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Diffusion behavior of implanted iron in fused silica glass
Journal of Non-Crystalline Solids 104 (1988) 81-84 North-Holland, Amsterdam
81
D I F F U S I O N B E H A V I O R OF I M P L A N T E D I R O N IN F U S E D SILICA G L A S S * P.W. W A N G **, Y.P. F E N G , W.L. R O T H and J.W. C O R B E T T Pttvsics Department, SUNY/Albany, N Y 12222, USA Received 4 September 1987 Revised manuscript received 7 December 1987
Diffusion behavior of implanted iron in fused silica after thermal treatments was investigated by the Rutherford Back Scattering (RBS) technique. Two-way diffusion of implanted iron ions in silica after 400 ° C, 500 o C, 600 ° C, 700 ° C and 8 0 0 ° C annealings has been found. Implanted iron ions diffuse towards the implanted surface after 500 ° C , one hour annealing; while iron ions towards the surface and the bulk of the sample after 6 0 0 ° C and higher than 6 0 0 ° C heat treatments. After 600 ° C half hour treatment ions diffuse towards surface slower than ions diffuse into the bulk. The diffusion behavior of the implanted iron ions during heat treatment is controlled by competitive processes, i.e., trapping of the implantation damage, and diffusion between amorphous bulk structure and crystallized implantation layer. Diffusion of implanted iron ions is also influenced by the changes of chemical states of implanted ions through magnetic interactions among them.
1. Introduction Heat treatment is a necessary process for the dopant diffusion, thermal oxidation, and recovery of implantation damage in semiconductor device fabrication, but unwanted transition metals may be introduced into semiconductors duing this process [1,2]. By means of Deep Level Transient Spectroscopy and Electron Paramagnetic Resonance measurements, it has been found that iron impurities were introduced into silicon lattice after quenching silicon samples from 900 ° C to 1200 ° C in silica glass tube furnace (see, for example [3,4]). Iron has a deep donor level in the bandgap of silicon at ( E v + 0.38 eV) and can form (Fe + acceptors) pairs [5,6]. These energy levels associated with iron act as traps or generation-recombination centers which cause a reduction of the minority carrier lifetime as well as an increase in
* This work was supported in part by the DOE-JPL Flat Solar Array Program, the Solar Energy Research Institute, the Mobil Foundation, and the US A r m y Research Office. * * Present Address: Department of Mechanical and Materials Engineering, Vanderbilt University. Nashville, Tennessee 37235, USA. 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
the leakage current. Iron precipitation formed at high temperature reduces the breakdown strength of thin silicon oxide on Metal Oxide-Semiconductor (MOS) capacitors [7]. Since these deleterious effects occur in silicon due to iron introduced by heat treatment in silica glass tubes, we have investigated the diffusion behavior of implanted iron in fused silica glass.
2. Experimental Fused silica glass containing about 1 part per thousand hydroxyl was optically polished and chemically cleaned. Then a 2.5 c m x 4 cm x 0.05 cm piece of this glass was implanted with 120 keV iron ions (Fe +) at room temperature. A 0.5 m m thick M4N7 (0.99997) iron foil was used as the source of iron ions, The implantation dose was set t o 5.5 X 1016 F e / c m z. In order to minimize thermal heating during implantation, the current was held at a low value (1 to 3 ttA) and a cold trap surrounded the sample holder. The temperature on the sample was therefore held at room temperature during implantation. Samples were cut from
82
P. IV. Wang et aL / Diffusion behavior of implanted iron
the same piece of glass and annealed at different temperatures, i.e., from 4 0 0 ° C to 8 0 0 ° C in a conventional furnace (Lindberg 54453 tube furnace) for half an hour or one hour. In order to eliminate iron contamination from the furnace, a mullite tube was installed outside the silica glass tube [8] and nitrogen was used to flush the inner glass tube during annealing. By monitoring the energy and the yield of backscattered 2 MeV 4He ions which are incident on the sample, we can determine the implantation dose, the projected range of implanted ions under the surface and the diffusion behavior of implanted ions in the silica glass after heat treatments. The charges build up by 4He incident on the silica samples were neutralized by the emission of electrons from a hot filament.
3. Results
The implanted dose was checked by surface height methods [9], where the surface height of silica glass in the spectrum of backscattered 4He from the sample is the sum of the silicon step edge height and oxygen step edge height. We found that the implantation dose, which is 5.34 x 1016 F e / c m 2, calculated by the surface height method, is quite close to the dose, 5.5 x 106 F e / c m 2, obtained from the current integrator of the ion implanter. From the energy difference of the backscattered 4He between pure iron and implanted iron as shown in fig. 1, we found that the projected range of the implanted iron is 67 nm below the silica glass surface and the range straggling is 51 nm. This measured projected range is consistent with the calculated result [10]. The diffusion behavior of implanted iron in fused silica glass was determined from the changes of the iron peaks which were determined from the middle point of the half width of the iron distribution in the backscattering spectra of 2 MeV 4He bombarding the silica samples as shown in figs. 1 and 2. It is clear that implanted ions tend to diffuse both ways, i.e., towards the silica surface and into the bulk of the silica glass. When the annealing temperature is below 600 o C, the diffu-
50 -Silicon 5.34 x I016Fe/cmz Edge 45-alSiO 2 After Irnplantalion Surface ........ After 400% 30min. Annealing 4.0 . . . . After 500% 60rain Annealing Implanted 55 Iron Peak 2
==
3O
A
2.5
o
Pur~
2.0 15
~ii/ 1on
1.0
I.I
2 ,t,
1.2
1.3 14 ENERGY (MeV)
1.5
1.6
Fig. 1. Backscattered spectra of 2 MeV 4He ions from silica implanted with 120 keV Fe + ions to a fluence of 5.34× 1016 per cm a. The iron peak after implantation is 67 nm below the silica surface and the implanted layer is about 102 nm thick. Diffusion of ions after 400 o C, half hour and 500 o C, one hour annealing is also shown.
5.0F Silicon Edge a SiO? 4.5 [Surface . . . . I ---4.0r / I ........ t
-
I I~ 35 I / -
x 1016Fe/cm2 After Implantation After 600 0 3UminAnnemmg After 700% 30rain,Annealing After 800% 30min Annealing Implanted Irai Peak
554
o
.
.
.
.
3.0
2.5 0
2.0 15 1.0
\
0.5 II
12
1.5
1.4
1.5
16
ENERGY (MeV) Fig. 2. Backscattering spectra show two-way diffusion behavior of implanted irons in the supersaturated region after 600 o C, 700 o C and 800 ° C heat treatments.
P. W. Wang et al. / Diffusion behavior of implanted iron
sion of ions towards the surface is dominant. Implanted ions diffuse both ways after 600°C, half hour and 700 ° C, half hour annealing. Once the annealing temperature reaches 800 ° C, diffusion of ions towards the bulk is more important than the outdiffusion of the ions. Diffusion of implanted ions towards the bulk is clearly shown in fig. 2 after 600 ° C, 700 ° C and 8 0 0 ° C annealings. The higher the temperature, the more the implanted ions diffuse into the bulk. We cannot determine the furthest position of the diffusing iron towards bulk because the background of silicon and oxygen is much higher than the yield of diffused iron ions in the backscattering spectrum. We estimate the diffusivity of iron diffusing towards the bulk at 6 0 0 ° C is larger or equal to 8.0 X 10 13 cm2/s by assuming the diffusion length is equal, or greater than, the distance between the iron peak position and the silicon edges.
4. Discussion
In order to understand these diffusion results, the initial state of the implanted iron ions in silica glass is a key point. The implanted iron concentration in the silica before heat treatment can be easily calculated from the implantation dose and the thickness of implanted layer, i.e., about 102 nm, yielding a very high concentration of iron ions, 5.2 x 102~ per cm 3, located in this very thin implantation layer. Since implanted Fe ÷ ions lost their energies in the silica through atomic collisions and electronic ionizations, Fe 2÷ and Fe 3+ were also produced in the implantation layer. That is to say the initial state of implanted ions is supersaturated with different chemical states in which they can precipitate [11], diffuse, form compounds with silicon and oxygen [12], or change the relative concentrations of different chemical states of Fe +, Fe 2÷, and Fe 3÷ [13,14] depending upon the annealing temperatures. As indicated by Crowder and Title [15], the highest concentration of implanted ion in silicon is located deeper than the peak position of the damage profile. We believe that the outdiffusion of iron ions in the supersaturated region is due to
83
the interaction between implantation damage and ions with different chemical states during annealing. Defects created by implantation influence the diffusion of implanted iron ions if the damage is not well recovered. We have reported that incompletely-recovered implantation damage induced by iron implantation into crystalline silicon slows down, e.g., after l l 0 0 ° C and 1200°C annealing, or blocks, e.g., after 900 ° C and 1000 ° C annealing, the implanted iron ions diffusing into the bulk of silicon [16]. This slow-down or blocking phenomenon due to incompletely-recovered implantation damage may also happen in silica glass. Arnold [17] reported that a crystalline quartz film formed after 800°C annealing of Li + implanted silica and the thickness of the crystallized layer was very close to the depth of implanted Li +. From a comparison of our data with Arnold's results we conclude that the diffusivity of iron into the bulk silica is larger in non-damaged silica glass than in silica glass with a partially crystallized layer. This faster diffusion of iron ions into the bulk silica glass, which is determined to be > 8.0 x 10-~3 cmZ/s at 600°C, can result in iron contamination in silicon during heat treatment in the silica glass tube of a conventional furnace. Therefore we believe that the crystallization of the implantation layer can also play an important role in the diffusion of the implanted iron ions in silica. As we mentioned before, implanted iron ions mainly diffuse towards the bulk after 800 ° C annealing as shown in fig. 2. This may result from complete crystallization of the implanted layer and more efficient recovery of the implantation damage of this temperature. Step-like recovery characteristics of implanted damage over a short range of annealing temperature have also been found in crystalline silicon and germanium [16,18]. The crystallization and increased recovery of the implantation damage at 6 0 0 ° C may cause iron ions to drift away from this implanted region. The result of 8 0 0 ° C annealing shown in fig. 2 indicates that once the crystallization is completed and more damage recovered all the iron ions diffuse into the bulk silica. That is to say that most ions diffuse into the amorphous bulk when the implantation damage is more efficiently recovered
84
P. w. Wang et al. / Diffusion behavior of implanted iron
o r the i m p l a n t a t i o n layer is crystallized d u r i n g 800 ° C annealing. T h e R u t h e r f o r d b a c k s c a t t e r i n g s p e c t r u m in fig. 2 also shows that the p e a k position of i m p l a n t e d i r o n is l o c a t e d 63 n m b e l o w the surface after 800 ° C annealing, which is close to the original p e a k p o s i t i o n of i m p l a n t e d iron before the a n n e a l i n g of 67 n m b e l o w surface. This result is similar to the result of the Li + i m p l a n t e d e x p e r i m e n t [17]. W e e k s et al. [13,14] have r e p o r t e d the changes of m a g n e t i c i n t e r a c t i o n s a n d r e d i s t r i b u t i o n of relative c o n c e n t r a t i o n s a m o n g different chemical states of i m p l a n t e d i r o n ions in silica u n d e r 700 ° C a n d 800 ° C a n n e a l i n g in the air. T h e y c o n c l u d e d that the d o m i n a n t c h e m i c a l states after 700 ° C, 0.3 h t r e a t m e n t are F e 2+ a n d F e 3+ from their E P R results [13]. T h e y m e a s u r e d U V s p e c t r a of i r o n i m p l a n t e d silica a n d f o u n d that F e 3+ ions are i n c r e a s e d after 700 ° C or 800 ° C, 20 h a n n e a l i n g while F e 2+ ions are d e c r e a s e d [14]. D i f f e r e n t diffusivities of F e 2+ a n d F e 3+ in silica have b e e n r e p o r t e d [19,20]. F r o m these results we c o n c l u d e d t h a t the diffusion b e h a v i o r of i m p l a n t e d i r o n ions u n d e r heat t r e a t m e n t s is also influenced b y the c h a n g e s of c h e m i c a l states, F e +, F e z+, F e 3+, t h r o u g h the m a g n e t i c i n t e r a c t i o n s a m o n g them.
5. Summary (1) T w o - w a y d i f f u s i o n of i m p l a n t e d irons in silica after 4 0 0 ° C , 5 0 0 ° C , 6 0 0 ° C , 7 0 0 ° C a n d 800 ° C a n n e a l i n g was found. (2) T h e diffusion b e h a v i o r of i m p l a n t e d irons und e r heat t r e a t m e n t is c o n t r o l l e d b y (a) t r a p p i n g of the i m p l a n t a t i o n d a m a g e , i.e., degree of recovery of the i m p l a n t a t i o n damage, (b) d i f f u s i o n b e t w e e n a m o r p h o u s b u l k a n d crystallized i m p l a n t a t i o n layer, i.e., crystallization o f the i m p l a n t a t i o n layer, a n d
(c) changes of c h e m i c a l states of i m p l a n t e d ions.
References [1] P.F. Schmidt and C.W. Pearce, J. Electrochem. Soc. 128 (1981) 630. [2] P.F. Schmidt, J. Electrochem. Soc. 130 (1983) 197. [3] J.D. Gerson, L.J. Cheng and J.W. Corbett, Appl. Phys. Lett. 48 (1977) 4821. [4] Y.H. Lee, R.L. Kleinhenz and J.W. Corbett, Appl. Phys. Lett. 31 (1977) 142. [5] K. Wiinstel, K.-H. Froehner and P. Wanger, Physica 116B (1983) 301. [6] R. Sauer and J. Webber, Physica l16B (1983) 195. [7] Kouichirou Honda, Akira Ohsawa and Nobuo Yoyokura, Appl. Phys. lett. 46 (1985) 582. [8] R. Krishnamurthy, B.E. Ramachandran and K. Kaliyamurphy, Solid State Technol. 28 (1985) 213. [9] W.A. Wenzel and W. Whaling, Phys. Rev. 87 (1952) 449. [10] J.F. Gibbs, W.S. Johnson and S.W. Mylroie, in: Projected Range Statistics, Semiconductors and Related Materials, 2nd ed. (Dowden, Hutchinson and Ross, Stroudsburg, PA, 1975). [11] A. Chatelain, Ph. Buffat, P. Stadelmann and R.A. Weeks, J. Non-Cryst. Solids 71 (1985) 335. [12] D.L. Griscom, E.J. Friebele and D.B. Shinn, J. Appl. Phys. 50 (1979) 2402. [13] R.A. Weeks, M.C. Silva, G. Kordas, D.L. Kinser, J. Martinelli and B.R. Appleton, in: Ion Beam Processes in Advanced Electronic Materials and Device Technology, eds. B.R. Appleton, F.H. Eisen and T.W. Sigmon (Materials Research Society, PA, 1985). [14] R.A. Weeks, Glenn Whichard, George Kordas and B.R. Appleton, in: Proc. XIV Int. Congr. on Glass, New Delhi, India, 1986, p. 236. [15] B.L. Crowder and R.S. Title, in: Ion Implantation, eds. F.H. Eisen and C.T. Chadderton (Gordon and Beach, London, 1971) p. 87. [16] P.W. Wang, H.S. Cheng, W.M. Gibson and J.W. Corbett, J. Appl. Phys. 60 (1986) 1336. [17] G.W. Arnold, Radiat. Eft. 47 (1980) 15. [18] J.W. Mayer, L. Eriksson, S.T. Picraux and J.A. Davies, Can. J. Phys. 46 (1968) 663. [19] R.N. Ghoshtagore, J. Appl. Phys. 40 (1969) 4373. [20] A. Atkinson and J.W. Gardner, Corr. Sci. 21 (1981) 49.
81
D I F F U S I O N B E H A V I O R OF I M P L A N T E D I R O N IN F U S E D SILICA G L A S S * P.W. W A N G **, Y.P. F E N G , W.L. R O T H and J.W. C O R B E T T Pttvsics Department, SUNY/Albany, N Y 12222, USA Received 4 September 1987 Revised manuscript received 7 December 1987
Diffusion behavior of implanted iron in fused silica after thermal treatments was investigated by the Rutherford Back Scattering (RBS) technique. Two-way diffusion of implanted iron ions in silica after 400 ° C, 500 o C, 600 ° C, 700 ° C and 8 0 0 ° C annealings has been found. Implanted iron ions diffuse towards the implanted surface after 500 ° C , one hour annealing; while iron ions towards the surface and the bulk of the sample after 6 0 0 ° C and higher than 6 0 0 ° C heat treatments. After 600 ° C half hour treatment ions diffuse towards surface slower than ions diffuse into the bulk. The diffusion behavior of the implanted iron ions during heat treatment is controlled by competitive processes, i.e., trapping of the implantation damage, and diffusion between amorphous bulk structure and crystallized implantation layer. Diffusion of implanted iron ions is also influenced by the changes of chemical states of implanted ions through magnetic interactions among them.
1. Introduction Heat treatment is a necessary process for the dopant diffusion, thermal oxidation, and recovery of implantation damage in semiconductor device fabrication, but unwanted transition metals may be introduced into semiconductors duing this process [1,2]. By means of Deep Level Transient Spectroscopy and Electron Paramagnetic Resonance measurements, it has been found that iron impurities were introduced into silicon lattice after quenching silicon samples from 900 ° C to 1200 ° C in silica glass tube furnace (see, for example [3,4]). Iron has a deep donor level in the bandgap of silicon at ( E v + 0.38 eV) and can form (Fe + acceptors) pairs [5,6]. These energy levels associated with iron act as traps or generation-recombination centers which cause a reduction of the minority carrier lifetime as well as an increase in
* This work was supported in part by the DOE-JPL Flat Solar Array Program, the Solar Energy Research Institute, the Mobil Foundation, and the US A r m y Research Office. * * Present Address: Department of Mechanical and Materials Engineering, Vanderbilt University. Nashville, Tennessee 37235, USA. 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
the leakage current. Iron precipitation formed at high temperature reduces the breakdown strength of thin silicon oxide on Metal Oxide-Semiconductor (MOS) capacitors [7]. Since these deleterious effects occur in silicon due to iron introduced by heat treatment in silica glass tubes, we have investigated the diffusion behavior of implanted iron in fused silica glass.
2. Experimental Fused silica glass containing about 1 part per thousand hydroxyl was optically polished and chemically cleaned. Then a 2.5 c m x 4 cm x 0.05 cm piece of this glass was implanted with 120 keV iron ions (Fe +) at room temperature. A 0.5 m m thick M4N7 (0.99997) iron foil was used as the source of iron ions, The implantation dose was set t o 5.5 X 1016 F e / c m z. In order to minimize thermal heating during implantation, the current was held at a low value (1 to 3 ttA) and a cold trap surrounded the sample holder. The temperature on the sample was therefore held at room temperature during implantation. Samples were cut from
82
P. IV. Wang et aL / Diffusion behavior of implanted iron
the same piece of glass and annealed at different temperatures, i.e., from 4 0 0 ° C to 8 0 0 ° C in a conventional furnace (Lindberg 54453 tube furnace) for half an hour or one hour. In order to eliminate iron contamination from the furnace, a mullite tube was installed outside the silica glass tube [8] and nitrogen was used to flush the inner glass tube during annealing. By monitoring the energy and the yield of backscattered 2 MeV 4He ions which are incident on the sample, we can determine the implantation dose, the projected range of implanted ions under the surface and the diffusion behavior of implanted ions in the silica glass after heat treatments. The charges build up by 4He incident on the silica samples were neutralized by the emission of electrons from a hot filament.
3. Results
The implanted dose was checked by surface height methods [9], where the surface height of silica glass in the spectrum of backscattered 4He from the sample is the sum of the silicon step edge height and oxygen step edge height. We found that the implantation dose, which is 5.34 x 1016 F e / c m 2, calculated by the surface height method, is quite close to the dose, 5.5 x 106 F e / c m 2, obtained from the current integrator of the ion implanter. From the energy difference of the backscattered 4He between pure iron and implanted iron as shown in fig. 1, we found that the projected range of the implanted iron is 67 nm below the silica glass surface and the range straggling is 51 nm. This measured projected range is consistent with the calculated result [10]. The diffusion behavior of implanted iron in fused silica glass was determined from the changes of the iron peaks which were determined from the middle point of the half width of the iron distribution in the backscattering spectra of 2 MeV 4He bombarding the silica samples as shown in figs. 1 and 2. It is clear that implanted ions tend to diffuse both ways, i.e., towards the silica surface and into the bulk of the silica glass. When the annealing temperature is below 600 o C, the diffu-
50 -Silicon 5.34 x I016Fe/cmz Edge 45-alSiO 2 After Irnplantalion Surface ........ After 400% 30min. Annealing 4.0 . . . . After 500% 60rain Annealing Implanted 55 Iron Peak 2
==
3O
A
2.5
o
Pur~
2.0 15
~ii/ 1on
1.0
I.I
2 ,t,
1.2
1.3 14 ENERGY (MeV)
1.5
1.6
Fig. 1. Backscattered spectra of 2 MeV 4He ions from silica implanted with 120 keV Fe + ions to a fluence of 5.34× 1016 per cm a. The iron peak after implantation is 67 nm below the silica surface and the implanted layer is about 102 nm thick. Diffusion of ions after 400 o C, half hour and 500 o C, one hour annealing is also shown.
5.0F Silicon Edge a SiO? 4.5 [Surface . . . . I ---4.0r / I ........ t
-
I I~ 35 I / -
x 1016Fe/cm2 After Implantation After 600 0 3UminAnnemmg After 700% 30rain,Annealing After 800% 30min Annealing Implanted Irai Peak
554
o
.
.
.
.
3.0
2.5 0
2.0 15 1.0
\
0.5 II
12
1.5
1.4
1.5
16
ENERGY (MeV) Fig. 2. Backscattering spectra show two-way diffusion behavior of implanted irons in the supersaturated region after 600 o C, 700 o C and 800 ° C heat treatments.
P. W. Wang et al. / Diffusion behavior of implanted iron
sion of ions towards the surface is dominant. Implanted ions diffuse both ways after 600°C, half hour and 700 ° C, half hour annealing. Once the annealing temperature reaches 800 ° C, diffusion of ions towards the bulk is more important than the outdiffusion of the ions. Diffusion of implanted ions towards the bulk is clearly shown in fig. 2 after 600 ° C, 700 ° C and 8 0 0 ° C annealings. The higher the temperature, the more the implanted ions diffuse into the bulk. We cannot determine the furthest position of the diffusing iron towards bulk because the background of silicon and oxygen is much higher than the yield of diffused iron ions in the backscattering spectrum. We estimate the diffusivity of iron diffusing towards the bulk at 6 0 0 ° C is larger or equal to 8.0 X 10 13 cm2/s by assuming the diffusion length is equal, or greater than, the distance between the iron peak position and the silicon edges.
4. Discussion
In order to understand these diffusion results, the initial state of the implanted iron ions in silica glass is a key point. The implanted iron concentration in the silica before heat treatment can be easily calculated from the implantation dose and the thickness of implanted layer, i.e., about 102 nm, yielding a very high concentration of iron ions, 5.2 x 102~ per cm 3, located in this very thin implantation layer. Since implanted Fe ÷ ions lost their energies in the silica through atomic collisions and electronic ionizations, Fe 2÷ and Fe 3+ were also produced in the implantation layer. That is to say the initial state of implanted ions is supersaturated with different chemical states in which they can precipitate [11], diffuse, form compounds with silicon and oxygen [12], or change the relative concentrations of different chemical states of Fe +, Fe 2÷, and Fe 3÷ [13,14] depending upon the annealing temperatures. As indicated by Crowder and Title [15], the highest concentration of implanted ion in silicon is located deeper than the peak position of the damage profile. We believe that the outdiffusion of iron ions in the supersaturated region is due to
83
the interaction between implantation damage and ions with different chemical states during annealing. Defects created by implantation influence the diffusion of implanted iron ions if the damage is not well recovered. We have reported that incompletely-recovered implantation damage induced by iron implantation into crystalline silicon slows down, e.g., after l l 0 0 ° C and 1200°C annealing, or blocks, e.g., after 900 ° C and 1000 ° C annealing, the implanted iron ions diffusing into the bulk of silicon [16]. This slow-down or blocking phenomenon due to incompletely-recovered implantation damage may also happen in silica glass. Arnold [17] reported that a crystalline quartz film formed after 800°C annealing of Li + implanted silica and the thickness of the crystallized layer was very close to the depth of implanted Li +. From a comparison of our data with Arnold's results we conclude that the diffusivity of iron into the bulk silica is larger in non-damaged silica glass than in silica glass with a partially crystallized layer. This faster diffusion of iron ions into the bulk silica glass, which is determined to be > 8.0 x 10-~3 cmZ/s at 600°C, can result in iron contamination in silicon during heat treatment in the silica glass tube of a conventional furnace. Therefore we believe that the crystallization of the implantation layer can also play an important role in the diffusion of the implanted iron ions in silica. As we mentioned before, implanted iron ions mainly diffuse towards the bulk after 800 ° C annealing as shown in fig. 2. This may result from complete crystallization of the implanted layer and more efficient recovery of the implantation damage of this temperature. Step-like recovery characteristics of implanted damage over a short range of annealing temperature have also been found in crystalline silicon and germanium [16,18]. The crystallization and increased recovery of the implantation damage at 6 0 0 ° C may cause iron ions to drift away from this implanted region. The result of 8 0 0 ° C annealing shown in fig. 2 indicates that once the crystallization is completed and more damage recovered all the iron ions diffuse into the bulk silica. That is to say that most ions diffuse into the amorphous bulk when the implantation damage is more efficiently recovered
84
P. w. Wang et al. / Diffusion behavior of implanted iron
o r the i m p l a n t a t i o n layer is crystallized d u r i n g 800 ° C annealing. T h e R u t h e r f o r d b a c k s c a t t e r i n g s p e c t r u m in fig. 2 also shows that the p e a k position of i m p l a n t e d i r o n is l o c a t e d 63 n m b e l o w the surface after 800 ° C annealing, which is close to the original p e a k p o s i t i o n of i m p l a n t e d iron before the a n n e a l i n g of 67 n m b e l o w surface. This result is similar to the result of the Li + i m p l a n t e d e x p e r i m e n t [17]. W e e k s et al. [13,14] have r e p o r t e d the changes of m a g n e t i c i n t e r a c t i o n s a n d r e d i s t r i b u t i o n of relative c o n c e n t r a t i o n s a m o n g different chemical states of i m p l a n t e d i r o n ions in silica u n d e r 700 ° C a n d 800 ° C a n n e a l i n g in the air. T h e y c o n c l u d e d that the d o m i n a n t c h e m i c a l states after 700 ° C, 0.3 h t r e a t m e n t are F e 2+ a n d F e 3+ from their E P R results [13]. T h e y m e a s u r e d U V s p e c t r a of i r o n i m p l a n t e d silica a n d f o u n d that F e 3+ ions are i n c r e a s e d after 700 ° C or 800 ° C, 20 h a n n e a l i n g while F e 2+ ions are d e c r e a s e d [14]. D i f f e r e n t diffusivities of F e 2+ a n d F e 3+ in silica have b e e n r e p o r t e d [19,20]. F r o m these results we c o n c l u d e d t h a t the diffusion b e h a v i o r of i m p l a n t e d i r o n ions u n d e r heat t r e a t m e n t s is also influenced b y the c h a n g e s of c h e m i c a l states, F e +, F e z+, F e 3+, t h r o u g h the m a g n e t i c i n t e r a c t i o n s a m o n g them.
5. Summary (1) T w o - w a y d i f f u s i o n of i m p l a n t e d irons in silica after 4 0 0 ° C , 5 0 0 ° C , 6 0 0 ° C , 7 0 0 ° C a n d 800 ° C a n n e a l i n g was found. (2) T h e diffusion b e h a v i o r of i m p l a n t e d irons und e r heat t r e a t m e n t is c o n t r o l l e d b y (a) t r a p p i n g of the i m p l a n t a t i o n d a m a g e , i.e., degree of recovery of the i m p l a n t a t i o n damage, (b) d i f f u s i o n b e t w e e n a m o r p h o u s b u l k a n d crystallized i m p l a n t a t i o n layer, i.e., crystallization o f the i m p l a n t a t i o n layer, a n d
(c) changes of c h e m i c a l states of i m p l a n t e d ions.
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