Quantum Transport Study Of Photo-induced Current Effects In Two-dimensional Wide-bandgap Semiconductor MgCl₂/ZnBr₂ Heterojunctions

Photodetectors based on two-dimensional(2D)semiconductor materials have shown excellent optoelectronic properties,such as high responsivity,ultra-fast responseand low dark current,attracting extensive research interest.However,it is currently difficult to simultaneously have multiple excellent properties,and therefore it is desirable to explore new mechanisms for photodetection.In this thesis,we study the photogalvanic effect(PGE)of the MgCl₂/ZnBr₂vertical and in-plane heterojunctions composed of two-dimensional wide-bandgap semiconductor monolayer MgCl₂and ZnBr₂,by using the non-equilibrium Green's function method combined with first-principles calculations,which proposes a mechanism for high-performance ultra-violet photodetection.The main research contents and results are as follows:Firstly,we investigate the PGE in the 2D MgCl₂/ZnBr₂vertical heterojunction is investigated.The band gaps of the monolayer MgCl₂and ZnBr₂are 6.0 and 3.55 eV,respectively,and the band gap of the vertical heterojunction of AA structure is 3.47 eV,within the ultraviolet(UV)range.This vertical heterostructure has a non-spatial inversion C_3vpoint group,and therefore the PGE can be excited under the irradiation of linearly polarized light,generating a persistent photocurrent without the need for any external or built-in electric field.The photocurrent flowing along the armchair direction is calculated at zero bias for photon energies from 3.8 to 6.0 eV.We found that the magnitude of the photocurrent varies with the polarization angle?,which is proportional to cos(2?).Then,the influence of mechanical stress on the photocurrent is studied.When a uniform tensile stress(5%stretch)was applied to the heterojunction,the photocurrent still has a cos(2?)behavior,and the magnitude of the photocurrent does not change significantly.In contrast,under inhomogenous stress,that is,partial stretching(5%stretch),gradient stretching(3%?5%),gradient compression(3%?5%),the photocurrent is largely enhanced by up to 3 orders of magnitude at some photon energies,while still have the cos(2?)form.The large enhancement of the photocurrent is attributed to the substantially reduced the symmetry of the device caused by inhomogenous mechanical stress.Besides,we found that the gate voltage can significantly improve the polarization sensitivity of the photocurrent,and the maximum extinction ratio can reach about 10³,while the magnitude of the photocurrent changes little.Secondly,the PGE in the 2D MgCl₂-ZnBr₂in-plane heterojunction with C_ssymmetry is investigated under the illumination of linearly polarized light,with photon energies from 5.5 to 7.0 eV.The photocurrent has a form of cos(2?).Compared with the vertical heterojunction,the maximum photocurrent can be increased by about 200 times at some photon energies.However,the extinction ratio of the photocurrent is lowered with the maximum about 2.8.The reason for the enhancement of the photocurrent is mainly due to its reduced asymmetry(C_s),as compared to that(C_3v)of the vertical heterojunction.In summary,inhomogenous mechanical stress can significantly enhance the PGE,and the in-plane heterojunction has a larger photocurrent than the vertical heterojunction.Besides,the gate voltage can largely improve the polarization sensitivity of the photocurrent.The proposed photodetector based on the 2D MgCl₂/ZnBr₂heterojunction driven by the photocurrent effect simultaneously has the self-powered,highly polarization-sensitive,broadband and solar-blind ultraviolet UV photodetection.Our results provide a theoretical insight on the design of 2D high-performance UV photodetectors.