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Damage buildup and the molecular effect in Si bombarded with PFn cluster ions
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 256 (2007) 207–210 www.elsevier.com/locate/nimb
Damage buildup and the molecular effect in Si bombarded with PFn cluster ions A.I. Titov a, A.Yu. Azarov b, L.M. Nikulina a, S.O. Kucheyev a
c,*
Department of Physical Electronics, St. Petersburg State Polytechnical University, St. Petersburg 195251, Russian Federation b Research and Production Company Electron-Optronic, Morisa Toreza 68, St. Petersburg 194223, Russian Federation c Lawrence Livermore National Laboratory, Livermore, CA 94550, USA Available online 22 January 2007
Abstract We study the molecular effect (ME) in damage accumulation in Si bombarded at room temperature with atomic P and F and cluster PFn (n = 2 and 4) ions with an energy of 2.1 keV/amu. Correct ion irradiation conditions for unambiguous studies of the ME are discussed. Rutherford backscattering/channeling spectrometry results show that the damage buildup behavior strongly depends on the cluster ion size, and the ME efficiency increases rapidly with increasing the number of atoms in cluster ions. Moreover, the ME efficiency decreases with increasing the defect generation rate, indicating that dynamic annealing processes, rather than nonlinear energy spikes, play a major role in the ME for these irradiation conditions. 2006 Elsevier B.V. All rights reserved. PACS: 61.80.Lj; 61.72.Yy; 68.49.Sf; 61.72.Tt Keywords: Ion implantation; Cluster ions; Molecular effect; Defects; Collision cascades; Silicon; Si
1. Introduction Interaction of cluster ions with solids has been studied for several decades (see, for example, an early review [1]). There has been a resurgence of cluster bombardment studies in recent years, primarily because of unique possibilities for material modification by cluster ions, including the fabrication of ultrashallow p–n junctions and efficient ion-etching and polishing of surfaces (see, for examples, [2–11]). It has been observed experimentally that the efficiency of many ion-beam-related phenomena (such as ion sputtering and mixing, secondary ion and electron emission, and radiation damage) often differs for atomic and cluster (molecular) ion bombardment regimes under correct irradiation conditions (see Section 2 for a definition of the correct conditions). This is the so-called molecular effect (ME). *
Corresponding author. Tel.: +1 9254225866; fax: +1 9254230785. E-mail address: [email protected] (S.O. Kucheyev).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.12.004
The ME in damage accumulation has generally been attributed to increased disordering in nonlinear energy spikes (such as thermal and displacement spikes), which form due to spatial overlap of collision cascades produced by the atoms comprising a molecular ion [1,7]. This concept can, at least qualitatively, explain experimental observations for amorphizable semiconductors bombarded with keV heavy ions or with cluster ions comprised of a large number of light atoms (e.g. C60), when the collision cascade density is large enough for nonlinear energy spikes to occur. However, for small cluster ions comprised of light atoms, the collision cascade as a whole cannot be considered nonlinear [4]. Therefore, the ME experimentally observed for light-ion bombardment has commonly been attributed to nonlinear energy spike processes resulting from a spatial overlap of relatively dense collision subcascades [4]. We have recently demonstrated that nonlinear energy spikes are not responsible for the ME in Si bombarded with keV N1 and N2 ions [10]. Instead, the ME in this case has been attributed to dynamic annealing processes
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A.I. Titov et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 207–210
(i.e. processes of annihilation and clustering of point defects during ion irradiation) [10]. In this contribution, we study the ME in Si irradiated with cluster ions which are made from different atomic species – PFn cluster ions. Correct ion irradiation conditions for unambiguous ME studies are discussed in Section 2. We also present results revealing strong effects of the collision cascade density and the displacement generation rate on damage buildup (Section 4.1) and ME efficiency (Section 4.2). 2. Correct irradiation conditions In order to study the underlying physical mechanisms of the ME, the only difference between atomic and molecular ion bombardment regimes should be the fact that atomic ions impinge on the sample surface at random locations, whereas the atoms comprising a molecular/cluster ion hit the surface at the same location. Hence, the following parameters should be kept the same for both atomic and molecular ion bombardment regimes: (i) ion velocity; (ii) the total number of atomic displacements and (iii) the displacement generation rate. For cluster ions made from the same atomic species (e.g. Cn, Aun, etc.), such conditions are satisfied if the energy, fluence, and ion beam flux, all three normalized per one incident atom, are kept constant. It is typically trivial to achieve such conditions experimentally. In contrast, it is much more challenging to satisfy correct irradiation conditions for cluster ions made from different elements. Generally, for XnYm cluster ions, this would require an experimental setup with possibilities of simultaneous irradiation with X and Y ions. However, when the profiles of ion-beam-generated displacements for the components of a cluster ion are similar, correct irradiation conditions are simplified to [11] Eamu ¼ const:; UDPA ¼ const:; F DPA ¼ const:;
ð1Þ
where Eamu is ion energy normalized to amu, and UDPA and FDPA are ion fluence and beam flux, respectively, normalized to the number of displacements per atom (DPA). Such a normalized fluence UDPA can be expressed as the average number of vacancies N max vac at the depth where the nuclear energy loss is maximum created by irradiation to fluence U normalized to the atomic concentration n0 UDPA ¼ N max vac U=n0 :
ð2Þ
The normalized ion flux FDPA could be estimated as the vacancy generation rate at the depth of the displacement profile maximum F DPA ¼
dUDPA ; dt
ð3Þ
where t is time. It is important to note that we are not aware of any previous ME studies (except for a recent brief report [11]) with cluster ions made from different elements where the correct conditions described above were followed.
N X nY m
nY m ¼ Nd X ;Y , where The ME efficiency can be defined as cXX ;Y X nY m Nd is the concentration of stable lattice defectsdproduced by XnYm, cluster ions and N Xd ;Y is the concentration of stable defects created by atomic X or Y ions under the conditions defined by Eq. (1).
3. Experimental Boron doped (1 0 0) Si samples (with a resistivity of 12 X cm) were implanted with F+, P+ and PFþ n (n = 2 and 4) ions over a wide fluence range. Implantation was carried out at room temperature (RT) at 7 off the [1 0 0] direction in order to minimize channeling. Simulations with the TRIM code [12] (with an effective threshold energy for atomic displacements of 13 eV, chosen based on experimental data from [13]) show that the depth profiles of vacancies generated in Si by P and F ions with an energy of 2.1 keV/amu essentially coincide when the profile for F ions is multiplied by a factor of k = 1.8 (figure not shown). Hence, to satisfy Eqs. (2) and (3), we used the following expressions to calculate fluences and beam fluxes of P and PFn ions: UPFn ¼ UF =ðk þ nÞ; F
PFn
F
¼ F =ðk þ nÞ:
ð4Þ ð5Þ
The implant conditions used in this work are summarized in Table 1. Implantation-produced disorder was measured by Rutherford backscattering/channeling (RBS/C) spectrometry with 0.7 MeV 4He2+ ions incident along the [1 0 0] direction and backscattered into a detector at 103 relative to the incident beam direction. All RBS/C spectra were analyzed using one of the conventional algorithms [14] for extracting the effective number of scattering centers (referred to below as ‘‘relative disorder’’). 4. Results and discussion 4.1. Damage buildup Fig. 1 shows depth profiles of stable disorder in Si bombarded at RT to different fluences of 2.1 keV/amu F (Fig. 1(a)), P (Fig. 1(b)) and PF4 (Fig. 1(c)) ions. For F ion bombardment (Fig. 1(a)), in addition to the bulk defect peak (at the maximum of the nuclear energy loss profile), a surface defect peak is clearly seen. Such a bimodal defect distribution is typical for Si irradiated with light ions at RT [15]. The surface defect peak, which typically reflects a thin surface amorphous layer [16], is also present but is less pronounced for irradiation with P ions (Fig. 1(b)). Interestingly, Fig. 1 clearly illustrates that the damage buildup is entirely different for cluster PF4 ions than for atomic P or F ions. Indeed, Fig. 1(c) shows that damage accumulates primarily in the near-surface region (for depth [30 nm), well below the depth of the maximum of nuclear energy loss of the molecular ion components. This effect is explained in the next section.
A.I. Titov et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 207–210
209
Table 1 Implant conditions used in this study Ion
Eamu (keV/amu)
Energy (keV)
FDPA (104 DPA s1)
Beam flux (1011 cm2 s1)
˚ 1) N max (vacancies ion1 A v
F+ P+ PFþ 2 PFþ 4
2.1 2.1 2.1 2.1
40 65 145 225
5.5 5.5 and 55 5.5 and 55 5.5 and 55
3.4 1.9 and 19 0.9 and 9.0 0.6 and 6.0
0.81 1.5 3.1 4.7
All implants were performed at room temperature. Calculated values of the number of vacancies at the maximum of the nuclear energy loss profile (N max v ) and the defect generation rate (FDPA) are also given.
0
a
b
depends on ion beam flux (i.e. on the displacement generation rate). Fig. 2 shows damage–depth profiles in Si bombarded with 2.1 keV PF2 ions for two beam flux values. It is seen from Fig. 2 that an increase in beam flux increases the bulk defect peak and decreases the surface defect peak. Therefore, despite the fact that masses of PF2 and PF4 molecules are comparable with masses of heavy ions such as Ga and Ag, the damage buildup behavior in Si at RT is entirely different for the cluster ions and heavy atomic ions of similar masses. Indeed, the dependence of damage buildup on ion flux observed for PFþ n cluster ions is typical for light ions but not for heavy ions [15]. 4.2. Molecular effect
c
Fig. 1. Depth profiles of relative disorder (extracted from RBS/C spectra) in Si bombarded at room temperature with 2.1 keV/amu F (a), P (b), and PF4 (c) ions. Normalized fluences are 0.29 (down triangles), 0.58 (circles), 0.87 (up triangles) and 1.3 DPA (stars). Ion fluences in 1014 cm2 are given in the legends. The displacement generation rate is 5.5 · 104 DPA s1 for all implants.
Fig. 2. Depth profiles of relative disorder (extracted from RBS/C spectra) in Si bombarded at room temperature with 2.1 keV/amu PF2 ions up to two different fluences and with two different beam fluxes, as indicated.
It has been demonstrated (see, for example [15]) that the damage buildup in Si under light-ion irradiation at RT
Fig. 3 shows damage–depth profiles for Si bombarded at RT with F, P, PF2 and PF4 ions up to a normalized fluence and flux of UDPA = 0.3 DPA and FDPA = 5.5 · 104 DPA/s. It is seen from Fig. 3 that, as compared to atomic ions, cluster ions create more damage in the near-surface region between the surface and bulk defect peaks. In other words, the ME is observed. The inset in Fig. 3 illustrates that the ME efficiency is significantly larger for PFn cluster ions than for small clusters of lighter elements such as N1 and N2 with a comparable displacement generation rate. Fig. 3 also shows that the ME efficiency is maximum at the sample surface and decreases with depth. This is attrib-
Fig. 3. Depth profiles of relative disorder (extracted from RBS/C spectra) in Si bombarded at room temperature with 2.1 keV/amu F, P, PF2 and PF4 ions. Normalized fluence and the displacement generation rate are 0.3 DPA and 5.5 · 104 DPA s1, respectively. For comparison, the inset shows depth profiles of relative disorder in Si bombarded at room temperature with 2.85 keV/amu N1 and N2 ions up to a normalized fluence of 2.2 DPA and with a displacement generation rate of 6.5 · 104 DPA s1.
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A.I. Titov et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 207–210
Fig. 4. Dependence of the relative efficiency of the molecular effect (ME) on the displacement generation rate at a depth of 30 nm in Si bombarded at room temperature with 2.85 keV/amu N1 and N2 ions and at a depth of 20 nm in Si bombarded at room temperature with 2.1 keV/amu P and PFn (n = 2 and 4) ions. Symbols are experimental results, and the dashed lines are shown to guide the reader’s eye.
uted to the fact that the average distance between atoms comprising the molecular ion increases (due to ion straggling) as the ion propagates through a solid. In particular, simulations with the TRIM code [12] of individual collision cascades created by P and F ions in Si show that, at the maximum of the nuclear energy loss profile, the average distance between the atoms forming a molecular ion is about 3 times larger than the average lateral size (at the same depth) of a collision cascade for a P or F ion. An increased damage level at the bulk defect peak for P ion bombardment as compared to the case of lighter F ions (with the same UDPA) revealed by Fig. 3 can be attributed to a lower cascade density at the bulk defect peak region for F ions. This can also explain why the bulk defect peak in Fig. 3 is slightly smaller for cluster PF4 than for atomic P ions. Fig. 4 shows displacement generation rate (FDPA) dependencies of the ME efficiency (c) at a depth of 20 nm in Si irradiated with PF2 and PF4 ions. For comparison, also shown in Fig. 4 is the c (FDPA) dependence (at a depth of 30 nm) for 2.85 keV/amu N1 and N2 ion bombardment of Si at RT, taken from [10]. The depths of 20 and 30 nm have been chosen so that they are between the bulk and surface defect peaks. At such depths, the ME is still strong, and we can neglect often complex behavior of the surface defect peak. It is seen from Fig. 4 that the ME efficiency depends on FDPA in all cases, and, for the highest FDPA value for N1 and N2 bombardment regimes, the ME is absent. As discussed in detail in [10], the dependence of the ME efficiency on beam flux indicates that defect accu-
mulation for N1 and N2 ion bombardment is primarily controlled not by nonlinear energy spikes but by dynamic annealing processes. A decrease in the ME efficiency with increasing FDPA for PFn ions revealed in Fig. 4 also suggests a significant role of dynamic annealing processes in this case. However, spatial overlap of collision cascades of the components of a PFn cluster ion could also create regions with densities of displacements large enough for energy spikes to occur. For example, TRIM simulations show that when cascades completely overlap (i.e. at the sample surface), the density of displacements created by a 2.1 keV/amu PF4 cluster ion is comparable to that for heavy As ions with an energy of 25 keV. For such 25 keV As ions, defect formation in collision cascades is believed to be described by nonlinear energy peak processes [1,7]. Hence, although the flux effect revealed in Figs. 2 and 4 suggests an important role of dynamic annealing in the ME for PFn ion bombardment of Si, a contribution from nonlinear energy spikes cannot be completely ruled out at the moment. Acknowledgements Work in St. Petersburg was supported by Grant RFFI 06-08-00989. Work at LLNL was performed under the auspices of the US DOE by the University of California, LLNL under Contract No. W-7405-Eng-48. References [1] D.A. Thompson, Rad. Eff. 56 (1981) 105. [2] P. Sigmund, I.S. Bitensky, J. Jensen, Nucl. Instr. and Meth. B 112 (1996) 1. [3] I. Yamada, Nucl. Instr. and Meth. B 148 (1999) 1. [4] A.I. Titov, S.O. Kucheyev, Nucl. Instr. and Meth. B 149 (1999) 129. [5] B. Canut, M. Fallavier, O. Marty, S.M.M. Ramos, Nucl. Instr. and Meth. B 164–165 (2000) 396. [6] X. Lu, L. Shao, X. Wang, J. Liu, W.-K. Chu, J. Bennett, L. Larson, P. Ling, J. Vac. Sci. Technol. B 20 (2002) 992. [7] A.I. Titov, V.S. Belyakov, S.O. Kucheyev, Nucl. Instr. and Meth. B 194 (2002) 323. [8] J. Peltola, K. Nordlund, Phys. Rev. B 68 (2003) 035419. [9] S. Bouneau, S. Della Negra, D. Jacquet, Y. Le Beyec, M. Pautrat, M.H. Shapiro, T.A. Tombrello, Phys. Rev. B 71 (2005) 174110. [10] A.I. Titov, A.Yu. Azarov, L.M. Nikulina, S.O. Kucheyev, Phys. Rev. B 73 (2006) 064111. [11] A.Yu. Azarov, A.I. Titov, Semiconductors, in press. [12] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [13] J.J. Loferski, P. Rappaport, Phys. Rev. 111 (1958) 432. [14] K. Schmid, Radiat. Eff. 17 (1973) 201. [15] A.I. Titov, G. Carter, Nucl. Instr. and Meth. B 119 (1996) 491. [16] D.I. Tetelbaum, E.I. Zorin, A.I. Gerasimov, P.V. Pavlov, Phys. Status Solidi A 12 (1972) 679.
Nuclear Instruments and Methods in Physics Research B 256 (2007) 207–210 www.elsevier.com/locate/nimb
Damage buildup and the molecular effect in Si bombarded with PFn cluster ions A.I. Titov a, A.Yu. Azarov b, L.M. Nikulina a, S.O. Kucheyev a
c,*
Department of Physical Electronics, St. Petersburg State Polytechnical University, St. Petersburg 195251, Russian Federation b Research and Production Company Electron-Optronic, Morisa Toreza 68, St. Petersburg 194223, Russian Federation c Lawrence Livermore National Laboratory, Livermore, CA 94550, USA Available online 22 January 2007
Abstract We study the molecular effect (ME) in damage accumulation in Si bombarded at room temperature with atomic P and F and cluster PFn (n = 2 and 4) ions with an energy of 2.1 keV/amu. Correct ion irradiation conditions for unambiguous studies of the ME are discussed. Rutherford backscattering/channeling spectrometry results show that the damage buildup behavior strongly depends on the cluster ion size, and the ME efficiency increases rapidly with increasing the number of atoms in cluster ions. Moreover, the ME efficiency decreases with increasing the defect generation rate, indicating that dynamic annealing processes, rather than nonlinear energy spikes, play a major role in the ME for these irradiation conditions. 2006 Elsevier B.V. All rights reserved. PACS: 61.80.Lj; 61.72.Yy; 68.49.Sf; 61.72.Tt Keywords: Ion implantation; Cluster ions; Molecular effect; Defects; Collision cascades; Silicon; Si
1. Introduction Interaction of cluster ions with solids has been studied for several decades (see, for example, an early review [1]). There has been a resurgence of cluster bombardment studies in recent years, primarily because of unique possibilities for material modification by cluster ions, including the fabrication of ultrashallow p–n junctions and efficient ion-etching and polishing of surfaces (see, for examples, [2–11]). It has been observed experimentally that the efficiency of many ion-beam-related phenomena (such as ion sputtering and mixing, secondary ion and electron emission, and radiation damage) often differs for atomic and cluster (molecular) ion bombardment regimes under correct irradiation conditions (see Section 2 for a definition of the correct conditions). This is the so-called molecular effect (ME). *
Corresponding author. Tel.: +1 9254225866; fax: +1 9254230785. E-mail address: [email protected] (S.O. Kucheyev).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.12.004
The ME in damage accumulation has generally been attributed to increased disordering in nonlinear energy spikes (such as thermal and displacement spikes), which form due to spatial overlap of collision cascades produced by the atoms comprising a molecular ion [1,7]. This concept can, at least qualitatively, explain experimental observations for amorphizable semiconductors bombarded with keV heavy ions or with cluster ions comprised of a large number of light atoms (e.g. C60), when the collision cascade density is large enough for nonlinear energy spikes to occur. However, for small cluster ions comprised of light atoms, the collision cascade as a whole cannot be considered nonlinear [4]. Therefore, the ME experimentally observed for light-ion bombardment has commonly been attributed to nonlinear energy spike processes resulting from a spatial overlap of relatively dense collision subcascades [4]. We have recently demonstrated that nonlinear energy spikes are not responsible for the ME in Si bombarded with keV N1 and N2 ions [10]. Instead, the ME in this case has been attributed to dynamic annealing processes
208
A.I. Titov et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 207–210
(i.e. processes of annihilation and clustering of point defects during ion irradiation) [10]. In this contribution, we study the ME in Si irradiated with cluster ions which are made from different atomic species – PFn cluster ions. Correct ion irradiation conditions for unambiguous ME studies are discussed in Section 2. We also present results revealing strong effects of the collision cascade density and the displacement generation rate on damage buildup (Section 4.1) and ME efficiency (Section 4.2). 2. Correct irradiation conditions In order to study the underlying physical mechanisms of the ME, the only difference between atomic and molecular ion bombardment regimes should be the fact that atomic ions impinge on the sample surface at random locations, whereas the atoms comprising a molecular/cluster ion hit the surface at the same location. Hence, the following parameters should be kept the same for both atomic and molecular ion bombardment regimes: (i) ion velocity; (ii) the total number of atomic displacements and (iii) the displacement generation rate. For cluster ions made from the same atomic species (e.g. Cn, Aun, etc.), such conditions are satisfied if the energy, fluence, and ion beam flux, all three normalized per one incident atom, are kept constant. It is typically trivial to achieve such conditions experimentally. In contrast, it is much more challenging to satisfy correct irradiation conditions for cluster ions made from different elements. Generally, for XnYm cluster ions, this would require an experimental setup with possibilities of simultaneous irradiation with X and Y ions. However, when the profiles of ion-beam-generated displacements for the components of a cluster ion are similar, correct irradiation conditions are simplified to [11] Eamu ¼ const:; UDPA ¼ const:; F DPA ¼ const:;
ð1Þ
where Eamu is ion energy normalized to amu, and UDPA and FDPA are ion fluence and beam flux, respectively, normalized to the number of displacements per atom (DPA). Such a normalized fluence UDPA can be expressed as the average number of vacancies N max vac at the depth where the nuclear energy loss is maximum created by irradiation to fluence U normalized to the atomic concentration n0 UDPA ¼ N max vac U=n0 :
ð2Þ
The normalized ion flux FDPA could be estimated as the vacancy generation rate at the depth of the displacement profile maximum F DPA ¼
dUDPA ; dt
ð3Þ
where t is time. It is important to note that we are not aware of any previous ME studies (except for a recent brief report [11]) with cluster ions made from different elements where the correct conditions described above were followed.
N X nY m
nY m ¼ Nd X ;Y , where The ME efficiency can be defined as cXX ;Y X nY m Nd is the concentration of stable lattice defectsdproduced by XnYm, cluster ions and N Xd ;Y is the concentration of stable defects created by atomic X or Y ions under the conditions defined by Eq. (1).
3. Experimental Boron doped (1 0 0) Si samples (with a resistivity of 12 X cm) were implanted with F+, P+ and PFþ n (n = 2 and 4) ions over a wide fluence range. Implantation was carried out at room temperature (RT) at 7 off the [1 0 0] direction in order to minimize channeling. Simulations with the TRIM code [12] (with an effective threshold energy for atomic displacements of 13 eV, chosen based on experimental data from [13]) show that the depth profiles of vacancies generated in Si by P and F ions with an energy of 2.1 keV/amu essentially coincide when the profile for F ions is multiplied by a factor of k = 1.8 (figure not shown). Hence, to satisfy Eqs. (2) and (3), we used the following expressions to calculate fluences and beam fluxes of P and PFn ions: UPFn ¼ UF =ðk þ nÞ; F
PFn
F
¼ F =ðk þ nÞ:
ð4Þ ð5Þ
The implant conditions used in this work are summarized in Table 1. Implantation-produced disorder was measured by Rutherford backscattering/channeling (RBS/C) spectrometry with 0.7 MeV 4He2+ ions incident along the [1 0 0] direction and backscattered into a detector at 103 relative to the incident beam direction. All RBS/C spectra were analyzed using one of the conventional algorithms [14] for extracting the effective number of scattering centers (referred to below as ‘‘relative disorder’’). 4. Results and discussion 4.1. Damage buildup Fig. 1 shows depth profiles of stable disorder in Si bombarded at RT to different fluences of 2.1 keV/amu F (Fig. 1(a)), P (Fig. 1(b)) and PF4 (Fig. 1(c)) ions. For F ion bombardment (Fig. 1(a)), in addition to the bulk defect peak (at the maximum of the nuclear energy loss profile), a surface defect peak is clearly seen. Such a bimodal defect distribution is typical for Si irradiated with light ions at RT [15]. The surface defect peak, which typically reflects a thin surface amorphous layer [16], is also present but is less pronounced for irradiation with P ions (Fig. 1(b)). Interestingly, Fig. 1 clearly illustrates that the damage buildup is entirely different for cluster PF4 ions than for atomic P or F ions. Indeed, Fig. 1(c) shows that damage accumulates primarily in the near-surface region (for depth [30 nm), well below the depth of the maximum of nuclear energy loss of the molecular ion components. This effect is explained in the next section.
A.I. Titov et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 207–210
209
Table 1 Implant conditions used in this study Ion
Eamu (keV/amu)
Energy (keV)
FDPA (104 DPA s1)
Beam flux (1011 cm2 s1)
˚ 1) N max (vacancies ion1 A v
F+ P+ PFþ 2 PFþ 4
2.1 2.1 2.1 2.1
40 65 145 225
5.5 5.5 and 55 5.5 and 55 5.5 and 55
3.4 1.9 and 19 0.9 and 9.0 0.6 and 6.0
0.81 1.5 3.1 4.7
All implants were performed at room temperature. Calculated values of the number of vacancies at the maximum of the nuclear energy loss profile (N max v ) and the defect generation rate (FDPA) are also given.
0
a
b
depends on ion beam flux (i.e. on the displacement generation rate). Fig. 2 shows damage–depth profiles in Si bombarded with 2.1 keV PF2 ions for two beam flux values. It is seen from Fig. 2 that an increase in beam flux increases the bulk defect peak and decreases the surface defect peak. Therefore, despite the fact that masses of PF2 and PF4 molecules are comparable with masses of heavy ions such as Ga and Ag, the damage buildup behavior in Si at RT is entirely different for the cluster ions and heavy atomic ions of similar masses. Indeed, the dependence of damage buildup on ion flux observed for PFþ n cluster ions is typical for light ions but not for heavy ions [15]. 4.2. Molecular effect
c
Fig. 1. Depth profiles of relative disorder (extracted from RBS/C spectra) in Si bombarded at room temperature with 2.1 keV/amu F (a), P (b), and PF4 (c) ions. Normalized fluences are 0.29 (down triangles), 0.58 (circles), 0.87 (up triangles) and 1.3 DPA (stars). Ion fluences in 1014 cm2 are given in the legends. The displacement generation rate is 5.5 · 104 DPA s1 for all implants.
Fig. 2. Depth profiles of relative disorder (extracted from RBS/C spectra) in Si bombarded at room temperature with 2.1 keV/amu PF2 ions up to two different fluences and with two different beam fluxes, as indicated.
It has been demonstrated (see, for example [15]) that the damage buildup in Si under light-ion irradiation at RT
Fig. 3 shows damage–depth profiles for Si bombarded at RT with F, P, PF2 and PF4 ions up to a normalized fluence and flux of UDPA = 0.3 DPA and FDPA = 5.5 · 104 DPA/s. It is seen from Fig. 3 that, as compared to atomic ions, cluster ions create more damage in the near-surface region between the surface and bulk defect peaks. In other words, the ME is observed. The inset in Fig. 3 illustrates that the ME efficiency is significantly larger for PFn cluster ions than for small clusters of lighter elements such as N1 and N2 with a comparable displacement generation rate. Fig. 3 also shows that the ME efficiency is maximum at the sample surface and decreases with depth. This is attrib-
Fig. 3. Depth profiles of relative disorder (extracted from RBS/C spectra) in Si bombarded at room temperature with 2.1 keV/amu F, P, PF2 and PF4 ions. Normalized fluence and the displacement generation rate are 0.3 DPA and 5.5 · 104 DPA s1, respectively. For comparison, the inset shows depth profiles of relative disorder in Si bombarded at room temperature with 2.85 keV/amu N1 and N2 ions up to a normalized fluence of 2.2 DPA and with a displacement generation rate of 6.5 · 104 DPA s1.
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A.I. Titov et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 207–210
Fig. 4. Dependence of the relative efficiency of the molecular effect (ME) on the displacement generation rate at a depth of 30 nm in Si bombarded at room temperature with 2.85 keV/amu N1 and N2 ions and at a depth of 20 nm in Si bombarded at room temperature with 2.1 keV/amu P and PFn (n = 2 and 4) ions. Symbols are experimental results, and the dashed lines are shown to guide the reader’s eye.
uted to the fact that the average distance between atoms comprising the molecular ion increases (due to ion straggling) as the ion propagates through a solid. In particular, simulations with the TRIM code [12] of individual collision cascades created by P and F ions in Si show that, at the maximum of the nuclear energy loss profile, the average distance between the atoms forming a molecular ion is about 3 times larger than the average lateral size (at the same depth) of a collision cascade for a P or F ion. An increased damage level at the bulk defect peak for P ion bombardment as compared to the case of lighter F ions (with the same UDPA) revealed by Fig. 3 can be attributed to a lower cascade density at the bulk defect peak region for F ions. This can also explain why the bulk defect peak in Fig. 3 is slightly smaller for cluster PF4 than for atomic P ions. Fig. 4 shows displacement generation rate (FDPA) dependencies of the ME efficiency (c) at a depth of 20 nm in Si irradiated with PF2 and PF4 ions. For comparison, also shown in Fig. 4 is the c (FDPA) dependence (at a depth of 30 nm) for 2.85 keV/amu N1 and N2 ion bombardment of Si at RT, taken from [10]. The depths of 20 and 30 nm have been chosen so that they are between the bulk and surface defect peaks. At such depths, the ME is still strong, and we can neglect often complex behavior of the surface defect peak. It is seen from Fig. 4 that the ME efficiency depends on FDPA in all cases, and, for the highest FDPA value for N1 and N2 bombardment regimes, the ME is absent. As discussed in detail in [10], the dependence of the ME efficiency on beam flux indicates that defect accu-
mulation for N1 and N2 ion bombardment is primarily controlled not by nonlinear energy spikes but by dynamic annealing processes. A decrease in the ME efficiency with increasing FDPA for PFn ions revealed in Fig. 4 also suggests a significant role of dynamic annealing processes in this case. However, spatial overlap of collision cascades of the components of a PFn cluster ion could also create regions with densities of displacements large enough for energy spikes to occur. For example, TRIM simulations show that when cascades completely overlap (i.e. at the sample surface), the density of displacements created by a 2.1 keV/amu PF4 cluster ion is comparable to that for heavy As ions with an energy of 25 keV. For such 25 keV As ions, defect formation in collision cascades is believed to be described by nonlinear energy peak processes [1,7]. Hence, although the flux effect revealed in Figs. 2 and 4 suggests an important role of dynamic annealing in the ME for PFn ion bombardment of Si, a contribution from nonlinear energy spikes cannot be completely ruled out at the moment. Acknowledgements Work in St. Petersburg was supported by Grant RFFI 06-08-00989. Work at LLNL was performed under the auspices of the US DOE by the University of California, LLNL under Contract No. W-7405-Eng-48. References [1] D.A. Thompson, Rad. Eff. 56 (1981) 105. [2] P. Sigmund, I.S. Bitensky, J. Jensen, Nucl. Instr. and Meth. B 112 (1996) 1. [3] I. Yamada, Nucl. Instr. and Meth. B 148 (1999) 1. [4] A.I. Titov, S.O. Kucheyev, Nucl. Instr. and Meth. B 149 (1999) 129. [5] B. Canut, M. Fallavier, O. Marty, S.M.M. Ramos, Nucl. Instr. and Meth. B 164–165 (2000) 396. [6] X. Lu, L. Shao, X. Wang, J. Liu, W.-K. Chu, J. Bennett, L. Larson, P. Ling, J. Vac. Sci. Technol. B 20 (2002) 992. [7] A.I. Titov, V.S. Belyakov, S.O. Kucheyev, Nucl. Instr. and Meth. B 194 (2002) 323. [8] J. Peltola, K. Nordlund, Phys. Rev. B 68 (2003) 035419. [9] S. Bouneau, S. Della Negra, D. Jacquet, Y. Le Beyec, M. Pautrat, M.H. Shapiro, T.A. Tombrello, Phys. Rev. B 71 (2005) 174110. [10] A.I. Titov, A.Yu. Azarov, L.M. Nikulina, S.O. Kucheyev, Phys. Rev. B 73 (2006) 064111. [11] A.Yu. Azarov, A.I. Titov, Semiconductors, in press. [12] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [13] J.J. Loferski, P. Rappaport, Phys. Rev. 111 (1958) 432. [14] K. Schmid, Radiat. Eff. 17 (1973) 201. [15] A.I. Titov, G. Carter, Nucl. Instr. and Meth. B 119 (1996) 491. [16] D.I. Tetelbaum, E.I. Zorin, A.I. Gerasimov, P.V. Pavlov, Phys. Status Solidi A 12 (1972) 679.