Study On The Formation Mechanism Of Ag-based P-GaN Ohmic Contact Layer With High Reflectivity

In recent years,with the rapid development of GaN-based LED lighting technology,the electro-optical conversion efficiency of LED chips has been continuously improved,and LED lighting products have also spread to thousands of households.The silicon substrate vertical structure LED chip has good heat dissipation performance and high light extraction efficiency,and is widely used in high power LED devices such as flashlights,automobile headlights and mobile phone flash lamps.The high reflectivity p-GaN ohmic contact electrode is the key to improve the electro-optical conversion efficiency and light extraction efficiency of the LED chip.Ag,which has the highest efficiency of visible light reflection,is the preferred metal material for p-GaN reflector,but it is difficult to form a good ohmic contact directly because of the large difference in work function with p-GaN.Metal Ni can improve the electrical contact performance between Ag and p-GaN,but the formation mechanism of ohmic contact is still unclear,and the problem of the influence of the presence of Ni on Ag reflectivity has not been solved.In this thesis,the GaN-based green LED chip on silicon substrate is taken as the experimental object.The influence mechanism of Ni on the contact performance of Ag/p-GaN interface is studied.The effect of Ni of Ag-based p-GaN reflective electrode on the photoelectric performance of LED device is discussed.The influence of Ni metal annealing temperature on the activation of Mg in p-GaN yielded the following research results:1.By comparing the reflectivity of different structure Ag-based p-GaN mirrors,it is found that the presence of Ni at the interface does hinder the reflection performance of Ag-based p-GaN reflector.By comparing the reflectance changes of the reflector before and after the anneal,it was confirmed that the alloy oxidized and reversed gradually leaving the Ag/p-GaN interface.Using secondary ion mass spectrometer?SIMS?and X-ray photoelectron spectroscopy?XPS?,it was found that after Ni-assisted treatment,a small amount of Ni remained on the surface of p-GaN,and it was confirmed to exist in the form of Ni₂O₃.It is also reported for the first time that the oxide of Ni on the surface of p-GaN is Ni₂O₃.The surface states of p-GaN before and after Ni-assisted treatment were characterized by XPS.It was found that the peak of Ga 2p3 binding energy on the surface of p-GaN shifted to 0.3 eV in the low energy direction after Ni-assisted treatment,indicating that the p-GaN surface Fermi energy The top-level valence band movement reduces the band bending of the Ag/p-GaN contact and reduces the Schottky barrier height of the contact,thereby improving the ohmic contact performance between Ag and p-GaN.It is concluded that the mechanism of Ni affecting the ohmic contact performance of Ag-based p-GaN is that the ohmic contact performance between Ag and p-GaN mainly depends on the surface state of p-GaN,especially the oxide layer dominated by Ga₂O₃.The effect of Ni on the p-GaN surface will take up O in Ga₂O₃ to form Ni₂O₃,which is consumed to reduce the surface Fermi level of p-GaN,thereby improving the ohmic contact performance between Ag and p-GaN.After alloying in oxygen,the p-GaN surface is re-oxidized to form a stable state of Ga₂O₃,and the interface ohmic contact performance is reduced to the native state level.2.By comparing the photoelectric properties of LED chips integrated with different Ag-based p-GaN reflectors,it is found that Ni at the Ag/p-GaN interface will reduce the reflectivity of Ag,which leads to the optical power and external quantum efficiency of the LED chip.Even if Ni₂O₃ remains after Ni-assisted treatment,it does not affect the optical performance of the LED chip due to the small amount.Though the LED chip integrated with the pure Ag-based p-GaN reflector without Ni has better optical performance,the operating voltage of that at the same large current is significantly higher.It is considered that this is because without the regulation of Ni2O3,the electrical properties of pure Ag based p-GaN mirror have been seriously destroyed in the subsequent high temperature process,resulting in the increase of the working voltage of LED chip.The electrical performance of the reflow process has been severely damaged,resulting in an increase in the operating voltage of the LED chip.The LED chip integrated with the Ag-based p-GaN reflector of the Ni-assisted treatment has not only the same high light integrated with the pure Ag reflector LED chip.The optical power and external quantum efficiency also have the same low operating voltage of the LED chip integrated with the NiAg-based reflector.Compared with the three LED chips,the LED chip integrated with the pure Ag reflector with the Ni-assisted treatment has the highest electro-optic efficiency.3.The effects of different activation temperatures on the photoelectric properties of LED chips under the action of Ni metal on p-GaN surface were investigated experimentally.The mechanism of Ni on Mg activation concentration was discussed.It is found that the annealing temperature is below 350°C,with the increase of temperature,Ni can promote the activation of Mg,generate more holes,and improve the photoelectric performance of the LED chip.When the annealing temperature is higher than 350°C,Ni promotes the increase of N vacancies in p-GaN,resulting in a decrease in hole concentration,thereby reducing the photoelectric performance of the LED chip.By comparing the working voltages of the LED chips integrated with Ni-coated annealing Ag-based reflectors and NiAg-based reflectors,the mechanism of Ni on the ohmic contact of Ag/p-GaN is verified.It is considered that the existence of Ni₂O₃ can improve the thermal stability of ohmic contact properties of Ag/p-GaN and ensure that the electrical properties of Ag-based reflectors are not affected by the high temperature process.This paper also improves the test method of ohmic contact performance between metal/p-GaN,and uses plasma etching process to assist ohmic contact resistance measurement.Insulating the p-GaN surface around the measurement area by plasma etching can avoid the lateral transmission of the test current on the p-GaN mesa,eliminate the parasitic resistance,reduce the measurement error,and improve the stability of the measurement data..