Study of ohmic contacts to n-type Ge: Snowplow and laser activation

Typical contacts to n-Ge result in high specific contact resistivity (ρC) or rectifying behaviour due to Fermi level pinning (resulting in high barrier heights) and low dopant activation generally observed for n-Ge. In this work, we fabricated and studied ohmic contacts to n-type Ge prepared by Ni germanidation of samples with As ion implantation for two different approaches: (1) dopant activation by sub-melt laser anneal (LA) prior to germanidation and (2) Snowplow, i.e., dopant segregation during Ni germanidation without previous thermal activation. Low resistivity ohmic contacts of 8 × 10−7 Ω cm2 for LA and 2 × 10−5 Ω cm2 for Snowplow were obtained using circular transfer length method to determine ρC. We show contacts to be thermally stable to 350 °C. Furthermore, by studying the temperature dependence of ρC, we clarify that the underlying cause of the low contact resistivity for both approaches is the high active dopant concentration obtained. We find the results in both cases to be consistent with the thermionic field-emission mechanism for a barrier height of 0.75 eV and with active dopant concentrations of 3 × 1019 cm−3 for LA and 1.5 × 1019 cm−3 for Snowplow samples.