Fermi-level unpinning and low resistivity in contacts to n-type Ge with a thin ZnO interfacial layer

Abstract

We report low resistance Ohmic contacts on n-Ge using a thin ZnO interfacial layer (IL) capped with Ti. A 350°C post metallization anneal is used to create oxygen vacancies that dope ZnO heavily n-type (n+). Rectifying Ti/n-Ge contacts become Ohmic with 1000× higher reverse current density after insertion of n+-ZnO IL. Specific resistivity of ∼1.4×10−7 Ω cm2 is demonstrated on epitaxial n+-Ge (2.5×1019 cm−3) layers. Low resistance with ZnO IL can be attributed to (a) low barrier height from Fermi-level unpinning, (b) good conduction band alignment between ZnO and Ge, and (c) thin tunneling barrier due to the n+ doping.