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High energy implants of aluminum in Czochralski and floating zone grown silicon substrates
Nuclear Instruments and Methods in Physics Research B74 (1993) 109-112 North-Holland
NUMB
Beam interactions with Materials & Atoms
High energy implants of aluminum in Czochralski and floating zone grown silicon substrates A, La Ferla, L. Torrisi, G. Galvagno and E. Rimini Dipartirnento di Fisica, Universitd di Catania, Corso Italia 57, 95129 Catania, Italy
G. Ciavola La~orato~o Nazionale de1 Sod-IAN,
via Santa So&,
9.5123 Catark,
It&y
A. Carnera and A. Gasparotto Dipartimento di Fisica, UniversitG di Padova - via Marzolo 8, 3.5131 Padova, Italy
~uminum ions at 100 MeV were implanted into floating zone (FZ) and Czochralski (CZ) grown Si substrates. At this energy the implanted ions are located at a depth of about 40 p,rn so to minimize the influence of the surface on the subsequent thermal treatment. In FZ samples the electrically active dose, as measured by spreading resistance profilometry, is independent of the annealing time at 12OO”C,but in the CZ samples it decreases considerably with time. Secondary ion mass spectrometry analysis in CZ samples has revealed the presence of a multipeak structure around the projected range region for both Al and 0 signals. In the FZ the structure is just detectable. The results imply that the Al-O complex formation is, of course, enhanced by the large content of oxygen but that it is catalyzed by the damage created during the implant. In contrast the carrier profiles coincide in both CZ and FZ substrates doped by prede~sit~on of Al from a solid source and subsequent diffusion; i.e. in damage free samples.
1. Introduction The growing interest for Al as a p-type dopant in Si is due to its high ~~i~~. Its vision coefficient is about three times higher than that of Ga and one order of magnitude higher than that of B [l]. In the manufacturing of discrete high power devices the junction termination requires doped layers several tens of microns deep and for this purpose the use of AI is mandatory [Z]. In fact the vision rate of Al permits a junction depth of 120 p,rn by a reasonable thermal drive-in process, e.g. T = 1250°C and t = 60 h. For these devices, aluminum is introduced by a solid or gas source sealed with the wafers in a diffusion tube. The extension of this dopant to other families of integrated power devices, such as power-MO& isolated gate bipolar transistors, etc. where a well defined control of conductivity over about a 10 urn bulk region is required, needs its introduction by implantation. A number of drawbacks have been found and the poor electrical activity after thermal processes, 0.1-l% of the fluence, is the main problem. High oxygen reactivity [3,4], out~ffusion from the surface 131, and precipitation at concentrations above the solubility limit [4-61 are all responsible of the low residual active dose. Moreover, the implantation damage produces prefer0168-583X/93/$06.00
ential sites for nucleation of metallic Al precipitates [5,7] or of Al-O complexes [3,4]. Finally capping layers such as silicon dioxide or silicon nitride are not quite effective in blocking the out~~sion due to a value higher than 1 of the segregation coefficient of Al at these interfaces [3]. To avoid the simultaneous occurrence of almost all of these phenomena, very high energy (about 100 MeV) Al implants were performed. The aluminum atoms are located at some tens of microns beneath the surface and any interface interaction (out-diffusion, segregation) is avoided or minimized. The influence of oxygen on the formation of complexes with the Al ions was investigated by using CZ and FZ grown Si substrates with different oxygen content (= 1016/cm3 and = 8 x 1017/cm3 respectively as determined by Fourier transform infrared measurements).
2. Experimental FZ (100) n-type Si of 1000 fz cm resistivity and CZ grown of 250 fl cm resist@& samples were implanted with Al at 100 MeV. The implants were performed at room temperature with a tandem accelerator of 15 MV terminal voltage and an ion current of about 100 nA. A
0 1993 - Elsevier Science Publishers B.V. All rights reserved
IV. MATERIALS SCIENCE
A. La Ferla et al. / High energy implantsof Al in Si
110
sample area of 3 X 7 mm2 was implanted while the Al beam was magnetically scanned over an area of 1 X 1 cm’. A rapid thermal annealing process at 1200°C for 5 s was adopted to activate the dopant. Several thermal process schedules were used to study the Al diffusion. The electrical profiles were determined by the Spreading Resistance technique bevelling the samples at 2’54’ or l”5’. The chemical profiles of Al and 0 atoms were obtained by Secondary Ion Mass Spectroscopy performed on a bevelled sample mounted perpendicular to the secondary ion detection direction by means of stepping motors.
3. Results and discussion The comparison between the electrical and chemical profiles for 100 MeV Al implants in Si is shown in fig. 1. The hole distribution obtained by spreading resistance analysis refers to a sample implanted at a dose of 1 X 1014/cm2 and annealed at 1200°C for 5 s. The peak concentration is almost two orders of magnitude lower than the solid solubility of Al in Si at 1200°C: C, = 2 X 101’/cm3 [8] and the electrical activation is complete. The thermal diffusion is negligible at this temperature-time ~mbination with respect to the extension of the as-implanted pro@e and the carrier distribution is therefore expected to be coincident with the chemical profile. The SIMS analysis was performed in a sample implanted at a dose of 3.5 X 10*5/cm2 due to the detection limit of this technique. In our experimental conditions, the fluence was chosen in such way to follow the SIMS profile for more than four orders of magnitude around the peak. The Al peak concentration is about
10
20 30 40 Depth [F]
50
60
Fig. 1. Electrical and chemical profiles of 100 MeV Al implantation in Si. The car&r ~~ibution (dashed line) corresponds to a dose of 1 X lO”/cm’ while the chemical profile (full line) was measured on a sample implanted at 3.5~ 101s/cm2. The left scale is aligned to the right scale by a shift corresponding to a reduction factor of l/3.5.
2 X 101’/cm3 and the SIMS detection limit is around 6 X 1014/cm3. The imposition of four different SIMS analyses results in the profile shown in fig. 1. Four craters about 12 pm deep were eroded in different positions of the bevelled sample corresponding to four different starting depths: 20, 27.5, 35 and 42.5 pm, respectively. This gives about 4-5 pm of superposition between two successive profiles and the perfect correspondence of the end ~stribution on one crater with the initial one of the next crater demonstrates the good reproducibility of the technique when the crater reaches lo-12 pm in depth. The scale on the right-hand side has been aligned to that of the hole concentration (left-hand side) by a reduction factor of l/35 corresponding to the ratio of the two fluences. The two profiles coincide quite well. The shape of the as-implanted aluminum distribution corresponds to that characteristic of the high energy regime. Near the projected range, about 38 km for 100 MeV implantation, the Al concentration rises ab~p~ for more than four orders of magnitude while the front region of the Si substrate is characterized by a flat tail due to the very small number of ions elastically scattered from the Si nuclei. At 100 MeV the electronic stopping power of Al in Si is about a factor lo3 higher than the nuclear stopping power; for this reason the projected range of the dist~bution corresponds to the depth where inelastic electronic collisions dominate the stopping process. Around the peak the profile is broadened mainly by electronic straggling; after the maximum the concentration falls off asymmetrically with a channeling tail due probably to ions that are fed into low index axes or planes [9]. The influence of the oxygen on the Al activation was investigated in CZ and FZ samples for different annealing times at 1200°C. The results are summarized in fig. 2 for 1 x 1014/cm2 implants. While in the FZ substrates the electrically active component of the Al fluence remains constant during diffusion ( q), in the CZ samples (x) it decreases with time. After 9 h at 12OO“Conly 20% of the implanted dose is electrically active in the CZ material, but the activation was almost complete after processing at 1200°C for 5 s. In some cases oxygen was implanted at a dose of 2 x 1015/cm2 and an energy of 55 MeV in FZ samples. The annealing performed at 1200°C distributes the oxygen throughout the sample thickness as a result of the very high diffusion coefficient, D = 3.5 X fO_” cm2/s. In these FZ implanted samples the electrical activity was reduced to a value close to that found in CZ substrates. The point marked 0 in fig. 2 represents the results. vacation of the carrier profiles, not reported here, show that the inactive Al atoms are located in the peak region in both FZ and CZ substrates. In these samples the as-implanted Al peak concentration is
A. La Ferla et al. / High energy implants of Al in Si 100 MeV Al 1~10’~ 1.25 - ‘,““‘, ’ “““‘1 “““‘1 l,~~~*~~~_~~~~~~___~_~ - 1200 Oc q
em-’ - 7““‘I
0 _c
0.75
: X
% ;
0.50
7
: ‘3
0.25
:
0.00
-
101
x-
FZ
0:
FZ+O * : x
0 4
cz
x x
0
102
103
Time
x
104
x
-
105
[s]
Fig. 2. Active dose (normalized) in the CZ and FZ samples implanted with 1 X1014/cm2 Al at 106 Mev for different annealing times at 1200°C (0 FZ, X CZ). The diamond (0) refers to a FZ sample implanted with 0 at 55 MeV. = 5 x 1017/cm3, much lower than the solid solubility, so that no precipitation is expected. The FZ and CZ substrates differ only in the bulk oxygen concentration. To investigate the oxygen influence on the Al electrical activity, SIMS analysis of Al and 0 was performed on FZ and CZ samples implanted at 100 MeV with a dose of 3 x 1015/cm2, and where the peak concentration is still below the solid solubility limit. The Al distribution was probed with a primary beam of oxygen, while that of oxygen with a Cs beam. The oxygen profile of the as-implanted CZ sample was quite flat all over the sample. Two 3 X 1015/cm2 Al implanted CZ and FZ samples were simultaneously diffused for 1 h at 1200°C and analyzed (figs. 3a and 3b, respectively). The SIMS analysis indicates the presence of a multipeak structure located around the depth of the projected Al range for both aluminum and oxygen atoms. This structure is more pronounced in the CZ sample but it is also present in the FZ substrate. The oxygen
1020
111
profile follows perfectly the Al distribution (in this region) and the Al atoms are not electrically active as shown by the superimposed carrier distributions (dashed lines). However while in the FZ sample the difference between the chemical and the electrically active dose is only about 2%, in the CZ sample the carrier profile corresponds to only 40% of the implanted dose. From the SIMS oxygen spectra we note in the inactive region an abrupt increase in the 0 concentration over the bulk level and a depletion before and after this region. It must be noted that the region of Al and 0 accumulation is located around the projected range where the maximum of the nuclear energy deposition occurs. Thus, we believe that the multi-peak structure with a large amount of inactive Al is due to the formation of Al-0 complexes, where the implantation damage acts as a nucleation center, or as a catalyst. Considering only the inactive Al (from the SIMS and the SR profiles) it is possible to observe that the ratio between Al and 0 is about one to one. We suggest that radiation damage reduces the kinetic barrier for the formation of Al-0 complexes by facilitating a large number of broken or dangling bonds. As soon as some complexes are formed, they may act as gettering sites for neighbouring Al and 0 atoms, thus depleting the surroundings regions and giving rise to the multipeak structure. The free energy of the Al-0 bonding is lower than that of Si-0, thus this reduction is the driving force for the process. During annealing, two competitive phenomena occur: diffusion of aluminum and precipitation in inactive complexes. The formation of complexes reduces the amount of Al available for the diffusion so that, in FZ material the distribution is broader than in CZ samples. Transmission electron analysis, not reported here, shows the presence of extended defects, like dislocation loops, at the (d E/dx) peak. These defects may act as nucleation centers of the Al-0 complexes. The amount of the defects is
I ”
;““,““,““I””
FZ
1015
-
”
I ”
“I”’ (b)
40 50 60 20 30 40 50 60 Depth km1 Depth km1 Fig. 3. Secondary Ion Mass Spectroscopy analysis of Al and 0 in the diffused (12Oo”C,1 h) CZ (a) and FZ (b) samples. The Al implantation was performed at 100 MeV with a dose of 3 X 101*/cm2. The dashed lines represent the hole profiles. 20
30
IV. MATERIALS SCIENCE
112
A. La Ferla et al. / High energy implants of Al in Si 1017 m‘@
1016
-
1015
I’ 7 ’ 77 ’ 7 ’ ’ ”
z’
3
is no implantation
dam-
4. Conclusions
g 3 1014 2 +d E 1013 3
active component since there age in these cases.
1012 1011
0
5
10 Depth
15
20
25
30
[pm]
Fig. 4. Electrical carrier profiles of CZ and FZ grown Si substrates diffused at 1250°C for 2 h with Al source in a sealed ampoule.
larger in CZ than in FZ substrates evidencing an interplay between the formation of complexes and the growth of dislocation. At this stage of the experimental work, only a qualitative discussion of the results is possible. Of course, a detailed investigation is required to understand the evolution of the multipeak structure as a function of the annealing temperature and time. The damage seems to play a fundamental role in the Al-O nucleation. To verify this point, the following experiment has been performed. Two FZ and CZ grown Si substrates were Al doped by furnace predeposition and drive-in diffusion process in a sealed tube under an Ar ambient at a temperature of 1250°C for 2 h. The electrical profiles of these samples are reported in fig. 3; the hole distributions coincide in the two samples up to the junction depth, which is different as a result of the different resistivity of the substrates. The different oxygen bulk concentration does not appear to influence the distribution of the electrically
We have demonstrated, for Al doping by implantation, a strong dependence of the amount of electrically active Al on the oxygen concentration level in Si, even for Al concentrations below the solid solubility limit. It is suggested that the defects induced by ion implantation act as nucleation centers for the formation of inactive A-0 complexes. The inactive Al region is formed around the peak of the disorder distribution curve and grows laterally with annealing time. The Al-O complexes appear stable throughout a thermal process at 1200°C for 9 h.
References 111B.J. Baliga, Modern Power Devices (Wiley, New York, 1987) p. 29. Dl E. Halder, P. Roggwiller and J. Gobrecht, Phys. Scripta 39 (1989) 406. 131P. Bruesch, E. Halder, P. Kluge, J. Rhyner, P. Roggwiller, Th. Stockmeier, F. Stucki and H.J. Wiesmann, J. Appl. Phys. 68 (1990) 2226. [41 H.R. Chang, N. Lewis, GA. Smith, E.L. Hall and V.A.K. Temple, J. Electrochem. Sot. 13.5(1988) 252. [51 D.K. Sadana, M.H. Norcott, R.G. Wilson and U. Dahmen, Appl. Phys. Lett. 49 (1986) 1169. 161R.G. Wilson, J. Appl. Phys. 61 (1987) 933. [71 E.L. Mathe’ and J.C. Desoyer, Phys. Stat. Sol. A 112 (1989) 511. 181S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1985) p. 69. [91 A. La Ferla, E. Rimini, A. Camera, A. Gasparotto, G. Ciavola and G. Ferla, Nucl. Instr. Meth. B55 (1991) 561.
NUMB
Beam interactions with Materials & Atoms
High energy implants of aluminum in Czochralski and floating zone grown silicon substrates A, La Ferla, L. Torrisi, G. Galvagno and E. Rimini Dipartirnento di Fisica, Universitd di Catania, Corso Italia 57, 95129 Catania, Italy
G. Ciavola La~orato~o Nazionale de1 Sod-IAN,
via Santa So&,
9.5123 Catark,
It&y
A. Carnera and A. Gasparotto Dipartimento di Fisica, UniversitG di Padova - via Marzolo 8, 3.5131 Padova, Italy
~uminum ions at 100 MeV were implanted into floating zone (FZ) and Czochralski (CZ) grown Si substrates. At this energy the implanted ions are located at a depth of about 40 p,rn so to minimize the influence of the surface on the subsequent thermal treatment. In FZ samples the electrically active dose, as measured by spreading resistance profilometry, is independent of the annealing time at 12OO”C,but in the CZ samples it decreases considerably with time. Secondary ion mass spectrometry analysis in CZ samples has revealed the presence of a multipeak structure around the projected range region for both Al and 0 signals. In the FZ the structure is just detectable. The results imply that the Al-O complex formation is, of course, enhanced by the large content of oxygen but that it is catalyzed by the damage created during the implant. In contrast the carrier profiles coincide in both CZ and FZ substrates doped by prede~sit~on of Al from a solid source and subsequent diffusion; i.e. in damage free samples.
1. Introduction The growing interest for Al as a p-type dopant in Si is due to its high ~~i~~. Its vision coefficient is about three times higher than that of Ga and one order of magnitude higher than that of B [l]. In the manufacturing of discrete high power devices the junction termination requires doped layers several tens of microns deep and for this purpose the use of AI is mandatory [Z]. In fact the vision rate of Al permits a junction depth of 120 p,rn by a reasonable thermal drive-in process, e.g. T = 1250°C and t = 60 h. For these devices, aluminum is introduced by a solid or gas source sealed with the wafers in a diffusion tube. The extension of this dopant to other families of integrated power devices, such as power-MO& isolated gate bipolar transistors, etc. where a well defined control of conductivity over about a 10 urn bulk region is required, needs its introduction by implantation. A number of drawbacks have been found and the poor electrical activity after thermal processes, 0.1-l% of the fluence, is the main problem. High oxygen reactivity [3,4], out~ffusion from the surface 131, and precipitation at concentrations above the solubility limit [4-61 are all responsible of the low residual active dose. Moreover, the implantation damage produces prefer0168-583X/93/$06.00
ential sites for nucleation of metallic Al precipitates [5,7] or of Al-O complexes [3,4]. Finally capping layers such as silicon dioxide or silicon nitride are not quite effective in blocking the out~~sion due to a value higher than 1 of the segregation coefficient of Al at these interfaces [3]. To avoid the simultaneous occurrence of almost all of these phenomena, very high energy (about 100 MeV) Al implants were performed. The aluminum atoms are located at some tens of microns beneath the surface and any interface interaction (out-diffusion, segregation) is avoided or minimized. The influence of oxygen on the formation of complexes with the Al ions was investigated by using CZ and FZ grown Si substrates with different oxygen content (= 1016/cm3 and = 8 x 1017/cm3 respectively as determined by Fourier transform infrared measurements).
2. Experimental FZ (100) n-type Si of 1000 fz cm resistivity and CZ grown of 250 fl cm resist@& samples were implanted with Al at 100 MeV. The implants were performed at room temperature with a tandem accelerator of 15 MV terminal voltage and an ion current of about 100 nA. A
0 1993 - Elsevier Science Publishers B.V. All rights reserved
IV. MATERIALS SCIENCE
A. La Ferla et al. / High energy implantsof Al in Si
110
sample area of 3 X 7 mm2 was implanted while the Al beam was magnetically scanned over an area of 1 X 1 cm’. A rapid thermal annealing process at 1200°C for 5 s was adopted to activate the dopant. Several thermal process schedules were used to study the Al diffusion. The electrical profiles were determined by the Spreading Resistance technique bevelling the samples at 2’54’ or l”5’. The chemical profiles of Al and 0 atoms were obtained by Secondary Ion Mass Spectroscopy performed on a bevelled sample mounted perpendicular to the secondary ion detection direction by means of stepping motors.
3. Results and discussion The comparison between the electrical and chemical profiles for 100 MeV Al implants in Si is shown in fig. 1. The hole distribution obtained by spreading resistance analysis refers to a sample implanted at a dose of 1 X 1014/cm2 and annealed at 1200°C for 5 s. The peak concentration is almost two orders of magnitude lower than the solid solubility of Al in Si at 1200°C: C, = 2 X 101’/cm3 [8] and the electrical activation is complete. The thermal diffusion is negligible at this temperature-time ~mbination with respect to the extension of the as-implanted pro@e and the carrier distribution is therefore expected to be coincident with the chemical profile. The SIMS analysis was performed in a sample implanted at a dose of 3.5 X 10*5/cm2 due to the detection limit of this technique. In our experimental conditions, the fluence was chosen in such way to follow the SIMS profile for more than four orders of magnitude around the peak. The Al peak concentration is about
10
20 30 40 Depth [F]
50
60
Fig. 1. Electrical and chemical profiles of 100 MeV Al implantation in Si. The car&r ~~ibution (dashed line) corresponds to a dose of 1 X lO”/cm’ while the chemical profile (full line) was measured on a sample implanted at 3.5~ 101s/cm2. The left scale is aligned to the right scale by a shift corresponding to a reduction factor of l/3.5.
2 X 101’/cm3 and the SIMS detection limit is around 6 X 1014/cm3. The imposition of four different SIMS analyses results in the profile shown in fig. 1. Four craters about 12 pm deep were eroded in different positions of the bevelled sample corresponding to four different starting depths: 20, 27.5, 35 and 42.5 pm, respectively. This gives about 4-5 pm of superposition between two successive profiles and the perfect correspondence of the end ~stribution on one crater with the initial one of the next crater demonstrates the good reproducibility of the technique when the crater reaches lo-12 pm in depth. The scale on the right-hand side has been aligned to that of the hole concentration (left-hand side) by a reduction factor of l/35 corresponding to the ratio of the two fluences. The two profiles coincide quite well. The shape of the as-implanted aluminum distribution corresponds to that characteristic of the high energy regime. Near the projected range, about 38 km for 100 MeV implantation, the Al concentration rises ab~p~ for more than four orders of magnitude while the front region of the Si substrate is characterized by a flat tail due to the very small number of ions elastically scattered from the Si nuclei. At 100 MeV the electronic stopping power of Al in Si is about a factor lo3 higher than the nuclear stopping power; for this reason the projected range of the dist~bution corresponds to the depth where inelastic electronic collisions dominate the stopping process. Around the peak the profile is broadened mainly by electronic straggling; after the maximum the concentration falls off asymmetrically with a channeling tail due probably to ions that are fed into low index axes or planes [9]. The influence of the oxygen on the Al activation was investigated in CZ and FZ samples for different annealing times at 1200°C. The results are summarized in fig. 2 for 1 x 1014/cm2 implants. While in the FZ substrates the electrically active component of the Al fluence remains constant during diffusion ( q), in the CZ samples (x) it decreases with time. After 9 h at 12OO“Conly 20% of the implanted dose is electrically active in the CZ material, but the activation was almost complete after processing at 1200°C for 5 s. In some cases oxygen was implanted at a dose of 2 x 1015/cm2 and an energy of 55 MeV in FZ samples. The annealing performed at 1200°C distributes the oxygen throughout the sample thickness as a result of the very high diffusion coefficient, D = 3.5 X fO_” cm2/s. In these FZ implanted samples the electrical activity was reduced to a value close to that found in CZ substrates. The point marked 0 in fig. 2 represents the results. vacation of the carrier profiles, not reported here, show that the inactive Al atoms are located in the peak region in both FZ and CZ substrates. In these samples the as-implanted Al peak concentration is
A. La Ferla et al. / High energy implants of Al in Si 100 MeV Al 1~10’~ 1.25 - ‘,““‘, ’ “““‘1 “““‘1 l,~~~*~~~_~~~~~~___~_~ - 1200 Oc q
em-’ - 7““‘I
0 _c
0.75
: X
% ;
0.50
7
: ‘3
0.25
:
0.00
-
101
x-
FZ
0:
FZ+O * : x
0 4
cz
x x
0
102
103
Time
x
104
x
-
105
[s]
Fig. 2. Active dose (normalized) in the CZ and FZ samples implanted with 1 X1014/cm2 Al at 106 Mev for different annealing times at 1200°C (0 FZ, X CZ). The diamond (0) refers to a FZ sample implanted with 0 at 55 MeV. = 5 x 1017/cm3, much lower than the solid solubility, so that no precipitation is expected. The FZ and CZ substrates differ only in the bulk oxygen concentration. To investigate the oxygen influence on the Al electrical activity, SIMS analysis of Al and 0 was performed on FZ and CZ samples implanted at 100 MeV with a dose of 3 x 1015/cm2, and where the peak concentration is still below the solid solubility limit. The Al distribution was probed with a primary beam of oxygen, while that of oxygen with a Cs beam. The oxygen profile of the as-implanted CZ sample was quite flat all over the sample. Two 3 X 1015/cm2 Al implanted CZ and FZ samples were simultaneously diffused for 1 h at 1200°C and analyzed (figs. 3a and 3b, respectively). The SIMS analysis indicates the presence of a multipeak structure located around the depth of the projected Al range for both aluminum and oxygen atoms. This structure is more pronounced in the CZ sample but it is also present in the FZ substrate. The oxygen
1020
111
profile follows perfectly the Al distribution (in this region) and the Al atoms are not electrically active as shown by the superimposed carrier distributions (dashed lines). However while in the FZ sample the difference between the chemical and the electrically active dose is only about 2%, in the CZ sample the carrier profile corresponds to only 40% of the implanted dose. From the SIMS oxygen spectra we note in the inactive region an abrupt increase in the 0 concentration over the bulk level and a depletion before and after this region. It must be noted that the region of Al and 0 accumulation is located around the projected range where the maximum of the nuclear energy deposition occurs. Thus, we believe that the multi-peak structure with a large amount of inactive Al is due to the formation of Al-0 complexes, where the implantation damage acts as a nucleation center, or as a catalyst. Considering only the inactive Al (from the SIMS and the SR profiles) it is possible to observe that the ratio between Al and 0 is about one to one. We suggest that radiation damage reduces the kinetic barrier for the formation of Al-0 complexes by facilitating a large number of broken or dangling bonds. As soon as some complexes are formed, they may act as gettering sites for neighbouring Al and 0 atoms, thus depleting the surroundings regions and giving rise to the multipeak structure. The free energy of the Al-0 bonding is lower than that of Si-0, thus this reduction is the driving force for the process. During annealing, two competitive phenomena occur: diffusion of aluminum and precipitation in inactive complexes. The formation of complexes reduces the amount of Al available for the diffusion so that, in FZ material the distribution is broader than in CZ samples. Transmission electron analysis, not reported here, shows the presence of extended defects, like dislocation loops, at the (d E/dx) peak. These defects may act as nucleation centers of the Al-0 complexes. The amount of the defects is
I ”
;““,““,““I””
FZ
1015
-
”
I ”
“I”’ (b)
40 50 60 20 30 40 50 60 Depth km1 Depth km1 Fig. 3. Secondary Ion Mass Spectroscopy analysis of Al and 0 in the diffused (12Oo”C,1 h) CZ (a) and FZ (b) samples. The Al implantation was performed at 100 MeV with a dose of 3 X 101*/cm2. The dashed lines represent the hole profiles. 20
30
IV. MATERIALS SCIENCE
112
A. La Ferla et al. / High energy implants of Al in Si 1017 m‘@
1016
-
1015
I’ 7 ’ 77 ’ 7 ’ ’ ”
z’
3
is no implantation
dam-
4. Conclusions
g 3 1014 2 +d E 1013 3
active component since there age in these cases.
1012 1011
0
5
10 Depth
15
20
25
30
[pm]
Fig. 4. Electrical carrier profiles of CZ and FZ grown Si substrates diffused at 1250°C for 2 h with Al source in a sealed ampoule.
larger in CZ than in FZ substrates evidencing an interplay between the formation of complexes and the growth of dislocation. At this stage of the experimental work, only a qualitative discussion of the results is possible. Of course, a detailed investigation is required to understand the evolution of the multipeak structure as a function of the annealing temperature and time. The damage seems to play a fundamental role in the Al-O nucleation. To verify this point, the following experiment has been performed. Two FZ and CZ grown Si substrates were Al doped by furnace predeposition and drive-in diffusion process in a sealed tube under an Ar ambient at a temperature of 1250°C for 2 h. The electrical profiles of these samples are reported in fig. 3; the hole distributions coincide in the two samples up to the junction depth, which is different as a result of the different resistivity of the substrates. The different oxygen bulk concentration does not appear to influence the distribution of the electrically
We have demonstrated, for Al doping by implantation, a strong dependence of the amount of electrically active Al on the oxygen concentration level in Si, even for Al concentrations below the solid solubility limit. It is suggested that the defects induced by ion implantation act as nucleation centers for the formation of inactive A-0 complexes. The inactive Al region is formed around the peak of the disorder distribution curve and grows laterally with annealing time. The Al-O complexes appear stable throughout a thermal process at 1200°C for 9 h.
References 111B.J. Baliga, Modern Power Devices (Wiley, New York, 1987) p. 29. Dl E. Halder, P. Roggwiller and J. Gobrecht, Phys. Scripta 39 (1989) 406. 131P. Bruesch, E. Halder, P. Kluge, J. Rhyner, P. Roggwiller, Th. Stockmeier, F. Stucki and H.J. Wiesmann, J. Appl. Phys. 68 (1990) 2226. [41 H.R. Chang, N. Lewis, GA. Smith, E.L. Hall and V.A.K. Temple, J. Electrochem. Sot. 13.5(1988) 252. [51 D.K. Sadana, M.H. Norcott, R.G. Wilson and U. Dahmen, Appl. Phys. Lett. 49 (1986) 1169. 161R.G. Wilson, J. Appl. Phys. 61 (1987) 933. [71 E.L. Mathe’ and J.C. Desoyer, Phys. Stat. Sol. A 112 (1989) 511. 181S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1985) p. 69. [91 A. La Ferla, E. Rimini, A. Camera, A. Gasparotto, G. Ciavola and G. Ferla, Nucl. Instr. Meth. B55 (1991) 561.