NIGE-BASED OHMIC CONTACTS TO N-TYPE GAAS .2. EFFECTS OF AU ADDITION

Our efforts have been continued to improve the electrical properties of NiGe ohmic contacts by adding a small amount of a third element to the NiGe contacts without deteriorating thermal stability and surface smoothness. In the present study, Au was chosen as the third element, and the optimum conditions to produce thermally stable, low resistance ohmic contacts were determined by preparing a variety of contacts with different thickness ratios of the Ni, Ge, and Au layers. The best ohmic contact was prepared by depositing sequentially Ni (40 nm), Au (5 nm), and Ge (100 nm) onto the n-type GaAs substrate, and annealing at 450-degrees-C for 5 s. This contact provided the contact resistance of about 0.2 OMEGA mm, which is lower than that of the NiGe(In) contacts. The present contact had smooth surface after contact formation and showed excellent thermal stability during isothermal annealing at 400-degrees-C. The cross-sectional observation using high-resolution electron microscopy indicated that the GaAs/metal interface was uniform and the diffusion depth of the contact metal to the GaAs substrate was shallow (approximately 20 nm). These contact properties are desirable for future GaAs very large scale integration devices.