Dislocation-induced nonuniform interfacial reactions of Ti∕Al∕Mo∕Au ohmic contacts on AlGaN∕GaN heterostructure

Transmission electron microscopy (TEM) is utilized to elucidate the postannealing interfacial microstructure of Ti∕Al∕Mo∕Au metallization with AlGaN∕GaN heterostructures to gain insight into the formation mechanism of low-resistance ohmic contacts. The reaction between the metal and the AlGaN layer does not proceed uniformly. Localized penetration through the AlGaN layer beyond the two-dimensional electron gas (2DEG) is observed while partial consumption of the AlGaN layer is noted in other areas. Analytical TEM analyses confirm that the main reaction product is TiN. A correlation between the appearance of TiN islands and threading dislocations is observed. Threading dislocations serve as short-circuit diffusion channels, and are responsible for the nonuniform reaction. TiN islands have a large total area of intimate contact with the 2DEG, and since no tunneling of electron through the AlGaN is required, a low-resistance ohmic contact is obtained.