Dephasing rates for weak localization and universal conductance fluctuations in two dimensional Si:P and Ge:P δ-layers.

We report quantum transport measurements on two dimensional (2D) Si:P and Ge:P δ-layers and compare the inelastic scattering rates relevant for weak localization (WL) and universal conductance fluctuations (UCF) for devices of various doping densities (0.3-2.5 × 1018 m-2) at low temperatures (0.3-4.2 K). The phase breaking rate extracted experimentally from measurements of WL correction to conductivity and UCF agree well with each other within the entire temperature range. This establishes that WL and UCF, being the outcome of quantum interference phenomena, are governed by the same dephasing rate.

Morphology and chemical composition of cobalt germanide islands on Ge(001).

The reactive growth of cobalt germanide on Ge(001) was investigated by means of in situ x-ray absorption spectroscopy photoemission electron microscopy (XAS-PEEM), micro-illumination low-energy electron diffraction (µ-LEED), and ex situ atomic force microscopy (AFM). At a Co deposition temperature of 670 °C, a rich morphology with different island shapes and dimensions is observed, and a correlation between island morphology and stoichiometry is found. By combining XAS-PEEM and µ-LEED, we were able to identify a large part of the islands to consist of CoGe2, with many of them having an unusual epitaxial relationship: CoGe2 [Formula: see text] [Formula: see text] Ge [Formula: see text]. Side facets with (112) and (113) orientation have been found for such islands. However, two additional phases were observed, most likely Co5Ge7 and CoGe. Comparing growth on Ge(001) single crystals and on Ge(001)/Si(001) epilayer substrates, the occurrence of these intermediate phases seems to be promoted by defects or residual strain.

Spontaneous breaking of time-reversal symmetry in strongly interacting two-dimensional electron layers in silicon and germanium.

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.

Stacking of 2D electron gases in Ge probed at the atomic level and its correlation to low-temperature magnetotransport.

Stacking of two-dimensional electron gases (2DEGs) obtained by δ-doping of Ge and patterned by scanning probe lithography is a promising approach to realize ultrascaled 3D epitaxial circuits, where multiple layers of active electronic components are integrated both vertically and horizontally. We use atom probe tomography and magnetotransport to correlate the real space 3D atomic distribution of dopants in the crystal with the quantum correction to the conductivity observed at low temperatures, probing if closely stacked δ-layers in Ge behave as independent 2DEGs. We find that at a separation of 9 nm the stacked-2DEGs, while interacting, still maintain their individuality in terms of electron transport and show long phase coherence lengths (∼220 nm). Strong vertical electron confinement is crucial to this finding, resulting in an interlayer scattering time much longer (∼1000 × ) than the scattering time within the dopant plane.

n-type doping of germanium from phosphine: early stages resolved at the atomic level.

To understand the atomistic doping process of phosphorus in germanium, we present a combined scanning tunneling microscopy, temperature programed desorption, and density functional theory study of the reactions of phosphine with the Ge(001) surface. Combining experimental and theoretical results, we demonstrate that PH(2) + H with a footprint of one Ge dimer is the only product of room temperature chemisorption. Further dissociation requires thermal activation. At saturation coverage, PH(2) + H species self-assemble into ordered patterns leading to phosphorus coverages of up to 0.5 monolayers.

Phosphorus atomic layer doping of germanium by the stacking of multiple δ layers.

In this paper we demonstrate the fabrication of multiple, narrow, and closely spaced δ-doped P layers in Ge. The P profiles are obtained by repeated phosphine adsorption onto atomically flat Ge(001) surfaces and subsequent thermal incorporation of P into the lattice. A dual-temperature epitaxial Ge overgrowth separates the layers, minimizing dopant redistribution and guaranteeing an atomically flat starting surface for each doping cycle. This technique allows P atomic layer doping in Ge and can be scaled up to an arbitrary number of doped layers maintaining atomic level control of the interface. Low sheet resistivities (280 Ω/ [symbol see text ) and high carrier densities (2 × 10(14) cm( - 2), corresponding to 7.4 × 10(19) cm( - 3)) are demonstrated at 4.2 K.

A complete fabrication route for atomic-scale, donor-based devices in single-crystal germanium.

Despite the rapidly growing interest in Ge for ultrascaled classical transistors and innovative quantum devices, the field of Ge nanoelectronics is still in its infancy. One major hurdle has been electron confinement since fast dopant diffusion occurs when traditional Si CMOS fabrication processes are applied to Ge. We demonstrate a complete fabrication route for atomic-scale, donor-based devices in single-crystal Ge using a combination of scanning tunneling microscope lithography and high-quality crystal growth. The cornerstone of this fabrication process is an innovative lithographic procedure based on direct laser patterning of the semiconductor surface, allowing the gap between atomic-scale STM-patterned structures and the outside world to be bridged. Using this fabrication process, we show electron confinement in a 5 nm wide phosphorus-doped nanowire in single-crystal Ge. At cryogenic temperatures, Ohmic behavior is observed and a low planar resistivity of 8.3 kΩ/□ is measured.

Preparation of the Ge(001) surface towards fabrication of atomic-scale germanium devices.

We demonstrate the preparation of a clean Ge(001) surface with minimal roughness (RMS ~0.6 Å), low defect densities (~0.2% ML) and wide mono-atomic terraces (~80-100 nm). We use an ex situ wet chemical process combined with an in situ anneal treatment followed by a homoepitaxial buffer layer grown by molecular beam epitaxy and a subsequent final thermal anneal. Using scanning tunneling microscopy, we investigate the effect on the surface morphology of using different chemical reagents, concentrations as well as substrate temperature during growth. Such a high quality Ge(001) surface enables the formation of defect-free H-terminated Ge surfaces for subsequent patterning of atomic-scale devices by scanning tunneling lithography. We have achieved atomic-scale dangling bond wire structures 1.6 nm wide and 40 nm long as well as large, micron-size patterns with clear contrast of lithography in STM images.