Electroluminescent properties of Tb-doped carbon-enriched silicon oxide
An electroluminescent device utilizing a heterostructure of amorphous terbium doped carbon-rich SiOx (a - SiOx : C : Tb) on silicon has been developed. The a - SiOx : C : Tb active layer was formed by RF magnetron sputtering of a - SiO₁₋x : Cx : H(:Tb) film followed by high-temperature oxidation....
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
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| Zitieren: | Electroluminescent properties of Tb-doped carbon-enriched silicon oxide / S.I. Tiagulskyi, A.N. Nazarov, S.O. Gordienko, A.V. Vasin, A.V. Rusavsky, T.M. Nazarova, Yu.V. Gomeniuk, V.S. Lysenko, L. Rebohle, M. Voelskow, W. Skorupa, Y. Koshka // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 1. — С. 34-40. — Бібліогр.: 30 назв. — англ. |
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Tiagulskyi, S.I. Nazarov, A.N. Gordienk, S.O. Vasin, A.V. Rusavsky, A.V. Nazarova, T.M. Gomeniuk, Yu.V. Rudko, G.V. Lysenko, V.S. Rebohle, L. Voelskow, M. Skorupa, W. Koshka, Y. 2017-05-30T05:29:27Z 2017-05-30T05:29:27Z 2014 Electroluminescent properties of Tb-doped carbon-enriched silicon oxide / S.I. Tiagulskyi, A.N. Nazarov, S.O. Gordienko, A.V. Vasin, A.V. Rusavsky, T.M. Nazarova, Yu.V. Gomeniuk, V.S. Lysenko, L. Rebohle, M. Voelskow, W. Skorupa, Y. Koshka // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 1. — С. 34-40. — Бібліогр.: 30 назв. — англ. 1560-8034 PACS 72.20.-i, 73.40.-c, 78.60.Fi, 81.15.Cd https://nasplib.isofts.kiev.ua/handle/123456789/118346 An electroluminescent device utilizing a heterostructure of amorphous terbium doped carbon-rich SiOx (a - SiOx : C : Tb) on silicon has been developed. The a - SiOx : C : Tb active layer was formed by RF magnetron sputtering of a - SiO₁₋x : Cx : H(:Tb) film followed by high-temperature oxidation. It was shown that, depending on the polarity of the applied voltage, the electroluminescence is either green or white, which can be attributed to different mechanisms of current transport through the oxide film – space charge limited bipolar double injection current for green electroluminescence and trap assisted tunneling or Fowler-Nordheim tunneling for white electroluminescence. We would like to appreciate Dr. Yu. Ishikawa (Japan Fine Ceramic Center, Nagoya, Japan) and Prof. Sh.Muto (Nagoya University, Japan) for the opportunity of FTIR and EELS measurements. en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics Electroluminescent properties of Tb-doped carbon-enriched silicon oxide Article published earlier |
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Electroluminescent properties of Tb-doped carbon-enriched silicon oxide |
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Electroluminescent properties of Tb-doped carbon-enriched silicon oxide Tiagulskyi, S.I. Nazarov, A.N. Gordienk, S.O. Vasin, A.V. Rusavsky, A.V. Nazarova, T.M. Gomeniuk, Yu.V. Rudko, G.V. Lysenko, V.S. Rebohle, L. Voelskow, M. Skorupa, W. Koshka, Y. |
| title_short |
Electroluminescent properties of Tb-doped carbon-enriched silicon oxide |
| title_full |
Electroluminescent properties of Tb-doped carbon-enriched silicon oxide |
| title_fullStr |
Electroluminescent properties of Tb-doped carbon-enriched silicon oxide |
| title_full_unstemmed |
Electroluminescent properties of Tb-doped carbon-enriched silicon oxide |
| title_sort |
electroluminescent properties of tb-doped carbon-enriched silicon oxide |
| author |
Tiagulskyi, S.I. Nazarov, A.N. Gordienk, S.O. Vasin, A.V. Rusavsky, A.V. Nazarova, T.M. Gomeniuk, Yu.V. Rudko, G.V. Lysenko, V.S. Rebohle, L. Voelskow, M. Skorupa, W. Koshka, Y. |
| author_facet |
Tiagulskyi, S.I. Nazarov, A.N. Gordienk, S.O. Vasin, A.V. Rusavsky, A.V. Nazarova, T.M. Gomeniuk, Yu.V. Rudko, G.V. Lysenko, V.S. Rebohle, L. Voelskow, M. Skorupa, W. Koshka, Y. |
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2014 |
| language |
English |
| container_title |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| format |
Article |
| description |
An electroluminescent device utilizing a heterostructure of amorphous terbium
doped carbon-rich SiOx (a - SiOx : C : Tb) on silicon has been developed. The
a - SiOx : C : Tb active layer was formed by RF magnetron sputtering of
a - SiO₁₋x : Cx : H(:Tb) film followed by high-temperature oxidation. It was shown that,
depending on the polarity of the applied voltage, the electroluminescence is either green
or white, which can be attributed to different mechanisms of current transport through the
oxide film – space charge limited bipolar double injection current for green
electroluminescence and trap assisted tunneling or Fowler-Nordheim tunneling for white
electroluminescence.
|
| issn |
1560-8034 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/118346 |
| citation_txt |
Electroluminescent properties of Tb-doped carbon-enriched silicon oxide / S.I. Tiagulskyi, A.N. Nazarov, S.O. Gordienko, A.V. Vasin, A.V. Rusavsky, T.M. Nazarova, Yu.V. Gomeniuk, V.S. Lysenko, L. Rebohle, M. Voelskow, W. Skorupa, Y. Koshka // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 1. — С. 34-40. — Бібліогр.: 30 назв. — англ. |
| work_keys_str_mv |
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| first_indexed |
2025-11-24T16:06:26Z |
| last_indexed |
2025-11-24T16:06:26Z |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 1. P. 34-40.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
34
PACS 72.20.-i, 73.40.-c, 78.60.Fi, 81.15.Cd
Electroluminescent properties of Tb-doped
carbon-enriched silicon oxide
S.I. Tiagulskyi1, A.N. Nazarov1, S.O. Gordienko1, A.V. Vasin1, A.V. Rusavsky1, T.M. Nazarova2,
Yu.V. Gomeniuk1, G.V. Rudko1, V.S. Lysenko1, L. Rebohle3, M. Voelskow3, W. Skorupa3, Y. Koshka4
1Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine,
45, prospect Nauky, 03028 Kyiv, Ukraine
2Department of Common and Inorganic Chemistry, National Technical University of Ukraine “KPI”, Kyiv, Ukraine
3Institut für Ionenstrahlphysik und Materialforschung,
Helmholtz Zentrum Dresden-Rossendorf e.V., Dresden, Germany
4Department of Electrical and Computer Engineering, Mississippi State University,
P.O. Box 9571, Mississippi 39762, USA
Phone/fax:+38 (044) 525 61 77; e-mail: nazarov@lab15.kiev.ua
Abstract. An electroluminescent device utilizing a heterostructure of amorphous terbium
doped carbon-rich SiOx (a - SiOx : C : Tb) on silicon has been developed. The
a - SiOx : C : Tb active layer was formed by RF magnetron sputtering of
a - SiO1–x : Cx : H(:Tb) film followed by high-temperature oxidation. It was shown that,
depending on the polarity of the applied voltage, the electroluminescence is either green
or white, which can be attributed to different mechanisms of current transport through the
oxide film – space charge limited bipolar double injection current for green
electroluminescence and trap assisted tunneling or Fowler-Nordheim tunneling for white
electroluminescence.
Keywords: electroluminescence, a-SiO:C/Si heterostructure, Tb, RF magnetron
sputtering, charge transport mechanisms.
Manuscript received 25.11.13; revised version received 23.01.14; accepted for
publication 20.03.14; published online 31.03.14.
1. Introduction
Silicon photonics is attractive field of the research in
recent decades that is associated with integration of
photonic component in silicon (Si) microelectronics
[1, 2]. Special attention is attracted by silicon-based
switchable devices that can change colour (wavelength)
of light emission by changing either applied electric field
[3] or bias polarity [4, 5]. Usually, for such kind devices
SiO2-Si structures are used with the oxide doped with
rear-earth impurities [3] or enriched with Si nanocrystals
[5], which results in colour changing from red to blue in
the former case and from red to infra-red in the latter
one. Recently, it was demonstrated that silicon
oxycarbide by itself can efficiently emit white photo-
[6, 7] and electroluminescence [8]. On the other hand,
embedding rear-earth (RE) metals into silicon
oxycarbide resulted in a more significant increase of
photoluminescence (PL) intensity at wavelengths
corresponding to intra-4f shell transitions of the impurity
in comparison with the PL intensity at the same
wavelengths for the RE embedded in SiO2 matrix [9]. In
this paper, for the first time a light-emitting device based
on Tb doped carbon-rich silicon oxide/silicon
heterostructure, which can emit green light at the
forward bias and white light at the reverse one, is
considered. Parameters of the light emission and current
transfer mechanisms are studied.
2. Sample preparation and methods of research
The first step in fabrication of the electroluminescent
structure was to deposit a - SiO1–x : Cx : H(:Tb) thin films
on Si(100) (p-type, 40 Ohm·cm) wafer by reactive radio-
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 1. P. 34-40.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
35
frequency magnetron sputtering of a polycrystalline
SiC+Tb targets in Ar (96 vol.%) + CH4 (4 vol.%) gas
mixture. The substrate temperature was about 200 °C. The
thickness of the a - SiO1–x : Cx : H(:Tb) layer was 750 nm.
After deposition, the samples were annealed in dry
oxygen for 15 min within the temperature range 450 to
700 °C.
The chemical composition and interatomic bonds
were analyzed using Fourier-transform infrared (FTIR)
transmittance spectroscopy (2000FT-IR, Perkin Elmer),
electron energy-loss spectroscopy (EELS) (JEM2100,
Jeol Co.), and Rutherford back-scattering spectroscopy
(RBS) at the Rossendorf Van-de-Graaf accelerator,
using 1.7 MeV He+ ions as incident particles and back-
scattering angle 170°.
Photoluminescence (PL) spectra were measured at
room temperature using typical PL setup with excitation
by LED with 375-nm excitation wavelength. The
maximum PL intensity was observed after annealing at
600 °C, and the corresponding samples were selected for
the fabrication of the electroluminescent device.
For electroluminescence (EL) and the electrical
measurements, an Au circular ring gate with ultrathin
semi-transparent inner part was deposited on the
oxidized Tb:H:C:SiO-a xx1 film, and an Al layer
was deposited on the back side of the silicon substrate
(see inset in Fig. 1). The EL spectra were measured
under a constant current for the forward and reverse
regimes of the applied voltage at room temperature. A
negative voltage applied to the gate electrode
corresponded to the forward-bias regime, and a positive
voltage – to the reverse-bias regime. The EL intensity
(ELI) versus the applied voltage was measured together
with the current-voltage ( VI ) characteristics by using
a Keithley 2410 high voltage sourcemeter. The EL
signal from the samples was collected with a Triax 320
monochromator and detected by a photomultiplier
(Hamamatsu H7732–100).
Fig. 1. Capacitance-voltage characteristic for the
a - SiOx : C : Tb/p - Si heterostructure. Inset: the schematic
view of the used devices.
High-frequency (1 MHz) capacitance-voltage
( VC ) characteristics were measured using precision
LCR meter Agilent 4284A at room temperature.
3. Experimental results
3.1. Chemical composition
3.1.1. FTIR
Transmittance spectra of as-deposited and oxidized
a - SiO1–x : Cx : H(:Tb) films are presented in Fig. 2. The
spectrum from as-deposited a – SiO1–x : Cx : H(:Tb)
contains several absorption bands within the spectral
range 400 to 4000 cm–1. Two main absorption bands are
at 780 and 1000 cm–1. The first one is due to CSi
stretching vibration with possible minor contribution
from CH3 rocking in 3CH-Si (780 cm–1) and
23CH-Si (800 cm–1) [10-13]. The absorption band at
1000 cm–1 in a - SiO1–x : Cx : H films is commonly
ascribed to rocking/waging vibration modes in CH2
radicals attached to silicon atoms [12]. Additional
absorption bands in as-deposited sample were observed
at 2100 cm–1 ( nHSi stretching), 2700…3000 cm–1
(C – Hn stretching).
Annealing of the film in oxygen drastically changed
the FTIR spectrum. FTIR spectrum is composed now of
four bands, all of them are related to oxygen. The bands at
450 cm–1 (rocking), 802 cm–1 (symmetrical stretching) and
1063 cm–1 (in-phase asymmetrical stretching) with the
shoulder at 1160 cm–1 (out-of-phase asymmetrical
stretching) are originated from vibration modes of
SiOSi bridges, while the very broad one at
3000…3700 cm–1 comes from HO stretching
vibrations. It is important to note that no sign of Si – C
related to the band at 780 cm–1 is detected after oxidation.
Also, after oxidation the Si – Hn and the C –Hn bonds
related correspondingly to the bands at 2100 and
2700…3000 cm–1 are not observed (see Fig. 2).
Fig. 2. FTIR spectra of the as-deposited a - SiO : H(: Tb)
film and after annealing in oxygen at 600 ºC for 15 min.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 1. P. 34-40.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
36
3.1.2. EELS
An energy-loss near edge structure of the Si-L2,3 spectra
of the as-deposited a - SiO1–x : Cx : H(:Tb) film and after
annealing in oxygen are represented in Fig. 3. The EELS
spectrum of the as-deposited samples is characterized by
ionization edge at about 103 eV typical for silicon atoms
bonded to carbon. Oxidation resulted in about 3 eV high-
energy shift of the ionization edge and development of
two peaks at 108 and 115 eV typical for silicon oxide
[14-16]. No lower energy loss due to CSi bonds is
now detected. Thus, this is in good agreement with FTIR
data and shows that after annealing in oxygen at 600 °C
the x1x CSi-a material is transformed into xSiO-a .
3.1.3. RBS
The Tb distributions in a-Si1-xCx:H film before and after
oxidation were measured by RBS technique and are
presented in Fig. 4. Tb concentration in the film was
composed about 2 at.% and distributed enough
uniformly through the film. After low temperature
oxidation, the Tb concentration was decreased
insignificantly.
Fig. 3. Electron energy-loss near edge structure of the Si-L2,3
spectra of the as-deposited a - SiO : H (:Tb) film and after
annealing in oxygen at 600 ºC for 15 min.
Fig. 4. Rutherford back-scattering spectra of the as-deposited
a - SiO : H(:Tb) film and after annealing in oxygen at 600 ºC
for 15 min.
3.2. Light-emission properties of a-SiOx:C:Tb
3.2.1. Photoluminescence
As-deposited a-Si1-xCx:H(:Tb) film did not show visible
luminescence, while luminescence well visible at day
light under argon laser excitation was observed after
oxidation (curve 1 in Fig. 5). The PL spectrum excited
by the 390-nm line of LED radiation contains four Tb3+
related lines at 488, 545, 590 and 621 nm and broad
background in all the visible spectral range. The Tb3+
related lines correspond to intra-4ƒ shell transitions of
Tb3+ ion such as 5D4–
7F6,
5D4–
7F5,
5D4–
7F4 and 5D4–
7F3.
The strongest line at 545 nm (green light) corresponds to
the 5D4–
7F5 transition [17, 18]. The nature of the wide
broad PL band is unclear up to date. But it can be
suggested that in green-red region the PL can be
associated with amorphous carbon nanoclusters [19, 20]
and/or carbon-related defects [21]. Some part of the PL
band in blue region could be attributed to luminescence
from defects located in silicon dioxide matrix such as
neutral oxygen vacancies [22].
Our previous study by using the Raman scattering
technique of this kind samples after low-temperature
oxidation above 600 °C [20] exhibited a broad Raman
scattering band within the range 1cm18001000
associated with disordered sp2-coordinated carbon
clusters. This band was a superposition of two broad G-
and D-bands inherent to graphite-like local arrangement
of amorphous carbon. Thus, from Raman scattering and
PL measurements one can conclude that our material
( xSiO-a ) consists of amorphous carbon nanoclusters
and oxidized Tb inclusions which show specific light-
emitting properties.
3.2.2. Electroluminescence
Fig. 5 demonstrates normalized EL spectra of the
heterostructure for two regimes of the applied voltage.
For the forward bias (curve 3), the sharp lines at 488,
545, 590 nm were observed. These lines correspond to
intra-4ƒ shell transitions of Tb3+ ion [18]. At the reverse
bias, white light emission was observed (curve 2 in
Fig. 5). The spectrum can be separated into two regions.
Within the spectral range 500…650 nm, a broad EL
band is observed with a narrow Tb3+ line at 545 nm. A
broad PL spectrum in the same region was also observed
in amorphous oxycarbide films fabricated by a similar
procedure [23]. By analogy with Refs. [20] and [21] and
our PL results described in the previous part, the origin
of the broad EL band in our structure can be attributed to
radiative transitions of the electronic states associated
with CSiO bonds or carbon nanoclusters.
Additionally, some contribution of the light emission
associated with the excitation of non-bridging oxygen
hole centers can be expected around 620 nm [24]. The
second band covering the range from 350 up to 430 nm
with the maximum at 370 nm can be associated with
oxygen deficiency related centers in oxide matrix [22].
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 1. P. 34-40.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
37
Fig. 5. Normalized photoluminescence (1) and
electroluminescence intensity spectra for the forward (2) and
reverse (3) biases of the a - SiOx : C : Tb/p - Si heterostructure.
It should be noted that application to the structure
different voltage polarity, besides of different EL
spectrum, shows strongly different electric field
corresponding to EL lighting (Fig. 6a), which can be
associated with different potential barriers for electrical
charge injection into a - SiOx : C : Tb layer.
3.3. Electrical properties of a-SiOx:C:Tb/p-Si
heterostructures
3.3.1. Capacitance-voltage characteristics
The capacitance-voltage (C – V) characteristic measured
at room temperature and at a frequency of 1 MHz for the
sample annealed in oxygen at 600 °C is shown in Fig. 1.
The C – V characteristic has a step-like form, which is
typical for metal-oxide-semiconductor (MOS) structures
in the case of high frequency, featuring the plateau Cmax at
negative voltages corresponding to accumulation of
majority carriers at the a - SiOx : C : Tb/p - Si interface,
the plateau Cmin, and deep depletion at positive gate
voltages. From the magnitude of Cmax, the dielectric
constant ε of the a - SiOx : C : Tb layer was determined as
7.5, that is larger than for SiO2 film (3.9) and can be
associated with terbium oxide clusters incorporation into
a - SiOx : C film having a high dielectric constant (near
13.3) [25].
3.3.2. Current transport through a-SiOx:C:Tb
Fig. 6 shows ELI at 545 nm versus the applied electric
field (E) and the I – E dependence for the forward and
reverse biases. For the forward bias, the I – E
characteristic exhibits three regions of current transport
(see inset Fig. 6b). Up to 0.1 MV/cm, the current
dependence on the electric field is linear; within the
range between 0.1 to 0.5 MV/cm, the current shows a
power low relationship (J ~ En) with n = 3; above
1 MV/cm, the current has an exponential dependence on
the electric field. At electric fields above 1 MV/cm, EL
was also observed.
Fig. 6. (a) Dependence of the electroluminescence intensity at
545 nm vs. the electric field applied to the a - SiOx : C : Tb
layer for the forward (1) and reverse (2) biases. Inset:
Dependence of the electroluminescence intensity vs. current
density. (b) Current vs. electric field applied to the a -
SiOx : C : Tb layer for the forward (1) and reverse (2) biases.
Inset: Current vs. electric field applied to the a - SiOx : C : Tb
layer for the forward bias in log – log coordinates.
In the reverse bias regime, the I-V curve consists of
two regions. In the 0…3 MV/cm region, the current varies
linearly with the electric field, which is indicative of
ohmic type conduction. This type of current transport can
be due to a hopping mechanism where current is carried
by thermally excited electrons moving between isolated
discrete defect states [26]. Above ~4 MV/cm, the
relatively high leakage current was observed. For the
reverse bias regime, EL is observed at high electric fields
(above 4 MV/cm), when the leakage current exponentially
depends on the electric field in this material.
4. Discussion
The current vs. electric field dependence have different
behaviour for forward and reverse biases. For the
forward bias, current changes are first linear, then cubic
and exponential vs. electric field (inset to Fig. 6b). The
cubic power law at high electric field following on a
linear dependence at low electric field is indicative of
different cases of the space charge limited current
(SCLC) [27]:
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 1. P. 34-40.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
38
i) unipolar injection in semiconductors and
dielectrics with exponentially distributed shallow traps.
In this case, the current vs. voltage dependence is
described as:
12
1
m
m
L
V
KJ , (1a)
where
T
T
m t , (1b)
1
1
12
)1(
mm
t
s
nc m
m
mN
m
eNK , (1c)
kT
EE
kT
N
E c
t
t
t exp)( . (1d)
L being the thickness of the film, T is the
temperature of the sample, Nc – effective density of
states, μn, εn – charge mobility and dielectric permittivity
of the film, Nt – density of the traps, Tt – characteristic
temperature of the distribution Nt(E) that is the density
of trapping states at the energy E;
ii) bipolar double injection in semiconductors and
dielectrics when L/Lа < 20, where La is ambipolar
diffusion length, L – thickness of the film:
)]/(1/[2~ LLl aVJ
, (2a)
)]/(1/[3~ LLl aVJ
, (2b)
where l is a constant close to 2…3. Expression (2a) is
used for semiconductors and (2b) – for dielectrics.
Exponential growth of the current in SCLC model
is observed in the following cases:
i) bipolar SCLC in semiconductors and dielectrics
at a high voltage and large ambipolar length – L/Lа< 12.
ii) unipolar SCLC in semiconductors and
dielectrics with uniform energy traps distribution
20
2
exp2
kTeLN
V
L
V
enJ
n
, (3a)
const)( nt NEN . (3b)
So, for the forward bias the current transport
through the oxide is determined by SCLC mechanism.
As it can be seen, the exact determination of the type of
SCLC and corresponded EL excitation mechanism in the
forward bias regime is complicated by different
mechanisms that have similar current-voltage behaviour.
To accurately determine the current transport
mechanism, we need to carry out current vs. temperature
measurements. But we try to suggest possible
mechanisms for EL excitation based on SCLC
measurements. In the case of bipolar injection, electrons
from the metal gate and holes from silicon injected into
the Tb:C:SiO-a x film (Fig. 8a, process 1) recombine
on defects of the dielectric matrix with subsequent
energy transfer to Tb3+ ions (Fig. 8a, processes 2, 3). We
can suggest that terbium ions are localized on the ways
of electron-hole transport.
In the case of unipolar injection, motion of holes
from silicon is complicated (probably by high energy
barrier or lack of states in the band-gap near valence
band), electrons from the Fermi level in metal gate
injected by thermally emission into a - SiOx : C : Tb
layer conduction band with subsequent trapping in
exponentially or uniform energy distributed states.
However, in case of unipolar SCLC no light emission
has to be observed. Thus, we can conclude that in our
case of the forward applied bias bipolar double injection
takes place.
For the reverse bias at high electric field (above
4 MV/cm), the leakage current exponentially depends on
the electric field in material (Fig. 6b). Such a strong
dependence on the electric field is indicative of Fowler-
Nordheim (FN) or trap assisted tunneling (TAT) of
electrons through a triangle barrier as it has been shown
to be the case in most of the dielectrics at high fields
[26, 28]. The current density of FN tunneling may be
expressed as
2/3
*
2
2
2
FN 3
28
exp
8
b
b E
qm
E
q
J
, (4)
where b is the barrier height, m – electron effective
mass. The plot of ln(J/E2) versus 1/E in Fig. 7 shows a
linear relationship. With an assumption of
052.0 mm , the barrier height between the
a - SiOx : C : Tb film and silicon wafer was determined
as 1.41 eV from the slope of the EEJ 1ln 2
dependence. It is known that the barrier height for
electrons in a-SiO2 is from 2.8 to 3.1 eV [29, 30]. The
determined small value of the barrier height for electrons
allows to suggest that in our case the TAT process takes
place. The TAT process through a triangle barrier is
described by the expression similar to (4):
2/3
*
3
28
exp~ tTAT E
qm
J
. (5)
According to this relationship, the trap energy level
Фt can be found from the plot of ln(J) versus 1/E (see
inset in Fig. 7), and for our case it equals to 0.86 eV.
Energetic hot electrons, probably, outgo from the
channels where terbium ions are located and excite
mainly carbon related defects and C – Si – O bonds by
direct impact ionization (Fig. 8b), which results in light
emission in a wide spectral range.
The influence of the excitation mechanisms on the
effective excitation cross-section σ for Tb luminescence
can be estimated from the ELI – current density
characteristic for the forward and reverse regimes of the
device operation (see inset Fig. 6a). If we take into
account the equality of the effective electron lifetime τ
for both regimes of operation, it is possible to conclude
that in the case of SCLC the excitation cross-section for
Tb is at least by a factor of four higher than in the case
of impact excitation.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 1. P. 34-40.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
39
Fig. 7. Dependence of the current density vs. electric field in
Fowler-Nordheim coordinates for the reverse bias at which EL
is observed. Inset: Dependence of the current density vs.
electric field in the TAT coordinates reverse bias.
Fig. 8. Energy band-gap diagrams of the a-SiOx:C:Tb/p-Si
heterostructure at the applied forward (a) and reverse (b)
biases. EFS is the Fermi level in p-Si semiconductor, EFM –
Fermi level Au metal, VG – gate bias. In Fig. 8a process 1 is
electron and 2 – hole emission, respectively, from metal and
silicon into silicon oxycarbide; 3 – electrons or holes exchange
between conduction or valance bands and traps; 4 – electron-
hole recombination on levels in the band gap with energy
transfer to Tb3+ ions; process 5 – light emission. In Fig. 8b
process 1 corresponds to high-field electron tunneling from Si
inversion layer into the conduction band of silicon oxycarbide
by either Fowler-Nogdheim effect or trap assisted tunneling;
process 2 is the energy scattering of electrons by luminescent
centers with excitation of these centers and following light
emission.
The question why excitation of centers
corresponding to green or white EL is different for
forward and reverse biases is unclear. Diversity of EL
for forward and reverse biases can be attributed to
energy or space separation charge delivery to EL
centers. Energy separation reaches by different charge
transport: SCLC for forward bias, and TAT for reverse
bias. In Ref. [23], it was demonstrated that in this
material perpendicular to substrate plane high
conductive carbon channels was created. If this channel
disposition mainly near surface of the film and Tb3+ ions
placed across the carbon channels for this kind of space
separation is realized. For the forward bias, electrons
transfer energy mainly to Tb3+ ions (Fig. 8a) and haven’t
enough energy to excite “white” centers. In the opposite
case, for the applied reverse bias, electrons excite mainly
“white” centers and partially the “green” ones.
5. Conclusions
In conclusion, an a-SiOx:C:Tb-based polarity controlled
white-green light-emitting device has been fabricated.
The different EL colours relates to different mechanisms
of charge transport: white EL due to impact excitation of
carbon-related bonds and defects by hot electrons and
green EL due to the double injected space charge limited
current.
Acknowledgements
We would like to appreciate Dr. Yu. Ishikawa (Japan
Fine Ceramic Center, Nagoya, Japan) and Prof. Sh.Muto
(Nagoya University, Japan) for the opportunity of FTIR
and EELS measurements.
References
1. D.J. Lockwood, L. Pavesi, Silicon Photonics.
Springer, New York, 2004.
2. J.M. Shainline, J. Xu, Silicon as an emissive optical
medium // Laser & Photon. Rev. 1, No. 4, p. 334-
348 (2007).
3. S. Pruchnal, J.M. Sun, W. Skorupa, M. Helm,
Switchable two-color electroluminescence based on
a Si metal-oxide-semiconductor structure doped
with Eu // Appl. Phys. Lett. 90, 181121 (2007).
4. S. Kuai, A. Meldrum, Rapid color-switching micro-
LEDs from silicon MIS diodes // Physica E, 41,
p. 916-920 (2009).
5. Z. Liu, J. Huang, P.C. Joshi, A.T. Voutsas,
J. Hartzell, F. Capasso, J. Bao, Polarity-controlled
visible/infrared electroluminescence in Si-
nanocrystal/Si light-emitting devices // Appl. Phys.
Lett. 97, 071112 (2010).
6. A. Karakuscu, R. Guider, L. Pavesi, G.D. Sorarù,
White luminescence from Sol–Gel-derived SiOC
thin films // J. Amer. Ceram. Soc. 92, p. 2969-2974
(2009).
7. Yi Ding, Hajime Shirai, White light emission from
silicon oxycarbide films prepared by using
atmospheric pressure microplasma jet // J. Appl.
Phys. 105, 043515 (2009).
8. Yi Ding, Hajime Shirai, Deyan He, White light
emission and electrical properties of silicon
oxycarbide-based metal–oxide–semiconductor
diode // Thin Solid Films, 519, p. 2513-2515
(2011).
9. S. Gallis, M. Huang, Al.E. Kaloyeros, Efficient
energy transfer from silicon oxycarbide matrix to
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 1. P. 34-40.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
40
Er ions via indirect excitation mechanisms // Appl.
Phys. Lett. 90, 161914 (2007).
10. C. Rau, W. Kulish, Mechanisms of plasma
polymerization of various silico-organic monomers
// Thin Solid Films, 249, p. 28-37 (1994).
11. H.G.P. Lewis, D.J. Edell, K.K. Gleason, Pulsed-
PECVD films from hexamethyl-cyclotrisiloxane
for use as insulating biomaterials // Chem. Mater.
12, p. 3488-3494 (2000).
12. H. Wieder, M. Cardona, R. Guarnieri, Vibrational
spectrum of hydrogenated amorphous SiC films //
Phys. Status Solidi (b), 92, p. 99-112 (1979).
13. A. Grill, D.A. Neumayer, Structure of low
dielectric constant to extreme low dielectric
constant SiCOH films: Fourier transform infrared
spectroscopy characterization // J. Appl. Phys. 94,
p. 6697-6709 (2003).
14. O. Lichtenberger, R. Schneider, J. Woltersdorf,
Analyses of EELS fine structures of different
silicon compounds // Phys. Status Solidi (a), 150,
p. 661-672 (1995).
15. L.A.J. Garvie, P.R. Buseck, Bonding in silicates;
investigation of the Si-L2,3 edge by parallel electron
energy-loss spectroscopy // Amer. Mineralogist, 84,
p. 946-961 (1999).
16. E. Pipel, O. Lichtenberger, J. Woltersdorf,
Identification of silicon oxycarbide bonding in Si-
C-O-glasses by EELS // J. Mater. Sci. Lett. 19,
p. 2059-2060 (2000).
17. H. Amekura, A. Eckau, R. Carius, and Ch. Buchal,
Room-temperature photoluminescence from Tb
ions implanted in SiO2 on Si // J. Appl. Phys. 84,
p. 3867-3873 (1998).
18. J.M. Sun, W. Skorupa, T. Dekorsy, M. Helm,
L. Rebohle, T. Gebel, Bright green electro-
luminescence from Tb3+ in silicon metal-oxide-
semiconductor devices // J. Appl. Phys. 97, 123513
(2005).
19. O. Gonzalez-Varona, A. Perez-Rodriguez,
B. Garrido, C. Bonafos, M. Lopez, J.R. Morante,
J. Montserrat, R. Rodriguez, Ion beam synthesis of
semiconductor nanoparticles for Si based
optoelectronic devices // Nucl. Instrum. and Meth.
in Phys. Res. B, pp. 161-163, 904-908 (2000).
20. Y. Ishikawa, A.V. Vasin, J. Salonen, S. Muto,
V.S. Lysenko, A.N. Nazarov, N. Shibata, and V.-
P. Lehto, Color control of white photoluminescence
from carbon-incorporated silicon oxide // J. Appl.
Phys. 104, 083522 (2008).
21. L. Rebohle, T. Gebel, H. Fröb, H. Reuther, W.
Skorupa, Ion-beam processing for Si/C rich SiO2
layers: Photoluminescence and microstructure //
Appl. Surf. Sci. 184, p. 156-161 (2001).
22. L. Skuja, Optically active oxygen-deficiency-
related centers in amorphous silicon dioxide // J.
Non-Cryst. Solids, 239, p. 16-48 (1998).
23. S.O. Gordienko, A.N. Nazarov, A.V. Rusavsky,
A.V. Vasin, Yu.V. Gomeniuk, V.S. Lysenko, V.V.
Strelchuk, A.S. Nikolenko, and S. Ashok, Influence
of oxidation temperature on photoluminescence
and electrical properties of amorphous thin film
SiC:H:O+Tb // Phys. Status Solidi (c), 8, p. 2749-
2751 (2011).
24. C. Itoh, T. Suzuki, N. Itoh, Luminescence and
defect formation in undensified and densified
amorphous SiO2 // Phys. Rev. B, 41, p. 3794-3799
(1990).
25. D. Xue, K. Betzler, and H. Hesse, Dielectric
constants of binary rare-earth compounds // J. Phys.:
Condens. Matter, 12, p. 3113-3118 (2000).
26. R. Perera, A. Ikeda, R. Hattori, Y. Kuroki, Trap
assisted leakage current conduction in thin silicon
oxynitride films grown by rapid thermal oxidation
combined microwave excited plasma nitridation //
Microelectron. Eng. 65, p. 357-370 (2003).
27. M.A. Lampert and P. Mark, Current Injection in
Solids. Academic, New York, 1970.
28. S.M. Sze, Physics of Semiconductor Devices.
Wiley, New York, 1981.
29. G. Krieger, R.M. Swanson, Fowler-Nordheim
electron tunnelling in thin Si-SiO2-Al structures //
J. Appl. Phys. 52, p. 5710-5717 (1981).
30. Z.N. Weinberg, On tunnelling in metal-oxide-
silicon structures // J. Appl. Phys. 53, p. 5052-5056
(1982).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 1. P. 34-40.
PACS 72.20.-i, 73.40.-c, 78.60.Fi, 81.15.Cd
Electroluminescent properties of Tb-doped
carbon-enriched silicon oxide
S.I. Tiagulskyi1, A.N. Nazarov1, S.O. Gordienko1, A.V. Vasin1, A.V. Rusavsky1, T.M. Nazarova2,
Yu.V. Gomeniuk1, G.V. Rudko1, V.S. Lysenko1, L. Rebohle3, M. Voelskow3, W. Skorupa3, Y. Koshka4
1Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine,
45, prospect Nauky, 03028 Kyiv, Ukraine
2Department of Common and Inorganic Chemistry, National Technical University of Ukraine “KPI”, Kyiv, Ukraine
3Institut für Ionenstrahlphysik und Materialforschung,
Helmholtz Zentrum Dresden-Rossendorf e.V., Dresden, Germany
4Department of Electrical and Computer Engineering, Mississippi State University,
P.O. Box 9571, Mississippi 39762, USA
Phone/fax:+38 (044) 525 61 77; e-mail: nazarov@lab15.kiev.ua
Abstract. An electroluminescent device utilizing a heterostructure of amorphous terbium doped carbon-rich SiOx (a - SiOx : C : Tb) on silicon has been developed. The
a - SiOx : C : Tb active layer was formed by RF magnetron sputtering of
a - SiO1–x : Cx : H(:Tb) film followed by high-temperature oxidation. It was shown that, depending on the polarity of the applied voltage, the electroluminescence is either green or white, which can be attributed to different mechanisms of current transport through the oxide film – space charge limited bipolar double injection current for green electroluminescence and trap assisted tunneling or Fowler-Nordheim tunneling for white electroluminescence.
Keywords: electroluminescence, a-SiO:C/Si heterostructure, Tb, RF magnetron sputtering, charge transport mechanisms.
Manuscript received 25.11.13; revised version received 23.01.14; accepted for publication 20.03.14; published online 31.03.14.
1. Introduction
Silicon photonics is attractive field of the research in recent decades that is associated with integration of photonic component in silicon (Si) microelectronics [1, 2]. Special attention is attracted by silicon-based switchable devices that can change colour (wavelength) of light emission by changing either applied electric field [3] or bias polarity [4, 5]. Usually, for such kind devices SiO2-Si structures are used with the oxide doped with rear-earth impurities [3] or enriched with Si nanocrystals [5], which results in colour changing from red to blue in the former case and from red to infra-red in the latter one. Recently, it was demonstrated that silicon oxycarbide by itself can efficiently emit white photo- [6, 7] and electroluminescence [8]. On the other hand, embedding rear-earth (RE) metals into silicon oxycarbide resulted in a more significant increase of photoluminescence (PL) intensity at wavelengths corresponding to intra-4f shell transitions of the impurity in comparison with the PL intensity at the same wavelengths for the RE embedded in SiO2 matrix [9]. In this paper, for the first time a light-emitting device based on Tb doped carbon-rich silicon oxide/silicon heterostructure, which can emit green light at the forward bias and white light at the reverse one, is considered. Parameters of the light emission and current transfer mechanisms are studied.
2. Sample preparation and methods of research
The first step in fabrication of the electroluminescent structure was to deposit a - SiO1–x : Cx : H(:Tb) thin films on Si(100) (p-type, 40 Ohm·cm) wafer by reactive radio-frequency magnetron sputtering of a polycrystalline SiC+Tb targets in Ar (96 vol.%) + CH4 (4 vol.%) gas mixture. The substrate temperature was about 200 °C. The thickness of the a - SiO1–x : Cx : H(:Tb) layer was 750 nm. After deposition, the samples were annealed in dry oxygen for 15 min within the temperature range 450 to 700 °C.
The chemical composition and interatomic bonds were analyzed using Fourier-transform infrared (FTIR) transmittance spectroscopy (2000FT-IR, Perkin Elmer), electron energy-loss spectroscopy (EELS) (JEM2100, Jeol Co.), and Rutherford back-scattering spectroscopy (RBS) at the Rossendorf Van-de-Graaf accelerator, using 1.7 MeV He+ ions as incident particles and back-scattering angle 170°.
Photoluminescence (PL) spectra were measured at room temperature using typical PL setup with excitation by LED with 375-nm excitation wavelength. The maximum PL intensity was observed after annealing at 600 °C, and the corresponding samples were selected for the fabrication of the electroluminescent device.
For electroluminescence (EL) and the electrical measurements, an Au circular ring gate with ultrathin semi-transparent inner part was deposited on the oxidized
(
)
Tb
:
H
:
C
:
SiO
-
a
x
x
1
-
film, and an Al layer was deposited on the back side of the silicon substrate (see inset in Fig. 1). The EL spectra were measured under a constant current for the forward and reverse regimes of the applied voltage at room temperature. A negative voltage applied to the gate electrode corresponded to the forward-bias regime, and a positive voltage – to the reverse-bias regime. The EL intensity (ELI) versus the applied voltage was measured together with the current-voltage (
V
I
-
) characteristics by using a Keithley 2410 high voltage sourcemeter. The EL signal from the samples was collected with a Triax 320 monochromator and detected by a photomultiplier (Hamamatsu H7732–100).
Fig. 1. Capacitance-voltage characteristic for the
a - SiOx : C : Tb/p - Si heterostructure. Inset: the schematic view of the used devices.
High-frequency (1 MHz) capacitance-voltage (
V
C
-
) characteristics were measured using precision LCR meter Agilent 4284A at room temperature.
3. Experimental results
3.1. Chemical composition
3.1.1. FTIR
Transmittance spectra of as-deposited and oxidized a - SiO1–x : Cx : H(:Tb) films are presented in Fig. 2. The spectrum from as-deposited a – SiO1–x : Cx : H(:Tb) contains several absorption bands within the spectral range 400 to 4000 cm–1. Two main absorption bands are at 780 and 1000 cm–1. The first one is due to
C
Si
-
stretching vibration with possible minor contribution from CH3 rocking in
3
CH
-
Si
(780 cm–1) and
(
)
2
3
CH
-
Si
(800 cm–1) [10-13]. The absorption band at 1000 cm–1 in a - SiO1–x : Cx : H films is commonly ascribed to rocking/waging vibration modes in CH2 radicals attached to silicon atoms [12]. Additional absorption bands in as-deposited sample were observed at 2100 cm–1 (n
H
Si
-
stretching), 2700…3000 cm–1 (C – Hn stretching).
Annealing of the film in oxygen drastically changed the FTIR spectrum. FTIR spectrum is composed now of four bands, all of them are related to oxygen. The bands at 450 cm–1 (rocking), 802 cm–1 (symmetrical stretching) and 1063 cm–1 (in-phase asymmetrical stretching) with the shoulder at 1160 cm–1 (out-of-phase asymmetrical stretching) are originated from vibration modes of
Si
O
Si
-
-
bridges, while the very broad one at 3000…3700 cm–1 comes from
H
O
-
stretching vibrations. It is important to note that no sign of Si – C related to the band at 780 cm–1 is detected after oxidation. Also, after oxidation the Si – Hn and the C –Hn bonds related correspondingly to the bands at 2100 and 2700…3000 cm–1 are not observed (see Fig. 2).
Fig. 2. FTIR spectra of the as-deposited a - SiO : H(: Tb)
film and after annealing in oxygen at 600 ºC for 15 min.
3.1.2. EELS
An energy-loss near edge structure of the Si-L2,3 spectra of the as-deposited a - SiO1–x : Cx : H(:Tb) film and after annealing in oxygen are represented in Fig. 3. The EELS spectrum of the as-deposited samples is characterized by ionization edge at about 103 eV typical for silicon atoms bonded to carbon. Oxidation resulted in about 3 eV high-energy shift of the ionization edge and development of two peaks at 108 and 115 eV typical for silicon oxide [14-16]. No lower energy loss due to
C
Si
-
bonds is now detected. Thus, this is in good agreement with FTIR data and shows that after annealing in oxygen at 600 °C the
x
1
x
C
Si
-
a
-
material is transformed into
x
SiO
-
a
.
3.1.3. RBS
The Tb distributions in a-Si1-xCx:H film before and after oxidation were measured by RBS technique and are presented in Fig. 4. Tb concentration in the film was composed about 2 at.% and distributed enough uniformly through the film. After low temperature oxidation, the Tb concentration was decreased insignificantly.
Fig. 3. Electron energy-loss near edge structure of the Si-L2,3 spectra of the as-deposited a - SiO : H (:Tb) film and after annealing in oxygen at 600 ºC for 15 min.
Fig. 4. Rutherford back-scattering spectra of the as-deposited a - SiO : H(:Tb) film and after annealing in oxygen at 600 ºC for 15 min.
3.2. Light-emission properties of a-SiOx:C:Tb
3.2.1. Photoluminescence
As-deposited a-Si1-xCx:H(:Tb) film did not show visible luminescence, while luminescence well visible at day light under argon laser excitation was observed after oxidation (curve 1 in Fig. 5). The PL spectrum excited by the 390-nm line of LED radiation contains four Tb3+ related lines at 488, 545, 590 and 621 nm and broad background in all the visible spectral range. The Tb3+ related lines correspond to intra-4ƒ shell transitions of Tb3+ ion such as 5D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3. The strongest line at 545 nm (green light) corresponds to the 5D4–7F5 transition [17, 18]. The nature of the wide broad PL band is unclear up to date. But it can be suggested that in green-red region the PL can be associated with amorphous carbon nanoclusters [19, 20] and/or carbon-related defects [21]. Some part of the PL band in blue region could be attributed to luminescence from defects located in silicon dioxide matrix such as neutral oxygen vacancies [22].
Our previous study by using the Raman scattering technique of this kind samples after low-temperature oxidation above 600 °C [20] exhibited a broad Raman scattering band within the range
1
cm
1800
1000
-
-
associated with disordered sp2-coordinated carbon clusters. This band was a superposition of two broad G- and D-bands inherent to graphite-like local arrangement of amorphous carbon. Thus, from Raman scattering and PL measurements one can conclude that our material (
x
SiO
-
a
) consists of amorphous carbon nanoclusters and oxidized Tb inclusions which show specific light-emitting properties.
3.2.2. Electroluminescence
Fig. 5 demonstrates normalized EL spectra of the heterostructure for two regimes of the applied voltage. For the forward bias (curve 3), the sharp lines at 488, 545, 590 nm were observed. These lines correspond to intra-4ƒ shell transitions of Tb3+ ion [18]. At the reverse bias, white light emission was observed (curve 2 in Fig. 5). The spectrum can be separated into two regions. Within the spectral range 500…650 nm, a broad EL band is observed with a narrow Tb3+ line at 545 nm. A broad PL spectrum in the same region was also observed in amorphous oxycarbide films fabricated by a similar procedure [23]. By analogy with Refs. [20] and [21] and our PL results described in the previous part, the origin of the broad EL band in our structure can be attributed to radiative transitions of the electronic states associated with
C
SiO
-
bonds or carbon nanoclusters. Additionally, some contribution of the light emission associated with the excitation of non-bridging oxygen hole centers can be expected around 620 nm [24]. The second band covering the range from 350 up to 430 nm with the maximum at 370 nm can be associated with oxygen deficiency related centers in oxide matrix [22].
Fig. 5. Normalized photoluminescence (1) and electroluminescence intensity spectra for the forward (2) and reverse (3) biases of the a - SiOx : C : Tb/p - Si heterostructure.
It should be noted that application to the structure different voltage polarity, besides of different EL spectrum, shows strongly different electric field corresponding to EL lighting (Fig. 6a), which can be associated with different potential barriers for electrical charge injection into a - SiOx : C : Tb layer.
3.3. Electrical properties of a-SiOx:C:Tb/p-Si heterostructures
3.3.1. Capacitance-voltage characteristics
The capacitance-voltage (C – V) characteristic measured at room temperature and at a frequency of 1 MHz for the sample annealed in oxygen at 600 °C is shown in Fig. 1. The C – V characteristic has a step-like form, which is typical for metal-oxide-semiconductor (MOS) structures in the case of high frequency, featuring the plateau Cmax at negative voltages corresponding to accumulation of majority carriers at the a - SiOx : C : Tb/p - Si interface, the plateau Cmin, and deep depletion at positive gate voltages. From the magnitude of Cmax, the dielectric constant ε of the a - SiOx : C : Tb layer was determined as 7.5, that is larger than for SiO2 film (3.9) and can be associated with terbium oxide clusters incorporation into a - SiOx : C film having a high dielectric constant (near 13.3) [25].
3.3.2. Current transport through a-SiOx:C:Tb
Fig. 6 shows ELI at 545 nm versus the applied electric field (E) and the I – E dependence for the forward and reverse biases. For the forward bias, the I – E characteristic exhibits three regions of current transport (see inset Fig. 6b). Up to 0.1 MV/cm, the current dependence on the electric field is linear; within the range between 0.1 to 0.5 MV/cm, the current shows a power low relationship (J ~ En) with n = 3; above 1 MV/cm, the current has an exponential dependence on the electric field. At electric fields above 1 MV/cm, EL was also observed.
Fig. 6. (a) Dependence of the electroluminescence intensity at 545 nm vs. the electric field applied to the a - SiOx : C : Tb layer for the forward (1) and reverse (2) biases. Inset: Dependence of the electroluminescence intensity vs. current density. (b) Current vs. electric field applied to the a - SiOx : C : Tb layer for the forward (1) and reverse (2) biases. Inset: Current vs. electric field applied to the a - SiOx : C : Tb layer for the forward bias in log – log coordinates.
In the reverse bias regime, the I-V curve consists of two regions. In the 0…3 MV/cm region, the current varies linearly with the electric field, which is indicative of ohmic type conduction. This type of current transport can be due to a hopping mechanism where current is carried by thermally excited electrons moving between isolated discrete defect states [26]. Above ~4 MV/cm, the relatively high leakage current was observed. For the reverse bias regime, EL is observed at high electric fields (above 4 MV/cm), when the leakage current exponentially depends on the electric field in this material.
4. Discussion
The current vs. electric field dependence have different behaviour for forward and reverse biases. For the forward bias, current changes are first linear, then cubic and exponential vs. electric field (inset to Fig. 6b). The cubic power law at high electric field following on a linear dependence at low electric field is indicative of different cases of the space charge limited current (SCLC) [27]:
i) unipolar injection in semiconductors and dielectrics with exponentially distributed shallow traps. In this case, the current vs. voltage dependence is described as:
1
2
1
+
+
=
m
m
L
V
K
J
,
(1a)
where
T
T
m
t
=
,
(1b)
1
1
1
2
)
1
(
+
÷
ø
ö
ç
è
æ
+
+
÷
÷
ø
ö
ç
ç
è
æ
+
e
m
=
m
m
t
s
n
c
m
m
m
N
m
e
N
K
,
(1c)
÷
÷
ø
ö
ç
ç
è
æ
-
=
N
kT
E
E
kT
N
E
c
t
t
t
exp
)
(
.
(1d)
L being the thickness of the film, T is the temperature of the sample, Nc – effective density of states, μn, εn – charge mobility and dielectric permittivity of the film, Nt – density of the traps, Tt – characteristic temperature of the distribution Nt(E) that is the density of trapping states at the energy E;
ii) bipolar double injection in semiconductors and dielectrics when L/Lа < 20, where La is ambipolar diffusion length, L – thickness of the film:
)]
/
(
1
/[
2
~
L
L
l
a
V
J
-
,
(2a)
)]
/
(
1
/[
3
~
L
L
l
a
V
J
-
,
(2b)
where l is a constant close to 2…3. Expression (2a) is used for semiconductors and (2b) – for dielectrics.
Exponential growth of the current in SCLC model is observed in the following cases:
i) bipolar SCLC in semiconductors and dielectrics at a high voltage and large ambipolar length – L/Lа < 12.
ii) unipolar SCLC in semiconductors and dielectrics with uniform energy traps distribution
2
0
2
exp
2
kTeL
N
V
L
V
en
J
n
e
m
=
,
(3a)
const
)
(
=
=
n
t
N
E
N
.
(3b)
So, for the forward bias the current transport through the oxide is determined by SCLC mechanism. As it can be seen, the exact determination of the type of SCLC and corresponded EL excitation mechanism in the forward bias regime is complicated by different mechanisms that have similar current-voltage behaviour. To accurately determine the current transport mechanism, we need to carry out current vs. temperature measurements. But we try to suggest possible mechanisms for EL excitation based on SCLC measurements. In the case of bipolar injection, electrons from the metal gate and holes from silicon injected into the
Tb
:
C
:
SiO
-
a
x
film (Fig. 8a, process 1) recombine on defects of the dielectric matrix with subsequent energy transfer to Tb3+ ions (Fig. 8a, processes 2, 3). We can suggest that terbium ions are localized on the ways of electron-hole transport.
In the case of unipolar injection, motion of holes from silicon is complicated (probably by high energy barrier or lack of states in the band-gap near valence band), electrons from the Fermi level in metal gate injected by thermally emission into a - SiOx : C : Tb layer conduction band with subsequent trapping in exponentially or uniform energy distributed states. However, in case of unipolar SCLC no light emission has to be observed. Thus, we can conclude that in our case of the forward applied bias bipolar double injection takes place.
For the reverse bias at high electric field (above 4 MV/cm), the leakage current exponentially depends on the electric field in material (Fig. 6b). Such a strong dependence on the electric field is indicative of Fowler-Nordheim (FN) or trap assisted tunneling (TAT) of electrons through a triangle barrier as it has been shown to be the case in most of the dielectrics at high fields [26, 28]. The current density of FN tunneling may be expressed as
÷
÷
÷
ø
ö
ç
ç
ç
è
æ
j
-
j
p
=
2
/
3
*
2
2
2
FN
3
2
8
exp
8
b
b
E
qm
E
q
J
h
h
,
(4)
where
b
j
is the barrier height,
*
m
– electron effective mass. The plot of ln(J/E2) versus 1/E in Fig. 7 shows a linear relationship. With an assumption of
0
52
.
0
m
m
=
*
, the barrier height between the
a - SiOx : C : Tb film and silicon wafer was determined as 1.41 eV from the slope of the
(
)
E
E
J
1
ln
2
-
dependence. It is known that the barrier height for electrons in a-SiO2 is from 2.8 to 3.1 eV [29, 30]. The determined small value of the barrier height for electrons allows to suggest that in our case the TAT process takes place. The TAT process through a triangle barrier is described by the expression similar to (4):
÷
÷
÷
ø
ö
ç
ç
ç
è
æ
F
-
2
/
3
*
3
2
8
exp
~
t
TAT
E
qm
J
h
.
(5)
According to this relationship, the trap energy level Фt can be found from the plot of ln(J) versus 1/E (see inset in Fig. 7), and for our case it equals to 0.86 eV. Energetic hot electrons, probably, outgo from the channels where terbium ions are located and excite mainly carbon related defects and C – Si – O bonds by direct impact ionization (Fig. 8b), which results in light emission in a wide spectral range.
The influence of the excitation mechanisms on the effective excitation cross-section σ for Tb luminescence can be estimated from the ELI – current density characteristic for the forward and reverse regimes of the device operation (see inset Fig. 6a). If we take into account the equality of the effective electron lifetime τ for both regimes of operation, it is possible to conclude that in the case of SCLC the excitation cross-section for Tb is at least by a factor of four higher than in the case of impact excitation.
Fig. 7. Dependence of the current density vs. electric field in Fowler-Nordheim coordinates for the reverse bias at which EL is observed. Inset: Dependence of the current density vs. electric field in the TAT coordinates reverse bias.
Fig. 8. Energy band-gap diagrams of the a-SiOx:C:Tb/p-Si heterostructure at the applied forward (a) and reverse (b) biases. EFS is the Fermi level in p-Si semiconductor, EFM – Fermi level Au metal, VG – gate bias. In Fig. 8a process 1 is electron and 2 – hole emission, respectively, from metal and silicon into silicon oxycarbide; 3 – electrons or holes exchange between conduction or valance bands and traps; 4 – electron-hole recombination on levels in the band gap with energy transfer to Tb3+ ions; process 5 – light emission. In Fig. 8b process 1 corresponds to high-field electron tunneling from Si inversion layer into the conduction band of silicon oxycarbide by either Fowler-Nogdheim effect or trap assisted tunneling; process 2 is the energy scattering of electrons by luminescent centers with excitation of these centers and following light emission.
The question why excitation of centers corresponding to green or white EL is different for forward and reverse biases is unclear. Diversity of EL for forward and reverse biases can be attributed to energy or space separation charge delivery to EL centers. Energy separation reaches by different charge transport: SCLC for forward bias, and TAT for reverse bias. In Ref. [23], it was demonstrated that in this material perpendicular to substrate plane high conductive carbon channels was created. If this channel disposition mainly near surface of the film and Tb3+ ions placed across the carbon channels for this kind of space separation is realized. For the forward bias, electrons transfer energy mainly to Tb3+ ions (Fig. 8a) and haven’t enough energy to excite “white” centers. In the opposite case, for the applied reverse bias, electrons excite mainly “white” centers and partially the “green” ones.
5. Conclusions
In conclusion, an a-SiOx:C:Tb-based polarity controlled white-green light-emitting device has been fabricated. The different EL colours relates to different mechanisms of charge transport: white EL due to impact excitation of carbon-related bonds and defects by hot electrons and green EL due to the double injected space charge limited current.
Acknowledgements
We would like to appreciate Dr. Yu. Ishikawa (Japan Fine Ceramic Center, Nagoya, Japan) and Prof. Sh.Muto (Nagoya University, Japan) for the opportunity of FTIR and EELS measurements.
References
1.
D.J. Lockwood, L. Pavesi, Silicon Photonics. Springer, New York, 2004.
2.
J.M. Shainline, J. Xu, Silicon as an emissive optical medium // Laser & Photon. Rev. 1, No. 4, p. 334-348 (2007).
3.
S. Pruchnal, J.M. Sun, W. Skorupa, M. Helm, Switchable two-color electroluminescence based on a Si metal-oxide-semiconductor structure doped with Eu // Appl. Phys. Lett. 90, 181121 (2007).
4.
S. Kuai, A. Meldrum, Rapid color-switching micro-LEDs from silicon MIS diodes // Physica E, 41, p. 916-920 (2009).
5.
Z. Liu, J. Huang, P.C. Joshi, A.T. Voutsas, J. Hartzell, F. Capasso, J. Bao, Polarity-controlled visible/infrared electroluminescence in Si-nanocrystal/Si light-emitting devices // Appl. Phys. Lett. 97, 071112 (2010).
6.
A. Karakuscu, R. Guider, L. Pavesi, G.D. Sorarù, White luminescence from Sol–Gel-derived SiOC thin films // J. Amer. Ceram. Soc. 92, p. 2969-2974 (2009).
7.
Yi Ding, Hajime Shirai, White light emission from silicon oxycarbide films prepared by using atmospheric pressure microplasma jet // J. Appl. Phys. 105, 043515 (2009).
8.
Yi Ding, Hajime Shirai, Deyan He, White light emission and electrical properties of silicon oxycarbide-based metal–oxide–semiconductor diode // Thin Solid Films, 519, p. 2513-2515 (2011).
9.
S. Gallis, M. Huang, Al.E. Kaloyeros, Efficient energy transfer from silicon oxycarbide matrix to Er ions via indirect excitation mechanisms // Appl. Phys. Lett. 90, 161914 (2007).
10.
C. Rau, W. Kulish, Mechanisms of plasma polymerization of various silico-organic monomers // Thin Solid Films, 249, p. 28-37 (1994).
11.
H.G.P. Lewis, D.J. Edell, K.K. Gleason, Pulsed-PECVD films from hexamethyl-cyclotrisiloxane for use as insulating biomaterials // Chem. Mater. 12, p. 3488-3494 (2000).
12.
H. Wieder, M. Cardona, R. Guarnieri, Vibrational spectrum of hydrogenated amorphous SiC films // Phys. Status Solidi (b), 92, p. 99-112 (1979).
13.
A. Grill, D.A. Neumayer, Structure of low dielectric constant to extreme low dielectric constant SiCOH films: Fourier transform infrared spectroscopy characterization // J. Appl. Phys. 94, p. 6697-6709 (2003).
14.
O. Lichtenberger, R. Schneider, J. Woltersdorf, Analyses of EELS fine structures of different silicon compounds // Phys. Status Solidi (a), 150, p. 661-672 (1995).
15.
L.A.J. Garvie, P.R. Buseck, Bonding in silicates; investigation of the Si-L2,3 edge by parallel electron energy-loss spectroscopy // Amer. Mineralogist, 84, p. 946-961 (1999).
16.
E. Pipel, O. Lichtenberger, J. Woltersdorf, Identification of silicon oxycarbide bonding in Si-C-O-glasses by EELS // J. Mater. Sci. Lett. 19, p. 2059-2060 (2000).
17.
H. Amekura, A. Eckau, R. Carius, and Ch. Buchal, Room-temperature photoluminescence from Tb ions implanted in SiO2 on Si // J. Appl. Phys. 84, p. 3867-3873 (1998).
18.
J.M. Sun, W. Skorupa, T. Dekorsy, M. Helm, L. Rebohle, T. Gebel, Bright green electroluminescence from Tb3+ in silicon metal-oxide-semiconductor devices // J. Appl. Phys. 97, 123513 (2005).
19.
O. Gonzalez-Varona, A. Perez-Rodriguez, B. Garrido, C. Bonafos, M. Lopez, J.R. Morante, J. Montserrat, R. Rodriguez, Ion beam synthesis of semiconductor nanoparticles for Si based optoelectronic devices // Nucl. Instrum. and Meth. in Phys. Res. B, pp. 161-163, 904-908 (2000).
20.
Y. Ishikawa, A.V. Vasin, J. Salonen, S. Muto, V.S. Lysenko, A.N. Nazarov, N. Shibata, and V.-P. Lehto, Color control of white photoluminescence from carbon-incorporated silicon oxide // J. Appl. Phys. 104, 083522 (2008).
21.
L. Rebohle, T. Gebel, H. Fröb, H. Reuther, W. Skorupa, Ion-beam processing for Si/C rich SiO2 layers: Photoluminescence and microstructure // Appl. Surf. Sci. 184, p. 156-161 (2001).
22.
L. Skuja, Optically active oxygen-deficiency-related centers in amorphous silicon dioxide // J. Non-Cryst. Solids, 239, p. 16-48 (1998).
23.
S.O. Gordienko, A.N. Nazarov, A.V. Rusavsky, A.V. Vasin, Yu.V. Gomeniuk, V.S. Lysenko, V.V. Strelchuk, A.S. Nikolenko, and S. Ashok, Influence of oxidation temperature on photoluminescence and electrical properties of amorphous thin film SiC:H:O+Tb // Phys. Status Solidi (c), 8, p. 2749-2751 (2011).
24.
C. Itoh, T. Suzuki, N. Itoh, Luminescence and defect formation in undensified and densified amorphous SiO2 // Phys. Rev. B, 41, p. 3794-3799 (1990).
25.
D. Xue, K. Betzler, and H. Hesse, Dielectric constants of binary rare-earth compounds // J. Phys.: Condens. Matter, 12, p. 3113-3118 (2000).
26.
R. Perera, A. Ikeda, R. Hattori, Y. Kuroki, Trap assisted leakage current conduction in thin silicon oxynitride films grown by rapid thermal oxidation combined microwave excited plasma nitridation // Microelectron. Eng. 65, p. 357-370 (2003).
27.
M.A. Lampert and P. Mark, Current Injection in Solids. Academic, New York, 1970.
28.
S.M. Sze, Physics of Semiconductor Devices. Wiley, New York, 1981.
29.
G. Krieger, R.M. Swanson, Fowler-Nordheim electron tunnelling in thin Si-SiO2-Al structures // J. Appl. Phys. 52, p. 5710-5717 (1981).
30.
Z.N. Weinberg, On tunnelling in metal-oxide-silicon structures // J. Appl. Phys. 53, p. 5052-5056 (1982).
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
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