Relaxation of photovoltage in ITO-Ge-Si heterojunction with Ge nanostructured thin films
The paper focuses on an experimental study of the photovoltage time decay in an ITO-Ge-Si heterojunction with a Ge nanostructured thin film. Kinetics under 650 nm excitation within the temperature range 80 to 290 K are successfully described by a single exponential function with temperature-dependen...
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| Опубліковано в: : | Semiconductor Physics Quantum Electronics & Optoelectronics |
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| Дата: | 2017 |
| Автори: | , , , , , |
| Формат: | Стаття |
| Мова: | Англійська |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2017
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| Цитувати: | Relaxation of photovoltage in ITO-Ge-Si heterojunction with Ge nanostructured thin films / S.A. Iliash, Yu.V. Hyrka, S.V. Kondratenko, V.S. Lysenko, Yu.M. Kozyrev, V.V. Lendel // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2017. — Т. 20, № 2. — С. 259-261. — Бібліогр.: 6 назв. — англ. |
Репозитарії
Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1860292447072944128 |
|---|---|
| author | Iliash, S.A. Hyrka, Yu.V. Kondratenko, S.V. Lysenko, V.S. Kozyrev, Yu.M. Lendel, V.V. |
| author_facet | Iliash, S.A. Hyrka, Yu.V. Kondratenko, S.V. Lysenko, V.S. Kozyrev, Yu.M. Lendel, V.V. |
| citation_txt | Relaxation of photovoltage in ITO-Ge-Si heterojunction with Ge nanostructured thin films / S.A. Iliash, Yu.V. Hyrka, S.V. Kondratenko, V.S. Lysenko, Yu.M. Kozyrev, V.V. Lendel // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2017. — Т. 20, № 2. — С. 259-261. — Бібліогр.: 6 назв. — англ. |
| collection | DSpace DC |
| container_title | Semiconductor Physics Quantum Electronics & Optoelectronics |
| description | The paper focuses on an experimental study of the photovoltage time decay in an ITO-Ge-Si heterojunction with a Ge nanostructured thin film. Kinetics under 650 nm excitation within the temperature range 80 to 290 K are successfully described by a single exponential function with temperature-dependent decay constants. Photovoltage relaxation is modeled taking into account the hopping nature of electron transport in the band of localized states.
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| first_indexed | 2026-03-21T02:22:28Z |
| format | Article |
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 2. P. 259-261.
doi: https://doi.org/10.15407/spqeo20.02.259
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
259
PACS 73.40.-c, 73.50.Pz
Relaxation of photovoltage in ITO-Ge-Si heterojunction
with Ge nanostructured thin films
S.A. Iliash1, Yu.V. Hyrka1, S.V. Kondratenko1, V.S. Lysenko2, Yu.M. Kozyrev3, V.V. Lendel1
1Taras Shevchenko National University of Kyiv, 64/13, Volodymyrs’ka str., 01601 Kyiv, Ukraine
E-mail: iliashsviatoslav@gmail.com
2Institute of Semiconductor Physics, 41, prospect Nauky, 03680 Kyiv, Ukraine
3O.O. Chuiko Institute of Surface Chemistry, 17, Generala Naumova str., 03164 Kyiv, Ukraine
Abstract. The paper focuses on experimental study of the photovoltage time decay in
ITO-Ge-Si heterojunction with Ge nanostructured thin film. Kinetics under 650 nm
excitation within the temperature range 80 to 290 K are successfully described by a
single exponential function with temperature-dependent decay constants. Photovoltage
relaxation is modeled taking into account the hopping nature of electron transport in the
band of localized states.
Keywords: heterostructure, ITO-Ge-Si heterojunction, thin films, photovoltage, transient
voltage.
Manuscript received 13.01.17; revised version received 27.04.17; accepted for
publication 14.06.17; published online 18.07.17.
1. Introduction
Semiconductor nanoclusters (NCs) attract interest due to
new possibilities for many electronic applications, e.g.,
field-effect transistors, IR photodetectors, and solar cells
[1-3]. The improvement in the device performance is
reached by decreasing the distance between the NCs,
narrowing the dispersion of NC sizes as well as
changing the NCs capping material.
The nanocrystalline Ge films grown on silicon
substrates are of particular interest, due to their
compatibility with silicon technology. Electronic
spectrum, charge carrier transport and optical properties
of SiGe-based nanostructures essentially depend on the
mechanical stress caused by mismatch of parameters of
Si and Ge crystal lattices as well as variations in the size
and composition of Ge nanostructures. This makes
possibilities for creating thin films with predetermined
properties for applications in optoelectronics, photo-
voltaics and sensors.
2. Experimental details
The molecular beam epitaxy (MBE) technique was
used to prepare monolayer and multilayer Ge–Si(100)
NC arrays with the clusters of various sizes and surface
density. The (100) oriented plates of p-Si with
resistivity of 7.5 Ω·cm and diameter of 76 mm were
used as substrates. The background pressure of residual
gases in the MBE set-up was 6·10–10 Torr. After
desorption of the passivating Si oxide film from the Si
substrate, which was exposed to a Si beam of a weak
intensity and kept at 800 °C in the course of this
process, a buffer Si film, about 0.1 to 0.5 µm thick, was
deposited onto the Si surface. This film produces a
high-contrast Si(100) 2×1 RHEED pattern typical of
clean Si. The Ge nanocrystalline layers were deposited
at 350 °С. The deposition rate was 0.15 nm/min for Si
and 5.0 nm/min for Ge.
The ITO films (95% In2O3 + 5% SnO2) with a
thickness of 50 nm were deposited on the p-Si(001)
boron-doped substrate or Ge thin film by vacuum
magnetron sputtering at 125 °C.
AFM measurements were performed with an NT-
MDT Ntegra microscope using cantilevers with a tip
radius of about 10 nm. Fig. 1a shows the AFM images
of the surface consisting of the nanocrystalline Ge film.
The photovoltage temporal dependences were
recorded on a Siglent 70-MHz-bandwidth and 1 MΩ
input impedance digital oscilloscope with a low-noise
amplifier (AD8138) that measured a voltage signal drop
across the series load resistance of 47 kΩ (see Fig. 1b).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 2. P. 259-261.
doi: https://doi.org/10.15407/spqeo20.02.259
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
260
Fig. 1. AFM images of ITO-Ge-Si heterojunction surface (a).
The scheme for transient photocurrent measurements (b).
3. Results and discussion
Morphology studies showed that low-temperature Ge
epitaxy on Si(001) surface at 350° C and high (about
5 nm/min) epitaxy rate leads to formation of nano-
crystalline films consisting of Ge crystallites, 5-6 nm in
size, which are pseudomorphic to the substrate.
Fig. 2 shows the dark current-voltage (I-V) curves
of ITO-Ge-pSi and ITO-pSi structures at room
temperature. The current in the reverse bias is the order
of magnitude lower than in forward bias due to the
potential barrier of ITO-Si heterojunction.
The shape of curves was analyzed within the
framework of the standard diode equation. If the
diffusion current and the recombination one are both
present in a single diode, the diode model usually
describes the dark I-V dependences. The current density
J is described as follows:
( ) ( )
0 exp 1S S
SH
e V IR V IRJ V J
nkT R
⎛ ⎞−⎛ ⎞ −
= − +⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠
, (1)
where J0 is the diode reverse saturation current, k –
Boltzmann’s constant, T – absolute temperature, RS and
RSH are the series and shunt resistances, respectively.
The diode ideality factor is a characteristic of the
generation-recombination (G-R) processes, which
dominate in the p-i-n diodes. They are mainly con-
sidered as active only in the space-charge region. It is
commonly accepted that n = 1, when the diffusion
current dominates, and n = 2, when the recombination
current is dominant. Analysis of the forward bias shows
that the value n ≈ 3 within the range from 0.2 to 0.4 V
due to significant impact of recombination in the space
charge heterojunction and limiting forward current by
series resistance. Under U > 0.4 V, the series resistance
has a substantial effect on the value of current, which is
found to be 193.4 and 204.9 Ω·cm2 at 290 K for ITO-Ge-
pSi and ITO-pSi structures, respectively. The inset to
Fig. 2 shows the I-V dependences measured under
AM1.5G illumination. The structure of Ge thin films
reveales higher values of short circuit current and photo-
EMF in comparsion with the reference ITO-pSi structure
due to accumulation of a positive charge by Ge NCs and
lower values of RS.
Fig. 3 shows the photovoltage temporal depen-
dences for ITO-Ge-pSi and ITO-pSi structures measured
after excitation by the laser diode with the wavelength
650 nm at different temperatures. Decay curves are
approximated by exponential dependence within the
investigated temperature range 80 to 290 K:
( )τexp~)( ttU − , (2)
where τ is a decay constant.
Fig. 4 shows temperature dependences of photo-
voltage decay constants τ(T) for ITO-Ge-pSi
heterojunction and reference ITO-pSi structure. The
structure with Ge nanostructured thin film reveales faster
relaxation of photo-EMF within the temperature range
80 to 290 K. The reason for this behavior is that Ge
nanoclusters are additional recombination centers.
The rate of recombination of nonequilibrium
carriers involving Ge states determines the value τ for
the structure ITO-Ge-pSi. The decay constant slightly
increases from 28 up to 33 ms under the temperature
decrease from 290 down to 220 K. However, the value τ
rises with decreasing temperature in the range
T < 220 K, which is described as follows:
( )0exp~)(τ TTT − , (3)
where T0 is the constant equal to 90.2 K (kT0 = 7.8 meV)
for structures with Ge nanoclasters.
The observed temperature dependence can be
explained by the peculiarities of photogenerated charge
carrier transport in a Ge thin film.
-0.4 0.0 0.4 0.8 1.2
10-3
10-2
10-1
100
0.0 0.2 0.4
0.0
0.5
1.0
1.5
2.0
2.5
(2)
(1)
(2)
Ph
ot
oc
ur
re
nt
, m
A
/c
m
2
Voltage, mV
ITO-pSi
ITO-Ge-pSi (1)
Ph
ot
oc
ur
re
nt
(m
A
/c
m
2 )
Voltage(mV)
Fig. 2. Dark I-V curves of the ITO-Ge-pSi (1) and ITO-pSi (2)
structures. The inset shows the I-V dependences measured
under AM1.5G illumination at 290 K.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 2. P. 259-261.
doi: https://doi.org/10.15407/spqeo20.02.259
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
261
0.0 0.2 0.4
0.01
0.1
1
0.01
0.1
1
0.01
0.1
1
(2)
(1)
Time(ms)
pSi
GepSi
T = 285K
(2)
(1)
Ph
ot
ov
ol
ta
ge
, a
rb
.u
n.
pSi
GepSi
T = 155K
pSi
GepSiT = 85K
(1)
(2)
Fig. 3. Time dependences of photovoltage decay for the ITO-
Ge-pSi (1) and ITO-pSi (2) structures, measured at different
temperatures: 85, 155 and 285 K.
50 100 150 200 250 300
0.05
0.1
0.15
0.2 reference
ITO-pSi
D
ec
ay
c
on
st
an
ts
(m
s)
Temperature (K)
ITO-Ge-pSi
Fig. 4. The temperature dependence of photovoltage decay
constants for the ITO-Ge-pSi heterojunction and reference
ITO-pSi structure.
The charge transport is defined by thermal
activation to the band of extended states (multiple
trapping) or by hopping via localized states in the film
consisting of crystalline Ge NCs of 5…6 nm in size,
where the space between them is filled with air and
contains amorphous phase of Ge. Under high
temperatures, T > 220 K, the recombination rate is
determined by times of electron capture/release from
traps. When temperature becomes lower, the Ge NCs
becomes a giant trap for holes accumulating a positive
charge causing downward band banding in the
underlying Si [4, 5]. On the basis of the foregoing, we
assume that holes trapped by Ge are practically
immobile. Electrons have a chance to meet a hole and
recombine only during their transport. In this case,
recombination rate depends on the total density of holes
and mobility of photoexcited electrons. Under low
temperatures, electron transport occurs by hopping
within the band of localized states and recombination
occurs between hopping electrons and holes trapped by
Ge NCs. The number of trapped holes increases under
the temperature decrease, while the average jump
distance of electrons becomes higher. The photo-
electrons will be more and more accumulated in more
isolated states with longer release time constants,
which will increase τ exponentially with decreasing
temperature [6].
4. Conclusions
The obtained results indicate that the Ge film
significantly affects the transport and recombination of
nonequilibrium carriers in barrier structures based on
heterojunction ITO-pSi. The increase in the short circuit
current value and photo-EMF in the structure of the Ge
thin film barrier is caused by a higher value as compared
with the reference ITO-pSi heterojunction. The lifetime
of photogenerated carrier is determined by the
recombination channel efficiency, which is associated
with Ge NCs. The hopping transport of photoexcited
electrons in Ge thin films determines the recombination
rate in ITO-Ge-pSi heterostructures.
References
1. Brunner K. Si/Ge nanostructures. Rep. Prog. Phys.
2002. 65. P. 27.
2. Liu J., Beals M., Pomerene A., Bernardis S., Sun
R., Cheng J., Kimerling L.C. & Michel J.
Waveguide-integrated, ultralow-energy GeSi
electro-absorption modulators. Nat. Photonics.
2008. 2. P. 433–437.
3. Cosentino S., Liu P., Le S.T. et al. High-efficiency
silicon-compatible photodetectors based on Ge
quantum dots. Appl. Phys. Lett. 2011. 98.
P. 221107.
4. Lysenko V.S., Gomeniuk Y.V., Kudina V.N. et al.
Hopping conduction and LF noise in structures with
Ge nanoclusters grown on oxidized Si (001).
J. Mater. Sci. 2016. 51, No. 19. P. 8799–8811.
5. Garbar N.P., Kudina V.N., Lysenko V.S., Kon-
dratenko S.V. & Kozyrev Yu.N. Effect of Ge-
nano-islands on the low-frequency noise in
Si/SiOx/Ge structures. Adv. Mater. Res. 2014. 854.
P. 21–27.
6. Iacchetti A., Natali D., Binda M., Beverina L. &
Sampietro M. Hopping photoconductivity in an
exponential density of states. Appl. Phys. Lett. 2012.
101, No. 10. P. 103307.
|
| id | nasplib_isofts_kiev_ua-123456789-214921 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1560-8034 |
| language | English |
| last_indexed | 2026-03-21T02:22:28Z |
| publishDate | 2017 |
| publisher | Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| record_format | dspace |
| spelling | Iliash, S.A. Hyrka, Yu.V. Kondratenko, S.V. Lysenko, V.S. Kozyrev, Yu.M. Lendel, V.V. 2026-03-04T12:47:44Z 2017 Relaxation of photovoltage in ITO-Ge-Si heterojunction with Ge nanostructured thin films / S.A. Iliash, Yu.V. Hyrka, S.V. Kondratenko, V.S. Lysenko, Yu.M. Kozyrev, V.V. Lendel // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2017. — Т. 20, № 2. — С. 259-261. — Бібліогр.: 6 назв. — англ. 1560-8034 PACS: 73.40.-c, 73.50.Pz https://nasplib.isofts.kiev.ua/handle/123456789/214921 https://doi.org/10.15407/spqeo20.02.259 The paper focuses on an experimental study of the photovoltage time decay in an ITO-Ge-Si heterojunction with a Ge nanostructured thin film. Kinetics under 650 nm excitation within the temperature range 80 to 290 K are successfully described by a single exponential function with temperature-dependent decay constants. Photovoltage relaxation is modeled taking into account the hopping nature of electron transport in the band of localized states. en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics Relaxation of photovoltage in ITO-Ge-Si heterojunction with Ge nanostructured thin films Article published earlier |
| spellingShingle | Relaxation of photovoltage in ITO-Ge-Si heterojunction with Ge nanostructured thin films Iliash, S.A. Hyrka, Yu.V. Kondratenko, S.V. Lysenko, V.S. Kozyrev, Yu.M. Lendel, V.V. |
| title | Relaxation of photovoltage in ITO-Ge-Si heterojunction with Ge nanostructured thin films |
| title_full | Relaxation of photovoltage in ITO-Ge-Si heterojunction with Ge nanostructured thin films |
| title_fullStr | Relaxation of photovoltage in ITO-Ge-Si heterojunction with Ge nanostructured thin films |
| title_full_unstemmed | Relaxation of photovoltage in ITO-Ge-Si heterojunction with Ge nanostructured thin films |
| title_short | Relaxation of photovoltage in ITO-Ge-Si heterojunction with Ge nanostructured thin films |
| title_sort | relaxation of photovoltage in ito-ge-si heterojunction with ge nanostructured thin films |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/214921 |
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