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Internal friction study of ion-implantation induced defects in silicon
Materials Science and Engineering A 442 (2006) 63–66
Internal friction study of ion-implantation induced defects in silicon Xiao Liu a,∗ , R.O. Pohl b , D.M. Photiadis a b
a Naval Research Laboratory, Washington, DC 20375, USA Department of Physics, Cornell University, Ithaca, NY 14853-2501, USA
Received 16 August 2005; received in revised form 6 February 2006; accepted 9 February 2006
Abstract Using a sensitive silicon paddle oscillator technique, we measured the internal friction of single-crystalline silicon substrate implanted with As+ , Si+ , B+ , and H+ ions with a dose of ∼1019 m−2 . A distinct internal friction peak at about 48 K is observed in all cases. The overall internal friction from 0.4 K to room temperature is also similar for the different species of ions used. The peak shifts to lower temperature at a reduced resonance frequency, characteristic of a thermally activated relaxation. The insensitivity of the peak to the ion species indicates that it is caused by a vacancy-type defect. Isochronal annealing study shows that the peak anneals away at a temperature between 200 and 300 ◦ C, and that is consistent with divacancy defects. By plasma etching the substrate layer by layer, we have identified that the defects are concentrated at a damaged crystalline layer right underneath the top amorphized layer in the Si+ implanted sample. The exact mechanism that caused the relaxation peak is still not clear, although our quantitative analysis points to an electronic origin. © 2006 Elsevier B.V. All rights reserved. Keywords: Internal friction; Ion-implantation; Tunneling states; Divacancy
1. Introduction Ion-implantation is widely used for precision doping of semiconductors. During ion-implantation, point defects are generated. Annealing is normally applied to remove the defects and to activate the dopants. The understanding of such irradiationinduced point defects in prototype elemental semiconductors has attracted both theoretical and experimental interests for decades [1–5]. The formation, migration and annealing of defects have been primarily studied by electron paramagnetic resonance [1], deep-level transient spectroscopy [5], infrared absorption spectroscopy [6] and positron annihilation spectroscopy [7]. In 1970’s, Berry and coworkers pioneered the use of internal friction measurements for the study of defects generated by ionimplantation [8–10]. The anelastic technique, which was used widely in studying structural disorder in metals [11], has proved to be a sensitive and selective probe to defects in semiconductors; for a review, see Ref. [12]. Some useful information, such as elastic moduli, defects density, defects symmetry and activation energies has been revealed. However, such exciting research has not been continued since then.
∗
Corresponding author. Tel.: +1 202 404 8065; fax: +1 202 404 1721. E-mail address: [email protected] (X. Liu).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.212
Using a mechanical double-paddle oscillator technique developed at Cornell University [13], we have carried out studies of elastic properties of thin film materials on silicon substrates at low temperatures. Some preliminary results on the internal friction of ion-implantation induced defects have been reported [14]. Using ion-implantation, we have successfully generated atomic tunneling states in crystalline silicon substrates as the ionimplantation dose varied from 7 × 1014 m−2 to 7 × 1019 m−2 [15]. We show, in this study, how we locate and identify the divacancy defects in ion-implanted silicon by internal friction measurements. 2. Experimental Measurements of internal friction were performed using the double-paddle oscillator technique [13]. The oscillators were fabricated out of high purity, n-type, float-zone silicon wafers, with resistivity > 5000 m. The overall dimension of the oscillators was 28 mm high, 20 mm wide and 0.3 mm thick. It consisted of a foot, a pair of wings, and a head. The head and the wings were connected by a narrow neck, and the wings were in turn connected to the foot by a long and thin leg. The oscillator was mounted in a He3 cryostat by adhering its foot on an invar block to minimize thermal strain during a cool down, using an epoxy resin. The oscillators were excited electrostatically. Two
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X. Liu et al. / Materials Science and Engineering A 442 (2006) 63–66
vibration modes were used in this work. They are the second antisymmetric mode (AS2), in which the head and the wings vibrate out of phase, twisting the neck, with a frequency of ∼5.5 kHz, and the symmetric mode (ST), in which the head and the wings vibrate in phase, twisting the leg, at ∼400 Hz. The AS2 mode has an exceptionally low internal friction, Q−1 ≈ 2 × 10−8 at low temperatures (T < 10 K), which is reproducible within ±10% for different oscillators. That enables an accurate determination of changes in internal friction. The ST mode has a higher internal friction, because it is more closely coupled to the cryostat than the AS2 mode, and also because it carries a metal film, which usually has a high internal friction [16], needed for electrostatic actuation and detection. Nevertheless, the ST mode is still useful to check for a shift of an internal friction peak with frequency. The double-paddle oscillators were ion-implanted at room temperature in the Cornell Nanofabrication Facility using a Varian/Extrion implanter. In order to achieve a uniform distribution of ion induced damage, implantation was done on one side, under an angle of 7◦ off the axis, at energies of 50, 120, and 180 keV for As+ , Si+ , and B+ , and at 15, 55, and 100 keV for H+ . The resulting thicknesses of damaged layer were expected to be 145, 400, 680, and 950 nm for As+ , Si+ , B+ , and H+ , respectively [17]. The ion-implantation dose was 7 × 1019 m−2 for Si+ and B+ , and 2 × 1019 m−2 for As+ and H+ . Subsequent structure analysis was done using cross-sectional transmission-electron microscopy (TEM) and Rutherford backscattering (RBS) and channeling on pieces of silicon wafers implanted at the same time with oscillators used for internal friction measurements. Reactive-ion etching was done on Si+ implanted oscillators to remove the top damaged layer in order to get a depth profile of ion-implantation induced defects. A Plasma Therm 72 RIE system in reactive SF6 and O2 plasma with a power of 50 W was used. The influence of reactive-ion etching on the damping of a bare oscillator was found to be negligible, leading to an increase of Q−1 by less than 10−8 . We also performed isochronal anneal for 1 h in a vacuum of ∼10−5 Pa on some of the Si+ implanted oscillators. 3. Results and discussion We show the internal friction of an oscillator implanted by Si+ in Fig. 1, together with a bare oscillator shown as the background. The measurements were done with the AS2 mode at 5.35 kHz. The most striking effect of the implantation on the internal friction is a narrow peak at 48 K. Considerable damping persists, however, to the lowest temperature (0.4 K) of the measurement. The low temperature internal friction below 10 K is as temperature independent as the background, which has been explained by the generation of atomic tunneling states in the amorphized region as well as in the heavily damaged crystalline region [15]. In order to track down the location of the defects that are responsible for the peak, we performed TEM and RBS channeling on similarly ion-implanted silicon wafers. The TEM picture shows a sharp boundary between amorphous and crystalline region at a depth of 330 nm, in agreement with the RBS channeling results. Underneath the amorphous layer, there is a heavily dam-
Fig. 1. The internal friction of Si+ self-implanted double-paddle oscillator, and of a similarly implanted oscillator with top implanted layer removed by 300 and 450 nm thick with reactive-ion etching. The resonance frequency is 5350 Hz. The solid curve is the background internal friction of a bare oscillator.
aged crystalline region about 70 nm thick and it fades away at a much larger depth. The total thickness of damaged region is in excellent agreement with a computer simulation [17]. We consecutively removed a 300 nm thick layer followed by another 150 nm thick layer by reactive-ion etching on an identically ionimplanted oscillator. The results are also shown in Fig. 1. Substrating the background, we find that the 48 K peak is reduced by about 10% after the first etching and about 80% after the second etching. This shows clearly that the defects responsible for the 48 K peak are concentrated in the heavily damaged region underneath the amorphized layer. The 10% reduction after the first etching could be explained by the inhomogeneity of the amorphized layer, which is due to the limited ion-implantation energy used. In addition to the 48 K peak, two small peaks become apparent at around 5–7 and 20 K after the first etching, which may have been covered by the large internal friction background in the as-implanted oscillator. They are removed, however, after the second etching, indicating the same structural location as the 48 K peak. Tan et al. [8] studied Si+ implanted silicon. They found a peak at 218 K at a frequency of 550 Hz, and concluded that the corresponding defects were located in the heavily damaged crystalline region based on the anisotropy of the peak. Although we do not find any peak above the one at 48 K (probably due to the rapidly rising background damping of our oscillators towards higher temperatures), our observation is a more direct proof as far as the location of the defects is concerned. In contrast, the internal friction below 10 K is reduced by 70% and 80% after the first and the second etching, respectively. This is consistent with its origin of atomic tunneling states, which are distributed almost uniformly no matter the material is amorphized or not [15]. Using the ST mode of the same oscillator, we find that the 48 K peak shifts to 42 K at 410 Hz. We further find that the internal friction of oscillators implanted with As+ , B+ , and even H+ shows exactly the same peaks, which can be described by a
X. Liu et al. / Materials Science and Engineering A 442 (2006) 63–66
Fig. 2. The normalized internal friction of As+ , Si+ , B+ , and H+ implanted double-paddle oscillators vs. the inverse of temperature. The results are shown for two resonance modes at 5350 and 410 Hz for the Si+ implanted oscillator, which varies ±2% among different oscillators. The solid curves are fits to Debye relaxation process described in the text.
single Debye relaxation process as ωτ Q−1 = 2Q−1 p 1 + ω2 τ 2
(1)
where Q−1 p is the height of the internal friction peak, ω the resonance frequency, and τ is the relaxation time. The internal friction has a peak when ωτ = 1. In a thermally activated relaxation process, τ takes an Arrhenius expression of the type V (2) τ = τ0 exp kT where k is the Boltzmann constant, T the temperature, V the activation energy for defect relaxation, and τ0 is a preexponential constant whose magnitude should be roughly the inverse of the mean lattice vibration frequency. Fig. 2 shows the normalized internal friction of As+ , Si+ , B+ , and H+ implanted oscillators versus the inverse of temperature at two resonance frequencies. The solid curves are fits of Eqs. (1) and (2) with V = 0.075 eV and τ0 = 4.6 × 10−13 s. Fig. 2 shows that the implanted ions are not part of the defects contributing to the Debye relaxation. Since single vacancies would anneal well below room temperature [18], the most likely defects are divacancies, or conceivably higher vacancy aggregates. To determine the size of the vacancies, we performed 1-h isochronal vacuum annealing from 200 to 700 ◦ C, and we found that the 48 K peak anneals away at between 200 and 300 ◦ C, see Fig. 3. Since divacancies are known to anneal at 250 ◦ C [6], we conclude that the 48 K peak has its origin of divacancy, although we are not sure of the exact mechanism that leads to the relaxation process. Fig. 3 also shows that, while the 48 K peak decreases considerably after an anneal at 200 ◦ C, two new peaks appear at 5 and 14 K. After the 300 ◦ C anneal, the 14 K peak disappears and the 5 K peak remains until after the 500 ◦ C anneal. Annealing at 700 ◦ C for 1 h is known to completely restore the crystallinity of an amorphous silicon film [19], and it
65
Fig. 3. The internal friction of the Si+ self-implanted double-paddle oscillator, and of similarly implanted oscillators after 1-h isochronal anneal in a vacuum. The resonance frequency is 5350 Hz. The solid curve is the background internal friction of a bare oscillator.
restores also the internal friction almost completely except for another peak appears at 24 K. The low temperature rise of the internal friction below 2 K for oscillators after 500 and 700 ◦ C anneal is known to be caused by the activation of contaminants by annealing [20]. The isochronal annealing behavior reveals the interesting structural transformation of the defects in ionimplanted silicon, such as one kind of defects transforms to other more stable kinds with each incremental increase of annealing temperature. Most likely, divacancies migrate upon annealing and some of them aggregate to larger vacancy clusters with reduced activation energy and higher annealing temperature. Cheng and Vajda [21] have determined the activation energy for the reorientation of the electronic configuration of the neutral divacancy to be 0.076 eV, in good agreement with the value determined in the present measurements. For atomic reorientation of the divacancy axis, Waktins and Corbett [1] obtained an activation energy of 1.3 eV. To support an electronic origin of divacancy in the 48 K internal fiction peak, however, measurements of internal friction in a magnetic field is needed. 4. Conclusions We have identified that a pronounced peak at 48 K in ionimplanted silicon is caused by divacancy defects concentrated in heavily damaged crystalline region just underneath the amorphized top layer. The peak can be well described by a thermally activated Debye relaxation process with an activation energy of 0.075 eV. This observation demonstrates the high sensitivity of the anelastic technique for the detection of defects in semiconductors. Acknowledgements We thank Dr. R.S. Crandall and Dr. K.M. Jones at the National Renewable Energy Laboratory for the TEM measurements and Dr. P. Revesz at the Cornell Center for Materials Research Ion
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Beam Analytical Facility for the RBS and channeling measurements. This work was supported by the Office of Naval Research. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10]
G.D. Watkins, J.W. Corbett, Phys. Rev. 138 (1965) A543–A555. M. Saito, A. Oshiyama, Phys. Rev. Lett. 73 (1994) 866–869. G.D. Watkins, Phys. Rev. Lett. 74 (1995) 4353. M. Saito, A. Oshiyama, Phys. Rev. Lett. 74 (1995) 4354. ˙ L. Dobaczewski, K. Go´sci´nski, Z.R. Zytkiewicz, K.B. Nielsen, L. Rubaldo, O. Andersen, A.R. Peaker, Phys. Rev. B 65 (2002) 113203–113204. H.J. Stein, F.L. Vook, J.A. Borders, Appl. Phys. Lett. 14 (1969) 328–330. S. Szpala, P.J. Simpson, J. Appl. Phys. 89 (2001) 5991–5996. S.I. Tan, B.S. Berry, B.L. Crowder, Appl. Phys. Lett. 20 (1972) 88–90. S.I. Tan, B.S. Berry, W.F.J. Frank, in: B.L. Crowder (Ed.), Ion Implantation in Semiconductors and Other Materials, Plenum, New York, 1973, pp. 19–30. B.S. Berry, W.C. Pritchet, in: F. Chernow, J.A. Borders, D.K. Brice (Eds.), Ion Implantation in Semiconductors, Plenum, New York, 1977, pp. 435–443.
[11] A.S. Nowick, B.S. Berry, Anelastic relaxation in crystalline solids, Academic Press, New York, London, 1972. [12] B.S. Berry, in: D. Gupta, P.S. Ho (Eds.), Diffusion Phenomena in Thin Films and Microelectronic Materials, Noyes, Park Ridge, NJ, 1988, pp. 73–145. [13] B.E. White Jr., R.O. Pohl, Mat. Res. Soc. Symp. Proc. 356 (1995) 567– 572. [14] X. Liu, R.O. Pohl, R.S. Crandall, K.M. Jones, Mat. Res. Soc. Symp. Proc. 469 (1997) 419–424. [15] X. Liu, P.D. Vu, R.O. Pohl, F. Schiettekatte, S. Roorda, Phys. Rev. Lett. 81 (1998) 3171–3174. [16] X. Liu, E. Thompson, B.E. White Jr., R.O. Pohl, Phys. Rev. B 59 (1999) 11767–11776. [17] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985 and software package: TRIM-98. [18] H.J. Stein, Rad. Effects 9 (1971) 195–210. [19] G.L. Olson, J.A. Roth, Mater. Sci. Rep. 3 (1988) 1–78. [20] X. Liu, R.O. Pohl, S. Asher, R.S. Crandall, J. Non-Cryst. Solids 227–230 (1998) 407–410. [21] L.J. Cheng, P. Vajda, Phys. Rev. 186 (1969) 816–823.
Internal friction study of ion-implantation induced defects in silicon Xiao Liu a,∗ , R.O. Pohl b , D.M. Photiadis a b
a Naval Research Laboratory, Washington, DC 20375, USA Department of Physics, Cornell University, Ithaca, NY 14853-2501, USA
Received 16 August 2005; received in revised form 6 February 2006; accepted 9 February 2006
Abstract Using a sensitive silicon paddle oscillator technique, we measured the internal friction of single-crystalline silicon substrate implanted with As+ , Si+ , B+ , and H+ ions with a dose of ∼1019 m−2 . A distinct internal friction peak at about 48 K is observed in all cases. The overall internal friction from 0.4 K to room temperature is also similar for the different species of ions used. The peak shifts to lower temperature at a reduced resonance frequency, characteristic of a thermally activated relaxation. The insensitivity of the peak to the ion species indicates that it is caused by a vacancy-type defect. Isochronal annealing study shows that the peak anneals away at a temperature between 200 and 300 ◦ C, and that is consistent with divacancy defects. By plasma etching the substrate layer by layer, we have identified that the defects are concentrated at a damaged crystalline layer right underneath the top amorphized layer in the Si+ implanted sample. The exact mechanism that caused the relaxation peak is still not clear, although our quantitative analysis points to an electronic origin. © 2006 Elsevier B.V. All rights reserved. Keywords: Internal friction; Ion-implantation; Tunneling states; Divacancy
1. Introduction Ion-implantation is widely used for precision doping of semiconductors. During ion-implantation, point defects are generated. Annealing is normally applied to remove the defects and to activate the dopants. The understanding of such irradiationinduced point defects in prototype elemental semiconductors has attracted both theoretical and experimental interests for decades [1–5]. The formation, migration and annealing of defects have been primarily studied by electron paramagnetic resonance [1], deep-level transient spectroscopy [5], infrared absorption spectroscopy [6] and positron annihilation spectroscopy [7]. In 1970’s, Berry and coworkers pioneered the use of internal friction measurements for the study of defects generated by ionimplantation [8–10]. The anelastic technique, which was used widely in studying structural disorder in metals [11], has proved to be a sensitive and selective probe to defects in semiconductors; for a review, see Ref. [12]. Some useful information, such as elastic moduli, defects density, defects symmetry and activation energies has been revealed. However, such exciting research has not been continued since then.
∗
Corresponding author. Tel.: +1 202 404 8065; fax: +1 202 404 1721. E-mail address: [email protected] (X. Liu).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.212
Using a mechanical double-paddle oscillator technique developed at Cornell University [13], we have carried out studies of elastic properties of thin film materials on silicon substrates at low temperatures. Some preliminary results on the internal friction of ion-implantation induced defects have been reported [14]. Using ion-implantation, we have successfully generated atomic tunneling states in crystalline silicon substrates as the ionimplantation dose varied from 7 × 1014 m−2 to 7 × 1019 m−2 [15]. We show, in this study, how we locate and identify the divacancy defects in ion-implanted silicon by internal friction measurements. 2. Experimental Measurements of internal friction were performed using the double-paddle oscillator technique [13]. The oscillators were fabricated out of high purity, n-type, float-zone silicon wafers, with resistivity > 5000 m. The overall dimension of the oscillators was 28 mm high, 20 mm wide and 0.3 mm thick. It consisted of a foot, a pair of wings, and a head. The head and the wings were connected by a narrow neck, and the wings were in turn connected to the foot by a long and thin leg. The oscillator was mounted in a He3 cryostat by adhering its foot on an invar block to minimize thermal strain during a cool down, using an epoxy resin. The oscillators were excited electrostatically. Two
64
X. Liu et al. / Materials Science and Engineering A 442 (2006) 63–66
vibration modes were used in this work. They are the second antisymmetric mode (AS2), in which the head and the wings vibrate out of phase, twisting the neck, with a frequency of ∼5.5 kHz, and the symmetric mode (ST), in which the head and the wings vibrate in phase, twisting the leg, at ∼400 Hz. The AS2 mode has an exceptionally low internal friction, Q−1 ≈ 2 × 10−8 at low temperatures (T < 10 K), which is reproducible within ±10% for different oscillators. That enables an accurate determination of changes in internal friction. The ST mode has a higher internal friction, because it is more closely coupled to the cryostat than the AS2 mode, and also because it carries a metal film, which usually has a high internal friction [16], needed for electrostatic actuation and detection. Nevertheless, the ST mode is still useful to check for a shift of an internal friction peak with frequency. The double-paddle oscillators were ion-implanted at room temperature in the Cornell Nanofabrication Facility using a Varian/Extrion implanter. In order to achieve a uniform distribution of ion induced damage, implantation was done on one side, under an angle of 7◦ off the axis, at energies of 50, 120, and 180 keV for As+ , Si+ , and B+ , and at 15, 55, and 100 keV for H+ . The resulting thicknesses of damaged layer were expected to be 145, 400, 680, and 950 nm for As+ , Si+ , B+ , and H+ , respectively [17]. The ion-implantation dose was 7 × 1019 m−2 for Si+ and B+ , and 2 × 1019 m−2 for As+ and H+ . Subsequent structure analysis was done using cross-sectional transmission-electron microscopy (TEM) and Rutherford backscattering (RBS) and channeling on pieces of silicon wafers implanted at the same time with oscillators used for internal friction measurements. Reactive-ion etching was done on Si+ implanted oscillators to remove the top damaged layer in order to get a depth profile of ion-implantation induced defects. A Plasma Therm 72 RIE system in reactive SF6 and O2 plasma with a power of 50 W was used. The influence of reactive-ion etching on the damping of a bare oscillator was found to be negligible, leading to an increase of Q−1 by less than 10−8 . We also performed isochronal anneal for 1 h in a vacuum of ∼10−5 Pa on some of the Si+ implanted oscillators. 3. Results and discussion We show the internal friction of an oscillator implanted by Si+ in Fig. 1, together with a bare oscillator shown as the background. The measurements were done with the AS2 mode at 5.35 kHz. The most striking effect of the implantation on the internal friction is a narrow peak at 48 K. Considerable damping persists, however, to the lowest temperature (0.4 K) of the measurement. The low temperature internal friction below 10 K is as temperature independent as the background, which has been explained by the generation of atomic tunneling states in the amorphized region as well as in the heavily damaged crystalline region [15]. In order to track down the location of the defects that are responsible for the peak, we performed TEM and RBS channeling on similarly ion-implanted silicon wafers. The TEM picture shows a sharp boundary between amorphous and crystalline region at a depth of 330 nm, in agreement with the RBS channeling results. Underneath the amorphous layer, there is a heavily dam-
Fig. 1. The internal friction of Si+ self-implanted double-paddle oscillator, and of a similarly implanted oscillator with top implanted layer removed by 300 and 450 nm thick with reactive-ion etching. The resonance frequency is 5350 Hz. The solid curve is the background internal friction of a bare oscillator.
aged crystalline region about 70 nm thick and it fades away at a much larger depth. The total thickness of damaged region is in excellent agreement with a computer simulation [17]. We consecutively removed a 300 nm thick layer followed by another 150 nm thick layer by reactive-ion etching on an identically ionimplanted oscillator. The results are also shown in Fig. 1. Substrating the background, we find that the 48 K peak is reduced by about 10% after the first etching and about 80% after the second etching. This shows clearly that the defects responsible for the 48 K peak are concentrated in the heavily damaged region underneath the amorphized layer. The 10% reduction after the first etching could be explained by the inhomogeneity of the amorphized layer, which is due to the limited ion-implantation energy used. In addition to the 48 K peak, two small peaks become apparent at around 5–7 and 20 K after the first etching, which may have been covered by the large internal friction background in the as-implanted oscillator. They are removed, however, after the second etching, indicating the same structural location as the 48 K peak. Tan et al. [8] studied Si+ implanted silicon. They found a peak at 218 K at a frequency of 550 Hz, and concluded that the corresponding defects were located in the heavily damaged crystalline region based on the anisotropy of the peak. Although we do not find any peak above the one at 48 K (probably due to the rapidly rising background damping of our oscillators towards higher temperatures), our observation is a more direct proof as far as the location of the defects is concerned. In contrast, the internal friction below 10 K is reduced by 70% and 80% after the first and the second etching, respectively. This is consistent with its origin of atomic tunneling states, which are distributed almost uniformly no matter the material is amorphized or not [15]. Using the ST mode of the same oscillator, we find that the 48 K peak shifts to 42 K at 410 Hz. We further find that the internal friction of oscillators implanted with As+ , B+ , and even H+ shows exactly the same peaks, which can be described by a
X. Liu et al. / Materials Science and Engineering A 442 (2006) 63–66
Fig. 2. The normalized internal friction of As+ , Si+ , B+ , and H+ implanted double-paddle oscillators vs. the inverse of temperature. The results are shown for two resonance modes at 5350 and 410 Hz for the Si+ implanted oscillator, which varies ±2% among different oscillators. The solid curves are fits to Debye relaxation process described in the text.
single Debye relaxation process as ωτ Q−1 = 2Q−1 p 1 + ω2 τ 2
(1)
where Q−1 p is the height of the internal friction peak, ω the resonance frequency, and τ is the relaxation time. The internal friction has a peak when ωτ = 1. In a thermally activated relaxation process, τ takes an Arrhenius expression of the type V (2) τ = τ0 exp kT where k is the Boltzmann constant, T the temperature, V the activation energy for defect relaxation, and τ0 is a preexponential constant whose magnitude should be roughly the inverse of the mean lattice vibration frequency. Fig. 2 shows the normalized internal friction of As+ , Si+ , B+ , and H+ implanted oscillators versus the inverse of temperature at two resonance frequencies. The solid curves are fits of Eqs. (1) and (2) with V = 0.075 eV and τ0 = 4.6 × 10−13 s. Fig. 2 shows that the implanted ions are not part of the defects contributing to the Debye relaxation. Since single vacancies would anneal well below room temperature [18], the most likely defects are divacancies, or conceivably higher vacancy aggregates. To determine the size of the vacancies, we performed 1-h isochronal vacuum annealing from 200 to 700 ◦ C, and we found that the 48 K peak anneals away at between 200 and 300 ◦ C, see Fig. 3. Since divacancies are known to anneal at 250 ◦ C [6], we conclude that the 48 K peak has its origin of divacancy, although we are not sure of the exact mechanism that leads to the relaxation process. Fig. 3 also shows that, while the 48 K peak decreases considerably after an anneal at 200 ◦ C, two new peaks appear at 5 and 14 K. After the 300 ◦ C anneal, the 14 K peak disappears and the 5 K peak remains until after the 500 ◦ C anneal. Annealing at 700 ◦ C for 1 h is known to completely restore the crystallinity of an amorphous silicon film [19], and it
65
Fig. 3. The internal friction of the Si+ self-implanted double-paddle oscillator, and of similarly implanted oscillators after 1-h isochronal anneal in a vacuum. The resonance frequency is 5350 Hz. The solid curve is the background internal friction of a bare oscillator.
restores also the internal friction almost completely except for another peak appears at 24 K. The low temperature rise of the internal friction below 2 K for oscillators after 500 and 700 ◦ C anneal is known to be caused by the activation of contaminants by annealing [20]. The isochronal annealing behavior reveals the interesting structural transformation of the defects in ionimplanted silicon, such as one kind of defects transforms to other more stable kinds with each incremental increase of annealing temperature. Most likely, divacancies migrate upon annealing and some of them aggregate to larger vacancy clusters with reduced activation energy and higher annealing temperature. Cheng and Vajda [21] have determined the activation energy for the reorientation of the electronic configuration of the neutral divacancy to be 0.076 eV, in good agreement with the value determined in the present measurements. For atomic reorientation of the divacancy axis, Waktins and Corbett [1] obtained an activation energy of 1.3 eV. To support an electronic origin of divacancy in the 48 K internal fiction peak, however, measurements of internal friction in a magnetic field is needed. 4. Conclusions We have identified that a pronounced peak at 48 K in ionimplanted silicon is caused by divacancy defects concentrated in heavily damaged crystalline region just underneath the amorphized top layer. The peak can be well described by a thermally activated Debye relaxation process with an activation energy of 0.075 eV. This observation demonstrates the high sensitivity of the anelastic technique for the detection of defects in semiconductors. Acknowledgements We thank Dr. R.S. Crandall and Dr. K.M. Jones at the National Renewable Energy Laboratory for the TEM measurements and Dr. P. Revesz at the Cornell Center for Materials Research Ion
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X. Liu et al. / Materials Science and Engineering A 442 (2006) 63–66
Beam Analytical Facility for the RBS and channeling measurements. This work was supported by the Office of Naval Research. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10]
G.D. Watkins, J.W. Corbett, Phys. Rev. 138 (1965) A543–A555. M. Saito, A. Oshiyama, Phys. Rev. Lett. 73 (1994) 866–869. G.D. Watkins, Phys. Rev. Lett. 74 (1995) 4353. M. Saito, A. Oshiyama, Phys. Rev. Lett. 74 (1995) 4354. ˙ L. Dobaczewski, K. Go´sci´nski, Z.R. Zytkiewicz, K.B. Nielsen, L. Rubaldo, O. Andersen, A.R. Peaker, Phys. Rev. B 65 (2002) 113203–113204. H.J. Stein, F.L. Vook, J.A. Borders, Appl. Phys. Lett. 14 (1969) 328–330. S. Szpala, P.J. Simpson, J. Appl. Phys. 89 (2001) 5991–5996. S.I. Tan, B.S. Berry, B.L. Crowder, Appl. Phys. Lett. 20 (1972) 88–90. S.I. Tan, B.S. Berry, W.F.J. Frank, in: B.L. Crowder (Ed.), Ion Implantation in Semiconductors and Other Materials, Plenum, New York, 1973, pp. 19–30. B.S. Berry, W.C. Pritchet, in: F. Chernow, J.A. Borders, D.K. Brice (Eds.), Ion Implantation in Semiconductors, Plenum, New York, 1977, pp. 435–443.
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