- Email: [email protected]
Oxygen adsorption on the LaB6(100) surface studied by UPS and LEED
Surface Science 92 (1980) 191-200 0 North-Holland Publishing Company
OXYGEN ADSORPTION
ON THE LaB6(100) SURFACE STUDIED BY UPS AND
LEED
Ryusuke NISHITANI,
Shichio KAWAI, Hiroshi IWASAKI and Shogo NAKAMURA
The Institute of Scientific and Industrial Research, Osaka University, Suita, Osaka 565, Japan
and Masakazu AONO and Takaho TANAKA National Institute for Researches in Inorganic Materials, Namiki I-I, Sakura-mura, Niihan’-gun, Ibaraki 300-31, Japan
Received 9 August 1979; accepted for publication 11 September 1979
The surface states (-2 eV below .!?F), which are originated mainly from the dangling bonds of boron atoms on the LaBs(100) clean surface, disappear at an oxygen exposure of -1.4 L. At the same exposure, an oxygen sticking-coefficient has a maximum value, -1.0. A change in the work function due to oxygen adsorption increases linearly with increasing surface oxygen and varies its slope at the above-mentioned exposure. At a low oxygenexposure of -0.38 L, the first peak appears at -6.6 eV below EF in UPS spectra. The second overlapping oxygen peak at -6.0 eV below _!?Fin UPS spectra, which grows around 1 L and overcomes the first peak, shifts to the low bindingenergy side above -1.4 L. The (1 X 1) LEED pattern does not disappear until an oxygen exposure of several hundreds langmuir. It is suggested that the results support the presence of more than two adsorption states. The results are principally interpreted on the basis of two kinds of chemisorption sites; one is a boron site, and another a lanthanum site.
1. Introduction
Lanthanum hexaboride, LaB6, which is a refractory compound with a low work function, has recently become of interest as an electron-beam source of high brightness for various purposes such as the microfabrication of super-large-scale integrated circuits [ 11. Aono et al. (1977) reported that photoemission from surface states was observed at about 2 eV below the Fermi level, EF, for the LaB6(1 00) clean surface and these energy band structures were successfully determined by angular resolved UPS [2]. They demonstrated that the outermost layer of the (100) surface is a lanthanum layer, which is in a (1 X 1) ordered state, using angular resolved XPS and LEED [3]. It was suggested from these results that the origin of the unusually low work func191
tion of the LaB,(lOO) surface was electric dipole moments produced by positive charges of such surface lanthanum ions. The adsorption of gases on the LaB6 surface forms a dipole layer at the surface, which modifies the surface potential and the work function. As oxygen has a high electronegativity of 3.5 141, the strong dipole should be formed by adsorbed oxygen. In fact, the oxidation of the La& surface causes an increase of about 1.6 eV in the work function, followed by a constant vaiue with increasing oxygen exposure [S]. Oxygen is the most poisoi~ing gas for a I.aBe cathode, and then the elucidation of the nature of the adsorbed oxygen is required for practical use such as a cathode, In the present study, the interaction of the LaB6(100) surface with oxygen has been investigated by means of UPS, LEED and work function lneasuremerlfs. It has been found that there are some correlations among the kinetics of oxygen adsorption, the disappearance of the surface state, the shift of an oxygen-derived peak at -6 eV below EF and the change in work function. 2. Experimental UPS spectra were measured by the V.G.ADES 400 photoelectron spectrometer equipped with an electron analyzer which can be rotated. For excitation of photoelectrons, He I (hv = 21.2 eV) radiation was used; the incident angle of the exciting Iight was always 45* from the surface normal All UPS spectra were measured in the normal direction to the sample surface. The vacuum chamber of the spectrometer could be evacuated down to -4 X 10-r’ Torr. During the UPS measurements using a windowless hehum lamp, the back ground pressure in the vacuum chamber was -3 X lo-’ Torr, the main residual gas component being helium. A single crystal of LaB6 was grown by the floating-zone method (molten zone pass was repeated three times), and a thin sample (-6 X 5 X 0.4 mm3) parallel to the (100) plane was cut from it and was polished by a suspension of alumina (-0.2 pm in particle size). In order to clean the sample surface, annealing by the electron beam heating from the behind of the sample (-1300-1500°C} was repeated under ultra-high vac~~um. The cleanliness of the sampie surface was checked by XPS, UPS and LEED; extra 01s and Cls XPS peaks and an oxygen-derived 02p UPS peak were not detected at ah, and a (1 X 1) LEED pattern was clearly observed. The work function was measured from the cut-off energy of the photo-excited secondary electron and the kinetic energy of the Fermi level in UPS spectra. The reliable accuracy of the work function measured was rtO.1 eV.
3. Results and discussion 3.1. LEELI studies LEED patterns of the LaB6(100) surface were observed after various oxygen exposure. The energy of primary electrons used is 25 eV. The observed sequence of
R. Nishitani et al. /Oxygen
193
adsorption on LaBg(100)
LEED patterns as oxygen exposure proceeds are shown in fig. 1. As we see in fig. 1, the intensity of the (1 1) spots decreases gradually by oxygen exposure, but the intensity of the (1 0) spots does not change till an oxygen exposure of -1 L (1 L = 10e6 Torr s). Upon further exposure, the (1 0) spots decrease gradually in intensity, and at an exposure of -2 L they have nearly the same intensity as the (1 1) spots. After that, the background intensity increases gradually with increasing oxygen exposure. (1 X 1) LEED patterns did not disappear until an oxygen exposure of several hundreds langmuir. 3.2. Kinetics of oxygen adsorption All adsorption experiments were done at room temperature by introducing oxygen of -1 X lo-’ Torr into the vacuum chamber. Fig. 2 shows UPS spectra for the (100) clean and oxygen-exposed surface. The peak appearing at -2 eV below the Fermi level EF is attributable to a surface state [2]. It seems reasonable to consider that the surface state is mainly originated from the dangling bonds of the boron framework by considering the structure of the LaB,(lOO) surface [3]. The surfacestate peak disappears at an exposure of -1.4 L. The peaks appearing at around 6 eV below EF are due to the 02p level of oxygen adsorbed on the surface [ 51. The oxygen-derived peak grows as oxygen exposure proceeds. At a low oxygen-exposure of
Fig. 1. LEED patterns of the LaBe surface clean,(b) 0.7 L, (c) 1.1 L, (d) 2.2 L.
at 25 eV before
and after
oxygen
exposure:
(a)
194
EXP(L)
WORK FUNCTION
5.12
3.91
2.16
3.81
192
3.65
1.66
3.57
1.08
3.06
(eV)
2 95
2.86 2.78
2.49
CLEAN
EF
2
BINDINC‘ENE6RGY
2.3
8
(eV)
Fig. 2. UPS spectra for the LaB,(lOO) surface measured before and after oxygen exposures.
-0.38 L, a small peak appears at -6.6 eV below E,, and at an oxygen exposure of -0.7 L, a left overlapping-peak at -6.0 eV starts to grow. These different states may be due to the different adsorption sites. The peak at -6.6 eV below EF seems to disappear as oxygen exposure proceeds. This state may be considered as a precursor state. The peak height of the surface-state peak and the area of the oxygenderived peaks are plotted as a function of oxygen exposure in fig. 3. These results were reproduced well. The structure of rare earth hexaborides is based on the three-dimensional boron frameworks and the rare-earth ions are situated in the large hole in the framework [6]. The cohesive energy results mainly from the covalently-bonded framework of the boron atoms, and the boron-metal bonding is relatively weak due to the shielding effect by conduction electrons [7]. This feature of bonding explains many properties of rare earth hexaborides such as high melting point and high hardness.
R. Nishitani et al. /Oxygen I
I
I
I
1
1
adsorption on LaBg(IO0)
195
-
OXYGEN
EXPOSURE 2 34 1 II
I.7
( L )
1.4 L A
PEAK STATE
I 0 0
1
2 3 OXYGEN
Fig. 3. Oxygenexposure UPS peak.
4 5 EXPOSURE
dependences
7
6 (L)
of the intensities
I
I
5
10
OXVGEN
15
20
UPS PEAK INTENSITY (arb. units )
of the surface-state
and oxygen-derived
Fig. 4. Oxygen sticking probability versus the intensity of the oxygen-derived UPS peak.
As to the LaB6, the conduction band consists of a mixing of rare-earth metal 5d and boron 2s, p states, which has been shown from a comparison of the experimental Fermi surface [8] with band structure calculation [9]. The principal difference of the electronic structure among rare-earth hexaborides may be due to the 4f level of a rare-earth metal. The surface sensitive peak, which disappears at an oxygen exposure of -0.5 L, is also observed for the CeB6(1 00) clean surface at the same energy of 2 eV below EF as that of the surface-state peak of the LaB6(100) surface [lo]. CeB6 has a 4f electron but LaB6 does not. Therefore, the appearance of the surface sensitive peak at the same energy in a UPS spectrum for the CeB,(lOO) clean surface as the LaB6(100) clean surface supports that both the surface states are mainly originated from the dangling bonds of the boron frameworks. Furthermore, this statement is also suggested by the fact that the surface-state peak observed at room temperature on the LaB,(lOO) surface undergoes no essential change even at -1400°C in spite of the vigorous thermal motion of the surface lanthanum ions, which has been indicated by LEED [ Ill. The sticking probability which is the differential of the oxygen peak intensity with respect to oxygen exposure and is derived from fig. 3, is shown in fig. 4 as a function of the amount of oxygen on the surface. It is interesting that the sticking probability increases up to the maximum value at the exposure of -1.4 L where the
196
R. Nishitani et al. / Oxygen adsorption on LaBg(100)
surface-state peak disappears. As far as we know, the sticking probability of gases on a metal surface has a large value at the initial stage of the adsorption and it decreases as the adsorption proceeds. While LaB6 is a metallic boride, the covalent bonding plays an important role in explaining the many properties of LaB6 as mentioned before. In this sense, it is not surprising that the kinetics of adsorption on the LaBh(100) surface differs from that on a metal surface. Nevertheless, it must be noted that the initial sticking probability of oxygen on the LaB,(lOO) surface has a low value while those for the LaBr, (1 10) and (111) clean surface have a large value which will be described elsewhere [12]. Oxygen adsorbed at first on LaB,(lOO) may be at a precursor state which corresponds to the peak at -6.6 eV below L,‘r:. With increasing the oxygen exposure, oxygen gas transfers the energy to oxygen adsorbed at first, and the accomodation coefficient increases. So, the sticking probability may increase with increasing oxygen exposure. The fact that the sticking probability decreases rapidly after the disappearance of the surface-state peak indicates the completion of the fast oxygen-adsorption on the surface boron atoms. If we assume that oxygen is adsorbed at all sites of surface boron atoms dissociatively (the number of surface boron atoms, N= 5.8 X 1014/cn~2) at the oxygen exposure of -1.4 L where the sticking probability has a maximum, the maximum value of the sticking probability is -1 .O. If we define that the coverage, 0, is 0.5 at this oxygen exposure, the saturated coverage is unity from fig. 3. Therefore it is suggested that oxygen is adsorbed mainly on the boron atoms at first, and subsequently mainly on the lanthanum atoms. On the contrary, Goldstein and Szostak [ 131 suggested that the oxygen atoms were located preferentially over the boron octahedra and bonded to four lanthanum atoms in a terminal plane. This model was derived because they used the metallic radius of 1.88 A as the size of the lanthanum atom. This radius is, however, unreasonably large. The band structure calculations of metal hexaborides by Longuet--Higgins and Loberts [14], and Yamazaki [ 151, using the tight-binding approaximation, showed that the valence bands contain 20 electrons per unit cell, leading to the strong covalent bonding for the simple cubic lattice of the B6 octahedra, The six B atoms in the unit cell provide only 18 electrons, thus, the remaining two electrons must be provided by the La atoms, resulting in the ionic nature between the lanthanum and boron. An analysis on temperature dependence of the electrical resistivity supported this ionic nature, since the electrons are scattered not only by acoustic phonons but also by polar optical phonons [ 161. According to a recent investigation of the experimental Fermi surface [8], LaB6 has one conduction electron per unit cell. Furthermore, LaB6 can be considered diamagnetic [ 171. These facts show that the La atoms in LaB6 are trivalent. Therefore, the size of the lanthanum atoms should be referred to the trivalent ionic radius of 1.14 A. In this case, the oxygen atoms can be bonded to boron atoms directly without bonding to lanthanum atoms.
197
3.3. Workfunc tion A change in the work function due to oxygen adsorption is plotted in fig. 5 as a function of the intensity of the oxygen-derived peaks. The change in the work function, A@,,increases linearly with increasing surface oxygen and changes its slope at A@= -1.0 eV, i.e. at the oxygen exposure of -1.4 L where the surface-state peak disappears. The linearity of the change in # with increasing oxygen exposure indicates that the change in the work function of the LaB6(100) surface is due to the change in the density of dipole moments. The change in the slope at -1.4 L, i.e. B = 0.5 indicates that another adsorption process arises at this oxygen exposure. An estimate of’ the dipole moment in each process can be made with the use of the equation, A$ = 4nnp A0,
(1)
where n is the number of adsorption site, n = 2 X 5.8 X 10’4/cm2 (= 2/a2, a = 4.16 a, lattice constant), A@ the change in coverage, and /J the dipole moment. For low coverage, if we take Ac#= 0.76 eV and A@= 0.32, then ,U= 0.54 D (1 D = 1O-‘* esu cm). This calculated value is nearly in agreement with that reported in ref. [13]. For higher coverage than B = 0.5, if we take A# = 0.65 eV and A@= 0.5, then p = 0.25 D. 3.4. Energy levels of adsorbed oxygen It can be seen from fig. 2 that the oxygen-derived peak shifts to the low binding side as oxygen exposure proceeds. A change in the maximum position of the oxy-
_ 1.6 % - 1.4 % %’12 z $ 1.0 U $0.8 F ” “, 0.6 CL z 0.4 P
0.2 0
0
5 OXYGEN
Fig. 5. Change
UPS
10 PEAK
INTENSITY
15
(arb. units)
in work function versus the intensity of the oxygen-derived UPS peak.
198
R. Nishitani
et al. /Oxygen
adsorption
on LaB,/lOO)
gen-derived peak is shown in fig. 6 against the work function. Below the work function of -2.7 eV, the peak positions are independent of the coverage. This may be due to the nonexistence of the second oxygen peak for these low exposures. In the range between @J= 2.7 eV and (p = 3.1 eV, there is a marked change in the peak position. It may be seen in fig. 2 that the peak in this range consists of two overlapped peaks whose binding energies are -6.6 eV and -6.0 eV below EF, respectively. So, the oxygen adsorption on boron sites is prominant, while the adsorption on another site may be mixed. For higher work function than 3.4 eV, i.e. at an exposure of 1.4 L, the peak position shifts to the low binding energy side linearly with the increase in the work function. The peak in this region has narrow FWHM and may consist of a unique state. The binding energy of orbitals within an adsorbed atom may be expressed by the equation [ 181, BEad = BE, - (b + fD + ABE,
(2)
where BEad is the binding energy of an adsorbate level with respect to EF, BE, is the binding energy of the equivalent level in the gas phase with respect to the vacuum level, # is the work function, ABE is the chemical shift, D is the total dipole barrier at the surface, and f is the fraction of the dipole potential which has been penetrated by the atom. The energy diagram for a molecule adsorbed at the surface of a metal crystal is depicted in fig. 7. The fact that the peak position varies linearly with $I above @= 3.4 eV could mean that the valence level of the adsorbate is not strongly fixed to the metal valence levels and the potential at the adsorption site varies with I# or coverage. That is, if the value off is small, BE,d increases
I
I
I
I
1
2.5
3.0
3.5
4.0
WORK
FUNGTION
( eV
)
Fig. 6. Peak position as a function of the work function for the UPS peak derived by oxygen adsorption.
199
Electrostatic
Fig. 7. The energy level diagram for an adsorbate on a metal surface.
linearly with (a according to eq. (2). Therefore, it may be said that the position of the adsorbed oxygen above the oxygen exposure of -1.4 L is far from the surface, compared with that below the exposure of -1.4 L. This is consistent with the statement that the oxygen is adsorbed mainly on a lanthanum atom above the exposure of -1.4 L.
4. Summary It has been demonstrated that there are some correlations among the kinetics of adsorption, the disappearance of the surface state, the change in the work function and the states of adsorbed’oxygen. The surface states, which are originated mainly from the dangling bonds of boron atoms, disappear at the oxygen exposure of -1.4 L where an oxygen sticking-coefficient has a maximum value, -1 .O. The dipole moment which is produced by oxygen adsorption has depended on the coverage; the value of the dipole moment is 0.55 or 0.25 D, according as 8 < 0.5 or 3 > 0.5. We have suggested that there are more than two adsorption states on the LaB&lOO) surface. The experimental results described in section 3 can be interpreted most reasonably by thinking that the oxygen is adsorbed mainly on the boron atom at first, and subsequently mainly on the lanthanum atom.
Acknowledgements One of the authers (R.N.) wishes to thank Dr. C. Oshima and Dr. Y. Ishizawa at NIRIM for heIpfu1 and stimulating discussions.
200
R. Nishitani
et al. /Oxygen
adsorption
on LaBe(lO0)
References [l]
[2] [3] [4] [5] [6] [7] [8] [9]
[lo] [ 1 l] [ 121 [13] [14] [15] [ 161 [17] [ 181
A.N. Broers and M. Hatzakis, Sci. Am. 227 (1972) 34; H. Ahmed and A.N. Broers, J. Appl. Phys. 43 (1972) 2185; R.T. Miller, Solid State Technol. 7 (1973) 25; J.D. Verhoven and E.D. Gibson, J. Phys. E9 (1976) 65; R. Shimizu, Y. Kataoka, S. Kawai and T. Tanaka, Japan. J. Appl. Phys. (1975) 1089; R. Shimizu, T. Shinike, S. lchimura, S. Kawai and T. Tanaka, J. Vacuum Sci. Technol. 15 (1978) 922. M. Aono, T. Tanaka, E. Bannai, C. Oshima and S. Kawai, Phys. Rev. B16 (1977) 3489. M. Aono, C. Oshima, T. Tanaka, E. Bannai and S. Kawai, J. Appl. Phys. 49 (1978) 2761. L. Pauling, The Nature of the Chemical Bond, 3rd ed. (Cornell Univ. Press, Ithaca, 1960). M. Aono, T. Tanaka, E. Bannai and S. Kawai, Appl. Phys. Letters 31 (1977) 323. K.E. Spear, Phase Diagram, Vol. IV (Academic Press, 1976) p. 91. M. Ishii, M. Aono, S. Muranaka and S. Kawai, Solid State Commun. 20 (1976) 437. Y. Ishizawa, T. Tanaka, E. Bannai and S. Kawai, J. Phys. Sot. Japan 42 (1977) 112. A. Hasegawa and A. Yanase, J. Phys. F (Metal Phys.) 7 (1977) 1245. Present authors, in preparation. M. Aono, R. Nishitani, C. Oshima, T. Tanaka, E. Bannai and S. Kawai, J. Appl. Phys. 50 (1979) 4802. Present authors, in preparation. B. Goldstein and D.J. Szostak, Surface Sci. 74 (1978)461. H.C. Longuet-Higgins and M. de V. Roverts, Proc. Roy. Sot. (London) 224 (1954) 336. M. Yamazaki, J. Phys. Sot. Japan 12 (1957) 1. T. Tanaka, T. Akahane, E. Bannai, S. Kawai, N. Tsuda and Y. Ishizawa, J. Phys. C9 (1976) 1235. J.R. Shannon and M.J. Sienko, Inorg. Chem. 11 (1972) 904. C.R. Brundle, in: Electronic Structure and Reactivity of Metal Surfaces, Eds. E.G. Derouane and A.A. Lucas (Plenum, 1976).
OXYGEN ADSORPTION
ON THE LaB6(100) SURFACE STUDIED BY UPS AND
LEED
Ryusuke NISHITANI,
Shichio KAWAI, Hiroshi IWASAKI and Shogo NAKAMURA
The Institute of Scientific and Industrial Research, Osaka University, Suita, Osaka 565, Japan
and Masakazu AONO and Takaho TANAKA National Institute for Researches in Inorganic Materials, Namiki I-I, Sakura-mura, Niihan’-gun, Ibaraki 300-31, Japan
Received 9 August 1979; accepted for publication 11 September 1979
The surface states (-2 eV below .!?F), which are originated mainly from the dangling bonds of boron atoms on the LaBs(100) clean surface, disappear at an oxygen exposure of -1.4 L. At the same exposure, an oxygen sticking-coefficient has a maximum value, -1.0. A change in the work function due to oxygen adsorption increases linearly with increasing surface oxygen and varies its slope at the above-mentioned exposure. At a low oxygenexposure of -0.38 L, the first peak appears at -6.6 eV below EF in UPS spectra. The second overlapping oxygen peak at -6.0 eV below _!?Fin UPS spectra, which grows around 1 L and overcomes the first peak, shifts to the low bindingenergy side above -1.4 L. The (1 X 1) LEED pattern does not disappear until an oxygen exposure of several hundreds langmuir. It is suggested that the results support the presence of more than two adsorption states. The results are principally interpreted on the basis of two kinds of chemisorption sites; one is a boron site, and another a lanthanum site.
1. Introduction
Lanthanum hexaboride, LaB6, which is a refractory compound with a low work function, has recently become of interest as an electron-beam source of high brightness for various purposes such as the microfabrication of super-large-scale integrated circuits [ 11. Aono et al. (1977) reported that photoemission from surface states was observed at about 2 eV below the Fermi level, EF, for the LaB6(1 00) clean surface and these energy band structures were successfully determined by angular resolved UPS [2]. They demonstrated that the outermost layer of the (100) surface is a lanthanum layer, which is in a (1 X 1) ordered state, using angular resolved XPS and LEED [3]. It was suggested from these results that the origin of the unusually low work func191
tion of the LaB,(lOO) surface was electric dipole moments produced by positive charges of such surface lanthanum ions. The adsorption of gases on the LaB6 surface forms a dipole layer at the surface, which modifies the surface potential and the work function. As oxygen has a high electronegativity of 3.5 141, the strong dipole should be formed by adsorbed oxygen. In fact, the oxidation of the La& surface causes an increase of about 1.6 eV in the work function, followed by a constant vaiue with increasing oxygen exposure [S]. Oxygen is the most poisoi~ing gas for a I.aBe cathode, and then the elucidation of the nature of the adsorbed oxygen is required for practical use such as a cathode, In the present study, the interaction of the LaB6(100) surface with oxygen has been investigated by means of UPS, LEED and work function lneasuremerlfs. It has been found that there are some correlations among the kinetics of oxygen adsorption, the disappearance of the surface state, the shift of an oxygen-derived peak at -6 eV below EF and the change in work function. 2. Experimental UPS spectra were measured by the V.G.ADES 400 photoelectron spectrometer equipped with an electron analyzer which can be rotated. For excitation of photoelectrons, He I (hv = 21.2 eV) radiation was used; the incident angle of the exciting Iight was always 45* from the surface normal All UPS spectra were measured in the normal direction to the sample surface. The vacuum chamber of the spectrometer could be evacuated down to -4 X 10-r’ Torr. During the UPS measurements using a windowless hehum lamp, the back ground pressure in the vacuum chamber was -3 X lo-’ Torr, the main residual gas component being helium. A single crystal of LaB6 was grown by the floating-zone method (molten zone pass was repeated three times), and a thin sample (-6 X 5 X 0.4 mm3) parallel to the (100) plane was cut from it and was polished by a suspension of alumina (-0.2 pm in particle size). In order to clean the sample surface, annealing by the electron beam heating from the behind of the sample (-1300-1500°C} was repeated under ultra-high vac~~um. The cleanliness of the sampie surface was checked by XPS, UPS and LEED; extra 01s and Cls XPS peaks and an oxygen-derived 02p UPS peak were not detected at ah, and a (1 X 1) LEED pattern was clearly observed. The work function was measured from the cut-off energy of the photo-excited secondary electron and the kinetic energy of the Fermi level in UPS spectra. The reliable accuracy of the work function measured was rtO.1 eV.
3. Results and discussion 3.1. LEELI studies LEED patterns of the LaB6(100) surface were observed after various oxygen exposure. The energy of primary electrons used is 25 eV. The observed sequence of
R. Nishitani et al. /Oxygen
193
adsorption on LaBg(100)
LEED patterns as oxygen exposure proceeds are shown in fig. 1. As we see in fig. 1, the intensity of the (1 1) spots decreases gradually by oxygen exposure, but the intensity of the (1 0) spots does not change till an oxygen exposure of -1 L (1 L = 10e6 Torr s). Upon further exposure, the (1 0) spots decrease gradually in intensity, and at an exposure of -2 L they have nearly the same intensity as the (1 1) spots. After that, the background intensity increases gradually with increasing oxygen exposure. (1 X 1) LEED patterns did not disappear until an oxygen exposure of several hundreds langmuir. 3.2. Kinetics of oxygen adsorption All adsorption experiments were done at room temperature by introducing oxygen of -1 X lo-’ Torr into the vacuum chamber. Fig. 2 shows UPS spectra for the (100) clean and oxygen-exposed surface. The peak appearing at -2 eV below the Fermi level EF is attributable to a surface state [2]. It seems reasonable to consider that the surface state is mainly originated from the dangling bonds of the boron framework by considering the structure of the LaB,(lOO) surface [3]. The surfacestate peak disappears at an exposure of -1.4 L. The peaks appearing at around 6 eV below EF are due to the 02p level of oxygen adsorbed on the surface [ 51. The oxygen-derived peak grows as oxygen exposure proceeds. At a low oxygen-exposure of
Fig. 1. LEED patterns of the LaBe surface clean,(b) 0.7 L, (c) 1.1 L, (d) 2.2 L.
at 25 eV before
and after
oxygen
exposure:
(a)
194
EXP(L)
WORK FUNCTION
5.12
3.91
2.16
3.81
192
3.65
1.66
3.57
1.08
3.06
(eV)
2 95
2.86 2.78
2.49
CLEAN
EF
2
BINDINC‘ENE6RGY
2.3
8
(eV)
Fig. 2. UPS spectra for the LaB,(lOO) surface measured before and after oxygen exposures.
-0.38 L, a small peak appears at -6.6 eV below E,, and at an oxygen exposure of -0.7 L, a left overlapping-peak at -6.0 eV starts to grow. These different states may be due to the different adsorption sites. The peak at -6.6 eV below EF seems to disappear as oxygen exposure proceeds. This state may be considered as a precursor state. The peak height of the surface-state peak and the area of the oxygenderived peaks are plotted as a function of oxygen exposure in fig. 3. These results were reproduced well. The structure of rare earth hexaborides is based on the three-dimensional boron frameworks and the rare-earth ions are situated in the large hole in the framework [6]. The cohesive energy results mainly from the covalently-bonded framework of the boron atoms, and the boron-metal bonding is relatively weak due to the shielding effect by conduction electrons [7]. This feature of bonding explains many properties of rare earth hexaborides such as high melting point and high hardness.
R. Nishitani et al. /Oxygen I
I
I
I
1
1
adsorption on LaBg(IO0)
195
-
OXYGEN
EXPOSURE 2 34 1 II
I.7
( L )
1.4 L A
PEAK STATE
I 0 0
1
2 3 OXYGEN
Fig. 3. Oxygenexposure UPS peak.
4 5 EXPOSURE
dependences
7
6 (L)
of the intensities
I
I
5
10
OXVGEN
15
20
UPS PEAK INTENSITY (arb. units )
of the surface-state
and oxygen-derived
Fig. 4. Oxygen sticking probability versus the intensity of the oxygen-derived UPS peak.
As to the LaB6, the conduction band consists of a mixing of rare-earth metal 5d and boron 2s, p states, which has been shown from a comparison of the experimental Fermi surface [8] with band structure calculation [9]. The principal difference of the electronic structure among rare-earth hexaborides may be due to the 4f level of a rare-earth metal. The surface sensitive peak, which disappears at an oxygen exposure of -0.5 L, is also observed for the CeB6(1 00) clean surface at the same energy of 2 eV below EF as that of the surface-state peak of the LaB6(100) surface [lo]. CeB6 has a 4f electron but LaB6 does not. Therefore, the appearance of the surface sensitive peak at the same energy in a UPS spectrum for the CeB,(lOO) clean surface as the LaB6(100) clean surface supports that both the surface states are mainly originated from the dangling bonds of the boron frameworks. Furthermore, this statement is also suggested by the fact that the surface-state peak observed at room temperature on the LaB,(lOO) surface undergoes no essential change even at -1400°C in spite of the vigorous thermal motion of the surface lanthanum ions, which has been indicated by LEED [ Ill. The sticking probability which is the differential of the oxygen peak intensity with respect to oxygen exposure and is derived from fig. 3, is shown in fig. 4 as a function of the amount of oxygen on the surface. It is interesting that the sticking probability increases up to the maximum value at the exposure of -1.4 L where the
196
R. Nishitani et al. / Oxygen adsorption on LaBg(100)
surface-state peak disappears. As far as we know, the sticking probability of gases on a metal surface has a large value at the initial stage of the adsorption and it decreases as the adsorption proceeds. While LaB6 is a metallic boride, the covalent bonding plays an important role in explaining the many properties of LaB6 as mentioned before. In this sense, it is not surprising that the kinetics of adsorption on the LaBh(100) surface differs from that on a metal surface. Nevertheless, it must be noted that the initial sticking probability of oxygen on the LaB,(lOO) surface has a low value while those for the LaBr, (1 10) and (111) clean surface have a large value which will be described elsewhere [12]. Oxygen adsorbed at first on LaB,(lOO) may be at a precursor state which corresponds to the peak at -6.6 eV below L,‘r:. With increasing the oxygen exposure, oxygen gas transfers the energy to oxygen adsorbed at first, and the accomodation coefficient increases. So, the sticking probability may increase with increasing oxygen exposure. The fact that the sticking probability decreases rapidly after the disappearance of the surface-state peak indicates the completion of the fast oxygen-adsorption on the surface boron atoms. If we assume that oxygen is adsorbed at all sites of surface boron atoms dissociatively (the number of surface boron atoms, N= 5.8 X 1014/cn~2) at the oxygen exposure of -1.4 L where the sticking probability has a maximum, the maximum value of the sticking probability is -1 .O. If we define that the coverage, 0, is 0.5 at this oxygen exposure, the saturated coverage is unity from fig. 3. Therefore it is suggested that oxygen is adsorbed mainly on the boron atoms at first, and subsequently mainly on the lanthanum atoms. On the contrary, Goldstein and Szostak [ 131 suggested that the oxygen atoms were located preferentially over the boron octahedra and bonded to four lanthanum atoms in a terminal plane. This model was derived because they used the metallic radius of 1.88 A as the size of the lanthanum atom. This radius is, however, unreasonably large. The band structure calculations of metal hexaborides by Longuet--Higgins and Loberts [14], and Yamazaki [ 151, using the tight-binding approaximation, showed that the valence bands contain 20 electrons per unit cell, leading to the strong covalent bonding for the simple cubic lattice of the B6 octahedra, The six B atoms in the unit cell provide only 18 electrons, thus, the remaining two electrons must be provided by the La atoms, resulting in the ionic nature between the lanthanum and boron. An analysis on temperature dependence of the electrical resistivity supported this ionic nature, since the electrons are scattered not only by acoustic phonons but also by polar optical phonons [ 161. According to a recent investigation of the experimental Fermi surface [8], LaB6 has one conduction electron per unit cell. Furthermore, LaB6 can be considered diamagnetic [ 171. These facts show that the La atoms in LaB6 are trivalent. Therefore, the size of the lanthanum atoms should be referred to the trivalent ionic radius of 1.14 A. In this case, the oxygen atoms can be bonded to boron atoms directly without bonding to lanthanum atoms.
197
3.3. Workfunc tion A change in the work function due to oxygen adsorption is plotted in fig. 5 as a function of the intensity of the oxygen-derived peaks. The change in the work function, A@,,increases linearly with increasing surface oxygen and changes its slope at A@= -1.0 eV, i.e. at the oxygen exposure of -1.4 L where the surface-state peak disappears. The linearity of the change in # with increasing oxygen exposure indicates that the change in the work function of the LaB6(100) surface is due to the change in the density of dipole moments. The change in the slope at -1.4 L, i.e. B = 0.5 indicates that another adsorption process arises at this oxygen exposure. An estimate of’ the dipole moment in each process can be made with the use of the equation, A$ = 4nnp A0,
(1)
where n is the number of adsorption site, n = 2 X 5.8 X 10’4/cm2 (= 2/a2, a = 4.16 a, lattice constant), A@ the change in coverage, and /J the dipole moment. For low coverage, if we take Ac#= 0.76 eV and A@= 0.32, then ,U= 0.54 D (1 D = 1O-‘* esu cm). This calculated value is nearly in agreement with that reported in ref. [13]. For higher coverage than B = 0.5, if we take A# = 0.65 eV and A@= 0.5, then p = 0.25 D. 3.4. Energy levels of adsorbed oxygen It can be seen from fig. 2 that the oxygen-derived peak shifts to the low binding side as oxygen exposure proceeds. A change in the maximum position of the oxy-
_ 1.6 % - 1.4 % %’12 z $ 1.0 U $0.8 F ” “, 0.6 CL z 0.4 P
0.2 0
0
5 OXYGEN
Fig. 5. Change
UPS
10 PEAK
INTENSITY
15
(arb. units)
in work function versus the intensity of the oxygen-derived UPS peak.
198
R. Nishitani
et al. /Oxygen
adsorption
on LaB,/lOO)
gen-derived peak is shown in fig. 6 against the work function. Below the work function of -2.7 eV, the peak positions are independent of the coverage. This may be due to the nonexistence of the second oxygen peak for these low exposures. In the range between @J= 2.7 eV and (p = 3.1 eV, there is a marked change in the peak position. It may be seen in fig. 2 that the peak in this range consists of two overlapped peaks whose binding energies are -6.6 eV and -6.0 eV below EF, respectively. So, the oxygen adsorption on boron sites is prominant, while the adsorption on another site may be mixed. For higher work function than 3.4 eV, i.e. at an exposure of 1.4 L, the peak position shifts to the low binding energy side linearly with the increase in the work function. The peak in this region has narrow FWHM and may consist of a unique state. The binding energy of orbitals within an adsorbed atom may be expressed by the equation [ 181, BEad = BE, - (b + fD + ABE,
(2)
where BEad is the binding energy of an adsorbate level with respect to EF, BE, is the binding energy of the equivalent level in the gas phase with respect to the vacuum level, # is the work function, ABE is the chemical shift, D is the total dipole barrier at the surface, and f is the fraction of the dipole potential which has been penetrated by the atom. The energy diagram for a molecule adsorbed at the surface of a metal crystal is depicted in fig. 7. The fact that the peak position varies linearly with $I above @= 3.4 eV could mean that the valence level of the adsorbate is not strongly fixed to the metal valence levels and the potential at the adsorption site varies with I# or coverage. That is, if the value off is small, BE,d increases
I
I
I
I
1
2.5
3.0
3.5
4.0
WORK
FUNGTION
( eV
)
Fig. 6. Peak position as a function of the work function for the UPS peak derived by oxygen adsorption.
199
Electrostatic
Fig. 7. The energy level diagram for an adsorbate on a metal surface.
linearly with (a according to eq. (2). Therefore, it may be said that the position of the adsorbed oxygen above the oxygen exposure of -1.4 L is far from the surface, compared with that below the exposure of -1.4 L. This is consistent with the statement that the oxygen is adsorbed mainly on a lanthanum atom above the exposure of -1.4 L.
4. Summary It has been demonstrated that there are some correlations among the kinetics of adsorption, the disappearance of the surface state, the change in the work function and the states of adsorbed’oxygen. The surface states, which are originated mainly from the dangling bonds of boron atoms, disappear at the oxygen exposure of -1.4 L where an oxygen sticking-coefficient has a maximum value, -1 .O. The dipole moment which is produced by oxygen adsorption has depended on the coverage; the value of the dipole moment is 0.55 or 0.25 D, according as 8 < 0.5 or 3 > 0.5. We have suggested that there are more than two adsorption states on the LaB&lOO) surface. The experimental results described in section 3 can be interpreted most reasonably by thinking that the oxygen is adsorbed mainly on the boron atom at first, and subsequently mainly on the lanthanum atom.
Acknowledgements One of the authers (R.N.) wishes to thank Dr. C. Oshima and Dr. Y. Ishizawa at NIRIM for heIpfu1 and stimulating discussions.
200
R. Nishitani
et al. /Oxygen
adsorption
on LaBe(lO0)
References [l]
[2] [3] [4] [5] [6] [7] [8] [9]
[lo] [ 1 l] [ 121 [13] [14] [15] [ 161 [17] [ 181
A.N. Broers and M. Hatzakis, Sci. Am. 227 (1972) 34; H. Ahmed and A.N. Broers, J. Appl. Phys. 43 (1972) 2185; R.T. Miller, Solid State Technol. 7 (1973) 25; J.D. Verhoven and E.D. Gibson, J. Phys. E9 (1976) 65; R. Shimizu, Y. Kataoka, S. Kawai and T. Tanaka, Japan. J. Appl. Phys. (1975) 1089; R. Shimizu, T. Shinike, S. lchimura, S. Kawai and T. Tanaka, J. Vacuum Sci. Technol. 15 (1978) 922. M. Aono, T. Tanaka, E. Bannai, C. Oshima and S. Kawai, Phys. Rev. B16 (1977) 3489. M. Aono, C. Oshima, T. Tanaka, E. Bannai and S. Kawai, J. Appl. Phys. 49 (1978) 2761. L. Pauling, The Nature of the Chemical Bond, 3rd ed. (Cornell Univ. Press, Ithaca, 1960). M. Aono, T. Tanaka, E. Bannai and S. Kawai, Appl. Phys. Letters 31 (1977) 323. K.E. Spear, Phase Diagram, Vol. IV (Academic Press, 1976) p. 91. M. Ishii, M. Aono, S. Muranaka and S. Kawai, Solid State Commun. 20 (1976) 437. Y. Ishizawa, T. Tanaka, E. Bannai and S. Kawai, J. Phys. Sot. Japan 42 (1977) 112. A. Hasegawa and A. Yanase, J. Phys. F (Metal Phys.) 7 (1977) 1245. Present authors, in preparation. M. Aono, R. Nishitani, C. Oshima, T. Tanaka, E. Bannai and S. Kawai, J. Appl. Phys. 50 (1979) 4802. Present authors, in preparation. B. Goldstein and D.J. Szostak, Surface Sci. 74 (1978)461. H.C. Longuet-Higgins and M. de V. Roverts, Proc. Roy. Sot. (London) 224 (1954) 336. M. Yamazaki, J. Phys. Sot. Japan 12 (1957) 1. T. Tanaka, T. Akahane, E. Bannai, S. Kawai, N. Tsuda and Y. Ishizawa, J. Phys. C9 (1976) 1235. J.R. Shannon and M.J. Sienko, Inorg. Chem. 11 (1972) 904. C.R. Brundle, in: Electronic Structure and Reactivity of Metal Surfaces, Eds. E.G. Derouane and A.A. Lucas (Plenum, 1976).