Strain-induced, self-assembled iron silicide nanostructures were grown on Si(001) substrate by conventional
Fe evaporation and subsequent annealing. The initial Fe thickness was in the 0.1-6.0 nm range and
the annealing temperature was 850 °C. The formed phases and structures were characterized by reflection
high energy electron diffraction, and scanning electron microscopy. The electrical characteristics were investigated
by I-V and C-V measurements, and by DLTS. The samples show silicide nanostructure formation
in the whole thickness range. The shape of the nanostructures varied from rod like to triangular
and quadratic depending on the initial Fe thickness. The size distribution of the formed iron silicide nanoobjects
was not homogeneous, but they were oriented in square directions on Si(001). Higher thickness
resulted in increased particles size.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3518

Dozsa, L.

Horvath, Z.J.

Molnar, G.

Vertesy, Z.

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 2 No 2, 02PCN21(4pp) (2013) 
 
 
2304-1862/2013/2(2)02PCN21(4) 02PCN21-1  2013 Sumy State University 
Thickness Dependent Growth of Epitaxial Iron Silicide Nanoobjects on Si (001) 
 
G. Molnár1,*, L. Dózsa†, Z. Vértesy‡, Zs.J. Horváth1,2,§ 
 
1 Institute of Technical Physics and Materials Science, Research Centre for Natural Sciences, HAS, Budapest,  
H-1525 P.O. Box 49, Hungary 
2 Óbuda University, Kandó Kálmán Faculty of Electrical Engineering, Institute of Microelectronics and Technology, 
H-1084 Budapest,Tavaszmező u. 15-17, Hungary 
 
(Received 05 June 2013; published online 31 August 2013) 
 
Strain-induced, self-assembled iron silicide nanostructures were grown on Si(001) substrate by conven-
tional Fe evaporation and subsequent annealing. The initial Fe thickness was in the 0.1-6.0 nm range and 
the annealing temperature was 850 °C. The formed phases and structures were characterized by reflection 
high energy electron diffraction, and scanning electron microscopy. The electrical characteristics were in-
vestigated by I-V and C-V measurements, and by DLTS. The samples show silicide nanostructure for-
mation in the whole thickness range. The shape of the nanostructures varied from rod like to triangular 
and quadratic depending on the initial Fe thickness. The size distribution of the formed iron silicide na-
noobjects was not homogeneous, but they were oriented in square directions on Si(001). Higher thickness 
resulted in increased particles size. 
 
Keywords: Iron silicide, Self-assembly, Nanostructures. 
 
 
                                               
* molnar.gyorgy@ttk.mta.hu  
†
 dozsa.laszlo@ttk.mta.hu  
‡
 vertesy.zofia@ttk.mta.hu  
§ horvath.zsolt@kvk.uni-obuda.hu  
1. INTRODUCTION 
 
Future generation thin film solar cells have to use 
abundantly available, non toxic and environmentally 
friendly chemical elements. Semiconducting -FeSi2 is 
a possible material, which has 23 % theoretical effi-
ciency in solar cells, and both its layer and nanoparticle 
forms have potential applications in photovoltaic tech-
nology [1]. Recently, Terasawa and coworkers proposed 
a composite -FeSi2 / Si film for photovoltaic use, where 
photocarriers generated in the iron silicide particles, 
which has high photoabsorption coefficient, and photo-
carrier transport takes place in silicon. This material 
may result a superior solar cell due to its high photoab-
sorption coefficient and high carrier mobility [2]. 
-FeSi2 is an indirect semiconductor, but in some 
epitaxial configurations on silicon substrate it has a 
direct band gap due to strain effects [3, 4]. The follow-
ing phases of the Fe-Si equilibrium phase diagram 
have found in thin film reactions, mainly on Si(111) 
substrates: The most Fe-rich silicide is Fe3Si (DO3 
type), with cubic structure. Two types of iron monosili-
cides may appear in thin film form. The first phase is ε-
FeSi with cubic structure and the second monosilicide 
phase is cesium-chloride type cubic FeSi. The iron disil-
icides, prepared in thin layers, might have three differ-
ent crystal structures. The high temperature, metasta-
ble, tetragonal α-FeSi2 phase may be epitaxially stabi-
lized in thin film form on Si substrates. The cubic γ-
FeSi2 phase is also a metastable structure. At the end, 
-FeSi2 has orthorhombic structure. All of the above 
phases, including metastable ones, may be epitaxially 
stabilized on the surface of Si substrates [5]. 
The fabrication of artificial low dimensional struc-
tures is one of the most challenging research fields in 
the solid state technology. These nanoobjects are pre-
pared by physical and chemical or by combined meth-
ods and they have attracted great interest, according to 
their scientific peculiarity and technical significance. 
One of the most challenging methods of nanostructure 
production is the phenomena of self-assembly, that has 
been applied in case of compound and group IV semi-
conductors and wide range of material and substrate 
combinations [6]. 
In case of self-assembly the natural laws are used 
as instruments to product nanostructures. The strain 
induced self-assembled growth is a basic physical 
method of the preparation of nanostructures. During 
the growth of strained layers, the film is able to remain 
planar up to a critical thickness that depends on the 
lattice mismatch between the film and the substrate. 
Above a critical thickness, three dimensional, disloca-
tion free islands may form [7]. This phenomenon is 
known as Stransky-Krastanov transition, which is an 
important way of the preparation of self-assembled 
quantum dots and wires.  
The motivation of the recent study was to find prop-
er methods for the preparation of iron silicide 
nanostructures. Actually, we have demonstrative ex-
amples of the existence of iron silicide nanostructures 
[8], but they have not been systematically grown on Si 
substrates with different initial Fe thicknesses at a 
same annealing temperature. Research on the above 
fields may contribute to gain new knowledge in epitax-
ial growth and morphology changes in iron silicides, 
and for practical side to prepare more effective solar 
cells. 
 
 PACS numbers: 64.75.Yz, 81.15.Np 
 G. MOLNÁR, L. DÓZSA, Z. VÉRTESY, ZS.J. HORVÁTH PROC. NAP 2, 02PCN21 (2013) 
 
 
02PCN21-2 
2. EXPERIMENTAL 
 
Pieces of (001) oriented Si (p-type, 12-20 Ωcm) wa-
fers were used as substrates. Before loading the sam-
ples into the oil free evaporation chamber their surface 
was etched in diluted HF. Prior to evaporation Si wa-
fers were annealed in situ for 5 min at 850 °C. Iron 
ingots of 99.9 % purity were evaporated using an elec-
tron gun, at an evaporation rate of 0.01-0.03 nm/s, at a 
pressure of 310 – 6 Pa. The film thickness was meas-
ured by vibrating quartz. The temperatures were moni-
tored by small-heat-capacity Ni-NiCr thermocouples. 
The initial Fe thicknesses were in the 0.1-6.0 nm range 
and the annealing temperature was 850 °C. During 
evaporation the substrates were held at room tempera-
ture (RT) and the subsequent annealings were carried 
out in situ.  
The phases and structures were characterized by 
reflection high energy electron diffraction (RHEED), 
scanning- electron microscopy (SEM). The electrical 
characteristics were investigated by I-V and C-V meas-
urements and the defects were measured by deep level 
transient spectroscopy (DLTS). 
 
3. RESULTS AND DISCUSSION  
 
During sample preparation the whole process was 
followed by RHEED. In Fig. 1(a) can be seen the 
RHEED image taken from the cleaned and annealed 
Si(001) substrate showing 2  1 reconstruction. The 
RHEED image of Fig. 1(b) shows state of the surface 
after 0.1 nm iron evaporation. As can be seen, the orig-
inal Si surface reconstruction disappeared and the 
lines weakened, as consequence of very thin Fe cover-
ing. After 60 min annealing at 850 ºC according to the 
RHEED image a new, reconstructed surface developed 
showing the epitaxial character of the iron silicide 
(Fig. 1(c)) 
 
 
 
 
 
 
Fig. 1 – RHEED images of (a) the Si(001) substrate, (b) after 
0.1 nm iron evaporation, (c) after 60 min annealing at 850 ºC  
 
In case of thicker films in Fig. 2(a) can be seen the 
RHEED image of the Si(001) substrate. The image of 
Fig. 2(b) shows the polycrystalline surface after 3 nm 
iron evaporation. After 60 min annealing at 850 ºC a 
new, dotted pattern developed showing the epitaxy of 
the iron silicide (Fig. 2(c)). 
 
 
 
 
 
 
 
Fig. 2 – RHEED images of (a) the Si(001) substrate, (b) after 
3 nm iron evaporation, (c) after 60 min annealing at 850 ºC for 
iron silicide formation 
 THICKNESS DEPENDENT GROWTH OF EPITAXIAL IRON SILICIDE… PROC. NAP 2, 02PCN21 (2013) 
 
 
02PCN21-3 
The scanning electron microscopy investigations 
present that all of the samples show aggregated iron 
silicide nanoobjects. The thickness dependence of iron 
silicide nanostructure formation is shown in Fig. 3. All 
of the samples were annealed at 850 °C for 60 minutes. 
The initial mass equivalent iron thicknesses were 0.1, 
0.3, 0.6, 1.0, 3.0, and 6.0 nm as are shown Fig. 3(a-e), 
respectively.  
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 3 – SEM images of thickness dependent formation of iron 
silicides at the same annealing (850 °C, 60 min). The nanoob-
jects developed from (a) 0.1 nm, (b) 0.3 nm, (c) 0.6 nm,  
(d) 1.0 nm, (e) 3.0 nm, (f) 6.0 nm initially evaporated iron film 
 
The size and the distribution of the islands depend 
on the initial Fe thickness. The thicker films resulted 
in nanostructures with enhanced size, and with de-
creased density taking into account both the maximum 
size and the smaller islands too. 
These phenomena might be a consequence of Ost-
wald ripening. In post deposition conditions where no 
additional material is being deposited, Ostwald ripen-
ing is considered the primary coarsening mechanism 
for island growth [9]. The primary role of the Ostwald 
ripening is strengthened by the depleted regions 
around the bigger objects. 
Samples that formed from 0.3-1.0 nm Fe show tri-
angular, elongated, and rectangular nanoobjects, which 
appeared in square directions adapted epitaxially to 
the Si(001) substrate. While, for samples formed from 
0.1, 3.0 and 6.0 nm randomly shaped structures  
appeared. 
The electrical characteristics in all samples are 
dominated by Fe-related defects in about 2 m depth 
below the surface. The apparent doping concentration 
determined from the C-V characteristics decreases near 
the surface, the deep level defects compensate the dop-
ing of the starting wafer. The I-V and C-V characteris-
tics in different junction has significant scatter, some of 
the junctions are dominated by leakage of the junctions 
in reverse bias. It is due to the rough silicides / silicon 
interface morphology and to very large defect concen-
tration in the vicinity of the interface. The defect con-
centration measured by DLTS is an underestimation of 
the defect concentration since the filling pulses cannot 
fill defects in the whole depleted layer volume. 
 G. MOLNÁR, L. DÓZSA, Z. VÉRTESY, ZS.J. HORVÁTH PROC. NAP 2, 02PCN21 (2013) 
 
 
02PCN21-4 
4. CONCLUSIONS  
 
Iron silicide nanostructuress are grown by conven-
tional Fe evaporation and subsequent annealing meth-
od on Si(001) substrate. The size distribution and 
shape of the formed islands depend on the initial 
thickness of evaporated iron. Initially thicker films 
produce bigger size iron silicide nanostructures. In case 
of 0.3-1.0 nm thick iron film rectangular and elongated 
nanoobjects form in square directions, while thicker 
films result randomly shaped silicide objects. The elec-
trical characteristics in all samples are dominated by 
Fe-related defects. 
 
ACKNOWLEDGEMENTS 
 
The authors would like to thank the support of OT-
KA Grant No. 81998. 
 
 
REFERENCES 
 
1. T. Buonassisi, A.A. Istratov, M.A. Marcus, B. Lai, Z. Cai, 
S.M. Heald, E.R. Weber, Nat. Mater. 4, 676 (2005). 
2. S. Terasawa, T. Inoue, M. Ihara, Sol. Energ. Mater. Sol. C. 
93, 215 (2009). 
3. N.E. Christensen, Phys. Rev. B 42, 7148 (1990). 
4. K. Yamaguchi, K. Mizushima, Phys. Rev. Lett. 86, 6006 
(2001). 
5. G. Molnár, L. Dózsa, G. Pető, Z. Vértesy, A.A. Koós, 
Z.E. Horváth, E. Zsoldos, Thin Solid Films 459, 48 (2004). 
6. A.L. Barabási, Appl. Phys. Lett. 70, 2565 (1997). 
7. J. Tersoff, F.K. LeGoues: Phys. Rev. Lett. 72, 3570 (1994). 
8. N. Vouroutzis, T.T. Zorba, C.A. Dimitriadis, K.M. Paras-
kevopoulos, L. Dózsa, G. Molnár. J. Alloy. Compd. 448, 
202 (2008). 
9. M. Zinke-Allmang, Thin Solid Films 346, 1 (1999). 
 


Thickness Dependent Growth of Epitaxial Iron Silicide Nanoobjects on Si (001)

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 2 No 2, 02PCN21(4pp) (2013)   2304-1862/2013/2(2)02PCN21(4) 02PCN21-1  2013 Sumy State University Thickness Dependent Growth of Epitaxial Iron Silicide Nanoobjects on Si (001)  G. Molnár1,*, L. Dózsa†, Z. Vértesy‡, Zs.J. Horváth1,2,§  1 Institute of Technical Physics and Materials Science, Research Centre for Natural Sciences, HAS, Budapest,  H-1525 P.O. Box 49, Hungary 2 Óbuda University, Kandó Kálmán Faculty of Electrical Engineering, Institute of Microelectronics and Technology, H-1084 Budapest,Tavaszmező u. 15-17, Hungary  (Received 05 June 2013; published online 31 August 2013)  Strain-induced, self-assembled iron silicide nanostructures were grown on Si(001) substrate by conven-tional Fe evaporation and subsequent annealing. The initial Fe thickness was in the 0.1-6.0 nm range and the annealing temperature was 850 °C. The formed phases and structures were characterized by reflection high energy electron diffraction, and scanning electron microscopy. The electrical characteristics were in-vestigated by I-V and C-V measurements, and by DLTS. The samples show silicide nanostructure for-mation in the whole thickness range. The shape of the nanostructures varied from rod like to triangular and quadratic depending on the initial Fe thickness. The size distribution of the formed iron silicide na-noobjects was not homogeneous, but they were oriented in square directions on Si(001). Higher thickness resulted in increased particles size.  Keywords: Iron silicide, Self-assembly, Nanostructures.                                                  * molnar.gyorgy@ttk.mta.hu  † dozsa.laszlo@ttk.mta.hu  ‡ vertesy.zofia@ttk.mta.hu  § horvath.zsolt@kvk.uni-obuda.hu  1. INTRODUCTION  Future generation thin film solar cells have to use abundantly available, non toxic and environmentally friendly chemical elements. Semiconducting -FeSi2 is a possible material, which has 23 % theoretical effi-ciency in solar cells, and both its layer and nanoparticle forms have potential applications in photovoltaic tech-nology [1]. Recently, Terasawa and coworkers proposed a composite -FeSi2 / Si film for photovoltaic use, where photocarriers generated in the iron silicide particles, which has high photoabsorption coefficient, and photo-carrier transport takes place in silicon. This material may result a superior solar cell due to its high photoab-sorption coefficient and high carrier mobility [2]. -FeSi2 is an indirect semiconductor, but in some epitaxial configurations on silicon substrate it has a direct band gap due to strain effects [3, 4]. The follow-ing phases of the Fe-Si equilibrium phase diagram have found in thin film reactions, mainly on Si(111) substrates: The most Fe-rich silicide is Fe3Si (DO3 type), with cubic structure. Two types of iron monosili-cides may appear in thin film form. The first phase is ε-FeSi with cubic structure and the second monosilicide phase is cesium-chloride type cubic FeSi. The iron disil-icides, prepared in thin layers, might have three differ-ent crystal structures. The high temperature, metasta-ble, tetragonal α-FeSi2 phase may be epitaxially stabi-lized in thin film form on Si substrates. The cubic γ-FeSi2 phase is also a metastable structure. At the end, -FeSi2 has orthorhombic structure. All of the above phases, including metastable ones, may be epitaxially stabilized on the surface of Si substrates [5]. The fabrication of artificial low dimensional struc-tures is one of the most challenging research fields in the solid state technology. These nanoobjects are pre-pared by physical and chemical or by combined meth-ods and they have attracted great interest, according to their scientific peculiarity and technical significance. One of the most challenging methods of nanostructure production is the phenomena of self-assembly, that has been applied in case of compound and group IV semi-conductors and wide range of material and substrate combinations [6]. In case of self-assembly the natural laws are used as instruments to product nanostructures. The strain induced self-assembled growth is a basic physical method of the preparation of nanostructures. During the growth of strained layers, the film is able to remain planar up to a critical thickness that depends on the lattice mismatch between the film and the substrate. Above a critical thickness, three dimensional, disloca-tion free islands may form [7]. This phenomenon is known as Stransky-Krastanov transition, which is an important way of the preparation of self-assembled quantum dots and wires.  The motivation of the recent study was to find prop-er methods for the preparation of iron silicide nanostructures. Actually, we have demonstrative ex-amples of the existence of iron silicide nanostructures [8], but they have not been systematically grown on Si substrates with different initial Fe thicknesses at a same annealing temperature. Research on the above fields may contribute to gain new knowledge in epitax-ial growth and morphology changes in iron silicides, and for practical side to prepare more effective solar cells.   PACS numbers: 64.75.Yz, 81.15.Np  G. MOLNÁR, L. DÓZSA, Z. VÉRTESY, ZS.J. HORVÁTH PROC. NAP 2, 02PCN21 (2013)   02PCN21-2 2. EXPERIMENTAL  Pieces of (001) oriented Si (p-type, 12-20 Ωcm) wa-fers were used as substrates. Before loading the sam-ples into the oil free evaporation chamber their surface was etched in diluted HF. Prior to evaporation Si wa-fers were annealed in situ for 5 min at 850 °C. Iron ingots of 99.9 % purity were evaporated using an elec-tron gun, at an evaporation rate of 0.01-0.03 nm/s, at a pressure of 310 – 6 Pa. The film thickness was meas-ured by vibrating quartz. The temperatures were moni-tored by small-heat-capacity Ni-NiCr thermocouples. The initial Fe thicknesses were in the 0.1-6.0 nm range and the annealing temperature was 850 °C. During evaporation the substrates were held at room tempera-ture (RT) and the subsequent annealings were carried out in situ.  The phases and structures were characterized by reflection high energy electron diffraction (RHEED), scanning- electron microscopy (SEM). The electrical characteristics were investigated by I-V and C-V meas-urements and the defects were measured by deep level transient spectroscopy (DLTS).  3. RESULTS AND DISCUSSION   During sample preparation the whole process was followed by RHEED. In Fig. 1(a) can be seen the RHEED image taken from the cleaned and annealed Si(001) substrate showing 2  1 reconstruction. The RHEED image of Fig. 1(b) shows state of the surface after 0.1 nm iron evaporation. As can be seen, the orig-inal Si surface reconstruction disappeared and the lines weakened, as consequence of very thin Fe cover-ing. After 60 min annealing at 850 ºC according to the RHEED image a new, reconstructed surface developed showing the epitaxial character of the iron silicide (Fig. 1(c))       Fig. 1 – RHEED images of (a) the Si(001) substrate, (b) after 0.1 nm iron evaporation, (c) after 60 min annealing at 850 ºC   In case of thicker films in Fig. 2(a) can be seen the RHEED image of the Si(001) substrate. The image of Fig. 2(b) shows the polycrystalline surface after 3 nm iron evaporation. After 60 min annealing at 850 ºC a new, dotted pattern developed showing the epitaxy of the iron silicide (Fig. 2(c)).        Fig. 2 – RHEED images of (a) the Si(001) substrate, (b) after 3 nm iron evaporation, (c) after 60 min annealing at 850 ºC for iron silicide formation  THICKNESS DEPENDENT GROWTH OF EPITAXIAL IRON SILICIDE… PROC. NAP 2, 02PCN21 (2013)   02PCN21-3 The scanning electron microscopy investigations present that all of the samples show aggregated iron silicide nanoobjects. The thickness dependence of iron silicide nanostructure formation is shown in Fig. 3. All of the samples were annealed at 850 °C for 60 minutes. The initial mass equivalent iron thicknesses were 0.1, 0.3, 0.6, 1.0, 3.0, and 6.0 nm as are shown Fig. 3(a-e), respectively.              Fig. 3 – SEM images of thickness dependent formation of iron silicides at the same annealing (850 °C, 60 min). The nanoob-jects developed from (a) 0.1 nm, (b) 0.3 nm, (c) 0.6 nm,  (d) 1.0 nm, (e) 3.0 nm, (f) 6.0 nm initially evaporated iron film  The size and the distribution of the islands depend on the initial Fe thickness. The thicker films resulted in nanostructures with enhanced size, and with de-creased density taking into account both the maximum size and the smaller islands too. These phenomena might be a consequence of Ost-wald ripening. In post deposition conditions where no additional material is being deposited, Ostwald ripen-ing is considered the primary coarsening mechanism for island growth [9]. The primary role of the Ostwald ripening is strengthened by the depleted regions around the bigger objects. Samples that formed from 0.3-1.0 nm Fe show tri-angular, elongated, and rectangular nanoobjects, which appeared in square directions adapted epitaxially to the Si(001) substrate. While, for samples formed from 0.1, 3.0 and 6.0 nm randomly shaped structures  appeared. The electrical characteristics in all samples are dominated by Fe-related defects in about 2 m depth below the surface. The apparent doping concentration determined from the C-V characteristics decreases near the surface, the deep level defects compensate the dop-ing of the starting wafer. The I-V and C-V characteris-tics in different junction has significant scatter, some of the junctions are dominated by leakage of the junctions in reverse bias. It is due to the rough silicides / silicon interface morphology and to very large defect concen-tration in the vicinity of the interface. The defect con-centration measured by DLTS is an underestimation of the defect concentration since the filling pulses cannot fill defects in the whole depleted layer volume.  G. MOLNÁR, L. DÓZSA, Z. VÉRTESY, ZS.J. HORVÁTH PROC. NAP 2, 02PCN21 (2013)   02PCN21-4 4. CONCLUSIONS   Iron silicide nanostructuress are grown by conven-tional Fe evaporation and subsequent annealing meth-od on Si(001) substrate. The size distribution and shape of the formed islands depend on the initial thickness of evaporated iron. Initially thicker films produce bigger size iron silicide nanostructures. In case of 0.3-1.0 nm thick iron film rectangular and elongated nanoobjects form in square directions, while thicker films result randomly shaped silicide objects. The elec-trical characteristics in all samples are dominated by Fe-related defects.  ACKNOWLEDGEMENTS  The authors would like to thank the support of OT-KA Grant No. 81998.   REFERENCES  1. T. Buonassisi, A.A. Istratov, M.A. Marcus, B. Lai, Z. Cai, S.M. Heald, E.R. Weber, Nat. Mater. 4, 676 (2005). 2. S. Terasawa, T. Inoue, M. Ihara, Sol. Energ. Mater. Sol. C. 93, 215 (2009). 3. N.E. Christensen, Phys. Rev. B 42, 7148 (1990). 4. K. Yamaguchi, K. Mizushima, Phys. Rev. Lett. 86, 6006 (2001). 5. G. Molnár, L. Dózsa, G. Pető, Z. Vértesy, A.A. Koós, Z.E. Horváth, E. Zsoldos, Thin Solid Films 459, 48 (2004). 6. A.L. Barabási, Appl. Phys. Lett. 70, 2565 (1997). 7. J. Tersoff, F.K. LeGoues: Phys. Rev. Lett. 72, 3570 (1994). 8. N. Vouroutzis, T.T. Zorba, C.A. Dimitriadis, K.M. Paras-kevopoulos, L. Dózsa, G. Molnár. J. Alloy. Compd. 448, 202 (2008). 9. M. Zinke-Allmang, Thin Solid Films 346, 1 (1999).  

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Thickness Dependent Growth of Epitaxial Iron Silicide Nanoobjects on Si (001)

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