Molecular beam deposition of Al2O3 on p-Ge(001)/Ge0.95Sn0.05 heterostructure and impact of a Ge-cap interfacial layer

We investigated the molecular beam deposition of Al2O3 on Ge0.95Sn0.05 surface with and without an ultra thin Ge cap layer in between. We first studied the atomic configuration of both Ge1−xSnx and Ge/Ge1−xSnx surfaces after deoxidation by reflection high-energy electron diffraction and resulted, respectively, in a c(4×2) and (2×1) surface reconstructions. After in situ deposition of an Al2O3 high-κ gate dielectric we evidenced using time-of-flight secondary ion mass spectroscopy analyses that Sn diffusion was at the origin of high leakage current densities in the Ge1−xSnx/Al2O3 gate stack. This damage could be avoided by inserting a thin 5-nm-thick Ge cap between the oxide and the Ge1−xSnx layer. Finally, metal-oxide-semiconductor capacitors on the Ge capped sample showed well-behaved capacitance-voltage (C-V) characteristics with interface trap density (Dit) in the range of 1012 eV−1 cm−2 in mid gap and higher close to the valence band edge.