Fermi-Level Unpinning Technique with Excellent Thermal Stability for n-Type Germanium

A metal-interlayer-semiconductor (M-I-S) structure with excellent thermal stability and electrical performance for a nonalloyed contact scheme is developed, and considerations for designing thermally stable M-I-S structure are demonstrated on the basis of n-type germanium (Ge). A thermal annealing process makes M-I-S structures lose their Fermi-level unpinning and electron Schottky barrier height reduction effect in two mechanisms: (1) oxygen (O) diffusion from the interlayer to the contact metal due to high reactivity of a pure metal contact with O and (2) interdiffusion between the contact metal and semiconductor through grain boundaries of the interlayer. A pure metal contact such as titanium (Ti) provides very poor thermal stability due to its high reactivity with O. A structure with a tantalum nitride (TaN) metal contact and a titanium dioxide (TiO₂) interlayer exhibits moderate thermal stability up to 400 °C because TaN has much lower reactivity with O than with Ti. However, the TiO₂ interlayer cannot prevent the interdiffusion process because it is easily crystallized during thermal annealing and its grain boundaries act as diffusion path. A zinc oxide (ZnO) interlayer doped with group-III elements, such as an aluminum-doped ZnO (AZO) interlayer, acts as a good diffusion barrier due to its high crystallization temperature. A TaN/AZO/n-Ge structure provides excellent thermal stability above 500 °C as it can prevent both O diffusion and interdiffusion processes; hence, it exhibits Ohmic contact properties for all thermal annealing temperatures. This work shows that, to fabricate a thermally stable and low resistive M-I-S contact structure, the metal contact should have low reactivity with O and a low work-function, and the interlayer should have a high crystallization temperature and a low conduction band offset to Ge. Furthermore, new insights are provided for designing thermally stable M-I-S contact schemes for any semiconductor material that suffers from the Fermi-level pinning problem.