173 research outputs found
Light-Hole Gate-Defined Spin-Orbit Qubit
The selective confinement of light-holes (LHs) is demonstrated by introducing
a low-dimensional system consisting of highly tensile-strained Ge quantum well
enabling the design of an ultrafast gate-defined spin qubit under the electric
dipole spin resonance. The qubit size-dependent -factor and dipole moment
are mapped, and the parameters inducing their modulation are discussed. It is
found that the LH qubit dipole moment is 2 to 3 orders of magnitude higher than
that of the canonical heavy-hole qubit. This behavior originates from the
significant spin splitting resulting from the combined action of large cubic
and linear Rashba spin-orbit interactions that are peculiar to LHs. The qubit
relaxation rate is also affected by the strong spin-orbit interaction and
follows typically a behavior. The proposed all-group IV, direct bandgap
LH qubit provides an effective platform for a scalable qubit-optical photon
interface sought-after for long-range entanglement distribution and quantum
networks
Mid-Infrared Optical Spin Injection and Coherent Control
The optical injection of charge and spin currents are investigated in
GeSn semiconductors as a function of Sn content. These emerging
silicon-compatible materials enable the modulation of these processes across
the entire mid-infrared range. Under the independent particle approximation,
the one- and two-photon interband absorption processes are elucidated, and the
evolution of the coherent control is discussed for three different polarization
configurations. To evaluate the contribution of high-energy transitions, a
full-zone 30-band kp is employed in the calculations. It was found that,
besides the anticipated narrowing of the direct gap and the associated shift of
the absorption to longer wavelengths, incorporating Sn in Ge also increases the
one-photon degree of spin polarization (DSP) at the resonance. Moreover,
as the Sn content increases, the magnitude of the response tensors near the
band edge exhibits an exponential enhancement. This behavior can be attributed
to the Sn incorporation-induced decrease in the carrier effective masses. This
trend appears to hold also at the resonance for pure spin current
injection, at least at low Sn compositions. The two-photon DSP at the band edge
exceeds the value in Ge to reach 60 % at a Sn content above 14 %. These results
demonstrate that GeSn semiconductors can be exploited to achieve
the quantum coherent manipulation in the molecular fingerprint region relevant
to quantum sensing.Comment: 8 pages, 9 figures, with a Supporting Material fil
Combined Iodine- and Sulfur-based Treatments for an Effective Passivation of GeSn Surface
GeSn alloys are metastable semiconductors that have been proposed as building
blocks for silicon-integrated short-wave and mid-wave infrared photonic and
sensing platforms. Exploiting these semiconductors requires, however, the
control of their epitaxy and their surface chemistry to reduce non-radiative
recombination that hinders the efficiency of optoelectronic devices. Herein, we
demonstrate that a combined sulfur- and iodine-based treatments yields
effective passivation of Ge and Ge0.9Sn0.1 surfaces. X-ray photoemission
spectroscopy and in situ spectroscopic ellipsometry measurements were used to
investigate the dynamics of surface stability and track the reoxidation
mechanisms. Our analysis shows that the largest reduction in oxide after HI
treatment, while HF+(NH4)2S results in a lower re-oxidation rate. A combined
HI+(NH4)2S treatment preserves the lowest oxide ratio <10 % up to 1 hour of air
exposure, while less than half of the initial oxide coverage is reached after 4
hours. These results highlight the potential of S- and I-based treatments in
stabilizing the GeSn surface chemistry thus enabling a passivation method that
is compatible with materials and device processing
3-D Atomic Mapping of Interfacial Roughness and its Spatial Correlation Length in sub-10 nm Superlattices
The interfacial abruptness and uniformity in heterostructures are critical to
control their electronic and optical properties. With this perspective, this
work demonstrates the 3-D atomistic-level mapping of the roughness and
uniformity of buried epitaxial interfaces in Si/SiGe superlattices with a layer
thickness in the 1.5-7.5 nm range. Herein, 3-D atom-by-atom maps were acquired
and processed to generate iso-concentration surfaces highlighting local
fluctuations in content at each interface. These generated surfaces were
subsequently utilized to map the interfacial roughness and its spatial
correlation length. The analysis revealed that the root mean squared roughness
of the buried interfaces in the investigated superlattices is sensitive to the
growth temperature with a value varying from about 0.2 nm (+/- 13%) to about
0.3 nm (+/- 11.5%) in the temperature range of 500-650 Celsius. The estimated
horizontal correlation lengths were found to be 8.1 nm (+/- 5.8%) at 650
Celsius and 10.1 nm (+/- 6.2%) at 500 Celsius. Additionally, reducing the
growth temperature was found to improve the interfacial abruptness, with 30 %
smaller interfacial width is obtained at 500 Celsius. This behavior is
attributed to the thermally activated atomic exchange at the surface during the
heteroepitaxy. Finally, by testing different optical models with increasing
levels of interfacial complexity, it is demonstrated that the observed
atomic-level roughening at the interface must be accounted for to accurately
describe the optical response of Si/SiGe heterostructures.Comment: 17 A4 pages of main manuscript, 2 table, 5 figures, 20 A4 pages of
supplementary informatio
Mid-infrared emission and absorption in strained and relaxed direct bandgap GeSn semiconductors
By independently engineering strain and composition, this work demonstrates
and investigates direct band gap emission in the mid-infrared range from GeSn
layers grown on silicon. We extend the room-temperature emission wavelength
above ~4.0 {\mu}m upon post-growth strain relaxation in layers with uniform Sn
content of 17 at.%. The fundamental mechanisms governing the optical emission
are discussed based on temperature-dependent photoluminescence, absorption
measurements, and theoretical simulations. Regardless of strain and
composition, these analyses confirm that single-peak emission is always
observed in the probed temperature range of 4-300 K, ruling out defect- and
impurity-related emission. Moreover, carrier losses into thermally-activated
non-radiative recombination channels are found to be greatly minimized as a
result of strain relaxation. Absorption measurements validate the direct band
gap absorption in strained and relaxed samples at energies closely matching
photoluminescence data. These results highlight the strong potential of GeSn
semiconductors as versatile building blocks for scalable, compact, and
silicon-compatible mid-infrared photonics and quantum opto-electronics
Decoupling the effects of composition and strain on the vibrational modes of GeSn
We report on the behavior of Ge-Ge, Ge-Sn, Sn-Sn like and disorder-activated
vibrational modes in GeSn semiconductors investigated using Raman scattering
spectroscopy. By using an excitation wavelength close to E1 gap, all modes are
clearly resolved and their evolution as a function of strain and Sn content is
established. In order to decouple the individual contribution of content and
strain, the analysis was conducted on series of pseudomorphic and relaxed
epitaxial layers with a Sn content in the 5-17at.% range. All vibrational modes
were found to display the same qualitative behavior as a function of content
and strain, viz. a linear downshift as the Sn content increases or the
compressive strain relaxes. Simultaneously, Ge-Sn and Ge-Ge peaks broaden, and
the latter becomes increasingly asymmetric. This asymmetry, coupled with the
peak position, is exploited in an empirical method to accurately quantify the
Sn composition and lattice strain from Raman spectra
Mid-infrared top-gated Ge/GeSn nanowire phototransistors
Achieving high crystalline quality GeSn semiconductors at Sn
content exceeding 10\% is quintessential to implementing the long sought-after
silicon-compatible mid-infrared photonics. Herein, by using sub-20 nm Ge
nanowires as compliant growth substrates, GeSn alloys with a Sn
content of 18\% exhibiting a high composition uniformity and crystallinity
along a few micrometers in the nanowire growth direction were demonstrated. The
measured bandgap energy of the obtained Ge/GeSn core/shell
nanowires is 0.322 eV enabling the mid-infrared photodetection with a cutoff
wavelength of 3.9 m. These narrow bandgap nanowires were also integrated
into top-gated field-effect transistors and phototransistors. Depending on the
gate design, these demonstrated transistors were found to exhibit either
ambipolar or unipolar behavior with a subthreshold swing as low as 228
mV/decade measured at 85 K. Moreover, varying the top gate voltage from -1 V to
5 V yields nearly one order of magnitude increase in the photocurrent generated
by the nanowire phototransistor under a 2330 nm illumination. This study shows
that the core/shell nanowire architecture with a super thin core not only
mitigates the challenges associated with strain buildup observed in thin films
but also provides a promising platform for all-group IV mid-infrared photonics
and nanoelectronics paving the way toward sensing and imaging applications
500-period epitaxial Ge/Si0.18Ge0.82 multi-quantum wells on silicon
Ge/SiGe multi-quantum well heterostructures are highly sought-after for
silicon-integrated optoelectronic devices operating in the broad range of the
electromagnetic spectrum covering infrared to terahertz wavelengths. However,
the epitaxial growth of these heterostructures at a thickness of a few microns
has been a challenging task due the lattice mismatch and its associated
instabilities resulting from the formation of growth defects. To elucidates
these limits, we outline herein a process for the strain-balanced growth on
silicon of 11.1 nm/21.5 nm Ge/Si0.18Ge0.82 superlattices (SLs) with a total
thickness of 16 {\mu}m corresponding to 500 periods. Composition, thickness,
and interface width are preserved across the entire SL heterostructure, which
is an indication of limited Si-Ge intermixing. High crystallinity and low
defect density are obtained in the Ge/Si0.18Ge0.82 layers, however, the
dislocation pile up at the interface with the growth substate induces
micrometer-longs cracks on the surface. This eventually leads to significant
layer tilt in the strain-balanced SL and in the formation of millimeter-long,
free-standing flakes. These results confirm the local uniformity of structural
properties and highlight the critical importance of threading dislocations in
shaping the wafer-level stability of thick multi-quantum well heterostructures
required to implement effective silicon-compatible Ge/SiGe photonic devices
Excitonic Aharonov-Bohm Effect in Isotopically Pure 70Ge/Si Type-II Quantum Dots
We report on a magneto-photoluminescence study of isotopically pure 70Ge/Si
self-assembled type-II quantum dots. Oscillatory behaviors attributed to the
Aharonov-Bohm effect are simultaneously observed for the emission energy and
intensity of excitons subject to an increasing magnetic field. When the
magnetic flux penetrates through the ring-like trajectory of an electron moving
around each quantum dot, the ground state of an exciton experiences a change in
its angular momentum. Our results provide the experimental evidence for the
phase coherence of a localized electron wave function in group-IV Ge/Si
self-assembled quantum structures.Comment: 4 pages, 4 figure
Coherent X-ray diffraction imaging and characterization of strain in silicon-on-insulator nanostructures
Coherent X-ray diffraction imaging (CDI) has emerged in the last decade as a promising high resolution lens-less imaging approach for the characterization of various samples. It has made significant technical progress through developments in source, algorithm and imaging methodologies thus enabling important scientific breakthroughs in a broad range of disciplines. In this report, we will introduce the principles of forward scattering CDI and Bragg geometry CDI (BCDI), with an emphasis on the latter. BCDI exploits the ultra-high sensitivity of the diffraction pattern to the distortions of crystalline lattice. Its ability of imaging strain on the nanometer scale in three dimensions is highly novel. We will present the latest progress on the application of BCDI in investigating the strain relaxation behavior in nanoscale patterned strained silicon-on-insulator (sSOI) materials, aiming to understand and engineer strain for the design and implementation of new generation semiconductor devices
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