The formation of a Sn monolayer on Ge(1 0 0) studied at the atomic scale

The growth of multi-layer germanium-tin (GeSn) quantum wells offers an intriguing pathway towards the integration of lasers in a CMOS platform. An important step in growing high quality quantum well interfaces is the formation of an initial wetting layer. However, key atomic-scale details of this process have not previously been discussed. We use scanning tunneling microscopy combined with density functional theory to study the deposition of Sn on Ge(1 0 0) at room temperature over a coverage range of 0.01 to 1.24 monolayers. We demonstrate the formation of a sub-2% Ge content GeSn wetting layer from three atomic-scale characteristic ad-dimer structural components, and show that small quantities of Sn incorporate into the Ge surface forming two atomic configurations. The ratio of the ad-dimer structures changes with increasing Sn coverage, indicating a change in growth kinetics. At sub-monolayer coverage, the least densely packing ad-dimer structure is most abundant. As the layer closes, forming a two-dimensional wetting layer, the more densely packing ad-dimer structure become dominant. These results demonstrate the capability to form an atomically smooth wetting layer at room temperature, and provide critical atomic-scale insights for the optimization of growth processes of GeSn multi-quantum-wells to meet the quality requirements of optical GeSn-based devices

Bottom-up assembly of metallic germanium

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, a light emitting medium in silicon-integrated lasers, and a plasmonic conductor for bio-sensing. 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 (1019 to 1020 cm-3) low-resistivity (10-4Ω ∙ cm) metallic germanium of precisely defined thickness, beyond the capabilities of diffusion-based doping technologies. 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

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 similar to 0.6 angstrom), low defect densities (similar to 0.2% ML) and wide mono-atomic terraces (similar to 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

New avenues to an old material: controlled nanoscale doping of germanium

We review our recent research into n-type doping of Ge for nanoelectronics and integrated photonics.We demonstrate a doping method in ultra-high vacuum to achieve high electron concentrations in Ge while maintaining atomic-level control of the doping process. We integrated this doping technique with ultrahigh vacuum scanning tunneling microscope lithography and femtosecond laser ablation micron-scale lithography, and demonstrated basic components of donor-based nanoelectronic circuitry such as wires and tunnel gaps. By repetition of controlled doping cycles we have shown that stacking of multiple Ge:P two-dimensional electron gases results in high electron densities in Ge (>1020 cm3). Because of the strong vertical electron confinement, closely stacked 2D layers – although interacting – maintain their individuality in terms of electron transport. These results bode well towards the realization of nanoscale 3D epitaxial circuits in Ge comprising stacked 2DEGs and/or atomic-scale Ge:P devices with confinement in more dimensions

Phosphorus atomic layer doping of germanium by the stacking of multiple delta layers

In this paper we demonstrate the fabrication of multiple, narrow, and closely spaced delta-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 Omega/square) and high carrier densities (2 x 10(14) cm(-2), corresponding to 7.4 x 10(19) cm(-3)) are demonstrated at 4.2 K

Dual-temperature encapsulation of phosphorus in germanium delta-layers toward ultra-shallow junctions

We have developed a dual-step encapsulation process for phosphorus in germanium delta-layers with initial low-temperature encapsulation to suppress dopant redistribution, followed by a higher temperature overgrowth to improve crystalline quality and electrical transport properties. Structural and electrical characterization shows that encapsulation of the delta-layer with a 2-nm-thick Ge layer deposited at 350 degrees C followed by Ge growth at 530 degrees C confines P donors into an atomically flat layer with limited dopant segregation, high carrier concentration and low resistivity. This doping method is promising for the fabrication of ultra-shallow junctions. (C) 2010 Elsevier B.V. All rights reserved

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 nano-electronic 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

ECS Transactions

In this paper we review the state of the art of high n-type doping techniques in germanium alternative to ion implantation. We discuss a novel technique for achieving ultra-high doping based on adsorption and thermal incorporation of P atoms from PH3 or P2 molecules into a Ge surface and subsequent encapsulation by Ge homoepitaxial growth. This process results in the formation of spatially-confined P δ-layers with planar electrically active densities as high as 1×1014 cm-2. Owing to the high morphological quality of the crystal matrix, it is possible to stack an arbitrary number of d-layers and tailor the thickness of spacer layers in between to build an electrically active donor density in excess of 1020 cm-3 in a bottom-up process

Alternative High n-Type Doping Techniques in Germanium

In this paper we review the state of the art of high n-type doping techniques in germanium alternative to ion implantation. We discuss a novel technique for achieving ultra-high doping based on adsorption and thermal incorporation of P atoms from PH3 or P2 molecules into a Ge surface and subsequent encapsulation by Ge homoepitaxial growth. This process results in the formation of spatially-confined P -layers with planar electrically active densities as high as 1×1014 cm-2. Owing to the high morphological quality of the crystal matrix, it is possible to stack an arbitrary number of -layers and tailor the thickness of spacer layers in between to build an electrically active donor density in excess of 1020 cm-3 in a bottom-up process