Study Of Reflector Electrodes In GaN-based Vertical-structure LEDs

As wide-band gap semiconductors,GaN which has outstanding physical and chemical properties and its ternary alloys have tunable directband gaps from 0.7eV to 6.7 eV which cover the whole visible spectral region and reaches to infrared and ultraviolet regions.GaN is widely used for semiconductor light emission devices such as light emitting diode or laser diode which LED as a kind of small volume,low energy consumption,long life new lighting source,more and more widely applied.Traditional GaN based LED withinsulating sapphire substrate and horizontal structure has current crowding effect caused by lateral extension of current inevitably which lead the device to heat dissipation problem,low light efficiency and so on.The flip-chip LED with uniform current distribution,low threshold voltage,high light extraction efficiency which can solve the problems above attracts great attention.However,the implementation of reflector electrode for flip-chip LED is hard due to thep-GaN whose work function is up to 7.5eV couldn't obtain a high concentration of holes.In this thesis,we use a new method based on the concept of decoupling the contact and mirror to fabricate NiAg metal combination as reflector electrode of blue-ray LED and NiAl metal combination as reflector electrode of ultraviolet LED,meanwhile we studied the key technique in this process.The main contents are summarized as follows:1.Separate the growth of contact layer and reflect layer,annealing twice for contact layer and entire electrode respectively.2.Using dot circular transmission line model to fitting the resistivity of the electrode and spectrophotometer to measure the reflectivity.The result has been analysed.3.For blue LED,NiAg based reflector electrode has been implemented.It was found that NiO result from contact layer annealing decrease the barrier between p-GaN and the electrode,the spreading of Ga improvehole density of p-GaN surface layer.The factors above reduce the resistivity.With long time contact layer annealing,the increasing of Ag2O around interface enhance the barrier which cause decreasing of resistivity.The reflectivity at 450nm of electrode with new process methodusing optimized conditionsreaches 94%and the resistivity was 1.31×10-3?·cm2.4.For UV-LED,the NiAl metal combine is unable to form ohmic contact with p-GaN,so we choose NiAg to implement contact layer.For the counteract between oxidation of A1 around interface and the spreading of Ga,entire electrode annealing have less infect on NiAl based electrode than NiAg ones.NiAl based reflector electrode has been implemented using the new process method,the reflectivity at 300nm reaches 84%and the resistivity was 1.34x10-3?·cm2.