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

Paper EM-MoA1
Metals are not Created Equal: Metal Barrier-layer Dependent Ohmic Performance of Ti/Al/Metal/Au Schemes on AlGaN/GaN Heterostructures

Monday, November 13, 2006, 2:00 pm, Room 2003

Session: Contacts, Interfaces and Defects in Semiconductors
Presenter: F.M. Mohammed, University of Illinois at Urbana-Champaign
Authors: F.M. Mohammed, University of Illinois at Urbana-Champaign
L. Wang, University of Illinois at Urbana-Champaign
I. Adesida, University of Illinois at Urbana-Champaign
Correspondent: Click to Email
AlGaN/GaN HEMTs are useful for application in microwave power amplification. Low-resistance ohmic contacts are required for enhanced performance of these devices. To this end, Ti/Al/MetalAu schemes, where Metal layer is a high melting-point element such as Ni, Ti, Mo, etc. have been investigated. In such a design, Ti participates in interfacial reactions yielding TiN and rendering the contact/GaN interface n-type doped. Coupling it with metals of high conductivity, such as Al, can enable direct Ohmic contact formation. However, the propensity of Al to easily oxidize upon annealing poses major reliability challenges. To solve this problem a Metal/Au bilayer cap is deposited. The Metal layer is believed to act as a barrier against the outdiffusion of Al and the indiffusion of Au. However, microstructural characterizations of Ti/Al/Mo/Au and Ta/Al/Mo/Au indicate that the Mo layer is not effective as a diffusion barrier. Therefore, a critical assessment of the presumed role of the Metal-layer acting as a diffusion barrier is needed. Here, we present the study of Ohmic performance and surface morphology of Ti/Al/Metal/Au schemes, where Metal is Ti, Mo, Pt, Ir, Ni, Ta, or Nb, carried out to identify the role of the barrier layer on contact formation and reliability of the metallizations. A strong dependence of contact resistance and specific contact resistivity, as well as surface morphology, on the type of Metal layer used was found. SEM/EDS characterization has shown evidence for alloy segregation, Metal layer fragmentation, Al-Au solid solution formation, and possible Au and/or Al reaction with Metal layer. The observed variations in Ohmic performance and surface morphology did not correlate with the properties of elemental Metal-layers. Contrary to the generally accepted opinion of the layer acting as a mere diffusion barrier layer, results from the present study provide new insights on the active role this layer plays on the characteristics of metallizations.