2, University of Nebraska-Lincoln, Lincoln, Nebraska, United States
3, Chinese Academy of Sciences, Beijing, , China
Germanium is an attractive alternative to silicon due to its substantially higher carrier mobility and good lattice match with GaAs that can facilitate materials integration for high-performance electronics and optoelectronics. But its oxides are unstable and have poor electronic properties. Surface passivation by chalcogens could potentially endow Ge surfaces with properties similar to those of 2D metal chalcogenides, in particular a very low chemical reactivity and complete elimination of dangling bonds. For transition metals, direct sulfurization has been reported as a way of producing high quality few- and single-layer MoS2 and WS2. In the case of Ge, a similar sulfurization might protect the surface against oxidation by formation of germanium sulfide (GeS), a layered chalcogenide semiconductor with a direct bandgap in the visible range and promising optoelectronic and electronic properties.
Here we report solid-state reactions of Ge with sulfur involving exposure to S vapor at elevated substrate temperatures and near atmospheric pressure to produce Ge-sulfides on extended Ge(100) and (111) surfaces. We analyze the reaction kinetics and derive the activation energy of the rate-limiting step of the sulfurization reaction through X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy on samples exposed to the same S dose at different temperatures. Our results show that the sulfurization process gives rise to a sequence of GeSx phases, terminating in few nanometer layers of GeS2. The thickness evolution is thermally activated with a barrier EA = 0.68 eV, attributed to a reaction-limiting diffusion process through the sulfide layer. GeSx terminated Ge exhibits excellent long-term stability against oxidation in air, demonstrating the ability of controllably producing Ge-chalcogenide passivation layers via solid-state reactions in S vapor .
In addition to these results for flat Ge surfaces, we discuss the sulfurization of Ge nanowires. Semiconductor/chalcogenide core-shell nanowires could become heterostructure materials for photovoltaics, photo-electrochemistry, and optoelectronics. Ge nanowires synthesized by the vapor-liquid-solid (VLS)  method and exposed to sulfur vapor at different temperatures and times show the controlled formation of GeS shells. Single nanowire cathodoluminescence measurements establish the optoelectronic properties resulting in these novel one-dimensional core-shell heterostructures .
 H. Chen, C. Keiser, S. Du, H-J. Gao, E. Sutter, and P. Sutter, “Termination of Ge Surfaces with Ultrathin GeS and GeS2 Layers via Solid-State Sulfurization”, under review (2017).
 E. Sutter, B. Ozturk, and P. Sutter,”Selective Growth of Ge Nanowires by Low-Temperature Thermal Evaporation”, Nanotechnol. 19, 435607 (2008).
 C. Keiser, P. Sutter, and E. Sutter, “Controlled Formation of Hybrid Germanium/Germanium Sulfide Core-Shell Nanowire Heterostructure”, in preparation (2017).