Study On Manufacturing Technology For GaN-based LEDs

Solid-state lighting source such as GaN-based LEDs for its energy-saving,environmental protection,long life-time,good spectral purity,strong mechanical vibration resistance,and many other advantages caused a wide range of research interests in the scientific community and industry.It is expected to replace incandescent lamp,fluorescent lamp,and become a new generation of lighting source However,there are still some challenges in GaN-LEDs.For example,low light extraction efficiency,high defect density due to heteroepitaxial growth of GaN,quantum confinement stark effect caused by the polarization of quantum wells,difficulty in ohmic contact with p-type GaN and the current crowding effect,the efficiency droop effect,etc.The main purpose of this thesis is to study how to further improve the efficiency and reliability of GaN-based LEDs,and promote its development in the field of general lighting.This thesis studies is based on processing technology of LED chip The main research conclusion summed up as follows1.By using the technique developed independently,that is,the surface modification of sapphire substrate,we solved the problem of photoresist often split from sapphire substrate successfully.On the other hand,via using multilayer metal mask technology,we solved the problem of the metal Ni mask easily shed or warp,and succeeded in the preparation of the micron scale patterned sapphire substrates.2.InGaN/GaN blue LED wafers have epitaxially grown on both PSS substrates and CSS substrates for comparison,and then low power LED chips have been fabricated.As PSS substrates improved the quality of GaN crystal,the reverse leakage current of LED was suppressed significantly.The reverse leakage current of PSS-LED is 4.8×10-8A when the reverse bias voltage is 10V,which is only about 1/5 of that of CSS-LED.Compared with CSS-LED,the light output power of PSS-LED increased significantly.The light output power is 11.39mW at 20mA and 17.05mW at 40mA,increase 32.1%and 37.5%,respectively3.The optical and electrical properties of ITO thin films as current spreading layers were studied systematically.Based on the influence of thermal annealing on optical and electrical properties of ITO thin films deposited via electron beam evaporation,optimized annealing condition of ITO films was obtained.And it is also useful to prevent the degradation of ITO thin films in LED electrode annealing process.The annealing time is uniformly 10 min,when the annealing temperature is 300?,the ITO thin film resistance reaches the minimum 6.67?/sq.However,when the annealing temperature is higher,the film resistance increases significantly.When the annealing temperature is 400?,the mean transmittance from 400nm to 800 nm reaches the maximum of 89.03%.Then the corresponding figure of merit is 17.79(unit:10-3?-1).With the increase of annealing temperature,the absorption edge of transmission spectrum shift to red gradually,it can be attributed to Burstein-Moss shift.4.The correlation between microstaructure,optical and electrical properties of ITO thin films was discussed systematically.It provides useful information to understand the aforementioned correlation of ITO thin films,and provides a reference to design optimized deposition condition of ITO thin films.The conductivity of ITO thin films is dominant by the carrier concentration,and carrier concentration is mainly influenced by the oxygen vacancy concentration.Hall mobility is mainly influenced by the lattice distortion,but grain size also plays an important role.Transmittance of ITO thin films is mainly affected by defect density and grain orientation consistency,and the defect density is dominant5.Thick photoresist was adopted as a mask layer in the dry etching process.During this process,due to the mask layer's expansion,with the increase of etching depth,mask areas enlarged gradually.Then the inclined sidewall formed naturally.The obliquity of sidewall was controlled by the ratio of the longitudinal etching rate and lateral expansion rate of mask layer.As the lateral expansion rate of mask layer can be controlled by hard-bake of lithography process.Therefore,a simple and controllable method etching GaN-base material to form the inclined side-wall was developed.6.Fabrication of HV-LED chips.As we adopted the technique developed independently to etch GaN-base materials to form inclined sidewall,HV-LEDs consisted of 10 sub-regions were fabricated successfully.According to the electrical measurement,10 sub-regions of the HV-LEDs emitted light uniformly.It means the inclined sidewall can effectively solve the problem of metal deposition of interconnected circuit.And at the same time,the quality of the sidewall insulation layer improved,it can also solve the p-n junction of sub-region shorted by sidewall metal interconnected circuit effectively.So it effectively improved the yield of HV-LED chip.When the forward current is 60mA,the corresponding forward voltage is 52V,so the input power is 3.12W.And the light output power reaches 1024.2mW.Under the same input power condition,the forward current of HV-LED is only about 1/16 of the high power DC-LED.