- Email: [email protected]
Angle-resolved inverse photoemission from W(001)
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Surface Science 303 (1994) 341-346
ELSEVIER
Angie-resolved
inverse ~hoto~mission from W( 001) A. Lamouri, I.L. Krainsky *
NASA LeRC, 21000 Brookpark Road, Cleveland, OH 44135, USA
(Received 24 August 1993;accepted for publication 11 October 1993)
Abstract Angle-resolved inverse photoelectron spectroscopy (IPS) in the isochromat mode was used --- to map the unoccupied energy levels of the W(O01) surface. Data were collected at room temperature along the IZM symmetry line of the surface Briliouin zone (SBZ) of the 1 X 1 structure. States observed near 0.5, 3.0 and 4.5 eV above the Fermi level (E,) showed strong sensitivity to surface contamination and were therefore assigned to surface states and/or resonances. These states showed little or no dispersion as expected for states with significant d character. The dispersion of the state near 0.5 eV above E, is in good agreement with recent band structure calculations. The implications of our results on the driving force leading to the reconstruction of the W(OO1) surface are discussed.
1. Introduction The W(OO1) surface has been the subject of many experimental and theoretical studies in the last few decades. The choice of the W(OOl) surface was motivated by many factors among which is the existing controversy concerning the temperature- and hydrogen-induced phase transitions. At high temperatures, a clean W(OO1) surface exhibits a p(1 X 1) LEED pattern as one would expect from a simple te~ination of the bulk structure. When cooled below room temperature, this surface undergoes a reversible temperatureinduced phase transition characterized by a c(2 x 2) LEED pattern [1,2]. A model which well describes this structure was proposed by Debe and King 131and consists of lateral displacements of the tungsten top layer atoms in the (110)
* Corresponding author. Fax: + 1 216 433 8705. ~39-6028/94/$07.~
0 1994
SSDI 0039-6028(93)E0758-M
direction forming zigzag chains. A similar hydrogen-induced reconstruction is also observed on the W(OO1) surface with atoms displaced in the (100) direction [4,5f. ~though the Debe-King model is now generaIly accepted to describe the low-temperature phase of the W(OO1) surface, the nature of the high-temperature phase and the nature of the driving force leading to the reconstruction still remain controversial. Early studies appeared to indicate that the room-temperature phase of the W(OO1)surface was unreconstructed [l]. The driving force for the reconstruction was attributed to a charge density wave (CDW) [6,7] in which nesting of the surface electronic bands near the Fermi levet could drive the reconstruction according to a Peierls type mechanism. Subsequently, a local bonding mechanism based on short-range JahnTeller-like forces between the surface atoms was proposed [8]. This model was based on photoemission studies which showed no significant
Else&r Science B.V. All rights reserved
342
A. Lamouri, I.L. Krainsky/Surface Science 303 (1994) 341-346
nesting of the surface states at the Fermi level [9]. The results of a high energy ion-scattering experiment performed in 1989 by Stensgaard et aI. [lo] confirmed this hypothesis by showing that the surface atomsowere laterally displaced by approximately 0.14 A at all temperatures. In addition, X-ray diffraction studies [ll] showed that the local structure of the W(OO1) surface was unaltered through the transition and led to the conclusion that the high temperature phase must be disordered, thus favoring local bonding over the CDW mechanism for the reconstruction. Recently, the CDW mechanism has received new support. Collins et al. [123 found a surface state ctose to the Fermi level near k,, = +FM and attributed its emergence in the IPS spectra to a possible Fermi level crossing. Using angle-resolved photoelectron spectroscopy (ARPES), Smith et al. [13] observed a surface state close to the Fermi level over a large portion of the SBZ, but were unable to determine whether or not this state crossed the Fermi level. They concluded that such a state can participate in a CDW-like mechanism and that a combination of both models is probably necessary to expIain the reconstruction. More recent band structure calcuiations by Yu et al. (141 appear, however, to be in favor of the local bonding being the dominant mechanism in the reconstruction. Based on their calculation of the energy difference between the reiaxed ideal surface and the reconstructed surface, Yu et al. concluded that it is unlikely that the high temperature phase is the unreconstructed W(OO1) surface. Their band structure for the equilibrium surface showed no well-defined surface states or resonances crossing the Fermi level and appears to be in favor of a model for reconstruction originating from the splitting of the surface bands into occupied and unoccupied subbands. The results of an angle-resolved inverse photoemission study of the W(OO1) surface at room temperature are reported here. Our measurements exhibit an almost flat unoccupied surface band in the vicinity -- of the Fermi level over a long segment of the TM line and appear to be consistent with the local bonding being the dominant mechanism in the reconstruction. However, our
results do not necessarily contradict the CDW mechanism and it is possibIe that a combination of both models is required to account for the reconstruction of the W~OOl)surface as suggested by Smith et al. [13]
2. Experimental The experimental work was carried out in an all stainless steel ultra high vacuum chamber which operated in the low lo-” Torr range. The vacuum system was equipped with a double pass cylindrical mirror analyzer (CMA) for performing Auger electron spectroscopy @ES), and a residual gas analyzer to monitor ambient gases. Sample impurities and surface orientation were monitored by AES and LEED. A detailed description of the vacuum system can be found elsewhere [15,16] and only its most important features wiI1 be described here. IPS measurements were performed using a spectrometer which consisted of a photon detector designed after that of Babbe et al. [171 and a low energy electron gun [IS] built for this experiment. The electron gun used a BaO cathode and was mounted at 40” relative to the photon detector. The angular divergence of the electron beam was estimated at better than 3” giving a, momentum resolution of approximately 0.1 A-‘. The measurements were performed in the isochromat mode by scanning the electron energy and collecting photons at a fixed energy of 9.8 eV. Count rates of several hundred counts per second were achieved with an electron beam current of 5 PA. The overall energy resolution of the spectrometer was approximately 0.6 eV. The sample was cleaned by repeated heating at 1700 K in an oxygen atmosphere of 6.0 x lo-* Torr followed by occasional flashing at 2500 K until cleanliness was confirmed by AES, LEED, and IPS. The target was also flashed before a new scan was started. The variation of the surface work function was monitored continuously using the retarding field method as described in Ref. EN. At our working base pressure, l-l.5 h were necessary for the surface to adsorb enough hydro-
A. Lamouri, I.L. Kraimky /Surface
gen to cause re~nstruction. This was indicated by the formation of a c(2 X 2) LEED structure and the appearance of a discontinuity (knee) in the derivative of the surface work function versus hydrogen coverage as described by Barker and Estrup [19]. Therefore, data collection was limited to the first forty minutes after flashing the sample which was a compromise between reasonable scan times and signal to noise ratios. All the spectra were collected at or above room temperature.
343
Science 303 (I 9941341-346
B 54.0°
P P ir-I
49.5*
,’
,’
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3. Results and discussion
31.s”
Fig. 1 shows IPS spectra collected at different angles of electron incidence 0 from the W(OO1) surface. Data for 0.5 L of oxygen adsorbed on W(OO1) are also included in this figure and represented by dashed lines. The spectra were collected in_--the (110) mirror plane for states with k,, in the I?T;M symmetry direction of the SBZ of the ideal surface. Their intensities were normalized to the primary current. We used the sensitivity of the observed peaks to hydrogen and oxygen adsorption as a criterion to distinguish between surface and bulk states. The adsorption of both species had similar effects and only spectra for oxygen adsorption are presented here. Details concerning both oxygen and hydrogen adsorption will be the subject of future publications. The Fermi edge was determined from the onset of photon emission. At normal incidence, a broad peak was observed around 3 eV above E,. Near normal incidence, this peak showed sensitivity to oxygen and hydrogen adsorption indicative of a strong surface component. Away from normal incidence, this peak changed character and was nearly insensitive to surface contamination as indicated by the solid and dashed lines of the scan corresponding to 0 = 9”. We therefore attributed the off-normal portion of this peak to transitions into bulk states. This peak showed very little dispersion as the angle of electron incidence was increased and disappeared when 8 reached 23”. A similar peak was observed by Collins et al. [123 and Drube et al. 1201. Near normal incidence, they attributed
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ENERGY (eV)
Fig. 1. Angle-resolved inverse photoemission spectra from W(OO1) at different angles of electron incidence B in the r%a direction of the surface Brillouin zone of the 1 x 1 structure. Solid and dashed lines represent clean W(OO1)and W@Ol) covered with 0.5 L of oxygen respectively.
this peak to transitions into the plateau region of the W(OO1) A, bulk band which extends from I,, to H,, 1211.Off normal, this peak was assigned by Collins et al. to transitions into the relatively flat bulk bands located near 3 eV above E, in the NP and PH directions of the bulk Brillouin zone [21]. The --- continuation of this state was observed in the TAX direction [15] (The actual direction --- in which the data of 1151were taken was the TAX symmetry line surface and not --- of the unreconstructed the I’XM direction as was reported). For 0 greater than 35”, a second non-dispersing peak which was also insensitive to surface contamination was observed in our spectra at 3 eV above E,. Because the bands originating from these peaks were flat in the vicinity of the T and a points,
344
A. Lamouri, I.L. Krair&y/Surface
these states contribute significantly to the density of states at 3 eV above E,. These results are in good agreement with the calculated density of states from the unreconst~cted surface performed by Mattheiss and Hamann f22] which showed a prominent peak near 3 eV above E, originating from the inner layers of the film. A similar peak was observed in the calculated densities of states performed by Fu et al. [23] for both the W(OOl) unreconstructed and reconstructed surfaces. As the angle of electron incidence was varied --to map states with different k,, along the TCM direction, several new peaks appeared. Near 8 = 15”, new peaks appeared at 0.5 and 4.4 eV above E,, which is 0.23 eV below the vacuum level (E,). The peak located immediately above E, showed a slight upward dispersion and disappeared when 8 reached 48”. The peak near E,, on the other hand, dispersed towards the vacuum level and appeared as a step at the vacuum level for 8 between 31 and 36“. For angles larger than 36”, this peak dispersed back towards the Fermi level and disappeared at 4.4 eV above E,. These two peaks were assigned to surface states and/or resonance since they were strongly affected by oxygen adsorption as indicated by the dashed lines of Fig. 1. It is important to note that the states observed here show little or no dispersion which is consistent with these states having a significant d character. The remainder of the paper is dedicated to the dispersion of the surface state located near the Fermi level since this is the state which is likely to play a role in the reconstruction. The experimental two-dimensional band structure of the W(OO1) --surface above the Fermi level in both the TAX --and the l?_CMdirections of the ~(1 X 1) structure is presented in detail elsewhere [241. The dispersion of the surface-sensitive feature observed near the Fermi level in the spectra of Fig. 1 is represented by open circles in Fig. 2. The even and odd bandgaps, reproduced from [251, are aiso included in Fig. 2 a0nd represented by dashed lines. Near k,, = 0.6 A-‘, this state is a true surface state, since it is located in an absolute bandgap. A similar band was recently observed above the Fermi level by Collins et al. [12]
Science 303 (1994) 341-346
k,,@-‘1 Fig. 2. Dispersion of---the surface state of W(OOl) near the Fermi level in the TPM direction of the unreconstructed surface. The IPS (this work) and ARPES [13] results are represented by open circles and crosses respectively. The results of recent band structure calculations (solid lines) [14] and the projected bulk even and odd bandgaps (dashed lines) i2.5) are also included.
Its emergence above the Fermi level near k,, = $?M led to the suggestion that this band was partially occupied. Their data for the dispersion of this band agree very poorly with ours. Using the technique of IPS in the fluorescent mode, Drube et al. [20] observed a non-dispersing state near 1.4 eV above E, at k ,, = O.SFiiiT and tentatively associated it with -- their calculated x2 band. Our state at k,, = 0.8 TM occurs at a much lower energy. Another surface state was also observed by Drube et al. near the ?; point at 0.3 eV above E,. They assigned it to the surface state calculated and observed at 0.3 eV below E,, arguing that this state extends above the Fermi levei due to broadening by mixing with bulk states. Our measurements did not show such a state and the reasons for the discrepancies between our results and those of Drube et al. are not understood at present. Possible explanation may include the
A. Lamouri, IL. fiainsky
/Surface
relatively poor resolution of 0.6 eV as discussed by Collins et al. [12]. Early theories of the reconstruction of the W(OO1) surface were based on the CDW model. The central point of this model is the presence of surface states crossing the Fermi level near the midpoint of the i?M line of the 1 X 1 structure. [26,27] Recently, a local bonding model based on the instability of the high temperature phase to the Debe-King reconstruction was proposed. In this model, the bands of the ideal surface split by the reconst~ction into bonding (occupied) and antibonding ~un~cupied) states in the direction of the reconstruction. [14,28] Our experimental measurements indicate that such a splitting is present even at room temperature and are in good agreement with the recent band structure calculations by Yu et al. 1141 based on the local bonding mechanism. Two bands, a %r and a &, are predicted by these calculations at 0.4 and 0.7 eV above E, respectively. The upper band evolved from the splitting of the 2, surface resonance of the ideal surface into bonding and antibonding surface bands. These calculations are reproduced in Fig. 2 and represented by solid lines. Furthermore, the band derived from the peak at 0.5 eV above E, in our spectra is well defined and flat over a long segment of the i?a line. Such a flat band produces a large peak in the density of states. This is in good agreement with the results of Fu et al. [23] where a sharp peak was observed near 0.5 eV above E, in the calculated density of states for the reconstructed surface. The symmet~ of the state in question is important for comparison with the theoretical results. Unfortunately, our experimental arrangement does not allow us to determine the symmetry of this state. In addition, this state falls between theoretically predicted even and odd states (solid lines in Fig. 2). However, there is evidence that this band comprises a large even component since it existed entirely in an odd bandgap and exhibited maximum intensity near the absolute bandgap. Furthermore, away from this bandgap, its intensity decreased presumably due to hybridization with even states since coupling to odd bulk states is symmetry forbidden.
Science 303 (‘19941341-346
345
Finally, we note that it is not likely that the surface state observed by Smith et al. (131 below the Fermi level is observed here above the Fermi level due to broadening caused by coupling with bulk states [20]. First, coupling to bulk states is forbidden in an absolute bandgap where our surface state is partially located; and second, if we compare the intensities of the signals between k ,, = 0.4 and k ,, = 0.7 A- ’ in direct and inverse photoemission assuming similar background signal strengths, we can conclude that the major part of this state is observed in IPS. In summa~, we have measured the unoccupied energy levels of W(OO1) at room temperature. Our results are in reasonably good agreement with previous IPS measurements 1121 and with the theoretically predicted dispersions of the surface states near the Fermi level [14]. The states observed here show very little or no dispersion which is consistent with these states having significant d character. We observed a surface-related band near 0.5 eV above the Fermi level over a large segment of the T&8 line. The contribution of this band to the density of states is in favor of the local bonding model. Although our results indicate that the local bonding is likely the dominant mechanism in the reconstruction as suggested by Yu et al. [14], the CDW model can not be totally ruled out since a Fermi level crossing of the band observed near --the Fermi level is possible along the I%M line. It is clear from our experimental measurements that if this crossing occurs at all, it takes place near k,, = 0.5 A-‘. This is consistent with the recent ARPES measurements by Smith et al. [13] where an occupied band was observed near the Fermi level over a large portion of the SBZ. This band, reproduced by crosses in Fig. 2, showedono dispersion between k,, = 0.31 and k,, = 0.64 A--r and it was not apparent whether it crossed or remained below the Fermi level as indicated by the large error bars (not reproduced here). Furthermore, Holmes and Gustafsson [29] observed an occupied band of even symometry approaching the Fermi level near k,, = 0.6 A- *. Our band appears as the continuation of this band which is further evidence that our band comprises a large even component.
346
A. Lamowi, I. L. Krainsky /Surface
Overall, our results indicate that a combination of both models is probably necessary for a correct description of the W(OO1) surface reconstruction as suggested by Smith et al. [133. 4. Acknowledgements This work was supported by NASA LeRC and the National Research Council. The authors wish to thank Dr. A. Petukhov for helpful discussions and comments.
5. References 111T.E. Felter, R.A. Barker and P.J. Es&up, Phys. Rev. Lett. 38 (1977) 1138. [21M.K. Debe and D.A. King, J. Phys. C: Solid State Phys. IO (1977) L303. [31 M.K. Debe and D.A. King, Phys. Rev. Lett. 39 (1977) 708. [41 D.A. King and G. Thomas, Surf. Sci. 92 (1980) ‘201. 151 R.A. Barker and P.J. Estrup, J. Chem. Phys. 74 (1981) 1442. [hl E. Tosatti, Solid State Commun. 2.5 (1978) h37. 171 H. Krakauer, M. Posternak and A.J. Freeman, Phys. Rev. Lett. 43 (1979) 1885. 181 D. Singh, Su-Huai Wei and H. Krakauer, Phys. Rev. Lett. 57 (1986) 3292; D. Singh and H. Krakauer, Phys. Rev. I3 37 (19881 3999. 191 J.C. Campuzano, J.E. Inglesfield, D.A. King and C. Somerton, J. Phys. C: Solid State Phys. 14 (1981) 3099. 1101 1. Stensgaard, K.G. Purcell and D.A. King, Phys. Rev. B 39 ( 1989) 897.
Science 303 (I 994) 341-346
fill I.K. Robinson, A.A. MacDowelI, MS. Ahman, P.J, Estrup, K. Evans-Lutterodt, J.D. Brock and R.J. Birgeneau, Phys. Rev. Lett. 62 (1989) 1294. 1121 I.R. Collins, A.D. Laine and P.T. Andrews, J. Phys. Condensed Matter 4 (1992) 2891. (131 K.E. Smith, G.S. Elliott and S.D. Kevan, Phys. Rev. B 42 (1990) 5385. 1141 R. Yu, H. Krakauer and D. Singh, Phys. Rev. B 45 (1992) 8671. 1151 I.L. Krainsky, J. Vat. Sci. Technol. A 5 (1987) 735. 1161A. Lamouri and IL. Krainsky, Surf. Sci. 278 (19921 286. 1171N. Babbe, W. Drube, I. Schafer and M. Skibowski, J. Phys. E 18 0985) t.58. 1181 I.L. Krainsky, Rev. Sci. Instrum. 62 (1991) 1746. 1191 R.A. Barker and P.J. Estrup, Phys. Rev. Lett. 41 (1978) 1307. r201 W. Drube, D. Straub, F.J. Himpsel, P. Soukiassian, CL. Fu and A.J. Freeman, Phys. Rev. B 34 (1986) 8989. 1211 N.E. Christensen and 8. Feuerbacher. Phys. Rev. B 10, 2349, (1974). 1221L.F. Mattheiss and D.R. Hamann, Phys. Rev. B 29 (1984) 5372. 1231 C.L. Fu, A.J. Freeman, E. Wimmer and M. Weinert, Phys. Rev. Lett. 54 (1985) 2261. (241 A. Lamouri and I.L. Krainsky, unpublished. 1251W.R. Grise, D.G. Dempsey, L. Kleinman and K. Mednick, Phys. Rev. B 20 (1979) 3045. 061 M. Posternak, H. Krakauer, A.J. Freeman and D.D. Koelling, Phys. Rev. R 21 (1980) 5601. 1271S. Ohnishi, A.J. Freeman and E. Wimmer, Phys. Rev. B 29 (1984) 5267. 1281 D.W. Bullett and PC. Stephenson, Solid State Commun. 45 (1983) 47; P.C. Stephenson and D.W. Bullett, Surf. Sci. 139 (1984) 1. f291MI. Holmes and T. Gustafsson, Phys. Rev. Lett. 47 (1981) 443.
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Surface Science 303 (1994) 341-346
ELSEVIER
Angie-resolved
inverse ~hoto~mission from W( 001) A. Lamouri, I.L. Krainsky *
NASA LeRC, 21000 Brookpark Road, Cleveland, OH 44135, USA
(Received 24 August 1993;accepted for publication 11 October 1993)
Abstract Angle-resolved inverse photoelectron spectroscopy (IPS) in the isochromat mode was used --- to map the unoccupied energy levels of the W(O01) surface. Data were collected at room temperature along the IZM symmetry line of the surface Briliouin zone (SBZ) of the 1 X 1 structure. States observed near 0.5, 3.0 and 4.5 eV above the Fermi level (E,) showed strong sensitivity to surface contamination and were therefore assigned to surface states and/or resonances. These states showed little or no dispersion as expected for states with significant d character. The dispersion of the state near 0.5 eV above E, is in good agreement with recent band structure calculations. The implications of our results on the driving force leading to the reconstruction of the W(OO1) surface are discussed.
1. Introduction The W(OO1) surface has been the subject of many experimental and theoretical studies in the last few decades. The choice of the W(OOl) surface was motivated by many factors among which is the existing controversy concerning the temperature- and hydrogen-induced phase transitions. At high temperatures, a clean W(OO1) surface exhibits a p(1 X 1) LEED pattern as one would expect from a simple te~ination of the bulk structure. When cooled below room temperature, this surface undergoes a reversible temperatureinduced phase transition characterized by a c(2 x 2) LEED pattern [1,2]. A model which well describes this structure was proposed by Debe and King 131and consists of lateral displacements of the tungsten top layer atoms in the (110)
* Corresponding author. Fax: + 1 216 433 8705. ~39-6028/94/$07.~
0 1994
SSDI 0039-6028(93)E0758-M
direction forming zigzag chains. A similar hydrogen-induced reconstruction is also observed on the W(OO1) surface with atoms displaced in the (100) direction [4,5f. ~though the Debe-King model is now generaIly accepted to describe the low-temperature phase of the W(OO1) surface, the nature of the high-temperature phase and the nature of the driving force leading to the reconstruction still remain controversial. Early studies appeared to indicate that the room-temperature phase of the W(OO1)surface was unreconstructed [l]. The driving force for the reconstruction was attributed to a charge density wave (CDW) [6,7] in which nesting of the surface electronic bands near the Fermi levet could drive the reconstruction according to a Peierls type mechanism. Subsequently, a local bonding mechanism based on short-range JahnTeller-like forces between the surface atoms was proposed [8]. This model was based on photoemission studies which showed no significant
Else&r Science B.V. All rights reserved
342
A. Lamouri, I.L. Krainsky/Surface Science 303 (1994) 341-346
nesting of the surface states at the Fermi level [9]. The results of a high energy ion-scattering experiment performed in 1989 by Stensgaard et aI. [lo] confirmed this hypothesis by showing that the surface atomsowere laterally displaced by approximately 0.14 A at all temperatures. In addition, X-ray diffraction studies [ll] showed that the local structure of the W(OO1) surface was unaltered through the transition and led to the conclusion that the high temperature phase must be disordered, thus favoring local bonding over the CDW mechanism for the reconstruction. Recently, the CDW mechanism has received new support. Collins et al. [123 found a surface state ctose to the Fermi level near k,, = +FM and attributed its emergence in the IPS spectra to a possible Fermi level crossing. Using angle-resolved photoelectron spectroscopy (ARPES), Smith et al. [13] observed a surface state close to the Fermi level over a large portion of the SBZ, but were unable to determine whether or not this state crossed the Fermi level. They concluded that such a state can participate in a CDW-like mechanism and that a combination of both models is probably necessary to expIain the reconstruction. More recent band structure calcuiations by Yu et al. (141 appear, however, to be in favor of the local bonding being the dominant mechanism in the reconstruction. Based on their calculation of the energy difference between the reiaxed ideal surface and the reconstructed surface, Yu et al. concluded that it is unlikely that the high temperature phase is the unreconstructed W(OO1) surface. Their band structure for the equilibrium surface showed no well-defined surface states or resonances crossing the Fermi level and appears to be in favor of a model for reconstruction originating from the splitting of the surface bands into occupied and unoccupied subbands. The results of an angle-resolved inverse photoemission study of the W(OO1) surface at room temperature are reported here. Our measurements exhibit an almost flat unoccupied surface band in the vicinity -- of the Fermi level over a long segment of the TM line and appear to be consistent with the local bonding being the dominant mechanism in the reconstruction. However, our
results do not necessarily contradict the CDW mechanism and it is possibIe that a combination of both models is required to account for the reconstruction of the W~OOl)surface as suggested by Smith et al. [13]
2. Experimental The experimental work was carried out in an all stainless steel ultra high vacuum chamber which operated in the low lo-” Torr range. The vacuum system was equipped with a double pass cylindrical mirror analyzer (CMA) for performing Auger electron spectroscopy @ES), and a residual gas analyzer to monitor ambient gases. Sample impurities and surface orientation were monitored by AES and LEED. A detailed description of the vacuum system can be found elsewhere [15,16] and only its most important features wiI1 be described here. IPS measurements were performed using a spectrometer which consisted of a photon detector designed after that of Babbe et al. [171 and a low energy electron gun [IS] built for this experiment. The electron gun used a BaO cathode and was mounted at 40” relative to the photon detector. The angular divergence of the electron beam was estimated at better than 3” giving a, momentum resolution of approximately 0.1 A-‘. The measurements were performed in the isochromat mode by scanning the electron energy and collecting photons at a fixed energy of 9.8 eV. Count rates of several hundred counts per second were achieved with an electron beam current of 5 PA. The overall energy resolution of the spectrometer was approximately 0.6 eV. The sample was cleaned by repeated heating at 1700 K in an oxygen atmosphere of 6.0 x lo-* Torr followed by occasional flashing at 2500 K until cleanliness was confirmed by AES, LEED, and IPS. The target was also flashed before a new scan was started. The variation of the surface work function was monitored continuously using the retarding field method as described in Ref. EN. At our working base pressure, l-l.5 h were necessary for the surface to adsorb enough hydro-
A. Lamouri, I.L. Kraimky /Surface
gen to cause re~nstruction. This was indicated by the formation of a c(2 X 2) LEED structure and the appearance of a discontinuity (knee) in the derivative of the surface work function versus hydrogen coverage as described by Barker and Estrup [19]. Therefore, data collection was limited to the first forty minutes after flashing the sample which was a compromise between reasonable scan times and signal to noise ratios. All the spectra were collected at or above room temperature.
343
Science 303 (I 9941341-346
B 54.0°
P P ir-I
49.5*
,’
,’
,<--.. ,_-P F .-,’
,’
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3. Results and discussion
31.s”
Fig. 1 shows IPS spectra collected at different angles of electron incidence 0 from the W(OO1) surface. Data for 0.5 L of oxygen adsorbed on W(OO1) are also included in this figure and represented by dashed lines. The spectra were collected in_--the (110) mirror plane for states with k,, in the I?T;M symmetry direction of the SBZ of the ideal surface. Their intensities were normalized to the primary current. We used the sensitivity of the observed peaks to hydrogen and oxygen adsorption as a criterion to distinguish between surface and bulk states. The adsorption of both species had similar effects and only spectra for oxygen adsorption are presented here. Details concerning both oxygen and hydrogen adsorption will be the subject of future publications. The Fermi edge was determined from the onset of photon emission. At normal incidence, a broad peak was observed around 3 eV above E,. Near normal incidence, this peak showed sensitivity to oxygen and hydrogen adsorption indicative of a strong surface component. Away from normal incidence, this peak changed character and was nearly insensitive to surface contamination as indicated by the solid and dashed lines of the scan corresponding to 0 = 9”. We therefore attributed the off-normal portion of this peak to transitions into bulk states. This peak showed very little dispersion as the angle of electron incidence was increased and disappeared when 8 reached 23”. A similar peak was observed by Collins et al. [123 and Drube et al. 1201. Near normal incidence, they attributed
27.0’ 22.5’
18.0’ 1 3s” 09.0’
.’
,’
i_/-“ P <-
,/-
.’
-\.___
.’
,’
,’
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*-
04.5’ 00.0’
I
-
W(OO1) -__ EM
,S(II(I,,LI,~L,I
i
0.0
1.5
3.0
4.5
6.0
ENERGY (eV)
Fig. 1. Angle-resolved inverse photoemission spectra from W(OO1) at different angles of electron incidence B in the r%a direction of the surface Brillouin zone of the 1 x 1 structure. Solid and dashed lines represent clean W(OO1)and W@Ol) covered with 0.5 L of oxygen respectively.
this peak to transitions into the plateau region of the W(OO1) A, bulk band which extends from I,, to H,, 1211.Off normal, this peak was assigned by Collins et al. to transitions into the relatively flat bulk bands located near 3 eV above E, in the NP and PH directions of the bulk Brillouin zone [21]. The --- continuation of this state was observed in the TAX direction [15] (The actual direction --- in which the data of 1151were taken was the TAX symmetry line surface and not --- of the unreconstructed the I’XM direction as was reported). For 0 greater than 35”, a second non-dispersing peak which was also insensitive to surface contamination was observed in our spectra at 3 eV above E,. Because the bands originating from these peaks were flat in the vicinity of the T and a points,
344
A. Lamouri, I.L. Krair&y/Surface
these states contribute significantly to the density of states at 3 eV above E,. These results are in good agreement with the calculated density of states from the unreconst~cted surface performed by Mattheiss and Hamann f22] which showed a prominent peak near 3 eV above E, originating from the inner layers of the film. A similar peak was observed in the calculated densities of states performed by Fu et al. [23] for both the W(OOl) unreconstructed and reconstructed surfaces. As the angle of electron incidence was varied --to map states with different k,, along the TCM direction, several new peaks appeared. Near 8 = 15”, new peaks appeared at 0.5 and 4.4 eV above E,, which is 0.23 eV below the vacuum level (E,). The peak located immediately above E, showed a slight upward dispersion and disappeared when 8 reached 48”. The peak near E,, on the other hand, dispersed towards the vacuum level and appeared as a step at the vacuum level for 8 between 31 and 36“. For angles larger than 36”, this peak dispersed back towards the Fermi level and disappeared at 4.4 eV above E,. These two peaks were assigned to surface states and/or resonance since they were strongly affected by oxygen adsorption as indicated by the dashed lines of Fig. 1. It is important to note that the states observed here show little or no dispersion which is consistent with these states having a significant d character. The remainder of the paper is dedicated to the dispersion of the surface state located near the Fermi level since this is the state which is likely to play a role in the reconstruction. The experimental two-dimensional band structure of the W(OO1) --surface above the Fermi level in both the TAX --and the l?_CMdirections of the ~(1 X 1) structure is presented in detail elsewhere [241. The dispersion of the surface-sensitive feature observed near the Fermi level in the spectra of Fig. 1 is represented by open circles in Fig. 2. The even and odd bandgaps, reproduced from [251, are aiso included in Fig. 2 a0nd represented by dashed lines. Near k,, = 0.6 A-‘, this state is a true surface state, since it is located in an absolute bandgap. A similar band was recently observed above the Fermi level by Collins et al. [12]
Science 303 (1994) 341-346
k,,@-‘1 Fig. 2. Dispersion of---the surface state of W(OOl) near the Fermi level in the TPM direction of the unreconstructed surface. The IPS (this work) and ARPES [13] results are represented by open circles and crosses respectively. The results of recent band structure calculations (solid lines) [14] and the projected bulk even and odd bandgaps (dashed lines) i2.5) are also included.
Its emergence above the Fermi level near k,, = $?M led to the suggestion that this band was partially occupied. Their data for the dispersion of this band agree very poorly with ours. Using the technique of IPS in the fluorescent mode, Drube et al. [20] observed a non-dispersing state near 1.4 eV above E, at k ,, = O.SFiiiT and tentatively associated it with -- their calculated x2 band. Our state at k,, = 0.8 TM occurs at a much lower energy. Another surface state was also observed by Drube et al. near the ?; point at 0.3 eV above E,. They assigned it to the surface state calculated and observed at 0.3 eV below E,, arguing that this state extends above the Fermi levei due to broadening by mixing with bulk states. Our measurements did not show such a state and the reasons for the discrepancies between our results and those of Drube et al. are not understood at present. Possible explanation may include the
A. Lamouri, IL. fiainsky
/Surface
relatively poor resolution of 0.6 eV as discussed by Collins et al. [12]. Early theories of the reconstruction of the W(OO1) surface were based on the CDW model. The central point of this model is the presence of surface states crossing the Fermi level near the midpoint of the i?M line of the 1 X 1 structure. [26,27] Recently, a local bonding model based on the instability of the high temperature phase to the Debe-King reconstruction was proposed. In this model, the bands of the ideal surface split by the reconst~ction into bonding (occupied) and antibonding ~un~cupied) states in the direction of the reconstruction. [14,28] Our experimental measurements indicate that such a splitting is present even at room temperature and are in good agreement with the recent band structure calculations by Yu et al. 1141 based on the local bonding mechanism. Two bands, a %r and a &, are predicted by these calculations at 0.4 and 0.7 eV above E, respectively. The upper band evolved from the splitting of the 2, surface resonance of the ideal surface into bonding and antibonding surface bands. These calculations are reproduced in Fig. 2 and represented by solid lines. Furthermore, the band derived from the peak at 0.5 eV above E, in our spectra is well defined and flat over a long segment of the i?a line. Such a flat band produces a large peak in the density of states. This is in good agreement with the results of Fu et al. [23] where a sharp peak was observed near 0.5 eV above E, in the calculated density of states for the reconstructed surface. The symmet~ of the state in question is important for comparison with the theoretical results. Unfortunately, our experimental arrangement does not allow us to determine the symmetry of this state. In addition, this state falls between theoretically predicted even and odd states (solid lines in Fig. 2). However, there is evidence that this band comprises a large even component since it existed entirely in an odd bandgap and exhibited maximum intensity near the absolute bandgap. Furthermore, away from this bandgap, its intensity decreased presumably due to hybridization with even states since coupling to odd bulk states is symmetry forbidden.
Science 303 (‘19941341-346
345
Finally, we note that it is not likely that the surface state observed by Smith et al. (131 below the Fermi level is observed here above the Fermi level due to broadening caused by coupling with bulk states [20]. First, coupling to bulk states is forbidden in an absolute bandgap where our surface state is partially located; and second, if we compare the intensities of the signals between k ,, = 0.4 and k ,, = 0.7 A- ’ in direct and inverse photoemission assuming similar background signal strengths, we can conclude that the major part of this state is observed in IPS. In summa~, we have measured the unoccupied energy levels of W(OO1) at room temperature. Our results are in reasonably good agreement with previous IPS measurements 1121 and with the theoretically predicted dispersions of the surface states near the Fermi level [14]. The states observed here show very little or no dispersion which is consistent with these states having significant d character. We observed a surface-related band near 0.5 eV above the Fermi level over a large segment of the T&8 line. The contribution of this band to the density of states is in favor of the local bonding model. Although our results indicate that the local bonding is likely the dominant mechanism in the reconstruction as suggested by Yu et al. [14], the CDW model can not be totally ruled out since a Fermi level crossing of the band observed near --the Fermi level is possible along the I%M line. It is clear from our experimental measurements that if this crossing occurs at all, it takes place near k,, = 0.5 A-‘. This is consistent with the recent ARPES measurements by Smith et al. [13] where an occupied band was observed near the Fermi level over a large portion of the SBZ. This band, reproduced by crosses in Fig. 2, showedono dispersion between k,, = 0.31 and k,, = 0.64 A--r and it was not apparent whether it crossed or remained below the Fermi level as indicated by the large error bars (not reproduced here). Furthermore, Holmes and Gustafsson [29] observed an occupied band of even symometry approaching the Fermi level near k,, = 0.6 A- *. Our band appears as the continuation of this band which is further evidence that our band comprises a large even component.
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A. Lamowi, I. L. Krainsky /Surface
Overall, our results indicate that a combination of both models is probably necessary for a correct description of the W(OO1) surface reconstruction as suggested by Smith et al. [133. 4. Acknowledgements This work was supported by NASA LeRC and the National Research Council. The authors wish to thank Dr. A. Petukhov for helpful discussions and comments.
5. References 111T.E. Felter, R.A. Barker and P.J. Es&up, Phys. Rev. Lett. 38 (1977) 1138. [21M.K. Debe and D.A. King, J. Phys. C: Solid State Phys. IO (1977) L303. [31 M.K. Debe and D.A. King, Phys. Rev. Lett. 39 (1977) 708. [41 D.A. King and G. Thomas, Surf. Sci. 92 (1980) ‘201. 151 R.A. Barker and P.J. Estrup, J. Chem. Phys. 74 (1981) 1442. [hl E. Tosatti, Solid State Commun. 2.5 (1978) h37. 171 H. Krakauer, M. Posternak and A.J. Freeman, Phys. Rev. Lett. 43 (1979) 1885. 181 D. Singh, Su-Huai Wei and H. Krakauer, Phys. Rev. Lett. 57 (1986) 3292; D. Singh and H. Krakauer, Phys. Rev. I3 37 (19881 3999. 191 J.C. Campuzano, J.E. Inglesfield, D.A. King and C. Somerton, J. Phys. C: Solid State Phys. 14 (1981) 3099. 1101 1. Stensgaard, K.G. Purcell and D.A. King, Phys. Rev. B 39 ( 1989) 897.
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fill I.K. Robinson, A.A. MacDowelI, MS. Ahman, P.J, Estrup, K. Evans-Lutterodt, J.D. Brock and R.J. Birgeneau, Phys. Rev. Lett. 62 (1989) 1294. 1121 I.R. Collins, A.D. Laine and P.T. Andrews, J. Phys. Condensed Matter 4 (1992) 2891. (131 K.E. Smith, G.S. Elliott and S.D. Kevan, Phys. Rev. B 42 (1990) 5385. 1141 R. Yu, H. Krakauer and D. Singh, Phys. Rev. B 45 (1992) 8671. 1151 I.L. Krainsky, J. Vat. Sci. Technol. A 5 (1987) 735. 1161A. Lamouri and IL. Krainsky, Surf. Sci. 278 (19921 286. 1171N. Babbe, W. Drube, I. Schafer and M. Skibowski, J. Phys. E 18 0985) t.58. 1181 I.L. Krainsky, Rev. Sci. Instrum. 62 (1991) 1746. 1191 R.A. Barker and P.J. Estrup, Phys. Rev. Lett. 41 (1978) 1307. r201 W. Drube, D. Straub, F.J. Himpsel, P. Soukiassian, CL. Fu and A.J. Freeman, Phys. Rev. B 34 (1986) 8989. 1211 N.E. Christensen and 8. Feuerbacher. Phys. Rev. B 10, 2349, (1974). 1221L.F. Mattheiss and D.R. Hamann, Phys. Rev. B 29 (1984) 5372. 1231 C.L. Fu, A.J. Freeman, E. Wimmer and M. Weinert, Phys. Rev. Lett. 54 (1985) 2261. (241 A. Lamouri and I.L. Krainsky, unpublished. 1251W.R. Grise, D.G. Dempsey, L. Kleinman and K. Mednick, Phys. Rev. B 20 (1979) 3045. 061 M. Posternak, H. Krakauer, A.J. Freeman and D.D. Koelling, Phys. Rev. R 21 (1980) 5601. 1271S. Ohnishi, A.J. Freeman and E. Wimmer, Phys. Rev. B 29 (1984) 5267. 1281 D.W. Bullett and PC. Stephenson, Solid State Commun. 45 (1983) 47; P.C. Stephenson and D.W. Bullett, Surf. Sci. 139 (1984) 1. f291MI. Holmes and T. Gustafsson, Phys. Rev. Lett. 47 (1981) 443.