Recombination of charge carriers in heterostructures with Ge nanoislands grown on Si(100)
In this paper, the study of the recombination of non-equilibrium charge carriers and determination of recombination mechanisms in Ge/Si heterostructures with nanoislands have been presented. The effects of long-term photoconductivity decay in Ge/Si heterostructures with Ge nanoislands have been foun...
Збережено в:
| Дата: | 2015 |
|---|---|
| Автор: | |
| Формат: | Стаття |
| Мова: | English |
| Опубліковано: |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2015
|
| Назва видання: | Semiconductor Physics Quantum Electronics & Optoelectronics |
| Онлайн доступ: | http://dspace.nbuv.gov.ua/handle/123456789/120734 |
| Теги: |
Додати тег
Немає тегів, Будьте першим, хто поставить тег для цього запису!
|
| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | Recombination of charge carriers in heterostructures with Ge nanoislands grown on Si(100) / S.V. Kondratenko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2015. — Т. 18, № 1. — С. 97-100. — Бібліогр.: 15 назв. — англ. |
Репозитарії
Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
irk-123456789-120734 |
|---|---|
| record_format |
dspace |
| spelling |
irk-123456789-1207342017-06-13T03:02:49Z Recombination of charge carriers in heterostructures with Ge nanoislands grown on Si(100) Kondratenko, S.V. In this paper, the study of the recombination of non-equilibrium charge carriers and determination of recombination mechanisms in Ge/Si heterostructures with nanoislands have been presented. The effects of long-term photoconductivity decay in Ge/Si heterostructures with Ge nanoislands have been found as caused by variations of the electrostatic potential in the near-surface region of Si(100) substrate and spatial separation of electron-hole pairs between localized states of Ge nanoislands and states of wetting layer and Si. It has been shown that the photoconductivity decay depends on the excitation energy and temperature, while Ge nanoislands are Shockley-Read recombination centers with a higher recombination rate as compared with Si. 2015 Article Recombination of charge carriers in heterostructures with Ge nanoislands grown on Si(100) / S.V. Kondratenko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2015. — Т. 18, № 1. — С. 97-100. — Бібліогр.: 15 назв. — англ. 1560-8034 DOI: 10.15407/spqeo18.01.097 PACS 62.23.Eg, 72.20.Jv, 73.40.-c http://dspace.nbuv.gov.ua/handle/123456789/120734 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| description |
In this paper, the study of the recombination of non-equilibrium charge carriers and determination of recombination mechanisms in Ge/Si heterostructures with nanoislands have been presented. The effects of long-term photoconductivity decay in Ge/Si heterostructures with Ge nanoislands have been found as caused by variations of the electrostatic potential in the near-surface region of Si(100) substrate and spatial separation of electron-hole pairs between localized states of Ge nanoislands and states of wetting layer and Si. It has been shown that the photoconductivity decay depends on the excitation energy and temperature, while Ge nanoislands are Shockley-Read recombination centers with a higher recombination rate as compared with Si. |
| format |
Article |
| author |
Kondratenko, S.V. |
| spellingShingle |
Kondratenko, S.V. Recombination of charge carriers in heterostructures with Ge nanoislands grown on Si(100) Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Kondratenko, S.V. |
| author_sort |
Kondratenko, S.V. |
| title |
Recombination of charge carriers in heterostructures with Ge nanoislands grown on Si(100) |
| title_short |
Recombination of charge carriers in heterostructures with Ge nanoislands grown on Si(100) |
| title_full |
Recombination of charge carriers in heterostructures with Ge nanoislands grown on Si(100) |
| title_fullStr |
Recombination of charge carriers in heterostructures with Ge nanoislands grown on Si(100) |
| title_full_unstemmed |
Recombination of charge carriers in heterostructures with Ge nanoislands grown on Si(100) |
| title_sort |
recombination of charge carriers in heterostructures with ge nanoislands grown on si(100) |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2015 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/120734 |
| citation_txt |
Recombination of charge carriers in heterostructures with Ge nanoislands grown on Si(100) / S.V. Kondratenko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2015. — Т. 18, № 1. — С. 97-100. — Бібліогр.: 15 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT kondratenkosv recombinationofchargecarriersinheterostructureswithgenanoislandsgrownonsi100 |
| first_indexed |
2025-07-08T18:29:18Z |
| last_indexed |
2025-07-08T18:29:18Z |
| _version_ |
1837104479904727040 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 1. P. 97-100.
doi: 10.15407/ spqeo18.01.097
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
97
PACS 62.23.Eg, 72.20.Jv, 73.40.-c
Recombination of charge carriers in heterostructures
with Ge nanoislands grown on Si(100)
S.V. Kondratenko
Taras Shevchenko Kyiv National University,
64/13, Volodymyrs’ka str., 01601 Kyiv, Ukraine; e-mail: kondr@univ.kiev.ua
Abstract. In this paper, the study of the recombination of non-equilibrium charge
carriers and determination of recombination mechanisms in Ge/Si heterostructures with
nanoislands have been presented. The effects of long-term photoconductivity decay in
Ge/Si heterostructures with Ge nanoislands have been found as caused by variations of
the electrostatic potential in the near-surface region of Si(100) substrate and spatial
separation of electron-hole pairs between localized states of Ge nanoislands and states of
wetting layer and Si. It has been shown that the photoconductivity decay depends on the
excitation energy and temperature, while Ge nanoislands are Shockley-Read
recombination centers with a higher recombination rate as compared with Si.
Keywords: photoconductivity, recombination, nanoislands.
Manuscript received 06.10.14; revised version received 14.12.14; accepted for
publication 19.02.15; published online 26.02.15.
1. Introduction
Low-dimensional Ge/Si heterostructures have attracted
considerable research interest in recent years due to
their significant potential to impact new electronic
devices which are compatible with the available silicon
technology. Interest in semiconductor heterostructures
with nanoscale objects is due to the size quantization
effects that lead to changes in the electronic spectrum.
Optoelectronic devices based on SiGe dots grown on a
Si substrate have been already proposed [3, 4]. In
particular, the use of interband or intraband transitions
involving localized states in the valence band of SiGe
can increase the photoconversion efficiency in the near
infrared range. The low-dimensional silicon-
germanium alloys have a wide range of applications,
including quantum dot IR photodetectors, memory cells
and spintronic devices. Widespread application of this
system is the arrangement of SiGe quantum dots in the
space-charge region of heterojunctions, Schottky
diodes, p-n junctions or metal-oxide-semiconductor
structures [1-5, 10].
It is known that Ge/Si heterostructures are related
to the type II heterostructures where the potential well
exists only for one type of charge carrier – holes in the
valence band of Ge nanoisland. The valence band offset
value in these structures depends on the composition of
the nanoislands and their environment, on the presence
of mechanical strain and on the influence of quantum
confinement effects. In heterostructures Ge/Si with SiGe
quantum dots, spatial separation of non-equilibrium
charge carriers takes place – holes in the valence band
states are captured by SiGe, and electrons are
accumulated in the potential well of Si surrounding. As a
result, the SiGe quantum dots at low temperatures can
accumulate positive charges. Under these conditions, the
presence of SiGe nanoislands on the surface of Si
substrate affects on redistribution of the charge density
along the epitaxial layers, which in turn affects the
processes of recombination of charge carriers.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 1. P. 97-100.
doi: 10.15407/ spqeo18.01.097
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
98
In this paper, we present the results of analyzing
the processes of photogeneration and recombination of
non-equilibrium charge carriers in heterostructures with
self-assembled Ge nanoislands on Si(001) substrate
grown using the molecular beam epitaxy (MBE)
technique as well as photocurrent spectroscopy and
photocurrent decay techniques.
2. Experiment
The Ge nanocluster structures were grown using a MBE
technique on boron doped (Na ~ 315 cm10 ) Si(001)
substrates with the resistivity of 7.5 Ω·cm is the
Stranski-Krastanov growth mode [12]. For deposition
monitoring, reflection high-energy electron diffraction
(RHEED) was employed. After oxide removal, a well-
defined Si(001) surface was achieved by depositing a Si
buffer layer onto the silicon wafer until a high-contrast
Si(001) 2×1 electron diffraction pattern was visible.
Subsequently, Ge was deposited at the deposition rate
close to ~0.05 Å/s and the substrate temperature 600 °C
resulting in a strain driven formation of Ge nanoislands
with the lateral extensions between 60 and 450 nm and
height between 60 and 450 nm. Topographic
measurements of Ge-Si heterostructure were performed
simultaneously under normal ambient conditions using
NTEGRA (NT-MDT) system equipped with an
15×15 µm closed-loop scanner and an 650 nm super
luminescent diode for readout of the cantilever bending.
Ohmic contacts separated by 10 mm from each
other were formed by annealing Au on the surface at
370 °C in N2 ambient, to provide that conductivity
measurements can be performed for in-plane, lateral
transport. The dark current and the photocurrent were
measured over the temperature range of 80…290 K
using a current amplifier and standard detection of the
direct current. The experimental current-voltage curves
were linear within the range from 77 to 290 K at low
applied voltages, less than 500 mV, and demonstrating
ohmic behavior. Transient photoconductivity (PC) and
spectral measurements were done using excitation from
a 250-W lamp spectrally resolved through a
monochromator. Photoconductivity spectra were
measured between 0.6 to 1.8 eV with normal incidence
light excitation and a low electric field of 10 mV/cm,
which was applied along [001] direction.
3. Results and discussion
Measurements of infrared photoconductivity in
Ge nanoislands / Si structures made it possible to
evaluate their electronic spectrum. Fig. 1 shows the
spectral dependences of in-plane photoconductivity of
Ge/Si structure with SiGe nanoislands measured at
different temperatures. The PC spectrum shape for the
structure with SiGe nanoislands indicates the small
(~10
3
cm/s) velocity of surface recombination. The in-
plane photocurrent in the range hν > εGe,Si is mainly
originated from band-to-band transitions in c-Si. For
light excitations with photon energy below the band gap
of Si hν < εGe,Si (εGe,Si = 1.17 eV at 77 K), the electronic
transitions from the valence band to conduction band of
SiGe nanoislands give their main contribution to PC.
However, generation of photocurrent within the range
0.8 < hν < εGe,Si for Ge/Si is also possible due to
transitions between tails of the density of states in the
near-surface c-Si, the optical absorption spectra of which
are described by the Urbach rule.
1
Fig. 1. In-plane PC spectra of the Si/Ge heterostructure with
nanoislands at different temperatures. The inset shows the set-
up for in-plane PC measurement.
0 25 50 75 100
0.0
0.2
0.4
0.6
0.8
1.0
1.2
100 150 200 250
3
4
80 K120 K
I P
C
,
A
Time (s)
50 K
ln
(
a
rb
.u
n
.)
1/kT (eV-1
)
a
0.8 1.0 1.2 1.4
13.0
13.5
14.0
14.5
15.0
120 K
(s
)
Energy (eV)
b
Fig. 2. (a) Time dependences of the photocurrent measured
after photoexcitation by the quanta with the energy hν = 0.9 еV
at different temperatures: 50, 80 and 120 K. (b) Spectral
dependence of photocurrent decay constant of the sample at the
temperature T = 120 K.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 1. P. 97-100.
doi: 10.15407/ spqeo18.01.097
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
99
Fig. 2 shows the temporal evolution of photo-
current measured after the excitation by the quanta with
the energy hν = 0.9 еV at different temperatures: 50, 80
and 120 K. After illumination, the current decays under
the law:
t
III expPC0 , (1)
where I0 and IPC are the values of dark current and
photocurrent, respectively, and τ is a decay constant. The
activation energy εa = 12 meV was extracted from the
Arrhenius-type plot of ln τ vs 1/(kT), shown in the inset
to Fig. 2a. The spectral dependence of the decay
constant is shown in Fig. 2b. It was found that the decay
constant increases with increasing the excitation
quantum energy. The relatively faster relaxation after
excitation of SiGe nanoislands, which is observed after
0.8-eV excitation is the evidence of the fact that
recombination involving the states of nanoislands is
more effective in comparison with recombination in the
wetting layer or Shockley-Read-Hall recombination
centres in the depletion layer of Si substrate.
Several models were proposed to explain the origin
of the observed long-term PC. In the microscopic local-
potential model, the local fields of inhomogeneity
separate photoexcited electrons and holes and thus delay
recombination [8]. The other dominant mechanism
involves macroscopic potential barriers, which prevent
recombination in the other way [9, 10]. One type of
carriers should be localized by traps, while the other one
are free and separated spatially. As reported in [8, 11],
both the long decay and high conductivity values cannot
be explained by usual models of Shockley-Read
recombination centers. After optical illumination is
removed, the long-term PC was observed due to
recombination-preventing potential barrier, which
separates spatially the regions with trapped carriers and
the conductivity channel in p-Si substrate. The described
macroscopic barrier originates from the space-charge
region of near-surface Si. In addition, availability of Ge
nanoislands would induce in-plane variations of the
electrostatic potential in the conductivity channel, which
favours to spatial separation of photoexcited electron-
hole pairs in lateral direction. Large-scale electrostatic
fluctuations could exist in the near-surface region of Si
substrate, which are the result of spatial distribution of
trapped electrons and holes, inhomogeneity of the
wetting layer, interface imperfections, and non-uniform
strains in Ge nanoislands and Si substrate. Moreover, the
strain-modified confinement potential for electrons in
underlying silicon can enhance in-plane fluctuations for
Ge nanoislands / Si structures [12]. In the work [13], for
example, the authors found that inhomogeneous
deformation can cause significant changes in the optical
properties due to the shift of the electron spectrum by the
order of 100 meV. Calculations of fields of mechanical
stress in Ge/Si structures with quantum dots of Ge [14]
showed that the most stressed area is located under the
nanoisland basis, and the value of the silicon lattice
deformation along the plane of the structure decreases
with the distance to nanoisland. As a result, the band
structure of silicon nanoislands surrounding is
characterized by the presence of variations in the plane
of the structure, so it is graded-band. The region of Si
with the minimal value of band gap, which is also less
than the band gap of the strained Si, is located near the
base nanoislands.
The decrease in the recombination rate of non-
equilibrium electrons and holes photogenerated in the Si
substrate is promoted by the electric field in the surface
layer of depletion in the p-Si substrate. This field has a
direction that facilitates the drift of non-equilibrium
holes, which are photoexcited due to band-to-band
transitions in Si, from the illuminated surface. At the
same time, non-equilibrium electrons fill the minimum
of the potential energy near the surface. Spatial
separation of non-equilibrium charge carriers, thus,
reduces the probability of Shockley-Read recombination
and delay PC kinetics at low temperatures. Fig. 2b
demonstrates the increase of τ with increasing the
quantum energy, while the PC fall in the shortwave
region of the photoconductivity spectrum at hν > 1.3 eV
was also observed (see Fig. 1). Usually, the decrease of
the photoconductivity in this spectral range is explained
by the fact that with increasing hν more and more
excitation radiation is absorbed in the surface region of
Si, where the recombination rate is higher in comparison
with that in bulk of Si. The observed changes in the
shape of the spectra indicate that the rate of
recombination of charge carrier photogenerated in the
depletion layer of the Si substrate decreases during
cooling when the spatial separation became more
pronounced.
It should be noted that described long-term PC
originates from spatial separation of electron-hole pairs
photoexcited in Ge nanoislands and Si substrate. Let us
analyze the peculiarities of long-term PC decay after
excitation by photons that cannot excite free electron-
hole pairs in Si. In this case, observed photoconductivity
is monopolar: electrons photoexcited in Ge nanoislands
are free to move. On the other hand, photoexcited holes
are localized in the deep potential well of Ge
nanoislands and can’t contribute to in-plane transport.
As a consequence, the Si regions under the base of the
Ge nanoislands would have additional electrons as
compared to the bulk Si, while the Ge nanoislands can
be considered as an efficient trap for holes. Under
conditions of effective hole trapping by Ge nanoislands,
recombination rate should be restricted by supply of
minority carriers – electrons, which makes
recombination more faster as compared with those after
excitation of electron-hole pairs in the Si substrate.
4. Conclusions
In this study, PC measurements were used for detection
of charge recombination in structures with Ge NCs
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 1. P. 97-100.
doi: 10.15407/ spqeo18.01.097
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
100
grown on Si (100) substrate. The in-plane photo-
conductivity, excited by light excitations with the photon
energy below the band gap of Si, is caused by interband
transitions involving localized states of the valence band
in the nanoislands. During investigation of the kinetics
of photoconductivity in Si/Ge nanoheterostructures,
long-term relaxation of the photocurrent has been
observed. Effects of the long-term photoconductivity
decay in Ge/Si heterostructures with Ge nanoislands has
been found to be caused by variations of the electrostatic
potential in the near-surface region of Si(100) substrate
and spatial separation of electron-hole pairs between
localized states of Ge nanoislands and states in the
wetting layer and Si. It has been found that, during
selective photoexcitation of the Ge nanoislands,
recombination of electron-hole pairs is defined by spatial
separation of non-equilibrium charge carriers, when
holes are trapped in the valence band states of Ge and
electrons are in their silicon surroundings. The PC decay
dependence on the excitation energy and temperature
shows that Ge nanoislands are Shockley-Read
recombination centers with a higher recombination rate
as compared with Si.
Acknowledgements
The work was carried out due to the support of the State
Agency on Science, Innovations and Informatization of
Ukraine (project number M/94–2014).
References
1. K. Bruner, Si/Ge nanostructures // Repts. Progr.
Phys. 65, p. 27-72 (2002).
2. A.V. Dvurechenskii, A.I. Yakimov, N.P. Stepina,
V.V. Kirienko, P.L. Novikov, SiGe nanodots in
electro-optical SOI devices, in: Nanoscaled
Semiconductor-on-Insulator Structures and
Devices. Springer, 2007, p. 113-128.
3. G. Abstreiter, P. Schittenhelm, C. Engel, E. Silveira,
A. Zrenner, D. Meertens, W. Jager, Growth and
characterization of self-assembled Ge-rich islands on
Si // Semicond. Sci Technol. 11, p. 1525 (1996).
4. A. Usami, N. Ujihara, T. Fujiwara, K. Sazaki, G.
Nakajima, K. Shiraki, Enhanced quantum
efficiency of solar cells with self-assembled Ge
dots stacked in multilayer structure // Appl. Phys.
Lett. 83, p. 1258 (2003).
5. N.T. Bagraev, A.D. Bouravleuv, L.E. Klyachkin,
A.M. Malyarenko, S.A. Rykov, Self-ordered
microcavities embedded in ultrashallow silicon p–n
junctions // Semiconductors, 34, p. 700-711 (2000).
6. J.M. Baribeau, X. Wu, N.L. Rowell, D.J. Lockwood,
Ge dots and nanostructures grown epitaxially on Si //
Condensed Matter, 18, p. 139-174 (2006).
7. O.P. Pchelyakov, Yu.B. Bolkhovityanov,
A.V. Dvurechenski, L.V. Sokolov, A.I. Nikiforov,
A.I. Yakimov, B. Voigtlander, Silicon-germanium
nanostructures with quantum dots: Formation
mechanisms and electrical properties //
Semiconductors, 34, p. 1229-1247 (2000).
8. M.K. Sheinkman and A.Ya. Shik, Long-term
relaxation and residual conductivity in
semiconductors // Fizika Tekhnika Poluprovodn.
10, p. 209-232 (1976), in Russian.
9. H.J. Queisser, D.E. Theodorou, Decay kinetics of
persistent photoconductivity in semiconductors //
Phys. Rev. B, 33, p. 4027-4033 (1986).
10. H.J. Queisser, Nonexponential relaxation of
conductance near semiconductor interfaces // Phys.
Rev. Lett. 54, p. 234-236 (1985).
11. T.N. Sitenko, I.P. Tyagulskii, V.I. Lyashenko, V.S.
Lysenko, Role of surface band bending in residual
conductivity formation in epitaxial GaAs films //
phys. status solidi (a), 30, p. 755-763 (1975).
12. V. Kuryliuk, O. Korotchenkov, A. Cantarero,
Carrier confinement in Ge/Si quantum dots grown
with an intermediate ultrathin oxide layer // Phys.
Rev. B, 85, 075406-075416 (2012).
13. A.V. Dvurechenskii, A.I. Yakimov, Type-II Ge/Si
quantum dots // Semiconductor Physics and
Technology, 35, p. 1143-1153 (2001), in Russian.
14. O.A. Shegai, K.S. Zhuravlev, V.A. Markov, A.I.
Nikiforov, O.P. Pchelyakov, Photoresistance of
Si/Ge/Si structures with germanium quantum dots
// Semiconductor Physics and Technology, 34,
p. 1363-1367 (2000), in Russian.
15. A.A. Mykytiuk, S.V. Kondratenko, V.S. Lysenko,
Yu.N. Kozyrev, Photocurrent spectroscopy of Ge
nanoclusters grown on oxidized silicon surface //
Proc. SPIE 9126, Nanophotonics, 9126, p. 3J
(2014).
|