Effect of microwave electromagnetic radiation on the structure, photoluminescence and electronic properties of nanocrystalline silicon films on silicon substrate
We studied the effect of microwave electromagnetic radiation on silicon low-dimensional structures. The nanocrystalline silicon (nc-Si) films on p-Si substrate were formed with pulsed laser ablation. The surface morphology of films was studied with atomic force microscopy. We made X-ray phase analys...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
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| Zitieren: | Effect of microwave electromagnetic radiation on the structure, photoluminescence and electronic properties of nanocrystalline silicon films on silicon substrate / E.B. Kaganovich, I.M. Kizyak, S.I. Kirillova, R.V. Konakova, O.S. Lytvyn, P.M. Lytvyn, E.G. Manoilov, V.E. Primachenko, I.V. Prokopenko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2003. — Т. 6, № 4. — С. 471-478. — Бібліогр.: 12 назв. — англ. |
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irk-123456789-1180902017-05-29T03:05:29Z Effect of microwave electromagnetic radiation on the structure, photoluminescence and electronic properties of nanocrystalline silicon films on silicon substrate Kaganovich, E.B. Kizyak, I.M. Kirillova, S.I. Konakova, R.V. Lytvyn, O.S. Lytvyn, P.M. Manoilov, E.G. Primachenko, V.E. Prokopenko, I.V. We studied the effect of microwave electromagnetic radiation on silicon low-dimensional structures. The nanocrystalline silicon (nc-Si) films on p-Si substrate were formed with pulsed laser ablation. The surface morphology of films was studied with atomic force microscopy. We made X-ray phase analysis of films and measured strains in the structures obtained using X-ray diffractometry. We also investigated the time-resolved photoluminescence (PL) spectra and temperature dependence of photovoltage for the nc-Si/p-Si and nc- Si<Au>/p-Si structures, both before and after exposure to magnetron microwave radiation of moderate (1.5 W/cm²) irradiance. It was shown that after microwave irradiation photovoltage in the nc-Si films, as well as electron trap concentration in both the films and p-Si substrates, decrease. After irradiation of the nc-Si/p-Si structures the density of interfacial electron states (IES) decreases, while both PL intensity and relaxation time increase. At the same time irradiation of the nc-Si<Au>/p-Si structures that had high values of PL intensities and relaxation times before irradiation results in decrease of these values, as well as somewhat increases the density of IES. Higher (7.5 W/cm2) irradiance of microwave field impairs the PL properties (to the point of complete disappearance of PL). In addition it induces changes in film structure resulting, in the course of time, in decrease of strains in the structures studied. We discuss some mechanisms for microwave field effect on the properties of these structures. 2003 Article Effect of microwave electromagnetic radiation on the structure, photoluminescence and electronic properties of nanocrystalline silicon films on silicon substrate / E.B. Kaganovich, I.M. Kizyak, S.I. Kirillova, R.V. Konakova, O.S. Lytvyn, P.M. Lytvyn, E.G. Manoilov, V.E. Primachenko, I.V. Prokopenko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2003. — Т. 6, № 4. — С. 471-478. — Бібліогр.: 12 назв. — англ. 1560-8034 PACS: 73.63.Bd, 78.55.Ap, 78.67.Bf http://dspace.nbuv.gov.ua/handle/123456789/118090 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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We studied the effect of microwave electromagnetic radiation on silicon low-dimensional structures. The nanocrystalline silicon (nc-Si) films on p-Si substrate were formed with pulsed laser ablation. The surface morphology of films was studied with atomic force microscopy. We made X-ray phase analysis of films and measured strains in the structures obtained using X-ray diffractometry. We also investigated the time-resolved photoluminescence (PL) spectra and temperature dependence of photovoltage for the nc-Si/p-Si and nc- Si<Au>/p-Si structures, both before and after exposure to magnetron microwave radiation of moderate (1.5 W/cm²) irradiance. It was shown that after microwave irradiation photovoltage in the nc-Si films, as well as electron trap concentration in both the films and p-Si substrates, decrease. After irradiation of the nc-Si/p-Si structures the density of interfacial electron states (IES) decreases, while both PL intensity and relaxation time increase. At the same time irradiation of the nc-Si<Au>/p-Si structures that had high values of PL intensities and relaxation times before irradiation results in decrease of these values, as well as somewhat increases the density of IES. Higher (7.5 W/cm2) irradiance of microwave field impairs the PL properties (to the point of complete disappearance of PL). In addition it induces changes in film structure resulting, in the course of time, in decrease of strains in the structures studied. We discuss some mechanisms for microwave field effect on the properties of these structures. |
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Article |
| author |
Kaganovich, E.B. Kizyak, I.M. Kirillova, S.I. Konakova, R.V. Lytvyn, O.S. Lytvyn, P.M. Manoilov, E.G. Primachenko, V.E. Prokopenko, I.V. |
| spellingShingle |
Kaganovich, E.B. Kizyak, I.M. Kirillova, S.I. Konakova, R.V. Lytvyn, O.S. Lytvyn, P.M. Manoilov, E.G. Primachenko, V.E. Prokopenko, I.V. Effect of microwave electromagnetic radiation on the structure, photoluminescence and electronic properties of nanocrystalline silicon films on silicon substrate Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Kaganovich, E.B. Kizyak, I.M. Kirillova, S.I. Konakova, R.V. Lytvyn, O.S. Lytvyn, P.M. Manoilov, E.G. Primachenko, V.E. Prokopenko, I.V. |
| author_sort |
Kaganovich, E.B. |
| title |
Effect of microwave electromagnetic radiation on the structure, photoluminescence and electronic properties of nanocrystalline silicon films on silicon substrate |
| title_short |
Effect of microwave electromagnetic radiation on the structure, photoluminescence and electronic properties of nanocrystalline silicon films on silicon substrate |
| title_full |
Effect of microwave electromagnetic radiation on the structure, photoluminescence and electronic properties of nanocrystalline silicon films on silicon substrate |
| title_fullStr |
Effect of microwave electromagnetic radiation on the structure, photoluminescence and electronic properties of nanocrystalline silicon films on silicon substrate |
| title_full_unstemmed |
Effect of microwave electromagnetic radiation on the structure, photoluminescence and electronic properties of nanocrystalline silicon films on silicon substrate |
| title_sort |
effect of microwave electromagnetic radiation on the structure, photoluminescence and electronic properties of nanocrystalline silicon films on silicon substrate |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2003 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/118090 |
| citation_txt |
Effect of microwave electromagnetic radiation on the structure, photoluminescence and electronic properties of nanocrystalline silicon films on silicon substrate / E.B. Kaganovich, I.M. Kizyak, S.I. Kirillova, R.V. Konakova, O.S. Lytvyn, P.M. Lytvyn, E.G. Manoilov, V.E. Primachenko, I.V. Prokopenko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2003. — Т. 6, № 4. — С. 471-478. — Бібліогр.: 12 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
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2025-07-08T13:20:56Z |
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2025-07-08T13:20:56Z |
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471© 2003, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Semiconductor Physics, Quantum Electronics & Optoelectronics. 2003. V. 6, N 4. P. 471-478.
PACS: 73.63.Bd, 78.55.Ap, 78.67.Bf
Effect of microwave electromagnetic radiation
on the structure, photoluminescence and
electronic properties of nanocrystalline
silicon films on silicon substrate
E.B. Kaganovich, I.M. Kizyak, S.I. Kirillova, R.V. Konakova, O.S. Lytvyn, P.M. Lytvyn,
E.G. Manoilov, V.E. Primachenko, I.V. Prokopenko
V. Lashkaryov Institute of Semiconductor Physics, NAS Ukraine, 41, prospect Nauky, Kyiv, 03028, Ukraine
Phone: +380 (44) 2656182; Fax: +380 (44) 2658342; E-mail: silitech@ukr.net
Abstract. We studied the effect of microwave electromagnetic radiation on silicon low-dimen-
sional structures. The nanocrystalline silicon (nc-Si) films on p-Si substrate were formed with
pulsed laser ablation. The surface morphology of films was studied with atomic force
microscopy. We made X-ray phase analysis of films and measured strains in the structures
obtained using X-ray diffractometry. We also investigated the time-resolved photolumines-
cence (PL) spectra and temperature dependence of photovoltage for the nc-Si/p-Si and nc-
Si<Au>/p-Si structures, both before and after exposure to magnetron microwave radiation of
moderate (1.5 W/cm2) irradiance. It was shown that after microwave irradiation photovoltage
in the nc-Si films, as well as electron trap concentration in both the films and p-Si substrates,
decrease. After irradiation of the nc-Si/p-Si structures the density of interfacial electron states
(IES) decreases, while both PL intensity and relaxation time increase. At the same time irra-
diation of the nc-Si<Au>/p-Si structures that had high values of PL intensities and relaxation
times before irradiation results in decrease of these values, as well as somewhat increases the
density of IES. Higher (7.5 W/cm2) irradiance of microwave field impairs the PL properties (to
the point of complete disappearance of PL). In addition it induces changes in film structure
resulting, in the course of time, in decrease of strains in the structures studied. We discuss
some mechanisms for microwave field effect on the properties of these structures.
Keywords: nanocrystalline silicon, microwave irradiation, photoluminescence, photovoltage,
residual strains.
Paper received 05.09.03; accepted for publication 11.12.03.
1. Introduction
Silicon nanocrystals (Si NC) dispersed in a dielectric
medium (mostly in silicon oxide SiOx with x ≤ 2) form
nanocomposites. The latter demonstrate efficient photo-
luminescence (PL) in the visible spectral range at room
temperature. Among such composites are porous silicon
(por-Si) formed with chemical etching of single-crystal-
line silicon (p-Si), as well as nanocrystalline silicon (nc-
Si) films obtained using sputtering, chemical deposition,
implantation, laser ablation and other techniques. These
materials offer promise as a basis for development of sili-
con light-emitting facilities integrable with elements of
microelectronics. Investigations of the features of the ef-
fect of electromagnetic radiation on the structural, PL
and electronic properties of nc-Si were performed to de-
velop technological processes for purposeful changes of
these properties, as well as for determination of such ir-
radiation conditions at which the above properties still
remain stable.
There exist some information on the physical proc-
esses in por-Si induced by a high-power laser beam (see,
e.g., [1]) and degradation of por-Si PL under 60Ñî γ-
irradiation [2]. However, the effect of microwave (radio-
frequency region) electromagnetic radiation on nc-Si and
its PL (in the visible spectral region) still remains practi-
cally unexplored. At the same time microwave treatments
are known to be promising for changing the structural,
physico-chemical and electrophysical properties of a
number of semiconductor materials and device structures
472
SQO, 6(4), 2003
E.B. Kaganovich et al.: Effect of microwave electromagnetic radiation on ...
[3]. The objective of our investigations was to determine
how microwave radiation affects the structure, PL spec-
trum and electronic properties of nc-Si films obtained with
pulsed laser deposition onto p-Si substrates.
2. Experimental procedure
We obtained PL nc-Si films using the pulsed laser deposi-
tion technique [4]. A Q-switched YAG:Nd3+-laser had
the following operating characteristics: wavelength λ =
= 1.06 µm, pulse irradiance of ~20 J/cm2, pulse duration
of 10 ns and repetition rate of 25 Hz. A laser beam
scanned a target (made of c-Si, grade ÊÄÁ-10) without
or with a deposited gold film ~80 nm thick (Fig.1). The
target and substrate (located in the target plane) were in
a vacuum chamber filled with Ar (pressure of ~13 Pa).
The interaction between argon atoms and erosion cone
particles led to nc-Si film deposition from the reverse par-
ticle flow onto the p-Si substrate. At that size separation
of Si NC occurs. At substrate sections located closer to
the erosion cone axis the thicker films with bigger Si NC
are growing. The film length was 12 mm, and the film
thickness varied from 500 down to 50 nm. The film depo-
sition rate varied at different substrate sections from 20
down to 2 nm/min. The film porosity ranged up to 20�
30%. For these films the PL spectrum ranged from 1.4 up
to 3.2 eV at room temperature.
It was found in earlier works [5,6] that visible PL of
these films is determined by the quantum confinement and
dielectric effects; the predominant radiation mechanism
is annihilation of electron-hole excitations. In [7] it was
shown that doping of these films with electropositive metal
(gold) leads to increase of PL intensity and shift of PL
spectrum to the red region, as well as to increase of the
PL relaxation time and stability. These effects were at-
tributed to passivation by gold silicon dangling bonds at
the Si NC surface, as well as gold action as a catalyst of
NC oxidation [8]. The efficiency of gold action was due
to higher values of the first ionization energy and elec-
tron affinity of gold atom, as compared to the correspond-
ing values of other metals.
Both undoped and Au-doped samples were exposed
to microwave radiation of the cm wavelength range
(magnetron, frequency of 2.45 GHz). The output irradi-
ance was 1.5 and 7.5 W/cm2; time of exposure te was var-
ied from 5 up to 15 s.
The film surface morphology was studied with the
atomic force microscopy (AFM) using a microscope
NanoScope IIIa (Digital Instruments) operating in the
tapping mode. Both before and after each measurement
the probes were tested using the test structures produced
by NT-MDT (Russia). We applied only those probes
which had high symmetry and tip radius below 7 nm.
The strains of the p-Si substrate near the nc-Si/p-Si
heteroboundary were determined from the results of X-
ray diffraction (XRD) measurements of the radius of cur-
vature R of the substrate near-surface crystallographic
planes. The strain value was estimated from the relation
ε ~ t/2R, t is the substrate thickness. The structure sur-
face profiles were also recorded with a profilometer
DEKTAK 3030. The results obtained correlated with
those of XRD measurements concerning the structure
bending sign.
An attempt to determine phase composition of the films
with X-ray technique (using CuKα-radiation and setup
with a focusing monochromator) has shown that most of
the film volume is an X-ray-amorphous phase. Weak dif-
fraction peaks of polycrystalline silicon, without preferred
orientation of Si NC, could be observed against a back-
ground of halo due to the above phase. The fraction of
silicon oxide was small.
For the nc-Si/p-Si systems we took (both before and
after microwave irradiation) time-resolved PL spectra [5-
7] and temperature dependencies of the photovoltage
appearing at nc-Si/p-Si system illumination with light
pulses of high intensity [8,9]. PL was excited with nitro-
gen laser radiation (λ = 337 nm, τ = 8 ns). Stroboscopic
registration of a signal was made in the photon-counting
mode. The minimal duration of the gate during which
photons were collected was 250 ns. The relaxation times
below that value were estimated from the oscillograph
patterns. Usually the successive gates were registered with
a retardation of the measuring gate relative to the laser
pulse by an integer number of gate duration. When meas-
uring the maximal (several tens of µs) relaxation times,
both gate duration and retardation were arbitrarily in-
creased.
To measure photovoltage, we have installed a meas-
uring capacitor in the vacuum cryostat. This capacitor
involved the p-Si/nc-Si system and mica (pressed to the
film) with a semitransparent conducting SnO2<Sb> layer.
We measured (i) the photovoltage Vph that appeared in
the substrate under illumination of the capacitor with
pulses of red light, and (ii) the total photovoltage in the
film and substrate that appeared under illumination with
pulses of white light. In the first case (red light), the
7
56
2
3
8
4
1
Fig. 1. Schematic of the vacuum chamber used in the PLA tech-
nique for nc-Si films: 1 � YAG:Nd3+-laser beam; 2 � gas leak-in;
3 � vacuum chamber; 4 � erosion cone; 5 � target; 6, 7 � substrates;
8 � vacuum pump.
E.B. Kaganovich et al.: Effect of microwave electromagnetic radiation on ...
473SQO, 6(4), 2003
photovoltage Vph (taken with opposite sign) is equal to
the surface potential ϕs of the p-Si substrate. The
photovoltage was registered using an oscillograph with
memory. A flash tube ÈÑØ-100 (pulse duration of 10 ms,
pulse intensity of 1021 photons/cm2⋅s) served as light
source.
3. Results and discussion
The results of our AFM studies of surface morphology of
nc-Si films are presented in Figs 2 and 3. The films were
a nanograin aggregates whose grains consisted of Si NC
enveloped with silicon oxide (Fig. 2). The grain size and
distribution character were determined by the distance
from the erosion cone and the fact if the film has been Au-
doped during it deposition. It was also found that if the
films were formed from the reverse flow of particles, then
their surface was more uniform and grains were smaller
as compared to the corresponding characteristics for the
case when the films were deposited from the direct flow.
The grains in film areas located farther from the erosion
cone are smaller, i.e., AFM studies confirmed variation
of Si NC sizes along the film (see Fig. 2a and b, c and d).
The film surfaces are presented in Fig. 2 as height
maps. The height values are reproduced with color gra-
dation in accordance with the scale to the right of the
Fig. 2. Shown in Fig. 3a, b are the quantitative charac-
teristics of these surfaces as relative distributions of the
relief heights and grain diameters. One can conclude from
Fig. 3a that the relief heights for thin film areas (obtained
at a distance of about 12 mm from the erosion cone axis)
have lower dispersion (± 2 nm), while for thick film areas
(obtained closer to the erosion cone axis) the height dis-
persion is higher (± 6 nm). At that Au-doping does not
lead to changes in the relief height distribution.
One can see from Fig. 3b that, as distance from the
erosion cone axis grows, the film grain diameters consid- erably decrease (as well as their dispersion). The undoped
films are characterized by more uniform size distribu-
tion of grains. Contrary to this, the Au-doped films dem-
onstrate nonuniform size distribution, namely, there are
several well-defined characteristic grain sizes in the right
section of the plot. This means that gold favors coagula-
tion of some Si NC into larger grains.
It should be noted that, when AFM-analyzing grains
several nm in diameter, the sizes obtained are overesti-
mated due to the effect of �superposition� of the tip form
on the relief details (the so-called erosion effect). Know-
ing tip form from the results of testing, we have made the
surface reconstruction procedure (subtraction of the ero-
sion effect). The grain sizes for the reconstructed surface
are presented in Table 1.
We believe that the reason for additional coagulation
of NC at doping with gold is different substrate condi-
tions at target sputtering with a laser beam. When a
substrate (which, after HF treatment, had mainly hydride
coating before being placed into the vacuum chamber) is
doped with gold, then gold ions and atoms are deposited
first of all. They form a surface phase with silicon [8].
This is supported by further data on decrease of the den-
a
b
c
d
0.0 nm
7.5 nm
15.0 nm
Fig. 2. Surface morphology of nc-Si films: a, b � undoped; c, d �
Au-doped; a, c � thin with smaller Si NC; b, d � thick with larger
Si NC.
�6 �4 �2 0 2 4 6 8
0.0
0.1
0.2
0.3
0.4
P
ro
b
a
b
ili
ty
,
a
rb
.
u
n
it
s
Height, nm
4
3
2
1 a
0 10 20 30 40 50
0.0
0.1
0.2
0.3
P
ro
b
a
b
il
it
y
,
a
rb
.
u
n
it
s
Diameter, nm
43
21
b
Fig. 3. Relative distribution of relief heights (a) and NC dia-
meters (b) in nc-Si films (from direct AFM measurements): 1 and
2 � undoped; 3 and 4 � Au-doped thin (1, 3) and thick (2, 4).
474
SQO, 6(4), 2003
E.B. Kaganovich et al.: Effect of microwave electromagnetic radiation on ...
sity of interfacial electron states (IES) at the nc-Si<Au>/
p-Si interface. Thus deposition of silicon particles from
the erosion cone onto the substrate with gold and without
it proceeds under different conditions. Previously depos-
ited gold favors more intense coagulation of some of Si
NC into larger grains.
Both doped and undoped nc-Si films deposited onto
the p-Si substrate are in a nonequilibrium state. Proc-
esses of structural ordering are to occur in them under
various external actions, in particular, microwave irra-
diation. AFM studies of the films exposed to microwave
radiation (even of the maximal � 7.5 W/cm2 � irradian-
ce) did not revealed noticeable changes in their surface
morphology. The results of high information ability have
been obtained, however, when measuring radius of cur-
vature for the p-Si, p-Si/nc-Si and p-Si/nc-Si<Au> sam-
ples with XRD technique (Fig. 4). For the samples con-
sisting of the p-Si substrate only, the radius of curvature
R was about 30 m; the samples were convex toward the
side at which the nc-Si films were later deposited (curva-
ture sign �plus�). The calculation gave for the strain ε
the value of about 5⋅10�6. Stressed state of the initial
substrate was due to different treatments of its sides. That
used for nc-Si films deposition was at first chemo-dynami-
cally treated, then treated in HF and washed in water. As
a result, it had a hydride coating that, being exposed to
air, gradually gave place to a thin oxide film. Another
side of the substrate experienced only mechanical lap-
ping before the HF treatment. One can see from Fig. 4
that the radius of curvature of the p-Si substrate practi-
cally did not change after microwave irradiation followed
by keeping out of doors.
After deposition of an undoped nc-Si film onto the
p-Si substrate, the strain in the system studied dropped
(R = 84 m). Microwave irradiation somewhat increased
strain (R = 75 m), but then, after keeping the sample out
of doors for 120 h, strains strongly relaxed (the radius of
curvature grew up to 1444 m). Very interesting results
were obtained after deposition of an nc-Si<Au> film onto
the substrate. In this case the radius of curvature changed
its sign, i.e., the nc-Si<Au>/p-Si system became concave
toward the film, and its strain considerably decreased
(R = �102 m). This fact can be related to both the above
mechanism for a gold-silicon phase formation at the film/
substrate interface and thermal strains caused by distinc-
tion between the film and substrate thermal expansion
coefficients.
Changes of sign and value of the radius of curvature
immediately after microwave irradiation (R = 50 m) seems
to result from both disordering of the above surface phase
and structure defects redistribution under microwave field
action. However, during keeping samples out of doors
that phase restores and metastable defect clusters disso-
ciate. As a result, the radius of curvature becomes nega-
tive again, and its magnitude substantially increases
(R = �289 m). Thus in both cases (undoped and Au-doped
films on the p-Si substrate) microwave irradiation induces
processes of structure relaxation proceeding after it. As a
result, the residual strains in the nc-Si/p-Si and nc-
Si<Au>/p-Si systems drop abruptly.
Fig. 5 shows the time-resolved PL spectra for undoped
(a) and Au-doped (b, c, d) films before (curve 1a and
Fig. 5b) and after (curves 2�4 in Fig. 5à and curves in
Figs. 5c and 5d) microwave irradiation. The spectra range
from 1.4 up to 3.2 eV. In our previous works [5�7] we gave
interpretation of the PL spectra of these films before mi-
crowave irradiation depending on the formation condi-
tion, as well as explained spectra transformation during
PL relaxation. For the undoped nc-Si/p-Si structure the
character of PL spectral dependence practically did not
change after microwave irradiation. The PL intensity,
however, at moderate (1.5 W/cm2) irradiance increased
by a factor of 2�3 with time of sample exposure te in mi-
crowave field (Fig. 5a, curves 2 and 3 te = 5 and 15 s). At
bigger irradiance (7.5 W/cm2, te = 15 s) the PL intensity
decreased (curve 4). The PL relaxation time t (which is
determined mainly by the time of nonradiative recombi-
nation of nonequilibrium electron-hole pairs) varied with
PL intensity. After exposure to moderate microwave irra-
diance the PL relaxation time grew from 50 up to 100�
�400
0
400
800
1200
1600
cba
R
a
d
iu
s
o
f
cu
rv
a
tu
re
,
m
�289
1444
365075
32
�102
84
30
321321321
Fig. 4. Variation dynamics for radii of curvature of structures: 1 �
substrate; 2 � substrate with undoped film; 3 � substrate with Au-
doped film. a � initial; b � after microwave irradiation for 5 s; c �
after further keeping out of doors for 120 h.
Table 1. Grain diameters determined by direct AFM measure-
ments and after surface reconstruction.
Grain diameter Grain diameter Degree
(direct AFM (reconstructed of diameter
measurements), surface), nm overestimation,
nm %
3.20 1.18 63.09
5.42 2.51 53.67
10.00 6.70 41.00
12.80 8.85 30.88
17.83 14.83 16.80
20.62 18.05 12.45
25.06 23.43 6.52
E.B. Kaganovich et al.: Effect of microwave electromagnetic radiation on ...
475SQO, 6(4), 2003
200 ns, and after strong (irradiance of 7.5 W/cm2) micro-
wave action it dropped to several tens of ns.
All the spectral curves in Fig. 5a were taken during
the first gate of PL relaxation (t < 250 ns). For Au-doped
structures (where the PL relaxation time increased al-
most by three orders of magnitude) the PL spectra before
(Fig. 5b) and after (Fig. 5c, d) microwave irradiation are
presented for two different gates. The spectra obtained
during the first gate (their intensities are decreased by a
factor of ten) are shown with broken curves. Full curves
corresponding to higher intensities IPL represent the spec-
tra obtained during the second gate (250 ns < τ < 500 ns),
while the curves corresponding to lower IPL values show
the spectra obtained during further gates. (In Figs. 5b, c,
d the curves corresponding to the lowest IPL values were
obtained for the PL relaxation times of 1.8, 1.2 and 12 µs,
respectively).
Contrary to undoped structure, irradiation of as-pre-
pared nc-Si<Au>/p-Si structure with microwave field
(1.5 W/cm2 , te = 15 s) results in impairment of PL pa-
rameters (Fig. 5ñ): (i) the integral (i.e., sum over all the
PL components with different relaxation times) PL in-
tensity becomes several times below; (ii) the PL relaxa-
tion time goes down to 1 µs; (iii) the spectrum is modified,
first of all, due to intensity drop for the low-energy band.
After irradiation with microwave field (irradiance of
7.5 W/cm2, te = 15 s) PL disappears completely.
Another situation was realized after microwave irra-
diation (irradiance of 1.5 W/cm2, te = 15 s) of an Au-
doped structure that previously was kept out of doors for
8 months, i.e., was aged. The spectra of such structure
after microwave irradiation are presented in Fig. 5d. In
this case the structure is more tolerant to action of micro-
wave field: the PL relaxation time remains sufficiently
high (~20 µs); the PL intensity dropped but slightly, es-
pecially in the low-energy region (~1.6 eV) where an in-
tense PL band was observed both before and after irra-
diation.
When measuring the temperature dependencies of the
capacitor photovoltage Vph, we illuminated the nc-Si/p-
Si structures alternately by red and white light pulses.
Figures 6à, b show the Vph(T) curves for undoped and
Au-doped structures, respectively (the photovoltage at the
semitransparent SnO2<Sb> electrode was negative). The
curves 1, 1′, 2 and 2′ were obtained before microwave
10
20
a
4
3
2
1
IPL, arb. units
10
20 ×0.1
b
10
20
×0.1
c
1.6 2.0 2.4 2.8 3.2
10
20 ×0.1
d
h n , eV
Fig. 5. Time-resolved PL spectra of undoped (a) and Au-doped
(b, c, d) nc-Si/p-Si structures before (curves 1a, b) and after (curves
2�4a, c, d) microwave irradiation with irradiance of 1.5 W/cm2
(curve 4a � 7.5 W/cm2). Time of exposure 15 s (curve 2a � 5 s).
Curves a and broken curves b, c, d were obtained in the τ <
< 250 ns gate. Full curves b, c, d are given for further gates; the
lowest curves correspond to the PL relaxation times of 1.8, 1.2
and 12 µs. The structure d was aged out of doors.
100 150 200 250 300
0.10
0.15
0.20
0.25
0.30
0.35
0.40
T, K
3'
4'
3
4
1'
2'
1
2
�Vph , V
a
Fig. 6. Photovoltage for undoped (a) and Au-doped (b) struc-
tures as function of temperature: 1, 1′, 2, 2′ � before, 3, 3′, 4, 4′ �
after microwave irradiation at the first (1, 2, 3, 4) and second (1′,
2′, 3′, 4′) pulses of red (1, 1′, 3, 3′) and white (2, 2′, 4, 4′) light.
T, K
�Vph, V
100 150 200 250 300
0.10
0.15
0.20
0.25
3 '
4 '
3
4
1 ' 2 '
1
2
b
476
SQO, 6(4), 2003
E.B. Kaganovich et al.: Effect of microwave electromagnetic radiation on ...
irradiation of the structures; the curves 3, 3′, 4 and 4′
were obtained after microwave irradiation, at the first (1,
2, 3 and 4) and second (1′, 2′, 3′ and 4′) pulses of red (1, 1′,
3 and 3′) and white (2, 2′, 4 and 4′) light.
The curves 1, 1′, 3 and 3′ show variation of the poten-
tial ϕs = �Vph at the p-Si substrate boundary with tem-
perature. The Vph values measured at the second pulse
(or any next pulse from their group) had lower magni-
tude than those measured at the first pulse. This effect
was observed starting from certain temperatures (which
were somewhat different for different structures) with their
further decreasing. This means that at low temperatures
the optical memory effects exist [10]. They are due to
trapping of electrons by traps located at the p-Si substrate/
nc-Si film interface (at illumination with red light) or by
both the above traps and those in the film (at illumination
with white light). In presence of the optical memory ef-
fects the measurements were made with warming the sam-
ples up (see [9, 11, 12].
One can see from Fig.6 that the Vph(T) curves change
as a result of microwave irradiation (1.5 W/cm2, 15 s).
The corresponding |Vph| values at the same temperatures
decrease, and the very character of the curves changes,
especially for the undoped structure. For all the curves at
T > 200�240 K the Vph values obtained at illumination
with red and white light are the same. This is evidence
that no considerable photovoltage appears at such tem-
peratures in the nc-Si films illuminated with pulses of
white light whose high-energy part is absorbed in them.
Increase of |Vph| at lowering temperature in that region is
due to charging of IES of the p-Si substrate with holes
when the Fermi level in the silicon bulk goes to the va-
lence band. The calculations (similar to those made in
[10]) showed that before microwave irradiation the Fermi
level in p-Si was practically pinned near the Si midgap Ei
due to high (over 1012 cm�2⋅eV�1) density of IES at the
p-Si/undoped nc-Si film interface. After microwave irra-
diation the density of IES decreases considerably and is
about 3⋅1011 cm�2⋅eV�1 near Ei.
One can see from the Vph(T) curves (Fig. 6à) obtained
at the first red light pulses (curves 1 and 3) that, starting
from certain temperature, the |Vph| values begin to de-
crease when temperature goes down. Such decrease of
|Vph| = ϕs is related to transformation of the IES system
due to reversible structure changes at the nc-Si/p-Si inter-
face [11]. For structures exposed to microwave irradia-
tion decrease of js gives place to its growth at T < 120�
140 K (curve 3). This results from the competition be-
tween the processes of transformation of the IES system
and charging IES with holes as temperature varies.
No transformation of the IES system was observed for
the aged Au-doped nc-Si/p-Si structure over the whole
temperature range studied (300�100 K), both before and
after microwave irradiation (Fig. 6b, curves 1 and 3).
This makes it possible to study, as temperature varies, a
wider portion of the silicon gap and calculate the density
of IES in that range [10]. Fig. 7 shows the density of IES
below Ei in the energy range 0.14�0.31 eV before (curve
1) and after (curve 2) microwave irradiation of the nc-
Si<Au>/p-Si structure. One can see that before micro-
wave irradiation two discrete levels � at Ei � 0.18 eV and
Ei � 0.23 eV � show themselves. The corresponding den-
sities of state are 3.2⋅1011 and 4⋅1010 cm�2⋅eV�1.
It should be noted that the density of IES after doping
wit Au is low. This is related to formation of a gold-sili-
con phase at the p-Si substrate surface [8]. After micro-
wave irradiation these levels shift from Ei. Their energy
positions become Ei � 0.23 eV and Ei � 0.27 eV, while the
corresponding densities increase up to 3.8⋅1011 and
5.8⋅1010 cm�2⋅eV�1, respectively. Thus moderate micro-
wave irradiation decreases the density of IES at the p-Si
substrate for the undoped structure and somewhat increase
it for the Au-doped structure.
One can see from Fig. 6 that the |Vph| values measured
at structure illumination with the first pulses of white and
red light begin to differ only at certain temperatures with
Fig. 7. Density of IES at Au-doped structure below the silicon
midgap Ei: 1 � before, 2 � after microwave irradiation.
100 150 200
0.00
0.05
0.10
T, K
1'
2'
1
2
�Vph
nc-Si
, V
Fig. 8. Photovoltage for nc-Si films of undoped (1, 1′) and Au-
doped (2, 2′) structures as function of temperature: 1, 2 � before,
1′, 2′ � after microwave irradiation.
E, eV
N
fs , 10
11
cm
�2
eV
�1
0
1
2
3
4
2
1
�0.30 �0.25 �0.20 �0.15
E.B. Kaganovich et al.: Effect of microwave electromagnetic radiation on ...
477SQO, 6(4), 2003
their going down. This indicates that at these low tem-
peratures a photovoltage Vph
nc-Si appears in the nc-Si
films. It is a difference between the Vph values obtained
under illumination with white and red light at the same
temperature. The photovoltage Vph
nc-Si is negative; its
sign is the same as that of the photovoltage in the substrate.
Shown in Fig. 8 are the temperature dependencies of
photovoltage Vph
nc-Si for undoped (curves 1 and 1′) and
Au-doped (curves 2 and 2′) structures measured before
(curves 1 and 2) and after (curves 1′ and 2′) microwave
irradiation. The very appearance of photovoltage in the
films as temperature decreases, as well as its variation
with temperature, evidence that appearance of photo-
voltage is due reversible stresses in the films that appear
as temperature goes down. These stresses induce a built-
in positive charge which increases when approaching the
outer surface of the film. The electron-hole pairs pro-
duced in the film by white light are separated by the elec-
tric field of the above charge, and this leads to appear-
ance of photovoltage in the film. The lower is photo-
voltage in the Au-doped films, the smaller are stresses in
the films. After microwave irradiation of the structures,
the magnitudes of photovoltage decrease for all films
(Fig. 8). This fact also indicates at reduction of stresses
in the films after microwave irradiation as temperature
goes down.
Fig. 9à presents temperature dependencies of the con-
centrations Np-Si of electrons trapped by the traps at the
p-Si substrate boundary. The Np-Si(T) curves were calcu-
lated (as in [9]) from the difference between the curves 1
and 1′, 3 and 3′ (Fig. 6) taken at the red light pulses. The
dependencies Np-Si(T) were obtained for undoped (curves
1 and 1′) and Au-doped (curves 2 and 2′) structures before
(curves 1 and 2) and after (curves 1′ and 2′) microwave
irradiation. The traps are completely filled with elec-
trons during the first light pulse. Therefore the Np-Si(T)
curves also describe temperature dependence of the elec-
tron trap concentrations at the substrate. Growth of these
concentrations as temperature goes down results from the
fact that shallow-lying traps (which are closer to the con-
duction band of silicon) begin to take part in electron
trapping. One can see from Fig. 9à that for all the struc-
tures studied the concentrations of traps at the substrate
dropped after microwave irradiation.
Shown in Fig. 9b are the temperature dependencies of
the concentrations Nnc-Si of nonequilibrium electrons cap-
tured by the traps in the nc-Si films. These dependencies
were calculated using the curves 2 and 2′, 4 and 4′ (Fig. 6)
taken at the white light pulses (the concentration of elec-
trons captured by the traps at the p-Si substrate was sub-
tracted). It should be noted that in our calculations of
Nnc-Si we determined the lower bound to the number of
electrons captured by the traps in the film, because drop
of potential (produced by the charge of trapped electrons)
takes place not only in the substrate but in the film as
well. The traps in the film, as those at the substrate, were
completely filled with electrons during the first pulse of
white light. Therefore the Nnc-Si(T) curves also show the
temperature dependencies of the electron trap concen-
tration in the film. One can see from Fig. 9b that for all
the structures studied the trap concentration in the nc-Si
film decreases after microwave irradiation over almost
the whole temperature range. Thus, for both undoped
and Au-doped nc-Si/p-Si structures, moderate microwave
irradiation decreases concentration of traps for nonequi-
librium electrons at the p-Si substrates, as well as in the
nc-Si films.
4. Some concluding remarks
Summing up the results obtained, we would like to note
that variation of the properties of nc-Si/p-Si structures
exposed to microwave irradiation depends, on the one
hand, on the microwave irradiance and time of exposure,
and, on the other hand, on the initial (before irradiation)
structure properties. High microwave irradiance results
in degradation of some structure properties (say, PL re-
duction for undoped structure and complete suppression
of PL for Au-doped structure at microwave irradiation
Fig. 9. Temperature dependencies of the concentrations of
trapped nonequilibrium electrons for p-Si substrate (a) and nc-Si
film (b) in undoped (1, 1′) and Au-doped (2, 2′) structures: 1, 2 �
before, 1′, 2′ � after microwave irradiation.
100 125 150 175 200 225
0.0
0.4
0.8
1.2
T, K
1'
2'
1
2
N
p-Si
, 10
10
cm
�2
a
T, K
N
nc-Si
, 10
10
cm
�2
100 150 200 250
0.0
0.4
0.8
1 '
2 '
1
2
b
478
SQO, 6(4), 2003
E.B. Kaganovich et al.: Effect of microwave electromagnetic radiation on ...
with irradiance of 7.5 W/cm2). At the same time moder-
ate (1.5 W/cm2) irradiance can impair, as well as im-
prove, structure parameters. For instance, after moder-
ate action of microwave field the following improvements
are observed:
· both PL intensity and PL relaxation time increase
for undoped structure;
· density of IES at the substrate decreases for
undoped structure;
· for all structures concentration of traps for non-
equilibrium electrons decreases, both at the substrate and
in the nc-Si film;
· strains in the nc-Si film and at the film/substrate
interface decrease as temperature goes down (this fol-
lows from photovoltage decreasing in the film and varia-
tion of the character of ϕs(T) curves for undoped structu-
re after microwave irradiation).
At the same time moderate microwave action impairs
some parameters of the Au-doped nc-Si/p-Si structure,
namely:
· both PL intensity and relaxation time decrease;
· density of IES at the p-Si substrate somewhat in-
creases.
These changes occur for the structure which (due to
Au-doping) demonstrated much better parameters than
the undoped structure, even before microwave irradia-
tion. It should be noted that PL of aged nc-Si<Au>/p-Si
structure is more tolerant to microwave action, due to
more complete oxidation of Si NC when keeping out of
doors.
The changes occurring in the nc-Si/p-Si structures
during microwave irradiation may result from several
factors. First, microwave field is heating up charge car-
riers in both the p-Si substrate and Si NC. At high ener-
gies and concentrations of these charge carriers (which
are obtained at high microwave irradiance) they produce
local defects, in particular, silicon dangling bonds. This
results in decrease, or even complete suppression, of PL.
Contrary to this, at moderate microwave irradiance (when
local defect production is insignificant) the principal role
of heated charge carriers is in transfer of their energy to
silicon crystal lattice as a whole. Heating of the lattice
leads to structure changes in the nc-Si film, as well as at
the film/substrate interface: the nonequilibrium structures
(obtained under laser ablation) become more ordered.
This can explain reduction of stresses in the films and at
their interfaces with the substrate, which can be seen im-
mediately from XRD studies.
Besides, structure annealing that occurs under mod-
erate microwave irradiation also leads to reduction of
the electron trap concentration in the film and at the
substrate, as well as to reduction of the density of IES. It
is likely that these positive actions of moderate micro-
wave irradiations are supplemented with afteroxidation
of Si NC that occurs at microwave action out of doors.
This results in PL growth for the undoped structure where
(contrary to the nc-Si<Au>/p-Si structure) both PL in-
tensity and relaxation time were small before microwave
irradiation.
In conclusion we would like to stress that consequences
of the action of any electromagnetic radiation of low ir-
radiance (dose, intensity) depend on the degree of order-
ing (equilibrium) of the system under action. If the system
is in equilibrium, then even moderate (in irradiance, dose
or intensity) radiation can impair its parameters. On the
contrary, moderate irradiation often improves parameters
of a non-equilibrium (disordered) system, as was observed
in [1�3]. The above statements are also supported by the
results of this work.
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