Electron transport through nanocomposite SiO₂(Si) films containing Si nanocrystals
The current transport through insulating SiO₂ films with silicon nanocrystals in Si/SiO₂(Si)/Al structures has been investigated in the wide range of temperatures (82…350 K). The nanocomposite SiO₂(Si) films containing the silicon nanoclusters embedded into insulating SiO₂ matrix have been obtained...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2016
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nasplib_isofts_kiev_ua-123456789-1215172025-02-09T16:29:11Z Electron transport through nanocomposite SiO₂(Si) films containing Si nanocrystals Bratus, O.L. Evtukh, A.A. Steblova, O.V. Prokopchuk, V.M. The current transport through insulating SiO₂ films with silicon nanocrystals in Si/SiO₂(Si)/Al structures has been investigated in the wide range of temperatures (82…350 K). The nanocomposite SiO₂(Si) films containing the silicon nanoclusters embedded into insulating SiO₂ matrix have been obtained by ion-plasma sputtering of silicon target and subsequent high-temperature annealing. Based on the detailed analysis of current-voltage characteristics, calculation of some electrical parameters has been performed and the mechanism of electron conductivity of nanocomposite SiO₂(Si) films has been ascertained. The electrical conductivity of the films is based on the mechanism of hopping conductivity with variable-range hopping through the traps near the Fermi level. 2016 Article Electron transport through nanocomposite SiO₂(Si) films containing Si nanocrystals / O.L. Bratus, A.A. Evtukh, O.V. Steblova, V.M. Prokopchuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2016. — Т. 19, № 1. — С. 9-13. — Бібліогр.: 11 назв. — англ. 1560-8034 DOI: 10.15407/spqeo19.01.009 PACS 72.20.Ее, 73.63.Bd https://nasplib.isofts.kiev.ua/handle/123456789/121517 en Semiconductor Physics Quantum Electronics & Optoelectronics application/pdf Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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The current transport through insulating SiO₂ films with silicon nanocrystals in Si/SiO₂(Si)/Al structures has been investigated in the wide range of temperatures (82…350 K). The nanocomposite SiO₂(Si) films containing the silicon nanoclusters embedded into insulating SiO₂ matrix have been obtained by ion-plasma sputtering of silicon target and subsequent high-temperature annealing. Based on the detailed analysis of current-voltage characteristics, calculation of some electrical parameters has been performed and the mechanism of electron conductivity of nanocomposite SiO₂(Si) films has been ascertained. The electrical conductivity of the films is based on the mechanism of hopping conductivity with variable-range hopping through the traps near the Fermi level. |
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Bratus, O.L. Evtukh, A.A. Steblova, O.V. Prokopchuk, V.M. |
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Bratus, O.L. Evtukh, A.A. Steblova, O.V. Prokopchuk, V.M. Electron transport through nanocomposite SiO₂(Si) films containing Si nanocrystals Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Bratus, O.L. Evtukh, A.A. Steblova, O.V. Prokopchuk, V.M. |
| author_sort |
Bratus, O.L. |
| title |
Electron transport through nanocomposite SiO₂(Si) films containing Si nanocrystals |
| title_short |
Electron transport through nanocomposite SiO₂(Si) films containing Si nanocrystals |
| title_full |
Electron transport through nanocomposite SiO₂(Si) films containing Si nanocrystals |
| title_fullStr |
Electron transport through nanocomposite SiO₂(Si) films containing Si nanocrystals |
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Electron transport through nanocomposite SiO₂(Si) films containing Si nanocrystals |
| title_sort |
electron transport through nanocomposite sio₂(si) films containing si nanocrystals |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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2016 |
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https://nasplib.isofts.kiev.ua/handle/123456789/121517 |
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Electron transport through nanocomposite SiO₂(Si) films containing Si nanocrystals / O.L. Bratus, A.A. Evtukh, O.V. Steblova, V.M. Prokopchuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2016. — Т. 19, № 1. — С. 9-13. — Бібліогр.: 11 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT bratusol electrontransportthroughnanocompositesio2sifilmscontainingsinanocrystals AT evtukhaa electrontransportthroughnanocompositesio2sifilmscontainingsinanocrystals AT steblovaov electrontransportthroughnanocompositesio2sifilmscontainingsinanocrystals AT prokopchukvm electrontransportthroughnanocompositesio2sifilmscontainingsinanocrystals |
| first_indexed |
2025-11-27T22:35:49Z |
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1849984766896504832 |
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 1. P. 009-013.
doi: 10.15407/spqeo19.01.009
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
9
PACS 72.20.Ее, 73.63.Bd
Electron transport through nanocomposite SiO2(Si) films
containing Si nanocrystals
O.L. Bratus1, A.A. Evtukh1, O.V. Steblova2, V.M. Prokopchuk2
1V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
Kyiv, Ukraine; e-mail: bratus1981@gmail.com
2Taras Shevchenko Kyiv National University, Institute of High Technologies,
Kyiv, Ukraine; e-mail: steblolia@gmail.com
Abstract. The current transport through insulating SiO2 films with silicon nanocrystals in
Si/SiO2(Si)/Al structures has been investigated in the wide range of temperatures
(82…350 K). The nanocomposite SiO2(Si) films containing the silicon nanoclusters
embedded into insulating SiO2 matrix have been obtained by ion-plasma sputtering of
silicon target and subsequent high-temperature annealing. Based on the detailed analysis
of current-voltage characteristics, calculation of some electrical parameters has been
performed and the mechanism of electron conductivity of nanocomposite SiO2(Si) films
has been ascertained. The electrical conductivity of the films is based on the mechanism
of hopping conductivity with variable-range hopping through the traps near the Fermi
level.
Keywords: silicon nanoclusters, electron transport, current-voltage characteristic,
variable-range hopping, trap.
Manuscript received 05.10.15; revised version received 23.01.16; accepted for
publication 16.03.16; published online 08.04.16.
1. Introduction
The properties of films with silicon nanocrystals
embedded into the dielectric matrix cause great attention
of investigators to build on their basis the electronic and
optoelectronic devices, namely: light emitting diodes,
single-electron transistors, resonant-tunnel diodes and
memory cells [1-5]. Nowadays, there are the number of
technologies for obtaining the nanocomposite SiO2(Si)
films, and each of them has its advantages and
disadvantages. Physical properties of SiO2(Si) films
obtained by different methods can differ significantly
and, accordingly, there is the need to study them in close
relation with the technological conditions of deposition.
Electron transport processes are not yet fully clarified,
and it indicates the necessity to further research
the electron processes in nanocomposite SiO2(Si)
films. The main purpose of this work is to investigate
the mechanisms of electron transport through
nanocomposite SiO2(Si) films containing Si nanoclusters
and obtained by ion-plasma sputtering for subsequent
use as the medium for the accumulation and storage of
electric charge in the cells of nanocrystal non-volatile
memory.
2. Experimental details
The metal-insulator-semiconductor (MIS) Al/n-
Si/SiO2(Si)/Al structures with nanocomposite SiO2(Si)
film as the insulator were prepared. At the beginning the
SiOx films were deposited on a n-type silicon wafer
(ρ = 4.5 Ohm·cm, (100)) by using the ion-plasma
sputtering (IPS) method. The silicon target was sputtered
with argon ions in the environment of Ar + O2 [6]. The
variables were the ratio of gas flows of Ar and O2. Other
deposition parameters were as follows: the pressure
during the deposition process P = 8·10–4 Torr, substrate
temperature T = 150 °C, heating cathode current IC =
140 A, anode voltage VA = 50 V, voltage on the target VT
= 1.2…1.25 kV, deposition current IS = 0.65 mA.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 1. P. 009-013.
doi: 10.15407/spqeo19.01.009
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
10
The nanocomposite SiO2(Si) film containing Si
nanocrystals in the insulating SiO2 matrix was formed
during the high-temperature annealing at T = 1100 °C
for 30 min in N2 atmosphere. Two types of nano-
composite SiO2(Si) films were investigated. At
formation of one of them the stoichiometry index of
initial SiOx film was x1 = 1.3 (sample 1), and for another
one it was x2 = 1.1 (sample 2).
MIS capacitors were formed to investigate the
electrical conductivity. Aluminum gates were obtained
by the magnetron sputtering of Al target. Solid
aluminum layer was also deposited on the back side of
wafer. The circle capacitors with the area 7·10–3 cm2
were formed using Al deposition through the metal
mask.
Measurements of current-voltage (I–V) charac-
teristics were carried out using the automated complex
consisting of the controlled voltage source and
ampermeter Keithley-6485. A personal computer was
used to control the measurement process. All the
electrical measurements were carried out in the dark. To
determine the current transport mechanism, I–V
characteristics were measured at various temperatures
within the range (83…350 K) and results were rebuilt in
different coordinates.
3. Results and discussion
Investigation of structural properties of the nano-
composite SiO2(Si) films obtained using the IPS method
with the following high-temperature annealing enabled
to establish the basic peculiarities of silicon nanocrystals
growth in dependence on technological regimes and to
determine their size and surface density [6]. The
technological regimes during formation of samples
under investigation provide the silicon nanocrystals with
the diameter of 3…4 nm [7]. In the assumption of
uniform distribution of the nanocrystals in the film, the
thickness of the dielectric layers between the
nanocrystals is 10 to 15 nm. The energy band diagram of
SiO2(Si) nanocomposite films is shown in Fig. 1.
Availability of a large number of defects in the SiO2 film
including three coordinated silicon atom, oxygen
vacancy, peroxide radicals and others leads to the
appearance of deep localized energy levels in the band
gap (Fig. 1) [8, 9].
The direct branches of the typical I–V cha-
racteristics of MIS structures with nanocomposite
SiO2(Si) films with different contents of excess silicon in
the initial SiOx films are shown using semi-logarithmic
coordinates in Fig. 2. As can be seen, the conductivity of
the film with the higher content of excess silicon is also
higher. It is caused by the large number of silicon
dangling bonds and other defects in the film, the greater
density of nanocrystals, and as a result the thinner
dielectric layers between nanocrystals.
To ascertain the mechanism of electrical
conductivity, the I–V characteristics were measured at
various temperatures, and the dependence of current on
voltage in different coordinates was analyzed. Fig. 3
shows the direct branches of I–V characteristics for the
samples 1 and 2, taken within the temperature range 83
to 350 K. As can be seen, the current through the
nanocomposite SiO2(Si) film strongly depends on the
temperature of measurements for both samples, and the
dependence on the electric field for the fields
E > 1·105 V/cm is rather weak. The dependence of
conductivity on temperature points out on the hopping
conductivity mechanism at carrier transport. For more
detail analysis of electron transport, the current-voltage
characteristics were rebuilt in the Mott coordinates
(σT1/2–T–1/4) for different values of electric fields.
Fig. 1. Energy band diagram of nanocomposite SiO2(Si) films.
Fig. 2. I–V characteristics of MIS structures with nano-
composite SiO2(Si) films (T = 350 K). The stoichiometry
indexes of initial SiOx films are: x = 1.3 (1), x = 1.1 (2).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 1. P. 009-013.
doi: 10.15407/spqeo19.01.009
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
11
Fig. 3. Variation of I–V characteristics with temperature in the
range from 83 to 350 K as a parameter: a) sample 1,
b) sample 2. The arrow shows the direction of the temperature
increase within the above range.
The dependences of the SiO2(Si) films conductivity
on temperature in the Mott coordinates at various
electric fields are shown in Fig. 4. It is seen that the
experimental data in these coordinates lie on the straight
line indicating the hopping conductivity at electron
transport through the film [10]. There is no dependence
of curve slope on electric field for the sample 1 (Fig. 4a).
On the contrary, for the sample 2 the field dependence of
the curves slopes is significant (Fig. 4b).
In the case of the conductivity according to the Mott
mechanism, the expression for it (σ) has the look [11]:
( ) .exp 4/10 ⎟
⎠
⎞
⎜
⎝
⎛−⋅σ=σ
T
BT
(1)
The preexponential factor is presented as
( )
( )
( ) ,
)8(2
8
9
6
2/1
F
2/1
2
2/1
F
2
0
⎟
⎠
⎞
⎜
⎝
⎛
α
ν
π
=
=⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
πα
ν
=σ
kT
ENe
kTEN
ENe
ph
Fph
(2)
where e is the electron charge, N(EF) – density of
localized states near the Fermi level, νph – hopping
probability per unit time.
( )
4/1
FB
3
0 ⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛ α
=
ENk
ВВ , (3)
where α is the electron wave function attenuation in the
localized states, kB – Boltzmann constant, and B0 =
2(3/2π)1/4 = 1.66.
The B parameter was determined from the slope of
the linear function in the coordinates of ln(σT1/2)–T–1/4.
Implementation of this dependence in the temperature
range T = 83…350 K indicates that the charge transport
in the nanocomposite SiO2(Si) films under investigation
is realized by hopping conductivity with variable-range
hopping through the traps near the Fermi level. These
states can be created by dangling bonds of Si and other
defects in the dielectric SiO2 matrix [8, 9].
Fig. 4. The conductivity of nanocomposite SiO2(Si) films
versus temperature in the Mott coordinates at various electric
fields: a) sample 1, b) sample 2.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 1. P. 009-013.
doi: 10.15407/spqeo19.01.009
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
12
The density of localized energy states N(EF) near the
Fermi level EF can be estimated from the slope of the con-
ductivity on T–1/4 dependence, assuming more-less accep-
table spatial extent of the localized wave functions (3):
( )
B
34
4
0
kB
B
EN F
α
= . (4)
The different influence of the electric field on the
curve slopes in (σT1/2–T–1/4) coordinates (Fig. 4) for two
samples can be explained by higher silicon excess in
initial SiOx film with stoichiometry index x = 1.1. In this
case, the higher concentration of the dangling bonds of
Si and other defects causes the higher quantity of the
localized electron energy states near the Fermi level.
In frame of the considered model, the average
length of charge carrier hopping on localized states near
the Fermi level at given temperature can be determined
from the expression [11]:
( ) .
8
9
4/1
BF
⎥
⎦
⎤
⎢
⎣
⎡
πα
=
TkEN
R (5)
Fig. 5. The temperature dependences of the carriers hopping
average length through the localized states near the Fermi
level: a) sample 1, b) sample 2. E = 6.3·103 (1), 5·104 (2),
1·105 (3), 5·105 (4), 1·106 V/cm (5).
If we take the value for the localization radius of
the electron wave function α = 0.8 nm, similar to the
data for amorphous semiconductors [11], the energy
density of states for the sample 1 (x = 1.3) is equal to
N(EF) = 7.88·1019 eV–1 cm–3
(4), and the average hopping
length R(T1) = 4.531·10–7 cm (T1 = 100 K), and R(T2) =
3.525·10–7 cm (T2 = 273 K) (5). As it follows from
Eq. (5), the average length of hopping R increases with
the decrease of temperature. In this case, the hoppings
are in the lower energy range. The temperature
dependences of the average hopping length at localized
states near the Fermi level are shown in Fig. 5.
As can be seen, the hopping length decreases with
the temperature growth. With lowering the temperature,
the energy and quantity of optical phonons, as the main
scatter, are decreased. As a result, the probability of the
phonon assisted hopping with a higher energy is lower.
The hoppings at long distances but in narrower energy
interval are more probable. It is the mechanism of
hopping conductivity with variable-range hopping [10].
In case of the sample 2, beside temperature
dependence of the hopping length, there is a strong
influence of electric field. The average hopping length
decreases with the increase of field (Fig. 5b). At higher
electric fields, electron obtains more energy, so it
becomes accessible to tunneling at higher energy but
closer in distance.
The energy interval including the local energy
states involved in conductivity is determined in accord to
the formula:
( )
.
π4
3
F
3 ENR
E =Δ (6)
As it was determined above, the value of the energy
of local states ΔE taking part in the electron transport
increases with increasing the temperature for both
samples. According to the equation (6), the value of ΔE
is determined by the average hopping length R and
density of states N(EF). The value N(EF) increases with
the temperature, while R decreases (5). Substitution of
the equation (5) into the equation (6) gives
( ) 4/1
F
4/3
EN
TaE =Δ , (7)
where a is the constant.
The number of traps involved in current transport
can be determined from the equation:
( ) .F EENNt Δ⋅= (8)
This value Nt increases with temperature T. The
field dependence of Nt for the sample 2 is also observed.
The growth of temperature T and electric field E
increases the charge carrier energy and decreases the
barrier height during tunneling. Both of these lead to the
increase in the concentration of traps Nt participating in
the current transport.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 1. P. 009-013.
doi: 10.15407/spqeo19.01.009
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
13
4. Conclusions
The investigations of electron transport through
nanocomposite SiO2(Si) films containing Si nanocrystals
embedded into the dielectric SiO2 matrix and prepared
by ion-plasma sputtering technology with the following
high temperature annealing enabled to determine the
electron transport mechanism. The current flow is
realized by variable-range hopping through the traps
near the Fermi level. The increase of the silicon
concentration in the initial SiOx film leads to the electric
field dependence of the curve slope in the Mott
coordinates.
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