Voids’ layer structures in silicon irradiated with high doses of high-energy helium ions
Structural and optical properties of single crystal silicon irradiated with 27.2 MeV helium ions by using fluences Ф ≥ 10¹⁶ ion/cm² were studied at various beam currents. It was found that at currents 0.25 to 0.45 μА, heavily damaged layers containing voids were formed in ion path in Si and behind...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
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irk-123456789-1211982017-06-14T03:07:32Z Voids’ layer structures in silicon irradiated with high doses of high-energy helium ions Starchyk, M.I. Marchenko, L.S. Pinkovska, M.B. Shmatko, G.G. Varnina, V.I. Structural and optical properties of single crystal silicon irradiated with 27.2 MeV helium ions by using fluences Ф ≥ 10¹⁶ ion/cm² were studied at various beam currents. It was found that at currents 0.25 to 0.45 μА, heavily damaged layers containing voids were formed in ion path in Si and behind it. The number of layers in the ion path region depends on the beam intensity. With increasing the beam current up to ~1 μA, the layer structures consisting of voids were observed only in the ion path. As helium is poorly soluble in Si, during implantation it collects in the gas-filled vacancy complexes. We consider that, like to the case of keV-ion implantation at fluences of Ф ≥ 10¹⁶ ion/cm² , an amorphous layer is created in the ion stopping region at annealing. Moving by recrystallization fronts on both sides of the amorphous layer, vacancy clusters are collected inside, coalesce and form voids. It is a combination of high energy and high fluence helium implantation that can form layered structure with voids in silicon, as observed by us. At present, there is no strict explanation of the mechanism of voids’ ordering (forming of superlattice of them). Especially, it concerns the void layer formation beyond helium ion path. The concept of mobile solitons is used. Formation of the “lattice” from the voids leads to swelling of the material. Further researches are necessary to understand these processes and control them. 2015 Article Voids’ layer structures in silicon irradiated with high doses of high-energy helium ions / M.I. Starchyk, L.S. Marchenko, M.B. Pinkovska, G.G. Shmatko, V.I. Varnina // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2015. — Т. 18, № 3. — С. 292-296. — Бібліогр.: 11 назв. — англ. 1560-8034 PACS 61.72.-y, 61.80.-x http://dspace.nbuv.gov.ua/handle/123456789/121198 DOI: 10.15407/spqeo18.03.292 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Structural and optical properties of single crystal silicon irradiated with 27.2 MeV helium ions by using fluences Ф ≥ 10¹⁶ ion/cm² were studied at various beam currents. It was found that at currents 0.25 to 0.45 μА, heavily damaged layers containing voids were formed in ion path in Si and behind it. The number of layers in the ion path region depends on the beam intensity. With increasing the beam current up to ~1 μA, the layer structures consisting of voids were observed only in the ion path. As helium is poorly soluble in Si, during implantation it collects in the gas-filled vacancy complexes. We consider that, like to the case of keV-ion implantation at fluences of Ф ≥ 10¹⁶ ion/cm² , an amorphous layer is created in the ion stopping region at annealing. Moving by recrystallization fronts on both sides of the amorphous layer, vacancy clusters are collected inside, coalesce and form voids. It is a combination of high energy and high fluence helium implantation that can form layered structure with voids in silicon, as observed by us. At present, there is no strict explanation of the mechanism of voids’ ordering (forming of superlattice of them). Especially, it concerns the void layer formation beyond helium ion path. The concept of mobile solitons is used. Formation of the “lattice” from the voids leads to swelling of the material. Further researches are necessary to understand these processes and control them. |
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Starchyk, M.I. Marchenko, L.S. Pinkovska, M.B. Shmatko, G.G. Varnina, V.I. |
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Starchyk, M.I. Marchenko, L.S. Pinkovska, M.B. Shmatko, G.G. Varnina, V.I. Voids’ layer structures in silicon irradiated with high doses of high-energy helium ions Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Starchyk, M.I. Marchenko, L.S. Pinkovska, M.B. Shmatko, G.G. Varnina, V.I. |
| author_sort |
Starchyk, M.I. |
| title |
Voids’ layer structures in silicon irradiated with high doses of high-energy helium ions |
| title_short |
Voids’ layer structures in silicon irradiated with high doses of high-energy helium ions |
| title_full |
Voids’ layer structures in silicon irradiated with high doses of high-energy helium ions |
| title_fullStr |
Voids’ layer structures in silicon irradiated with high doses of high-energy helium ions |
| title_full_unstemmed |
Voids’ layer structures in silicon irradiated with high doses of high-energy helium ions |
| title_sort |
voids’ layer structures in silicon irradiated with high doses of high-energy helium ions |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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2015 |
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http://dspace.nbuv.gov.ua/handle/123456789/121198 |
| citation_txt |
Voids’ layer structures in silicon irradiated with high doses of high-energy helium ions / M.I. Starchyk, L.S. Marchenko, M.B. Pinkovska, G.G. Shmatko, V.I. Varnina // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2015. — Т. 18, № 3. — С. 292-296. — Бібліогр.: 11 назв. — англ. |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 3. P. 292-296.
doi: 10.15407/spqeo18.03.292
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
292
PACS 61.72.-y, 61.80.-x
Voids’ layer structures in silicon irradiated with high doses
of high-energy helium ions
M.I. Starchyk, L.S. Marchenko, M.B. Pinkovska, G.G. Shmatko, V.I. Varnina
Institute for Nuclear Research, National Academy of Science of Ukraine,
47, prospect Nauky 03028 Kyiv, Ukraine; phone 38(044) 525 3749, e-mail: myrglory@yahoo.com
Abstract. Structural and optical properties of single crystal silicon irradiated with
27.2 MeV helium ions by using fluences Ф ≥ 1016 ion/cm2 were studied at various beam
currents. It was found that at currents 0.25 to 0.45 μА, heavily damaged layers containing
voids were formed in ion path in Si and behind it. The number of layers in the ion
path region depends on the beam intensity. With increasing the beam current up to
~1 μA, the layer structures consisting of voids were observed only in the ion path. As
helium is poorly soluble in Si, during implantation it collects in the gas-filled vacancy
complexes. We consider that, like to the case of keV-ion implantation at fluences of
Ф ≥ 1016 ion/сm2, an amorphous layer is created in the ion stopping region at annealing.
Moving by recrystallization fronts on both sides of the amorphous layer, vacancy clusters
are collected inside, coalesce and form voids. It is a combination of high energy and high
fluence helium implantation that can form layered structure with voids in silicon, as
observed by us. At present, there is no strict explanation of the mechanism of voids’
ordering (forming of superlattice of them). Especially, it concerns the void layer
formation beyond helium ion path. The concept of mobile solitons is used. Formation of
the “lattice” from the voids leads to swelling of the material. Further researches are
necessary to understand these processes and control them.
Keywords: silicon, helium implantation, high defect concentration, layer structures,
voids.
Manuscript received 06.04.15; revised version received 16.06.15; accepted for
publication 03.09.15; published online 30.09.15.
1. Introduction
Theoretical and experimental data received in the field
of radiation solid state physics indicate the possibility of
self-organization in the ensemble of radiation defects,
especially under ion implantation [1-3]. An important
factor for the radiation modification of material
properties is information on the distribution of ions and
structural defects in the irradiated material, their size,
thermal stability and so on. The results are of funda-
mental importance for better understanding the nature of
interaction of radiation with solids at high concentration
of defects.
Interest in the behaviour of helium implanted in
silicon is related with the fact of voids’ formation at high
fluences of irradiation [4, 5]. Helium is poorly soluble in
Si and during irradiation collects in the gas-filled
vacancy complexes. When silicon is heated at
temperatures above T ≥ 400 °C, these complexes grow
and helium are released, leaving voids. Surface of voids
remains clean, making them places of possible gettering
of impurities from the surrounding matrix, as well as
centres of strain relaxation in the silicon lattice.
We present the results of studying the structural
and optical properties of monocrystalline silicon
irradiated with helium ions possessing the energy close
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 3. P. 292-296.
doi: 10.15407/spqeo18.03.292
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
293
to 27.2 MeV at fluences ≥5·1016 ion/сm2 by varying the
beam current. The goal of the work is to obtain infor-
mation about the nature and parameters of the defect
structure and optical properties of irradiated silicon.
2. Experimental
Silicon samples were made using the Czochralsky
method (〈111〉, p-type, ρ = 10 Ohm·cm) and irradiated
with helium ions with an energy of 27.2 MeV at fluences
of 1016 to 1017 ion/сm2 by varying the beam current from
0.25 to 1.0 μA in the cyclotron U-120 of Institute for
Nuclear Research, NAS of Ukraine. During irradiation,
the samples were cooled with running water. The
projection length of the path of ions with a given energy
is about 360 μm. The irradiated samples were cut along
the direction of irradiation, allowing to study the
properties of silicon in the region of ions’ path, braking
and behind stopping parts.
3. Results
X-ray topography of irradiated silicon sample, after
polishing and chemical etching, displayed significant
disruption of the structure in the region of ion stopping
at the depth approximately 360 μm (Fig. 1).
In the micrographs of silicon surface obtained by
scanning electron microscopy (Fig. 2) after irradiation of
two different samples (a, b) with the fluence 1017 cm–2 at
a beam current of ~0.25…0.45 μA, the lines of strains
are visible. Distances of lines from the irradiated surface
are shown in Table. Lines (III-IV) are associated with
the end of the path of helium ions. Other lines are shown
within the path and behind the stopping part of the
sample (long-range effect). When the beam current
increases up to 1 μA, lines of the strain appeared only in
the path region of ions. Long-range effect was not
observed. The number of strain lines in the path region
of the sample was determined likely by the temperature
distribution over the irradiated surface, declining from
the centre (c) to the edge (a, b) of the sample, where the
maximum cooling took place (Fig. 3).
Table. Distances of “defect walls” from the sample surface.
Distances of “defect walls”
from the sample surface, μm Number
of the “wall” Metallography Scanning electron
microscopy
I 132 150
II 242 282
III 341 362
IV 380 385
V 423 –
VI 627 636
VII 720 674
VIII 764 –
Fig. 1. X-ray topogram of the irradiated silicon sample
(18×4 mm).
a
b
Fig. 2. The micrographs of silicon surface obtained by
scanning electron microscopy after irradiation of two different
samples (a, b) with the fluence 1017 ion/cm2 at the beam
current 0.3 μA. Arrows indicate the direction of irradiation.
Microprofilogram of the sample surface (Fig. 4)
shows the swelling of silicon in the region of the helium
ions’ path. The difference in the profile of the irradiated
(a) and unexposed (b) parts of the sample is about 8 μm.
The dot-dashed line indicates the position of straggling
(stopping) of ions.
Images of the surfaces of the initial and irradiated
Si samples (1017 ion/сm2) obtained using the atomic
force microscope (Fig. 5) confirm the swelling of Si in
the path region of ions: roughness of the irradiated
surface greatly increased in comparison with the initial
one [7].
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 3. P. 292-296.
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© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
294
a b c
Fig. 3. Picture of the selective etching of Si irradiated with the fluence 1017 ion/cm2 at the beam current 1.0 μA (a, b – at the edge,
c – in the centre of irradiated region of the sample).
Results of metallographic study for the Si sample
irradiated up to the dose 1017 сm–2 at the ion beam
current close to 1 μA after selective etching are shown in
Fig. 6. It is seen that the strain lines in the path region of
the sample are the layers consisting of voids with
different shapes and sizes. Voids were observed in the
layers (a) and isolated from each other (b). The number
of layers depends on the intensity of the ion beam and,
hence, the heating temperature of the sample during
irradiation. Fig. 3c shows two well-observed etched
layers: upper one – the width 30 to 45 μm (borderline of
ion stopping); lower one – the width 10 to 15 μm located
closer to the irradiated surface. Besides, voids near the
stacking faults and dislocations were observed in the
vicinity of the layers. Strain lines at a distance greater
than the mean free path of ions (long-range effect) were
observed at the beam current 0.25 to 0.45 μA and disap-
peared as the current increases to ~1 μA. IR absorption
spectra of silicon studied using the Fourier spectrometer
FIS-113V at room temperature with a spectral resolution
1.0 cm–1 are shown in Fig. 7. In the differential
absorption spectra of silicon irradiated with the ion
fluences of 5·1016 and 1·1017 ion/сm2 (curves 2, 3) an
additional absorption associated with the irradiation was
not observed within the spectral range 400…5000 cm–1.
Fig. 4. Microprofilogram of Si suface after irradiation (a – ion
path region, b – behind it). The dashed line indicates struggling.
a b
Fig. 5. AFM image of the Si surface before (a) and after irradiation (b).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 3. P. 292-296.
doi: 10.15407/spqeo18.03.292
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
295
a)
b)
Fig. 6. Voids and defects in the ion path region of irradiated Si,
which were revealed using the selective etching.
Fig. 7. Differential absorption spectra of Si irradiated with
helium ions: 1 (Si-5) – Ф = 5·1016 ion/cm2; 2 (Si-6) – Ф =
1·1017 ion/cm2; 3 (Si-7) – Ф = 0.
4. Discussion
It is known [5] that, in silicon irradiated at room
temperature with ions possessing the energy of tens keV
and the fluence of Ф≥ 1016 ion/сm2, an amorphous layer
is created in the ion stopping region. The centre of the
amorphous region lies in the range with the highest
number of displaced atoms. The thickness of the
amorphous layer increases with increasing the irradiation
fluence. Amorphization can be avoided while increasing
the irradiation temperature up to 250 °C and above.
Clustering of vacancies in the case of He
implantation manifested itself in the form of formation
of gas-vacancies complexes, and excess interstitial
atoms form extended defects (stacking faults, rod-like
defects and ribbon-like defects) [8]. Increasing the
irradiation temperature promotes releasing of gas from
the vacancy complexes, and then voids remain.
Formation of the vacancy complexes is possible with a
high concentration of vacancies, which is determined by
the concentration of disintegrated Frenkel pairs. When in
our experiment we used helium ions with MeV-energies
instead of keV, these conditions are performed better.
It is believed that the ion fluence Ф = 1016 ion/сm2
is the threshold fluence to form gas-vacancy complexes
in silicon. A more accurate value of the fluence is
determined by temperature of implantation, energy and
mass of ions. Upon annealing of irradiated silicon at
800 °C, the restore of disintegrated amorphous structure
occurs through epitaxial recrystallization on both sides
of the amorphous layer [9]. The process begins at the
boundary between the amorphous layer and Si matrix.
Recrystallization promotes acceleration of the gas
segregation, the restoring of the defect structure
(amorphous layer) and formation of a polycrystalline
layer.
In addition, in the case of large ion fluence,
formation of periodic structures was observed in
irradiated materials [10, 11].
At the beam current ~1 μA and fluence
1017 ion/сm2, in path (for helium ions) region of Si we
observe two heavily damaged layers consisting of voids
(Fig. 3c): upper one – in the region of ion stopping at a
depth of ~360 μm; lower one – closer to the irradiated
surface. Behind the stopping part of silicon such defects
were not detected.
When reducing the beam current to 0.25…0.45 μA,
the strain lines are observed in the ion path region of
silicon and beyond. It shows the possibility to obtain
different layer picture of the defect distribution by
changing the current of the ion beam (irradiation
intensity).
It is assumed [4] that during the annealing of
silicon irradiated with helium ions of keV-energy,
helium-filled vacancy complexes are moving in the
direction of the irradiated surface. After reaching it,
helium goes of them, leaving large empty “pockets” of
voids near surface. The authors of [5] explain this
movement by the effect of restructuring, which is caused
by the movement of the recrystallization front through
the amorphous material.
Moving by the recrystallization fronts on both sides
of the amorphous layer, vacancy clusters are collected
inside, coalesce and form voids. It is a combination of a
high energy and a high fluence implantation of helium
that can form multi-layered structure of voids in silicon,
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 3. P. 292-296.
doi: 10.15407/spqeo18.03.292
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
296
which is observed by us. Upon annealing of silicon
irradiated with helium ions of the keV energy at
~800 °С, the amorphous region at the end of the ion path
recrystallizes forming a polycrystalline region with
stacking faults and dislocations [4]. For this reason, to
avoid amorphization step implantation is carried out at
elevated temperatures. High-energy ion irradiation
provides such defect structuring, i.e., formation of
enriched or depleted defect regions by varying the beam
current (irradiation intensity).
The mechanisms of amorphization and recrys-
tallization under ion irradiation of silicon are still under
discussion [5]. Because of the lack of reliable data on the
nature and parameters of the altered layers, it is
impossible to determine the precise threshold fluence of
irradiation, period and size of these superstructures.
In order to explain the observed by us the spread of
ordered structures behind the stopping for helium
ions region of silicon in the case of the current
0.25…0.45 μA and fluence Ф ≥ 1016 ion/сm2, the
concept of mobile solitons is used nowadays [8].
Solitons originate in the planes of cascades of atoms
displacements and propagating along close-packed
directions in the crystal: they are thermostable and
introduced by the focusing collisions of atoms. Defects
that form these structures are likely to be interstitial
atoms.
5. Conclusion
Irradiation of monocrystalline silicon with helium ions
possessing the energy 27.2 MeV and fluence Ф ≥
1016 ion/сm2 leads to formation of linear structures
within the region of ion path as well as behind them. The
number of structures depends on the intensity of
irradiation (beam current). At currents ~0.25…0.45 μA,
linear structures are observed in ion path of Si and
behind it. With increasing beam current to ~1 μA, linear
structures consisting of voids were observed only in the
path region of ions.
At present, there is no other explanation of the
mechanism of pores’ ordering (forming of superlattice of
them). Especially, it concerns the “shrinkage” or
compression of pores beyond their limits. It is assumed
that, when pores are along the path of propagation of
solitons, the conditions should create of ordering up to
creation of the “lattice” of them; for all other directions
there are compression and “sweeping”.
Formation of the “lattice” from the pores leads to
swelling of material. Further researches are necessary to
understand these processes and to control them.
Acknowledgement
The authors are grateful to Corresponding Member of
NASU Vladimir Sugakov for a fruitful discussion of the
results and help. The authors are also grateful to referee
for careful reading the manuscript and discussion.
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