Doping and Nanostructuring of Germanium with Atomic Precision

Abstract

In this thesis we demonstrate phosphorus (P)-atomic layer doping of germanium (Ge)-based materials in ultra high vacuum (UHV). We achieve high electron concentrations in the range of 10^19-10^20 cm^-3 and low resistivities (~120W/Ohm) with atomic-level control over the doping process. Using scanning tunnelling microscopy (STM), secondary ion mass spectrometry (SIMS) and magnetotransport measurements at 4 K, we detail each doping step, focusing on atomically precise doping for the fabrication of nano-electronic devices in single crystal Ge. By combining ex-situ wet processing with in-situ homoepitaxy we produce pristine Ge(001) surfaces with low defect densities (~0.2 %) and wide mono-atomic terraces (80-100 nm), which represent ideal starting surfaces to obtain uniform hydrogen resist layers suitable for STM lithography of nano-scale devices. Next, we study the adsorption and dissociation behavior of PH3 and P2 molecules on Ge(001) at the atomic level using STM. At low coverages we characterize Ge-P heterodimers and ejected Ge atoms as clear signatures for P incorporation into Ge(001) between 200 °C and 250 °C for both PH3 and P2. At saturation coverages we observe for PH3 a maximum density of incorporated P of 0.5 monolayer, whereas doping from P2 molecules can yield nearly full monolayer coverages. We then present P d-doping of bulk Ge from PH3 gas and P2 molecules to obtain confined Ge:P d-layers with active electron sheet densities between 10^13-10^14 cm-2. The low thermal budget (250 °C) required allows us to stack multiple P d-layers to increase the active carrier density in thin Ge:P films to ~2x10^20 cm^-3 with abrupt dopant profiles. Using this method we are able to dope thin epilayers of Ge-on-insulator, whilst preserving the integrity and stain (~0.35 %) of the substrates. Finally, we integrate d-doping from PH3 with STM lithography to a full fabrication route for atomic-scale, donor-based devices and explore its potential to form 3D nano-electronic architectures in Ge. We fabricate a P-doped nano-wire (5x130nm^2) in Ge with a planar resistivity of 8.3W/Ohm at 4 K. Our results pave the way for the future assembly of individual atomic-scale components such as nano-wires, tunnel gaps or quantum dots into 3D circuits in all-epitaxial germanium.