The low energy features of the IPES conform tolerably well to the band structure for SnO, calculated by
Sn
3
250
300
350
400
450
01
500
Kinetic Energy / eV
Fig. 2. N(E) Auger spectra of (a) cleaned polycrystalline SnO, and (b) cleaned polycrystalline Sn,,Sb,,,O,.
6-4 annealed
mained after this treatment, but the peak height was less than 5% of that of the oxygen peak and the contamination did not influence the IPES. As found in previous XPS experiments [11,12], there is marked Sb segregation at annealed Sb-doped SnO, surfaces and the Sb/SnM,N,N, Auger area ratio was around 0.27, very much greater than the value of about 0.03 expected from the bulk doping level (fig. 2). Note however that the previous work has shown that the segregated Sb ions do not act as donor centres so that the carrier concentration close to the surface is essentially the same as in the bulk. This was confirmed in the present work by observation of a plasmon loss peak at 0.55 eV in energy loss spectra excited with a 25 eV electron beam from the bulk doping level [lO,ll]. Oxygen deficient SnO, surfaces were prepared by 500 eV ion bombardment [191 of the cleaned SnO, surface at a current of order 5 PA. The decrease in surface oxygen content with time was
tc)
I
2 hour
3jo
3eo 4jo Kinetic Energy / eV
4ho
5 i0
Fig. 3. Influence of progressively increasing exposure to 500 eV ion beam at 5 ~A/cm’ on N(E) Auger spectra of SnO,. (a) unexposed, (b) 1 h exposure, (c) 2 h exposure, and (d) 3 h exposure.
P.C. Hol~mby
-I--
.-
r
hv= 9.8eV
IPES
J
et al. / The influence of 0 and Sb on IFE spectra of SnO,
Sn02
--
(a) :
(b) i 0
7
2
4
6
6
IO
12
14
16
Energy relative to 4 / eV
Fig. 4. Inverse photoemission spectra of (a) clean polycrystalline SnOz and (b) cleaned polycrystalline Sn,,Sb0,,302. In (b) the dots show the spectrum of undoped SnO,, highlighting the small shift to lower energy in empty state structure upon Sb doping.
Robertson [S] which shows a very small density of states at the bottom of the conduction band associated with the strongly dispersing Sn5s states and a much higher density of states between 4.5 and 8 eV above the Fermi energy associated with Sn5p states. There is however quantitative disagreement in that the experimental Sn5p peak appears at lower energy than in the band structure calculation. Note also that matrix element effects will exert a very strong influence on the spectra, the one electron cross section relevant to photoemission being calculated [20] as 1.45 X 10e4 Mb (Sn5.s) and 1.3 Mb (Sn5p> at hv = 16.7 eV for atomic Sn (atomic SnSs states are not ionised at the lower energy of 10.2 eV, which is closer to the energy of the IPES detector). The same matrix elements govern the radiative decay in IPES, although of course the cross sections are
scaled down by a factor of order l/a2, where LYis the fine structure constant [21]. Band structure calculations do not extend to the energy of the 12 eV peak. In our experience so far with IPES of metal oxides, structure at this high energy is usuahy broad and ill defined and depends very strongly on the details of surface preparation. It is therefore difficult to offer an assignment in terms of empty densities of states. An alternative interpretation can be given in terms of sequential electron scattering followed by radiative decay. The probability of electron scattering via interband excitation is very high at oxide surfaces [19] and will contribute to a high kinetic energy background in IPE spectra, possibly with a discrete structure. Sb doping is known to populate states in the Sn5s conduction band [lo-121. From photoemission experiments on ceramic Sn,,g,Sb,,,,O, samples similar to that studied here, the Fermi level is estimated to be 0.42 eV above the bottom of the conduction band [ lo,1 11.In a rigid band model this should lead to a downward shift in the energy of the empty states. Comparison between spectra (fig. 4) for undoped SnO, and Sn,,,Sb,,,,O, does indeed reveal a downward shift in spectral weight, although due to the relatively poor experimental resofution and broad spectral features it is not possible to make quantitative comparison with the photoemission spectra. We consider next the effects of oxygen deficiency on the IPES. Prolonged ion bombardment reduces the O/Sri Auger intensity ratio from the value of 0.84 characteristic of stoichiometric, annealed surfaces to the lower value of 0.66. We thus estimate that the effective surface composition is SnO,,s, and that roughly half of the tin ions sampled by Auger spectroscopy find themself in an SnO-like environment. The electron pathlength is likely to be longer at the very low electron energies used in the IPES experiment. The changes in IPES are therefore rather more muted than in the Auger spectra. Nonetheless, comparison between spectra before and after ion bombardment (fig. 5) shows the emergence of new structures to the high energy side of the 4 eV peak, filling in the vaIley between this peak and the 12 eV peak. These new states are attributed
F.C. Halls
IPES
et al. / The influence of 0 and Sb on IPE spectra of SnO,
397
Acknowledgements
hv= 9.0eV
The equipment used was funded by SERC and the Royal Society. P.C.H. is grateful to BP Research for the award of CASE studentship. References 111V.T. Agekyan, Phys. Status Solidi (A) 43 (1977) 11.
(d) !
3 hour
0
2
10 4 6 I3 Energy relative to Ef / eV
12
14
Fig. 5. Inverse phot~mi~ion spectra of SnO, as a function of exposure to 500 eV argon ion beam at 5 ~Afcm’ (a) unexposed, (b) 1 h exposure, (cl 2 h exposure, and (d) 3 h exposure. In (d), the original spectrum (a) is shown with dots, emphasising the creation of new empty states about 6-8 eV above the Fermi energy as a result of ion bombardment,
to the antibonding 5s5p hybrids anticipated in fig. lb and are the out of phase counterparts of the in phase 5sSp states seen in photoemission studies [9,13] of ion bombarded SnO, just above the valence band edge. In summary IPES confirms the conclusion from earlier photoemission work that ion beam induced oxygen deficiency and Sb doping exert very different influences on the electronic structure of tin(W) oxide.
[2] D. Frohlich, R. Klenklies and R. Helbig, Phys. Rev, Lett. 41(19781 1750. [3] M. Nagasawa and S. Shionoya, J. Phys. Sot. Jpn 30 (1971) 158. [4] Z.M. Jarzebski and J.P. Marton, J. Electrochem. Sot. 123 (1976) pp. 199C, 299C. 151J. Robertson, J. Phys. C 12 (1979) 4767. [6] J.L. Jacquemin and G. Bordure, J. Phys. Chem. Solids 36 (1975) 1081. [7] D.F. Cox, S. Semancik and D.P. Szuromi, J. Vat. Sci. Technol. A 4 (1986) 627. [8] D.F. Cox, T.B. Fryberger and S. Semancik, Phys. Rev. B 38 (1988) 2072. [9] J.M. Themlin, R. Sporken, J. Darville, R. Caudano, J.M. Gilles and R.L. Johnson, Phys. Rev. B 42 (1990) 11914. [lo] P.A. Cox, R.G. Egdell, A.F. Orchard, C. Harding, W.R. Patterson and P.J. Tavener, Solid State Commun. 44 (1982) 837. [ll] P.A. Cox, R.G. Egdell, C. Harding, W.R. Patterson and P. Tavener, Surf. Sci. 123 (1982) 179. 1121 R.G. Egdell, W.R. Ravel1 and P. Tavener, J. Solid State Chem. 51(1984) 345. 1131 R.G. Egdell, S. Eriksen and W.R. Flavell, Solid State Commun. 60 (1986) 835. [14] J. Terra and D. Guenzburger, Phys. Rev. B 44 (1991) 8584. Il.51 N.G. Stoffel and P.D. Johnson, Nucl. Instrum. Methods A 234 (1985) 230. [16] P.L. Wincott, K.L. H&ansson and L.I. Johansson, Surf. sci. 211/212 (19891404. [17] A. Kovacs, P.O. Nilsson and J. Kanski, Phys. Ser. 25 (1982) 791. [18] T.S. Bush, C.R.A. Catlow, A.V. Chadwick, M. Cole, R.M. Geatches, G.N. Greaves and SM. Tomlinson, J. Mater. Chem. 2 (1992) 309. 1191 R.G. Egdell, S. Eriksen and W.R. Flavell, Surf. Sci. 192 (1987) 265. [20] J.J. Yeh and I. Lindau, At. Data Nucl. Data Tables 32 (1985) 1. [21] N.V. Smith and D.P. Woodruff, Prog. Surf. Sci. 21 (1986) 295.
....“”.‘::Y.?;;;,,: “:“... ..i ..A.. ” ‘* “%..:‘....‘ .......c .. . . . . .. .. . . . . .. . .. ,.,:.:.:: :,.:y .:,. $&~~:~~:~..~ _ ” ‘. .. ....:.y .:..: .....,.,:.:. pv
surface science :.:b. . .,,,) ..::::::>f:# ..c.:.::::::::g~~~,~~:::. ‘I‘:’‘9’.7: ....A... .,.,,,,,,,,j,,n, /\,
Surface Science 280 (1993) 393-397 North-Holland
““..+“G’. “.‘....A%‘... .....“.~.... ,,,,,,:.,,,,,,,. Q.‘..-.w. .,...A. .,.,.... ,,....,j:.,, ,,,.,./.~:,~’ ::.~:,‘.~.:‘.‘.‘..~..i.~ ....\...A ‘.“...‘~:::,~~:~,:~:~::::~~::~,~.~.~.::~ ,:,:.:.:,,,,,. ,_
_
The influence of oxygen deficiency and Sb doping on inverse photoemission spectra of SnO, P.C. Hollamby, P.S. Aldridge, G. Moretti ‘, R.G. Egdell Inorganic Chemistry Laboratory, South Parks Road, Oxford OX2 3QR, UK
and W.R. Flaveil Department of Chemistry, VMIST, P.O. Box 88, Manchester M60 lQD, UK
Received 14 July 1992; accepted for publication 22 September 1992
The changes in the empty electronic states in SnO, produced by ion-beam induced oxygen deficiency and by Sb doping have been studied by inverse photoemission spectroscopy. Inverse photoemission in SnO, itself is dominated by peaks 4 and 12 eV above the Fermi level, the former associated with empty states of dominant Sn 5p atomic character. Sb doping populates states in the Sn 5s conduction band, shifting the empty state structure closer to the Fermi energy. By contrast oxygen deficiency introduces new states above the main Sn5p peak. These are tentatively described as 5s-5p hybrids pushed up in energy from the Sp band by mixing between atomic orbitals of different parity in the non-centrosymmetric cation environment of oxygen deficient SnO,.
1. Introduction
Stoichiometric tin(W) oxide is a semiconductor with a direct forbidden bandgap of 3.6 eV adopting the tetragonal rutile structure [l-4]. Due to the essentially centrosymmetric environment of the tin cations within the rutile structure, there is little mixing between Sn5s and 5p states at the zone centre, and the lowest energy states are therefore of almost pure SnSs atomic character: the maximum in the Sn 5p partial density of states is at much higher energy [5,61. Electrons may be introduced into the conduction band by substitutional replacement of Sn with Sb or of 0 with F, the metal-to-non-metal transition occurring at carrier concentrations around 1.8 x 10”/cm3 [41. ’ On leave from: Centro di Studio de1 CNR “SASCO”, Dipartimento di Chimica, Universita “La Sapienza, Piazzale Aldo Moro 5, 00185 Roma, Italy. 0039-6028/93/$06.00
There have been several previous studies of polycrystalline and single crystal tin(W) oxide by photoemission techniques [7-g], including our own direct observations of the conduction electrons in Sb-doped material [lo-121. Photoemission shows that oxygen deficiency in SnO, produced either by removal of top-atomic-layer bridging oxygen atoms at SnO, (110) surfaces by thermal annealing in UHV [7-91 or by preferential sputtering of oxygen at single-crystal or polycrystalline surfaces under argon ion bombardment [133 does not introduce electrons into the SnSs conduction band. Instead, new occupied states are observed just above the 02p valence band edge, some 3 eV below the bottom of the conduction band. The stabilisation of these states has been discussed previously in terms of crystal field mixing between SnSs and 5p levels at noncentrosymmetric sites at the surface or adjacent to an 0 vacancy [8,13], giving a local density of
0 1993 - Elsevier Science Publishers B.V. All rights reserved
states similar to that of SnO where there is pronounced mixing of this sort [14]. Fig. 1 shows a schematic view of the pattern of ionic energy levels expected at centrosymmetric and noncentros~metric sites. Recent resonant photoemission studies have confirmed the cationic nature of the bandgap states [9], but the idea of 5s-5p mixing has been called into question. In the present communication we extend previous work on filled electronic states in SnO, to the study of empty states, using the technique of inverse photoemission spectroscopy. Comparison between the effects on the spectra of Sb doping and ion beam induced oxygen deficiency confirms the essential features of the levels illustrated in fig. 1.
2. Experimental Experiments were performed in a diffusion and titanium sublimation pumped UHV chamber (base pressure 1.5 x 10ei" mbar) equipped with a 50 mm-mean-radius spherical-sector electron energy analyser and a 5 keV electron gun for measurement of N(E) Auger spectra and energy loss spectra. Electrons for the IPES experiments were produced by a low energy electron gun with an
(a)
W
Fig. 1. Schematic ionic energy level diagram for Sn cations (a) in a centros~m~tric coordination environment and (b) in a non-centrosymmetric environment. Note strong mixing between 5s and 5p levels in the latter.
indirectly heated BaO/W dispenser cathode whose design was similar to that of Stoffel and Johnson [15]. With the aid of the electron energy analyser, it was established that the energy hal~idth in the emission from the electron gun under typical operating conditions was of the order 0.25-0.30 eV. Photons were detected with a bandpass detector comprising a 2 mm thick CaF, window 25 mm in diameter and a grounded Ta truncated conical photoemitter foil [Xl. Electrons emitted from the foil were counted with a Gahleo CEM4503 channeltron operating under 3 kV bias. The bandpass of this design of detector is centred around hv = 9.8 eV, with a theoretical energy halfwidth in the bandpass of 0.8 eV [17]. Electrons were incident normally on the surfaces of the polyc~stalIine samples studied in the present work and the photon detector was mounted at 4.5” to the axis of the electron gun. Channeltron count rates were of the order 500 s-l for incident gun currents of around 1 PA. The position of the Fermi energy was established from measurements on a polyc~stalline tantalum foil in electrical contact with the sample pellets. The full width at half maximum height of the differentiated Fermi edge signal was 1.2 eV. Comparison between IPE spectra of the metallic conductor Sn,,,Sb,,O, and semiconducting SnO, indicated that the conductivity of the latter was high enough to prevent problems in the calibration associated with sample charging. Polycrystalline SnO, (Johnson Matthey Specpure Grade) was pressed at 10 tonnes/cm2 into a 13 mm disc and sintered for several days at 1000°C to give a mechanically robust ceramic pellet. Similar ceramic samples of the doped material Sn 0,97Sb0.0302were prepared by coprecipitation of Sn and Sb from a solution with aqueous NH,, as described in detail elsewhere [lO,lS]. The undoped tin oxide samples were cleaned by cycles of bombardment with 500 eV argon ions and annealing in UHV or lop5 mbar oxygen at temperatures up to 500°C until Auger spectra were free of signals due to carbon contamination and the ratio between 0 KLL and Sn M,N,,N,, peak heights assumed a constant value of 0.84. The doped sample was cleaned by annealing alone. A very weak CKLL Auger feature re-
P.C. Hollamby et al. / The injluence of 0 and Sb on IPE spectra of SnO,
395
monitored by the progressive decrease O/Sri Auger ratio, as shown in fig. 3.
in the
3. Results and discussion The inverse photoemission spectrum of SnO, shown in fig. 4 contains two broad peaks on a rising background respectively 4 and 12 eV above the Fermi energy. The intensity of the structure immediately above the Fermi energy is very low, appearing as a broad and ill-defined shoulder to the low energy side of the 4 eV peak. The low energy features of the IPES conform tolerably well to the band structure for SnO, calculated by
Sn
3
250
300
350
400
450
01
500
Kinetic Energy / eV
Fig. 2. N(E) Auger spectra of (a) cleaned polycrystalline SnO, and (b) cleaned polycrystalline Sn,,Sb,,,O,.
6-4 annealed
mained after this treatment, but the peak height was less than 5% of that of the oxygen peak and the contamination did not influence the IPES. As found in previous XPS experiments [11,12], there is marked Sb segregation at annealed Sb-doped SnO, surfaces and the Sb/SnM,N,N, Auger area ratio was around 0.27, very much greater than the value of about 0.03 expected from the bulk doping level (fig. 2). Note however that the previous work has shown that the segregated Sb ions do not act as donor centres so that the carrier concentration close to the surface is essentially the same as in the bulk. This was confirmed in the present work by observation of a plasmon loss peak at 0.55 eV in energy loss spectra excited with a 25 eV electron beam from the bulk doping level [lO,ll]. Oxygen deficient SnO, surfaces were prepared by 500 eV ion bombardment [191 of the cleaned SnO, surface at a current of order 5 PA. The decrease in surface oxygen content with time was
tc)
I
2 hour
3jo
3eo 4jo Kinetic Energy / eV
4ho
5 i0
Fig. 3. Influence of progressively increasing exposure to 500 eV ion beam at 5 ~A/cm’ on N(E) Auger spectra of SnO,. (a) unexposed, (b) 1 h exposure, (c) 2 h exposure, and (d) 3 h exposure.
P.C. Hol~mby
-I--
.-
r
hv= 9.8eV
IPES
J
et al. / The influence of 0 and Sb on IFE spectra of SnO,
Sn02
--
(a) :
(b) i 0
7
2
4
6
6
IO
12
14
16
Energy relative to 4 / eV
Fig. 4. Inverse photoemission spectra of (a) clean polycrystalline SnOz and (b) cleaned polycrystalline Sn,,Sb0,,302. In (b) the dots show the spectrum of undoped SnO,, highlighting the small shift to lower energy in empty state structure upon Sb doping.
Robertson [S] which shows a very small density of states at the bottom of the conduction band associated with the strongly dispersing Sn5s states and a much higher density of states between 4.5 and 8 eV above the Fermi energy associated with Sn5p states. There is however quantitative disagreement in that the experimental Sn5p peak appears at lower energy than in the band structure calculation. Note also that matrix element effects will exert a very strong influence on the spectra, the one electron cross section relevant to photoemission being calculated [20] as 1.45 X 10e4 Mb (Sn5.s) and 1.3 Mb (Sn5p> at hv = 16.7 eV for atomic Sn (atomic SnSs states are not ionised at the lower energy of 10.2 eV, which is closer to the energy of the IPES detector). The same matrix elements govern the radiative decay in IPES, although of course the cross sections are
scaled down by a factor of order l/a2, where LYis the fine structure constant [21]. Band structure calculations do not extend to the energy of the 12 eV peak. In our experience so far with IPES of metal oxides, structure at this high energy is usuahy broad and ill defined and depends very strongly on the details of surface preparation. It is therefore difficult to offer an assignment in terms of empty densities of states. An alternative interpretation can be given in terms of sequential electron scattering followed by radiative decay. The probability of electron scattering via interband excitation is very high at oxide surfaces [19] and will contribute to a high kinetic energy background in IPE spectra, possibly with a discrete structure. Sb doping is known to populate states in the Sn5s conduction band [lo-121. From photoemission experiments on ceramic Sn,,g,Sb,,,,O, samples similar to that studied here, the Fermi level is estimated to be 0.42 eV above the bottom of the conduction band [ lo,1 11.In a rigid band model this should lead to a downward shift in the energy of the empty states. Comparison between spectra (fig. 4) for undoped SnO, and Sn,,,Sb,,,,O, does indeed reveal a downward shift in spectral weight, although due to the relatively poor experimental resofution and broad spectral features it is not possible to make quantitative comparison with the photoemission spectra. We consider next the effects of oxygen deficiency on the IPES. Prolonged ion bombardment reduces the O/Sri Auger intensity ratio from the value of 0.84 characteristic of stoichiometric, annealed surfaces to the lower value of 0.66. We thus estimate that the effective surface composition is SnO,,s, and that roughly half of the tin ions sampled by Auger spectroscopy find themself in an SnO-like environment. The electron pathlength is likely to be longer at the very low electron energies used in the IPES experiment. The changes in IPES are therefore rather more muted than in the Auger spectra. Nonetheless, comparison between spectra before and after ion bombardment (fig. 5) shows the emergence of new structures to the high energy side of the 4 eV peak, filling in the vaIley between this peak and the 12 eV peak. These new states are attributed
F.C. Halls
IPES
et al. / The influence of 0 and Sb on IPE spectra of SnO,
397
Acknowledgements
hv= 9.0eV
The equipment used was funded by SERC and the Royal Society. P.C.H. is grateful to BP Research for the award of CASE studentship. References 111V.T. Agekyan, Phys. Status Solidi (A) 43 (1977) 11.
(d) !
3 hour
0
2
10 4 6 I3 Energy relative to Ef / eV
12
14
Fig. 5. Inverse phot~mi~ion spectra of SnO, as a function of exposure to 500 eV argon ion beam at 5 ~Afcm’ (a) unexposed, (b) 1 h exposure, (cl 2 h exposure, and (d) 3 h exposure. In (d), the original spectrum (a) is shown with dots, emphasising the creation of new empty states about 6-8 eV above the Fermi energy as a result of ion bombardment,
to the antibonding 5s5p hybrids anticipated in fig. lb and are the out of phase counterparts of the in phase 5sSp states seen in photoemission studies [9,13] of ion bombarded SnO, just above the valence band edge. In summary IPES confirms the conclusion from earlier photoemission work that ion beam induced oxygen deficiency and Sb doping exert very different influences on the electronic structure of tin(W) oxide.
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