AVS 53rd International Symposium
    Electronic Materials and Processing Thursday Sessions
       Session EM-ThP

Paper EM-ThP17
High Thermal Stability Ag-based Ohmic Contacts for InAlAs/InGaAs/InP High Electron Mobility Transistors

Thursday, November 16, 2006, 5:30 pm, Room 3rd Floor Lobby

Session: Electronic Materials and Processing Poster Session
Presenter: I. Adesida, University of Illinois at Urbana-Champaign
Authors: L. Wang, University of Illinois at Urbana-Champaign
W.F. Zhao, University of Illinois at Urbana-Champaign
I. Adesida, University of Illinois at Urbana-Champaign
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InAlAs/InGaAs/InP HEMTs are promising for microwave applications due to their high-frequency and low-noise characteristics. Low-resistive and high thermal stable ohmic contacts are desirable to fully exploit their potential. The Au-Ge-Ni contacts used for GaAs are also applied in InP-based devices with further optimization. However, the optimum annealing temperatures of these contacts are low (~250 °C). Due to the high diffusivity of Au and low eutectic point of AuGe alloy, these contacts are intrinsically unstable when subjected to thermal processes of fabrication or electrical stresses during operation. We showed that the optimum contact resistance of Au-Ge-Ni is achievable only within a narrow window of ~50 °C which makes the reliable control of device performance problematic. Ge/Ag/Ni ohmic contacts with excellent contact resistance of 0.07 Ω-mm were obtained after annealing at 425 °C. These contacts have a large processing window of >130 °C. A SiNx layer deposited prior to annealing could suppress the degradation of the devices. Storage tests at 215 and 250 °C indicate that this metallization is far superior to the Au-based contacts in term of thermal stability. TEM studies confirm that metal spikes successfully link the 2DEG layer with the metal layer to produce excellent ohmic characteristics. The spikes are mainly Ag and Au, respectively, as identified by EDS. For the optimum annealed samples, the Ag and Au spikes have similar area density and penetration depth. This is a logical result since the injection properties/efficiencies of the metal spikes, which are dictated by the density, size, and shape of the spikes, determine the ohmic performance. The formation of liquid Au-Ge eutectic phase in Au-Ge-Ni at 300 °C and the fast diffusion of Au are believed to be the reason of overannealing. The eutectic temperature of Ag-Ge is 300 °C higher rendering Ag-contacts a higher annealing temperature and a wider processing window.