Hot Electron Effects in Excitation of
Visible Photoluminescence in Si Nanowires
T.V. TORCHYNSKAa), L.Y. KHOMENKOVAb), V.N. ZAKHARCHENKOc),
R.V. ZAKHARCHENKOc,d), J. GONZÁLEZ-HERNÁNDEZd), Y.V. VOROBIEVd)
a)
Instituto Politecnico Nacional, U.P.A.L.M., 07738, México D.F., MÉXICO
b)
Institute of Semiconductor Physics, National Academy of Sciences, Kiev, UKRAINE
c)
National Politechnic Institute KPI, Dept. of Gen. Eksp. Physics, Kiev, UKRAINE
d)
CINVESTAV-IPN, Unidad Querétaro, Juriquilla, Querétaro, QRO., MÉXICO
Abstract: - Visible photoluminescence of Si nanowires prepared by electrochemical
etching was studied with the use of photoluminescence spectral measurements, Atomic
Force Microscopy and Magnetic Field effect. The photoluminescence spectra had no
dependence upon the nanoparticle size, in contradiction with the model based on the
quantum confinement effect. The magnetic field of 0.52 T has a noticeable influence on
the luminescence intensity, but not on the emission spectra. The features observed
confirm the hipothesis about the emission excitation by hot electrons created by the
light absorption in nanowires which energy band structure remains practically the same
as in the bulk Si. The effective mobility of electrons involved in the excitation process
estimated from the effect value is around 30000 cm2/V·s, which greatly exceeds the
normal electron mobility in Si at room temperature.
Key-Words: - Si nanowires, Visible photoluminescence, Magnetic field effect, Hot
electrons excitation
1 Introduction
The room temperature strong visible
luminescence (PL) of Si wire
nanostructures (the most frequently
used name for them is porous Si, or PSi)
has attracted much attention because of
its evident incompatibility with the
normal Si band structure, and due to its
many
potential
applications
in
optoelectronics. The most popular
explanation of this emission is based on
the quantum confinement effect [1],
although there are many other models
[2-5] which only number is the reason
to say that no one is good enough.
Recently a new approach was
introduced by one of the authors
(Torchynska [6,7]) considering the
luminescence excitation process as the
ballistic effect caused by the hot
electrons
produced
by
optical
absorption in the Si with practically
unaltered band structure, and the hot
electrons of the c-band participating in
the process. Here we present the new
experimental data confirming this
model, and the estimation of average
mobility of the electrons exciting the
emission, which directly indicates that
these are hot electrons.
2 Experimental
Investigated PSi samples were prepared
by electrochemical etching from (100)
oriented p-type silicon wafers with
resistivity of 1.0 Ω·cm. The HF-ethanol
solution was used (HF:H2O:C2H5OH =
1:1:2), and the anodization current
density was 20 mA/cm with the
duration between 5 and 60 min.
Depending on the process duration, the
details of the surface relief varied in
size, on average, from 30 to 300 nm.
Fig. 1 shows the AFM picture of the
sample surface after 15 min of etching,
with the details of the orders of 100 nm,
which proved to be optimum for the PL
intensity.
Fig.1. AFM image of the PSi layer
fabricated at anodization current density
20 mA/cm2 during 15 min.
The PL spectra of the samples with
different etching time are shown in Fig.
2; it is evident that all three spectra have
practically the same shape, without any
dependence upon the characteristic size
of the details of the nanostructure.
developed (whereas the details of the
smaller size, which are able to produce
really quantum effects, are oxidized
quickly, and for that reason are not
observable). On the other hand, for
these effects, the dependence of the
spectrum shape upon the characteristic
size ought to be very strong, which is
definitely not the case.
On the contrary, the ballistic model
introduced in [6,7] well agrees with the
stable emission spectrum, which, from
the positions of the model, is
determined by the radiative defects at
Si/SiOx interface. The excitation of
these defects is produced by the hot
electrons generated by light-induced
transitions from the valence band of Si
into the Si c-band [6,7]. In this case, the
magnetic field effect on the emission
intensity is to be expected, since the
high energy (“hot”) electrons have
relatively high velocity and therefore
the magnetic field ought to have a
noticeable influence on their motion,
reducing the mean free path and
correspondingly the probability to reach
the surface where the radiative centers
are situated.
1.00
B
PL(0.5T)/PL(0)
0.95
PL intensity, arb.un.
40
30
20
0.90
0.85
0.80
1.68
1.72
1.76
1.80
1.84
1.88
h , eV
10
0
1,4
1,5
1,6
1,7
1,8
1,9
2,0
2,1
Fig. 3. Relative reduction of emission
intensity caused by magnetic field.
h , eV
Fig.2. The PL spectra for samples made
with 5 (squares), 15 (triangles) and 45
(rhombs) min etching time.
The data presented already show that
the quantum confinement effects are not
responsible for the emission observed:
for the characteristic size of the surface
observed, these effects are not well
In the Fig. 3 the results of the emission
measurements with the magnetic field
are shown. The magnetic field of 0.52 T
was applied to the Si sample in such a
way that the field direction was parallel
to the sample surface (i.e. normal to the
axes of the nanopiramides created by
etching, see Fig. 1). The luminescence
was excited with the Hg arc lamp, and
the emission in different spectral
intervals was selected with the
monochromator MDR, and emission
intensity was measured with the
photomultiplier. It is seen from the
figure 3, that the reduction of
luminescence intensity does not depend
on the emission wavelength, and is
equal to 10 % of the initial intensity (in
other words, the PL intensity with the
magnetic field applied is 0.9 of the
initial PL intensity in a given spectral
region).
3 Discussion
It was already mentioned that the
spectral measurements favor the idea
about the photoluminescence excitation
with the hot electrons transferred by
light to the upper levels of the Si c-band
(for the details, see [7]). The effect of
the magnetic field reported here
strongly supports this point of view. In
case of quantum confined systems, the
possible magnetic field influence upon
the emission may have a form of the
Zeeman effect, which (i) demands
larger fields to be well pronounced, and
(ii) affects, first of all, the emission
spectrum, and to less extent the
emission intensity. However, we
observe quite noticeable effect caused
by relatively small field, and no
dependence of the effect on the
emission wavelength.
On the basis of the hot electron
excitation model, the magnetic field
effect is explained by the reduction of
the electron mean free path caused by
the Lorents force and the corresponding
curving of electron’s trajectory. If we
denote the mean free path in absence of
magnetic field as L (the corresponding
time between the scattering processes as
), in the magnetic field this path will
correspond to the part of a circular
trajectory caused by the Lorents force
effect determined by the angle of
rotation of the radius R of circular
trajectory during the time interval and
the effective mean free path in magnetic
field with induction B will be
L* = 2 R sin /2 ≈ R (1  /24) (1)
Since L = R we obtain the relative
reduction of the mean free path as
(L – L*)/L ≈ /24
(2)
We take that this value should define
the relative reduction of the emission
intensity (the smaller is the free path,
the smaller amount of electrons get to
the surface and excite the emission).
The value of angle  is determined by
the frequency  of rotation of electrons
in magnetic field (the cyclotron
frequency):  = e B/m, where “e” is the
electron charge, and “m” – its mass.
The angle  is equal to
 = e B /m =  B,
(3)
where  = e/m is the effective
mobility of electron.

Taking (L – L*)/L = 0.1, from (2)
we obtain  = (2.4)0.5 ≈ 1.55, and from
(3) we find an electron effective
mobility  = 30000 cm2/V·s. This
value is more than an order of
magnitude higher than the normal
electron mobility in Si, which also
confirms the concept of the hot electron
excitation of the photoluminescence.
3 Conclusions
The influence of the magnetic field of
0.5 T upon the photoluminescence of Si
nanowires
was
observed:
the
luminescence intensity in magnetic field
constitutes 90 % of the intensity in
absence of field. The effect has no
dependence
on
the
emission
wavelength.
This effect could not be explained on
the basis of the quantum confinement
model, and is supporting the hypothesis
about the luminescence excitation by
hot electrons transferred by the
illumination from the v-band to the
upper levels of the Si c-band, while the
Si band structure for the nanowires size
observed is pracrtically the same as for
the bulk Si.
From the value of the magnetic field
effect, we estimated the effective
mobility of the electrons participating in
the emission excitation process as
30000 cm2/V·s. This value is much
higher than the standard electron
mobility in Si, which also support the
idea about the ballistic mechanism of
the PL excitation, with the hot electrons
exciting the surface emission centers.
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