AVS 53rd International Symposium
    Electronic Materials and Processing Monday Sessions
       Session EM-MoA

Paper EM-MoA2
Tailoring the Interfacial Reactions between Ti/Al/Mo/Au Ohmic Contacts and AlGaN/GaN by Si lAyers: An Investigation by Cross-Sectional TEM

Monday, November 13, 2006, 2:20 pm, Room 2003

Session: Contacts, Interfaces and Defects in Semiconductors
Presenter: L. Wang, University of Illinois at Urbana-Champaign
Authors: L. Wang, University of Illinois at Urbana-Champaign
F.M. Mohammed, University of Illinois at Urbana-Champaign
I. Adesida, University of Illinois at Urbana-Champaign
Correspondent: Click to Email
The interfacial reactions activated during annealing between Ti-containing metallizations and GaN or AlGaN are believed to be the ohmic contact formation mechanism. During annealing, Ti extracts N from the nitride layers and forms Ti-nitrides, such as TiN, AlTi2N, etc, triggered by their low enthalpies of formation as compared to that of GaN or AlGaN. It is believed this reaction generates a highly n-type layer in GaN or AlGaN which facilitates carrier tunneling. It was widely observed that the resulting Ti-nitride layers are continuous. However, our observation of the Ti/Al/Mo/Au contacts on AlGaN/GaN indicated that TiN did not always form as a continuous layer. Discrete TiN islands were observed to grow preferentially along dislocations and penetrate through AlGaN. The TiN islands, having an electrical resistivity as low as 13 μΩ-cm depending on N content being smaller than Ti, electrically link the metal layer with the 2DEG at the AlGaN/GaN interface and make direct transport of carriers possible. This mechanism is also observed in the contacts to GaAs-based devices. In this paper, we demonstrate, with the aid of analytical TEM, that the interfacial reaction of Ti/Al/Mo/Au contacts on AlGaN/GaN heterostructures could be tailored by adding Si layers. Of particular importance is the Si/Al ratio. At low Si/Al ratio [0.138 and 0.276 (atomic ratio)], discrete metal islands rich in Al and Au form along dislocations and penetrate through the AlGaN layer. At medium ratio of 0.415, no island formation along dislocation is observed, rather a continuous layer of AlN forms at the interface. In both low and medium Si/Al ratio cases, Si dissolves in the metal and no silicide is perceived. At high Si/Al ratio (0.829), silicides, such as TiSi and MoSi2, are observed to precipitate. The observed reaction pathways are discussed in light of the Al-Si phase diagram. Meanwhile, the effects of the reactions on ohmic characteristics of different contacts are also presented.