- Email: [email protected]
Evolution of low-fluence heavy-ion damage in Si under high energy ion irradiation
Nuclear Instruments and Methods in Physics Research B55 (1991) 611-614 North-Holland
611
Evolution of low-fluence heavy-ion damage in Si under high energy ion irradiation A. Battaglia a, F. Priolo b, C. Spinella a and E. Rimini b a Istituto di Metodologie e Tecnologie per la ~~icr~elettronic~, CNR Corso Italia b
~~~~tirne~to
57, 195129 Catania,
Ifa@
di Fisica, Corso Italia 57, 193129 Catania, Ita&
The annealing of low-fluence heavy-ion damaged Si crystals induced by ion-assisted treatments is reported. Damage was produced by 150 keV Au implantations at a dose of 2 X 10 I3 ions/cm2 onto (100) oriented Si single crystals and resulted in small amorphous-like regions surrounded by crystal material. The interaction of these damaged structures with defects induced by energetic ions (600 keV I@+) was investigated. Kr post-irradiation resulted in either damage accumulation or annealing, depending on the substrate temperature. A transition temperature of about 420 K was found between these two different regimes. Ion-assisted processes are discussed and explained on the basis of the damage morphology.
2. Experimental The epitaxial recrystallization of continuous amorphous Si (a-Si) layers under both thermal [l] and ion-assisted [2,3] treatments has been the subject of extensive investigations during the last decade. Thermal heating of a-% layers onto single crystal substrates results in layer-by-layer growth at temperatures above 750 K. At lower temperatures, however, the process is kinetically inhibited and the amorphous phase exists under metastable conditions. In the temperature range between 400 and 650 K e&axial recrystallization can also occur if stimulated by ion beam irradiation [2,3]. This process has been attributed [4,5] to the generation of a nonequilibrium defect concentration which enhances the kinetics of the phase transition. Moreover, as soon as the t~mperat~e is further decreased the process can be reversed and ion irradiation can produce a layer-by-layer amorphization [6]. Heavy-ion damaging of Si crystals at low doses is not able to fully amorphize a surface layer and produces instead several damage clusters embedded in a crystal matrix. The nature and annealing behavior of these clusters raised great interest in the last years [7-lo]. The question concerning the “amorphicity” of these clusters, however, is still unresolved. In this paper we have studied the evolution of lowdose Au damaged regions in Si under ion-assisted treatments, in order to investigate the difference, if any, with the behavior of continuous amorphous layers and to obtain a better understanding on the morphology of this damage. 0168-583X/91/$03.50
(100) oriented Si single crystals were predarnaged by 150 keV Au implantations at a dose of 2 X 1013 ions/cm’ and at an average current density of 5 nA/cm’. Au implantations were performed maintaining the samples either at room temperature (RT) or at 77 K (LN,T) by means of a liquid nitrogen cooled sample holder. Fig. 1 shows 2.0 MeV He’ Rutherford backscattering (RBS) spectra in random (continuous line) and with the He+ beam aligned along the (100) direction, for the as-implanted samples. A grazing angle detection was used to enhance the depth resolution. Open circles refer to
4 150 keV Au 2x1d~rn2
Fig. 1. RBS spectra in random (continuous line) and in channeling along the (100) direction (symbols) for Au damaged Si single crystals at LN,T (closed circles) and at RT (open circles).
Q 1991 - Elsevier Science Publishers B.V. born-Holl~d)
VI. MATERIALS
SCIENCE
612
A. Buttaglia et al. / Low-fluence
samples damaged at RT, closed circles to those damaged at LNzT. In both cases a damage peak at a depth of 40 nm is present. From an analysis of these spectra we can estimate that the total number of displaced Si atoms per unit area is about 9 X 1Ol6 at/cm* for RT damaged samples and 1.4 X 1Or7 at/cm2 for LNZT damaged samples, i.e., about 4500 and 7000 atoms per collision cascade respectively. We can correlate this difference to dynamic annealing processes which occur already during RT ~mpl~tations. This result is in qu~ita~ve agreement with the amorphization model proposed by Morehead and Crowder [ll] where the size of stable amorphous clusters formed around a given ion track strongly depends on the substrate temperature. The morphology of RT as-damaged samples has been also investigated by plan-view transmission electron microscopy (TEM), as shown by the image in fig. 2. Bright regions represent undamaged crystal material, black dots are associated to clusters of defects, whilst the diffused grey regions are probably composed by amorphous material. The presence of the amorphous phase is confirmed by the diffraction pattern shown as an inset in the same figure. Ion-assisted treatments were performed on RT damaged samples by irradiation with a 600 keV Kr2+ beam at different doses and for different substrate temperatures in the range 350-500 K. The beam was electrostatically scanned onto a 1 in diam sample area and the average dose rate was maintained at 1 X 1Ol2 ions/cm2 s.
3. Results and discussion Fig. 3 shows the RBS spectra in random (~on~uous line) and with the He beam aligned along the (100)
Fig. 2. TEM plan-tiew micrograph of RT Au damaged samples. The diffraction pattern is shown as an inset.
heavy-ion damage in Si
4M
T=378K
(b)
T=438K
1.0
1.2
1.4
Energy(MeV) Fig. 3. RBS spectra in random (continuous line) and in (100) aligned direction of Au damaged Si crystals before and after post-irradiation with 600 keV Kr ions. Spectra refer to irradiation performed at a substrate temperature of (a) 378 K, and (b) 438 K at different Kr doses.
direction (symbols), before and after IQ irradiations at different doses and for different substrate temperatures. At a temperature of 378 K (fig. 3a) an increase of the damage peak, corresponding to disorder accumulation, is observed. At a dose of 5 X 1014 ions/cm* a continuous amorphous layer is obtained. A completely opposite behavior is shown in fig. 3b where at a substrate temperature of 438 K the damage peak is seen to decrease, i.e., a reordering of the displaced atoms is taking place. The structure of both unirradiated as-damaged samples and Kr irradiated samples was investigated by cross-sectional TEM analyses. Fig. 4a shows the morphology of the non-irradiated as-damaged sample. At a depth af about 40 nm damage structures are present. The darker areas can probably be attributed to clusters of defects which are generated at the boundaries of the Au collision subcascades whilst the grey regions are amorphous. This picture is consistent with the plan-view image shown in fig. 2. In fig. 4b the effect of Kr post irradiation on the damaged samples at a temperature of 378 K and, at a dose of 5 x 1014 ions/cm2 (RBS spectrum in fig. 3a) is clearly shown. A continuous amorphous layer, with a wavy interface, is present. The c-a interface is composed by a band of defects (darker
613
A. Battaglia et al. / Low-flueme heavy-ion damage in Si regions) accumulated during Kr irradiation and swept on the border during the growth of the amorphous layer. Fig. 4c, finally, shows the damage morphology in
T(K) 450
500
400
l/T(d3
Crystallization
\
0
GOOkeV
\
K”)
\
Kf+
-301 2.0
I 2.2
I 2.4
I 2.6
l/T (u 1O-3K-‘)
Fig. 5. Radial rate as a function of the reciprocal temperature (on a linear scale). The transition temperature between the amorphization regime and the crystallization regime corresponds to a nul1 rate. In the inset, in a logarithmic plot, the annealing rate of isolated damage clusters (closed circles) is shown together with the ion-induced epitaxial crystallization rate of continuous amorphous layers (triangles).
Fig. 4. TEM cross-sectional images of Au damaged samples (a) before and (b) after 600 keV Kr irradiation at 378 K to a dose of 5 ~10’~ Kr/cm2 and (c) at 438 K to a dose of 3 X 1015 Kr/cm’.
the samples after Kr irradiation at a temperature of 43X K and at a dose of 3 X IO’* Kr/cm’ (RRS spectrum in fig. 3b). Amorphous regions are not present, few residual damage clusters (dislocation loops) can be evidenced within a good quality single-crystal matrix. This residual damage being below the detection limit was not observed by channeling mesurements. The kinetics of damage accumulation or annealing under ion beam irradiation has been characterized in some details as a function of the substrate temperature. In fig. 5 we report the measured value for the radial rate of castigation or foliation, k, as a function of the reciprocal temperature. These values are extracted from fits to the experimental channeling data assuming that the pre-existing amorphous-like regions have an almost spherical shape and that they shrink or grow linearly along the radius with the irradiating dose, with a radial rate independent of the clusters dimension. The error bars in the experimental points refer to the uncertainty due to the fits (*lo%). The two regimes, amorphization and crystallization, are well separated by the horizontal line, and a transition temperature of 420 VI. MATERIALS SCIENCE
614
A. Battaglia et al. / Low-fluence heavy-ion damage in Si
K is measured at a dose rate of 1 X 101* Kr/cm* s. This value is in good agreement with the one observed during ion-beam induced layer-by-layer crystallization [6]. In the inset of fig. 5 the values for the crystallization rate of damage clusters (circles) and continuous surface amorphous layers (triangles) irradiated under identical conditions are compared. It should be noted that the two sets of data correlate to one another very well; as soon as the transition temperature is approached we can observe a sudden decrease of the curve towards zero. This behavior is well-known for ion-beam induced epitaxy and has been attributed [4,5] to a balance between an athermal amorphization term and a temperature-dependent crystallization term. As soon as the temperature is lowered the crystallization rate decreases and the athermal amorphization regime becomes suddenly important producing the sharp fall in the net rate. In conclusion we have studied the annealing behavior of low-fluence heavy-ion damage in Si under ion-assisted treatments. Ion beam irradiation of this damage can produce either crystallization or amorphization depending on the substrate temperature. The strong similarities between this behavior and that observed for planar c-a interfaces support the idea that these damage structures are composed mainly by amorphous material.
Acknowledgements We wish to acknowledge Mr. 0. Parasole and Mr. A. Marino for technical assistance. This work was supported in part by Progetto Finalizzato Materiali e Dispositivi per PElettronica a Stato Solido, CNR, and in part by the Esprit Project 2016 TIP BASE.
References [l] G.L. Olson and J.A. Roth, Mater. Sci. Rep. 3 (1988) 1. [2] J.S. Williams, R.G. Elliman, W.L. Brown and T.E. Seidel, Phys. Rev. Lett. 55 (1985) 1482. [3] F. Priolo, A. La Ferla and E. Rimini, J. Mater. Res. 3 (1988) 1212. [4] K.A. Jackson, J. Mater. Res. 3 (1988) 1218. [5] F. Priolo, C. Spinella and E. Rimini, Phys. Rev. B41 (1990) 5235. [6] J. Linnros, R.G. ElIiman, W.L. Brown, J. Mater. Res. 3 (1988) 1208. [7] L.M. Howe and M.H. Rainville, Nucl. Instr. and Meth. 182/183 (1981) 143. [8] D.A. Thompson, A. Golanski, H.K. Haugen, L.M. Howe and J.A. Davies, Radiat. Eff. Lett. 50 (1980) 125. [9] L.M. Howe and M.H. Rainville, Nucl. Instr. and Meth. B19/20 (1987) 61. [lo] A. Battaglia, F. Priolo, E. Rimini and G. Ferla, Appl. Phys. Lett. 56 (1990) 2622. [ll] F.F. Morehead and B.L. Crowder, Radiat. Eff. 6 (1970) 27.
611
Evolution of low-fluence heavy-ion damage in Si under high energy ion irradiation A. Battaglia a, F. Priolo b, C. Spinella a and E. Rimini b a Istituto di Metodologie e Tecnologie per la ~~icr~elettronic~, CNR Corso Italia b
~~~~tirne~to
57, 195129 Catania,
Ifa@
di Fisica, Corso Italia 57, 193129 Catania, Ita&
The annealing of low-fluence heavy-ion damaged Si crystals induced by ion-assisted treatments is reported. Damage was produced by 150 keV Au implantations at a dose of 2 X 10 I3 ions/cm2 onto (100) oriented Si single crystals and resulted in small amorphous-like regions surrounded by crystal material. The interaction of these damaged structures with defects induced by energetic ions (600 keV I@+) was investigated. Kr post-irradiation resulted in either damage accumulation or annealing, depending on the substrate temperature. A transition temperature of about 420 K was found between these two different regimes. Ion-assisted processes are discussed and explained on the basis of the damage morphology.
2. Experimental The epitaxial recrystallization of continuous amorphous Si (a-Si) layers under both thermal [l] and ion-assisted [2,3] treatments has been the subject of extensive investigations during the last decade. Thermal heating of a-% layers onto single crystal substrates results in layer-by-layer growth at temperatures above 750 K. At lower temperatures, however, the process is kinetically inhibited and the amorphous phase exists under metastable conditions. In the temperature range between 400 and 650 K e&axial recrystallization can also occur if stimulated by ion beam irradiation [2,3]. This process has been attributed [4,5] to the generation of a nonequilibrium defect concentration which enhances the kinetics of the phase transition. Moreover, as soon as the t~mperat~e is further decreased the process can be reversed and ion irradiation can produce a layer-by-layer amorphization [6]. Heavy-ion damaging of Si crystals at low doses is not able to fully amorphize a surface layer and produces instead several damage clusters embedded in a crystal matrix. The nature and annealing behavior of these clusters raised great interest in the last years [7-lo]. The question concerning the “amorphicity” of these clusters, however, is still unresolved. In this paper we have studied the evolution of lowdose Au damaged regions in Si under ion-assisted treatments, in order to investigate the difference, if any, with the behavior of continuous amorphous layers and to obtain a better understanding on the morphology of this damage. 0168-583X/91/$03.50
(100) oriented Si single crystals were predarnaged by 150 keV Au implantations at a dose of 2 X 1013 ions/cm’ and at an average current density of 5 nA/cm’. Au implantations were performed maintaining the samples either at room temperature (RT) or at 77 K (LN,T) by means of a liquid nitrogen cooled sample holder. Fig. 1 shows 2.0 MeV He’ Rutherford backscattering (RBS) spectra in random (continuous line) and with the He+ beam aligned along the (100) direction, for the as-implanted samples. A grazing angle detection was used to enhance the depth resolution. Open circles refer to
4 150 keV Au 2x1d~rn2
Fig. 1. RBS spectra in random (continuous line) and in channeling along the (100) direction (symbols) for Au damaged Si single crystals at LN,T (closed circles) and at RT (open circles).
Q 1991 - Elsevier Science Publishers B.V. born-Holl~d)
VI. MATERIALS
SCIENCE
612
A. Buttaglia et al. / Low-fluence
samples damaged at RT, closed circles to those damaged at LNzT. In both cases a damage peak at a depth of 40 nm is present. From an analysis of these spectra we can estimate that the total number of displaced Si atoms per unit area is about 9 X 1Ol6 at/cm* for RT damaged samples and 1.4 X 1Or7 at/cm2 for LNZT damaged samples, i.e., about 4500 and 7000 atoms per collision cascade respectively. We can correlate this difference to dynamic annealing processes which occur already during RT ~mpl~tations. This result is in qu~ita~ve agreement with the amorphization model proposed by Morehead and Crowder [ll] where the size of stable amorphous clusters formed around a given ion track strongly depends on the substrate temperature. The morphology of RT as-damaged samples has been also investigated by plan-view transmission electron microscopy (TEM), as shown by the image in fig. 2. Bright regions represent undamaged crystal material, black dots are associated to clusters of defects, whilst the diffused grey regions are probably composed by amorphous material. The presence of the amorphous phase is confirmed by the diffraction pattern shown as an inset in the same figure. Ion-assisted treatments were performed on RT damaged samples by irradiation with a 600 keV Kr2+ beam at different doses and for different substrate temperatures in the range 350-500 K. The beam was electrostatically scanned onto a 1 in diam sample area and the average dose rate was maintained at 1 X 1Ol2 ions/cm2 s.
3. Results and discussion Fig. 3 shows the RBS spectra in random (~on~uous line) and with the He beam aligned along the (100)
Fig. 2. TEM plan-tiew micrograph of RT Au damaged samples. The diffraction pattern is shown as an inset.
heavy-ion damage in Si
4M
T=378K
(b)
T=438K
1.0
1.2
1.4
Energy(MeV) Fig. 3. RBS spectra in random (continuous line) and in (100) aligned direction of Au damaged Si crystals before and after post-irradiation with 600 keV Kr ions. Spectra refer to irradiation performed at a substrate temperature of (a) 378 K, and (b) 438 K at different Kr doses.
direction (symbols), before and after IQ irradiations at different doses and for different substrate temperatures. At a temperature of 378 K (fig. 3a) an increase of the damage peak, corresponding to disorder accumulation, is observed. At a dose of 5 X 1014 ions/cm* a continuous amorphous layer is obtained. A completely opposite behavior is shown in fig. 3b where at a substrate temperature of 438 K the damage peak is seen to decrease, i.e., a reordering of the displaced atoms is taking place. The structure of both unirradiated as-damaged samples and Kr irradiated samples was investigated by cross-sectional TEM analyses. Fig. 4a shows the morphology of the non-irradiated as-damaged sample. At a depth af about 40 nm damage structures are present. The darker areas can probably be attributed to clusters of defects which are generated at the boundaries of the Au collision subcascades whilst the grey regions are amorphous. This picture is consistent with the plan-view image shown in fig. 2. In fig. 4b the effect of Kr post irradiation on the damaged samples at a temperature of 378 K and, at a dose of 5 x 1014 ions/cm2 (RBS spectrum in fig. 3a) is clearly shown. A continuous amorphous layer, with a wavy interface, is present. The c-a interface is composed by a band of defects (darker
613
A. Battaglia et al. / Low-flueme heavy-ion damage in Si regions) accumulated during Kr irradiation and swept on the border during the growth of the amorphous layer. Fig. 4c, finally, shows the damage morphology in
T(K) 450
500
400
l/T(d3
Crystallization
\
0
GOOkeV
\
K”)
\
Kf+
-301 2.0
I 2.2
I 2.4
I 2.6
l/T (u 1O-3K-‘)
Fig. 5. Radial rate as a function of the reciprocal temperature (on a linear scale). The transition temperature between the amorphization regime and the crystallization regime corresponds to a nul1 rate. In the inset, in a logarithmic plot, the annealing rate of isolated damage clusters (closed circles) is shown together with the ion-induced epitaxial crystallization rate of continuous amorphous layers (triangles).
Fig. 4. TEM cross-sectional images of Au damaged samples (a) before and (b) after 600 keV Kr irradiation at 378 K to a dose of 5 ~10’~ Kr/cm2 and (c) at 438 K to a dose of 3 X 1015 Kr/cm’.
the samples after Kr irradiation at a temperature of 43X K and at a dose of 3 X IO’* Kr/cm’ (RRS spectrum in fig. 3b). Amorphous regions are not present, few residual damage clusters (dislocation loops) can be evidenced within a good quality single-crystal matrix. This residual damage being below the detection limit was not observed by channeling mesurements. The kinetics of damage accumulation or annealing under ion beam irradiation has been characterized in some details as a function of the substrate temperature. In fig. 5 we report the measured value for the radial rate of castigation or foliation, k, as a function of the reciprocal temperature. These values are extracted from fits to the experimental channeling data assuming that the pre-existing amorphous-like regions have an almost spherical shape and that they shrink or grow linearly along the radius with the irradiating dose, with a radial rate independent of the clusters dimension. The error bars in the experimental points refer to the uncertainty due to the fits (*lo%). The two regimes, amorphization and crystallization, are well separated by the horizontal line, and a transition temperature of 420 VI. MATERIALS SCIENCE
614
A. Battaglia et al. / Low-fluence heavy-ion damage in Si
K is measured at a dose rate of 1 X 101* Kr/cm* s. This value is in good agreement with the one observed during ion-beam induced layer-by-layer crystallization [6]. In the inset of fig. 5 the values for the crystallization rate of damage clusters (circles) and continuous surface amorphous layers (triangles) irradiated under identical conditions are compared. It should be noted that the two sets of data correlate to one another very well; as soon as the transition temperature is approached we can observe a sudden decrease of the curve towards zero. This behavior is well-known for ion-beam induced epitaxy and has been attributed [4,5] to a balance between an athermal amorphization term and a temperature-dependent crystallization term. As soon as the temperature is lowered the crystallization rate decreases and the athermal amorphization regime becomes suddenly important producing the sharp fall in the net rate. In conclusion we have studied the annealing behavior of low-fluence heavy-ion damage in Si under ion-assisted treatments. Ion beam irradiation of this damage can produce either crystallization or amorphization depending on the substrate temperature. The strong similarities between this behavior and that observed for planar c-a interfaces support the idea that these damage structures are composed mainly by amorphous material.
Acknowledgements We wish to acknowledge Mr. 0. Parasole and Mr. A. Marino for technical assistance. This work was supported in part by Progetto Finalizzato Materiali e Dispositivi per PElettronica a Stato Solido, CNR, and in part by the Esprit Project 2016 TIP BASE.
References [l] G.L. Olson and J.A. Roth, Mater. Sci. Rep. 3 (1988) 1. [2] J.S. Williams, R.G. Elliman, W.L. Brown and T.E. Seidel, Phys. Rev. Lett. 55 (1985) 1482. [3] F. Priolo, A. La Ferla and E. Rimini, J. Mater. Res. 3 (1988) 1212. [4] K.A. Jackson, J. Mater. Res. 3 (1988) 1218. [5] F. Priolo, C. Spinella and E. Rimini, Phys. Rev. B41 (1990) 5235. [6] J. Linnros, R.G. ElIiman, W.L. Brown, J. Mater. Res. 3 (1988) 1208. [7] L.M. Howe and M.H. Rainville, Nucl. Instr. and Meth. 182/183 (1981) 143. [8] D.A. Thompson, A. Golanski, H.K. Haugen, L.M. Howe and J.A. Davies, Radiat. Eff. Lett. 50 (1980) 125. [9] L.M. Howe and M.H. Rainville, Nucl. Instr. and Meth. B19/20 (1987) 61. [lo] A. Battaglia, F. Priolo, E. Rimini and G. Ferla, Appl. Phys. Lett. 56 (1990) 2622. [ll] F.F. Morehead and B.L. Crowder, Radiat. Eff. 6 (1970) 27.