Effect of Temperature Annealing on Electrical and Optical Properties of Ni/Ag/ITO p-Type contacts to p-GaN for Blue Light-Emitting Diodes Applications

The transmittance of thin metal contacts on p-GaN plays a crucial role in the performance of conventional GaN based light emitting diodes (LEDs).In this paper, the effect of temperature annealing on electrical and optical properties of Ni/Ag/ITO (3/2/100 nm) contacts were studied and investigated. Ni/Ag/ITO multi layer metals were deposited on patterned TLM structures at average base pressure of ~ 2 x 10-6 torr on metal organic chemical vapor deposition (MOCVD) grown LED structure with top p-GaN (2 x 1017 cm-3) by electron beam evaporation. The size of TLM pads were 200 x 200 µm2 with a gap spacing of 5, 10, 15, 20, 25, 30 and 35 µm, respectively. The as-deposited Ni/Ag/ITO contacts show non-linear current-voltage (I-V) characteristics. However, the I-V characteristics of the annealed contacts in the temperature range of 500 0C to 600 0C in N2+O2 ambient for 5 min. in rapid thermal annealing system (RTA) become linear i.e. ohmic. The lowest specific contact resistance (ρc) was obtained ~ 1.9 x 10-2 Ω-cm2 for contacts annealed at 500 0C and highest (1.02 x 10-1 Ω-cm2) for as-deposited. The transparency of as-deposited Ni/Ag/ITO film was measured using) spectrophotometer and found 0-3% in the wavelength range of 350-550 nm. The highest transparency of the contacts was 85.8 %, when the contact was annealed at 550 0C at wavelength of 460 nm. On the other hand, transparency was ~ 83.5 % and ~ 81% for contacts annealed at 500 0C and 600 0C, respectively. High resolution X-ray diffraction (HRXRD) was performed to analyze the transmittance of the contacts. The increase in transparency of Ni/Ag/IT contacts is attributed to increase in the crystalline property of the contacts during annealing and confirmed by increase in intensity counts of diffraction peak ITO (222)in XRD measurements.