J. Phys. Chem. C 2008, 112, 9861–9864
9861
Structural, Electronic, and Optical Properties of N-doped SnO2 Xueqin Sun,† Run Long,‡ Xiufeng Cheng,† Xian Zhao,*,† Ying Dai,*,‡ and Baibiao Huang† State Key Laboratory of Crystal Materials, and School of Physics, Shandong UniVersity, Jinan 250100, People’s Republic of China ReceiVed: NoVember 2, 2007; ReVised Manuscript ReceiVed: April 1, 2008
Structural, electronic, and optical properties of various N-doped SnO2 were investigated using firstprinciples calculations. The calculated formation energies show that both the substitutional and the interstitial N atoms are preferentially occupied in anion sites, while the N defect formation energies in the O-rich conditions are much lower than that in the Sn-rich ones. The electronic structures demonstrate that three mechanisms are possible with regard to the red-shift of photoluminescence. The first is that the band gap width reduces because of N2p repulsing O2p states and raising up the top of valence band (EV) with N substituting for Sn; the second is that some N2p gap states are induced by N substituting for O resulting in the band gap reducing; and the third is N2p states lowering the bottom of the conduction band (EC) leading to the reduction of band gap by introducing a interstitial N. On the basis of the calculated formation energy and experimental results, the red-shift phenomenon should not be the transition from band to band but the band to gap states. The red-shift mechanism should be N2p gap states to band transition. TABLE 1: Formation Energy of N-Doped SnO2 48-Atom Supercells Where the Unit is Electronvolts
1. Introduction As a direct band gap semiconductor, SnO2 is a promising material for applications in short-wavelength light-emitting diode and laser diode due to its large band gap of 3.6 eV and high exciton binding energy of 130 meV at room temperature.1,2 Undoped and F (Sb)-doped SnO2 thin films have been widely used in transparent electrodes in solar cells,3 flat plane displays,4 or gas sensors.5 To make SnO2-based optoelectronic devices, the production of high quality p-type and n-type thin films is crucial. The undoped SnO2 thin film shows n-type conduction due to native oxygen vacancy defects.6 Experimentally, the p-type SnO2 has been obtained by doping Al,7 In,8 or Li.9 In the theoretical study by Yan et al.,10 nitrogen was predicted to be a good p-type dopant source. At the same time, only a few experimental and theoretical investigations were performed on N-doped SnO2.10–14 An adequate understanding of structural, electronic, and optical properties of N-doped SnO2 certainly is necessary for the explication of the mechanism of the N dopant arising red-shift phenomenon. However, there is a lack of theoretical investigation in this regard. Therefore, it is important to exert first-principles calculations on N-doped SnO2 to obtain an exact knowledge of the physical nature of the red-shift of N-doped SnO2 and the performance of p-type conduction. To further clarify the relationship between structural and electronic properties in N-doped SnO2, density functional theory (DFT) calculations were performed using different N-doped structures of SnO2. It is found that substitutional N can lead to p-type conductivity, while the N interstitials may induce p-type or n-type conduction depending on the N atom’s location. Additionally, this study reveals that the red-shift can occur through three different mechanisms. The first is that the N2p repulses the O2p states which results in an upward shift of valence band in N substituting for Sn structure. The second is that, for the substitutional N for O structures, some gap states * Corresponding author. E-mail:
[email protected] and daiy60@ sdu.edu.cn. † State Key Laboratory of Crystal Materials. ‡ School of Physics.
N-doped SnO2
O-rich condition
Sn-rich condition
NSn NO Nin-3 Nin-4
3.7 0.68 4.4 6.3
12.9 2.0 8.3 10.2
are introduced nearly up the middle of the band gap, and the third is N2p states lowering the bottom of the conduction band by introducing an interstitial N. On the basis of the theoretical analysis and experiment result,11 we predict that the N substituting for O is responsible for the red-shift. The calculated formation energies indicate that the N atom preferentially occupies the anion site, while the formation energies in the O-rich conditions are much higher than that in the Sn-rich conditions. Our theoretical findings thus may provide a better interpretation of Pang and co-workers’ results.11 2. Computational Method All the spin-polarized DFT calculations were performed using the program package DMol3,15–17 in which wave functions are expanded in terms of accurate numerical basis sets. The doublenumeric quality basis set with polarization functions (DNP) was 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.15–17 AE, all electron pseudopotential of Perdew-Burke-Ernzerhof’s (PBE) version of the gradient-corrected GGA function18 was employed in our calculations. A fine real-space cutoff of 4.9 Å was used. The special points sampling integration over the Brillouin zone was employed by using the Monkhorst-Pack method with a 2 × 2 × 3 special k-points mesh,19 and a finer k-points mesh 3 × 3 × 4 was used for electronic properties calculations. The differential charge density was calculated at the gamma 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.
10.1021/jp710564g CCC: $40.75 2008 American Chemical Society Published on Web 05/30/2008
9862 J. Phys. Chem. C, Vol. 112, No. 26, 2008
Sun et al.
Figure 1. Supercell and partial geometry of the SnO2 models for (a) the supercell, (b) one substitutional N atom to Sn (NSn), (c) one substitutional N atom to O (NO), (d) one interstitial N atom (Nin-3), and (e) one interstitial N atom (Nin-4). The Sn and O atoms replaced by substitutional N are marked by 1 and 2, and the two types of interstitial N positions with grey ball are labeled as 3 and 4 in part a, respectively. The black and light grey balls represent the Sn and O atoms, respectively. The bond length is in angstroms.
Figure 2. Deformation charge density contours for (a) NSn, (b) NO, (c) Nin-3, and (d) Nin-4, respectively. The unit is 0.0625 e/Å3.
3. Results and Discussions 3.1. Structures and Stability of Models. The optimized 48atom SnO2 supercell is shown in Figure 1a. In principle, two types of N defects, substitutional and interstitial, may exist in the lattice. For the type of substitutional defect, two models were introduced, that is, an N atom substituting for a lattice Sn atom (NSn) [marked by 1 in Figure 1a] and for an O atom (NO) [marked by 2 in Figure 1a], respectively. Two models with interstitial N (Nin) were constructed too. The location of Nin is marked by 3 and 4 in Figure 1a in light gray balls, respectively. The site 3 corresponds to the center of the two neighboring O atoms, while the site 4 locates at the center of the Sn-O bond. Four of the partial geometry structures are shown in Figure 1b-e, respectively. In the structure of NSn, the optimal N-O bond length is 1.945 Å which relaxes inward about 5.5% of the Sn-O bond length (2.058 Å) [see Figure 1b]. In the case of NO, the optimal bond length of the N atom and adjacent Sn atom is 2.070 Å and 2.091 Å which slightly relaxes outward about 0.023 Å and 0.033 Å in comparison with the Sn-O bond length of 2.047 Å and 2.058 Å, respectively. This is caused by the N atomic radius which is slightly larger than that of O [see Figure 1c]. In the cases of Nin-3 models, the shortest distance between N and its nearest
neighbors O and Sn are 1.784 Å and 1.995 Å, respectively [see Figure 1d]. As to the cases of Nin-4 models, the distances between N and the nearest O and Sn are 1.150 Å and 1.585 Å, respectively [see Figure 1e]. The properties of valence electrons of N dopant can be directly described by the deformation charge densities for the four N-doped models. The deformation charge density is the self-consistent density minus the sum of superposed atomic spherical densities. Figure 2 shows deformation charge densities for different N-dopant structures. In the case of NSn [see Figure 2a], a strong interaction between O and N makes electrons around O move to N, which decreases the overlap between O and Sn (only one plane is shown). The substitutional N atom is bound to six O atoms in a neutral charge state. Its formal charge is 3, while the formal charge of its first neighbor for N atoms increases to about -1. For the model NO [Figure 2b], the neighboring Sn atoms around N atom lose a lot of electrons, and these electrons accumulated around the N atom. The N atom is bound to three Sn atoms, and its formal charge is -3. Furthermore, the degree of localization of electrons around the N atom is much stronger in the NO model [Figure 2b] than that in the NSn model [Figure 2a]. In the case of Nin, the charge of N at site 3 is formally -1 [see Figure 2c] and the charge of the
Properties of N-doped SnO2
J. Phys. Chem. C, Vol. 112, No. 26, 2008 9863
Figure 3. Total density of states (DOS) of (a) NSn, (b) NO, (c) Nin-3, (d) Nin-4, and (e) pure. The corresponding predicted density of states (PDOS) are also shown (right graph) as a′ to e′, respectively. Energy is relative to the Fermi level EF, and the dashed lines denote EF. Units in the DOS and PDOS are 1/eV.
two neighboring O atoms increased to -1. Differently, the charge of N at site 4 is formally -2, and the binding O increases to -1 [see Figure 2d]; these are ascribed to the redistribution of electron among N and O atoms. The accumulation degree of electrons in the former case [see Figure 2c] is stronger than that in the latter case [see Figure 2d]. To study the relative stability of different N-doped structures, the formation energies Eform for the four different N-doped models were also calculated. The formation energies of the substitutional N to O model and substitutional N to Sn model can be defined as follows, respectively.20
Eform ) E(Sn16O31N) - E(Sn16O31) + E(O) - E(N) (1) Eform ) E(Sn15O32N) - E(Sn16O31) + E(Sn) - E(N) (2) For the interstitial N-doped model, the formation energy is calculated by
Eform ) E(Sn16O32N) - E(Sn16O32) - E(N)
(3)
where E(Sn16O31N), E(Sn15O32N), E(Sn16O32N), and E(Sn16O32) denote the total energy of the supercell with and without N, 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(Sn) is the energy of the one Sn atom in bulk Sn. Different formation energies which were calculated using the above method are presented in Table 1. It is clear that N is preferentially incorporated to the anion site. NO has the lowest formation energy in both the O-rich and the Sn-rich conditions. It can be predicted that NO is the primary defect among the four defects, and its concentration is high enough to affect the electronic and optical properties. As to NSn, the formation energy increases significantly about 9.2 eV from that in the O-rich conditions to that in the Sn-rich. Formation energies for the N interstitials are large in both O-rich and Sn-rich conditions,
which could be explained by the presence of residual strain energy when the mismatch between the host and the impurity atoms is large. The formation energies for Nin-3 are 4.4 and 8.3 eV in the neutral charge state under both conditions, respectively. In comparison, the formation energies (i.e., 6.2 and 10.2 eV) for the Nin-4 under the same condition rises by 1.9 eV. This means that a very large energy barrier must be overcome; therefore, it is very difficult to occur, when an interstitial N atom is incorporated into the O-Sn bond in the Nin-3 model. 3.2. Electronic and Optical Properties. As known, analysis of photon transition from electronic structures has been widely used.21 To understand the effect of N doping on the electronic and optical properties of SnO2, the total density of states (TDOS) and the projected density of states (PDOS) of the N-doped systems were calculated and shown in Figure 3. To facilitate the comparison, the TDOS of undoped model and its corresponding PDOS of near band edge states are also shown in Figure 3e,e′, respectively. The results show that the occupied frontier states mainly result from O2p states, while the unoccupied frontier states mainly arise from the hybridization Sn5s and Sn5p states. Therefore, the electronic properties near the EV and the EC of SnO2 are tightly connected with Sn5s5p and O2p states. In Figure 3a-d, it indicates that NSn, NO, and Nin-3 models show p-type character, which are consistent with the theoretical predications by Yan et al.10 In contrast, the Nin-4 model shows n-type character. By considering that the Nin-4 model has high formation energy and large activation energy, the n-type conduction may be difficult to take effect. Compared with the undoped system, the TDOS of NSn model in Figure 3a indicates that the EV raises about 0.2 eV with the immobility of EC, leading to the band gap reduction and then the red-shift of photoluminescence (PL). To further discuss the origin of the red-shift, the PDOS of N2p and its neighboring
9864 J. Phys. Chem. C, Vol. 112, No. 26, 2008 O2p states are presented in Figure 3a′. It shows that the N2p states do not directly upward shift the top of the valence band but repulse O2p states moving to high energy region. In the conduction band, the N2p states are localized at the EC, showing no movement in comparison with that in the pure case. Therefore, the mechanism in this red-shift might be a transition from band to band. Figure 3b clearly shows that some gap states (ED) appearing in the midgap in the NO model, which are from N2p states, as shown in Figure 3b′. The transition from EC to ED might be possible. Among the aforementioned formation energy results, the NO defect has the lowest value of 0.68 eV that it is the dominating defect and energetically high enough to affect the photoluminescence. It thus could be speculated that the substitutional N for O is contributory to the red-shift, which is consistent with other experimental findings.11 The band gaps in Figure 3c,d show results distinctive from those in NSn and NO models. In the Nin-3 model, the EV does not move, the EC shifts to the low energy region, and the edge states of conduction band are composed of N2p (see Figure 3c′). As to the Nin-4 model, all energy levels move to the low energy region because of the interstitial N. The N2p states dominate the edge of conduction edge and downshift it. The above analyses thus suggest that the N interstitials induce the band to band transition. Consequently, NSn, Nin-3, and Nin-4 reduce the band gap through either upshifting the top of the valence band or lowering the bottom of conduction band, which leads to a band to band transition. Furthermore, the substitutional N for O reduces the band gap because of the introduction of some gap states resulting in a band to gap states transition. It thus could be inferred that the NO defect is responsible for the experimental observations of red-shift origin reported by Pan et al.11 4. Conclusions Density function theory calculations were performed to investigate the structural, electronic, and optical properties of N-doped SnO2. The calculated formation energies show that N is energetically favorable to be incorporated to the O site, while the interstitial N has large formation energy in both O-rich and Sn-rich growth conditions. The electronic structures demonstrate that substituting N for O may be an explanation to the red-shift phenomenon as was found in the experiment,11 whereas TDOS and PDOS analyses suggest that the red-shift of luminescence spectrum in N-doped SnO2 result from the band to localized states transition.
Sun et al. Acknowledgment. The work was supported by the Fund for Doctoral Program of National Education 20070422060 and 20060422023, the National Science Foundation of Shandong Province under Grant Y2007B08 and Y2007A18, the National Basic Research Program of China (973 program, Grant No. 2007CB613302), National Natural Science Foundation of China under Grant 10774091. Note Added after ASAP Publication. This article was published ASAP on May 30, 2008. Two co-authors have been added to the paper, and changes have been made to the Acknowledgment. The correct version was published on June 5, 2008. References and Notes (1) Batzill, M.; Diebold, U. Prog. Surf. Sci. 2005, 79, 47. (2) Yu, B.; Zhu, C.; Gan, F. Opt. Mater. 1997, 7, 15. (3) Nuruddin, A.; Abelson, J. R. Thin Solid Films 2001, 394, 49. (4) Gordon, R. G. MRS Bull. 2000, 25, 52. (5) Tamura, S.; Ishida, T.; Magara, H.; Tabaka, O.; Tatsuta, T. Thin Solid Films 1999, 343–344, 142. (6) Kilic, C.; Zunger, A. Phys. ReV. Lett. 2002, 88, 095501. (7) Bagheri-Mohagheghi, M. M.; Shokooh-Saremi, M. J. Phys. D 2004, 37, 1248. (8) Zhao, Z. Ji. L.; He, Z.; Zhou, Q.; Chen, C. Mater. Lett. 2006, 60, 1387. (9) Bagheri-Mohagheghi, M. M.; Shokooh-Saremi, M. Semicond. Sci. Technol. 2004, 19, 764. (10) Yan, Y.; Zhang, S. B.; Pantelides, S. T. Phys. ReV. Lett. 2001, 86, 5723. (11) Pan, S. S.; Ye, C.; Teng, X. M.; Li, L.; Li, G. H. Appl. Phys. Lett. 2006, 89, 251911. (12) Pan, S. S.; Ye, C.; Teng, X. M.; Li, L.; Li, G. H. Appl. Phys. A: Mater. Sci. Process. 2006, 85, 21. (13) Luo, S.; Chu, P. K.; Di, Z.; Zhang, M.; Liu, W.; Lin, C.; Fan, J.; Wu, X. Appl. Phys. Lett. 2006, 88, 013109. (14) Korotkov, R. Y.; Farran, A. J. E.; Culp, T.; Russo, D.; Roger, C. J. Appl. Phys. 2004, 96, 6445. (15) Delley, B. J. Chem. Phys. 1990, 92, 508. (16) Delley, B. J. Phys. Chem. 1996, 100, 6107. (17) Delley, B. J. Chem. Phys. 2000, 113, 7756. (18) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (19) Monkhorst, H.; Pack, J. Phys. ReV. B 1976, 13, 5188. (20) Laks, D. B.; Van de Walle, C. G.; Neumark, G. F.; Pantelides, S. T. Phys. ReV. Lett. 1991, 66, 648. (21) Di Valentin, C.; Pacchioni, G.; Selloni, A.; Livraghi, S.; Giamello, E. J. Phys. Chem. B 2005, 109, 11414.
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