Modeling and Numerical Simulation of Material Science, 2013, 3, 13-15
Published Online January 2013 (http://www.SciRP.org/journal/mnsms)
Copyright © 2013 SciRes. MNSMS
First Principle Study on the Electric Structure of β-FeSi2
with Native Point Defects
L.P. Peng1, A.L. He2
1Science and Technology on Plasma Physics Laboratory, Reasch Center of Laser Fusion, CAEP, P.O.Box 919-987,Mianyang 621900,
PR China
2Science and Tichnology Information Center, CAEP, P.O.Box 919-983,Mianyang 621900, PR China
Email: healing08@163.com, pengliping2005@126.com
Received 2012
ABSTRACT
The projector-augmented plane wave potentials method under the density functional theory (DFT ) was used to calcu-
late the electronic structure of perfect and native point defective β-FeSi2 crystal. The calculated band structure shows
that the band gap of perfect crystal is about 0.74eV, which is a little smaller than the experimental of about 0.9eV. The
density of states results predicts that β-FeSi2 with Fe vacancies behaves n-type, and that with Si vacancies will shows
p-type, which is in accordant with the experimental results.
Keywords: β-FeSi2; First Principle Calculation; Electronic Struct ure
1. Introduction
In recent years there has been an increasing effort in the
development of new silicon based optoelectronic mate-
rials due to their possible implementation in integrated
opto- and micro-electronic devices. Due to its lumines-
cent properties corresponding to a direct band gap of
about 0.875eV and strong optical absorption
( α=105cm-1), β-FeSi2 is an attractive silicon based op-
toelectronic materials expected for use in optoelectronic
device applications such as infrared detectors or light
emitters integrated in silicon technology [1-3]. More over
high abundance of its non-toxic co nstituents Fe and Si.
This opens new fields of applications, namely, high effi-
cient solar cells, photo-detectors, and thermoelectric de-
vices. On the other hand, β-FeSi2 has been studied as a
material for the thermoelectric conversion application
due to its superior features such as its larger Seebeck
coefficient, low electrical resistivity, and chemical stabil-
ity. The quality of a good thermoelectric material is
usually characterized by the dimensionless figure of me-
rit ZT [4], which is defined as
ZT =
2
ST
σ
κ
(1)
where κ, σ, and S represent the thermal conductivity,
electrical conductivity, and Seebeck coefficient, respec-
tively, and S2σ is generally defined as the power factor.
To achieve higher ZT, it is required to increase S and/or σ,
and/ or decrease κ. But for simple material, κ, σ, and S
are dependent on one another, a simple effective method
is to improve the transport properties of the material. The
transport properties of the materials depend on structural
properties such as the microstructure and defects as well
as the kinds of dopants. For β-FeSi2, there are always
some point defects in the crystal, which was suggested to
affect the semiconductor types and the transport proper-
ties of carrier significantly [5-6].
In this work, we performed first-principle density
function calculations to get the information on the elec-
tronic structure of the perfect and native point defective
β-FeSi2. The aim is to investigate the effect of point de-
fect of Fe and Si on the electrical properties of β-FeSi2
crystal.
2. Computational Method
Our calculations are performed based on the density
functional theory (DFT) within the generalized gradient
approximation implemented in the VIENNA AB INITIO
SIMULATION PACKAGE. The projector-augmented
plane wave potentials are used to represent the elec-
tron-ion interactions. Atomic coordinates were fully op-
timized by using the conjugate gradient technique. A
kinetic energy cuto ff of 520 eV is used to ensure a con-
vergence better than 1 meV for total energy per atom.
We constructed a conventional unites containing 48
atoms ( 8 Fe, 8 Fe, 32 Si and 32 Si) with the
space group Cmca and the lattice constants length
A=9.8632 Å, length B= 7.7916 Å, length C= 7.8278 Å
for β-FeSi2, and the structure is shown in Fig.1. For the
calculatio n of electronic structure of perfect and defec-
L. P. PENG, A. L. HE
Copyright © 2013 SciRes. MNSMS
tive β-FeSi2 crystal, a supercell containing 172 atoms
were used. All supercells adoped are of vacuum layer of
15 Å in order to guarantee negligible interactions be-
tween the neighboring atoms. We replace one of the 172
site of Fe, Fe, Si and Siatom by vacancy, and
signed as VFe, VFe, VSi, and VFe, respective-
l y.
Fig.1. Unite cell of β-FeSi2.
3. Results and discussion
The calculated band structures of β-FeSi2 crystal are
shown in Fig.2. It indicates that the valence band maxi-
mum (VBM ) and the conduction band minimum (CBM )
are located at G, suggesting a direct band gap semicon-
ductor of β-FeSi2 crystal, with the calculated band gap of
about 0.74eV. This result is accordant with the calculated
result band gaps of Pan et al., who used Win2K [7]. But
the calculated band gap is smaller than the experimental
band gap of about 0.9 eV. Generally, an underestima-
tion of the calculated band gaps is an intrinsic feature of
the ab initio method due to the DFT limitations, no tak-
ing into account the discontinuity in the ex-
chan ge-correlation potential.
Fig.2. Calculated band structures of perfect crystal of
β-FeSi2
Fig.3.Total DOS the perfect crystal and defective crystal of
β-FeSi2
Fig.3 shows the calculated total DOS for the perfect
and defective β-FeSi2 crystal. Compared with the perfect
crystal, there is some different in DOS of the defective
crystal. For DOS of the defective crystal with VFe and
VFe, it can be seem clearly that there is sharp level pro-
duced between the valence band and the conduction band.
The EF exists at the sharp defective level below the bot-
tom of the conduction band. As a result that electrons in
the levels can exists to the conduction band by thermal
energy, and the electrons can be produced in the conduc-
tion band. This calculated results indicate that the defec-
tive β-FeSi2 crystal with VFe and VFe will behaves as
n-type semiconductor. For DOS of the defective crystal
with VSi and VSi, the EF level shifts to the top of the
valence band. In this condition, the electrons in the top of
the valence band can excite to the defective level by
thermal energy, and holes would be produced in the va-
lence band. J.Tani et al. obtained a similar calculated
results using CASTEP [8]. It suggested that the defective
β-FeSi2 crystal with VSi and VSi will behaves as
p-type semiconductor. It is strange for compound semi-
conductor that the defective crystal containing metal va-
cancies shows n-type, while the crystal containing
non-metal vacancies shows p-type. But it e xist s in the
defective β-FeSi2 crystal, and the calculated results are
consistent with the experime ntal results [9].
4. Conclusions
The projector-augmented plane wave potentials method
under the density functional theory (DFT) was used to
calculate the electronic structure of perfect and native
point defective β-FeSi2 crystal. The calculated band
structure shows that the band gap of perfect crystal is
14
L. P. PENG, A. L. HE
Copyright © 2013 SciRes. MNSMS
about 0.74eV, which is a little smaller than the experi-
mental result of about 0.9eV. The density of states results
predicts that β-FeSi2 with Fe vacancies behaves n-type,
and that with Si vacancies will shows p-type, which is in
accordant with the experimental results.
REFERENCES
[1] M.Q. Wang, Q. Xie, Q. Luo, Review on Doped β-FeSi2
Mater. Sci. A, 25 (2011) 26-30.
[2] Z.J. Pan, Study of Electronic Structure of Thermoelectric
Material Effects of Doping on Thernoelectric Properties
of Single Crystla MaterialPH.D. Thesis, Shanghai Jiao-
tong University, Shanghai, 2007.
[3] W.J. Yan,The calculation of Band Structure and elec-
tronic and optical properties ofβ-FeSi2PH.D. Thesis,
Guizhou University, 2007.
[4] D.M. Rowe, CRC Handbook of Thermoelectrics, CRC
press, New York, 1995.
[5] N.K. Liu, B.S.Zhu, J.S. Luo, Semiconductor Physics,
Electronic industry press, Beijing, 2008
[6] S.I. Kurganskil, N.S. Pereslavtseva, “ ” Phys. Solid State.
44 (2002) 704-711.
[7] Z.J. Pan, L.T. Zhang, J.S. Wu, A First-principle study of
electronic and geometrical structures of semiconducting
β-FeSi2 with dopingActa. Phys. Sin. 54 (2005)
5208-5313
[8] J. Tani, H. Kido, First Principle study of native point
defects in β-FeSi2J. Alloys and Comp. 352 (2003)
153-157.
[9] X.Y. Jiang, The structure and thermoelectronical proper-
ties of β-FeSi2 prepared by laser induced CVD Jpn.
Appl.Phys.45 (2006) 1351.
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