Study On Annealing Process Of Ag/p-GaN Ohmic Contacts

GaN-based blue and green LEDs have been widely used in solid-state lighting(SSL),backlights display,large-screen display,and signal lights.GaN-based ultraviolet and deep ultraviolet LEDs have important application value and broad market demand in the fields of sterilization,biochemical detection,medical treatment,and high-density optical recording,etcWith the development of LEDs to higher drive current density and higher efficiency,GaN-based light-emitting devices have increasingly higher requirements for lower absorption in the light-emitting wave band and lower contact resistance,thus the fabrication of p-type ohmic contact with lower light absorption and lower contact resistance is extremely essential for GaN-based light-emitting devices.The Ag/p-GaN contact is an optional ohmic contact for GaN-based blue and green LEDs and ultraviolet LEDs.However,the previous research of Ag/p-GaN ohmic contact,especially of the technological process,is not systematic enough,which is not conducive to obtaining stable and repeated ohmic contacts with low absorption and low resistanceThis paper mainly studies the contact performance of Ag/p-GaN ohmic contact under different parameters in annealing process,and analyzes the optimization of process conditions.The main research contents and results of the paper are as follows1.The Ag/p-GaN ohmic contact between the LED structure and the non-LED structure is compared.The p-GaN layer in the LED structure is thinner,and the abnormality of the Rtot～In(Rn/r0)linear fitting curve with a negative Y-axis intercept is observed using the D·CTLM model,which can be attributed to the decrease in the bulk resistivity of the contact region of p-GaN during the annealing;2.When the annealing temperature is between 350℃ and 450℃ and O2:N2 ≤1:10,the contact resistance generally decreases with the increase of oxygen concentration.Oxygen has significant influences on the formation of ohmic contact and the reduction of contact resistance,which can be attributed to the proper amount of oxygen promoting the activation of Mg acceptor.Annealing under pure nitrogen results in poor ohmic contact performance,but even at 450℃ its surface morphology is relatively flat,indicating that oxygen also plays an important role in Ag agglomeration3.There is a coupling relation between the effects of annealing temperature,oxygen concentration,and annealing time on the ohmic contact.Under the premise of forming a low contact resistance,the effect of annealing temperature on the surface morphology and reflectivity of Ag layer exceeds the oxygen concentration and annealing time,and the reflectivity is lowest at 450℃.For blue and green LEDs,on the basis of ensuring low ohmic contact,combined with the size of the process window and reflectivity,we recommend that the optimal annealing condition is 400℃,the concentration of 02:N2 is 1:10,and the annealing time is between 60～90s.Through the experiment with the annealing temperature of 400℃,the O2:N2 ratio of 1:10 and the annealing time of 60s,the contact resistance as low as 1.21 ×10-4/Ω·cm2 is achieved.