Theoretical study on Schottky regulation of WSe<sub>2</sub>/graphene heterostructure doped with nonmetallic elements
Authors: Ma Hao-Hao , Zhang Xian-Bin , Wei Xu-Yan , Cao Jia-Meng ,   School of Science, Xi’an University of Technology, Xi’an 710048, China
Published: December 2020

In order to effectively control the type and height of Schottky barrier, it is crucial to appropriately select the material and method of controlling the type and height of the Schottky barrier effectively. Two-dimensional materials exhibit massive potential in research and development due to their unique electrical, optical, thermal and mechanical properties. Graphene is a two-dimensional material found earliest, which has many excellent properties, such as high carrier mobility and large surface area. However, single-layered graphene has a zero band gap, which limits its response in electronic devices. Unlike the graphene, the transition metal sulfides have various band structures and chemical compositions, which greatly compensate for the defect of zero gap in graphene. From among many two-dimensional transition metal sulfides, we choose WSe<sub>2</sub>. The reason is that the single-layered WSe<sub>2</sub> possesses the photoelectric excellent performance, band gap that can meet the majority of requirements in electronic and photoelectric devices, and transport properties that can be adjusted to p-type or bipolar which is first found in semiconductor materials. And compared with metal, the graphene at room temperature has superior properties such as high electron mobility, resistivity of 10<sup>-6</sup> Ω·m lower than copper and silver, coefficient of thermal conductivity 5300 W/(m·K) large than 10 times that of copper, aluminum and other metal, and hardness exceeding the diamond, fracture strength up to 100 times more than that of iron and steel. The Two-dimensional semiconductors along with semimetallic graphene are seen as the basic building blocks for a new generation of nanoelectronic devices, in this sense, the artificially designed transition metal sulfide heterostructure is a promising option for ultrathin photodetectors. At present, most researchers focus on the control of the type and height of Schottky via heterojunction doped metallic element. However, there are few Schottky that are doped by nonmentallic element. Therefore, our work provides the interaction between WSe<sub>2</sub> and graphene, which are described by the first principles effectively. The results show that there is the van der Waals interaction between the interface of WSe<sub>2</sub> and that of graphene, and thus forming a stable structure. Through the analysis of energy band, it is found that the semiconductor properties of WSe<sub>2</sub> are changed by the coupling between WSe<sub>2</sub> and graphene, making the WSe<sub>2</sub> transform from direct band gap into indirect band gap semiconductor. Furthermore, the total density of states and corresponding partial density of states of WSe<sub>2</sub>/graphene heterostructure are investigated, and the results show that the valence band is composed of hybrid orbitals of W 5d and Se 4p, whereas the conduction band is comprised of W 5d and C 2p orbitals, the orbital hybridization between W 5d and Se 4p will cause the photo generated electrons to transfer easily from the internal W atoms to the external Se atoms, thereby forming a build-in internal electric field from graphene to WSe<sub>2</sub>. Finally, for ascertaining the effect of doping WSe<sub>2</sub> with nonmetallic elements, the WSe<sub>2</sub>/graphene Schottky is investigated by using the plane-wave ultrasoft pseudo potentials in detail. Besides, the lattice mismatch rate and lattice mismatch can prove the rationality of doping WSe<sub>2</sub> by non-metallicelement. The stability of the combination between the doped WSe<sub>2</sub> and graphene is demonstrated by the interface binding energy. The influence of nonmetallic atoms on WSe<sub>2</sub> is analyzed before investigating the heterojunction of the doped WSe<sub>2</sub> and graphene. The results show that the band gap of WSe<sub>2</sub> doped by O atoms changes from 1.62 to 1.66 eV and the leading band moves upward by 0.04 eV. This indicates that O atom doping has little effect on the band gap of WSe<sub>2</sub>. When WSe<sub>2</sub> is doped with N and B atoms, the impurity energy level appears near the Fermi level of WSe<sub>2</sub>, which results in the band gap being zero, and then it presents severe metallization. This is due to the Fermi level of WSe<sub>2</sub> shifting. When the C atom is doped, the impurity level appears at the bottom of the guide band of WSe<sub>2</sub>, and the band gap is 0.78 eV. Furthermore, we analyze the effect of doping on heterojunction. In the W<sub>9</sub>Se<sub>17</sub>O<sub>1</sub>/graphene heterojunction, the Schottky barrier height of n-type and p-type are 0.77 eV and 0.79 eV respectively. It shows that the heterojunction type transforms form p-type into n-type, whose Schottky barrier height is reduced effectively. Due to the W<sub>9</sub>Se<sub>17</sub>N<sub>1</sub> as well as W<sub>9</sub>Se<sub>17</sub>B<sub>1</sub> with metallic properties combining with graphene, the Fermi energy level of graphene is shifted, its Dirac point is located above the Fermi energy level and its conduction band has a filling energy level. When doped with N and B atoms, WSe<sub>2</sub>/graphene belongs to the type of ohmic contact. When W<sub>9</sub>Se<sub>17</sub>C<sub>1</sub> contacts the graphene, the graphene Dirac point is on the Fermi surface, and the Fermi energy level of W<sub>9</sub>Se<sub>17</sub>C<sub>1</sub> is shifted by 0.59 eV. And then, the height of Schottky barrier of type-n for the heterojunction is 0.14 eV, the height of type-p is 0.59 eV and overall type of heterojunction is type-n. Therefore, by doping WSe<sub>2</sub> with O, N, C and B, the WSe<sub>2</sub>/graphene Schottky type and barrier height can be adjusted. These will provide guidance for designing and manufacturing the 2D FET.