Research On Improving External Quantum Efficiency Of AlGaN-based Deep Ultraviolet Light Emitting Diodes

AlGaN,as a ternary alloy for III-nitride material system,is a semiconductor with wide and direct band gap.The band gap is continuously adjustable between 3.4-6.2e V with variation of Al content.It is an ideal material for preparing high-efficiency ultraviolet(UV)light-emitting diodes(LEDs)with wavelength range between 210-365 nm.The AlGaN-based UV LEDs have incomparable advantages,including Hg-free material,controllable wavelength,compact size,fast response,resistant heat and radiation,low operating voltage,long lifetime and so on.Therefor,they can be applied in the fields of lighting,disinfection,sterilization,purification,biomedicine,printing,storage and communication.Although significant progress has been made on the research for AlGaN UV LED,the external quantum efficiency(EQE)of LED decreases sharply with the increase of Al content.Therefore,it is still difficult to achieve a breakthrough for the performance of AlGaN DUV LED with high Al content.In this thesis,the investigations on the internal quantum efficiency(IQE)and light extraction efficiency(LEE)for AlGaN DUV LEDs have been carried out to enhance the EQE.In order to enhance the injection efficiency of hole,and increase the concentration of holes in multiple quantum wells(MQWs),an AlGaN DUV LED with the built-in structure of p⁺-Al_0.55Ga_0.45N/Al_xGa_1-xN/n⁺-Al_0.55Ga_0.45N polarization tunnel junction(PTJ)is proposed,with Al content of x=0.4,0.55 and 0.7.In order to enhance the injection efficiency and radiation recombination efficiency of electrons and holes in MQWs,and reduce the forward voltage,an AlGaN DUV LED with the structure of Al content graded polarization p-type electron blocking layer(p-EBL)and n-type hole blocking layer(n-HBL)is proposed.In order to enhance the LEE of an AlGaN DUV LED,a technique of preparing the porous Distributed Bragg Reflector(DBR)by decomposing Ga N in Al N/Ga N DBR at high temperature is proposed.The main innovative achievements of this thesis are summarized as follows.(1)The AlGaN DUV LEDs with conventional p-type Structure A,conventional TJ Structure B(homojunction,x=0.55),PTJ Structure C(heterojunction,x=0.4)and PTJ Structure D(heterojunction,x=0.7)were simulated and compared by using APSYS software.Compared with Structure A,B and C,Structure D has the highest electron tunneling probability,the highest concentration and radiation recombination rate of electrons and holes,and the highest transverse current expansion.It means that Structure D can enhance the hole injection efficiency through electron tunneling,increase the concentration of electrons and holes in MQWs,and improve the optical output power and IQE of AlGaN DUV LED.(2)The AlGaN DUV LEDs with conventional p-EBL structures,polarization p-EBL and n-HBL structure,and MQW structures with different thickness were simulated and compared by using APSYS software,epitaxially grown by metal-organic chemical vapor deposition(MOCVD),and characterized by X-ray diffraction(XRD)and electroluminescence(EL)measurements.(1)For the AlGaN DUV LEDs with conventional p-EBL structures,the concentration of electrons and holes in MQWs can be increased by increasing the Al content in p-EBL,and the optical output power and IQE of AlGaN DUV LED can be improved.However,the barrier height of p-EBL increases with the increase of Al content in p-EBL,which results in the increase of forward voltage and the decrease of wall-plug efficiency(WPE).(2)Compared with the conventional p-EBL structures,the polarization p-EBL and n-HBL structure can not only increase the concentration of electrons and holes in MQWs,and enhance the injection efficiency of electrons and holes,but also block the diffusion of electrons from MQW to p-type region and holes from MQW to n-type region,and enhance the radiation recombination efficiency of electrons and holes in MQWs.Besides,it can also reduce the forward voltage and enhance the WPE.(3)In the case of the same thickness of quantum barrier(QB),with the increase of quantum well(QW)thickness,the effective state density of electrons and holes in QW increases,so the concentrations of electrons and holes in QW will increase;Under the same QW thickness,with the decrease of QB thickness,the work of electric field on electrons and holes decreases,and the transport rates of electrons and holes in MQWs decrease,which will increase the concentration uniformities of electrons and holes in MQWs.(3)The Al N/air porous DBR was prepared by decomposing Ga N in Al N/Ga N DBR at high temperature.Even if Al N/Ga N DBR has incomplete periodic structure and uneven thickness of Al N and Ga N,compared with Al N/Ga N DBR without annealing at high temperature,the reflectivity of Al N/air porous DBR annealed at high temperature increases from 10%to 50%(5 times),with bandwidth of 114.0 nm.It means that the approach of preparing Al N/air porous DBR structure by decomposing Ga N in Al N/Ga N DBR at high temperature is very effective,which can efficiently improve the reflectivity and bandwidth of DBR structure at the central wavelength.