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KRYPTON OVERLAYERS ON GRAPHITE : LOW
ENERGY ELECTRON DIFFRACTION AND AUGER
ELECTRON SPECTROSCOPY MEASUREMENTS
S. Fain, Jr, M. Chinn
To cite this version:
JOWRNAL DE PHYSIQUE Colloque C4, supplkment au no 10, Tome 38, octobre 1977, page C4-99
KRYPTON OVERLAYERS ON GRAPHITE
:LOW ENERGY ELECTRON
DIFFRACTION AND AUGER ELECTRON SPECTROSCOPY MEASUREMENTS
(*)S.C. FAIN, Jr. and M.D. CHINN Physics Department, University of Washington, Seattle, Washington, 98195 U.S.A.
Rhsumh.
-
La diffraction d'Blectrons lents et la spectroscopie des Blectrons Auger ont BtB utiliskes pour Btudier I'adsorption de krypton sur la face (0001) du graphite. Les pressions et temperatures de changement d'ktat des phases bidimensionnelles h p < torr sont comparBes avec les isothermes d'adsorptionB
p > torr.Abstract. - Low energy electron diffraction and Auger electron spectroscopy were used to study the adsorption of krypton on the (0001) plane of graphite. The pressures and temperatures for two dimensional phase transitions at p < torr are compared with adsorption isotherms at p > torn.
1 . Introduction.
-
The adsorption of Kr on graphite has been extensively studied by isotherm measurements for equilibrium vapor pressures greater than torr [I-61. Adsorption at pressures less than torr was first investigated by Kramer and Suzanne using low-energy electron diffraction (LEED) and Auger spectroscopy [7]. Monolayer condensation was explored by both LEED and Auger measurements, but the transition from an in-registry solid to a compressed, out-of-registry solid first inferred from isotherms at pressures near 1 torr [2] was not observed [7]. We have been able to observe this transition with a high resolution, low incident current LEED apparatus [S]. We present elsewhere [9] lattice parameter measurements near this transition in a form directly comparable to a theory of monolayer epitaxy [lo]. Venables and Schabes-Retchkiman also discuss our measurements in these proceedings [I 11.In this paper we present some LEED and Auger determinations of pressures
(lo-' torr < p
<
torr)and temperatures for various two-dimensional phase transitions for Kr on the basal plane of a graphite single crystal. We compare these results with vapor pressure isotherm results [2-41 and earlier LEED
(*) Research supported by National Science Foundation, Research Corporation, and University of Washington Graduate School Research Fund.
and Auger results [7]. In addition we compare the shape of the deregistry transition observed by LEED with that observed by isotherms near 1 torr 121 and present our lattice parameter determination of a two layer film.
2. Procedures. - The LEED apparatus has been described elsewhere
[a].
The most important modification made for this work was to connect an AC preamplifier between a 0-5 kV high voltage supply and the phosphor screen to permit Auger measurements to be made while observing LEED patterns. This was done to be certain that both LEED and Auger observations were being made for the same area of the sample and for nearly the same desorption rate associated with electron effects on the adsorbed layers. In the work of Kramer and Suzanne, the pressure required for monolayer condensation was definitely higher as determined by Auger measurements than by LEED, due to the much higher energy incident electrons of the former t73.Our Auger spectra were obtained with a 342 eV, 2
x
lo-' A primary electron beam and a modulation of 3 eV peak-to-peak. Long data collection times were necessary because of the low signal to noise ratio due to shot noise [l2]. The Auger curves presented here are smoothed averages of three scans taking 5 minutes each. The LEED pattern at 342 eV was sufficiently clear to ensure that the electron beam was focused on the same graphite crystallite area as at 144 eV. LEED patterns were obtainedC4-100 S. C. FAIN, Jr. AND M. D. CHINN
with a 144 eV, 5 x lo-' A primary beam between Auger scans. Except for the photographs taken between Auger scans, the LEED observations were made with a 144 eV, 3
x
A, 0.2 mm diameter electron beam [9].Temperatures were taken from the calibration of
Rosenbaum for hard-worked Au-0.07 % Fe-
Chrome1 P thermocouples [13]. The considerable pressure used to clamp the thermocouple to the front surface of the graphite and the high in-plane thermal conduction of graphite assure that the thermocouple
indication is representative of the sample
temperature to better than
*
1 K. Small changes in temperature could be measured to a greater accuracy of -+ 0.3 K. The pressures reported here are mostly taken from a glass Bayard-Alpert guage(Consolidated Vacuum Corporation, GIC-017)
calibrated on a dynamic standard which utilized a capillary flow meter and a fixed orifice (Boeing Technology Services, Seattle, Washington). This method permits an absolute accuracy of 2 10 % for pressures between 10" and torr. The results reported here were taken with the ion pumps off and the entire chamber filled with Kr. Normally a constant pressure was maintained with the valve to the Kr bottle essentially closed (static conditions). An exception was the highest pressure experiment at 47 K where the pressure dropped very quickly from 3.6
x
lo-' to 2.0x
lo-' torr when the Kr valve was closed. We believe this was due to formation of bulk Kr layers on the coldest surfaces.3. Results and discussion.
-
The pressures and temperatures of two-dimensional phase transitions observed in this work are given in Table I and as the points for p<
lop4 torr in figure 1. The points for p>
torr are from volumetric isotherm measurements [2-41 as explained below and in the figure caption.Temperature and pressures for two -dimensional phase transitions of Kr on graphite (not including possible systematic errors discussed in text).
a ) Fluid-solid condensation
6 1 t l K 5.0 2 0.2 x lo-' torr 62.2 r 0.3 K 1.5 2 0.2 X torr
65.7 2 0.3 K 7.3 r 0.6 x torr b ) Transition from in-registry solid to out-of-registry solid 54.3 r 0.3 K 2.8 r 0.9 x lo-' torr
57.1 & 0 . 3 K 1.65 r 0.4 x torr 59.3 r 0.3 K 7.4 -t 0.5 x torr
c ) Second layer condensation
46.8 K t 0.5 K 2.8 2 0.8 x torr d ) Third layer condensation
46.9 K t 0.5 K 9.8 2 1.3 x torr
e ) Bulk condensation
46.8 K r 0.5 K 2.3 r 1.2 X lo-' torr
FIG. 1.
-
Pressure of Kr versus inverse temperature of graphitesubstrate for fluid-solid condensation ( a ) , onset of in-registry to
out-of-registry solid-solid transition ( b ) , second layer condensation ( c ) , third layer condensation (d), and bulk condensation (e). The data from this experiment (p < torr) are listed in table I and discussed in the text. The points above torr are taken from references [2-41. The 2-D triple point deduced by Larher [3] is shown as an asterisk. Equations for line ( a ) above and below the asterisk are from Larher [3].
Equations for lines ( b ) and (c) are taken from Regnier, Thorny,
Duval [2]. The bulk vapor pressure equation for line (e) is taken from Pollack [14]. A vertical dashed line marks 77 K.
3.1 FLUID-SOLID CONDENSATION.
-
Our measurements of the condensation of a 2-D solid monolayer from a low density 2-D gas were made by LEED observations of the appearance of first orderdiffraction beams of the KF
'
overlayer.Measurements made at 61 K with a spot photometer showed an increase of overlayer LEED intensity from background level to full intensity for a gradual, continuous increase of the Kr pressure from 4.8 to 5.2
x
lo-' torr in 100 seconds. The width of the transition observed in this way is narrower than that observed by Kramer and Suzanne using Auger measurements [7]. Monolayer condensation was reversible for temperatures above about 58 K.The pressures and temperatures we obtain for monolayer condensation (squares in Fig. 1) are in
reasonable agreement with the LEED
determinations of Kramer and Suzanne [7] and with the extrapolation (line a in Fig. 1) of Larher's measurements [3].
3 . 2 TRANSITION FROM IN-REGISTRY SOLID TO OUT-OF-REGISTRY SOLID.
-
Our measurements of Kr-Kr distance near the deregistry transi- tion [9] are presented in figure 2 a as the square ofKRYPTON OVERLAYERS ON GRAPHITE C4-101
FIG. 2. - Comparison of LEED measurements and vapor pressure isotherms near the transition from an in-registry solid to a compressed, out-of-registry solid. a ) do is the registered layer Kr-Kr distance and d is the mean observed Kr-Kr distance ;
temperatures shown are 54 K (circles), 57 K (squares), and 59 K (triangles) ; the same solid line is drawn through each set of experimental data. b ) 0 is the coverage measured by Thomy, Regnier, and Duval [2] at 77.3 K (circles), 90.0 K (squares), and 96.3 K (triangles) ; 00 = 0.930,0.940, and 0.945, respectively ; the
error bars are the size of the points given in figure 4 of reference [2] ; the solid lines are guides to the eye.
observed mean Kr-Kr distance d. The data from Thomy, Regnier, and Duval[2] presented in figure 2 b would be directly comparable to that in figure 2 a (except for temperature differences) only for a per- f ect monolayer with no vacancies, dislocations, or second layer atoms. For such a perfect monolayer, the coverage 8 normalized to the coverage 6, at the onset of the transition would be simply given by (6/8,) = (do/d)2. However, from figure 1 it is evi- dent that at the higher temperatures the in-registry to out-of-registry transition is much closer in pressure to second layer condensation ; thus the number of atoms in the second layer will be higher for the data in figure 2 b than for figure 2a. If a correction for second layer adsorption is applied to the 77.3 K data in figure 2 b , the shape of the corrected data is more similar, to that of the curves in figure 2 a
[G. D. Halsey, University of Washington, private communication].
Our pressures and temperatures for onset of the transition are shown as circles in figure 1. The circles at higher pressure are taken from Thomy, Regnier, and Duval [2] and from Putnam and
Fort [4 and F. A. Putnam, Massachusetts Institute of Technology, private communication]. Our onset data are consistent with the extrapolation (line b of Fig. 1) of the work of Thomy, Regnier, and Duval [ 2 ] .
The Auger measurements shown in figures 3 a and 3 b are taken at pressures well below and well above the deregistry transition. The increase in Auger signal in figure 3 b over 3 a is due to the
combined effects of completing a registered
monolayer, adding atoms to compress the
monolayer, and putting some atoms in the second layer. An Auger spectrum taken just above the deregistry transition (at 53.5 K, 3.6 x torr) had a peak-to-peak height midway between those of figures 3 a and 3 6 . From this measurement we conclude that the increase in Kr coverage from monolayer condensation to the onset of our deregistry transition is about 10 %, presumably due to filling in vacancies in the layer.
FIG. 3.
-
Smoothed Auger electron spectra obtained with a 342 eV, 2 x lo-' A primary electron beam. a ) In-registry layer (near fluid-solid condensation ; 60.5 K, 1.7 x torr), 6 ) Out- of-registry layer (at a coverage just below second layer condensation ; 47 K, 2.0 x torr), c ) Two layer film (at a coverage just above second layer condensation; 47K, 3.6 xlod6
torr), d ) Three or four layer film (47 K, 1.1 X to-' torr).3 . 3
MULTILAYER
CONDENSATION. - AS thepressure was increased from 2.0 x torr
C4- 102 S . C. FAIN, Jr. AND M. D. CHINN
in the intensity of LEED spots and an increased a triangle in figure 1. Again, our data agrees well diffuse scattering was also seen (compare Fig. 4 a with the extrapolation (line c in Fig. 1) of the data of and 4 b ) . As this is the first abrupt change in LEED Thomy, Regnier, and Duval [2].
patterns observed past the deregistry transition, Auger electron spectra and LEED photographs both LEED and Auger seem to indicate that second taken at 47 K and 9 x lop6 torr are essentially the layer condensation occurred between the pressures same as in figures 3c and 4b. When the pressure of figure 3 b and 3 c. This point at 47 K is shown as was further increased to 1.1
x
torr, the secondary electron background flattened even further as shown in figure 3d, the nravhite LEEDFIG. 4. - LEED patterns obtained with a 144 eV, 5 X lo-' A primary electron beam. Due to non-normal incidence, only 4 of the 6 graphite first order diffraction beams can be seen near the edges of t h e pattern. Two graphite crystallites with slightly different orientations are present in the electron beam. The six triplets of spots arise from an out-of-registry Kr overlayer as explained in 191. ~ h e ' t e m ~ e r a t u r e s and pressures for a and b are
the same as for figure 3 b and 3c.
-
-
spots diminished in intensity, and a further increase in diffuse scattering in LEED was observed. We believe that three or four Kr layers were present at this pressure ; our observation is consistent with the one observation of third layer condensation reported by Thomy and Duval[2] (see points d in Fig. 1).
For a further increase in pressure to
3.6 x torr, the Auger spectra were shifted toward lower energies as expected for an insulator which is charging up due to a high secondary electron yield. The LEED pattern showed only 6 first order Kr diffraction spots as expected for a thick, bulk-like Kr film. As mentioned in section 2, shutting the Kr valve produced an immediate drop in pressure, suggesting that thick Kr films were accumulating on the coldest surfaces. The steady state pressure observed agrees well with the extrapolation (line e in Fig. 1) given by Pollack [14].
3 . 4 KRYPTON BILAYER LATTICE CONSTANT.
-
The lateral nearest-neighbor distance in the bilayer film determined from figure 4b is 4.02-
% 0.02A.
measured relative to the in-registry spacing of 4.26
h;.
Our agreement with the bulk Kr-Kr distance of 4.02A
at 47 K [14] may be due to a cancellation of various substrate effects. In order to compare our results with work done at higher temperatures, we take the bulk expansion coefficient as a first estimate for the layer expansion. Thus we would expect the bilayer film to have a Kr-Kr spacing of 4.06h;
at 79 K and 4.08A
at 90 K.Regnier, Thomy, and Duval [6] deduce from
volumetric isotherms a Kr-Kr spacing of
3.97 2 0.05
A
at 90 K, a value much smaller than our estimate. Moreover, since their coverage is below second layer condensation, their value should be greater than for a bilayer film. Their arguments depend crucially on an interpretation of isosteric heat measurements [IS] and on the assumption of no second layer coverage.A neutron diffraction peak observed by Marti, et
a / . [16] can be interpreted as arising from a triangular lattice with Kr-Kr spacing of 4.03
h;
(no error stated) for a bilayer at 79 K. However, Marti etKRYPTON OVERLAYERS ON GRAPHITE C4-103 4. Conclusions. - Although the agreement of our
observations with extrapolations of vapor pressure
isotherms taken above torr appears from
figure 1 to be satisfactory, several points should be kept in mind.
a ) There is no a priori reason to expect the differential entropy and isosteric heat at the various two-dimensional phase transitions to be temperature independent.
b ) Systematic errors in the temperature
measurements of 2 1 K could significantly affect the agreement.
C ) For comparison with truly isothermal
measurements the correction to the pressure measured at the ionization gauge may be as large as d ~ ~ / 3 0 0 K where Ts is the substrate tempe- rature [17].
d ) Due to the many orders of magnitude of pressure shown in figure' 1 , disagreements of a factor of 2 in pressure seem small.
The most satisfactory result of the measurements reported here is to confirm directly by diffraction techniques most of the inferences made by Thomy, Regnier, and Duval [ 2 ] from vapor pressure isotherms at much higher pressures. Interpretations of the lattice constant data near the in-registry to out-of-registry transition are discussed else- where [9, 111. The idea that dislocations or struc- tural modulations are important for out-of- registry layers 19, 11, 181 will certainly receive more experimental and theoretical attention in the next few years.
AcknowIedgments.
-
We wish t o thankG.D. Halsey for many useful discussions,
A. Thomy, F. Putnam, and P. Thorel for sending data and articles prior to publication, and R. Diehl for assistance in data analysis.
References
[I] THOMY, A., DUVAL, X., J. Chim. Phys. 66 (1969) 1966, 67 (1970) 286, 1101.
[2] THOMY, A., REGNIER, J., DUVAL, X., in Thermochimie,
Colloques Internationaux du CNRS (CNRS, Paris) 201 (1972) 511.
[3] LARHER, Y., J. Chem. Soc. Faraday Trans. I70 (1974) 320. [4] PUTNAM, F. A., FORT, T., Jr., J. Phys. Chem. 79 (1975) 459
and in press ; PUTNAM, F. A., FORT, T., Jr., GRIFFITHS, R. B., J. Phys. Chem. (in press).
[5] DUVAL, X., THOMY, A., Carbon 13 (1975) 242.
[6] REGNIER, J., THOMY, A., DUVAL, X., 1. Chim. Phys. (in
press).
[7] KRAMER, H. M., SUZANNE, J., Surf. Sci. 54 (1976) 659. [8] CHINN, M. D. and FAIN, S. C., Jr., J. Vac. Sci. Technol. 14
(1977) 314.
191 CHINN, M. D., FAIN, S. C., Jr., Phys. Rev. Lett. 39 (1977) 146.
[lo] FRANCK, F. C., VAN DER MERWE, J. H., h ~ . R. SOC. A 198
(1949) 205, 216.
[I 11 VENABLES, J. A., SCHABES-RETCHKIMAN, P., These procee- dings.
[12] TAYLOR, N. J., Rev. Sci. Instrum. 40 (1969) 792. [13] ROSENBAUM, R. L., Rev. Sci. Instrum. 40 (1969) 577. [14] POLLACK, G. L., Revs. Mod. Phys. 36 (1964) 748. 1151 REGNIER, J . , ROUQUEROL, J . , THOMY, A., J. Chim. Phys. 3
(1975) 327.
[I61 MARTI, C. L., CROSET, B., THOREL, P., COULOMB, J. P.,
Surf. Sci. 65 (1977).
[I71 EDMONDS, T., HOBSON, J . P., 3. Vac. Sci. Technol. 2 (1965)
182.
[18] N o v ~ c o , A., MCTAGUE, J. P., Phys. Rev. Lett. 38 (1977)
1286 and these proceedings.
DISCUSSION
X. DUVAL.
-
How did you estimate the correction due to the second layer ?S. C. FAIN.
-
We extrapolate the linear part of the isotherm between B, and the onset of second layer condensation back to zero pressure. The dif- ference between this line and the intercept at zero pressure is what we call a second layer correction. This method was suggested to us by Halsey and is mentioned in his paper to be published in J. Phys. Chem. The correction also includes adsorption on edges and defects.J. SUZANNE.
-
We studied the intensity of superstructure spots Kr/Gr versus temperature at constant pressure. It appeared that starting from the temperature of formation of the first solid layer, the intensity is first constant then decreased abruptly (published in Surf.Sci.
54 (1976) 659). We gave as an explanation the in-registry -+ out-of registry transition. We could not see the spot splitting but what you found could explain what we measured and our explanation would be right.C4-104 S. C. FAIN, Jr. AND M. D. CHINN such a measurement. In some preliminary measure-
ments, in which a photometer was focused on an area that included all three spots for an out-of- registry layer, M. D. Chinn and I found a larger total intensity at constant temperature for an out-of- registry overlayer with large misfit compared to an in-registry overlayer. (We did not do measurements close to the transition). As Bienfait pointed out, if your photometer saw only the center of the in-registry spot, then a decrease in intensity for an out-of-registry layer would be expected as the spots move away from the in-registry position.
C. MARTI.
-
What happens on the part of your surface that does not give an ordered LEED pattern ?S. C. FAIN.
-
We can of course provide no information on the lattice constant for the disordered parts of the surface. Auger spectroscopy will detect atoms in both the ordered and disordered parts of our surface. For example, Auger spectra from an area which showed a well defined registered LEED pattern still showed some Kr after the pressure was reduced enough to eliminate the ordered LEED pattern. This could be explained by a higher adsorption energy on the parts of the surface that give no overlayer LEED pattern, as expected for steps and kinks.F. A. PUTNAM.
-
Do you have plans to do LEED intensity measurements to determine the overlayer substrate spacing ?S. C. FAIN. -Yes. The amount of measurements and calculations necessary to determine this distance can be quite extensive. We have made measurements of the energy variation of the specular beam (at one angle) for a clean graphite crystal and for a Kr overlayer on graphite. The changes in the energy dependence are rather subtle, as expected if the overlayer-substrate distance is close to the graphite interlayer spacing. This result is
consistent with conclusions of Marti et al. regarding overlayer-substrate spacing.
M. NIELSEN.
-
Your LEED results on Kr filmsshow that Kr has structures with a continuous range of lattice parameter just above the registered phase density. Can you (as we did for D,, H, layers) correlate the lattice spacing and the coverage (filling) ?
