Study of Structural and Electronic properties of Non-metal Elements doped SnO2

The crystal structure, electronic properties of F, S, C, B and N doped SnO2 were studied with the First-Principle Method. The theoretical results show that doping of non-metal elements did not change the structure of SnO2 but result in slight lattice volume expansion. The dope of the non-metal elements of B, F, and S cause the Fermi level to shift up. The most obvious finding to emerge from the analysis is that F-doped SnO2 has the lowest defect binding energy, stable crystal structure, and the easiest doping. The B, F, and S element doped SnO2 can modulate the fermi level. The doping of the B and S elements introduced additional defect energy levels to appear within the forbidden band-gap,which improved the crystal conductivity. Analysis of the energy band structure of SnO2 crystals doped with C and N elements shows that the Fermi level has crossed the impurity level. The Fermi level of F doped SnO2 is inside the conduction band, and the doped crystal has metallicity. The optical properties of SnO2 crystals doped with non-metallic elements were analyzed and calculated. The SnO2 crystal doped with F element had the highest reflectivity in the infrared region, and the reflectance of the crystals doped with N, C, S, and B elements decreased sequentially. Based on this theoretical calculations, F doped SnO2 is found to be the best photoelectric material for for preparing low-e thin film.


Introduction
Thin-film solar cells are devices that convert solar energy into electrical energy. The transparent conductive films (TCF) is a thin film material that has both conductive capabilities and high transmittance in the visible light range (300 ~ 800nm) [1]- [3] . The TCFs serve as the front electrode of the thin-film solar cell. Up to now, the solar energy conversion efficiency is about 23.3% [ 4 ] ,it is important to increase the photovoltaiv power generation efficiency, as well as the performance of the front electrode.
The intrinsic SnO2 semiconductor is not conductive due the absent of free carriers.
However, the band gap of 3.6eV of SnO2 made it a potential ideal material for transparent electrode film. It had been proved that the doping of heteroatoms to replace Sn or O elements can lead to more carriers or holes, due to which, many research works have been done using different elements. Non-metallic atoms, such as fluorine, nitrogen etc. were proved to be proper elements for this purpose. After doping heteroatom, the preferred orientation, optical properties and electrical properties of SnO2 films are improved.
Analysis of the electrical properties of SnO2 thin films doped with different non-metallic elements showed that the resistance of SnO2 thin films doped with N element was higher [5] [6][7] than other doped SnO2.
Nguyen successfully prepared p-type N-doped SnO2 film using magnetron sputtering technology [9]. The research results showed that the intrinsic SnO2 thin film was an n-type semiconductor, and the concentration of free carrier in the thin film increased as the temperature for sedimentation increases. However, P-type semiconductors were successfully prepared with N-doped SnO2 thin films. Through Al / N double doping, a p-type SnO2 semiconductor thin film with excellent electrical properties was prepared.
The resistivity, hole concentration and hole mobility of the thin film were 7.1 × 10 -3 Ωcm, 6.24 × 10 19 cm -3 and 14.1 cm 2 V −1 s −1 [7]. Doping SnO2 with F element (substituting element O) can effectively increase the carrier concentration and improve the conductivity of the film. R. Majumder successfully prepared SnO2: F thin film using spray pyrolysis method with SnF2 as the precursor. By adjusting the concentration of the precursor solution, doped-SnO2 films with different properties 3 were obtained. When the concentration of the precursor solution was adjusted to 0.15M and the substrate temperature was 773K, a film with a resistivity of 1.2 × 10 − 4 Ω· cm was prepared [8] .
Theoretically, based on the First-Principles, the doping of N to the SnO2 crystal structure can introduce oxygen vacancies, and thus, increase the charge density of Sn sites. The replacement of O with N can simultaneously decrease the release of CO2 gas.
There have been reports about the experimental research of non-metallic doped SnO2, but the mechanism of the effect of non-metallic element doping on SnO2 performance is not clear enough. In this paper, the Density Functional Theory is used to analyze the electronic structure and optical properties of SnO2 doped with non-metallic elements.

Crystal structure model and calculation method
The SnO2 crystal has the tetragonal structure, its international space symmetry group symbol is P42 / MNM. There are 6 O -2 ions in the nearest neighbor around each Sn 4 + site, and 3 Sn 4+ ions in the nearest neighbor around each O -2 ion, so the coordination number for Sn and O are6 and 3, respectively. The CASTEP package was used to construct a 3 × 2 × 1 tin dioxide supercellular structure system. In order to study the effect of doped non-metallic element on SnO2 structure, N, C, B, F, or S was used to replace one O atom in the supercell, respectively. The model diagram of doped SnO2 was shown in Figure 1. The red spheres were oxygen atoms, the gray spheres were tin atoms, and the light blue oneswere non-metal atoms. The crystal structure optimization and electronic structure of the doped SnO2 cell were obtained through the CASTEP program based on density functional theory. The Perdew-Burke-Ernzerhof (PBE) of generalized gradient approximation (GGA) was selected as the exchange association functional. The interaction between inner electrons and valence electrons was described by OTFG ultra-soft pseudopotential.
The valence electronic configuration in the system are Sn 5s 2 5p 2 , O 2s 2 2p 4 , N 2s 2 2p 3 , C2s 2 2p 2 , B2s 2 2p 1 , F2s 2 2p 6 , S3s 2 3p 4 . The calculation parameters of the doping system were set as follows, the plane waves cut-off energy is 571.4eV, the B-Brillouin zone K space was divided by 2 × 3 × 8, the convergence accuracy of each atom was 10 -5 eV / atom, the internal stress between atoms waz less than 0.05GPa, atoms displacement was less than 0.0001 nm. Table 1 shows the results of geometric optimization of the crystal structure of SnO2 doped with non-metallic elements. After replacing O in SnO2 with non-metallic elements (F, S, C, B, N), the crystal structure is distorted and the volume expands.

Structural optimization
Since the S atom is in the third period of the periodic table, its atomic radius is much larger than other doping atoms, the resulted volume expansion is 8.315%. According to the periodic table, the atomic radius of an element decreases from left to right, the lattice expansion rate also decreases in sequence. However, it can be noticed that the SnO2 doped with F atoms has larger volume expansion. Therefore, it is necessary to analyze the bond formation before and after F atom doping, as shown in Table 3 Therefore, SnO2 doped with F element exhibits an abnormal lattice expansion rate. To evaluate the stability of the crystal structure of the doped lattice, it is necessary to calculate the defect binding energy of the lattice. It can be calculated according to equation below [9] .

Band structure and density of states
Due to the addition of non-metallic atoms, not only the crystal structure was distorted, but also the electronic structure of the SnO2 crystal is changed. The doping atoms introduce impurity levels in the band gap of SnO2, which changes the electronic structure of the system. The different properties of each doping atom make SnO2 electronic structure different changes. The SnO2 crystal shows metallicity when the introduction of non-metallic atoms causes the Fermi level to enter the conduction band. The electronic structure Including the energy band structure, total state density and partial wave state density of the non-metallic element doping system are shown in  For SnO2, the Fermi energy level is at the top of the valence band, indicating that the conductivity of SnO2 is low.. The conduction band bottom (CBM) and the valence band top (VBM) are located at the same G point, which is consistent with the calculation results [10] [11] , indicating that SnO2 is a direct band gap semiconductor. In this paper, the calculated band gap is 1.28eV, which is consistent with the previous calculation results [12][13] [14] . However, the band gap is lower than the experimental value of 3.6eV [15] , which is caused by the underestimation of the cross-correlation energy by the GGA function.
The energy of Sn5s orbits and the interaction between Sn5s orbitals and O2p orbitals are overestimated, resulting in a wider valence band and narrower band gap. Band gap and energy band changes do not affect the electronic structure analysis results of SnO2 crystals. The bandgap value can be modified by the complex variable function method 9 (DFT + U) [16] to obtain a more accurate bandgap value. Although there are still some issues need to be solved with this method, since we mainly discuss the photoelectric properties of element-doped SnO2, it is sufficient for us.
The total state density of the SnO2 crystal shown in Figure 2 illustratesthe valence band of the system is divided into two parts, -19.1eV ~ 14.9eV and -8.1eV ~ 0eV.
According to the density of partial wave states, the deep energy level is contributed by Sn5s and O2s; the shallow energy level is mainly contributed by O2p, and Sn5s and partially contribute by Sn5p. The shallow energy level -8.1eV ~ -5.8eV is mainly due to Sn5s orbit, and O2p orbit is responsible to the part of -5.8eV ~ 0eV. The conduction band is mainly caused by Sn5s and Sn5p orbits.
It is worth noting that the energy band of SnO2 doped with non-metallic elements In order to obtain the information of charge transfer after doping, the secondary differential charge distribution of element doping is calculated and the results were shown in Figure 3. Compared with the O element, the ability of doped atoms to accumulate charge is reduced. This provides more electrons as free carriers, making the system appear metallic. The analysis result is consistent with the analysis result of the energy band structure.

Optical properties
SnO2 is widely used as a transparent conductive film material, especially in doors and windows, it is also preferred for low-e film glass. The most important performance of low-e glass is the reflectivity in the infrared region. According to the reflectance spectrum of the material, a good adiabatic performance of the material requires the plasma frequency to be close to the visible region. The reflectance spectrum of SnO2 doped with non-metallic elements are shown in Figure 4. Analysis of the reflected light spectrum shows that the plasma frequency of SnO2 doped with F atoms is similar to the visible region, in consequence, it has the highest reflectivity in infrared region.
Therefore, SnO2 doped with F element is the most suitable material for preparing low-e glass thin film. Fig4. The reflection spectrum of SnO2 of O atom replaced by nonmetal atom

Conclusions
Based on first-principles density functional theory, the crystal structure, electronic structure and optical properties of SnO2 system doped with non-metallic elements were theoretically analyzed. The calculation results confirmed that the doping caused the crystal lattice to expand. The F-doped SnO2 lattice has the lowest binding energy and is prone to replacement doping. S-doping forms p-type semiconductors, and F-doping forms n-type semiconductors. The optical analysis results revealed that F-doped SnO2 has the highest reflectivity in the infrared region, and is most suitable as a low-e thin film material to block the ambient temperature.