Initial carrier relaxation dynamics in ion-implanted Si nanocrystals: Femtosecond transient absorption study

Article abstract of DOI:10.1063/1.122519

Transient absorption spectra of ion-implanted Si nanocrystals (NCs) exhibit two picosecond photoinduced absorption features, attributed to carriers in NC quantized states (high-energy band) and Si/SiO₂ interface states (low-energy band). Fast relaxation of the high-energy band indicates that populations of quantized states are short lived and decay on the sub-10-ps time scale due to efficient surface trapping. This shows that the red emission in our samples is not due to carriers in quantized states but rather is a result of deactivation of surface traps.

Full text of DOI:10.1063/1.122519

Initial carrier relaxation dynamics in ion-implanted Si nanocrystals: Femtosecond
transient absorption study
V. I. Klimov, Ch. J. Schwarz, D. W. McBranch, and C. W. White
Citation: Applied Physics Letters 73, 2603 (1998); doi: 10.1063/1.122519
View online: http://dx.doi.org/10.1063/1.122519
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APPLIED PHYSICS LETTERS
VOLUME 73, NUMBER 18
2 NOVEMBER 1998
Initial carrier relaxation dynamics in ion-implanted Si nanocrystals:
Femtosecond transient absorption study
V. I. Klimov,a) Ch. J. Schwarz, and D. W. McBranch
Chemical Sciences and Technology Division, CST-6, MS-J585, Los Alamos National Laboratory,
Los Alamos, New Mexico 87544
C. W. White
Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
͑Received 10 April 1998; accepted for publication 31 August 1998͒
Transient absorption spectra of ion-implanted Si nanocrystals ͑NCs͒ exhibit two picosecond
photoinduced absorption features, attributed to carriers in NC quantized states ͑high-energy band͒
and Si/SiO2 interface states ͑low-energy band͒. Fast relaxation of the high-energy band indicates
that populations of quantized states are short lived and decay on the sub-10-ps time scale due to
efficient surface trapping. This shows that the red emission in our samples is not due to carriers in
quantized states but rather is a result of deactivation of surface traps. © 1998 American Institute
of Physics. ͓S0003-6951͑98͒00544-0͔
1͒ and 6ϫ1017 cmϪ2 ͑sample 2͒. After implantation the
Observations of efficient photoluminescence ͑PL͒ from
nanoporous Si1 and Si nanoparticles2 has stimulated exten-
samples were annealed at 1100 °C in flowing Arϩ4% H2 to
sive research into Si nanostructures, promising candidates
for applications in Si-based optoelectronics. In addition to
electrochemical etching, which is used to fabricate nanopo-
rous Si,1 several other techniques have been developed for
preparation of Si nanomaterials, such as plasma deposition
from silane,3 crystallization of amorphous silicon,4 gas py-
rolysis of Si2H6 and O2,5 and ion implantation.2 Despite nu-
merous PL studies, the mechanism for light emission in Si
nanostructures remains controversial. Ion-implanted nanoc-
rystals ͑NCs͒ exhibit a number of PL bands in the visible
spectral range.2,6 Emission from as-implanted samples has
been identified as originating from defects in the SiO2 ma-
trix, generated during ion implantation.2,6 Annealing at tem-
peratures above 1000 °C leads to quenching of the defect-
related PL,2,6 and to the development of a red emission at
1.6–1.7 eV, which is correlated with the formation of a large
number of Si NCs.7,8 This correlation indicates that the red
emission is due to Si NCs, although it remains unclear
whether it originates from quantized states extended over the
NC volume, or from localized surface/interface states.
The mechanisms for light emission can be clarified by
studies of carrier dynamics at early stages after photoexcita-
tion. Time-resolved PL has been applied to probe the slow
␮s–ms recombination dynamics of relaxed carriers.9 Fast
initial carrier relaxation can be studied using femtosecond
͑fs͒ transient absorption ͑TA͒.10 In this letter, we report fs
TA studies of ion-implanted Si NCs which exhibit efficient
red emission. We observe fast sub-10-ps relaxation of the
photoinduced absorption ͑PA͒ associated with populations of
quantized states, indicating efficient carrier trapping at
surface/interface states. These states are most likely respon-
sible for the red emission in our samples.
initiate formation of NCs. Transmission electron microscopy
indicates that the mean NC diameters in the annealed
samples are 2.5–3 nm ͑sample 1͒ and 5–6 nm ͑sample 2͒.7
As implanted, both samples show a defect-related PL at
ϳ1.9 eV, usually attributed to the E center2 or to the non-
Ј
bridging oxygen hole center.6 Annealing at 1100 °C leads to
quenching of the 1.9 eV band and gives rise to a new band
͑red emission͒ at ϳ1.65 eV.
To probe the initial carrier dynamics, we monitored
carrier-induced absorption changes using a high-sensitivity
fs TA experiment in the novel chirp-free configuration.11 The
samples were pumped at 3.1 eV by frequency-doubled
100-fs pulses from an amplified Ti-sapphire laser. The trans-
mission of the photoexcited spot was probed by variably-
delayed pulses of a white light fs continuum. To evaluate the
effect of nonlinear interactions on carrier dynamics, the
pump fluence (wp) was varied over the range
0.1–5.5 mJ cmϪ2 which allowed tuning the carrier density
from Ͻ1 electron-hole ͑e-h͒ pair per NC on average to the
regime of multiple-pair excitations.
Figure 1 shows spectra of the pump-induced absorption
changes (⌬␣d; d is the thickness of the ion-implanted layer͒
of sample 2 recorded for different delay times (⌬t) between
pump and probe pulses. At ⌬tϽ10 ps, the TA spectra ex-
hibit two PA bands with maxima at ϳ2.1 eV (PA1) and be-
low 1.6 eV (PA2). PA1 decays exponentially to a constant
background with an 8-ps relaxation time ͑Fig. 2͒. In addition
to the same 8 ps component, PA2 shows an initial fast com-
ponent with a 1.5 ps relaxation time. The difference in PA1
and PA2 dynamics is less pronounced in sample 1, for which
the PA1 initial relaxation time shortens down to 2.5 ps,
whereas the initial relaxation constant of PA2 remains
ϳ1.5 ps.
Si NCs were made by Siϩ implantation into fused silica
͑ion energy 400 keV͒ with doses of 1.5ϫ1017 cmϪ2 ͑sample
In addition to different dynamics, PA1 and PA2 show
different pump dependence. The PA1 signal grows linearly
with the pump fluence, and the PA1 dynamics remain almost
unchanged over the pump range 0.1–5 mJ cmϪ2. The PA2
a͒
Electronic mail: klimov@lanl.gov
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0003-6951/98/73(18)/2603/3/$15.00 2603 © 1998 American Institute of Physics
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2604
Appl. Phys. Lett., Vol. 73, No. 18, 2 November 1998
Klimov et al.
FIG. 1. Time-resolved chirp-free TA spectra of sample
2 (wp
ϭ5.3 mJ cmϪ2). Inset: Comparison of TA spectra (⌬tϭ4 ps) of samples 1
͑dashed line͒ and 2 ͑solid line͒.
FIG. 3. Pump-intensity dependent dynamics of PA2 ͑1.65 eV; thick lines͒,
fit to a double-exponential decay ͑thin solid lines͒. Inset: Pump dependence
for the fast (Af) and slow (As) components of the PA2 decay, fit to the
dynamics exhibit obvious pump dependence ͑Fig. 3͒, indicat-
ing an increasing relative contribution of the fast component
at high pump intensities. For all pump fluences, the PA2
relaxation can be fit to a double exponential decay with
pump-independent constants of 1.5 and 8 ps. The amplitude
of the slow component (As) grows linearly with the pump
intensity, whereas the growth of the fast component (Af) is
superlinear ͑slope of ϳ1.7) ͑Fig. 3, inset͒, which results in
the apparent intensity-dependent decay of the PA2 feature.
Although PA1 and PA2 have a similar sub-ps rise time,
the buildup dynamics for them are different. PA1 shows an
dependence Af(As)ϰwmp
.
extremely steep rise, with a negative curvature, whereas the
PA2 buildup is slightly delayed, which is reflected in the
positive curvature of the PA2 signal ͑Fig. 2, inset͒.
The difference in PA1 and PA2 dynamics and pump de-
pendence indicates that these bands are due to different spe-
cies. The ultrafast build-up of PA1, along with its linear
pump dependence, suggest that this band is associated with
primary photoexcitations, directly generated by the pump
pulse. On the other hand, the presence of a superlinear fast
component in PA2 and its delayed growth show that this
feature is due to secondary species generated as a result of a
nonlinear excitation process which, we believe, is associated
with reexcitation of the primary species. A high reexcitation
absorption cross section is indicated by TA measured at the
pump spectral energy. The 3.1 eV TA signal is photoinduced
absorption with dynamics which are almost identical to
those of PA1. The assumption about re-excitation as a
mechanism for generation of PA2 allows also a close fit to
the PA2 buildup ͑crosses in the inset to Fig. 2͒. This fit was
obtained by integrating the product of the pump pulse and
the true PA1 growth dynamics, assuming an instrument re-
sponse time of 100 fs.
Although there is no consensus regarding the mecha-
nisms for light emission in Si nanostructures, it is now
widely accepted that the light-absorption process takes place
inside Si nanoclusters, suggesting that the primary photoex-
citations are carriers in quantized states. In our TA data,
these carriers give rise to PA1, identified as due to the pri-
mary species. The PA1 maximum is blueshifted in NCs of
smaller sizes ͑Fig. 1, inset͒, consistent with the increased
spacing between intraband levels resulting from quantum
confinement. The ultrafast buildup of PA1 might seem to
FIG. 2. TA dynamics for sample 2 at 2.2 eV (PA1; open circles͒ and 1.65
eV (PA2; crosses͒, fit to single- and double-exponential decays, respec-
tively. Inset: Short-term dynamics of PA1 ͑dashed line͒ and PA2 ͑solid line͒;
crosses are a fit to the PA2 buildup assuming that PA2 is due to re-excitation
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of primary species ͑dotted line is the pump-probe cross correlation͒.
contradict the prediction of a ‘‘phonon bottleneck’’ in quan-
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Appl. Phys. Lett., Vol. 73, No. 18, 2 November 1998
Klimov et al.
2605
tum confined systems, which should lead to a slowing down
in the energy relaxation.12 However, our recent studies13 in-
dicate the absence of a ‘‘phonon bottleneck’’ in NCs, likely
due to an enhancement of Auger-type energy transfer pro-
cesses.
riers are trapped at the surface. This shows that even in small
Si NCs (ϳ2.5 nm diameter͒, the interband transition remains
indirect, in agreement with data of absorption studies.5 The
nonlinear optical response in indirect-gap semiconductors is
dominated by free-carrier absorption due to intraband tran-
sitions. Relaxation in the wave-vector conservation leads to
the enhancement of this process in NCs, which is likely re-
sponsible for the intense PA1 signal in our samples.
In conclusion, we have performed fs TA studies of ion-
implanted Si NCs in correlation with studies of their micro-
structural and photoluminescence properties. Post-
implantation annealing leads to suppression of the defect-
related emission at ϳ1.9 eV and the development of a red
emission at ϳ1.65 eV, which is correlated with formation of
a large number of NCs. The TA spectra of annealed samples
reveal the presence of two ps PA bands attributed to carriers
in NC quantized (PA1) and NC interface (PA2) states. The
sub-10-ps NC-size-dependent decay of PA1 is indicative of
the fast depopulation of quantized states due to efficient sur-
face trapping. This observation strongly suggests that the red
emission in our samples is not due to carriers in quantized
states, but rather due to deactivation of surface traps.
The TA data indicate that PA2 is due to population of
secondary states which are optically-coupled to NC quan-
tized states. These secondary states are likely located at the
Si/SiO2 NC interface, since this would provide a nonvanish-
ing overlap with the wavefunctions of quantized states. The
fast 1.5 ps decay of PA2 can be attributed to the internal
energy relaxation of the interface states, which should be
NC-size independent, in agreement with our data. The oscil-
lations in the PA2 spectrum ͑Fig. 1͒ indicate a spacing of
800–900 cmϪ1, consistent with vibrational coupling to
Raman-active modes of Si–O with a range of terminating
atoms. A near-IR fast-relaxing PA band with molecular-like
properties was previously observed by us in nanoporous Si.14
The fast relaxation of PA1 indicates that populations of
quantized states are short-lived and decay on the ps time
scale. Since this decay does not show pump-intensity depen-
dence, it cannot be explained in terms of nonlinear recombi-
nation processes ͑such as the Auger recombination͒, and is
most likely due to efficient surface trapping, analogous to
that in II–VI NCs with incomplete surface passivation.10,13
The increase in the surface-to-volume ratio in NCs of smaller
sizes, leading to enhanced surface trapping,13 can explain the
shortening of the PA1 relaxation time in sample 1. The broad
TA spectrum, observed after the decay of PA1 and PA2
(⌬tϾ10 ps) ͑Fig. 1͒, can be attributed to surface-trapped
carriers. The observation of a fast depopulation of quantized
states strongly indicates that the red emission in our samples
is not due to recombination of carriers in quantized states,
but is rather due to surface-trapped carriers as was suggested
previously for Si NCs6 and nanoporous Si.15,16
In II–VI NCs, the dominant channel for carrier decay at
high pump densities is non-radiative Auger recombination.10
However, in our samples we did not observe any signatures
of nonlinear decay even at carrier densities of Ͼ10 e-h pairs
per NC. At the same pump fluences, in bulk-Si samples, we
observed Auger-process dominated recombination with
sub100 ps time constants. These observations provide addi-
tional evidence for the extremely fast depopulation of NC
quantized states with a rate exceeding that for the Auger
process.
In direct gap NCs, resonant optical nonlinearities are
dominated by state-filling-induced bleaching of optical
transitions.10 Si is an indirect-gap material in the bulk form.
However, confinement-induced mixing of states can lead to
an indirect-to-direct gap transformation in Si NCs.17 For NCs
studied in this work, the lowest interband transition is lo-
cated between 1.3 and 1.9 eV.17,18 Our TA data indicate that
in the range from 1.2 to 2.6 eV no bleaching is observed in
our samples even at short times after excitation, before car-
This research was supported by Los Alamos Directed
Research and Development funds, under the auspices of the
U.S. Department of Energy. Oak Ridge National Laboratory
is managed by Lockheed Martin Energy Corp. for the U.S.
Department of Energy under Contract No. DE-AC05-
96OR22464.
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