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Radiation damage and amorphization of silicon by 2 MeV nitrogen ion implantation
Materials Science and Engineering, B I 2 (1992) 7-11
/
Radiation damage and amorphization of silicon by 2 MeV nitrogen ion implantation J. K. N. Lindner, R. Zuschlag and E. H. te Kaat Institute of Physics, University of Dortmund, POB 500 500, W-4600Dortmund 50 (FR G)
Abstract Radiation damage and amorphization of (111) silicon after 2 MeV 14N ion implantation at temperatures from 125 to 450 K is studied in a dose range of 2 x 10 ~3to 5 x 10 ~7N cm -2 mainly by optical reflectivity depth profiling. Over a wide range of doses and temperatures, damage can be described by a model (N. Hecking et al., Nucl. Instr. and Meth., B15 (1986) 760). A comparison of model parameters for nitrogen and heavier ions yields insight in the different weights of defect production and interaction processes involved. A favoring influence of nitrogen on the defect stabilization and the amorphization is discussed.
1. Introduction
Recently, radiation damage and amorphization of silicon owing to MeV implantation of heavy ions as 197Au o r 58Ni has been studied [1, 2] by optical reflectivity depth profiling [3] and transmission electron microscopy (TEM). A damage model proposed by Hecking et al. [4, 5] to describe the dose and temperature dependence of the optically detected damage for 2 MeV 2ssi self-irradiations has been successfully applied also for 6 MeV 58Ni implantations [2], using an altered set of model parameters. Supported by the model considerations, a suppression of the silicon amorphization occurring at high nickel concentrations [6, 7] has been interpreted in terms of an influence of the metallic impurities on the covalent character of silicon bonds [2, 7]. In this article, our studies on MeV ion implantation damage are extended towards lower ion masses. Nitrogen has been chosen as ion species, since from studies on the keV ion beam synthesis of buried silicon nitride layers it is known [8] that even at very high implantation temperatures, amorphous layers are obtained after high dose nitrogen implantations, possibly indicating a stimulating influence of nitrogen on the amorphization of silicon. 2. Experimental details
The 2 MeV t4N+ ions have been implanted from a 7 ° off-normal direction into (111) oriented, n-phosphorus doped ( 8 5 - 1 1 0 Q c m ) silicon crystals at constant target temperatures between 125 and 450 K and doses of 2 x 1013 to 5 x 1017 N cm -2. Implanta0921-5107/92/$5.00
tions were carried out at the Dynamitron-TandemLaboratory at Bochum (FRG), as described elsewhere [7] in more detail. Irradiated samples were bevelled to angles of approximately 0.5 ° in order to reveal the damage depth profile at the sample surface for measurements of the optical reflectivity change [3] at a wavelength of 650 nm, At sufficiently low doses, the change of reflectivity R compared with the reflectivity R c of crystalline silicon (c-Si) is mainly a result of distorted bonds. The relative reflectivity change AR/Rc, normalized to its maximum value ARmJ R ~ obtained at amorphization, can be used [1, 2, 4, 5, 9] as a measure of damage at the depth z
AR(z)/ARmax -
R(z)-Rc R,a-Rc
(1)
where Ria is the reflectivity of implantation amorphized silicon (ia-Si).
3. Results and discussion
For doses below the amorphization thresholds, Figs. l(a) and l(b) show semi-logarithmic depth profiles of the radiation damage s for temperatures of 125 and 300 K, respectively. In Fig. 1(c), depth profiles of the normalized reflectivity change are presented for 450 K. In the interpretation of the high dose profiles in Fig. 1(c), the definition of s (eqn. (1)) is no longer valid, since in addition to the damage, the contribution of implanted nitrogen to the reflectivity change has to be taken into account. © 1992--Elsevier Sequoia. All rights reserved
8
J . K . N . Lituhzer el a/.
/'
2 Me l'nitrogen ion implantation o/Si
For the same temperatures and doses above the amorphization dose D*, optical profiles are shown in Fig. 2. As expected for low doses, the buried amorphous layers formed at 125 K are characterized by a constant reflectivity change AR/ARm,x = 1 (Fig. 2(a)). At higher temperatures, the crystalline/amorphous interfaces become increasingly steeper, leading to wedge interference fringes [3] being predominantly produced between the upper c-Si/ia-Si interface and the bevelled sample surface. Moreover, at the high doses necessary to produce buried amorphous layers at these temperatures, the evolution of a local minimum induced by the deposited nitrogen atoms is clearly visible within the amorphous profile parts. The depth of maximum nitrogen concentration of 2.27 + 0.10~m is in good agreement with a former result of Bussmann et al. [10] using the same technique. Basing on measurements at amorphous layers, the contribution of nitrogen to the optical reflectivity of non-amorphized samples can be roughly estimated, assuming that the
effect of impurities is the same in damaged c-Si and in ia-Si. In Fig. 3, damage at profile maximum is displayed as a function of dose for different temperatures. I~)r 350 K and above, the estimated influence of nitrogen atoms on the optical reflectivity change has been taken into account, leading to considerably higher damage values at doses above 1015 N cm 2. For 450 K this correction is indicated by the original data points and arrows in Fig. 3. The shapes of the damage curves are similar to the shape obtained for MeV silicon, nickel and gold ion implantations [1, 2, 4] and therefore may be interpreted mainly on the basis of the same micromechanisms. For doses up to 5 x 1013 N cm-2 damage increases linearly with dose for all temperatures. Since at low doses collision cascades predominantly develop in a not predamaged crystal, each cascade leads on average to the same amount of damage. The decreasing damage efficiency s/D obtained in the low dose region for
Ti = t,50 K
Ti = 300 K
Ti =125 K
I
D{ 101t'cm4 ] 50O 2O0 100 50
D[IO" cm"2] 50
50 30
30 20
10 -I
10
t,~ c3 10-2
10-3
(a)
0
I 2 DEPTH z [pm]
t: 3
I
/3
70. x
104
L Lz
r
o
Q
p
/b)
~
1
1
I 2 DEPTH z [pm]
(c)
0 2
4 3
DEPTH z [pro]
Fig. 1. Depth profiles of the radiation damage at: (a) 125 K; (b) 300 K, and of the normalized reflectivity change at (c) 450 K after 2 MeV nitrogen ion implantation. Ti =125 K
l~=t+50K
Ti =300K
O110~6cm-2l
Ol101Scm-21
10
1
i
1
(a)
2 DEPTH z lpm]
3
(b)
DEPTHzlpml
3
I
(c)
2 DEPTHz [pro]
Fig. 2. Depth profiles of the normalized reflectivity change of silicon amorphized by 2 MeV N ion implantation at: (a) 125 K; (b) 300 K and (c) 450 K. Profiles for different doses are vertically shifted by AR/ARma× = 1.
J. K. N. Lindner et al. / 2 Me V nitrogen ion implantation of Si ,
,
~
,
, ~
;
- :!
9
1018
10-1
'E 10 u,
w (,9
SaNi
•
El 10_2 14 + ~
10-3 1012
I 1013
I 1014
i 10 TM DOSE
si
I 1016
~
I 1017
1015
f~ 1
(N/cm 2)
Fig. 3. Dose dependence of the opticallydetected damage at profile maximum after 2 MeV N-Si implantation at different temperatures.
~
1014
1013 /
0
,
I
,
L ,
TEMPERATURE
increasing implantation temperatures reflects a stronger defect annihilation within single collision cascades at higher target temperatures. At doses above approximately 5 x 1013 N cm -2 the generation of collision cascades in predamaged regions favors the interaction of point defects from different collision cascades, leading to deviations from a dose proportional damage behavior. At temperatures T i >t 300 K point defect recombination obviously predominates the stabilization of point defects into clusters, resulting in an underproportional dose dependence of damage. Recombination differs from silicon or nickel implantations [2, 4] in that it does not lead to a plateaulike damage behavior at temperatures as high as 450 K (discussed later). For all implantation temperatures there is a dose interval where damage increases overproportionally with dose. This stronger increase is attributed to point defect clustering and the stimulated growth [4, 5] of amorphous regions, It continues until damage saturation occurs owing to the formation of a continuous amorphous layer (s = 1 ). The critical dose DN* for amorphization of (111) silicon of 2 MeV N ions is shown in Fig. 4 as a function of temperature and is compared with corresponding data for other MeV ions. Up to temperatures of approximately 300 K critical doses for nitrogen ions are reasonably higher than those for the heavier ion species; the ratio DN*/Dsi* is increasing. Using the TRIM [11] nuclear stopping power of 2 MeV N ions (10 eV A-1) at profile maximum, a critical energy
I
,
I
,
I
100 200 300 4-00 500 Ti I'K3
Fig. 4. Temperature dependence of the amorphization dose D* of (111) silicon after implantation of 2 MeV HN, 2 MeV 2sSi[4], 6 MeV 5SNi[2], and 5 MeV '"TAu[ 1] ions.
increasing temperatures, which indicates a favoring influence of nitrogen on the amorphization of silicon. At T i = 450 K the critical dose for nitrogen ions is even smaller than that for the heavier silicon projectiles. For temperatures up to 300 K damage accumulation can be described up to amorphization by the model of Hecking et al. [4, 5] (solid lines in Fig. 3). For higher temperatures a satisfactory description is not obtained in the dose intervals marked by dashed lines in Fig. 3, where damage increases under-proportionally with dose but does not pass a transient saturation, as in the case of silicon or nickel implantations. Since this stronger damage increase is most distinct at the depth of the profile maximum, it may be concluded that implanted nitrogen contributes to a stabilization of defects. The same assumption has been made [14] to explain the increased crystallization temperature of nitrogen implanted ia-Si layers. Moreover, TEM investigations on 450 K samples reveal the presence of extended defects, which might contribute to the stronger damage increase, but are not included in the damage model. In the model [4, 5] damage is considered as a sum s = Sp+ sa of the relative fractions damaged by isolated or clustered point defects sp and amorphization Sa. The dose dependence of damage is given by
densityforamorphization[12]of(5.6+O.2)xlO14eV~D=pp[eXp(_R2D2)](l_sa)+Csp(l_ cm -3 is calculated from a T--*0 extrapolation of DN*, in good agreement with the results for other MeV and keV implantations [2, 12, 13]. At temperatures above 300 K, i.e. critical doses above 1016 N cm -2 and nitrogen peak concentrations of more than 1 at.% according to TRIM [11], the amorphization dose for nitrogen ions increases comparatively slowly with
Sp) sp*( 1 - sa)
dsa sp dD 1 - s ,
-dsa - = ( P a +A~Sa)(1-Sa) dD
(2)
(3)
I0
J . K . N . Lindner eta/.
,/
2 Me V nitrogen ion implantation ojSi
TABLE 1. Model parameters for the calculation of solid lines in Fig. 3 by eqns. (2) and (3) 7~
1'
R
C"
P,
, t,
[K]
[lP0-~4cm2]
[10 '4cm21
ll0 '4cm2]
se*
11'0 '4cm:l
IJ0 '4cm2I
125 210 300 350 400 450
0.033 0.032 0.030 0.027 0.020 0.012
0 0.20 0.65 0.95 1.00 0.68
0.70 (1.45 0.35 0.28 0.26 0.19
1 0.15 0.068 0.055 0.044 0.038
0.027 0.014 9E - 4 2E - 5 1E - 7 1E - 9
U.55 0.48 0.08 ().06 0.04 0.03
1043
"
1 104!
104-~ •
Ni~Si
E
r
i
104z
• Si~Si •
i
N~Si 104!
104~. 104~"
~C ~
~
04
P~
104~ 10-17
1045
104~
8
IE
r I~
i
1
i
1
100 200 30o ~00 TEMPE RATURE [K ]
"N"
i
5oo
10-15
A Si ~ S i • N ~Si
104s
I
i
i
0-2 liT2t
i
100 200 300 bOO TEMPERATURE [K]
500
i
i
i
l
100 200 300 4.00 500 TEMPERATURE [ K ]
Fig. 5. Temperature dependence of defect production and interaction parameters, used to describe lattice damage by 2 MeV nitrogen, 2 MeV silicon [4] and 6 MeV nickel [2] ions with eqns. (2) and (3).
and temperature dependent model parameters (Table 1) obtained by a fit. They are compared with parameters for the damage by heavier ions in Fig. 5. Primary production of point defects Pp by nitrogen ions is smaller than that for silicon ions by a factor of 2.5 at temperatures up to 300 K and by a factor of up to 3.7 above. These factors have to be compared with the ratio of nuclear stopping powers at the profile maximum, which is 2,5 according to T R I M [11]. Recombination R of point defects from different collision cascades is smaller than for the heavier ions, most probably a result of the smaller size of the cascades produced by nitrogen ions. Parameter C describing the clustering of point defects from different cascades is nearly the same for all ion species considered, while parameter Sp* for the saturation of point defects differs f r o m that obtained for silicon and nitrogen ions. The primary production Pa N of amorphous regions by nitrogen ions is significantly smaller than that for silicon ions, where the ratio PaN/Pa s~ decreases with increasing temperatures. The parameter A N, which describes the stimulated growth of amorphous regions and which is determined in the dose region of overproportional damage increase, is con-
siderably smaller than that for silicon ions at temperatures up to 300 K. At higher temperatures, where higher doses are required to achieve the transition to amorphization, A.N decreases less than expected with increasing temperature. Therefore the small critical doses DN* at elevated temperatures have not to be referred to an enhanced primary production but to a nitrogen enhanced growth of amorphous regions, i.e. to a diminished crystallization of spike volumes owing to the presence of nitrogen. The opposite behavior of A, Ni has been observed and the reverse argumentation has been used to explain the nickel-induced suppression of amorphization [2].
References
1 J. K. N. Lindner, N. Hecking and E. te Kaat, Nucl. lnstrum. Methods, B26 (1987) 551. 2 J.K.N. Lindner, R. Domres and E. H. te Kaat, Nucl. lnstrum. Methods, B39 (1989) 306. 3 K.F. Heidemann, Philos. Mag., B44 (1981) 465. 4 N. Hecking, K. F. Heidemann and E. te Kaat, Nucl. lnstrum. Methods, B15 (1986) 760.
J. K. N. Lindner et al.
/
2 Me V nitrogen ion implantation of Si
5 N. Hecking and E. H. te Kaat, Appl. Surf. Sci., 43 (1989) 87. 6 J. K. N. Lindner and E. H. te Kaat, Mater. Res. Soc. Syrup. Proc., 107(1988) 275. 7 J. K. N. Lindner and E. H. te Kaat, J. Mater. Res., 3 (1988) 1238. 8 J. Petruzzello, T. F. McGee, M. H. Frommer, V. Rumennik, P. A. Waiters and C. J. Chou, J. Appl. Phys., 58(1985) 4605. 9 K. F. Heidemann and H. F. Kappert, Defects and Radiation Effects in Semiconductors, Conf. Ser. 46 (Institute of Physics, London) 1979, p. 492. 10 U. Bussmann, N. Hecking, K. F. Heidemann and E. H. te
11
Kaat, Nucl. lnstrum. Methods, B15 (1986) 105. 11 J. F. Ziegler, J. P. Biersack and U. Littmark, in J. F. Ziegler (ed.), The Stopping and Range of Ions in Matter, Vol. l, Pergamon, New York, 1985. 12 E L. Vook, in J. E. Whitehouse (ed.), Radiation Damage and Defects in Semiconductors, Institute of Physics, 1972, pp. 60-71. 13 J. Narayan, D. Fathy, O. S. Oen and O. W. Holland, J. Vac. Sci. Technol., A2 (1984) 1303. 14 F. E Komarov, I. A. Rogalevich and V. S. Tishkov, Radiat. Eft., 39(1978) 163.
/
Radiation damage and amorphization of silicon by 2 MeV nitrogen ion implantation J. K. N. Lindner, R. Zuschlag and E. H. te Kaat Institute of Physics, University of Dortmund, POB 500 500, W-4600Dortmund 50 (FR G)
Abstract Radiation damage and amorphization of (111) silicon after 2 MeV 14N ion implantation at temperatures from 125 to 450 K is studied in a dose range of 2 x 10 ~3to 5 x 10 ~7N cm -2 mainly by optical reflectivity depth profiling. Over a wide range of doses and temperatures, damage can be described by a model (N. Hecking et al., Nucl. Instr. and Meth., B15 (1986) 760). A comparison of model parameters for nitrogen and heavier ions yields insight in the different weights of defect production and interaction processes involved. A favoring influence of nitrogen on the defect stabilization and the amorphization is discussed.
1. Introduction
Recently, radiation damage and amorphization of silicon owing to MeV implantation of heavy ions as 197Au o r 58Ni has been studied [1, 2] by optical reflectivity depth profiling [3] and transmission electron microscopy (TEM). A damage model proposed by Hecking et al. [4, 5] to describe the dose and temperature dependence of the optically detected damage for 2 MeV 2ssi self-irradiations has been successfully applied also for 6 MeV 58Ni implantations [2], using an altered set of model parameters. Supported by the model considerations, a suppression of the silicon amorphization occurring at high nickel concentrations [6, 7] has been interpreted in terms of an influence of the metallic impurities on the covalent character of silicon bonds [2, 7]. In this article, our studies on MeV ion implantation damage are extended towards lower ion masses. Nitrogen has been chosen as ion species, since from studies on the keV ion beam synthesis of buried silicon nitride layers it is known [8] that even at very high implantation temperatures, amorphous layers are obtained after high dose nitrogen implantations, possibly indicating a stimulating influence of nitrogen on the amorphization of silicon. 2. Experimental details
The 2 MeV t4N+ ions have been implanted from a 7 ° off-normal direction into (111) oriented, n-phosphorus doped ( 8 5 - 1 1 0 Q c m ) silicon crystals at constant target temperatures between 125 and 450 K and doses of 2 x 1013 to 5 x 1017 N cm -2. Implanta0921-5107/92/$5.00
tions were carried out at the Dynamitron-TandemLaboratory at Bochum (FRG), as described elsewhere [7] in more detail. Irradiated samples were bevelled to angles of approximately 0.5 ° in order to reveal the damage depth profile at the sample surface for measurements of the optical reflectivity change [3] at a wavelength of 650 nm, At sufficiently low doses, the change of reflectivity R compared with the reflectivity R c of crystalline silicon (c-Si) is mainly a result of distorted bonds. The relative reflectivity change AR/Rc, normalized to its maximum value ARmJ R ~ obtained at amorphization, can be used [1, 2, 4, 5, 9] as a measure of damage at the depth z
AR(z)/ARmax -
R(z)-Rc R,a-Rc
(1)
where Ria is the reflectivity of implantation amorphized silicon (ia-Si).
3. Results and discussion
For doses below the amorphization thresholds, Figs. l(a) and l(b) show semi-logarithmic depth profiles of the radiation damage s for temperatures of 125 and 300 K, respectively. In Fig. 1(c), depth profiles of the normalized reflectivity change are presented for 450 K. In the interpretation of the high dose profiles in Fig. 1(c), the definition of s (eqn. (1)) is no longer valid, since in addition to the damage, the contribution of implanted nitrogen to the reflectivity change has to be taken into account. © 1992--Elsevier Sequoia. All rights reserved
8
J . K . N . Lituhzer el a/.
/'
2 Me l'nitrogen ion implantation o/Si
For the same temperatures and doses above the amorphization dose D*, optical profiles are shown in Fig. 2. As expected for low doses, the buried amorphous layers formed at 125 K are characterized by a constant reflectivity change AR/ARm,x = 1 (Fig. 2(a)). At higher temperatures, the crystalline/amorphous interfaces become increasingly steeper, leading to wedge interference fringes [3] being predominantly produced between the upper c-Si/ia-Si interface and the bevelled sample surface. Moreover, at the high doses necessary to produce buried amorphous layers at these temperatures, the evolution of a local minimum induced by the deposited nitrogen atoms is clearly visible within the amorphous profile parts. The depth of maximum nitrogen concentration of 2.27 + 0.10~m is in good agreement with a former result of Bussmann et al. [10] using the same technique. Basing on measurements at amorphous layers, the contribution of nitrogen to the optical reflectivity of non-amorphized samples can be roughly estimated, assuming that the
effect of impurities is the same in damaged c-Si and in ia-Si. In Fig. 3, damage at profile maximum is displayed as a function of dose for different temperatures. I~)r 350 K and above, the estimated influence of nitrogen atoms on the optical reflectivity change has been taken into account, leading to considerably higher damage values at doses above 1015 N cm 2. For 450 K this correction is indicated by the original data points and arrows in Fig. 3. The shapes of the damage curves are similar to the shape obtained for MeV silicon, nickel and gold ion implantations [1, 2, 4] and therefore may be interpreted mainly on the basis of the same micromechanisms. For doses up to 5 x 1013 N cm-2 damage increases linearly with dose for all temperatures. Since at low doses collision cascades predominantly develop in a not predamaged crystal, each cascade leads on average to the same amount of damage. The decreasing damage efficiency s/D obtained in the low dose region for
Ti = t,50 K
Ti = 300 K
Ti =125 K
I
D{ 101t'cm4 ] 50O 2O0 100 50
D[IO" cm"2] 50
50 30
30 20
10 -I
10
t,~ c3 10-2
10-3
(a)
0
I 2 DEPTH z [pm]
t: 3
I
/3
70. x
104
L Lz
r
o
Q
p
/b)
~
1
1
I 2 DEPTH z [pm]
(c)
0 2
4 3
DEPTH z [pro]
Fig. 1. Depth profiles of the radiation damage at: (a) 125 K; (b) 300 K, and of the normalized reflectivity change at (c) 450 K after 2 MeV nitrogen ion implantation. Ti =125 K
l~=t+50K
Ti =300K
O110~6cm-2l
Ol101Scm-21
10
1
i
1
(a)
2 DEPTH z lpm]
3
(b)
DEPTHzlpml
3
I
(c)
2 DEPTHz [pro]
Fig. 2. Depth profiles of the normalized reflectivity change of silicon amorphized by 2 MeV N ion implantation at: (a) 125 K; (b) 300 K and (c) 450 K. Profiles for different doses are vertically shifted by AR/ARma× = 1.
J. K. N. Lindner et al. / 2 Me V nitrogen ion implantation of Si ,
,
~
,
, ~
;
- :!
9
1018
10-1
'E 10 u,
w (,9
SaNi
•
El 10_2 14 + ~
10-3 1012
I 1013
I 1014
i 10 TM DOSE
si
I 1016
~
I 1017
1015
f~ 1
(N/cm 2)
Fig. 3. Dose dependence of the opticallydetected damage at profile maximum after 2 MeV N-Si implantation at different temperatures.
~
1014
1013 /
0
,
I
,
L ,
TEMPERATURE
increasing implantation temperatures reflects a stronger defect annihilation within single collision cascades at higher target temperatures. At doses above approximately 5 x 1013 N cm -2 the generation of collision cascades in predamaged regions favors the interaction of point defects from different collision cascades, leading to deviations from a dose proportional damage behavior. At temperatures T i >t 300 K point defect recombination obviously predominates the stabilization of point defects into clusters, resulting in an underproportional dose dependence of damage. Recombination differs from silicon or nickel implantations [2, 4] in that it does not lead to a plateaulike damage behavior at temperatures as high as 450 K (discussed later). For all implantation temperatures there is a dose interval where damage increases overproportionally with dose. This stronger increase is attributed to point defect clustering and the stimulated growth [4, 5] of amorphous regions, It continues until damage saturation occurs owing to the formation of a continuous amorphous layer (s = 1 ). The critical dose DN* for amorphization of (111) silicon of 2 MeV N ions is shown in Fig. 4 as a function of temperature and is compared with corresponding data for other MeV ions. Up to temperatures of approximately 300 K critical doses for nitrogen ions are reasonably higher than those for the heavier ion species; the ratio DN*/Dsi* is increasing. Using the TRIM [11] nuclear stopping power of 2 MeV N ions (10 eV A-1) at profile maximum, a critical energy
I
,
I
,
I
100 200 300 4-00 500 Ti I'K3
Fig. 4. Temperature dependence of the amorphization dose D* of (111) silicon after implantation of 2 MeV HN, 2 MeV 2sSi[4], 6 MeV 5SNi[2], and 5 MeV '"TAu[ 1] ions.
increasing temperatures, which indicates a favoring influence of nitrogen on the amorphization of silicon. At T i = 450 K the critical dose for nitrogen ions is even smaller than that for the heavier silicon projectiles. For temperatures up to 300 K damage accumulation can be described up to amorphization by the model of Hecking et al. [4, 5] (solid lines in Fig. 3). For higher temperatures a satisfactory description is not obtained in the dose intervals marked by dashed lines in Fig. 3, where damage increases under-proportionally with dose but does not pass a transient saturation, as in the case of silicon or nickel implantations. Since this stronger damage increase is most distinct at the depth of the profile maximum, it may be concluded that implanted nitrogen contributes to a stabilization of defects. The same assumption has been made [14] to explain the increased crystallization temperature of nitrogen implanted ia-Si layers. Moreover, TEM investigations on 450 K samples reveal the presence of extended defects, which might contribute to the stronger damage increase, but are not included in the damage model. In the model [4, 5] damage is considered as a sum s = Sp+ sa of the relative fractions damaged by isolated or clustered point defects sp and amorphization Sa. The dose dependence of damage is given by
densityforamorphization[12]of(5.6+O.2)xlO14eV~D=pp[eXp(_R2D2)](l_sa)+Csp(l_ cm -3 is calculated from a T--*0 extrapolation of DN*, in good agreement with the results for other MeV and keV implantations [2, 12, 13]. At temperatures above 300 K, i.e. critical doses above 1016 N cm -2 and nitrogen peak concentrations of more than 1 at.% according to TRIM [11], the amorphization dose for nitrogen ions increases comparatively slowly with
Sp) sp*( 1 - sa)
dsa sp dD 1 - s ,
-dsa - = ( P a +A~Sa)(1-Sa) dD
(2)
(3)
I0
J . K . N . Lindner eta/.
,/
2 Me V nitrogen ion implantation ojSi
TABLE 1. Model parameters for the calculation of solid lines in Fig. 3 by eqns. (2) and (3) 7~
1'
R
C"
P,
, t,
[K]
[lP0-~4cm2]
[10 '4cm21
ll0 '4cm2]
se*
11'0 '4cm:l
IJ0 '4cm2I
125 210 300 350 400 450
0.033 0.032 0.030 0.027 0.020 0.012
0 0.20 0.65 0.95 1.00 0.68
0.70 (1.45 0.35 0.28 0.26 0.19
1 0.15 0.068 0.055 0.044 0.038
0.027 0.014 9E - 4 2E - 5 1E - 7 1E - 9
U.55 0.48 0.08 ().06 0.04 0.03
1043
"
1 104!
104-~ •
Ni~Si
E
r
i
104z
• Si~Si •
i
N~Si 104!
104~. 104~"
~C ~
~
04
P~
104~ 10-17
1045
104~
8
IE
r I~
i
1
i
1
100 200 30o ~00 TEMPE RATURE [K ]
"N"
i
5oo
10-15
A Si ~ S i • N ~Si
104s
I
i
i
0-2 liT2t
i
100 200 300 bOO TEMPERATURE [K]
500
i
i
i
l
100 200 300 4.00 500 TEMPERATURE [ K ]
Fig. 5. Temperature dependence of defect production and interaction parameters, used to describe lattice damage by 2 MeV nitrogen, 2 MeV silicon [4] and 6 MeV nickel [2] ions with eqns. (2) and (3).
and temperature dependent model parameters (Table 1) obtained by a fit. They are compared with parameters for the damage by heavier ions in Fig. 5. Primary production of point defects Pp by nitrogen ions is smaller than that for silicon ions by a factor of 2.5 at temperatures up to 300 K and by a factor of up to 3.7 above. These factors have to be compared with the ratio of nuclear stopping powers at the profile maximum, which is 2,5 according to T R I M [11]. Recombination R of point defects from different collision cascades is smaller than for the heavier ions, most probably a result of the smaller size of the cascades produced by nitrogen ions. Parameter C describing the clustering of point defects from different cascades is nearly the same for all ion species considered, while parameter Sp* for the saturation of point defects differs f r o m that obtained for silicon and nitrogen ions. The primary production Pa N of amorphous regions by nitrogen ions is significantly smaller than that for silicon ions, where the ratio PaN/Pa s~ decreases with increasing temperatures. The parameter A N, which describes the stimulated growth of amorphous regions and which is determined in the dose region of overproportional damage increase, is con-
siderably smaller than that for silicon ions at temperatures up to 300 K. At higher temperatures, where higher doses are required to achieve the transition to amorphization, A.N decreases less than expected with increasing temperature. Therefore the small critical doses DN* at elevated temperatures have not to be referred to an enhanced primary production but to a nitrogen enhanced growth of amorphous regions, i.e. to a diminished crystallization of spike volumes owing to the presence of nitrogen. The opposite behavior of A, Ni has been observed and the reverse argumentation has been used to explain the nickel-induced suppression of amorphization [2].
References
1 J. K. N. Lindner, N. Hecking and E. te Kaat, Nucl. lnstrum. Methods, B26 (1987) 551. 2 J.K.N. Lindner, R. Domres and E. H. te Kaat, Nucl. lnstrum. Methods, B39 (1989) 306. 3 K.F. Heidemann, Philos. Mag., B44 (1981) 465. 4 N. Hecking, K. F. Heidemann and E. te Kaat, Nucl. lnstrum. Methods, B15 (1986) 760.
J. K. N. Lindner et al.
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