124 research outputs found
Suppression of low-frequency noise in two-dimensional electron gas at degenerately doped Si:P \delta-layers
We report low-frequency 1/f noise measurements of degenerately doped Si:P
\delta-layers at 4.2K. The noise was found to be over six orders of magnitude
lower than that of bulk Si:P systems in the metallic regime and is one of the
lowest values reported for doped semiconductors. The noise was found to be
nearly independent of magnetic field at low fields, indicating negligible
contribution from universal conductance fluctuations. Instead interaction of
electrons with very few active structural two-level systems may explain the
observed noise magnitudeComment: 4 pages, 4 figure
Measurement of phosphorus segregation in silicon at the atomic-scale using STM
In order to fabricate precise atomic-scale devices in silicon using a
combination of scanning tunnelling microscopy (STM) and molecular beam epitaxy
it is necessary to minimize the segregation/diffusion of dopant atoms during
silicon encapsulation. We characterize the surface segregation/diffusion of
phosphorus atoms from a -doped layer in silicon after encapsulation at
250C and room temperature using secondary ion mass spectrometry
(SIMS), Auger electron spectroscopy (AES), and STM. We show that the surface
phosphorus density can be reduced to a few percent of the initial
-doped density if the phosphorus atoms are encapsulated with 5 or 10
monolayers of epitaxial silicon at room temperature. We highlight the
limitations of SIMS and AES to determine phosphorus segregation at the
atomic-scale and the advantage of using STM directly
Effective mass theory of monolayer \delta-doping in the high-density limit
Monolayer \delta-doped structures in silicon have attracted renewed interest
with their recent incorporation into atomic-scale device fabrication strategies
as source and drain electrodes and in-plane gates. Modeling the physics of
\delta-doping at this scale proves challenging, however, due to the large
computational overhead associated with ab initio and atomistic methods. Here,
we develop an analytical theory based on an effective mass approximation. We
specifically consider the Si:P materials system, and the limit of high donor
density, which has been the subject of recent experiments. In this case,
metallic behavior including screening tends to smooth out the local disorder
potential associated with random dopant placement. While smooth potentials may
be difficult to incorporate into microscopic, single-electron analyses, the
problem is easily treated in the effective mass theory by means of a jellium
approximation for the ionic charge. We then go beyond the analytic model,
incorporating exchange and correlation effects within a simple numerical model.
We argue that such an approach is appropriate for describing realistic,
high-density, highly disordered devices, providing results comparable to
density functional theory, but with greater intuitive appeal, and lower
computational effort. We investigate valley coupling in these structures,
finding that valley splitting in the low-lying \Gamma band grows much more
quickly than the \Gamma-\Delta band splitting at high densities. We also find
that many-body exchange and correlation corrections affect the valley splitting
more strongly than they affect the band splitting
3-Dimensional Tuning of an Atomically Defined Silicon Tunnel Junction
A requirement for quantum information processors is the in-situ tunability of
the tunnel rates and the exchange interaction energy within the device. The
large energy level separation for atom qubits in silicon is well suited for
qubit operation but limits device tunability using in-plane gate architectures,
requiring vertically separated top-gates to control tunnelling within the
device. In this paper we address control of the simplest tunnelling device in
Si:P, the tunnel junction. Here we demonstrate that we can tune its conductance
by using a vertically separated top-gate aligned with +-5nm precision to the
junction. We show that a monolithic 3D epitaxial top-gate increases the
capacitive coupling by a factor of 3 compared to in-plane gates, resulting in a
tunnel barrier height tunability of 0-186meV. By combining multiple gated
junctions in series we extend our monolithic 3D gating technology to implement
nanoscale logic circuits including AND and OR gates
Superexchange coupling of donor qubits in silicon
Atomic engineering in a solid-state material has the potential to
functionalize the host with novel phenomena. STM-based lithographic techniques
have enabled the placement of individual phosphorus atoms at selective lattice
sites of silicon with atomic precision. Here, we show that by placing four
phosphorus donors spaced 10-15 nm apart from their neighbours in a linear
chain, it is possible to realize coherent spin coupling between the end dopants
of the chain, analogous to the superexchange interaction in magnetic materials.
Since phosphorus atoms are a promising building block of a silicon quantum
computer, this enables spin coupling between their bound electrons beyond
nearest neighbours, allowing the qubits to be spaced out by 30-45 nm. The added
flexibility in architecture brought about by this long-range coupling not only
reduces gate densities but can also reduce correlated noise between qubits from
local noise sources that are detrimental to error correction codes. We base our
calculations on a full configuration interaction technique in the atomistic
tight-binding basis, solving the 4-electron problem exactly, over a domain of a
million silicon atoms. Our calculations show that superexchange can be tuned
electrically through gate voltages where it is less sensitive to charge noise
and donor placement errors
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