Phosphorus molecules on Ge(001): a playground for controlled n-doping of germanium at high densities

The achievement of controlled high n-type doping in Ge will enable the fabrication of a number of innovative nanoelectronic and photonic devices. In this work, we present a combined scanning tunneling microscopy, secondary ions mass spectrometry, and magnetotransport study to understand the atomistic doping process of Ge by P2 molecules. Harnessing the one-dimer footprint of P2 molecules on the Ge(001) surface, we achieved the incorporation of a full P monolayer in Ge using a relatively low process temperature. The consequent formation of P-P dimers, however, limits electrical activation above a critical donor density corresponding to P-P spacing of less than a single dimer row. With this insight, tuning of doping parameters allows us to repeatedly stack such 2D P layers to achieve 3D electron densities up to ∼2 × 10(20) cm(-3).