Effects Of The Interlayer In The N-type Region On Characteristics Of AlGaN-based UV-LEDs

With the development of gallium nitride(GaN)materials,ultraviolet light-emitting diodes(UV-LEDs)based on AlGaN band modulation can cover almost the entire ultraviolet spectrum(210-400 nm),making GaN-based UV-LEDs suitable for many fields such as environment,industry and medicine.However,due to the polarization effect of the group III nitride material with high Al composition,the external quantum efficiency(EQE)of GaN-based UV-LEDs has not yet been compared with similar products at visible wavelengths,but this technology has great potential.Therefore,as a fundamental issue in this field,improving the optoelectronic performance of GaN-based UV-LEDs has always been highlighted in the research.It is a common optimization method to reduce the influence of polarization effect by adjusting the structure of conventional(Al)GaN-based UV-LED.The use of an interlayer to change the inherent characteristics of the original structure,thereby improving the performance of UV-LEDs has been widely used.From n-to p-type region,different interlayer designs can play an optimal role in different positions of the UV-LED structure.In this paper,an interlayer is introduced into the AlGaN-based UV-LED electron layer,and the effect of the interlayer on the photoelectric and crystal characteristics of the device is studied.The influence of the n-type Al_xGa_1-xN interlayer on the characteristics of AlGaN-based UV-LEDs under four compositions(Al composition 0.1/0.16/0.23/0.3)and three thicknesses(10/20/50-nm)was studied.According to the simulation results,when the 20-nm Al_0.23Ga_0.77N interlayer is used,the photoelectric characteristics of the UV-LED are enhanced.Since the net polarization charge at the interface between the electron layer and the interlayer heterojunction makes electrons accumulate in the conductive band,which expands the current in the n-type region,the radiative recombination rate of the active region is increased,and the photoelectric characteristics of the UV-LED are improved.Then,according to the simulation results,two UV-LED samples containing 20-nm Al_0.16Ga_0.84N and Al_0.23Ga_0.77N interlayers were epitaxially grown by metal organic chemical vapor deposition technology.The experimental results show that with the current spreading effect of the interlayer,the LOP of the sample increases from 24.3 to 43 m W at 200 m A,and the EQE increases from 2.69%to 5.04%.The experimental results are consistent with the simulation results,indicating that the interlayer improves the photoelectric performance of UV-LEDs.On the other hand,the interlayer with a higher Al composition may introduce greater lattice mismatch to the heterojunction interface,but the results of the crystal characteristics show that the crystal quality of the UV-LED is not degraded.A design of n-type interlayer to further optimize the optoelectronic properties of Ga-and N-polar AlGaN-based UV-LEDs is proposed.The thickness of the new interlayer is 20 nm,and the type of the interlayer is changed to In_xAl_1-xN and lattice-matched Ga_(1-y-z)Al_yIn_zN.According to the simulation results,the In_0.4Al_0.6N insertion layer introduces a potential well into the energy bands of the Ga-and N-polar AlGaN-based UV-LED electron layers,and the electrons in the conductive band are trapped by the potential well,which reduces the number of electrons injected into the active region.Meanwhile,the deeper hole potential well in the valence band causes holes leakage from the active region to electron layer,the radiative recombination rate of the UV-LED decreases,and the photoelectric characteristics are degraded.On the other hand,when Ga_(1-y-z)Al_yIn_zN maintains the composition ratio of y:z=83:17,it matches the GaN lattice.When the lattice-matched Ga_0.4Al_0.498In_0.102N interlayer is used in the electron layer of the Ga-polar UV-LED,electrons accumulate at the left interface of the heterojunction.This process expands the electron current in the n-type region,which is beneficial to increase the injection of electrons into the active region so that improves the photoelectric characteristics of device.The internal quantum efficiency(IQE)droop degree is significantly reduced.In addition,for N-polar UV-LEDs,when the lattice-matched Ga_0.6Al_0.332In_0.068N interlayer is introduced into the electron layer,electrons accumulate at the right interface of the interlayer,and the electron level is significantly increased.The injection of electrons into the active region does not need to cross the interlayer barrier,which is more conducive to improving the radiation recombination rate of the quantum well,and the droop degree of IQE is significantly reduced as well.The simulation results show that the Ga_(1-y-z)Al_yIn_zN interlayer can significantly improve the characteristics of AlGaN-based UV-LEDs.The lattice-matched design also brings new possibilities for reducing negative crystal effects such as new threading dislocations caused by mismatch stress to the interface of the interlayer during the subsequent actual epitaxial growth process.