Cross-architecture tuning of silicon and SiGe-based quantum devices using machine learning.

The potential of Si and SiGe-based devices for the scaling of quantum circuits is tainted by device variability. Each device needs to be tuned to operation conditions and each device realisation requires a different tuning protocol. We demonstrate that it is possible to automate the tuning of a 4-gate Si FinFET, a 5-gate GeSi nanowire and a 7-gate Ge/SiGe heterostructure double quantum dot device from scratch with the same algorithm. We achieve tuning times of 30, 10, and 92 min, respectively. The algorithm also provides insight into the parameter space landscape for each of these devices, allowing for the characterization of the regions where double quantum dot regimes are found. These results show that overarching solutions for the tuning of quantum devices are enabled by machine learning.

Nanostructured 3C-SiC on Si by a network of (111) platelets: a fully textured film generated by intrinsic growth anisotropy.

In this paper, we address the unique nature of fully textured, high surface-to-volume 3C-SiC films, as produced by intrinsic growth anisotropy, in turn generated by the high velocity of the stacking fault growth front in two-dimensional (111) platelets. Structural interpretation of high resolution scanning electron microscopy and transmission electron microscopy data is carried out for samples grown in a hot-wall low-pressure chemical vapour deposition reactor with trichlorosilane and ethylene precursors, under suitable deposition conditions. By correlating the morphology and the X-ray diffraction analysis we also point out that twinning along (111) planes is very frequent in such materials, which changes the free-platelet configuration.

Ge-rich graded-index Si1-xGex waveguides with broadband tight mode confinement and flat anomalous dispersion for nonlinear mid-infrared photonics.

This work explores the use of Ge-rich graded-index Si1-xGex rib waveguides as building blocks to develop integrated nonlinear optical devices for broadband operation in the mid-IR. The vertical Ge gradient concentration in the waveguide core renders unique properties to the guided optical mode, providing tight mode confinement over a broadband mid-IR wavelength range from λ = 3 µm to 8 µm. Additionally, the gradual vertical confinement pulls the optical mode upwards in the waveguide core, overlapping with the Ge-rich area where the nonlinear refractive index is larger. Moreover, the Ge-rich graded-index Si1-xGex waveguides allow efficient tailoring of the chromatic dispersion curves, achieving flat anomalous dispersion for the quasi-TM optical mode with D ≤ 14 ps/nm/km over a ~1.4 octave span while retaining an optimum third-order nonlinear parameter, γeff. These results confirm the potential of Ge-rich graded-index Si1-xGex waveguides as an attractive platform to develop mid-IR nonlinear approaches requiring broadband dispersion engineering.

Anisotropic extended misfit dislocations in overcritical SiGe films by local substrate patterning.

In this work we will show how local substrate patterning leads to a long range controlled propagation of dislocations in SiGe films grown on Si(001) substrates. Dislocations preferentially nucleate in the inhomogeneous strain field associated with the patterned pits, and then partialize on the local (111) surfaces which form the pit sidewalls. The resulting V-shaped defects extend for several microns and effectively block the propagation of randomly nucleated dislocations which propagate in the perpendicular direction. The surface morphology and strain fields associated with the extended defects have been characterized by atomic force microscopy and µRaman spectroscopy, and the defects have been directly observed with high resolution transmission electron microscopy.

Extending the emission wavelength of Ge nanopillars to 2.25 µm using silicon nitride stressors.

The room temperature photoluminescence from Ge nanopillars has been extended from 1.6 µm to above 2.25 µm wavelength through the application of tensile stress from silicon nitride stressors deposited by inductively-coupled-plasma plasma-enhanced chemical-vapour-deposition. Photoluminescence measurements demonstrate biaxial equivalent tensile strains of up to ∼ 1.35% in square topped nanopillars with side lengths of 200 nm. Biaxial equivalent strains of 0.9% are observed in 300 nm square top pillars, confirmed by confocal Raman spectroscopy. Finite element modelling demonstrates that an all-around stressor layer is preferable to a top only stressor, as it increases the hydrostatic component of the strain, leading to an increased shift in the band-edge and improved uniformity over top-surface only stressors layers.

Ordered arrays of embedded Ga nanoparticles on patterned silicon substrates.

We fabricate site-controlled, ordered arrays of embedded Ga nanoparticles on Si, using a combination of substrate patterning and molecular-beam epitaxial growth. The fabrication process consists of two steps. Ga droplets are initially nucleated in an ordered array of inverted pyramidal pits, and then partially crystallized by exposure to an As flux, which promotes the formation of a GaAs shell that seals the Ga nanoparticle within two semiconductor layers. The nanoparticle formation process has been investigated through a combination of extensive chemical and structural characterization and theoretical kinetic Monte Carlo simulations.

Patterning-induced strain relief in single lithographic SiGe nanostructures studied by nanobeam x-ray diffraction.

The continued downscaling in SiGe heterostructures is approaching the point at which lateral confinement leads to a uniaxial strain state, giving high enhancements of the charge carrier mobility. Investigation of the strain relaxation as induced by the patterning of a continuous SiGe layer is thus of scientific and technological importance. In the present work, the strain in single lithographically defined low-dimensional SiGe structures has been directly mapped via nanobeam x-ray diffraction. We found that the nanopatterning is able to induce an anisotropic strain relaxation, leading to a conversion of the strain state from biaxial to uniaxial. Its origin is fully compatible with a pure elastic deformation of the crystal lattice without involving plastic relaxation by injection of misfit dislocations.

Ordered Arrays of SiGe Islands from Low-Energy PECVD.

SiGe islands have been proposed for applications in the fields of microelectronics, optoelectronics and thermoelectrics. Although most of the works in literature are based on MBE, one of the possible advantages of low-energy plasma-enhanced chemical vapor deposition (LEPECVD) is a wider range of deposition rates, which in turn results in the possibility of growing islands with a high Ge concentration. We will show that LEPECVD can be effectively used for the controlled growth of ordered arrays of SiGe islands. In order to control the nucleation of the islands, patterned Si (001) substrates were obtained by e-beam lithography (EBL) and dry etching. We realized periodic circular pits with diameters ranging from 80 to 300 nm and depths from 65 to 75 nm. Subsequently, thin films (0.8-3.2 nm) of pure Ge were deposited by LEPECVD, resulting in regular and uniform arrays of Ge-rich islands. LEPECVD allowed the use of a wide range of growth rates (0.01-0.1 nm s(-1)) and substrates temperatures (600-750°C), so that the Ge content of the islands could be varied. Island morphology was characterized by AFM, while µ-Raman was used to analyze the Ge content inside the islands and the composition differences between islands on patterned and unpatterned areas of the substrate.

Photoluminescence Study of Low Thermal Budget III-V Nanostructures on Silicon by Droplet Epitaxy.

We present of a detailed photoluminescence characterization of high efficiency GaAs/AlGaAs quantum nanostructures grown on silicon substrates. The whole process of formation of the GaAs/AlGaAs active layer was realized via droplet epitaxy and migration enhanced epitaxy maintaining the growth temperature ≤350°C, thus resulting in a low thermal budget procedure compatible with back-end integration of the fabricated materials on integrated circuits.

Ge-rich islands grown on patterned Si substrates by low-energy plasma-enhanced chemical vapour deposition.

Si(1-x)Ge(x) islands grown on Si patterned substrates have received considerable attention during the last decade for potential applications in microelectronics and optoelectronics. In this work we propose a new methodology to grow Ge-rich islands using a chemical vapour deposition technique. Electron-beam lithography is used to pre-pattern Si substrates, creating material traps. Epitaxial deposition of thin Ge films by low-energy plasma-enhanced chemical vapour deposition then leads to the formation of Ge-rich Si(1-x)Ge(x) islands (x > 0.8) with a homogeneous size distribution, precisely positioned with respect to the substrate pattern. The island morphology was characterized by atomic force microscopy, and the Ge content and strain in the islands was studied by µRaman spectroscopy. This characterization indicates a uniform distribution of islands with high Ge content and low strain: this suggests that the relatively high growth rate (0.1 nm s(-1)) and low temperature (650 °C) used is able to limit Si intermixing, while maintaining a long enough adatom diffusion length to prevent nucleation of islands outside pits. This offers the novel possibility of using these Ge-rich islands to induce strain in a Si cap.

Gate-controlled rectifying barrier in a two-dimensional hole gas.

The current flowing in a homogeneous low-dimensional conductor is shown to be rectified by a gate-controlled asymmetric barrier resembling a Schottky barrier. The barrier shape is set by varying the potential along a nanofabricated nonequipotential gate which allows simple external control over the device function independent of material properties. A forward-to-reverse current ratio of more than 10(4) is obtained. The merits of diodes fabricated in this way with respect to conventional diodes are discussed.