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Low energy electron diffraction study of the adsorption of oxygen on a (100) Tungsten surface
Low Energy
Electron D@i-action
Study of the Adsorption of Oxygen on a (100) Tungsten Surface* by
J.
ANDERSON
AND
W.
El. DANFOR+
The Barb1 Research Foundation of The Franklin Institute Swarthmore, Pentasylvaniu Low energy electron digraction studies of a (100) face of a tungsten crystal hea;tedto about 1860°K in oxygen at a pressure of about IO-’ torr, show that the adsorption proceeds through three distinct stages, each accompanied by some surface reconstruction. Diflraction patterns are interpreted to indicate that the first stage results in a surface structure containing one-half a monolayer of oxygen and the second stage in a structure containing three-quarters of a monolayer of oxygen. Di$raction features from the terminal structure strongly suggest a model wherein the surface ha8 become faceted, the facets being planes of type (110). Beams from the individual (110) planes can be observed and, at certain electron voltages, all the (110) planes cooperate to produce a sharp bright beam. This latter beam is shown to be equivalent to a Bragg reflection from the bulk crystal. On this model we calculate the lattice parameter of the crystal to be 3306 =t 0.01 A, close to but slightly larger than that of bulk tungsten. The adsorbed oxygen can be removed from the suface and the three structures traversed in the reverse order by heating the crystal to higher temperatures. No observable pressure increase accompanies the removal of the oxygen, for which fact possible explanations are discussed. ABSTRACT:
Introduction
adsorption of oxygen on tungsten has been the subject of many investigations over the past few decades. This system has been frequently selected because of its occurrence in the practical field of thermionic emission and because of its status as representative of the chemisorption of gases on metals. Tungsten has been chosen as a substrate because of its high melting point, its ease of cleaning, and because it is a convenient material for field emission microscope tips. For some earlier work in this field, see (I-5). More recently this system has been studied by flash filament techniques (6, 7, 8), by field emission microscopy (6, 9, lo),by adsorption pumping techniques (7), by adsorption pumping combined with mass spectrometer techniques (1l),and by low energy electron diffraction (12, 13). In explaining and analyzing the results of these (and similar) experiments The
* Tti
work was supported in part by the U. S. Atomic Energy Commission.
160
Low Energy
Electron
Diflraction
Study
investigators have sometimes assumed atomic models of the adsorption process, depicting hypothetical arrangements of substrate and adsorbate atoms. However, low energy electron diffraction (LEED) affords a direct means of inferring such arrangements and mutual positions. The application of LEED to surface studies and the results of recent experiments have been reviewed recently (14,
15, 16).
The present paper treats the adsorption of oxygen on the (100) face of a tungsten crystal heated to about 1250°K. It is found that the surface goes through three different structural stages upon oxygen adsorption and simultaneous heat treatment. The observed diffraction patterns are interpreted to indicate that the first stage corresponds to one-half monolayer oxygen coverage. (A monolayer is herein defined as a density of adsorbate atoms equal to the density of atoms in the idealized substrate surface plane; in this case 1.0 X 1016 per sq. cm. for the (X00) tungsten plane.) The second stage of adsorption results in a structure corresponding to three quarters of a monolayer, and finally the third state results in a structure which is no longer parallel to the (100) surface but is faceted, the facets being planes of type (110). Apparatus
The LEED apparatus used is a Varian instrument, whose design incorporates certain developments accomplished at the Bell Telephone Laboratories. It is of the post-acceleration display type described at length in several papers (17, IS). The vacuum chamber is of stainless steel, pumped by an 80 l/set VacIon pump and the pressure, under most operating conditions, is in the lOWlo torr range. There are provisions for admitting oxygen (from Linde Co., 99.99 per cent pure) through a variable leak valve. Oxygen pressures were inferred from the pump current ; the ion gauge was not operated during periods of oxygen exposure. The tungsten specimen was a ribbon (20 X 2.2 X 0.08) mm, cut from an ingot obtained from the Linde Co., ground thin, and electropolished in NaOH to its final thickness. The major impurity in the crystal as quoted by the manufacturer was MO at 60 parts per million. Others were less than 20 ppm, carbon in particular amounting to 4 ppm. Its orientation was one degree off the (100) plane. The ribbon could be rotated about its long axis and heated by passage of current. Relative intensities of the diffracted electron beams were determined with a Photo Research Corp., “‘Spectra” spot photometer. Experimental
Changes
Results insurfacestructure
Upon
Oxygen
Adsorption
Heating the ribbon to about 2500°K in vacuum cleaned it immediately yielding a sharp diffraction pattern, Fig. 1 (a), of the tungsten (100) surface. This surface is a square array of atoms of spacing 3.16 A. Carbon contamination is to be expected with tungsten but there was no evidence of this of the kind reported by other investigators (12, 13). Exposure to oxygen at room temperature increased the background and the pattern showed some evidence
Vd.
279, No.
3. March
1965
161
J. Anderson
and W. E. Danforth
of furrowing, but no well-ordered structure was found until the ribbon was heated to incandescent temperatures. The following is a description and discussion of the diffraction patterns as the ribbon is heated to a temperature T = 1250°K in oxygen at a pressure In approximately P = 10e7 torr (the ribbon is cooled for LEED observations). two minutes there appears a pattern, as in Fig. 1 (b), showing the normal beams m as well as extra beams which can be indexed n F and F n (n and m integral with m odd). This will be called the first pattern and the diffracting surface the first structure. Two more minutes of exposure produces the second structure, its pattern containing
in addition the 2 y beams with n, m/odd,
Fig. 1 (c).
The appearance of the half-integral beams indicates that the presence of the oxygen has enlarged the unit mesh. A simple and highly probable structure that could yield the first pattern would be a surface with two types of domains, one double double
spaced
spaced
in the x-direction
in the
y-direction
giving
yielding
the 2
the n 7
n beams beams.
and the The
high
other beam
FzQ. 1. (a) Diffraction pattern from clean tungsten (100) surface, 160 volts (some spots hidden by crystal holder). (b> Diffraction pattern from first structure 125 volts. (c) Diffraotion pattern from second structure 79 volta. (d, e) Diffraction pattern from the faceted structure at 101 and 115 volt+ respectively. Approximately same 61m exposure resulta in exaggerated spot size in (d). One of the four equivaIent bright apota in (d) would be indexed (510). Figure (e) shows the splitting of this beam into four diEuse beams. (f) Faceted structure, 82 volts. The ribbon hae been rotated 45” to display diffraction features from an individual (110) plane.
162
Journal of The Franklin
kstkuk
Low Energy
ElecCon
Diflrcsction Study
intensity indicates that tungsten atoms are involved in the scattering, not oxygen only. Accordingly it is assumed that in either type of domain we have rows of tungsten atoms with double the original spacing between rows and that the oxygen atoms occupy the positions half way between. This assumption implies, therefore, a coverage of one half monolayer, 5.0 X 1014 atoms per sq. cm. Similarly we can identify the second structure as double spaced in both directions; the oxygen occupies alternate rows and columns. This would amount to three-quarters of a monolayer of oxygen. Further Ot exposure and heating causes a diffuseness in all beams and their gradual disappearance. In five or ten minutes further change ceases and the resulting pattern indicates that the surface has been faceted, the facets being (1 IO)-type planes. This is herein referred to as the faceted structure, described in the following section. The
Fuceted
Structure
Reports of diffraction features characteristic of planes other than the nominal substrate surface plane have been published by others. MacRae reports the growth of NiO pyramids upon the (111) face of nickel (19). Lander and Morrison (16, 20) have observed the development of etch pits with (111) faces when a silicon surface with one-haJf a monolayer of aluminum wa8 heated to 700°C. Taylor (13) has observed the (111) face of tungsten to form (211) facets when heated in oxygen. In the present case, the faceted structure is characterized by a complete disappearance of the normal tungsten (100) features. Instead there is a fourfold symmetric pattern of relatively weak diffuse beams, Fig. 1 (e), whose behavior indicates that the diffracting surface is not the (100) plane but a faceted structure containing the four crystallographic planes (of type) (1 lo} inclined at 45” to the (100) surface. The pattern of diffuse beams, then, is the superposition of four sets of diffraction beams, one from each (100) plane. With increasing electron voltage, the beams converge, not on the normal 00 position (in the center of the screen), but towards the 00 position of the appropriate (110) plane. Thus, one set of beams moves across the screen from left to right, one set from right to left, one up, and one down. By rotating the sample 45O so that the primary beam is normal to a (110) plane it is possible to see the expected diffraction pattern for normal incidence on a (110) plane, Fig. l(f). The diffuse beams do not move independently, since all beams are diffracted from a single monolithic structure. At certain electron energies beams from all four (110) planes are diffracted in the same direction and produce a single bright sharp beam. With increasing voltage the beam will again split into four diffuse spots. Figures 1 (d) and I (e) show this effect. This is the identical situation reported by Lander and Morrison in the aluminum on silicon-(100) system _ A convenient way of expressing the situation is to make use of the reciprocal lattice and Ewald sphere construction. Let us assume that the surface is simply four (110) planes inclined at 45O to the (100). Figure 2 shows the mutual posi-
Vol. 279. No. 3, March
1965
163
J.
Anderson
and
W.
E.
Danforth
tions of the four planes. The reciprocal lattice for a regular planar array is a series of rods perpendicular to the reciprocal net at the reciprocal net points. The reciprocal unit mesh (non-primitive) for the (110) tungsten plane is a centered rectangle of base vectors ~/CL and G/a, where a. is 3.16 A, the tungsten lattice constant. Figure 2 is in fact a picture of the four reciprocal nets. The intersection of the reciprocal lattice rods with the Ewald sphere defines the diffraction angle of a beam from a single (110) plane; these are the diffuse beams that move across the screen in accordance with the motion of the intersection point as the sphere expands with increasing electron voltage. Because of the mutual arrangement of the reciprocal nets, the reciprocal lattice rods intersect each other, each intersection point being common to four and only four rods, one from each net. F’igure 2 shows two such intersections.
FI :a. 2. The nm rec&mocfbl (110) interee ction pain would be indexed
reciprorcslla*&ice
neta fshowing
some of the reeSpr0callat ,tice rods.
(10 0 0) and
(820)
with
respect
The to the 3-d imensi Ional
When the Ewald sphere touches such a point, all four planes are scattering in phase in the same direction and a sharp spot results. It can be seen then, how this spot will split into four as the sphere expands past that point. There is an interesting observation associated with this phenomenon. On this model, when the four planes are scattering coherently in a bright beam, the coherent area is four times that of a single plane. Therefore, from kinematical considerations, the beam intensity should be sixteen-fold greater and the resolution correspondingly better. The size and brightness of the bright beams of Fig. 1 (d) as compared with the diffuse beams of Fig. 1 (e) are roughly consistent with these deductions. The intensity of the bright beam of Fig. 1 (d) is greater, by at least a factor of two, than four times the intensity of a diffuse beam of Fig. 1 (e). Likewise the spot size is considerably smaller, although this
164
Joumd
of The Franklin bstitute
Electron Diflraction Study
Low Energy
is not evident in the illustration; the extreme film exposure necessary to show the relative beam intensities of Figs. 1 (d) and 1 (e) caused an apparent increase in spot size in the former. The set of rod intersection points is a three-dimensional array of points and this array is just the lattice which is reciprocal to the b.c.c. tungsten lattice; it is a f.c.c. lattice of periodicity Z/a+ Thus the conditions for occurrence of a bright spot happen to be simply those for diffraction from the bulk tungsten. Consideration of this structure and its relation to the Ewald sphere accounts for all the diffraction features seen. In this treatment we take no account of the fact that the surface is surely covered with oxygen. The role of this layer of oxygen with respect to diffraction effects is not apparent. The surface is, of course, not one big pit as the illustration suggests. The elongation of the diffraction spots indicates that the pits are also elongated; the resolution in one direction is greater than in the other, and we suppose that the surface is broken up into grooves several atoms deep and perhaps tens of atoms long. As noted above the occurrence of sharp diffraction beams from the faceted structure can be considered as being equivalent to diffraction from a bulk crystal of the proper orientation. Accordingly, it should be possible unambiguously to identify these features and assign to them three-dimensional diffracted beams within a Miller indices. Of the 21 possible non-equivalent diffraction angle of 50” at electron energies below 400 volts, fourteen were sufficiently well-developed to determine the intensity maxima with precision. For these indexed beams it is possible to write the Bragg equation X =
2d(h, k, Z) sin 8.
For a cubic crystal and for the primary following relations hold :
beam
d = d,/dh2 sin 8 = h/dh2
+ +
normal
h2 + k2 +
(1) to the
(100)
plane the
Z2
(2)
%
(3)
12),
(4)
so that x = 2d,h/(h2 where do is the lattice length X
parameter
A=
F
+
k2 t
of the cubic crystal.
I!
%Land V,,, =
Vt +
For the electron Vc,
where V, is the measured electron voltage at maximum the true voltage of the electron incident on the surface, potential. Combination of these expressions yields V,, =
Vol. 279, No. 3, March
196.5
150.4(h2 +
k2 +
I”)”
wave-
2+
beam intensity, V, is and V, is the contact
V,.
(6)
165
J. Anderson
and W. E. Danforth
A plot of V, vs 150.4 (h2 + k2 + Z”)2/4h2 should be a straight line where the intercept on the V,,, axis is the contact potential and d, is related to the slope by the equation d, = (slope)-*. Figure 3 is such a plot and the best fit as determined by the method of least squares yields d, =
v. =
3.206 4.0 +
+ 0.01 A 1.5 volts.
Although the difference between d, and the tungsten lattice constant is very small it would appear that the difference is real and that a slight dilation of the lattice occurs at the surface. The presence of some unrecognized systematic error is certainly possibIe, however. As for the contact potential between the
400
350
250
200
150
100
50
6
12 150.4
FIG. 3. The fourteen
bright
18 .-@qp2
24
30
36
42
x lo-2
beams from the faceted structure: a plot of the electron the parameter 150.4 (hs + k* + 12)%/4hp.
energy
vs
oxygen covered tungsten and the bariated nickel cathode, the experimental uncertainty is quite large. Nevertheless, four volts would be about right if the work function of the ribbon were 6.6 eV, the value given by Becker and Brandes (6) for an oxygen-covered 110 surface, and the work function of the gun cathode were that of barium, about 2.5 eV.
166
Journa.Iof The PrHnklinInstitute
Low Energy Desorption
Electron Bi$kzction
Study
of Oxygen
After the faceted structure has been developed the process may be reversed by heating the ribbon to higher temperatures in a vacuum. Heating at 1375°K for a few seconds will destroy the faceted structure leaving the second structure, which is stable at that temperature. Heating at 1525°K will produce the first structure in a few seconds while a few minutes at 185O*K is required to bring back the clean tungsten pattern. It would appear that desorption of oxygen is the cause of these transformations. Thus the faceted structure is stable only with an adsorbed layer of oxygen and we suggest that its greater stability relative to that of the (100) is due to the larger quantity of oxygen adsorbed on this hill-and-valley structure thsn is possible on the planar (100) surface. The transformations of structure : faceted + second and second + first can be correlated with data of Becker and Brandes (6). They observed the work function of an oxygen-covered tungsten tip in a field emission microscope as the tip was heated to successively higher temperatures. For the (110) plane there was a sudden decrease in work function as the tip was heated to between 13OOOK and 1400°K. For the (100) plane there was another decrease in the region of 1550°K. They observed no change, however, in the region of 1850°K. The amount of desorbed gas can be calculated from the time integral of the pump current, using the current-pressure calibration and the speed of the pump. Some doubt has been cast on the quantitative accuracy of this method. We note, however, that the tungsten ribbon, if allowed to take up a layer of gas from the ambient at the nominal vacuum, will, when flashed, cause a pulse of pump current which will indicate the desorption of an appropriate quantity of gas (a large fraction of a monolayer). When the ribbon in any of the three stages of oxygen adsorption is flashed, less than 1 per cent of a monolayer is observed to be desorbed, That oxygen is actually on the surface seems inescapable, for none of the three structures appear in the absence of oxygen. We have also noticed that none of the structures will allow the adsorption of the residual gases, and such immunity would be hard to understand if the surface was not covered with oxygen. Experiments on the desorption of oxygen from tungsten have been made by several investigators. Becker and Brandes (6), and Eisinger (7) report bursts of pressure on flashing which they attribute to Oz. Schlier (S), analyzing the desorption products with an omegatron, reports no O2 produced but observes the generation of CO. He believes that CO is responsible for the pressure bursts observed in such systems, the carbon originating as an impurity in the tungsten. Results similar to ours for different tungsten surfaces have been reported by others (12, 13). The question of the desorption mechanism has been the subject of considerable discussion and it would appear that the matter is still not settled. The most prevalent viewpoint is that it leaves the surface as a tungsten oxide which, condensing on the walls, is not recorded by any pressure measuring device. We would suggest an alternative explanation whereby the oxygen is desorbed as atoms. That oxygen can desorb as 0 has been established experimentally by Moore and Unterwald (21), although they give no
Vol. 279, No.
3, March 1965
167
.I. Aadersom
aad
W. E. Danforth
quantitative data. Atomic oxygen, being very active chemically, could then combine with the chamber walls thereby not yielding any observabIe pressure increase. Acknowledgment For many helpful discussions concerning this experiment the authors are grateful Elmgren of Swarthmore College, L. H. Germer and J. W. May of Cornell University, Lander of The Bell Telephone Laboratories and P. J. Estrup of the Bartol laboratory.
to J. J. J.
References (1) K. H. Kingdon, (2) (3) (4) (5)
(6) (7) (8)
(9) (10) (11) (12) (13)
(14) (15) (16) (17) (18) (19) (20) (21)
“Electron Emission from Adsorbed Films on Tungsten,” Phys. Rev. Vol. 24, p. 510, 1924. I. Langmuir and D. S. Villars, “Oxygen Films on Tungsten,” .i. Amer. Chem. Sot. Vol. 53, p. 488, 1931. J. A. Becker, “Use of Thermionics in the Study of Adsorption of Vapours and Gases,” Tram. Farathy. Sot. Vol. 28, p. 148, 1932. J. K. Roberts, “Some Properties of Adsorbed Films of Oxygen on Tungsten,” Proc. Roy. Sot. Vol. A152, p. 464, 1935. J. L. Marrison and J. K. Roberts, “New Methods for Studying Adsorption of Gases at Very Low Pressures and Properties of Adsorbed Films of Oxygen on T rgsten,” Proc. Roy. Sot. Vol. A173, p. 1, 1939. J. A. Becker and R. G. Brandes, “On the Adsorption of Oxygen on Tungsten as Revealed in the Field Emission Microscope,” J. Chem. Phys. Vol. 23, p. 1323, 1955. J. Eisinger, “Adsorption of Oxygen on Tungsten,” J. Chem. Phys. Vol. 30, p. 412, 1959. R. E. S&her, “Adsorption of Oxygen and Carbon Monoxide on Tungsten,” J. Appl. Phys. Vol. 29, p. 1162, 1958. and Diffusion of Oxygen on Tungsten,” Robert Gomer and J. K. Hulm, “Adsorption J. Chew. Phys. Vol. 27, p. 1363, 1957. of Oxygen on Ordered Tungsten Surfaces,” T. H. George and P. M. Stier, “Chemisorption J. Chem. Phys. Vol. 37, p_ 1935, 1962. J. A. Becker, E. J. Becker, and R. G. Brandes, “Reactions of Oxygen with Pure Tungsten and Tungsten Containing Carbon,” J. Appl. Phys. Vol. 32, p. 411, 1961. J. W. May and L. H. Germer, to be published. Study of the Structural Effect of N. J. Taylor, “A Low Energy Electron Diffraction Inter. Conf. Physics and Chemistry Oxygen on the (111) Face of a Tungsten Crystal,” of Solid Surfaces, Brown Univ., June 1964, Surface Science Vol. 2, p. 544, 1964. “Low Energy Electron Diffraction,” Science Vol. 139, p. 379, 1963. A. U. MacRae, and Ordered Surface Structures,” Surface Science Vol. 1, J. J. Lander, “Chemisorption p. 125, 1964. to J. J. Lander, “Low Energy Electron Diffraction and Surface Structural Chemistry,” be published in Recent Progress in Solid State Chemistry, editor H. Reiss. E. D. Schiebner, L. H. Germer, and C. D. Hartman, “Apparatus for Direct Observation Rsv. Sci. Xmt. Vol. 31, p. 112, 1960. of Low-Energy Electron Diffraction Patterns,” “Improved Low Energy Electron Diffraction ApL. H. Germer and C. D. Hartman, paratus,” Rev. Sci. Inst. Vol. 31, p. 784, 1960. “The Epitaxial Growth of NiO on a (111) Nickel Surface,” AppE. Phys. A. U. MacRae, Letters Vol. 2, p. 88, 1963. J. J. Lander and J. Morrison, to be published. “Adsorption-Desorption of Hydrogen on Tungsten G. E. Moore and F. C. Unterwald, and Molybdenum,” J. Chem. Phys. Vol. 40, p- 2626, 1964.
168
Journal of The Franklin Institute
Electron D@i-action
Study of the Adsorption of Oxygen on a (100) Tungsten Surface* by
J.
ANDERSON
AND
W.
El. DANFOR+
The Barb1 Research Foundation of The Franklin Institute Swarthmore, Pentasylvaniu Low energy electron digraction studies of a (100) face of a tungsten crystal hea;tedto about 1860°K in oxygen at a pressure of about IO-’ torr, show that the adsorption proceeds through three distinct stages, each accompanied by some surface reconstruction. Diflraction patterns are interpreted to indicate that the first stage results in a surface structure containing one-half a monolayer of oxygen and the second stage in a structure containing three-quarters of a monolayer of oxygen. Di$raction features from the terminal structure strongly suggest a model wherein the surface ha8 become faceted, the facets being planes of type (110). Beams from the individual (110) planes can be observed and, at certain electron voltages, all the (110) planes cooperate to produce a sharp bright beam. This latter beam is shown to be equivalent to a Bragg reflection from the bulk crystal. On this model we calculate the lattice parameter of the crystal to be 3306 =t 0.01 A, close to but slightly larger than that of bulk tungsten. The adsorbed oxygen can be removed from the suface and the three structures traversed in the reverse order by heating the crystal to higher temperatures. No observable pressure increase accompanies the removal of the oxygen, for which fact possible explanations are discussed. ABSTRACT:
Introduction
adsorption of oxygen on tungsten has been the subject of many investigations over the past few decades. This system has been frequently selected because of its occurrence in the practical field of thermionic emission and because of its status as representative of the chemisorption of gases on metals. Tungsten has been chosen as a substrate because of its high melting point, its ease of cleaning, and because it is a convenient material for field emission microscope tips. For some earlier work in this field, see (I-5). More recently this system has been studied by flash filament techniques (6, 7, 8), by field emission microscopy (6, 9, lo),by adsorption pumping techniques (7), by adsorption pumping combined with mass spectrometer techniques (1l),and by low energy electron diffraction (12, 13). In explaining and analyzing the results of these (and similar) experiments The
* Tti
work was supported in part by the U. S. Atomic Energy Commission.
160
Low Energy
Electron
Diflraction
Study
investigators have sometimes assumed atomic models of the adsorption process, depicting hypothetical arrangements of substrate and adsorbate atoms. However, low energy electron diffraction (LEED) affords a direct means of inferring such arrangements and mutual positions. The application of LEED to surface studies and the results of recent experiments have been reviewed recently (14,
15, 16).
The present paper treats the adsorption of oxygen on the (100) face of a tungsten crystal heated to about 1250°K. It is found that the surface goes through three different structural stages upon oxygen adsorption and simultaneous heat treatment. The observed diffraction patterns are interpreted to indicate that the first stage corresponds to one-half monolayer oxygen coverage. (A monolayer is herein defined as a density of adsorbate atoms equal to the density of atoms in the idealized substrate surface plane; in this case 1.0 X 1016 per sq. cm. for the (X00) tungsten plane.) The second stage of adsorption results in a structure corresponding to three quarters of a monolayer, and finally the third state results in a structure which is no longer parallel to the (100) surface but is faceted, the facets being planes of type (110). Apparatus
The LEED apparatus used is a Varian instrument, whose design incorporates certain developments accomplished at the Bell Telephone Laboratories. It is of the post-acceleration display type described at length in several papers (17, IS). The vacuum chamber is of stainless steel, pumped by an 80 l/set VacIon pump and the pressure, under most operating conditions, is in the lOWlo torr range. There are provisions for admitting oxygen (from Linde Co., 99.99 per cent pure) through a variable leak valve. Oxygen pressures were inferred from the pump current ; the ion gauge was not operated during periods of oxygen exposure. The tungsten specimen was a ribbon (20 X 2.2 X 0.08) mm, cut from an ingot obtained from the Linde Co., ground thin, and electropolished in NaOH to its final thickness. The major impurity in the crystal as quoted by the manufacturer was MO at 60 parts per million. Others were less than 20 ppm, carbon in particular amounting to 4 ppm. Its orientation was one degree off the (100) plane. The ribbon could be rotated about its long axis and heated by passage of current. Relative intensities of the diffracted electron beams were determined with a Photo Research Corp., “‘Spectra” spot photometer. Experimental
Changes
Results insurfacestructure
Upon
Oxygen
Adsorption
Heating the ribbon to about 2500°K in vacuum cleaned it immediately yielding a sharp diffraction pattern, Fig. 1 (a), of the tungsten (100) surface. This surface is a square array of atoms of spacing 3.16 A. Carbon contamination is to be expected with tungsten but there was no evidence of this of the kind reported by other investigators (12, 13). Exposure to oxygen at room temperature increased the background and the pattern showed some evidence
Vd.
279, No.
3. March
1965
161
J. Anderson
and W. E. Danforth
of furrowing, but no well-ordered structure was found until the ribbon was heated to incandescent temperatures. The following is a description and discussion of the diffraction patterns as the ribbon is heated to a temperature T = 1250°K in oxygen at a pressure In approximately P = 10e7 torr (the ribbon is cooled for LEED observations). two minutes there appears a pattern, as in Fig. 1 (b), showing the normal beams m as well as extra beams which can be indexed n F and F n (n and m integral with m odd). This will be called the first pattern and the diffracting surface the first structure. Two more minutes of exposure produces the second structure, its pattern containing
in addition the 2 y beams with n, m/odd,
Fig. 1 (c).
The appearance of the half-integral beams indicates that the presence of the oxygen has enlarged the unit mesh. A simple and highly probable structure that could yield the first pattern would be a surface with two types of domains, one double double
spaced
spaced
in the x-direction
in the
y-direction
giving
yielding
the 2
the n 7
n beams beams.
and the The
high
other beam
FzQ. 1. (a) Diffraction pattern from clean tungsten (100) surface, 160 volts (some spots hidden by crystal holder). (b> Diffraction pattern from first structure 125 volts. (c) Diffraotion pattern from second structure 79 volta. (d, e) Diffraction pattern from the faceted structure at 101 and 115 volt+ respectively. Approximately same 61m exposure resulta in exaggerated spot size in (d). One of the four equivaIent bright apota in (d) would be indexed (510). Figure (e) shows the splitting of this beam into four diEuse beams. (f) Faceted structure, 82 volts. The ribbon hae been rotated 45” to display diffraction features from an individual (110) plane.
162
Journal of The Franklin
kstkuk
Low Energy
ElecCon
Diflrcsction Study
intensity indicates that tungsten atoms are involved in the scattering, not oxygen only. Accordingly it is assumed that in either type of domain we have rows of tungsten atoms with double the original spacing between rows and that the oxygen atoms occupy the positions half way between. This assumption implies, therefore, a coverage of one half monolayer, 5.0 X 1014 atoms per sq. cm. Similarly we can identify the second structure as double spaced in both directions; the oxygen occupies alternate rows and columns. This would amount to three-quarters of a monolayer of oxygen. Further Ot exposure and heating causes a diffuseness in all beams and their gradual disappearance. In five or ten minutes further change ceases and the resulting pattern indicates that the surface has been faceted, the facets being (1 IO)-type planes. This is herein referred to as the faceted structure, described in the following section. The
Fuceted
Structure
Reports of diffraction features characteristic of planes other than the nominal substrate surface plane have been published by others. MacRae reports the growth of NiO pyramids upon the (111) face of nickel (19). Lander and Morrison (16, 20) have observed the development of etch pits with (111) faces when a silicon surface with one-haJf a monolayer of aluminum wa8 heated to 700°C. Taylor (13) has observed the (111) face of tungsten to form (211) facets when heated in oxygen. In the present case, the faceted structure is characterized by a complete disappearance of the normal tungsten (100) features. Instead there is a fourfold symmetric pattern of relatively weak diffuse beams, Fig. 1 (e), whose behavior indicates that the diffracting surface is not the (100) plane but a faceted structure containing the four crystallographic planes (of type) (1 lo} inclined at 45” to the (100) surface. The pattern of diffuse beams, then, is the superposition of four sets of diffraction beams, one from each (100) plane. With increasing electron voltage, the beams converge, not on the normal 00 position (in the center of the screen), but towards the 00 position of the appropriate (110) plane. Thus, one set of beams moves across the screen from left to right, one set from right to left, one up, and one down. By rotating the sample 45O so that the primary beam is normal to a (110) plane it is possible to see the expected diffraction pattern for normal incidence on a (110) plane, Fig. l(f). The diffuse beams do not move independently, since all beams are diffracted from a single monolithic structure. At certain electron energies beams from all four (110) planes are diffracted in the same direction and produce a single bright sharp beam. With increasing voltage the beam will again split into four diffuse spots. Figures 1 (d) and I (e) show this effect. This is the identical situation reported by Lander and Morrison in the aluminum on silicon-(100) system _ A convenient way of expressing the situation is to make use of the reciprocal lattice and Ewald sphere construction. Let us assume that the surface is simply four (110) planes inclined at 45O to the (100). Figure 2 shows the mutual posi-
Vol. 279. No. 3, March
1965
163
J.
Anderson
and
W.
E.
Danforth
tions of the four planes. The reciprocal lattice for a regular planar array is a series of rods perpendicular to the reciprocal net at the reciprocal net points. The reciprocal unit mesh (non-primitive) for the (110) tungsten plane is a centered rectangle of base vectors ~/CL and G/a, where a. is 3.16 A, the tungsten lattice constant. Figure 2 is in fact a picture of the four reciprocal nets. The intersection of the reciprocal lattice rods with the Ewald sphere defines the diffraction angle of a beam from a single (110) plane; these are the diffuse beams that move across the screen in accordance with the motion of the intersection point as the sphere expands with increasing electron voltage. Because of the mutual arrangement of the reciprocal nets, the reciprocal lattice rods intersect each other, each intersection point being common to four and only four rods, one from each net. F’igure 2 shows two such intersections.
FI :a. 2. The nm rec&mocfbl (110) interee ction pain would be indexed
reciprorcslla*&ice
neta fshowing
some of the reeSpr0callat ,tice rods.
(10 0 0) and
(820)
with
respect
The to the 3-d imensi Ional
When the Ewald sphere touches such a point, all four planes are scattering in phase in the same direction and a sharp spot results. It can be seen then, how this spot will split into four as the sphere expands past that point. There is an interesting observation associated with this phenomenon. On this model, when the four planes are scattering coherently in a bright beam, the coherent area is four times that of a single plane. Therefore, from kinematical considerations, the beam intensity should be sixteen-fold greater and the resolution correspondingly better. The size and brightness of the bright beams of Fig. 1 (d) as compared with the diffuse beams of Fig. 1 (e) are roughly consistent with these deductions. The intensity of the bright beam of Fig. 1 (d) is greater, by at least a factor of two, than four times the intensity of a diffuse beam of Fig. 1 (e). Likewise the spot size is considerably smaller, although this
164
Joumd
of The Franklin bstitute
Electron Diflraction Study
Low Energy
is not evident in the illustration; the extreme film exposure necessary to show the relative beam intensities of Figs. 1 (d) and 1 (e) caused an apparent increase in spot size in the former. The set of rod intersection points is a three-dimensional array of points and this array is just the lattice which is reciprocal to the b.c.c. tungsten lattice; it is a f.c.c. lattice of periodicity Z/a+ Thus the conditions for occurrence of a bright spot happen to be simply those for diffraction from the bulk tungsten. Consideration of this structure and its relation to the Ewald sphere accounts for all the diffraction features seen. In this treatment we take no account of the fact that the surface is surely covered with oxygen. The role of this layer of oxygen with respect to diffraction effects is not apparent. The surface is, of course, not one big pit as the illustration suggests. The elongation of the diffraction spots indicates that the pits are also elongated; the resolution in one direction is greater than in the other, and we suppose that the surface is broken up into grooves several atoms deep and perhaps tens of atoms long. As noted above the occurrence of sharp diffraction beams from the faceted structure can be considered as being equivalent to diffraction from a bulk crystal of the proper orientation. Accordingly, it should be possible unambiguously to identify these features and assign to them three-dimensional diffracted beams within a Miller indices. Of the 21 possible non-equivalent diffraction angle of 50” at electron energies below 400 volts, fourteen were sufficiently well-developed to determine the intensity maxima with precision. For these indexed beams it is possible to write the Bragg equation X =
2d(h, k, Z) sin 8.
For a cubic crystal and for the primary following relations hold :
beam
d = d,/dh2 sin 8 = h/dh2
+ +
normal
h2 + k2 +
(1) to the
(100)
plane the
Z2
(2)
%
(3)
12),
(4)
so that x = 2d,h/(h2 where do is the lattice length X
parameter
A=
F
+
k2 t
of the cubic crystal.
I!
%Land V,,, =
Vt +
For the electron Vc,
where V, is the measured electron voltage at maximum the true voltage of the electron incident on the surface, potential. Combination of these expressions yields V,, =
Vol. 279, No. 3, March
196.5
150.4(h2 +
k2 +
I”)”
wave-
2+
beam intensity, V, is and V, is the contact
V,.
(6)
165
J. Anderson
and W. E. Danforth
A plot of V, vs 150.4 (h2 + k2 + Z”)2/4h2 should be a straight line where the intercept on the V,,, axis is the contact potential and d, is related to the slope by the equation d, = (slope)-*. Figure 3 is such a plot and the best fit as determined by the method of least squares yields d, =
v. =
3.206 4.0 +
+ 0.01 A 1.5 volts.
Although the difference between d, and the tungsten lattice constant is very small it would appear that the difference is real and that a slight dilation of the lattice occurs at the surface. The presence of some unrecognized systematic error is certainly possibIe, however. As for the contact potential between the
400
350
250
200
150
100
50
6
12 150.4
FIG. 3. The fourteen
bright
18 .-@qp2
24
30
36
42
x lo-2
beams from the faceted structure: a plot of the electron the parameter 150.4 (hs + k* + 12)%/4hp.
energy
vs
oxygen covered tungsten and the bariated nickel cathode, the experimental uncertainty is quite large. Nevertheless, four volts would be about right if the work function of the ribbon were 6.6 eV, the value given by Becker and Brandes (6) for an oxygen-covered 110 surface, and the work function of the gun cathode were that of barium, about 2.5 eV.
166
Journa.Iof The PrHnklinInstitute
Low Energy Desorption
Electron Bi$kzction
Study
of Oxygen
After the faceted structure has been developed the process may be reversed by heating the ribbon to higher temperatures in a vacuum. Heating at 1375°K for a few seconds will destroy the faceted structure leaving the second structure, which is stable at that temperature. Heating at 1525°K will produce the first structure in a few seconds while a few minutes at 185O*K is required to bring back the clean tungsten pattern. It would appear that desorption of oxygen is the cause of these transformations. Thus the faceted structure is stable only with an adsorbed layer of oxygen and we suggest that its greater stability relative to that of the (100) is due to the larger quantity of oxygen adsorbed on this hill-and-valley structure thsn is possible on the planar (100) surface. The transformations of structure : faceted + second and second + first can be correlated with data of Becker and Brandes (6). They observed the work function of an oxygen-covered tungsten tip in a field emission microscope as the tip was heated to successively higher temperatures. For the (110) plane there was a sudden decrease in work function as the tip was heated to between 13OOOK and 1400°K. For the (100) plane there was another decrease in the region of 1550°K. They observed no change, however, in the region of 1850°K. The amount of desorbed gas can be calculated from the time integral of the pump current, using the current-pressure calibration and the speed of the pump. Some doubt has been cast on the quantitative accuracy of this method. We note, however, that the tungsten ribbon, if allowed to take up a layer of gas from the ambient at the nominal vacuum, will, when flashed, cause a pulse of pump current which will indicate the desorption of an appropriate quantity of gas (a large fraction of a monolayer). When the ribbon in any of the three stages of oxygen adsorption is flashed, less than 1 per cent of a monolayer is observed to be desorbed, That oxygen is actually on the surface seems inescapable, for none of the three structures appear in the absence of oxygen. We have also noticed that none of the structures will allow the adsorption of the residual gases, and such immunity would be hard to understand if the surface was not covered with oxygen. Experiments on the desorption of oxygen from tungsten have been made by several investigators. Becker and Brandes (6), and Eisinger (7) report bursts of pressure on flashing which they attribute to Oz. Schlier (S), analyzing the desorption products with an omegatron, reports no O2 produced but observes the generation of CO. He believes that CO is responsible for the pressure bursts observed in such systems, the carbon originating as an impurity in the tungsten. Results similar to ours for different tungsten surfaces have been reported by others (12, 13). The question of the desorption mechanism has been the subject of considerable discussion and it would appear that the matter is still not settled. The most prevalent viewpoint is that it leaves the surface as a tungsten oxide which, condensing on the walls, is not recorded by any pressure measuring device. We would suggest an alternative explanation whereby the oxygen is desorbed as atoms. That oxygen can desorb as 0 has been established experimentally by Moore and Unterwald (21), although they give no
Vol. 279, No.
3, March 1965
167
.I. Aadersom
aad
W. E. Danforth
quantitative data. Atomic oxygen, being very active chemically, could then combine with the chamber walls thereby not yielding any observabIe pressure increase. Acknowledgment For many helpful discussions concerning this experiment the authors are grateful Elmgren of Swarthmore College, L. H. Germer and J. W. May of Cornell University, Lander of The Bell Telephone Laboratories and P. J. Estrup of the Bartol laboratory.
to J. J. J.
References (1) K. H. Kingdon, (2) (3) (4) (5)
(6) (7) (8)
(9) (10) (11) (12) (13)
(14) (15) (16) (17) (18) (19) (20) (21)
“Electron Emission from Adsorbed Films on Tungsten,” Phys. Rev. Vol. 24, p. 510, 1924. I. Langmuir and D. S. Villars, “Oxygen Films on Tungsten,” .i. Amer. Chem. Sot. Vol. 53, p. 488, 1931. J. A. Becker, “Use of Thermionics in the Study of Adsorption of Vapours and Gases,” Tram. Farathy. Sot. Vol. 28, p. 148, 1932. J. K. Roberts, “Some Properties of Adsorbed Films of Oxygen on Tungsten,” Proc. Roy. Sot. Vol. A152, p. 464, 1935. J. L. Marrison and J. K. Roberts, “New Methods for Studying Adsorption of Gases at Very Low Pressures and Properties of Adsorbed Films of Oxygen on T rgsten,” Proc. Roy. Sot. Vol. A173, p. 1, 1939. J. A. Becker and R. G. Brandes, “On the Adsorption of Oxygen on Tungsten as Revealed in the Field Emission Microscope,” J. Chem. Phys. Vol. 23, p. 1323, 1955. J. Eisinger, “Adsorption of Oxygen on Tungsten,” J. Chem. Phys. Vol. 30, p. 412, 1959. R. E. S&her, “Adsorption of Oxygen and Carbon Monoxide on Tungsten,” J. Appl. Phys. Vol. 29, p. 1162, 1958. and Diffusion of Oxygen on Tungsten,” Robert Gomer and J. K. Hulm, “Adsorption J. Chew. Phys. Vol. 27, p. 1363, 1957. of Oxygen on Ordered Tungsten Surfaces,” T. H. George and P. M. Stier, “Chemisorption J. Chem. Phys. Vol. 37, p_ 1935, 1962. J. A. Becker, E. J. Becker, and R. G. Brandes, “Reactions of Oxygen with Pure Tungsten and Tungsten Containing Carbon,” J. Appl. Phys. Vol. 32, p. 411, 1961. J. W. May and L. H. Germer, to be published. Study of the Structural Effect of N. J. Taylor, “A Low Energy Electron Diffraction Inter. Conf. Physics and Chemistry Oxygen on the (111) Face of a Tungsten Crystal,” of Solid Surfaces, Brown Univ., June 1964, Surface Science Vol. 2, p. 544, 1964. “Low Energy Electron Diffraction,” Science Vol. 139, p. 379, 1963. A. U. MacRae, and Ordered Surface Structures,” Surface Science Vol. 1, J. J. Lander, “Chemisorption p. 125, 1964. to J. J. Lander, “Low Energy Electron Diffraction and Surface Structural Chemistry,” be published in Recent Progress in Solid State Chemistry, editor H. Reiss. E. D. Schiebner, L. H. Germer, and C. D. Hartman, “Apparatus for Direct Observation Rsv. Sci. Xmt. Vol. 31, p. 112, 1960. of Low-Energy Electron Diffraction Patterns,” “Improved Low Energy Electron Diffraction ApL. H. Germer and C. D. Hartman, paratus,” Rev. Sci. Inst. Vol. 31, p. 784, 1960. “The Epitaxial Growth of NiO on a (111) Nickel Surface,” AppE. Phys. A. U. MacRae, Letters Vol. 2, p. 88, 1963. J. J. Lander and J. Morrison, to be published. “Adsorption-Desorption of Hydrogen on Tungsten G. E. Moore and F. C. Unterwald, and Molybdenum,” J. Chem. Phys. Vol. 40, p- 2626, 1964.
168
Journal of The Franklin Institute