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Band structure engineering and transport properties of aluminium phosphide nanoribbon – A first-principles study
Superlattices and Microstructures 76 (2014) 135–148
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Band structure engineering and transport properties of aluminium phosphide nanoribbon – A first-principles study R. Chandiramouli ⇑, S. Rubalya Valantina, V. Nagarajan School of Electrical & Electronics Engineering, SASTRA University, Tirumalaisamudram, Thanjavur 613 401, India
a r t i c l e
i n f o
Article history: Received 22 June 2014 Received in revised form 4 September 2014 Accepted 10 October 2014 Available online 16 October 2014 Keywords: Aluminium phosphide Nanoribbon Transport property Electron density Density of states
a b s t r a c t The band structure and transport properties of pristine, boron, gallium and arsenic substituted AlP nanoribbon are studied using density functional theory. The band structure of pristine, boron, gallium and arsenic substituted AlP nanoribbon exhibits semiconducting behavior. The substitution of boron decreases the band gap of AlP nanoribbon. The substitution of group-III semiconductor has much influence in density of states. The major contribution is observed in p and d orbitals. The electron density increases with boron substitution and there is a slight decrease in electron density for gallium substitution. The transmission of AlP nanoribbon molecular device is analyzed with two probe method. The substitution impurity and bias voltage influence the transmission across AlP nanoribbon. From the results, it is inferred that the band structure and electronic transport properties can be fine-tuned with substitution impurity along AlP nanoribbon. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Nanoscience and technology opened a new era in fundamental research in nanoscale, the economic nanoscale manufacturing of devices leads to more coherent science and engineering applications. The recent advancement in the group-III and V compound semiconductor attracted the scientific ⇑ Corresponding author. Tel.: +91 9489566466; fax: +91 4362 264120. E-mail address: [email protected] (R. Chandiramouli). http://dx.doi.org/10.1016/j.spmi.2014.10.013 0749-6036/Ó 2014 Elsevier Ltd. All rights reserved.
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community due to their applications in semiconductor industries. This motivates scientific community for numerous theoretical and experimental studies in the group III–V compound semiconductors such as gallium arsenide, indium phosphide, aluminium phosphide and gallium phosphide, etc. Among these compound semiconductors aluminium phosphide (AlP) receive a considerable attention due to lower mass, which leads to higher vibrational frequencies [1]. Aluminium phosphide (AlP) is a wide-band compound semiconductor with a band gap of around 2.5 eV. Usually, AlP is alloyed with binary materials for applications in optoelectronic devices [2]. To this time, even though numerous experimental and theoretical studies are reported in AlP and their counter parts group III–V semiconductors [3–5]; to the best of our knowledge; the band structure engineering and transport properties of aluminium phosphide nanoribbon with substitution impurities have not been investigated. The systematic computational study could be a pioneer step for the experimental characterization of nanostructures. Ling Guo studied carbon monoxide adsorption on cationic, neutral, and anionic aluminium phosphide clusters [6] and also reported the electronic structure and properties of neutral and charged aluminium phosphide clusters [7]. Mirzaei et al., reported boron doped and carbon doped AlP nanotubes [8,9]. Rezaei-Sameti reported SiC-doped aluminium phosphide nanotubes [10]. Density functional theory (DFT) is an effective method to study the structural and electronic transport properties of nanostructures [11–13]. The motivation of the present work is to tailor the band structure and electronic transport properties of AlP nanostructures with improved performance in optoelectronic devices with the incorporation of substitution impurities. Moreover, controlling the transport phenomena along nanoelectronic devices is a challenging task. To date, there are numerous reports in molecular electronic devices, which are used for rectification, switching, optoelectronics and spintronics applications [14–23]. The novel aspect of the work is to design AlP nanoribbon with substitution impurities and AlP nanoribbon is used as a molecular device. The molecular electronic device may have a single molecule between the electrodes which acts as source and drain. The state-of-the-art of the present work is to construct AlP nanoribbon and to fine-tune the band structure and electronic properties with substitution impurity and the designed AlP nanoribbon are used as a molecular device between the electrodes. In the present work an attempt has been made to fine-tune, the properties of AlP with substitution impurities such as boron, gallium and arsenic and the results are reported. 2. Computational details The present investigation on AlP nanoribbon is carried out by DFT method utilizing TranSIESTA module in SIESTA package [24]. DFT method explores the band structure, density of states and transmission coefficient for the present system. In the reported work, generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) exchange correlation functional, (GGA–PBE) functional is employed throughout the work [25,26]. The sampling of Brillouin zone is carried out by 1 1 1 k points. By reducing the atomic forces of atoms as small as 0.05 eV/Å, the molecular geometry is optimized. The real-space grid for electrostatic potentials is considered in the mesh cut-off energy of 10 5 eV that realizes the balance between the efficiency and accuracy in calculation. The electronic properties of AlP nanoribbon are determined with vacuum padding of 10 Å modeled along x and y directions to remove the interaction of AlP nanoribbon with its periodic images. The atoms along the nanoribbon are free to move in their position till the convergence with a force less than 0.05 eV/Å on each individual atom in AlP nanoribbon is achieved. The complete optimization of AlP nanoribbon is supported by double zeta polarization (DZP) basis set [27,28]. 3. Results and discussion 3.1. Structures of AlP nanoribbon Initially, AlP nanoribbon is designed from International Centre for Diffraction Data (ICDD) card number 79-2500. For pristine AlP nanoribbon, there are twenty-four aluminium atoms and twentyfour phosphorus atoms forming a hexagonal structured AlP nanoribbon and both ends of AlP nanorib-
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bon are terminated with four hydrogen atoms each. In the case of boron substituted AlP nanoribbon, there are twenty-four phosphorus atoms, twenty aluminium atoms and four boron atoms are substituted in place of aluminium atoms, and four hydrogen atoms are terminated at both ends. Looking at gallium substituted AlP nanoribbon, there are twenty aluminium atoms with twenty-four phosphorus atoms. Four gallium atoms are substituted in place of aluminium at the bottom end of AlP nanoribbon. Observing the structure of arsenic substituted nanoribbon, there are twenty-four aluminium atoms with twenty phosphorus atoms, four atoms of arsenic replace phosphorus atoms in the top end of AlP nanoribbon. In order to eliminate the passivation effects both the ends of AlP nanoribbon are terminated with hydrogen atoms. The reason behind the selection of boron and gallium as substitution impurities is both the elements; boron and gallium belong to group-III elements like aluminium; the substitution may change the band structure and electronic properties of AlP nanoribbon. Likewise, arsenic is a group-V element as that of phosphorus; this may also change the band structure and transport property of AlP nanoribbon. These are the reasons behind the selection of boron, gallium and arsenic as substitution impurities in AlP nanoribbon. Figs. 1(a)–(d) represents pristine AlP nanoribbon, boron substituted AlP nanoribbon, gallium substituted AlP nanoribbon and arsenic substituted AlP nanoribbon respectively. 3.2. Band structure analysis of AlP nanoribbon The electronic band structures provide the insight for the materials properties involved in band structure engineering. The band structures of AlP are discussed in terms of conducting channels (lines) crossing the Fermi energy (EF) across the valence band and conduction band; the conducting property of nanostructures also depends on the conducting channels [29–31]. If lines cross the Fermi level, it infers the metallic nature of the material. Moreover, the band gap is inferred from the gap across the C point in the band structure diagram. In the case of pristine AlP nanoribbon as in Fig. 2a, the band gap for pristine AlP nanoribbon is found to be around 2.45 eV. The calculated band gap is in agreement with the reported band gap of bulk AlP, which is 2.5 eV [32]. Fig. 2b shows the band structure of boron substituted AlP nanoribbon, the substitution of boron in the bottom terminating end of AlP nanorib-
Fig. 1a. Schematic diagram of pristine AlP nanoribbon.
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Fig. 1b. Schematic diagram of boron substituted AlP nanoribbon.
Fig. 1c. Schematic diagram of gallium substituted AlP nanoribbon.
bon drastically decreases the band gap to 1.5 eV. This arises due to the electronic configuration of boron [He] 2s2 2p1. The boron is substituted in place of aluminium atoms at the bottom end, this consequence to the reduction in the band gap of AlP nanoribbon. Fig. 2c illustrates the gallium substituted AlP nanoribbon. Interestingly, the substitution of gallium instead of aluminium in the bottom end also
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Fig. 1d. Schematic diagram of arsenic substituted AlP nanoribbon.
Fig. 2a. Band structure of pristine AlP nanoribbon.
decreases the band gap. In this case, the band gap decreases to 2.1 eV. Comparing with its pristine counterpart the decrease in the band gap arises due to electronic configuration of gallium [Ar] 3d10 4s2 4p1. Fig. 2d depicts the arsenic substituted AlP nanoribbon; in this case, the band gap is found to be around 2.4 eV. There is not much variation observed for arsenic substitution. Since arsenic is a fifth group element like that of phosphorus, the termination of arsenic in the top end does not have much impact in the band gap when compared with pristine AlP nanoribbon. Du et al. reported first-principles studies of AlN nanoribbon terminated with hydrogen atoms [33]. The trend in the band structure of AlP nanoribbon in the present work resembles with that of AlN nanoribbon. Dai et al. studied the electronic structures of AlN, GaN nanoribbons and AlxGa1 xN nanoribbon heterojunctions using first-principles studies [34]. The band structure fashions of AlP nanoribbon are in reasonable
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Fig. 2b. Band structure of boron substituted AlP nanoribbon.
Fig. 2c. Band structure of gallium substituted AlP nanoribbon.
Fig. 2d. Band structure of arsenic substituted AlP nanoribbon.
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closeness with the reported work. From the results, it can be observed that the substitution of third group elements in AlP nanoribbon shows a considerable variation in the band gap. Moreover, the substitution of boron drastically changes the band gap of AlP nanoribbon. The conducting properties can be fine-tuned with substitution impurities in AlP nanoribbon at the different site.
3.3. Density of states of AlP nanoribbon The density of states (DOS) spectrum provides the insight for visualization of charges in the energy interval of valence band and conduction band [35–37]. The projected density of states (PDOS) gives a clear insight to the localized electronic structure of AlP nanoribbon. It plays an important role in discussing the transport properties of AlP nanoribbon. The PDOS of an atom or a group of atoms is acquired by summing the individual orbital PDOS contributions of an atom or group of atoms [20]. Fig. 3a shows the PDOS spectrum of pristine AlP nanoribbon. Looking at the spectrum it is clearly observed that the major contribution is from p orbitals in the valence band; likewise, localization of charges is seen on p and d orbital in the conduction band. The major contribution is observed only in the p and d orbitals in PDOS spectrum of pristine AlP nanoribbon. The PDOS spectrum of boron substituted AlP nanoribbon is shown in Fig. 3b. The termination with boron in AlP nanoribbon alters the p and d orbitals. The substitution of boron on the bottom end results in peak maximums in p and d orbital along the valence band. However, the peak maximums’ decrease in p and d orbitals in the conduction band of AlP nanoribbon. Fig. 3c depicts PDOS spectrum of gallium substituted AlP nanoribbon. Moreover, the peak maximums are observed both in valence band and conduction band in p and d orbitals of AlP nanoribbon. Since the atomic number of gallium is thirty-one, more electrons in gallium atoms results in peak maximums in p and d orbitals. Fig. 3d represents PDOS spectrum of arsenic substituted AlP nanoribbon. In this case, there is no drastic variation when comparing the PDOS spectrum of pristine AlP nanoribbon. Only a small variation is observed in the conduction band of p and d orbitals. Fig. 4 illustrates the density of states (DOS) spectrum of pristine, boron, gallium and arsenic substituted AlP nanoribbon. Ting-Ge et al. studied aluminium vacancy in wurtzite aluminium nitride [38]. A similar PDOS spectrum is observed as that of the present work. It is evident in the observation that the termination of AlP nanoribbon with group-III semiconductor such as boron; gallium shows much variation in DOS spectrum, in contrast there is no significant variation observed with the substitution of arsenic in AlP nanoribbon.
Fig. 3a. PDOS spectrum of pristine AlP nanoribbon.
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Fig. 3b. PDOS spectrum of boron substituted AlP nanoribbon.
Fig. 3c. PDOS spectrum of gallium substituted AlP nanoribbon.
3.4. Electron density of AlP nanoribbon The electron density of pristine AlP nanoribbon is shown in Fig. 5a; the electron density is found to be more in the phosphorus site than in the aluminium site, the electronegativity property of phosphorus leads to accumulation of more electrons near phosphorus atoms. The electronegativity of phosphorus is one of the chemical properties, which represents the tendency of the phosphorus atoms to attract electrons towards it. Moreover, the electronegativity of atoms in AlP nanostructure is influenced by the atomic number and the distance of valence electrons residing from the nucleus in the nanostructure. Fig. 5b denotes the electron density of boron substituted AlP nanoribbon. The substitution of boron in the bottom end slightly increases the electron density across phosphorus atoms compared with electron density of pristine AlP nanoribbon (color bar gradient). Fig. 5c illustrates
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Fig. 3d. PDOS spectrum of arsenic substituted AlP nanoribbon.
Fig. 4. Density of states spectrum of AlP nanoribbon.
the electron density of gallium substituted AlP nanoribbon. In this case, there is a slight decrease in electron density across phosphorus atoms (color bar gradient). The substitution of gallium atoms in the bottom terminating end decreases the electron density in AlP nanoribbon. Even though, both boron and gallium belong to group-III semiconductor, the electronic configuration of boron and gallium atoms influences the electron density across the phosphorus site. Fig. 5d represents the electron density of arsenic substituted AlP nanoribbon. There is not much variation noticed in electron density when compared with electron density of pristine AlP nanoribbon. Zheng et al. studied the vacancy defect for AlN nanoribbon using the first-principles method [39]. In the reported work, the charge density is found to be more near nitrogen atoms due to electronegative property of nitrogen. In the present work, the electronegative property of phosphorus atoms influences the accumulation of electrons
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Fig. 5a. Electron density of pristine AlP nanoribbon.
Fig. 5b. Electron density of boron substituted AlP nanoribbon.
near the phosphorus site. From the observations, it is inferred that substitution of group-III semiconductor shows the variation in the electron density across the phosphorus site. 3.5. Transport properties of AlP nanoribbon The electronic transmission of AlP nanoribbon can be explored in terms of transport property [40,41]. The transmission of AlP nanoribbon is determined with two probe method. The length of the scattering region along z-direction is around 12.6 Å. The two ends of AlP nanoribbon are held within the electrodes. The width of left electrode and right electrode is 3.15 Å each. The potential across the left electrode is varied in terms of 0.5 V, 1 V and 1.5 V and the right electrode is kept at constant ground potential. The electrons near the Fermi level contribute to electronic transport property in AlP nanoribbon. The orbital delocalization near the Fermi level results in high mobility of electrons which corresponds to certain peak amplitudes in the transmission spectrum [42,43]. The electronic transmission is invoked with magnitude of transmission in a particular energy interval. Fig. 6a represents the transmission spectrum of pristine AlP nanoribbon. In the valence band, peaks are observed
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Fig. 5c. Electron density of gallium substituted AlP nanoribbon.
Fig. 5d. Electron density of arsenic substituted AlP nanoribbon.
for 0.5 V, 1 V and 1.5 V. For 0.5 V bias, the applied bias is not sufficient to cause the transition of charges from the valence band to the conduction band. Moreover, for 1 V bias peaks are observed only beyond 1 eV. The applied bias of 1.5 V makes the electrons to transit from the valence band to the conduction band. This is clearly observed with the shift in peaks, the peaks move towards Fermi level. Fig. 6b denotes the transmission spectrum of boron substituted AlP nanoribbon. A drastic change in transmission is observed for boron substitution. More peak maximums are observed in the valence band and in the conduction band. The valence electrons of boron atoms result in more peak maximums in the valence band. This is also in agreement with the electron density of boron substituted AlP nanoribbon, where the electron density increases with boron substitution. However, when the bias voltage is increased, the peak maximums move towards the Fermi level. This confirms the transition of electrons from the valence band to the conduction band. Fig. 6c illustrates the transmission spectrum of gallium substituted AlP nanoribbon. In this case, a sharp decrease in transmission is observed near 2 eV. This arises due to the electronic configuration of gallium atoms when attached to its neighboring atoms. However, the decrease in transmission diminishes with the increase in bias voltage. For 1.5 V bias, the sharp decrease in transmission near 2 eV is considerably reduced. This
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Fig. 6a. Transmission spectrum of pristine AlP nanoribbon.
Fig. 6b. Transmission spectrum of boron substituted AlP nanoribbon.
confirms the movement of charge towards the conduction band across AlP nanoribbon. Fig. 6d denotes the transmission spectrum of arsenic substituted AlP nanoribbon. For 0.5 V bias, a sharp decrease in transmission near 1.5 eV is observed. In contrast, for 1 V and 1.5 V bias, the sharp decrease in peak maximums is not observed. The applied voltage is sufficient to make the transition of electrons from the valence band to the conduction band. From all the observation, it is inferred that the transmission across AlP nanoribbon can be fine-tuned with substitution impurity as well as by the applied bias voltage. The transport property can be enhanced by proper substitution impurity and bias voltage across AlP nanoribbon.
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Fig. 6c. Transmission spectrum of gallium substituted AlP nanoribbon.
Fig. 6d. Transmission spectrum of arsenic substituted AlP nanoribbon.
4. Conclusions Using density functional theory, the band structure and transport property of AlP nanoribbon is optimized and studied. The band structure of pristine, boron, gallium and arsenic substituted AlP nanoribbon shows semiconducting behavior. The substitution of group-III semiconductors such as boron and gallium decreases the band gap whereas the substitution of arsenic does not have much change in the band gap of AlP nanoribbon. Observing the density of states spectrum, density of charges varies in different energy interval for boron and gallium substitution in AlP nanoribbon. In contrast, there is not much variation noticed for arsenic substitution in density of states spectrum. The electron
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density increases for boron substitution and a slight decrease in electron density are observed for gallium substitution. There is no change in electron density for arsenic substitution. The transmission spectrum of AlP nanoribbon is discussed with two probe method. When the bias voltage increases between the electrodes, the electrons transit from the valence band to conduction band. This is confirmed with the peak maximums moving towards Fermi level. Moreover, there is a drastic variation in transmission is noticed for boron substituted AlP nanoribbon. From the observation, the impurity substitution and the bias voltage influence the transmission across AlP nanoribbon. In conclusion, the substitution impurity and the location of substitution impurity play an important role in tuning the electronic transport properties of AlP nanoribbon. The band structure and transport property can be fine-tuned and enhanced with proper substitution impurity, which finds potential application in the semiconductor industry. References [1] H. Gomez, T.R. Taylor, D.M. Neumark, J. Phys. Chem. A 105 (2001) 6886–6893. [2] D.E.C. Corbridge, Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology, fifth ed., Elsevier, Amsterdam, 1995. ISBN 0-444-89307-5. [3] S. Sriram, R. Chandiramouli, Eur. Phys. J. Plus 128 (2013) 116. [4] M. Takikawa, T. Ohori, M. Takechi, M. Suzuki, J. Komeno, J. Cryst. Growth 107 (1991) 942–946. [5] S.J. Chen, Y.C. Liu, H. Jiang, Y.M. Lu, J.Y. Zhang, D.Z. Shen, X.W. Fan, J. Cryst. Growth 285 (2005) 24–30. [6] Ling. Guo, Comp. Theor. Chem. 984 (2012) 102–107. [7] Ling. Guo, J. Alloys Compd. 490 (2010) 78–83. [8] Maryam Mirzaei, Mahmoud Mirzaei, Comp. Theor. Chem. 963 (2011) 294–297. [9] Maryam Mirzaei, Mahmoud Mirzaei, Solid State Sci. 13 (2011) 244–250. [10] Mahdi Rezaei-Samet, Physica E 44 (2012) 1770–1775. [11] R. Chandiramouli, S. Sriram, D. Balamurugan, Mol. Phys. 112 (2014) 151–164. [12] S.C. Hsieh, S.M. Wang, F.Y. Li, CARBON 49 (2011) 955–965. [13] S. Sriram, R. Chandiramouli, Res. Chem. Intermed. (2013), http://dx.doi.org/10.1007/s11164-013-1334-6. [14] Z.H. Zhang, X.Q. Deng, X.Q. Tan, M. Qiu, J.B. Pan, Appl. Phys. Lett. 97 (2010) 183105. [15] S. V Aradhya, L. Venkataraman, Nat. Nanotechnol. 8 (2013) 399–410. [16] J.B. Pan, Z.H. Zhang, X.Q. Deng, M. Qiu, C. Guo, Appl. Phys. Lett. 98 (2011) 013503. [17] J.B. Pan, Z.H. Zhang, K.H. Ding, X.Q. Deng, C. Guo, Appl. Phys. Lett. 98 (2011) 092102. [18] Z. Zhang, J. Zhang, G. Kwong, J. Li, Z. Fan, X. Deng, G. Tang, Sci. Rep. 3 (2013) 2575. [19] J.J. Zhang, Z.H. Zhang, G.P. Tang, X.Q. Deng, Z.Q. Fan, Org. Electron. 15 (2014) 1338–1346. [20] Z. Zhang, C. Guo, D.J. Kwong, J. Li, X. Deng, Z. Fan, Adv. Funct. Mater. 23 (2013) 2765–2774. [21] Y. Cho, W.Y. Kim, K.S. Kim, J. Phys. Chem. A 113 (2009) 4100–4104. [22] W.Y. Kim, K.S. Kim, Acc. Chem. Res. 43 (2010) 111–120. [23] W.Y. Kim, Y.C. Choi, S.K. Min, Y. Cho, K.S. Kim, Chem. Soc. Rev. 38 (2009) 2319–2333. [24] J.M. Soler, E. Artacho, J.D. Gale, A. Garcia, J. Junquera, P. Ordejon, D.S. Portal, J. Phys.: Condens. Matter 14 (2002) 2745–2779. [25] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Phys. Rev. B 46 (1992) 6671–6687. [26] J.P. Perdew, K. Burke, Y. Wang, Phys. Rev. B 54 (1996) 16533–16539. [27] Z.H. Zhang, C. Guo, G. Kwong, X.Q. Deng, CARBON 51 (2013) 313–321. [28] Q.Y. Hua, G.D. Ren, L.C. Bu, Chin. J. Chem. 24 (2006) 326–330. [29] N.K. Jaiswal, P. Srivastava, Solid State Commun. 152 (2012) 1489–1492. [30] R. Chandiramouli, S. Sriram, NANO 9 (2014) 1450020. [31] A.F. Kuloglu, B. Sarikavak-Lisesivdin, S.B. Lisesivdin, E. Ozbay, Comput. Mater. Sci. 68 (2013) 18–22. [32] LevI Berger, Semiconductor Materials, CRC Press, 1996. [33] A.J. Du, Z.H. Zhu, Y. Chen, G.Q. Lu, S.C. Smith, Chem. Phys. Lett. 469 (2009) 183–185. [34] Y. Dai, X. Chen, C. Jiang, Phys. B Condens. Matter 407 (2012) 515–518. [35] R. Chandiramouli, Struct. Chem. (2014), http://dx.doi.org/10.1007/s11224-014-0434-2. [36] R. Chandiramouli, S. Sriram, J. Inorg. Organomet. Polym. (2014), http://dx.doi.org/10.1007/s10904-014-0041-0. [37] R. Chandiramouli, S. Sriram, Mol. Phys. 112 (2014) 1954–1962. [38] G. Ting-Ge, Y. Jue-Min, Z. Zi-Yao, H. Xiao-Dong, Chin. Phys. Lett. 25 (2008) 2989–2992. [39] F.L. Zheng, J.M. Zhang, Y. Zhang, V. Ji, Phys. B Condens. Matter 405 (2010) 3775–3781. [40] R. Chandiramouli, S. Sriram, Superlattices Microstruct. 65 (2014) 22–34. [41] R. Chandiramouli, Ceram. Int. 40 (2014) 9211–9216. [42] C.J. Xia, D.S. Liu, C.F. Fang, P. Zhao, Physica E 42 (2010) 1763–1768. [43] F. Hiroyuki, K. Yoshikazu, T. Kazuyoshi, Sci. China Chem. 55 (2012) 796–801.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Band structure engineering and transport properties of aluminium phosphide nanoribbon – A first-principles study R. Chandiramouli ⇑, S. Rubalya Valantina, V. Nagarajan School of Electrical & Electronics Engineering, SASTRA University, Tirumalaisamudram, Thanjavur 613 401, India
a r t i c l e
i n f o
Article history: Received 22 June 2014 Received in revised form 4 September 2014 Accepted 10 October 2014 Available online 16 October 2014 Keywords: Aluminium phosphide Nanoribbon Transport property Electron density Density of states
a b s t r a c t The band structure and transport properties of pristine, boron, gallium and arsenic substituted AlP nanoribbon are studied using density functional theory. The band structure of pristine, boron, gallium and arsenic substituted AlP nanoribbon exhibits semiconducting behavior. The substitution of boron decreases the band gap of AlP nanoribbon. The substitution of group-III semiconductor has much influence in density of states. The major contribution is observed in p and d orbitals. The electron density increases with boron substitution and there is a slight decrease in electron density for gallium substitution. The transmission of AlP nanoribbon molecular device is analyzed with two probe method. The substitution impurity and bias voltage influence the transmission across AlP nanoribbon. From the results, it is inferred that the band structure and electronic transport properties can be fine-tuned with substitution impurity along AlP nanoribbon. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Nanoscience and technology opened a new era in fundamental research in nanoscale, the economic nanoscale manufacturing of devices leads to more coherent science and engineering applications. The recent advancement in the group-III and V compound semiconductor attracted the scientific ⇑ Corresponding author. Tel.: +91 9489566466; fax: +91 4362 264120. E-mail address: [email protected] (R. Chandiramouli). http://dx.doi.org/10.1016/j.spmi.2014.10.013 0749-6036/Ó 2014 Elsevier Ltd. All rights reserved.
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community due to their applications in semiconductor industries. This motivates scientific community for numerous theoretical and experimental studies in the group III–V compound semiconductors such as gallium arsenide, indium phosphide, aluminium phosphide and gallium phosphide, etc. Among these compound semiconductors aluminium phosphide (AlP) receive a considerable attention due to lower mass, which leads to higher vibrational frequencies [1]. Aluminium phosphide (AlP) is a wide-band compound semiconductor with a band gap of around 2.5 eV. Usually, AlP is alloyed with binary materials for applications in optoelectronic devices [2]. To this time, even though numerous experimental and theoretical studies are reported in AlP and their counter parts group III–V semiconductors [3–5]; to the best of our knowledge; the band structure engineering and transport properties of aluminium phosphide nanoribbon with substitution impurities have not been investigated. The systematic computational study could be a pioneer step for the experimental characterization of nanostructures. Ling Guo studied carbon monoxide adsorption on cationic, neutral, and anionic aluminium phosphide clusters [6] and also reported the electronic structure and properties of neutral and charged aluminium phosphide clusters [7]. Mirzaei et al., reported boron doped and carbon doped AlP nanotubes [8,9]. Rezaei-Sameti reported SiC-doped aluminium phosphide nanotubes [10]. Density functional theory (DFT) is an effective method to study the structural and electronic transport properties of nanostructures [11–13]. The motivation of the present work is to tailor the band structure and electronic transport properties of AlP nanostructures with improved performance in optoelectronic devices with the incorporation of substitution impurities. Moreover, controlling the transport phenomena along nanoelectronic devices is a challenging task. To date, there are numerous reports in molecular electronic devices, which are used for rectification, switching, optoelectronics and spintronics applications [14–23]. The novel aspect of the work is to design AlP nanoribbon with substitution impurities and AlP nanoribbon is used as a molecular device. The molecular electronic device may have a single molecule between the electrodes which acts as source and drain. The state-of-the-art of the present work is to construct AlP nanoribbon and to fine-tune the band structure and electronic properties with substitution impurity and the designed AlP nanoribbon are used as a molecular device between the electrodes. In the present work an attempt has been made to fine-tune, the properties of AlP with substitution impurities such as boron, gallium and arsenic and the results are reported. 2. Computational details The present investigation on AlP nanoribbon is carried out by DFT method utilizing TranSIESTA module in SIESTA package [24]. DFT method explores the band structure, density of states and transmission coefficient for the present system. In the reported work, generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) exchange correlation functional, (GGA–PBE) functional is employed throughout the work [25,26]. The sampling of Brillouin zone is carried out by 1 1 1 k points. By reducing the atomic forces of atoms as small as 0.05 eV/Å, the molecular geometry is optimized. The real-space grid for electrostatic potentials is considered in the mesh cut-off energy of 10 5 eV that realizes the balance between the efficiency and accuracy in calculation. The electronic properties of AlP nanoribbon are determined with vacuum padding of 10 Å modeled along x and y directions to remove the interaction of AlP nanoribbon with its periodic images. The atoms along the nanoribbon are free to move in their position till the convergence with a force less than 0.05 eV/Å on each individual atom in AlP nanoribbon is achieved. The complete optimization of AlP nanoribbon is supported by double zeta polarization (DZP) basis set [27,28]. 3. Results and discussion 3.1. Structures of AlP nanoribbon Initially, AlP nanoribbon is designed from International Centre for Diffraction Data (ICDD) card number 79-2500. For pristine AlP nanoribbon, there are twenty-four aluminium atoms and twentyfour phosphorus atoms forming a hexagonal structured AlP nanoribbon and both ends of AlP nanorib-
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bon are terminated with four hydrogen atoms each. In the case of boron substituted AlP nanoribbon, there are twenty-four phosphorus atoms, twenty aluminium atoms and four boron atoms are substituted in place of aluminium atoms, and four hydrogen atoms are terminated at both ends. Looking at gallium substituted AlP nanoribbon, there are twenty aluminium atoms with twenty-four phosphorus atoms. Four gallium atoms are substituted in place of aluminium at the bottom end of AlP nanoribbon. Observing the structure of arsenic substituted nanoribbon, there are twenty-four aluminium atoms with twenty phosphorus atoms, four atoms of arsenic replace phosphorus atoms in the top end of AlP nanoribbon. In order to eliminate the passivation effects both the ends of AlP nanoribbon are terminated with hydrogen atoms. The reason behind the selection of boron and gallium as substitution impurities is both the elements; boron and gallium belong to group-III elements like aluminium; the substitution may change the band structure and electronic properties of AlP nanoribbon. Likewise, arsenic is a group-V element as that of phosphorus; this may also change the band structure and transport property of AlP nanoribbon. These are the reasons behind the selection of boron, gallium and arsenic as substitution impurities in AlP nanoribbon. Figs. 1(a)–(d) represents pristine AlP nanoribbon, boron substituted AlP nanoribbon, gallium substituted AlP nanoribbon and arsenic substituted AlP nanoribbon respectively. 3.2. Band structure analysis of AlP nanoribbon The electronic band structures provide the insight for the materials properties involved in band structure engineering. The band structures of AlP are discussed in terms of conducting channels (lines) crossing the Fermi energy (EF) across the valence band and conduction band; the conducting property of nanostructures also depends on the conducting channels [29–31]. If lines cross the Fermi level, it infers the metallic nature of the material. Moreover, the band gap is inferred from the gap across the C point in the band structure diagram. In the case of pristine AlP nanoribbon as in Fig. 2a, the band gap for pristine AlP nanoribbon is found to be around 2.45 eV. The calculated band gap is in agreement with the reported band gap of bulk AlP, which is 2.5 eV [32]. Fig. 2b shows the band structure of boron substituted AlP nanoribbon, the substitution of boron in the bottom terminating end of AlP nanorib-
Fig. 1a. Schematic diagram of pristine AlP nanoribbon.
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Fig. 1b. Schematic diagram of boron substituted AlP nanoribbon.
Fig. 1c. Schematic diagram of gallium substituted AlP nanoribbon.
bon drastically decreases the band gap to 1.5 eV. This arises due to the electronic configuration of boron [He] 2s2 2p1. The boron is substituted in place of aluminium atoms at the bottom end, this consequence to the reduction in the band gap of AlP nanoribbon. Fig. 2c illustrates the gallium substituted AlP nanoribbon. Interestingly, the substitution of gallium instead of aluminium in the bottom end also
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Fig. 1d. Schematic diagram of arsenic substituted AlP nanoribbon.
Fig. 2a. Band structure of pristine AlP nanoribbon.
decreases the band gap. In this case, the band gap decreases to 2.1 eV. Comparing with its pristine counterpart the decrease in the band gap arises due to electronic configuration of gallium [Ar] 3d10 4s2 4p1. Fig. 2d depicts the arsenic substituted AlP nanoribbon; in this case, the band gap is found to be around 2.4 eV. There is not much variation observed for arsenic substitution. Since arsenic is a fifth group element like that of phosphorus, the termination of arsenic in the top end does not have much impact in the band gap when compared with pristine AlP nanoribbon. Du et al. reported first-principles studies of AlN nanoribbon terminated with hydrogen atoms [33]. The trend in the band structure of AlP nanoribbon in the present work resembles with that of AlN nanoribbon. Dai et al. studied the electronic structures of AlN, GaN nanoribbons and AlxGa1 xN nanoribbon heterojunctions using first-principles studies [34]. The band structure fashions of AlP nanoribbon are in reasonable
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Fig. 2b. Band structure of boron substituted AlP nanoribbon.
Fig. 2c. Band structure of gallium substituted AlP nanoribbon.
Fig. 2d. Band structure of arsenic substituted AlP nanoribbon.
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closeness with the reported work. From the results, it can be observed that the substitution of third group elements in AlP nanoribbon shows a considerable variation in the band gap. Moreover, the substitution of boron drastically changes the band gap of AlP nanoribbon. The conducting properties can be fine-tuned with substitution impurities in AlP nanoribbon at the different site.
3.3. Density of states of AlP nanoribbon The density of states (DOS) spectrum provides the insight for visualization of charges in the energy interval of valence band and conduction band [35–37]. The projected density of states (PDOS) gives a clear insight to the localized electronic structure of AlP nanoribbon. It plays an important role in discussing the transport properties of AlP nanoribbon. The PDOS of an atom or a group of atoms is acquired by summing the individual orbital PDOS contributions of an atom or group of atoms [20]. Fig. 3a shows the PDOS spectrum of pristine AlP nanoribbon. Looking at the spectrum it is clearly observed that the major contribution is from p orbitals in the valence band; likewise, localization of charges is seen on p and d orbital in the conduction band. The major contribution is observed only in the p and d orbitals in PDOS spectrum of pristine AlP nanoribbon. The PDOS spectrum of boron substituted AlP nanoribbon is shown in Fig. 3b. The termination with boron in AlP nanoribbon alters the p and d orbitals. The substitution of boron on the bottom end results in peak maximums in p and d orbital along the valence band. However, the peak maximums’ decrease in p and d orbitals in the conduction band of AlP nanoribbon. Fig. 3c depicts PDOS spectrum of gallium substituted AlP nanoribbon. Moreover, the peak maximums are observed both in valence band and conduction band in p and d orbitals of AlP nanoribbon. Since the atomic number of gallium is thirty-one, more electrons in gallium atoms results in peak maximums in p and d orbitals. Fig. 3d represents PDOS spectrum of arsenic substituted AlP nanoribbon. In this case, there is no drastic variation when comparing the PDOS spectrum of pristine AlP nanoribbon. Only a small variation is observed in the conduction band of p and d orbitals. Fig. 4 illustrates the density of states (DOS) spectrum of pristine, boron, gallium and arsenic substituted AlP nanoribbon. Ting-Ge et al. studied aluminium vacancy in wurtzite aluminium nitride [38]. A similar PDOS spectrum is observed as that of the present work. It is evident in the observation that the termination of AlP nanoribbon with group-III semiconductor such as boron; gallium shows much variation in DOS spectrum, in contrast there is no significant variation observed with the substitution of arsenic in AlP nanoribbon.
Fig. 3a. PDOS spectrum of pristine AlP nanoribbon.
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Fig. 3b. PDOS spectrum of boron substituted AlP nanoribbon.
Fig. 3c. PDOS spectrum of gallium substituted AlP nanoribbon.
3.4. Electron density of AlP nanoribbon The electron density of pristine AlP nanoribbon is shown in Fig. 5a; the electron density is found to be more in the phosphorus site than in the aluminium site, the electronegativity property of phosphorus leads to accumulation of more electrons near phosphorus atoms. The electronegativity of phosphorus is one of the chemical properties, which represents the tendency of the phosphorus atoms to attract electrons towards it. Moreover, the electronegativity of atoms in AlP nanostructure is influenced by the atomic number and the distance of valence electrons residing from the nucleus in the nanostructure. Fig. 5b denotes the electron density of boron substituted AlP nanoribbon. The substitution of boron in the bottom end slightly increases the electron density across phosphorus atoms compared with electron density of pristine AlP nanoribbon (color bar gradient). Fig. 5c illustrates
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Fig. 3d. PDOS spectrum of arsenic substituted AlP nanoribbon.
Fig. 4. Density of states spectrum of AlP nanoribbon.
the electron density of gallium substituted AlP nanoribbon. In this case, there is a slight decrease in electron density across phosphorus atoms (color bar gradient). The substitution of gallium atoms in the bottom terminating end decreases the electron density in AlP nanoribbon. Even though, both boron and gallium belong to group-III semiconductor, the electronic configuration of boron and gallium atoms influences the electron density across the phosphorus site. Fig. 5d represents the electron density of arsenic substituted AlP nanoribbon. There is not much variation noticed in electron density when compared with electron density of pristine AlP nanoribbon. Zheng et al. studied the vacancy defect for AlN nanoribbon using the first-principles method [39]. In the reported work, the charge density is found to be more near nitrogen atoms due to electronegative property of nitrogen. In the present work, the electronegative property of phosphorus atoms influences the accumulation of electrons
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Fig. 5a. Electron density of pristine AlP nanoribbon.
Fig. 5b. Electron density of boron substituted AlP nanoribbon.
near the phosphorus site. From the observations, it is inferred that substitution of group-III semiconductor shows the variation in the electron density across the phosphorus site. 3.5. Transport properties of AlP nanoribbon The electronic transmission of AlP nanoribbon can be explored in terms of transport property [40,41]. The transmission of AlP nanoribbon is determined with two probe method. The length of the scattering region along z-direction is around 12.6 Å. The two ends of AlP nanoribbon are held within the electrodes. The width of left electrode and right electrode is 3.15 Å each. The potential across the left electrode is varied in terms of 0.5 V, 1 V and 1.5 V and the right electrode is kept at constant ground potential. The electrons near the Fermi level contribute to electronic transport property in AlP nanoribbon. The orbital delocalization near the Fermi level results in high mobility of electrons which corresponds to certain peak amplitudes in the transmission spectrum [42,43]. The electronic transmission is invoked with magnitude of transmission in a particular energy interval. Fig. 6a represents the transmission spectrum of pristine AlP nanoribbon. In the valence band, peaks are observed
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Fig. 5c. Electron density of gallium substituted AlP nanoribbon.
Fig. 5d. Electron density of arsenic substituted AlP nanoribbon.
for 0.5 V, 1 V and 1.5 V. For 0.5 V bias, the applied bias is not sufficient to cause the transition of charges from the valence band to the conduction band. Moreover, for 1 V bias peaks are observed only beyond 1 eV. The applied bias of 1.5 V makes the electrons to transit from the valence band to the conduction band. This is clearly observed with the shift in peaks, the peaks move towards Fermi level. Fig. 6b denotes the transmission spectrum of boron substituted AlP nanoribbon. A drastic change in transmission is observed for boron substitution. More peak maximums are observed in the valence band and in the conduction band. The valence electrons of boron atoms result in more peak maximums in the valence band. This is also in agreement with the electron density of boron substituted AlP nanoribbon, where the electron density increases with boron substitution. However, when the bias voltage is increased, the peak maximums move towards the Fermi level. This confirms the transition of electrons from the valence band to the conduction band. Fig. 6c illustrates the transmission spectrum of gallium substituted AlP nanoribbon. In this case, a sharp decrease in transmission is observed near 2 eV. This arises due to the electronic configuration of gallium atoms when attached to its neighboring atoms. However, the decrease in transmission diminishes with the increase in bias voltage. For 1.5 V bias, the sharp decrease in transmission near 2 eV is considerably reduced. This
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Fig. 6a. Transmission spectrum of pristine AlP nanoribbon.
Fig. 6b. Transmission spectrum of boron substituted AlP nanoribbon.
confirms the movement of charge towards the conduction band across AlP nanoribbon. Fig. 6d denotes the transmission spectrum of arsenic substituted AlP nanoribbon. For 0.5 V bias, a sharp decrease in transmission near 1.5 eV is observed. In contrast, for 1 V and 1.5 V bias, the sharp decrease in peak maximums is not observed. The applied voltage is sufficient to make the transition of electrons from the valence band to the conduction band. From all the observation, it is inferred that the transmission across AlP nanoribbon can be fine-tuned with substitution impurity as well as by the applied bias voltage. The transport property can be enhanced by proper substitution impurity and bias voltage across AlP nanoribbon.
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Fig. 6c. Transmission spectrum of gallium substituted AlP nanoribbon.
Fig. 6d. Transmission spectrum of arsenic substituted AlP nanoribbon.
4. Conclusions Using density functional theory, the band structure and transport property of AlP nanoribbon is optimized and studied. The band structure of pristine, boron, gallium and arsenic substituted AlP nanoribbon shows semiconducting behavior. The substitution of group-III semiconductors such as boron and gallium decreases the band gap whereas the substitution of arsenic does not have much change in the band gap of AlP nanoribbon. Observing the density of states spectrum, density of charges varies in different energy interval for boron and gallium substitution in AlP nanoribbon. In contrast, there is not much variation noticed for arsenic substitution in density of states spectrum. The electron
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density increases for boron substitution and a slight decrease in electron density are observed for gallium substitution. There is no change in electron density for arsenic substitution. The transmission spectrum of AlP nanoribbon is discussed with two probe method. When the bias voltage increases between the electrodes, the electrons transit from the valence band to conduction band. This is confirmed with the peak maximums moving towards Fermi level. Moreover, there is a drastic variation in transmission is noticed for boron substituted AlP nanoribbon. From the observation, the impurity substitution and the bias voltage influence the transmission across AlP nanoribbon. In conclusion, the substitution impurity and the location of substitution impurity play an important role in tuning the electronic transport properties of AlP nanoribbon. The band structure and transport property can be fine-tuned and enhanced with proper substitution impurity, which finds potential application in the semiconductor industry. References [1] H. Gomez, T.R. Taylor, D.M. Neumark, J. Phys. Chem. A 105 (2001) 6886–6893. [2] D.E.C. Corbridge, Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology, fifth ed., Elsevier, Amsterdam, 1995. ISBN 0-444-89307-5. [3] S. Sriram, R. Chandiramouli, Eur. Phys. J. Plus 128 (2013) 116. [4] M. Takikawa, T. Ohori, M. Takechi, M. Suzuki, J. Komeno, J. Cryst. Growth 107 (1991) 942–946. [5] S.J. Chen, Y.C. Liu, H. Jiang, Y.M. Lu, J.Y. Zhang, D.Z. Shen, X.W. Fan, J. Cryst. Growth 285 (2005) 24–30. [6] Ling. Guo, Comp. Theor. Chem. 984 (2012) 102–107. [7] Ling. Guo, J. Alloys Compd. 490 (2010) 78–83. [8] Maryam Mirzaei, Mahmoud Mirzaei, Comp. Theor. Chem. 963 (2011) 294–297. [9] Maryam Mirzaei, Mahmoud Mirzaei, Solid State Sci. 13 (2011) 244–250. [10] Mahdi Rezaei-Samet, Physica E 44 (2012) 1770–1775. [11] R. Chandiramouli, S. Sriram, D. Balamurugan, Mol. Phys. 112 (2014) 151–164. [12] S.C. Hsieh, S.M. Wang, F.Y. Li, CARBON 49 (2011) 955–965. [13] S. Sriram, R. Chandiramouli, Res. Chem. Intermed. (2013), http://dx.doi.org/10.1007/s11164-013-1334-6. [14] Z.H. Zhang, X.Q. Deng, X.Q. Tan, M. Qiu, J.B. Pan, Appl. Phys. Lett. 97 (2010) 183105. [15] S. V Aradhya, L. Venkataraman, Nat. Nanotechnol. 8 (2013) 399–410. [16] J.B. Pan, Z.H. Zhang, X.Q. Deng, M. Qiu, C. Guo, Appl. Phys. Lett. 98 (2011) 013503. [17] J.B. Pan, Z.H. Zhang, K.H. Ding, X.Q. Deng, C. Guo, Appl. Phys. Lett. 98 (2011) 092102. [18] Z. Zhang, J. Zhang, G. Kwong, J. Li, Z. Fan, X. Deng, G. Tang, Sci. Rep. 3 (2013) 2575. [19] J.J. Zhang, Z.H. Zhang, G.P. Tang, X.Q. Deng, Z.Q. Fan, Org. Electron. 15 (2014) 1338–1346. [20] Z. Zhang, C. Guo, D.J. Kwong, J. Li, X. Deng, Z. Fan, Adv. Funct. Mater. 23 (2013) 2765–2774. [21] Y. Cho, W.Y. Kim, K.S. Kim, J. Phys. Chem. A 113 (2009) 4100–4104. [22] W.Y. Kim, K.S. Kim, Acc. Chem. Res. 43 (2010) 111–120. [23] W.Y. Kim, Y.C. Choi, S.K. Min, Y. Cho, K.S. Kim, Chem. Soc. Rev. 38 (2009) 2319–2333. [24] J.M. Soler, E. Artacho, J.D. Gale, A. Garcia, J. Junquera, P. Ordejon, D.S. Portal, J. Phys.: Condens. Matter 14 (2002) 2745–2779. [25] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Phys. Rev. B 46 (1992) 6671–6687. [26] J.P. Perdew, K. Burke, Y. Wang, Phys. Rev. B 54 (1996) 16533–16539. [27] Z.H. Zhang, C. Guo, G. Kwong, X.Q. Deng, CARBON 51 (2013) 313–321. [28] Q.Y. Hua, G.D. Ren, L.C. Bu, Chin. J. Chem. 24 (2006) 326–330. [29] N.K. Jaiswal, P. Srivastava, Solid State Commun. 152 (2012) 1489–1492. [30] R. Chandiramouli, S. Sriram, NANO 9 (2014) 1450020. [31] A.F. Kuloglu, B. Sarikavak-Lisesivdin, S.B. Lisesivdin, E. Ozbay, Comput. Mater. Sci. 68 (2013) 18–22. [32] LevI Berger, Semiconductor Materials, CRC Press, 1996. [33] A.J. Du, Z.H. Zhu, Y. Chen, G.Q. Lu, S.C. Smith, Chem. Phys. Lett. 469 (2009) 183–185. [34] Y. Dai, X. Chen, C. Jiang, Phys. B Condens. Matter 407 (2012) 515–518. [35] R. Chandiramouli, Struct. Chem. (2014), http://dx.doi.org/10.1007/s11224-014-0434-2. [36] R. Chandiramouli, S. Sriram, J. Inorg. Organomet. Polym. (2014), http://dx.doi.org/10.1007/s10904-014-0041-0. [37] R. Chandiramouli, S. Sriram, Mol. Phys. 112 (2014) 1954–1962. [38] G. Ting-Ge, Y. Jue-Min, Z. Zi-Yao, H. Xiao-Dong, Chin. Phys. Lett. 25 (2008) 2989–2992. [39] F.L. Zheng, J.M. Zhang, Y. Zhang, V. Ji, Phys. B Condens. Matter 405 (2010) 3775–3781. [40] R. Chandiramouli, S. Sriram, Superlattices Microstruct. 65 (2014) 22–34. [41] R. Chandiramouli, Ceram. Int. 40 (2014) 9211–9216. [42] C.J. Xia, D.S. Liu, C.F. Fang, P. Zhao, Physica E 42 (2010) 1763–1768. [43] F. Hiroyuki, K. Yoshikazu, T. Kazuyoshi, Sci. China Chem. 55 (2012) 796–801.