Monolayer Group III Phosphides (XP; X = B, Al, Ga, and In): Structural Stability to Thermoelectrics and Transport Properties

The study explores the structural, electronic, thermoelectric, elastic, and electron transport characteristics of a two-dimensional (2D) hexagonal phase monolayer X-phosphides (XP; X = B, Al, Ga, and In) compounds through first principle calculations using density functional theory (DFT), along with non-equilibrium Green's functions formalism (NEGF) approach. Phonon dispersion analysis confirms the stability of 2D XP compounds, except for AlP and InP. Furthermore, from the detailed investigation of lattice parameters, density of states and energy bands, it is demonstrated that only BP exhibits direct bandgap (0.92 eV) amongst all. Because of the strong in-plane covalent bonding in pristine BP, it has the highest Young's modulus value of similar to 137 Nm(-1) among the four XP compounds as studie. It is worth noting the excellent thermoelectric performance of InP, which exhibits a good figure of merit (ZT similar to 0.9) throughout the temperature range of 100-800 K. Interestingly, from the electron transport properties it is observed that BP also possess better current-voltage characteristics compared to GaP and InP, which makes BP a suitable compound for two or more terminal electronic devices, while GaP and InP devices maybe applicable for negative differential resistance (NDR) driven devices.