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Swift heavy ion beam induced recrystallization of amorphous Si layers
Nuclear Instruments and Methods in Physics Research B 240 (2005) 239–244 www.elsevier.com/locate/nimb
Swift heavy ion beam induced recrystallization of amorphous Si layers P.K. Sahoo a
a,*
, T. Som b, D. Kanjilal c, V.N. Kulkarni
a
Department of Physics, Indian Institute of Technology, Kanpur-208016, India b Institute of Physics Bhubaneswar-751005, India c Nuclear Science Center, New Delhi-110067, India Available online 28 July 2005
Abstract This paper focuses on the role of electronic energy loss in ion beam induced epitaxial crystallization for which swift heavy ions (100 MeV Ag7+) have been used. We observed good epitaxial crystallization at 473–623 K, which is a much lower temperature regime as compared to the one needed for conventional solid phase epitaxial growth. A systematic planar recrystallization has been observed as a function of temperature giving rise to an activation energy of 0.25 ± 0.02 eV. A possible mechanism of recrystallization is discussed on the basis of the production of vacancies along the track of the swift heavy ion and their migration at elevated temperatures. 2005 Elsevier B.V. All rights reserved. PACS: 61.85.+P; 61.80.Jh Keywords: IBIEC; Swift heavy ions; RBS-channeling; Recrystallization
1. Introduction In device fabrication technology, removal of defects and recovery of original crystalline structure
* Corresponding author. Present address: II. Physikalisches Institut, Universita¨t Go¨ttingen, Friedrich-Hund-Plaz 1, D37077 Go¨ttingen, Germany. Tel.: +49 551 397652, fax: +40 551 394493. E-mail addresses: [email protected] (P.K. Sahoo), [email protected] (V.N. Kulkarni).
of the device material is necessary to get the electrically active doped layer and also for suppression of electrically active defect structures. The most simple and well-established method of achieving original crystalline structure is furnace annealing. In the past two decades several alternative methods have been explored, viz. ion beam annealing, rapid thermal annealing, high power pulsed laser and electron beam annealing [1]. Ion beam induced epitaxial crystallization (IBIEC) of an amorphous Si (a-Si) layer crystalline matrix is being currently
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.06.123
240
P.K. Sahoo et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 239–244
investigated as a potential processing step for epitaxial recovery of ion damaged layers. Attention towards IBIEC is focused due to its unique characteristics such as low processing temperature, layerby-layer and selective area recrystallization, and dynamic defect annealing [2,3]. Linear dependence of the regrowth rate on nuclear energy deposition by low energy ions (up to a few MeV) or vacancy concentration has been reported [4–7]. The existing models attribute the point defects generated by nuclear collisions as the main cause for IBIEC. The effect of inelastic scattering processes on IBIEC in which the projectile loses its energy mostly by collisions with electrons without causing direct atomic displacements remains to be investigated fully. The possible role of electronic energy deposition on the crystallization process has been reported in case of silicon-on-insulator structures [8], which indirectly suggests that swift heavy ion (SHI) irradiation through an amorphous/crystalline (a/c) interface causes solid phase epitaxy. However, till date, there is no direct evidence of crystallization process induced in a-Si in the electronic energy loss dominated regime. In this work, we have demonstrated that electronic energy loss (Se) processes can lead to epitaxial crystallization of ion amorphized silicon and the mechanism of epitaxial growth is different as compared to the one based on nuclear energy loss (Sn) processes. As an example we have chosen 100 MeV Ag7+ ions for IBIEC for which the ratio of Se to Sn is 180. Activation energy for layer-bylayer recrystallization process has been calculated from Rutherford backscattering spectrometrychanneling (RBS-C) data. The tilt angle scanning from RBS-C measurements shows the quality of the recrystallization. Production of vacancies and its migration during SHI irradiation at elevated temperatures has been attributed to be the origin to stimulate the recrystallization in a-Si.
2. Experimental Well-polished and ultrasonically cleaned p-type Si (100) samples (size 10 mm · 10 mm) were amorphized by 200 keV Xe4+ ions at room temperature (RT). The amorphization was done with a fluence
of 5 · 1014 cm2 using an electron cyclotron resonance (ECR) ion source facility. The samples were kept at a glancing angle of 7 to the incident beam to produce amorphous layers in the thickness range of 150–220 nm, as calculated by SRIM2000 code [9]. The amorphized samples were kept on a high temperature target holder and irradiated by Ag7+ ions with a fluence of 2 · 1014 cm2 using the 15UD Pelletron accelerator at Nuclear Science Centre, New Delhi (India), to stimulate IBIEC. The target temperatures were stabilized within ±2 C using a lakeshore temperature controller. It can be mentioned here that the calculated value [9] (from SRIM-2000 code) of Se for 100 MeV Ag ions in Si is 11.2 keV/nm whereas the Sn is 59.3 eV/nm. The irradiation flux was always kept quite low (<1010 ions cm2 s1) to avoid any additional sample heating due to irradiation. A secondary-electron-suppressed geometry was used during irradiation and the uniform irradiation was achieved by using a 1 cm · 1 cm scanned beam. The thickness of the amorphous layer produced by Xe4+ ions and the thickness of recrystallized layer after Ag7+ ion-irradiation were measured by RBS-C experiments using 1.6 MeV He+ ions using the Van-de-Graaff accelerator and ion beam analysis facilities at IIT, Kanpur (India). The apparent damage distributions D(z) as a function of depth (z) of the Si matrix were calculated from the computer code DAMAGE [10] which is based on an algorithm given by Walker and Thompson [11] and allow for proper treatment of dechanneled fraction of the analyzing beam.
3. Results and discussion The RBS-C spectra for the as-amorphized and 100 MeV Ag7+ ion-induced recrystallized samples for a fluence of 2 · 1014 cm2 are presented in Fig. 1. Random and aligned spectra of the pristine c-Si are also shown for comparison. The pristine sample shows a minimum yield (vmin) of 3% indicating a very good crystalline quality. The amorphous layer thickness reduces as the temperature increases to 623 K during swift heavy ion beam induced epitaxial crystallization (SHIBIEC) process. It is important to note that almost no epitaxial
P.K. Sahoo et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 239–244
Fig. 1. RBS-C spectra obtained from as-amorphized and SHIBIEC Si samples for each temperature. The aligned spectrum of thermally annealed sample (673 K, Tth) along with random and aligned spectra of c-Si is also shown for comparison. The Xe profiles are multiplied by a factor of 10 for clarity. The inset shows the integrated Xe concentration as a function of sample temperature.
recovery was observed for RT swift heavy ion irradiated and 673 K furnace annealed samples. The RBS-C spectrum of the thermally annealed sample is also shown in Fig. 1. The Xe profiles are multiplied by a factor of 10 for clarity. The reduction of the Xe signal both from aligned and random spectra indicates its out-diffusion during the SHIBIEC process. The inset shows the integrated Xe concentration as a function of sample temperature. The normalized damage profiles D(z) have been calculated from the RBS-C spectra and are shown in Fig. 2. These damage profiles describe the fraction of the atoms distributed randomly in the matrix. These fractions are calculated using the DAMAGE code which uses the ratio of the yield of the aligned spectrum (after subtracting the dechanneling yield) to the one in the random spectrum of the sample. The thicknesses of the amorphous layers were determined by multiplying the damage profile full width at half maximum (in 1015 cm2) by the density of the amorphous Si. The thickness of as-amorphized sample comes out as 220 nm. The damage profile at 623 K shows that D(z) reduces to 55% of the total damage, which can be interpreted as isolated damage region in the crystallized layer of 60 nm thicknesses.
241
Fig. 2. The damage profile D(z) extracted from the aligned spectra of SHIBIEC samples shown in Fig. 1.
Fig. 3 shows the Arrhenius plot of the regrowth rate as a function of the inverse of SHI irradiation temperature, which gives the activation energy (Ea) of 0.25 ± 0.02 eV for 100 MeV Ag7+ induced crystallization. The value of the activation energy is comparable to those available in the literature corresponding to the low energy IBIEC [2–6]. Fig. 4 shows the angular scans for h1 0 0i axial channeling. The window was set just beneath the surface peak. The defects and impurity atoms af-
Fig. 3. Arrhenius plot of regrowth rates for 100 MeV Ag7+ SHIBIEC as a function of inverse of temperature.
242
P.K. Sahoo et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 239–244
Fig. 4. RBS-Channeling angular scan along the h1 0 0i axis of SHIBIEC Si samples as function of irradiation temperature. The inset shows the FWHM w1/2 as a function of SHIBIEC temperature.
fect the impact parameter of the projectile ions in case of axial channeling, which is reflected in the full width at half maximum (w1/2) of angular scan results. The value of w1/2 continuously reduces as a function of temperature and at 623 K it is 0.56 . For comparison, the value of w1/2 for pristine Si is 0.3 . Thus it can be inferred that the quality of epitaxy improves continuously as a function of irradiation temperature. Our investigations clearly indicate that SHI irradiation leads to recrystallization of amorphous Si layer over the crystalline Si matrix. The reduction in the amorphous layer thickness in Fig. 1 shows the movement of a/c interface towards the surface. In order to understand the effect of SHI irradiation on a/c interface of Si we first consider the models proposed by various authors [3–5,12– 15] for the case of low energy (up to a few MeV) ion-induced epitaxial crystallization process. In most of these models, the point defects produced during the displacement collisions are assumed to be responsible for the low temperature recrystallization under ion-irradiation. It was pointed out by Nakata [5] that the electronic energy loss Se might give rise to larger number of doubly negative vacancies V2 (produced in case of MeV ion beam irradiation due to ionization), which would play a crucial role in IBIEC. He also proposed that the
V2 vacancies are more mobile as compared to the neutral vacancies because of their higher migration energy, which is 0.33 eV. Since then, this is the first experimental attempt in the Se dominated regime to achieve IBIEC in Si, which we have termed as SHIBIEC. When a swift heavy ion passes through a material, it loses energy by ionization and atomic collision. The rapid energy transfer makes the system abnormally excited and the region around the ion track gets suddenly heated to a very high temperature within a small time scale [14–18]. The temperature evolution in the spike can be calculated by assuming that the initial temperature distribution has the form of a d-function along the ion track [14,15]. The temperature distribution then evolves according to the laws of classical heat conduction. The temperature evolution of the ion track as a function of time for 100 MeV Ag7+ ions in Si for different track radii are calculated using the simple function from [14] and is shown in Fig. 5. The maximum temperature of the track is higher for small track radius. Following the work of Toulemonde [16,18], it is reasonable to assume a track radius of 10 nm. The maximum temperature of the track for 1 MeV N ions is 420 K while for all SHIs it is above 800 K. It should be noted that the actual values may be slightly different than
Fig. 5. Temperature of the ion track (for different track radii) as a function of time.
P.K. Sahoo et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 239–244
the calculated ones but the dependence of the maximum temperature of the track on the value of Se should follow the same trend. First let us calculate the number of excess vacancies (in addition to those formed by the nuclear energy loss and eventually survives) created by this temperature rise and then we will deliberate on the mechanism of layer-by-layer growth under SHI irradiation. Although the vacancies/defects are produced all along the track, all of them cannot reach the interface and participate in the growth process. Assuming generation of vacancies of the order of 1019 cm3 along the track, a typical diffusion length of 25 nm [13,16] for the vacancies, and the track diameter of 10 nm, the number of vacancies reaching per unit area at the a/c interface is approximately 1014 cm2. These additional vacancies reaching the interface help in enhancing the regrowth process. The point to be noted here is that the thermal spikes only generate vacancies/defects, the migration of vacancies occur due to the ambient temperature since the life time of thermal spike is very small. This also explains as to why, SHIBIEC does not occur at RT. For low energy ions the maximum track temperature is 360 K. The enhancement in IBIEC starts, when Se value goes above 20 eV/nm. When the track temperature goes beyond 1000 K, the effect of the enhancement is clearly seen in the recrystallization process. In this way the enhancement in the growth rate and the threshold can be understood. Williams et al. [4], for example, used 0.6–3 MeV Ne ions at fluences from 5 · 1015 to 1.2 · 1017 cm2 to achieve IBIEC in Si matrix. At these energies, the Sn value ranges from 0.15 eV/nm to 0.48 eV/nm. However, in our experiment the value of Sn (59.3 eV/nm) and the irradiation fluence is 2 · 1014 cm2. Both these parameters are at least one order of magnitude less than that in the former case. This further fortifies the role of Se in SHIBIEC process of Si in our case.
4. Conclusion In conclusion, we have investigated the possibilities of SHIBIEC of pre-amorphized Si. The acti-
243
vation energy for recrystallization process has been calculated as 0.25 ± 0.02 eV, which is comparable to that reported using low energy ion beams. RBS-C angular scans show a good quality of epitaxy in SHIBIEC process. The enhancement in the regrowth rate is proposed to be occurring over a hot region around the ion track, which is created due to the very large electronic energy loss. Vacancies created in this region migrate, increase the vacant spaces at the a/c interface, and enhance the regrowth rate due to irradiation performed at elevated temperatures.
Acknowledgements The authors would like to thank Prof. N.C. Mishra, and Prof. K.P. Lieb for useful discussions and also gratefully acknowledge the helps received from the Van-de-Graaff accelerator group of IIT Kanpur, Pelletron and the low energy ion beam facility of NSC group during the experiments.
References [1] J.S. Williams, in: J.M. Poate, G. Foti, D.C. Jacobson (Eds.), Surface Modification and Alloyings by Laser, Ion and Electron Beams, Plenum Press, New York, 1983. [2] A. Kinomura, A. Chayahara, N. Tsubouchi, J.S. Williams, Nucl. Instr. and Meth. B 148 (1999) 370. [3] J. Linnros, G. Holmen, B. Sevensson, Phys. Rev. B 32 (1985) 2770. [4] J.S. Williams, R.G. Elliman, W.L. Brown, T.E. Seidel, Phys. Rev. Lett. 55 (1985) 1482. [5] J. Nakata, Phys. Rev. B 43 (1991) 14643. [6] J. Nakata, J. Appl. Phys. 79 (1996) 682. [7] K.A. Jackson, J. Mater. Res. 3 (1988) 1218. [8] G.S. Virdi, B.C. Pathak, D.K. Avasthi, D. Kanjilal, Nucl. Instr. and Meth. B 187 (2002) 189. [9] http://www.srim.org/. [10] J. Conrad, Ph.D. Thesis, Universita¨t Go¨ttingen, 1996 (unpublished). [11] R.S. Walker, D.A. Thompson, Nucl. Instr. and Meth. 135 (1976) 489. [12] V. Heera, T. Henkel, R. Ko¨gler, W. Skorupa, Phys. Rev. B 52 (1995) 15776. [13] G. Lulli, P.G. Merli, M.V. Antisari, Phys. Rev. B 36 (1987) 8038. [14] G.H. Vineyard, Radiat. Eff. Defects in Solids 29 (1976) 254.
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P.K. Sahoo et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 239–244
[15] M.L. Brongersma, E. Snoeks, A. Polman, Appl. Phys. Lett. 71 (1997) 1628. [16] M. Toulemonde, J.M. Costantini, C.h. Dufour, A. Meftah, E. Paumier, F. Studer, Nucl. Instr. and Meth. B 116 (1996) 37.
[17] A. Meftah, F. Brisard, J.M. Costantini, M. Hage-Ali, M. Hervieu, J.P. Stoquert, F. Studer, M. Toulemonde, Phys. Rev. B 49 (1994) 12457. [18] M. Toulemonde, C. Dufour, E. Paumier, Phys. Rev. B 46 (1992) 14362.
Swift heavy ion beam induced recrystallization of amorphous Si layers P.K. Sahoo a
a,*
, T. Som b, D. Kanjilal c, V.N. Kulkarni
a
Department of Physics, Indian Institute of Technology, Kanpur-208016, India b Institute of Physics Bhubaneswar-751005, India c Nuclear Science Center, New Delhi-110067, India Available online 28 July 2005
Abstract This paper focuses on the role of electronic energy loss in ion beam induced epitaxial crystallization for which swift heavy ions (100 MeV Ag7+) have been used. We observed good epitaxial crystallization at 473–623 K, which is a much lower temperature regime as compared to the one needed for conventional solid phase epitaxial growth. A systematic planar recrystallization has been observed as a function of temperature giving rise to an activation energy of 0.25 ± 0.02 eV. A possible mechanism of recrystallization is discussed on the basis of the production of vacancies along the track of the swift heavy ion and their migration at elevated temperatures. 2005 Elsevier B.V. All rights reserved. PACS: 61.85.+P; 61.80.Jh Keywords: IBIEC; Swift heavy ions; RBS-channeling; Recrystallization
1. Introduction In device fabrication technology, removal of defects and recovery of original crystalline structure
* Corresponding author. Present address: II. Physikalisches Institut, Universita¨t Go¨ttingen, Friedrich-Hund-Plaz 1, D37077 Go¨ttingen, Germany. Tel.: +49 551 397652, fax: +40 551 394493. E-mail addresses: [email protected] (P.K. Sahoo), [email protected] (V.N. Kulkarni).
of the device material is necessary to get the electrically active doped layer and also for suppression of electrically active defect structures. The most simple and well-established method of achieving original crystalline structure is furnace annealing. In the past two decades several alternative methods have been explored, viz. ion beam annealing, rapid thermal annealing, high power pulsed laser and electron beam annealing [1]. Ion beam induced epitaxial crystallization (IBIEC) of an amorphous Si (a-Si) layer crystalline matrix is being currently
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.06.123
240
P.K. Sahoo et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 239–244
investigated as a potential processing step for epitaxial recovery of ion damaged layers. Attention towards IBIEC is focused due to its unique characteristics such as low processing temperature, layerby-layer and selective area recrystallization, and dynamic defect annealing [2,3]. Linear dependence of the regrowth rate on nuclear energy deposition by low energy ions (up to a few MeV) or vacancy concentration has been reported [4–7]. The existing models attribute the point defects generated by nuclear collisions as the main cause for IBIEC. The effect of inelastic scattering processes on IBIEC in which the projectile loses its energy mostly by collisions with electrons without causing direct atomic displacements remains to be investigated fully. The possible role of electronic energy deposition on the crystallization process has been reported in case of silicon-on-insulator structures [8], which indirectly suggests that swift heavy ion (SHI) irradiation through an amorphous/crystalline (a/c) interface causes solid phase epitaxy. However, till date, there is no direct evidence of crystallization process induced in a-Si in the electronic energy loss dominated regime. In this work, we have demonstrated that electronic energy loss (Se) processes can lead to epitaxial crystallization of ion amorphized silicon and the mechanism of epitaxial growth is different as compared to the one based on nuclear energy loss (Sn) processes. As an example we have chosen 100 MeV Ag7+ ions for IBIEC for which the ratio of Se to Sn is 180. Activation energy for layer-bylayer recrystallization process has been calculated from Rutherford backscattering spectrometrychanneling (RBS-C) data. The tilt angle scanning from RBS-C measurements shows the quality of the recrystallization. Production of vacancies and its migration during SHI irradiation at elevated temperatures has been attributed to be the origin to stimulate the recrystallization in a-Si.
2. Experimental Well-polished and ultrasonically cleaned p-type Si (100) samples (size 10 mm · 10 mm) were amorphized by 200 keV Xe4+ ions at room temperature (RT). The amorphization was done with a fluence
of 5 · 1014 cm2 using an electron cyclotron resonance (ECR) ion source facility. The samples were kept at a glancing angle of 7 to the incident beam to produce amorphous layers in the thickness range of 150–220 nm, as calculated by SRIM2000 code [9]. The amorphized samples were kept on a high temperature target holder and irradiated by Ag7+ ions with a fluence of 2 · 1014 cm2 using the 15UD Pelletron accelerator at Nuclear Science Centre, New Delhi (India), to stimulate IBIEC. The target temperatures were stabilized within ±2 C using a lakeshore temperature controller. It can be mentioned here that the calculated value [9] (from SRIM-2000 code) of Se for 100 MeV Ag ions in Si is 11.2 keV/nm whereas the Sn is 59.3 eV/nm. The irradiation flux was always kept quite low (<1010 ions cm2 s1) to avoid any additional sample heating due to irradiation. A secondary-electron-suppressed geometry was used during irradiation and the uniform irradiation was achieved by using a 1 cm · 1 cm scanned beam. The thickness of the amorphous layer produced by Xe4+ ions and the thickness of recrystallized layer after Ag7+ ion-irradiation were measured by RBS-C experiments using 1.6 MeV He+ ions using the Van-de-Graaff accelerator and ion beam analysis facilities at IIT, Kanpur (India). The apparent damage distributions D(z) as a function of depth (z) of the Si matrix were calculated from the computer code DAMAGE [10] which is based on an algorithm given by Walker and Thompson [11] and allow for proper treatment of dechanneled fraction of the analyzing beam.
3. Results and discussion The RBS-C spectra for the as-amorphized and 100 MeV Ag7+ ion-induced recrystallized samples for a fluence of 2 · 1014 cm2 are presented in Fig. 1. Random and aligned spectra of the pristine c-Si are also shown for comparison. The pristine sample shows a minimum yield (vmin) of 3% indicating a very good crystalline quality. The amorphous layer thickness reduces as the temperature increases to 623 K during swift heavy ion beam induced epitaxial crystallization (SHIBIEC) process. It is important to note that almost no epitaxial
P.K. Sahoo et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 239–244
Fig. 1. RBS-C spectra obtained from as-amorphized and SHIBIEC Si samples for each temperature. The aligned spectrum of thermally annealed sample (673 K, Tth) along with random and aligned spectra of c-Si is also shown for comparison. The Xe profiles are multiplied by a factor of 10 for clarity. The inset shows the integrated Xe concentration as a function of sample temperature.
recovery was observed for RT swift heavy ion irradiated and 673 K furnace annealed samples. The RBS-C spectrum of the thermally annealed sample is also shown in Fig. 1. The Xe profiles are multiplied by a factor of 10 for clarity. The reduction of the Xe signal both from aligned and random spectra indicates its out-diffusion during the SHIBIEC process. The inset shows the integrated Xe concentration as a function of sample temperature. The normalized damage profiles D(z) have been calculated from the RBS-C spectra and are shown in Fig. 2. These damage profiles describe the fraction of the atoms distributed randomly in the matrix. These fractions are calculated using the DAMAGE code which uses the ratio of the yield of the aligned spectrum (after subtracting the dechanneling yield) to the one in the random spectrum of the sample. The thicknesses of the amorphous layers were determined by multiplying the damage profile full width at half maximum (in 1015 cm2) by the density of the amorphous Si. The thickness of as-amorphized sample comes out as 220 nm. The damage profile at 623 K shows that D(z) reduces to 55% of the total damage, which can be interpreted as isolated damage region in the crystallized layer of 60 nm thicknesses.
241
Fig. 2. The damage profile D(z) extracted from the aligned spectra of SHIBIEC samples shown in Fig. 1.
Fig. 3 shows the Arrhenius plot of the regrowth rate as a function of the inverse of SHI irradiation temperature, which gives the activation energy (Ea) of 0.25 ± 0.02 eV for 100 MeV Ag7+ induced crystallization. The value of the activation energy is comparable to those available in the literature corresponding to the low energy IBIEC [2–6]. Fig. 4 shows the angular scans for h1 0 0i axial channeling. The window was set just beneath the surface peak. The defects and impurity atoms af-
Fig. 3. Arrhenius plot of regrowth rates for 100 MeV Ag7+ SHIBIEC as a function of inverse of temperature.
242
P.K. Sahoo et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 239–244
Fig. 4. RBS-Channeling angular scan along the h1 0 0i axis of SHIBIEC Si samples as function of irradiation temperature. The inset shows the FWHM w1/2 as a function of SHIBIEC temperature.
fect the impact parameter of the projectile ions in case of axial channeling, which is reflected in the full width at half maximum (w1/2) of angular scan results. The value of w1/2 continuously reduces as a function of temperature and at 623 K it is 0.56 . For comparison, the value of w1/2 for pristine Si is 0.3 . Thus it can be inferred that the quality of epitaxy improves continuously as a function of irradiation temperature. Our investigations clearly indicate that SHI irradiation leads to recrystallization of amorphous Si layer over the crystalline Si matrix. The reduction in the amorphous layer thickness in Fig. 1 shows the movement of a/c interface towards the surface. In order to understand the effect of SHI irradiation on a/c interface of Si we first consider the models proposed by various authors [3–5,12– 15] for the case of low energy (up to a few MeV) ion-induced epitaxial crystallization process. In most of these models, the point defects produced during the displacement collisions are assumed to be responsible for the low temperature recrystallization under ion-irradiation. It was pointed out by Nakata [5] that the electronic energy loss Se might give rise to larger number of doubly negative vacancies V2 (produced in case of MeV ion beam irradiation due to ionization), which would play a crucial role in IBIEC. He also proposed that the
V2 vacancies are more mobile as compared to the neutral vacancies because of their higher migration energy, which is 0.33 eV. Since then, this is the first experimental attempt in the Se dominated regime to achieve IBIEC in Si, which we have termed as SHIBIEC. When a swift heavy ion passes through a material, it loses energy by ionization and atomic collision. The rapid energy transfer makes the system abnormally excited and the region around the ion track gets suddenly heated to a very high temperature within a small time scale [14–18]. The temperature evolution in the spike can be calculated by assuming that the initial temperature distribution has the form of a d-function along the ion track [14,15]. The temperature distribution then evolves according to the laws of classical heat conduction. The temperature evolution of the ion track as a function of time for 100 MeV Ag7+ ions in Si for different track radii are calculated using the simple function from [14] and is shown in Fig. 5. The maximum temperature of the track is higher for small track radius. Following the work of Toulemonde [16,18], it is reasonable to assume a track radius of 10 nm. The maximum temperature of the track for 1 MeV N ions is 420 K while for all SHIs it is above 800 K. It should be noted that the actual values may be slightly different than
Fig. 5. Temperature of the ion track (for different track radii) as a function of time.
P.K. Sahoo et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 239–244
the calculated ones but the dependence of the maximum temperature of the track on the value of Se should follow the same trend. First let us calculate the number of excess vacancies (in addition to those formed by the nuclear energy loss and eventually survives) created by this temperature rise and then we will deliberate on the mechanism of layer-by-layer growth under SHI irradiation. Although the vacancies/defects are produced all along the track, all of them cannot reach the interface and participate in the growth process. Assuming generation of vacancies of the order of 1019 cm3 along the track, a typical diffusion length of 25 nm [13,16] for the vacancies, and the track diameter of 10 nm, the number of vacancies reaching per unit area at the a/c interface is approximately 1014 cm2. These additional vacancies reaching the interface help in enhancing the regrowth process. The point to be noted here is that the thermal spikes only generate vacancies/defects, the migration of vacancies occur due to the ambient temperature since the life time of thermal spike is very small. This also explains as to why, SHIBIEC does not occur at RT. For low energy ions the maximum track temperature is 360 K. The enhancement in IBIEC starts, when Se value goes above 20 eV/nm. When the track temperature goes beyond 1000 K, the effect of the enhancement is clearly seen in the recrystallization process. In this way the enhancement in the growth rate and the threshold can be understood. Williams et al. [4], for example, used 0.6–3 MeV Ne ions at fluences from 5 · 1015 to 1.2 · 1017 cm2 to achieve IBIEC in Si matrix. At these energies, the Sn value ranges from 0.15 eV/nm to 0.48 eV/nm. However, in our experiment the value of Sn (59.3 eV/nm) and the irradiation fluence is 2 · 1014 cm2. Both these parameters are at least one order of magnitude less than that in the former case. This further fortifies the role of Se in SHIBIEC process of Si in our case.
4. Conclusion In conclusion, we have investigated the possibilities of SHIBIEC of pre-amorphized Si. The acti-
243
vation energy for recrystallization process has been calculated as 0.25 ± 0.02 eV, which is comparable to that reported using low energy ion beams. RBS-C angular scans show a good quality of epitaxy in SHIBIEC process. The enhancement in the regrowth rate is proposed to be occurring over a hot region around the ion track, which is created due to the very large electronic energy loss. Vacancies created in this region migrate, increase the vacant spaces at the a/c interface, and enhance the regrowth rate due to irradiation performed at elevated temperatures.
Acknowledgements The authors would like to thank Prof. N.C. Mishra, and Prof. K.P. Lieb for useful discussions and also gratefully acknowledge the helps received from the Van-de-Graaff accelerator group of IIT Kanpur, Pelletron and the low energy ion beam facility of NSC group during the experiments.
References [1] J.S. Williams, in: J.M. Poate, G. Foti, D.C. Jacobson (Eds.), Surface Modification and Alloyings by Laser, Ion and Electron Beams, Plenum Press, New York, 1983. [2] A. Kinomura, A. Chayahara, N. Tsubouchi, J.S. Williams, Nucl. Instr. and Meth. B 148 (1999) 370. [3] J. Linnros, G. Holmen, B. Sevensson, Phys. Rev. B 32 (1985) 2770. [4] J.S. Williams, R.G. Elliman, W.L. Brown, T.E. Seidel, Phys. Rev. Lett. 55 (1985) 1482. [5] J. Nakata, Phys. Rev. B 43 (1991) 14643. [6] J. Nakata, J. Appl. Phys. 79 (1996) 682. [7] K.A. Jackson, J. Mater. Res. 3 (1988) 1218. [8] G.S. Virdi, B.C. Pathak, D.K. Avasthi, D. Kanjilal, Nucl. Instr. and Meth. B 187 (2002) 189. [9] http://www.srim.org/. [10] J. Conrad, Ph.D. Thesis, Universita¨t Go¨ttingen, 1996 (unpublished). [11] R.S. Walker, D.A. Thompson, Nucl. Instr. and Meth. 135 (1976) 489. [12] V. Heera, T. Henkel, R. Ko¨gler, W. Skorupa, Phys. Rev. B 52 (1995) 15776. [13] G. Lulli, P.G. Merli, M.V. Antisari, Phys. Rev. B 36 (1987) 8038. [14] G.H. Vineyard, Radiat. Eff. Defects in Solids 29 (1976) 254.
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