Stemless InSb nanowire networks and nanoflakes grown on InP.

Among the experimental realization of fault-tolerant topological circuits are interconnecting nanowires with minimal disorder. Out-of-plane indium antimonide (InSb) nanowire networks formed by merging are potential candidates. Yet, their growth requires a foreign material stem usually made of InP-InAs. This stem imposes limitations, which include restricting the size of the nanowire network, inducing disorder through grain boundaries and impurity incorporation. Here, we omit the stem allowing for the growth of stemless InSb nanowire networks on an InP substrate. To enable the growth without the stem, we show that a preconditioning step using arsine (AsH3) is required before InSb growth. High-yield of stemless nanowire growth is achieved by patterning the substrate with a selective-area mask with nanohole cavities, containing restricted gold droplets from which nanowires originate. Interestingly, these nanowires are bent, posing challenges for the synthesis of interconnecting nanowire networks due to merging failure. We attribute this bending to the non-homogeneous incorporation of arsenic impurities in the InSb nanowires and the interposed lattice-mismatch. By tuning the growth parameters, we can mitigate the bending, yielding large and single crystalline InSb nanowire networks and nanoflakes. The improved size and crystal quality of these nanostructures broaden the potential of this technique for fabricating advanced quantum devices.

Mid-infrared Imaging Using Strain-Relaxed Ge1-xSnx Alloys Grown on 20 nm Ge Nanowires.

Germanium-tin (Ge1-xSnx) semiconductors are a front-runner platform for compact mid-infrared devices due to their tunable narrow bandgap and compatibility with silicon processing. However, their large lattice parameter has been a major hurdle, limiting the quality of epitaxial layers grown on silicon or germanium substrates. Herein, we demonstrate that 20 nm Ge nanowires (NWs) act as effective compliant substrates to grow extended defect-free Ge1-xSnx alloys with a composition uniformity over several micrometers along the NW growth axis without significant buildup of the compressive strain. Ge/Ge1-xSnx core/shell NWs with Sn content spanning the 6-18 at. % range are achieved and processed into photoconductors exhibiting a high signal-to-noise ratio at room temperature with a cutoff wavelength in the 2.0-3.9 µm range. The processed NW devices are integrated in an uncooled imaging setup enabling the acquisition of high-quality images under both broadband and laser illuminations at 1550 and 2330 nm without the lock-in amplifier technique.

Nuclear Spin-Depleted, Isotopically Enriched 70 Ge/28 Si70 Ge Quantum Wells.

The p-symmetry of the hole wavefunction is associated with a weaker hyperfine interaction, which makes hole spin qubits attractive candidates to implement quantum processors. However, recent studies demonstrate that hole qubits are still very sensitive to nuclear spin bath, thus highlighting the need for nuclear spin-free germanium (Ge) qubits to suppress this decoherence channel. Herein, this work demonstrates the epitaxial growth of 73 Ge- and 29 Si-depleted, isotopically enriched 70 Ge/silicon-germanium (SiGe) quantum wells. The growth is achieved by reduced pressure chemical vapor deposition using isotopically purified monogermane 70 GeH4 and monosilane 28 SiH4 with an isotopic purity higher than 99.9% and 99.99%, respectively. The quantum wells consist of a series of 70 Ge/SiGe heterostructures grown on Si wafers. The isotopic purity is investigated using atom probe tomography (APT) following an analytical procedure addressing the discrepancies caused by the overlap of isotope peaks in mass spectra. The nuclear spin background is found to be sensitive to the growth conditions with the lowest concentration of 73 Ge and 29 Si is below 0.01% in the Ge well and SiGe barriers. The measured average distance between nuclear spins reaches 3-4 nm in 70 Ge/28 Si70 Ge, which is an order of magnitude larger than in natural Ge/SiGe heterostructures. The spread of the hole wavefunction and the residual nuclear spin background in APT voluminals comparable to the size of realistic quantum dots are also discussed.

Short-wave infrared cavity resonances in a single GeSn nanowire.

Nanowires are promising platforms for realizing ultra-compact light sources for photonic integrated circuits. In contrast to impressive progress on light confinement and stimulated emission in III-V and II-VI semiconductor nanowires, there has been no experimental demonstration showing the potential to achieve strong cavity effects in a bottom-up grown single group-IV nanowire, which is a prerequisite for realizing silicon-compatible infrared nanolasers. Herein, we address this limitation and present an experimental observation of cavity-enhanced strong photoluminescence from a single Ge/GeSn core/shell nanowire. A sufficiently large Sn content ( ~ 10 at%) in the GeSn shell leads to a direct bandgap gain medium, allowing a strong reduction in material loss upon optical pumping. Efficient optical confinement in a single nanowire enables many round trips of emitted photons between two facets of a nanowire, achieving a narrow width of 3.3 nm. Our demonstration opens new possibilities for ultrasmall on-chip light sources towards realizing photonic-integrated circuits in the underexplored range of short-wave infrared (SWIR).

Polarization-Tuned Fano Resonances in All-Dielectric Short-Wave Infrared Metasurface.

The short-wave infrared (SWIR) is an underexploited portion of the electromagnetic spectrum in metasurface-based nanophotonics despite its strategic importance in sensing and imaging applications. This is mainly attributed to the lack of material systems to tailor light-matter interactions in this range. Herein, this limitation is addressed and an all-dielectric silicon-integrated metasurface enabling polarization-induced Fano resonance control at SWIR frequencies is demonstrated. The platform consists of a 2D Si/Ge0.9 Sn0.1 core/shell nanowire array on a silicon wafer. By tuning the light polarization, it is shown that the metasurface reflectance can be efficiently engineered due to Fano resonances emerging from the electric and magnetic dipoles competition. The interference of optically induced dipoles in high-index nanowire arrays offers additional degrees of freedom to tailor the directional scattering and the flow of light while enabling sharp polarization-modulated resonances. This tunablity is harnessed in nanosensors yielding an efficient detection of 10-2 changes in the refractive index of the surrounding medium.

Atomic fluctuations lifting the energy degeneracy in Si/SiGe quantum dots.

Electron spins in Si/SiGe quantum wells suffer from nearly degenerate conduction band valleys, which compete with the spin degree of freedom in the formation of qubits. Despite attempts to enhance the valley energy splitting deterministically, by engineering a sharp interface, valley splitting fluctuations remain a serious problem for qubit uniformity, needed to scale up to large quantum processors. Here, we elucidate and statistically predict the valley splitting by the holistic integration of 3D atomic-level properties, theory and transport. We find that the concentration fluctuations of Si and Ge atoms within the 3D landscape of Si/SiGe interfaces can explain the observed large spread of valley splitting from measurements on many quantum dot devices. Against the prevailing belief, we propose to boost these random alloy composition fluctuations by incorporating Ge atoms in the Si quantum well to statistically enhance valley splitting.

Electronic Signature of Subnanometer Interfacial Broadening in Heterostructures.

Interfaces are ubiquitous in semiconductor low-dimensional systems used in electronics, photonics, and quantum computing. Understanding their atomic-level properties has thus been crucial to controlling the basic behavior of heterostructures and optimizing the device performance. Herein, we demonstrate that subnanometer interfacial broadening in heterostructures induces localized energy states. This phenomenon is predicted within a theory incorporating atomic-level interfacial details obtained by atom probe tomography. The experimental validation is achieved using heteroepitaxial (Si1-xGex)m/(Si)m superlattices as a model system demonstrating the existence of additional paths for hole-electron recombination. These predicted interfacial electronic transitions and the associated absorptive effects are evaluated at variable superlattice thickness and periodicity. By mapping the energy of the critical points, the optical transitions are identified between 2 and 2.5 eV, thus extending the optical absorption to lower energies. This phenomenon is shown to provide an optical fingerprint for a straightforward and nondestructive probe of the subnanometer broadening in heterostructures.

A Light-Hole Germanium Quantum Well on Silicon.

The quiet quantum environment of holes in solid-state devices is at the core of increasingly reliable architectures for quantum processors and memories. However, due to the lack of scalable materials to properly tailor the valence band character and its energy offsets, the precise engineering of light-hole (LH) states remains a serious obstacle toward coherent optical photon-spin interfaces needed for a direct mapping of the quantum information encoded in photon flying qubits to stationary spin processors. Herein, to alleviate this long-standing limitation, an all-group-IV low-dimensional system is demonstrated, consisting of a highly tensile strained germanium quantum well grown on silicon allowing new degrees of freedom to control and manipulate the hole states. Wafer-level, high bi-isotropic in-plane tensile strain (<1%) is achieved using strain-engineered, metastable germanium-tin alloyed buffer layers yielding quantum wells with LH ground state, high g-factor anisotropy, and a tunable splitting of the hole sub-bands. The epitaxial heterostructures display sharp interfaces with sub-nanometer broadening and show room-temperature excitonic transitions that are modulated and extended to the mid-wave infrared by controlling strain and thickness. This ability to engineer quantum structures with LH selective confinement and controllable optical response enables manufacturable silicon-compatible platforms relevant to integrated quantum communication and sensing technologies.

Electronic Structure and Epitaxy of CdTe Shells on InSb Nanowires.

Indium antimonide (InSb) nanowires are used as building blocks for quantum devices because of their unique properties, that is, strong spin-orbit interaction and large Landé g-factor. Integrating InSb nanowires with other materials could potentially unfold novel devices with distinctive functionality. A prominent example is the combination of InSb nanowires with superconductors for the emerging topological particles research. Here, the combination of the II-VI cadmium telluride (CdTe) with the III-V InSb in the form of core-shell (InSb-CdTe) nanowires is investigated and potential applications based on the electronic structure of the InSb-CdTe interface and the epitaxy of CdTe on the InSb nanowires are explored. The electronic structure of the InSb-CdTe interface using density functional theory is determined and a type-I band alignment is extracted with a small conduction band offset ( ⩽0.3 eV). These results indicate the potential application of these shells for surface passivation or as tunnel barriers in combination with superconductors. In terms of structural quality, it is demonstrated that the lattice-matched CdTe can be grown epitaxially on the InSb nanowires without interfacial strain or defects. These shells do not introduce disorder to the InSb nanowires as indicated by the comparable field-effect mobility measured for both uncapped and CdTe-capped nanowires.

Atomic Pathways of Solute Segregation in the Vicinity of Nanoscale Defects.

Using GeSn semiconductor as a model system, this work unravels the atomic-level details of the behavior of solutes in the vicinity of a dislocation prior to surface segregation in strained, metastable thin layers. The dislocations appear in the 3D atom probe tomography maps as columnar regions, 3.5-4.0 nm wide, with solute concentrations 3-4 times higher than the sounding matrix. During the initial stage of phase separation, the migration of solute atoms toward the dislocation is associated with a gradual increase in Sn concentration and in density of atomic clusters, which reach 175-190 per 103 nm3 with 12-15 atoms/cluster close to dislocations. The latter provide, at advanced stages, fast diffusive channels for Sn mass-transport to the surface, thus bringing the matrix around the dislocation to the equilibrium concentration. In parallel, an increase in solute concentration (∼0.05 at. %/nm) and in the number of atomic clusters (12-16 clusters/33 nm) is observed along the dislocation core.

Moiré patterns of twisted bilayer antimonene and their structural and electronic transition.

Interlayer twisting in two-dimensional (2D) van der Waals (vdW) heterostructures often leads to a periodic moiré pattern which is a superlattice structure on top of the original atomic lattice of the 2D layers. The formation of a moiré superlattice can be accompanied by a significant structural reconstruction and ultra-flat electronic bands. The moiré superlattice is typically built with a tunable scale by controlling the rotation angle θ between the individual 2D layers. In this paper, we report the structural reconstruction and electronic transition in moiré patterns of twisted bilayer antimonene, based on Kohn-Sham density functional theory calculations. Starting from rigid moiré structures, the atomic relaxation leads to an array of high-symmetry stacking domains with soliton boundaries through a vortex-like reconstruction. For twist angle θ≤ 6.01°, the impact of the structural reconstruction on the electronic bands becomes very significant, in the appearance of flat bands at the valence band edge, and no magic angle is required for the flat bands to appear in the 2D Sb moiré patterns. Both inhomogeneous interlayer hybridization and local strain are found to be responsible for the formation of these flat electronic bands.

Pnictogens Allotropy and Phase Transformation during van der Waals Growth.

With their ns2 np3 valence electronic configuration, pnictogens are the only system to crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout the group. Light pnictogens crystallize in the A17 phase, and bulk heavier elements prefer the A7 phase. Herein, we demonstrate that the A17 of heavy pnictogens can be stabilized in antimonene grown on weakly interacting surfaces and that it undergoes a spontaneous thickness-driven transformation to the stable A7 phase. At a critical thickness of ∼4 nm, A17 antimony transforms from AB- to AA-stacked α-antimonene by a diffusionless shuffle transition followed by a gradual relaxation to the A7 phase. Furthermore, the competition between A7- and A17-like bonding affects the electronic structure of the intermediate phase. These results highlight the critical role of the atomic structure and substrate-layer interactions in shaping the stability and properties of layered materials, thus enabling a new degree of freedom to engineer their performance.

2D Antimony-Arsenic Alloys.

Alloying in group V 2D materials and heterostructures is an effective degree of freedom to tailor and enhance their physical properties. Up to date, black arsenic-phosphorus is the only 2D group V alloy that has been experimentally achieved by exfoliation, leaving all other possible alloys in the realm of theoretical predictions. Herein, the existence of an additional alloy consisting of 2D antimony arsenide (2D-Asx Sb1- x ) grown by molecular beam epitaxy on group IV semiconductor substrates and graphene is demonstrated. The atomic mixing of As and Sb in the lattice of the grown 2D layers is confirmed by low-energy electron diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. The As content in 2D-Asx Sb1- x is shown to depend linearly on the As4 /Sb4 deposition rate ratio and As concentrations up to 15 at% are reached. The grown 2D alloys are found to be stable in ambient conditions in a timescale of weeks but to oxidize after longer exposure to air. This study lays the groundwork for a better control of the growth and alloying of group V 2D materials, which is critical to study their basic physical properties and integrate them in novel applications.

3D 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 three-dimensional (3D) atomic-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, 3D atom-by-atom maps were acquired and processed to generate isoconcentration 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 0.17 ± 0.02 to 0.26 ± 0.03 nm in the temperature range of 500-650 °C. The estimated horizontal correlation lengths were found to be 8.11 ± 0.5 nm at 650 °C and 10.09 ± 0.6 nm at 500 °C. Additionally, reducing the growth temperature was found to improve the interfacial abruptness, with a 30% smaller interfacial width is obtained at 500 °C. 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.

Dynamics of Antimonene-Graphene Van Der Waals Growth.

Van der Waals (vdW) heterostructures have recently been introduced as versatile building blocks for a variety of novel nanoscale and quantum technologies. Harnessing the unique properties of these heterostructures requires a deep understanding of the involved interfacial interactions and a meticulous control of the growth of 2D materials on weakly interacting surfaces. Although several epitaxial vdW heterostructures have been achieved experimentally, the mechanisms governing their synthesis are still nebulous. With this perspective, herein, the growth dynamics of antimonene on graphene are investigated in real time. In situ low-energy electron microscopy reveals that nucleation predominantly occurs on 3D nuclei followed by a self-limiting lateral growth with morphology sensitive to the deposition rate. Large 2D layers are observed at high deposition rates, whereas lower growth rates trigger an increased multilayer nucleation at the edges as they become aligned with the Z2 orientation leading to atoll-like islands with thicker, well-defined bands. This complexity of the vdW growth is elucidated based on the interplay between the growth rate, surface diffusion, and edges orientation. This understanding lays the groundwork for a better control of the growth of vdW heterostructures, which is critical to their large-scale integration.

Alignment of semiconducting graphene nanoribbons on vicinal Ge(001).

Chemical vapor deposition of CH4 on Ge(001) can enable anisotropic growth of narrow, semiconducting graphene nanoribbons with predominately smooth armchair edges and high-performance charge transport properties. However, such nanoribbons are not aligned in one direction but instead grow perpendicularly, which is not optimal for integration into high-performance electronics. Here, it is demonstrated that vicinal Ge(001) substrates can be used to synthesize armchair nanoribbons, of which ∼90% are aligned within ±1.5° perpendicular to the miscut. When the growth rate is slow, graphene crystals evolve as nanoribbons. However, as the growth rate increases, the uphill and downhill crystal edges evolve asymmetrically. This asymmetry is consistent with stronger binding between the downhill edge and the Ge surface, for example due to different edge termination as shown by density functional theory calculations. By tailoring growth rate and time, nanoribbons with sub-10 nm widths that exhibit excellent charge transport characteristics, including simultaneous high on-state conductance of 8.0 µS and a high on/off conductance ratio of 570 in field-effect transistors, are achieved. Large-area alignment of semiconducting ribbons with promising charge transport properties is an important step towards understanding the anisotropic nanoribbon growth and integrating these materials into scalable, future semiconductor technologies.

Highly tensile-strained Ge/InAlAs nanocomposites.

Self-assembled nanocomposites have been extensively investigated due to the novel properties that can emerge when multiple material phases are combined. Growth of epitaxial nanocomposites using lattice-mismatched constituents also enables strain-engineering, which can be used to further enhance material properties. Here, we report self-assembled growth of highly tensile-strained Ge/In0.52Al0.48As (InAlAs) nanocomposites by using spontaneous phase separation. Transmission electron microscopy shows a high density of single-crystalline germanium nanostructures coherently embedded in InAlAs without extended defects, and Raman spectroscopy reveals a 3.8% biaxial tensile strain in the germanium nanostructures. We also show that the strain in the germanium nanostructures can be tuned to 5.3% by altering the lattice constant of the matrix material, illustrating the versatility of epitaxial nanocomposites for strain engineering. Photoluminescence and electroluminescence results are then discussed to illustrate the potential for realizing devices based on this nanocomposite material.

Evidence of sub-10 nm aluminum-oxygen precipitates in silicon.

In this research, ultraviolet laser-assisted atom-probe tomography (APT) was utilized to investigate precisely the behavior at the atomistic level of aluminum impurities in ultrathin epitaxial silicon layers. Aluminum atoms were incorporated in situ during the growth process. The measured average aluminum concentration in the grown layers exceeds by several orders of magnitude the equilibrium bulk solubility. Three-dimensional atom-by-atom mapping demonstrates that aluminum atoms precipitate in the silicon matrix and form nanoscopic precipitates with lateral dimensions in the 1.3 to 6.2 nm range. These precipitates were found to form only in the presence of oxygen impurity atoms, thus providing clear evidence of the longhypothesized role of oxygen and aluminum-oxygen complexes in facilitating the precipitation of aluminum in a silicon lattice. The measured average aluminum and oxygen concentrations in the precipitates are ∼10 ± 0.5 at.% and ∼4.4 ± 0.5 at.%, respectively. This synergistic interaction is supported by first-principles calculations of the binding energies of aluminum-oxygen dimers in silicon. The calculations demonstrate that there is a strong binding between aluminum and oxygen atoms, with Al-O-Al and O-Al-Al as the energetically favorable sequences corresponding to precipitates in which the concentration of aluminum is twice as large as the oxygen concentration in agreement with APT data.

Dynamics and Mechanisms of Exfoliated Black Phosphorus Sublimation.

We report on real time observations of the sublimation of exfoliated black phosphorus layers throughout annealing using in situ low energy electron microscopy. We found that sublimation manifests itself above 375 ± 20 °C through the nucleation and expansion of asymmetric, faceted holes with the long axis aligned along the [100] direction and sharp tips defined by edges consisting of alternating (10) and (11) steps. This thermally activated process repeats itself via successive sublimation of individual layers. Calculations and simulations using density functional theory and kinetic Monte Carlo allowed to determine the involved atomic pathways. Sublimation is found to occur via detachments of phosphorus dimers rather than single atoms. This behavior and the role of defects is described using an analytical model that captures all essential features. This work establishes an atomistic-level understanding of the thermal stability of exfoliated black phosphorus and defines the temperature window available for material and device processing.

Laser-Assisted Field Evaporation and Three-Dimensional Atom-by-Atom Mapping of Diamond Isotopic Homojunctions.

It addition to its high evaporation field, diamond is also known for its limited photoabsorption, strong covalent bonding, and wide bandgap. These characteristics have been thought for long to also complicate the field evaporation of diamond and make its control hardly achievable on the atomistic-level. Herein, we demonstrate that the unique behavior of nanoscale diamond and its interaction with pulsed laser lead to a controlled field evaporation thus enabling three-dimensional atom-by-atom mapping of diamond (12)C/(13)C homojunctions. We also show that one key element in this process is to operate the pulsed laser at high energy without letting the dc bias increase out of bounds for diamond nanotip to withstand. Herein, the role of the dc bias in evaporation of diamond is essentially to generate free charge carriers within the nanotip via impact ionization. The mobile free charges screen the internal electric field, eventually creating a hole rich surface where the pulsed laser is effectively absorbed leading to an increase in the nanotip surface temperature. The effect of this temperature on the uncertainty in the time-of-flight of an ion, the diffusion of atoms on the surface of the nanotip, is also discussed. In addition to paving the way toward a precise manipulation of isotopes in diamond-based nanoscale and quantum structures, this result also elucidates some of the basic properties of dielectric nanostructures under high electric field.