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Investigation of surface reactions by the static method of secondary ion mass spectrometry
SURFACE
SCIENCE 41 (1974) 493-503 0 North-Holland
INVESTIGATION METHOD
OF
SURFACE
OF SECONDARY
V. THE OXIDATION
Institut
REACTIONS
BY
THE
STATIC
ION MASS SPECTROMETRY
OF TITANIUM, NICKEL, MONOLAYER RANGE
A. MULLER I, Physikalisches
Publishing Co.
AND COPPER
IN THE
and A. BENNINGHOVEN
der Uttiversitiit
K&I,
Universithtsstr.
14, 5 KC% 41, Germany
Received 11 July 1973; revised manuscript received 21 September 1973 By means of the static method of secondary ion mass spectrometry the oxidation of a polycrystalline titanium, nickel, and copper surface cleaned by ion bombardment, was investigated in the oxygen dose range up to 1200 L at room temperature. On titanium and nickel two different oxide phases were found, one covering the other. The thickness of the oxide layer on copper did not exceed one monolayer.
1. Introduction The chemical properties of a metal are important for the formation of an oxide layer on its surface. In earlier paperslp 2) it was demonstrated that metals with similar chemical properties may show a similar behaviour during surface oxidation. This paper is concerned with the monolayer range oxidation of the metals titanium, nickel, and copper which were chosen because of their entirely different chemical properties. There are only a few investigations of the monolayer oxidation of titanium. By means of Auger spectroscopy, Bishop and Rivieres) found saturation after admission of about 60 L* oxygen to the cleaned surface. Escaig and Sella4) observed the oxidation of thin titanium and copper films in an electron microscope and found different stages of the nucleation of oxides. By photoemission energy level measurements, Eastmans) discovered that an exposure to 100 L oxygen resulted in the formation of titanium compounds on the surface. A number of papers are concerned with the interaction of oxygen with nickel surfaces. By a kinetic investigation, Horgan and Kings) discovered three distinct adsorption stages and found the sticking probability for oxygen * 1 L = 10-a torr sec. 493
494
A. MiiLLER
AND
A. BENNINGHOVEN
on the clean surface to be very close to unity. In a number of LEED measurements (e.g. refs. 7-9) of single crystals different oxide structures were found. MacRae9), too, found a sticking probability close to unity. Park and FarnsworthlO) observed work function changes resulting from the interaction of oxygen with clean nickel surfaces. By EID measurements, Klopferrl) discovered several adsorption phases. Eastman and Cashionrs) found work function changes during the interaction of oxygen with nickel. The interaction of oxygen with copper crystal faces has been examined in a number of studies using LEED (e.g. refs. 13 and 14) or RHEED (e.g. refs 15 and 16) techniques. Several structures have been described. Wilson 17)and Delcharrs) found changes of the work function of copper during admission of oxygen. By microgravimetry, Jardinier-Offergeld and Bouillon ls) observed the growth of copper I oxide on copper single crystals. In this paper the oxidation of poly~rystalline samples of titanium, nickel, and copper in the monolayer and submonolayer range is described. The oxidation was observed by the aid of the static method of secondary ion mass spectrometry (SIMS)20). The incipient adsorption layer was not investigated because of the reasons mentioned in an earlier paper2r). The experiments were conducted at room temperature. 2. The apparatus The spectrometer used in this work was described earliersr). The residual pressure amounted to some IO-” torr. So the oxidation by the residual gas could be neglected. During the oxidation experiments oxygen was admitted into the reaction chamber with a partial pressure of lO-8-lO-6 torr. The primary ions (Arf , 3 keV) hit the target at an angle of 20” towards the surface. 3. The secondary ion emission from cleaned and oxidized surfaces Similar to the first original monolayer on chromium shown in an earlier papersr) the original first monolayer on titanium, nickel, and copper consisted of hydrocarbons, oxides, hydroxides, different anions, and other contaminants, represented in the secondary ion spectra by their typical secondary ions. Fig. 1 shows the yields S’ (number of emitted secondary ions per incident primary ion) of some positive and negative secondary ions emitted from cleaned and oxidized surfaces. The cleaning was carried out by ion bombardment (0.1 A set/cm’), the oxidation by admission of oxygen. The remaining oxygen and oxide secondary ions in the spectrum of the cleaned surface are due to bulk impurities.
INVESTIGATION
OF SURFACE
REACTIONS.
495
V
1
*t u)
w-’
p
w-2
w-3
w-‘
1
w-1
iy I
10-Z
W+
IO-‘
moss number
Fig. 1.
Absolute yields for positive and negative secondary and oxidized surfaces.
ions emitted from cleaned
4. Changes of the secondary ion emission during the formation of an oxide layer In figs. 2-4 the yields of some typical secondary are plotted against the oxygen dose. For titanium
ions with high intensities as well as for nickel and
496
A. MiiLLER
AND A. BENNINGHOVEN
10-J
x1-2
10-3
Fig. 2.
Absolute yields for some secondary ions from a titanium target as a function of the oxygen dose.
Fig. 3.
Absolute yields for some secondary ions from a nickel target as a function of the oxygen dose.
INVESTIGATION
.,
1
I
I'
I
I
OF SURFACE
I'
REACTIONS.
I
I
491
V
I
I
I
I
j
to-2
lo-’
Fig. 4.
Absolute yields for some secondary ions from a copper target as a function of the oxygen dose.
I
200 Fig. 5.
I
400
I
l
600
I
I
800
I
I
I
IGCO oxygen dose (L )
Relative intensities of some secondary ions from a titanium target as a function of the oxygen dose.
copper all these curves were measured during one oxidation process. When oxygen was admitted, almost all yields changed by more than one order of magnitude. As a function of the oxygen dose, figs. 5, 6, and 7 show the relative intensities of all secondary ions, which contain only one metal atom and
498
A. MiiLLER
AND
A. BENNINGHOVEN
l
Ni’
v NiOA NiOi
Fig. 6.
Relative intensities of some secondary ions from a nickel target as a function of the oxygen dose.
“0
2m
LW
600
Boo
lcm oxygen
Fig. 7.
dose
1.m IL)
Relative intensities of some secondary ions from a copper target as a function of the oxygen dose.
yields above 5 x 10m3 (titanium and nickel) or 1 x low3 (copper). At sufficiently high oxygen doses all yields and relative intensities reach a saturation, some of them after passing through a maximum. The oxidation experiments were carried out at an oxygen pressure of 1O-7-1O-5 torr. In reach
INVESTIGATION
OF SURFACE
REACTIONS.
V
499
this pressure range the yield and relative intensity curves depended only on the oxygen dose (pressure x time)1t20-22). 5. Changes of the secondary ion emission during the removal of an oxide layer Further information about the oxide layers could be obtained by observ-
1
m-3
Fig. 8.
Absolute yields for some secondary ions from a titanium target as a function of the primary current exposure density.
ing the secondary ion yields, when oxide layers, formed by different oxygen exposures were removed by enhanced ion bombardment. Figs. 8, 9, and 10 show the yields of some secondary ions as a function of the primary ion current exposure density for different oxide layers.
5ocl
A. MiiLLER
AND A. BENNINGHOVEN
Fig. 9.
Absolute yields for some secondary ions from a nickel target as a function of the primary current exposure density.
Fig. 10.
Absolute yields for some secondary ions from a copper target as a function of the primary current exposure density.
primary current
exposure
dem
ty
(ASK /cm
2,
INVESTIGATION
OF SURFACE
RJiACTIONS.
V
501
6. I3iscussion A model for the secondary ion emission and the oxidation process has been described in an earlier paper 21).If an initial sticking probability close to unity is assumed (e.g. refs. 6 and 9) an oxygen dose of about 1 L would form a monomolecular adsorption layer. As figs. 2-7 show, a high excess dose is necessary for an oxidation, i.e. an incorporation of oxygen atoms into the lattice. These considerations are in good agreement with the results Eastman 5, obtained for titanium. During the oxidation of titanium a saturation is reached after admission of about 500 L oxygen (fig. 5). The difference between this value and the results of Bishop and Rivieres) may be ascribed to radiation damages due to the cleaning by ion bombardment. The yield and intensity curves for titanium are similar to those obtained for chromium2r), vanadium, niobium, and tantaluml). The order of appearance of the oxide-specific ions TiO;, TiO;, and TiO+ (fig. 5) corresponds to the results for these four metals. The TiO, curves show a similar course as the CrO;, VO;, NbO,, and Tao; curves. During the formation of an oxide layer a rise, a maximum, and a decrease to a saturation value can be observed. During the removal of an oxide layer formed by 130 L oxygen (fig. 8), the TiO, yield follows an exponential law similar to those other four metals after low oxygen exposures. This indicates that the thickness of this layer does not exceed one monolayer so, 21). The TiO; removal curves obtained after admission of higher oxygen doses were also similar to the corresponding curves from those other metals. A fast increase followed by a broad maximum and a subsequent decrease was observed. The incipient rise of the TiO; curve coincides with the fast decrease of the TiO+ curve. Evidently an upper layer emitting TiO+ ions (phase II), at most one monolayer thick, must vanish before the TiO; emission from the lower layer (phase I) can reach its maximum value. Similar to chromium, vanadium, niobium and tantalum the thickness of the phase I amounts to more than one monolayer. Because of the similarity between the TiO+ and TiO, curve and the TiO, and Ti+ curve during formation (fig. 5) and removal (fig. 8) it can be concluded that TiO, ions are emitted mainly from the phase II and Ti+ ions from the phase I. As demonstrated in an earlier papersi) the sputtering rate (number of emitted secondary particles per incident primary particle) can be calculated by the results shown in fig. 8. For the phase I, S,z 1, and for the phase II, St,%20 was found. The high value Stt may possibly be due to mere changes of the topmost layer caused by the ion bombardment or to the removal of a very weakly bound oxygen adsorption layer.
502
A. MliLLER
AND
A, BENNINGHOVEN
In contrast to titanium and the other metals mentioned above, an oxidized nickel surface emits the following secondary ions containing one nickel atom with yields above lo-“: Ni+, NiO-, and NiO; (fig. 1). The relative intensities of NiO- and NiO; (fig. 6) during the formation of an oxide layer can be described by one curve. Similar to the TiO, curve (fig. 5) an initial rise, a maximum and a following decrease can be observed. The Ni+ curve reaches its final maximum and saturation value at a higher oxygen dose than the NiO- and NiO; curve. During the removal of an oxide layer the NiO, and TiO; intensities show a similar behaviour (figs. 8 and 9). At low oxygen doses an exponential decrease, and at high doses an initial rise, a maximum, and a subsequent decrease can be observed. The initial rise of the high oxygen dose (1200 L) NiO; curve coincides with the initial decrease of the Ni+ curve. So it can be concluded that simiIar to titanium the oxide layer on nickel consists of two phases, one covering the other. The lower phase (phase I) is represented by NiO, secondary ions, the upper phase (phase II) by Ni+ ions. The yields were found to be S,%2,5 for the phase I and S,i~~20 for the phase II. The reasons for this high value Sn may be the same as mentioned for titanium. Similar to nickel, an oxidized copper surface emits the following secondary ions containing one metal atom with yields above 10m4: CuO-, CuO,, and Cu’ (fig. 1). In addition Cu- ions can be found. During the oxidation of copper the relative intensities of the CuO-, CuO,, and Cuf ions can be described by only one curve (fig. 7), which is completely unlike the secondary ion curves from titanium, nickel, and other metals described in earlier papersrFs?sr). Only the Ni+ intensity curve tends to a similar behaviour. The CuO-, CuO;, and Cuf curve rises with a monotonously decreasing slope, whereas the corresponding curves of the other metals mentioned above start with a slope of zero and pass through a point of inflexion before reaching their maximum. In the oxygen dose range below 1200 L the CuO-, CuO,, and Cu+ relative intensity curve reaches its saturation value without passing through a maximum. No curve similar to the TiO, or NiO, curves has been found. During the removal of an oxide layer from a copper surface, a different behaviour was observed too (fig. 10). All yields decrease monotonously. By the aid of the Cu+ curve the sputtering rate was found to be S~2.5. About the same sputtering rate was calculated for the first part of the CUOand CuO; curves. So it can be concluded that the oxide layer formed on a copper surface in this oxygen dose range does not exceed one monolayer. By comparing the monolayer range oxidation of titanium, nickel, and copper it becomes evident that the different chemical properties of these metals lead to a different behaviour during surface oxidation.
INVESTIGATION
OF SURFACE
REACTIONS,
V
503
Acknowledgements We wish to thank the “Landesamt fiir Forschung des Landes NordrheinWestfalen” for financial support of this work. References 1) 2) 3) 4) 5) 6) 7) 8)
9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22)
A. Mtiller and A. Benninghoven, Surface Sci. 39 (1973) 427. A. Benninghoven and L. Wiedmann, Surface Sci. 41(1974) 483. H. E. Bishop and J. C. Rivibre, Surface Sci. 24 (1971) 1. J. Escaig and C. Sella, Compt. Rend. (Paris) B 274 (1972) 27. D. E. Eastman, Solid State Commun. 10 (1972) 933. A. M. Horgan and D. A. King, Surface Sci. 23 (1970) 259. J. W. May and L. H. Germer, Surface Sci. ll(1968) 443. L. H. Germer, J. W. May and R. J. Szostak, Surface Sci. 7 (1967) 430. A. U. MacRae, Surface Sci. l(l964) 319. R. L. Park and H. E. Farnsworth, Surface Sci. 3 (1965) 287. A. Klopfer, Ber. Bunsen Ges. 75 (1971) 1070. D. E. Eastman and J. K. Cashion, Phys. Rev. Letters 27 (1971) 1520. G. Ertl, Surface Sci. 6 (1967) 208. L. Lafourcade, A. Oustry, A. Escant and M. Provincial, Compt. Rend. (Paris) B 272 (1971) 1176. F. Grnnlund and P. E. H@jlund Nielsen, Colloque Intern. CNRS No. 187 (1969) 169. F. Grenlund and P. E. Hojlund Nielsen, Surface Sci. 30 (1972) 388. R. G. Wilson, Surface Sci. 7 (1967) 157. T. A. Delchar, Surface Sci. 27 (1971) 11. M. Jardinier-Offergeld and F. Bouillon, J. Vacuum Sci. Technol. 9 (1972) 770. A. Benninghoven, Surface Sci. 28 (1971) 541; 35 (1973) 427. A. Benninghoven and A. Mtiller, Surface Sci. 39 (1973) 416. A. Benninghoven, Chem. Phys. Letters 6 (1970) 626.
SCIENCE 41 (1974) 493-503 0 North-Holland
INVESTIGATION METHOD
OF
SURFACE
OF SECONDARY
V. THE OXIDATION
Institut
REACTIONS
BY
THE
STATIC
ION MASS SPECTROMETRY
OF TITANIUM, NICKEL, MONOLAYER RANGE
A. MULLER I, Physikalisches
Publishing Co.
AND COPPER
IN THE
and A. BENNINGHOVEN
der Uttiversitiit
K&I,
Universithtsstr.
14, 5 KC% 41, Germany
Received 11 July 1973; revised manuscript received 21 September 1973 By means of the static method of secondary ion mass spectrometry the oxidation of a polycrystalline titanium, nickel, and copper surface cleaned by ion bombardment, was investigated in the oxygen dose range up to 1200 L at room temperature. On titanium and nickel two different oxide phases were found, one covering the other. The thickness of the oxide layer on copper did not exceed one monolayer.
1. Introduction The chemical properties of a metal are important for the formation of an oxide layer on its surface. In earlier paperslp 2) it was demonstrated that metals with similar chemical properties may show a similar behaviour during surface oxidation. This paper is concerned with the monolayer range oxidation of the metals titanium, nickel, and copper which were chosen because of their entirely different chemical properties. There are only a few investigations of the monolayer oxidation of titanium. By means of Auger spectroscopy, Bishop and Rivieres) found saturation after admission of about 60 L* oxygen to the cleaned surface. Escaig and Sella4) observed the oxidation of thin titanium and copper films in an electron microscope and found different stages of the nucleation of oxides. By photoemission energy level measurements, Eastmans) discovered that an exposure to 100 L oxygen resulted in the formation of titanium compounds on the surface. A number of papers are concerned with the interaction of oxygen with nickel surfaces. By a kinetic investigation, Horgan and Kings) discovered three distinct adsorption stages and found the sticking probability for oxygen * 1 L = 10-a torr sec. 493
494
A. MiiLLER
AND
A. BENNINGHOVEN
on the clean surface to be very close to unity. In a number of LEED measurements (e.g. refs. 7-9) of single crystals different oxide structures were found. MacRae9), too, found a sticking probability close to unity. Park and FarnsworthlO) observed work function changes resulting from the interaction of oxygen with clean nickel surfaces. By EID measurements, Klopferrl) discovered several adsorption phases. Eastman and Cashionrs) found work function changes during the interaction of oxygen with nickel. The interaction of oxygen with copper crystal faces has been examined in a number of studies using LEED (e.g. refs. 13 and 14) or RHEED (e.g. refs 15 and 16) techniques. Several structures have been described. Wilson 17)and Delcharrs) found changes of the work function of copper during admission of oxygen. By microgravimetry, Jardinier-Offergeld and Bouillon ls) observed the growth of copper I oxide on copper single crystals. In this paper the oxidation of poly~rystalline samples of titanium, nickel, and copper in the monolayer and submonolayer range is described. The oxidation was observed by the aid of the static method of secondary ion mass spectrometry (SIMS)20). The incipient adsorption layer was not investigated because of the reasons mentioned in an earlier paper2r). The experiments were conducted at room temperature. 2. The apparatus The spectrometer used in this work was described earliersr). The residual pressure amounted to some IO-” torr. So the oxidation by the residual gas could be neglected. During the oxidation experiments oxygen was admitted into the reaction chamber with a partial pressure of lO-8-lO-6 torr. The primary ions (Arf , 3 keV) hit the target at an angle of 20” towards the surface. 3. The secondary ion emission from cleaned and oxidized surfaces Similar to the first original monolayer on chromium shown in an earlier papersr) the original first monolayer on titanium, nickel, and copper consisted of hydrocarbons, oxides, hydroxides, different anions, and other contaminants, represented in the secondary ion spectra by their typical secondary ions. Fig. 1 shows the yields S’ (number of emitted secondary ions per incident primary ion) of some positive and negative secondary ions emitted from cleaned and oxidized surfaces. The cleaning was carried out by ion bombardment (0.1 A set/cm’), the oxidation by admission of oxygen. The remaining oxygen and oxide secondary ions in the spectrum of the cleaned surface are due to bulk impurities.
INVESTIGATION
OF SURFACE
REACTIONS.
495
V
1
*t u)
w-’
p
w-2
w-3
w-‘
1
w-1
iy I
10-Z
W+
IO-‘
moss number
Fig. 1.
Absolute yields for positive and negative secondary and oxidized surfaces.
ions emitted from cleaned
4. Changes of the secondary ion emission during the formation of an oxide layer In figs. 2-4 the yields of some typical secondary are plotted against the oxygen dose. For titanium
ions with high intensities as well as for nickel and
496
A. MiiLLER
AND A. BENNINGHOVEN
10-J
x1-2
10-3
Fig. 2.
Absolute yields for some secondary ions from a titanium target as a function of the oxygen dose.
Fig. 3.
Absolute yields for some secondary ions from a nickel target as a function of the oxygen dose.
INVESTIGATION
.,
1
I
I'
I
I
OF SURFACE
I'
REACTIONS.
I
I
491
V
I
I
I
I
j
to-2
lo-’
Fig. 4.
Absolute yields for some secondary ions from a copper target as a function of the oxygen dose.
I
200 Fig. 5.
I
400
I
l
600
I
I
800
I
I
I
IGCO oxygen dose (L )
Relative intensities of some secondary ions from a titanium target as a function of the oxygen dose.
copper all these curves were measured during one oxidation process. When oxygen was admitted, almost all yields changed by more than one order of magnitude. As a function of the oxygen dose, figs. 5, 6, and 7 show the relative intensities of all secondary ions, which contain only one metal atom and
498
A. MiiLLER
AND
A. BENNINGHOVEN
l
Ni’
v NiOA NiOi
Fig. 6.
Relative intensities of some secondary ions from a nickel target as a function of the oxygen dose.
“0
2m
LW
600
Boo
lcm oxygen
Fig. 7.
dose
1.m IL)
Relative intensities of some secondary ions from a copper target as a function of the oxygen dose.
yields above 5 x 10m3 (titanium and nickel) or 1 x low3 (copper). At sufficiently high oxygen doses all yields and relative intensities reach a saturation, some of them after passing through a maximum. The oxidation experiments were carried out at an oxygen pressure of 1O-7-1O-5 torr. In reach
INVESTIGATION
OF SURFACE
REACTIONS.
V
499
this pressure range the yield and relative intensity curves depended only on the oxygen dose (pressure x time)1t20-22). 5. Changes of the secondary ion emission during the removal of an oxide layer Further information about the oxide layers could be obtained by observ-
1
m-3
Fig. 8.
Absolute yields for some secondary ions from a titanium target as a function of the primary current exposure density.
ing the secondary ion yields, when oxide layers, formed by different oxygen exposures were removed by enhanced ion bombardment. Figs. 8, 9, and 10 show the yields of some secondary ions as a function of the primary ion current exposure density for different oxide layers.
5ocl
A. MiiLLER
AND A. BENNINGHOVEN
Fig. 9.
Absolute yields for some secondary ions from a nickel target as a function of the primary current exposure density.
Fig. 10.
Absolute yields for some secondary ions from a copper target as a function of the primary current exposure density.
primary current
exposure
dem
ty
(ASK /cm
2,
INVESTIGATION
OF SURFACE
RJiACTIONS.
V
501
6. I3iscussion A model for the secondary ion emission and the oxidation process has been described in an earlier paper 21).If an initial sticking probability close to unity is assumed (e.g. refs. 6 and 9) an oxygen dose of about 1 L would form a monomolecular adsorption layer. As figs. 2-7 show, a high excess dose is necessary for an oxidation, i.e. an incorporation of oxygen atoms into the lattice. These considerations are in good agreement with the results Eastman 5, obtained for titanium. During the oxidation of titanium a saturation is reached after admission of about 500 L oxygen (fig. 5). The difference between this value and the results of Bishop and Rivieres) may be ascribed to radiation damages due to the cleaning by ion bombardment. The yield and intensity curves for titanium are similar to those obtained for chromium2r), vanadium, niobium, and tantaluml). The order of appearance of the oxide-specific ions TiO;, TiO;, and TiO+ (fig. 5) corresponds to the results for these four metals. The TiO, curves show a similar course as the CrO;, VO;, NbO,, and Tao; curves. During the formation of an oxide layer a rise, a maximum, and a decrease to a saturation value can be observed. During the removal of an oxide layer formed by 130 L oxygen (fig. 8), the TiO, yield follows an exponential law similar to those other four metals after low oxygen exposures. This indicates that the thickness of this layer does not exceed one monolayer so, 21). The TiO; removal curves obtained after admission of higher oxygen doses were also similar to the corresponding curves from those other metals. A fast increase followed by a broad maximum and a subsequent decrease was observed. The incipient rise of the TiO; curve coincides with the fast decrease of the TiO+ curve. Evidently an upper layer emitting TiO+ ions (phase II), at most one monolayer thick, must vanish before the TiO; emission from the lower layer (phase I) can reach its maximum value. Similar to chromium, vanadium, niobium and tantalum the thickness of the phase I amounts to more than one monolayer. Because of the similarity between the TiO+ and TiO, curve and the TiO, and Ti+ curve during formation (fig. 5) and removal (fig. 8) it can be concluded that TiO, ions are emitted mainly from the phase II and Ti+ ions from the phase I. As demonstrated in an earlier papersi) the sputtering rate (number of emitted secondary particles per incident primary particle) can be calculated by the results shown in fig. 8. For the phase I, S,z 1, and for the phase II, St,%20 was found. The high value Stt may possibly be due to mere changes of the topmost layer caused by the ion bombardment or to the removal of a very weakly bound oxygen adsorption layer.
502
A. MliLLER
AND
A, BENNINGHOVEN
In contrast to titanium and the other metals mentioned above, an oxidized nickel surface emits the following secondary ions containing one nickel atom with yields above lo-“: Ni+, NiO-, and NiO; (fig. 1). The relative intensities of NiO- and NiO; (fig. 6) during the formation of an oxide layer can be described by one curve. Similar to the TiO, curve (fig. 5) an initial rise, a maximum and a following decrease can be observed. The Ni+ curve reaches its final maximum and saturation value at a higher oxygen dose than the NiO- and NiO; curve. During the removal of an oxide layer the NiO, and TiO; intensities show a similar behaviour (figs. 8 and 9). At low oxygen doses an exponential decrease, and at high doses an initial rise, a maximum, and a subsequent decrease can be observed. The initial rise of the high oxygen dose (1200 L) NiO; curve coincides with the initial decrease of the Ni+ curve. So it can be concluded that simiIar to titanium the oxide layer on nickel consists of two phases, one covering the other. The lower phase (phase I) is represented by NiO, secondary ions, the upper phase (phase II) by Ni+ ions. The yields were found to be S,%2,5 for the phase I and S,i~~20 for the phase II. The reasons for this high value Sn may be the same as mentioned for titanium. Similar to nickel, an oxidized copper surface emits the following secondary ions containing one metal atom with yields above 10m4: CuO-, CuO,, and Cu’ (fig. 1). In addition Cu- ions can be found. During the oxidation of copper the relative intensities of the CuO-, CuO,, and Cuf ions can be described by only one curve (fig. 7), which is completely unlike the secondary ion curves from titanium, nickel, and other metals described in earlier papersrFs?sr). Only the Ni+ intensity curve tends to a similar behaviour. The CuO-, CuO;, and Cuf curve rises with a monotonously decreasing slope, whereas the corresponding curves of the other metals mentioned above start with a slope of zero and pass through a point of inflexion before reaching their maximum. In the oxygen dose range below 1200 L the CuO-, CuO,, and Cu+ relative intensity curve reaches its saturation value without passing through a maximum. No curve similar to the TiO, or NiO, curves has been found. During the removal of an oxide layer from a copper surface, a different behaviour was observed too (fig. 10). All yields decrease monotonously. By the aid of the Cu+ curve the sputtering rate was found to be S~2.5. About the same sputtering rate was calculated for the first part of the CUOand CuO; curves. So it can be concluded that the oxide layer formed on a copper surface in this oxygen dose range does not exceed one monolayer. By comparing the monolayer range oxidation of titanium, nickel, and copper it becomes evident that the different chemical properties of these metals lead to a different behaviour during surface oxidation.
INVESTIGATION
OF SURFACE
REACTIONS,
V
503
Acknowledgements We wish to thank the “Landesamt fiir Forschung des Landes NordrheinWestfalen” for financial support of this work. References 1) 2) 3) 4) 5) 6) 7) 8)
9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22)
A. Mtiller and A. Benninghoven, Surface Sci. 39 (1973) 427. A. Benninghoven and L. Wiedmann, Surface Sci. 41(1974) 483. H. E. Bishop and J. C. Rivibre, Surface Sci. 24 (1971) 1. J. Escaig and C. Sella, Compt. Rend. (Paris) B 274 (1972) 27. D. E. Eastman, Solid State Commun. 10 (1972) 933. A. M. Horgan and D. A. King, Surface Sci. 23 (1970) 259. J. W. May and L. H. Germer, Surface Sci. ll(1968) 443. L. H. Germer, J. W. May and R. J. Szostak, Surface Sci. 7 (1967) 430. A. U. MacRae, Surface Sci. l(l964) 319. R. L. Park and H. E. Farnsworth, Surface Sci. 3 (1965) 287. A. Klopfer, Ber. Bunsen Ges. 75 (1971) 1070. D. E. Eastman and J. K. Cashion, Phys. Rev. Letters 27 (1971) 1520. G. Ertl, Surface Sci. 6 (1967) 208. L. Lafourcade, A. Oustry, A. Escant and M. Provincial, Compt. Rend. (Paris) B 272 (1971) 1176. F. Grnnlund and P. E. H@jlund Nielsen, Colloque Intern. CNRS No. 187 (1969) 169. F. Grenlund and P. E. Hojlund Nielsen, Surface Sci. 30 (1972) 388. R. G. Wilson, Surface Sci. 7 (1967) 157. T. A. Delchar, Surface Sci. 27 (1971) 11. M. Jardinier-Offergeld and F. Bouillon, J. Vacuum Sci. Technol. 9 (1972) 770. A. Benninghoven, Surface Sci. 28 (1971) 541; 35 (1973) 427. A. Benninghoven and A. Mtiller, Surface Sci. 39 (1973) 416. A. Benninghoven, Chem. Phys. Letters 6 (1970) 626.