Electrical and light-emitting properties of silicon dioxide co-implanted by carbon and silicon ions
In this paper we explore the electrophysical and electroluminescence (EL) properties of thermally grown 350 nm thick SiO₂ layers co-implanted with Si⁺ and C⁺ ions. The implanting fluencies were chosen in such a way that the peak concentration of excess Si and C of 5-10 at.% were achieved. Ef...
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| Cite this: | Electrical and light-emitting properties of silicon dioxide co-implanted by carbon and silicon ions / A.N. Nazarov, I.N. Osiyuk , S.I. Tiagulskyi, V.S. Lysenko, I.P. Tyagulskyy , V.N. Torbin, V.V. Omelchuk, T.M. Nazarova, L.Rebohle, W.Skorupa // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2008. — Т. 11, № 4. — С. 319-323. — Бібліогр.: 27 назв. — англ. |
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Nazarov, A.N. Osiyuk, I.N. Tiagulskyi, S.I. Lysenko, V.S. Tyagulskyy, I.P. Torbin, V.N. Omelchuk, V.V. Nazarova, T.N. Rebohle, L. Skorupa, W. 2017-06-03T05:04:44Z 2017-06-03T05:04:44Z 2008 Electrical and light-emitting properties of silicon dioxide co-implanted by carbon and silicon ions / A.N. Nazarov, I.N. Osiyuk , S.I. Tiagulskyi, V.S. Lysenko, I.P. Tyagulskyy , V.N. Torbin, V.V. Omelchuk, T.M. Nazarova, L.Rebohle, W.Skorupa // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2008. — Т. 11, № 4. — С. 319-323. — Бібліогр.: 27 назв. — англ. 1560-8034 PACS 68.35,78.55 https://nasplib.isofts.kiev.ua/handle/123456789/119072 In this paper we explore the electrophysical and electroluminescence (EL) properties of thermally grown 350 nm thick SiO₂ layers co-implanted with Si⁺ and C⁺ ions. The implanting fluencies were chosen in such a way that the peak concentration of excess Si and C of 5-10 at.% were achieved. Effect of hydrogen plasma treatment on electroluminescent and durability of SiO₂ (Si,C) - Si-system is studied. Combined measurements of charge trapping and EL intensity as a function of the injected charge and current have been carried out with the aim of clarifying the mechanisms of electroluminescence. EL was demonstrated to have defect-related nature. Cross sections of both electron traps and hole traps were determined. EL quenching at a great levels of injected charge is associated with strong negative charge capture, following capture of positive charge leading to electrical breakdown of SiO₂ structures. en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics Electrical and light-emitting properties of silicon dioxide co-implanted by carbon and silicon ions Article published earlier |
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Electrical and light-emitting properties of silicon dioxide co-implanted by carbon and silicon ions |
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Electrical and light-emitting properties of silicon dioxide co-implanted by carbon and silicon ions Nazarov, A.N. Osiyuk, I.N. Tiagulskyi, S.I. Lysenko, V.S. Tyagulskyy, I.P. Torbin, V.N. Omelchuk, V.V. Nazarova, T.N. Rebohle, L. Skorupa, W. |
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Electrical and light-emitting properties of silicon dioxide co-implanted by carbon and silicon ions |
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Electrical and light-emitting properties of silicon dioxide co-implanted by carbon and silicon ions |
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Electrical and light-emitting properties of silicon dioxide co-implanted by carbon and silicon ions |
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Electrical and light-emitting properties of silicon dioxide co-implanted by carbon and silicon ions |
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electrical and light-emitting properties of silicon dioxide co-implanted by carbon and silicon ions |
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Nazarov, A.N. Osiyuk, I.N. Tiagulskyi, S.I. Lysenko, V.S. Tyagulskyy, I.P. Torbin, V.N. Omelchuk, V.V. Nazarova, T.N. Rebohle, L. Skorupa, W. |
| author_facet |
Nazarov, A.N. Osiyuk, I.N. Tiagulskyi, S.I. Lysenko, V.S. Tyagulskyy, I.P. Torbin, V.N. Omelchuk, V.V. Nazarova, T.N. Rebohle, L. Skorupa, W. |
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2008 |
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English |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Article |
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In this paper we explore the electrophysical and electroluminescence (EL)
properties of thermally grown 350 nm thick SiO₂ layers co-implanted with Si⁺
and C⁺
ions. The implanting fluencies were chosen in such a way that the peak concentration of
excess Si and C of 5-10 at.% were achieved. Effect of hydrogen plasma treatment on
electroluminescent and durability of SiO₂ (Si,C) - Si-system is studied. Combined
measurements of charge trapping and EL intensity as a function of the injected charge
and current have been carried out with the aim of clarifying the mechanisms of
electroluminescence. EL was demonstrated to have defect-related nature. Cross sections
of both electron traps and hole traps were determined. EL quenching at a great levels of
injected charge is associated with strong negative charge capture, following capture of
positive charge leading to electrical breakdown of SiO₂ structures.
|
| issn |
1560-8034 |
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https://nasplib.isofts.kiev.ua/handle/123456789/119072 |
| citation_txt |
Electrical and light-emitting properties of silicon dioxide co-implanted by carbon and silicon ions / A.N. Nazarov, I.N. Osiyuk , S.I. Tiagulskyi, V.S. Lysenko, I.P. Tyagulskyy , V.N. Torbin, V.V. Omelchuk, T.M. Nazarova, L.Rebohle, W.Skorupa // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2008. — Т. 11, № 4. — С. 319-323. — Бібліогр.: 27 назв. — англ. |
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| first_indexed |
2025-11-26T06:24:07Z |
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2025-11-26T06:24:07Z |
| _version_ |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 4. P. 319-323.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
319
PACS 68.35,78.55
Electrical and light-emitting properties of silicon dioxide
co-implanted by carbon and silicon ions
A.N. Nazarov, I.N. Osiyuk , S.I. Tiagulskyi, V.S. Lysenko, I.P. Tyagulskyy
V.N. Torbin, V.V. Omelchuk, T.M. Nazarova,* L.Rebohle**, W.Skorupa**
V.E. Lashkarev Institute of Semiconductor Physics NAS Ukraine,45, prospekt Nauki,03028 Kiev, Ukraine
*National Technical University “KPI”,37, prospekt Peremogy,03056 Kyiv, Ukraine
**Institute of Ion Beam Physics and Materials Research, Forschungzentrum Rossendorf e.V., POB 510119, D-01314,
Dresden, Germany
Abstract. In this paper we explore the electrophysical and electroluminescence (EL)
properties of thermally grown 350 nm thick SiO2 layers co-implanted with Si+ and C+
ions. The implanting fluencies were chosen in such a way that the peak concentration of
excess Si and C of 5-10 at.% were achieved. Effect of hydrogen plasma treatment on
electroluminescent and durability of SiO2 (Si,C) - Si-system is studied. Combined
measurements of charge trapping and EL intensity as a function of the injected charge
and current have been carried out with the aim of clarifying the mechanisms of
electroluminescence. EL was demonstrated to have defect-related nature. Cross sections
of both electron traps and hole traps were determined. EL quenching at a great levels of
injected charge is associated with strong negative charge capture, following capture of
positive charge leading to electrical breakdown of SiO2 structures.
Keywords: electroluminescence, MOS – structure, implantation, EL quenching.
Manuscript received 08.10.08; accepted for publication 20.10.08; published online 11.11.08.
1. Introduction
Silicon-based light emission is a promising approach to
design and realize optoelectronic inter- and intrachip
communication in future computer systems.
Additionally, there is a great interest in multifunctional
microsystems (e.g. lab-on-chip applications). Since a lot
of these microsystems are based on standard SiO2-Si
technology, there is also a strong demand for Si-based
light emitters, especially to achieve low-cost production
processes. One method for the formation of the Si-based
light-emitting structures is ion beam synthesis.
Electroluminescence (EL) from the Ge-rich SiO2 layers
was found to be in the red/infrared and in the blue/violet
spectral regions [1]. The Si-rich SiO2 layer emits red
light [2]. Ion–beam implantation using a few co-implants
is of interest from the two standpoints. At first, this
procedure allows to expand spectral range of light
emission. Secondly, co-implantation can create more
stable precipitates and/or stable impurity distribution
inside SiO2. Using this approach, white
photoluminescence and electroluminescence of co-
implanted SiO2 by silicon and carbon ions have been
observed in [3] and [4] respectively, however, from our
knowledge the study of electrical characteristics and
degradation of electrical and luminescent properties
during operation of such light-emitting devices was
performed insufficiently. Thus, the present paper
addresses the electroluminescence, electrical
characterization and their degradation in the Si/C co-
implanted SiO2 structures.
2. Samples and Experimental Technique
The SiO2 layers with 350 nm thickness were thermally
grown on ‹100›-oriented n-type Si-substrates at 1000°C.
First, the oxide films were doubly-implanted with Si+
ions at the energy of 90 keV followed by a the second
Si+ implantation at 47 keV. Three sets of samples with
different implanted dose were prepared. The doses were
chosen in such a way that a broad implant profile with a
nearly constant concentration of excess Si about 5%,
7.5% and 10% at the depth of 60–180 nm below the
oxide surface was formed. Fig. 1 shows the implantation
profile calculated with the TRIM code [5]. After such
implantations the devices were furnace annealed at
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 4. P. 319-323.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
320
1100°C for 30 min in a N2 ambient. This annealing step
was carried out in order to initiate the formation of first
Si nanoclusters that have to be acted as seeds for the
final clusters. Then, C+ ions at the energy 43 keV were
implanted, then followed by the second C+ implantation
at 22 keV. A post-implantation heat treatment at 800° C
for 60 min followed by a final annealing step at 1100° C
for 60 min was employed.
The investigation of the EL requires transparent
and conductive gate electrodes. On the top of oxide, an
indium-tin-oxide (ITO) layer, a special kind of
transparent conductive oxide, was deposited using a
sputtering process. The thickness of the layers was 80
nm. The gate electrode was processed using standard
lithography. The sizes of the devices were 0.5 mm in
diameter in a periodic pattern of 2 mm pitch.
A portion of so fabricated MOS Light Emitting
Devices (MOSLEDs) had been subjected to RF plasma
treatment (13.56 MHz) in a low pressure, diode type
reactor, with the ITO electrode being exposed to the
plasma discharge. The plasma working gas was a
mixture of 90% nitrogen and 10% hydrogen. The plasma
power density was in the range of 0.5 – 1.5 W cm-2.
Additional substrate pre-heating from a heat source
independently of the plasma discharge was used over the
temperature range 100o to 300oC, and elevated
temperature was maintained during the plasma
treatment. The plasma treatment duration was 15 min.
Details of the plasma reactor and the used processing
parameters were published in [6].
EL spectra were measured on MOSLEDs with a
circular ITO at a constant current supplied by a
sourcemeter Keithley 2410. The EL signal was recorded
at room temperature with a monochromator Jobin Yvon
Triax 320 and a photosensor module Hamamatsu
H7732-10 in the range of wavelengths between 300 and
750 nm. The measurement was performed with electron
injection in the constant-current mode from ITO
electrode into the SiO2. The typical current density for
EL excitation was 10-3 down to 10-5 A/cm2. The charge
trapping during EL excitation was studied by analysing
the applied voltage at constant current injection (VCC)
from the ITO to the SiO2.
3. Results and Discussion
3.1 Current-voltage characteristics of SiO2 co-
implanted by C+ and Si+ ions
Current-voltage (I-V) characteristics for the case of
electron injection from ITO into implanted SiO2 are
represented in Fig. 2. The constant current regime used
for the EL excitation corresponds to high-field portions
in the I-V characteristics of the MOS structures. As it
can be seen, cut-off injection voltage (manifested as a
sharp bend in the I-V curve) shifts to lower value when
dose of implantation increases; that is, EL emission
arises at lower electric fields.
Actually, section of the I-V curve corresponding to
the EL processes is rectified well in coordinates J/E2-
1/E, where E is the electric field in oxide (see Fig. 2 b).
Therefore, charge transport in Si/C implanted SiO2 is
governed by Fowler-Nordheim (FN) tunneling, that is to
say, by tunnel injection of carriers in the conduction
band of oxide through the triangular shaped barrier. The
current density of FN tunneling may be expressed, as
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
ϕ
π
−
ϕπ
= 232
3
3
*28
exp
8 B
B
FN hE
qm
E
h
qJ (3.1)
where φВ is the barrier height, m* is the effective mass
of electron, other quantities are common accepted.
For the Si-SiO2 structures with 5% of Si/C
implants, from the slope of linear part of the
( ) ( )EfEJ /1/ln 2 − - dependence the barrier height
has been obtained as 3,05 эВ (in assumption of m* =
0.52m0). When the dose of Si/C implantation increases
up to 10% the barrier height falls up to 2.8 eV (at the
same value of the effective mass). It is worth noting that
the value is significantly lower than the injection barrier
for electrons in pure SiO2 (3.15 eV [7]). This
phenomenon is evidence of enhancement of the electron
injection into SiO2 due to the increasing concentration of
SiC-related complexes and associated with trap-assisted
tunneling mechanisms [8].
3.2 Charge trapping in the SiO2 co-implanted by C+
and Si+ ions
Changes of the voltage (VCC), applied to the MOS
structure, in the constant current regime of high-field
electron injection from ITO as a function of injected
charge are depicted in Fig. 3. The decrease of the voltage
during high-field electron injection (up to ~ 9·1014 e/cm2
) suggests positive charge trapping in the oxide at the
distance longer that the tunneling length from the
injected ITO-SiO2 interface. Then, the voltage tends to
steady increasing, which is indicative for negative
charge trapping.
Assuming the first-order trapping kinetics the
trapped charge (both negative and positive ones) versus
injected charge can be described by following the
expression [9]
)]exp(1[max
injitt QQQ σ−−= , (3.2)
where Qt is the trapped charge, Qt
max is the maximal
trapped charge and σi is the effective capture cross-
section of the trap.
If the trapping efficiency (P) is presented as the
first derivation with respect to the injected charge in Eq.
(3.2), then we obtain
( )injiti
inj
t QQ
dQ
dQ
P σ−σ== expmax . (3.3)
From Eq. (3.3), it is easy to understand that for
every trap the plot of ln(P) vs. Qinj will consist of linear
part with the slope corresponding to σi. The
extrapolation to Qinj=0 for this plot provides σiQt
max.
Thus, using the proposed method [10], the number and
main trap parameters can be estimated.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 4. P. 319-323.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
321
Our calculations have demonstrated that three kind
of electron traps with the average value of the capture
cross-section (σe
1 = 3.6×10-16 cm2, σe
2 = 8×10-18 cm2 and
σe
3 = 2×10-19 cm2,) and two kind of hole traps (σ1
h>1х10-
14 cм2, σ2
h=6.6х10-15 cm2) can be found by the proposed
method. It should be noted that hole traps with
σ2
h=6.6х10-15 cм2 are typical for Si-SiO2 structures with
excess Si content in SiO2, which subjected to rapid
thermal annealing [11].
3.3 The features of the EL spectra
Fig. 4 represents EL spectra for the MOSLEDs
fabricated at different implanted doses. In this
experiment, the MOSLEDs were operated under electron
injection from ITO at the current density 2.5·10-3 A·cm-2.
As seen from Fig. 4a, the EL spectra are relatively broad
and consist of several peaks. There is visible correlation
between EL spectra and the concentration of implants.
The multy-Gaussian deconvolution of all the spectra was
performed, and the set of band positions have been
determined. For the lowest concentration (5%), clear
double-peak structure is observed with maxima close
425…435 nm and 515…525 nm. The latter maximum
belongs to a relatively broad peak that consists of two
sub-peaks on its shoulders. The integral intensity of EL
spectra falls down when the concentration of implanted
Si and C increases, the high-energy peak decreases too,
but relative intensities af broad peaks are nearly constant
(Fig. 4a). Similar characteristics of photoluminescence
(PL), showing a decrease of the high energy tail
(340….410 nm) for higher Si/C concentration, have
been observed earllier [4].
Additional measurements were carried out in order
to investigate the influence of the injection current
density on the shape of the EL spectrum (Fig. 5).
Basically, no changes occur in the spectrum with
increasing the current density and consequently also
with increasing the electric field. This means that the
difference in the shape of the spectra for different Si/C
concentrations is only related to the different
microstructure and not to the distribution of hot
electrons, but effect of the injected charge on the EL
spectra demands further investigations.
Thus, we have analyzed variation of the EL spectra
at high levels of injected current employing the multi-
scanning regime: that is EL spectra have been recorded
repetitively one after another (Fig. 6). It should be noted
that about 3.725·1018 e/cm-2 are injected during one EL
scan. As one can see, the high energy band (within the
range 390 to 400 nm) is suppressed strongly when the
number of EL scans is increased, during which the
integral EL intensity decreased just linearly (see insert in
Fig. 6). That is, EL quenching is observed at the used
current density.
The mechanism of the MOSLED operation is
believed to be based upon impact ionization of specific
defect-type luminescent centers (LCs) by hot electrons
moving in the conduction band of oxide [1]. It follows
from the literature that the observed line of 520 nm is
attributed to Eδ center [12], lines of 385 nm and 490 nm
are related to the Si-C bonds [13] and the Si/C/O
complexes [14] correspondingly, and the line at 390-400
nm - to the oxygen deficiency centre (≡Si-Si≡) [15] or
two-fold coordinated silicon (=Si) [16]. The line of
440... 450 nm can be ascribed both to SiC pricipitats [17,
18] and neutral oxygen vacancy [12]. On the low energy
shoulder, one can see increase of the line 620 nm for the
largest current density. This line is attributed to the
nonbridging oxygen hole center [19].
3.4 Effect of RF plasma treatment on light-emitting
characteristics
As it was considered earlier in [20-22], the radio-
frequency (RF) plasma treatment (PT) of ion-implanted
MOS structures may result in considerable modification
of their structural and electrical properties. Especially, it
is regarded to post implanted defects both in Si and SiO2.
Low temperature defect annealing in SiO2 under PT can
be explained within the framework of the recombination
enhanced defect reactions [23, 24] caused by alternate
injection both electrons and holes and there
recombination at the defects.
Analysis of the hydrogen plasma effect on the SiO2
MOS structures co-implanted by C+ and Si+ ions allows
us to reveal the following peculiarities in light-emitting
and electrical properties.
1. There is an optimal regime of the hydrogen plasma
treatment (~0.7 W/сm2) that results in improvement
of the lifetime inherent to the device without
changing its EL intensity and spectrum (see Fig. 7);
2. Exceeding the optimal power density resulted in
reduction of the integral EL intensity and
transformation of the EL spectrum, namely, the
high energy shoulder is considerably suppressed;
3. The increase in the power density resulted in an
increase of device operation durability (Fig. 8).
The observed phenomena can be explained taking
into account an effective annealing and hydrogen
passivation of point defects and defect complexes in
SiO2 during hydrogen plasma treatment [6]. The
complicated EL spectrum of SiO2 (Si,C)-Si-structures in
a wide range of a wavelength is associated with both
nanoinclusions (such as SiXCYOZ [14] ) and different
kinds of defects, as mentioned above. Probably,
availability of high concentrations for different types of
defects in the SiO2 matrix shifts the reactions ordering of
amorphous network towards the higher power densities
used in the hydrogen plasma treatment, and ordering the
SiO2 network occurs simultaneously with annealing of
the defects determining the EL spectrum in our devices.
3.5 Interrelation between EL and charge trapping in the
SiO2 (Si,C) - Si-structure
In order to study the relationship between the EL
intensity and charge trapping in the dielectric, combined
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 4. P. 319-323.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
322
measurements of the EL intensity at the settled
wavelength and the ∆VCC versus the injected charge
(Qinj) at high constant current levels were studied. The
injected charge is simply calculated from the
measurement time using the expression:
∫=
t
inj dttJQ
0
)( . (3.4)
Fig. 9 demonstrates direct correlation between the
EL intensity and value of the constant current voltage up
to the injected charge ~ 2·1018 e/cm2 , that is, excitation
of EL is governed by impact ionization. When the
injected charge reaches 3·1019 e/cm2, the EL intensity
tends to decrease, though the applied voltage increases.
The dependence Vcc as a function of Qinj manifests
strong electron capture by the traps with the cross -
section as estimated above: σe
3 = 2×10-19 cm2. This
process is clear seen on the Vcc ─ Qinj curve for 7.5%
Si/C co-implantation (see Fig. 9).
It was shown in the paper [25], that small capture
cross-sections of the negative charge correlating with
quenching some EL lines, are associated with a
probability of defect luminescent centers reconstruction
during their excitation by electron impact ionization and
have to be designated for clarity as the quenching cross-
section, σq, for given EL line. So, the value of σe
3≡ σq,
440nm has to be linked with a probability of reconstruction
and/or destroying the EL center, revealing a light
emission at 440 nm, with following electron trapping in
the new generated defect. Since the EL at 440 nm in
SiO2 can be associated with excitation of the neutral
vacancy (O3≡Si-Si≡O3) or two-fold coordinated silicon
(=Si), the quenching process can be described by the
following reactions
33 OSiSiO ≡−≡ + e(hot) →
→ O3≡Si(-) ••• Si≡O4 (3.5)
OSiO −−
••
+ 33 OSiOSiO ≡−−≡ +
+ 2e(hot) → )(
3
−≡ SiO + )(
3
−≡ SiO
(3.6)
The reaction (3.5) assumes a breakup of Si-Si bond
by a hot electron and relaxation of Si atom at the
distance longer than 4Å with bonding to the neighboring
oxygen atom. These four-fold coordinated Si and the
silicon dangling bond can represent the electron trap
[26]. In case of the reaction (3.6) two-fold coordinated
Si interacts with broken Si-O bond in neighbouring Si-
O-Si fragment that results in formation of two silicon
dangling bonds (O3≡Si) that can work as electron traps.
Further electron injection (more than 3x1019 e/cm2)
leads to positive charge generation and drastic EL
quenching (see Fig. 9). It should be noted, that
capacitance-voltage characteristics of these structures
demonstrate a shift along the voltage axis towards
negative values, which corresponds to positive charge
accumulation in oxide near the SiO2/Si interface (not
shown here). Obtained results testify to positive charge
accumulation/generation in the bulk and oxide interface
[27]. The observed positive charge accumulation occurs
jointly with electrical breakdown of the MOSLEDs.
4. Conclusion
Fowler-Nordheim tunnelling offers the main mechanism
of charge transport in the Si/C co-implanted Si-SiО2
structures in actual for the EL voltage range.
Combined studies of the MOSLED EL intensity
and charge trapping vs. time for high field electron
injection into the Si/C implanted SiO2 have shown for
the first time that EL correlates well with charge
trapping governed by that three kind of electron traps
with the average value of the capture cross-sec tion (σe
1
= 3.6×10-16 cm2, σe
2 = 8×10-18 cm2 and σe
3 = 2×10-19
cm2,) and two kinds of hole traps (σ1
h > 1х10-14 cm2,
σ2
h=6.6х10-15 cm2 ). The EL quenching caused by
electron impact ionization resulting to reconstruction of
defect LC with following electron trap generation. The
quenching cross-section for the observed process is
σq,440nm
= 2×10-19 cm2. Positive charge generation arises
at high levels of charge injection leading to electrical
breakdown.
Hydrogen plasma treatment of implanted SiO2
layers containing of the nanoinclusions results in
increase of the lifetime inherent to light-emitting devices
and improvement of their stability without reduction in
EL intensity.
References
1. L.Rebohle, J. Von Borany, H. Fröb and
W.Skorupa, Blue photo- and electroluminescence
of silicon dioxide layers ion-implanted with group
IV elements // Appl.Phys. B 70(1), pp.1-21 (2000).
2. D.Garrido Fernandez, M.López, C.Garsía, A.Pérez-
Rodríguez, J.R.Morante, C.Bonafos, MCarrada and
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