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EPR, XRD and optical reflectivity studies of radiation damage in silicon after high energy implantation of Ni ions
& *H -_
Nuclear Instruments
and Methods in Physics Research B 94 (1994) 240-244
__
Beam Interactions with Materials 8 Atoms
I!!!!
ELSEVIER
EPR, XRD and optical reflectivity studies of radiation damage in silicon after high energy implantation of Ni ions V.S. Varichenko b, A.M. Zaitsev “*l, J.K.N. Lindner a,2, R. Domes a,3, N.M. Penina b, D.P. Erchak bFc,A.R. Chelyadinskii b, V.A. Martinovitsh b a University of Dortmund, Dortmund Germany ’ Belarussian State University, Minsk, Belarus ’ Moscow State lJniversi@, Moscow, Russia Received 10 February
1994; revised form received 13 July 1994
Abstract in a dose range from 5 X 101’ to (111) silicon has been implanted with 6 MeV 68Ni ions at various temperatures 2 X 10” Ni/cm’. Quantitative depth resolved optical studies on the radiation damage are compared with EPR and XRD measurements, giving direct information on existing defect species and changes of the lattice period, respectively. Results of EPR investigations support the interpretation of the dose and temperature dependence of the optically detected damage given by Lindner and te Kaat [J. Mater. Res. 3 (1988) 12381, who base themselves on the damage model of Hecking et al. [Nucl. Instr. and Meth. B 15 (1986) 7601. EPR lines of five different point defect (Si-A4, Si-Pl, Si-A6, Si-B2, Si-P3) and of amorphous material are observed. The use of different experimental methods allows for a comparison of the individual sensitivities for several kinds of damage.
1. Introduction Optical methods are known to be a powerful tool [l] for determination of damage in silicon. Optical reflectivity measurements have been performed for depth resolved damage investigations using small angle beveled samples [2-41 as well as for the in situ determination of amorphization doses during ion implantation [5]. Important features of the optical reflectivity depth profiling technique developed by Heidemann [2,3] and used in the present article are the capability to determine damage value over a range of three orders of magnitude and the suitability for deep implantations with high energy ions, when the use of other techniques like RBS/channeling is limited. It has been observed [3] that at wavelengths between 410 and 1000 nm the optical reflectivity change AR/R,, related to the reflectivity R, of undamaged crystalline silicon, increases with increasing damage until saturation is reached due to amorphization. In systematic damage studies [2,6-111 of silicon after MeV ion implantations with
* Corresponding 2331 987321. 1Present address: * Present address: 3 Present address:
author.
Tel.
+49
2331 9872567,
Fax
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University of Hagen, Germany. University of Augsburg, Germany. Fa. Schott Glaswerke, Mainz, Germany.
0168-583X/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0168-583X(94)00280-0
different ion species and at different target temperatures, the optical reflectivity change AR/R, normalized to the saturation value RJR, has been used as a measure of the optically detected damage S: SC_
AR/R, Ri,/R,
.
According to this definition, S = 0 and S = 1 describe undamaged crystalline and completely amorphous state, respectively. The optically detected damage characterised by parameter S is observed to be a complicated function of dose. However, a damage model proposed by Hecking et al. [6,8] has been applied for different ion species and yields a satisfactory description of the dose and temperature dependences of S at the depth of the damage profile maximum over a large interval of implantation parameters [6-8,111. Most prominent for 2 MeV Si [6] and 6 MeV Ni [7] implantations into silicon, it is observed that at elevated implantation temperatures, extended dose intervals exist beneath the amorphization dose, where the optically detected damage S (or reflectivity change) at the damage profile maximum is nearly constant. According to the model, such a behaviour is attributed to the recombination of point defects produced within the individual collision cascades. It is expected that mainly point defect clusters with a negligibly small density of amorphous nuclei exist
VS. Varichenko et al. /Nucl. Instr. and Meth. in Phys. Res. B 94 (1994) 240-244
over these dose intervals. With temperature rise up to about 425 K, the dose interval of small damage increases. Then at higher doses the formation and “stimulated growth” of amorphous regions lead to a steep overproportional increase of the optically detected damage S until saturation occurs through the formation of a continuous amorphous layer. Since the optical reflectivity change of silicon is expected to be dependent on the density of the various types of defects which cannot be distinguished, it is worthwhile to correlate the optically detected damage S with the data obtained by other techniques. In this paper optically measured damage data are compared with results of X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) measurement, giving direct notation about the lattice structure and existing types of defects, respectively.
0
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241
3
4
5
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Fig. 1. Depth profiles of the optically detected damage parameter 6) from silicon implanted with 2X 1014 Ni/cm* at different target temperatures.
2. Experimental (111) oriented silicon samples were impl~ted with 6 MeV @Nizc ions over a dose range of 5 X lOi to 2 x 10” cm-’ at temperatures from 125 K to 450 K. All implantations were carried out at the Dynamitron Tandem Laboratory at Bochum (Germany), as has been described elsewhere [12] in more detail. The samples were investigated by means of doublecrystal X-ray diffraction (Cu K&44)) [13]. EPR measurements in the X-band region were carried out using a Varian spectrometer. Optical reflectivity depth profiling [2] was performed on samples beveled at an angle of 1.3” using a wavelength of 650 nm.
3. Resuits and discussion Fig. 1 shows depth profiles of the optically detected damage S obtained for implantations at various temperatures and a dose of 2 X 10z4 cm-‘. Damage is observed in a depth interval extending from the surface to a depth of about 4.0 pm, reaching a maximum at about 3.0 pm. With decreasing temperature the damage produced by the Ni ions increases over the whole depth interval. The pronounced differences observed in the shape of the depth profile can be mainly referred to different production rates of diverse defects and their interaction processes which have been already described to explain the dose and temperature dependences of damage at the depth of the damage profile maximum after 6 MeV Ni implantation into silicon [7] and which obviously occur at different depths with altered strength. These processes are the production of isolated point defects and clusters of point defects as well as (i) the prounion of amo~hous regions, (ii) the recombination and clustering of point defects originating from different collision cascades and (iii) the stimulated growth of amorphous regions [6-81. At the lowest
temperature (125 K) the damage saturation is reached between 2 and 3.5 pm due to the fo~ation of a buried continuous amorphous Iayer (Fig. 1). Defect specific information on the damage state is obtained by evaluating of EPR spectra recorded from the same samples. After implantation at 450 K different paramagnetic centers could be resolved and five centres have been identi~ed (Fig. 2a). The Si-A4 center - (110) planar three vacancy ciuster [15]; the Si-Pl center - non planar five vacancy cluster in negative charge state [ 141; the Si-P3 center - tetra vacancy in neutral charge state I.141as well as the Si-A6 center [15] and the Si-B2 center [16], the atomic structure of which have not yet been identified. The S&A4 and Si-A6 centers are we11 known from neutron irradiated silicon. In the EPR spectra obtained from the samples irradiated at 350 K, it is hardly possible to distinguish the signals
Fig. 2. Room temperature EPR spectra from Si implanted with a dose of 2 X 1O’4cm- ’ at a temperature of 450 K (a) or 125 K (b). Point centers resolved are marked with arrows. The unknown centers are marked by NC1 and NCZ.
VS. Varichenko et al. /Nucl. In& and Meth. in Phys. Res. B 94 (1994) 240-244
242
from different paramagnetic centers because of the line broadening and their overlapping. Instead of this a characteristic C-line [17] with an anisotropic behaviour of width and shape is detected. The appearence of the X-line indicates the beginning of the formation of strongly disordered (but still not amorphous) regions in which point defects lose their individual features due to interactions [17]. For comparison, the optically detected damage (S) reaches a value of 0.13 at the depth profile maximum (Fig. 1) and the dose dependence of damage at this depth changes from an underproportional to an overproportional behaviour [7]. After the implantation at 300 K a singlet line with an isotropic g-factor of 2.0059 f 0.0002 and a slightly anisotropic width (AH= 6 G by HlI(111) and AH=55 G by H I[(110)) is observed, showing that amorphous material has been formed. A further decrease of the implantation temperature down to 210 K and 125 K results in the appearance of a quite isotropic line arising from the paramagnetic centers associated with a continuous amorphous phase of silicon (Fig. 2b). The Fig. 3 shows the total intensity of the radiation paramagnetic centers Npc as a function of implantation temperature compared with the data obtained by optical reflectivity depth profiling at the depth of the damage profile maximum. There is an agreement between the two curves in the temperature range 125-210 K, where the amorphous phase is dominant according to EPR data. It should be noted that the optically detected damage (S) is larger than 0.1 almost across the whole implanted layer at 125 K and at 210 K (Fig. 1). At the temperatures 300 K and higher the optically detected damage (S) decreases stronger than the total number of the paramagnetic centers. It may be concluded that the optical reflectivity technique is a method particularly sensitive to the amorphous phase. In Fig. 4 the dose dependences of damage Npc measured with the EPR and optical method at the depth of the
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Fig. 3. Temperature dependence of the total concentration of all paramagnetic centers, Npc, and the optically detected damage parameter (S) at the depth of the damage profile maximum in silicon implanted with 2 X 1014 Ni/cm’.
D.
m/an2
Fig. 4. Optically detected damage parameter (S) at the depth of the damage profile maximum (01, total number of all paramagnetic centers ( n ), and number of paramagnetic centers resulting from amorphization (0) in silicon implanted at 400 K as a function of dose.
damage profile maximum are compared for the temperature 400 K. As would be expected from simple collision theories, where damage production is considered within non-interacting collision cascades, at low doses (< 1013 Ni/cm’) optical measurements show a dose proportional damage increase. The markedly reduced damage efficiency dS/dD at a dose of 1013 Ni/cm* has been attributed [7] to the interaction of point defects produced along the paths of different ions, which may occur if the cascades are produced in predamaged material. At elevated temperatures the recombination of point defects is predominant compared to the formation of point defect clusters, leading to a plateau like behaviour of damage S. As can be seen in Fig. 4 there is a good agreement between the dose dependence of the optically detected damage (S) and the total number of paramagnetic centers (N,) within the dose range 5 X lo”-2 X 101’ Ni/cm’. At higher doses a considerable increase in the optically detected damage (S) occurs. Based on the damage model of Hecking et al. [6] this is ascribed [7] mainly to a stimulated growth of amorphous regions. At corresponding doses the EPR line assigned to the paramagnetic centers of the amorphous phase can be clearly observed. The intensity of this line follows the increase of the optically detected damage (S), showing that the strong increase of the optically detected damage at doses above 5 X 1015 Ni/cm’ is caused by the appearence of amorphous regions. It can be derived from Fig. 3 that the formation of the EPR signal due to the presence of the amorphous phase starts at an optically detected damage level of about 0.1. This is the same value as it has been reported above in the study of the temperature dependence of damage (Fig. 3). For comparison, signals related to the amorphous phase were observed in electron diffraction patterns taken from the samples having damage level (S) above 0.23 formed by 5 MeV Au implantation [9]. The dose dependence of the intensity of the paramagnetic centers, Nr,,_, reveals that there is no additional
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VS. Varichenko et al. / Nucl. Instr. and Meth. in Phys. Res. B 94 (1994) 240-244
increase of paramagnetically active damage due to amorphization (Fig. 4). This means that the conversion of damaged silicon comaining mainly point defects and point defect clusters into silicon contaiuing amorphous regions does not change the total number of unpaired electrons, at least in this case of 6 MeV Ni ~pl~tation. In contrast, the optical reflectivity method, which is very sensitive to the rearrangement of electron bonds involved in phase transformations, displays a remarkable increase of the damage parameter (S) when ~o~h~ation takes place. For samples implanted at 300 K with Ni ions in the dose range between 1013 and 10” Ni/cm’ the change of lattice parameter, ha, in the damaged region has been determined using XRD, by considering the Bragg case pendulous fringes. From the angular distance, A B, between the fringes the thickness of the nonamorphous subsurface layer, t, has been evaluated as 1.5 pm using [l&191: t= (h/2Ae)cose, where A is the X-ray wavelength and @is the Bragg angle. In the case of higher doses when a buried continous amorphous layer is formed the pendulous fringes disappear while the diffraction peak due to the change of lattice constant in the damaged but still crystalline top subsurface layer remains (Fig. Sb). Thus it is possible to gain information on the subsurface layer from XRD measurements even in those cases where the EPR spectra would be governed only by Z or the amorphous line due to the presence of buried heavily damaged zone.
,LWk
+e,
Fig. 5. X-ray diffraction pattern from silicon implanted at 300 K with a dose of 1014cm-’ (a) or lOi cmW2(bf.
243
Fig. 6. Recovery of the lattice period change (ho) in silicon implanted with 6 MeV 1X1014 Ni/cm* at 300 K (0) or with 200 keV 1X 1014 S/cm’ annealing (15 min).
at 300 K (0)
during isochronal
The XRD measurements reveal an expansion (Au) of the lattice constant, which increases with increasing dose and reach the value 2 X 10v4 nm at the dose of 1 X 10”’ cm -2. At this dose the interference maxima vanish and one broad diffraction line appears (Fig. 5b) indicating the beginning of the formation of a continous amorphous layer at the depth of the damage profile maximum. At this dose, also, the optically detected damage (Sf increases from 0.1 to 0.8 in the depth interval between the surface and a depth of 1.5 pm and it is equal to 1 (complete amorphization) at greater depths. For the dose 1 X 1014 cm-’ the recovery of the subsurface lattice period during isochronal annealing has been studied (Fig. 6). The recovery curve displays two stages at temperatures of about 350-550 K and 650-850 K. A very similar annealing behaviour has been reported for silicon implanted with Si ions at lower energies [13], indicating that the set of radiation defects near the surface after high energy Ni ion implantation does not differ much from that formed in the layer where the nuclear stopping power is a rn~irn~ for low energy ions. This might be interpreted as a further proof that during 6 MeV Ni ion implautation into silicon, even in the near surface region, the defect production is governed by the nuclear stopping rather than by the electronic energy deposition. The recovery step at 350-550 K is explained by the annealing of divacaucies and the step at 650-850 K by the dominant annealing of polivacancy complexes [13,20]. It has been shown earlier [17] that piateau-like behaviour of the damage versus dose, as shown in Fig. 2 for the target temperature 400 K, also exists at higher temperatures, while it is less pronounced at 350 K and completely disappears at lower temperatures. It is known that at temperatures of about 350 K annealing of stable interstitial complexes occur [21]. It was also shown that the concentration of these complexes is comparable with that of the vacancy-type defects 1131.Evidently when the implan~tion
244
VS. Vurichenko et al. /Nucl.
I&r.
and Meth. in Phys. Res. B 94 (1994f240-244
temperature reaches a value of 3.50 K the single interstitials cannot form these stable interstitial compiexes any more but participate in annihilation processes with vacancy-type defects. At tem~ratures above 350 K a contribution due to the ordinary annealing of divacancies becomes significant. Probably these two processes are the reason for the appearence of the dose plateau.
4. S~rna~
and cone~usions
The radiation damage of silicon implanted with 6 MeV 68Ni ions has been studied by the methods of optical reflectivity depth profiling, EPR and XRD in a dose range from 5 X 1012 to 2 X 1017 cm-a for different implantation temperatures. In the low dose samples, the Si-A4, the Si-Pl, the Si-P3, the Si-B2 and the %-A6 paramagnetic centers were found for the implantation temperature of 450 K. For lower temperatures the EPR spectra show the existence of amorphized material. The occurrence of the anisotropic line assigned to amorphous silicon in EPR spectra is correlated with the optically detected damage parameter (5) exceeding a value of about 0.1 at the depth profile maximum. From the XRD measurements the annealing behaviour of the lattice period has been determined and it is concluded that the defect production by MeV nickel ions is analogous to that of lower energy Si ions of tens to hundreds of keV energy. The low dose rate of the defect production at the doses beneath the amo~hi~tion dose at temperatures greater than 300 K is proposed to be governed by the annihilation of interstitial complexes and divacancies. Qualitatively, the results of the EPR investigations support the inte~retation of the dose and temperature dependences of the optically detected damage which was given earlier 1121and which is based on the damage model of Hecking et al. [6]. As in the optical investigations, the dose dependent EPR measurements for an impl~tation temperature of 400 K reveal the presence of an extended dose interval beneath the amorphization dose, where the EPR radiation damage measured as the total number of paramagnetic centers, Npc, is nearly unchanged. The value of NW remains constant even at higher doses where amorphization occurs, while the optically detected damage parameter (S) increases by about one order of magnitude in the corresponding dose range. The high sensitivity of the optical reflectivity method to damage in the form of amorphized material reflects an advantage of this technique.
The authors would like to thank Prof. E.H. te Kaat (bin-~eitner Institut, Berlin, ~e~~y~ for fruitful discussions. Dr. V.S. Varichenko and Dr. A.M. Zaitsev are very obliged to DAAD and Alexander van Humboldt Foundation for support to work in the University of Dortmund and the University of Hagen (Germany). References
[II V.M. Gusev, L.N. Strehsov, K.V. Starinin and LB. Klraibullin, SOY.Phys. Semiconductors 6 (1972) 1049.
E21K.F. ~eidemann and H.F. Kappert, in: Defects and Radiation Effects in Semiconductors, The Institute of Physics, London, Conf. Ser. 46 (1979) p. 492. [31 K.F. Heidemann, Phil. Mag. B 44 (1981) 46.5. 141 E.H. te Kaat, N. Hecking and J.K.N. Lindner, Materials Science and Engineering B2 (1989) 145. PI P.V. Zhykovskii, S.B. Kantorov, K. Kistchak, D. Mouchka and V.F. Stelmakh, Fiz. Techn. Poluprov. 24 (1990) 1475 (in Russian). I61 N. Heckiug, K.F. Heidemann and E.H. te Kaat, Nucl. Instr. and Meth. B 15 (1986) 760. [71 J.K.N. Lindner, R. Domres and E.H. te Kaat, Nucl. Instr. and Meth. B 39 (1989) 306. 181 N. Hecking and E.H. te Kaat, Applied Surface Science 43 (1989) 87. PI J.K.N. Lindner, N. Hecking and E. te Kaat, Nucl. instr. and Meth. B 26 (1987) 5.51, DO1 N. Hecking and E. te Kaat, in: EPM 87 Energy Pulse and Particle Beam Modifications of Materials, Ed. K. Henning (Akademie Verlag, Berlin, 1988) pp. 164. [III J.K.N. Lindner, R. Zuschlag and E.H. te Kaat, Nucl. Instr. and Meth. B 62 (1992) 314. D21 J.K.N. Lindner and E.H. te Kaat, J. Mater. Res. 3 (1988) 1238. 1131 N.I. Berezhnov, V.F. Stelm~ and A.R. Cheiyadins~i, Phys. Stat. Solidi A 78 (1983) K121. D41 Wun Jung and G.S. Newell, Phys. Rev. 132 (1963) 648. !%I Y.H. Lee, Y.M. Kim and J.W. Corbett, Rad. Effects 15 (1972) 77.
D.F. Daly and K.A. Pickar, Appl. Phys. Letters 15 (1969) 267. 1171 K.L. Brower and W.S. Beezhold, J. Appl. Phys. 43 (1972) 3499. [181P.V. Petrashel, Sov. Phys.: Solid State Physics 16 (1974) 2168. [191 E.H. te Kaat and G.H. Schwuttke, Advances in X-Ray Analysis 504 (1972). [201 H.J. Stein, F.L. Vook and J.A. Borders, Appl. Phys. Letters 14 (1969) 328. t211 Y.M. Lee, J.W. Corhett, Solid State Communications 15 (1974) 1781. ml
Nuclear Instruments
and Methods in Physics Research B 94 (1994) 240-244
__
Beam Interactions with Materials 8 Atoms
I!!!!
ELSEVIER
EPR, XRD and optical reflectivity studies of radiation damage in silicon after high energy implantation of Ni ions V.S. Varichenko b, A.M. Zaitsev “*l, J.K.N. Lindner a,2, R. Domes a,3, N.M. Penina b, D.P. Erchak bFc,A.R. Chelyadinskii b, V.A. Martinovitsh b a University of Dortmund, Dortmund Germany ’ Belarussian State University, Minsk, Belarus ’ Moscow State lJniversi@, Moscow, Russia Received 10 February
1994; revised form received 13 July 1994
Abstract in a dose range from 5 X 101’ to (111) silicon has been implanted with 6 MeV 68Ni ions at various temperatures 2 X 10” Ni/cm’. Quantitative depth resolved optical studies on the radiation damage are compared with EPR and XRD measurements, giving direct information on existing defect species and changes of the lattice period, respectively. Results of EPR investigations support the interpretation of the dose and temperature dependence of the optically detected damage given by Lindner and te Kaat [J. Mater. Res. 3 (1988) 12381, who base themselves on the damage model of Hecking et al. [Nucl. Instr. and Meth. B 15 (1986) 7601. EPR lines of five different point defect (Si-A4, Si-Pl, Si-A6, Si-B2, Si-P3) and of amorphous material are observed. The use of different experimental methods allows for a comparison of the individual sensitivities for several kinds of damage.
1. Introduction Optical methods are known to be a powerful tool [l] for determination of damage in silicon. Optical reflectivity measurements have been performed for depth resolved damage investigations using small angle beveled samples [2-41 as well as for the in situ determination of amorphization doses during ion implantation [5]. Important features of the optical reflectivity depth profiling technique developed by Heidemann [2,3] and used in the present article are the capability to determine damage value over a range of three orders of magnitude and the suitability for deep implantations with high energy ions, when the use of other techniques like RBS/channeling is limited. It has been observed [3] that at wavelengths between 410 and 1000 nm the optical reflectivity change AR/R,, related to the reflectivity R, of undamaged crystalline silicon, increases with increasing damage until saturation is reached due to amorphization. In systematic damage studies [2,6-111 of silicon after MeV ion implantations with
* Corresponding 2331 987321. 1Present address: * Present address: 3 Present address:
author.
Tel.
+49
2331 9872567,
Fax
i-49
University of Hagen, Germany. University of Augsburg, Germany. Fa. Schott Glaswerke, Mainz, Germany.
0168-583X/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0168-583X(94)00280-0
different ion species and at different target temperatures, the optical reflectivity change AR/R, normalized to the saturation value RJR, has been used as a measure of the optically detected damage S: SC_
AR/R, Ri,/R,
.
According to this definition, S = 0 and S = 1 describe undamaged crystalline and completely amorphous state, respectively. The optically detected damage characterised by parameter S is observed to be a complicated function of dose. However, a damage model proposed by Hecking et al. [6,8] has been applied for different ion species and yields a satisfactory description of the dose and temperature dependences of S at the depth of the damage profile maximum over a large interval of implantation parameters [6-8,111. Most prominent for 2 MeV Si [6] and 6 MeV Ni [7] implantations into silicon, it is observed that at elevated implantation temperatures, extended dose intervals exist beneath the amorphization dose, where the optically detected damage S (or reflectivity change) at the damage profile maximum is nearly constant. According to the model, such a behaviour is attributed to the recombination of point defects produced within the individual collision cascades. It is expected that mainly point defect clusters with a negligibly small density of amorphous nuclei exist
VS. Varichenko et al. /Nucl. Instr. and Meth. in Phys. Res. B 94 (1994) 240-244
over these dose intervals. With temperature rise up to about 425 K, the dose interval of small damage increases. Then at higher doses the formation and “stimulated growth” of amorphous regions lead to a steep overproportional increase of the optically detected damage S until saturation occurs through the formation of a continuous amorphous layer. Since the optical reflectivity change of silicon is expected to be dependent on the density of the various types of defects which cannot be distinguished, it is worthwhile to correlate the optically detected damage S with the data obtained by other techniques. In this paper optically measured damage data are compared with results of X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) measurement, giving direct notation about the lattice structure and existing types of defects, respectively.
0
I
2
241
3
4
5
=%im
Fig. 1. Depth profiles of the optically detected damage parameter 6) from silicon implanted with 2X 1014 Ni/cm* at different target temperatures.
2. Experimental (111) oriented silicon samples were impl~ted with 6 MeV @Nizc ions over a dose range of 5 X lOi to 2 x 10” cm-’ at temperatures from 125 K to 450 K. All implantations were carried out at the Dynamitron Tandem Laboratory at Bochum (Germany), as has been described elsewhere [12] in more detail. The samples were investigated by means of doublecrystal X-ray diffraction (Cu K&44)) [13]. EPR measurements in the X-band region were carried out using a Varian spectrometer. Optical reflectivity depth profiling [2] was performed on samples beveled at an angle of 1.3” using a wavelength of 650 nm.
3. Resuits and discussion Fig. 1 shows depth profiles of the optically detected damage S obtained for implantations at various temperatures and a dose of 2 X 10z4 cm-‘. Damage is observed in a depth interval extending from the surface to a depth of about 4.0 pm, reaching a maximum at about 3.0 pm. With decreasing temperature the damage produced by the Ni ions increases over the whole depth interval. The pronounced differences observed in the shape of the depth profile can be mainly referred to different production rates of diverse defects and their interaction processes which have been already described to explain the dose and temperature dependences of damage at the depth of the damage profile maximum after 6 MeV Ni implantation into silicon [7] and which obviously occur at different depths with altered strength. These processes are the production of isolated point defects and clusters of point defects as well as (i) the prounion of amo~hous regions, (ii) the recombination and clustering of point defects originating from different collision cascades and (iii) the stimulated growth of amorphous regions [6-81. At the lowest
temperature (125 K) the damage saturation is reached between 2 and 3.5 pm due to the fo~ation of a buried continuous amorphous Iayer (Fig. 1). Defect specific information on the damage state is obtained by evaluating of EPR spectra recorded from the same samples. After implantation at 450 K different paramagnetic centers could be resolved and five centres have been identi~ed (Fig. 2a). The Si-A4 center - (110) planar three vacancy ciuster [15]; the Si-Pl center - non planar five vacancy cluster in negative charge state [ 141; the Si-P3 center - tetra vacancy in neutral charge state I.141as well as the Si-A6 center [15] and the Si-B2 center [16], the atomic structure of which have not yet been identified. The S&A4 and Si-A6 centers are we11 known from neutron irradiated silicon. In the EPR spectra obtained from the samples irradiated at 350 K, it is hardly possible to distinguish the signals
Fig. 2. Room temperature EPR spectra from Si implanted with a dose of 2 X 1O’4cm- ’ at a temperature of 450 K (a) or 125 K (b). Point centers resolved are marked with arrows. The unknown centers are marked by NC1 and NCZ.
VS. Varichenko et al. /Nucl. In& and Meth. in Phys. Res. B 94 (1994) 240-244
242
from different paramagnetic centers because of the line broadening and their overlapping. Instead of this a characteristic C-line [17] with an anisotropic behaviour of width and shape is detected. The appearence of the X-line indicates the beginning of the formation of strongly disordered (but still not amorphous) regions in which point defects lose their individual features due to interactions [17]. For comparison, the optically detected damage (S) reaches a value of 0.13 at the depth profile maximum (Fig. 1) and the dose dependence of damage at this depth changes from an underproportional to an overproportional behaviour [7]. After the implantation at 300 K a singlet line with an isotropic g-factor of 2.0059 f 0.0002 and a slightly anisotropic width (AH= 6 G by HlI(111) and AH=55 G by H I[(110)) is observed, showing that amorphous material has been formed. A further decrease of the implantation temperature down to 210 K and 125 K results in the appearance of a quite isotropic line arising from the paramagnetic centers associated with a continuous amorphous phase of silicon (Fig. 2b). The Fig. 3 shows the total intensity of the radiation paramagnetic centers Npc as a function of implantation temperature compared with the data obtained by optical reflectivity depth profiling at the depth of the damage profile maximum. There is an agreement between the two curves in the temperature range 125-210 K, where the amorphous phase is dominant according to EPR data. It should be noted that the optically detected damage (S) is larger than 0.1 almost across the whole implanted layer at 125 K and at 210 K (Fig. 1). At the temperatures 300 K and higher the optically detected damage (S) decreases stronger than the total number of the paramagnetic centers. It may be concluded that the optical reflectivity technique is a method particularly sensitive to the amorphous phase. In Fig. 4 the dose dependences of damage Npc measured with the EPR and optical method at the depth of the
0.8
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1 1. I
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1
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Fig. 3. Temperature dependence of the total concentration of all paramagnetic centers, Npc, and the optically detected damage parameter (S) at the depth of the damage profile maximum in silicon implanted with 2 X 1014 Ni/cm’.
D.
m/an2
Fig. 4. Optically detected damage parameter (S) at the depth of the damage profile maximum (01, total number of all paramagnetic centers ( n ), and number of paramagnetic centers resulting from amorphization (0) in silicon implanted at 400 K as a function of dose.
damage profile maximum are compared for the temperature 400 K. As would be expected from simple collision theories, where damage production is considered within non-interacting collision cascades, at low doses (< 1013 Ni/cm’) optical measurements show a dose proportional damage increase. The markedly reduced damage efficiency dS/dD at a dose of 1013 Ni/cm* has been attributed [7] to the interaction of point defects produced along the paths of different ions, which may occur if the cascades are produced in predamaged material. At elevated temperatures the recombination of point defects is predominant compared to the formation of point defect clusters, leading to a plateau like behaviour of damage S. As can be seen in Fig. 4 there is a good agreement between the dose dependence of the optically detected damage (S) and the total number of paramagnetic centers (N,) within the dose range 5 X lo”-2 X 101’ Ni/cm’. At higher doses a considerable increase in the optically detected damage (S) occurs. Based on the damage model of Hecking et al. [6] this is ascribed [7] mainly to a stimulated growth of amorphous regions. At corresponding doses the EPR line assigned to the paramagnetic centers of the amorphous phase can be clearly observed. The intensity of this line follows the increase of the optically detected damage (S), showing that the strong increase of the optically detected damage at doses above 5 X 1015 Ni/cm’ is caused by the appearence of amorphous regions. It can be derived from Fig. 3 that the formation of the EPR signal due to the presence of the amorphous phase starts at an optically detected damage level of about 0.1. This is the same value as it has been reported above in the study of the temperature dependence of damage (Fig. 3). For comparison, signals related to the amorphous phase were observed in electron diffraction patterns taken from the samples having damage level (S) above 0.23 formed by 5 MeV Au implantation [9]. The dose dependence of the intensity of the paramagnetic centers, Nr,,_, reveals that there is no additional
.. l.O# ‘i:: 6
d$
m
VS. Varichenko et al. / Nucl. Instr. and Meth. in Phys. Res. B 94 (1994) 240-244
increase of paramagnetically active damage due to amorphization (Fig. 4). This means that the conversion of damaged silicon comaining mainly point defects and point defect clusters into silicon contaiuing amorphous regions does not change the total number of unpaired electrons, at least in this case of 6 MeV Ni ~pl~tation. In contrast, the optical reflectivity method, which is very sensitive to the rearrangement of electron bonds involved in phase transformations, displays a remarkable increase of the damage parameter (S) when ~o~h~ation takes place. For samples implanted at 300 K with Ni ions in the dose range between 1013 and 10” Ni/cm’ the change of lattice parameter, ha, in the damaged region has been determined using XRD, by considering the Bragg case pendulous fringes. From the angular distance, A B, between the fringes the thickness of the nonamorphous subsurface layer, t, has been evaluated as 1.5 pm using [l&191: t= (h/2Ae)cose, where A is the X-ray wavelength and @is the Bragg angle. In the case of higher doses when a buried continous amorphous layer is formed the pendulous fringes disappear while the diffraction peak due to the change of lattice constant in the damaged but still crystalline top subsurface layer remains (Fig. Sb). Thus it is possible to gain information on the subsurface layer from XRD measurements even in those cases where the EPR spectra would be governed only by Z or the amorphous line due to the presence of buried heavily damaged zone.
,LWk
+e,
Fig. 5. X-ray diffraction pattern from silicon implanted at 300 K with a dose of 1014cm-’ (a) or lOi cmW2(bf.
243
Fig. 6. Recovery of the lattice period change (ho) in silicon implanted with 6 MeV 1X1014 Ni/cm* at 300 K (0) or with 200 keV 1X 1014 S/cm’ annealing (15 min).
at 300 K (0)
during isochronal
The XRD measurements reveal an expansion (Au) of the lattice constant, which increases with increasing dose and reach the value 2 X 10v4 nm at the dose of 1 X 10”’ cm -2. At this dose the interference maxima vanish and one broad diffraction line appears (Fig. 5b) indicating the beginning of the formation of a continous amorphous layer at the depth of the damage profile maximum. At this dose, also, the optically detected damage (Sf increases from 0.1 to 0.8 in the depth interval between the surface and a depth of 1.5 pm and it is equal to 1 (complete amorphization) at greater depths. For the dose 1 X 1014 cm-’ the recovery of the subsurface lattice period during isochronal annealing has been studied (Fig. 6). The recovery curve displays two stages at temperatures of about 350-550 K and 650-850 K. A very similar annealing behaviour has been reported for silicon implanted with Si ions at lower energies [13], indicating that the set of radiation defects near the surface after high energy Ni ion implantation does not differ much from that formed in the layer where the nuclear stopping power is a rn~irn~ for low energy ions. This might be interpreted as a further proof that during 6 MeV Ni ion implautation into silicon, even in the near surface region, the defect production is governed by the nuclear stopping rather than by the electronic energy deposition. The recovery step at 350-550 K is explained by the annealing of divacaucies and the step at 650-850 K by the dominant annealing of polivacancy complexes [13,20]. It has been shown earlier [17] that piateau-like behaviour of the damage versus dose, as shown in Fig. 2 for the target temperature 400 K, also exists at higher temperatures, while it is less pronounced at 350 K and completely disappears at lower temperatures. It is known that at temperatures of about 350 K annealing of stable interstitial complexes occur [21]. It was also shown that the concentration of these complexes is comparable with that of the vacancy-type defects 1131.Evidently when the implan~tion
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VS. Vurichenko et al. /Nucl.
I&r.
and Meth. in Phys. Res. B 94 (1994f240-244
temperature reaches a value of 3.50 K the single interstitials cannot form these stable interstitial compiexes any more but participate in annihilation processes with vacancy-type defects. At tem~ratures above 350 K a contribution due to the ordinary annealing of divacancies becomes significant. Probably these two processes are the reason for the appearence of the dose plateau.
4. S~rna~
and cone~usions
The radiation damage of silicon implanted with 6 MeV 68Ni ions has been studied by the methods of optical reflectivity depth profiling, EPR and XRD in a dose range from 5 X 1012 to 2 X 1017 cm-a for different implantation temperatures. In the low dose samples, the Si-A4, the Si-Pl, the Si-P3, the Si-B2 and the %-A6 paramagnetic centers were found for the implantation temperature of 450 K. For lower temperatures the EPR spectra show the existence of amorphized material. The occurrence of the anisotropic line assigned to amorphous silicon in EPR spectra is correlated with the optically detected damage parameter (5) exceeding a value of about 0.1 at the depth profile maximum. From the XRD measurements the annealing behaviour of the lattice period has been determined and it is concluded that the defect production by MeV nickel ions is analogous to that of lower energy Si ions of tens to hundreds of keV energy. The low dose rate of the defect production at the doses beneath the amo~hi~tion dose at temperatures greater than 300 K is proposed to be governed by the annihilation of interstitial complexes and divacancies. Qualitatively, the results of the EPR investigations support the inte~retation of the dose and temperature dependences of the optically detected damage which was given earlier 1121and which is based on the damage model of Hecking et al. [6]. As in the optical investigations, the dose dependent EPR measurements for an impl~tation temperature of 400 K reveal the presence of an extended dose interval beneath the amorphization dose, where the EPR radiation damage measured as the total number of paramagnetic centers, Npc, is nearly unchanged. The value of NW remains constant even at higher doses where amorphization occurs, while the optically detected damage parameter (S) increases by about one order of magnitude in the corresponding dose range. The high sensitivity of the optical reflectivity method to damage in the form of amorphized material reflects an advantage of this technique.
The authors would like to thank Prof. E.H. te Kaat (bin-~eitner Institut, Berlin, ~e~~y~ for fruitful discussions. Dr. V.S. Varichenko and Dr. A.M. Zaitsev are very obliged to DAAD and Alexander van Humboldt Foundation for support to work in the University of Dortmund and the University of Hagen (Germany). References
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