Change of the material properties of Ni, Pt and Au thin films and thin film stacks for GaN Schottky contacts during thermal processing

Abstract

For GaN HEMTs the gate metallization is a critical element regarding leakage current, barrier height and reliability of the devices. Therefore, it is important to analyze the material properties and changes during thermal treatment. For GaN HEMTS Ni/Au and Ni/Pt/Au contacts are frequently used. Variations of different Ni, Ni/Pt, NiAu and Ni/Pt/Au layers on silicon nitride were tested under different rapid thermal annealing conditions. The metals were investigated using resistance measurements, secondary ion mass spectrometry and x-ray diffraction analysis. It was shown that Au diffusion takes place through the Ni and Ni/Pt layers already after 20 min at 400 °C. Appropriate modifications of the Ni/Pt stack processing resulted in a decreased Au diffusion to the silicon nitride. After annealing for 20 min at 400 °C the normalized resistance of the stabilized Ni/Pt stack was 30% lower than the one of the reference Ni/Pt stack. The formation of different Ni–Pt binary alloys was proven and controlled by adjusting layer thicknesses. The impact on normalized resistance was investigated for different ratios of Ni and Pt, variations in the thermal processing and the number of deposited layers per metal species. The effects are shown to be caused by self- and inter-diffusion processes.