11 research outputs found
Utilizing multimodal microscopy to reconstruct Si/SiGe interfacial atomic disorder and infer its impacts on qubit variability
SiGe heteroepitaxial growth yields pristine host material for quantum dot
qubits, but residual interface disorder can lead to qubit-to-qubit variability
that might pose an obstacle to reliable SiGe-based quantum computing. We
demonstrate a technique to reconstruct 3D interfacial atomic structure spanning
multiqubit areas by combining data from two verifiably atomic-resolution
microscopy techniques. Utilizing scanning tunneling microscopy (STM) to track
molecular beam epitaxy (MBE) growth, we image surface atomic structure
following deposition of each heterostructure layer revealing nanosized SiGe
undulations, disordered strained-Si atomic steps, and nonconformal uncorrelated
roughness between interfaces. Since phenomena such as atomic intermixing during
subsequent overgrowth inevitably modify interfaces, we measure post-growth
structure via cross-sectional high-angle annular dark field scanning
transmission electron microscopy (HAADF-STEM). Features such as nanosized
roughness remain intact, but atomic step structure is indiscernible in ~nm-wide intermixing at interfaces. Convolving STM and HAADF-STEM data
yields 3D structures capturing interface roughness and intermixing. We utilize
the structures in an atomistic multivalley effective mass theory to quantify
qubit spectral variability. The results indicate (1) appreciable valley
splitting (VS) variability of roughly owing to alloy disorder, and
(2) roughness-induced double-dot detuning bias energy variability of order
meV depending on well thickness. For measured intermixing, atomic steps
have negligible influence on VS, and uncorrelated roughness causes spatially
fluctuating energy biases in double-dot detunings potentially incorrectly
attributed to charge disorder.Comment: 12 pages, 6 figure
Quantifying Local Structure of Complex Oxides Using Accurate and Precise Scanning Transmission Electron Microscopy
Comparison of thermoelectric properties of nanostructured Mg2Si, FeSi2, SiGe, and nanocomposites of SiGe–Mg2Si, SiGe–FeSi2
Thermoelectric properties of nanostructured FeSi2, Mg2Si, and SiGe are compared with their nanocomposites of SiGe–Mg2Si and SiGe–FeSi2. It was found that the addition of silicide nanoinclusions to SiGe alloy maintained or increased the power factor while further reduced the thermal conductivity compared to the nanostructured single-phase SiGe alloy. This resulted in ZT enhancement of Si0.88Ge0.12–FeSi2 by ∼30% over the broad temperature range of 500-950 °C compared to the conventional Si0.80Ge0.20 alloy. The Si0.88Ge0.12–Mg2Si nanocomposite showed constantly increasing ZT versus temperature up to 950 °C (highest measured temperature) reaching ZT ∼ 1.3. These results confirm the concept of silicide nanoparticle-in-SiGe-alloy proposed earlier by Mingo et al. [Nano Lett. 9, 711–715 (2009)]
Structure and Chemistry of Oxide Surface Reconstructions in III-Nitrides Observed using STEM EELS
Structure of Ultrathin Native Oxides on III–Nitride Surfaces
When pristine material
surfaces are exposed to air, highly reactive broken bonds can promote
the formation of surface oxides with structures and properties differing
greatly from bulk. Determination of the oxide structure is often elusive
through the use of indirect diffraction methods or techniques that
probe only the outermost layer. As a result, surface oxides forming
on widely used materials, such as group III-nitrides, have not been
unambiguously resolved, even though critical properties can depend
sensitively on their presence. In this study, aberration corrected
scanning transmission electron microscopy reveals directly, and with
depth dependence, the structure of ultrathin native oxides that form
on AlN and GaN surfaces. Through atomic resolution imaging and spectroscopy,
we show that the oxide layers are comprised of tetrahedra–octahedra
cation–oxygen units, in an arrangement similar to bulk θ-Al<sub>2</sub>O<sub>3</sub> and β-Ga<sub>2</sub>O<sub>3</sub>. By
applying density functional theory, we show that the observed structures
are more stable than previously proposed surface oxide models. We
place the impact of these observations in the context of key III-nitride
growth, device issues, and the recent discovery of two-dimensional
nitrides