First principle study of Germanium and Selenium doped monolayer black phosphorus

C.S. Tejaswi Issarapu, R.C. Junghare, G.C. Patil
Visvesvaraya National Institute of Technology Nagpur,

Keywords: monolayer-Black Phosphorus, density functional theory, Band structure


Recently, due to 2D nature, thickness variable bandgap and anisotropic effective mass, black phosphorus (BP) has attracted the attention of various researchers for using it as a channel material for sub-5 nm field effect transistors. Since the layers interact through weak van Der Waals (vdW) forces, it is easy to exfoliate single layer of BP from bulk material. Further, the electronic properties of phosphorene can be enhanced by doping foreign atoms, which results in better device performance. Selenium (Se) doped multilayer-BP has been proven to be having superior photo responsivity resulting in usage of photodetectors. In addition, several studies have been carried out to investigate the effect on electronic properties of phosphorene with defects and to find ideal dopants for monolayer-BP (m-BP). However, to the best of our knowledge first principle study on Germanium (Ge) and Se as dopants in m-BP has not been reported. In this paper, we carry out first principle calculations to explore the impact Ge and Se dopants on electronic properties of phosphorene. First principle calculations have been carried out with the help of density functional theory (DFT) with general gradient approximation (GGA) implemented in Quantum Espresso and using projector augmented wave method (PAW) pseudopotentials for electron-ion interaction. The exchange correlation potential uses GGA schemes of Perdew-Burke-Ernzerhof (PBE). A vaccum space of 15 Å is added in structure of unit cell along the thickness direction to avoid vdW interactions between repeated unit cells. A supercell of 2 × 2 is formed from unit cell to carry out dopant study calculations. A 8 × 8 × 1 Gamma centered Monkhorst-Pack scheme is used for structural relaxation with a cut-off energy of 100 Ry for both unit cell and dopant induced supercell. The relatively stable chemisorption doping configuration is used for doping Ge and Se. Substitutional doping configuration is also being used for doping Ge. From band structure and the density of states (DOS) m-BP it has been found that, m-BP has direct bandgap of 1.01eV which matches with the reported band gap value. Since DFT underestimates bandgap, the bandgap is less than the practical value. Further, due to availability of free electrons, the Fermi level of un-doped m-BP is found to be closer to the conduction band. The Ge doped m-BP with chemisorption configuration structure has direct bandgap of 0.88 eV and the DOS introduced by the Ge dopant are at conduction band which is close to the calculated Fermi level. Further, the band structure and DOS of Ge doped m-BP of substitution configuration has direct bandgap of 0.8eV and DOS introduced by dopant are at valence band. Further, the Se doped m-BP structure has direct bandgap of 1.01 eV and the DOS introduced by the Se dopant are well into the valance band. Thus, due to lower ionization energies, Ge seems to be a better dopant for n-type and p-type m-BP. Further, since dopant states in the case of Se are well into the valence band, Se is not optimal dopant for m-BP.