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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 FeⅠ, 8 FeⅡ, 32 SiⅠ and 32 SiⅡ) 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 FeⅠ, FeⅡ, SiⅠ and SiⅡatom by vacancy, and signed as VFeⅠ, VFeⅡ, VSiⅠ, and VFeⅡ, 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 VFeⅠ and VFeⅡ, 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 VFeⅠ and VFeⅡ will behaves as n-type semiconductor. For DOS of the defective crystal with VSiⅠ and VSiⅡ, 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 Ⅰ and VSi Ⅱ 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 Material” PH.D. Thesis, Shanghai Jiao- tong University, Shanghai, 2007. [3] W.J. Yan, “The calculation of Band Structure and elec- tronic and optical properties ofβ-FeSi2” PH.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 doping” Acta. Phys. Sin. 54 (2005) 5208-5313 [8] J. Tani, H. Kido, “First Principle study of native point defects in β-FeSi2” J. 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. 15 |