Materials Chemistry and Physics

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1 Materials Chemistry and Physics 125 (2011) Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: Pressure induced phase transformation and electronic properties of AlAs Anurag Srivastava a,, Neha Tyagi a, U.S. Sharma b, R.K. Singh c a Advance Materials Research Lab, Indian Institute of Information Technology & Management, Gwalior, M.P , India b Department of Physics, RJIT, BSF Academy, Tekanpur, Gwalior , India c School of Basic Sciences, ITM University, Gurgaon, HRY , India article info abstract Article history: Received 8 June 2010 Received in revised form 26 July 2010 Accepted 18 August 2010 PACS: ±p Rh Nr Mb Kb We have performed the first-principle study to analyze the structural and electronic properties of aluminum arsenide under the application of pressure. The computations have been carried out using the ground state total energy calculation approach of the system. The first-principle approach has been used to compute the stability of various phases of AlAs, like original zinc blende (B3), intermediate NiAs (B8), NaCl (B1) and CsCl (B2) type as a function of pressure. The study observes a B3 B8, B3 B1 and B3 B2 transitions at 6.99 GPa, 8.18 GPa and GPa. The computed phase transition pressures, lattice parameters, bulk modulus, and energy gaps are in good agreement with their experimental as well as theoretical counterparts. Band structure and density of states analysis have also been performed and results have been discussed in detail Elsevier B.V. All rights reserved. Keywords: Phase transition High pressure Electronic properties AlAs 1. Introduction The operating characteristics of the electronic and optoelectronic devices not only talks about the materials engineering at a practical level but they are also required for better understanding of the properties of materials and associated fundamental science behind them. Theoretical investigations as well as experimental researches are therefore of vital interest to all those working in this area of research. The electronic and structural properties of the complex systems have attracted considerable interest in both fundamental and applied physics. A large amount of work has been focused on theoretical understanding of a variety of compound semiconductors and their related properties. These compounds play an important role in microelectronics, for example, in the development of light emitting diodes and high frequency low noise devices for mobile telephones and advanced materials for spintronics [1 13]. The most remarkable aspect of tetrahedrally coordinated structures is their low density. The openness of these semiconductors is highlighted by the fact that for the homopolar members, the ratio of the volume of touching atom spheres to that of the Corresponding author at: Advance Materials Research Lab, Indian Institute of Information Technology & Management, E-110, First Floor, IIITM Campus Morena Link Road, Gwalior, M.P , India. Tel.: addresses: profanurag@gmail.com, anurags@iiitm.ac.in (A. Srivastava). unit cell is 0.34 which is less than half for the close-packed element structure (0.74). It is not surprising, because under pressure tetrahedrally coordinated semiconductors can be transformed to the structure with higher density [14]. Froyen and Cohen [15] and Martin [16] reported first on the phase transition in AlAs which was based on ab initio pseudopotential calculation and suggested that the high pressure structure could be either rocksalt (B1) or NiAs (B8). Weinstein et al. [17] reported the pressure induced structural transition by microscopic examination at 12.3 GPa on loading but the structure was unknown. Greene et al. [18] have performed an EDXD study on AlAs up to 46 GPa and found that AlAs transforms to NiAs structure. This was the first experimental observation of III V compound transforming into the NiAs structure. The equilibrium transformation pressure was found to be 7 ± 5 GPa, averaging the large hysteresis. Onodera et al. [19] have reported B3 B8 phase transition in AlAs at 14.2 GPa by high pressure X-ray diffraction and electrical resistivity measurements. Many methods of calculations have been used to confirm these results. One of them is to relate the high pressure behavior of these semiconductors to the type of chemical bonding between the nearest atoms by examining the electronic charge density evolution, which has been correlated to the empirical qualitative concept as ionicity [20,21]. The first-principles electronic structure calculations have allowed detailed studies of the energetics of the group IVA elements and the groups IIIA VA and IIB VIA compounds under high pressures [22]. Theoretically, III-arsenide /$ see front matter 2010 Elsevier B.V. All rights reserved. doi: /j.matchemphys

2 A. Srivastava et al. / Materials Chemistry and Physics 125 (2011) compounds have been studied by employing different approaches; from phenomenological methods such as k.p. theory or empirical pseudo-potentials methods [23] to atomistic ab initio methods, such as the full-potential linear augmented plane wave (FP-LAPW) method within local density approximation (LDA) or generalized gradient approximation (GGA) and pseudopotential methods [24]. Recently, Wang et al. [25] have reported the first-principle study of the phase transition of AlAs in three crystallographic structures, i.e., B3 (zinc blende), B8 (nickel arsenide) and B1 (rock salt) at high pressures using the full-potential linearized muffin-tin orbital (FP-LMTO) scheme within the generalized gradient approximation (GGA) correction in the framework of the density functional theory (DFT) and based on the condition of equal enthalpies. Singh et al. [26 28] have successfully applied three-bodypotential approach to describe the high pressure phase transition and other properties of Al based compound semiconductors. The effect of pressure on the structural stability of some III V and IV VI compound semiconductor based alloys has also been investigated successfully with three-body-potential (TBP) approach [29,30]. Cai and Chen [31] have reported a possible mechanism for B3 B8 transition, characterized by the space group of C222 1, and observed that there are relatively small values of activation enthalpy and strain anisotropy for B3 B8 transition of AlAs in comparison to the B3 B1 case. The calculated transition pressure from B3 to B8 ranges from 6.1 GPa [32] to 9.15 GPa [31], and B3 to B1 phase, from 7.4 GPa [32] to GPa [31]. Not much information is available on B3 B2 transition in this compound. Looking to the technological importance of this material and success of first-principle methods, we thought it pertinent to analyze the transitions due to the application of pressure and thereby its material characteristics. Particularly three transitions B3 B8, B3 B1 and B3 B2 have been studied. Further the present work also computes lattice constant, bulk modulus and its pressure derivative, band structure and density of state in different phases of AlAs. These calculations provide a one stop shop for fundamental understanding of the structural and electronic properties of the aluminum arsenide. 2. Computational details The present computations of the structural and electronic properties of aluminum arsenide have been performed using ATK tool [33]. Atomistix ToolKit (ATK) is a further development of TranSIESTA-C [34,35] which, in turn, is based on the technology, models and algorithms developed in the academic code TranSI- ESTA and, in part, McDCal [36], employing localized basis sets as developed in SIESTA [37]. The density functional theory (DFT) is, in principle a very good theory to predict ground state properties (e.g. total energy, atomic structure, Bulk modulus, etc.). However, DFT is not a theory to address efficiently the excited state properties and hence DFT typically underestimates the band gap of semiconductors and insulators by 20 30%. The ATK has been proved to be a very efficient tool in predicting the transport properties [38 40] of variety of bulk as well as nanostructured materials, where the electronic properties have also been discussed in detail. The normconserving pseudopotential is used in density function theory for total energy calculation of polyatomic systems. The electronic configuration of AlAs is Al: Ne 3s 2 3p 1, and As: Ar 3d 10 4s 2 4p 3.Inthe calculation of pseudopotential, the inner-cell configurations for Al (1s 2 2s 2 2p 6 ), and As (1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 ) have been distinguished from the valence electrons of Al (3s 2 3p 1 ) and As (4s 2 4p 3 ) shells, respectively. The Perdew Zunger (PZ) type parameterized local density approximation (LDA) exchange correlation functional (LDA-PZ) [41], Perdew, Burke and Ernzerhof (PBE) [42] type parameterized generalized gradient approximation (GGA-PBE) and Zhang and Yang revised PBE (rev PBE) [43,44] type GGA have been used Fig. 1. Energy vs volume curve for B3, B8, B1 and B2 type phases of AlAs. for the present computations. In self-consistent manner, the calculation is performed using steepest descent geometric optimization technique with Pulay algorithm [45] for iteration mixing. The mesh cut-off is taken as 150Ryd with a k-mesh of LDA-PZ type potential computes total energy much lower than that of GGArevPBE and GGA-PBE approaches. The total energy for original B3 type AlAs using LDA-PZ potential is ev and with GGA rev PBE potential ev, which indicates that LDA-PZ potential, is quite good for the calculation of energies of AlAs in different structural phases like B3, B1, B2 and B8. To get better understanding of fundamental physics associated with different phases of AlAs, the Fermi energies, binding energies and band energies have also been computed using LDA-PZ potential and given in Table 3. Classical understanding on the phase stability of solids suggests that as the pressure is applied, a particular phase of the solid becomes unstable and causes a change in the density and the volume, which in turn leads to the overlapping of the electron shells (charge transfer mechanism) and thus the phase transition takes place. Under the application of pressure, the B3 type III V semiconductors are expected to transform into the NaCl (B1) structure and there is possibility of some intermediate NiAs (B8) type phase and further increase in pressure may cause stability of CsCl (B2) type structure. The stability of the phase of the solid can be defined in terms of its Gibbs free energy (G=U+PV TS). This free energy at T = 0 K corresponds to the cohesive energy due to the mutual interaction of the ions. S is the vibrational entropy at absolute temperature T and V is the volume of the unit cell at pressure P. 3. Results and discussions 3.1. Energy vs volume (E V) curve, lattice parameter and bulk modulus To test the stability of various phases of AlAs, like B3, B8, B1 and B2 under the ambient condition as well as under compression, the calculated total energies have been plotted as a function of volume in Fig. 1. In ambient condition B3 structure has been found to be with minimum energy and same under compression first stabilizes in B8 type, then B1 and finally to the CsCl (B2) type with the lowest energy. The positive lattice energy difference of the two competitive structures at zero pressure very well explains the relative stability criterion given by Sangster et al. [46]. The lattice parameter corresponding to minima of the E V curves corresponds to zero pressure, termed as the equilibrium or theoretical lattice constant.

3 68 A. Srivastava et al. / Materials Chemistry and Physics 125 (2011) Table 1 The lattice constants (a), bulk modulus B 0 and its pressure derivative B calculated using LDA scheme for AlAs. 0 Material Lattice constant a (in Å) B 0 (GPa) B 0 PW a Exp. Others PW a Exp. Others PW a Exp. Others AlAs-B (LDA) 5.72 (GGA-PBE) 5.77 (GGArev PBE) 5.66 [18,48] 5.61 [4] 5.63 [7] 5.65 [49] AlAs-B8 3.72(LDA) 3.79 [18] 3.77 [28] 3.72 [32] [50] [4] [49] 64.5 [25] ± 7 [18] 101 ± 26 [19] 84.9 [25] 93.3 [32] ± 1 [18] 10.7 ± 1.4 [19] ± 0.7 [18] 4.0 [19] 4.18 [4] 4.53 [49] 3.88 [25] 4.29 [25] 4.58 [32] AlAs-B (LDA) 5.31 [28] 5.22 [32] 5.19 [15] AlAs-B2 3.22(LDA) 4.97 [52] 3.56 [51] [25] 91.5 [32] [25] 4.3 [32] a PW: present work. In our observations, the equilibrium lattice constant 5.64 Å has been computed using LDA-PZ scheme and is found to be lower as compared to those revealed from GGA-PBE (5.72 Å) and GGArevPBE (5.77 Å) approaches for the original B3 type phase of AlAs. The calculated lattice parameter for B3 type phase is in close agreement with the reported experimental value 5.66 Å [18,48] and other theoretical values [4,7,49]. Similarly the lattice parameters for B1, B8 and B2 type phases have also been in close match with the other theoretical as well as experimental values. The stability analysis of AlAs in B8 type phase, corresponds to the c/a ratio of 1.59 Å, where the equilibrium lattice parameters a and c are 3.72 Å and 5.91 Å, respectively. The c/a ratio, and individual lattice parameters a and c are in close agreement with their theoretical [25,32] as well as experimental [18] counterparts as shown in Table 1. The bulk modulus and its pressure derivative in all the tested phases B3, B8, B1 and B2 type have been computed using Murnaghan equation of state [47] and their values are compared with their experimental and theoretical counterparts. The computed bulk modulus B 0 for B3, B8, B1 and B2 type phases has also been calculated and are in good agreement with the reported values. However in the case of B8 type phase B 0 is comparatively less and the pressure derivative B is slightly higher than the other reported 0 values in Table High pressure phase transition In order to study the high pressure phase transition in AlAs, we have obtained converged values in B3, B1, B2 and B8 type phases. These values have been utilized to determine the total energy at different lattice parameters and plots of energy vs volume for all the tested phases (B3, B8, B1 and B2) as shown in Fig. 1, where, total energies correspond to each phase have been plotted as a function of volume and by drawing tangent of E V curves of two competitive structure, the transition pressure has been calculated. Another method of calculation of transition pressure is by observing the crossover of free energies of two competitive structures as a function of pressure. It is clear from the E V curve that under the ambient condition, AlAs crystallizes in B3 structure. Under the application of pressure original B3 type phase of AlAs first transforms to NiAs (B8) type phase at around 6.61 GPa and further compression leads to the B1 phase stable at around 8.18 GPa and finally the most stable hypothetical CsCl (B2) type phase at around GPa. The computed values of transition pressure are in good agreement with their experimental [18,19] as well as theoretical [25,28,31,32,51,52] counterpart and compared in Table Band structure and density of state (DOS) In order to have more insight towards understanding the change in behavior of the material from one particular type of phase to another under the influence of pressure, the electronic properties have been studied. In the electronic properties, the band structure and density of state for AlAs have been analyzed at the theoretical equilibrium lattice constants in all the stable phases obtained in the present study. For the B3 type phase, band structure has been computed with GGA-revPBE, GGA-PBE and LDA-PZ exchange correlation functional approaches. In Fig. 2(a), it is seen that the various bands have prominent maxima at the central point and minima at the point X of the Brillouin zone. AlAs is an indirect band gap semiconductor because the top of the valence band and the bottom of the conduction band are not at the same center point ( )inthe Brillouin zone. Using LDA-PZ, GGA-PBE and GGA-revPBE exchange correlation functional, the study computes the indirect band gap (E g ) of AlAs as 1.33 ev, 1.53 ev and 1.65 ev respectively. The band below the 7,8 point constitutes the valence band and those above the point X6 form the conduction band. The band structure also shows a direct gap of around 2.28 ev at point 7,8 in valence band to 6 in conduction band, hence, X6 is the lowest energy point of the conduction band and 7,8 is the highest point of the valence band. At room temperature, the gap is sufficiently small so that few electrons are thermally excited from the valence band to the conduction band and few excited electrons gather in the region of the conduction band immediately above its minimum at X6, a region that is termed as Valley. In Fig. 2(b), the valence band is crossing the Fermi level at the point 6,7,8 and conduction band is coming below the Fermi level at the point X6 that clearly indicates B1 type phase of AlAs is metallic in nature. In Fig. 2(c), the valence band is crossing the Fermi level between the points and X, while the conduction band is coming below the Fermi level at the point Table 2 Phase transition pressure in AlAs. Phase transition pressure (GPa) Transition type PW Exp Others B3 B ± 5 [18], 14.2 [19] 5.34 [25], 6.1 [32], 7.12 [31], 9.15 [31] B3 B [25], 7.4 [32], 8.25 [31], [31], 7.5 [28] B3 B [51], 76.8 [52]

4 A. Srivastava et al. / Materials Chemistry and Physics 125 (2011) Fig. 2. (a) Band structure plot for B3 type phase of AlAs, (b) band structure plot for B1 type phase of AlAs, (c) band structure plot for B2 type phase of AlAs, and (d) band structure plot for B8 type phase of AlAs. 6 that shows again the metallic nature in case of B2 type phase. In Fig. 2(d), the valence bands are crossing the Fermi level at the point 8 and the conduction band is also touching the Fermi level. This shows that alike B1 and B2 phases, B8 type phase of AlAs is also metallic. Due to compression, the change of material characteristics from semiconductor to metallic can be seen very well from their band structure, DOS plots and through the numerical values of band gap, Fermi energy and band energy as reported in Table 3. The computed values of indirect band gaps of AlAs are comparatively less than the experimental value and other reported values; however, it is close to some reported values. The densities of states for all the four tested phases (B3, B1, B2 and B8) have been shown in Fig. 3(a) (d). For B3 type phase of AlAs, the nature of peaks is shown in Fig. 3(a), where one can notice that there is a band gap at the Fermi level. In this phase, the highest magnitude peak appears in the conduction band region at around 2.44 ev. The DOS for B1 type phase of AlAs is shown in Fig. 3(b), there appears one prominent peak near 1.72 ev and two peaks in the conduction band region among which the highest peak is around 4.92 ev. The DOS for B2 type phase of AlAs is depicted in Fig. 3(c), where there is no prominent peak in the valence band region but there are two peaks in the conduction band region out Table 3 Band gap (E g), Fermi energy and band energy for AlAs. Material Bandgap E g(ev) Fermi energy (ev) Band energy (ev) AlAs-B3 PW a Exp. Others 1.33(I)[LDA] 1.53(I) [GGA] 1.65(I)[GGArev] 1.88[GGArev] 1.72[GGA] 2.28[LDA] 2.16 [48] 2.05 [4], 2.36 [49], 1.84 [9], 1.29 [5], 2.18 [5], 1.56 [10] 3.92 (LDA) (LDA) AlAs-B (LDA) (LDA) AlAs-B (LDA) (LDA) AlAs-B (LDA) (LDA) a PW: present work.

5 70 A. Srivastava et al. / Materials Chemistry and Physics 125 (2011) Fig. 3. (a) Density of state plot for B3 type phase of AlAs, (b) density of state plot for B1 type phase of AlAs, (c) density of state plot for B2 type phase of AlAs, and (d) density of state plot for B8 type phase of AlAs. of which one is at 2.8 ev and the second one is near 4.24 ev. For B8 type phase of AlAs, we have shown DOS in Fig. 3(d). There are three prominent peaks appearing in the valence band region out of which the highest peak appears around 4.72 ev and also there is one peak in the conduction band region which is occurring around 4.48 ev. 4. Conclusion The present first-principle study computes the pressure induced phase transitions, band structure, density of states, lattice parameter and bulk modulus of AlAs, using the LDA-PZ exchange correlation scheme. It is clearly noted from the band structure plots as well as the density of states that the original semiconducting nature of AlAs in B3 type phase has been transformed to a metallic one in B1, B8 and B2 type phases due to compression. The calculated bulk modulus for all the tested phases of AlAs compared with other reported values shows a close match, except in case of B8 type phase, where bulk modulus is less and its pressure derivative is slightly higher to their experimental as well as theoretical counterparts. As not much information is available about the B3 B2 transition, bulk modulus and pressure derivative in B2 type phase, the present result will certainly serve as a guide to the investigators in future. Acknowledgments The authors are thankful to ABV-Indian Institute of Information Technology and Management, Gwalior for the financial support provided to the work as the Faculty initiation grant. References [1] O. Berolo, J.C. Woolley, J.A. Van Vechten, Phys. Rev. B 8 (1973) [2] S. Kalvoda, B. Paulus, P. Fulde, H. Stoll, Phys. Rev. B 55 (1997) [3] G. Klimeck, R.C. Bowen, T.B. Boykin, T.A. Cwik, Superlatt. Microstruct. 27 (2000) 519. [4] S.Q. Wang, H.Q. Ye, Phys. Rev. B 66 (2002) [5] K.A. Johnson, N.W. Ashcroft, Phys. Rev. B 58 (1998) [6] M.D. Ventra, P. Fernández, Phys. Rev. B 56 (1997) R [7] S.Zh. Karazhanov, L.C.L.Y. Voon, Fiz. Tekh. Poluprovodn. 39 (2) (2005) 177. [8] C.B. Geller, W. Wolf, S. Picozzi, et al., Appl. Phys. Lett. 79 (2001) 368. [9] S.H. Wei, A. Zunger, Phys. Rev. B 39 (1989) [10] R.W. Jansen, O.F. Sankey, Phys. Rev. B 36 (1987) [11] W.Y. Ching, M.-Z. Huang, Phys. Rev. B 47 (1993) [12] J.-Q. Wang, Z.-Q. Gu, M.-F. Li, W.-Y. Lai, Phys. Rev. B 46 (1992) [13] M.-Z. Huang, W.Y. Ching, Phys. Rev. B 47 (1993) 9449.

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