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Thermal and ion beam diffusion constants of Sb impurity implanted into 〈100〉 Ni single crystal
a
Nuclear Instruments
and Methods in Physics Research B 101 (1995) 388-393
__
NONIB
Beam Interactions with Materials 6 Atoms
gi II!!! ELSEVIER
Thermal and ion beam diffusion constants of Sb impur ,ity imp lanted into ( 100) Ni single crystal A. Belattar, G.A. Stephens
*,
P.D. Cardwell
Department of Electronic and Electrical Engineering, University of Salford, Salford A45 4W,
UK
Received 17 August 1994; revised form received 24 March 1995
Abstract In previous investigations (A. Belattar et al., Nucl. Instr. and Meth. B 88 (1994) 394; B 93 (1994) 261) the annealing of the surface amorphous layer produced in (100) Ni crystal by implantation of 40 keV Sb*+ to a fluence of 1017 ionscm-* was observed to occur in two distinct stages irrespective of whether the anneal process was due to an isochronal anneal in the temperature range 250 to 1100” C or whether the process involved the high energy irradiation with 1.5 MeV Ar or Xe ions at a constant temperature of 350” C. In this study the diffusion of the Sb was monitored by observing the Sb profile using the Rutherford backscattering technique. A computer fitting procedure was adopted to quantitatively determine the standard deviation of the width of the Sb profile (R 1, the results of which showed that this is a good parameter for monitoring the various diffusion processes. The results of the analysis show that in the case of the first anneal stage the diffusion is relatively slow, but that there is evidence that some form of segregation of the Sb impurity occurs; this is most pronounced during a prolonged isothermal anneal carried out at 350” C. The second rapid anneal stage is accompanied by an equally rapid diffusion of the Sb impurities. In the case of the isochronal anneal, the activation energy for the diffusion process was measured to be 2.0 eV, which is not incompatible with a vacancy diffusion mechanism. In the case of the ion beam annealing processes, the monitoring of R clearly showed a considerable enhancement of the main diffusion process of the Sb impurity which is directly related to the vacancies produced by the incident high energy irradiation.
1. Introduction In previous investigations [1,2], it was found that the annealing of the surface amorphous layers of Sb implanted (100) Ni was similar for both thermal and ion beam annealing processes and was observed to occur in two stages, the second of which happened rapidly with no visible evidence of an epitaxial growth as reported in semiconductors such as Si [7]. This second annealing stage was accompanied with a rapid diffusion of the Sb impurity in the lattice. The Sb concentration would appear to play a major role in the annealing process since it was observed to occur only when it fell below the critical concentration of 8-9 at.% [1,2]; hence it was considered important to make a more quantitative assessment of the diffusion mechanism during both thermal and ion beam assisted processes by monitoring the variation of the width of the Sb profile as measured by the standard deviation 0.
* Corresponding 7455999.
author.
Tel.
+44
61 7455000,
fax +44
61
The dechannelling level x was also monitored to observe any possible correlation between the variation of 0 and annealing of defects.
2. Experimental techniques The initial preparation of the (100) Ni crystals and also the ion implantation with 1017 ionscm-’ Sb*’ at 40 kV has already been described in detail elsewhere [1,2]. In the case of the isochronal studies, the annealing of the amorphous surface damage was observed particularly in the temperature range of 250 to 700” C in steps of 50” C
ill. Similar annealing effects were also observed during the high energy irradiation of the samples with 1.5 MeV ions in the fluence range of 2.5 X 1015 to 2.4 X 1017 ionscm-’ and 8 X 1014 to 1.4 X 10” ionscm-* for Ar+ and Xe+ ions, respectively. These irradiations were performed at a sample temperature of 350” C [2]. RBS techniques were used to investigate the diffusion process of Sb atoms through the Ni lattices by monitoring the profile of the implanted Sb peak.
0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)00504-8
A. Belattar et al./Nucl. lnstr. and Meth. in Phys. Rex B 101 (1995) 388-393
3. The diffusion process
to ion beam radiation annealing approach.
Since the diffusion of the Sb impurity plays a major role in both thermal and ion beam annealing, an attempt was made to estimate the diffusion constants following the procedure suggested by Bottiger et al. [3] which was originally applied to the study of impurity diffusion in ion-beam-mixed amorphous films. Basically, the measured implanted impurity profile is assumed to have a Gaussian distribution both before and after diffusion with the condition that there is no loss of impurity from the surface. However, the equation used for the fitting procedure makes a correction close to the surface after diffusion. The distribution will be mainly determined by the mean position of the peak of the Sb profile X,,, and its variance R’. These parameters were determined using a computer program [4] which fitted the following equation to the RBS profile spectra: ,,xj=&[exp(
+exp(
--Jj(+))
-i(
%jij],
where x is the depth or channel number and R, X, and A are fitting parameters. The program also took into account the finite detector resolution of the system. Assuming a linear relationship between the variance and diffusion time, the diffusion constant for a particular temperature can be determined according to the following equation: 0;
= 2Dt + 0;,
(2)
where fl: and 0: are the variances before and after the diffusion, t is the diffusion time and D is the diffusion constant. This method of analysis was used to monitor the diffusion of the Sb impurity for both the thermal and the radiation enhanced processes. In the case of the thermal process, a plot of the variance against the annealing time will give a value for the diffusion constant D at a particular temperature according to Eq. (2). However, in the case of the irradiated samples the high energy beam considerably enhances the diffusion process to such an extent that the diffusion may be assumed, to a first approximation, to be entirely beam dependent. This was observed to be particularly true in the case of the Xe irradiation. Therefore, assuming a linear relationship between the measured variance R ’ and the corresponding fluence, Eq. (2) can be modified to: 0:
= 2D,+ + a;,
where 0: and 0: are the variances before and after the irradiation, 4 is the irradiated fluence and Di is a measure of the diffusion effect per incident ion. All samples subject
were analysed using this
4. Results and discussions 4.1. The thermal diffusion process during the isochronal anneaf By using the computer based method outlined above, the different values of R before and after each isochronal anneal were determined along with the diffusion constant D using Eq. (2) for temperatures in the range of 250 to 650” C. For temperatures of 700” C and above, the Sb atoms began to disperse, except for a small amount which was trapped in the near surface [l] and therefore this does not meet the criteria on which Eq. (3) is based. Typical RBS spectra of the Sb profile for the two extreme cases, i.e. the as-implanted and after the anneal at 650” C, are shown in Fig. 1 and this illustrates the excellent agreement between both the computer prediction (solid lines) and the measured profiles. The different values obtained for fl are plotted against the corresponding anneal temperatures along with surface lattice damage in Fig. 2. It is clear that there are three annealing stages. Stage I. There is a small increase in R, which is mirrored by a correspondingly apparent increase in the surface lattice damage. This is possibly due to a slight redistribution of the Sb atoms within the damaged layer for temperatures up to 300” C causing a slight increase in the lattice strain. Stage II. This corresponds to a small but definite contraction of the Sb profile which is also associated with the observation of the first stage in the isochronal anneal-
8000
2600
B a
a10 (3)
389
820
200 240 Channel numbor
060
800
Fig. 1. RBS spectra and fitted curves (solid lines) for the Sb profile in Sb implanted (100) Ni before and after isochronal annealing at 650°C for 45 min.
390
A. Belattar et al./Nucl.
Instr. and Meth. in Phys. Res. B 101 (1995) 388-393
eoa
600
Fig. 2. The variation of R of the Sb RBS depth profile as a function of the isochronal temperature.
ing during which there is a partial recovery of the surface damage layer in the temperature range between 300 and 400” C. Above this temperature, the value of L? continues to increase slightly as the damage gradually anneals out further. This behaviour is possibly consistent with the segregation of the Sb atoms in the lattice occurring at this stage of the anneal process [l]. Stage ZZZ.In this region, there is a rapid increase in the diffusion of the Sb atoms through the Ni lattice, particularly between 500 and 650” C. This is accompanied by the
second major annealing stage associated with the surface damage layer during which the Sb atoms moved through the good crystalline Ni lattice beyond the surface damage region [ 11. From the measured variance f12, it is possible to calculate the diffusion constant for each temperature in Stage III using Eq. (2). A logarithmic plot of the diffusion constant D as a function of lo3 K/T, where T is the diffusion temperature in Kelvin, is shown in Fig. 3. The slope of the linear fit to the data, indicated by a solid line,
Fig. 3. Sb diffusion constants in (100) Ni as a function of 103/T,
T being the diffusion temperature
in Kelvin.
A.
aio
a20
Belattar
et al. /Nucl. Instr. and Meth. in Phys. Res. B 101 (1995) 388-393
a40
aao
020
diffusion at 350” C being negligible as compared with the beam assisted one. This is typically illustrated by the RBS spectra shown in Fig. 4, along with the fitted curves (solid line) of the Sb profile before and after irradiation with Xe ions to a fluence of 4.8 X 1016 ionscm-’ at 350” C. Similar curves were also obtained for the case of Ar ion irradiation. The variation in 0 of the Sb profile as a function of fluence for both Ar and Xe irradiations are plotted along with the associated surface damage in Fig. 5. There is a strong similarity between both sets of data which confirms there are two annealing stages as previously reported [2]; this is particularly apparent in the case of the Ar irradiation where the 0 curve attains a plateau before the sharp rise corresponding to the rapid diffusion process. In the case of the Xe, the corresponding plateau is not so evident. This is probably due to a combination of beam enhancing effects and also the slightly different initial Sb peak concentrations, which in the case of the Ar and Xe ion irradiated samples were - 14 and 11 at.%, respectively. The onset of the second annealing stage is known to be concentration dependent [2] and therefore in the case of the Xe irradiated sample, the critical Sb peak concentration of - 8-9 at.% was attained more rapidly thus foreshortening the observed plateau in Fig. 5. Another interesting feature is in both cases the observed slight contraction of the Sb profile just prior to the main rapid diffusion process which could be attributed to some form of segregation of Sb impurities similar to that observed during the isochronal annealing process [ 11. During the irradiation studies, an unirradiated area of the sample was also continually monitored using the same
a20
Channel number Fig. 4. RBS spectra and fitted curves (solid lines) for the Sb profile in Sb implanted (100) Ni before and after 1.5 MeV Xe+ irradiation with a fluence of 4.8 X 1016 ionscm-’ at 350°C.
gives an activation energy for this main diffusion process of - 2.0 eV. This is not inconsistent with that required by a vacancy diffusion mechanism [5,6]. 4.2. The diffusion process irradiation at 350°C
391
during both Ar and Xe ion
In this case, the main rapid diffusion is considerably enhanced by the ion beam irradiation process, the thermal
600 I-
600 /
-
400
-
* E sr ‘@oo
- 1.6
E 200
100
0
I
I
I
I
I
I
I
I I
I
I I I , I I II
10’6 Irradiated
doe0
IO” ( lon8.cms2)
’
‘0
Fig. 5. The variation of R of the Sb RBS depth profile and the associated surface damage as a function of the irradiated doses of both 1.5 MeV Ar+ and Xe+ at 350°C.
392
A. Belattar et al. /Nucl.
Instr. and Meth. in Phys. Res. B 101 (1995) 388-393
RBS techniques in order to determine any possible long term temperature effects which might occur after each irradiation dose. This was equivalent to a series of isothermal anneals at 3.50” C in which the analysis between each anneal was carried out at room !emperature. The unirradiated areas of both Ar and Xe irradiated samples showed similar annealing behaviour during the first annealing stage, but in the case of Xe irradiation, because of the extensive annealing time of N 28 h involved [2], the second rapid annealing stage was also eventually observed. The width of the Sb profile R of the unirradiated area of the sample is plotted as a function of Xe fluence along with both the normalised surface damage and the dechannelling level x in Fig. 6. It is clear from this data that prior to the onset of the second rapid anneal stage, 0 shows an oscillatory trend which could be attributed to the successive expansion and contraction of the Sb impurity profile with increasing dose. This is particularly noticeable during the latter stage of the first annealing process between the doses 1016 and 10” ionscm-*, which corresponds to an anneal time of between 4 and 20 h. This phenomenum is also accompanied by a steady increase in the dechannelling level x and this would suggest that the contraction and expansion of the Sb profile induces a succession of strain and relaxation processes which causes a change in either the density or the character of the extended defects within the surface damage region. Fig. 7 shows the measured values of the variance Q* versus the irradiated fluence in the region where rapid diffusion occurred for both the Ar and Xe ion irradiations together with that obtained from the unirradiated area of the specimen. The fitted lines to the data are indicated by solid lines, the slopes of which give the values of the ion beam diffusion constant Di as defined by Eq. (3). The
?
“c
0
6
10
Irrmdhhd
16
PO
dooo (10’@lon~/cm2
J PI
60
)
Fig. 7. The variance fin2 of the Sb RBS profile following both 1.5 MeV Ar and Xe ion irradiations at 350°C together with that of the
unirradiated area.
values obtained were II,(Ar) = 6.08 X 10e3’ and Di(Xe) = 39.3 X 10m3’ m4/ion for the Ar and Xe ion irradiations, respectively. The unirradiated area, which corresponds to the isothermal anneal at 350” C, yielded only a value of D&m) = 1.16 X 10K3’ m4/ion. Thus, the ratio of the diffusion parameters of D,(Xe>/D,(un) of N 34 shows that, in this case in particular, the thermal activated diffusion process is negligible compared with that of the radiation enhanced process which is a necessary criteria for Eq.
- 0.6
- 0.4
5. Conclusion
- 0.2
do00 (
f
(3) to apply. The measure of the relative effectiveness of the Ar+ and Xe+ beam for this enhanced process is given by the ratio Di(Xe)/Di(Ar), which is N 6.5. In fact, a TRIM91 [8] calculation predicts for the surface region of 500 A the average ratio for production of vacancies for Xe and Ar incident ions to be _ 7. The excellent agreement emphasises both the enhancement of the diffusion process by high energy ion irradiation as compared with the thermal one, and also the importance of the vacancy production rate in such a process.
n
Irrdl6hd
S
kno.ci’)
Fig. 6. The variation of R of the Sb RBS depth profile, the normalised damage and the dechannelling level ,y as a function of the irradiated dose for the unirradiated area of the sample at 350°C.
These results confirm in a more quantitative way that there is a strong correlation between the diffusion of the Sb and the anneal behaviour of the amorphous surface layer caused by the implantation of 40 keV Sb2+ to a fluence of 10” ionscmm2. This investigation shows that the diffusion process occurs in two main stages, the second of which takes place very rapidly during the observed corresponding anneal stage of the surface amorphous layer. The onset of this stage has been previously reported to be concentration
A. Belattar et al. /Nucl. Instr. and Meth. in Phys. Res. B 101 (1995) 388-393 driven [1,2] and occurs when the peak Sb concentration falls below 8 at.%. The effect of beam enhancement on the process has been confirmed, particularly for the second stage where the ratio of the ion beam diffusion constants D&Xe)/D@r) of about 6.5 was in good agreement with the vacancy production ratio in the region of the surface damage as simulated by TRIM91 [8]. This indicates that both the Sb diffusion and the crystal lattice recovery are strongly linked to the density of the vacancies, independently of whether their production is by thermal or radiation enhanced processes. During the first stage of the annealing process, the Sb profile gives possible evidence for some form of segregation process occurring. For example, it is noted that for the isochronal anneal and also for both Xe and Ar irradiations there is a slight contraction of the Sb profile just prior to the onset of the rapid anneal process. The oscillatory behaviour of the Sb profile during this first anneal stage was particularly noticeable during the prolonged isothermal annealing carried out at 350” C which could also be attributed to some form of segregation taking place. One possible interpretation of the observed RBS data in a previous paper [l] is that the recovery of the surface amorphous layer did not grow uniformly from the crystal/amorphous interface but that the epitaxial growth was confined to smaller regions where the Sb concentration had fallen below the critical value of 8 at.%. This effect could be due to the fact that the non-uniform nature of the segregation process would lead to a localised recrystallisation process in regions where the Sb peak concentration has fallen below the critical value. This, in turn, would result in a large concentration of extended defects:
393
thus explaining the high dechannelling level observed even after annealing to an extremely high temperature of 1100” C
111. The processes involved are clearly of a complex nature and these measurements should be treated with some caution, in that if the above anneal process is as described, then the concentration of the Sb in the lattice would have a significant effect on the measured diffusion coefficients and the values quoted in this paper are really an average of the various processes involved. These findings obtained using RBS and channelling techniques should be confirmed using complementary techniques such as TEM and STM.
References [l] A. Belattar, G.A. Stephens and P.D. Cardwell, Nucl. Instr. and Meth. B 88 (1994) 394. [2] A. Belattar, G.A. Stephens and P.D. Cardwell, Nucl. Instr. and Meth. B 93 (1994) 261. [3] J. Bottiger, N.J. Nikkelsen, SK. Nielsen and K. Pampus, J. Non-tryst. Solids 83 (1986) 35. [4] P.D. Cardwell and G.A. Stephens, University of Salford, private communication, 1993. [5] M.W. Thompson, Defects and Radiation Damage in Metals, eds. A. Herzenberg, J.M. Ziman and D. Phil (Cambridge University Press, Cambridge, 1969) p. 67. [6] P. Lucasson, F. Maury and P. Moser, Ann. Chim. Fr. 9 (1984) 1.5. [7] R.G. Elliman, J.S. Williams, W.L. Brown, A. Leiberich, D.M. Maher and R.V. Knoell, Nucl. Instr. and Meth. B 19/20 (1987) 435. [8] J.P. Biersack and R.B. Eckstein, Appl. Phys. A 34 (1984) 73.
Nuclear Instruments
and Methods in Physics Research B 101 (1995) 388-393
__
NONIB
Beam Interactions with Materials 6 Atoms
gi II!!! ELSEVIER
Thermal and ion beam diffusion constants of Sb impur ,ity imp lanted into ( 100) Ni single crystal A. Belattar, G.A. Stephens
*,
P.D. Cardwell
Department of Electronic and Electrical Engineering, University of Salford, Salford A45 4W,
UK
Received 17 August 1994; revised form received 24 March 1995
Abstract In previous investigations (A. Belattar et al., Nucl. Instr. and Meth. B 88 (1994) 394; B 93 (1994) 261) the annealing of the surface amorphous layer produced in (100) Ni crystal by implantation of 40 keV Sb*+ to a fluence of 1017 ionscm-* was observed to occur in two distinct stages irrespective of whether the anneal process was due to an isochronal anneal in the temperature range 250 to 1100” C or whether the process involved the high energy irradiation with 1.5 MeV Ar or Xe ions at a constant temperature of 350” C. In this study the diffusion of the Sb was monitored by observing the Sb profile using the Rutherford backscattering technique. A computer fitting procedure was adopted to quantitatively determine the standard deviation of the width of the Sb profile (R 1, the results of which showed that this is a good parameter for monitoring the various diffusion processes. The results of the analysis show that in the case of the first anneal stage the diffusion is relatively slow, but that there is evidence that some form of segregation of the Sb impurity occurs; this is most pronounced during a prolonged isothermal anneal carried out at 350” C. The second rapid anneal stage is accompanied by an equally rapid diffusion of the Sb impurities. In the case of the isochronal anneal, the activation energy for the diffusion process was measured to be 2.0 eV, which is not incompatible with a vacancy diffusion mechanism. In the case of the ion beam annealing processes, the monitoring of R clearly showed a considerable enhancement of the main diffusion process of the Sb impurity which is directly related to the vacancies produced by the incident high energy irradiation.
1. Introduction In previous investigations [1,2], it was found that the annealing of the surface amorphous layers of Sb implanted (100) Ni was similar for both thermal and ion beam annealing processes and was observed to occur in two stages, the second of which happened rapidly with no visible evidence of an epitaxial growth as reported in semiconductors such as Si [7]. This second annealing stage was accompanied with a rapid diffusion of the Sb impurity in the lattice. The Sb concentration would appear to play a major role in the annealing process since it was observed to occur only when it fell below the critical concentration of 8-9 at.% [1,2]; hence it was considered important to make a more quantitative assessment of the diffusion mechanism during both thermal and ion beam assisted processes by monitoring the variation of the width of the Sb profile as measured by the standard deviation 0.
* Corresponding 7455999.
author.
Tel.
+44
61 7455000,
fax +44
61
The dechannelling level x was also monitored to observe any possible correlation between the variation of 0 and annealing of defects.
2. Experimental techniques The initial preparation of the (100) Ni crystals and also the ion implantation with 1017 ionscm-’ Sb*’ at 40 kV has already been described in detail elsewhere [1,2]. In the case of the isochronal studies, the annealing of the amorphous surface damage was observed particularly in the temperature range of 250 to 700” C in steps of 50” C
ill. Similar annealing effects were also observed during the high energy irradiation of the samples with 1.5 MeV ions in the fluence range of 2.5 X 1015 to 2.4 X 1017 ionscm-’ and 8 X 1014 to 1.4 X 10” ionscm-* for Ar+ and Xe+ ions, respectively. These irradiations were performed at a sample temperature of 350” C [2]. RBS techniques were used to investigate the diffusion process of Sb atoms through the Ni lattices by monitoring the profile of the implanted Sb peak.
0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)00504-8
A. Belattar et al./Nucl. lnstr. and Meth. in Phys. Rex B 101 (1995) 388-393
3. The diffusion process
to ion beam radiation annealing approach.
Since the diffusion of the Sb impurity plays a major role in both thermal and ion beam annealing, an attempt was made to estimate the diffusion constants following the procedure suggested by Bottiger et al. [3] which was originally applied to the study of impurity diffusion in ion-beam-mixed amorphous films. Basically, the measured implanted impurity profile is assumed to have a Gaussian distribution both before and after diffusion with the condition that there is no loss of impurity from the surface. However, the equation used for the fitting procedure makes a correction close to the surface after diffusion. The distribution will be mainly determined by the mean position of the peak of the Sb profile X,,, and its variance R’. These parameters were determined using a computer program [4] which fitted the following equation to the RBS profile spectra: ,,xj=&[exp(
+exp(
--Jj(+))
-i(
%jij],
where x is the depth or channel number and R, X, and A are fitting parameters. The program also took into account the finite detector resolution of the system. Assuming a linear relationship between the variance and diffusion time, the diffusion constant for a particular temperature can be determined according to the following equation: 0;
= 2Dt + 0;,
(2)
where fl: and 0: are the variances before and after the diffusion, t is the diffusion time and D is the diffusion constant. This method of analysis was used to monitor the diffusion of the Sb impurity for both the thermal and the radiation enhanced processes. In the case of the thermal process, a plot of the variance against the annealing time will give a value for the diffusion constant D at a particular temperature according to Eq. (2). However, in the case of the irradiated samples the high energy beam considerably enhances the diffusion process to such an extent that the diffusion may be assumed, to a first approximation, to be entirely beam dependent. This was observed to be particularly true in the case of the Xe irradiation. Therefore, assuming a linear relationship between the measured variance R ’ and the corresponding fluence, Eq. (2) can be modified to: 0:
= 2D,+ + a;,
where 0: and 0: are the variances before and after the irradiation, 4 is the irradiated fluence and Di is a measure of the diffusion effect per incident ion. All samples subject
were analysed using this
4. Results and discussions 4.1. The thermal diffusion process during the isochronal anneaf By using the computer based method outlined above, the different values of R before and after each isochronal anneal were determined along with the diffusion constant D using Eq. (2) for temperatures in the range of 250 to 650” C. For temperatures of 700” C and above, the Sb atoms began to disperse, except for a small amount which was trapped in the near surface [l] and therefore this does not meet the criteria on which Eq. (3) is based. Typical RBS spectra of the Sb profile for the two extreme cases, i.e. the as-implanted and after the anneal at 650” C, are shown in Fig. 1 and this illustrates the excellent agreement between both the computer prediction (solid lines) and the measured profiles. The different values obtained for fl are plotted against the corresponding anneal temperatures along with surface lattice damage in Fig. 2. It is clear that there are three annealing stages. Stage I. There is a small increase in R, which is mirrored by a correspondingly apparent increase in the surface lattice damage. This is possibly due to a slight redistribution of the Sb atoms within the damaged layer for temperatures up to 300” C causing a slight increase in the lattice strain. Stage II. This corresponds to a small but definite contraction of the Sb profile which is also associated with the observation of the first stage in the isochronal anneal-
8000
2600
B a
a10 (3)
389
820
200 240 Channel numbor
060
800
Fig. 1. RBS spectra and fitted curves (solid lines) for the Sb profile in Sb implanted (100) Ni before and after isochronal annealing at 650°C for 45 min.
390
A. Belattar et al./Nucl.
Instr. and Meth. in Phys. Res. B 101 (1995) 388-393
eoa
600
Fig. 2. The variation of R of the Sb RBS depth profile as a function of the isochronal temperature.
ing during which there is a partial recovery of the surface damage layer in the temperature range between 300 and 400” C. Above this temperature, the value of L? continues to increase slightly as the damage gradually anneals out further. This behaviour is possibly consistent with the segregation of the Sb atoms in the lattice occurring at this stage of the anneal process [l]. Stage ZZZ.In this region, there is a rapid increase in the diffusion of the Sb atoms through the Ni lattice, particularly between 500 and 650” C. This is accompanied by the
second major annealing stage associated with the surface damage layer during which the Sb atoms moved through the good crystalline Ni lattice beyond the surface damage region [ 11. From the measured variance f12, it is possible to calculate the diffusion constant for each temperature in Stage III using Eq. (2). A logarithmic plot of the diffusion constant D as a function of lo3 K/T, where T is the diffusion temperature in Kelvin, is shown in Fig. 3. The slope of the linear fit to the data, indicated by a solid line,
Fig. 3. Sb diffusion constants in (100) Ni as a function of 103/T,
T being the diffusion temperature
in Kelvin.
A.
aio
a20
Belattar
et al. /Nucl. Instr. and Meth. in Phys. Res. B 101 (1995) 388-393
a40
aao
020
diffusion at 350” C being negligible as compared with the beam assisted one. This is typically illustrated by the RBS spectra shown in Fig. 4, along with the fitted curves (solid line) of the Sb profile before and after irradiation with Xe ions to a fluence of 4.8 X 1016 ionscm-’ at 350” C. Similar curves were also obtained for the case of Ar ion irradiation. The variation in 0 of the Sb profile as a function of fluence for both Ar and Xe irradiations are plotted along with the associated surface damage in Fig. 5. There is a strong similarity between both sets of data which confirms there are two annealing stages as previously reported [2]; this is particularly apparent in the case of the Ar irradiation where the 0 curve attains a plateau before the sharp rise corresponding to the rapid diffusion process. In the case of the Xe, the corresponding plateau is not so evident. This is probably due to a combination of beam enhancing effects and also the slightly different initial Sb peak concentrations, which in the case of the Ar and Xe ion irradiated samples were - 14 and 11 at.%, respectively. The onset of the second annealing stage is known to be concentration dependent [2] and therefore in the case of the Xe irradiated sample, the critical Sb peak concentration of - 8-9 at.% was attained more rapidly thus foreshortening the observed plateau in Fig. 5. Another interesting feature is in both cases the observed slight contraction of the Sb profile just prior to the main rapid diffusion process which could be attributed to some form of segregation of Sb impurities similar to that observed during the isochronal annealing process [ 11. During the irradiation studies, an unirradiated area of the sample was also continually monitored using the same
a20
Channel number Fig. 4. RBS spectra and fitted curves (solid lines) for the Sb profile in Sb implanted (100) Ni before and after 1.5 MeV Xe+ irradiation with a fluence of 4.8 X 1016 ionscm-’ at 350°C.
gives an activation energy for this main diffusion process of - 2.0 eV. This is not inconsistent with that required by a vacancy diffusion mechanism [5,6]. 4.2. The diffusion process irradiation at 350°C
391
during both Ar and Xe ion
In this case, the main rapid diffusion is considerably enhanced by the ion beam irradiation process, the thermal
600 I-
600 /
-
400
-
* E sr ‘@oo
- 1.6
E 200
100
0
I
I
I
I
I
I
I
I I
I
I I I , I I II
10’6 Irradiated
doe0
IO” ( lon8.cms2)
’
‘0
Fig. 5. The variation of R of the Sb RBS depth profile and the associated surface damage as a function of the irradiated doses of both 1.5 MeV Ar+ and Xe+ at 350°C.
392
A. Belattar et al. /Nucl.
Instr. and Meth. in Phys. Res. B 101 (1995) 388-393
RBS techniques in order to determine any possible long term temperature effects which might occur after each irradiation dose. This was equivalent to a series of isothermal anneals at 3.50” C in which the analysis between each anneal was carried out at room !emperature. The unirradiated areas of both Ar and Xe irradiated samples showed similar annealing behaviour during the first annealing stage, but in the case of Xe irradiation, because of the extensive annealing time of N 28 h involved [2], the second rapid annealing stage was also eventually observed. The width of the Sb profile R of the unirradiated area of the sample is plotted as a function of Xe fluence along with both the normalised surface damage and the dechannelling level x in Fig. 6. It is clear from this data that prior to the onset of the second rapid anneal stage, 0 shows an oscillatory trend which could be attributed to the successive expansion and contraction of the Sb impurity profile with increasing dose. This is particularly noticeable during the latter stage of the first annealing process between the doses 1016 and 10” ionscm-*, which corresponds to an anneal time of between 4 and 20 h. This phenomenum is also accompanied by a steady increase in the dechannelling level x and this would suggest that the contraction and expansion of the Sb profile induces a succession of strain and relaxation processes which causes a change in either the density or the character of the extended defects within the surface damage region. Fig. 7 shows the measured values of the variance Q* versus the irradiated fluence in the region where rapid diffusion occurred for both the Ar and Xe ion irradiations together with that obtained from the unirradiated area of the specimen. The fitted lines to the data are indicated by solid lines, the slopes of which give the values of the ion beam diffusion constant Di as defined by Eq. (3). The
?
“c
0
6
10
Irrmdhhd
16
PO
dooo (10’@lon~/cm2
J PI
60
)
Fig. 7. The variance fin2 of the Sb RBS profile following both 1.5 MeV Ar and Xe ion irradiations at 350°C together with that of the
unirradiated area.
values obtained were II,(Ar) = 6.08 X 10e3’ and Di(Xe) = 39.3 X 10m3’ m4/ion for the Ar and Xe ion irradiations, respectively. The unirradiated area, which corresponds to the isothermal anneal at 350” C, yielded only a value of D&m) = 1.16 X 10K3’ m4/ion. Thus, the ratio of the diffusion parameters of D,(Xe>/D,(un) of N 34 shows that, in this case in particular, the thermal activated diffusion process is negligible compared with that of the radiation enhanced process which is a necessary criteria for Eq.
- 0.6
- 0.4
5. Conclusion
- 0.2
do00 (
f
(3) to apply. The measure of the relative effectiveness of the Ar+ and Xe+ beam for this enhanced process is given by the ratio Di(Xe)/Di(Ar), which is N 6.5. In fact, a TRIM91 [8] calculation predicts for the surface region of 500 A the average ratio for production of vacancies for Xe and Ar incident ions to be _ 7. The excellent agreement emphasises both the enhancement of the diffusion process by high energy ion irradiation as compared with the thermal one, and also the importance of the vacancy production rate in such a process.
n
Irrdl6hd
S
kno.ci’)
Fig. 6. The variation of R of the Sb RBS depth profile, the normalised damage and the dechannelling level ,y as a function of the irradiated dose for the unirradiated area of the sample at 350°C.
These results confirm in a more quantitative way that there is a strong correlation between the diffusion of the Sb and the anneal behaviour of the amorphous surface layer caused by the implantation of 40 keV Sb2+ to a fluence of 10” ionscmm2. This investigation shows that the diffusion process occurs in two main stages, the second of which takes place very rapidly during the observed corresponding anneal stage of the surface amorphous layer. The onset of this stage has been previously reported to be concentration
A. Belattar et al. /Nucl. Instr. and Meth. in Phys. Res. B 101 (1995) 388-393 driven [1,2] and occurs when the peak Sb concentration falls below 8 at.%. The effect of beam enhancement on the process has been confirmed, particularly for the second stage where the ratio of the ion beam diffusion constants D&Xe)/D@r) of about 6.5 was in good agreement with the vacancy production ratio in the region of the surface damage as simulated by TRIM91 [8]. This indicates that both the Sb diffusion and the crystal lattice recovery are strongly linked to the density of the vacancies, independently of whether their production is by thermal or radiation enhanced processes. During the first stage of the annealing process, the Sb profile gives possible evidence for some form of segregation process occurring. For example, it is noted that for the isochronal anneal and also for both Xe and Ar irradiations there is a slight contraction of the Sb profile just prior to the onset of the rapid anneal process. The oscillatory behaviour of the Sb profile during this first anneal stage was particularly noticeable during the prolonged isothermal annealing carried out at 350” C which could also be attributed to some form of segregation taking place. One possible interpretation of the observed RBS data in a previous paper [l] is that the recovery of the surface amorphous layer did not grow uniformly from the crystal/amorphous interface but that the epitaxial growth was confined to smaller regions where the Sb concentration had fallen below the critical value of 8 at.%. This effect could be due to the fact that the non-uniform nature of the segregation process would lead to a localised recrystallisation process in regions where the Sb peak concentration has fallen below the critical value. This, in turn, would result in a large concentration of extended defects:
393
thus explaining the high dechannelling level observed even after annealing to an extremely high temperature of 1100” C
111. The processes involved are clearly of a complex nature and these measurements should be treated with some caution, in that if the above anneal process is as described, then the concentration of the Sb in the lattice would have a significant effect on the measured diffusion coefficients and the values quoted in this paper are really an average of the various processes involved. These findings obtained using RBS and channelling techniques should be confirmed using complementary techniques such as TEM and STM.
References [l] A. Belattar, G.A. Stephens and P.D. Cardwell, Nucl. Instr. and Meth. B 88 (1994) 394. [2] A. Belattar, G.A. Stephens and P.D. Cardwell, Nucl. Instr. and Meth. B 93 (1994) 261. [3] J. Bottiger, N.J. Nikkelsen, SK. Nielsen and K. Pampus, J. Non-tryst. Solids 83 (1986) 35. [4] P.D. Cardwell and G.A. Stephens, University of Salford, private communication, 1993. [5] M.W. Thompson, Defects and Radiation Damage in Metals, eds. A. Herzenberg, J.M. Ziman and D. Phil (Cambridge University Press, Cambridge, 1969) p. 67. [6] P. Lucasson, F. Maury and P. Moser, Ann. Chim. Fr. 9 (1984) 1.5. [7] R.G. Elliman, J.S. Williams, W.L. Brown, A. Leiberich, D.M. Maher and R.V. Knoell, Nucl. Instr. and Meth. B 19/20 (1987) 435. [8] J.P. Biersack and R.B. Eckstein, Appl. Phys. A 34 (1984) 73.