4 years ago

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

Fermi-Level Unpinning Technique with Excellent Thermal Stability for n-Type Germanium
Jiyoung Kim, Changhwan Choi, Joon Hyung Shim, Hyun-Yong Yu, Tae In Lee, Gwang-Sik Kim, Seung-Hwan Kim, Changhwan Shin, Byung Jin Cho
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.

Publisher URL: http://dx.doi.org/10.1021/acsami.7b10346

DOI: 10.1021/acsami.7b10346

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