Structural properties of Fe ion implanted and ruby laser annealed Si layers

Abstract

The processes in the synthesis of iron silicide thin films (FeSi and FeSi2) on a single-crystal Si substrate implanted with different doses of Fe+ ions (D = 1015–2 × 1017 cm−2) and subjected to pulsed laser annealing (λ = 0.69 µm, τ = 80 ns, W = 0.6–1.4 J cm−2) are investigated. Using x-ray diffraction, transmission electron microscopy and Rutherford backscattering spectrometry, the structure and phase composition of the synthesized films and the depth profile of Fe atoms in the Si are studied. It is shown that laser annealing (W = 0.6–1.1 J cm−2) of high-dose implanted Si (D > 1017 cm−2) results in the formation of epitaxial iron monosilicide (FeSi) layers. Increasing the pulse energy up to 1.4 J cm−2 leads to a redistribution of Fe atoms in the Si and formation of a mixture of silicide phases (FeSi+FeSi2) with the cellular structure of a synthesized layer. In the case of low-dose implanted Si (D ~ 1016 cm−2), the formation of cellular structures takes place at lower energy densities (W ~ 0.8 J cm−2), with segregation of Fe atoms to the Si surface.

Full text

INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 37 (2004) 468–471 PII: S0022-3727(04)68945-5
Structural properties of Fe ion implanted
and ruby laser annealed Si layers
R M Bayazitov1,4, R I Batalov1, I B Khaibullin1, G D Ivlev2,
ID
´
ezsi3andEK
´
otai3
1Kazan Physical-Technical Institute of RAS, Sibirsky trakt 10/7, 420029 Kazan, Russia
2Institute of Electronics of NAS, Logoisky trakt 22, 220090 Minsk, Belarus
3KFKI Research Institute for Particle and Nuclear Physics, PO Box 49,
H-1525 Budapest 114, Hungary
E-mail: bayaz@kfti.knc.ru
Received 15 September 2003
Published 14 January 2004
Online at stacks.iop.org/JPhysD/37/468 (DOI: 10.1088/0022-3727/37/3/026)
Abstract
The processes in the synthesis of iron silicide thin films (FeSi and FeSi2)on
a single-crystal Si substrate implanted with different doses of Fe+ions
(D=1015–2 ×1017 cm−2)and subjected to pulsed laser annealing
(λ=0.69 µm, τ=80 ns, W=0.6–1.4 J cm−2)are investigated. Using
x-ray diffraction, transmission electron microscopy and Rutherford
backscattering spectrometry, the structure and phase composition of the
synthesized films and the depth profile of Fe atoms in the Si are studied. It is
shown that laser annealing (W=0.6–1.1 J cm−2)of high-dose implanted Si
(D>1017 cm−2)results in the formation of epitaxial iron monosilicide
(FeSi) layers. Increasing the pulse energy up to 1.4 J cm−2leads to a
redistribution of Fe atoms in the Si and formation of a mixture of silicide
phases (FeSi + FeSi2)with the cellular structure of a synthesized layer. In
the case of low-dose implanted Si (D∼1016 cm−2), the formation of
cellular structures takes place at lower energy densities (W∼0.8Jcm
−2),
with segregation of Fe atoms to the Si surface.
1. Introduction
Single-crystal Si is the main material for microelectronics;
however, its application for optoelectronic purposes is limited
due to its indirect band gap (Eg=1.1 eV), which makes
it an inefficient light emitter at room temperature. In the
last decade, the formation of Si-based structures emitting
light in the visible and infrared regions has attracted much
attention (Si-based optoelectronics) in order to overcome
this limitation. One of the approaches to creating Si-based
structures emitting in the 1.5 µm region (a transparency
band for Si and SiO2)is the formation of continuous and
precipitate layers of semiconducting iron disilicide (β-FeSi2)
having a direct band structure with an optical gap of about
0.85 eV according to the optical absorption measurements [1].
Nowadays, by means of ion-beam synthesis and reactive
deposition techniques, several diode structures incorporating
β-FeSi2films in the conventional Si p–n junction and emitting
4Author to whom any correspondence should be addressed.
light at about 1.5 µm at 300 K are obtained [2–5]. However,
both methods of synthesis of β-FeSi2films include a prolonged
high-temperature furnace annealing (T∼900 ◦C, t∼20 h)
undesirable in microelectronic technology due to uncontrolled
diffusion of impurity atoms in the bulk of a Si crystal,
resulting in the spread of the profiles of electrically active
dopants (boron, phosphorus) and in degradation of the main
electrophysical parameters of the Si substrate.
An alternative to furnace annealing of whole Si wafers
could be pulsed treatment of the implanted Si layers by
powerful laser, electron and ion beams that are characterized
by localization (in area and depth) and short duration (typically
<1µs) of processing. At present, a limited number of works
have been published relating to pulsed treatments of the Fe–Si
system in order to form iron silicide films on Si [6–9]. In [6],
it was shown that treatment of epitaxial β-FeSi2films on an
Si(111) substrate with 25 ns ruby laser pulses with an energy
density of about 0.5 J cm−2led to a transformation from the
orthorhombic β-FeSi2phase to the cubic γ-FeSi2one. In [7],
irradiation of 57Fe/Si(100) layers by excimer XeCl laser pulses
0022-3727/04/030468+04$30.00 © 2004 IOP Publishing Ltd Printed in the UK 468

Structural properties of Fe ion implanted Si layers
at different energy densities (0.5 and 1.0Jcm−2)and pulse
numbers (2, 4, 8, 16 and 32) resulted in the formation of various
silicide phases (FeSi, β-FeSi2,γ-FeSi2), depending on the
laser processing regime. However, to synthesize single-phase
β-FeSi2films, additional furnace annealing (750 ◦C, 1 h) was
employed. It is necessary to note that in the above works iron
films have been deposited onto the Si substrate in ultra-vacuum
chambers (base pressure 10−10 Torr), making the deposition
rather expensive. Ion implantation into the Si substrate
is usually performed under moderate vacuum conditions
(∼10−5Torr) and is well compatible with Si microelectronics
technology.
Earlier, we studied the formation of iron silicide films in
Fe-implanted Si layers subjected to pulsed ion-beam treatment
(C+,H
+)[8] or Nd : YAG laser (λ=1.06 µm) irradiation [9].
We showed that pulsed ion-beam treatment of the implanted
Si layers with an energy density of more than 1 J cm−2
leads to the formation of single-phase, textured and stressed
β-FeSi2films. Pulsed laser annealing (PLA) with an energy
density of 3 J cm−2gives similar results but is characterized
by severe damage of the Si surface due to the features of
energy absorption in the substance under ion and laser beams
[10]. In this work, the formation of iron silicide films in
Fe-implanted and ruby laser (λ=0.69 µm)-annealed Si layers
is investigated.
2. Experimental
Czochralski-grown single-crystal Si wafers with n-type
conductivity (4–5 cm) and (100) orientation were implanted
at 300 K by 40 keV Fe+ions with doses (D) in the range of
1016–2 ×1017 cm−2at an ion current density j5µAcm
−2.
After implantation, the samples were subjected to PLA in
open air by a single shot of a ruby laser (λ=0.69 µm)
with 80 ns pulse duration. Energy density values varied
between 0.6 and 1.4 J cm−2. An optical set-up allowed
us to obtain high uniformity of intensity (±5%) of the
laser spot (6 mm in diameter). The structure and phase
composition of the implanted and laser-annealed Si :Fe layers
were investigated by grazing incidence x-ray diffraction
(GIXRD) at an angle of incidence ϕ=1–3◦and plan-view
transmission electron microscopy (PVTEM) at an acceleration
voltage of 90 kV. Depth profiles of Fe atoms in the Si matrix
after implantation and laser annealing were deduced from
Rutherford backscattering spectrometry (RBS) data. The
measurements were carried out using two combinations of
the energy of the analysing 4He beam and the detection angle
of the backscattered particles (E=1 MeV, θ=165◦and
E=3 MeV, θ=97◦). The second geometry is a grazing
exit mode that allowed one to achieve a depth resolution of
about 10 nm (as calculated by DEPTH code [11]) within the
Fe-implanted region (Rp=37 nm, Rp=14 nm according
to the TRIM code [12]).
3. Results and discussion
Figure 1 shows GIXRD spectra of high-dose (D=1.8×
1017 cm−2)implanted Si(100) after PLA with an energy
density W=1.1Jcm
−2measured under different angles
of x-ray incidence (ϕ=1◦,2
◦and 3◦). It can be seen
that no diffraction peaks are present at the incidence angles
of 1◦and 2◦, while a sufficiently intense diffraction peak
corresponding to Bragg reflection from the (210) plane of the
iron monosilicide (FeSi) appears at the angle of 3◦. An azimuth
scanning of this peak (2θ=56.3◦for λFeKα=1.93 Å) in the
angle range χ=0–360◦(figure 1, inset) shows four intense
maxima every 90◦. These results indicate the formation of
epitaxial FeSi layers on the Si(100) substrate. The formation
of single-crystalline FeSi layers is also corroborated by the
electron diffraction pattern, where the spots corresponding
both to single-crystal Si and FeSi were observed.
The Fe depth profiles in the Si (figure 2) deduced
from RBS spectra (not shown) evidence the synthesis
of iron monosilicide films at the given regime of PLA
(W=1.1Jcm
−2)with a composition very close to the
°
°
°
°
°
Figure 1. GIXRD spectra at different angles of incidence
(ϕ=1–3◦) of high-dose implanted Si (E=40 keV,
D=1.8×1017 Fe cm−2)after PLA (W=1.1Jcm
−2). In the inset,
the azimuth dependence of the FeSi(210) diffraction peak is shown.
Figure 2. Fe depth profiles in Si deduced from RBS spectra (4He,
E=1 MeV, θ=165◦) before and after PLA of the implanted Si
(E=40 keV, D=1.8×1017 Fe cm−2)at the energy densities (W)
shown. The arrows show the stoichiometric compositions
corresponding to FeSi and FeSi2phases.
469

R M Bayazitov et al
stoichiometric one (Fe : Si =1 : 1). In the as-implanted state
(before PLA), the Fe depth profile is in the narrow Si layer
70 nm thick with the maximum of the atomic concentration
of about 75%. As a result of PLA (1.1 J cm−2), a gradual
redistribution of the implanted Fe atoms in the Si takes place,
and within a depth of about 40 nm a flat part in the Fe
profile with 50% atomic concentration can be seen. Under
PLA, with an energy density of 1.4 J cm−2, the Fe profile
has a flat part, with the Fe atomic fraction about 30%. The
electron diffraction pattern of this sample indicates formation
of a mixture of silicide phases (FeSi, γ-FeSi2)with the
polycrystalline structure. These findings correlate well with
the results of [7], where the same phase mixture, determined
from conversion electron M¨
ossbauer spectroscopy (CEMS)
measurements, was formed under excimer laser treatment of
57Fe/Si layers with an energy density of 0.5 J cm−2using many
laser pulses (16 and more).
PVTEM studies of high-dose (D=1.8×1017 cm−2)
implanted and laser-annealed (W=1.4Jcm
−2)Si layers
(figure 3) indicate the formation of the cellular structures
characteristic of liquid-phase crystallization. These cellular
structures are columns of single-crystalline Si 30–40 nm in
Figure 3. Bright field PVTEM micrograph of high-dose implanted
Si (E=40 keV, D=1.8×1017 Fe cm−2)after PLA
(W=1.4Jcm
−2). Black lines intersecting the picture are due to the
extinction contrast.
(a) (b)
Figure 4. Bright field PVTEM micrograph of low-dose implanted Si (E=40 keV, D=7×1015 Fe cm−2)after PLA (W=0.8Jcm
−2)(a)
and its electron diffraction pattern (b).
diameter surrounded by walls of the mixture of silicides (FeSi,
γ-FeSi2). Therefore, the average Fe atomic concentration in Si
is slightly reduced relative to the stoichiometric composition of
FeSi and FeSi2. Note that similar cellular structures attributed
to rapid solidification of liquid Si enriched by low-soluble
impurities were observed earlier both on Me/Si layers (Me =
Co, Ni, Mo, Pd) and In+-implanted Si layers irradiated by high-
power pulsed laser or ion beams [13–15].
PLA of low-dose (D∼1016 cm−2)Fe-implanted Si layers
also leads to the formation of cellular structures with 40–50 nm
column size (figure 4(a)). However, this phenomenon takes
place under lower energy densities (∼0.8Jcm
−2)due to the
difference in the melting thresholds of high-dose and low-dose
implanted Si (∼1.2Jcm
−2and ∼0.6Jcm
−2, respectively).
The electron diffraction pattern (figure 4(b)) revealed that the
iron impurity after PLA is in the bound state as a mixture
of polycrystalline silicides (FeSi and α-FeSi2). The absence
of the semiconducting β-FeSi2phase was corroborated by
measurements of the Raman scattering [16] due to the absence
of characteristic peaks at 190 and 247 cm−1[17]. In contrast to
the case of high-dose implantation, the Fe impurity profile has
a different behaviour (figure 5). In the as-implanted state, the
Fe depth profile has a Gaussian-like shape with a maximum at
about 30 nm close to Rp=37 nm according to the TRIM code.
As the pulse energy increases, the redistribution of Fe atoms
to the Si surface (segregation) characteristic for low-soluble
impurities in Si (such as Bi, Pt, Fe) [18–20] is observed. So
the ruby laser processing of the implanted Si in the energy
density range (W=0.6–1.4 J cm−2)investigated does not
lead to direct synthesis of the β-FeSi2phase, which is of
most interest for Si-based optoelectronics. It is possible that
in order to synthesize single-phase β-FeSi2layers either a
higher energy density (W>2Jcm
−2)of the laser processing
[9] or additional short-term furnace annealing (T∼800 ◦C,
t∼30 min) [7,8] is required.
4. Conclusions
By means of ion implantation of Fe ions into Si and nanosecond
laser treatments, heterostructures of iron silicides on Si
have been formed, and the processes of pulsed synthesis of
470

Structural properties of Fe ion implanted Si layers
Figure 5. Fe depth profiles in Si deduced from RBS spectra (4He,
E=3 MeV, θ=97◦) before and after PLA of the implanted Si
(E=40 keV, D=7×1015 Fe cm−2)at the energy densities (W)
indicated.
submicron iron silicide layers have been studied. It has been
established that depending on the initial Fe concentration in
the implanted Si layers, the spread by diffusion of the Fe
distribution profiles or preferable segregation-induced pushing
of Fe atoms to the Si surface takes place accompanied by the
formation of cellular structures with a 30–50 nm column size.
Depending on the energy density of the laser pulse, poly- or
single-crystalline iron silicide compounds are produced.
Acknowledgments
This work was supported by the Russian Foundation for
Basic Research (Grant Nos 01-02-16649 and 02-02-16838),
Scientific programme of RAS ‘Thermophysics and mechanics
of intense energetic treatments’ and Grant No T 34332
(OTKA).
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