SCIENCE CHINA Physics, Mechanics & Astronomy. Electronic structure and optical properties of N-Zn co-doped -Ga 2 O 3

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1 SCIENCE CHINA Physics, Mechanics & Astronomy Article April 2012 Vol.55 No.4: doi: /s Electronic structure and optical properties of N-Zn co-doped -Ga 2 O 3 YAN JinLiang * & ZHAO YinNv School of Physics, Ludong University, Yantai , China Received October 17, 2011; accepted December 13, 2011; published online March 1, 2012 The electronic structure and optical properties of N-doped -Ga 2 O 3 and N-Zn co-doped -Ga 2 O 3 are investigated by the first-principles calculation. In the N-Zn co-doped -Ga 2 O 3 system, the lattice parameters of a, b, c, V decrease and the total energy E total increases in comparison with N-doped -Ga 2 O 3. The calculated ionization energy of N-Zn co-doped -Ga 2 O 3 is smaller than that of N-doped -Ga 2 O 3. Two shallower acceptor impurity levels are introduced in N-Zn co-doped -Ga 2 O 3. Compared with N-doped -Ga 2 O 3, the major absorption peak is red-shifted and the impurity absorption edge is blue-shifted for N-Zn co-doped -Ga 2 O 3. The results show that the N-Zn co-doped -Ga 2 O 3 is found to be a better method to push p-type conductivity in -Ga 2 O 3. electronic structure, optical properties, -Ga 2 O 3, co-doped -Ga 2 O 3 PACS number(s): Nr, Ci, Je, Ap, Mb Citation: Yan J L, Zhao Y N. Electronic structure and optical properties of N-Zn co-doped -Ga 2 O 3. Sci China-Phys Mech Astron, 2012, 55: , doi: /s *Corresponding author ( yanjinliang@yahoo.cn) Wideband gap (WBG) semiconductors, such as -Ga 2 O 3 and ZnO, are essential materials for making short wavelength lasers and light-emitting diodes. Unfortunately, because WBG semiconductors either have a low valence band maximum (VBM) or high conduction band minimum (CBM), they experience the doping asymmetry problem; i.e., they can be easily doped n-type, but not p-type [1]. These doping asymmetry problems are the major obstacles for potential applications of many WBG materials, even though they hold excellent properties [2]. In general, there are mainly three reasons for the doping asymmetry: (a) the desirable dopants have limited solubility, (b) the desirable dopants have sufficient solubility, but they produce deep levels, which are not ionized at working temperature, and (c) the self-compensation defects are spontaneously formed. Great efforts have been devoted to overcoming the doping asymmetric problem in WBG semiconductors [3]. The co-doping concept has received special and extensive attention. The original co-doping concept suggests that the solubility of the desirable dopants can be enhanced through the Coulomb coupling of the donor-acceptor pairs and the defect levels can be reduced through the donors and acceptors level repulsion [4]. Further studies find that co-doping increases the doping concentration and reduces the defect transition energy levels [5]. Furthermore, experimental studies also show that the co-doping can drastically reduce the defect levels in some WBG materials. The p-type doping in ZnO has been realized by co-doping Ga with N [6], and the measured activation energy is less than 150 mev [7]. In this paper, the electronic structure and optical properties of N-doped -Ga 2 O 3 and N-Zn co-doped -Ga 2 O 3 are investigated by the first-principles calculation. 1 Details of the calculation All the calculation is completed by CASTEP [8,9] software Science China Press and Springer-Verlag Berlin Heidelberg 2012 phys.scichina.com

2 Yan J L, et al. Sci China-Phys Mech Astron April (2012) Vol. 55 No package in Materials Studio (MS) 5.0 software. It is a quantum mechanics program ab initio Simulation Packag based on the density functional theory (DFT). When making use of the plane wave pseudo-potential technology of general energy where an ion potential is replaced by ultra-soft pseudo-potential, electronic wave function is unfolded with plane wave base groups. The generalized gradient approximation (GGA) in Perdew, Burke, and Ernzerhof (PBE) scheme [10] is adopted for describing the exchange correlation interactions. Basic parameters are chosen as follows: kinetic energy cut-off=380 ev, SCF tolerance= ev atm 1. The special K points of monkorst-parks scheme [11] are summed in all Brillouin zone in the reciprocal space. The maximum displacement is Å and the force per atom is lower than 0.03 ev nm 1. At the same time, the tolerance deviation and stress deviation are 0.02 nm and 0.05 GPa, respectively. The electronic states 2s 2 2p 4, 3d 10 4s 2 4p 1, 3d 10-4s 2 and 2s 2 2p 3 are considered as the valence states for O, Ga, Zn and N, respectively. In the present work, a super-cell containing four monoclinic unit cells (namely 40 Ga and 60 O atoms) is modeled. One gallium is substituted by a zinc atom and one oxygen atom is substituted by a nitrogen atom. The supercell structures of N-doped -Ga 2 O 3 and N-Zn co-doped -Ga 2 O 3 are shown in Figures 1(a) and (b). 2 Results and discussion 2.1 Geometrical structures The geometry optimization is performed using Broyden- Fletcher-Goldfarb-Shanno minimization algorithm [9]. The equilibrium structure is obtained after the cell geometry and volume are fully relaxed by minimizing the total energy and forces. The equilibrium lattice parameters of N-doped -Ga 2 O 3 and N-Zn co-doped -Ga 2 O 3 are shown in Table 1. The lattice parameters of -Ga 2 O 3 are mainly dependent on the kinds of dopants as well as the sites of doping. In N-Zn co-doped -Ga 2 O 3, the value of a, b, c, V decreases and the total energy E total increases in comparison with N-doped -Ga 2 O 3. The covalent radius of Ga (1.81) is larger than that of Zn (1.51), resulting in a smaller value of a, b, c, V in the N-Zn co-doped -Ga 2 O 3 system. Acceptor ionization is the process of holes escaping from the binding of acceptor impurities. Acceptor ionization energy is the energy required to remove a hole from the Figure 1 (Color online) The supercell structure of N-doped -Ga 2 O 3 (a) and N-Zn co-doped -Ga 2 O 3 (b). binding of acceptor impurities to the valence band. The ionization energy of an acceptor with respect to the VBM is calculated by [12] K (0 / q) [ E(, q) ( E(,0) q D (0))] q[ (0) (host)], (1) D VBM where E(,q) and E(,0) are the total energy of the super-cell at charge state q or neutral for defect, K D (0) and D (0) are the defect levels at the special k-points (averaged) and at -point, respectively. q is the defect charge state, (host) VBM is the VBM energy of the host super-cell Table 1 The lattice parameters of N-doped -Ga 2 O 3 and N-Zn co-doped -Ga 2 O 3 a (Å) b (Å) c (Å) V (Å 3 ) E total E ion Intrinsic N-doped N-Zn co-doped

3 656 Yan J L, et al. Sci China-Phys Mech Astron April (2012) Vol. 55 No. 4 at -point. The first term on the right hand side of eq. (1) determines the energy parameter (including both the Coulomb contribution and the atomic relaxation contribution) of the charged defects calculated at the special k-points, which is the extra cost of energy after moving ( q) charge from the VBM of the host to the neutral defect level. The second term gives the single electron defect level at the -point. Compared with the N-doped -Ga 2 O 3, the ionization energy (E ion ) of N-Zn co-doped -Ga 2 O 3 decreases largely. Based on the first-principles calculations we find that the ionization energy of N-doped -Ga 2 O 3 is 0.76 ev, the ionization energy of N-Zn co-doped -Ga 2 O 3 is 0.15 ev. N-Zn impurities require little energy to give off holes to the valence band, and hence can lead to hole conductivity in -Ga 2 O 3. The result indicates that the N-Zn co-doped is a better method to improve the p-type conductivity of -Ga 2 O 3 than N mono-doping. 2.2 Charge density Figures 2 and 3 show the contour plots of charge density of N-doped -Ga 2 O 3 and N-Zn co-doped -Ga 2 O 3. The density difference distributions are used to analyze the bonding characteristics in crystal. The ionic or covalent character in -Ga 2 O 3 is a somewhat controversial topic and has important information to interpret the transport properties in these materials [13,14]. The figure clearly reflects the charge distribution between the impure atom and its neighbor atoms. Significant changes in the charge density distribution are observed by introducing N and Zn impurity. The covalent characteristics can be clearly seen in the bonds Ga(1) O(2). Indeed, the nuclei are bound by the charge density which is shared between them. The electro-negativity difference of N Zn atom (1.41 ev) is larger than that of N Ga atom (1.23 ev), which indicates that the interaction between N and Zn atom is stronger than that of N Ga(1). The Zn atom releases more electrons and forms holes around impurity levels. Figure 3 (Color online) Contour map of difference charge density on the (100) plane of N-Zn co-doped -Ga 2 O Electronic structure Figures 4 and 5 show the calculated band structures, where the Fermi-level is specified to be zero in this paper. It can be seen that the bottom of the conduction band and the top of the valence band are at the same k-point ( ), which indi- Figure 4 (Color online) Band structure of N-doped -Ga 2 O 3. Figure 2 (Color online) Contour map of difference charge density on the (100) plane of N-doped -Ga 2 O 3. Figure 5 (Color online) Band structure of N-Zn co-doped -Ga 2 O 3.

4 Yan J L, et al. Sci China-Phys Mech Astron April (2012) Vol. 55 No cates that N-doped -Ga 2 O 3 and N-Zn co-doped -Ga 2 O 3 are still direct gap semiconductors. The calculated band gap values are smaller than the experimental ones. This is because DFT theory is based on the ground state theory, in which the exchange correlation potential of the excited electrons has been underestimated. However, in the same computing conditions, it would not affect the analysis of the electronic structure. The impurity level of N-doped -Ga 2 O 3 lies above the valence band (about ev at the R point) and intersects with the Fermi level. It plays the role of acceptors of electrons excited from the valence band to make the N-doped -Ga 2 O 3 a p-type semiconductor. However, the acceptor impurity level is too deep to be thermally excited at room temperature. That can be used to explain why we cannot prepare a high conductive N-doped -Ga 2 O 3 in the experiment. There are two impurity levels above the valence band about ev and ev at the same R point for N-Zn co-doped -Ga 2 O 3. It indicates that the impurity level of N-Zn co-doped -Ga 2 O 3 is shallower than that of N-doped -Ga 2 O 3. Therefore, the N-Zn co-doped -Ga 2 O 3 is a very promising method to get p-type -Ga 2 O 3. Figures 6 and 7 show the density of states (DOS) and the partial density of states (PDOS) of N-doped -Ga 2 O 3 and N-Zn co-doped -Ga 2 O 3. Corresponding to the band structure, there are three valence bands. N-doped -Ga 2 O 3 has the upper valence band, the middle valence band and the lower valence band located at 0.71 ev 7.66 ev, ev ev and ev ev. N-Zn co-doped -Ga 2 O 3 has the upper valence band, the middle valence band and the lower valence band located at 0.34 ev 7.44 ev, ev ev and ev ev. The upper valence band is mainly formed by the states of Ga 4s, 4p and N 2p orbits in N-doped -Ga 2 O 3. However, the upper valence band of N-Zn co-doped -Ga 2 O 3 is formed by Zn 4s, 4p and N 2p states. The characteristics of -Ga 2 O 3 are mainly depended on the top of upper valence band and the bottom of the conduction band. So we will not discuss the middle valence band and the lower valence band. The conduction band of N-doped -Ga 2 O 3 is located between 1.73 ev and 5.68 ev, which is mainly formed by the Ga 4s, 4p states, and the N 2p states have a minor contribution. However, the conduction band of N-Zn co-doped -Ga 2 O 3 is located between 1.54 ev and 5.43 ev, which is mainly formed by the Zn 4s states and the Ga 4s, 4p states, the N 2p states also have a minor contribution. The characteristic peak of N atom in N-Zn co-doped -Ga 2 O 3 is stronger than that of N-doped -Ga 2 O 3. The N 2p states and the Zn 4s, 4p states display extensive overlap which indicates the strong interaction between the N atom and the Zn atom. Figure 6 (Color online) The total density of states (DOS) and partial density of states (PDOS) for N-doped -Ga 2 O 3. Figure 7 (Color online) The total density of states (DOS) and partial density of states (PDOS) for N-Zn co-doped -Ga 2 O Optical properties The optical properties can be obtained from the dielectric function ( ) = 1 ( ) + i 2 ( ), which is calculated by the approach of Ehrenreich and Cohen ( is the light frequency), the real part 1 ( ) of the dielectric function can be derived from the imaginary part 2 ( ) using the Kramers- Kronig dispersion equation [13,14]. In order to get a better optical constant, we use the scissor operator to implement a band gap correction ( E g ) 2.4 ev according to the experimental value of intrinsic -Ga 2 O 3 band gap [15]. The real part 1 and imaginary part 2 for N-doped -Ga 2 O 3 and N-Zn co-doped -Ga 2 O 3 are plotted in Figures 8 and 9 in a wide range of energy 0 30 ev. The optical properties are investigated by means of the complex dielectric function, which are explained by the selection rule of the band-to- band transitions. The major peak (peak 1) of N-doped -Ga 2 O 3 is located at 8.9 ev, which is attributed to the inter-bands transition from the excitation of N-2p electrons in the valence band to the conduction band of Ga-4s. However, the major peak (peak 3) of N-Zn co-doped -Ga 2 O 3 is located at 8.4 ev, which is attributed to the inter-bands transition from the excitation of N-2p electrons in the valence band to the Zn-4s in the conduction band. There are two impurity peaks (peak 2) appearing at 3.8 ev for N-doped -Ga 2 O 3 and 5.6 ev for N-Zn co-doped -Ga 2 O 3, which represents the transition from the acceptor impurity

5 658 Yan J L, et al. Sci China-Phys Mech Astron April (2012) Vol. 55 No. 4 levels to the conduction band bottom. The absorption spectra of N-doped -Ga 2 O 3 and N-Zn co-doped -Ga 2 O 3 are shown in Figure 10. The major absorption peak is red-shifted and the impurity absorption edge is blue-shifted for N-Zn co-doped -Ga 2 O 3 in comparison with N-doped -Ga 2 O 3. Figures 4 and 5 show that the energy gap between the top of the valence band and the conductive band bottom is ev for N-doped -Ga 2 O 3 and ev for N-Zn co-doped -Ga 2 O 3. Compared with N-doped -Ga 2 O 3, the major absorption peak of N-Zn co-doped -Ga 2 O 3 is red-shifted. The energy gap between the acceptor impurity levels and the conductive band bottom is ev for N-Zn co-doped -Ga 2 O 3 and ev for N-doped -Ga 2 O 3, the impurity absorption edge of N-Zn co-doped -Ga 2 O 3 is blue-shifted in comparison with N-doped -Ga 2 O 3. 3 Conclusions Figure 8 (Color online) Real part of the dielectric function for N-doped -Ga 2 O 3 and N-Zn co-doped -Ga 2 O 3. We have studied the electronic structure and optical properties of N-doped -Ga 2 O 3 and N-Zn co-doped -Ga 2 O 3 by the first-principles calculation. The covalent radius of Ga (1.81) is larger than that of Zn (1.51), resulting in the N-Zn co-doped -Ga 2 O 3 having a smaller lattice parameter of a, b, c and V than that of N-doped ones. The ionization energy of N-Zn co-doped -Ga 2 O 3 is 0.15 ev, the co-doped can largely reduce the ionization energy. There is one acceptor impurity level above the valence band about ev in N-doped -Ga 2 O 3. However, there are two impurity levels above the valence band about ev and ev in N-Zn co-doped -Ga 2 O 3. Compared with N-doped -Ga 2 O 3, the major absorption peak is red-shifted and the impurity absorption edge is blue-shifted for N-Zn co-doped -Ga 2 O 3. All the results indicate that N-Zn co-doped -Ga 2 O 3 is a very promising method to get the p-type -Ga 2 O 3. Figure 9 (Color online) Imaginary part of the dielectric functions for N-doped -Ga 2 O 3 and N-Zn co-doped -Ga 2 O 3. This work was supported by the National Natural Science Foundation of China (Grant No ), the Natural Science Foundation of Shandong Province, China (Grant No. 2009ZRB01702), and the Project of Shandong Province Higher Educational Science and Technology Program (Grant No. J10LA08). Figure 10 (Color online) Absorption spectra of N-doped -Ga 2 O 3 and N-Zn co-doped -Ga 2 O 3. 1 Zhang S B, Wei S H, Zunger A. A phenomenological model for systematization and prediction of doping limits in II-VI and I-III-VI 2 compounds. J Appl Phys, 1998, 83(6): Shigeo O, Norihito S. Characterization of transparent and conducting Sn-doped -Ga 2 O 3 single crystal after annealing. Thin Solid Films, 2008, 516(17): Nishimatsu T, Katayama-Yoshida H, Orita N. Ab initio study of donor hydrogen complexes for low-resistivity n-type diamond semiconductor. Jpn J Appl Phys, 2002, 41(4A): Katayama-Yoshida H, Yamamoto T. Materials design of the co-doping for the fabrication of low-resistivity p-type ZnSe and GaN by ab-initio electronic structure calculation. Phys Status Solidi B, 1997, 202(2): Wei S H, Zhang S B. Chemical trends of defect formation and doping limit in II-VI semiconductors: The case of CdTe. Phys Rev B, 2002, 66(15):

6 Yan J L, et al. Sci China-Phys Mech Astron April (2012) Vol. 55 No Look D C, Claflin B, Alivov Y I, et al. The future of ZnO light emitters. Phys Status Solidi A, 2004, 201(10): Zeuner A, Alves H, Hofmann D M, et al. Optical properties of the nitrogen acceptor in epitaxial ZnO. Phys Status Solidi B, 2002, 234(3): R7 R9 8 Segall A M D, Lindan P L D, Probert M J. First-principles simulation: ideas, illustrations and the CASTEP code. J Phys-Condensed Matter, 2002, 14(11): Feng J, Xiao B, Chen J C, et al. Theoretical study on the stability and electronic property of Ag 2 SnO 3. Solid State Sci, 2009, 11(1): Ueda N, Hosono H, Waseda R, et al. Synthesis and control of conductivity of ultraviolet transmitting -Ga 2 O 3 single crystals. Appl Phys Lett, 1997, 70(26): Litimeina F, Racheda D, Khenatab R, et al. FPLAPW study of the structural, electronic, and optical properties of Ga 2 O 3 : Monoclinic and hexagonal phases. J Alloys Compd, 2009, 488(1): Wei S H. Overcoming the doping bottleneck in semiconductors. Comput Mater Sci, 2004, 30(3-4): Feng J, Xiao B, Chen J C. Optical properties of new photovoltaic materials: AgCuO 2 and Ag 2 Cu 2 O 3. Solid State Commun, 2009, 149(37-38): Xu J, Huang S P, Wang Z S. First principles study on the electronic structure of fluorine-doped SnO 2. Solid State Commun, 2009, 149(13-14): Passlacki M, Schubert E, Hobson W S, et al. Ga 2 O 3 films for electronic and optoelectronic applications. J Appl Phys, 1995, 77(2):