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LEED studies of argon crystals on niobium (100) surface
SURFACE SCIENCE 23 (1970) 448-452 8 North-Holland Publishing Co.
LEED STUDIES OF ARGON CRYSTALS ON NIOBIUM (100) SURFACE * Received 27 July 1970
We report here some initial LEED studies of argon crystals grown on the (100) surface of niobium. The Nb crystal was cooled in a cryostat designed to do LEED work at cryogenic temperatures, and the argon was then condensed onto the surface. These initial studies show that the Ar crystal grows in register with the Nb substrate. In the apparatus used for these experiments, the sample can be heated above 1500 “C, and can then be cooled to cryogenic temperatures. The high temperatures necessary for cleaning the surfaces are achieved by ohmic heating of the sample. With our apparatus, we can pass 50 A through the sample. A system of ballasting has been developed so that the current leads are not the major heat leak at low temperatures. The primary heat leak is radiation from the LEED optics which is at room temperature. This is accentuated by large apertures in the 4.2 and 78 “K shields, which are necessary to allow the diffracted electrons to reach the screen. Another exceedingly d&cult problem is thermometry. It would, of course, be desirable to have a thermometer on the sample. However, because of the high outgassing temperatures necessary to clean the surface, it is impossible to place a thermometer in contact with the sample that will survive the high temperatures and still be a sensitive device at cryogenic temperatures. At present, we have used the resistance of the sample itself as a thermometer, but this is extremely insensitive below about 30”K, and so we can only give rather crude estimates of the sample temperature. The Ar that we used was commercial grade and had a nominal purity of 99.99%. The crystals were grown by condensing the Ar onto the cold substrate [Nb( loo)]. During crystal growth, the pressure was kept at about 1 x 10m6 torr of argon for a minute, and during the experiments, the pressure in the chamber was about 2 x 10-l’ torr. From our earlier studies of Nb, we know that the substrate was not perfectly clean since this requites heating at -2000 “C. Hence, even though we did not see any additional diffraction features due to oxides, we think the surface was still slightly contaminated, and we cannot exclude the unlikely possibility * This work was performed under the auspices of the U.S. Atomic Energy Commission. 448
LEED
STUDIES
OF ARGON
CRYSTALS
ON NIOBIUM
(1 c@)
SURFACE
449
that the argon structure is somewhat dependent on these impurities. The major result of this work is the observation of a LEED pattern from a rare gas crystal. With present experimental conditions the argon crystals that we studied could only be maintained for a short time in the presence of the electron beam. In figs. la and lb, we show photographs of a LEED pattern of Ar on the Nb(lO0) face. The two parts of the figure show the pattern with Ar on the surface taken immediately when the electron beam was turned on (fig. la) and that after a short exposure to the electron beam (fig. lb). After the argon pattern had disappeared, it could be seen again by
(4
(b)
Fig. 1. Diffraction pattern observed for an electron energy of 119 eV. (a) Immediately after switching the electron beam to a new position on the crystal, extra spots due to Ar are visible. (b) About 1 min later, extra spots have disappeared leaving the clean Nb pattern.
moving the beam to a different part of the crystal with a small magnet. Eventually, the argon was exhausted by this process. The rate at which the pattern disappeared after exposure to the electron beam varied with the electron energy, and in fig. 2 we show some photometer curves of how the intensity of a diffraction spot in the argon pattern falls off at different electron energies. At present, it is not clear whether the argon was removed due to a heating effect, i.e., the electron energy was transferred to the lattice, or whether the Ar was removed via a collision process. At 30”K, the vapor pressure of Ar is about 5 x IO-’ torr (ref. 1) and at this pressure, the Ar should be coming off the surface at about a monolayer in a few seconds. Hence, the entire crystal would evaporate in a few minutes. This was not the case, as crystals have been maintained at least an hour in the absence of the electron beam, indicating that our substrate temperature must have been lower. At about 28°K the vapor pressure is about 3 x lOma torr, and we suspect that to have kept the crystal for any length of time, the pressure must
450
J. M. DICKEY,
Ii. H.FARRELL
AND
M. STRONGIN
have been below this value. In general, a temperature change of about 2°K in this regime raises the vapor pressure by an order of magnitude. Hence, if our ambient temperature was from about 26 to 38°K and the electron beam raised the temperature of the argon crystal several degrees, the vapor pressure would have been high enough to cause rapid sublimation of the
TIME
Fig. 2.
(SECONDS)
Intensity of Ar diffraction spot measured on a photometer. The electron beam boils off the Ar atoms so the spot rapidly fades away.
entire crystal. A new sample mounting which should be capable of providing somewhat lower temperatures has been constructed and we hope to provide more information on this problem of whether the Ar is removed from the surface by a temperature rise, or by direct collisions with the incoming electrons. It is worth mentioning that transmission experiments on polycrystalline films at higher electron energies where the substrate was held at a lower temperature did not indicate any evaporation of the argon filmz). In the above discussion, we have assumed that the Ar was evaporated or sputtered from the surface rather than having formed an amorphous structure. Of course, at some stage while the pattern was disappearing, the Ar crystal may have been liquid-like, but we think the Ar was finally removed from the surface. This seems reasonable since the Nb spots grow sharper as the Ar pattern disappeared. Also, after many switchings of the electron beam, the Ar pattern could no longer be found on the substrate, whereas if the Ar was only becoming liquid-like, we would expect it to recrystallize when the beam
LEED
STUDIES
OF ARGON
CRYSTALS
ON NIOBIUM
(100)
451
SURFACti
was removed. No Ar pattern was observed when the substrate was held at liquid nitrogen temperature. Similar experiments with nitrogen in the 20 to 30°K region gave a diffuse pattern. Also, neon did not form observable crystallites at these temperatures when the substrate was exposed to I x 10e6 torr of Ne for 1 min. This should be expected because of the high vapor pressure of neon at these temperatures. Fig. 1 shows the observed diffraction pattern for Nb and for Ar on Nb. Note that the crystallographic directions for the Ar are aligned relative to those for the Nb substrate and that the (no) [and (On)] diffraction spots for the two crystals coincide. This suggests that the Ar crystal grows in register, in a manner to be discussed below, with the Nb substrate. The diffraction patterns, in general, show a twelvefold symmetry. At 20”K, the lattice spacing of fee Ar is 5.43 A (ref. 3). The corresponding distance between the close packed rows in the (111) plane is 3.32 A and this is very close to the lattice parameter of bee Nb, which is 3.29 A at 20”Ks). Thus, a (111) plane of Ar atoms may be stacked on a (100) Nb face with the closed packed (110) rows lying in the troughs formed by the rows of Nb atoms, as shown in fig. 3a. From the symmetry, two configurations at right angles to each other are possible. Nucleation of the Ar at several points would result in both orientations being present. Normal Ar has a face centered cubic structure, but there is some experimental evidence for the existence of a metastable hexagonal close packed structure4). The (0001) face of the hcp lattice has true hexagonal symmetry while the (111) face of D
.
.X.
.X. X
.*. .
X
.
0x0 0x0
X
a
a >
.x. X
n,, c/l _
a
. .
. a
I
X
,D-.y
a D
*I
aDI
X .X.
.
. X
.
ax*
x
D
.
X
l X0
X .
X l
x. a
Fig. 3. fraction
D
(a) Suggested structure of the first Ar layer (x) on the Nb substrate (a). (b) Difpatterns which would result from (a): (0) Nb spot; (4) Nb and Ar spot; (v, A) Ar spots.
452
J. M. DICKEY,
I-I. H. FARRELL
AND
M. STRONGIN
the fee lattice has trigonal symmetry. However, as there are two ways of building up the fee lattice on the first layer, the existence of domains would give rise to pseudohexagonal symmetry. Either fee or hcp can give a pseudo twelvefold symmetry when the Crst layer is split into islands oriented with respect to the Nb substrate in the two ways described previously. Fig. 3b shows schematically the diffraction pattern that would result from the structure suggested here. Note that it is identical with the observed diffraction pattern of Ar (fig. la). It is not possible to distinguish between the hcp and fee structures solely on the basis of the symmetry of the LEED pattern unless the structure is fee and a large enough single crystal could be grown to exhibit trigonal symmetry. J. M. DICKEY, H. H. FARRELL and M. STRONGIN Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973, U.S.A. References 1) For a review of the properties of the rare gases, see G. L. Pollack, Rev. Mod. Phys. 36 (1964) 748. 2) S. I. Kovalenko and N. N. Bagrov, Fiz. Tverd. Tela 11(1969) 2724 [Soviet Phys.-Solid State 11 (1970) 22071. 3) C. Kittel, Introduction fo Solid State Physics (Wiley, New York, 1963). 4) L. Meyer, C. S. Barrett and P. Haasen, J. Chem. Phys. 40 (1964) 2744.
LEED STUDIES OF ARGON CRYSTALS ON NIOBIUM (100) SURFACE * Received 27 July 1970
We report here some initial LEED studies of argon crystals grown on the (100) surface of niobium. The Nb crystal was cooled in a cryostat designed to do LEED work at cryogenic temperatures, and the argon was then condensed onto the surface. These initial studies show that the Ar crystal grows in register with the Nb substrate. In the apparatus used for these experiments, the sample can be heated above 1500 “C, and can then be cooled to cryogenic temperatures. The high temperatures necessary for cleaning the surfaces are achieved by ohmic heating of the sample. With our apparatus, we can pass 50 A through the sample. A system of ballasting has been developed so that the current leads are not the major heat leak at low temperatures. The primary heat leak is radiation from the LEED optics which is at room temperature. This is accentuated by large apertures in the 4.2 and 78 “K shields, which are necessary to allow the diffracted electrons to reach the screen. Another exceedingly d&cult problem is thermometry. It would, of course, be desirable to have a thermometer on the sample. However, because of the high outgassing temperatures necessary to clean the surface, it is impossible to place a thermometer in contact with the sample that will survive the high temperatures and still be a sensitive device at cryogenic temperatures. At present, we have used the resistance of the sample itself as a thermometer, but this is extremely insensitive below about 30”K, and so we can only give rather crude estimates of the sample temperature. The Ar that we used was commercial grade and had a nominal purity of 99.99%. The crystals were grown by condensing the Ar onto the cold substrate [Nb( loo)]. During crystal growth, the pressure was kept at about 1 x 10m6 torr of argon for a minute, and during the experiments, the pressure in the chamber was about 2 x 10-l’ torr. From our earlier studies of Nb, we know that the substrate was not perfectly clean since this requites heating at -2000 “C. Hence, even though we did not see any additional diffraction features due to oxides, we think the surface was still slightly contaminated, and we cannot exclude the unlikely possibility * This work was performed under the auspices of the U.S. Atomic Energy Commission. 448
LEED
STUDIES
OF ARGON
CRYSTALS
ON NIOBIUM
(1 c@)
SURFACE
449
that the argon structure is somewhat dependent on these impurities. The major result of this work is the observation of a LEED pattern from a rare gas crystal. With present experimental conditions the argon crystals that we studied could only be maintained for a short time in the presence of the electron beam. In figs. la and lb, we show photographs of a LEED pattern of Ar on the Nb(lO0) face. The two parts of the figure show the pattern with Ar on the surface taken immediately when the electron beam was turned on (fig. la) and that after a short exposure to the electron beam (fig. lb). After the argon pattern had disappeared, it could be seen again by
(4
(b)
Fig. 1. Diffraction pattern observed for an electron energy of 119 eV. (a) Immediately after switching the electron beam to a new position on the crystal, extra spots due to Ar are visible. (b) About 1 min later, extra spots have disappeared leaving the clean Nb pattern.
moving the beam to a different part of the crystal with a small magnet. Eventually, the argon was exhausted by this process. The rate at which the pattern disappeared after exposure to the electron beam varied with the electron energy, and in fig. 2 we show some photometer curves of how the intensity of a diffraction spot in the argon pattern falls off at different electron energies. At present, it is not clear whether the argon was removed due to a heating effect, i.e., the electron energy was transferred to the lattice, or whether the Ar was removed via a collision process. At 30”K, the vapor pressure of Ar is about 5 x IO-’ torr (ref. 1) and at this pressure, the Ar should be coming off the surface at about a monolayer in a few seconds. Hence, the entire crystal would evaporate in a few minutes. This was not the case, as crystals have been maintained at least an hour in the absence of the electron beam, indicating that our substrate temperature must have been lower. At about 28°K the vapor pressure is about 3 x lOma torr, and we suspect that to have kept the crystal for any length of time, the pressure must
450
J. M. DICKEY,
Ii. H.FARRELL
AND
M. STRONGIN
have been below this value. In general, a temperature change of about 2°K in this regime raises the vapor pressure by an order of magnitude. Hence, if our ambient temperature was from about 26 to 38°K and the electron beam raised the temperature of the argon crystal several degrees, the vapor pressure would have been high enough to cause rapid sublimation of the
TIME
Fig. 2.
(SECONDS)
Intensity of Ar diffraction spot measured on a photometer. The electron beam boils off the Ar atoms so the spot rapidly fades away.
entire crystal. A new sample mounting which should be capable of providing somewhat lower temperatures has been constructed and we hope to provide more information on this problem of whether the Ar is removed from the surface by a temperature rise, or by direct collisions with the incoming electrons. It is worth mentioning that transmission experiments on polycrystalline films at higher electron energies where the substrate was held at a lower temperature did not indicate any evaporation of the argon filmz). In the above discussion, we have assumed that the Ar was evaporated or sputtered from the surface rather than having formed an amorphous structure. Of course, at some stage while the pattern was disappearing, the Ar crystal may have been liquid-like, but we think the Ar was finally removed from the surface. This seems reasonable since the Nb spots grow sharper as the Ar pattern disappeared. Also, after many switchings of the electron beam, the Ar pattern could no longer be found on the substrate, whereas if the Ar was only becoming liquid-like, we would expect it to recrystallize when the beam
LEED
STUDIES
OF ARGON
CRYSTALS
ON NIOBIUM
(100)
451
SURFACti
was removed. No Ar pattern was observed when the substrate was held at liquid nitrogen temperature. Similar experiments with nitrogen in the 20 to 30°K region gave a diffuse pattern. Also, neon did not form observable crystallites at these temperatures when the substrate was exposed to I x 10e6 torr of Ne for 1 min. This should be expected because of the high vapor pressure of neon at these temperatures. Fig. 1 shows the observed diffraction pattern for Nb and for Ar on Nb. Note that the crystallographic directions for the Ar are aligned relative to those for the Nb substrate and that the (no) [and (On)] diffraction spots for the two crystals coincide. This suggests that the Ar crystal grows in register, in a manner to be discussed below, with the Nb substrate. The diffraction patterns, in general, show a twelvefold symmetry. At 20”K, the lattice spacing of fee Ar is 5.43 A (ref. 3). The corresponding distance between the close packed rows in the (111) plane is 3.32 A and this is very close to the lattice parameter of bee Nb, which is 3.29 A at 20”Ks). Thus, a (111) plane of Ar atoms may be stacked on a (100) Nb face with the closed packed (110) rows lying in the troughs formed by the rows of Nb atoms, as shown in fig. 3a. From the symmetry, two configurations at right angles to each other are possible. Nucleation of the Ar at several points would result in both orientations being present. Normal Ar has a face centered cubic structure, but there is some experimental evidence for the existence of a metastable hexagonal close packed structure4). The (0001) face of the hcp lattice has true hexagonal symmetry while the (111) face of D
.
.X.
.X. X
.*. .
X
.
0x0 0x0
X
a
a >
.x. X
n,, c/l _
a
. .
. a
I
X
,D-.y
a D
*I
aDI
X .X.
.
. X
.
ax*
x
D
.
X
l X0
X .
X l
x. a
Fig. 3. fraction
D
(a) Suggested structure of the first Ar layer (x) on the Nb substrate (a). (b) Difpatterns which would result from (a): (0) Nb spot; (4) Nb and Ar spot; (v, A) Ar spots.
452
J. M. DICKEY,
I-I. H. FARRELL
AND
M. STRONGIN
the fee lattice has trigonal symmetry. However, as there are two ways of building up the fee lattice on the first layer, the existence of domains would give rise to pseudohexagonal symmetry. Either fee or hcp can give a pseudo twelvefold symmetry when the Crst layer is split into islands oriented with respect to the Nb substrate in the two ways described previously. Fig. 3b shows schematically the diffraction pattern that would result from the structure suggested here. Note that it is identical with the observed diffraction pattern of Ar (fig. la). It is not possible to distinguish between the hcp and fee structures solely on the basis of the symmetry of the LEED pattern unless the structure is fee and a large enough single crystal could be grown to exhibit trigonal symmetry. J. M. DICKEY, H. H. FARRELL and M. STRONGIN Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973, U.S.A. References 1) For a review of the properties of the rare gases, see G. L. Pollack, Rev. Mod. Phys. 36 (1964) 748. 2) S. I. Kovalenko and N. N. Bagrov, Fiz. Tverd. Tela 11(1969) 2724 [Soviet Phys.-Solid State 11 (1970) 22071. 3) C. Kittel, Introduction fo Solid State Physics (Wiley, New York, 1963). 4) L. Meyer, C. S. Barrett and P. Haasen, J. Chem. Phys. 40 (1964) 2744.