Formation, Evolution And Thermal Stability Of Interstitial
Clusters In Ion Implanted c-Si
Sebania Libertino
CNR-IMM – Sez. Catania, Stradale Primosole 50, I-95121, Catania, Italy
Abstract. The increasing knowledge on the evolution, upon annealing, of the defects generated during ion implantation
in crystalline Si helps scientists in the understanding and modelling many phenomena such as transient enhanced
diffusion of dopants an extended defect evolution. Nevertheless, it is not fully clear how point-like defects agglomerate
forming defect clusters and how they evolve into extended defects. Aim of this work is to provide an interpretation of
damage evolution in ion implanted Si using optical, electrical and structural measurements. Low temperature (300500°C) annealing, causes the formation of I-type point-like defects. Annealing at intermediate temperatures (550-650°C)
produces the formation of I-clusters, experimentally identified observing the effects of the lattice induced strain. High
temperatures cause the I-cluster transition to {311} defects. It takes place only if the I supersaturation exceeds a certain
value (implantation doses ≥1x1013Si/cm2 in pure Si). Moreover, {311} form only after annealing at T≥650°C, thus
showing the existence of a temperature threshold. These results suggest the presence of a strong structural rearrangement
during the transition.
diffusion vehicles mediating the transport of matters in
Si [2]. Both, the final dopant distribution and the
defects configuration strongly depend on the thermal
budget provided to the sample. For a light ion, such as
Si, implanted in Si, the residual damage depends on
the implantation dose and annealing temperature. A
schematic summarizing our current knowledge of
residual damage structure after a MeV Si ion
implantation in Si is plotted in Fig. 1. The implantation
dose–annealing temperature graph is divided in
regions, and each of them is labeled according to the
dominant defect type in that domain. Of course, the
positions of the borders are only tentative. Si low dose
implants, ≤1x1010/cm2, and annealing up to ~ 300 °C,
form point-like defects. Their properties have been
widely studied with deep level transient spectroscopy
(DLTS) [3] and Photoluminescence (PL) [4]. For
annealing in the range 300-550 °C and implantation
doses of 109-1011/cm2 the defects in trap form. They
are second order point defects [3]. A further increase
in temperature will cause their dissociation and the
lattice is fully recovered. The borders of this region
strongly depend on the sample impurity content, that
determines the annealing temperature. Implantation
doses ≥ 1x1013/cm2, produce amorphous pockets or
INTRODUCTION
When an energy as high as ~ 5 eV is transferred to
a Si atom, it is displaced from its lattice position
creating a vacant site (vacancy, V) while it will occupy
an interstitial position (Si self-interstitial I) in the
lattice. V and I are highly mobile with an estimated
activation energy for migration of ~ 0.3eV and ∼ 1eV,
respectively [1]. The wide use of ion implantation as
method of choice to dope Si in the semiconductor
industry, has renewed the scientific interest in the
defect study, since, the stringent requirements in the
miniaturization of Si devices, need a complete control
of the processing steps. Once generated, defects
migrate until they either recombine, according to
relation I+V=∅, or are stored in stable defect
complexes. They strongly interact among themselves,
with impurities (e.g. C and O) and dopant atoms (e.g.
B, P) of the substrate, forming point-like defects stable
at room temperature (RT). These defects have been
widely characterized and all of them anneal at
temperatures below 450 °C, releasing the I and V they
stored. Upon annealing, the I and V will interact
among themselves and with the dopant atoms. In fact,
dopant migration is assisted by V or I which act as
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continuous amorphous layers in the as implanted. For
annealing at T ≥ 600 °C extended defects, such as
{311} defects or dislocations form. These defects have
been widely studied with techniques for structural
characterization like transmission electron microscopy
(TEM). The lattice reconstruction around them makes
such defects very stable in temperature and they
represent, therefore, a big problem in the device
fabrication. Although extremely powerful, the TEM
fails to monitor small defect clusters. On the other
hand, the high implantation doses (>1013cm-2) used in
device fabrication are outside the domain usually
explored by DLTS and PL. Therefore, it is extremely
difficult to link the results of the different techniques
into a coherent picture. Due to the lack of experimental
techniques to detect such defects, there is a wide
region labeled as Defect clusters, where only recently
experimental data [5] and simulations [6] have been
provided. They are formed after implantation to doses
≥ 1x1012 Si/cm2 and annealing temperatures ≥ 550 °C.
Their study will help in the comprehension of the
damage evolution path from point to extended defects.
FIGURE 2. Total concentration of I and V stored in defects,
as a function of annealing temperature for epi Si implanted
with 1.2 MeV Si, 1x109/cm2, I („) V (V), and electron
irradiated with 3.5x1015/cm2, 9.2 MeV, I (.) and V (◊).The
lines refers to ion implantation generated defects.
The concentration of both I and V stored into the
defects, is given for ion implanted (filled symbols) and
electron irradiated samples (open symbols). The right
hand scale of the plot is the number of defects per
implanted ion, N. Although each ion produces ~ 2500
Frenkel pairs (as calculated by simulations), only ∼ 60
per ion escape recombination and form RT stable
defects. This result is in perfect agreement with data
showing that long range defect migration and
recombination [8] occurs already at RT if the sample
impurity content is low, like in epi Si. Annealing at
temperatures up to 300 °C produces a concomitant
reduction of I and V-type defects. The defect released
upon dissociation will anneal other type defects (e.g. I
anneal V-type defects and vice-versa). This process
results in defect recombination in the bulk thus
maintaining the balance between I and V. Indeed, an
equal number of V-type and I-type defects remain
although their absolute concentrations have been
reduced by approximately one order of magnitude. For
T>300 °C all of the V-type defects are annealed out,
but a measurable number of I (~ 2-3 per implanted ion)
is still detectable. This unbalance between I- and Vtype defects is due to the extra incorporated ion which
only becomes experimentally detectable when most of
the defects have recombined. An identical annealing
behavior for T<300°C is observed for electron
irradiated samples, as shown in figure. The imbalance
in the two concentrations observed for ion
implantation is not detected in this case. The results
clearly demonstrate that the imbalance between V and
I in ion implanted samples has to be attributed to the
extra ion introduced in the implantation process,
confirming, at low doses, the “+1” phenomenological
model used to describe the transient enhanced
FIGURE 1. Schematic showing the principal damage
features as a function of the ion dose and annealing
temperature for MeV Si self-implantation.
SECONDARY–DEFECTS
To study the defect evolution Si was implanted in
Si in the dose range 1x108-5x1013cm-2 and monitored
by DLTS, PL and TEM after annealing from RT to
800 °C. The first step was to monitor, by DLTS, the
evolution of both I-type and V-type point defects upon
annealing from 100°C to 600°C in Si-implanted and
electron irradiated epitaxial (epi) Si samples. DLTS
allows one to determine the concentration of V-type
and I-type electrically active defects. Hence, using the
counting procedure described elsewhere [7] the
number of I and V was monitored as a function of the
annealing and the results are plotted in Fig. 2.
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The PL spectra of a sample implanted to 1013
Si/cm2 annealed at 600 °C for 30 min (dashed line) and
of a sample implanted to 1011Si/cm2 annealed at 400
°C 30 min (solid line) are compared in Fig. 4. The last
sample exhibits PL lines belonging to I-type point-like
defects. The W line, associated to small I-rich clusters,
at 1218 nm (1,018 eV) is the dominant center [4]. The
shoulder observed at 1279 nm (0.969 eV) is the G-line.
Quite different is the PL of the 1013Si/cm2 sample. It
shows two broad peaks centered at 1320 nm (0.94 eV)
and 1390 nm (0.89 eV). They are broad luminescence
bands and do not arise from the convolution of
narrower peaks [10]. Several sharp lines in the range
1200-1280 nm are superimposed to these broad peaks.
All of them are associated to point-like defect and
defect-impurity complexes: the W’ at 1233 nm (1.0048
eV) which is a perturbed form of the W line; the C line
at 1570 nm (0.789 eV). Moreover, two peaks at 1620
nm (0.765 eV) and 1660 nm (0.7466 eV) associated to
O thermal donors were detected.
diffusion (TED) at high dose [9]. The I-type defects
store the I “excess” caused by the presence of the extra
ion. Their full annealing is achieved for T< 600 °C.
I-CLUSTERS
As mentioned in the introduction, the more
interesting area to monitor is the region labelled in Fig.
1 as defect in cluster region. The DLTS analysis of the
residual damage after implants ≥ 1x1012 Si/cm2, show
that the same class of defects is formed regardless of
the annealing conditions in this case. In particular, the
DLTS spectrum of a sample implanted with 145 keV
Si to a dose of 2x1013 cm-2 on Czochralski Si (CZ,
[O]~ [C] 1018cm-3, 5x1016 B/cm3) and annealed at 680
°C 1h (solid line) and of a sample implanted with 1.2
MeV Si to a dose of 1012cm-2 on epi Si ([O]~ [C]
<1016, 1015B/cm3) and annealed at 600 °C 30 min
(dashed line, multiplied by 10) are compared in Fig. 3.
The residual damage is mainly given by two
signatures, with activation energies of EV + 0.33 eV
and EV + 0.52 eV and labeled as B1 and B2,
respectively. The same kind of defects was detected
regardless of the implantation energy, dose, impurity
and dopant content of the sample. Moreover,
differently from low dose residual damage, the
annealing does not determine the final defect
characteristics. The two samples underwent treatments
different in temperature and time. These results and
more extensive studies [5] show that neither impurities
nor the dopant are the main constituents of these
defects, confirming their nature of Si I-clusters.
FIGURE 4. PL spectra of Si implanted with 1.2 MeV Si to a
dose of 1011 cm-2 annealed at 400°C 30 min (solid line), 1012
cm-2 annealed at 600°C 30 min (dashed line) and 2x1013 cm-2
annealed at 680°C 1h (dot-dashed line)
DLTS and PL measurements show the presence of
a stable class of damage not depending on the impurity
or dopant concentration or type. The measurements,
carried out in the dose range 1012–2x1013 cm-2, for T in
the range 550-750 °C and times from 10 min up to 15h
show the same signatures. Finally, TEM analyses do
not show the formation of extended defects in these
samples. Broad features in PL spectra were associated
to the quantum confinement of carriers in regions with
the high strain region surrounding the defects. Since
no extended defects are detected in our samples, we
believe that these signatures are associated to the
carrier recombination in the strained region
surrounding the I-clusters embedded in the Si matrix
[5]. Also the cluster electrical signatures exhibit a
broadening that could be associated to the same cause.
FIGURE 3. DLTS spectra measured on p-type CZ Si
implanted with 145 keV Si to a dose of 2x1013 cm-2 and
annealed at 680°C 1h (solid line); epi Si implanted with 1.2
MeV Si to a dose of 1012 cm-2 and annealed at 600°C 30 min
(dashed line, multiplied by a factor of 10).
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(dot-dashed line) and annealed at 680 °C 1h is plotted
in Fig. 4. The lower dose sample (dashed line, see
before) shown in figure exhibits the typical I-clusters
features. When {311} form (detected by TEM) a sharp
peak at 1376 nm (0.9007 eV) dominates the spectrum.
This line is associated to optical transition occurring at
or close to these defects. PL measurements turned out
to be extremely powerful in observing the transition
from I-cluster to {311} defects.
I-Clusters Dissociation
In order to verify if the I-clusters might be the
source of TED at low dose, we studied their annealing
behavior, calculating their dissociation energy. The
data for a given temperature at different annealing
times allows one to obtain the characteristic time τ0 at
that temperature. The τ0 values are summarized in the
Arrhenius plot shown in Fig. 5 for both B1 () and B2
(). The best fits of the data are plotted in the figure as
a solid and dashed line for B1 and B2, respectively. The
slope of the fits provides an estimation of the
activation energy for dissociation. The same energy
value, ~ 2.3 eV, within the experimental errors, was
obtained for B1 and B2. It is consistent with the TED
characteristic energy value in absence of extended
defects [11]. In addition, the B-line dissociation was
monitored as a function of the implantation dose.
Increasing the dose, the time needed for cluster
dissociation increases. Since a higher dose implies a
higher I supersaturation, the results suggest that bigger
cluster, more stable in temperature, are formed.
Since the sample impurity content is known to play
a role on the extended defects formation, implantation
at doses of 1x1013 Si/cm2 in pure Si and annealing at
680 °C 1h were performed. PL measurements show
the formation of {311}, as observed by the presence of
their sharp peak at 1376nm. This result is in agreement
with the literature data, confirming the lowering of the
threshold dose for {311} formation in highly pure
materials [9]. This has been attributed to the C
efficiency in storing I thus preventing their clustering
and the extended defects formation. More interesting is
the study of {311} defects formation as a function of
annealing. Si samples were implanted to doses of 1012
cm-2 (dotted line), 1013 cm-2 (solid line), 2x1013 cm-2
(dashed line) and 5x1013cm-2 (dot-dashed line) and
annealed at 600°C for times up to 15h. The PL spectra
of samples annealed at 600°C 4h are compared in Fig.
6. Although the intensity of the spectra increases with
ion dose, the major features remain unchanged. TEM
analyses performed on these samples revealed that no
extended defects are formed even at the highest dose,
as shown in the inset. Only a weak contrast at the
projected range (at ~ 1.35 µm from the surface) is
observed, showing the presence of a heavily damaged
region, probably consisting of small defect aggregates
with a large strain.
FIGURE 5. Arrhenius plot of the B1 ( ) and B2 (Ο)
characteristic times. The solid lines are linear fits of the data.
TRANSITION FROM I-CLUSTERS TO
EXTENDED DEFECTS
At high implantation doses, ≥ 5x1013 Si/cm2, and
annealing temperatures ≥ 680°C extended defects form
(extended defect region in Fig. 1), starting from {311}
rod like defects, which we characterized by DLTS and
PL [5]. DLTS measurements showed an electrical
signature at EV+0.50 eV when {311} are detected in
the sample. Also PL measurements show major
modifications in the optical properties of the residual
damage when {311} defects are present. The spectrum
of a sample implanted with Si to a dose of 2x1013 cm-2
FIGURE 6. PL spectra on samples implanted with 1.2 MeV
Si to doses of 1012/cm2 (dotted line), 1013/cm2 (solid line),
2×1013/cm2 (dashed line) and 5×1013/cm2 (dash-dot-dashed
line) and annealed at 600 °C for 4 h. In the inset, TEM cross
section of the last sample.
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small clusters in storing the I excess. Finally,
thresholds for both ion dose (1x1013/cm2) and
annealing temperature (600 °C) exist for the transition
from I-cluster to {311} defects due to a structural
transformation accompanying the defects growth.
DLTS measurements on the same samples only
reveal the two I-clusters characteristic signatures. The
data clearly show the presence of a threshold
temperature. At T ≤ 600 °C regardless of ion dose,
{311} defects are not formed at all: the I stored into
small clusters are eventually released and anneal out,
probably at the surface. The results reported, indicate
that major transformations in the optical, electrical and
structural properties of Si occur when {311} are
formed. It is therefore tempting to speculate that the
early stage of I nucleation produce small I-clusters
whose structures significantly differ from those of the
planar {311} defects. A transition between the two
structures requires a morphological transformation. T
low temperature annealing, the probability to
overcome this nucleation barrier is small and most of
the I will remain stored in the I-clusters until their
annealing occur. These clusters dominate the optical,
electrical and structural properties. On the other hand,
when temperature is increased and dose is high enough
to provide large I-clusters, the probability of transition
is very large and {311} defects are soon formed. These
results show that the I-clusters evolution into extended
defects cannot occur through a simple Ostwald
Ripening mechanism, in perfect agreement with recent
Monte Carlo simulations on lattice [6] and the “magic
numbers” model recently proposed [12]. According to
such model, a very stable I-clusters configuration (with
about 8 I) exist, hence a large potential barrier must be
overcome before the OR can take place. We believe
that the “magic number” found arises from the fact
that at a certain size the clusters must undergo a
structural transformation in order to grow bigger. Only
after this transformation occurs the “traditional” OR
takes place and the extended defect regime is entered.
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In this paper experimental data on formation,
evolution and annealing of I-cluster are shown. Low
dose implants, 109-1011 Si/cm2, result in the formation
of I-related second order point-defects that dissociate
at low temperatures (≤ 550 °C). Increasing the dose,
1012-1013 Si/cm2, and the annealing temperature (550700 °C) I-clusters form. They exhibit both electrical
and optical signatures: two DLTS peaks at EV+0.33eV
and EV+0.52eV, and two broad features centered at
1320nm (0.94eV) and 1390nm (0.89eV) in PL. Their
dissociation energy, ~ 2.3 eV, is in good agreement
with those observed for low dose TED. At high doses,
≥ 5x1013Si/cm2, and annealing temperatures, ≥ 680°C
1h, {311} extended defects form. They exhibit an
electrical signature, at EV+0.50 eV, and an optical
signature at 1376 nm. Extended defects compete with
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