1,720,970 research outputs found
Phosphorus atomic layer doping of germanium by the stacking of multiple delta layers
No full textIn 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
Alternative High n-Type Doping Techniques in Germanium
No full textIn 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
Atomic layer doping of strained Ge-on-insulator thin films with high electron densities
No full textWe demonstrate that phosphorous atomic layer doping in ultra-high vacuum is a viable method to obtain n-type doping of strained germanium-on-insulator thin films. By engineering single and multiple, closely-spaced P d-layers, we obtain high active electron concentrations (11020 cm3)and low electrical resistivity (120X/square) whilst keeping control over doping profile, structural integrity, and tensile strain levels (e1⁄40.35%). Investigation of magnetotransport over a large temperature range (1.7-290 K) allows observation of two-dimensional electrons’ weak localization
16 up to 30 K
Preparation of the Ge(001) surface towards fabrication of atomic-scale germanium devices
No full textWe 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
No full textWe 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 Molecules on Ge(001): A Playground for Controlled N-Doping of Germanium at High Densities
No full textThe 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
Dual-temperature encapsulation of phosphorus in germanium delta-layers toward ultra-shallow junctions
No full textWe 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
N-type doping of germanium from phosphine: early stages resolved at the atomic level
No full textTo understand the atomistic doping process of phosphorus in germanium, we present a combined scanning tunneling microscopy, temperature programmed desorption, and density functional theory study of the reactions of phosphine with the Ge(001) surface. Combining experimental and theoretical results, we demonstrate that PH2+H with a footprint of one Ge dimer is the only product of room temperature chemisorption. Further dissociation requires thermal activation. At saturation coverage, PH2+H species self-assemble into ordered patterns leading to phosphorus coverages of up to 0.5 monolayers
A Complete Fabrication Route for Atomic-Scale, Donor-Based Devices in Single-Crystal Germanium
No full textDespite 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 k Omega/square is measured
Stacking of 2D electron gases in Ge probed at the atomic-level and its correlation to low temperature magnetotransport
No full textStacking of two-dimensional electron gases (2DEGs) obtained by δ-doping of Ge and patterned by scanning probe lithography is a promising approach to realize ultra-scaled 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, whilst 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 plan
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