Research On P-type Electrode And AlN Template For Group ?-Nitride-based Deep Ultraviolet Light-Emitting Diode

Group ? nitride materials?AlN,Ga N,In N?have the advantages of long carrier life,excellent chemical and thermal stability etc.,which are the only known luminescent materials with wide and direct band gap,continuously adjustable emitting wavelength currently.Deep Ultraviolet Light-emitting diodes?DUV-LEDs?based on Group ? nitride materials have the advantages of small size,long working life,energy saving,and environmental friendly,thus have huge application potential in UV curing,disinfection,biomass testing etc.With the official implementation of the Minamata Convention in 2020,the production and sale of mercury-containing products will be banned globally.Therefore,DUV-LEDs are the most promising alternative to traditional ultraviolet?UV?mercury lamps.However,currently the external quantum efficiency?EQE?of Al Ga N DUV-LEDs is as low as?20%,and there is still much room for improvement.This is mainly caused by three reasons:First,The increase of the threading dislocations density?TDD?and the polarization field inside the multiple-quantum-well?MQW?during the growth of the high Al component ? nitride materials which greatly reduces the internal quantum efficiency?IQE?of the LED;Secondly,the decrease of the doping efficiency inside semiconductor materials and the increase of carrier activation energy for ? nitride materials with high Al component leads to the decrease of the electrical injection efficiency;Finally,the strong absorption of ultraviolet light by the semiconductor,electrode materials and total reflection make DUV-LED difficult to extract ultraviolet light,and consequently reduces the light extraction efficiency?LEE?of LED.Among them,LEE has the greatest impact on luminous efficiency of DUV-LED,and thus the most urgent to be improved.Since the metal electrode is opaque to the emitted ultraviolet light,flip-chip structure is generally used for DUV-LED,where the light is extracted from the substrate side.With regard to the flip-chip structure DUV-LED,high reflectivity and high transmittance is required for p-type electrode and substrate,respectively.Therefore,this article will mainly focus on the design and preparation of p-type high reflective electrode,AlN single crystal substrate that can be used for DUV-LED.As a transparent metal oxide,Mo O_xmaterial has advantages of high work function,high light transmittance and excellent hole transmission ability.Metal Al has a reflectivity of more than 90%in the DUV wavelength range.Combine these two materials,a new type of Mo O_x/Al oxide/metal heterojunction is designed to be used as the p-type high reflectivity electrode for DUV-LED.After optimizing the electrode preparation process,the reflectivity and ohmic contact performance of this electrode was characterized.When the Mo O_xthickness is 2 nm and after rapid thermal annealing at 300?,N₂atmosphere for 1 min,a reflectivity of 89.9%at a wavelength of 280 nm and a specific contact resistivity of 0.22?*cm²to the p-Ga N layer was successfully prepared for flip-chip DUV-LED.Under the input current of 6m A,the DUV-LED using Mo O_x/Al as p-type electrode has a backside luminous efficiency at 271 nm that is 2.1 times that of a conventional DUV-LED with Ni/Au p-type electrode.Both AlN films and sapphire materials have high transmittance in deep ultraviolet wavelength region.Therefore,AlN single crystal films grown on sapphire substrates can be used as epitaxial growth templates for flip-chip DUV-LEDs.What's more,the lattice constant and thermal expansion coefficient of AlN are matched with Al Ga N,so,high-quality Al Ga N epitaxial layer can be grown on a single crystal AlN template.Up to now,the production of high-quality AlN single crystal templates is mostly prepared by metal organic chemical vapor deposition?MOCVD?,physical vapor transmission?PVT?and other methods.The cost is very high and it is difficult to achieve large-scale commercial use.The magnetron sputtering technology has advantages of high growth rate,low cost,and allowing films growth with large size.High temperature thermal annealing?HTTA??>1500??can significantly improve the crystal quality of magnetron sputtered AlN films.We have combined magnetron sputtering and HTTA technology and optimized the growth parameters of AlN thin film prepared by magnetron sputtering.The optimal parameters of total pressure 0.2 Pa,Ar/N₂ratio of 65%/35%,580?substrate temperature,500 nm film thickness was obtained.AlN films was deposited on the polar C?0001?,semi-polar R?10-12?,and non-polar A?11-12?sapphire substrate with the optimal parameters.The AlN thin film before and after HTTA was characterized by powder X-ray diffraction,X-ray rocking curve,transmission spectrum,raman and infrared spectroscopy,which further enriched the theory and experimental system for AlN thin films preparation by magnetron sputtering technology,and help to accelerate the commercialization process of high quality AlN thin films.