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Some properties of the rhenium (0001) surface
SURFACE
SOME
SCIENCE
19 (1970) 1-8 8 North-Holland
PROPERTIES GEORGE
Publishing Co., Amsterdam
OF THE RHENIUM J. DOOLEY
(0001) SURFACE
III and T. W. HAAS
Aerospace Research Laboratories (OA R,l, Wright Patterson Air Force Base, Ohio 45433, U.S.A.
Received 22 August 1969; revised manuscript received 25 September 1969 The Re(OO01) surface has been studied using low energy electron diffraction
(LEED) and Auger electron spectroscopy. Auger electron spectroscopy verifies that the surface can be cleaned by simple heating in vacuum. The LEED results show that the clean surface has the structure expected of a (OOOI)plane. Gas adsorption studies showed this surface to be inert to all gases studied except oxygen. Oxygen gives a (2 x 2) diffraction pattern.
1.Introduction Relatively few studies of hexagonal metal surfaces have been carried out on clean well characterized surfaces. In the area of low energy electron diffraction (LEED) research the only hexagonal metal that has been studied thus far is titaniumr-3). Rhenium, on the other hand, is an interesting candidate for such a study. The pure metal has a high melting point, low vapor pressure, apparently does not form carbides and its oxides are reiatively volatile4). In this study the basal plane, (OOOl), was studied using LEED and Auger electron spectroscopy. A number of unexpected results were obtained in gas adsorption studies.
2. Experimental The samples used in these studies were cut from electron beam zone refined crystals obtained from Materials Research Corp. They were prepared in several ways, the most successful technique being to cut by spark erosion, rough polish on a diamond wheel, final polish on vibratory polishers, and in some case an e~ectropolish in concentrated H,SO, at 0°C. The first samples were cut thin enough so that they could be heated in vacua by passing a current through the sample. This caused some problem in that rhenium is easily deformed by twinning or simple slip and single crystal slices may become polycrystalline. There seemed to be no way to heat the thin samples by resistance heating and still maintain the single crystal integrity. The physical constraints necessary to hold the sample in place did not permit
2
G. J. DOOLEY
I11 AND
T. W. HAAS
unrestrained expansion of the sample during the Joule heating. This effect caused sufficient thermal stresses to be set up in the sample so that the critical resolved shear stresses for either twinning or slip were exceeded. These stresses then caused the observed deformation. It is interesting to observe, however, that those samples which became polycrystalline from this treatment still gave similar results in gas adsorption studies, suggesting that grain boundaries were relatively unimportant in determining reactivities and structures. The final samples used were heated by electron bombardment and no difficulty with deformation was then experienced. For a bombarding filament, pure rhenium wire was used in order to avoid contamination by evaporation. Temperatures were measured as well as possible using an optical pyrometer, but are not considered very accurate as light from the bombarding filaments produced interference. The LEED system was a three concentric grid display type obtained from Varian associates. The use of getters enabled base pressures of 2 x 10-l’ Torr to be attained during clean surface studies, thus minimizing contamination. All voltages were measured with a calibrated digital voltmeter and are considered accurate to better than f 0.5 V. The Auger electron spectroscopy experiments were made using techniques which are well described in the literatures>s). The one novel approach is the use of a positive bias voltage on the sample in order to shift high voltage peaks to lower voltages?) where the resolution of the three grid system remains high 8). 3. Results and discussion 3.1. THE CLEANSURFACE The first problem is the production of the clean surface. For rhenium this turns out to be remarkably easy to do. The oxides, being volatile, are easy to remove by simple heating in vacuum. Carbides do not form and sulfur compounds are either volatile or decompose. Thus clean surfaces of rhenium can be produced by simple heating in vacuum to a temperature of about 1500°C. Auger electron spectra were taken from the surface in order to detect impurities, if such exist. A careful search was made for peaks at voltages of 151 (sulfur), 272 (carbon), 384 (nitrogen), and 510 (oxygen). Within the sensitivity of the instrument no such peaks were found for what was felt to be the clean surface. A typical spectrum is shown in fig. 1. Among metallic impurities one might expect to find copper and molybdenum as rhenium is refined from these oresa). Auger spectra from copper have been published by Palmberg and Rhodins) and show strong peaks at 7, 62, and 109 V. Spectra from clean molybdenum surfaces have been takeng) and strong peaks are found at 15,28,42, 100, 125, 150, 166, 188, and 226 V. For
PROPERTXESOF Re(OOO1) SURFACE
3
rhenium, on the other hand, the principal transitions are at 8.5, 13, 22, 35, 170, 182, and 217 V. It might be argued that the 182 and 217 V transitions correspond to the MO 188 and 226 V transitions. However, the 125 V peak in MO is stronger than the 226 peak and nearly as strong as the 188, and it does not appear. We feel, therefore, that the pure Re has peaks at 182 and 217 V and does not show extensive and nonremovable segregation of MO impurities on the surface. This viewpoint is substantiated by the fact that gas adsorption studies do not give results similar to those carried out on MO surfacesaJ0). If a substantial fraction of the surface were covered with MO one would expect some reactivity like that of clean MO surfaces. In one
E (eV) Fig. 1. Derivative of the energy distribution, dN(E)/dE, ofinelastically scatteredelectrons versus energy, E, from clean (solid line) and contaminated (dashed line) Re(0001). The Auger peak at 150 V. on the contaminated surface is due to S; those at 120, 225, and possibly 180 V are apparently due to MO.
of the early Auger spectra taken after the sample had been only briefly heated, we did find some peaks which were due to impurities. One of these is due to sulfur at 1.52V and can be seen in fig. 1, the dotted curve. Two other peaks at 120 and 225 V as well as some possible change in the 180 V region may very well be due to some traces of Mo on the surface. These impurities were removed by further heating in vacuum to temperatures around 1500°C and did not reappear later from other treatment, Only the faintest
4
G. 1. DOOLEY
III AND
T. W. HAAS
hint of a carbon peak was ever found. This could not have represented more than about 2% of a monolayer of either carbon or carbon monoxide. It should be noted that at elevated temperatures rhenium dissolves carbon, and the diffusion into the bulk is rapida). In any Auger study it is important to have spectra taken under a variety of experimental conditions. This is necessary in order to lessen the possibility that impurity peaks are being overlooked. In this case an Re polycrystalline foil of 99.99% purity was mounted in the system. The foil was heated to near the melting point (the actual temperature was estimated from an extrapolated power curve to be near 3000°C) several times to remove gaseous impurities and possibly segregate non-metallic impurities on the surface. A spectrum taken after this treatment was identical to the spectrum taken from what was felt to be the clean (0001) single crystal surface. It is possible, however, that the same impurity was present on both surfaces. This point was checked by ion bombarding the foil and taking an Auger spectrum without annealing out the bombardment damage. This technique has been used by us in other Auger studies and whenever surface impurities are present, significant changes in relative peak heights occur following ion bombardment7). In this case no such changes were observed indicating that a clean surface had been obtained. Further substantiation of this point can be inferred from a comparison of the spectrum of Re with that of elements adjacent to it in the periodic chart. We have found that the spectra for such elements are very similar except for shifts in energy7). In this case the spectrum of Re was compared with those from W, Ta, and Ir and found to be very similar. Thus the spectrum shown in fig. 1 (solid line) is felt to be characteristic of an Re surface essentially free of contamination. The clean surface gave the six fold symmetrical diffraction pattern expected from a (0001) plane. Some examples are shown in fig. 2. The patterns observed show a high spot to background ratio and no splitting the diffraction spots (possibly indicative of steps as proposed by Ellis and Schwoebel 11)) was observed on the single crystal sample. No rearrangement of the surface, such as postulated for Atlases) and Pt14) was expected or found. The distribution of intensity of the (00) (or specularly reflected) spot was followed as a function of voltage using photometric techniques. This information is useful in attempting to elucidate structures from LEED information15). The spacing of equivalent * layers for HCP (0001) surfaces is 2, i.e. ABAB. Using the criteria discussed by McRae16) we should expect to find fractional order Bragg peaks in these intensity curves; however, the analysis of which peaks are expected is complicated by the fact that the c/a * Equivalent layers are ones which have the same arrangement with one another when viewed along an axis normal to the layer.
and are in exact registry
PROPERTIESOF
5
(b)
(a) Fig. 2.
Re(OOO1) SURFACE
LEED patterns from the clean Re(OOO1)surface. (a) 200 V, (b) 480 V.
is non-integral. The positions of expected peaks are shown in the intensity curve, fig. 3, the arrows representing Bragg (integral order)reflections. It can be seen that many features of the curve are explained using this simple analysis (and including a fixed inner potential correction), but an accounting for relative heights and other fine structure features are not explained. Note that many of the Bragg peaks have shoulders on their high voltage side. This behavior had been interpreted in terms of steps by Germer and MacRael’) in earlier work on Ni (11 l), but in view of the uncertainties in interpretation of these curves we hesitate to make such an assignment here. ratio
3.2. GAS ADSORPTION
STUDIES
Rhenium has been suggested for use in electronic devices which require a material of high strength at elevated temperature. It has been pointed out, however, that this material suffers from easy oxidation when heated in any oxidizing atmosphere. Accordingly, we felt a study of the interaction of Re with gases commonly found in vacuum systems would be of interest. The gases studied were H,, N,, CO, and 0,. It was somewhat surprising to find that only 0, gave any effect which could be detected. It is possible that small amounts of H,, N,, and CO adsorbed but their effect on the Auger spectra or on the LEED pattern or LEED intensity curves were very small at best. These techniques are very sensitive to most kinds of adsorption and accordingly we would expect the coverage of these gases to be less than 1% of a
6
G. J. DOOLEY
III AND
T. W. HAAS
monolayer. This same result was obtained whether the sample was heated or held at room temperature and for very high exposures to all these gases. This is substantiated by an experiment where the system was cycled to atmospheric pressures several times, the LEED gun was turned on with no further sample treatment and diffraction patterns characteristic of perhaps one monolayer of oxygen were obtained. No other material studied in this laboratory has ever given LEED patterns without at least mild heating after exposure to the atmosphere. An additional factor that must be considered is the possibility that these gases did adsorb but were electron beam desorbed during the LEED or Auger measurements. We cannot rule out this possibility entirely but no pressure burst was observed after exposing the surface to
I
0
100
300
200
400
5 0
Ep (eV1
Fig. 3. Intensity, 2, of the (00) beam versus energy of primary beam, &, from the clean Re(UOO1) surface at near normal incidence. The arrows represent locations of expected Bragg maxima; no corrections were made for inner potential.
these gases and then turning on the electron beam, nor were there differences in the LEED patterns or intensity measurements taken when the gas being studied was left in the chamber at a pressure of - 1 x 10m7 Torr during the measurement. This remarkable inertness of the clean well annealed Re (0001) surface suggests that it might be a good candidate for surface studies in systems where bakeout and ultra high vacuum conditions are not
PROPERTIESOF
Re(OOO1) SURFACE
7
possible or practical. However, we should note that not all Re surfaces are this inert. The Re foil was relatively reactive towards CO as could be seen in the Auger spectrum where strong C and 0 peaks could be detected after the foil remained in the ambient vacuum for some time. This CO could be easily removed from the foil by flashing to about 1000 “C. Some effects due to oxygen adsorption were found. Slight heating in an oxygen environment produces a (2 x 2) pattern as shown in fig. 4. In some cases a (3 x 3) pattern was also obtained. No further change in the pattern
Fig. 4. LEED pattern resulting from interactions of oxygen with Re(OOO1) surface. The pattern is indexed as a (2 x 2); the outer 6 spots are due to the substrate while the inner 6 spots are due to a (2 x 2) mesh from the oxygen.
was observed regardless of exposure and temperature treatments. The oxygen was easily removed by heating to above 1500°C. It is possible that oxygen or carbon monoxide in the presence of water vapor might have produced more of an effect, but this experiment was not tried for practical reasons. 4. Conclusions The clean Re (0001) surface can be produced under favorable circumstances by simple heating in ultra high vacuum. Auger electron spectroscopy gives added evidence for the clean surface condition. No traces of carbon were observed and small amounts of sulfur and possibly molybdenum are easily removed.
8
G. J. DOOLEY
The Re (0001) surface is at elevated temperatures to It does, however, react with The oxide is easily removed by
III AND T. W. HAAS
quite inert both at room temperature and hydrogen, nitrogen, and carbon monoxide. oxygen, giving a (2 x 2) diffraction pattern. heating in vacua.
References 1) H. E. Farnsworth, R. E. Schlier, T. H. George and R. M. Burger, J. Appl. Phys. 29 (1958) 1150. 2) T. H. George, H. E. Farnsworth, R. E. Schlier, J. Chem. Phys. 31(1959) 89. 3) A. M. Russell, Appl. Phys. Letters 8 (1966) 12 4) A. D. Melaven, in: Rare Metals Handbook, Ed. C. A. Hampel (Reinhold, New York, 1961). 5) P. W. Palmberg and T. N. Rhodin, J. Appl. Phys. 39 (1968) 2425. 6) R. E. Weber and W. T. Peria, J. Appl. Phys. 38 (1969) 4355. 7) T. W. Haas, J. T. Grant and G. J. Dooley, to be published. 8) P. W. Palmberg, Appl. Phys. Letters 13 (1968) 183. 9) G. J. Dooley and T. W. Haas, to be published. IO) T. W. Haas and A. G. Jackson, J. Chem. Phys. 44 (1966) 2921. 11) W. P. Ellis and R. L. Schwoebel, Surface Sci ll(1968) 82. 12) D. G. Fedak and N. A. Gjostein, Acta Met. 15 (1967) 827. 13) P. W. Palmberg and T. N. Rhodin, Phys. Rev. 161(1967) 586. 14) S. Hagstrum, H. B. Lyon, and G. A. Somorjai, Phys. Rev. Letters 5 (1965) 491. 15) J. J. Lander, Progr. Solid State Chem. 2 (1965) 26. 16) E. G. McRae, in: Fundamentals of Gas-Surface Interactions, Eds. H. Saltsburg, J. N. Smith, Jr. and M. Rogers (Academic Press, New York, 1967) p. 116. 17) L. H. Germer and A. U. MacRae, Ann. N.Y. Acad. Sci. lOl(l963) 605.
SOME
SCIENCE
19 (1970) 1-8 8 North-Holland
PROPERTIES GEORGE
Publishing Co., Amsterdam
OF THE RHENIUM J. DOOLEY
(0001) SURFACE
III and T. W. HAAS
Aerospace Research Laboratories (OA R,l, Wright Patterson Air Force Base, Ohio 45433, U.S.A.
Received 22 August 1969; revised manuscript received 25 September 1969 The Re(OO01) surface has been studied using low energy electron diffraction
(LEED) and Auger electron spectroscopy. Auger electron spectroscopy verifies that the surface can be cleaned by simple heating in vacuum. The LEED results show that the clean surface has the structure expected of a (OOOI)plane. Gas adsorption studies showed this surface to be inert to all gases studied except oxygen. Oxygen gives a (2 x 2) diffraction pattern.
1.Introduction Relatively few studies of hexagonal metal surfaces have been carried out on clean well characterized surfaces. In the area of low energy electron diffraction (LEED) research the only hexagonal metal that has been studied thus far is titaniumr-3). Rhenium, on the other hand, is an interesting candidate for such a study. The pure metal has a high melting point, low vapor pressure, apparently does not form carbides and its oxides are reiatively volatile4). In this study the basal plane, (OOOl), was studied using LEED and Auger electron spectroscopy. A number of unexpected results were obtained in gas adsorption studies.
2. Experimental The samples used in these studies were cut from electron beam zone refined crystals obtained from Materials Research Corp. They were prepared in several ways, the most successful technique being to cut by spark erosion, rough polish on a diamond wheel, final polish on vibratory polishers, and in some case an e~ectropolish in concentrated H,SO, at 0°C. The first samples were cut thin enough so that they could be heated in vacua by passing a current through the sample. This caused some problem in that rhenium is easily deformed by twinning or simple slip and single crystal slices may become polycrystalline. There seemed to be no way to heat the thin samples by resistance heating and still maintain the single crystal integrity. The physical constraints necessary to hold the sample in place did not permit
2
G. J. DOOLEY
I11 AND
T. W. HAAS
unrestrained expansion of the sample during the Joule heating. This effect caused sufficient thermal stresses to be set up in the sample so that the critical resolved shear stresses for either twinning or slip were exceeded. These stresses then caused the observed deformation. It is interesting to observe, however, that those samples which became polycrystalline from this treatment still gave similar results in gas adsorption studies, suggesting that grain boundaries were relatively unimportant in determining reactivities and structures. The final samples used were heated by electron bombardment and no difficulty with deformation was then experienced. For a bombarding filament, pure rhenium wire was used in order to avoid contamination by evaporation. Temperatures were measured as well as possible using an optical pyrometer, but are not considered very accurate as light from the bombarding filaments produced interference. The LEED system was a three concentric grid display type obtained from Varian associates. The use of getters enabled base pressures of 2 x 10-l’ Torr to be attained during clean surface studies, thus minimizing contamination. All voltages were measured with a calibrated digital voltmeter and are considered accurate to better than f 0.5 V. The Auger electron spectroscopy experiments were made using techniques which are well described in the literatures>s). The one novel approach is the use of a positive bias voltage on the sample in order to shift high voltage peaks to lower voltages?) where the resolution of the three grid system remains high 8). 3. Results and discussion 3.1. THE CLEANSURFACE The first problem is the production of the clean surface. For rhenium this turns out to be remarkably easy to do. The oxides, being volatile, are easy to remove by simple heating in vacuum. Carbides do not form and sulfur compounds are either volatile or decompose. Thus clean surfaces of rhenium can be produced by simple heating in vacuum to a temperature of about 1500°C. Auger electron spectra were taken from the surface in order to detect impurities, if such exist. A careful search was made for peaks at voltages of 151 (sulfur), 272 (carbon), 384 (nitrogen), and 510 (oxygen). Within the sensitivity of the instrument no such peaks were found for what was felt to be the clean surface. A typical spectrum is shown in fig. 1. Among metallic impurities one might expect to find copper and molybdenum as rhenium is refined from these oresa). Auger spectra from copper have been published by Palmberg and Rhodins) and show strong peaks at 7, 62, and 109 V. Spectra from clean molybdenum surfaces have been takeng) and strong peaks are found at 15,28,42, 100, 125, 150, 166, 188, and 226 V. For
PROPERTXESOF Re(OOO1) SURFACE
3
rhenium, on the other hand, the principal transitions are at 8.5, 13, 22, 35, 170, 182, and 217 V. It might be argued that the 182 and 217 V transitions correspond to the MO 188 and 226 V transitions. However, the 125 V peak in MO is stronger than the 226 peak and nearly as strong as the 188, and it does not appear. We feel, therefore, that the pure Re has peaks at 182 and 217 V and does not show extensive and nonremovable segregation of MO impurities on the surface. This viewpoint is substantiated by the fact that gas adsorption studies do not give results similar to those carried out on MO surfacesaJ0). If a substantial fraction of the surface were covered with MO one would expect some reactivity like that of clean MO surfaces. In one
E (eV) Fig. 1. Derivative of the energy distribution, dN(E)/dE, ofinelastically scatteredelectrons versus energy, E, from clean (solid line) and contaminated (dashed line) Re(0001). The Auger peak at 150 V. on the contaminated surface is due to S; those at 120, 225, and possibly 180 V are apparently due to MO.
of the early Auger spectra taken after the sample had been only briefly heated, we did find some peaks which were due to impurities. One of these is due to sulfur at 1.52V and can be seen in fig. 1, the dotted curve. Two other peaks at 120 and 225 V as well as some possible change in the 180 V region may very well be due to some traces of Mo on the surface. These impurities were removed by further heating in vacuum to temperatures around 1500°C and did not reappear later from other treatment, Only the faintest
4
G. 1. DOOLEY
III AND
T. W. HAAS
hint of a carbon peak was ever found. This could not have represented more than about 2% of a monolayer of either carbon or carbon monoxide. It should be noted that at elevated temperatures rhenium dissolves carbon, and the diffusion into the bulk is rapida). In any Auger study it is important to have spectra taken under a variety of experimental conditions. This is necessary in order to lessen the possibility that impurity peaks are being overlooked. In this case an Re polycrystalline foil of 99.99% purity was mounted in the system. The foil was heated to near the melting point (the actual temperature was estimated from an extrapolated power curve to be near 3000°C) several times to remove gaseous impurities and possibly segregate non-metallic impurities on the surface. A spectrum taken after this treatment was identical to the spectrum taken from what was felt to be the clean (0001) single crystal surface. It is possible, however, that the same impurity was present on both surfaces. This point was checked by ion bombarding the foil and taking an Auger spectrum without annealing out the bombardment damage. This technique has been used by us in other Auger studies and whenever surface impurities are present, significant changes in relative peak heights occur following ion bombardment7). In this case no such changes were observed indicating that a clean surface had been obtained. Further substantiation of this point can be inferred from a comparison of the spectrum of Re with that of elements adjacent to it in the periodic chart. We have found that the spectra for such elements are very similar except for shifts in energy7). In this case the spectrum of Re was compared with those from W, Ta, and Ir and found to be very similar. Thus the spectrum shown in fig. 1 (solid line) is felt to be characteristic of an Re surface essentially free of contamination. The clean surface gave the six fold symmetrical diffraction pattern expected from a (0001) plane. Some examples are shown in fig. 2. The patterns observed show a high spot to background ratio and no splitting the diffraction spots (possibly indicative of steps as proposed by Ellis and Schwoebel 11)) was observed on the single crystal sample. No rearrangement of the surface, such as postulated for Atlases) and Pt14) was expected or found. The distribution of intensity of the (00) (or specularly reflected) spot was followed as a function of voltage using photometric techniques. This information is useful in attempting to elucidate structures from LEED information15). The spacing of equivalent * layers for HCP (0001) surfaces is 2, i.e. ABAB. Using the criteria discussed by McRae16) we should expect to find fractional order Bragg peaks in these intensity curves; however, the analysis of which peaks are expected is complicated by the fact that the c/a * Equivalent layers are ones which have the same arrangement with one another when viewed along an axis normal to the layer.
and are in exact registry
PROPERTIESOF
5
(b)
(a) Fig. 2.
Re(OOO1) SURFACE
LEED patterns from the clean Re(OOO1)surface. (a) 200 V, (b) 480 V.
is non-integral. The positions of expected peaks are shown in the intensity curve, fig. 3, the arrows representing Bragg (integral order)reflections. It can be seen that many features of the curve are explained using this simple analysis (and including a fixed inner potential correction), but an accounting for relative heights and other fine structure features are not explained. Note that many of the Bragg peaks have shoulders on their high voltage side. This behavior had been interpreted in terms of steps by Germer and MacRael’) in earlier work on Ni (11 l), but in view of the uncertainties in interpretation of these curves we hesitate to make such an assignment here. ratio
3.2. GAS ADSORPTION
STUDIES
Rhenium has been suggested for use in electronic devices which require a material of high strength at elevated temperature. It has been pointed out, however, that this material suffers from easy oxidation when heated in any oxidizing atmosphere. Accordingly, we felt a study of the interaction of Re with gases commonly found in vacuum systems would be of interest. The gases studied were H,, N,, CO, and 0,. It was somewhat surprising to find that only 0, gave any effect which could be detected. It is possible that small amounts of H,, N,, and CO adsorbed but their effect on the Auger spectra or on the LEED pattern or LEED intensity curves were very small at best. These techniques are very sensitive to most kinds of adsorption and accordingly we would expect the coverage of these gases to be less than 1% of a
6
G. J. DOOLEY
III AND
T. W. HAAS
monolayer. This same result was obtained whether the sample was heated or held at room temperature and for very high exposures to all these gases. This is substantiated by an experiment where the system was cycled to atmospheric pressures several times, the LEED gun was turned on with no further sample treatment and diffraction patterns characteristic of perhaps one monolayer of oxygen were obtained. No other material studied in this laboratory has ever given LEED patterns without at least mild heating after exposure to the atmosphere. An additional factor that must be considered is the possibility that these gases did adsorb but were electron beam desorbed during the LEED or Auger measurements. We cannot rule out this possibility entirely but no pressure burst was observed after exposing the surface to
I
0
100
300
200
400
5 0
Ep (eV1
Fig. 3. Intensity, 2, of the (00) beam versus energy of primary beam, &, from the clean Re(UOO1) surface at near normal incidence. The arrows represent locations of expected Bragg maxima; no corrections were made for inner potential.
these gases and then turning on the electron beam, nor were there differences in the LEED patterns or intensity measurements taken when the gas being studied was left in the chamber at a pressure of - 1 x 10m7 Torr during the measurement. This remarkable inertness of the clean well annealed Re (0001) surface suggests that it might be a good candidate for surface studies in systems where bakeout and ultra high vacuum conditions are not
PROPERTIESOF
Re(OOO1) SURFACE
7
possible or practical. However, we should note that not all Re surfaces are this inert. The Re foil was relatively reactive towards CO as could be seen in the Auger spectrum where strong C and 0 peaks could be detected after the foil remained in the ambient vacuum for some time. This CO could be easily removed from the foil by flashing to about 1000 “C. Some effects due to oxygen adsorption were found. Slight heating in an oxygen environment produces a (2 x 2) pattern as shown in fig. 4. In some cases a (3 x 3) pattern was also obtained. No further change in the pattern
Fig. 4. LEED pattern resulting from interactions of oxygen with Re(OOO1) surface. The pattern is indexed as a (2 x 2); the outer 6 spots are due to the substrate while the inner 6 spots are due to a (2 x 2) mesh from the oxygen.
was observed regardless of exposure and temperature treatments. The oxygen was easily removed by heating to above 1500°C. It is possible that oxygen or carbon monoxide in the presence of water vapor might have produced more of an effect, but this experiment was not tried for practical reasons. 4. Conclusions The clean Re (0001) surface can be produced under favorable circumstances by simple heating in ultra high vacuum. Auger electron spectroscopy gives added evidence for the clean surface condition. No traces of carbon were observed and small amounts of sulfur and possibly molybdenum are easily removed.
8
G. J. DOOLEY
The Re (0001) surface is at elevated temperatures to It does, however, react with The oxide is easily removed by
III AND T. W. HAAS
quite inert both at room temperature and hydrogen, nitrogen, and carbon monoxide. oxygen, giving a (2 x 2) diffraction pattern. heating in vacua.
References 1) H. E. Farnsworth, R. E. Schlier, T. H. George and R. M. Burger, J. Appl. Phys. 29 (1958) 1150. 2) T. H. George, H. E. Farnsworth, R. E. Schlier, J. Chem. Phys. 31(1959) 89. 3) A. M. Russell, Appl. Phys. Letters 8 (1966) 12 4) A. D. Melaven, in: Rare Metals Handbook, Ed. C. A. Hampel (Reinhold, New York, 1961). 5) P. W. Palmberg and T. N. Rhodin, J. Appl. Phys. 39 (1968) 2425. 6) R. E. Weber and W. T. Peria, J. Appl. Phys. 38 (1969) 4355. 7) T. W. Haas, J. T. Grant and G. J. Dooley, to be published. 8) P. W. Palmberg, Appl. Phys. Letters 13 (1968) 183. 9) G. J. Dooley and T. W. Haas, to be published. IO) T. W. Haas and A. G. Jackson, J. Chem. Phys. 44 (1966) 2921. 11) W. P. Ellis and R. L. Schwoebel, Surface Sci ll(1968) 82. 12) D. G. Fedak and N. A. Gjostein, Acta Met. 15 (1967) 827. 13) P. W. Palmberg and T. N. Rhodin, Phys. Rev. 161(1967) 586. 14) S. Hagstrum, H. B. Lyon, and G. A. Somorjai, Phys. Rev. Letters 5 (1965) 491. 15) J. J. Lander, Progr. Solid State Chem. 2 (1965) 26. 16) E. G. McRae, in: Fundamentals of Gas-Surface Interactions, Eds. H. Saltsburg, J. N. Smith, Jr. and M. Rogers (Academic Press, New York, 1967) p. 116. 17) L. H. Germer and A. U. MacRae, Ann. N.Y. Acad. Sci. lOl(l963) 605.