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Epitaxial Growth of n-Type -FeSi2 Thin Films on p-Type Si(111) Substrates by Radio-Frequency Magnetron Sputtering and Rectifying Action of Heterojunctions
Tarek M. Mostafa1, Motoki Takahara1, Ryuji Baba1, Suguru Funasaki1, Mahmoud Shaban2*, Nathaporn Promros3**, Aki Tominaga1,4, Maiko Nishibori4, and Tsuyoshi Yoshitake1,4***
1Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga,
Fukuoka 816-8580, Japan 2Department of Electrical Engineering, Aswan Faculty of Engineering, Aswan University,
Aswan 81542, Egypt 3Department of Physics, Faculty of Science, King Mongkut’s Institute of Technology
Ladkrabang, Bangkok 10520, Thailand 4Research Center for Synchrotron Light Applications, Kyushu University, Kasuga,
Fukuoka 816-8580, Japan
E-mail: *m_shaban@kyudai.jp; **nathaporn_promros@kyudai.jp;
***tsuyoshi_yoshitake@kyudai.jp
(Received July 31, 2014)
n-Type β-FeSi2 thin films were deposited on p-type Si(111) substrates by conventional radio
frequency magnetron sputtering at substrate temperatures of 500 - 600 ºC without post-annealing.
The epitaxial growth of β-FeSi2 on Si(111) initiates at substrate temperatures of higher than 560
ºC, and it was found that the epitaxial growth is indispensable for the n-type β-FeSi2/p-type Si
heterojunctions having rectifying action.
1. Introduction
In the last few years, the orthorhombic semiconducting phase of iron disilicide (β-FeSi2) has
received much attention owing to its potential application to optoelectronics such as near infrared
detectors and photovoltaics [1,2]. β-FeSi2 has a large absorption coefficient, which is 200-fold
larger than that of crystalline silicon at 1.5 eV, and possesses indirect and direct optical band gaps
of 0.78 eV and 0.85 eV, respectively, which is relevant to an optical fiber for telecommunication
wavelengths (1.3 and 1.5 μm) [3-5]. It is also compatible with silicon technology due to small
lattice mismatches of 2 - 5% [6,7]. From the ecological point of view, β-FeSi2 is a nontoxic
material, and its elements (Fe and Si) are abundant in nature [8]. -FeSi2 is a potential
semiconductor applicable to silicon-based optoelectronic devices. However, there have been a few reports on heterojunction photodiodes comprising -FeSi2 and Si thus far because it is difficult to
fabricate diodes that exhibit good rectifying action.
In order to form β-FeSi2/Si heterojunctions, several techniques such as ion beam synthesis
[9,10], molecular beam epitaxy [11], chemical vapor deposition [12], and reactive deposition
epitaxy [11] have been employed. However, the heterojunctions prepared by these techniques
seems not to exhibit good rectifying action as photodiodes. The most probable reason might be the
diffusion of Fe atoms into Si substrates. It results in the generation of deep trap levels in the Si
layer, which should act as leakage centers in the rectifying action and trap centers for photogenerated carriers [14]. The Fe diffusion is enhanced with increasing temperature. Although
post-annealing at temperatures higher than 800 C is generally applied for growing -FeSi2 and for
JJAP Conf. Proc. (2015) 011102©2015 The Japan Society of Applied Physics
3Proc. Int. Conf. and Summer School on Advanced Silicide Technology 2014
011102-1
enhancing the crystalline quality of -FeSi2, it accelerates the diffusion of Fe atoms into Si
substrates. In order to avoid such a situation, we have ever employed facing target direct current
sputtering for as-growing -FeSi2 thin films on Si(111) epitaxially at substrate temperatures as low
as possible [15]. On the other hand, although a conventional magnetron sputtering, which enable us
to deposit large-area films and is suitable for industrial applications, has ever been employed for
the deposition of -FeSi2 thin films, there has been a few reports on the demonstration of
-FeSi2/Si heterojunctions as photodiodes [16].
In this work, we employed a conventional radio-frequency magnetron sputtering (RFMS) for
fabricating -FeSi2/Si heterojunctions, wherein -FeSi2 thin films were deposited on Si(111)
substrates without post-annealing and studied the formation of the -FeSi2/Si heterojunctions. It
was found that the epitaxial growth of -FeSi2 is an important key factor for producing rectifying
action of the heterojunctions.
2. Experimental Processes
300-nm β-FeSi2 thin films were grown on a p-type Si (111) substrates with an electrical resistivity
of 10 Ωcm and thickness of 260 µm at substrate temperatures range of 500 - 600 ºC by RFMS with
an FeSi2 alloy targets (purity: 4N), with an atomic ratio of Fe:Si = 1:2. The deposition rate was
approximately 0.5 nm/min at an applied radio-frequency power of 20 W. In order to remove their
native oxide layers, the Si substrates were initially immersed in diluted hydrofluoric acid (HF)
solution (concentration: 1%) and then rinsed in deionized water. After that, they were immediately
mounted in a film preparation chamber evacuated down to a base pressure of less than 10-4 Pa.
During the sputtering process, a pressure inside the chamber was fixed to be 2.66 × 10-1 Pa by
introducing Ar gas (purity: 6N) at a flow rate of 15 sccm. The applied RF power was 20 W. After
that, they were transferred to another RFMS apparatus in order to deposit the electrodes. As shown in Fig. 1, Pd (purity, 4N) was deposited on the top of Si surface in finger-shaped pattern, while Al
(purity, 4N) was deposited on the entire β-FeSi2 back surface. These depositions were carried out at
room temperature. In this device structure, NIR light, which is transmitted through the front-side Si
substrate, directly reaches a depletion region in the β-FeSi2 thin film. The crystalline structure was characterized by X-ray diffraction (XRD) (Rigaku, RINT
2000/PC) using Cu-Kα radiation. The current-voltage (J-V) characteristics of the heterojunctions
were measured using a source meter (Keithley 2400) in the dark and under illumination with a 6
mW, 1.31 μm laser diode (Neoark, TC20).
Fig. 1. Schematic diagram of n-type β-FeSi2/p-type Si heterojunction.
011102-2JJAP Conf. Proc. (2015) 0111023
3. Results and discussion
Fig. 2 displays the XRD patterns of β-FeSi2 thin films deposited on Si(111) substrates at substrate
temperatures between 500 and 600 ºC, measured in (a) 2θ-θ scan and (b) grazing incidence (2θ scan) with an incidence angle of 4 °. The films deposited at 560, 580, and 600 C exhibit 202/220
peaks due to β-FeSi2 in the 2θ-θ patterns, and they are strengthened with increasing substrate
temperature. At substrate temperatures lower than 540 C, the -202/220 peaks are not clearly
observed. The 2θ patterns exhibit several peaks due to -FeSi2, which indicates the existence of
polycrystalline grains of β-FeSi2 in the films.
25 30 35 40
2 (deg)
Inte
nsi
ty (
arb
. u
nit
)
Si
111
-FeSi2 202/220
600 °C
580 °C
560 °C
540 °C
500 °C
(a)
20 40 60 80
2 (deg)
Inte
nsi
ty (
arb
. u
nit
)
600 °C
580 °C
560 °C
2
02
/22
0
540 °C
500 °C
3
12
/32
1
3
31
/04
0
1
04
/14
0
03
3/4
22
3
23
0
24
2
24
(b)
Fig. 2. XRD patterns of β-FeSi2 films deposited on Si(111) substrates at substrate temperatures of
500 - 600 ºC, measured in (a) 2θ-θ scan and (b) 2θ scan with an incidence angle of 4 °.
To confirm the in-plane orientation of -FeSi2, pole figure measurements were examined.
The pole figure patterns concerning -440/404 peak are shown in Fig. 3. Whereas the films
deposited at substrate temperatures of higher than 560 C clear exhibit the existence of three types
of epitaxial variant that are rotated at an angle 120 with respect to each other [16,17]. From these
results, the β-FeSi2 thin films deposited at substrate temperatures of higher than 560 C are
epitaxially grown on Si(111) with the typical epitaxial relationships with three variants although
they contain polycrystalline grains of -FeSi2 in the films. On the other hand, at lower than 540 C,
the pole figure patterns do not have diffraction spots due to -FeSi2 clearly, which indicates that the
-FeSi2 thin films are hardly epitaxially grown on Si(111). It was found that the epitaxial growth of
-FeSi2 on Si(111) initiates from approximately 560 C.
011102-3JJAP Conf. Proc. (2015) 0111023
Fig. 3. XRD pole figure pattern concerning β-440/404 diffraction peak, of β-FeSi2 thin film
deposited at substrate temperatures of (a) 600, (b) 560, (c) 540, and (d) 500 C.
J-V characteristics in the dark and under illumination with a 1.31 m light, of the
heterojunctions are shown in Fig. 4. The epitaxially-grown films, which were deposited at higher
than 560 C, exhibit rectifying action. In addition, they slightly exhibit photocurrents for the
illumination. On the other hands, the non-epitaxial films, which were deposited at lower than 540
C, hardly exhibit reifying action, and the behavior of the junctions is nearly ohmic. Of course,
they rarely indicate photodetection. While the heterojunctions that employ p-type Si substrates
exhibit the rectifying action as diodes, heterojunctions comprising n-type Si substrates and -FeSi2
thin films exhibited ohmic behaviors, which evidently indicates that the -FeSi2 thin films grown
by RFMS in this work have n-type conduction, similarly to those prepared by facing targets
direct-current sputtering (FTDCS) in our previous work. It was demonstrated that the epitaxial
growth of -FeSi2 is necessary for producing rectifying action in the heterojunctions. The carrier
concentration of the non-epitaxial -FeSi2 thin films, wherein the crystalline growth is insufficient
due to the low substrate temperatures, should be too large to generate a depletion region in the
heterojunctions, and a huge number of grain boundaries might act as leakage centers.
Among the epitaxial films, the 560 C film exhibits the best rectifying behavior. The
rectification ratio is gradually degraded with increasing substrate temperature. This implies that the
011102-4JJAP Conf. Proc. (2015) 0111023
diffusion of Fe atoms into the Si substrates might be enhanced by increasing substrate temperature,
which results in the degraded diode performances. Whereas an increase in the substrate temperature
enhances the crystalline growth of -FeSi2 and epitaxial growth of -FeSi2, it degrades the junction
quality, particularly around the interfaces. In the film deposition by RFMS, films receive plasma damage since substrates are located in
plasma. A reason for the heterojunctions prepared by conventional RFMS being inferior to those
prepared by facing targets direct-current sputtering in our previous study [17], wherein substrates
are located away from plasma distinctly differently from a situation in conventional RFMS and
films receive less damage from plasma, might be a difference in the plasma damage. The film
quality of -FeSi2 should be degraded by plasma damage. In addition, the existence of films in
plasma makes the effective surface temperature of films be increased, which might enhance the
diffusion of Fe atoms into the Si substrates.
Fig. 4. J-V characteristics in the dark and under illumination with 1.31 m monochromatic lamp, of
n-type β-FeSi2/p-type Si heterojunctions deposited at substrate temperatures of (a) 600, (b) 560, (c)
540, and (d) 500 C
10-6
10-5
10-4
10-3
10-2
10-1
-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1
darkillumination
Cu
rren
t d
ensi
ty (
A/c
m2)
Voltage (V)
(a) 600 oC
10-6
10-5
10-4
10-3
10-2
10-1
-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1
darkillumination
Cu
rren
t d
ensi
ty (
A/c
m2)
Voltage (V)
(c) 560 oC
10-6
10-5
10-4
10-3
10-2
10-1
-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1
darkillumination
Cu
rren
t d
ensi
ty (
A/c
m2)
Voltage (V)
(c) 540 oC
10-6
10-5
10-4
10-3
10-2
10-1
-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1
dark
Cu
rren
t d
ensi
ty (
A/c
m2)
Voltage (V)
(f) 500 oC
(b) 560 ºC
(d) 500 ºC
011102-5JJAP Conf. Proc. (2015) 0111023
4. Conclusion
Heterojunction diodes, wherein n-type β-FeSi2 thin films were epitaxially grown on p-type Si(111)
substrates by RFMS, were fabricated, and their rectifying action and photodetection were
investigated. The epitaxial growth of n-type β-FeSi2 thin films on p-type Si(111) substrates was
realized at substrate temperatures of 560 - 600 ºC, and heterojunctions comprising their
epitaxially-grown -FeSi2 thin films exhibit rectifying action. It was experimentally demonstrated
that the epitaxial growth of -FeSi2 thin films is indispensable for the formation of the
heterojunction diodes. In the substrate temperate range of 560-600 C, the rectification ratio was
gradually degraded with increasing substrate temperature. It is considered that an enhancement in
the diffusion of Fe atoms into Si substrates with increasing substrate temperature might degrade the
junction quality. Both rectifying action and photodetection were inferior to those of similar
heterojunctions prepared by facing targets direct-current sputtering in our previous study. Since an
obvious difference between RFMS and FTDCS is whether substrates are located in plasma or away
from plasma, -FeSi2 thin films prepared by RFMS might be damaged by plasma.
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