Theoretical Design And Performance Study Of Two-Dimensional Wide Bandgap Semiconductors

Two dimensional wide bandgap semiconductors are the ideal choice for the high temperature,high power and high frequency devices.Compared to the two dimensional narrow bandgap semiconductors,they have wider bandgap,better thermal conductivity,and higher breakdown field strength.These advantages result in the better performance for two dimensional wide bandgap under extreme environment.In this paper,we first introduce the development history of the two dimensional wide banddgap materials.Then,we introduce the relative theoretical knowledge and calculation methods which are used in our paper.Last,from chapter three to chapter five,three work contents are introduced.We predict one ultra-wide bandgap semiconductor Ca FCl,one wide bandgap semiconductor Pb FCl and one ultra-wide bandgap semiconductor Ba FCl.In addition,we explore the stability and photoelectricity properties of them.Details are as follows:Based on the First principles calculations,we design a two dimensional ultra-wide bandgap semiconductor Ca FCl with good kinetic and thermal stability.The interlayer van der Waals(vd W)interaction exists in its bulk form.The monolayer,bilayer and few-layer Ca FCl can be easily obtained through mechanical exfoliated or liquid phase exfoliation method.Monolayer Ca FCl is an ultra-wide bandgap semiconductor with the bandgap of 5.97 e V.A direct-to-indirect bandgap transition can be induced by external strain.The bandgap of monolayer Ca FCl increases linearly with decreasing the layer number and an indirect-to-direct bandgap transition occurs,indicating that the strong quantum confinement effect exists in this system.In addition,it is noteworthy that monolayer Ca FCl exhibits a prominent ultraviolet absorption,indicating its potential application in optoelectronic devices.Bulk Pb FCl has been successfully synthesized in 1932.It has van der Waals layered structure and wide bandgap.We simulate the exfoliation of monolayer Pb FCl,confirming that the cleavage energy is about 0.21 J/m~2.Meanwhile,monolayer Pb FCl shows excellent thermal stability.It is an indirect semiconductor with a bandgap of 3.20e V and shows a strong optical absorption in ultraviolet region.Based on the first principles molecular dynamics calculation,monolayer Pb FCl can remain thermally stable at 1000 K,suitable for operation in high-temperature environments.The calculated hole(0.34)and electron(0.13)effective masses are relatively small,indicating that monolayer Pb FCl has high carrier mobility and good electrical conductivity.Bandgap can be effectively adjusted by biaxial strain and layer thickness.These properties of monolayer Pb FCl render its potential application in photoelectron devices.We predict a monolayer Ba FCl which can be exfoliated from its bulk phase.The obtained monolayer Ba FCl is an ultra-wide bandgap semiconductor with a bandgap of5.58 e V.It has a small interlayer binding energy,indicating that it is feasible to exfoliate monolayer structure in experiment.The calculations of cohesive energy and phonon spectrum confirm that monolayer Ba FCl has good kinetic stability.Applying external strain or electric field can effectively adjust the bandgap of monolayer Ba FCl.Monolayer Ba FCl undergoes a transition from an indirect bandgap semiconductor to a direct bandgap semiconductor when the uniaxial compressive strain beyond 2%and the biaxial compressive strain beyond 4%.A semiconductor-to-metal transition occurs under an electric field of 0.8 V/?.In addition,monolayer Ba FCl exhibits strong optical absorption in deep ultraviolet spectrum.These results render monolayer Ba FCl as an attractive 2D material for application in flexible nanoelectronic and optoelectronic devices.