2012, Rodriguez, R.D., Sheremet, E., Thurmer, D.J., Lehmann, D., Gordan, O.D., Seidel, F., Milekhin, A., Schmidt, O.G., Hietschold, M., Zahn, D.R.T.

Large arrays of multifunctional rolled-up semiconductors can be mass-produced with precisely controlled size and composition, making them of great technological interest for micro- and nano-scale device fabrication. The microtube behavior at different temperatures is a key factor towards further engineering their functionality, as well as for characterizing strain, defects, and temperature-dependent properties of the structures. For this purpose, we probe optical phonons of GaAs/InGaAs rolled-up microtubes using Raman spectroscopy on defect-rich (faulty) and defect-free microtubes. The microtubes are fabricated by selectively etching an AlAs sacrificial layer in order to release the strained InGaAs/GaAs bilayer, all grown by molecular beam epitaxy. Pristine microtubes show homogeneity of the GaAs and InGaAs peak positions and intensities along the tube, which indicates a defect-free rolling up process, while for a cone-like microtube, a downward shift of the GaAs LO phonon peak along the cone is observed. Formation of other type of defects, including partially unfolded microtubes, can also be related to a high Raman intensity of the TO phonon in GaAs. We argue that the appearance of the TO phonon mode is a consequence of further relaxation of the selection rules due to the defects on the tubes, which makes this phonon useful for failure detection/prediction in such rolled up systems. In order to systematically characterize the temperature stability of the rolled up microtubes, Raman spectra were acquired as a function of sample temperature up to 300°C. The reversibility of the changes in the Raman spectra of the tubes within this temperature range is demonstrated.

2012, Picco, A., Bonera, E., Pezzoli, F., Grilli, E., Schmidt, O.G., Isa, F., Cecchi, S., Guzzi, M.

In this work, we present an experimental procedure to measure the composition distribution within inhomogeneous SiGe nanostructures. The method is based on the Raman spectra of the nanostructures, quantitatively analyzed through the knowledge of the scattering efficiency of SiGe as a function of composition and excitation wavelength. The accuracy of the method and its limitations are evidenced through the analysis of a multilayer and of self-assembled islands.

2012, Plumhof, J.D., Trotta, R., Rastelli, A., Schmidt, O.G.

Deterministic sources of polarization entangled photon pairs on demand are considered as important building blocks for quantum communication technology. It has been demonstrated that semiconductor quantum dots (QDs), which exhibit a sufficiently small excitonic fine structure splitting (FSS) can be used as triggered, on-chip sources of polarization entangled photon pairs. As-grown QDs usually do not have the required values of the FSS, making the availability of post-growth tuning techniques highly desired. This article reviews the effect of different post-growth treatments and external fields on the FSS such as thermal annealing, magnetic fields, the optical Stark effect, electric fields, and anisotropic stress. As a consequence of the tuning of the FSS, for some tuning techniques a rotation of the polarization of the emitted light is observed. The joint modification of polarization orientation and FSS can be described by an anticrossing of the bright excitonic states.