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The initial oxidation of molybdenum
Surface Science 48 (1975)617-623 © North-Holland Publishing Company
THE INITIAL OXIDATION OF MOLYBDENUM III. Oxide nucleation rates on (100) and (110) molybdenum surfaces H.M. KENNETT and A.E. LEE Physics and Chemistry of Solids, Cavendish Laboratory, Madingley Road, Cambridge, England Received 26 March 1974; revised manuscript received 14 August 1974
The induction periods for the nucleation of epitaxial oxide on (100) and (110) surfaces of molybdenum have been determined. The role of impurities has been investigated by AES. It is found that only gross amounts of impurity are likely to be significant. Under isobaric conditions the activation energy associated with nucleation is 0.11 +-0.2 MJ mole-1 on both faces between 700 and 1050 K. At 850 K the induction rate is proportional to the pressure. Under isoposal conditions on the (110) surface an activation energy of 0.22 +0.03 MJ mole-1 is found.
1. Introduction The growth of oxide nuclei over a wide range of conditions is a discontinuous process. The RHEED pattern before nucleation occurs is generally changing very slowly with exposure when quite suddenly reflexions arising from epitaxially grown oxide appear. At a constant temperature and pressure of oxygen it is thus possible to measure the exposure time required to convert a clean surface to one on which oriented oxide nuclei have just formed; this time has been called the induction period t n. The induction period has been measured for both Mo (100) and (110) surfaces under different conditions of temperature and pressure, in order to obtain information on the rate determining processes occurring in the region leading up to nuclei formation. A particular feature of this work is that care was taken to eliminate errors arising from impurity effects and anomalous gauge readings. Thus the starting surfaces were prepared as described in KL I [1]. Experiments were also carried out on surfaces which were known to be contaminated by sulphur and carbon, the results from these experiments are described below. During exposure to oxygen the gas purity was continually monitored. In spite of these precautions however it is impossible to completely * Present address: Department of Applied Physics, University of California, La JoUa, California 92037, U.S.A.
618
H.M. Kennett, A.E. Lee/Initial oxidation o f molybdenum. IH
o.,J\Y 2
°'1
4
6
ti~-e (minuteS]
W (minutes)
Fig. l. Variation of the oxygen/molybdenum (o) and carbon/molybdenum (e) Auger signal ratios as a function of time. (a) A Mo (100) surface exposed at 970 K to 1.3 x 10 - 6 Pa oxygen. (b) An oxygen covered Mo (100) surface heated to 1220 K. remove all possibility of impurity effects. For instance, in the isoposal (constant dosage) experiments the crystal is exposed to 1.3 x 10-3 Pa oxygen for 8 rain. With the stated purity of the oxygen as 99.96% a monolayer of CO may be present. Within necessary limitations however we were unable to observe any systematic errors due to unsure starting conditions.
2. Impurity effects Oxides nucleate on metals with concentrations that are typically below 1014 m -2. We report below that the exposures needed to cause nucleation may be as high as 0.8 Pa s of oxygen. Thus it is not possible to study oxide nucleation on flat surfaces which do not have a contaminant concentration greater than the concentration of nuclei. To evaluate the importance of impurities as nucleation centres several adsorption cycles were carried out on (100) surfaces of m o l y b d e n u m which had not been cleaned. It was hoped to observe discontinuous changes in the impurity signals as the oxide present nucleated.
H.M. Kennett, A.E. L ee/lnitial oxidation o f molybdenum. 111
619
A typical experiment on a surface contaminated by sulphur and carbon gave the results in fig. 1. The surface initially gave an Auger electron spectrum similar to that of fig. 2a in KL I [1]. On admitting oxygen with the crystal at 970 K the carbon signal falls rapidly and has completely disappeared soon after the oxygen signal saturates. The time taken for the oxygen signal to saturate is greater than that observed under comparable circumstances from clean surfaces. Thus it seems likely that carbon is being replaced as CO by oxygen at these temperatures. If the surface is heated to 1220 K in the absence of oxygen gas the oxygen signal falls and the carbon signal reappears. On surfaces which are initially cleaner oxygen is not removed under these conditions. In each of these experiments the 150 V signal did not change at all. Moreover at the high exposure associated with nucleation of oxide this peak remained unchanged. It seems therefore that any carbon present is rapidly removed as CO in the presence of oxygen and that oxide does not nucleate in a manner which masks the 150 V signal.
3. Measurement of induction periods under isobaric conditions The time required for oriented MoO 2 to be observable in the RHEED patterns was measured at a constant oxygen pressure for different temperatures on Mo (100) and (110) surfaces. Determinations were made by flashing the crystal to 2000 K and then exposing it to oxygen at a given temperature for a chosen time. On removal of the oxygen and simultaneous cooling to room temperature, the diffraction pattern was viewed for the presence of oxide and the results recorded photographically. It was not possible to view the diffraction pattern during the actual exposure to oxygen because of the detrimental effect of the gas on the lanthanum hexaboride filament used in the electron gun. If no oxide was observable in the diffraction pattern the crystal was flashed clean and the procedure repeated for a greater exposure of oxygen. In this way the time taken for oxide to become detectable was measured. The results of these experiments are shown in figs. 2 and 3 where lOgl0t n is shown plotted against l I T (where T is the absolute temperature) for various pressures. The error bars indicate the uncertainty in determining when the oxide diffraction features are present. The lower limit represents the time at which oxide is definitely not detectable and the upper limit the time of exposure at which oxide has certainly formed; more difficulty was experienced in determining t n at the lower temperatures due to the higher background intensity in the diffraction patterns. The activation energy (Q) associated with the linear regions of the graphs in figs. 2 and 3 have been determined. On the (100) molybdenum surface the average value obtained for the pressure range 6.6 x 10-6-1.3 x 10--4 Pa of oxygen was found to be Q(100) = 0.11 + 0.02 MJ mo1-1 . On the (I 10) molybdenum surface in the pressure range 1.3 x 10-4-1.3 x 10-3 Pa of oxygen the average value was found to be Q(uo) = 0.10 + 0.02 MJ mo1-1 .
620
H.M. Kennett, A.E. Lee/Initial oxidation of molybdenum. III
/
I0050-
C
I05"
+
,6.o
9.2
,6.B
,,:6
104/T (K -I)
Fig. 2. Temperature dependence of the oxide nucleation times (tn) on a (100) Mo surface; log t n against l I T for different oxygen pressures. (+)6.6 X 10-6 Pa oxygen, (X) 1.3 × 10- 5 Pa oxygen, (o) 1.3 X 10-4 Pa oxygen, (,~)ltypical error on each point.
I0-
~
5-
E
1.0-
0.5"
2 o.s
'
,b
,:2
,.'4 103IT (K-I)
Fig. 3. Temperature dependence of the oxide nucleation times on a (110) Mo surface; log t n against 1/Tin: (a) 1.3 X 10--4 Pa oxygen, (b) 1.3 × 10-3 Pa oxygen.
H.M. Kennett, A.E. Lee/Initial oxidation of molybdenum. III
Q.
621
\
x x
IG 4 x
ro-5
0.3
,.b
~
,b
sb
tn(min utes)
Fig. 4. Pressure dependence of oxide nucleation times on a (110) Mo surface at 850 K; log (oxygen pressure) aginst log t n. within the limits of experimental error there is no difference between the activation energies for isobaric nucleation on these two surfaces.
4. Measurement of induction periods under other conditions on (110) surfaces In a discussion presented previously [2] the epitaxy observed in nucleation on the (100) molybdenum surface is described as being derived from the epitaxy on the (110) molybdenum surface. For this reason further experiments were made on the nuclea. tion of oxide on the (110) surface. The results of an isothermal experiment of 850 K in which the induction time was determined as a function of pressure are given in fig. 4. Clearly the dependence of the reaction rate on pressure is high. The dashed line in this figure indicates the condition that the rate is directly proportional to the oxygen pressure. The experimental points lie close to this line and the calculated gradient gives the result: - l°gl0 tn =/~850 + (0.94 + 0.08) lOgl0 p,
(2)
where ~850 is a constant and p is the oxygen pressure, In the course of the oxidation studies it was noticed that oxide nuclei could be formed by adsorbing oxygen at a low temperature followed by a heat in vacuum. Measurements of a modified induction time t'n were made under these conditions. The
622
H.M. Kennett, A.E. Lee/Initial oxidation of molybdenum. III
10(3-
/
50 ¸
/+
~1© .a 5-
IO-
/
05-
10.2
II.O
I Ii~
12.6 104IT (K-I)
Fig. 5. Heating times required to produce oxide nucleation on a (t 10) M© surface after exposure to: (a) 1.3 × 10-3 Pa oxygen at 800 K for 1 min; (b) 1.3 × 10-3 Pa oxygen at 670 K for 8 min. Log t n against lIT. (110) M© crystal was first exposed to oxygen for 8 min at 1.3 x 10 -3 Pa and 670 K. The crystal was then heated in vacuum to a chosen temperature. The reading from the thermocouple inserted into the crystal was continuously recorded on an X - T plotter and showed that the specimen took about 30 sec to reach the required temperature. The heating time needed for the first signs of oxide to appear in the diffraction pattern. which was continuously viewed, was recorded. The estimated heating time was subtracted from this value to obtain t n' . Values for t'n were obtained for a range of temperatures between 750 and 950 K. The experiment was repeated using a smaller initial oxygen exposure at 800 K. The results are shown in fig. 5 the error bars again indicate the uncertainty in determining when oxide features are present in the diffraction pattern. There are several other errors associated with this method for the determination of induction times in addition to those arising from the lack of precision in deciding when oxide is present on the surface. There is likely to be some variability in the starting conditions for the various runs and also some oxidation will be occurring during the heating up period. The effect of the latter will become more noticeable at high temperatures when the warming up time is comparable to t'n "
H.M. Kennett, A.E. Lee/Initial oxidation of molybdenum. III
623
Notwithstanding these errors an estimate of the activation energy associated with this procedure can be made from fig. 5. The results indicate that Q* = 0.21 -+ 0.04 MJ mo1-1 Q* = 0.23 + 0.04 MJ mo1-1
for preadsorption at 800 K, for preadsorption at 670 K.
These values are considerably higher than those found for the reaction at constant oxygen pressure. The activation energies obtained for preadsorption at 800 and 670 K are obviously similar. However, a marked temperature dependence was noticed in this effect. If oxygen was adsorbed at too low a temperature no amount of heating led to the formation of oxide nuclei. Thus room temperature adsorption followed by heating in vacuum did not produce oxide nuclei even after oxygen exposures well in excess of 1 Pa s. This contrasts with the observations on the (111) surface. These experimental results are rather unexpected. A noticeable feature is that preadsorption at 670 K leads to a far shorter induction time than preadsorption at 800 K.
5. Summary (1) Under conditions of constant oxygen pressure between 6.6 x 10 -6 and 1.3 × 10 -4 Pa and for temperatures between about 700 and 1050 K, (100) and (110) surfaces of molybdenum have activated induction periods for epitaxial oxide nucleation with similar activation energies of 0.11 -+ 0.2 MJ mole -1 . (2) At a temperature of 850 K the induction period on (110) molybdenum is directly proportional to the reciprocal of the pressure. (3) Under isoposal conditions with dosing at 670 or 800 K the activation energy for nucleation on the (110) surface is found to be 0.22 -+ 0.03 MJ mole -1 . (4) Dosing at 670 K leads to a subsequent induction period which is about one order of magnitude shorter than that found after dosing at 800 K.
References [1] H.M. Kennett and A.E. Lee, Surface Sci. 48 (1975) 591,606,624,633. [2] H.M. Kennett, A.E. Lee and J.M. Wilson, Proc. Roy. Soc. (London) A 331 (1972) 429.
THE INITIAL OXIDATION OF MOLYBDENUM III. Oxide nucleation rates on (100) and (110) molybdenum surfaces H.M. KENNETT and A.E. LEE Physics and Chemistry of Solids, Cavendish Laboratory, Madingley Road, Cambridge, England Received 26 March 1974; revised manuscript received 14 August 1974
The induction periods for the nucleation of epitaxial oxide on (100) and (110) surfaces of molybdenum have been determined. The role of impurities has been investigated by AES. It is found that only gross amounts of impurity are likely to be significant. Under isobaric conditions the activation energy associated with nucleation is 0.11 +-0.2 MJ mole-1 on both faces between 700 and 1050 K. At 850 K the induction rate is proportional to the pressure. Under isoposal conditions on the (110) surface an activation energy of 0.22 +0.03 MJ mole-1 is found.
1. Introduction The growth of oxide nuclei over a wide range of conditions is a discontinuous process. The RHEED pattern before nucleation occurs is generally changing very slowly with exposure when quite suddenly reflexions arising from epitaxially grown oxide appear. At a constant temperature and pressure of oxygen it is thus possible to measure the exposure time required to convert a clean surface to one on which oriented oxide nuclei have just formed; this time has been called the induction period t n. The induction period has been measured for both Mo (100) and (110) surfaces under different conditions of temperature and pressure, in order to obtain information on the rate determining processes occurring in the region leading up to nuclei formation. A particular feature of this work is that care was taken to eliminate errors arising from impurity effects and anomalous gauge readings. Thus the starting surfaces were prepared as described in KL I [1]. Experiments were also carried out on surfaces which were known to be contaminated by sulphur and carbon, the results from these experiments are described below. During exposure to oxygen the gas purity was continually monitored. In spite of these precautions however it is impossible to completely * Present address: Department of Applied Physics, University of California, La JoUa, California 92037, U.S.A.
618
H.M. Kennett, A.E. Lee/Initial oxidation o f molybdenum. IH
o.,J\Y 2
°'1
4
6
ti~-e (minuteS]
W (minutes)
Fig. l. Variation of the oxygen/molybdenum (o) and carbon/molybdenum (e) Auger signal ratios as a function of time. (a) A Mo (100) surface exposed at 970 K to 1.3 x 10 - 6 Pa oxygen. (b) An oxygen covered Mo (100) surface heated to 1220 K. remove all possibility of impurity effects. For instance, in the isoposal (constant dosage) experiments the crystal is exposed to 1.3 x 10-3 Pa oxygen for 8 rain. With the stated purity of the oxygen as 99.96% a monolayer of CO may be present. Within necessary limitations however we were unable to observe any systematic errors due to unsure starting conditions.
2. Impurity effects Oxides nucleate on metals with concentrations that are typically below 1014 m -2. We report below that the exposures needed to cause nucleation may be as high as 0.8 Pa s of oxygen. Thus it is not possible to study oxide nucleation on flat surfaces which do not have a contaminant concentration greater than the concentration of nuclei. To evaluate the importance of impurities as nucleation centres several adsorption cycles were carried out on (100) surfaces of m o l y b d e n u m which had not been cleaned. It was hoped to observe discontinuous changes in the impurity signals as the oxide present nucleated.
H.M. Kennett, A.E. L ee/lnitial oxidation o f molybdenum. 111
619
A typical experiment on a surface contaminated by sulphur and carbon gave the results in fig. 1. The surface initially gave an Auger electron spectrum similar to that of fig. 2a in KL I [1]. On admitting oxygen with the crystal at 970 K the carbon signal falls rapidly and has completely disappeared soon after the oxygen signal saturates. The time taken for the oxygen signal to saturate is greater than that observed under comparable circumstances from clean surfaces. Thus it seems likely that carbon is being replaced as CO by oxygen at these temperatures. If the surface is heated to 1220 K in the absence of oxygen gas the oxygen signal falls and the carbon signal reappears. On surfaces which are initially cleaner oxygen is not removed under these conditions. In each of these experiments the 150 V signal did not change at all. Moreover at the high exposure associated with nucleation of oxide this peak remained unchanged. It seems therefore that any carbon present is rapidly removed as CO in the presence of oxygen and that oxide does not nucleate in a manner which masks the 150 V signal.
3. Measurement of induction periods under isobaric conditions The time required for oriented MoO 2 to be observable in the RHEED patterns was measured at a constant oxygen pressure for different temperatures on Mo (100) and (110) surfaces. Determinations were made by flashing the crystal to 2000 K and then exposing it to oxygen at a given temperature for a chosen time. On removal of the oxygen and simultaneous cooling to room temperature, the diffraction pattern was viewed for the presence of oxide and the results recorded photographically. It was not possible to view the diffraction pattern during the actual exposure to oxygen because of the detrimental effect of the gas on the lanthanum hexaboride filament used in the electron gun. If no oxide was observable in the diffraction pattern the crystal was flashed clean and the procedure repeated for a greater exposure of oxygen. In this way the time taken for oxide to become detectable was measured. The results of these experiments are shown in figs. 2 and 3 where lOgl0t n is shown plotted against l I T (where T is the absolute temperature) for various pressures. The error bars indicate the uncertainty in determining when the oxide diffraction features are present. The lower limit represents the time at which oxide is definitely not detectable and the upper limit the time of exposure at which oxide has certainly formed; more difficulty was experienced in determining t n at the lower temperatures due to the higher background intensity in the diffraction patterns. The activation energy (Q) associated with the linear regions of the graphs in figs. 2 and 3 have been determined. On the (100) molybdenum surface the average value obtained for the pressure range 6.6 x 10-6-1.3 x 10--4 Pa of oxygen was found to be Q(100) = 0.11 + 0.02 MJ mo1-1 . On the (I 10) molybdenum surface in the pressure range 1.3 x 10-4-1.3 x 10-3 Pa of oxygen the average value was found to be Q(uo) = 0.10 + 0.02 MJ mo1-1 .
620
H.M. Kennett, A.E. Lee/Initial oxidation of molybdenum. III
/
I0050-
C
I05"
+
,6.o
9.2
,6.B
,,:6
104/T (K -I)
Fig. 2. Temperature dependence of the oxide nucleation times (tn) on a (100) Mo surface; log t n against l I T for different oxygen pressures. (+)6.6 X 10-6 Pa oxygen, (X) 1.3 × 10- 5 Pa oxygen, (o) 1.3 X 10-4 Pa oxygen, (,~)ltypical error on each point.
I0-
~
5-
E
1.0-
0.5"
2 o.s
'
,b
,:2
,.'4 103IT (K-I)
Fig. 3. Temperature dependence of the oxide nucleation times on a (110) Mo surface; log t n against 1/Tin: (a) 1.3 X 10--4 Pa oxygen, (b) 1.3 × 10-3 Pa oxygen.
H.M. Kennett, A.E. Lee/Initial oxidation of molybdenum. III
Q.
621
\
x x
IG 4 x
ro-5
0.3
,.b
~
,b
sb
tn(min utes)
Fig. 4. Pressure dependence of oxide nucleation times on a (110) Mo surface at 850 K; log (oxygen pressure) aginst log t n. within the limits of experimental error there is no difference between the activation energies for isobaric nucleation on these two surfaces.
4. Measurement of induction periods under other conditions on (110) surfaces In a discussion presented previously [2] the epitaxy observed in nucleation on the (100) molybdenum surface is described as being derived from the epitaxy on the (110) molybdenum surface. For this reason further experiments were made on the nuclea. tion of oxide on the (110) surface. The results of an isothermal experiment of 850 K in which the induction time was determined as a function of pressure are given in fig. 4. Clearly the dependence of the reaction rate on pressure is high. The dashed line in this figure indicates the condition that the rate is directly proportional to the oxygen pressure. The experimental points lie close to this line and the calculated gradient gives the result: - l°gl0 tn =/~850 + (0.94 + 0.08) lOgl0 p,
(2)
where ~850 is a constant and p is the oxygen pressure, In the course of the oxidation studies it was noticed that oxide nuclei could be formed by adsorbing oxygen at a low temperature followed by a heat in vacuum. Measurements of a modified induction time t'n were made under these conditions. The
622
H.M. Kennett, A.E. Lee/Initial oxidation of molybdenum. III
10(3-
/
50 ¸
/+
~1© .a 5-
IO-
/
05-
10.2
II.O
I Ii~
12.6 104IT (K-I)
Fig. 5. Heating times required to produce oxide nucleation on a (t 10) M© surface after exposure to: (a) 1.3 × 10-3 Pa oxygen at 800 K for 1 min; (b) 1.3 × 10-3 Pa oxygen at 670 K for 8 min. Log t n against lIT. (110) M© crystal was first exposed to oxygen for 8 min at 1.3 x 10 -3 Pa and 670 K. The crystal was then heated in vacuum to a chosen temperature. The reading from the thermocouple inserted into the crystal was continuously recorded on an X - T plotter and showed that the specimen took about 30 sec to reach the required temperature. The heating time needed for the first signs of oxide to appear in the diffraction pattern. which was continuously viewed, was recorded. The estimated heating time was subtracted from this value to obtain t n' . Values for t'n were obtained for a range of temperatures between 750 and 950 K. The experiment was repeated using a smaller initial oxygen exposure at 800 K. The results are shown in fig. 5 the error bars again indicate the uncertainty in determining when oxide features are present in the diffraction pattern. There are several other errors associated with this method for the determination of induction times in addition to those arising from the lack of precision in deciding when oxide is present on the surface. There is likely to be some variability in the starting conditions for the various runs and also some oxidation will be occurring during the heating up period. The effect of the latter will become more noticeable at high temperatures when the warming up time is comparable to t'n "
H.M. Kennett, A.E. Lee/Initial oxidation of molybdenum. III
623
Notwithstanding these errors an estimate of the activation energy associated with this procedure can be made from fig. 5. The results indicate that Q* = 0.21 -+ 0.04 MJ mo1-1 Q* = 0.23 + 0.04 MJ mo1-1
for preadsorption at 800 K, for preadsorption at 670 K.
These values are considerably higher than those found for the reaction at constant oxygen pressure. The activation energies obtained for preadsorption at 800 and 670 K are obviously similar. However, a marked temperature dependence was noticed in this effect. If oxygen was adsorbed at too low a temperature no amount of heating led to the formation of oxide nuclei. Thus room temperature adsorption followed by heating in vacuum did not produce oxide nuclei even after oxygen exposures well in excess of 1 Pa s. This contrasts with the observations on the (111) surface. These experimental results are rather unexpected. A noticeable feature is that preadsorption at 670 K leads to a far shorter induction time than preadsorption at 800 K.
5. Summary (1) Under conditions of constant oxygen pressure between 6.6 x 10 -6 and 1.3 × 10 -4 Pa and for temperatures between about 700 and 1050 K, (100) and (110) surfaces of molybdenum have activated induction periods for epitaxial oxide nucleation with similar activation energies of 0.11 -+ 0.2 MJ mole -1 . (2) At a temperature of 850 K the induction period on (110) molybdenum is directly proportional to the reciprocal of the pressure. (3) Under isoposal conditions with dosing at 670 or 800 K the activation energy for nucleation on the (110) surface is found to be 0.22 -+ 0.03 MJ mole -1 . (4) Dosing at 670 K leads to a subsequent induction period which is about one order of magnitude shorter than that found after dosing at 800 K.
References [1] H.M. Kennett and A.E. Lee, Surface Sci. 48 (1975) 591,606,624,633. [2] H.M. Kennett, A.E. Lee and J.M. Wilson, Proc. Roy. Soc. (London) A 331 (1972) 429.