Modulation Effect Of External Stress On The Electronic And Optical Properties Of H-BN

Hexagonal boron nitride (h-BN) is a wide-bandgap layered material. It has attracted increasing interest due to its potential application in the future opto-electron devices. Although the pure h-BN has been successfully fabricated in experiments under high temperature and high pressure, some basic natures of h-BN are still controversial in both experimental and theoretical studies, such as the stacking sequence and band structure. Moreover, as the miniaturization becomes a development trend of electronic devices, monolayer or multilayer boron nitride based opto-electron device is potentially used in nano-electronics after its successful preparation in experiments. Furthermore, h-BN is a soft layered material, and its structural, electronic and optical properties can be easily affected by the external stress. In present work, based on the first-principles method, we systematically investigate the effect of the external stress on the electronic and optical properties of h-BN. The magnetotunnel junction (MTJs) based on bilayer h-BN is constructed and then its tunneling magnetoresistance (TMR) is also studied using the first-principles method within the non-equilibrium Green's function. The corresponding conclusions and results are as follows:We systematically study the electronic and optical properties of h-BN with five relative stable stackings (AA, AB, AE, AD, and AF) under the hydrostatic pressure. Using the density of states, bandgap, difference of the charge density, and frequency-dependent dielectric function, we explore the mechanism of the effect of the hydrostatic pressure on the electronic and optical properties of h-BN. The results show that the bandgap of h-BN of five stackings decreases as the increase in hydrostatic pressures. Moreover, the band structure of AA stacking changes from indirect to direct at the hydrostatic pressure of 9.19 GPa. As the increase in hydrostatic pressures, the adsorption threshold of h-BN except for AB is red shifted.Using the first-principles method, the effect of the uniaxial stress on the electronic and optical properties of h-BN is studied. By analyzing the band structures and density of states, the evolution of bandgap of h-BN with five relative stable stackings is investigated. The results indicate that the bandgap of h-BN decreases as the increase in the uniaxial stress. The type of the band structure of AA can be modulated by the uniaxial stress. Furthermore, the electrons of h-BN transfer from the in-plane to the inter-layer space gradually as the increase in the uniaxial stress. We use the imaginary part of frequency-dependent dielectric function to study the optical properties of h-BN under the uniaxial compression. The results show that when the uniaxial stress increases, the band-edge absorption threshold of AB stacking is red shifted initially, and blue shifted from 10.53 GPa. For the other stackings, their band-edge absorption thresholds are red shifted as the increase in the uniaxial stress.We investigate the tunneling magnetoresistance effect of the bilayer h-BN based magnetotunnel junction under the uniaxial strain due to its potential application. The results show that the TMR of the corresponding system linearly increases as the increase in the uniaxial strain. In particular, TMR of the system can reach to 95% as the uniaxial strain increases to 2.51%, which approach that of a perfect spin filter.