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Formation and electronic structure of terbium silicide epitaxially grown on Si(111)
0038-1098/91$3.00+.00 Pergamon Press plc
Solid State Communications, Vol. 79, No. 10, pp. 795-798, 1991. Printed in Great Britain.
FORMATION
AND
ELECTRONIC
STRUCTURE GROWN ON
OF
TERBIUM
Si(ll1)
SILICIDE
EPITAXIALLY
J-Y. Veuillen, S. Kennou* and T.A. Nguyen Tan LEPES/CNRS, BP 166X, F-38042 Grenoble Cedex, France *Department of Physics, University of Ioannina, P.O. Box 1186, Gr-451 10 Ioannina Greece (Received 25 February 1991 by M. Tosi)
The growth of epitaxial TbSi, (x-1.7) on Si(lll)-7x7 was studied using Low Energy Electron Diffraction (LEED), Photoelectron Spectroscopies (XPS, UPS, AES) and Work Function (WF) measurements. The slllcide was formed by depositing a 70 1 Tb layer on Si at RT and subsequent annealing. Up to 900 K a 1x1 LEED pattern is observed and with further heating up to 1lOOK a 43x d3-R30° superstructure appears. The XPS and AES results indicate that charge transfer between Tb and Si upon silicide formation is very weak, while Si derived states contribute to the valence band DOS cf TbSix close to the Fermi level. Angle resolved UPS for the two structures shows a band located just below the Fermi level in the vicinity of the M point of the hexagonal Surface Brillouin Zone.
The films of metalllc slliddes epitaxlally grown on Si substrates are of great technological interest. Among them, the rare earth (RE) slllcldes exhibit hlgh electrical conductivity and unusually low (0.3 to 0.4 eV) Schottky barrier on n-type slllconl. It has been shown by Knapp et al2 that the heavy RE (from Gd to Lu) sllicides can form epitaxial films on Si(111). These films form an &x=~o;z phase based on the AlB;! structure. actual composition of the epitaxlal RE-silldde is close to RESi1.7, 1520% vacancies should be
and heating to Ez a&ti;n-EED diagram appeared With spots of a 43x J3-R30° superstmcture. Upon further annealing up to 1lOOK the superstructure became sharp and much more intense. The very 43~ 43-R30° pattern ls commonly observed for heavy rare-earth slllcldes with an approxlmate composition RESl1.7 epitaxlally grown on Si(111)3J. The observation of this diagram indicates that the surface of the slliclde is ordered (in the sense of LEED). although the crystalllnlty of the film may not be very good, as it Is reDorted ln Ref. 18. _ Fig. 1 a,b shows the core level (CL) spectra (Si 21, and Tb 4d) for the uure elements and for the compound. Ori going from the pure elements to the slllcldes the core level binding energy shifts (positive for Tb 4d and negative for Si 2p) are very weak (-0.2eV). The Sl KLL Auger peak was also recorded and was found at a kinetic energy of 1610.9eV for Sl and 1612.2eV for the TbSix, referenced to the vacuum level. From the Auger parameter6 we estimate the initial state shift for the Si 2p level to be -(O.lQ50.15)eV in the sllidde with respect to dean Si. This indicates that the charge transfer between Tb and Si in TbSi, is quite small, although Tb is more electropositive than Si. We also fouird that the extra-atomic relaxation on Si atoms increases bv 0.4eV in the sllldde. which implies a better sc&nlng of the Si 2p hole in the metallic TbSi, than in the semlconducting Si. The Auger Si L23W line for the sllldde and for the clean Si(lll)-7x7 surface is shown in fig. 2. The position of the arrows corresponds to Zhe Si 2p binding energy corrected for the analyzer work function. The analysis of the Si L23W
present ln the Si sublattice2. Although the slllcldes of transition metals (Ni, Co, Pt, Pd) have been extensively studied, only llttle attentlon has been paid to the electronic structure of the rare-earth ~illcides~~~.We have studied the formation and electronic structure of the Tb-slliclde epitaxially grown on Si(ll1). Terbium-silicide is espedally attractive among the. heavy rare-earth silicides since it has the smallest lattice mismatch to Si(lll>, (0.5%)2. The experiments were car&d out in a VSW Photoelectron system multitechique Spectroscopies (XPS, UPS, AES), Low Energy Electron Diffraction (LEED), and Work Function (WF) measurements. The substrate was cut from a n-!g Si(ll1) wafer with a resistlvlty of 1.8-3.2 The clean Si(lll)-7x7 substrate was prepared by cycles of A.r+ sputtering and annealing at 11OOK. The crystal was heated by electronbombardment and its temperature was controlled with an optical pyrometer. Terbium was evaporated by electron-bombardment of a Tb tip at a base pressure of 2x10-10 mbar. The evaporation rate, 2 &nin, was measured by a quartz mlcrobalance. After evaporation of a -70 x thick de-posit 795
TERBIUM SILICIDE EPITAXIALLY GROWN
vol.
ON Si(lll)
79, No.
10
Q
Si 2p
AL TbSi,
Si LVV
\
Si
i
I
I
105 103
101 99 BINDING ENERGY (&I
75
97
Fig. 2
I
I
I
I
I
’
b
spectra I
I
153 BINDING
I
I
149 ENERGY (eV)
145
XPS spectra (excited with the MgKa line) of the Si 2p (a) and the ‘lb 4d (b) core levels for the pure elements and the sllidde. The arrow in the fig.lb indicates the position of the Si 2s core level ln the TbSi,
l&shape has been used by several authors to probe the Sl-states contribution to the valenceband DOS for Pd2Si7, Ni-slliddes*, Ca-sillcidesg It has been shown9 that this and bulk Silo. l&shape is adequately described by a selfconvolution of the densi of states on Si atoms in the ground state weiz ted by the appropriate matrix elements for p-p, s-p and s-s contributions. Fig. 2 shows that for TbSi, a structure appears just below the Si 2p core level position. Since the Si 2p level ls the right energy reference for the Si L23W line7-10, this indicates that some Si derived states contribute to the valence-band DOS of TbSi, in the vicinity of the Fermi level. Experimental data on the density of states (DOS) of trivalent rere earth slllddes are rather scarce. In a recent study of the Gd slllddes using energy dependent photoemission, it has been found that both Si 3s 3p and Gd 5d states partidpate in the valence band DOSl9, with an increase of the metal d contribution close to the Fermi level ~~19 (within - 1 eV of EF). Our Si LW Auger results are ln agreement with this finding and moreover show that the Si3s3p partial DOS remains finite close to EF.
The
He-I
and
He-II
UPS velence
band
80
85
ENERGY
(eV)
90
b3W lineshape 9(111)-7x7 surface and position of the arrow the Si 2p binding correction for our function. Si
95
for a clean for TbSi,. The corresponds to energy after analyser work
epitaxial TbSix exhibiting the 43x 43-
R30° LEED pattern and of polycrystalllne Tb metal are shown in flg. 3. The He-II spectrum for. Tb exhibits a triangular shape between 0 and 2eV of binding energies typical of the heavy rare-earth metals which is due to the 5d-6s states. The Tb 4f states, located between 2 and 3eV binding energies are much more romlnent in the He-II than in the He-I spectrum Por polycrystalllne lb as expected from the computed energy dependence of the 4f cross-sectionl6. These 4f states exhibit surface core level shift of 0.5eV1*. The He-I spectrum of the silicide resembles closely those of GdSix3, ErSix13 and YSix14, with two structures at about 1eV and a broad peak extending from 2 to 4eV centered at around 2SeV. ‘lhls resemblance is a consequence of their identical crystallographic structure (defective AlB2) and of a similar valence electronic configuration (6s-5d)3 of the three rareearth metals. The He-II spectrum of TbSi, is less structured than the He-I one, and the quasi-atomic 4f states contribute sWfk.antl~ to the peak centered at 2SeV. The -work function, mea&red from the low cut-off of the UPS snectra for Tb and TbSix was found 3.44 and 4.65eV, respectively. Fig. 4a and b shows the He-I UPS spectra for various take-off angles 8, with respect to the sample surface normal. They have been obtained in the two lnequivalent directions lKM and FMT of the hexagonal Surface Brillouin Zone (SBZ). The rKM and I’MT refer to the 1x1 surfaces, whereas I?vi’l?M’ and I”K’M’K’T’ refer to the 43x 43R30° reconstructed surface. Note that this reconstruction exchanges the FK and TM directions due to the 30° rotation. The value of 8 corresoondina to the various special ~0i.m of the SBZ ‘along -the two inequivdent directions for states at the Fermi level and at 4eV of bindine energy are listed ln Table 1 for the two surf& structures. The angular resolution of the analyser
Vol. 79, No.
TERBIUM SILICIDE
10
EPITAXIALLY
GROWN ON Si(ll1)
I
I
‘MT He1
I
I
TbSi,
797
I I
TKM
He1
b,
I
4 BINDING
3
4
2
ENERGY
1
C.&l
UPS spectra, with two dlfferent photon energies (He-I:21.2eV; He-II40.8eV). of clean polycrystahlne ‘I% and of an epitaxlal TbSix surface exhibiting a
Fig. 3
I
3 2 (eV)
I
I I
1
0
Anale resolved UPS snectra obtained wit6 He-I along the - IMI (a) and I’KM @) directions of the hexagonal Surface Brlllouln Zone.
Fip. _ 4a.b
0
ENERGY
I
Tb.5,
decide on the surface or bulk origin of the d3x 43R 30° superstructure only from this experhnent, since it is well-known that the relative intensity of the structures of the ARUPS spectra strongly depends on the collection angle. A common feature of figs. 4a and 4b is the presence of a band just below the Fermi level in the vicinity of the M point. Finally. we remark that our spectra are very similar to those obtained for YSix17, GdSlx3 and ErSlx (under investigation at the present time ln our group).. Until now there has not been any report in the literature on band structure calculations for the rare-earth slhcldes with the AlB2 structure. However, we have already mentioned that our UPS spectra look much like those of the Yttrium slllclde. This resembance was also found ln the angular dependence of the He-I UPS spectra14. Thus, we conclude that the electronic structure of the heavy rare-earth sillcldes and of the epitaxlal YSI, are quite similar. This is not unexpected since epitaxlal YSix crystallizes ln the defective AlB2 structure and Y is often considered as a rtue-
43x 43-R300 superstructure. S and B indicate the Tb 4f binding energy for surface and bulk atoms respectively. was of the order of So. The results are discussed for convenience ln terms of the SBZ since the projection of the first Brlllouln zone of the ALB;! structure perpendicular to the surface ls an hexagon identical to the 1x1 SBZ. One can see from fig. 4 ln connection with table 1 that the_ energy location of the prominent peaks is near the zone boundaries of the 1x1 SBZ. Moreover, the positions of the main structures are symmetric with respect to the M point along IMI. as expected. In the IKM direction no symmetry ls anticipated for a 1x1 structure, but since the K point is ln principle equivalent to the r’ point for the 43~ d3-R30° superstructure, one should ln this case observe an additional symmetry ln the position of the peaks relative to the M’ point. This does not seem to be actually the case for all the structures in fig. 4. Of course it is dlfflcult to TABLE
1
Take-off angles 8 corresponding to special points of the Surface Brillouln Zone (for the spectra of fig. 4) at two BE’s for the 1x1 and 4343-R30 superstructures SInface
1x1
Structure Dire&on
;pBt k,,
PohtJ
(1-l)
I
I,
43d3-R30 r'M'r'M' (id.
l-Ml-
l-ml
rKM)
l-xM’K’r’
I
r
K
M
r
M
r
r’
M’
0
1.09
1.63
0
0.94
1.89
0
0.55
1.09
1.63
[ 0 [ 0
15.4’
32’
52’
10
17.70
17.70
370
64O
1 0
20.4’
0 (E@
W
1 0
320
520
0
270
65.4’
e (E+
ev)
10
370
64O
0
31.5’
-
l-‘(rK)
M’(id.M)
l-’
K
0
0.63
(id.
M’(id.M) 0.94
rw) K 1.26
270
37.40
31.50
44.20
T’(id.l-) 1.a9 65.4’ -
798
TERBIUM SILICIDE
EPITAXIALLY
earth in its physico-chemical properties. This aiiows us to compare our experimental data to the calculated DOS for stoichiometric YSi2 with the AiB;! structurel5. In this calculation, the Si atoms are found in the spz hybridization state, with the pz orbitais perpendicuiar to both Si and metal planes. In the binding energy range of O-5eV the metal-d and Si-pz derived states give the main contribution to the valence band DOS, and Y-Si interaction is mainly due to coupling between those states. Our Si LW spectra can thus be taken as an evidence for the presence of such Si-pz states, close to the Fermi level. Both metal-d and Si-pz should contribute to the UPS spectra since their weights in the valence band DOS in the binding energy range of 0-5eV are of the same or&r of magnitude. From computed atomic photoemission cross-sections16, the metal contribution to the valence band UPS spectra should, however, be enhanced relative to that of the Si states, at least
GROWN ON Si (111)
Vol. 79, No. 10
for He-I. It is difficuit to compare our exuerimentai data to the theoretical DOS of ref.15 since our spectra are taken at rather low photon energy in a partially angle resolved mode. In conclusion, epitaxiai TbSix (x-1.7) on Si(ll1) was formed by heating above 900K a 7Ob: Tb layer deposited on Si(lll)-7x7 at RT. After annealing at 1lOOK the LEED diagram shows a 43~ J3-R30° superstructure. XPS remits indicate that the charge transfer between Tb and Si upon silicide formation is very weak. The analysis of the Si I23W iineshape reveals that Si derived states contribute to the valence band DOS close to the Fermi level. From a comparison with theoretical calculation, these states should have a Si3pz character. Angie resolved UPS establishes that the series of epitaxiai trivalent rare-earth siiicides (YSix, GdSix, TbSiz. ErSix) have a very simiiar electronic structure. It also shows a band located just below the Fermi level in the vicinity of the h4 point of the surface Briiiouin Zone.
References
J.Y. Duboz et al., Appl. Surf. Sci. & 171 (1989) and K.N. Tu et ai., Appl. Phys. Lett. s 626 (1981). 2. J.A. Knapp et ai., Appl. Phys. Lett. 4& 466 (1986) 3. W.A. Henle et al., Soiid State Comm. z1, 657 (1989) 4. I Abbati et ai., Solid State Comm. 62, 35 (1987) 5. F. Amaud d’ Avitaya et al., Thin Solid Films 184. 283 (1990); F.H. Kaatz et ai., ibid p.325 6. S. Kohiki et al., Appiied Surface Science 28, 163 (1987) 7. P.S. Ho et al., Phys. Rev. B 2 4784 (1980) 8. U. de1 Pennino et ai., J. Phys. C .I& 6309 (1983) 9. L. Caiiiari et ai., Phys. Rev. B &I, 7569 (19W 10. D.E. Ramaker et al., Phys. Rev. B j& 2574 1.
11. 12. 13. 14. 15. 16. 17. 18. _^ IY.
(1976); D.R. Jennison, Phys. Rev. Lett. $6, 807 (1978) F. Gerken et ai.. Surf. Sci. 117. 468 (1982) F.P. Netzer et al., Rep. Prog. Phys. 4a 621 (1986) J-Y. Veuilien et al., to be pubiished in Surface Science R. Baptist et al.. Soiid Sate Comm. 68, 555 (1988)L. Martinage, J. Phys. Condens. Matter 1, 2593 (1989) J.J. Yeh et ai.. Atomic Data and Nuclear Data Tables 12, .l (1985) R. Baptist et al., private communication and to be pubiished F.H. .___..Kaatz et ai., J. Appl. Phys. a 514 !‘?).. . . . L. Bratcovrcn et ai., Phys. Rev. B a 3123 (1990).
Solid State Communications, Vol. 79, No. 10, pp. 795-798, 1991. Printed in Great Britain.
FORMATION
AND
ELECTRONIC
STRUCTURE GROWN ON
OF
TERBIUM
Si(ll1)
SILICIDE
EPITAXIALLY
J-Y. Veuillen, S. Kennou* and T.A. Nguyen Tan LEPES/CNRS, BP 166X, F-38042 Grenoble Cedex, France *Department of Physics, University of Ioannina, P.O. Box 1186, Gr-451 10 Ioannina Greece (Received 25 February 1991 by M. Tosi)
The growth of epitaxial TbSi, (x-1.7) on Si(lll)-7x7 was studied using Low Energy Electron Diffraction (LEED), Photoelectron Spectroscopies (XPS, UPS, AES) and Work Function (WF) measurements. The slllcide was formed by depositing a 70 1 Tb layer on Si at RT and subsequent annealing. Up to 900 K a 1x1 LEED pattern is observed and with further heating up to 1lOOK a 43x d3-R30° superstructure appears. The XPS and AES results indicate that charge transfer between Tb and Si upon silicide formation is very weak, while Si derived states contribute to the valence band DOS cf TbSix close to the Fermi level. Angle resolved UPS for the two structures shows a band located just below the Fermi level in the vicinity of the M point of the hexagonal Surface Brillouin Zone.
The films of metalllc slliddes epitaxlally grown on Si substrates are of great technological interest. Among them, the rare earth (RE) slllcldes exhibit hlgh electrical conductivity and unusually low (0.3 to 0.4 eV) Schottky barrier on n-type slllconl. It has been shown by Knapp et al2 that the heavy RE (from Gd to Lu) sllicides can form epitaxial films on Si(111). These films form an &x=~o;z phase based on the AlB;! structure. actual composition of the epitaxlal RE-silldde is close to RESi1.7, 1520% vacancies should be
and heating to Ez a&ti;n-EED diagram appeared With spots of a 43x J3-R30° superstmcture. Upon further annealing up to 1lOOK the superstructure became sharp and much more intense. The very 43~ 43-R30° pattern ls commonly observed for heavy rare-earth slllcldes with an approxlmate composition RESl1.7 epitaxlally grown on Si(111)3J. The observation of this diagram indicates that the surface of the slliclde is ordered (in the sense of LEED). although the crystalllnlty of the film may not be very good, as it Is reDorted ln Ref. 18. _ Fig. 1 a,b shows the core level (CL) spectra (Si 21, and Tb 4d) for the uure elements and for the compound. Ori going from the pure elements to the slllcldes the core level binding energy shifts (positive for Tb 4d and negative for Si 2p) are very weak (-0.2eV). The Sl KLL Auger peak was also recorded and was found at a kinetic energy of 1610.9eV for Sl and 1612.2eV for the TbSix, referenced to the vacuum level. From the Auger parameter6 we estimate the initial state shift for the Si 2p level to be -(O.lQ50.15)eV in the sllidde with respect to dean Si. This indicates that the charge transfer between Tb and Si in TbSi, is quite small, although Tb is more electropositive than Si. We also fouird that the extra-atomic relaxation on Si atoms increases bv 0.4eV in the sllldde. which implies a better sc&nlng of the Si 2p hole in the metallic TbSi, than in the semlconducting Si. The Auger Si L23W line for the sllldde and for the clean Si(lll)-7x7 surface is shown in fig. 2. The position of the arrows corresponds to Zhe Si 2p binding energy corrected for the analyzer work function. The analysis of the Si L23W
present ln the Si sublattice2. Although the slllcldes of transition metals (Ni, Co, Pt, Pd) have been extensively studied, only llttle attentlon has been paid to the electronic structure of the rare-earth ~illcides~~~.We have studied the formation and electronic structure of the Tb-slliclde epitaxially grown on Si(ll1). Terbium-silicide is espedally attractive among the. heavy rare-earth silicides since it has the smallest lattice mismatch to Si(lll>, (0.5%)2. The experiments were car&d out in a VSW Photoelectron system multitechique Spectroscopies (XPS, UPS, AES), Low Energy Electron Diffraction (LEED), and Work Function (WF) measurements. The substrate was cut from a n-!g Si(ll1) wafer with a resistlvlty of 1.8-3.2 The clean Si(lll)-7x7 substrate was prepared by cycles of A.r+ sputtering and annealing at 11OOK. The crystal was heated by electronbombardment and its temperature was controlled with an optical pyrometer. Terbium was evaporated by electron-bombardment of a Tb tip at a base pressure of 2x10-10 mbar. The evaporation rate, 2 &nin, was measured by a quartz mlcrobalance. After evaporation of a -70 x thick de-posit 795
TERBIUM SILICIDE EPITAXIALLY GROWN
vol.
ON Si(lll)
79, No.
10
Q
Si 2p
AL TbSi,
Si LVV
\
Si
i
I
I
105 103
101 99 BINDING ENERGY (&I
75
97
Fig. 2
I
I
I
I
I
’
b
spectra I
I
153 BINDING
I
I
149 ENERGY (eV)
145
XPS spectra (excited with the MgKa line) of the Si 2p (a) and the ‘lb 4d (b) core levels for the pure elements and the sllidde. The arrow in the fig.lb indicates the position of the Si 2s core level ln the TbSi,
l&shape has been used by several authors to probe the Sl-states contribution to the valenceband DOS for Pd2Si7, Ni-slliddes*, Ca-sillcidesg It has been shown9 that this and bulk Silo. l&shape is adequately described by a selfconvolution of the densi of states on Si atoms in the ground state weiz ted by the appropriate matrix elements for p-p, s-p and s-s contributions. Fig. 2 shows that for TbSi, a structure appears just below the Si 2p core level position. Since the Si 2p level ls the right energy reference for the Si L23W line7-10, this indicates that some Si derived states contribute to the valence-band DOS of TbSi, in the vicinity of the Fermi level. Experimental data on the density of states (DOS) of trivalent rere earth slllddes are rather scarce. In a recent study of the Gd slllddes using energy dependent photoemission, it has been found that both Si 3s 3p and Gd 5d states partidpate in the valence band DOSl9, with an increase of the metal d contribution close to the Fermi level ~~19 (within - 1 eV of EF). Our Si LW Auger results are ln agreement with this finding and moreover show that the Si3s3p partial DOS remains finite close to EF.
The
He-I
and
He-II
UPS velence
band
80
85
ENERGY
(eV)
90
b3W lineshape 9(111)-7x7 surface and position of the arrow the Si 2p binding correction for our function. Si
95
for a clean for TbSi,. The corresponds to energy after analyser work
epitaxial TbSix exhibiting the 43x 43-
R30° LEED pattern and of polycrystalllne Tb metal are shown in flg. 3. The He-II spectrum for. Tb exhibits a triangular shape between 0 and 2eV of binding energies typical of the heavy rare-earth metals which is due to the 5d-6s states. The Tb 4f states, located between 2 and 3eV binding energies are much more romlnent in the He-II than in the He-I spectrum Por polycrystalllne lb as expected from the computed energy dependence of the 4f cross-sectionl6. These 4f states exhibit surface core level shift of 0.5eV1*. The He-I spectrum of the silicide resembles closely those of GdSix3, ErSix13 and YSix14, with two structures at about 1eV and a broad peak extending from 2 to 4eV centered at around 2SeV. ‘lhls resemblance is a consequence of their identical crystallographic structure (defective AlB2) and of a similar valence electronic configuration (6s-5d)3 of the three rareearth metals. The He-II spectrum of TbSi, is less structured than the He-I one, and the quasi-atomic 4f states contribute sWfk.antl~ to the peak centered at 2SeV. The -work function, mea&red from the low cut-off of the UPS snectra for Tb and TbSix was found 3.44 and 4.65eV, respectively. Fig. 4a and b shows the He-I UPS spectra for various take-off angles 8, with respect to the sample surface normal. They have been obtained in the two lnequivalent directions lKM and FMT of the hexagonal Surface Brillouin Zone (SBZ). The rKM and I’MT refer to the 1x1 surfaces, whereas I?vi’l?M’ and I”K’M’K’T’ refer to the 43x 43R30° reconstructed surface. Note that this reconstruction exchanges the FK and TM directions due to the 30° rotation. The value of 8 corresoondina to the various special ~0i.m of the SBZ ‘along -the two inequivdent directions for states at the Fermi level and at 4eV of bindine energy are listed ln Table 1 for the two surf& structures. The angular resolution of the analyser
Vol. 79, No.
TERBIUM SILICIDE
10
EPITAXIALLY
GROWN ON Si(ll1)
I
I
‘MT He1
I
I
TbSi,
797
I I
TKM
He1
b,
I
4 BINDING
3
4
2
ENERGY
1
C.&l
UPS spectra, with two dlfferent photon energies (He-I:21.2eV; He-II40.8eV). of clean polycrystahlne ‘I% and of an epitaxlal TbSix surface exhibiting a
Fig. 3
I
3 2 (eV)
I
I I
1
0
Anale resolved UPS snectra obtained wit6 He-I along the - IMI (a) and I’KM @) directions of the hexagonal Surface Brlllouln Zone.
Fip. _ 4a.b
0
ENERGY
I
Tb.5,
decide on the surface or bulk origin of the d3x 43R 30° superstructure only from this experhnent, since it is well-known that the relative intensity of the structures of the ARUPS spectra strongly depends on the collection angle. A common feature of figs. 4a and 4b is the presence of a band just below the Fermi level in the vicinity of the M point. Finally. we remark that our spectra are very similar to those obtained for YSix17, GdSlx3 and ErSlx (under investigation at the present time ln our group).. Until now there has not been any report in the literature on band structure calculations for the rare-earth slhcldes with the AlB2 structure. However, we have already mentioned that our UPS spectra look much like those of the Yttrium slllclde. This resembance was also found ln the angular dependence of the He-I UPS spectra14. Thus, we conclude that the electronic structure of the heavy rare-earth sillcldes and of the epitaxlal YSI, are quite similar. This is not unexpected since epitaxlal YSix crystallizes ln the defective AlB2 structure and Y is often considered as a rtue-
43x 43-R300 superstructure. S and B indicate the Tb 4f binding energy for surface and bulk atoms respectively. was of the order of So. The results are discussed for convenience ln terms of the SBZ since the projection of the first Brlllouln zone of the ALB;! structure perpendicular to the surface ls an hexagon identical to the 1x1 SBZ. One can see from fig. 4 ln connection with table 1 that the_ energy location of the prominent peaks is near the zone boundaries of the 1x1 SBZ. Moreover, the positions of the main structures are symmetric with respect to the M point along IMI. as expected. In the IKM direction no symmetry ls anticipated for a 1x1 structure, but since the K point is ln principle equivalent to the r’ point for the 43~ d3-R30° superstructure, one should ln this case observe an additional symmetry ln the position of the peaks relative to the M’ point. This does not seem to be actually the case for all the structures in fig. 4. Of course it is dlfflcult to TABLE
1
Take-off angles 8 corresponding to special points of the Surface Brillouln Zone (for the spectra of fig. 4) at two BE’s for the 1x1 and 4343-R30 superstructures SInface
1x1
Structure Dire&on
;pBt k,,
PohtJ
(1-l)
I
I,
43d3-R30 r'M'r'M' (id.
l-Ml-
l-ml
rKM)
l-xM’K’r’
I
r
K
M
r
M
r
r’
M’
0
1.09
1.63
0
0.94
1.89
0
0.55
1.09
1.63
[ 0 [ 0
15.4’
32’
52’
10
17.70
17.70
370
64O
1 0
20.4’
0 (E@
W
1 0
320
520
0
270
65.4’
e (E+
ev)
10
370
64O
0
31.5’
-
l-‘(rK)
M’(id.M)
l-’
K
0
0.63
(id.
M’(id.M) 0.94
rw) K 1.26
270
37.40
31.50
44.20
T’(id.l-) 1.a9 65.4’ -
798
TERBIUM SILICIDE
EPITAXIALLY
earth in its physico-chemical properties. This aiiows us to compare our experimental data to the calculated DOS for stoichiometric YSi2 with the AiB;! structurel5. In this calculation, the Si atoms are found in the spz hybridization state, with the pz orbitais perpendicuiar to both Si and metal planes. In the binding energy range of O-5eV the metal-d and Si-pz derived states give the main contribution to the valence band DOS, and Y-Si interaction is mainly due to coupling between those states. Our Si LW spectra can thus be taken as an evidence for the presence of such Si-pz states, close to the Fermi level. Both metal-d and Si-pz should contribute to the UPS spectra since their weights in the valence band DOS in the binding energy range of 0-5eV are of the same or&r of magnitude. From computed atomic photoemission cross-sections16, the metal contribution to the valence band UPS spectra should, however, be enhanced relative to that of the Si states, at least
GROWN ON Si (111)
Vol. 79, No. 10
for He-I. It is difficuit to compare our exuerimentai data to the theoretical DOS of ref.15 since our spectra are taken at rather low photon energy in a partially angle resolved mode. In conclusion, epitaxiai TbSix (x-1.7) on Si(ll1) was formed by heating above 900K a 7Ob: Tb layer deposited on Si(lll)-7x7 at RT. After annealing at 1lOOK the LEED diagram shows a 43~ J3-R30° superstructure. XPS remits indicate that the charge transfer between Tb and Si upon silicide formation is very weak. The analysis of the Si I23W iineshape reveals that Si derived states contribute to the valence band DOS close to the Fermi level. From a comparison with theoretical calculation, these states should have a Si3pz character. Angie resolved UPS establishes that the series of epitaxiai trivalent rare-earth siiicides (YSix, GdSix, TbSiz. ErSix) have a very simiiar electronic structure. It also shows a band located just below the Fermi level in the vicinity of the h4 point of the surface Briiiouin Zone.
References
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