N. Pauc

Surface recombination velocity measurements of efficiently passivated gold-catalyzed silicon nanowires by a new optical method.

The past decade has seen the explosion of experimental results on nanowires grown by catalyzed mechanisms. However, few are known on their electronic properties especially the influence of surfaces and catalysts. We demonstrate by an optical method how a curious electron-hole thermodynamic phase can help to characterize volume and surface recombination rates of silicon nanowires (SiNWs). By studying the electron-hole liquid dynamics as a function of the spatial confinement, we directly measured these two key parameters. We measured a surface recombination velocity of passivated SiNWs of 20 cm s(-1), 100 times lower than previous values reported. Furthermore, the volume recombination rate of gold-catalyzed SiNWs is found to be similar to that of a high-quality three-dimensional silicon crystal; the influence of the catalyst is negligible. These results advance the knowledge of SiNW surface passivation and provide essential guidance to the development of efficient nanowire-based devices.

Micro-ultracapacitors with highly doped silicon nanowires electrodes

Highly n-doped silicon nanowires (SiNWs) with several lengths have been deposited via chemical vapor deposition on silicon substrate. These nanostructured silicon substrates have been used as electrodes to build symmetrical micro-ultracapacitors. These devices show a quasi-ideal capacitive behavior in organic electrolyte (1 M NEt4BF4 in propylene carbonate). Their capacitance increases with the length of SiNWs on the electrode and has been improved up to 10 μFcm−2 by using 20 μm SiNWs, i.e., ≈10-fold bulk silicon capacitance. This device exhibits promising galvanostatic charge/discharge cycling stability with a maximum power density of 1.4 mW cm−2.

Direct quantification of gold along a single Si nanowire.

The presence of gold on the sidewall of a tapered, single silicon nanowire is directly quantified from core-level nanospectra using energy-filtered photoelectron emission microscopy. The uniform island-type partial coverage of gold determined as 0.42+/-0.06 (approximately 1.8 ML) is in quantitative agreement with the diameter reduction of the gold catalyst observed by scanning electron microscopy and is confirmed by a splitting of the photothresholds collected from the sidewall, from which characteristic local work functions are extracted using a model of the full secondary electron distributions.

1.9% bi-axial tensile strain in thick germanium suspended membranes fabricated in optical germanium-on-insulator substrates for laser applications

High tensile strains in Ge are currently studied for the development of integrated laser sources on Si. In this work, we developed specific Germanium-On-Insulator 200 mm wafer to improve tolerance to high strains induced via shaping of the Ge layers into micro-bridges. Building on the high crystalline quality, we demonstrate bi-axial tensile strain of 1.9%, which is currently the highest reported value measured in thick (350 nm) Ge layer. Since this strain is generally considered as the onset of the direct bandgap in Ge, our realization paves the way towards mid-infrared lasers fully compatible with CMOS fab technology.

Recombination Dynamics of Spatially Confined Electron−Hole System in Luminescent Gold Catalyzed Silicon Nanowires

We study by time-resolved low temperature photoluminescence (PL) experiments of the electronic states of silicon nanowires (SiNWs) grown by gold catalyzed chemical vapor deposition and passivated by thermal SiO(2). The typical recombination line of free carriers in gold-catalyzed SiNWs (Au-SiNWs) is identified and studied by time-resolved experiments. We demonstrate that intrinsic Auger recombination governs the recombination dynamic of the dense e-h plasma generated inside the NW. In a few tens of nanoseconds after the pulsed excitation, the density of the initial electronic system rapidly decreases down to reach that of a stable electron-hole liquid phase. The comparison of the PL intensity decay time of Au-SiNWs with high crystalline quality and purity silicon layer allows us to conclude that the Au-SiNW electronic properties are highly comparable to those of bulk silicon crystal.

Tensile Strained Germanium Nanowires Measured by Photocurrent Spectroscopy and X-ray Microdiffraction

Applying tensile strain in a single germanium crystal is a very promising way to tune its bandstructure and turn it into a direct band gap semiconductor. In this work, we stress vapor-liquid-solid grown germanium nanowires along their [111] axis thanks to the strain tranfer from a silicon nitride thin film by a microfabrication process. We measure the Γ-LH direct band gap transition by photocurrent spectrometry and quantify associated strain by X-ray Laue microdiffraction on beamline BM32 at the European Synchrotron Radiation Facility. Nanowires exhibit up to 1.48% strain and an absorption threshold down to 0.73 eV, which is in good agreement with theoretical computations for the Γ-LH transition, showing that the nanowire geometry is an efficient way of applying tensile uniaxial stress along the [111] axis of a germanium crystal.

Accurate strain measurements in highly strained Ge microbridges

Ge under high strain is predicted to become a direct bandgap semiconductor. Very large deformations can be introduced using microbridge devices. However, at the microscale, strain values are commonly deduced from Raman spectroscopy using empirical linear models only established up to e100 = 1.2% for uniaxial stress. In this work, we calibrate the Raman-strain relation at higher strain using synchrotron based microdiffraction. The Ge microbridges show unprecedented high tensile strain up to 4.9% corresponding to an unexpected Δω = 9.9 cm−1 Raman shift. We demonstrate experimentally and theoretically that the Raman strain relation is not linear and we provide a more accurate expression.

Structural and optical properties of 200 mm germanium-on-insulator (GeOI) substrates for silicon photonics applications

Integrated laser sources compatible with microelectronics represent currently one of the main challenges for silicon photonics. Using the Smart CutTM technology, we have fabricated for the first time 200 mm optical Germanium-On-Insulator (GeOI) substrates which consist of a thick layer of germanium (typically greater than 500 nm) on top of a thick buried oxide layer (around 1 µm). From this, we fabricated suspended microbridges with efficient Bragg mirror cavities. The high crystalline quality of the Ge layer should help to avoid mechanical failure when fabricating suspended membranes with amounts of tensile strain high enough to transform Ge into a direct bandgap material. Optical GeOI process feasibility has successfully been demonstrated, opening the way to waferscale fabrication of new light emitting devices based on highly-tensely strained (thanks to suspended membranes) and/or doped germanium.

Lattice strain and tilt mapping in stressed Ge microstructures using X-ray Laue micro-diffraction and rainbow filtering

Micro-Laue diffraction and simultaneous rainbow-filtered micro-diffraction were used to measure accurately the full strain tensor and the lattice orientation distribution at the sub-micron scale in highly strained, suspended Ge micro-devices. A numerical approach to obtain the full strain tensor from the deviatoric strain measurement alone is also demonstrated and used for faster full strain mapping. We performed the measurements in a series of micro-devices under either uniaxial or biaxial stress and found an excellent agreement with numerical simulations. This shows the superior potential of Laue micro-diffraction for the investigation of highly strained micro-devices.

Ultra-high amplified strain on 200 mm optical Germanium-On-Insulator (GeOI) substrates: towards CMOS compatible Ge lasers

Currently, one of the main challenges in the field of silicon photonics is the fabrication of efficient laser sources compatible with the microelectronic fabrication technology. An alternative to the complexity of integration of group III-V laser compounds is advancing from high tensile strains applied to germanium leading to improved emission properties by transforming the material from an indirect to a direct bandgap semiconductor. Theory predicts this transformation occurs at around 4.7% uniaxial tensile strain or 2.0% bi-axial tensile strain. Here, we report on ultrahigh strains obtained by amplifying the residual strain from novel optical Germanium-On-Insulator (GeOI) substrates fabricated by Smart CutTM technology and patterned with micro-bridges and micro-crosses. The high crystalline quality of the GeOI layers dramatically declined the mechanical failure limits when liberating the Ge microbridges. Record level Raman shift of 8.1 cm-1 for biaxial (micro-crosses) and 8.7 cm-1 for uniaxial stress (micro-bridges) were reached by carefully designing the geometry of the micro-structures. The photoluminescence (PL) evolution is compared to theoretical calculations based on the tight-binding model revealing a detailed understanding of the influence of strain on the germanium optical properties.