Extending chip performance beyond current limits of miniaturisation requires new materials and functionalities that integrate well with the silicon platform. Germanium fits these requirements and has been proposed as a high-mobility channel material,
[1] a light emitting medium in silicon-integrated lasers,[2,3] and a plasmonic conductor for bio-sensing.[4,5] Common to these diverse applications is the need for homogeneous, high electron densities in three-dimensions (3D). Here we use a bottom-up approach to demonstrate the 3D assembly of atomically sharp doping profiles in germanium by a repeated stacking of two-dimensional (2D) high-density phosphorus layers. This produces high-density (10^19 to 10^20 cm-3) low-resistivity (10^-4 Ohmcm) metallic germanium of precisely defined thickness, beyond the capabilities of diffusion-based doping technologies.[6] We demonstrate that free electrons from distinct 2D dopant layers coalesce into a homogeneous 3D conductor using anisotropic quantum interference measurements, atom probe tomography, and density functional theory.
We report experimental evidence of a remarkable spontaneous time reversal symmetry breaking in two dimensional electron systems formed by atomically confined doping of phosphorus (P) atoms inside bulk crystalline silicon (Si) and germanium (Ge). Weak
localization corrections to the conductivity and the universal conductance fluctuations were both found to decrease rapidly with decreasing doping in the Si:P and Ge:P $delta-$layers, suggesting an effect driven by Coulomb interactions. In-plane magnetotransport measurements indicate the presence of intrinsic local spin fluctuations at low doping, providing a microscopic mechanism for spontaneous lifting of the time reversal symmetry. Our experiments suggest the emergence of a new many-body quantum state when two dimensional electrons are confined to narrow half-filled impurity bands.
We present a systematic study of the influence of the encapsulation temperature on dopant confinement and electrical properties of Ge:P delta-doped layers. For increasing growth temperature we observe an enhancement of the electrical properties accom
panied by an increased segregation of the phosphorous donors, resulting in a slight broadening of the delta-layer. We demonstrate that a step-flow growth achieved at 530 C provides the best compromise between high crystal quality and minimal dopant redistribution, with an electron mobility ~ 128 cm^2/Vs at a carrier density 1.3x10^14 cm-2, and a 4.2 K phase coherence length of ~ 180 nm.
