Study On The Light Field Distribution And Extraction Efficiency Of AlGaN Nanocolumns

AlGaN material can be widely used in LEDs,lasers,photodetectors and other devices by adjusting the aluminum composition to continuously change the band gap from 3.4 eV(GaN)to 6.2 eV(AIN),which covering the near ultraviolet to deep ultraviolet bands.However,the high refractive index makes the AIGaN material has a smaller exit angle,so that the conventional flat plate structure device has a large amount of total reflection and thus the external quantum efficiency is very low.In order to improve the light extraction efficiency by changing the exit angle,various surface structures have been proposed,among which the nano-pillar structure has attracted much attention.Studies have shown that the nanocolumn structure changes the exit angle of the device and increases the surface area of the structure,thereby increasing the external quantum efficiency.The refractive index of AlGaN varies with the composition of the aluminum.The higher the aluminum composition,the larger the band gap of the material and the smaller the refractive index.The influence of the aluminum component and nano-column size parameters on the optical properties of nano-pillars requires further in-depth study.In recent years,plasmons generated by the interaction of metals with electromagnetic waves have attracted attention.When the free electrons in the metal resonate with the incident electromagnetic wave,the electric field component of the electromagnetic wave is confined to a very small area near the metal.This characteristic can break through the optical geometrical limit.The plasmon is closely related to the shape and size of the metal and the surrounding medium.As a typical nanostructure,the thin film metal influences the optical properties of the nitride semiconductor.In this paper,the light field distribution and extraction efficiency of AlGaN nanocolumn structure are studied.The variation of the light field distribution of nanocolumn with the aluminum composition,the height of nanocolumn and the diameter of the nanocolumn is studied by finite-difference time-domain method.The diameter,height,and Effect of Cycle on Extraction Efficiency of Nano-pillar Structures.In addition,an aluminum metal thin film was uniformly added to the existing nanocolumn structure to study the influence of the metal thin film on the light field distribution of the nanocolumn.The main contents of the study and the main results obtained are as follows:Firstly,the light field distribution and electric field intensity of AlGaN nanocolumns were studied.First,the relationship between the refractive index and the aluminum composition of AlGaN materials is given,and the relationship between the electric field strength and the aluminum composition is obtained by calculation.The results show that the field strength of the nanopillar significantly differs by a factor of two between the different components.Then,the variation of field intensity inside and outside the nano-columns at different heights was analyzed.In general,the field strength changes with the height of thediameter oscillation,and the influence of the diameter is more significant than the height.The relationship between the waveguide mode and the diameter of the nanocylinder waveguide was studied for the diameter.The diameters of the fundamental modes of the AlGaN cylindrical waveguide at three wavelengths were obtained.The effective refractive index of the fundamental mode in the range of 60-240 nm was calculated.Secondly,the distribution of the optical field and the electric field strength of the AlGaN nanocolumn structure with an additional metallic aluminum film were calculated.The influence of aluminum metal film thickness on the intensity of light field inside and outside the nano-column was studied.The results show that with the increase of the thickness of the metal film,the internal electric field strength of the nano-pillar structure decreases as a whole,and the external field intensity decreases with the increase of the thickness of the aluminum film at most wavelengths.At a wavelength of 490 nm and a film thickness of 10 um,the field strength above the nanocolumns has an extreme value,and the electric field is distributed in a tiny area on the edge of the metal film,which may be a local enhancement effect of the metal on the light.In addition,the relationship between the electric field strength of the metal thin film nanopillar and the height of the nanocolumn was calculated in this chapter.The results show that the electric field intensity distribution of the nanocolumn with a 10 nm aluminum film is approximately the same as that of a nanocolumn without an aluminum film,and the electric field intensity is reduced.This is due to the absorption of electric fields by metals.Thirdly,the extraction efficiency of periodic AlGaN nanocolumn array structure is improved compared with the planar structure,and the effects of height,diameter,period and diameter cycle ratio on the extraction efficiency of nanocolumn structure are analyzed.The results show that,overall,the nano-pillar structure significantly improves the extraction efficiency in both TM mode and TE mode.Among them,the enhancement effect of the TM mode is more obvious and the maximum is 8.08 times,and the extraction efficiency of the TE mode is 1.6 times that of the planar structure.Lower.For the TM mode of light,the height does not significantly affect the extraction efficiency;for the TE mode,the extraction efficiency increases with height,and when the height is 100-300 nm,the TE mode extraction efficiency is only 1.1 times that of the planar structure.The following shows that the height of the nanocolumn structure should be at least 300 nm.The effect of diameter and cycle on extraction efficiency is essentially achieved by changing the diameter to cycle ratio.For both TM and TE polarization modes,increasing the diameter at a fixed period will result in a significant increase in extraction efficiency,and a fixed diameter increase period will significantly reduce the extraction efficiency.When the duty cycle is fixed,the variation of the extraction efficiency with the diameter as a whole is not significant except in some regions.When the diameter-to-cycle ratio reaches 1:1,the extraction efficiency in the TM mode is even lower,indicating that the duty cycle has a maximum value,and the nano-column extraction efficiency is higher than that of the nano-pit structure.For TM-polarized light,the extraction efficiency of the nanocolumn structure increases at shorter wavelengths;for TE-polarized light,the extraction efficiency of the nanocolumn structure increases at the long-wavelength side.