Electro-optical effects in 2D macroporous silicon structures with nanocoatings
The near-IR light absorption oscillations in 2D macroporous silicon structures with microporous silicon layers, CdTe, surface nanocrystals and SiO₂ nanocoatings have been investigated. The electro-optical effect was taken into account within the strong electric field approximation. Oscillations with...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2015
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| Цитувати: | Electro-optical effects in 2D macroporous silicon structures with nanocoatings / L.A. Karachevtseva, O.O. Lytvynenko, K.P. Konin, K.A. Parshyn, O.Yu. Sapelnikova, O.J. Stronska // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2015. — Т. 18, № 4. — С. 377-384. — Бібліогр.: 24 назв. — англ. |
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irk-123456789-1212572017-06-14T03:07:16Z Electro-optical effects in 2D macroporous silicon structures with nanocoatings Karachevtseva, L.A. Lytvynenko, O.O. Konin, K.P. Parshyn, K.A. Sapelnikova, O.Yuю Stronska, O.J. The near-IR light absorption oscillations in 2D macroporous silicon structures with microporous silicon layers, CdTe, surface nanocrystals and SiO₂ nanocoatings have been investigated. The electro-optical effect was taken into account within the strong electric field approximation. Oscillations with a giant amplitude were observed in the spectral ranges of surface level absorption. This process is because of resonance electron scattering on the surface impurity states with the difference between two resonance energies equal to the Wannier–Stark ladder due to big scattering lifetime as compared to the electron oscillation period in the strong surface electric field. The electron transitions and free electron motion are realized due to additional change in the local electric field as a result of grazing light incidence and quasi-guided mode formation. 2015 Article Electro-optical effects in 2D macroporous silicon structures with nanocoatings / L.A. Karachevtseva, O.O. Lytvynenko, K.P. Konin, K.A. Parshyn, O.Yu. Sapelnikova, O.J. Stronska // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2015. — Т. 18, № 4. — С. 377-384. — Бібліогр.: 24 назв. — англ. 1560-8034 DOI: 10.15407/spqeo18.04.377 PACS 78.20.Jq, 78.60.Bf http://dspace.nbuv.gov.ua/handle/123456789/121257 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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The near-IR light absorption oscillations in 2D macroporous silicon structures with microporous silicon layers, CdTe, surface nanocrystals and SiO₂ nanocoatings have been investigated. The electro-optical effect was taken into account within the strong electric field approximation. Oscillations with a giant amplitude were observed in the spectral ranges of surface level absorption. This process is because of resonance electron scattering on the surface impurity states with the difference between two resonance energies equal to the Wannier–Stark ladder due to big scattering lifetime as compared to the electron oscillation period in the strong surface electric field. The electron transitions and free electron motion are realized due to additional change in the local electric field as a result of grazing light incidence and quasi-guided mode formation. |
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Article |
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Karachevtseva, L.A. Lytvynenko, O.O. Konin, K.P. Parshyn, K.A. Sapelnikova, O.Yuю Stronska, O.J. |
| spellingShingle |
Karachevtseva, L.A. Lytvynenko, O.O. Konin, K.P. Parshyn, K.A. Sapelnikova, O.Yuю Stronska, O.J. Electro-optical effects in 2D macroporous silicon structures with nanocoatings Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Karachevtseva, L.A. Lytvynenko, O.O. Konin, K.P. Parshyn, K.A. Sapelnikova, O.Yuю Stronska, O.J. |
| author_sort |
Karachevtseva, L.A. |
| title |
Electro-optical effects in 2D macroporous silicon structures with nanocoatings |
| title_short |
Electro-optical effects in 2D macroporous silicon structures with nanocoatings |
| title_full |
Electro-optical effects in 2D macroporous silicon structures with nanocoatings |
| title_fullStr |
Electro-optical effects in 2D macroporous silicon structures with nanocoatings |
| title_full_unstemmed |
Electro-optical effects in 2D macroporous silicon structures with nanocoatings |
| title_sort |
electro-optical effects in 2d macroporous silicon structures with nanocoatings |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2015 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/121257 |
| citation_txt |
Electro-optical effects in 2D macroporous silicon structures with nanocoatings / L.A. Karachevtseva, O.O. Lytvynenko, K.P. Konin, K.A. Parshyn, O.Yu. Sapelnikova, O.J. Stronska // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2015. — Т. 18, № 4. — С. 377-384. — Бібліогр.: 24 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
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2025-07-08T19:28:43Z |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 4. P. 377-384.
doi: 10.15407/spqeo18.04.377
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
377
PACS 78.20.Jq, 78.60.Bf
Electro-optical effects in 2D macroporous silicon structures
with nanocoatings
L.A. Karachevtseva
*
, O.O. Lytvynenko, K.P. Konin, K.A. Parshyn,
O.Yu. Sapelnikova, O.J. Stronska
V. Lashkaryov Institute of Semiconductor Physics of NAS of Ukraine;
41, prospect Nauky, 03028 Kyiv, Ukraine
*
Corresponding author: phone +38(044) 525-23-09, e-mail: lakar@isp.kiev.ua
Abstract. The near-IR light absorption oscillations in 2D macroporous silicon structures
with microporous silicon layers, CdTe, surface nanocrystals and SiO2 nanocoatings have
been investigated. The electro-optical effect was taken into account within the strong
electric field approximation. Oscillations with a giant amplitude were observed in the
spectral ranges of surface level absorption. This process is because of resonance electron
scattering on the surface impurity states with the difference between two resonance
energies equal to the Wannier–Stark ladder due to big scattering lifetime as compared to
the electron oscillation period in the strong surface electric field. The electron transitions
and free electron motion are realized due to additional change in the local electric field as
a result of grazing light incidence and quasi-guided mode formation.
Keywords: macroporous silicon, nanocoatings, Wannier–Stark ladder.
Manuscript received 27.05.15; revised version received 10.09.15; accepted for
publication 28.10.15; published online 03.12.15.
1. Introduction
One of promising materials to develop 2D photonic
structures is macroporous silicon that can be formed
using the photoanodic etching. It is related with
formation of structures with necessary geometry and
high ratio between the cylindrical macropore depth and
diameter [1, 2]. Presence of periodically located
cylindrical pores separated by silicon columns provides
large effective surface of the samples. This determines
optical and photo-physical characteristics of macro-
porous silicon structures [3-6]. For wavelengths below
the optical period of structures, the reduction of light
absorption is observed owing to the guided and radiation
optical modes formed by macroporous silicon as a short
waveguide [6]. The absolute absorption maxima are
determined by the guided optical mode position. The
results obtained were explained by specificity of a
macroporous silicon surface [3]. The existence of an
intrinsic electric field FS = (5…9)10
5
V/cm is confirmed
by electrorefectance study of macroporous silicon
surfaces [7]. In view of the potential barrier on a
macropore surface, one should take into account
recharging of the local surface centers at energies below
that of the indirect interband transition. The near-IR
optical absorption in 2D photonic macroporous silicon
structures was investigated with allowance made for the
linear electro-optical effect [8]. The spectral dependence
of optical absorption of macroporous silicon structures
in the near-IR (impurity absorption) has oscillating
structure and varies under the “3/2” law at long
wavelengths. It correlates with the frequency
dependence of the imaginary part of permittivity for
optical transitions between impurity levels and the
allowed bands of a crystal in an electric field (the
impurity Franz–Keldysh effect). The experimental
absorption spectra of macroporous silicon agree well
with the corresponding spectral dependences of the
electro-optical energy and the imaginary part of
permittivity in the weak electric field approximation,
thus confirming realization of the impurity Franz–
Keldysh effect. The electric field of the reflected
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 4. P. 377-384.
doi: 10.15407/spqeo18.04.377
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
378
electromagnetic wave at the grazing angle of light
incidence onto macropore surface changes effectively a
local electric field on the macropore surface. In this
paper, the near-IR light absorption oscillations of 2D
macroporous silicon structures with microporous silicon
layers, CdTe surface nanocrystals and SiO2 nanocoatings
are investigated taking into account the electro-optical
effect within the strong electric field approximation. An
analysis of the experimental absorption spectra is carried
out within the model of the resonant electron scattering
on impurity states in strong electric field, with the
difference between two resonant energies equal to the
Wannier–Stark ladder. An additional electric field
intensity growth due to the quasi-guided mode formation
in silicon matrix with SiO2 nanocoatings was considered.
A comparison of results obtained for SiO2 nanocoatings
and nanocrystal surface was made.
2. Methodology
The samples to be studied were made of silicon wafers
characterized by the [100] orientation and n-type of
conductivity (the electron concentration n0 = 10
15
cm
–3
).
We used the technique of electrochemical etching at
illumination of the back side of a silicon substrate
(thickness H = 520 m) [3]. The square-lattice periodic
structures, as well as those with arbitrary distribution of
macropores, were fabricated. The initial specimens are
complex micropore-macropore silicon structures
consisting of 100-nm micropore layers on macropore
walls. In addition, the anisotropic etching in 10%
solution of KOH was used to remove the microporous
layers from macropore walls. Macropores with the depth
hp = 80…100 m, diameter Dp = 2…5 m and period
ap = 4…7 m were formed.
CdTe nanocrystals of 20 nm in size were grown on
the modified installation of metal dispersion using “a hot
wall” molecular epitaxy on macroporous silicon substrates
[9]. The undoped CdTe sputtered at the substrate
temperature 475 K and source temperature 650 K served
as substance for evaporation. The thickness of the
deposited films (200 nm) was set by time of structure stay
above the source of evaporation [10]. The layers of oxide
(thickness within 70…200 nm) have been formed on
macroporous silicon samples in the dry oxygen for
40(60) min at the temperature close to 1050 °С (1200 °С).
The 800-nm oxide layer was formed for 50 min at the
temperature 1100 °С in wet oxygen using a steam
generator with deionized water. The thickness of oxide
layer was measured using ellipsometry.
We performed optical investigations within the
spectral range 1.3…25 m using the IR Fourier
spectrometer “Perkin Elmer” Spectrum BXII. The
optical absorption spectra were measured at normal
incidence of IR radiation on a sample (along the
cylindrical macropores). The experiments were carried
out in air at room temperature. The error of spectral
measurements did not exceed 2 сm
–1
.
3. Experimental
Microporous silicon layers and CdTe nanocrystals. For
the macroporous silicon structures with microporous
layers and surface nanocrystals, light absorption increases,
and an oscillating structure occurs (Fig. 1). The absorption
spectra of macroporous silicon structures with CdTe
surface nanocrystals (Fig. 1a, curve 2) and without
nanocoatings (Fig. 1a, curve 3) have similar shapes.
The amplitude of oscillations is maximal in the
spectral ranges of surface levels (organic species, Si-H,
C–H and O–H bonds [11-14]) in the absorption spectra
of macroporous silicon structures with CdTe surface
nanocrystals. And Si–O–Si, Si–Si, Si–H2, Si–O, SiCH3,
C=O bonds were observed only in absorption spectra of
macroporous silicon structures with the microporous
silicon layer (Fig. 1a, curve 1) or without nanocoatings
(Fig. 1a, curve 3). The form of oscillations (Fig. 1b)
indicates their resonant character.
60 80 100 200 400 600 800 1000
10
100
1000
10000
2
S
i-
H
S
i-
O
O
H
C
-H
SiCH
3
S
i-
S
i
S
i-
O
-S
i
3
A
b
s
o
rb
a
n
c
e
,
a
.u
.
Photon Energy, meV
1
O
rg
an
ic
co
m
pounds
Fig. 1a. Absorption spectra of macroporous silicon structures
with microporous layers (1), CdTe surface nanocrystals (2) and
without coatings (3).
260 270 280 290 300 310
200
400
600
800
1000
A
b
s
o
rb
a
n
c
e
,
a
rb
.u
n
.
Photon energy, meV
2
1
3
2
Fig. 1b. Fragments of absorption spectra of macroporous
silicon structures with microporous layers (1), CdTe surface
nanocrystals (2) and without coatings (3) in the vicinity of Si-H
bonds.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 4. P. 377-384.
doi: 10.15407/spqeo18.04.377
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
379
The spectral positions of oscillation maxima
inherent to macroporous silicon structures with surface
nanocrystals and microporous layers vs oscillation
number curves (Fig. 2a, curves 1, 2) are straight lines,
and the oscillation period is almost constant (Fig. 2b).
The oscillation energies E lie between 0.7 to 2 meV for
microporous layers and lie between 0.7 to 2 and 3 to
5 meV for CdTe surface nanocrystals. The oscillations
of small amplitude and other spectral position of
oscillation maxima (Fig. 2a, curve 3) have been
investigated for macroporous silicon structures without
nanocoatings [8]. The experimental absorption spectra of
macroporous silicon agree well with the corresponding
spectral dependences of the electro-optical energy and
the imaginary part of permittivity in the weak electric
field approximation, thus confirming realization of the
impurity Franz–Keldysh effect.
SiO2 nanocoatings. For macroporous silicon
structures with SiO2 nanocoatings, light absorption
increases and oscillating structure occurs, too (Fig. 3a,
curves 1 and 2). We observed the essential absorption
growth in the spectral range of Si–O, Si–H, O–H bonds
and organic compounds. The amplitude of oscillations is
maximal within the spectral ranges of surface level
absorption (Fig. 3b).
0 50 100 150 200
0
200
400
600
800
1000
O
s
c
il
la
ti
o
n
s
p
e
c
tr
a
l
p
o
s
it
io
n
,
m
e
V
Oscillation number, m
1
3
2
Fig. 2a. The spectral position of oscillation maxima of
macroporous silicon structures with microporous layers (1),
CdTe nanocrystals (2) and without coatings (3) [7] as a
function of oscillation number.
200 400 600 800 1000
0
2
4
6
O
s
c
il
la
ti
o
n
e
n
e
rg
y
E
,
m
e
V
Photon energy, meV
1
2
1
Fig. 2b. Spectral dependences of the oscillation period of
macroporous silicon structures with microporous layers (1),
CdTe (2).
60 80 100 200 400 600 800 1000
100
1000
10000
100000
A
b
s
o
rb
a
n
c
e
,
a
rb
.u
.
Photon Energy, meV
1
2
S
i-
H
S
i-
O
O
-HO
rg
an
ic
co
m
pounds
3
Fig. 3a. Absorption spectra of macroporous silicon structures
with SiO2 nanocoating 70 nm (1) and 800 nm (2) thick and
without coating (3).
380 390 400 410 420 430 440 450 460 470
1000
10000
A
b
s
o
rb
a
n
c
e
,
a
rb
.u
.
Photon energy, meV
1
2
3
1
O
-H
Fig. 3b. Fragment of absorption spectra of macroporous silicon
structures with SiO2 nanocoatings 70 nm (1) and 800 nm (2)
thick and without coating (3).
The spectral positions of oscillation maxima in the
macroporous silicon structures with SiO2 nanocoatings
and without coating are rather different (Fig. 4a). The
dependence of oscillation maxima of macroporous
silicon structures with SiO2 nanocoating on the
oscillation number has peculiarities at energies of 0.25,
0.4 and 0.7 eV. In addition, the oscillation period
fluctuates about a constant value at low spectral energies
and becomes quadratic in the photon energy depending
on the SiO2 nanocoating thickness (Fig. 4b).
4. Discussion
We observed the oscillating structure in the absorption
spectra of macroporous silicon structures with surface
nanocrystals. The amplitude of oscillations is maximal in
spectral ranges of organic species, Si–H, C–H and O–H
bonds. The obtained results indicate strong effect of
impurity states on the surface of macroporous silicon
structures with nanocoatings. It may result from
scattering of both electromagnetic radiation and
electrons on the impurity states. The form of oscillations
(Fig. 1b, 3b) indicates resonant character of scattering.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 4. P. 377-384.
doi: 10.15407/spqeo18.04.377
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
380
0 50 100 150 200 250 300 350 400 450
200
400
600
800
1000
P
h
o
to
n
e
n
e
rg
y
,
m
e
V
Oscillation number, m
1 2 3
Fig. 4a. The spectral position of oscillation maxima in the
macroporous silicon structures with SiO2 nanocoatings of
70 nm (1), 200 nm (2) and 800 nm (3) thickness.
40 60 80 100 200 400 600 800 1000
1
10
2
1
O
s
c
il
la
ti
o
n
e
n
e
rg
y
,
m
e
V
Photon energy, meV
1
Fig. 4b. Spectral dependences of the oscillation energy in the
macroporous silicon structures with SiO2 nanocoatings of
200 nm (1) and 800 nm (2) thickness.
The oscillations of small amplitude in macroporous
silicon structures without nanocoatings correspond to the
weak electric field approximation [8]. The macroporous
silicon structures with surface nanocrystals investigated
in this work have higher surface potential of
nanocoatings with surface bonds. Therefore, the onset of
oscillations with a giant amplitude can be attributed to
the electro-optical processes in strong electric fields.
Moreover, the constant oscillation period (Fig. 2b) may
specify the realization of the Wannier–Stark effect on
randomly distributed surface bonds in nanocrystals. The
method for experimental observation of Wannier–Stark
ladder was proposed by Berezhkovskii and
Ovchinnikov [15]. It was shown that the scattering
amplitude has resonant behavior in the case of electron
scattering by impurities. If the electric field is directed
along the х-axis of the crystal, then electron scattering
occurs in the plane (y, z), and the difference between two
resonant energies is approximately equal to Wannier–
Stark ladder. In our case, an electric field of “silicon-
nanocoating” heterojunctions on the macropore surface
is directed perpendicularly to the surface, too (Fig. 3),
and surface states that scatter electrons are concentrated
perpendicularly to the х-direction in the plane (y, z) that
is the plane of resonant scattering.
Let us consider a semiconductor with the dispersion
law E(k) = Е0 – (cosky a + coskz a), where k is a quasi-
momentum with components ky, kz, Е0 – energy
corresponding to the midgap, – energy equal to 1/6 of
the band gap, a – lattice parameter. The wave function in
the Wannier representation was written as [15]:
0)(ˆ01
00)(ˆ
00
00
EGV
VEGj
jj
E
EE
.
Here, the first (second) term describes the incident
wave (scattered waves); j numbers the lattice site,
)(ˆ
0 EG is the Green operator, V0 – impurity potential.
The complex energies for which the denominator of the
second term becomes zero correspond to the resonances
in electron scattering
0)(ˆ0/1 00 EGV
at E = ε – iΓ (Γ > 0). The difference of two neighbouring
resonance energies is approximately equal to the value
of the step in the Wannier–Stark ladder.
The fact is that the levels of the Wannier–Stark
ladder have a certain width Г, while its detection
requires that this width should be less than the difference
of energies of adjacent levels, < Fd. The contributions
to the width Г come from the interband interaction,
electron-phonon interaction, and interaction with
impurity atoms. The interband [16] and the electron-
phonon [17] interactions have been studied, and such
interactions do not break the Wannier–Stark ladder. The
influence of impurities on the Wannier–Stark ladder and
calculation of the width of the Wannier–Stark ladder
levels E due to scattering from impurities were
considered by Berezhkovskii and Ovchinnikov [18]. The
Wannier–Stark ladder is not broken by impurities, if the
intervals between transitions due to scattering by
impurity atoms with the lifetime are bigger than the
period of electron oscillations in external field,
TB ( /TB > 1), where TB = 2h/E, is equal to 1/W (W
is the probability for an electron to leave the state per
unit time due to scattering by an impurity atom at lattice
site). The following estimate of the probability W for
electron to leave the state per unit time due to scattering
by an impurity atom at lattice site was obtained:
W < 2V0 Ni /
(Nh), where V0 is the impurity potential, Ni –
impurity concentration and N (a
2
)
-1
– density of
states. As a result, the inequality /TB > 1 passes to
Ni < E/(4a
2
V0). Using the latter inequality, we found a
numerical estimate for the impurity concentration.
We obtained the surface impurity concentration in
macroporous silicon structures by using the method of
the photoconductivity dependence on the distance
between macropores [19] and the temperature
dependences of the photocarrier lifetime in macroporous
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 4. P. 377-384.
doi: 10.15407/spqeo18.04.377
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
381
silicon [20]. From the experimental temperature
dependences of the photocarrier lifetime in 2D
macroporous silicon structures, the dimensionless
surface potential 0y is about 12 at room temperature,
which corresponds to the equilibrium surface band
bending of about 0.31 eV and to the surface impurity
concentration Ni = 510
10
cm
–2
for the electron
concentration n0 = 10
15
cm
–3
of the investigated
macroporous silicon samples. In this case, the Wannier–
Stark ladder is preserved within the whole spectral range
for macroporous silicon structures with microporous
layers and surface nanocrystals (Fig. 6). The Bloch
oscillation time is equal to TB (4…8)10
–12
s for
macroporous silicon structures with surface
nanocrystals, and TB (1…4)10
–11
s for macroporous
silicon structures with microporous layers. The lifetime
ratio is /TB > 1 within the whole spectral region studied
for macroporous silicon structures with surface
nanocrystals. And the inequality /TB > 1 for this
lifetime ratio is satisfied within the whole investigated
spectral region for macroporous silicon structures with
SiO2 nanocoatings (Fig. 7), if taking into account that the
surface impurity concentration for macroporous silicon
structures, Ni, is less than 10
11
cm
–2
.
Air
Si0
n
2
a
X
Y
Z
Si
n
R
d
Fig. 5. Fragment of system considered.
40 60 80 100 200 400 600 800 1000
1
10
100
T
h
e
li
fe
ti
m
e
r
a
ti
o
/T
B
,
a
rb
.u
.
Photon energy, meV
1
2
Fig. 6. Spectral dependence of the lifetime ratio /TB for
macroporous silicon structures with microporous layers (1) and
CdTe (2) surface nanocrystals.
40 60 80 100 200 400 600 800 1000
1
10
2T
h
e
l
if
e
ti
m
e
r
a
ti
o
/T
B
,
a
rb
.u
.
Photon energy, meV
1
Fig. 7. Spectral dependence of the lifetime ratio /TB for
macroporous silicon structures with SiO2 nanocoatings of
200 nm (1) and 800 nm (2) thickness.
The big scattering time is needed to make possible
generation of well-separated oscillations. The oscillation
periods E for macroporous silicon structures with
surface nanocrystals and microporous layers depend
mainly on the band gap of the nanocrystal material (see
Fig. 2b and Table). The electric field intensity F is equal
to 10
4
…10
5
V/cm for F = E/a, and (3…8)10
3
V/cm
for F = E/d (see Table). The latter value of electric
field intensity is too small, the Bloch time is bigger than
the relaxation time, and therefore neither Bloch
oscillations nor Wannier–Stark ladders have been
observed yet. This proves the validity of the model of
electron oscillations in the atomic lattice.
Usually, the basic sources of external electric field
at semiconductor surface are the charge of electron
levels and built-in charge in semiconductor surface
oxide [7]. The oscillation period and electric field
intensity of macroporous silicon structures with SiO2
nanocoatings fluctuate around constant value at low
spectral light energies and become quadratic in the
photon energy depending on geometrical sizes of silicon
matrix and SiO2 nanocoatings (see Fig. 8). The electric
field intensity growth corresponds to the quasi-guided
mode formation [21] in the silicon matrix (minimal
distance between macropores) with
2SiOSi2 dDa pp and in silicon column with
2SiOSi 4.12 dDa pp . The mode parameter [22]
QSi ~ kSi is determined by the beginning of the photon
energy quadratic growth (Fig. 8, curves 1–4).
In general, at grazing angle of light incidence [23],
the electric field of the reflected electromagnetic wave
changes the local electric field in the near-surface region
of the macropore walls with the thickness d ≈ 0.1 for
the wavelength . Let us consider that d is determined by
the electric component of electromagnetic wave with ħ
and by the change of built-in electric field Fs (d =
ħ/(eFs)). Indeed, under our experimental condition of
the grazing angle of light incidence onto the macropore
surface, the electric field intensity on macroporous
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 4. P. 377-384.
doi: 10.15407/spqeo18.04.377
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
382
Table. Structure parameters and the electric field intensity.
Nanocrystal
material
Oscillation
period, meV
Lattice
parameter,
a, nm
Band
gap, eV
Nanocrystal
size,
d, nm
Electric field
intensity
F = E/a, V/cm
Electric field
intensity
F = E/d, V/cm
Si 1.4 0.6 0.54 1.1 2…3 (1.5…4)104 (3…9)103
CdTe 3.9 0.5 0.65 1.6 20 (6…8)104 (2…3)103
silicon surface for structures with SiO2 nanocoatings is
about Fs + Fs with Fs ≈ ħ
/(0.11) ~ ħ
2
according to
the experiment (Fig. 4). The light wavelength is equal to
1 = /ni (ni is effective refractive index of pores with
SiO2 nanocoatings or refractive index of SiO2
nanocoatings).
The electric component of electromagnetic wave
changes from E = ħ to zero when going from the silicon
matrix to macropore. It corresponds to a reverse bias at
the surface barrier, and its band bending grows. This
effect is strong for light energy corresponding to the
spectral range of quasi-guided (leakage) mode formation
in the silicon matrix (see Fig. 8). This result differs from
that for macroporous silicon structures without
nanocoatings [8], where the Fs change was observed
over for all the spectral range investigated due to the low
surface barrier. On the other hand, the electric field
intensity in the macroporous silicon structures with
microporous layer and surface nanocrystals [21] does not
change in the short wavelength spectral range (Fig. 8,
curve 5). Constant oscillation period (and electric field
intensity) in macroporous silicon structures with
nanocrystals may be attributed to absence of the quasi-
guided mode formation in the silicon matrix due to strong
light scattering by nanocrystals. As a result, the local
electric field variation in the heterojunction area became
negligible in comparison with macroporous silicon
structures with SiO2 nanocoating. In addition, the
nanocrystals form local contact with silicon surface, thus
increasing the constant local electric field intensity and
decreasing the surface level concentration. That is why
harmonic oscillations with low period deviations were
measured in absorption spectra of the macroporous silicon
structures with the contacted CdTe nanocrystals (Fig. 1).
In general, the absorption spectra of macroporous
silicon without nanocoatings agree well with the
corresponding spectral dependences of the electro-
optical energy and the imaginary part of permittivity in
the weak electric field approximation, thus confirming
realization of the impurity FranzKeldysh effect [8]. At
a higher light intensity, the impurity absorption bands
become wider, the broadening parameter grows, and
oscillations are not observed (Fig. 1a, curve 3). But the
oscillator structure was restored in the absorption spectra
of macroporous silicon with nanocoatings. In the latter
case, recharging the local surface centers at energies
below those of the indirect interband transition (the
impurity FranzKeldysh effect) was added by resonance
electron scattering (the WannierStark effect) due to a
higher electric field intensity at the silicon–nanocoating
interface and electron motion from silicon matrix to the
surface states in the heterojunction area. Really, the
near-IR absorption is a result of electron transitions from
the v-band to empty acceptor surface levels, and the
Fermi level shifts to the c-band. It increases the free
electron concentration in the c-band. The electron
transitions and free electron motion are realized due to
additional change in the local electric field as a result of
grazing light incidence and quasi-guided mode
formation (for SiO2 nanocoatings only). Observation of
coherent electronic wave-packet oscillations in a
semiconductor heterostructure at room temperature was
reported in [24], too.
5. Conclusions
We have observed well-separated oscillations in
absorption spectra of macroporous silicon structures
with surface nanocrystals and with SiO2 nanocoatings.
The amplitude of oscillations is maximal within the
spectral ranges of organic species, Si–H, C–H and O–H
bonds. The results obtained indicate strong influence of
impurity states on the surface of macroporous silicon
60 80 100 200 400 600 800 1000
5000
10000
50000
100000
500000
F
,
V
/c
m
Photon energy, meV
1
2
34
5
Fig. 8. Spectral dependences of the electric field intensity F
on macroporous silicon surface for structures with SiO2
nanocoatings of 200 nm (1) and 800 nm (2) thickness; and
spectral dependences of the electric field intensity as a result
of quasi-guided mode formation in the silicon matrix (3, 4);
spectral dependences of the electric field intensity in the
macroporous silicon structures with CdTe surface
nanocrystals (5).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 4. P. 377-384.
doi: 10.15407/spqeo18.04.377
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
383
structures with nanocoatings. It is caused by resonant
electron scattering by the impurity states in the electric
field of “silicon-nanocoating” heterojunction on
macropore surface. The constant oscillation period
specifies realization of the Wannier–Stark effect on the
randomly distributed surface bonds on nanocrystals. The
amplitude of scattering has resonant behavior in the case
of electron scattering by impurity, and the difference
between two resonant energies is equal to the Wannier–
Stark ladder. The oscillation periods E of macroporous
silicon structures with surface nanocrystals and
microporous layers mainly depend on the band gap of
the nanocrystal material. The electric field intensity F is
10
4
…10
5
V/cm for F = E/a, thus indicating validity of
the model of electron oscillations in the atomic lattice.
The Wannier–Stark ladder is not broken by
impurities, if the intervals between the transitions due to
scattering by impurity atoms with the lifetime are
wider than the period of electron oscillations in the
external field, TB. The lifetime ratio is /TB > 1 in all the
spectral regions considered for macroporous silicon
structures with CdTe surface nanocrystals, taking into
account that the surface impurity concentration for
macroporous silicon structures is less than Ni ≈
510
10
cm
–2
. And the inequality /TB > 1 holds over the
whole spectral regions considered for macroporous
silicon structures with SiO2 nanocoatings, taking into
account that the surface impurity concentration Ni for
macroporous silicon structures is less than 5∙10
11
cm
–2
.
The oscillation period and electric field intensity in
the macroporous silicon structures with SiO2 nanocoatings
fluctuate around constant value at low photon energies
and become quadratic in photon energy depending on the
geometrical sizes of silicon matrix and SiO2 nanocoatings.
The relevant electric field intensity growth corresponds to
quasi-guided mode formation in the silicon matrix
(minimal distance between the macropores) and in the
silicon column. The local electric field variation was
observed in macroporous silicon structures without
nanocoatings within all the investigated spectral range due
to the low surface barrier. The electric field intensity in
the macroporous silicon structures with microporous layer
and surface CdTe nanocrystals does not change due to the
high surface barrier, strong light scattering by
nanocrystals and absence of quasi-guided mode formation
in the silicon matrix.
In general, for macroporous silicon structures with
nanocoatings, the near-IR absorption is a result of
electron transitions from the v-band to empty acceptor
surface levels, and the Fermi level shifts to the c-band. It
increases the free electron concentration in the c-band.
The electron transitions and free electron motion are
realized due to additional change in the local electric
field as a result of grazing light incidence and quasi-
guided mode formation (for SiO2 nanocoatings only).
Resonant electron scattering gives rise to the resonances
of the permittivity at room temperature and a
corresponding change in absorption.
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