Engineering Of Interface Optical Field In AlGaN Based IR/UV Photoelectric Devices

AlGaN material is a kind of direct wide bandgap semiconductor,which is continuously adjustable from 3.4 eV?GaN?to 6.2 eV?AlN?.With stable physical and chemical properties,it can resist high temperature and strong radiation.For AlGaN-based photoelectronic devices such as near ultraviolet light-emitting diodes and ultraviolet photodetectors,fruitful research results have been obtained and even been commercialized.Currently,in the field of AlGaN-based optoelectronic devices,researchers mainly focus on the AlGaN/GaN quantum well infrared detector?QWIP?based on intersubband transition?ISBT?and deep ultraviolet light-emitting diode?DUV-LED?based on interband transition.In order to obtain efficient vertical optical coupling of QWIP and efficient light extraction of DUV-LED,how to effectively regulate the electric field vibration direction and light energy flow direction at the dielectric interface is a problem that needs to be solved.Focusing on the problem of interface optical field regulation in AlGaN-based QWIP and DUV-LED,both simulation and experiment have been carried out to make thorough research,and some results have been achieved.These details are as follows:?1?A novel quasi-one-dimensional square array gold grating was designed by means of finite element method.It can strongly change the energy flow direction of the normally incident infrared light and generated a large amount of electric field component along the growth direction of the multi-quantum wells.At the wavelength of 4.6?m,the defined vertical optical coupling efficiency of QWIP was as high as 85%,which was 1.3 times and 2times of two-dimensional and one-dimensional gold grating respectively.The quasi-one-dimensional gold grating had three advantages:both surface plasmon polaritons?SPP?and localized surface plasmons?LSP?can be generated under normally incident infrared light;there were more SPP excitation sources;the Fresnel reflection of the gold grating was weak.?2?The long-range surface plasmon?LRSPP?excitation structure—500-nm-thick Si₃N₄/two-dimensional square-hole square array gold grating,insensitive to the polarization of light source,was designed by means of finite element method.The refractive index of Si₃N₄ dielectric material and the upper electrode layer of QWIP was effectively matched.On the upper and lower surfaces of the gold grating,the symbols of the electric field component in the z-direction were symmetrically distributed,and the current density symbols were in the opposition.At the wavelength of 4.6?m,the defined vertical optical coupling efficiency of QWIP was as high as 151%.There were two reasons for the enhancement:LRSPP had small absorption loss and large penetration depth;the 500-nm-thick Si₃N₄ layer effectively weakened the reflection of the gold grating.?3?The novel DUV-LED with detection structure was successfully fabricated,which can eliminate the absorption of p-type GaN layer.And the integrated intensity of the spectrum at 7.8457?A was the same as that of general LED at 1.87 mA.Its external quantum efficiency and wall plug efficiency were 240 times and 60 times enhancement respectively.The core process of efficiency enhancement was as follows:Electron-hole pairs in the quantum well region recombined and emitted photons.Some of the photons directly escaped to the n-type AlGaN material,and some propagated to the depletion region and absorbed to excite new electron-hole pairs.Under the build-in electric field,electrons drifted towards the heavily doped n-type GaN layer and were finally collected by the electrodes.The holes drifted and again injected into the quantum well region to participate in the radiative recombination process.When the excited electrons and holes drifted towards different sides,they will collide with the atoms and new electrons-holes pairs can be ionized.This meant the formation of PD/EBL interface with an internal gain,greatly improving the efficiency of the DUV-LED.?4?The DUV-LED with moth-eye microstructure on the surface of sapphire was prepared by step projection exposure process.Compared to the flat sapphire LED,the light extraction efficiency was nearly doubled.Based on the self-constructed TE/TM polarization measurement system,polarization measurements were performed and the spatial distribution of the TE/TM polarized light was obtained by a specific algorithm for the first time in experiment.The degree of polarization of the moth-eye micro-structure DUV-LED was as high as 81.8%,much larger than that of 64.7%in the flat sapphire LED.The transmittance of moth-eye microstructure and flat sapphire were obtained by FDTD simulation.It was confirmed that the moth-eye microstructure can effectively reduce the total reflection of the sapphire surface/air interface,and improve the light extraction efficiency mainly by enhancing the TE polarized light.?5?The sidewall of sapphire in DUV-LEDs was effectively roughened by optimized stealth lasering dicing.The maximum light extraction efficiency was enhanced by 22%.By analyzing the distribution of TE/TM polarized light of DUV-LEDs under four different dicing conditions,it was found that the roughened sapphire sidewall formed by stealth lasering dicing had a certain enhancement on the extraction of both TE polarized light and TM polarized light.It was insensitive to the light polarization.Further analysis confirmed that the collected TM polarized light was basically emitted from the lateral side of sapphire into the air,while the TE polarized light can be emitted from both the surface and the lateral side.?6?The nanopatterned sapphire substrate?NPSS?was prepared with wet etching and high quality AlN material was epitaxially grown by MOCVD.Based on the raised triangular pyramid pattern substrate,a 10-?m-thick AlN was epitaxially grown without any crack in entire 2-inch wafer,and surface roughness was only 0.14 nm.The XRD half width of maximum:?002?was 164 arcsec and?102?was 185 arcsec.Its crystal quality kept ahead in the world.During the process of lateral epitaxy before AlN material healing,a number of air voids can be formed,which can effectively destroy the waveguide structure in DUV-LED.It will lay a foundation for the design of DUV-LED with high light extraction efficiency.