Ultrafast carrier dynamics in germanium nanoparticles
P. Tognini
a)
and A. Stella
INFM, Dipartimento di Fisica ‘‘A. Volta,’’ Universita
`
degli Studi di Pavia, Via Bassi 6,
I-27100 Pavia, Italy
S. De Silvestri, M. Nisoli, and S. Stagira
INFM, Dipartimento di Fisica Politecnico di Milano, Centro di Elettronica Quantistica e Strumentazione
Elettronica, CNR, Piazza L. da Vinci 32, I-20133 Milano, Italy
P. Cheyssac and R. Kofman
Laboratoire de Physique de la Matie
`
re Condense
´
e, URA 190, Universite
´
de Nice-Sophia Antipolis,
Nice Cedex, France
͑Received 10 February 1999; accepted for publication 18 May 1999͒
It is shown that the ultrafast carrier dynamics in Ge nanoparticles involving the electronic density
of states well above the gap can be separated in two regimes: a faster one ͑ϳ1ps͒characterized by
bleaching of the absorption around 490 nm and a slower one ͑up to a few hundred picoseconds͒
governed by band gap renormalization. There is also a clear correspondence between the spectral
response in both regimes and a bulk-like band structure for sizes below the exciton Bohr radius.
Quantum confinement is manifested through the measurable blueshift of E
1
ϩ⌬
1
spectral structure
in the ultrafast optical response. © 1999 American Institute of Physics. ͓S0003-6951͑99͒03028-4͔
The investigation of ultrafast carrier dynamics has
yielded new information on quantum dots, mainly the III–V
and II–VI compounds.
1,2
Much less is known about the re-
laxation mechanisms in nanocrystals of indirect-gap materi-
als. In particular in the case of germanium, previous works
have been essentially restricted to the study of the electron
dynamics in the bulk: femtosecond photoluminescence
upconversion,
3
pump–probe,
4,5
four-wave mixing,
6
and
time-resolved Raman scattering.
7
Time-resolved four-wave-
mixing measurements were recently performed on Ge nanoc-
rystals to study the intensity-dependent refractive index.
8
In this work we report on the ultrafast electron relaxation
processes in two samples with Ge nanoparticle sizes compa-
rable and below the exciton Bohr radius ͓22–39 nm ͑Ref.
9͔͒. By using the pump–probe experimental technique we
studied the relaxation processes of electrons excited well
above the gap in a wide probe energy range. We will show
that the electron dynamics is to a great extent intrinsic ͑i.e.,
related to the properties of the semiconductor dots͒, with
negligible influence from the environment ͑embedding ma-
trix͒. Moreover, it will be shown that the bulk band structure
represents a good starting reference not only for the ‘‘static’’
optical properties
10
but also for the ultrafast response.
The investigated Ge nanocrystals are embedded in a film
of alumina sitting on a sapphire substrate. The growth is
based on an evaporation-condensation self-organized process
in the liquid phase, and on the partially wetting character of
Ge with respect to the matrix.
10
The balance of the surface
tensions of Ge and alumina, characterized by a wetting angle
of 106.5°, allows to obtain nanoparticles with truncated
spherical shape and relatively low size dispersion ͑р20%͒.
The average radius of the investigated nanoparticles is, re-
spectively, ϳ16 nm ͑Ge16͒ and ϳ4nm͑Ge4͒. Transient
transmissivity measurements were performed by using a con-
ventional pump–probe configuration. The Ti:sapphire laser
system employed for the experiments has been described
elsewhere.
11
Pump–probe experiments ͑pump wavelength at
390 nm͒, with a time resolution of 150 fs, were carried out in
the visible ͑450–750 nm͒ using as a probe the white light
supercontinuum generated in a thin sapphire plate.
In Fig. 1 we report the difference between the transmit-
tances of a reference sample without nanoparticles and the
investigated samples. The spectral structures in the region
450–600 nm originate from transitions between nearly par-
allel bands along the ⌳ direction ͑inset Fig. 1͒ giving rise to
the doublet E
1
and E
1
ϩ⌬
1
.
12
The ⌬T/T spectra of the two
Ge samples at room temperature are shown in Fig. 2 for
different pump–probe delays
D
. The spectra at
D
ϭ0 are
dominated by a prominent bleaching (⌬T Ͼ0) with a maxi-
mum at ϳ480 and ϳ490 nm in Ge4 and Ge16, respectively,
which corresponds to the position of the E
1
ϩ⌬
1
spectral
structure. The measured blueshift of this band in the smaller
nanoparticles can be interpreted in terms of quantum
confinement.
10
In both samples the bleaching band com-
pletely disappears in about 1 ps, as evidenced in Fig. 3,
a͒
Electronic mail: tognini@fisav.unipv.it
FIG. 1. Difference between the transmittance of a reference sample without
nanoparticles and the two investigated samples ͑Ge16: continuous line; Ge4:
dashed line͒. Inset: Ge band structure.
APPLIED PHYSICS LETTERS VOLUME 75, NUMBER 2 12 JULY 1999
2080003-6951/99/75(2)/208/3/$15.00 © 1999 American Institute of Physics