127 research outputs found
Encapsulation of phosphorus dopants in silicon for the fabrication of a quantum computer
The incorporation of phosphorus in silicon is studied by analyzing phosphorus
delta-doped layers using a combination of scanning tunneling microscopy,
secondary ion mass spectrometry and Hall effect measurements. The samples are
prepared by phosphine saturation dosing of a Si(100) surface at room
temperature, a critical annealing step to incorporate phosphorus atoms, and
subsequent epitaxial silicon overgrowth. We observe minimal dopant segregation
(5 nm), complete electrical activation at a silicon growth temperature of 250
degrees C and a high two-dimensional electron mobility of 100 cm2/Vs at a
temperature of 4.2 K. These results, along with preliminary studies aimed at
further minimizing dopant diffusion, bode well for the fabrication of
atomically precise dopant arrays in silicon such as those found in recent
solid-state quantum computer architectures.Comment: 3 pages, 4 figure
Microwave Properties of 2D CMOS Compatible Co-Planar Waveguides Made from Phosphorus Dopant Monolayers in Silicon
Low-dimensional microwave interconnects have important applications for nanoscale electronics, from complementary metalâoxide-semiconductor (CMOS) to silicon quantum technologies. Graphene is naturally nanoscale and has already demonstrated attractive electronic properties, however its application to electronics is limited by available fabrication techniques and CMOS incompatibility. Here, the characteristics of transmission lines made from silicon doped with phosphorus are investigated using phosphine monolayer doping. S-parameter measurements are performed between 4â26 GHz from room temperature down to 4.5 K. At 20 GHz, the measured monolayer transmission line characteristics consist of an attenuation constant of 40 dB mmâ1 and a characteristic impedance of 600 Ω. The results indicate that Si:P monolayers are a viable candidate for microwave transmission and that they have a.c. properties similar to graphene, with the additional benefit of extremely precise, reliable, stable, and inherently CMOS compatible fabrication
2D-3D crossover in a dense electron liquid in silicon
Doping of silicon via phosphene exposures alternating with molecular beam
epitaxy overgrowth is a path to Si:P substrates for conventional
microelectronics and quantum information technologies. The technique also
provides a new and well-controlled material for systematic studies of
two-dimensional lattices with a half-filled band. We show here that for a dense
(\,cm) disordered two-dimensional array of P
atoms, the full field angle-dependent magnetostransport is remarkably well
described by classic weak localization theory with no corrections due to
interaction effects. The two- to three-dimensional cross-over seen upon warming
can also be interpreted using scaling concepts, developed for anistropic
three-dimensional materials, which work remarkably except when the applied
fields are nearly parallel to the conducting planes.Comment: 9 pages, 4 figures, supplementary informatio
STM characterization of the Si-P heterodimer
We use scanning tunneling microscopy (STM) and Auger electron spectroscopy to
study the behavior of adsorbed phosphine (PH) on Si(001), as a function
of annealing temperature, paying particular attention to the formation of the
Si-P heterodimer. Dosing the Si(001) surface with 0.002 Langmuirs of
PH results in the adsorption of PH (x=2,3) onto the surface and
some etching of Si to form individual Si ad-dimers. Annealing to 350C
results in the incorporation of P into the surface layer to form Si-P
heterodimers and the formation of short 1-dimensional Si dimer chains and
monohydrides. In filled state STM images, isolated Si-P heterodimers appear as
zig-zag features on the surface due to the static dimer buckling induced by the
heterodimer. In the presence of a moderate coverage of monohydrides this static
buckling is lifted, rending the Si-P heterodimers invisible in filled state
images. However, we find that we can image the heterodimer at all H coverages
using empty state imaging. The ability to identify single P atoms incorporated
into Si(001) will be invaluable in the development of nanoscale electronic
devices based on controlled atomic-scale doping of Si.Comment: 6 pages, 4 figures (only 72dpi
Split-off dimer defects on the Si(001)2x1 surface
Dimer vacancy (DV) defect complexes in the Si(001)2x1 surface were
investigated using high-resolution scanning tunneling microscopy and first
principles calculations. We find that under low bias filled-state tunneling
conditions, isolated 'split-off' dimers in these defect complexes are imaged as
pairs of protrusions while the surrounding Si surface dimers appear as the
usual 'bean-shaped' protrusions. We attribute this to the formation of pi-bonds
between the two atoms of the split-off dimer and second layer atoms, and
present charge density plots to support this assignment. We observe a local
brightness enhancement due to strain for different DV complexes and provide the
first experimental confirmation of an earlier prediction that the 1+2-DV
induces less surface strain than other DV complexes. Finally, we present a
previously unreported triangular shaped split-off dimer defect complex that
exists at SB-type step edges, and propose a structure for this defect involving
a bound Si monomer.Comment: 8 pages, 7 figures, submitted to Phys. Rev.
Magnetic Anisotropy of Single Mn Acceptors in GaAs in an External Magnetic Field
We investigate the effect of an external magnetic field on the physical
properties of the acceptor hole states associated with single Mn acceptors
placed near the (110) surface of GaAs. Crosssectional scanning tunneling
microscopy images of the acceptor local density of states (LDOS) show that the
strongly anisotropic hole wavefunction is not significantly affected by a
magnetic field up to 6 T. These experimental results are supported by
theoretical calculations based on a tightbinding model of Mn acceptors in GaAs.
For Mn acceptors on the (110) surface and the subsurfaces immediately
underneath, we find that an applied magnetic field modifies significantly the
magnetic anisotropy landscape. However the acceptor hole wavefunction is
strongly localized around the Mn and the LDOS is quite independent of the
direction of the Mn magnetic moment. On the other hand, for Mn acceptors placed
on deeper layers below the surface, the acceptor hole wavefunction is more
delocalized and the corresponding LDOS is much more sensitive on the direction
of the Mn magnetic moment. However the magnetic anisotropy energy for these
magnetic impurities is large (up to 15 meV), and a magnetic field of 10 T can
hardly change the landscape and rotate the direction of the Mn magnetic moment
away from its easy axis. We predict that substantially larger magnetic fields
are required to observe a significant field-dependence of the tunneling current
for impurities located several layers below the GaAs surface.Comment: Non
Reaction paths of phosphine dissociation on silicon (001)
Using density functional theory and guided by extensive scanning tunneling microscopy (STM) image data, we formulate a detailed mechanism for the dissociation of phosphine (PH3) molecules on the Si(001) surface at room temperature. We distinguish between a main sequence of dissociation that involves PH2+H, PH+2H, and P+3H as observable intermediates, and a secondary sequence that gives rise to PH+H, P+2H, and isolated phosphorus adatoms. The latter sequence arises because PH2 fragments are surprisingly mobile on Si(001) and can diffuse away from the third hydrogen atom that makes up the PH3 stoichiometry. Our calculated activation energies describe the competition between diffusion and dissociation pathways and hence provide a comprehensive model for the numerous adsorbate species observed in STM experiments
Towards the fabrication of phosphorus qubits for a silicon quantum computer
The quest to build a quantum computer has been inspired by the recognition of
the formidable computational power such a device could offer. In particular
silicon-based proposals, using the nuclear or electron spin of dopants as
qubits, are attractive due to the long spin relaxation times involved, their
scalability, and the ease of integration with existing silicon technology.
Fabrication of such devices however requires atomic scale manipulation - an
immense technological challenge. We demonstrate that it is possible to
fabricate an atomically-precise linear array of single phosphorus bearing
molecules on a silicon surface with the required dimensions for the fabrication
of a silicon-based quantum computer. We also discuss strategies for the
encapsulation of these phosphorus atoms by subsequent silicon crystal growth.Comment: To Appear in Phys. Rev. B Rapid Comm. 5 pages, 5 color figure
Evolution of breeding plumages in birds: A multiple-step pathway to seasonal dichromatism in New World warblers (Aves: Parulidae)
Ecology and Evolution published by John Wiley & Sons Ltd Many species of birds show distinctive seasonal breeding and nonbreeding plumages. A number of hypotheses have been proposed for the evolution of this seasonal dichromatism, specifically related to the idea that birds may experience variable levels of sexual selection relative to natural selection throughout the year. However, these hypotheses have not addressed the selective forces that have shaped molt, the underlying mechanism of plumage change. Here, we examined relationships between life-history variation, the evolution of a seasonal molt, and seasonal plumage dichromatism in the New World warblers (Aves: Parulidae), a family with a remarkable diversity of plumage, molt, and life-history strategies. We used phylogenetic comparative methods and path analysis to understand how and why distinctive breeding and nonbreeding plumages evolve in this family. We found that color change alone poorly explains the evolution of patterns of biannual molt evolution in warblers. Instead, molt evolution is better explained by a combination of other life-history factors, especially migration distance and foraging stratum. We found that the evolution of biannual molt and seasonal dichromatism is decoupled, with a biannual molt appearing earlier on the tree, more dispersed across taxa and body regions, and correlating with separate life-history factors than seasonal dichromatism. This result helps explain the apparent paradox of birds that molt biannually but show breeding plumages that are identical to the nonbreeding plumage. We find support for a two-step process for the evolution of distinctive breeding and nonbreeding plumages: That prealternate molt evolves primarily under selection for feather renewal, with seasonal color change sometimes following later. These results reveal how life-history strategies and a birds\u27 environment act upon multiple and separate feather functions to drive the evolution of feather replacement patterns and bird coloration
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