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Substantial band-gap narrowing of α-Si3N4 induced by heavy Al doping
Physics Letters A 375 (2011) 2874–2877
Contents lists available at ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
Substantial band-gap narrowing of
α -Si3 N4 induced by heavy Al doping
W. Xiao, W.T. Geng ∗ School of Materials Science & Engineering, University of Science & Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 24 March 2011 Received in revised form 7 June 2011 Accepted 9 June 2011 Available online 14 June 2011 Communicated by R. Wu Keywords: Silicon nitride Band-gap narrowing First-principles calculation Oxynitride phosphors
a b s t r a c t Our first-principles study on the structural and electronic properties of Al-doped α -Si3 N4 predict a significant band-gap narrowing, which makes this material a more efficient phosphor. Strong attraction of substitutional and interstitial Al atoms leads to the formation of stable (3 + 1) complexes that behave as isoelectronic traps. The near-mid-gap states of the interstitials reduce nearly half of the band-gap of α -Si3 N4 . Such a new nitride-based semiconductor could be a promising photocatalyst with high reactivity in solar irradiation or interior lighting in visible spectrum. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nitride-based materials have revolutionized solid-state microelectronics over the past few decades as bright solid-state sources with many advantages such as thermal stability, high efficiency, long lifetime, security, nontoxicity. Most researches have been focused on group III nitrides, especially GaN, InN and Al–Ga–In–N alloys whose band-gaps range from 1.9 eV (red) for InN to 3.4 eV (ultraviolet) for GaN [1]. Both Ga and In are scarce resources, which raises greatly the cost of these nitride-based semiconductors. By comparison, Si is an abundant IV element which contributes 26.30% in total weight of lithosphere. Usually, Si3 N4 have been considered as a prototypical material used in cutting tools and antifriction bearings in mechanical applications, because of its unique combination of high density, high melting temperature, low mechanical stress, and strong resistance against thermal shock [2, 3]. It has also been exploited as memory layer and charge storage medium in microelectronics industry for its large band-gap, high dielectric constant, high-energy barrier for impurity diffusion, and high resistance against radiation [4,5]. However, little effort has been made to try to turn them into solid-state electrical or optical materials because of the large 5.1 eV band-gap [6]. For its mature applications in mechanical devices and potential applications in electronic devices, the electronic structures of Si3 N4 have been extensively studied by XPS, X-ray emission, and ELS experimentally [7]. Ren and Ching [8] calculated the elec-
*
Corresponding author. Tel.: +86 10 6233 4143; fax: +86 10 6233 4143. E-mail address: [email protected] (W.T. Geng).
0375-9601/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2011.06.019
tronic energy bands of α and β phases of silicon nitride using a first-principles orthogonalized linear combination of atomic orbitals method. De Brito Mota et al. [9] developed an empirical potential for interactions between Si and N to describe Si3 N4 , and explored the structural properties of α -Si3 N4 using that empirical model through Monte Carlo simulations. Using accurate GW approach, Giacomazzi and Umari [10] have recently found that α -Si3 N4 displays a great amount of edge-sharing SiN4 tetrahedral. For its luminescence properties after doping, an inspiring phenomenon has been discovered by Benco et al. using first-principles calculations in Y- and O-doped α -SiAlONs [11], which has been extensively studied by experiments [12,13] as yellow oxynitride phosphors with Eu2+ doping. In their calculations, a new band originates from Y 4d states appears in the band-gap, thus doping with yttrium leads to an appreciable band-gap narrowing (BGN) of this material. Other additions such as Dy and Gd were found to exert similar effects, leading to good optical transparency [14]. Al as a doping element has always been used to dope semiconductors for tailoring band-gap in ZnO [15]. A recent work by Ding et al. [16] indicated that substitutional Al in spinel silicon nitride (γ -Si3 N4 ) makes the material dielectric at low concentration and metallic at high concentration. However, its performance in changing the band-gap of α -Si3 N4 and the role of the impurity complex of interstitial and substitutional Al are still unknown. We report in this Letter a first-principles density functional theory (DFT) [17] study of α -Si3 N4 :Al system. Our calculations demonstrate that Al will reduce substantially the band-gap of α -Si3 N4 . In comparison to Modavis and Hall’s finding [18], that Al and N, in the form of impurity complex, give rise to isoelectronic traps and produces deep energy level in the narrow energy gap.
W. Xiao, W.T. Geng / Physics Letters A 375 (2011) 2874–2877
Fig. 1. (Color online.) The supercell containing 3 primitive α -Si3 N4 unit cells (a × b × 3c). Large and small solid circles represent respectively Si and N. Unequivalent Si sites in the top atomic layer are labeled by numbers 1 and 2. The 28-atom primitive unit cell is hexagonal with a = 7.753 Å and c = 5.618 Å. Open circles denote the interstitial sites.
We reveal that the impurity complex of interstitial Al and substitutional Al will also produce a deep energy level in the wide energy gap, and hence a much reduced band-gap of the material. 2. Methodology We have modeled the Al-doped α -Si3 N4 system using a supercell consisting of 84 atoms illustrated in Fig. 1. Our DFT calculations were carried out using Vienna Ab initio Simulation Package (VASP) [19]. The electron–ion interaction was described using projector augmented wave method [20], the exchange correlation potential using the generalized gradient approximation (GGA) in the Perdew–Burke–Ernzerhof form [21]. The energy cutoff for the plane wave basis set was 400 eV for calculated systems. We employed a (3 × 3 × 1) mesh to perform the Brillouin zone integration. The geometry optimization for each system was continued until the forces on all the atoms were no larger than 0.005 eV/Å. 3. Results and discussion For Al-doped cases, we replaced one or two Si atoms by Al and with/without an interstitial Al. In view of the fact that Al3+ has a similar size as Si4+ and thus the volume change of this material upon Al doping shall be negligible, we have fixed the lattice constants a and c at the experimental values (a = 7.753 Å and c = 5.618 Å) and only optimized the internal freedoms. Since the details of Al distribution in α -Si3 N4 might influence its electronic and optical properties in a significant way, we have launched a systematic total binding energy study of many a few alignments of Al in α -Si3 N4 . The formation heat of Al-doped α -Si3 N4 , defined as the energy needed in forming such a compound in reference to the pure α -Si3 N4 , elemental crystals of Al and Si, is the quantity we follow in order to find the stable structures.
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There are two unequivalent (in the sense of symmetry) substitutional and three unequivalent interstitial sites for Al atoms in α -Si3 N4 , as are labeled in Fig. 1. Our calculations demonstrate that the formation heat for Al locating at the substitutional position sub-1 is 1.45 eV, and it is 1.54 eV at sub-2. By comparison, all the interstitial sites have the same formation heat for Al, 5.73 eV. This indicates that in dilute limit, the substitutional sites are over 3 eV more stable than the interstitial sites. Next, we examined the nonuniform distribution of the doped atoms, which is almost always the case in reality. The interaction between substitutional Al atoms is slightly repulsive, as evidenced by the fact that the configuration with two Al atoms far apart from each other is 0.19 eV more favorable than the configuration with two neighboring Al atoms. Moreover, we have investigated the possibility of sub + int complex [hereafter denoted as complex (1 + 1)]. We have explored all the six combinations and find that in all cases an interstitial Al would be strongly attracted toward a substitution Al atom and the formation heat per Al drops to 1.38–1.62 eV. Clearly, the interstitial site becomes much more stable for Al than the substitutional one when an Al atom has substituted a neighboring Si atom in α Si3 N4 . Meanwhile, with the combination of the substitutional and interstitial Al dopants, the material recovers a semiconductor with greatly reduced band-gaps of 2.05–2.15 eV, from metallic states induced by sole substitutional or interstitial Al. As the concentration of Al rises, larger aggregation of doped Al in α -Si3 N4 might occur. Surprisingly, our calculations show that the combination of one interstitial Al and two neighboring substitutional Al [complex (2 + 1)] will further lower the formation heat of the solid solution to about 0.66–0.84 eV/Al. Meanwhile, an array of such a complex of Al atoms in α -Si3 N4 will again turn the material into a metallic system. We went on to investigate the stability of complex (3 + 1) and find that the formation heat decreases to 0.25–0.38 eV/Al. However, the beneficial combinatory effect cannot go further. Our calculations show that putting another substitutional Al to the vicinity of a (3 + 1) complex is energetically unfavorable. This set of detailed calculations thus demonstrate clearly the (3 + 1) complex, an isoelectronic trap in which four Al3+ replacing three Si4+ , is a stable local structure of Al dopants. To illustrate the changes in electronic properties of α -Si3 N4 with Al doping, we plot in Fig. 2 the calculated density of states (DOS) for a supercell containing (i) no Al, (ii) one substitutional Al, (iii) one interstitial Al, or (iv) one substitutional plus one interstitial Al atoms. The band-gap yielded by GGA-PBE for α -Si3 N4 is 4.61 eV, about 0.49 eV smaller than experiment, in accordance with the well-known underestimation of band-gap of insulators by local density approximation within the framework of DFT. Obviously, substitutional Al atoms alone make α -Si3 N4 a p-type conductor, as expected from the substitution of Al3+ for Si4+ . When alone, interstitial Al atoms form no chemical bonds with the matrix and will turn the material into n-type conductor as a result of the addition of abundant free electrons. Note that since the interaction between substitutional Al atoms is quite weak, only slightly attractive as mentioned above, we do not plot here all the DOS for different (1 + 1) configurations, but only show those for the most stable one instead. We emphasize that the system of Al-doped α -Si3 N4 cannot be viewed simply as a mixing of α -Si3 N4 and AlN. Otherwise, one would expect a wide-gap semiconductor for Al-doped α -Si3 N4 at any Al concentration, for both Si3 N4 and AlN are wide-gap semiconductors. The reason is quite straightforward: Si and Al ions in this material bear different valence states. Comparing the local DOS of Al in Si35 AlN48 and Si36 AlN48 , we find the interstitial Al produces two energy bands. One is in the middle of the energy gap (Al-3s), sharp and fully occupied, and the other is in the tail of the conduction band (Al-3p) which makes it a donor. The
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W. Xiao, W.T. Geng / Physics Letters A 375 (2011) 2874–2877
Fig. 2. (Left) Total density of states (DOS) for various Al-doped α -Si3 N4 configurations containing (i) no Al (Si36 N48 ), (ii) one substitutional Al (Si35 AlN48 ), (iii) one interstitial Al (Si36 AlN48 ), (iv) or one substitutional plus one interstitial Al atoms (Si35 Al2 N48 ). Light solid vertical lines denote the Fermi energy. (Right) Local DOS for individual Al atoms in each doped system.
Fig. 3. The calculated density of states for Si34 Al3 N48 (Al complex 2 + 1, left panel) and Si33 Al4 N48 (Al complex 3 + 1, right panel). Light solid vertical lines denote the Fermi energy.
appearance of these new states diminishes the original band-gap by about 50%. By contrast, the substitutional Al acts as an acceptor bearing a hole. The addition of a substitutional Al shifts the Fermi level to the left due to the loss of one valence electron and the material turns to be a semiconductor. The electron and hole generated by the interstitial and substitutional Al atoms are bound to each other forming an exciton, which would make the pair of Al atoms energetically more stable. It is worth mentioning that the addition of another substitutional Al to the above (1 + 1) Al pair would turn the mid-gap band half filled as one valence electron is missing. When there are two electron–hole pairs (structure of Si34 Al3 N48 ), the structure with the deep energy level will get more stable (Fig. 3). The energy level in the gap of semiconductor is used as intermediate state through which the electron can combine with the hole. Also, the Fermi level will decrease with the increasing of the number of acceptors
[22]. That means the increase of substitutional Al will reduce the Fermi level, in good agreement with our computational results. As a consequence, the Fermi level of the system Si34 Al3 N48 (2 + 1) appears right at the deep energy level. On the other hand, in the case of complex (3 + 1), the interstitial Al and the three substitutional ones form an isoelectronic trap, a special structure to generate a deep energy level. In other words, there is strong electron–hole recombination at electrically neutral impurity centre, thereby lowering the total energy of the whole system. Similar nonradiative impurity centers [23] have been discovered in Si, which recombine the electron–hole pairs. 4. Summary As a final remark, we want to note that depending on Al concentration and synthetic conditions (temperature, for instance), all
W. Xiao, W.T. Geng / Physics Letters A 375 (2011) 2874–2877
of the local structures of Al, i.e. the (1 + 1), (2 + 1), (3 + 1) complexes can be present. So, experimental realization of semiconducting Al-doped α -Si3 N4 demands careful tuning of Al contents. Such a new nitride-based semiconductor could be promising in solar irradiation or interior lighting as a photocatalyst with high reactivity under visible light.
[6] [7] [8] [9] [10] [11] [12]
Acknowledgements
[13]
We thank Y.C. Zhu for helpful discussions. The work was supported by NSFC (No. 90922027) and NHTRDP (No. 2009AA03Z432).
[14] [15] [16]
References [1] [2] [3] [4]
F.A. Ponce, D.P. Bour, Nature 386 (1997) 351. R.N. Katz, Science 208 (1980) 841. A.Y. Liu, M.L. Cohen, Phys. Rev. B 41 (1990) 10727. V.A. Gritsenko, in: Structure and Electronic Properties of Amorphous Insulator in Silicon MIS Structures, Science, Novosibirsk, 1993, p. 280. [5] M. Pepper, in: G.G. Roberts, M.J. Morant (Eds.), Insulating Films on Semiconductors, Institute of Physics, London, 1980, p. 193.
[17] [18] [19] [20] [21] [22] [23]
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D.J. Dimaria, P.C. Arnett, Appl. Phys. Lett. 26 (1975) 711. V.A. Gritsenko, Y.N. Morokov, Y.N. Novikov, Appl. Surf. Sci. 114 (1997) 417. S.Y. Ren, W.Y. Ching, Phys. Rev. B 23 (1981) 5454. F. de Brito Mota, J.F. Justo, A. Fazzio, Phys. Rev. B 58 (1998) 8323. L. Giacomazzi, P. Umari, Phys. Rev. B 80 (2009) 144201. L. Benco, J. Hafner, Z. Lences, P. Sajgalik, J. Eur. Ceram. Soc. 28 (2008) 995. R.J. Xie, N. Hirosaki, K. Sakuma, Y. Yamamoto, M. Mitomo, Appl. Phys. Lett. 84 (2004) 5404. R.J. Xie, N. Hirosaki, M. Mitomo, Y. Yamamoto, T. Suehiro, K. Sakuma, J. Phys. Chem. B 108 (2004) 12027. H.M. Zhong, Q. Liu, J. Jiang, G.Y. Sun, Y. Luo, J. Appl. Phys. 106 (2009) 093514. B.E. Sernelius, K.F. Berggren, Z.C. Jin, I. Hamberg, C.G. Granqvist, Phys. Rev. B 37 (1988) 10244. Y.C. Ding, A.P. Xiang, J. Luo, X.J. He, Q. Cai, X.F. Hu, Physica B 405 (2010) 828. Y.X. Li, D.W. Brenner, Phys. Rev. Lett. 92 (2004) 4. R.A. Modavis, D.G. Hall, J. Appl. Phys. 67 (1990) 545. G. Kresse, J. Furthmuller, Comput. Mater. Sci. 6 (1996) 15. P.E. Blochl, Phys. Rev. B 50 (1994) 17953. J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. A.S. Grove, Physics and Technology of Semiconductor Device, Wiley, New York, 1967. S.S. Iyer, Y.H. Xie, Science 260 (1993) 280.
Contents lists available at ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
Substantial band-gap narrowing of
α -Si3 N4 induced by heavy Al doping
W. Xiao, W.T. Geng ∗ School of Materials Science & Engineering, University of Science & Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 24 March 2011 Received in revised form 7 June 2011 Accepted 9 June 2011 Available online 14 June 2011 Communicated by R. Wu Keywords: Silicon nitride Band-gap narrowing First-principles calculation Oxynitride phosphors
a b s t r a c t Our first-principles study on the structural and electronic properties of Al-doped α -Si3 N4 predict a significant band-gap narrowing, which makes this material a more efficient phosphor. Strong attraction of substitutional and interstitial Al atoms leads to the formation of stable (3 + 1) complexes that behave as isoelectronic traps. The near-mid-gap states of the interstitials reduce nearly half of the band-gap of α -Si3 N4 . Such a new nitride-based semiconductor could be a promising photocatalyst with high reactivity in solar irradiation or interior lighting in visible spectrum. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nitride-based materials have revolutionized solid-state microelectronics over the past few decades as bright solid-state sources with many advantages such as thermal stability, high efficiency, long lifetime, security, nontoxicity. Most researches have been focused on group III nitrides, especially GaN, InN and Al–Ga–In–N alloys whose band-gaps range from 1.9 eV (red) for InN to 3.4 eV (ultraviolet) for GaN [1]. Both Ga and In are scarce resources, which raises greatly the cost of these nitride-based semiconductors. By comparison, Si is an abundant IV element which contributes 26.30% in total weight of lithosphere. Usually, Si3 N4 have been considered as a prototypical material used in cutting tools and antifriction bearings in mechanical applications, because of its unique combination of high density, high melting temperature, low mechanical stress, and strong resistance against thermal shock [2, 3]. It has also been exploited as memory layer and charge storage medium in microelectronics industry for its large band-gap, high dielectric constant, high-energy barrier for impurity diffusion, and high resistance against radiation [4,5]. However, little effort has been made to try to turn them into solid-state electrical or optical materials because of the large 5.1 eV band-gap [6]. For its mature applications in mechanical devices and potential applications in electronic devices, the electronic structures of Si3 N4 have been extensively studied by XPS, X-ray emission, and ELS experimentally [7]. Ren and Ching [8] calculated the elec-
*
Corresponding author. Tel.: +86 10 6233 4143; fax: +86 10 6233 4143. E-mail address: [email protected] (W.T. Geng).
0375-9601/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2011.06.019
tronic energy bands of α and β phases of silicon nitride using a first-principles orthogonalized linear combination of atomic orbitals method. De Brito Mota et al. [9] developed an empirical potential for interactions between Si and N to describe Si3 N4 , and explored the structural properties of α -Si3 N4 using that empirical model through Monte Carlo simulations. Using accurate GW approach, Giacomazzi and Umari [10] have recently found that α -Si3 N4 displays a great amount of edge-sharing SiN4 tetrahedral. For its luminescence properties after doping, an inspiring phenomenon has been discovered by Benco et al. using first-principles calculations in Y- and O-doped α -SiAlONs [11], which has been extensively studied by experiments [12,13] as yellow oxynitride phosphors with Eu2+ doping. In their calculations, a new band originates from Y 4d states appears in the band-gap, thus doping with yttrium leads to an appreciable band-gap narrowing (BGN) of this material. Other additions such as Dy and Gd were found to exert similar effects, leading to good optical transparency [14]. Al as a doping element has always been used to dope semiconductors for tailoring band-gap in ZnO [15]. A recent work by Ding et al. [16] indicated that substitutional Al in spinel silicon nitride (γ -Si3 N4 ) makes the material dielectric at low concentration and metallic at high concentration. However, its performance in changing the band-gap of α -Si3 N4 and the role of the impurity complex of interstitial and substitutional Al are still unknown. We report in this Letter a first-principles density functional theory (DFT) [17] study of α -Si3 N4 :Al system. Our calculations demonstrate that Al will reduce substantially the band-gap of α -Si3 N4 . In comparison to Modavis and Hall’s finding [18], that Al and N, in the form of impurity complex, give rise to isoelectronic traps and produces deep energy level in the narrow energy gap.
W. Xiao, W.T. Geng / Physics Letters A 375 (2011) 2874–2877
Fig. 1. (Color online.) The supercell containing 3 primitive α -Si3 N4 unit cells (a × b × 3c). Large and small solid circles represent respectively Si and N. Unequivalent Si sites in the top atomic layer are labeled by numbers 1 and 2. The 28-atom primitive unit cell is hexagonal with a = 7.753 Å and c = 5.618 Å. Open circles denote the interstitial sites.
We reveal that the impurity complex of interstitial Al and substitutional Al will also produce a deep energy level in the wide energy gap, and hence a much reduced band-gap of the material. 2. Methodology We have modeled the Al-doped α -Si3 N4 system using a supercell consisting of 84 atoms illustrated in Fig. 1. Our DFT calculations were carried out using Vienna Ab initio Simulation Package (VASP) [19]. The electron–ion interaction was described using projector augmented wave method [20], the exchange correlation potential using the generalized gradient approximation (GGA) in the Perdew–Burke–Ernzerhof form [21]. The energy cutoff for the plane wave basis set was 400 eV for calculated systems. We employed a (3 × 3 × 1) mesh to perform the Brillouin zone integration. The geometry optimization for each system was continued until the forces on all the atoms were no larger than 0.005 eV/Å. 3. Results and discussion For Al-doped cases, we replaced one or two Si atoms by Al and with/without an interstitial Al. In view of the fact that Al3+ has a similar size as Si4+ and thus the volume change of this material upon Al doping shall be negligible, we have fixed the lattice constants a and c at the experimental values (a = 7.753 Å and c = 5.618 Å) and only optimized the internal freedoms. Since the details of Al distribution in α -Si3 N4 might influence its electronic and optical properties in a significant way, we have launched a systematic total binding energy study of many a few alignments of Al in α -Si3 N4 . The formation heat of Al-doped α -Si3 N4 , defined as the energy needed in forming such a compound in reference to the pure α -Si3 N4 , elemental crystals of Al and Si, is the quantity we follow in order to find the stable structures.
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There are two unequivalent (in the sense of symmetry) substitutional and three unequivalent interstitial sites for Al atoms in α -Si3 N4 , as are labeled in Fig. 1. Our calculations demonstrate that the formation heat for Al locating at the substitutional position sub-1 is 1.45 eV, and it is 1.54 eV at sub-2. By comparison, all the interstitial sites have the same formation heat for Al, 5.73 eV. This indicates that in dilute limit, the substitutional sites are over 3 eV more stable than the interstitial sites. Next, we examined the nonuniform distribution of the doped atoms, which is almost always the case in reality. The interaction between substitutional Al atoms is slightly repulsive, as evidenced by the fact that the configuration with two Al atoms far apart from each other is 0.19 eV more favorable than the configuration with two neighboring Al atoms. Moreover, we have investigated the possibility of sub + int complex [hereafter denoted as complex (1 + 1)]. We have explored all the six combinations and find that in all cases an interstitial Al would be strongly attracted toward a substitution Al atom and the formation heat per Al drops to 1.38–1.62 eV. Clearly, the interstitial site becomes much more stable for Al than the substitutional one when an Al atom has substituted a neighboring Si atom in α Si3 N4 . Meanwhile, with the combination of the substitutional and interstitial Al dopants, the material recovers a semiconductor with greatly reduced band-gaps of 2.05–2.15 eV, from metallic states induced by sole substitutional or interstitial Al. As the concentration of Al rises, larger aggregation of doped Al in α -Si3 N4 might occur. Surprisingly, our calculations show that the combination of one interstitial Al and two neighboring substitutional Al [complex (2 + 1)] will further lower the formation heat of the solid solution to about 0.66–0.84 eV/Al. Meanwhile, an array of such a complex of Al atoms in α -Si3 N4 will again turn the material into a metallic system. We went on to investigate the stability of complex (3 + 1) and find that the formation heat decreases to 0.25–0.38 eV/Al. However, the beneficial combinatory effect cannot go further. Our calculations show that putting another substitutional Al to the vicinity of a (3 + 1) complex is energetically unfavorable. This set of detailed calculations thus demonstrate clearly the (3 + 1) complex, an isoelectronic trap in which four Al3+ replacing three Si4+ , is a stable local structure of Al dopants. To illustrate the changes in electronic properties of α -Si3 N4 with Al doping, we plot in Fig. 2 the calculated density of states (DOS) for a supercell containing (i) no Al, (ii) one substitutional Al, (iii) one interstitial Al, or (iv) one substitutional plus one interstitial Al atoms. The band-gap yielded by GGA-PBE for α -Si3 N4 is 4.61 eV, about 0.49 eV smaller than experiment, in accordance with the well-known underestimation of band-gap of insulators by local density approximation within the framework of DFT. Obviously, substitutional Al atoms alone make α -Si3 N4 a p-type conductor, as expected from the substitution of Al3+ for Si4+ . When alone, interstitial Al atoms form no chemical bonds with the matrix and will turn the material into n-type conductor as a result of the addition of abundant free electrons. Note that since the interaction between substitutional Al atoms is quite weak, only slightly attractive as mentioned above, we do not plot here all the DOS for different (1 + 1) configurations, but only show those for the most stable one instead. We emphasize that the system of Al-doped α -Si3 N4 cannot be viewed simply as a mixing of α -Si3 N4 and AlN. Otherwise, one would expect a wide-gap semiconductor for Al-doped α -Si3 N4 at any Al concentration, for both Si3 N4 and AlN are wide-gap semiconductors. The reason is quite straightforward: Si and Al ions in this material bear different valence states. Comparing the local DOS of Al in Si35 AlN48 and Si36 AlN48 , we find the interstitial Al produces two energy bands. One is in the middle of the energy gap (Al-3s), sharp and fully occupied, and the other is in the tail of the conduction band (Al-3p) which makes it a donor. The
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W. Xiao, W.T. Geng / Physics Letters A 375 (2011) 2874–2877
Fig. 2. (Left) Total density of states (DOS) for various Al-doped α -Si3 N4 configurations containing (i) no Al (Si36 N48 ), (ii) one substitutional Al (Si35 AlN48 ), (iii) one interstitial Al (Si36 AlN48 ), (iv) or one substitutional plus one interstitial Al atoms (Si35 Al2 N48 ). Light solid vertical lines denote the Fermi energy. (Right) Local DOS for individual Al atoms in each doped system.
Fig. 3. The calculated density of states for Si34 Al3 N48 (Al complex 2 + 1, left panel) and Si33 Al4 N48 (Al complex 3 + 1, right panel). Light solid vertical lines denote the Fermi energy.
appearance of these new states diminishes the original band-gap by about 50%. By contrast, the substitutional Al acts as an acceptor bearing a hole. The addition of a substitutional Al shifts the Fermi level to the left due to the loss of one valence electron and the material turns to be a semiconductor. The electron and hole generated by the interstitial and substitutional Al atoms are bound to each other forming an exciton, which would make the pair of Al atoms energetically more stable. It is worth mentioning that the addition of another substitutional Al to the above (1 + 1) Al pair would turn the mid-gap band half filled as one valence electron is missing. When there are two electron–hole pairs (structure of Si34 Al3 N48 ), the structure with the deep energy level will get more stable (Fig. 3). The energy level in the gap of semiconductor is used as intermediate state through which the electron can combine with the hole. Also, the Fermi level will decrease with the increasing of the number of acceptors
[22]. That means the increase of substitutional Al will reduce the Fermi level, in good agreement with our computational results. As a consequence, the Fermi level of the system Si34 Al3 N48 (2 + 1) appears right at the deep energy level. On the other hand, in the case of complex (3 + 1), the interstitial Al and the three substitutional ones form an isoelectronic trap, a special structure to generate a deep energy level. In other words, there is strong electron–hole recombination at electrically neutral impurity centre, thereby lowering the total energy of the whole system. Similar nonradiative impurity centers [23] have been discovered in Si, which recombine the electron–hole pairs. 4. Summary As a final remark, we want to note that depending on Al concentration and synthetic conditions (temperature, for instance), all
W. Xiao, W.T. Geng / Physics Letters A 375 (2011) 2874–2877
of the local structures of Al, i.e. the (1 + 1), (2 + 1), (3 + 1) complexes can be present. So, experimental realization of semiconducting Al-doped α -Si3 N4 demands careful tuning of Al contents. Such a new nitride-based semiconductor could be promising in solar irradiation or interior lighting as a photocatalyst with high reactivity under visible light.
[6] [7] [8] [9] [10] [11] [12]
Acknowledgements
[13]
We thank Y.C. Zhu for helpful discussions. The work was supported by NSFC (No. 90922027) and NHTRDP (No. 2009AA03Z432).
[14] [15] [16]
References [1] [2] [3] [4]
F.A. Ponce, D.P. Bour, Nature 386 (1997) 351. R.N. Katz, Science 208 (1980) 841. A.Y. Liu, M.L. Cohen, Phys. Rev. B 41 (1990) 10727. V.A. Gritsenko, in: Structure and Electronic Properties of Amorphous Insulator in Silicon MIS Structures, Science, Novosibirsk, 1993, p. 280. [5] M. Pepper, in: G.G. Roberts, M.J. Morant (Eds.), Insulating Films on Semiconductors, Institute of Physics, London, 1980, p. 193.
[17] [18] [19] [20] [21] [22] [23]
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