Non-Alloyed Schottky and Ohmic Contacts to As-Grown and Oxygen-Plasma Treated n-Type SnO₂(110) and (101) Thin Films

Ohmic and Schottky contacts have been investigated on high quality epitaxial n-type tin oxide thin films that were grown by plasma-assisted molecular beam epitaxy. Non-alloyed Ti/Au, In, Ta, Al, Hg, Pt/Au, Pd/Au, and Au contacts were characterized by current–voltage measurements. Schottky contacts were realized using Hg, Pt/Au, Pd/Au, and Au only after an oxygen plasma treatment of the SnO2 surface with only Au showing a pronounced Schottky barrier (0.53 to 0.56 eV). On the as-grown surface, all metal contacts except Au showed ohmic properties. Specific contact resistances were measured for Ti/Au, Al, and Pt/Au. Using Ti/Au on an as-grown surface, the lowest specific contact resistance of 10-6 Ω cm2 was measured. The difference of contact properties between a plasma-treated and an untreated surface is explained in terms of different near surface free electron concentration.