Phosphorus precursors reactivity versus hydrogenated Ge surface: towards a reliable self-limited monolayer doping

Three different phosphorus compounds are tested as precursors for monolayer formation on Ge (1 0 0) surface to be used as the nanoscale-controlled dopant source. By applying different deposition methodologies, the role of several deposition parameters is evaluated employing X-ray Photoelectron Spectroscopy and Grazing Incidence Extended X-Ray Absorption Fine Structure Spectroscopy. The self-limiting physisorption or chemisorption process has proved to be strongly influenced by deposition ambient conditions for phosphonates or phosphonic acids, while the adsorption of allyl-phosphine occurs through a competitive oxidation reaction instead of hydrogermylation process, even in ultra-dry conditions. The produced phosphine oxide monolayer is structurally characterized, and an explanation of its formation is presented, based on the chemical features of both the hydrogenated Ge surface and P-based molecule. The ability of the adsorbed layers in releasing P to dope Ge is tested, revealing strong thermal stability of the deposited layers that is disclosed to be directly associated with the adsorption chemistry. However, the use of the Pulsed Laser Melting technique allows achieving a homogeneous fully active doped region with a high concentration level, thus pointing to allyl-diphenyl phosphine as the best precursor used here for Ge doping purposes for a n/p junction formation.