Impact of ammonium sulfide solution on electronic properties and ambient stability of germanium surfaces: towards Ge-based microelectronic devices

Abstract

The monolayer adsorption of sulfur on Ge(100) surfaces from aqueous (NH₄)₂S solution is an approach to saturate, i.e., to passivate broken surface bonds and hence to reduce the electrical and chemical activity of this semiconductor surface. Despite its importance in view of developing Ge-based microelectronic devices, we still lack a fundamental understanding of how this treatment modifies the electrical and chemical properties of the Ge surface. In this study, the electronic properties and ambient stability of sulfurized p-type Ge surfaces are investigated using a variety of complementary spectroscopic techniques. Based on these results we evaluate the degree of electrical and chemical passivation that can be achieved by sulfur adsorption from (NH₄)₂S solution. We find that sulfur atoms chemically bind to Ge surface atoms within the first few seconds after immersion in solution. Saturation is achieved after approximately 30 s at a maximum sulfur coverage below half of a monolayer. The Ge–S bonds have a partial ionic character, causing depletion of the majority charge carriers near the surface. The band gap measured at the surface exhibits a lower density of surface states compared to the clean Ge surface, indicating that the S/Ge surface is electrically passivated. The Ge–S bonds are preserved upon moderate exposure to ambient conditions (ca. 2 hours), but a small fraction of the sulfur is oxidized. The steady increase of the oxygen coverage with increasing exposure time suggests a growth of Ge oxides, indicating limited resistance of the sulfurized Ge surfaces to oxidizing conditions.