J. Phys. Chem. B 2007, 111, 3379-3383
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Structural, Electronic, and Optical Properties of Oxygen Defects in Zn3N2 Run Long, Ying Dai,* Lin Yu, Meng Guo, and Baibiao Huang School of Physics and Microelectronics, State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, People’s Republic of China ReceiVed: October 16, 2006; In Final Form: February 5, 2007
The structural, electronic, and optical properties of oxygen-defective Zn3N2 were studied by means of density functional theory. The geometry optimization result shows that pure Zn3N2 is cubic in structure, which is in agreement with experiment. Our results indicate that Zn3N2 with nitrogen replaced by oxygen is more stable than that with interstitial oxygen and the substitutional oxygen for nitrogen in defective Zn3N2 is responsible for the n-type conduction character. The possible optical transition mechanisms are discussed.
1. Introduction The group III nitride semiconductors have been recognized as one of the most useful materials in both fundamental and practical fields. However, many fewer studies about group II nitride semiconductors have been reported than those about group III nitrides. In the present work, we choose Zn3N2 from among the group II semiconductors because Zn has an interesting electronic structure and the 3d orbital of Zn affects its valence band. Experiments showed that Zn3N2 nanowires exhibit a UV and blue emission1 and Zn3N2 films can be thermally oxidized to fabricate p-type ZnO materials.2 It is known that Zn3N2 powders, films, and nanowires have been achieved by different synthesis methods during the past decade1,3-5 and its nanowires can be synthesized by oxide-assisted growth.6 Nevertheless, to our knowledge its practical use, such as whether it is a potential candidate competitive with GaN for fabricating blue light emitting diodes, has been a riddle. Zn3N2 powder was first synthesized by Juza and Hahn7 in 1940; this material has rarely been studied for over 50 years. The early studies indicated that it is of the anti-scandium oxide (Sc2O3) structure, where the Zn atoms occupy three-fourths of the F positions and N atoms occupy the Ca positions in the CaF2 structure. In 1997, the structure of Zn3N2 was experimentally refined by Partin et al.,8 who found it has a cubic structure with the lattice constant a ) 9.7691(1) Å. Although the properties of Zn3N2 have been studied experimentally recently,1-5 many issues remain uninvestigated. For example, the optical band gap of Zn3N2 is still controversial.1,3-5 Futsuhara’s studies indicated that the synthetic polycrystalline Zn3N2 films exhibit a high electron mobility of about 100 cm2 V-1 s-1 at room temperature and Zn3N2 is of n-type character with a direct band gap of 1.23 eV.3 On the other hand, other experiments show the optical band gap between 2.12 and 3.2 eV.1,4,5 Since little theoretical work has been reported, it is essential to clarify the related properties of Zn3N2 materials by means of theoretical investigation. Oxygen can be easily incorporated into zinc nitride films during the synthesis process and may usually hardly be detected, because zinc nitride X-ray diffraction patterns are quite similar to those of zinc oxide. In this paper, we carried out density functional theory calculations on pure Zn3N2 and oxygendefective Zn3N2 to investigate both their geometric and electronic structures as well as the origin of optical transitions. The results indicate the cubic structure of pure Zn3N2 is consistent
with that refined by Partin et al.8 The solid with substitutional O is more stable than that with interstitial O. The electronic band structure shows that the oxygen defect plays an important role in the n-type character of the synthetic zinc nitride films. The origin of the different optical band gaps observed from experiments is qualitatively discussed based on the calculated results. 2. Methods All the spin-polarized density functional theory (DFT) calculations were performed using the program package DMol3,9-11 in which wavefunctions are expanded in terms of accurate numerical basis sets. The double-numeric quality basis sets with polarization functions (DNP) were used. The DNP basis sets are comparable to 6-31G** sets, and the numerical basis set is much more accurate than a Gaussian basis set of the same size.9-11 DSPP, the density functional semicore pseudopotential12 of Perdew-Wang 1991 (PW91) version of the gradient-corrected GGA functionals,13,14 were employed in our calculations. A real-space cutoff of 3.9 Å was used. Increasing the real-space cutoff to 4.4 Å did not change the results. For the numerical integration, we used the medium quality mesh size predefined in the program. The single-point energy tolerance of the SCF convergence was set to 1.0 × 10-6 Ha (1 Ha ) 27.2114 eV). The Zn3d shell was treated as valence. The Monkhorst-Pack15 grid with 2 × 2 × 2 k-points was used for structural optimization and self-consistent energy computations, and 4 × 4 × 4 k-points were used for the electronic band structures and density of states calculations. The density of states was calculated by the parallelepipeds method with no broadening. The differential charge density was calculated at the γ-point. For the geometry optimization, the tolerances of energy, gradient, and displacement convergence were 2 × 10-5 Ha, 4 × 10-3 Ha/Å, and 5 × 10-3 Å, respectively. In this work, we adopted one 80-atom supercell for cubic zinc nitride. Two possible substitutional O-defective structures were modeled by replacing a nitrogen atom in the lattice with an oxygen atom, and one interstitial O defect structure was modeled by putting an O atom at an interstitial position. The Zn3N2 supercell was optimized without restriction, and the defective models were optimized for only the atoms around the O atom.
10.1021/jp0667902 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/09/2007
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Figure 1. Supercell and partial geometry of the Zn3N2 models for (a) the supercell, (b) one substitutional O atom to N(1) (ON(1)), (c) one O atom substituting N(2) (ON(2)), (d) one O atom substituting Zn (OZn), and (e) one interstitial O atom (OIn). The bond length is in angstroms.
Figure 2. Differential charge density maps for (a) ON(1), (b and c) ON(2), (d) OZn, and (e and f) OIn models. The unit for bond length is angstroms. The contour interval is 0.05 e/Å3.
The results of optimal geometry and electronic structures by DMol3 are in agreement with that by CASTEP16 for pure Zn3N2, implying that our calculated results are reliable. 3. Results and Discussion 3.1. Optimized Structure and Formation Energy. We first performed calculations for the pure Zn3N2 in a cubic structure with an 80-atom supercell (see Figure 1a). The optimized lattice constant is 9.769 Å, and the Zn-N bond lengths are 2.122 Å (Zn-N(1)), 2.002, 2.060, and 2.287 Å (Zn-N(2)), respectively, which are in good agreement with experiment.8 In principle, two types of possible O atom defects, substitutional and interstitial, may exist in the lattice. For the type of substitutional defect, either a lattice N atom or a Zn atom may be substituted by an O atom, so we considered two models for substitutional defect. One model starts with an O substituting an N atom at different sites [N(1), N(2)] denoted by ON(1) (see Figure 1b) and ON(2) (see Figure 1c), respectively, and another model with an O atom substituting a Zn atom denoted by OZn (see Figure 1d). The geometry optimization shows that the Zn-O bond lengths are 2.210 Å in ON(1), and those in ON(2) are 2.030, 2.159, and 2.406 Å, respectively. The calculations
also indicate that for the model OZn, a large distortion with obvious displacement of intrinsic lattice position occurs, where the distance between the O atom and the other side adjacent N and Zn are 1.397 and 1.945 Å, respectively. The local structure of the interstitial O model signed with OIn is shown in Figure 1e. The calculated optimal bond lengths of Zn-O are 2.018 and 2.304 Å, respectively. The properties of valence electrons of defective O atoms can be directly described by the differential charge density (DCD), which is also called deformation density, namely, the selfconsistent density minus the sum of superposed atomic spherical densities. Figure 2a shows the DCD around the O atom of ON(1). Considering another plane, the O is coordinated with six nearest Zn atoms and the charge on O is about -2. Parts b and c of Figure 2 display the DCD of ON(2) in a different plane, which indicates that the 6-fold-coordinated O atom with three different Zn-O bond lengths may carry negative charge, e.g., formal -2. In the OZn model, Figure 2d shows that the O atom is bound to an N atom in π form. The electron cloud leans to the N atom from the O atom, whereas there is rare interaction between the O atom and the Zn atom, increasing the charge on O to about +1. Parts e and f of Figure 2 are the DCD of two planes for the
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Figure 3. Total density of states of Zn3N2. Units in the DOS curves are 1/eV.
OIn model, and in another plane, the O atom is bound to six Zn atoms, and its charge should be -2. To investigate the stability of O-defective Zn3N2 materials, we need to examine the formation energies for various models. For the O-defective models, the formation energy Eform for the substitutional O is calculated as
Eform ) E(Zn48N31O) - [E(Zn48N32) + E(O) - E(N)]
(1)
Eform ) E(Zn47N32O) - [E(Zn48N32) + E(O) - E(Zn)] (2) The formation energy of interstitial O is defined as
Eform ) E(Zn48N32O) - E(O) - E(Zn48N32)
(3)
where E(Zn48N32O), E(Zn47N32O), E(Zn48N31O), and E(Zn48N32) denote the total energy of the supercell with and without O, respectively, and E(N) and E(O) are determined by the energy of an N atom in N2 and an O atom in an O2 molecule, respectively, (E(N) ) 1/2E(N2), E(O) ) 1/2E(O2)). E(Zn) is the energy of the one Zn atom in bulk Zn. The calculated results show that the formation energy of ON(1), ON(2), OZn, and OIn are -3.62, -3.35, 1.99, and 1.66 eV, respectively. The negative formation energies for the O-defective system imply an easy formation of O defects during the synthesis of zinc nitride films. The formation energies of ON(1) and ON(2) are nearly equivalent and smaller than those of the OIn structure, which indicates the substitutional structures are relatively more stable than the OIn structure. Therefore, interstitial O is less favorable in energy than substitutional O in the lattice due to the large O atom radius. In contrast, the formation energy of OZn is larger than that of OIn, suggesting that substituting O for a Zn atom is harder than introducing an interstitial O. Consequently, the substitutional O for N is more favorable in energy. 3.2. Electronic Properties. The total density of state (TDOS) for pure Zn3N2 is given in Figure 3, showing the semiconductor character of Zn3N2. The valence band extends down to about -15.43 eV below the top of the valence band and divides into two main parts. The upper valence bands range from 0 to -7.97 eV, major components of which are Zn3d, Zn4s, Zn4p, and N2p; the lower valence bands vary from about -15.43 to -13.03 eV, mainly from N2s. The conduction bands are composed of Zn4s, Zn4p, and N2p. In addition, a remarkable band ranges from about 1.0 to 1.8 eV near the conduction band edge. To
Figure 4. Partial DOS of Zn3N2 of (a) Zn4s, Zn4p, Zn3d and (b) N2s, N2p. The solid, dashed, and dotted lines correspond to s, p, and d bands, respectively. Units in the DOS curves are 1/eV.
study more detail of the frontier bands, we give the partial density of state (PDOS) of the Zn4s, Zn4p, Zn3d, N2p, and N2s components in Figure 4, parts a and b, respectively, for Zn3N2 and the TDOS in Figure 3. It shows that the occupied frontier bands mainly result from the overlapping of N2p, Zn3d, and Zn4p, and the unoccupied frontier bands arise mainly from Zn4s and N2p. The isolated band near the conduction band edge is contributed mainly by the Zn4s and N2p states. Therefore, the valence band of Zn3N2 is tightly related to N atoms. To interpret the effects of O defect on the electronic properties of Zn3N2, we performed calculations on the substitutional and interstitial O-defective models, i.e., the ON(1), ON(2), OZn, and OIn structures, respectively. Their TDOS near EF and PDOS are shown, respectively, in Figure 5. When compared to part e of Figure 5, parts a and b of Figure 5 clearly indicate that both the valence band and conduction band shift to a lower energy region in the cases of both ON(1) and ON(2) and the Fermi level EF locates near the bottom of the conduction band, which is of n-type conduction character according to the semiconductor energy band theory. This can be ascribed to the effect of O2p on the frontier orbitals. Unlike pure Zn3N2 (Figure 4), parts a′ and b′ of Figure 5 show that an O substituting an N atom introduces an O2p gap spreading out of the frontier band edge. In the cases of both OZn (Figure 5c) and OIn (Figure 5d), there is little change on the gap and frontier bands and the EF pinned around the top of the valence band showing p-type conduction characteristic. Figure 5, parts c′ and d′, indicates that both O replacing Zn and interstitial O result in most of the O2p bands spreading out of the top valence band. However, there is an obvious difference between the two casessthe major components of the localized gap in OZn are Zn4s and O2p and those in OIn are N2p and O2p. One can conclude that the defective Zn3N2 with O substituting N may induce n-type conduction and Zn3N2 with others may not. Consequently, combining the results from the formation energy calculations, the experimental measuring sample with n-type conduction should incidentally involve a substitutional O defect. This is consistent with what Futsuhara et al. detected of the O component in their samples.3 That is, the unintentional substitutional O defect is responsible for the n-type conductivity of zinc nitride samples, and the ON(1) structure is the most possible configuration. 3.3. Optical Properties. There has been a debate on the value of the optical band gap of zinc nitride (1.23 to ∼3.22 eV).1,3-5 The zinc nitride films reported in ref 3 were determined to be an n-type semiconductor with a gap of 1.23 eV, whereas the gap reported in ref 4 is 2.12 eV and that in ref 5 is 3.2 eV. The
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Figure 5. Total density of states of (a) ON(1), (b) ON(2), (c) OZn, (d) OIn, and (e) pure Zn3N2. The corresponding O2p band is shown in (a′), (b′), (c′), and (d′), respectively. The insets in (c′) and (d′) represent the nearest neighbor Zn4s (N2p) and O2p states, respectively, in the range of 0.3 to ∼1.0 eV. Energy is relative to the Fermi level EF (for pure Zn3N2, the top of valence band is taken as the zero-energy point). The dashed line represents EF. Units in the DOS curves are 1/eV.
origin of the various gaps that appeared in the samples may be interpreted by the possible optical transition discussed below in O-defective Zn3N2 according to the DOS of model ON(1) shown in Figure 5a. It is well-known that DFT calculations usually underestimate the excitation energies due to the GGA approximation, but the discussion of optical properties will not be affected because only the relative energy changes are considered and the errors induced in DFT may be canceled out. Figure 5a illustrates three possible transitions: First, the electron transition from the top of the valence band (EV) to the states (ED) near EF; second, from the bands ED near EF to the bottom of the conduction band (EC). Both of the transitions correspond to the relatively small optical gap. Third, the band-band transition from the top of the valence band (EV) to the bottom of the conduction band (EC) corresponds to the biggest optical gap. Therefore, different optical band gaps can be detected depending on the experimental conditions. The 1.23 eV optical gap energy observed in Futsuhara et al.’s samples with n-type conduction3,17 may correspond to the transition between the states near EF and the band states, and the optical gap energy of 2.12 to ∼3.2 eV measured in refs 4 and 5 should correspond to the band-band transitions.
4. Conclusions A theoretical investigation of pure and O-defective Zn3N2 was performed. The optimized lattice constant is 9.769 Å for cubic pure Zn3N2 and is in good agreement with experiment. The formation energy results of O defects indicate that the structure with N substituted by O is more stable than the structure with Zn replaced by O or with interstitial O in the lattice, so the formation of O defects is easy during the synthesis of zinc nitride films. The electronic structure study proves that substituting N with O should be responsible for the n-type conductivity of zinc nitride samples. It is also presumed that introducing an effective defect, such as interstitial O, is a way to obtain the applicable p-type semiconductor Zn3N2. We also present three possible transitions for the O-defective Zn3N2, which explains the various values of the optical band gap detected in experiments. Acknowledgment. The work was supported by the National Science Foundation of China under Grants 10374060 and 60377041 and the Fund for Doctoral Program of National Education 20060422023.
Oxygen Defects in Zn3N2 References and Notes (1) Zong, F. J.; Ma, H. L.; Ma, J.; Du, W.; Zhang, X. J.; Xiao, H. D.; Ji, F.; Xue, C. S. Appl. Phys. Lett. 2005, 87, 233104. (2) Wang, D.; Liu, Y. C.; Mu, R.; Zhang, J. Y.; Lu, Y. M.; Shen, D. Z.; Fan, X. W. J. Phys.: Condens. Matter 2004, 16, 4635. (3) Futsuhara, M.; Yoshioka, K.; Takai, O. Thin Solid Films 1998, 322, 274. (4) Zong, F. J.; Ma, H. L.; Du, W.; Ma, J.; Zhang, X. J.; Xiao, H. D.; Ji, F.; Xue, C. S. Appl. Surf. Sci. 2006, 252 (22), 7583. (5) Kuriyama, K.; Takahashi, Y.; Sunohara, F. Phys. ReV. B 1993, 48, 2781. (6) Zhang, R. Q.; Lifshitz, Y. H.; Lee, S. T. AdV. Mater. 2003, 15, 635. (7) Juza, R.; Hahn, H. Z. Anorg. Allg. Chem. 1940, 224, 125. (8) Partin, D. E.; Williams, D. J.; O’Keeffe, M. J. Solid State Chem. 1997, 132, 56.
J. Phys. Chem. B, Vol. 111, No. 13, 2007 3383 (9) Delley, B. J. Chem. Phys. 1990, 92, 508. (10) Delley, B. J. Phys. Chem. 1996, 100, 6107. (11) Delley, B. J. Chem. Phys. 2000, 113, 7756. (12) Delley, B. Phys. ReV. B 2002, 66, 155125. (13) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244. (14) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (15) Monkhorst, H.; Pack, J. Phys. ReV. B 1976, 13, 5188. (16) Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. J. Phys.: Condens. Matter 2002, 14 (11), 2717. (17) Futsuhara, M.; Yoshioka, K.; Takai, O. Proc. Asia-Pacific Symp. Plasma Sci. Technol., 3rd 1996, 2, 511.
