# Defect Processes in Halogen Doped SnO2
## 1 Introduction  
SnO2 is a wide bandgap, n-type semiconductor that adopts a tetragonal crystal structure [1, 2, 3, 4, 5, 6]. Doping is also used to increase further its electrical conductivity as well as its transparency in the visible region [4, 5, 6]. SnO2 is mainly used as an opacifier of glazes [1], polishing powder [2], protective polymer coating [3], as a functional material in the solar cell technology [4], transparent conducting oxide [5, 6] and gas sensor [7]. Recently [PERSON] et al. [8] demonstrated that a NO gas sensor based on Sn2O3 has an improved selectivity to NO, NO2, and CO gases [8]. [PERSON] et al. [9] have shown that Sn3O4 in ultrathin SnO2 nanosheets exhibits great performance for Li-ion storage for battery technology. Furthermore, there are many experiments that examine the effect of halogen doping in SnO2 thin films with fluorine (F) being the most commonly used dopant because it significantly increases the conductivity and transparency of SnO2, hence making it appropriate for use as the transparent electrode in several classes of optoelectronics [10, 11, 12, 13, 14]. These include the sol-gel preparation of F:SnO2 nanostructures [11], studies on the structural characteristics, and the changes through the fluorine doping using X-ray diffraction (XRD) techniques [12] as well as deposition methods, such us spray pyrolysis [13] and chemical vapor deposition [14, 15] for producing the F:SnO2. Regarding the F doping mechanism, several studies on F-doped SnO2 nanopowders using X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transformedinfrared spectroscopy (FTIR) [16] revealed that at low concentration, F atoms occupy oxygen vacancy sites as substitutional dopants, but for higher concentrations F atoms site in interstitial positions in the host lattice. Moreover, few reports have also demonstrated the effects of chlorine (Cl), bromine (Br), and I doping in SnO\({}_{2}\). [PERSON] et al. [17; 18; 19], using several experiments with emphasis on optical and electrical properties, found pronounced improvements in these properties by halogen doping. [PERSON] et al. [20] also studied the electrical properties of sprayed deposited SnO\({}_{2}\) films and showed that when replacing F dopants with Br, conductivity was reduced. Except for experimental studies, there are also many theoretical research efforts using DFT that examine the role of defects such as F, Sb, and Cl in substitutional positions in SnO\({}_{2}\)[21; 22; 23; 24; 25; 26; 27]. They revealed that such doping contributes to the formation of single donor states; however, the effect of interstitially doped SnO\({}_{2}\) has not been studied yet. Additionally, the influence of other halogen dopants (besides F, Cl) in SnO\({}_{2}\) and the corresponding changes in the oxide's band structure and optical properties have not been systematically investigated yet. In most previously published DFT studies, the bandgap is significantly underestimated within the range 1 eV-2 eV [27; 28; 29]. This is a common problem when DFT calculations on wide bandgap semiconductors are conducted [30; 31]. As a result, a number of methods have been developed in order to calculate more accurately the bandgap of these materials, with the Hubbard + U correction method and the computationally demanding but more accurate hybrid pseudopotentials being the most commonly used. Many researchers have applied the Hubbard + U method for the d orbitals of Sn, but still, the band gap was substantially underestimated (2.5 eV). In the case of hybrid pseudopotentials, although the band gap reaches the correct experimental value of 3.7 eV [32], the computational cost is very high, and this is why the majority of these researchers use a small number of atoms in their unit cells [33]. In the present study, we investigate the halogen doping of the bulk SnO\({}_{2}\) using DFT in order to examine changes in the electronic structure as well as improvements in the photocatalytic properties of SnO\({}_{2}\).