Study On Stress And I-V Characteristics Of Gan-basedvertical LED On Si Substrate

Because GaN-based LED is the coredevice ofsolid state lighting,it hasbeenwidely studiedrecently.GaN bulk single crystal is expensive and difficult togrow,so heterogeneous substrate is commonly used for GaN-based LED expitaxy.Atpresent,three technical routesof solid state lighting on different substrateshave beendeveloped,i.e.sapphire,silicon carbide and silicon.With the latebeginningofreaserch,GaN-based LEDon silicon substrate has become a hotspot inacademia and industrydue to its low cost,and good performance.Because the visiblelight can be absorbed bysilicon substrate,sosubstrate transfer technology is commonlyused to fabricateGaN-based vertical thin-film LED on silicon substrate.Verticalthin-film LEDhas many advantages,such as high light extraction efficiency,good heatdissipation,homogeneous current distribution.Nevertheless,there are some problemshaven’t gotdeeply understanded because of the short developing history.Forexample:(1)Silver mirror is commonlyused in vertical thin-filmLEDpreparation.Whether the silver atomsmigrate to the active regionunder workingcondition,and then affects its performance?(2)Substrate transfer technologyis usedinvertical thin-film LED preparation.Change of stress is very complex.Doesit influencethedevice performance.(3)The current distribution of vertical LEDis different fromthe traditional lateral LED,What’s the difference between their I-V characteristics,anditseffect ondevice reliability?And so on.In this thesis,theseissues werestudied.Some meaningful and innovative researchresultswereobtainedas following:1)The diffusion of silver and platinuminto GaN-based LED epitaxial filmonsilicon substrate with different defect densitywere tested by high-resolution X-raydiffraction(HRXRD) and secondary ion mass spectrometry(SIMS).we found thatsilvercan diffuseinto the epitaxial film,The higher the defectdensity,the moresilverdiffused.Platinumalmost didn’t diffuse into the epitaxial film regardless thedefect density.These observations were meaningfulto understand thedegradation mechanism of GaN-based vertical thin-film LED on silicon substrate.2)GaN-basedLEDthin films were transferred from the original Si(111) substrateto new Si substrateusing the epitaxial wafer bonding method and wetetching.Theadhesion materialused in my study weresilver andbeeswax,respectively.GaN-basedLEDthin filmswere testedby micro-ramanandphotoluminescence spectra before and after substratetransfer and atthe process ofchips fabricated technologies.The results showed that the stress of GaN filmswasextremelyreduced and uniform after substratetransfer whenthe adhesionmaterialwasbeeswax.Full width at half maximum(FWHM)of photoluminescencespectra was narrower.The results showed that flexible material is betteradhesionmaterial.3)The I-V characteristics and the external quantum efficiency ofdifferenttypeGaN-based LED are tested by variable temperature electroluminescence(VTEL)system.The results showed thattheir I-V characteristics and the external quantumefficiencywere different becsuse of different defect densityand chip structure.Idealityfactorwasclosely related to theinjection leveland temperature.Idealityfactor decreasedfirst and then increased with the increase of injection level.Idealityfactordecreasedwith thetemperatureincreasing.Minimumof ideality factor at differenttemperature can reflect the defect density level of epitaxial film.