A. Bernardi

On-chip Si/SiOx microtube refractometer

The authors fabricate rolled up microtubes consisting of Si/SiOx on Si substrate and analyze the possibility to use them as a refractometric sensor. An aqueous sugar solution is inserted into the microtube, which leads to a change in refractive index and, as a result, to a detectable spectral shift of the whispering gallery modes. Experimental results can fit well with finite-difference time-domain simulations, which are used to determine the sensitivity of this tube refractometer. The ratio of spectral sensitivity to channel cross-sectional area of the refractometer is particularly striking and allows analysis of fluid volumes in the range of femtoliters. A comparative discussion with other existing refractometer schemes concludes this work.

Composition dependence of the phonon strain shift coefficients of SiGe alloys revisited

By combining Raman scattering from the cleaved edge and under hydrostatic pressure, we have accurately determined the tetragonal phonon deformation potentials of strained Si1−xGex alloys in the entire compositional range for the Ge-like, Si-like, and mixed Si–Ge optical modes. A known biaxial strain is induced on thin alloy layers by pseudomorphic epitaxial growth on silicon and subsequent capping. We also determine the strain shift coefficient of the three modes, which are essentially independent of Ge content between 0.4 and 1. This is key information for an effective use of Raman scattering as strain-characterization tool in SiGe nanostructures.

Probing residual strain in InGaAs/GaAs micro-origami tubes by micro-Raman spectroscopy

We have experimentally investigated nanostructures consisting of free-standing microtubes with diameters in the micrometer range fabricated by rolling-up InGaAs∕GaAs bilayers grown by molecular-beam epitaxy on a GaAs substrate. The formation of the microtubes is powered by the built-in strain in the InGaAs layer and they develop after releasing the bilayer structure from the substrate by selective etching. Through micro-Raman spectroscopy we were able to detect the residual strain of the microtube, which results in a frequency shift of phonon modes measured on the tube as compared with reference unstrained material. We developed a simple elastic model to describe the measured phonon frequency shifts, from which we estimate the strain status of the microtube. Results demonstrate the power of Raman spectroscopy as a diagnostic tool for engineering of strain-driven self-positioning microelectromechanical systems.

Cross-plane thermal conductivity reduction of vertically uncorrelated Ge∕Si quantum dot superlattices

A drastic reduction in temperature dependent cross-plane thermal conductivity κ⊥ occurs in Ge quantum dot superlattices (QDSLs), depending on the vertical correlation between dots. Measurements show at least a twofold decrease of κ⊥ in uncorrelated dot structures as compared to structures with the same Si spacer of 20nm but good vertical dot alignment. The observed impact of disorder on the conductivity provides an alternative route to reduce the thermal conductivity of QDSLs. The results of this work have implications for the development of highly efficient thermoelectric materials and on-chip nanocooling devices.

Phonon pressure coefficient as a probe of the strain status of self-assembled quantum dots

The built-in strain in self-assembled quantum dots has large impact on their physical properties, but both its average value and degree of anisotropy are often unknown. The authors demonstrate that the pressure coefficient of optical phonons might be used as probe for the strain status of the dots. This method was applied to the case of Ge dots capped with Si layers of different thicknesses. The authors observe a transition from a strictly biaxial stress situation for uncapped dots to a status of quasihydrostatic strain for cap-layer thicknesses larger than a critical value of the order of the dot height.

Density control on self-assembling of Ge islands using carbon-alloyed strained SiGe layers

The authors show that by deposition of 0.1 ML of carbon prior to the self-assembled growth of Ge quantum dots on a strained Si1−xGex buffer layer a striking decrease in dot density by two orders of magnitude from about 1011to109cm−2 occurs when the Ge content of the buffer layer increases from 0% to 64%. Their results give experimental evidence for a kinetically limited growth mechanism in which Ge adatom mobility is determined by chemical interactions among C, Si, and Ge. Thus, by adjusting the Ge content of the SiGe buffer layer onto which a carbon submonolayer is deposited they are able to fine tune the density of the carbon-induced Ge quantum dots.

Evolution of strain and composition during growth and capping of Ge quantum dots with different morphologies

We follow the growth of islands with different shapes by monitoring the strain relaxation by reflection high energy electron diffraction (RHEED). Comparing a bimodal ensemble of pyramids and domes with a monomodal distribution of C-induced domes, we observe different relaxation pathways and a growth mode change from Stranski‐Krastanow to Volmer‐Weber. We also study the changes induced by the capping process with Si. Small strains in thin cap layers are revealed by spectroscopic ellipsometry. Raman spectroscopy is employed to probe the built-in strain and silicon intermixing in different types of islands, evidencing that smaller islands are enriched in Si and effectively recompressed, whereas bigger relaxed dots remain substantially unaffected. (Some figures in this article are in colour only in the electronic version)

Numerical investigation of optical response from rolled-up microtube resonator and its application

Summary form only given. We preformed a numerical investigation of the optical mode in rolled-up microtube resonators. The optical responses of the self-rolling semiconductor-based resonators are calculated by solving Maxwellpsilas equations using the finite-difference time-domain (FDTD) method. Observed mode patterns characterized by whispering-gallery-mode-like profile are presented with various sets of structural parameters (tube diameter, tube wall thickness, and the number of rotation). The degeneracy breaking results in splitting of the mode peaks with the same azimuthal mode index is observed and we confirmed that it causes by structural asymmetry of the rolled-up structure. For the rolled-up tube with thick tube wall, the quasi-whispering gallery mode pattern due to very high degree of structural asymmetry is shown. In this presentation, we also propose an application of the rolled-up microtube as a refractometric sensor. For this proposal, we experimentally fill the rolled-up tube with liquid and measure the mode peaks shift due to the change of refractive index inside the tube. A FDTD simulation with realistic structural parameters fits well with the experimental spectra. The field shows more leaky to the inner part of the tube due to the increase of the refractive index. High spectral sensitivity is obtained from the tube with thin-wall structure at the long wavelength peak with low azimuthal mode index. This is due to the large overlap between the leakage field and the liquid for the lower mode index. However, the quality factor considerably drops when the wall is very thin. Rolled-up tube with larger number of rotation has lower sensitivity as effectively similar to the tube with thicker wall.