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Dynamic annealing study of SiC epilayers implanted with Ni ions at different temperatures
Nuclear Instruments and Methods in Physics Research B 267 (2009) 1097–1100
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Dynamic annealing study of SiC epilayers implanted with Ni ions at different temperatures J. García López a,*, Y. Morilla a, J.C. Cheang-Wong a,b, G. Battistig c, Z. Zolnai c, J.L. Cantin d a
Centro Nacional de Aceleradores, Av. Thomas A. Edison n° 7, Isla de La Cartuja, E-41092 Sevilla, Spain Instituto de Física, Universidad Nacional Autónoma de México, A.P. 20-364, México, D.F. 01000, Mexico c Research Institute for Technical Physics and Materials Science, Konkoly Thege Miklós út 29-33, H-1121 Budapest, Hungary d Institut des NanoSciences de Paris, Campus Boucicaut, 140 rue de Lourmel, 75015 Paris, France b
a r t i c l e
i n f o
Article history: Received 26 September 2008 Received in revised form 10 December 2008 Available online 27 January 2009 PACS: 29.27. a 61.72.Tt 82.80.Yc 61.85.+p Keywords: Ion implantation RBS channeling SiC Magnetic semiconductors
a b s t r a c t SiC epilayers grown on 4H-SiC single crystals were implanted with 850 keV Ni+ ions with fluences in the 0.5–9 1016 Ni+/cm2 range. Most of the samples were implanted at 450 °C, but for comparison some implantations were performed at room temperature (RT). In addition, a post-implantation annealing was performed in N2 at 1100 °C in order to recover from the implantation-induced structural damage. The disorder produced by the implantation at 450 °C and the effect of the post-implantation annealing on the recrystallization of the substrates have been studied as a function of the fluence by Backscattering Spectrometry in channeling geometry (BS/C) with a 3.45 MeV He2+ beam. RT as-implanted samples showed a completely amorphous region which extends until the surface when irradiated with the highest dose, whereas in the case of 450 °C implantation amorphization does not occur. In general, partial recovery of the crystal lattice quality was found for the less damaged samples, and the dynamic recovery of the crystalline structure increases with the irradiation temperature. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Silicon carbide is a promising wide-bandgap semiconductor material (3.26 eV for the 4H-SiC polytype) suited for potential applications in spin-based electronics at practical operating temperatures. Other unique properties are its high thermal conductivity, high electron saturation drift velocity, high breakdown electric field and high radiation tolerance [1,2]. Due to the low diffusivity of dopants in this material, ion implantation is the only realistic way to dope SiC with magnetic ions at high fluences (>1016 ions/ cm2) to induce ferromagnetism in the so-called dilute magnetic semiconductor. However, ion implantation usually produces a certain degree of damage in the crystalline structure, and therefore a post-implantation annealing at high temperature is necessary to anneal out the damage in SiC. Thus, the mechanisms concerning the defect production by ion implantation are a very attractive subject of study because of the necessity to control the damage induced by ion implantation on materials with potential technological applications. Recently, we have studied the physical properties of SiC crystalline wafers implanted at room temperature * Corresponding author. Tel.: +34 954460553. E-mail address: [email protected] (J. García López). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.01.030
(RT) with Al+ ions and the recovery of the ion-induced structural damage after a high-temperature annealing [3,4]. On the other hand, it has been observed that the dynamic recovery of the crystalline structure increases with the irradiation temperature, i.e. the reduction of disorder is more efficient for implantations at elevated temperatures compared with RT implantations followed by subsequent thermal treatments [5,6]. This paper addresses the study by Backscattering Spectrometry in channeling geometry (BS/C) of the structural damage induced in SiC epilayers (grown on semi-insulating 4H-SiC single crystals) by 850 keV Ni+ implantation at 450 °C, and the effect of a 1100 °C thermal annealing on the damage recovery and recrystallization as a function of the fluence. 2. Experimental Highly-doped (n-type 5 1019 N/cm3) SiC epilayers grown on 4H-SiC single crystals (Cree Res. Inc.) were implanted with 850 keV Ni+. Under the assumption of a sample density of 3.2 g/ cm3, the projected range of the Ni ions corresponds approximately to 0.5 lm [7], i.e. half of the epilayer thickness (1 lm). Most of the samples were implanted at 450 °C, but for comparison some implantations were performed at room temperature, and the fluences ranged from 0.5 1016 Ni/cm2 to 9 1016 Ni/cm2. In order
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to keep a small virgin area as a crystalline reference in the irradiated samples, an adequate mask was used to restrict the beam exposed area. A post-implantation annealing was performed in a N2 atmosphere at 1100 °C, for 40 min. The crystalline quality of the samples was studied by Backscattering Spectrometry in channeling geometry (BS/C) along the [0 0 0 1] axial channeling direction before and after the annealing. The BS/C measurements were done with a 3.45 MeV 4He2+ beam, and the random and aligned spectra were recorded with a surface barrier semiconductor detector placed at 165° in Cornell geometry. We determined the crystal axes of the samples in the virgin areas, and then the He beam was just translated to the irradiated area to avoid deviations from the crystal axes. The BS spectra in aligned geometry were taken for the same integrated beam charge. Ion implantation and BS/C analysis were performed at the 3 MV tandem accelerator facilities at the Centro Nacional de Aceleradores. In all the cases the implanted fluence was experimentally determined by integrating the Ni peak area by means of the RBX simulation code [8]. 3. Results and discussion Fig. 1 shows the 3.45 MeV 4He BS/C aligned spectra corresponding in this particular case to a single-crystalline 4H-SiC sample implanted at room temperature with 850 keV Ni+ ions at a fluence of 1 1016 ions/cm2. For comparison, the aligned and random spectra of the unirradiated area (virgin) are also shown. An increase in the yield of the aligned spectrum corresponds to an increase in damage or number of displaced atoms. Thus, the amount of disorder increases continuously with the implanted fluence until amorphization occurs, when the aligned spectrum reaches the random level. Here, it is important to stress that even if a poly-crystalline sample would give a similar spectrum, we have previously demonstrated by transmission electron microscopy (TEM) measurements that as-implanted samples reaching the BS random level are really amorphous [3]. Therefore, from Fig. 1 it can be observed that the near-surface region of the implanted samples is completely amorphous after a 1 1016 Ni/cm2 fluence. Similar results have been recently found by other groups in the case of Fe implantations at 200 °C, where complete amorphization of the SiC samples occurred [9]. After the 1100 °C thermal annealing, the crystalline structure of the RT-implanted sample is still heavily damaged, and only a decrease of the damaged layer thickness is observed (see Fig. 1). This result suggests that a partial lattice recovery or recrystallization process merely starts at the boundary between amorphous and crystalline zones [3].
Fig. 1. 3.45 MeV He2+ BS/C spectra of a 4H-SiC sample implanted at RT with 850 keV Ni+ ions at a fluence of 1 1016 ions/cm2 before and after the 1100 °C thermal annealing. Aligned and random spectra of the virgin sample are also shown.
On the other hand, Fig. 2 shows the 3.45 MeV 4He BS/C aligned spectra corresponding to the SiC epilayers implanted at high temperature (450 °C) at several Ni fluences, as well as the aligned and random spectra of the virgin sample. In the near-surface region of the virgin sample a minimum yield (ratio of aligned to random yields) of vmin 2% is achieved in the 1865–1892 keV energy range (DE 27 keV energy window), indicating the excellent quality of the SiC epilayers. We can notice that in the case of high-temperature implantation the amount of disorder increases with the fluence, but no complete amorphization occurs (as in the case of RT implantation), even for a fluence as high as 9.1 1016 Ni/cm2. Thus, while the critical fluence /c necessary to achieve the full amorphization of SiC with 850 keV Ni ions at room temperature is about 1 1016 Ni/cm2, in the case of 450 °C implantation /c is, at least, one order of magnitude higher. Moreover, the disorder depth distribution in the silicon sublattice is presented in Fig. 3. The defect depth profile was extracted from the experimental BS/ C spectra of the irradiated samples using the RBX code [10], and assuming a density of 3.2 g/cm3 for SiC. Considering the two-beam model, the dechanneling fraction of the beam was calculated by an iterative procedure [11]. One can observe that the damage profiles extracted from the BS/C spectra increase with the fluence and exhibit asymmetrical distributions, with an important contribution into larger depths (0.5–1 lm) for all the studied fluences, but a complete buried amorphous layer is not formed. In principle, further implantations at higher fluences should result in a continuous broadening of the buried damage layer towards both the surface and into larger depths. Although a highly defective zone can be observed, peaking at a depth larger than the Ni projected range (Rp 0.5 lm) and exhibiting only a partial damage, we can affirm that the high-temperature implantation is effective to reduce the amorphization process of the samples, even for the highest fluence (9.1 1016 Ni/cm2). Fig. 4 shows the BS/C spectra of the 450 °C Ni-implanted epilayers before and after the 1100 °C thermal annealing, for three different fluences (0.5, 1.5 and 9.1 1016 Ni/cm2). One can observe that for a given fluence the recovery of the defects created during the implantation is not complete, persisting a residual damage after the high-temperature annealing. Moreover, the recrystallization is more noticeable at the boundaries than at the core of the damaged layer, producing a decrease in the thickness of the partially amorphous buried layer. Considering Figs. 1 and 4, it is clear that the extent of lattice recrystallization after the annealing is negligible in the RT-implanted sample compared with the damage
Fig. 2. 3.45 MeV 4He BS/C aligned spectra of SiC epilayers implanted at 450 °C with 850 keV Ni+ ions for various fluences. The aligned and random spectra of the virgin sample are also included.
J. García López et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1097–1100
Fig. 3. Extracted damage profiles obtained from the BS/C aligned spectra of SiC epilayers implanted at 450 °C with 850 keV Ni+ ions for various fluences.
Fig. 4. 3.45 MeV 4He BS/C aligned spectra of SiC epilayers implanted at 450 °C with 850 keV Ni+ ions, before and after the post-implantation thermal annealing, for three different fluences. The aligned and random spectra of the virgin sample are also included.
recovery of the 450 °C-implanted epilayers. Therefore, the reduction of disorder is more efficient in the case of implantations at elevated temperatures compared with RT implantations followed by subsequent thermal treatments [5,6]. It is important to stress that the main purpose of this two-step process implantation + annealing is to eliminate the disorder of the as-implanted samples, and it is clear that the extent of the structural recovery will depend on these two parameters: the implantation temperature and the annealing temperature. Therefore, the determination of these two critical temperatures can lead us not only to the lattice recrystallization by annealing out the residual damage, but also to finely tune the desired magnetic properties. Now, we would like to consider the relative disorder in the Si sublattice. In general, the main defects introduced by ion implantation into SiC are vacancy-type defects, i.e. vacancies, divacancies, and complexes involving both vacancies and impurities [12]. As discussed above, we can assume that the displaced atoms cause the direct backscattering of the channeled analyzing ions and extract from the BS/C spectra the value of the relative disorder within the Si sublattice at an energy window corresponding to a depth interval around the maximum of the damage peak, roughly 0.6 lm). Thus, Fig. 5 represents the Ni fluence dependence of the relative disorder concentration around the maximum of the Si damage profiles of the irradiated SiC epilayers, before and after
1099
Fig. 5. Relative disorder concentration ND/N in the near-surface region of SiC epilayers implanted at 450 °C with 850 keV Ni+ ions as a function of the fluence, before and after the thermal annealing. The solid lines are just a guide for the eye.
the thermal annealing. Full amorphization corresponds to a relative disorder of 1.0. The relative disorder increases with the fluence for both the as-implanted and the annealed samples, but it seems to saturate at a value of 0.8 and 0.7, respectively. Thus, we can notice that no complete amorphization occurs in the as-implanted epilayers, even for fluences as high as 9.1 1016 Ni/cm2. Moreover, we can say that the high-temperature implantation prevents the full amorphization of the Si sublattice, even if the amount of disorder increases as a function of the Ni fluence. On the other hand, this result exhibits also the recrystallization process due to the post-implantation annealing. In all the cases, the relative disorder decreases by an amount of the order of 0.13 after the annealing. The concentration of these vacancy-related centers decreases with the implantation temperature, because the rate of recombination of interstitial atoms with vacancies is favored by an increase in the mobility [12]. In no way our 1100 °C thermal annealing brings about the complete recrystallization of the implanted layer. Moreover, in the case of 4H-SiC[0 0 0 1] Al-implanted layers, we have recently reported that, if the amorphized region extends until the top surface, annealing at 1100 °C leads to a poly-crystalline structure which contains 3C-SiC inclusions [3]. In that case the long distance disorder remains, and no difference is observed in the BS/C spectra for as-implanted (quasi-amorphous) and annealed (poly-crystalline) samples, as we mentioned before. We have also shown that during annealing the solid phase regrowth of SiC started from two directions, i.e. from both the crystalline surface and the interface region. Therefore the crystallization during annealing started as the growth of small regions from the amorphous/crystalline interface and by the nucleation of crystalline grains in the top amorphous region [3]. Complementary experiments by spectroscopic ellipsometry and transmission electron microscopy are being carried out in order to determine the actual structure of our post-annealed SiC Ni-implanted samples. One of the important aspects of the post-implantation annealing process is not only to anneal out the defects and to recover the crystalline structure, but also to activate the implanted ions at the right places in the lattice. Then, the location of the Ni atoms in the SiC lattice should be studied somehow. From the backscattering spectra one can observe that the Ni signal is completely separated from the SiC one, so in this way it is possible to determine as a first approximation the minimum yield vmin(Ni) (ratio of aligned to random yields for Ni). Fig. 6 shows the minimum yield vmin(Ni) for the SiC epilayers as a function of the fluence after the post-implantation annealing. This graph indicates that the vmin(Ni) increases systematically with the fluence, and no saturation is
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Fig. 6. Minimum yield vmin(Ni) as a function of the fluence of SiC epilayers implanted with 850 keV Ni+ ions at 450 °C, after the 1100 °C thermal annealing. The solid line is just a guide for the eye.
the post-implantation annealing on the recrystallization process. The high-temperature implantation at 450 °C is effective to reduce the amorphization process in the samples, even for the highest fluence used in this work. Concerning the lattice recrystallization after the annealing, it is negligible in the RT-implanted sample compared with the damage recovery of the 450 °C-implanted epilayers. Therefore, the reduction of disorder is more efficient in the case of implantations at elevated temperatures compared with RT implantations followed by subsequent thermal treatments. On the other hand, the decrease in the BS aligned yield seems to indicate that a fraction of the implanted Ni atoms occupy actually substitutional sites in the SiC lattice after the annealing. The high-temperature implantation decreases the generation rate for induced defects in the Si sublattice and increases the critical radiation fluence that leads to amorphization. This finding indicates that it is possible to increase the radiation resistance of devices based on SiC at increased temperatures of operation and also to tune up the desired magnetic properties.
Acknowledgments observed in the studied range, i.e. vmin(Ni) < 100% in all the cases. In other words, for all the fluences the BS Ni yield in aligned geometry is always lower than the corresponding random value, and in the best case vmin(Ni) 62% for the lowest fluence. The decrease in the BS aligned yield seems to indicate that a fraction of the implanted Ni atoms occupy actually substitutional sites in the SiC lattice. The precise location of the Ni atoms is under study by means of angular scans through low index axial and planar channels, but according to atomic size considerations, they can be preferentially incorporated into Si lattice sites. The magnetic properties of the samples as a function of the ion fluence, structural damage and degree of substitutional Ni will be reported in a forthcoming paper. Finally, it is clear that in order to use SiC to create magnetic semiconductors for device applications, the structural damage induced by the implantation process must be reduced. The best way to achieve this goal is the implantation into heated targets, because the dynamic annealing decreases the generation rate for radiation defects. 4. Conclusions We have studied the structural disorder produced by the implantation of 850 keV Ni ions in SiC epilayers and the effect of
This work is partially supported by the Projects MAT200603519 (Spanish MEC), P06-TEP-01739 (Junta de Andalucía) and SAB2006-0041 (Spanish MEC).
References [1] W.J. Choyke, H. Matsunami, G. Pensl (Eds.), Silicon Carbide: Recent Major Advances, Springer, Berlin, 2004. [2] Z.C. Feng (Ed.), SiC Power Materials – Devices and Applications, Springer, Berlin, 2004. [3] G. Battistig, N.Q. Khánh, P. Petrik, T. Lohner, L. Dobos, B. Pécz, J. García López, Y. Morilla, J. Appl. Phys. 100 (2006) 093507. [4] Y. Morilla, J. García López, G. Battistig, J.L. Cantin, J.C. Cheang-Wong, H.J. von Bardeleben, M.A. Respaldiza, Mater. Sci. Forum 483–485 (2005) 291. [5] R. Héliou, J.L. Brebner, S. Roorda, Semicond. Sci. Technol. 16 (2001) 836. [6] A. Audren, A. Benyagoub, L. Thomé, F. Garrido, Nucl. Instr. and Meth. B 266 (2008) 2810. [7] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985; SRIM-2008,
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Dynamic annealing study of SiC epilayers implanted with Ni ions at different temperatures J. García López a,*, Y. Morilla a, J.C. Cheang-Wong a,b, G. Battistig c, Z. Zolnai c, J.L. Cantin d a
Centro Nacional de Aceleradores, Av. Thomas A. Edison n° 7, Isla de La Cartuja, E-41092 Sevilla, Spain Instituto de Física, Universidad Nacional Autónoma de México, A.P. 20-364, México, D.F. 01000, Mexico c Research Institute for Technical Physics and Materials Science, Konkoly Thege Miklós út 29-33, H-1121 Budapest, Hungary d Institut des NanoSciences de Paris, Campus Boucicaut, 140 rue de Lourmel, 75015 Paris, France b
a r t i c l e
i n f o
Article history: Received 26 September 2008 Received in revised form 10 December 2008 Available online 27 January 2009 PACS: 29.27. a 61.72.Tt 82.80.Yc 61.85.+p Keywords: Ion implantation RBS channeling SiC Magnetic semiconductors
a b s t r a c t SiC epilayers grown on 4H-SiC single crystals were implanted with 850 keV Ni+ ions with fluences in the 0.5–9 1016 Ni+/cm2 range. Most of the samples were implanted at 450 °C, but for comparison some implantations were performed at room temperature (RT). In addition, a post-implantation annealing was performed in N2 at 1100 °C in order to recover from the implantation-induced structural damage. The disorder produced by the implantation at 450 °C and the effect of the post-implantation annealing on the recrystallization of the substrates have been studied as a function of the fluence by Backscattering Spectrometry in channeling geometry (BS/C) with a 3.45 MeV He2+ beam. RT as-implanted samples showed a completely amorphous region which extends until the surface when irradiated with the highest dose, whereas in the case of 450 °C implantation amorphization does not occur. In general, partial recovery of the crystal lattice quality was found for the less damaged samples, and the dynamic recovery of the crystalline structure increases with the irradiation temperature. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Silicon carbide is a promising wide-bandgap semiconductor material (3.26 eV for the 4H-SiC polytype) suited for potential applications in spin-based electronics at practical operating temperatures. Other unique properties are its high thermal conductivity, high electron saturation drift velocity, high breakdown electric field and high radiation tolerance [1,2]. Due to the low diffusivity of dopants in this material, ion implantation is the only realistic way to dope SiC with magnetic ions at high fluences (>1016 ions/ cm2) to induce ferromagnetism in the so-called dilute magnetic semiconductor. However, ion implantation usually produces a certain degree of damage in the crystalline structure, and therefore a post-implantation annealing at high temperature is necessary to anneal out the damage in SiC. Thus, the mechanisms concerning the defect production by ion implantation are a very attractive subject of study because of the necessity to control the damage induced by ion implantation on materials with potential technological applications. Recently, we have studied the physical properties of SiC crystalline wafers implanted at room temperature * Corresponding author. Tel.: +34 954460553. E-mail address: [email protected] (J. García López). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.01.030
(RT) with Al+ ions and the recovery of the ion-induced structural damage after a high-temperature annealing [3,4]. On the other hand, it has been observed that the dynamic recovery of the crystalline structure increases with the irradiation temperature, i.e. the reduction of disorder is more efficient for implantations at elevated temperatures compared with RT implantations followed by subsequent thermal treatments [5,6]. This paper addresses the study by Backscattering Spectrometry in channeling geometry (BS/C) of the structural damage induced in SiC epilayers (grown on semi-insulating 4H-SiC single crystals) by 850 keV Ni+ implantation at 450 °C, and the effect of a 1100 °C thermal annealing on the damage recovery and recrystallization as a function of the fluence. 2. Experimental Highly-doped (n-type 5 1019 N/cm3) SiC epilayers grown on 4H-SiC single crystals (Cree Res. Inc.) were implanted with 850 keV Ni+. Under the assumption of a sample density of 3.2 g/ cm3, the projected range of the Ni ions corresponds approximately to 0.5 lm [7], i.e. half of the epilayer thickness (1 lm). Most of the samples were implanted at 450 °C, but for comparison some implantations were performed at room temperature, and the fluences ranged from 0.5 1016 Ni/cm2 to 9 1016 Ni/cm2. In order
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to keep a small virgin area as a crystalline reference in the irradiated samples, an adequate mask was used to restrict the beam exposed area. A post-implantation annealing was performed in a N2 atmosphere at 1100 °C, for 40 min. The crystalline quality of the samples was studied by Backscattering Spectrometry in channeling geometry (BS/C) along the [0 0 0 1] axial channeling direction before and after the annealing. The BS/C measurements were done with a 3.45 MeV 4He2+ beam, and the random and aligned spectra were recorded with a surface barrier semiconductor detector placed at 165° in Cornell geometry. We determined the crystal axes of the samples in the virgin areas, and then the He beam was just translated to the irradiated area to avoid deviations from the crystal axes. The BS spectra in aligned geometry were taken for the same integrated beam charge. Ion implantation and BS/C analysis were performed at the 3 MV tandem accelerator facilities at the Centro Nacional de Aceleradores. In all the cases the implanted fluence was experimentally determined by integrating the Ni peak area by means of the RBX simulation code [8]. 3. Results and discussion Fig. 1 shows the 3.45 MeV 4He BS/C aligned spectra corresponding in this particular case to a single-crystalline 4H-SiC sample implanted at room temperature with 850 keV Ni+ ions at a fluence of 1 1016 ions/cm2. For comparison, the aligned and random spectra of the unirradiated area (virgin) are also shown. An increase in the yield of the aligned spectrum corresponds to an increase in damage or number of displaced atoms. Thus, the amount of disorder increases continuously with the implanted fluence until amorphization occurs, when the aligned spectrum reaches the random level. Here, it is important to stress that even if a poly-crystalline sample would give a similar spectrum, we have previously demonstrated by transmission electron microscopy (TEM) measurements that as-implanted samples reaching the BS random level are really amorphous [3]. Therefore, from Fig. 1 it can be observed that the near-surface region of the implanted samples is completely amorphous after a 1 1016 Ni/cm2 fluence. Similar results have been recently found by other groups in the case of Fe implantations at 200 °C, where complete amorphization of the SiC samples occurred [9]. After the 1100 °C thermal annealing, the crystalline structure of the RT-implanted sample is still heavily damaged, and only a decrease of the damaged layer thickness is observed (see Fig. 1). This result suggests that a partial lattice recovery or recrystallization process merely starts at the boundary between amorphous and crystalline zones [3].
Fig. 1. 3.45 MeV He2+ BS/C spectra of a 4H-SiC sample implanted at RT with 850 keV Ni+ ions at a fluence of 1 1016 ions/cm2 before and after the 1100 °C thermal annealing. Aligned and random spectra of the virgin sample are also shown.
On the other hand, Fig. 2 shows the 3.45 MeV 4He BS/C aligned spectra corresponding to the SiC epilayers implanted at high temperature (450 °C) at several Ni fluences, as well as the aligned and random spectra of the virgin sample. In the near-surface region of the virgin sample a minimum yield (ratio of aligned to random yields) of vmin 2% is achieved in the 1865–1892 keV energy range (DE 27 keV energy window), indicating the excellent quality of the SiC epilayers. We can notice that in the case of high-temperature implantation the amount of disorder increases with the fluence, but no complete amorphization occurs (as in the case of RT implantation), even for a fluence as high as 9.1 1016 Ni/cm2. Thus, while the critical fluence /c necessary to achieve the full amorphization of SiC with 850 keV Ni ions at room temperature is about 1 1016 Ni/cm2, in the case of 450 °C implantation /c is, at least, one order of magnitude higher. Moreover, the disorder depth distribution in the silicon sublattice is presented in Fig. 3. The defect depth profile was extracted from the experimental BS/ C spectra of the irradiated samples using the RBX code [10], and assuming a density of 3.2 g/cm3 for SiC. Considering the two-beam model, the dechanneling fraction of the beam was calculated by an iterative procedure [11]. One can observe that the damage profiles extracted from the BS/C spectra increase with the fluence and exhibit asymmetrical distributions, with an important contribution into larger depths (0.5–1 lm) for all the studied fluences, but a complete buried amorphous layer is not formed. In principle, further implantations at higher fluences should result in a continuous broadening of the buried damage layer towards both the surface and into larger depths. Although a highly defective zone can be observed, peaking at a depth larger than the Ni projected range (Rp 0.5 lm) and exhibiting only a partial damage, we can affirm that the high-temperature implantation is effective to reduce the amorphization process of the samples, even for the highest fluence (9.1 1016 Ni/cm2). Fig. 4 shows the BS/C spectra of the 450 °C Ni-implanted epilayers before and after the 1100 °C thermal annealing, for three different fluences (0.5, 1.5 and 9.1 1016 Ni/cm2). One can observe that for a given fluence the recovery of the defects created during the implantation is not complete, persisting a residual damage after the high-temperature annealing. Moreover, the recrystallization is more noticeable at the boundaries than at the core of the damaged layer, producing a decrease in the thickness of the partially amorphous buried layer. Considering Figs. 1 and 4, it is clear that the extent of lattice recrystallization after the annealing is negligible in the RT-implanted sample compared with the damage
Fig. 2. 3.45 MeV 4He BS/C aligned spectra of SiC epilayers implanted at 450 °C with 850 keV Ni+ ions for various fluences. The aligned and random spectra of the virgin sample are also included.
J. García López et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1097–1100
Fig. 3. Extracted damage profiles obtained from the BS/C aligned spectra of SiC epilayers implanted at 450 °C with 850 keV Ni+ ions for various fluences.
Fig. 4. 3.45 MeV 4He BS/C aligned spectra of SiC epilayers implanted at 450 °C with 850 keV Ni+ ions, before and after the post-implantation thermal annealing, for three different fluences. The aligned and random spectra of the virgin sample are also included.
recovery of the 450 °C-implanted epilayers. Therefore, the reduction of disorder is more efficient in the case of implantations at elevated temperatures compared with RT implantations followed by subsequent thermal treatments [5,6]. It is important to stress that the main purpose of this two-step process implantation + annealing is to eliminate the disorder of the as-implanted samples, and it is clear that the extent of the structural recovery will depend on these two parameters: the implantation temperature and the annealing temperature. Therefore, the determination of these two critical temperatures can lead us not only to the lattice recrystallization by annealing out the residual damage, but also to finely tune the desired magnetic properties. Now, we would like to consider the relative disorder in the Si sublattice. In general, the main defects introduced by ion implantation into SiC are vacancy-type defects, i.e. vacancies, divacancies, and complexes involving both vacancies and impurities [12]. As discussed above, we can assume that the displaced atoms cause the direct backscattering of the channeled analyzing ions and extract from the BS/C spectra the value of the relative disorder within the Si sublattice at an energy window corresponding to a depth interval around the maximum of the damage peak, roughly 0.6 lm). Thus, Fig. 5 represents the Ni fluence dependence of the relative disorder concentration around the maximum of the Si damage profiles of the irradiated SiC epilayers, before and after
1099
Fig. 5. Relative disorder concentration ND/N in the near-surface region of SiC epilayers implanted at 450 °C with 850 keV Ni+ ions as a function of the fluence, before and after the thermal annealing. The solid lines are just a guide for the eye.
the thermal annealing. Full amorphization corresponds to a relative disorder of 1.0. The relative disorder increases with the fluence for both the as-implanted and the annealed samples, but it seems to saturate at a value of 0.8 and 0.7, respectively. Thus, we can notice that no complete amorphization occurs in the as-implanted epilayers, even for fluences as high as 9.1 1016 Ni/cm2. Moreover, we can say that the high-temperature implantation prevents the full amorphization of the Si sublattice, even if the amount of disorder increases as a function of the Ni fluence. On the other hand, this result exhibits also the recrystallization process due to the post-implantation annealing. In all the cases, the relative disorder decreases by an amount of the order of 0.13 after the annealing. The concentration of these vacancy-related centers decreases with the implantation temperature, because the rate of recombination of interstitial atoms with vacancies is favored by an increase in the mobility [12]. In no way our 1100 °C thermal annealing brings about the complete recrystallization of the implanted layer. Moreover, in the case of 4H-SiC[0 0 0 1] Al-implanted layers, we have recently reported that, if the amorphized region extends until the top surface, annealing at 1100 °C leads to a poly-crystalline structure which contains 3C-SiC inclusions [3]. In that case the long distance disorder remains, and no difference is observed in the BS/C spectra for as-implanted (quasi-amorphous) and annealed (poly-crystalline) samples, as we mentioned before. We have also shown that during annealing the solid phase regrowth of SiC started from two directions, i.e. from both the crystalline surface and the interface region. Therefore the crystallization during annealing started as the growth of small regions from the amorphous/crystalline interface and by the nucleation of crystalline grains in the top amorphous region [3]. Complementary experiments by spectroscopic ellipsometry and transmission electron microscopy are being carried out in order to determine the actual structure of our post-annealed SiC Ni-implanted samples. One of the important aspects of the post-implantation annealing process is not only to anneal out the defects and to recover the crystalline structure, but also to activate the implanted ions at the right places in the lattice. Then, the location of the Ni atoms in the SiC lattice should be studied somehow. From the backscattering spectra one can observe that the Ni signal is completely separated from the SiC one, so in this way it is possible to determine as a first approximation the minimum yield vmin(Ni) (ratio of aligned to random yields for Ni). Fig. 6 shows the minimum yield vmin(Ni) for the SiC epilayers as a function of the fluence after the post-implantation annealing. This graph indicates that the vmin(Ni) increases systematically with the fluence, and no saturation is
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J. García López et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1097–1100
Fig. 6. Minimum yield vmin(Ni) as a function of the fluence of SiC epilayers implanted with 850 keV Ni+ ions at 450 °C, after the 1100 °C thermal annealing. The solid line is just a guide for the eye.
the post-implantation annealing on the recrystallization process. The high-temperature implantation at 450 °C is effective to reduce the amorphization process in the samples, even for the highest fluence used in this work. Concerning the lattice recrystallization after the annealing, it is negligible in the RT-implanted sample compared with the damage recovery of the 450 °C-implanted epilayers. Therefore, the reduction of disorder is more efficient in the case of implantations at elevated temperatures compared with RT implantations followed by subsequent thermal treatments. On the other hand, the decrease in the BS aligned yield seems to indicate that a fraction of the implanted Ni atoms occupy actually substitutional sites in the SiC lattice after the annealing. The high-temperature implantation decreases the generation rate for induced defects in the Si sublattice and increases the critical radiation fluence that leads to amorphization. This finding indicates that it is possible to increase the radiation resistance of devices based on SiC at increased temperatures of operation and also to tune up the desired magnetic properties.
Acknowledgments observed in the studied range, i.e. vmin(Ni) < 100% in all the cases. In other words, for all the fluences the BS Ni yield in aligned geometry is always lower than the corresponding random value, and in the best case vmin(Ni) 62% for the lowest fluence. The decrease in the BS aligned yield seems to indicate that a fraction of the implanted Ni atoms occupy actually substitutional sites in the SiC lattice. The precise location of the Ni atoms is under study by means of angular scans through low index axial and planar channels, but according to atomic size considerations, they can be preferentially incorporated into Si lattice sites. The magnetic properties of the samples as a function of the ion fluence, structural damage and degree of substitutional Ni will be reported in a forthcoming paper. Finally, it is clear that in order to use SiC to create magnetic semiconductors for device applications, the structural damage induced by the implantation process must be reduced. The best way to achieve this goal is the implantation into heated targets, because the dynamic annealing decreases the generation rate for radiation defects. 4. Conclusions We have studied the structural disorder produced by the implantation of 850 keV Ni ions in SiC epilayers and the effect of
This work is partially supported by the Projects MAT200603519 (Spanish MEC), P06-TEP-01739 (Junta de Andalucía) and SAB2006-0041 (Spanish MEC).
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