Effect of chemical surface treatments on interfacial and electrical characteristics of atomic-layer-deposited Al2O3 films on Ge substrates

The Ge surfaces were cleaned and passivated by two kinds of chemical pretreatments: conventional combination of HF + (NH4)2S, and new one of HBr + (NH4)2S. The chemical states and stability at passivated Ge surfaces were carefully characterized. The influence of chemical surface treatments on the interface and electrical properties of Al2O3 gate dielectric films on Ge grown by atomic layer deposition (ALD) has been investigated deeply. It is found that the combination of HBr and (NH4)2S can remove more Ge-O bonds on the Ge surface compared to that of conventional HF and (NH4)2S with excellent stability. X-ray photoelectron spectroscopy (XPS) reveals that HBr and (NH4)2S treated Ge surface has a mixture states of GeOx (9.25%) and GeS (7.40%) while HF and (NH4)2S treated Ge surface has a mixture states of GeOx (16.45%) and GeS (3.37%). And the Ge-S peak from the surface of Ge substrates decreases a little after the HBr and (NH4)2S treated Ge surface was exposed in the ambient for 300 min, which suggests the Ge surface is stable to oxidants. The Al2O3 films on HBr and (NH4)2S treated Ge substrates exhibits better electrical properties such as large capacitance, decreased leakage current density by ∼two orders of magnitude, and less C-V hysteresis. This indicates that a reduction in charge traps possibly at the interface and more interface traps are terminated by sulfur. The surface treatment of HBr and (NH4)2S seems to be very promising in improving the quality of high-k gate stack on Ge substrates.