Research On Optoelectronic Properties Of AlGaN Cathode Materials Based On First Principles

With the development of detector applications in the military,civilian and other aspects,people are constantly exploring different response wavelength detectors.Because of the property of low response to visible light and infrared band,soalr blind ultraviolet?UV?detector with the wavelength response in the ultraviolet region is widely used in ultraviolet astronomy,space communication,environmental monitoring,fire monitoring and so on.As the main component of the detector,photocathode decides a numerous properties of the detector.As the solar blind UV detectors develop,people gradually turn their attention to the research of the UV photocathode.For the Ga N photocathode with wavelength response in the range of 150nm⁶00nm has high response in the visible and infrared wavelengths,it is not suitable for UV blind detection,therefore,the researchers begin to study the AlGaN photocathode with variable Al composition,by changing the Al composition to adjust the wavelength response ranges,finally achieve the goal of solar blind UV detection.In order to optimize the fabrication process of the NEA AlGaN photocathode,it is necessary to study the properties of the AlGaN material from the prospect of atomic structure.In the paper,by the simulation calculation based on the first-principles starting from the material atomic structure,the optoelectric properties of Al_x Ga_1-x-x N photocathode with different Al composition and the p-type Mg doping in Al_x Ga_1-x-x N bulk material were studied,providing theoretical reference for AlGaN photocathode experimental studies.The research contents are as follows:Using the Materials Studio?MS?software,the zinc-blende Ga N,Al_0.25Ga_0.75N,Al_0.5Ga_0.5N,Al_0.75Ga_0.25N and Al N supercell models with Al composition at 0,0.25,0.5,0.75,1 were established,the electronic structure properties,such as dielectric constant,refractive index,absorption coefficient and energy loss spectrum of the Al_x Ga_1-x-x N bulk materials with five Al compositions were calculated.The calculation results show that with the increase of Al composition,the formation energy of Al_x Ga_1-x-x N decreases and the structure becomes more stable,and from Ga N to Al N,the energy band gap gradually increases,and the detection wavelength ran ge is changing.With Al composition increasing,the absorption edge of Al_x Ga_1-x-x N absorption coefficient curve gradually moves toward the high energy end,corresponding to the phenomenon that band gap gradually enlarges.The widder the band gap is,the transition of electrons is more difficult,and then the peak of absorption coefficient moves to the high energy end,the region of metal reflection characteristic shifts to the right and the region narrows.The peak of the energy loss spectrum of Al _x Ga_1-x-x N also increases with the increase of the Al composition,the peak area becomes narrower and the peak gradually moves toward the high-energy end.To research the influence of Mg doping on Al _0.25Ga_0.75N bulk material,zinc-blende Al_0.25Ga?Mg?_0.75N?2×2×2?and Al?Mg?_0.25Ga_0.75N?2×2×2?bulk models were established,the electronic structure properties of band structure and density of states,and optical properties of dielectric constant,refractive index,absorption coefficient and energy loss spectrum were calculated.The calculation results indicate that Mg is easier to replace Al atoms in doping Al_0.25Ga_0.75N.After Mg doping,a new band appears in the band structure,the Fermi level enters the valence band and the conductivity of the material becomes p-type,but the material is still direct-bandgap semiconductor,and band gap becomes narrowwer,then electrons can transit more easily.The dielectric constant and refractive index have changes at low frequencies,new absorption peaks appear at the range of 0e V².49e V,37.12e V⁴0.00e V and44.89e V⁴7.92e V,and the absorption peak appear ing at-8.15e V disappears after Mg doping,the two absorption peaks at 10.23 e V and 18.70 e V decrease after doping,and the absorption peak moves toward high energy,the energy loss peak value decreases and the energy loss frequency range increases.