Structural Design And Performance Improvement Of AlGaN-based Deep Ultraviolet Light-emitting Diodes

The emission of the AlGaN material system with direct bandgap can be tuned via varying the Al component to almost cover the whole ultraviolet range(210-400 nm).Besides,AlGaNbased deep ultraviolet light-emitting diodes(DUV LEDs)featured with lower power loss,longer lifetime,smaller footprint and less hazard to the environment are recognized as promising candidates to replace the conventional mercury-based ultraviolet light sources in extensive fields,including air/water purification,sterilization,medical treatment and biochemistry.However,in most cases,the external quantum efficiency(EQE)of AlGaN-based DUV LEDs is still less than 10%,degrading with emission wavelength shortening.The EQE of the DUV LEDs is limited by various factors,including poor crystalline quality,current crowding,low carrier injection efficiency,inferior light extraction efficiency,as well as reduced electron and hole wavefunction overlap due to the polarization-induced electric field in the active region.Redesign and optimization of the epitaxial structures of the active region,electron blocking layer(p-EBL),structure of substrate,and electrode materials have been proposed to enhance the luminous efficiency of DUV LEDs.In this study,aimed at boosting the carrier injection efficiency and improving the current spreading effect,we redesigned the interface between the last quantum barrier(LQB)and the electron blocking layer as well as the structure of the hole injection layer(HIL)via semiconductor device simulation to improve the optical and electrical properties of AlGaNbased DUV LEDs.Three proposed approaches are as follows:(1)We propose to insert Al-rich AlxGa1-xN/AlyGa1-yN superlattices at the LQB/p-EBL interface to promote the hole injection efficiency and EQE of DUV LEDs.Besides,the Al components of the AlxGa1-xN/AlyGa1-yN superlattices are systematically investigated and optimized to enhance the overall optical and electrical characteristics of the proposed DUV LEDs to the maximum degree.By comparing the EQE,optical power,energy band,carrier concentration,and polarization charge density,it can be concluded that negative polarizationinduced sheet charges generated at the interface between the Al0.67Ga0.33N/Al0.7Ga0.3N superlattice and p-EBL can attract holes and enhance the hole concentration in p-EBL.Furthermore,the holes can be injected into the active region either through the thermionic process or intraband tunneling process from p-EBL,thus augmenting the hole concentration and radiative recombination rates in the active region.(2)The physical mechanism behind the fact that the introduction of Al-rich Al0.75Ga0.25N layer between the LQB and p-EBL contributes to enhanced carrier injection efficiency is numerically analyzed.Moreover,we propose to replace Al-rich Al0.75Ga0.25N insertion layer with an Al-component-increasing Al0.57～0.75Ga0.43～0.25N layer to further boost the carrier injection efficiency via regulating the interface sheet charges.The Al0.75Ga0.25N insertion layer induces negative sheet charges at the Al0.75Ga0.25N/p-EBL interface and positive sheet charges at the LQB/Al0.75Ga0.25N interface,which is beneficial to boosting the hole injection efficiency yet unfavorable for electron blocking.On the contrary,the inserted Al0.57～0.75Ga0.43～0.25N layer can eliminate the positive sheet charges at the LQB/Al0.57-0.75Ga0.43～0.25N interface and induce hole accumulation near the n-side interface of p-EBL via negative sheet charges at the Al0.57～0.75Ga0.43～0.25N/p-EBL interface,due to the fact that the Al0.57～0.75Ga0.43～0.25N layer possesses an identical initial Al component with LQB and a higher terminated Al composition than p-EBL.Thus,enhanced hole injection efficiency and simultaneously suppressed electron leakage can be obtained for DUV LEDs with Al0.57～0.75Ga0.43～0.25N insertion layer.Finally,the EQE and optical power of this DUV LED are improved without degrading the forward Ⅰ-Ⅴcharacteristics.(3)The conventional p-AlGaN HIL with a constant Al composition was replaced by Aldecreasing(along the direction of[0001])p-Al0.5～0.4Ga0.5-0.6N/n-Al0.4～0.3Ga0.6～0.7N PN junction HIL,in which the polarization-induced three-dimensional hole gas(3DHG)can elevate the average hole concentration in HIL.In addition,the barrier height for holes at the p-EBL/HIL interface is reduced,thus facilitating holes being injected from HIL into p-EBL and the active region as well as promoting the radiative recombination rates of DUV LEDs.Furthermore,by comparing and analyzing the horizontal hole concentration in the active region,Ⅰ-Ⅴcharacteristics,equivalent circuit diagram and energy band diagram,it can be observed that the n-Al0.4～0.3Ga0.6～0.7N layer can introduce an extra potential barrier for holes and modulate the resistance of HIL,which is favorable for alleviating the current crowding.