Optimization of ohmic contacts for reliable heterostructure GaAs materials

We studied silver barrier ohmic (Ni/AuGe/Ag/Au) contacts to the GaAs based HEMT structures and observed strong dependence of the cleaning procedures on the ohmic con-tact resistance (R_c), its stability and reliability. The chemical profiles of the metal con-tacts before and after alloying were measured by SIMS. Samples cleaned with the com-bined plasma O₂ and NH₄OH process exhibited excellent results:R _c ∼ 0.1–0.12 ohm-mm when alloyed in the temperature range of 440–540° C and remained stable when subjected to a 200° C and 600mA/mm stress condition for 1000 hr.