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A DLTS study of defects formed in silicon during ion beam mixing
234
Nuclear
A DLTS STUDY OF DEFECTS F.D. AURET
a), J.B. MALHERBE
FORMED
Instruments
and Methods
in Physics
Research B35 (1988) 234-237 North-Holland, Amsterdam
IN SILICON DURING ION BEAM MIXING
b), M. NEL a) and G. MYBURG
b,
‘) Physics Department, University of Port Elizabeth, PO Box, 1600, Port Elizabeth, 6000, Republic of South Africa h, Physics Department, University of Pretoria, Pretoria, Republic of South Africa
Ion beam mixing in Schottky barrier diodes (SBDs), achieved by implanting ions through the metal gate of the SBD, has been shown to cause substantial changes in its electrical properties. During the implantation structural damage is caused, both in the metal gate and in the Si substrate. In the latter this damage gives rise to electrically active defects. We report here on these defects caused by implanting 100 keV Si+ ions through 400 A thick Ni Schottky barrier diodes on n- and p-Si with doses ranging between lo’* and lOi cm-*. The results obtained using deep level transient spectroscopy (DLTS) showed the presence of several implantation-induced defects. For some of them the DLTS “signatures” correspond to those of defects caused by high energy (1 Mev) electron irradiation. Further, IV and CV measurements in conjunction with isochronal annealing revealed a definite trend between the properties of the Schottky barrier diodes and the deep level defects caused by ion beam mixing.
2. Experimental procedure
1. Introduction
It is well known that particle implantation
into semiconductors causes structural damage which in turn results in electrically active defects. In most cases these defects (commonly referred to as traps) adversely affect the electrical properties of semiconductor devices, but in some cases they may also have a beneficial effect on device parameters. Recently ion implantation through the Ni/n-Si contacts has been studied in detail [l], with the emphasis on the Schottky barrier diode (SBD) properties as a function of implantation dose and postimplantation annealing. The contacts were analysed using Auger electron spectroscopy, Rutherford backscattering and a-particle channeling [l]. The results showed that after implantation, as well as after subsequent annealing, changes were observed in the barrier height &,, series resistance R, and ideality factor n. These changes in the SBD characteristics were attributed to the ion beam (IB) mixing between the Ni and Si in the substrate, amorphization of the substrate and the creation of energy states in the bandgap of Si [1,2]. This article reports on the electrical properties studied by means of deep level transient spectroscopy (DLTS) [3] of the defects in the Si below the Ni contacts after 100 keV Si+ implantation through the contacts. The SBD properties Gb, R, and n as well as the IB-induced defects in the substrate were also studied for these samples as a function of isochronal -annealing at temperatures of up to 450 o C. The results show that there is a relationship between the deep level defects caused by ion beam mixing and the SBD and substrate properties. 0168-583X/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
The experimental procedure consisted of four steps: Schottky barrier diode (SBD) fabrication, implantation through the SBDs, post-implantation isochronal annealing and electrical evaluation of the SBD and substrate material after each processing step. This electrical evaluation consisted of the determination of the Schottky barrier height, ideality factor and series resistance of the SBD using IV measurements. CV measurements yielded the free carrier concentration of the substrate as a function of depth below the SBD, and DLTS was used to detect deep level defects (traps) in the substrate material. SBDs were fabricated on n- (No - NA = 3 X 1Or5 cm-3) and p-type (NA - No = 1 X 1015 cme3) Czochralski-grown Si by resistively evaporating 400 A thick circular Ni contacts of 0.52 mm diameter on the Si through a metal contact mask. Some of these SBDs were implanted with Si+ at an energy of 100 keV and doses of lo’*, 10i3, 1014, 1015 and 1016 cm-*. The samples (implanted and unimplanted) were isochronally annealed in Ar for 20 ruin periods at temperatures of up to 450 o c.
3. Results and discussion 3.1. SBD properties In general, a large
mately barrier
implantation R,,
(without
annealing)
caused
while & remained approxiconstant. Figs. 1 and 2 show the IV-measured height +b and series resistance R,, respectively, increase
in
235
F.D. A vet et al. / Defects formed in Si during ion beam mixing BARRIER
HEIGHT
(eb 1 vs ANNEAL
TEMPERATURE
e
lr
I
L/J
0.72.
2 -
.z
-
0
P ;:
DLTS
SPECTRA
(a)
Unannealed
(b)
450
‘C
FOR n-Si
(20
Ni
SBD’s
min.)
0.64.
5 w I
0.60.
k
0.56.
c
Q---J-IE,
2 /cm 2
--1E
14/m:
-1
E
/cm2
2 m
v
1
HZ\
,
0
100
,.““‘.““,“.‘..,..,,I,,,.’
0
100
200
TEMPERATURE
Fig. 1. implanted with
100
keV Si+ ions
at the indicated doses.
as a function of annealing temperature. For the sake of clarity the results for the 1013 and lOi cm-* doses were omitted in figs. 1 and 2. From fig. 1 it is evident that the general trend is the same for all the doses. For n-Si SBDs, +,, increases upon annealing, reaches a maximum at 350°C and then decreases. For samples implanted at 10’4-1016 cm-* these maximum & values are almost identical. Within this dose range the &, value after implantation is 0.59 eV, and after annealing at 350 o C and 400 o C changes to 0.72 eV and 0.68 eV, respectively. For comparison the annealing curves for unimplanted samples is included, and it is obvious that it does not follow the same trend as those for implanted samples. The R, values of n-Si showed a sharp increase after annealing up to 450 QC. These increases ranged between 1 and 2 orders of magnitude, depending on the dose, reaching a maximum value of 320 kQ for the 1016 cm-* doses at 450°C. Note that the R, values of unimplanted samples also increase with annealing temperature, but at 450° C it is still more than an order of
SERIES
RESISTANCE
($1
“s ANNEAL
TEMPERATURE
Fig. 3. Typical DLTS spectra of n-Si recorded at 46 Hz (a) after 100 keV Si+ implantation at a dose of 1 x 1015 cm-’ and (b) after annealing this SBD at 450°C for 20 min. Note that the spectrum to the left of 150 K has been enlarged by a factor of 10, and that the base line offsets were adjusted.
magnitude lower than that of the lowest implanted dose (lo’* cm-* ). In the case of implanted p-Si, R, remained constant within experimental errors at all anneal temperatures. 3.2. Substrate and deject properties The DLTS analyses of unimplanted n- and p-Si showed that Si substrates contained defects only in very low concentrations (< lo’* cme3). However, after the Sic implantation through the Ni SBDs several electron and hole defects were detected in the n- and p-Si respectively. Accurate DLTS depth profiling using small (- 0.2-0.5 V) pulses superimposed on a varying reverse bias was not possible, since at the relatively low carrier densities of these samples not all the defects could be filled completely. A filling pulse larger than the flatband forward bias was required to fill these defects. Therefore, the exact defect distribution below the SBDs could not be obtained. However, the qualitative trend observed revealed that before annealing at 300°C the defects were all located close (< 0.5 urn) to the metal-Si junction. This is to be expected since the channeling measurements [l], which have a much better depth resolution than DLTS, revealed that the damaged region extended to approximately 800 A beneath the interface. Figs. 3 and 4 show typical DLTS spectra for 1 X 10” cme3 implanted n- and p-Si respectively. From fig. 3, curve unannealed
(a), it is evident n-Si are El-E4.
that the main defects
100
200 TEMPERATURE
300
400
500
(“Cl
Fig. 2. Series resistance R, as a function of annealing temperature for 100 keV Si+ implanted devices at different doses.
samples are H5 and H6.
Because
the H6 peak could not be satisfactorily
voluted
from
that
of the
could not be determined. are summarized Identification comparing
their
in
For p-Si (fig. 4, curve (a))
the main defects in unannealed
0
300
(K)
H5,
its DLTS
The properties
decon-
“signature”
of these defects
in table 1. of the defects DLTS
was
signatures
accomplished (energy
level
by and
I. RANGES, DAMAGE AND MIXING
F. D. A wet et al. / Defects formed in Si during ion beam mixing
DLTS
SPECTRA
FOR
p-Si
Ni SBD’s
H2 \ (a)
Unannealed
(b)
450
‘C
(20
d ot.~..............“‘.““““‘~ 0
100
200
TEMPERATURE
min.)
300
(K)
Fig. 4. Typical DLTS spectra of p-Si recorded at 46 Hz (a) after 100 keV Si+ implantation at a dose of 1 X 10” cm-* and (b) after annealing this SBD at 450 .a C for 20 min.
capture cross section) [3] to those of defects formed during high energy electron irradiation, a process which is known to form point defects and complexes thereof with impurities such as C, 0 and various dopants [4]. From the literature it is clear that the DLTS signatures of most of the defects caused by IB mixing, observed in this study, closely correspond to those of defects formed during high energy electron irradiation. However, it should be noted that some of the defects observed in this study were not present when using 1 MeV electrons. The 1 MeV defects have been studied in great detail and most of them have been positively identified, a summary of which is given in refs. [5,6]. In table 1 the observed defects are correlated with high energy electron irradiation induced defects. Isochronal annealing for 20 min periods of the IBmixed samples showed that the concentrations of all defects changed with annealing. In the case of n-Si the main defect E4 as well as the other defects were all
Table 1 Properties of defects detected in IB-mixed strates Label
n- and p-Si sub-
after implantation
WI
Energy
a) orna, (x lo-r6
T46 b, [K]
Possible origin
El E2 E3 E4
0.14 0.18 0.23 0.39
15 I 3 1
13 91 128 214
1 2 5
50 95 201 221
unidentified V-O complex [4] divacancy [8] single charged divacancy [81 Ni related? Ni related [9] v-o-c [4] -
Hl H2 H5 H6
0.10 0.15 0.34 -
cm*)
a) Defect energies of electron traps below the conduction band and those of the hole traps above the valence band. b, Peak temperatures measured at a lock-in amplifier frequency of 46 Hz.
annealed out at 400” C. No other electron traps were formed in the process. However, the DLTS technique employing minority carrier injection [7] (fig. 3, curve (b)) showed the presence of the H2 hole trap (also observed in the p-Si as will be discussed below). This clearly indicates the H2 defect is not a p-dopant complex, which could only be present in the p-Si. For p-Si the initial stages of annealing followed the same qualitative trend as those for 1 MeV electron irradiated p-Si [6]. Above 300 o C, however, the situation changed drastically. Two new hole defects, Hl and H2, which act as acceptors, appeared and reached concentrations of at least 1 x 1015 cmm3 and 5 x 1015 cmp3 respectively. Because these defects are not seen in electron irradiated Si, we speculate that they are presumably Ni-related. Due to its poor depth resolution these DLTS measured defect concentrations represent the average concentrations of the defects in the first few urn below the junction. Therefore, it is reasonable to expect that their concentrations should reach maximum values close to the junction, where most of the Ni-related damage is caused [l]. This argument was substantiated by CV profiling on p-Si SBDs, where a carrier concentration of 2 X lOi cme3 was reached within the first 0.5 l.trn from the interface. This profile showed that the concentration gradually returned to normal within l-2 urn from the interface. As mentioned above the same defect H2 could also be detected in the n-Si SBDs after using a minority carrier injection technique [7]. Further, it is instructive to note that after annealing the p-SBDs an electron trap seems to be present over approximately the same temperature range as the E4 in n-Si. Due to the close proximity of the hole traps HS and H6, its DLTS characteristics could not be measured. It is, however, unlikely that it is the E4, because in the n-Si the E4 was absent after the 450 o C annealing step. This in turn implies that this defect in the annealed p-Si has to be a complex involving impurities (or dopants) that are present in the p-, but not in the n-Si. The increase in @,, of n-Si SBDs with increasing annealing temperature can possibly be explained by the DLTS results. The H2 defect which starts to appear above 300 ’ C acts as a p-dopant. This can lead to the formation of a local p-n junction in n-Si, which in turn leads to an increase in +b [lo]. The formation of such a p-n junction is only possible if the concentration of the H2 defect exceeds that of the n-dopant (3 x lOI cmp3). As mentioned above, CV profiling on the p-Si showed that this was indeed the case within the first 0.5 urn from the Ni-Si interface. The DLTS data for annealed p-Si SBDs can also possibly explain the high resistivities of the n-Si devices. In particular, it was found that the concentration of the H5 defect increased in magnitude after the 200 o C annealing step. Since such an increase causes a diminished free carrier concentration due to electron step-
F. D. A wet et al. / Defects formed in Si during ion beam mixing
in turn causes an increase in R, above 250 o C for all implantation doses. Although the H5 diminished in magnitude at temperatures above 350°C two new defects (H2 at 300 o C and Hl at 400 o C) appeared. The magnitude of the H2 defect concentration increased with increasing annealing temperature to exceed that of the H5 concentration above 400°C annealing. This explains the sharp increase in R, in n-Si at annealing temperatures in the region 350-450 o C because the H2 centre acts as a compensation centre.
4. Conclusions Three main conclusions can be drawn from this study. Firstly, the damage caused by ion beam mixing in the region below the Ni SBD contained electrically active defects. Some of these defects have energies and capture cross sections and an annealing behaviour which are, within the experimental error, the same as those created by high energy (1 Mev) electron irradiation. These defects are point defects and complexes of those defects with impurity atoms. They can be caused either by incident Si’ + ions that penetrate through the Ni contacts, or by Ni atoms IB-mixed in the Si. Secondly, the Si implantation led to modifications in the SBD properties. The barrier height of n-Si devices increased with increasing annealing temperature as well as implantation dose. This, however, was at the expense of an increased series resistance. It was further established that, provided a high series resistance can be tolerated, the barrier height could be optimized by the choice of an appropriate annealing cycle.
231
Thirdly, after annealing at 350° C, two Ni-related hole traps were detected which acted as acceptors. Their magnitudes were high enough to explain the increased series resistance of the n-Si SBDs in this temperature range, where they act as compensating centers. Their presence may also explain the observed increase in the barrier height.
The financial support of the FRD of the South African CSIR is greatfully acknowledged, and thanks are due to Dr. J.F. Prins for the implantations.
References [l] G. Myburg, J.B. Malherbe and E. Friedland, these Proceedings (IBA ‘88). Nucl. Instr. and Meth. B35 (1988) 451. [2] G. Myburg, J.B. Malherbe and E. Friedland, submitted to Jpn. J. Appl. Phys. [3] D.V. Lang, J. Appl. Phys. 45 (1974) 3023. [4] L.C. Kimerling, Inst. Phys. Conf. 31 (1977) 221. [5] F.D. Auret and P.M. Mooney, J. Appl. Phys. 55 (1984) 988. [6] F.D. Auret and P.M. Mooney, J. Appl. Phys. 55 (1984) 984. [7] F.D. Auret and M. Nel, J. Appl. Phys. 61 (1987) 2546. [8] A.O. Evwaraye and E. Sun, J. Appl. Phys. 47 (1976) 3775. [9] K. Graff and H. Pieper, in: Semiconductor Silicon, eds. H.R. Huff, R.J. Kriegler and Y. Takeishi (1981) p. 331. [lo] T.M. Reith and J.D. Schick, Appl. Phys. Lett. 25 (1974) 524.
I. RANGES, DAMAGE AND MIXING
Nuclear
A DLTS STUDY OF DEFECTS F.D. AURET
a), J.B. MALHERBE
FORMED
Instruments
and Methods
in Physics
Research B35 (1988) 234-237 North-Holland, Amsterdam
IN SILICON DURING ION BEAM MIXING
b), M. NEL a) and G. MYBURG
b,
‘) Physics Department, University of Port Elizabeth, PO Box, 1600, Port Elizabeth, 6000, Republic of South Africa h, Physics Department, University of Pretoria, Pretoria, Republic of South Africa
Ion beam mixing in Schottky barrier diodes (SBDs), achieved by implanting ions through the metal gate of the SBD, has been shown to cause substantial changes in its electrical properties. During the implantation structural damage is caused, both in the metal gate and in the Si substrate. In the latter this damage gives rise to electrically active defects. We report here on these defects caused by implanting 100 keV Si+ ions through 400 A thick Ni Schottky barrier diodes on n- and p-Si with doses ranging between lo’* and lOi cm-*. The results obtained using deep level transient spectroscopy (DLTS) showed the presence of several implantation-induced defects. For some of them the DLTS “signatures” correspond to those of defects caused by high energy (1 Mev) electron irradiation. Further, IV and CV measurements in conjunction with isochronal annealing revealed a definite trend between the properties of the Schottky barrier diodes and the deep level defects caused by ion beam mixing.
2. Experimental procedure
1. Introduction
It is well known that particle implantation
into semiconductors causes structural damage which in turn results in electrically active defects. In most cases these defects (commonly referred to as traps) adversely affect the electrical properties of semiconductor devices, but in some cases they may also have a beneficial effect on device parameters. Recently ion implantation through the Ni/n-Si contacts has been studied in detail [l], with the emphasis on the Schottky barrier diode (SBD) properties as a function of implantation dose and postimplantation annealing. The contacts were analysed using Auger electron spectroscopy, Rutherford backscattering and a-particle channeling [l]. The results showed that after implantation, as well as after subsequent annealing, changes were observed in the barrier height &,, series resistance R, and ideality factor n. These changes in the SBD characteristics were attributed to the ion beam (IB) mixing between the Ni and Si in the substrate, amorphization of the substrate and the creation of energy states in the bandgap of Si [1,2]. This article reports on the electrical properties studied by means of deep level transient spectroscopy (DLTS) [3] of the defects in the Si below the Ni contacts after 100 keV Si+ implantation through the contacts. The SBD properties Gb, R, and n as well as the IB-induced defects in the substrate were also studied for these samples as a function of isochronal -annealing at temperatures of up to 450 o C. The results show that there is a relationship between the deep level defects caused by ion beam mixing and the SBD and substrate properties. 0168-583X/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
The experimental procedure consisted of four steps: Schottky barrier diode (SBD) fabrication, implantation through the SBDs, post-implantation isochronal annealing and electrical evaluation of the SBD and substrate material after each processing step. This electrical evaluation consisted of the determination of the Schottky barrier height, ideality factor and series resistance of the SBD using IV measurements. CV measurements yielded the free carrier concentration of the substrate as a function of depth below the SBD, and DLTS was used to detect deep level defects (traps) in the substrate material. SBDs were fabricated on n- (No - NA = 3 X 1Or5 cm-3) and p-type (NA - No = 1 X 1015 cme3) Czochralski-grown Si by resistively evaporating 400 A thick circular Ni contacts of 0.52 mm diameter on the Si through a metal contact mask. Some of these SBDs were implanted with Si+ at an energy of 100 keV and doses of lo’*, 10i3, 1014, 1015 and 1016 cm-*. The samples (implanted and unimplanted) were isochronally annealed in Ar for 20 ruin periods at temperatures of up to 450 o c.
3. Results and discussion 3.1. SBD properties In general, a large
mately barrier
implantation R,,
(without
annealing)
caused
while & remained approxiconstant. Figs. 1 and 2 show the IV-measured height +b and series resistance R,, respectively, increase
in
235
F.D. A vet et al. / Defects formed in Si during ion beam mixing BARRIER
HEIGHT
(eb 1 vs ANNEAL
TEMPERATURE
e
lr
I
L/J
0.72.
2 -
.z
-
0
P ;:
DLTS
SPECTRA
(a)
Unannealed
(b)
450
‘C
FOR n-Si
(20
Ni
SBD’s
min.)
0.64.
5 w I
0.60.
k
0.56.
c
Q---J-IE,
2 /cm 2
--1E
14/m:
-1
E
/cm2
2 m
v
1
HZ\
,
0
100
,.““‘.““,“.‘..,..,,I,,,.’
0
100
200
TEMPERATURE
Fig. 1. implanted with
100
keV Si+ ions
at the indicated doses.
as a function of annealing temperature. For the sake of clarity the results for the 1013 and lOi cm-* doses were omitted in figs. 1 and 2. From fig. 1 it is evident that the general trend is the same for all the doses. For n-Si SBDs, +,, increases upon annealing, reaches a maximum at 350°C and then decreases. For samples implanted at 10’4-1016 cm-* these maximum & values are almost identical. Within this dose range the &, value after implantation is 0.59 eV, and after annealing at 350 o C and 400 o C changes to 0.72 eV and 0.68 eV, respectively. For comparison the annealing curves for unimplanted samples is included, and it is obvious that it does not follow the same trend as those for implanted samples. The R, values of n-Si showed a sharp increase after annealing up to 450 QC. These increases ranged between 1 and 2 orders of magnitude, depending on the dose, reaching a maximum value of 320 kQ for the 1016 cm-* doses at 450°C. Note that the R, values of unimplanted samples also increase with annealing temperature, but at 450° C it is still more than an order of
SERIES
RESISTANCE
($1
“s ANNEAL
TEMPERATURE
Fig. 3. Typical DLTS spectra of n-Si recorded at 46 Hz (a) after 100 keV Si+ implantation at a dose of 1 x 1015 cm-’ and (b) after annealing this SBD at 450°C for 20 min. Note that the spectrum to the left of 150 K has been enlarged by a factor of 10, and that the base line offsets were adjusted.
magnitude lower than that of the lowest implanted dose (lo’* cm-* ). In the case of implanted p-Si, R, remained constant within experimental errors at all anneal temperatures. 3.2. Substrate and deject properties The DLTS analyses of unimplanted n- and p-Si showed that Si substrates contained defects only in very low concentrations (< lo’* cme3). However, after the Sic implantation through the Ni SBDs several electron and hole defects were detected in the n- and p-Si respectively. Accurate DLTS depth profiling using small (- 0.2-0.5 V) pulses superimposed on a varying reverse bias was not possible, since at the relatively low carrier densities of these samples not all the defects could be filled completely. A filling pulse larger than the flatband forward bias was required to fill these defects. Therefore, the exact defect distribution below the SBDs could not be obtained. However, the qualitative trend observed revealed that before annealing at 300°C the defects were all located close (< 0.5 urn) to the metal-Si junction. This is to be expected since the channeling measurements [l], which have a much better depth resolution than DLTS, revealed that the damaged region extended to approximately 800 A beneath the interface. Figs. 3 and 4 show typical DLTS spectra for 1 X 10” cme3 implanted n- and p-Si respectively. From fig. 3, curve unannealed
(a), it is evident n-Si are El-E4.
that the main defects
100
200 TEMPERATURE
300
400
500
(“Cl
Fig. 2. Series resistance R, as a function of annealing temperature for 100 keV Si+ implanted devices at different doses.
samples are H5 and H6.
Because
the H6 peak could not be satisfactorily
voluted
from
that
of the
could not be determined. are summarized Identification comparing
their
in
For p-Si (fig. 4, curve (a))
the main defects in unannealed
0
300
(K)
H5,
its DLTS
The properties
decon-
“signature”
of these defects
in table 1. of the defects DLTS
was
signatures
accomplished (energy
level
by and
I. RANGES, DAMAGE AND MIXING
F. D. A wet et al. / Defects formed in Si during ion beam mixing
DLTS
SPECTRA
FOR
p-Si
Ni SBD’s
H2 \ (a)
Unannealed
(b)
450
‘C
(20
d ot.~..............“‘.““““‘~ 0
100
200
TEMPERATURE
min.)
300
(K)
Fig. 4. Typical DLTS spectra of p-Si recorded at 46 Hz (a) after 100 keV Si+ implantation at a dose of 1 X 10” cm-* and (b) after annealing this SBD at 450 .a C for 20 min.
capture cross section) [3] to those of defects formed during high energy electron irradiation, a process which is known to form point defects and complexes thereof with impurities such as C, 0 and various dopants [4]. From the literature it is clear that the DLTS signatures of most of the defects caused by IB mixing, observed in this study, closely correspond to those of defects formed during high energy electron irradiation. However, it should be noted that some of the defects observed in this study were not present when using 1 MeV electrons. The 1 MeV defects have been studied in great detail and most of them have been positively identified, a summary of which is given in refs. [5,6]. In table 1 the observed defects are correlated with high energy electron irradiation induced defects. Isochronal annealing for 20 min periods of the IBmixed samples showed that the concentrations of all defects changed with annealing. In the case of n-Si the main defect E4 as well as the other defects were all
Table 1 Properties of defects detected in IB-mixed strates Label
n- and p-Si sub-
after implantation
WI
Energy
a) orna, (x lo-r6
T46 b, [K]
Possible origin
El E2 E3 E4
0.14 0.18 0.23 0.39
15 I 3 1
13 91 128 214
1 2 5
50 95 201 221
unidentified V-O complex [4] divacancy [8] single charged divacancy [81 Ni related? Ni related [9] v-o-c [4] -
Hl H2 H5 H6
0.10 0.15 0.34 -
cm*)
a) Defect energies of electron traps below the conduction band and those of the hole traps above the valence band. b, Peak temperatures measured at a lock-in amplifier frequency of 46 Hz.
annealed out at 400” C. No other electron traps were formed in the process. However, the DLTS technique employing minority carrier injection [7] (fig. 3, curve (b)) showed the presence of the H2 hole trap (also observed in the p-Si as will be discussed below). This clearly indicates the H2 defect is not a p-dopant complex, which could only be present in the p-Si. For p-Si the initial stages of annealing followed the same qualitative trend as those for 1 MeV electron irradiated p-Si [6]. Above 300 o C, however, the situation changed drastically. Two new hole defects, Hl and H2, which act as acceptors, appeared and reached concentrations of at least 1 x 1015 cmm3 and 5 x 1015 cmp3 respectively. Because these defects are not seen in electron irradiated Si, we speculate that they are presumably Ni-related. Due to its poor depth resolution these DLTS measured defect concentrations represent the average concentrations of the defects in the first few urn below the junction. Therefore, it is reasonable to expect that their concentrations should reach maximum values close to the junction, where most of the Ni-related damage is caused [l]. This argument was substantiated by CV profiling on p-Si SBDs, where a carrier concentration of 2 X lOi cme3 was reached within the first 0.5 l.trn from the interface. This profile showed that the concentration gradually returned to normal within l-2 urn from the interface. As mentioned above the same defect H2 could also be detected in the n-Si SBDs after using a minority carrier injection technique [7]. Further, it is instructive to note that after annealing the p-SBDs an electron trap seems to be present over approximately the same temperature range as the E4 in n-Si. Due to the close proximity of the hole traps HS and H6, its DLTS characteristics could not be measured. It is, however, unlikely that it is the E4, because in the n-Si the E4 was absent after the 450 o C annealing step. This in turn implies that this defect in the annealed p-Si has to be a complex involving impurities (or dopants) that are present in the p-, but not in the n-Si. The increase in @,, of n-Si SBDs with increasing annealing temperature can possibly be explained by the DLTS results. The H2 defect which starts to appear above 300 ’ C acts as a p-dopant. This can lead to the formation of a local p-n junction in n-Si, which in turn leads to an increase in +b [lo]. The formation of such a p-n junction is only possible if the concentration of the H2 defect exceeds that of the n-dopant (3 x lOI cmp3). As mentioned above, CV profiling on the p-Si showed that this was indeed the case within the first 0.5 urn from the Ni-Si interface. The DLTS data for annealed p-Si SBDs can also possibly explain the high resistivities of the n-Si devices. In particular, it was found that the concentration of the H5 defect increased in magnitude after the 200 o C annealing step. Since such an increase causes a diminished free carrier concentration due to electron step-
F. D. A wet et al. / Defects formed in Si during ion beam mixing
in turn causes an increase in R, above 250 o C for all implantation doses. Although the H5 diminished in magnitude at temperatures above 350°C two new defects (H2 at 300 o C and Hl at 400 o C) appeared. The magnitude of the H2 defect concentration increased with increasing annealing temperature to exceed that of the H5 concentration above 400°C annealing. This explains the sharp increase in R, in n-Si at annealing temperatures in the region 350-450 o C because the H2 centre acts as a compensation centre.
4. Conclusions Three main conclusions can be drawn from this study. Firstly, the damage caused by ion beam mixing in the region below the Ni SBD contained electrically active defects. Some of these defects have energies and capture cross sections and an annealing behaviour which are, within the experimental error, the same as those created by high energy (1 Mev) electron irradiation. These defects are point defects and complexes of those defects with impurity atoms. They can be caused either by incident Si’ + ions that penetrate through the Ni contacts, or by Ni atoms IB-mixed in the Si. Secondly, the Si implantation led to modifications in the SBD properties. The barrier height of n-Si devices increased with increasing annealing temperature as well as implantation dose. This, however, was at the expense of an increased series resistance. It was further established that, provided a high series resistance can be tolerated, the barrier height could be optimized by the choice of an appropriate annealing cycle.
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Thirdly, after annealing at 350° C, two Ni-related hole traps were detected which acted as acceptors. Their magnitudes were high enough to explain the increased series resistance of the n-Si SBDs in this temperature range, where they act as compensating centers. Their presence may also explain the observed increase in the barrier height.
The financial support of the FRD of the South African CSIR is greatfully acknowledged, and thanks are due to Dr. J.F. Prins for the implantations.
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I. RANGES, DAMAGE AND MIXING