Second-harmonic generation as characterization tool for Ge/high-k dielectric interfaces

Because the germanium native oxide constitutes a poor dielectric, building metal oxide semiconductors (MOS) gate stacks on Ge requires passivation of the interface between the dielectric and the Ge channel. Different approaches to perform this passivation are available: GeO2 growth prior to high-k depositing, sulphur passivation, etc. The interface properties of these MOS stacks are important, because they determine the electrical properties of the whole structure. Dangling bonds introduce extra energy levels within the band gap, which results in a loss of efficiency in switching a MOS - field effect transistor on and off. Fixed charges near the interface enlarge the voltage needed for switching between on and off state as well. Hence, characterizing these interfaces is a key challenge in semiconductor fabrication. This can for example be achieved using Second Harmonic Generation (SHG) to probe the interface, because SHG is an inherent surface and interface sensitive technique. In this work, we present SHG as an promising surface and interface characterization tool for semiconductors for passivated germanium samples. Different SHG responses are shown for germanium samples with a sulphur passivated Ge or high-k dielectric on top of Si. We show that the oxide layer as such is not probed by SHG and that different bonds over the Ge/oxide interface result in a difference SHG response.