Thermal stability of tungsten ohmic contacts to the graded‐gap InGaAs/GaAs/AlGaAs heterostructure

Nonalloyed tungsten ohmic contacts to the Inx Ga1−x As/GaAs/Aly Ga1−y As (x=0.4, y=0.25) graded‐gap heterostructure showed excellent surface morphology after annealing of up to 1000 °C for 10 s. The minimum value for the specific contact resistance (1.3×10−6 Ω cm2 ) was obtained following annealing at 600 °C for 10 s. The increase in contact resistivity at higher temperatures was related to the increase in the heterostructure sheet resistance due to layer intermixing and In and Ga outdiffusion. The W/InGaAs graded‐gap refractory ohmic contact can be used as a self‐aligned emitter in heterojunction bipolar transistors.