| Summary: | 碩士 === 國立交通大學 === 材料科學與工程學系 === 101 === The characteristics of Ohmic contacts on GaN high electron mobility transistors (HEMTs) have profound influence to the DC and RF performance. The most critical issue for Ohmic contacts on GaN HEMTs is the roughness of the surface morphology caused by interdiffusion of metals under high temperature annealing. The rough surface morphology will cause difficulty in E-beam lithography aligner and potential degradation in device performance and reliability.
In this study, I optimized the surface morphology of the Ohmic contact on GaN HEMTs with improved barrier layer thickness. The Ti/Al/Ti/Ni/Au (20/120/40/60/50 nm) metal scheme shows a low specific contact resistivity (ρc) of 3.9x10-6Ωcm2 and a smooth surface morphology of 27.6nm root mean square roughness (Rrms) after annealed at 870℃for 35 seconds. This metal scheme is then analyzed by transmission electron microscope (TEM) and X-ray diffractormeter (XRD) to figure out the formation mechanism of it. According to the analytic results, formation of large flat AlNi grains that lay on the TiN stripe rather than surrounded by Al-Au might be the reason for improved surface morphology. With moderate Ti and Ni barrier layer thickness, the formation of Al-based intermetallics could be confined. Moreover, the second Ti layer would form thick TiN above AlGaN that might facilitate the tunneling mechanism of Ohmic contact.
Finally, the optimized Ohmic contact was applied to make the HEMT devices, and the DC characteristics are as shown below: VDmax about 490mA/mm at Vg =0V, peak gm about 170mS/mm and VBR larger than 100V.
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