Investigation On Mechanism And Method Of Improving Wall-plug Efficiency Of Deep Ultraviolet LED

The present development progress of ultraviolet(UV)light-emitting diodes(LEDs)towards a target of shorter wavelength and higher light output power.However,the overall photoelectric conversion efficiency of deep UV LED is still at a low level,which is commonly no more than 10%,thus,90% of the energy is wasted.This greatly limits the applications of deep UV LED,and makes the process,method and technology to improve its Wall-plug efficiency(WPE)as well as overall performance even more difficult.In order to obtain the deep UV LED device with high WPE,the WPE is decomposed into some basic efficiencies and analyzed in this dissertation.According to its physical mechanism,different improvement methods and processes are used for different basic efficiency to carry out relevant research.Combined with simulation and experiment more in-depth research is carried out,and finally the optimized technologies are applied to the design and fabrication of deep UV LED with high WPE.The main contents of this paper include:(1)Based on the polarization self-screening effect and polarization bulk charge theory,through the gradual design of quantum well components,the polarization bulk hole doping of quantum well can be realized without external Mg doping,which can improve the low hole carrier concentration of quantum well in the conventional deep UV LED.The relationship between the gradient slope of quantum well components and the overlapping of carriers wavefunction,the radiation recombination efficiency is established.The simulation results are verified by experimental.The results show that composition design of quantum well with aluminum composition changing from high to low can compensate the polarization electric field in the conventional quantum well,reduce the QCSE(Quantum Confinement Stark Effect)caused by spontaneous polarization and piezoelectric polarization.The wavefunction overlapping of electron and hole in the quantum well increased from 40.4% to 58.6%,the radiation recombination efficiency more than 50% is achieved under the large current density.(2)Starting from the design of epitaxial layer structure of deep UV LED,the main factors affecting carrier injection efficiency in the energy band are investigated.Different epitaxial layer structure is used to adjust the energy band that improve carrier injection efficiency.The methods include using superlattice structure to replace the last quantum barrier(LB),and using gradual electronic blocking layer(EBL)structure to replace the conventional EBL.Based on the simulation,the special epitaxial layer structure is designed precisely.The analysis and research are carried out in many aspects,such as band bending,barrier height of carriers,carrier concentration in the quantum well,and leakage carrier concentration.The simulation results show that the LB/EBL interface is the key factor to determine the carrier injection efficiency.On one hand,the superlattice structure is used to confine the electron carriers,which can increase the height of the electronic barrier from 737 me V to 852 me V,then reduce the electron leakage.On the other hand,the gradual EBL is used to decrease the height of the hole barrier from 395 me V to 303 me V,enhance the hole carrier injection ability.Finally,by adopted the two methods,the carrier injection efficiency is enhanced by 20% ? 30%.(3)Aiming at the low extraction efficiency of deep UV LED,the research on sidewall etching angle is put forward,explores the relationship between deep UV light of different polarization modes and sidewall etching angle,to realizes the effective sidewall extraction of TM mode.The optimal etching angle of sidewall is about 38 ° for 280 nm deep UV LED based on experimenta.In addition,on the surface of the flip-chip deep UV LED device,the influence of the double-layer nanoarrays on the LEE is studied.The enhancement mechanism of each layer of nanoarray on the LEE of different polarization modes is explored.The results show that the first layer of nanoarray on the sapphire substrate has a significant enhancement effect on TE mode,almost no enhancement on TM mode.But,the second layer nanoarray film has no selectivity for different modes of deep UV light,which both has obvious enhancement.The LEE almost doubled under the double-layer nanoarrays structure.(4)The current injection efficiency of deep UV LED affects the emission uniformity and stability of the device.By optimizing the electrode layout of LED devices,the mechanism of different electrode position,shape on current injection and current spreading is explored.Combined with the simulation of finite element analysis,the current density distribution under different electrode layout shapes is studied,and the experiment of current distribution on temperature and wavelength stability is carried out.The experiment shows it is more desirable to place the n-electrode on the device edge,and more edge contacts between N / P electrodes will be conducive to the current spreading and reducing the current crowding effect,which can lead to a large area of more uniform emission.